Chapter 17 Applications to Process Control Analysis

Chapter 17 Applications to Process Control Analysis

689 CHAPTER 1 7 APPLICATIONS TO PROCESS CONTROL ANALYSIS TABLE OF CONTENTS Introduction ........................... 1. The Analytical Unit ...

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689

CHAPTER 1 7

APPLICATIONS TO PROCESS CONTROL ANALYSIS TABLE OF CONTENTS Introduction

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1. The Analytical Unit

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b. The Pneumatic System

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2. The Control Unit

I I. Methodology . . . . . . . . 1. Selection of the Expe 2. Determination of the

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an Analysis

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4. Selection of the Deferred Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............................... 5. Calibration ..................... 6. Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Sample Probe with a Lateral Slit . . . . . . . . . . . . . . . . . . . . . . . . .......... c. Long Distance Transfer of the Sample .................... .......... d. Sample Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... 7. Location of the Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................................... 8. Start-up of the Analyzer . . . . . . . 9. Maintenance Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............. 111. The Deferred Standard Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Present Status of Process Control Gas Chro y ........................ a. Reliability ..................... ......................... b. Credibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c. Response Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... .............................. d. Maintenance . . . . . . . . . . . . . . . . .................................. e. Conclusion . . . . . . . 2. Definition of the Deferr rd . . . . . . . . . . . . . . . . 3. Implementation of the Deferred St ................ 4. Alarm Function of the Deferred St ................ a. PrimaryAlarm.. ........................ b. SecondaryAlarm . . . . . . . . . . . . ............................... .......... 5. Calibration Function of the Deferred Standard . . . . . . . . . . . . . . . . . 6. Predictive Maintenance with the Deferred Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... 7. Advantages of the Deferred Standard . . . . . . . . . . . . . . . . . . . ............... 8. Applications of the Deferred Standard Functions . . . . . . . . . . 9. Integration of the Analyzer in the Workshop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............................... 10. Conclusion . . . . . . . . . . . . . . . . . . . IV. Examples of On-Line Industrial Analyses . ............................... 1. Analysis of Hydrogen in a Catalytic Reforming Process . . . . . . . . . . . . . . . . . . . 2. Synthesis of Vinyl Chloride ...................... ................... a. Experimental Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. ColumnLifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

694 694 696 696 696 697 698 699 701 702 702 702 703 704 704 705 705

708 708 709 713 713 713 714 716 716 718 718 720 720 720

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c. Gascircuit ..................................................... d. LkferredStandard ................................................ e. Calibration ..................................................... f. SampleLine ..................................................... g. Results ........................................................ 3. Analysis of the Gas Evolved During a Chloration Process ....................... 4. Analysis of Gaseous Ammonia. ......................................... 5. Synthesisof Phthalic Anhydride. ........................................ 6. Analysis of Recycled Styrene ........................................... 7. Airborne Pollution Analysis in a Polymerization Plant ......................... 8. Analysis of Chloral .................................................. 9. On-Line Control of a DichlorodifluoromethaneProcess ........................ 10. Conclusion ....................................................... Literature Cited ..........................................................

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INTRODUCTION

Once the qualitative composition of the analyte has been elucidated, the separation scheme has been determined and the calibration completed, routine quantitative analysis may be performed. This may be done either in the laboratory or in the plant, using a process control analyzer. In both cases, the requirements of proper production control have major consequences on the implementation of the analysis. They are more drastic in the case of process control analysis. This last chapter deals essentially with the problems encountered when designing and operating an on-line chromatographic analysis. The methodology and part of the procedures may also be applied to routine laboratory analyses. Process control chromatographic analysis is very popular and extremely useful in petroleum refineries, petrochemical plants and most plants manufacturing fine chemicals, where it has been used for nearly 30 years. It has been discussed in detail in general papers dealing with industrial analyzers (1-4), in specialized papers (5-15) and in a few fundamental papers dealing with the specific problems of on-line gas chromatography, notably those of Pine (16), McWilliam (17) and Martin (18). I. DESCRIPTION OF AN ON-LINE PROCESS GAS CHROMATOGRAPH

Unlike laboratory gas chromatographs, which are usually integrated as a single unit, process control gas chromatographs are made of three different modules (see Figure 17.1): - the analytical unit, located at the plant site, - the control unit, and - the recording unit, or data display unit. These last two units are located in the control room.

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Figure 17.1. Schematic of an on-line process gas chromatograph. AU, analytical unit (with its three modules, oven, pneumatic circuit and electrical and electronic compartment). CU, control unit. RU, recording or display unit.

1. The Analytical Unit

The analytical unit incorporates the gas chromatograph proper, but with a design which is very different from that of a conventional laboratory instrument. It includes: - the temperature controlled oven, including the injection, the separation and the detection systems, - the pneumatic system, - the electronics. a. The Oven

The oven contains the sampling system (for gas or liquid samples), the column(s), the switching valves and an explosion-proof model of the detector. The oven is designed to easily accommodate several columns and valves and its size permits easy access to them for maintenance. The oven is designed to operate isothermally. It can be simple, well insulated, with a rather large thermal inertia. It does not usually include a fan or blower. This simplifies maintenance, but requires a very homogeneous thermal insulation of the walls to limit the temperature gradient in the oven. The standard temperature range extends from 50 to 250 O C. Temperature control at lower temperatures, between 0 and 5OoC,requires the use of a vortex fed with compressed air. Columns have too short a lifetime above 250 O C to justify the use of such temperatures. Temperature programming has not been successfully implemented in process control analysis, because of a lack of reliability and an excessive complexity requiring too much maintenance. It is difficult to achieve exactly the same temperature at the beginning of each cycle, which prevents the achievement of the required References on p. 739.

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level of repeatability, and the stationary phase does not tolerate frequent thermal shocks, which reduce the column lifetime. b. The Pneumatic System

An automatic gas chromatograph requires several sources of gases at controlled pressure: main carrier gas stream, auxiliary carrier gas, e.g., for rapid backpurging, hydrogen and air for the flame ionization detector, servo-air delivery to the solenoid valves which actuate the switching and sampling valves. In process control analysis, pressure control is adequate. Since temperature programming is not used and flow rate programming is too sophisticared, flow rate control is superfluous. c. The Electronics

This explosion-proof unit contains the various electrical and electronic circuits required for the temperature control of the oven (power supply to heat the oven, temperature sensor and control device), the temperature control of the sampling valve, the operation of the detector (power supply, bridge (TCD), electrometer (FID), impedance transformer, etc.), and the commands of the solenoid valves. 2. The Control Unit

The purpose of this unit is to program the cycle of operations during one analysis and its indefinite repetition. Its sends continuous voltages or pulses to the different subunits, usually solenoid valves, but also detector controls, at predetermined times, to order appropriate action. The design of this unit has changed considerably since the construction of the first process gas chromatographs. Originally electromechanical (rotary cams or disks actuating microswitches, punched tapes read by photoelectric cells, etc.), it incorporated more and more electronic circuits, then digital electronic boards. Now, the control unit is organized around a microcomputer. It is easily programmable, more reliable and much faster. Progress in this area as well as in the design of fast gas chromatographic columns permit the achievement of very fast analysis in process control, if needed. The control unit permits the achievement of the proper analytical cycle: automatic zero of the detector base line, connection of the sampling valve to the appropriate sampling line, filling of the sample loop, sampling, column commutation, injection of a deferred standard, adjustment of the detector signal for proper, on-scale display, integration of the detector signal corresponding to the key components of the mixture, and injection of a calibration mixture when needed. Finally, the control unit calculates the composition of the sample and sends the analysis report to the display unit and to a central computer for further decision in the closed loop control, if required, and possibly for archiving. A new task is progressively implemented on the control unit: the self-control of the analyzer (see below).

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3. The Data Display Originally a potentiometric recorder, the display unit is increasingly replaced by a monitor. The chromatogram is displayed as such or in a condensed format, the base line segments between successive peaks not being recorded, or as a bar graph, or as a trend recording, each peak being represented by a point corresponding to its maximum height (see Figure 17.2). The development of large computers as the heart of the control room permits the use of sophsticated programs resulting in more informative displays, making the task of the operators easier. For example, the computer may determine and display the difference between the current analysis and the previous one(s), illustrating trends, or between the current analysis and a typical or standard analysis, showing the difference between the composition of the product and the ideal specifications. It may as well display an OK message as long as the product is within the specifications and warn the operator when an adverse trend is developing. The basic requirement here is to avoid the display of useless information, resulting in an overloading of the mind of the operator. Gas chromatography easily produces a large amount of information, most of which is irrelevant as long as the plant works correctly .

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Figure 17.2. Schematic of the different modes of display of chromatographic data. 1 . Regular chromatograms, too complex for the plant operators. 2, Condensed chromatograms, still too complex. 3, Bar graph record of each peak. 4, Trend lines, displaying only the summit of each peak.

References o n p. 139.

