Design and Optimization of Fully Thermally Coupled Distillation Columns

Design and Optimization of Fully Thermally Coupled Distillation Columns

0960–3085/01/$10.00+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 79, Part A, October 2001 DESIGN AND OPTIMIZATION OF FULLY THERMALLY CO...

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0960–3085/01/$10.00+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 79, Part A, October 2001

DESIGN AND OPTIMIZATION OF FULLY THERMALLY COUPLED DISTILLATION COLUMNS Part 2: Application of Dividing Wall Columns in RetroŽ t K. A. AMMINUDIN 1 and R. SMITH Department of Process Integration, UMIST, Manchester, UK.

T

his paper addresses the application of dividing wall columns in retroŽ t. It emphasizes the need to take maximum advantage of the existing hardware with minimum capital outlay. Based on this study, several practical issues associated with the application of the dividing wall column in retroŽ t have been identiŽ ed and as a result, its thermodynamically equivalent arrangements, such as the prefractionator arrangement and the Petyluk column, are often recommended instead. A case study involving the improvement of energy efŽ ciency and capacity expansion of the NGL separation train has been illustrated to demonstrate the analysis involved. Keywords: dividing wall column; thermal coupling; prefractionator; Petlyuk column; debottlenecking; energy efŽ ciency; retroŽ t.

INTRODUCTION

ments, especially when investigating the use of the dividing wall column in retroŽ t. Most retroŽ t practice in distillation has emphasized column internals. As many retroŽ t projects now aim to increase throughput, column internals, which not only promote separation but also govern the column hydraulic performance, have been identiŽ ed in many cases to be at their maximum hydraulic capacity. This requires the column internals be debottlenecked in order to allow more vapour and liquid trafŽ c to be handled without a loss of separation efŽ ciency. Such column revamping on the internals gives rise to the use of high efŽ ciency trays, such as Nye trays1 and Multiple Downcomer trays2, and high efŽ ciency packing or structured packing. Installing these new internals can be costly and may require substantial changes, such as removing the tray support rings in the case of structured packing and modifying the downcomer to accommodate high efŽ ciency trays. Replacing existing internals with better ones can be expected to boost the capacity by up to 40%. Beyond this value, an additional column in parallel with the existing one must be considered3. Bravo4 considered retroŽ t of existing internals using either structured packing or high performance trays, as well as cost estimation to assist the selection. Following Bravo’s procedure, Williams5 provides a comprehensive approach to the revamp of the existing internals, ranging from the site investigation to analyse existing performance to the selection of the internals. He also suggested that it is not always necessary to switch to better internals if the existing internals can be signiŽ cantly improved. This

RetroŽ t of an existing plant is typically carried out to achieve the following objectives:

· process debottlenecking for an increase in plant throughput; · energy saving measures; · product and feedstock changes; · environmental compliance. RetroŽ t projects are particularly important for process expansion. The key to a successful retroŽ t lies in exploiting the existing hardware by maximizing the utilization of existing equipment while, at the same time, minimizing the new hardware to minimize the capital cost. For an existing distillation system, a retroŽ t project on such hardware can be a major undertaking. Adding a new column or placing a new column in parallel with an existing distillation system is often not economic in debottlenecking projects. What is really needed is to tap the hidden potential of the existing distillation system. To achieve this, requires an awareness of the practical issues associated with the column retroŽ t, such as the column hydraulics, plant down time and the feasibility of re-arranging the existing system. Of the three retroŽ t issues, re-arrangement of the existing column has not been given as much attention as column hydraulics. In this paper, the authors shall only consider rearranging existing columns to complex column arrangePresent address: 1Universiti Teknologi Petronas, Perak, Malaysia.

