The reduction of extractive agent in extractive distillation and auto-extractive distillation

The reduction of extractive agent in extractive distillation and auto-extractive distillation

Chemical Engineering and Processing 41 (2001) 673– 679 www.elsevier.com/locate/cep The reduction of extractive agent in extractive distillation and a...

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Chemical Engineering and Processing 41 (2001) 673– 679 www.elsevier.com/locate/cep

The reduction of extractive agent in extractive distillation and auto-extractive distillation Nidal Hilal a,*, George Yousef b, Paul Langston a a

School of Chemical, En6ironmental and Mining Engineering, The Uni6ersity of Nottingham, Uni6ersity Park, Nottingham NG7 2RD, UK b Department of Chemical Engineering, Al-Baath Uni6ersity, Homs, Syria Received 1 August 2001; received in revised form 9 November 2001; accepted 9 November 2001

Abstract Methanol–acetone, methanol–methyl acetate and methanol– chloroform were used as binary systems in extractive distillation and water–methanol–acetone, water–methanol–methyl acetate and water– methanol– chloroform were used as ternary systems in auto-extractive distillation, to study the possibility of reducing the specific consumption of extractive agent (q) and therefore, to reduce energy consumption of the separation process. Water was used in both binary and ternary systems as the extractive agent. Increasing the distance between the mixture feed and extractive agent feed along the separation column led to a 35 – 40% reduction in the specific consumption of extractive agent, while splitting the extractive agent feed into two branches on the column led to a further reduction of 25–30%. Computer simulation with HYSYS supported the conclusions for the binary methanol–acetone system. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Auto-extractive distillation; Extractive distillation; Azeotropes

1. Introduction The separation of liquid and gas mixtures into their constituent components is one of the major unit operations in the chemical, food and pharmaceutical industries. The most widely used processes are extraction, absorption, adsorption, membrane separation and distillation [1–3]. Throughout the chemical industry, the demand for purer products, coupled with the relentless pursuit of greater efficiency, has necessitated continued research into the techniques of distillation. There are many methods of distillation, such as simple distillation, partial distillation, flash or equilibrium distillation, rectification, azeotropic distillation and extractive distillation [4–6]. Extractive distillation is one of the most important of the distillation processes, since it combines two processes at the same time (distillation and extraction). This combination has many advantages in the chemical and food industries. Food and

* Corresponding author E-mail address: [email protected] (N. Hilal).

chemical applications have benefited from this type of separation process, for example, in the separation of volatile components of fruit [7], edible oils [8,9], wine aroma compounds [10] and polycyclic aromatic hydrocarbons, phenols and aromatic amines in particulate phase cigarette smoke [11]. Azeotropes are complex mixtures that require special methods for separation, such as pressure distillation and azeotropic distillation [12,13], as well as pervaporation membrane separation processes [14,15]. When the components of a system have low relative volatilities (0.95B hB 1.05) separation becomes difficult and expensive because a large number of trays are required and, usually, a high reflux ratio as well. Both equipment and utilities costs increase markedly and the operation can become uneconomic. If the system forms azeotropes, the azeotropic composition limits the separation and, for a better separation, this azeotrope must be bypassed in some way. For many systems, the vapor –liquid equilibrium of the feed mixture can be altered by deliberately adding a new material. In extractive distillation, the extractive agent is a higher-boiling material that affects the heavier components in the feed and exits with the bottoms. The choice