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11. METHODOLOGY

The proper implementation of an industrial analysis requires the prior completion of a certain number of tasks. These tasks are to be carried out under the joint responsibility of the chromatographer (department of analysis) and his colleagues in the department of plant control. Close cooperation between members of the two groups is required for the achievement of satisfactory performance. These tasks are listed in the order of increasing involvement of the regulation people (and decreasing involvement of the chromatographer). - Study of the conditions permitting a proper separation of the key components of the sample. - Determination of the column lifetime. - Design and operation of a column system permitting a satisfactory analysis (including column commutation if needed). - Selection of the deferred standard and of the proper time for its injection. - Quantitative calibration. Determination of the response factors of the components of the analyte relative to the deferred standard, or preparation of the required calibration mixtures or purchase of these mixtures. - Determination of the best method for the sampling of the product stream to be analyzed. - Check that the process control gas chromatograph works under laboratory conditions. - Start-up of the process gas chromatograph in the plant. - Maintenance evaluation. Writing the maintenance manual and the final report. 1. Selection of the Experimental Conditions for an Analysis

This first step requires the identification of all the components of the analyte, those whch are normally present as well as those which may appear in the case of some expectable malfunctions of the plant unit. This is usually an easy task, the process and the feed being known. Then a column or a column system permitting a resolution of the key components, whose concentration in the plant effluent has to be determined must be found (see Chapters 6, 7 and 8). The separation must be performed under constant temperature, but several column commutations may be performed. 2. Determination of the Column Lifetime A process control gas chromatograph works continuously, 24 hours per day,

often more than 350 days per year. It performs several analyses per hour, i.e., 20,000 analyses per year or more. It must work with little maintenance and no failure. The selection of the stationary phase cannot be made on its selectivity alone, however important this characteristic is. The column lifetime is a very important feature. It should be at least many months, if possible more than a year. A standard 1,000 hour test should be performed to assess the column lifetime. A

695 TABLE 17.1 Results of a 1000 hour test of a stationary phase* Over the 1000 hours of the test, a maximum deviation of 3% has been observed for absolute retention times, and a maximum change of 0.7% of the relative retention. The 3% fluctuation is probably explained more by fluctuation of parameters of the chromatograph than by a change of the properties of the stationary phase. Since the test is positive, the stationary phase may be used in process gas chromatography. Time

Absolute retention time

(h)

CCI,

240 336 432 552 600 840 936 1104

16.2 16.4 16.4 16.45 16.10 16.55 16.10 16.10

Change (%)

+ 1.20 0 + 0.30 -2.10 + 2.80 - 2.70

0

C6H6

27.3 27.6 27.8 27.9 27.3 27.95 27.15 27.3

Change

+ 1.10 + 0.70

+ 0.35 -2.15

+ 2.40

- 2.90 +0.55

Relative reten tion ('RC6H6/'RcCI,)

1.685 1.683 1.695 1.696 1.695 1.688 1.686 1.695

1,2,3,4,5,6-Hexakis(2-cyano)ethoxyhexane, 10% on 75 m2/g silica (Spherosil).

couple of components of the studied mixture are selected and they are injected periodically on the column, which is kept at the constant, selected temperature, in a process gas chromatograph. Their uncorrected retention times and their resolution are measured and plotted as a function of time, over at least 1,000 hours. The drift should be less than the experimental error (3% on the retention times). Table 17.1 shows typical results obtained for this test. The stationary phase used is 1,2,3,4,5,6-hexakis(2-cyano)ethoxyhexane,coated on silica. Experience shows that a column which passes t h s test has a lifetime of at least several months, often exceeding one year. Since it is imperative that columns used in process control analysis pass the 1,000 hour test, there will be cases where the selected column is not the one which offers the best separation of the mixture or the largest resolution of its components, i.e., the shortest analysis time. This type of compromise is frequent in industrial applications.

3. Design of the Switching Valve System The basic operations have been discussed in Chapter 9. Most process control analyses use a combination of several such operations. All industrial analyses must involve at least a back-purging step, as described by Villalobos et al. as early as 1961 (19), in order to eliminate the heavy components of the mixture analyzed, and avoid the base line drifts which would eventually appear and would interfere in an unpredictable way with quantitative analysis. In most cases, with conventional process control gas chromatographs using packed columns, the backpurging operation is carried out using valves on-line with the column (20,21), not the Deans method (see Chapter 9, Sections IV.1 and IV.2), References on p. 739.

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in spite of the better performance obtained with the latter method, its better suitability to the requirements of process control analysis and the lesser maintenance which is required for the valves used in the Deans implementation. It is interesting to observe, however, that the situation is very different for the instruments using open tubular columns. They are still very few, and we are certainly at the development stage. But all process control gas chromatographs which will operate with open tubular columns will certainly use the Deans method of column switching. The times at which the bands pass through the various valves are easily calculated using the values reported in Tables 9.3-9.6. 4. Selection of the Deferred Standard

This is discussed in more detail in the following sections. 5. Calibration

Calibration problems are discussed in Chapter 14. Conventional methods as well as the deferred standard approach are described and compared. Specific problems of quantitative analysis encountered in process control analysis have been discussed by different authors. Villalobos (22-24) dealt with the selection of the detector; Turner and Crum (25) with calibration methods and the pitfalls to avoid. Smith et al. (26) discussed the specifications for the achievement of a high degree of precision, and Villalobos and Turner (27) compared the use of peak height and peak area. When the gas density balance is used for calibration, it is placed temporarily in the oven, so that the relative response factors may be determined under the exact same experimental conditions that will be used later for the industrial analysis. 6. Sampling

Sampling requires extremely careful attention. Each case requires specific, tailored solutions (28-32). The accuracy of the analytical results much depends on the representativity of the sample. Laboratory simulation of a sample line is almost always doomed to failure and should not be attempted, except as a topic of research. One of the major reasons for this observation is found in the extreme difficulties encountered in scaling up or down the behavior of a system which depends on non-linear phenomena, such as molecular diffusion. Sample lines, sample pretreatment, and sample representativity should be studied at scale 1, in the plant. There are, however, some simple rules to follow (33). The sampling system must: - Deliver a representative sample of the plant stream. - Make the sample compatible with its introduction in the gas chromatograph (e.g., cool it, warm it, compress it, decompress it, etc.). There should be no change in composition of the sample, however, or minor and easily accounted for (e.g., filtering out suspended particulates).

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- Deliver the sample with small delay and very little mixing.

- Recycle to the plant or send to waste the excess sample, to maintain constant flow in the sample line. - Require little maintenance or none at all. The reliability of the results supplied by an on-line analyzer depends very much on the care taken to design and build a suitable sample line.

a.

Procedures

The various operations applied to the sample before introduction into the chromatographic columns are: Operation

Technical solutions

Sampling

Various types of probes Selection of material Backflush of the sample line

Adjustment of temperature

Condensers Coolers Heaters (steam or electric)

Adjustment of pressure

Pressure controllers Compressors Pumps

Sample treatment

Filters, separators Scrubbers, vaporizers Chemical treatment

Adjustment of flow rate

Flow rate controller

Flow meters Valves

Utilities

Tubings, unions, valves Materials Steam, water, servo-air

Safety

Fire proofing Explosion proofing Various protections

For obvious reasons, the selection of the sampling point in the plant should be decided by common agreement among all the interested parties: plant manager, chemical engineers, control specialists and analysts. The sampling probe should be located at a place where the sample is representative of the process, and preferably in a place where the stream is highly turbulent. Sometimes a primary loop has to be installed to ensure a short response time for the sampling system. Finally the probe should be designed so that its maintenance and its cleaning are easy. Often industrial streams contain a wide range of components, from gases to tars. Some of these components may condense somewhere in the probe and plug it. Various technical solutions have been described and used. They are not always completely satisfactory. References on p. 739.

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Figure 17.3. Schematic of the lateral slit probe for sampling a stream containing tars.

b. Sample Probe with a Lateral Slit

In most cases, a concentric probe is used. When the stream to be sampled contains tars or sticky suspended particles, a different probe is used. A small cylindrical tube, with its axis perpendicular to the direction of the stream, is introduced across the effluent tubing. This probe is closed at its end, but open along a slit parallel to its axis. The probe is oriented so that the slit is placed in the direction opposed to the incoming stream (see Figure 17.3). The impinging tar particles tend to stick on the sides of the probe and few of them may penetrate through the slit into the probe, because of their inertia and of the turbulent eddies inside the stream. A periodic backflush of the probe by a suitable gas or liquid, by steam or by a solvent may also help in keeping it clean and unplugged (see Figure 17.4).

c.g.

Figure 17.4. Principle of the method used to clean the sampling line with a dry gas or a steam jet. a. Probe. b, Filter. c, Valve permitting isolation of the sample loop, to let it reach the atmospheric pressure. d. Sampling valve. e, Stop valve, giving access to a stream of dry gas or steam, to periodically clean the sampling line.

699 c. Long Distance Transfer of the Sample

This original idea was tried first by Konrad (34) in 1960. It has permitted a satisfactory solution to very difficult sampling problems, resulting in a considerable reduction of maintenance problems. The idea is to shorten the sampling line whde increasing the length of the transfer line, by placing the sample valve of the gas chromatograph close to the stream to be analyzed, far from the gas chromatograph (35). A schematic is given in Figure 17.5, where the classical solution (sampling valve in the gas chromatograph) is compared to this original approach (sampling valve close to the plant stream). The transfer line is now reduced to a few tens of centimeters (a couple of feet) and can easily be purged automatically after each injection. The price to pay is in the considerably longer transfer line. It results in long injection times but surprisingly not in the injection of wide, diffused bands of sample into the gas chromatograph. The band width is increased and the resolution diminished. Longer, more efficient columns must be used and the detection limits are lugher. Selective adsorption of some components may take place on the wall of the transfer line. A compromise is acheved by using 1 mm i.d. stainless steel or Teflon tubing. The transfer line is heated electrically, or with steam flowing through a concentric tube. The temperature required is lower than it would be for a comparable sampling line, however. Figure 17.6 shows two chromatograms obtained for the same plant stream containing ethylene, ethyl chloride and vinyl chloride. In the first chromatogram a

Figure 17.5. Principle of the long transfer line. A. Conventional technique. The sampling line is long, the transfer line short. The sample is brought to the gas chromatograph. B. Long transfer line. The sampling line is as short as possible. The gas chromatograph inlet is brought close to the process.

