Separation of acetic acid and water by complex extractive distillation

Separation of acetic acid and water by complex extractive distillation

Separation and Purification Technology 36 (2004) 131–138 Separation of acetic acid and water by complex extractive distillation Zhigang Lei, Chengyue...

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Separation and Purification Technology 36 (2004) 131–138

Separation of acetic acid and water by complex extractive distillation Zhigang Lei, Chengyue Li∗ , Yingxia Li, Biaohua Chen The Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing University of Chemical Technology, Box 35, Beijing 100029, China Received 11 December 2002; received in revised form 23 June 2003; accepted 24 June 2003

Abstract A new separation method, complex extractive distillation, was put forward in this work for separating acetic acid and water. Tributylamine was selected as the separating agent. The reversible chemical interaction between acetic acid and tributylamine was verified through infrared spectra (IR) and mass chromatogram (MS) technique. The mathematics models of equilibrium (EQ) stage with and without incorporating chemical equilibrium equations were, respectively, established to simulate the extractive distillation column. From the comparison of simulated results with experimentally observed results, it was concluded that the EQ stage model was accurate whether chemical equilibrium equation was incorporated or not because the chemical equilibrium constant was small under the operation condition. © 2003 Elsevier B.V. All rights reserved. Keywords: Complex extractive distillation; Acetic acid; Water; Tributylamine; Equilibrium stage model

1. Introduction It is well-known that acetic acid is an important raw material in the chemical industries. But in the production of acetic acid, it often exists with much water. As a high-purity of acetic acid is needed in industry, the development of effective methods for separating acetic acid and water is an urgent and very important challenge. By now, there are three methods commonly used for this separation, i.e. ordinary distillation, azeotropic distillation and extractive distillation [1–3]. Although the ordinary distillation is simple ∗ Corresponding author. Tel.: +86-10-64436787; fax: +86-10-64419619. E-mail address: [email protected] (C. Li).

and easy to be operated, its energy consumption is large and a lot of column trays are required. The number of column trays for azeotropic distillation is fewer than that for ordinary distillation. But the amount of azeotropic agent is great, which leads to much energy consumption because the azeotropic agents must be vaporized in the column. However, in the extractive distillation process, the separating agents are not vaporized and thus the energy consumption is relatively little. Therefore, extractive distillation is an attractive method for separating acetic acid and water, and has been studied by some researchers. In the extractive distillation, the selection of a suitable solvent is fundamental to ensure an effective and economical design. The reported separating agents [4,5] are sulfolane, adiponitrile, pelargonic acid,

1383-5866/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1383-5866(03)00208-9

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Nomenclature å c C exp. F H Kp L M P Q r rj R Rk,j S t T v V x y z

reaction volume (m3 ) number of components molar concentration in the liquid phase (kmol m−3 ) experimental value feed flowrate (kmol h−1 ) molar enthalpy (kJ kmol−1 ) chemical equilibrium liquid flowrate (kmol h−1 ) hold-up in the stage (kmol) stage pressure (kPa) heat duty (kJ h−1 ) number of reactions ratio of sidestream flowrate to interstage flowrate gas constant (kJ kmol−1 K−1 ) reaction rate (kmol m−3 s−1 ) flowrate of the sidestream (kmol h−1 ) time (h) temperature (◦ C or K) stoichiometric coefficient vapor flowrate (kmol h−1 ) mole fraction in the liquid phase mole fraction in the vapor phase mole fraction in the feed

Greek symbol ν stoichiometric coefficient Subscripts I component number j stage number k reaction number salt product produced by the reaction 1 water 2 acetic acid 3 tributylamine Superscripts F feed stream L liquid phase V vapor phase

heptanoic acid, isophorone, neodecanoic acid, acetophenone, nitrobenzene, and so on. It is evident that the interaction between acetic acid or water and these separating agents is mainly physical force including the van der Waals bonding and hydrogen bonding. Herein, a new term, complex extractive distillation, is put forward, and a new solvent is selected as the separating agent.