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DESIGN AND OPTIMIZATION OF FULLY THERMALLY COUPLED DISTILLATION COLUMNS: PART 2 717

approach which has been further developed by Sloley6 aims at squeezing more from the existing column. Litzen and Bravo7 proposed a debottlenecking procedure for the overall process plant. They suggested that it would be unwise to focus too soon on speciŽ c equipment such as the distillation column for debottlenecking purposes, but rather to look at the overall process and interactions among the units. This could give rise to the re-arrangement of the existing columns in an attempt to synergize effects among existing columns. Manley8 noted that in some cases the use of better internals to debottleneck distillation columns could not improve the energy efŽ ciency of the system, and this could prevent a large increase in capacity. He proposed several process improvements such as incorporating interreboilers, feed preheating and intercondensing to improve capacity and energy efŽ ciency. Similarly, Liu9 used these process improvement options to reduce hydraulic loads in existing distillation columns after identifying which stage along the column is a bottleneck from a hydraulic analysis. Liebmann10 highlighted that an additional column, such as post-fractionator or prefractionator, could provide process debottlenecking options for a crude oil distillation unit. However, as these options require a signiŽ cant capital expenditure, he suggested that they should be considered after other methods, such as redistribution of stages or operational changes, fail to meet the debottlenecking target. RETROFIT USING DIVIDING WALL COLUMN Triantafyllou11 investigated the use of the dividing wall column for retroŽ t. He stressed that the dividing wall column could not be used for capital savings but for energy savings or process debottlenecking by using existing columns more efŽ ciently. He suggested several arrangements involving the use of the dividing wall column on two existing conventional columns. He Ž rst proposed two dividing wall columns in parallel, as shown in Figure 1. In this arrangement, the feed can be divided into two smaller feeds and dividing walls placed inside the two columns. Each of the two columns could then have three products and perform similar separations. The major drawback of this split arrangement is that in each dividing wall column only a fraction of the total number of stages are affectively used in

order to achieve the same separation as before. To accommodate such restrictions, the columns must be operated at a much higher re ux ratio and this could jeopardize energy savings. In the case of process debottlenecking, where energy savings may not be the top priority, the higher re ux ratio required to meet the intended product speciŽ cation results in a much higher volumetric  ow rate in the column. However, a Ž xed column diameter may not be able to handle such an increase in volumetric  ow rate. To fully exploit the columns and to overcome potential problems with the split arrangement, he suggested two dividing wall columns linked together as illustrated in Figure 2. It essentially maintains a single dividing wall column but splits it into two column shells. The column can now be operated at a much lower re ux ratio and utilizes all column trays to achieve the desired separation. Finally, he proposed a Petlyuk column conŽ guration, equivalent to the dividing wall column, to take advantage of the two existing columns. With this arrangement, one column with fewer trays becomes a prefractionator and the other turns into a main column. No dividing wall is needed but new nozzles are required to connect the two columns. Using his short cut method, Triantafyllou11 carried out a screening calculationon the optionsused to identify promising options for retroŽ t purposes. Based on his work, he identiŽ ed that the dividing wall column options could generate substantial energy savings but in the case of an increase in throughput, one of the two dividing wall column options results in unfeasible designs due to restrictions from the existing column diameter in handling the higher loads.

Issues in Using Dividing Wall Column in RetroŽ t Even though Triantafyllou11 indicated promising dividing wall column options in retroŽ t, he raised potential practical difŽ culties associated with the use of dividing wall column options in retroŽ t. Apart from the existing column constraints, such as Ž xed column diameter, most of these concerns focused on the need to modify column shells to accommodate the dividing wall, and to have another nozzle Ž tted to withdraw the middle product. Such modiŽ cations to install the dividing wall, which may involve the removal of the tray support rings and replacement of the existing internals, can be a major undertaking. In addition, the mechanical design aspects of the column, such as mechanical stress, will be affected as a result of changing the feed or side-draw locations. Thus, these

E1

E1

E1

E1

E2

E2

E2 E2

Figure 1. Exploiting two existing columns by incorporating two dividing wall columns in parallel.

Trans IChemE, Vol 79, Part A, October 2001

Figure 2. Three possible options for the two dividing wall columns linked together.

