Process Design and Optimization of an Acetic Acid Recovery System

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Process Design and Optimization of an Acetic Acid Recovery System in Terephthalic Acid Production via Hybrid Extraction-Distillation Using a Novel Mixed Solvent Gregorius R. Harvianto, Ki J. Kang, and Moonyong Lee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04586 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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Industrial & Engineering Chemistry Research

Process Design and Optimization of an Acetic Acid Recovery System in Terephthalic Acid Production via Hybrid Extraction-Distillation Using a Novel Mixed Solvent Gregorius Rionugroho Harviantoa, Ki Joon Kangb, Moonyong Leea, † a

School of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Republic of

Korea b

Benit M Co., Ltd., Ulsan 44992, Republic of Korea

Running title: Mixed Solvent Based Extraction-Distillation Hybrid Process for Acetic Acid Recovery

Submitted to Industrial & Engineering Chemistry Research

†

Correspondence concerning this article should be addressed to:

Prof. Moonyong Lee Email: [email protected] Telephone: +82-53-810-2512

1

ACS Paragon Plus Environment


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ABSTRACT An extraction and distillation hybrid process using a novel mixed solvent was proposed for enhancing acetic acid recovery during terephthalic acid production. Feasible hybrid extraction-distillation schemes were designed, simulated, and optimized for two mixed solvents, p-xylene + methyl acetate, and p-xylene + ethyl acetate, based on liquid-liquid equilibrium data for a quaternary system containing methyl acetate or ethyl acetate with pxylene, acetic acid, and water. A hybrid process using the p-xylene + methyl acetate mixed solvent was found to generate the desired purity of acetic acid with a lower energy consumption and higher product yield than conventional extraction processes. Results showed that the proposed hybrid extraction-distillation process improves the process economics remarkably: the hybrid process using the p-xylene + methyl acetate and p-xylene + ethyl acetate mixed solvent reduced a total annual cost by 14% and 6%, respectively compared with the conventional hybrid process using the ethyl acetate single solvent. With the important advantage of p-xylene and methyl acetate as internally existing components, the proposed mixed solvent is promising for safe and cost-effective recovery of acetic acid, particularly during terephthalic acid production.

Keywords: acetic acid recovery, terephthalic acid production, TPA, mixed solvent, extraction distillation hybrid process

1. INTRODUCTION Purified terephthalic acid (TPA) is an essential raw chemical material that is mainly used to create polyester fibers and polyethylene terephthalate. The TPA market is expected to expand in the future because of the growing demand for polyester. A high consumption of TPA is expected on the basis of its applications, which include textiles, bottles, packaging,

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and furnishings. The Asia-Pacific sector is the largest market on account of the high concentration of manufacturing industries, particularly in China and India. Specifically, the demand for TPA in Asia was 55.7 m ton/year at the end of 2013. Mordor Intelligence forecasts an annual growth rate of greater than 5% for the global TPA market.1 Since TPA is usually manufactured via catalytic liquid phase oxidation of p-xylene (PX) in acetic acid (HAc) in the presence of air,2 HAc and PX are necessary as the main reactants in TPA production processes. To minimize the loss of the valuable HAc solvent, HAc is generally recovered from water during this process; this demonstrates the economic significance of HAc recovery. Figure 1 shows a diagram of TPA production and its conventional recovery system, which involves an entrainer or a solvent. The crude TPA at the outlet of the reactor is subsequently purified by selective hydrogenation. This exothermic reaction also produces water as a by-product, which can be removed by an HAc recovery system. Products from the condensed liquid stream of the reactor and other dilute HAc streams, which comprise mostly the oxidation by-products such as methyl acetate (MA) and water, are mixed with the underflow from the absorber (condensed liquid from the vapor stream of the oxidation reactor) and fed to a dehydration unit to recover high-purity HAc.3, 4 As a solvent for TPA production, the purity of HAc is extremely important: the purity of HAc recovered from the HAc dehydration columns directly influences the quality of the produced TPA. In particular, in the HAc recovery system, a reduction in the amount of HAc lost at the recovery stream of the dehydration column results in higher energy consumption.4 The main component of this solvent recovery system is a dehydration column, which separates HAc and water. In terms of these applications, it is essential that HAc is recovered at a high rate. Most of the entrainer or solvent is recirculated inside the HAc recovery unit with the addition of small amounts to supplement for losses.