2. Mechanism of complex extractive distillation If we select the solvent, tributylamine (bp: 213.5 ◦ C), as the separating agent, then the following reversible chemical reaction may take place in the non-aqueous solution: HAc + R3 N  R3 NH+ · − OOCCH3

(a)

where HAc, R3 N and R3 NH+ · − OOCCH3 represent acetic acid, tributylamine and the salt or complex formed by the reaction, respectively. The complex can dissociate in the aqueous solution: R3 NH+ · − OOCCH3 → R3 NH+ + − OOCCH3

(b)

The reaction (a) may be reversible because weak acid (acetic acid) and weak base (tributylamine) are used as reactants. That is, for the extractive distillation process the forward reaction occurs in the extractive distillation column and the reverse reaction occurs in the solvent recovery column. Therefore, this new separation method is different from traditional extractive distillation, and based on the reversible chemical interaction between weak acid (acetic acid) and weak base (separating agent). So, we call this type of distillation complex extractive distillation. A new substance, R3 NH+ · − OOCCH3 , is produced in this reaction, which can be verified by infrared spectra (IR) technique in the absence of water. The IR used was the type, Mattson Cygnus-100 (made in USA) equipped with a KBr disk. The IR diagrams for different acetic acid concentrations in the solvent tributylamine are obtained and shown in Fig. 1, from which it can be seen that a new characteristic peak in the range of 1550–1600 cm−1 appears in the mixture of acetic acid and tributylamine, and is assigned to the carboxylic–salt functional group, –COO− [6,7]. This indicates that chemical reaction between HAc and R3 N indeed takes place.

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Fig. 1. IR diagrams for the mixture of acetic acid and tributylamine. (1) Acetic acid; (2) acetic acid 10 wt.% + tributylamine 90 wt.%; (3) acetic acid 20 wt.% + tributylamine 80 wt.%; (4) acetic acid 30 wt.% + tributylamine 70 wt.%; (5) tributylamine.

On the other hand, this chemical reaction is reversible, which can be verified by the GC–MS. The instrument used was the type GCMS-QP5000 (Shimadzu, Japan). The total ion chromatogram (TIC)

of the mixture of acetic acid 10 wt.% and tributylamine 90 wt.% is illustrated in Fig. 2, where nos. 1 and 2 peaks denote acetic acid and tributylamine, respectively.

Fig. 2. TIC of the mixture of: acetic acid 10 wt.% and tributylamine 90 wt.%.

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So, it indicates that HAc, R3 N and the product produced by them can all be detected by the combination of IR and MS techniques. In general, the reaction rate between weak acid and weak base is very quick. So, the chemical reaction between HAc and R3 N may be reversible. The further proof of reversible reaction is supported by calculating the chemical equilibrium constant in the following section. In order to verify the effect of tributylamine as the separating agent, the vapor liquid equilibrium (VLE) was measured by experiment. Fig. 3 shows the equilibrium data for the ternary system of water (1) + acetic acid (2) + tributylamine (3), plotted on a solvent free basis. It may be observed that the solvent, tributylamine, enhances the relative volatility of water to acetic acid in such a way that the composition of the more volatile component (water) is higher in the liquid than in the vapor phase. The reason may be that the interaction force between acetic acid and tributylamine is stronger than that between water and tributylamine because in the former the reversible chemical

1. The chemical reaction is reversible. The reaction product (here as R3 NH+ · − OOCCH3 ) is used as carrier to carry the separated materials back and forth. 2. One of the reactants (here acetic acid) is a low boiling-point component. It can be easily removed by distillation, so that the separating agent (here tributylamine) can be regenerated and recycled. 3. There are no other side reactions between the separating agent and the component to be separated. Otherwise, the separation process will be complicated and some extra equipment may be added, which results in no economy of this technique. It is obvious that the system consisting of water, acetic acid and tributylamine, meets these requirements. So, it seems to be advisable to separate water and acetic acid by complex extractive distillation. However, we should be cautious that not dialkylamine (dibutylamine) and monoalkylamine (butylamine), but trialkylamine (tributylamine) can be selected as the separating agent because the reaction between acetic acid and dibutylamine or butylamine is irreversible, and some amides will be produced.