References on p. 739.

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Figure 17.6. Example of application of a long transfer line. Analysis of a mixture of ethylene (l), chloroethane(2) and vinyl chloride (3) (see Section IV.2). A, Sampling valve close to the column inlet. B, Sample injected at 38 m from the gas chromatograph.

normal injection is made. In the second one the sampling valve is placed 38 m away from the gas chromatograph. Comparison between the two chromatograms shows a very long delay but very little band broadening, demonstrating the rapid radial diffusion in the 1 mm i.d. transfer tubing. We have found that very long transfer lines may be used and have been unable to determine an upper limit to the length which may reasonably be used in practice. Figure 17.7 shows an analysis of air on a Molecular Sieve 5A column. The use of a 50 m long transfer line does not result in a significant increase of the band width. Note that the volume of the line (1 mm i.d., 50 m long) is 40 mL. The 4a band width of such a tube ( O K with k’ = 0, see p. 105) at the appropriate flow rate (ca 5-10 mL/min) is about 0.4 mL. This is quite a reasonable sample volume for a conventional column and is at least 7 times smaller than the band width of an unretained zone. The most spectacular applications of this method we are aware of are in the

701 Long distance sample valve

Figure 17.7. Comparison between the width of an air peak injected just at column inlet (top), at the inlet of a 50 m long transfer line (middle) and at the inlet of a 140 m long transfer line (bottom).

analysis of the effluents of steam cracking units or of Claus process units, concerning the determination of H,S and SO,. d. Sample Treatment

Compatibility between the sample and the analyzer may need some adjustment of the characteristics of the sample, such as its temperature, pressure, flow rate, water content or some chemical treatment. The composition of the sample should not be altered, but, obviously, in these last two cases this is impossible. The change should be at least predictable. Although some preliminary work can be done in the laboratory, the study of the performance of the sample treatment devices can be carried out usefully only on site. Simulation in the laboratory does not afford a solution which can be trusted and merely amounts to a waste of time, energy and money.

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7. Location of the Analyzer The analytical unit of the process control chromatograph should ideally be located rather near the sampling point. Also, it should be housed in a protected cabin, where fluctuations in ambient conditions during the day and over the seasons are minimized, to ensure a good repeatability of the analytical results. When a plant requires the use of several chromatographs on the same site, a compromise becomes necessary, as for obvious economical reasons it is preferred to locate them in the same cabin, minimizing the costs of investment (electrical cables, cabin, sources of carrier gas, auxiliary gases, steam, servo-air, etc.) and functioning (air conditioning, etc.). The use of long transfer lines becomes a very useful and economical solution (see above).

8. Start-up of the Analyzer It is important that this critical operation be carried out by a team representing the analysts and the control department. They will have to write detailed instructions for proper start-up and stopping procedures of the instrument, for the determination and control of the optimum chromatographic conditions and for the operation and maintenance of the sampling line. 9. Maintenance Evaluation Maintenance evaluation can be done only after the instrument has been working in the plant for a certain time. It is always observed that the failure frequency decreases steadily with increasing time, after the first start-up. The frequency of calibration depends on the process controlled and the specifications, i.e., the importance in having accurate results. It can be daily or weekly. The use of microcomputers in the control unit permits tying the calibration to the occurrence of certain events, such as an analytical result out of specification, or a rapid variation in the concentration of a certain compound. The systematic use of the deferred standard and the proper assessment of its results permit serious economies on the maintenance by helping in scheduling preventive maintenance operations. Maintenance can be drastically reduced by observing some simple rules discovered through long term experience. For example, it is strongly recommended to systematically dry all the gases used in a process control gas chromatograph. Each gas line includes two drying cartridges, packed with a suitable adsorbent and a switching valve, permitting the regeneration of one while the other is operating. Water vapor in the carrier gas results in a slow hydrolysis of many stationary phases and in a slow decrease in the performance of nearly all columns. The response of the TCD varies with the composition of the carrier gas, including its water content. The response of the FID depends to some extent on the water content of the gases fed to it. Villalobos (36) has shown that the concentration of water in the gas delivered by a pressurized cylinder varies considerably with time, increasing steadily with decreasing pressure in the cylinder. As an example, if the pressure inside the cylinder

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is 150 atm, and the water content is 150 ppm, the water content of the gas stream rises to 8,150 ppm when the pressure has decreased to 2 atm. Figure 17.8 shows a plot of the water content of the gas stream as a function of the pressure inside the cylinder (36). 111. THE DEFERRED STANDARD CONCEPT

Designed by Guillemin in 1965, the deferred standard concept aims at controlling the process gas chromatograph itself. It was originally implemented in the on-line control of a unit for the synthesis of vinyl chloride. Since the first publication in 1971 (37), the deferred standard concept has gained wide acceptance in the field of process control analysis and of high precision analysis (38). Despite the recognized capability of the process gas chromatograph and a proven record of reliability, it is always considered suspicious by plant technical management, chemical engineers and even instrumentation engineers when it is incorporated in a closed loop control. Because most of them do not fully understand the operation of what seems to them a rather sophisticated piece of equipment, because the instrument gives results periodically, not continuously and its status at a given time is difficult to figure out by the non-specialist, its results are immediately questioned as soon as they point to another answer than the gut feelings of the plant operators. In order to be acceptable for the use in a closed loop control, an industrial sensor must meet the following fundamental criteria: - Reliability, or high probability that the results are accurate, or long mean time between failures. - Credibility, or confidence in the accuracy of the results. - Continuous response. - Low maintenance cost. References on p. 739.

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We discuss the present level of performance of the process gas chromatograph with regard to these criteria. 1. Present Status of Process Control Gas Chromatography

The process gas chromatograph marginally satisfies the basic requirements of control engineers for its incorporation in a closed loop control. On the other hand, it readily supplies information that no other sensor can. Something must be done to improve the confidence in its results. a. Reliability

Experience has demonstrated that the process gas chromatograph is indeed a very reliable instrument. The problem, however, is that it is very difficult to demonstrate to the satisfaction of the plant manager that the gas chromatograph is working reliably at a given instant, when confidence in unexpected results is vital for the soundness of impending decisions, to be taken for the operation of the plant; decisions which, if unwisely made, may cost huge amounts of money to the company and put the said manager in serious trouble. When the analyzer shows that the composition of the plant stream changes, one wants to be sure that this result is real and not a consequence of a faulty analyzer or a malfunctioning sample line. In conventional instruments the proof that the analyzer is working soundly can be obtained by injecting a calibration mixture, usually once, in difficult cases several times. The complete loss of information on the process behavior during the time necessary to process these calibration samples may be unacceptable in certain circumstances, however, especially when the gas chromatograph is included in a closed control loop. This situation generates justified fear and anxiety in the mind of the plant managers and needless hostility towards the chromatographic sorcerers. 6. Credibility

Confidence in the analytical results of the process gas chromatograph requires a satisfactory answer to the following conditions: - Accurate calibration mixtures must be constantly available. - The analytical results obtained for calibration mixtures must be in agreement with their known composition. - The sample must be representative of the analyzed stream. - The detector must be linear and reliable. The method lacks credibility especially when it is not possible to prepare accurate calibration mixtures, for example in the case of mixtures of ethylene and hydrochloric acid, gases which can coexist at low concentration in a plant effluent, but react in a metal bottle.

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c. Response Time The gas chromatograph is a sequential analyzer. Its response is periodic. Much time and effort has been spent in trying to design chromatographic methods supplying continuous responses, for example by deconvolution of the signal obtained with a pulsed input. Results have not justified the implementation of these methods in industrial practice. Chromatographic analyses are fast. In practice they can be performed in a time sufficiently short for the response to be considered to be semi-continuous, i.e., in most cases it is not very difficult to achieve an analysis time which is about three times shorter than the response time of the controlled unit in the plant. Still faster analyses can be performed, permitting a statistical study of the data.

d. Maintenance Routine, preventive maintenance is typically performed on the control instruments in almost every plant. Scheduled replacement of parts such as sampling and switching valves, columns, etc., is systematically performed at a frequency chosen arbitrarily, often as a function of the importance of the analysis performed. This frequency has not changed while the quality of the parts has improved markedly. Calibration are also performed at constant frequency. Such preventive maintenance is very costly. e. Conclusion This review shows that the conventional process gas chromatograph is not yet really a true process monitor, but a highly sophisticated analyzer. This justifies the frustrations of the plant managers and unit operators. who expect the same kind of reliability and service from a gas chromatograph as they are used to obtain from temperature sensors or infrared absorption analyzers. The implementation of the deferred standard, together with the use of microcomputers permits a definitive solution to this problem.

2. Definition of the Deferred Standard A second injection is performed during each analytical cycle (37-42). This injection is made a constant time after the injection of the sample. The second injection is of a pure compound, a gas or a liquid, chosen to be cheap, stable and easy to elute with a symmetrical peak. The delay between the two injections is chosen so that the peak of the deferred standard is eluted well resolved from all the components of the analyzed mixture. The deferred standard doubles conveniently as the reference compound for quantitative analysis, i.e., it replaces the internal standard (see Chapter 15). Figure 17.9 dramatically illustrates the major advantage of the deferred standard. It is immediately obvious that the gas chromatograph is working fine and the process is going wrong. References on p. 739.