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Figure 1. Diagram of the main units in the TPA production process. As discussed earlier, the HAc recovered from the aqueous mixture must be of sufficient purity for TPA production to ensure the purity of TPA in the product stream.4 The economics of the process are contingent on the development of an effective HAc recovery system. An HAc-water mixture is very difficult to separate because of the low relative volatility of HAcwater mixtures with high water concentrations.5 Traditionally, heterogeneous azeotropic distillation with an entrainer has been widely applied for HAc dehydration.6-8 Acetic esters, such as isobutyl acetate (IBA), n-propyl acetate (NPA), and ethyl acetate (EA), are most commonly used as the entrainers for heterogeneous azeotropic distillation.7, 9-14 However, the heterogeneous azeotropic distillation is energy intensive to generate high-purity HAc. Moreover, this technique involves the introduction of a third component into the process to break the azeotrope. Furthermore, this solution is neither environmentally friendly nor costeffective.15 For example, small amounts of the additional components, such as IBA and NPA, as impurities in the streams of recovered HAc can impede separation and increase the

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consumption of steam and cooling water in the dehydration system.16 Also, heterogeneous azeotropic distillation often becomes unstable and difficult to control.4 To solve this HAcwater separation problem, several researchers have evaluated liquid-liquid extraction as an effective alternative method for HAc separation from an aqueous solution.5, 17-19 Liquid-liquid extraction, which is also well-known as solvent extraction, is a technique to separate mixtures based on their relative solubilities in two different immiscible liquids, commonly water and an organic solvent.20 Choosing the appropriate solvent is a key factor in extraction process design.21 With respect to HAc extraction, Kürüm et al.18 examined numerous solvents in terms of selectivity, distribution coefficients, boiling points, recoverability, density, vapor pressure, toxicity, cost, etc. For HAc recovery, the extraction column is combined with two distillation columns (usually termed hybrid extractiondistillation) to separate HAc and water at the required product purity specifications for TPA production plants. According to the plant cost evaluation by Wennersten22, solvent extraction successfully introduces savings both in plant investment and operating cost for HAc dehydration among the other techniques. In particular, as described by QVF company23, in the case of HAc concentrations of below 40 wt%, it will be an economic way the initial extraction of HAc from the aqueous solution with a suitable extraction agent before pure recovery occurs during the rectification of the azeotropic mixture. HAc separation by extraction is promising because it provides significant energy savings for the separation of difficult mixtures without the need for a reboiler.24, 25 Previous works24, 25 also discussed and showed the apparent advantage of hybrid extraction and distillation (HED) over the conventional heterogeneous azeotropic distillation and other processes in terms of energy efficiency. However, similar to the heterogeneous azeotropic distillation process, the additional solvent component (such as ethyl acetate or methyl-tert-butyl ether) often acts as an impurity, thereby reducing the

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effectiveness of the process. HAc recovery and water removal in subsequent distillation columns must also be accounted for in the operating- and capital-cost evaluation of this process. In this study, a novel mixed solvent using PX and MA was proposed and examined for the extraction of HAc from the HAc-water aqueous feed in a TPA purification unit. PX has a unique advantage as a solvent because of its availability as a main reactant in the industrial TPA production process. Accordingly, PX has been investigated as an entrainer for heterogeneous azeotropic distillation;3 however, using PX as a single solvent for HAc dehydration results in a limited entrainer performance and can also cause PX imbalances in the column.26 Moreover, using PX as a single solvent for extraction is not suitable because its liquid-liquid equilibrium (LLE) tie-lines are not tilted to the favorable direction although it induces a large LLE envelope. On the other hand, MA has a desirable distribution feature for a HAc-water system. However, using MA as a single solvent for extraction is also not suitable because of its narrow LLE envelope. Recently, Harvianto et al.27 studied the LLE behavior of the HAc+water+PX+MA quaternary system and found the potential of a PX+MA mixed solvent for HAc extraction. MA is a less popular solvent and is relatively inexpensive;28 however, as can be seen in Figure 1, MA is already included in the residue stream because it is an oxidation by-product of the TPA unit. Furthermore, the MA impurity level does not need to be tightly controlled in the recovered HAc because it reduces the loss of HAc in the oxidation reaction. 29,30 Thus, if the PX+MA mixed solvent results in efficient HAc extraction, these advantageous features will make it more attractive as a solvent. In this work, a PX+EA mixed solvent was also investigated for comparison because EA is widely used as a single solvent for HAc extraction in the conventional HAc recovery system.17, 19

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The performances of the two mixed solvents were examined to explore their potential for industrial applications. Rigorous process simulations were carried out using Aspen Plus by employing previously reported LLE data.27, 31

2.