1.00

0.80

y1

reaction takes place. That is, water would be obtained as the overhead product in the extractive distillation column, and acetic acid and tributylamine as the bottom product. In terms of the mechanism of complex extractive distillation, the following criteria should be satisfied to ensure that this method can be implemented.

0.60

3. Chemical equilibrium constant 0.40

0.20

0.00 0.00

0.20

0.40

0.60

0.80

1.00

x1 Fig. 3. VLE curves on the solvent free basis for the ternary system of: water (1) + acetic acid (2) + tributylamine (3) at 101.33 kPa. (䉬) Solvent/feed volume ratio, 2/1; (䊉) solvent/feed volume ratio, 1/1; () no solvent.

In general, the kinetic property of a chemical reaction, which is carried out in a distillation column, should be measured separately. However, it is very difficult for us to obtain the chemical equilibrium constant of this reaction between acetic acid and tributylamine by experiment because the amount of salt produced by them is not easy to be determined by ordinary analysis. By analyzing the reaction system, it is found that the new group –NH is formed and the old group –OH is disappeared during the reaction. This reaction is exothermic and the heat generated can be obtained from the reference [7], i.e. −2.17 kJ mol−1 . In

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addition, the ionization constant pKa of acetic acid and tributylamine at 25 ◦ C can be found from [8], i.e. 4.76 and 10.87, respectively. Therefore, the chemical equilibrium constant KP of the overall reaction of reactions (a) and (b) in the aqueous solution at 25 ◦ C can be deduced, and thus the relation of chemical equilibrium constant with temperature is expressed by the following equation.   2 Csalt 261.01 Kp = = exp −4.6284 + (1) CHAc CR3 N T

135

Vj

Lj+1

SVj

Fj

Vapor

Qj

Stage j

liquid Pj, Tj Reaction SLj

Vj-1

4. Mathematics model of complex extractive distillation For the design of distillation process, two types of models have been developed: the equilibrium (EQ) stage model and the non-equilibrium (NEQ) stage model [9–12]. However, building an NEQ model for a distillation process is not as straightforward as it is for the EQ stage model in which we need to simply add an equation to take account of the effect of chemical reaction equilibrium on a tray or section of packing. As we know, the NEQ model is more complicated than the EQ model. In the NEQ model, the design information on the column configuration must be specified, so that the mass transfer coefficients, interfacial areas, liquid hold-ups, etc. can be calculated. Therefore, for any new invented configuration of the column, many experiments have to be done in advance to obtain the necessary model parameters. Evidently, it is too tedious, and much time will be spent on the design of extractive distillation process. Fortunately, as pointed out by Lee and Dudukovic [9], a close agreement between the predictions of the EQ and NEQ models can be found if the tray efficiency or height equal to a theoretical plate (HETP) is known. A schematic representation of the EQ stage model is shown in Fig. 4. This EQ stage may represent a tray or a section of packing. The assumptions used in this work are summarized as follows. (1) Operation reaches steady state. (2) System reaches mechanical equilibrium. (3) The vapor and liquid bulks are mixed perfectly and assumed to be at phase equilibrium. (4) Heat of mixing can be neglected. (5) Reactions take place in the liquid bulk. (6) The condenser and re-boiler are considered as an equilibrium stage.

Lj

Fig. 4. Schematic representation of an EQ stage.

(7) Chemical reaction equilibrium is achieved in every stage (if chemical reaction is considered in the EQ stage model). (8) Chemical reaction equilibrium is not included in every stage (if chemical reaction is not considered in the EQ stage model). The equations that model EQ stages are known as the MESHR equations [10]. MESHR is an acronym referring to the different types of equation. The M equations are the material balance equations. The total material balance takes the form: dMj = Vj+1 + Lj−1 + Fj − (1 + rjV )Vj dt c r   − (1 + rjL )Lj + vi,k Rk,j εj ·