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concerns must be fully addressed during the retroŽ t design even though initially the dividing wall column may be the best option for retroŽ t objectives. It is important to remember that these modiŽ cations require plant downtime. In many retroŽ t projects, downtime is the largest economic factor. SigniŽ cant downtime can lead to a loss of production and interruption of product supply to the customers. In an integrated facility where plants depend on each other, such plant downtime could disrupt the rest of the downstream processes. Apart from plant downtime, the cost of replacing internals and installing the dividing wall must be properly assessed. A cost estimate for replacing column internals has been presented by Bravo4. Other retroŽ t costs to be incurred will include instrumentation and control modiŽ cations, and the pipe work modiŽ cations associated with the column re-arrangement. Finally, let us assume that one of the two columns can be retroŽ tted successfully with the dividing wall column option. Since the dividing wall column option can replace two columns with a single column, the other column becomes redundant. This should be avoided unless the second column cannot be appropriately utilized somehow. Having identiŽ ed the issues regarding the use of the dividing wall column in retroŽ t scenarios, a clear strategy must be developed to accomplish a successful retroŽ t project, which will:

· minimize plant downtime and capital cost with as little disturbance as possible to the surrounding infrastructure; · maximize the use of existing equipment,avoid unnecessary capital expenses and early equipment decommissioning. PROPOSED MODIFICATIONS Rather than using the dividing wall column in retroŽ t, the two existing columns can be exploited and rearranged to become either a Petlyuk column arrangement or a prefractionator arrangement. These proposed arrangements, which aim to meet the retroŽ t strategy, are thermodynamically equivalent to the dividing wall column and hence, the energy performance from these two arrangements is expected to be similar. Furthermore, the improvement of energy efŽ ciency from these arrangements can be translated into process debottlenecking beneŽ ts. In each arrangement, the authors will explore two scenarios:

· utilize existing columns; · add a new column, which can also imply either re-tray an existing column or use high efŽ ciency internals (packing or trays).

Figure 3. Two possibilities of re-arranging the two existing columns into a prefractionator arrangement.

Add a new column=re-tray the existing columns=replace with high efŽ ciency internals In the event that use of existing columns could not meet the retroŽ t objectives, another option is to consider adding a new column to the existing columns. Adding a new column to the existing columns may not be justiŽ ed due to excessive cost or plant layout restrictions. Thus, several approaches can be adopted to accommodate the new column. These options can be taken in the form of either re-traying the existing columns or replacing the existing internals with high efŽ ciency internals, such as structured packing or the high performance trays. However, these options might require major modiŽ cations to the existing columns to accommodate these new internals. The cost implications from the purchase of new internals and removal of the existing internals may be substantial and should be minimized. Figure 4 presents six potential conŽ gurations of the prefractionator arrangement involving an additional column. These conŽ gurations cover all possibilities of having a new column and the two existing columns to be re-arranged into three column sections corresponding to a prefractionator and a main column in two parts.

E1 NEW

E2 NEW

E1 E2

E2

E1

NEW

E2

NEW

NEW

Prefractionator Arrangement Option Utilize existing columns Figure 3 illustrates two possibilities of using the existing columns in the prefractionator arrangement. It highlights the maximum use of existing hardware, such as the complete use of reboilers and the two columns but with some external modiŽ cations. These modiŽ cations involve a change from total condenser to partial condenser in the prefractionator column and the necessary pipe work to re-connect the feed stage and product side-draws.

E1

E1 NEW

E2 E2

E1

Figure 4. Six potential conŽ gurations involving an additional column and two existing columns.

Trans IChemE, Vol 79, Part A, October 2001

DESIGN AND OPTIMIZATION OF FULLY THERMALLY COUPLED DISTILLATION COLUMNS: PART 2 719 Petlyuk Arrangement Option Utilise existing columns The existing columns can also be transformed into a Petlyuk column arrangement. A thermal coupling link must be made between the prefractionator and the main column. As a result, both the condenser and reboiler in the prefractionator section will be bypassed in order to make way for the thermal coupling streams. Thus, more pipe work is required in addition to the pipe work associated with the relocation of feed stage and the generation of new side draw product streams. Figure 3 also highlights the presence of the thermal coupling link between the prefractionator and the main column.

Add a new column=re-tray the existing columns=replace with high efŽ cient internals Figure 4 can be re-drawn to form six conŽ gurations involving thermally coupled columns. Similar arguments and concerns as discussed earlier in the prefractionator arrangement option for adding a new column or retraying existing trays, apply to these conŽ gurations, but with additional complexity regarding the pipe work associated with the thermal coupling link. In this paper, the relative merits of the two options, the Petlyuk and prefractionator arrangements, will be discussed through a case study.