EVALUATION

OF

THE

PROPOSED

MIXED

SOLVENT

FOR

HAC

EXTRACTION 2.1 Data correlation A quaternary system comprising HAc-water and the mixed solvent components shows very complicated vapor-liquid-liquid equilibrium (VLLE) behavior with multiple boiling points, homogeneous and heterogeneous azeotropes, and strongly non-ideal vapor phase characteristics

caused

by

the

association

of

HAc.4

LLE

data

for

quaternary

HAc+water+PX+MA and HAc+water+PX+EA mixtures 27, 31 were used in this study, and the regression was carried out using Aspen Plus v8.4. The universal quasi-chemical (UNIQUAC) model32 was chosen as a thermodynamic model for process design. The Aspen Plus physical parameter regression system has proven to be powerful for the regression and correlation of experimental data to obtain the binary parameters.33, 34 The UNIQUAC model parameters were regressed by least squares minimization of the differences between the calculated and experimental mass fraction of the components in the two liquid phases. Since Aspen Plus has built-in UNIQUAC parameters, the binary interaction parameters of water-PX, water-MA, and water-EA were already available, and the missing binary parameters for LLE were obtained from reported experimental data. 27, 31 Tables 1 and 2 lists the resulting UNIQUAC model parameters for the HAc+water+PX+MA and HAc+water+PX+EA quaternary systems, respectively.

Table 1. LLE parameter values for the UNIQUAC model of HAc+water+PX+MA

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Component i

Water

Water

Water

HAc

HAc

PX

Component j

PX

MA

HAc

PX

MA

MA

Source

Aspen

Aspen

Regression

Regression

Regression

Regression

aij

-42.4943

-1.4256

0

0

0

0

aji

37.1468

2.4986

0

0

0

0

bij (K)

1588.46

364.553

-1980

164.31

463.95

-334.111

bji (K)

-1764.09

-1053.42

499.843

-537.598

-1545.64

150.909

Table 2. LLE parameter values for the UNIQUAC model of HAc+water+PX+EA Component i

Water

Water

Water

HAc

HAc

PX

Component j

PX

EA

HAc

PX

EA

EA

Source

Aspen

Aspen

Regression

Regression

Regression

Regression

aij

-42.4943

-2.0532

0

0

0

0

aji

37.1468

2.7214

0

0

0

0

bij (K)

1588.46

531.033

-1099.46

122.478

412.579

-321.165

bji (K)

-1764.09

-1212.89

499.257

-446.491

-3043.65

151.181

The mass-based root-mean-square deviation (RMSD) between the experimental and calculated LLE compositions was evaluated to quantify the correlation accuracy. The calculated RMSD values were 0.0168 and 0.0232 for the quaternary systems with MA and EA, respectively; these are sufficiently low to indicate that the UNIQUAC model accurately correlates with the experimental LLE data.

2.2. Extraction process analysis

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During the extraction process, the solvent and broth are fed from the bottom and top, respectively, because the mass separation agent or solvent has a lower density than the aqueous mixture. The raffinate phase contains most of the water with traces of HAc and solvent, whereas the extract phase includes most of the solvent, HAc, and traces of water. The amount and composition of the mixed solvent are very important for the process economy because it directly affects the composition of HAc in the extract phase. A good solvent should have a high distribution coefficient for HAc and a high selectivity between HAc and water. Further, the solvent must be easy to separate from the HAc mixture. Since the solvent used in this study is a low-boiling-point extraction agent, the energy consumption of the process also depends largely on the vaporization enthalpy of the mixture in the subsequent distillation columns. Since the ratio of the extraction solvent to the fresh feed flow rate (S/F) is the most important design variable on the HED process, its effect on the HED process was investigated in several related works 21, 35, 36. Higher S/F results in larger HAc recovery. But the amount of inlet solvent which needs to be recovered in the recovery column also increases. Since the overall energy efficiency is totally based on the total sum of the reboiler duties of subsequent columns, the amount of solvent must be optimized for a given feed flow rate in the HED process. Figure 2 plots the number of extraction stages required to achieve an HAc recovery of at least 99.5 wt%, as required for the extraction column, versus the corresponding S/F for the studied solvents. The feed containing 30 wt% of HAc was introduced to the top of the extractor, while the solvent was added to the bottom. The extract and raffinate phases were taken from the top and bottom of the extractor, respectively. The initial analysis involved a mixed solvent containing 50 wt% PX and 50 wt% MA or EA. As shown in Figure 2, the required solvent amount decreases with increasing number of stages. For the same separation