(2)

k=1 i=1

Mj is the hold-up on stage j. With very few exceptions, Mj is considered to be the hold-up only of the liquid phase. It is more important to include the hold-up of the vapor phase at higher pressures. The component material balance (neglecting the vapor hold-up) is dMj xi,j = Vj+1 yi,j+1 + Lj−1 xi,j−1 + Fj zi,j dt − (1 + rjV )Vj yi,j − (1 + rjL )Lj xi,j +

r 

vi,k Rk,j εj ·

(3)

k=1

In the material balance equations given above, rj is the ratio of sidestream flow to interstage flow: rjV =

SjV Vj

,

rjL =

SjL Lj

(4)

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where vi,k represents the stoichiometric coefficient of component i in the reaction k, and εj represents the reaction volume. The E equations are the phase equilibrium equations: yi,j = Ki,j xi,j ·

(5)

The S equations are the summation equations: c 

xi,j = 1,

c 

i=1

yi,j = 1·

(6)

is calculated by the modified Peng–Robinson (MPR) equation, and the sum of every vapor enthalpy multiplied by every mole fraction is the total vapor enthalpy. The liquid phase enthalpy is deduced from vapor phase enthalpy and evaporation heat. Thermodynamic data for this reaction system are taken from [16,17]. The simulation of extractive distillation column was performed on a PC (Pentium 4, 1.5 GB). It took a few minutes for the EQ stage model for an extractive distillation column with 25 stages.

i=1

The enthalpy balance is given by dMj Hj V L = Vj+1 Hj+1 + Lj−1 Hj−1 + Fj HjF dt − (1 + rjV )Vj HjV − (1 + rjL )Lj HjL − Qj (7) There is no need to take separate account in Eq. (7) of the heat generated due to chemical reaction since the computed enthalpies include the heats of formation. The R equations are the reaction rate equations. According to the assumption no. 7 given in Table 1, R is equal to zero. That is to say, 2 R = Kp CHAc CR3 N − Csalt =0

(8)

Under steady-state conditions all of the time derivatives in the MESH equations are equal to zero. The modified relaxation method where the MESH equations are written in unsteady-state form and are integrated numerically until the steady-state solution has been found, is used to solve the above equations [13–15]. In the EQ model, The Wilson model is used for description of liquid phase non-ideality, while the ideal equation of state is used for the vapor phase. The extended Antoine equation is used for calculating the vapor pressure. The vapor enthalpy of every component

5. Comparison of experimental and simulation results The experimental flow sheet of extractive distillation process with two columns, extractive distillation column and solvent recovery column, has been established in the laboratory and illustrated in Fig. 5 where components A (water) and B (acetic acid) to be separated are, respectively, obtained from the top of two columns, and the solvent S (tributylamine) is recovered in the solvent recovery column. The extractive distillation column, 30 mm in diameter, was composed of three sections: the rectifying section (700 mm height), the stripping section (700 mm height) and the scrubbing section (200 mm height). The recovery column, 30 mm in diameter, was composed of two sections, the rectifying section (600 mm height) and the stripping section (600 mm height). The

(A)

S

(B)

S+B A+B

1

2

Table 1 The operation condition of extractive distillation column No.

Solvent/feed volume ratio

Reflux ratio

1 2 3 4

1.0 1.0 2.0 2.0

1.0 0.0 1.0 0.0

S

Fig. 5. The experimental flow sheet of extractive distillation process with two columns: (1) extractive distillation column; (2) solvent recovery column.

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two columns were all packed by a type of ring-shaped packing with the size 3 mm (width) × 3 mm (height). The number of theoretical plate including a re-boiler and a condenser was determined by use of the n-heptane–methylcyclohexane system at infinite reflux ratio for each column. The number of theoretical plate of the extractive distillation column was determined as 25, and that of the solvent recovery column as 20. The extractive distillation column was operated at normal pressure, and the solvent recovery column was operated at a reduced pressure of 0.080 MPa. The solvent is still recycled from the recovery column to the extractive distillation column. Before the extractive distillation experiment was run, the mixture of acetic acid 72.1 wt.% and water 27.9 wt.% was separated in the extractive distillation column by ordinary distillation without adding any solvent. The product was obtained at the top with the concentration of water 94.87 wt.% for the reflux ratio 1.0, and 96.18 wt.% for the reflux ratio 4.0. In the extractive distillation process, the extractive distillation column is the key. The operation conditions of extractive distillation column are given in Table 1, where the feed was supplied to the column at room temperature and normal pressure, and the concentrations of acetic acid and water were 72.1 and 27.9 wt.%, respectively. At the same time, the solvent recovery column is operated in batch way. The experimental data as well as the calculated results from the EQ stage

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Table 2 Comparison of experimental and calculated values No.