106 WHiCH ONE

iS WRONG: THE P G C

I

I

I

I

1

-+

7 I

THE PC

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Figure 17.9. Illustration of the main application of the deferred standard: “What is going wrong, my nice process or your cursed machine?” (After ref. 42.) A, There is no deferred standard. The answer requires at least one analysis of a calibration mixture. B, The peak of the deferred standard is perfectly stable, the process is drifting, corrective action has to and may be taken immediately: as soon as frame 2, as a matter of fact.

The deferred standard has three functions in process control analysis: an alarm function, informing the operator of the status of the chromatograph, good or bad. It is a self-control function. - a calibration function, permitting ready achievement of accurate results. - a maintenance need function, quickly alerting the operator to the advisability of early service, before anything has gone wrong in the analytical results supplied by the instrument. Together, these functions permit the transformation of the process gas chromatograph into a true industrial sensor (43), as we show in the following four sections. -

3. Implementation of the Deferred Standard In order to minimize the difficulty in placing its band between the peaks of the components of the sample, the deferred standard is preferably a gas with a very small retention, eluted near or at the gas hold-up time. Air and nitrogen are most often used with the TCD and ethylene with the FID. Two different procedures may be used to inject the deferred standard: - a second sampling valve can be introduced in the gas line, upstream of the regular sampling valve, to prevent its accidental or progressive pollution by components of the sample. The standard is then pure and the volume of standard injected is selected to provide a peak having the proper size, depending whether one is interested in trace or main component analysis. - a single sampling valve is used, which alternately injects the sample and the standard. Since the volume injected is the same for both standard and sample, it may be necessary to use as standard a gas diluted in the carrier gas, or preferably in nitrogen, which has the same molecular weight and density and very similar physico-chemical characteristics as ethylene. As a consequence, the mixture of N,

t

DS ( g a s )

S

(liquid)

0

Figure 17.10. Schematic of different possible implementation of the deferred standard. a, One injection valve for the sample, another one for the standard, preferably placed upstream. b, The same valve is used for the sample and the standard. c, The deferred standard is a gas and the sample a liquid. atm indicates that the pressure in the gas loop is brought to atmospheric before the injection is carried out.

References on p. 739.

708

and C,H, behaves as nearly ideal and there is no need for compressibility correction when the mixture is prepared in a compressed gas cylinder. Also the mixture remains homogeneous after long periods of storage in poor weather conditions, another precious property for its use as deferred standard, since the cylinder must not be replaced often, but must supply a gas of constant composition. Various possible combinations are illustrated in Figure 17.10. 4. Alarm Function of the Deferred Standard This is illustrated in Figure 17.9. Figure 17.9A shows the result of four consecutive analyses of a process stream. It is not possible to immediately answer the critical question: “What is wrong, the process or the cursed chromatograph?” A calibration would be necessary to make sure the chromatograph is working properly. This would require at least the time of one sequence, which would be lost before proper action be taken. In Figure 17.9B it is now immediately obvious that the gas chromatograph is working fine but that either the sampling line, or the process is going wrong, since the repeatability of the deferred standard peak is excellent. In fact the second analysis should alert the operator and the third trigger some corrective action. The deferred standard does not check the proper functioning of the sampling line, through which it does not pass. It flows only through the transfer line. In the case when the chromatograph malfunctions, the deferred standard peak size would fluctuate, alerting the operator to its failure and the need for urgent maintenance, an alert which would be triggered as early as the failure appears. Thus the deferred standard is a constant, on-line check of the gas chromatograph. Its availability makes the gas chromatograph much more reliable and trustworthy. In the early implementations, the height of the deferred standard peak was determined and served as a visual basis for decisions by the operator. Things have improved. The area of the peak is measured during each analysis, by the microcomputer used for signal integration (see Chapter 15). The program available to the microcomputer permits a detailed analysis of the characteristics of the peak of the deferred standard, supplying useful information regarding a possible malfunction. Those are called the primary and secondary alarm functions. a. Primary Alarm

The repeatability of the area of the peak of the deferred standard is a measure of the reliability of the results of the process gas chromatograph. If the area of the deferred standard peak is within the range of experimental error, indicated to the microcomputer of the control unit from previous measurements, the gas chromatograph is considered as operative and reliable. The analytical report determined by the control unit of the gas chromatograph is accepted by the plant computer which takes proper action to keep the process working. If the area supplied to the computer is off limits an alarm is triggered, while the computer shifts to the proper alternate subroutine available to it, in order to

709

safeguard the process, the products and if possible continue operation without the information on the composition of the plant stream normally supplied by the gas chromatograph. b. Secondary Alarm

During each analysis the microcomputer of the control unit of the gas chromatograph measures the following characteristics of the peak of the deferred standard: its retention time, its height and its area. This permits the early detection of potential sources of trouble, while the area of the peak is still within the specifications for satisfactory results. The algorithm of the secondary alarm detection is summarized in Figure 17.11. The deferred standard is identified from its elution time, measured from the begmning of the analytical sequence, i.e., from the injection of the sample, not from its own injection. If no peak is observed during the proper time window an alarm, S a m p l e ,injection

DS

injection

Alarm : detection

1

Calculations carried out

?=*

Alarm : injection

Alarm :Separation

Displayed

messages

-

Alarm

D S not found

(END

)

Figure 17.11. Schematic of the algorithm used for the alarm functions of the deferred standard (44).

References on p. 739.

710

I

L T N

I

4 I

I I

Figure 17.12. Alarm function of the deferred standard. Definition of the parameters measured for the standard peak (44).

with a message: “Deferred standard not found” is issued. The gas chromatograph is declared inoperative. If the standard is found, its peak area is determined. The peak area is a function of the amount injected and the values of the experimental parameters determining the detector response. Over the years, the reliability of the sampling valves has proven to be excellent, much better than that of the detectors. If the area is off specifications, it is most probably due to a variation of the bridge current, of the detector temperature or of the flow rate for a TCD, a variation of the flame temperature, i.e. of the flow rates of air or hydrogen for a FID. An alarm with a message “Detection error” is issued and the gas chromatograph is declared inoperative. If the area is within the preset limits the concentrations of the various components are derived from the area of their peaks, the area of the standard peak and the relative response factors. The results are accepted by the plant computer. Then the microcomputer calculates the height of the deferred standard peak. Important variations of the peak height are sometimes observed, up to 50%, when the peak area changes by a few percent or less (see Chapter 13). They are due to faulty injection, such as a drop in the temperature of the vaporizer or in the pressure of the servo-air of the pneumatic sampling valve. Finally, fluctuations of the carrier gas flow rate cause changes in the retention times. Since the deferred standard has been identified, the pneumatic system is operating satisfactorily. Early signs of wear or other malfunction may result in short term, small fluctuations. Erratic fluctuations of the column temperature will have the same effect. These fluctuations may be detected by measuring the time interval between two closely eluted peaks (see Figure 17.12).

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-

I.

u,

Figure 17.13. BASIC program used to implement the alarm function of the deferred standard on a programmable integrator (44). The instruction "RUN" initializes the program. The operator is asked the reference of the analysis, the peak rank of the deferred standard, its concentration, the rank and name of each peak, the response factor relative to the standard, the upper and lower limits for the area, height and retention time of the standard and for the distance to the next peak, TN. An example is given in Section 111.8 and Figure 17.14.

References on p. 739.

712

TABLE 17.2 Alarm Function of the Deferred Standard Failure Diagnosis (44) TDS

0 1

S

H

TN

Actions

-

Alarm “DS not found”. Calculations are not made. Good working order - Concentrations are calculated with the DS used as standard. Good working order Calculations are made. Caution, “injection function”. G o d working order Calculations are made. Caution, “injection, separation functions”. Good working order - Calculations are made. Caution, “separation function”. Alarm - Calculations are not made. Failure on detection and injection systems. Alarm Calculations are not made. Failure on detection and separation system. Alarm Calculations are not made. Failure on detection and injection systems. Alarm Calculations are not made. Failure on detection, injection, and separation systems.

-

1 1 1

-

The diagnosis of the process gas chromatograph is carried out after each analysis, combining the results of all these measurements. A warning message is issued if the deferred standard peak is identified and has the proper area but its height or its distance to a close neighbor are out of specifications. Figure 17.11 and Table 17.2 explain the various combinations possible. A BASIC program (44),written for a Spectra-Physics4100 integrator, is given in Figure 17.13. This integrator was used to simulate a process gas chromatograph control unit in the laboratory. The determination of other characteristics of the deferred standard peak, such as its width at half height, its efficiency, its asymmetry, the resolution with a closely eluted peak permit more refined diagnosis of beginning malfunctions, before they have any serious effect. This, in turn, allows a more effective and cheaper maintenance, by avoiding a number of preventive maintenance steps. An example of application of these functions of the deferred standard is given later, Section 111.8. As with any other automatic system, the deferred standard is not a perfect zero-risk solution. The excellent reliability of microcomputers and of gas sampling valves, however, permits the achievement of an extremely high degree of performance. The method has been accepted by many industrial analysts in a large number of companies. The deferred standard gives an indisputable test of the chromatograph and gives total credibility to analytical results which would otherwise be hotly disputed between analysts and plant engineers. The use of the deferred standard restores confidence between these partners in the operation of the plant.