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task, the required amount of PX+MA is about 80% of that of PX+EA with the same number of stages. For example, for 15 stages, the required S/F ratios were 2.8 and 3.02 for PX+MA and PX+EA, respectively.

Figure 2. The mass ratio of the solvent-to-feed (S/F) plotted against the number of separation stages to recover HAc at ≥99.5 wt%. Note: the blue and red lines represent reductions from stage 6 to 15 and 15 to 20, respectively.

To reduce the energy requirements of the subsequent distillation columns, the amount of solvent must be minimized. However, this must be balanced with the higher capital cost involved in an increased number of extractor column stages. As described in Figure 2, as the number of stages increased from 6 to 15 (blue line in Figure 2), there is a significant reduction in the amount of solvent needed. Further increasing the number of stages from 15 to 20 (red line in Figure 2) resulted in a relatively insignificant reduction of solvent required (from 2.78 to 2.62 for PX+MA and 3.02 to 2.78 for PX+EA). Based on these quantitative and

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qualitative analyses, the optimal number of stages for the extractor was determined to be 15 for all case studies. Further, since a mixed solvent is being used, the effect of the flow rate of PX and MA (or EA) in the solvent stream must also be taken into account. A larger amount of HAc can be extracted by increasing the flow rate of PX and MA (or EA) in the extractor column. However, not all of the HAc in the feed is recoverable; therefore, some of the HAc remains dissolved in the aqueous phase. In addition, a higher PX and MA (or EA) flow rate increases the energy and capital costs of handling and separating the mixed solvent from HAc. Therefore, each component flow rate of the solvent stream must be optimized to minimize the total cost of the process. For this purpose, the effects of the PX and MA (or EA) flow rates on several important process parameters were investigated to determine the optimal PX and MA (or EA) flow rates. The amounts of HAc recovered were determined as the dependent variables in both systems. Figures 3, 4, and 5 show the effect of the PX and MA (or EA) flow rates on the HAc recovery, water flow rate of the extract stream, and reboiler duty of the regeneration column, respectively. The amount of solvent used was varied from 100–8000 kg/h for PX and 4000–8000 kg/h for MA (or EA). As seen from the sensitivity analysis results in the figures, the PX and MA (or EA) flow rates in the solvent stream significantly impact HAc recovery in the extract stream. From Figures 3(a) (or 3(b)), it is evident that HAc recovery is more dependent on the MA (or EA) flow rate than the PX flow rate in the solvent stream. In addition, a high PX flow rate enhances the MA (or EA) performance to generate a high HAc flow rate in the extract stream. These results are consistent with the previously reported LLE results27,31: the separation efficiency increases with increasing amount of PX solvent. A comparison of Figures 3(a) and (b) reveals that MA showed slightly better performance than EA on the extractor: it produced a high HAc flow rate in the extract stream with the same amount of