1 2 3 4

Ttop (◦ C)

Water concentration at the top (wt.%)

Exp.

EQ1

EQ2

Exp.

EQ1

EQ2

99.9 100.2 99.6 99.9

99.7 99.8 100.0 100.5

99.7 100.0 99.6 99.8

99.74 99.81 99.77 99.86

98.83 98.15 99.63 98.36

98.95 97.04 99.19 98.05

models without (EQ1) and with (EQ2) incorporating chemical equilibrium equations are listed in Table 2. It can be found from Table 2 that: (1) high-purity of water can be obtained at the top of extractive distillation column in the case of small solvent/feed volume ratio and reflux ratio, which indicates that tributylamine is effective for the separation of water and acetic acid; (2) by comparison of the calculated results with the experimental data, it can be seen that the EQ stage model was accurate irrespective of whether chemical equilibrium equation was incorporated or not. The reason may be that the value of chemical equilibrium constant was small, about 0.02 under the operation condition. This means that this reaction is reversible, and the chemical interaction between acetic acid and tributylamine is weak, which leads to no apparent influence of

Table 3 The calculated composition and temperature distributions along the extractive distillation column for the EQ1 and EQ2 models No.

1 2 3 5 7 9 11 13 15 17 19 21 23 24 25

x1

x2

x3

x4

T

EQ1

EQ2

EQ1

EQ2

EQ1

EQ2

EQ1

EQ2

EQ1

EQ2

0.0004 0.0029 0.0068 0.0301 0.1200 0.3430 0.6184 0.4862 0.4885 0.6334 0.6612 0.7236 0.7939 0.8717 0.9730

0.0002 0.0016 0.0039 0.0176 0.0743 0.2453 0.5097 0.5184 0.5876 0.6560 0.6903 0.7375 0.8499 0.8777 0.9693

0.4970 0.8910 0.9350 0.9168 0.8270 0.6015 0.3255 0.3006 0.3061 0.2318 0.2029 0.1386 0.0722 0.0328 0.0115

0.4645 0.8630 0.8997 0.9057 0.8510 0.6867 0.4277 0.3528 0.2832 0.2274 0.1835 0.1027 0.0422 0.0207 0.0103

0.5029 0.1058 0.0582 0.0531 0.0530 0.0553 0.0561 0.2133 0.2054 0.1348 0.1359 0.1378 0.1340 0.0955 0.0155

0.4711 0.0950 0.0628 0.0613 0.0599 0.0557 0.0430 0.1158 0.1048 0.0959 0.1188 0.1427 0.0965 0.0946 0.0203

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0641 0.0403 0.0336 0.0155 0.0148 0.0123 0.0196 0.0131 0.0243 0.0206 0.0074 0.0172 0.0094 0.0070 0.0002

404.83 393.75 392.51 391.41 388.34 382.65 377.76 378.06 378.73 376.89 376.44 376.44 374.12 373.30 372.84

405.37 393.84 392.70 391.98 389.60 384.33 378.90 378.57 378.15 376.67 376.26 374.79 373.61 373.11 372.80

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chemical reaction on solving the MESH equations of distillation column. In order to further investigate the difference between the EQ1 and EQ2 models, the composition and temperature distributions along the extractive distillation column under the no. 1 operation condition are given in Table 3, where subscripts 1, 2, 3 and 4 represent water, acetic acid, tributylamine and salt produced by the reaction, respectively. The tray is numbered from the bottom to the top. It can be seen from Table 3 that the calculated results of EQ1 model correspond well with those of EQ2 model on a large part of trays. However, in the vicinity of the feed tray, i.e. from nos. 9 to 15, the difference of liquid composition distribution is evident, which may be due to the relatively great influence of the feed mixture on the chemical reaction between acetic acid and tributylamine. So, the EQ1 model is more suitable for the preliminary design of the extractive distillation process disregarding the effect of chemical reaction.