713

5. Calibration Function of the Deferred Standard This results from a combination of the properties of the deferred standard already described and the use of the gas density balance as a calibration detector (see Chapter 14). This detector permits 2 rapid determination of relative response factors for any compound, using any reference standard. The deferred standard is the ideal reference for quantitative analysis. The use of thz deferred standard as a reference for quantitative analysis, with calibration using the GDB, permits the total elimination of the use of calibration mixtures which are costly and impractical. As a consequence, more frequent calibrations are possible, more accurate results are obtained and the process gas chromatograph credibility is further enhanced. 6. Predictive Maintenance with the Deferred Standard As explained above (Section 111.4), the use of the deferred standard permits more predictive maintenance and less preventive maintenance, resulting in a better reliability of the analytical results and a lower cost. As long as the test results, retention time of the standard, peak area, peak height, retention time difference, and peak width, are within the specifications, it is unnecessary to change parts, to recalibrate the detector or to proceed to other maintenance actions. The drift of one of the secondary parameters out of specifications does not require stopping the chromatograph. Some corrective actions may often be taken and it is possible to delay the required maintenance until a more favorable time. The alarm function of the deferred standard permits savings on the operator time and the cost of calibration gas mixtures. The calibration function of the standard permits additional savings on the preparation or purchase of calibration mixtures which are no longer necessary. The down time of a conventional process gas chromatograph is approximately 150 hours per year. With the use of the deferred standard we have been able to decrease it to approximately 50 hours per year. This reduction and the savings just mentioned combine to make the implementation of the method a very profitable investment. In the worst case, the cost of implementation would be: - a second sampling valve. - the pressure release in the sample loop, to atmospheric, using a solenoid valve (see Chapter 13). - the operation of these valves by the control unit. - one cylinder per year of either air or ethylene.

7. Advantages of the Deferred Standard

The only competition to the deferred standard method is the internal standard method. The automatic preparation of the sample to be injected is extremely costly, however, much more complicated than the injection of a pure compound. In order References on p. 739.

714

TABLE 17.3 Comparison between Internal Standard and Deferred Standard Internal standard

Deferred standard

Principle

Needs addition of known amount of Does not need any addition of com-

Choice

Standard must have a retention time different from all the components of the mixture analyzed. ID is almost always a Liquid.

Apparatus

Needs automatic preparation by ad- Needs 2 additional valves. The DS ding known amounts of IS and sam- does not increase the response time ple and mixing. Increases the re- of the analytical system. sponse time of the analytical system.

Cost of standard

Depends on the compound selected Consumption of one bottle of air, as internal standard. N, or ethylene during one year of operation.

Cost of apparatus

Coefficient 10 (10 times more than Coefficient 1. DS).

a very pure compound in the mix- ponent. Injection of pure standard is deferred by respect to sample injecture to be analyzed. tion. No retention time constraint. Deferred injection allows choice of any gas or liquid compound (air, N,, ethylene, benzene etc.). May or may not be a component of analyte.

to meet the accuracy requirements, it needs a very carefully prepared mixture of sample and standard, which may be nearly impossible with difficult samples. Table 17.3 contains a comparison between the main features of the two methods. The self-control of the analyzer can be performed to some extent by placing pressure and temperature sensors in critical locations of the instrument. The development of microcomputers and of data acquisition techniques certainly permits the use of enough sensors to be able to recognize malfunctions and issue the proper alarms. The system becomes very complex and expensive. By contrast, the deferred standard is a global test, which integrates all the effects of the chromatograph parts on a peak, from injection to detection, and which addresses the reproducibility of the very phenomena that are used to perform the required analysis. Furthermore, a fine analysis of the standard peak shape proves very suitable for troubleshooting investigations, while remaining simple and inexpensive. 8. Applications of the Deferred Standard Functions The analysis of air (oxygen and nitrogen) was performed as a test. One of the great advantages of this sample is that the result is well known in advance, so bias can be determined as well as experimental errors, and it is difficult not to grab a representative sample.

715 NAME

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TIME 18 1 3 : 2 8 : 5 1

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NAME

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106.

.

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I

4

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11176. 110.

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I

107.

GOOD

T"=

1

W O R K I N G ORDER

Figure 17.14. Example of implementation of the deferred standard. Analysis of air: oxygen (1) and nitrogen (2). Standard, nitrogen (3).

716

TABLE 17.4 Quantitative Analysis of Air by Different Methods Theoretical

Concentrationof oxygen in air (average of 10 analyses)

composition of the air

1 Internal normalization of areas 0,(W) 24.48 f 0.80% volume: 1.12 ml

Oxygen + argon 24.27%weight

2 Deferred standard 1 valve S and DS

3 Deferred standard 2 valves S and DS

0 2

0 2

($1

24.41 f 0.40% volume: 1.12 ml

(%)

24.17 & 0.57% volumes: Vol. S : 1.385 ml Vol. DS: 0.264 ml

Peak areas are measured with a computing integrator. Same experimental conditions for the 3 analyses. Elapsed time at atmospheric pressure prior to injection: 30 seconds.

The analysis was performed with the following experimental conditions: Column: i.d., 4 mm. Length: 2 m. Stationary phase: Molecular Sieve 5A, 26/29 mesh. Temperature: 72.5 O C. Carrier gas: helium, 3 L/hour. Detector: TCD,two parallel, direct passage cells, current: 250 mA. Sampling: two different procedures were used successively, with two sampling valves (see Figure 17.10a) and with one single valve (see Figure 17.10b). Pressure released to atmosphere during 30 seconds prior to injection. The threshold limits for the various characteristics of the deferred standard peak (nitrogen) were: peak area 1%;peak height 3%; elution time of the standard 2%; time between the two nitrogen peaks 25%. Table 17.4 gives the results of the quantitative analysis for the two injection modes. Figure 17.14 illustrates the analytical results and shows simplified analytical reports and messages regarding the status of the instrument. 9. Integration of the Analyzer in the Workshop

This task is carried out by the control and regulation department, with the help of the analysts. During its accomplishment, the deferred standard gives precious information regarding the status of the gas chromatograph and the origin of the troubles encountered. It permits a rapid check-up of the entire system, and the separation of problem sources between the three main types: those associated with the gas chromatograph, with the sample line and with the process. 10. Conclusion

Recent development in process control analysis by gas chromatography has focused along the following lines: adoption of the deferred standard and implementation of its different functions, incorporation of powerful microcomputers and sophisticated software, use of faster columns and column systems to accelerate the

717

analysis. The process gas chromatograph has become an industrial sensor, as shown by a comparison between the criteria detailed above (see Section 111) and the solution now adopted: Criteria

Technical Solutions

Reliability

Primary and secondary alarm functions of the deferred standard

Credibility

Calibration function of the deferred standard Fast analysis, permitting an estimate of the precision by averaging the results of several analyses, thus enhancing the credibility

Continuous response

Fast analysis permits a quasi-continuous response

Low maintenance cost

Predictive maintenance function of deferred standard

It should be emphasized at this point that, although the use of fast packed columns can be contemplated on conventional process control gas chromatographs without any significant modification of the analytical unit, this is not true of the short columns made available by modified gas-solid chromatography nor of open tubular columns. For these columns, which could permit analysis times within a minute or so, the entire design of the analytical unit has to be rethought. Work is in progress in this area in several companies, but no system of proven design and performance has yet been described. All control units should use a microcomputer. These devices have performance levels, in terms of speed, memory capacity and reliability that well exceed the requirements of the fastest, most complex gas chromatograph we can imagine. Its main tasks should certainly be the programming of the analysis (injection of sample and deferred standard, valve switching, zero base line, etc.) and the acquisition and handling of the detector signal (calculation of the deferred standard peak characteristics, integration of the sample component peak areas and calculation of the concentrations of these compounds in the monitored stream). But the tasks of the microcomputer should also include the control of the analytical unit (from the characteristics of the peak of the deferred standard) and the preparation of the analytical report for the display unit and the central plant computer. Because the power of the available microcomputers so much exceeds the needs of the process gas chromatograph, the question arises whether it is better to use a dedicated control unit for each process gas chromatograph, or one single control unit for a number of gas chromatographs. There is no general answer. It depends whether safety or cost is considered to be the most important factor. With a dedicated control unit, each process gas chromatograph is autonomous. If one control unit is used for several gas chromatographs, important savings are made, but if it fails, the information supplied by all the chromatographs it controls will be lost. It is certainly unwise to use the central plant computer to control all the gas chromatographs in the plant. This idea, which originated before the appearance of References on p. 739.

718

microcomputers, has proved utopian and dangerous. It has been abandoned. It could be conceivable to have fewer control units than gas chromatographs and to interconnect them, so that if control unit a, which normally works for the gas chromatographs A, B and C, fails, control unit p will take charge of the gas chromatographs A and B and control unit y of the gas chromatograph C (in addition to the gas chromatographs control units p and y already control normally).