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MA and EA input in the solvent stream. Note that if the PX to MA mass ratio on the mixed solvent stream was below ~0.25 (i.e., 1000 kg/h of PX in this study), only a single phase is generated, which means that no separation occurs in the extraction column, as observed previously.27 Formerly, to elucidate the separation efficiency of HAc from water, it is necessary to determine the amount of water in the extract stream. As can be seen in Figure 4; a higher PX flow rate results in decreased water flow in the extract stream, which leads to a higher HAc concentration in the extract stream. However, in contrast with the HAc recovery performance, EA has a slightly better performance with respect to decreasing the water flow rate in the extract stream than MA, as shown in Figure 4; this occurs because MA is more soluble in water during this liquid-liquid separation. This HAc recovery performance correlates well with the reported ternary diagram of mixed solvent+HAc+water27,31, in which the two-phase LLE region of the PX+EA system was larger than that of the PX+MA system. In a typical extraction-distillation process, the liquid-liquid extractor is followed by distillation columns (i.e., a recovery column and stripping column)35-37. Lucia et al.17 reported that a stripping column has only a minor effect on the total energy requirements of this process by removing the water from the bottom stream. Since the bottom product is water, low-pressure steam is used to supply the energy for reboiling in the bottom of the column. This minimizes both the capital and operating costs of the reboiler.38 Consequently, the reboiler duty in the recovery column was analyzed to confirm the total duty required in the subsequent distillation columns. This recovery column produced HAc and PX in the bottom stream to supply as a feed to the reactor. With the same separation task in a recovery column, higher PX and MA (or EA) flow rates generate higher duty requirements in the distillation column. Thus, Figure 5 can be used to find the minimum duty required to meet the design specifications in the extraction and distillation columns.

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Figure 3. Effect of the solvent flow rate on the HAc flow rate in the extract stream for the PX+MA system (a) and PX+EA system (b). The arrow indicates the HAc recovery specifications.

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To optimize the process to fulfill the high specifications for HAc recovery (99.5 wt%), optimization was done using the built-in optimization function in Aspen Plus. The minimum amount of solvent needed in the solvent feed was 4257.54 kg/h of PX and 6167.95 kg/h of MA for the PX+MA solvent and 393.92 kg/h of PX and 6698.12 kg/h of EA for the PX+EA solvent. In conclusion, the PX+MA solvent required less energy than the PX+EA solvent, although slightly more PX+MA solvent was required to extract a given amount of HAc than PX+EA solvent.

Figure 4. Effect of the solvent flow rate on the water flow rate in the extract stream for both systems.

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Figure 5. Effect of the solvent flow rate on the reboiler duty in the recovery column for both systems.

3. SIMULATION OF HYBRID EXTRACTION-DISTILLATION PROCESS 3.1 Thermodynamic model The hybrid extraction-distillation sequences were subjected to rigorous optimization using Aspen Plus process models. Note that these process models were robust and thermodynamically rigorous. In this process design, it was essential to apply different thermodynamic models for the LLE in the extraction column and decanter and for the vaporliquid equilibrium (VLE) in the distillation column. The detailed VLE binary parameters for vapor-liquid separation derived from the Aspen Plus built-in parameters are provided in Supporting Information. These binary parameters were obtained from the Aspen Plus v.8.4 databank to model liquid and vapor behavior. For analyzing the non-ideal behavior in the vapor phase, the second virial coefficient and fugacity coefficient were calculated using the Hayden–O’Connell (HOC) 15



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equation.39 Meanwhile, the non-ideal behavior in the liquid phase was separately represented by activity coefficients using the UNIQUAC model.32 Furthermore, the LLE quaternary parameters obtained from the experimental data (see Section 2) were used to model the liquid-liquid separation in the extractor and decanter.

3.2 Process design for the hybrid extraction-distillation process Figures 6 and 7 show the configurations of the hybrid extraction-distillation process with heat and material balances using the PX+MA and PX+EA mixed solvents, respectively. The extract phase from the top of the extractor was fed to a recovery column (RC), and HAc and PX were recovered from the bottom stream. Also, the raffinate phase from the extractor (E) bottom was fed to the stripping column (SC) where most of the water was removed as the bottom product. The top streams from both distillation columns were fed to the decanter (D) to undergo single-stage liquid-liquid separation into solvent-rich and water-rich phases. The lower phase, which contains the most water, was fed back to the stripping column together with the raffinate stream. To recycle the solvent, it was taken from the upper phase, mixed with a make-up solvent, and fed as the solvent stream into the extractor. Recycling a solvent greatly reduces the amount of fresh solvent required. Thus, the energy requirements for the recovery and stripping columns must also be accounted for in the selection of the solvent. This hybrid process eliminates the need of azeotropic distillation, which makes the process more competitive. Similar schematic configurations were considered and compared for those of the three different solvents in this study.

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Figure 6. Hybrid extraction-distillation process using PX+MA mixed solvent.

Figure 7. Hybrid extraction-distillation process using PX+EA mixed solvent.