APPLYING THE NEW DESIGN PROCEDURE IN RETROFIT The new design procedure developed in Part 1 of this paper to perform process screening and provide initialization for rigorous simulation, can be tailored to accommodate the retroŽ t situation by incorporating two limits, corresponding to the hydraulic capacity of an existing column and the given number of trays.

Hydraulic Capacity The hydraulic capacity is primarily in uenced by the upper operational limit of the operating column. This limit is characterized by the onset of  ooding in the column. Two types of  ooding mechanisms normally occur in trayed columns; entrainment or jet  ooding and downcomer backup. Both types of  ooding are affected by the internal column loadings of the vapour and liquid  ows. At moderate and high liquid  ow rates, the entrainment  ood limit is reached when the internal vapour  ow is raised, while the downcomer backup limit is reached when the internal liquid  ow is raised12. Of the two, the entrainment  ooding is normally used in staged columns as a practical design limit. Various  ooding correlations have been reported in the literature to predict such a  ooding mechanism. One of these correlations, Fair’s entrainment  ooding correlation has been widely used by the industry for entrainment  ood prediction for sieve and bubble cap trays12. The detailed calculation involving the use of Fair’s correlation, mainly used to determine the column diameter, Trans IChemE, Vol 79, Part A, October 2001

can be found in any standard distillation text, such as King13. Similarly, the onset of  ooding is used as a capacity limit for packed columns. Flooding correlations for packed columns have been presented by Kister12. The internal column loadings, which affect the hydraulic capacity, are in uenced directly by the re ux ratio of a column. To determine the hydraulic capacity limit of a particular column, a simulation run in a column rating mode, which incorporates column internal speciŽ cations such as the column diameter, fraction of  ooding, downcomer dimensions, type of trays and tray spacing, packing type, etc. must be performed by manipulating the re ux ratio. The approach is to keep the feed rate constant and raise both re ux and reboil rate, while maintaining the product purity14. As the re ux ratio is raised in the simulation, there will be a point where the increase in the internal vapour  ow reaches the  ood point, above which, the column loses its operability. Once this point is reached, the upper limit of the re ux ratio is deŽ ned. Using any appropriate hydraulic correlation in the simulation, it is possible to determine whether the internal vapour  ows at a given re ux ratio will cause  ooding in the column. Various parameters from the simulation can be used to indicate  ooding in the column, such as the fraction of entrainment  ooding, the percentage of downcomer backup  ooding and the pressure drop. These correlations can be used to check the  ooding against the maximum speciŽ ed values of these parameters, such as the fraction of  ooding. If the calculated value exceeds the maximum speciŽ ed value in a certain section of the column, then this section of column will  ood.

Number of Stages The total number of stages for the columns involved in retroŽ t should be Ž xed to allow the investigation of complete reuse of existing columns. In the event that a new column is needed, this can simply be incorporated into the number of stages as part of the optimization to minimize the number of stages. Since the purchase of the new column can be substituted by re-traying the existing column or replacing the existing internals with better performance internals, a similar approach as that for minimization of the number of trays when considering a new column to analyse alternatives can be used. Note that the cost of removal of the internals and other associated costs relating to the piping are not included at this conceptual stage.

CASE STUDY IMPROVEMENT OF ENERGY EFFICIENCY AND CAPACITY OF AN EXISTING NGL SEPARATION TRAIN This case study concentrates on the application of complex column arrangements such as the dividing wall column, prefractionator arrangement and the Petlyuk column in a retroŽ t of an existing NGL separation train15. The objective of the case study is to retroŽ t the exist-