IV. EXAMPLES OF ON-LINE INDUSTRIAL ANALYSES We present here several applications, which have been selected from real problems, in order to illustrate the various techniques and concepts presented in the first part of this chapter and emphasize their usefulness, or to describe the serious difficulties encountered by the analyst working in this challenging field, together with the special constraints of process control analysis: reliability, precision, rapidity and low maintenance cost. 1. Analysis of Hydrogen in a Catalytic Reforming Process The hydrogen concentration in these units, which produce high octane gasolines, is high, between 70 and 90%; the rest of the gas contains all hydrocarbons up to hexanes. It is important for a long catalyst life to accurately control the hydrogen concentration. The determination by the analyzer of the hydrogen concentration must be made with an absolute accuracy of 0.4%, i.e., with a relative accuracy on this concentration of 0.5%. Two methods have been tried by Follain (45): - a direct method uses a thermal conductivity detector, with argon as carrier gas, and compares the area of the hydrogen peak to the area of the peak of a deferred hydrogen standard. - an indirect method uses a flame ionization detector to measure the total amount of hydrocarbons in the sample, using methane as a deferred standard, and calculates the concentration of hydrogen by difference. The use of the TCD, whose response depends on the carrier gas flow rate and on the block temperature, and is not linear under the experimental conditions which have to be used, does not allow the required accuracy. On the other hand, the response of the FID is less sensitive to the fluctuations of experimental parameters, is more closely linear, and permits the achievement of an absolute error on the total amount of hydrocarbons which is the same as the absolute error made by the first method on the hydrogen concentration. The relative error of the hydrogen concentration derived by the second method is thus four times smaller. The second method demands, however, that all hydrocarbons be eluted from the column and that an accurate calibration be made. The hydrocarbons must be resolved because their relative responses are too different to permit the use of a single response factor for all of them. Both conditions have been satisfied, the second one because of the use of the deferred standard method (45). A mixture of

719

7

L

Figure 17.15. Quantitative analysis of hydrocarbons in the effluent of a catalytic reforming process. After

Follain (45). Column and experimental conditions, see text. 1, Methane. 2, Ethane. 3, Propane. 4, Isobutane. 5, n-Butane. 6, Isopentane. 7, n-Pentane. 8.2.2-Dimethylbutane. 9, 2-Methylpentane. 10, 3-Methylpentane. 11, n-Hexane. 12, Methylcyclopentane. 13, Benzene. 14,Isoheptane. 15, Isoheptane. 16, Isoheptane. 17, Deferred standard (methane).

hydrogen (90%) and methane (10%)is used and response factors of hydrocarbons relative to methane have been determined. Figure 17.15 shows the typical chromatogram obtained during this analysis, TABLE 17.6 Hydrogen Analysis by Direct* and Indirect** Methods (45) Theoretical composition

Hydrogen Methane Ethane Propane Isobutane n-Butane

79.45 i-0.5 10.70 5.40 2.89 0.53 1.02

Indirect method** (average of 10 runs) vol. % 79.805 f0.15 10.66 f0.07 5.20 f0.06 2.86 f0.045 0.494 f 0.02 0.994 f 0.04

Direct method* (average of 10 runs) vol. % 82.01 f 0.635

TCD, H, as deferred standard. methane as deferred standard (10% CH, in hydrogen). Concentration of H, equals to the balance of the total concentration of hydrocarbons.

** FID,

References on p. 739.

720

exhibiting complete resolution between the mixture components and with the deferred standard. The column used is 6 m long, 1/8 inch i.d.; it is packed with Chromosorb P, coated with 20% squalane. The carrier gas is helium (1.25 L/hour). The column temperature is 100 C. The results obtained by the two methods described above have been compared using a synthetic gas mixture (Air Liquide, Paris, France). The results are reported in Table 17.6.

2. Synthesis of Vinyl Chloride The process effluent stream contains hydrochloric acid, ethylene and vinyl chloride. The analytical difficulties are obvious. It is impossible, for example, to prepare calibration mixtures because ethylene and hydrochloric acid would react rapidly under pressure in a metal cylinder. It is for the solution of these problems that the concept of the deferred standard was first applied and the use of the gas density balance as a calibration detector investigated (37). a. Experimental Conditions

The instrument used is a Car10 Erba (Milan, Italy) Fractomatic. The column used is made of Teflon tubing, 4 mm id., 4.2 m long, divided in two sections (0.80 and 3.40 m long, respectively). It is packed with Teflon 6, coated with 15%SF-96 (General Electric), prepared using the procedure described by Saint-Yriex (46) (see Chapter 6). The column and the packing material are cooled at 0 O C prior to packing, which hardens the support particles, facilitates their handling and packing and avoids destroying the particle structure. The column temperature is 55 O C. The carrier gas is hydrogen, the flow rate 2 L/hour. The detector is a TCD, with serial, semi-diffusion cells, using W X wires (GowMac, Bound Brook, NJ, U.S.A.). The bridge current is 250 mA. The sample volume is approximately 1.5 mL. b. Column Lifetime

The columns easily passed the 1,000 hour test. In the field, the column lifetime proved to exceed 1 year, with continuous (24 hour/day) operation. The column is changed every year. c. Gas Circuit

The gas circuit includes two valves, a sampling valve and a switching valve for back-purging (see Figure 17.16). The only materials with which the sample and its components are in contact during the analysis are Hastelloy C (sampling and

721

*-&

I

11 ' Selector

5

b Detector

Figure 17.16. Schematic of the gas circuit for the analysis of the effluent stream in the vinyl chloride

synthesis. The selector valve selects the analyte, stream effluent or deferred standard.

switching valves, unions, detector block), Teflon (column, packing support, connecting tubes), SF-96 and the detector wires. Figure 17.17 shows the chronology of events during the analysis.

d. Deferred Standard Ethylene was chosen as deferred standard, since it is a component of the sample analyzed, and certainly the least reactive and the safest to store and handle. Pure ethylene is injected with the same valve as the sample (see Figure 17.17). TABLE 17.7

Relative Response Factors of Ethylene, Hydrochloric acid and Vinyl Chloride (37)

Components

Relative response factor (w/w)

Ethylene Hydrochloric acid Vinyl chloride

1.oo 1.38 1.555

Relative response factors on a TCD, with semi-diffusion measuring cells in series (Pretzel Gow-Mac cells). Carrier gas: hydrogen. References on p. 739.

122

v1

0

-

Figure 17.17. Chronology of events during the analysis of the effluent stream in the synthesis of vinyl chloride. Identification of the compounds in the chromatogram. 1, Ethylene (sample). 2, Hydrochloric acid. 3, Ethylene (standard). 4, Vinyl chloride.

e. Calibration

The response factors of hydrochloric acid and vinyl chloride relative to ethylene (w/w) were determined using the GDB as a calibration detector (see Chapter 14). Because of the chemical aggressiveness of hydrochloric acid a solid nickel balance was used for the calibration. The response factors are reported in Table 17.7.

f: Sample Line A long transfer line is used for this analysis, because of the presence of tars. They can be more easily backflushed from the transfer line, with a stream of nitrogen, than from the column (see Figure 17.4). The line is 10 m long, made of 1 mm i.d. Teflon tubing. Its temperature is kept at 170°C by steam flowing in a concentric metal tube. The sample and the deferred standard are injected successively by the same valve. Prior to injection the flow is stopped and the end of the sample line connected to atmospheric pressure for 20 seconds. A selecting valve alternates filling the sample loop with the sample and the standard.

123

15 rnin

Figure 17.18. Typical chromatogram of the effluent stream of the vinyl chloride synthesis plant (37).

g. Results

Figure 17.18 shows a typical chromatogram, with the successive elution of a tiny amount of air, ethylene (from the sample), hydrochloric acid, ethylene (deferred standard) and vinyl chloride. Also shown is the small disturbance resulting from the injection of the deferred standard. The base line is automatically set to zero twice during each analysis, after elution of the air peak and before elution of the standard. All peaks tail, probably a consequence of the nature of the analysis. Figure 17.19 shows a bar graph record corresponding to a few hours of operation of the process. I t illustrates another possible use of the alarm function of the deferred standard. After a few hours of satisfactory operation, with a constant response for the standard, a severe thunderstorm seriously perturbed the electrical power supply of the plant and the operation of the process. This is illustrated by a kick in the standard response, followed by oscillations for a couple of hours. The References on p. 139.

724 7h

6h

5h I

4h

3h I

lh

2h

I

time

-1

i

Figure 17.19. Bar graph record of the composition of the effluent stream of the vinyl chloride synthesis plant. A drop of concentration of the three main components, with a drop in yield, takes place during the first hour. Everything remains stable for four hours, when a thunderstorm perturbs the plant electrical power supply, causing the kick in the deferred standard peak height and in the yield. The gas chromatograph and the plant recover (37).

TABLE 17.8 Comparison between Chromatographic and Volumetric Analysis of Hydrochloric Acid Essays

Chromatography

(W weight)

Volumetric titration (I% weight)

Relative difference

(W) 1 2 3 4 5

66.34 65.71 61.04 62.55 63.40

68.33 68.07 65.09 65.67 62.51

3.0 3.6

- 2.9 5.0 - 1.4

Hydrochloric acid is absorbed by bubbling a known volume of gas in water, followed by titration.

125

response of the standard remains within the specifications, however, except during the transient, allowing a record of the excursion of the process and its recovery. Quantitative analyses have been performed by chemical methods on the effluent stream. Hydrochloric acid is analyzed by absorption of the content of a known volume of gas in an alkaline solution and titration. Differences between 3 and 5% (relative) for HC1 concentrations around 60% have been observed (47). Data are reported in Table 17.8. 3. Analysis of the Gas evolved During a Chloration Process

One of the analyses developed for this process required the separation of the following gases in less than 5 minutes: oxygen, nitrogen, carbon monoxide, carbon dioxide, ethylene and chloroethane. The difficulty of this problem lies in the need to use several columns to achieve the separation of oxygen and nitrogen, the separation of permanent gases and the analysis of organic vapors.