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3.3 Design variables, feed conditions, and constraints The objective of optimization was chosen to minimize the total annual cost (TAC), which is mainly proportional to the heat duty and column sizes. Minimization of this objective was subject to the required HAc recovery and water purities in each outlet stream. There are three optimizing variables associated with the extraction column in the proposed design: (1) the ratio of extraction solvent to feed flow rate, (2) the ratio of PX to MA (or EA) in the extraction solvent, and (3) the total number of extraction column stages. The optimal values of these variables are discussed in Section 2. Further, there are also three optimizing variables in the subsequent distillation columns: (1) the number of stages, (2) feed location for the recovery column, and (3) the number of stages for the stripping column. For the design of the proposed process, the temperature and pressure of the extraction column and decanter were chosen as 40 °C and 1 atm, respectively. The pressure drop for each tray in the distillation column was set to be 0.007 atm. The combination of sensitivity analysis and Aspen Plus built-in optimization function was used to determine the optimal design. The built-in optimization function which is based on SQP algorithm.40, 41 is capable of minimizing the objective function by manipulating the decisive variable. Aspen Plus optimization approach is well-proven for numerous processes.42-47 The optimal HED process was obtained by the minimization of total annual cost (TAC) with the optimizing variables that mentioned earlier. Figure 8 outlines the optimization procedure used for the proposed design. By neglecting the existence of MA in the feed stream, Lucia et al.17 proposed an optimal design with 4535.9 kg/h of feed containing 30 wt% HAc. For a fair comparison, this study used the same feed conditions. As constraints, two specifications were considered for all simulations: an HAc+PX mixture of 99.9 wt% was considered to be the final product that enters the TPA unit and the water stream was assumed to require a purity of 99.6 mol% for the water treatment unit; these values are the same as those used by Lucia et al.17 The design

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specification function in the “RadFrac” column in Aspen Plus was utilized for this task. Water purity in the bottom stream of the stripping column was obtained by employing reflux ratio as the manipulating variable. In addition, the overall recovery of HAc and MA (or EA) in the recovery column were 95.0 and 99.9 wt%, respectively. Reflux ratio and the distillate rate of recovery column were used as the manipulating variables to fulfill the recovery specifications.

Figure 8. Optimization procedures for the hybrid extraction-distillation process

4. HYBRID EXTRACTION-DISTILLATION PROCESS AND PERFORMANCE In previous studies regarding the HAc recovery process, EA was reported to be an efficient solvent;17 in these studies, optimizing the energy efficiency of the designs 19



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corresponded to the distillation line method of the ternary system. Further discussions and conclusions require a comparison of the solvent performance, total energy requirements, and TAC. Table 3 sums up the overall performance of different solvents for the hybrid extractiondistillation of HAc. In comparison with other systems, the PX+MA mixed solvent resulted in the lowest reboiler duty, which enhances TPA production; this is because the PX+MA system has a higher relative volatility than other systems. Although most solvents are circulated in the HAc recovery system, as mentioned in Section 1, the solvent loss is inevitable. As can be seen in Table 3, the PX+EA and PX+MA mixed solvent requires less make-up solvent than EA.

Table 3. Comparison of different solvents for hybrid extraction-distillation of HAc Variables

EA 17

PX+MA

PX+EA

Reboiler duty (KW) in

1650.9

1349.2

1535.3

8.1

30.2

18.1

41.3

329.3

198.7

EA: 0.19

MA: 0.08

EA: 0.03

PX: 36.88*

PX: 1.89*

HAc: 22.4

HAc: 21.8

HAc: 21.5

Water: 0.0011

PX: 36.9

PX: 1.89

EA: 0.0011

MA: 0.07

EA: 0.015

Water: 179.9

Water: 206.9

Water: 194.4

HAc: 0.47

HAc: 0.21

HAc: 0.19

recovery column Low pressure steam inlet (kmol/hr) Equivalent duty (KW) in stripping column Make-up solvent (kmol/hr)

Product (kmol/hr)

Water stream (kmol/hr)

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EA: 0.19

MA: 0.01

EA: 0.01

Number of stage in RC

28

15

28

Number of stage in SC

14

5

4

*) Note: PX is fed to the oxidation reactor as the reactant of TPA production.