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ing separation train for energy efŽ ciency and capacity improvement. Existing Process ConŽ guration Figure 5 illustrates the existing separation train and its current operating conditions. There are two opportunities for replacing pairs of the existing columns in sequence with dividing wall columns. The dividing wall column can be used to replace either, the depropanizer and the debutanizer, or the debutanizer and the deisobutanizer. Considering the substantial pressure difference between the depropanizer and the debutanizer, in this case 12 bar and 3.5 bar respectively, the dividing wall column needs to be operated at high pressure, which makes the separation more difŽ cult in terms of relative volatility and requires a high energy consumption. If the dividing wall column is to be operated at a low pressure to accommodate the debutanizer, the condenser for the dividing wall column will require refrigeration. Thus, the use of a dividing wall column to replace the depropanizer and the debutanizer is rejected. Instead, the other option involving the debutanizer and deisobutanizer indicates a relatively small pressure difference between the two columns (3.5 bar and 4.4 bar), and both columns require cooling water and steam heating. This allows the existing columns to be replaced with a dividing wall column. The above discussions only involve the potential replacement of the existing column pairs with the dividing wall column. Similar arguments will follow as to which column pair should be selected for prefractionator arrangement and the Petlyuk column arrangement.

Feed Composition, Conditions and Product SpeciŽ cations Table 1 lists the feed composition to the debutanizer in the existing column, showing a range of light hydrocarbons from propane to hexane+ with the presence of a signiŽ cant amount of the middle key, n-butane. In the absence of any feed condition information reported for the case study by Lestak and Collins15, the feed conditions were assumed as follows: Feed  ow rate: 600 kmol h71; Feed pressure: 8 bar; Feed temperature: 83° C. The product speciŽ cations are as follows:

· C5 + : 99 mol% (bottom of the debutanizer); · iC4: 99 mol% (top of deisobutanizer); · nC4: 95 mol% (bottom of deisobutanizer) Based on the feed condition and the product speciŽ cations from the debutanizer and deisobutanizer, simulation was performed to quantify the energy consumption and the  ow rates of the existing arrangement. This was necessary as a basis for comparison in evaluating other column arrangements. Table 2 summarizes the energy performance of the debutanizer and the deisobutanizer, and the product  ow rates from the columns. It shows that the current energy consumption of 3057 kW and 6029 kW for debutanizer and the deisobutanizer respectively.

Existing Column Hydraulics For the purposes of illustration, the following column parameters, as listed in Table 3, were assumed to deŽ ne the existing hydraulic features of the columns. Using these

C3

iC4 P=3.5bar T=34 C

1

1

10 P=12.1 bar

P=4.4bar T=33 C

Table 2. Energy performance and the product  ow rates for debutanizer and the deisobutanizer.

40

1

Columns

23

Reboiler duty, kW

19

40

P=4.1bar T=95 C

92

P=5.5bar T=55 C

nC4 C5+

Depropaniser

Debutaniser

Deisobutaniser

Figure 5. A simpliŽ ed  ow sheet illustrates the existing NGL separation train showing two pairs of columns for possible substitution for the dividing wall column.

Table 1. Feed composition to debutanizer shows about 50% of the feed is the middle key, n-butane. Component Propane i-butane n-butane i-pentane n-pentane Hexane+

Mole fraction 0.0008 0.1943 0.4841 0.0849 0.0972 0.1387

Debutanizer Deisobutanizer

3057 6029 Flow rate, kmol h-

Product streams C5+ (debutanizer) nC4 (deisobutanizer) iC4 (deisobutanizer)

1

192 305 103

Table 3. Prime parameters assumed to deŽ ne the existing column hydraulics.

Column diameter, m Tray type Number of  ow paths Fraction of entrainment  ooding, % Downcomer backup, % Pressure drop=tray, mm liq

Debutanizer

Deisobutanizer

2.15 Sieve 1 (single pass tray)

2.75 Sieve 2 (two pass tray)

85 50 203

85 50 203

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DESIGN AND OPTIMIZATION OF FULLY THERMALLY COUPLED DISTILLATION COLUMNS: PART 2 721

parameters, the upper limit of the re ux ratio could be determined.

r2 = 10

102 kmol/h

Rig. simulation (mole frac.)

iC4 : 0.99 (mol frac)

iC4 : 0.99

1

r1 = 1

Options Considered

42 330 kmol/h

Three complex column options were investigated corresponding to the prefractionator arrangement, Petlyuk column and the dividing wall column. Since the scope of the retroŽ t study on the existing NGL separation train will focus on the re-arrangement of the existing columns into complex column arrangements, the most obvious selection for retroŽ t on this system is to fully exploit the two existing columns, the debutanizer and the deisobutanizer. As such, this choice must be investigated Ž rst because, in principle, it allows the cheapest retroŽ t option as no new major hardware will be required. If the two columns cannot be exploited completely, adding a new column, re-traying the existing columns or replacing with better internals should then be analysed.