0

E

0 .-c 2

0 I

I

I

\

m n Q

v)

H ~ u x 2

I

I

I

\

I

I

Figure 17.20. Schematic of the gas circuit used for the analysis of the gas effluent stream in the chloration process.

References on p. 739.

126

l3I

0 2 N2

CO

W

Figure 17.21. Chronology of events in the analysis of the effluent of the chloration process.

The following experimental conditions were selected: Instrument: Car10 Erba (Milan, Italy) Fractomatic CT. Column 1: stainless steel, 2 mm i.d., 1 m long; packed with 100-200 pm particles of 80 m2/g silica (Spherosil, Rhone Poulenc), coated with 10% diethylene glycol succinate. Column 2: stainless steel, 2 mm i.d., 1.40 m long; packed with 50-80 mesh Porapak Q (Waters), treated with 0.01%phosphoric acid (48). Column 3: stainless steel, 2 mm id., 1 m long; packed with Molecular Sieve 5A. Temperature of the columns: 80°C. Carrier gas: hydrogen, flow rate 5.8 L/hour. Detector: TCD with serial, semi-dipfusion cells. Bridge current: 250 mA. The requirement that the entire analysis be performed within 5 minutes led to a rather complex gas circuit, with several switching valves (see Figure 17.20). Two sampling valves are used, for the sample and for the deferred standard (nitrogen). Each one is completed by a second valve to adjust the pressure in the sample loop to atmosphere. Three switching valves are used, permitting the carrier gas to bypass of any of the columns. Figure 17.21 shows the chronology of events taking place

5 6

I

721

2

4

DS ( N z )

3

7

Lsd It

5I min

I

I

I

I

Figure 17.22. Typical chromatogram supplied by the process gas chromatograph analyzing the effluent of the chloration process. 1, Nitrogen (deferred standard). 2, Chloroethane. 3, Oxygen. 4, Nitrogen (sample). 5 , Carbon dioxide. 6, Ethylene. 7 , Carbon monoxide.

during an analysis. Finally, Figure 17.22 shows a typical chromatogram. The resolution between all the bands is excellent; it would probably be easy to halve the analysis time if needed. The response factors of the various components relative to nitrogen have been determined using the GDB as a calibration detector. 4. Analysis of Gaseous Ammonia The components of this industrial stream are: air, carbon dioxide, ammonia and water. Since there is no other component, the quantitative analysis is derived from the peak area ratios and there is no deferred standard. The following experimental conditions were selected. Column: Teflon tubing, 4 mm i.d., 4 m long; packed with 80-100 mesh Chromapore (poly-2,6-dimethyl-para-phenylene oxide) (48). Column temperature: 70 O C. Carrier gas: helium, flow rate: 2.5 L/hour. Detector: TCD, with two serial, semi-diffusion cells (Pretzel, Gow-Mac), bridge current 210 mA. Sample size: 1.5 mL. A long transfer line is used. Figure 17.23 shows a typical analysis. The relative response factors have been measured with the GDB. The results are given in Table 17.9. References on p. 739.

728 H2O

Figure 17.23. Analysis of an ammoniacal gas stream (37).

The accuracy of the quantitative results has been checked by comparison with the results of two other methods, a chemical method and the method of O t h e r and Frolich (50). The results of this comparison are reported in Table 17.10. The excellent agreement observed confirms the validity of the chromatographic method, the soundness of the sampling procedure used, as well as the validity of the calibration performed with a GDB. TABLE 17.9 Response Factors of Some Compounds Relative to Ammonia (37) Components

Response factor

1.o 1.34 1.65 0.97 1.015 ~~

~

~

Factors determined with the gas density balance (seeChapter 14, Section 11.3), assuming the TCD to be linear in this concentration range. Helium carrier gas.

729

TABLE 17.10 Chromatographic Results Compared to the Other-Frohlich Method in the Case of NH,, C02, H20 Analysis (37) Control

ChromatotOPPhY

OthmerFrohlich Method

Deviation between the two methods - Ref. chromatography (%)

50.8 34.0 15.2

50.9 33.6 15.5

- 1.2

50.5 35.0 14.5

50.9 33.7 15.4

- 3.7 + 6.2

51.8 33.7 14.5

52.6 31.9 15.5

- 5.3 + 7.0

51.4 33.8 14.8

52.9 31.5 15.6

50.9 32.3 16.8

52.8 31.6 15.6

+0.2

+ 2.0

+0.8

+ 1.5

+ 3.0 - 6.0 + 5.4 + 3.7

- 2.2 - 7.0

NH3/C02 Ratio Obtained by Chromatography and Chemical Method Control

Chromatography

Chemical analysis

Deviation between the two methods (%)

1 2 3

1.51 1.50 1.44

1.57 1.52 1.51

+ 1.3 + 4.85

+4

Figure 17.24. Analysis of the effluent of the reactor preparing phthalic anhydride. 1-3. Unknowns. 4, Benzaldehyde. 5, ortho-Tolualdehyde. 6, Maleic anhydride. 7, Citraconic anhydride. 8, Unknown. 9, Ethylene (deferred standard). 10, Benzoic acid. 11, Phthalic anhydride.

References on p. 739.

730

5 i/ i I'

20 hours

5

10

1

Figure 17.25. Bar graph record of the composition of the effluent of the reactor for the synthesis of phthalic anhydride. 1, orrho-Tolualdehyde.2, Maleic anhydride. 3, Ethylene (deferred standard). 4, Benzoic acid. 5, Phthalic anhydride. The deferred standard has a constant area. The concentration of the main product oscillates slightly. The concentration of other products, especially tolualdehyde, varies much more widely.

5. Synthesis of Phthalic Anhydride The control of the synthesis of phthalic anhydride is an excellent example of a problem where the conventional calibration methods are difficult or even impossible to apply (37,41). The deferred standard solves the calibration problem easily. TABLE 17.11 Quantitative Results Obtained by the Deferred Standard Technique in Phthalic Anhydride Synthesis (37) Date

September 8th. 1969 9th 10th 16th 18th 19th 26th 29th 30th

Results expressed in g/m3 Benzaldehyde

o-Tolualdehyde

Maleic anhydride

Citraconic anhydride

Benzoic acid

Phthalic anhydride

0.020 0.020 0.020 0.020 0.025 0.025 0.020 0.020 0.020

0.055 0.070 0.035 0.110 0.110 0.090 0.170 0.195 0.210

5.815 5.515 5.670 5.630 5.665 5.730 5.110 5.315 5.060

1.250 1.205 1.245 1.140 1.200 1.200 1.080 1.145 1.100

0.400 0.430

56.00 56.20 57.30 55.30 55.10 55.00 55.00 55.00 54.65

0.445 0.445 0.430 0.390 0.370 0.370

731

The column is packed with Chromosorb PAW (80-100 mesh), coated with 4% phosphoric acid and 10% LAC 446. Figure 17.24 shows a typical chromatogram. The various components of the effluent are completely resolved. The stream sampled is at 180°C. The column is at the same temperature. The sample size is 512 pL. A 1.29 pL volume of ethylene is injected as a standard, 12.50 minutes after the sample. Figure 17.25 shows a 20-hour bar graph record of the analytical results. The stability of the deferred standard peak demonstrates the proper functioning of the gas chromatograph. The results show a good stability of the process, but some significant fluctuations, and a rather large excursion of the concentration of ortho-tolualdehyde, a reaction by-product. In this case there is no other analytical method available to test the accuracy of the results. A comparison with the material balance of the plant shows an excellent agreement with the integration of the concentration data supplied by the gas chromatograph (see Table 17.11).

6. Analysis of Recycled Styrene This stream contains a variety of aromatic hydrocarbons (35). The following experimental conditions were selected: Instrument: Carlo Erba (Milan, Italy) Fractomatic. Column: stainless steel, 1 mm i.d., 6 m long (1 m + 5 m); packed with 160-200 pm, 80 m2/g silica particles (Spherosil) coated with 10%diethylene glycol succinate. The short segment is back-purged at the end of the analysis.

0,

aJ C 0,

9

c h Y

w

+0

0

0 x

0

2

hI

15 rnin

I

10

I

5

Figure f7.26. Chromatogram of a recycled styrene stream.

References on p. 739

132

Column temperature: 155 C. Carrier gas: nitrogen, flow rate 0.36 L/hour. Detector: FID, polarized jet. Sample size: 0.5 pL, liquid, with a Siemens sampling valve. Deferred standard: ethylene, injected with a gas sampling valve placed upstream the sample injection valve (see Figure 17.10~). The analysis time of about 15 minutes could be markedly reduced by increasing the flow velocity, or reducing the column length. Figure 17.26 shows a typical chromatogram. Quantitative analyses were performed by using the response factors of the key compounds relative to the standard, as measured with the GDB. The use of narrow bore columns, or modified gas-solid chromatography does not raise any serious difficulty in adapting the instrument. The liquid sampling valve operates very satisfactorily. 7. Airborne Pollution Analysis in a Polymerization Plant

In this case the plant management wanted to follow the concentration in the working area of the following compounds: vinyl acetate, ethyl acrylate, styrene and butyl acrylate (42). The following experimental conditions were selected: Instrument: Car10 Erba (Milan, Italy) Fractomatic. 1

3 min

4'

Figure 17.27. Analysis of atmospheric pollutants in a plant (42). Experimental conditions, see text. 1, Vinyl acetate (93 vpm). 2, Ethyl acrylate (95 vprn). 3, Styrene (58 vpm). 4, Deferred standard, ethylene (51 vpm). 5, Butyl acrylate (74 vpm).