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of the extractive agent must affect the vapor– liquid equilibrium relations and relative volatilities, and it should be easily separable from the components with which it leaves the column. The principle of solvent extraction distillation lies in the addition of a new substance to the mixture in order to increase the relative volatility of the two key components and thus make separation relatively easier [16]. The substance added forms an azeotrope with one or more of the components in the mixture and as a result, is present on most of the plates of the column. The substance added is relatively non-volatile compared with the components to be separated and is therefore fed continuously near the top of the column. This extractive agent runs down the column as reflux and is present in appreciable concentration on all the plates. Separation of azeotropes is considered to be a complicated process. The complexity depends on liquid– liquid, vapor–liquid equilibrium of the system under study, in other words, it depends on the type and nature of azeotropic structure in the system [13,16]. The relationship of relative volatility for non-ideal mixtures has the following form: hAB =

kA p 0A kB p 0B

(1)

where kA and kB are the activity coefficients of A and B and p 0A, p 0B is the vapor pressure of pure components, A and B. The solvent added to the mixture (extractive agent) increases the activities of the two components kA and kB and hence, the relative volatility hAB [16]. The light product of the mixture is obtained from the top of the column, while the extractive agent and the heavy component from the base of the column [12,17]. The extractive agent should have the following characteristics: lower volatility than the volatilities of the mixture components, increases the relative volatility of the mixture, preferably does not form an azeotrope with any of the mixture components, is not appreciably vaporized in the fractionator, is readily available and cheap [12,16]. Water has most of the above characteristics, therefore, it has used been as an extractive agent in many systems. Previous studies have concentrated on the effect of the extractive agent on the separation process. For example, its effect on the relative volatility of the mixture, the effect of feed concentration on specific consumption of extractive agent and the effect of adding the extractive agent as one of the components of the mixtures [18,19]. The possibility of reducing the costs of energy consumption in auto-extractive distillation has also been studied [20]. In this paper, we describe an experimental study that concentrates upon methods to reduce the specific consumption of extractive agent (q) in both extractive and

auto-extractive distillation, in order to reduce energy consumption in the separation process. The specific consumption of extractive agent (q) is defined as: q=

FEA F

(2)

where FEA is the flow rate of extractive agent (mol/h) and F is the flowrate of the feed mixture (mol/h). Methanol–acetone, methanol–methyl acetate and methanol–chloroform were used as binary systems in extractive distillation, while water–methanol–acetone, water–methanol–methyl acetate and water–methanol– chloroform were used as ternary systems in auto-extractive distillation.

2. Experimental equipment and procedures Solvent-extraction distillation was carried out in a packed column of eight practical stages, each 300 mm high and 50 mm diameter with a sampling point. The column was packed with small random glass pieces of 1–2 mm average dimensions. The column was operated at atmospheric pressure. Fig. 1 shows a schematic diagram of the column. Operating conditions, such as flow rate and position of both feed and extractive agent and composition of azeotrope, were fixed before carrying out each experiment. During the experiments, the flow and tempera-

Fig. 1. Schematic diagram of the column.

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Fig. 2. Equilateral-triangular phase diagrams for systems used in the study.

tures at the top and bottom of the column were measured every 15 min. Samples were taken from each stage and from the top and bottom of the column every 1 h for analysis. After reaching steady state, the column was kept under strict observation for 3– 4 h, samples were taken from all stages to confirm steady state operation and the final data analysis was then recorded. The mixture feed was introduced into the second stage in all experiments, water was used as the extractive agent and introduced into different stages along the column. 3. Results and discussion The systems reported in this study were chosen due to the availability of information on specific consumption of extractive agent and for comparison with other studies reported in the literature on similar columns [18,19] under similar conditions but at a fixed position of extractive agent feed, stage 6. The component mixtures chosen in this work have different types of equilateral-triangular phase diagrams, as shown in Fig. 2(a –c). Methanol–acetone consists of one binary positive azeotrope and two separation regions, methanol– methyl acetate consists of two positive binary azeotropes and three separation regions and chloroform– methanol consists of two binary positive azeotropes, one ternary positive azeotrope and four separation regions [4,21].