6. Conclusion A new separation method, complex extractive distillation, was proposed for the separation of acetic acid and water. It was verified by the experiments that the complex extractive distillation is effective for separating acetic acid and water if tributylamine is used as a separating agent. Tributylamine is a chemically and physically stable solvent and can be easily available in the market. The separation mechanism of using tributylamine is based on the reversible chemical interaction between weak acid and weak base. Moreover, the mathematics models of equilibrium (EQ) stage with and without incorporating chemical equilibrium equations were, respectively, established to simulate the extractive distillation column. It has been proved by experiment that the EQ stage model is accurate whether chemical equilibrium equation is incorporated or not in the model, due to the small value of chemical equilibrium constant under the operation condition. It should be mentioned that although in this work complex extractive distillation was only used for the separation of acetic acid and water, it is not difficult to extend to such systems as acid + water and base

+ water systems because similar reversible chemical reactions can be used in the effective separations of these systems. Acknowledgements The authors are grateful to China Petroleum and Chemical Corporation for financial support of this work. References [1] R.W. Helsel, Removing carboxylic acids from aqueous wastes, Chem. Eng. Prog. 5 (1977) 55–59. [2] J. Golob, V. Grilc, B. Zadnik, Extraction of acetic acid from dilute aqueous solutions with trioctylphosphine oxide, Ind. Eng. Chem. Process Des. Dev. 20 (1981) 435–440. [3] Y. Kuo, H.P. Gregor, Acetic acid extraction by solvent membrane, Sep. Sci. Technol. 18 (1983) 421–440. [4] L. Berg, Dehydration of acetic acid by extractive distillation, US 5167774 (1992). [5] L. Berg, Dehydration of acetic acid by extractive distillation, US 4729818 (1987). [6] B. Dil, Y.Y. Yang, Y.Y. Dai, Extraction of acetic acid from dilution solution by reversible chemical complexation, J. Chem. Eng. Chin. Univ. 7 (1993) 174–179. [7] J.H. Ma, S.X. Sun, Extraction of acetic acid with primary amine N1923 , Chin. J. Appl. Chem. 14 (1997) 70–73. [8] L.L. Chen, Solvent Handbook, Chemical Industry Press, Beijing, 1997. [9] J.H. Lee, M.P. Dudukovic, A comparison of the equilibrium and non-equilibrium models for a multi-component reactive distillation column, Comput. Chem. Eng. 23 (1998) 159–172. [10] R. Taylor, R. Krishna, Modelling reactive distillation, Chem. Eng. Sci. 55 (2000) 5183–5229. [11] R. Baur, A.P. Higler, R. Taylor, R. Krishna, Comparison of equilibrium stage and non-equilibrium stage models for reactive distillation, Chem. Eng. J. 76 (2000) 33–47. [12] R. Baur, R. Taylor, R. Krishna, Dynamic behaviour of reactive distillation columns described by a non-equilibrium stage model, Chem. Eng. Sci. 56 (2001) 2085–2102. [13] H. Komatsu, Application of the relaxation method of solving reacting distillation problems, J. Chem. Eng. Jpn. 10 (1977) 200–205. [14] H. Komatsu, C.D. Holland, A new method of convergence for solving reacting distillation problems, J. Chem. Eng. Jpn. 10 (1977) 292–297. [15] J. Jelinek, V. Hlavacek, Steady-state countercurrent equilibrium stage separation with chemical reaction by relaxation method, Chem. Eng. Commun. 2 (1976) 79–85. [16] J.S. Tong, The Fluid Thermodynamics Properties, Petroleum Technology Press, Beijing, 1996. [17] R.C. Reid, J.M. Prausnitz, T.K. Sherwood, The Properties of Gases and Liquids, McGraw-Hill, New York, 1987.