1 600 kmol/h

92

DEBUTANISER

C3

Rig. simulation (mole frac.) C5+ : 0.99

DEISOBUTANISER

iC4: 0.99 (mole frac) 102 kmol/h

partial condenser

1

10 P = 12.1 bar

42 330 kmol/h

23

74 270 kmol/h

nC4: 0.95 (mole frac) 304 kmol/h

81

new pipings 92

40

C5+: 0.99 (mole frac) 194 kmol/h

DEBUTANISER 2172

Figure 7. RetroŽ t to a prefractionator arrangement fully utilizes the existing hardware.

Trans IChemE, Vol 79, Part A, October 2001

194 kmol/h C5+ : 0.99 (mol frac)

Prefractionator arrangement Figure 7 illustrates the modiŽ cations to the existing column arrangement to produce the prefractionator arrangement. It highlights the simplicity involved in achieving such a design. The original total condenser from the debutanizer can be readily converted into a partial condenser without incurring any substantial cost. However, it is expected that more pipe work would be needed due to the re-arrangement of the columns. Pipe work is needed for new streams such as the link between the new bottom stream of the debutanizer (prefractionator) and the deisobutanizer (main column), and to re-route the feed and product streams from the debutanizer to become intermediate feed streams to the deisobutanizer. In addition, a new side draw product stream to yield the nC4 product will be needed from the deisobutanizer. The pipe work costs depend heavily on the layout constraints that exist in the plant. In terms of control and operation of the prefractionator arrangement, it is easy to control the external  ows from the reboiler and condenser of the debutanizer. The energy savings generated from this arrangement are 27% compared with the performance of the existing column arrangement. Another signiŽ cant energy beneŽ t will be the hot utility consumption being distributed between the two reboilers that reduce the use of high temperature hot utility

Based on the rigorous simulations, the relative merits of the options can be compared.

Heat Duty (kW)

nC4 : 0.95

Figure 6. Applying the new design procedure shows good agreement with simulation.

Discussion of the Options

14

Rig. simulation (mole frac.)

81

To test the feasibility of the arrangement, the new design methodology introduced in Part 1 of the paper was used to carry out the investigation. This was accomplished by modifying the model to account for the Ž xed number of stages for both the debutanizer and the deisobutanizer, and the upper limit of the re ux ratios. The new design conŽ guration involving the prefractionator arrangement is shown in Figure 6. This was used to perform a rigorous simulation. The results show consistency in terms of the product distribution. Similarly, the second option involving the use of Petlyuk column can be analysed using the new design procedure and the results from the procedure used to initialize the simulation.

1

304 kmol/h nC4 : 0.95 (mol frac)

40

Analysis of the Options Using New Design Procedure and Simulation

1

74

14

DEISOBUTANISER 4500

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AMMINUDIN and SMITH C3 iC4: 0.99 (mole frac)

condenser bypass 415 kmol/h

1

10

1

new piping

P = 12.1 bar

1

39 40 72

22

79 80

23

reboiler 40 bypass

nC4: 0.95 (mole frac)

new piping 92 C5+: 0.99 (mole frac)

454 kmol/h DEBUTANISER Heat Duty (kW)

DEISOBUTANISER

0

6634

Figure 8. RetroŽ t to a Petyluk column arrangement shows the reboiler from the deisobutanizer supplies the heat load for both columns.