3.1. Extracti6e distillation 3.1.1. Optimum position of extracti6e agent feed Water is a relatively non-volatile component compared to acetone and methanol and it is therefore usually fed continuously near the top of the column. Both acetone and methanol separately form non-ideal liquid solutions with water, but the extent of non-ideality with acetone is greater than with methanol. When all three substances are present, the acetone and methanol themselves behave as a non-ideal mixture and then relative volatilities become higher. It is the custom that the extracting agent is added at or near the top of the tower and exits at the bottom. If this agent is not added at the top of the column, the plates from the top down to the extracting agent feed plate serve to recover this agent. To determine the optimum position of the extractive agent feed, a 50% mol composition of acetone– methanol was fed to the column at stage 2, while the extractive agent feed was introduced to the column at stage 3. The experiments started by increasing the flow rate of the extractive agent when it was fed to stage 3 until the azeotrope began to shift at the ratio of water feed to methanol–acetone feed (q) of 4 mol/mol. The position of extractive agent feed was then moved to successive stages, maintaining this value of q, until a top product of almost pure acetone was obtained when the extractive agent was introduced at stage 8, as shown in Fig. 3.

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Fig. 3 shows the strong effect of the distance between the methanol–acetone feed and the feed of the extractive agent. There was a gradual decrease of methanol in the top product from 17.7% mol when extractive agent was fed at stage 3 to 0% mol when that feed was at section 8. This shows that the extractive agent requires a sufficient distance along the column to fully separate methanol from the system. This distance plays an important role on the effect of the extractive agent on the activity coefficients ratio (kA/kB) and therefore, the relative volatility (hAB), enabling full shift of the azeotrope under study to obtain pure final product. Previous work [18] was carried out on a similar column in which the extractive agent feed was introduced to stage 6 at q = 6.4 mol/mol, to obtain a high purity acetone from a 50% mol methanol– acetone feed. Comparison of the findings of this work with the results of Ref. [18] shows a 38% reduction of specific consumption of extractive agent when the feed position of the extractive agent was raised from stage 6 to stage 8, as shown in Fig. 4. The results of Hanina et al. [18] on a similar column shows an inflection point when the extractive agent was introduced to the column at stage 6. This inflection was not as clear in the experimental results found in this work when the extractive agent feed was at stage 8 or as multiple feed at stage 5 and stage 8. Similar experiments were repeated at different compositions in the range 10– 90% by mole of methanol for three systems, acetone– methanol, methyl acetate– methanol and chloroform– methanol as shown in Fig. 4. A 35–40% reduction of specific consumption of extractive agent was obtained. It is clear that the relative decrease in specific consumption depends on feed tray location.

Fig. 3. Effect of extractive agent position.

Fig. 4. Specific consumption of extractive agent in binary systems. (a) Methanol– acetone, (b) methanol – methylacetate and (c) methanol – chloroform.

3.1.2. Effect of multi-extracti6e agent feed The extractive agent was then introduced to the column at two positions, stage 5 and stage 8, in equal proportions for a 50% mol methanol–acetone mixture. This resulted in a 28% further reduction of q compared to that of single feed at stage 8. Similar experiments were repeated at different compositions of methanol in the range 10–90% for three systems, acetone–methanol, methylacetate–methanol and chloroform–methanol. A 25–30% further reduction of q was obtained compared to that of single feed at stage 8. This further reduction due to splitting the extractive agent feed into two branches on the column was also inversely proportional to the methanol concentration in the mixture. The splitting of extractive agent feed increased the ability of the extractive agent to increase the relative volatility of the mixture, hAB, which led to a reduction of q. Fig. 4 shows the reduction of extractive agent feed and a comparison with previous work.