733

Column: stainless steel, 4 mm i.d., 30 cm long; packed with 150-200 pm particles of 200 m2/g silica (Spherosil), coated with 25% of diethylene glycol succinate. Column temperature: 90 O C. Carrier gas: nitrogen, flow rate 7 L/hour. Detector: FID, polarized jet. Sample size: 0.58 mL of air. Deferred standard: ethylene. In this case the deferred standard proved extremely useful. In addition to its classical advantages, already discussed above, it gave confidence to the.workers in the ability of the instrument to properly control the pollution inside the plant. 10

12

-9

o1 - - I 6

1

4

2iFe 1

5 vprn

-styrene

24 &vinyl

acetate 8 vprn

E 23

20

10

-9 -8

I

-4

-3

I

2

4 -~ D.S.

Deferred Standard Ethylene

Figure 17.28. Bar graph record of the atmospheric pollution in the plant. The number and the step identify each location where a sample is grabbed. Pollution levels are low and erratic. The stability of the deferred standard peak validates the results (42).

734

The requirement for the analysis was to check 10 different locations in the plant and to make a measurement at any given place every 30 minutes. The analysis should thus be completed in 3 minutes. The chromatogram on Figure 17.27 shows how this is possible, using a short column packed with a large surface area, modified adsorbent (see Chapter 7). Figure 17.28 shows a typical bar graph record. The analyzed stream is identified by a bar having a height proportional to its rank. The deferred standard peak demonstrates that the gas chromatograph is working well, although in most cases there is no detectable level of pollution. Without the deferred standard the confidence in the results given by the same instrument would not be very high. With conventional methods, calibration of the detector for those types of compound, in the concentration range investigated (0-20 ppm) is quite impossible, or at best unreliable. Calibration mixtures are impossibly difficult to prepare as these compounds would readily adsorb on the surface of the container walls. Again, the use of the deferred standard offers an easy solution, the response factors relative to the deferred standard being most conveniently and rapidly determined with the GDB (see Chapter 14). The good resolution and fast analyses allowed by the modified gas-solid chromatographic columns have permitted the use of a portable gas chromatograph to perform the same analysis. The column is made with the same material, but it is shorter and now operates at ambient temperature. A chromatogram is shown in Figure 17.29.

Figure 17.29. Analysis of the atmospheric pollutants in a polymerization plant, using a portable gas chromatograph (Century).

735

8. Analysis of Chloral

The control analysis of chloral (CC1,-CHO) is usually performed by chemical methods, which require the intervention of a chemist to grab a sample and perform the analysis in the laboratory. These methods are not readily adaptable to automation. The use of gas chromatography with steam as a component of the mobile phase (see Chapter 7) is an elegant solution to a difficult problem, since chloral is usually found in waste water, from which it must be recovered before sending the water to the sewage. The analysis is carried out directly on the waste water, without any previous treatment, by injecting a known volume into the gas chromatograph operating under the following conditions. Column: stainless steel, 4 mm i.d., 0.50 m long; packed with Porapak P. Column temperature: 132' C. Carrier gas: nitrogen, 2 L/min, steam, 1 L/hour. Detector: FID, with polarized jet. Sample volume: 0.5 to 1 pL. Figure 17.30 shows three chromatograms obtained with different matrices: - a mixture of chlorinated hydrocarbons (Figure 17.30A),

5 min

d 7 rnin

Figure 17.30. Quantitative analysis of chloral in various matrices, using steam as a component of the carrier gas (see text). A, In chlorinated hydrocarbons. Chloroform, carbon tetrachloride, 1,2-dichloroelhane, trichloroethylene. 1,1,2-trichloroethane. B, In 1.2 dichloroethane, at trace level. C, In waste water, at trace level.

References on p. 739.

136

in 1,2-dichloroethane (Figure 17.30B), in waste water (Figure 17.30C). In this last case, concentrations well below 1 ppm can be easily detected with the equipment used. -

9. On-Line Control of a Dicldorodifluoromethane Process The control of the synthesis process of dichlorodifluoromethane (Freon F12) requires the continuous analysis of the following trace components: - trichlorofluoromethane (Fll), - dichlorofluoromethane (F21), - chlorodifluoromethane (F22). The concentration of these impurities i s of the order of a few hundred ppm, so a process control gas chromatograph equipped with a flame ionization detector was selected, rather than one using a thermal conductivity detector. For reasons explained above, a packed column is used. The main component (F12) tails and this interferes with accurate quantitation of the impurities. Accordingly, a combination of several successive valve switchings was chosen. Heartcutting of the impurities band and their transfer to a second column permits the elimination of the interference with the main component band. Backflushing out of the second column of the bands of the impurities, which have been excessively broadened, followed by the injection of the backflushed band in a third column, permits the elution of much narrower peaks of the impurities. Finally the backpurging of the first column t

:0 sec

'Dm t

:65

sec

Delector

Backflush Coi 2

t

:109 sec

0

Vf VZ

0

v1

0

Backpurging

F21

DS

F22

Figure 17.31. Principle of the multi-columnon-line analysis of the impurities in dichlorodifluoromethane.

737

-

A u x CG

Figure 17.32. Schematics of the columns and valves setup for the on-line analysis of impurities in dichlorodifluoromethane. S, sample. DS, deferred standard. CG, carrier gas. Aux CG, auxiliary carrier gas. Atm 1, Atm 2, atmospheric pressure.

permits the elimination of any possible heavy component. Figure 17.31 illustrates the principle of this three step analysis. The schematic of the actual set-up is shown on Figure 17.32. For the analysis, a modified gas-solid chromatography packing (see Chapter 7) is used. The three stainless steel columns (4 mm id., 1, 0.5 and 3 m long, respectively) are packed with 200-250 pm porous silica particles (Spherosil, Rhone-Poulenc), 35 m2/g, coated with 2.5% (w/w) of Carbowax 20M. The column temperature is 65 O C, the carrier gas (nitrogen) flow rate is 3 L/hour. The hydrogen flow rate to the FID is 2 L/hour and the air flow rate is 15 L/hour. The sample size is 397 pL. Trichlorofluoromethane (F11) is used as deferred standard. The volume injected (0.54 pL) is small enough that the impurities contained in this product are not detected. Table 17.12 gives the timing of the events during this analysis and Figure 17.33 shows a typical chromatogram. The weight response factors of F21 and F22 relative to Fl1 are 0.41 and 0.55, respectively, under the experimental conditions described. The repeatability of the retention times is excellent. The average retention time calculated for 20 successive chromatograms has changed by less than 1%after 2,230 cycles. The deferred standard proved useful not only for an on-line check of the reliability of the analytical performance of the gas chromatograph, but also in considerably simplifying the calibration procedure. Calibration of the detector response is much easier and more accurate through the use of the response factors References on p. 739.

738 F 22

h

I

F 12

1, I

7minutes a

I

Figure 17.33. Typical chromatogram in the on-line analysis of dichlorodifluoromethane.

TABLE 17.12 Event Timing in the Analysis of Freon Time

Function

(set)

0 65

109 110 140 150 173 212 352 384 390 420

Sample injection Heartcut Col. 2 Backflush Col. 2 Separation Col. 3 Backpurge Col. 1 DS line put to atm. DS injection Sample line to atm. Detection of F22 Detection of F11 Detection of DS Detection of F21 Refill sample loop Refill DS loop Sample injection Heartcut Col. 2

Valves open: white (W) or black (B) Atm 2

DS

Atm 1

S

v1

v2

B

B

W

W

B

B

B

B

W

W

B

W

B W W W

B B W W

W W W B

W W

W W

W

B

B

B

B

B

B

W

W

W

W W W

B

B

W

W

B

B

W

relative to the deferred standard than when using conventional methods, such as the injection of samples of standard calibration mixtures. In the case of the Freon mixture, continuous vaporization of a stream of liquid sample would be necessary (see Chapter 14, Section 1.2). 10. Conclusion

These are but a few examples selected among the most significant applications that we have studied during the last twenty years. The combination of the calibration techniques developed around the use of the GDB, of the deferred standard, of the Deans techniques of column switching and of modern digital electronics and

739

microcomputers permits the solution of almost any process control analysis problem. Among the most spectacular results are the solution to difficult plant mass balance problems. For various reasons dealing with economics and financial problems, with material auditing and with pollution control, it is important to know in detail what happens to the products which enter the plant. Trying to solve this problem permits the discovery of all kinds of interesting problems, phenomena and stones. Process gas chromatography offers a very powerful tool in investigations of that kind. We can give here two examples, those of a chloration process giving a complex mixture of C1, C2 and C3 chlorinated hydrocarbons and an oxidation process giving mixtures of alcohols, aldehydes, acids, etc. The material balance calculated after the chromatographic analysis of the effluents of the reactor gives an account of the fate of 98%of the different feeds of the chloration process (see Section IV.3). Because pure compounds, especially the C3, C4, etc aldehydes could neither be purchased nor prepared at a sufficient degree of purity to proceed with conventional calibration procedures, the material balance of the process had to be calculated from approximate chromatographic results obtained by conventional methods. The mass balance results were erratic and did not account for more than 86% of the reagents. After implementation of the deferred standard method and using the GDB for determination of the response factors with the impure compounds available (see Chapter 14), the mass balance of the plant reached 95% of the feeds. These results explain why a good chromatography team and high quality equipment is a very profitable investment.

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