in the reboiler for the deisobutanizer (main column). Furthermore, by distributing the heat load to the prefractionator arrangement, the thermal bottleneck in the deisobutanizer can be alleviated, which in turn promotes process debottlenecking for the option (see later). Petlyuk column arrangement Figure 8 indicates more pipe work modiŽ cations are needed to implement the Petlyuk column arrangement. The pipe work modiŽ cations are primarily needed to create the thermal coupling links between the debutanizer and the deisobutanizer. As a result, the condenser and the reboiler from the debutanizer are bypassed. This highlights a drawback of this option; existing hardware is not fully utilized. In terms of utility consumption, even though the energy saving is similar to that for the prefractionator arrangement, the Petlyuk column utilizes both heating and cooling at extreme levels as the arrangement requires a complete shift of utility consumption from the debutanizer (prefractionator) to the deisobutanizer (main column). Extreme level utilities require expensive hot and cold utilities. In addition, as the load is now shifted to the deisobutanizer (main column) the debottlenecking option, particularly in the deisobutanizer, may be more restricted compared to the prefractionator arrangement option (see later) and the reboiler and condenser may need additional area. In terms of control and operation, it is useful to manipulate the liquid split (liquid side draw to the prefractionator) and the vapour split (vapour side draw to the prefractionator) of the thermal coupling  ows16. The Petlyuk arrangement would not be preferred because of the practical difŽ culty of

taking and controlling a vapour sidedraw from the main column. Hydraulic performance of the two options So far the discussion on the relative merits of the two options have concentrated on the external hardware. The internals for the two options will now be investigated based on the hydraulic parameters given in Table 3. Table 4 summarizes the maximum performance of the two options and it shows the existing internals can be accommodated for both options without any need for modiŽ cation. However, the table indicates that the hydraulic performance of the Petlyuk column arrangement is much closer to maximum speciŽ ed operating limits, as shown in Table 3, compared with the performance of the prefractionator arrangement. This conŽ rms that the prefractionator arrangement, which provides load distribution between the two reboilers, offers greater additional capacity to the separation train compared with the Petlyuk column arrangement. Dividing wall column option in the deisobutanizer By exploiting the presence of the two pass trays in the deisobutanizer, a dividing wall column can be constructed by inserting a dividing wall through the centre section of the two pass trays. Since the wall separates the middle section into two parts, corresponding to the prefractionator and the main column sections, the vapour and liquid trafŽ c in these sections may be restricted due to the smaller column diameter in these sections. Thus, the  ood point may be reached in this section of the dividing wall column.

Table 4. The deisobutanizer is limiting in the Petlyuk column. Prefractionator arrangement Hydraulic parameters Fraction of entrainment  ooding, % Downcomer backup, % Pressure drop, mm liquid

Petyluk column arrangement

Debutanizer

Deisobutanizer

Debutanizer

Deisobutanizer

53 27 54

58 22 51

62 28 57

77 26 58

Trans IChemE, Vol 79, Part A, October 2001

DESIGN AND OPTIMIZATION OF FULLY THERMALLY COUPLED DISTILLATION COLUMNS: PART 2 723 C3

iC4 Debutaniser column bypass

1 10

1 Double pass trays

P = 12.1 bar 1

23

X

Dividing wall

X

nC4

25

40

Not use of existing hardware (Debutaniser).

92

Extensive work on the column internals to install the dividing wall, may require a larger column shell C5+

Extensive plant downtime results in a loss of production

Figure 9. Possible implications of using the dividing wall column option in retroŽ t.

To avoid the problem, a larger column shell may be necessary. All this will require a signiŽ cant amount of work to be done on the column and lead to costlier modiŽ cations. Additionally, the work done on the column, either to install the wall or to replace with a new shell, will require considerable plant downtime and loss of production. Since the dividing wall column from the deisobutanizer can accomplish ternary separation in one unit, the dividing wall column arrangement results in a bypass around the debutanizer. Unless the debutanizer can be used for another separation service, the debutanizer will be redundant. Figure 9 summarizes the possible implications of having a dividing wall column in the deisobutanizer. It demonstrates the concerns regarding use of the dividing wall column in retroŽ t.

arrangements, the new design procedure can be easily adapted to include the hydraulic capacity and the total number of stages of the existing columns involved in retroŽ t. The hydraulic capacity can be deŽ ned by setting an upper limit for the re ux ratio. The investigation of using the dividing wall column option in retroŽ t has been demonstrated through a case study involving the retroŽ t of the NGL separation train for energy efŽ ciency and process expansion. Even though the dividing wall column option can still be used, it is achieved at the expense of greater number of modiŽ cations. Instead, similar beneŽ ts can be gained by maximizing the use of existing hardware to mimic the dividing wall column arrangement with lesser retroŽ t work.