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3.2. Auto-extracti6e distillation In this part of the study, the experimental findings in extractive distillation for binary systems were tested and applied for ternary systems, which have the extractive agent (water) as one of the components. Therefore, the study concentrated on the sections Xratio =xMethanol/ x = constant on the equilateral-triangular phase diagrams, where x was the molar concentration of the light component. Then, the effect of water concentration in the mixture feed, in the range between 0 and 80% on the specific consumption of extractive agent, was investigated. Two sets of experiments were run for each ternary system; methanol concentration to the light component was high in the first set Xratio =4 and low in the second set Xratio = 0.25 for acetone and methylacetate systems and Xratio =0.65 for the chloroform system. No values below Xratio =0.65 were available. The experimental results obtained in auto-extractive distillation for ternary systems are shown in Figs. 5–7 and clearly show the same behaviour as that for extractive distillation. The reduction of q at Xratio =4 for all systems was :35% due to changing the position of extractive agent feed from stages 6 to 8. This was further reduced by :25% from the value of q at stage

Fig. 6. Specific consumption of extractive agent in system of methanol – methylacetate – water, (I) xMethanol/xMethyacetate = 4; (II) xMethanol/xMethyacetate =0.25.

8 when splitting the extractive agent feed. At Xratio = 0.25 and 0.65, the reduction of q was : 40% when the extractive agent feed was introduced to stage 8. This was further reduced by : 30% from the value of q at stage 8 when splitting the extractive agent feed into two branches.

4. HYSYS simulation

Fig. 5. Specific consumption of extractive agent in system of methanol – acetone– water, (I) xMethanol/xAcetone = 4; (II) xMethanol/ xAcetone = 0.25.

After the experimental work had been completed, it was considered that theoretical simulation would be helpful to support the findings of this work. This will be the subject of a future study along with an overall economic optimisation. Some preliminary simulation results are shown here. The simulation was undertaken with the HYSYS model version 2.1 under license from Hyprotech [22]. The acetone–methanol binary feed with water solvent was studied with the scenario described in Fig. 3. That is a 50% by mole mixture fed into stage 2 and water fed into stage 3 with a q value of 4 for the first simulation. Then the water was fed into stage 4 for the next simulation and so on, up to stage 8. The feed streams were at 25 °C and atmospheric pressure. Various activity models available in HYSYS were tried and

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all gave similar results. The Van Laar package was used for the results shown here. The rigorous column was used and the main uncertainty was how many ideal trays to use. The experimental column contained packing. From the simulation results obtained, it is clear that this can be represented at the extreme by 8–32 ideal trays. The column solution converged in all cases using a distillate flow (47.8% of binary feed) specification and reflux ratio (R) specification. The value of R was not critical for the results. Fig. 8 shows the results obtained for these two cases. The experimental results in Fig. 3 are reproduced in Fig. 8 to facilitate comparison with the simulation. The simulation results for the 8-tray column are in reasonable agreement with experiment, except that when the water enters at tray 8, there is a significant amount of methanol remaining in the distillate. This is not so for the 32-tray column simulation, but here, there is overall a much bigger difference between the results. It is conjectured that the column is nearer to 8 ideal trays, but that the region above the feed for the ‘8-solvent-entry’ case is not adequately represented in the 8-tray simulation. The simulation results also show how the water composition in the distillate changes. Clearly, the solvent entry position significantly affects this value and

Fig. 8. HYSYS simulation of effect of extractive agent position on binary methanol – acetone system: (I) 8-ideal-tray column and (II) 32-ideal-tray column.

its importance must be evaluated in the future economic study.

5. Conclusions

Fig. 7. Specific consumption of extractive agent in system of methanol – chloroform– water, (I) xMethanol/xChloroform = 4; (II) xMethanol/xChloroform = 0.65.

The paper describes an experimental investigation on extractive and auto-extractive distillation of binary and ternary systems. The distance along the column between extractive agent feed and mixture feed plays an important role in both extractive and auto-extractive distillation because it affects the relative volatility of the mixture. Increasing this distance led to a reduction of specific consumption of extractive agent q; the reduction was inversely proportional to the methanol concentration in the mixture. The water concentration in the mixture had no effect on the reduction of q. A further reduction of q was achieved by splitting the extractive agent feed into two branches on the column. This enhanced the ability of the extractive agent to increase the relative volatility of the mixture, which lead

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to a reduction of q. The reduction of q made the separation process more economical. Some preliminary computer simulations have supported the experimental conclusion of the effect of the solvent feed location.

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