Overall Comments on the Case Study This case study has demonstrated that the applicationof the dividing wall column in retroŽ t is very limited and should be used with caution. Instead, the use of thermodynamically equivalent arrangements, such as the prefractionator arrangement and the Petlyuk column, can prove beneŽ cial, not only in terms of achieving energy savings or capacity improvement, but also in the ability to exploit the existing hardware to the maximum. Of the two options, the prefractionator arrangement provides a better option than the Petlyuk column as the former arrangement is much simpler to design with complete use of existing equipment. Furthermore, the prefractionator arrangement, which allows the heat load to be distributed between the reboilers, offers much greater added capacity to the separation train. CONCLUSIONS In this paper, the use of dividing wall column in retroŽ t has been discussed as part of the use of complex columns for retroŽ t. However, its application seems to be restricted in retroŽ t due to possible complications associated with installing the dividing wall into the existing column, the possibility of needing a bigger column shell and the longer downtime incurred. Instead, other options have been proposed by using thermodynamically equivalent conŽ gurations such as the Petlyuk and the prefractionator arrangements, which meet the retroŽ t objectives to exploit the existing columns and minimize the need to have another unit. With these new Trans IChemE, Vol 79, Part A, October 2001

REFERENCES 1. Nye, J. O. and Gangriwala, H. A., 1992, Nye trays, AIChE National Meeting (March, New Orleans, USA). 2. Delnicki, W. V. and Wagner, J. L., 1970, Performance of multiple downcomer trays, Chem Eng Prog, 66(3): 50. 3. Fair, J. R. and Seibert, A. F., 1996, Understand distillation-column debottlenecking options, Chem Eng Prog, June: 42. 4. Bravo, J. L., 1997, Select structured packings or trays?, Chem Eng Prog, 93(7): 36. 5. Williams, J. A., 1998, Optimize distillation system revamps, Chem Eng Prog, 94(3): 23. 6. Sloley, A. W., 1999, Should you switch to high capacity trays?, Chem Eng Prog, 95(1): 23. 7. Litzen, D. B. and Bravo, J. L., 1999, Uncover low-cost debottlenecking opportunities, Chem Eng Prog, 95(3): 25. 8. Manley, D. B., 1998, Capacity expansion options for NGL fractionation, Proc 77th GPA Annual Convention, Gas Processors Association, pp 114. 9. Liu, Z.-Y. and Jobson, M., 1999, Hydraulic analysis of distillation columns for retroŽ t design, AIChE Spring Meeting (14–18 March, Houston Texas, USA). 10. Liebmann, K., 1997, Integrated Crude Oil Distillation Design, Ph.D. Thesis (UMIST, Manchester, UK). 11. Triantafyllou, C., 1991, The Design Optimization and Integration of Dividing Wall Distillation Columns, Ph. D. Thesis (UMIST, Manchester, UK). 12. Kister, H. Z., 1992, Distillation Design (McGraw-Hill, New York, USA). 13. King, C. J., 1980, Separation Processes (McGraw-Hill, New York, USA). 14. Kister, H. Z., 1990, Distillation Operation (McGraw-Hill, New York, USA). 15. Lestak, F. and Collins, C., 1997, Advanced distillation saves energy and capital, Chem Eng, July: 72. 16. Christiansen, A. C. and Skogestad, S., 1997, Energy savings in

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integrated Petlyuk distillation arrangements: Importance of using preferred separation, AIChE Annual Meeting (Los Angeles, USA).

ACKNOWLEDGEMENTS The authors would like to thank the European Union under Joule Project (JOU3-CT 95-0035), and the member companies of the UMIST Process Integration Research Consortium for their Ž nancial support of the work described in this paper.

ADDRESS Correspondence concerning this paper should be addressed to Professor R. Smith, Department of Process Integration, UMIST, P.O. Box 88, Manchester M60 1QD, UK. E-mail: [email protected] The manuscript was received 21 July 2000 and accepted for publication after revision 23 January 2001.

Trans IChemE, Vol 79, Part A, October 2001