The product impurities are inherent and influence the quality of TPA-generated in the oxidation reaction process. To solve real industrial problems with respect to TPA production, the proposed solvent (PX+MA), which comprises components that are readily available in the process, is simpler than other entrainers or solvents. Moreover, in contrast to the results for EA and other entrainers or solvents, increasing the concentration of MA in the oxidation reactor reduces the loss of HAc solvent. The results show that recycling MA in the plant does not affect the oxidation reactor, but does selectively accelerate ester molecule decomposition to HAc.29,30 As can be seen in Table 3, the hybrid extraction distillation process using the PX+MA mixed solvent resulted in a greater amount of inlet low-pressure steam than that reported by Lucia et al.17 This phenomenon is mainly due to the water contents in the feeds of the stripping column, which were 98.8 and 83.4 wt% for the EA and PX+MA systems, respectively. However, the proposed PX+MA system required the lowest total duty among the three systems. Furthermore, to fully discuss the benefit of this design, the total operating cost, including the utility and solvent costs, must be determined. Utility cost includes water as a cooling medium in the heat exchanger and as steam for the reboiler and stripping column. Details regarding the utility and solvent costs are available in Supporting Information. Figure 9 compares the operating costs of different hybrid extraction-distillation processes. Compared to the conventional EA process, the operating costs of the PX+MA and PX+EA processes were up to 15% and 7% lower, respectively. In addition to the operating costs, the capital costs were also included in the economic calculations to determine the

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actual benefit of using a mixed solvent in this HAc recovery system. The capital cost of this process includes the cost of all major equipment units such as the extractor column, distillation columns, reboilers, condensers, and decanter. The Chemical Engineering’s Plant Cost Index of 587.6 in 2014 was used to estimate the time escalation. Figure 10 compares the TACs of the three different hybrid processes. All the calculations were based on the optimized results. The TAC calculations included the annual total operating costs and the annual investment costs over the plant lifetime of 10 years. The operating cost includes the cost of steam, cooling water, and solvent, while the capital cost includes the cost of the columns, vessels, and heat exchangers. The results correlated with those of the operating cost calculations: as can be seen in Figure 10, the PX+MA mixed solvent resulted in the lowest TAC of the investigated processes. As a final result, the optimized design of a hybrid extraction-distillation system using the mixed PX+MA solvent resulted in a significant reduction of the TAC by 14% compared to that of the single EA solvent.17 Figure 11 shows a diagram of TPA production and the HAc recovery system using the mixed PX+MA solvent. It should be noted that the important advantage of this process is that PX and MA, which are a reactant and by-product in TPA production, respectively, are used as an extraction solvent without the addition of a third component to the HAc recovery system. Moreover, the proposed design requires one less distillation column than the conventional system. The product from the HAc recovery system, which contains HAc, PX, and a trace of MA, can be directly fed to the oxidation reactor without further separation.

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Figure 9. Comparison of the total operating costs for the different solvents in the hybrid extraction-distillation (HED) process for HAc recovery.

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Figure 10. Comparison of the operating costs, capital costs, and TACs of the optimized hybrid extraction-distillation (HED) system using three different solvents: EA, PX+MA, and PX+EA.

Figure 11. Diagram of TPA production using the proposed PX+MA mixed solvent. 24


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5. CONCLUSIONS The HAc recovery system is an essential and core unit to minimize the loss of HAc in the TPA production process. In this study, PX+MA and PX+EA mixed solvents were proposed and evaluated to enhance the recovery of HAc in the TPA production process. Based on the LLE experimental results for the corresponding quaternary system, a hybrid extraction-distillation process using the mixed solvents was designed and optimized. Technoeconomic evaluation results showed that the proposed hybrid extraction-distillation process, particularly with the PX+MA mixed solvent, remarkably improved the process economics: the TACs of the PX+MA and PX+EA processes were up to 14% and 6% lower, respectively than that of the conventional process using a single EA extraction solvent. In addition to its apparent economic benefits, the proposed PX+MA mixed solvent offers an important practical advantage: PX and MA are already available in the process as the main reactant and by-product, respectively, in TPA production whereas conventional HAc recovery processes require the third component either as an entrainer for the azeotropic distillation process or as an extractant solvent for the extraction-distillation process. PX is a main reactant for the synthesis of TPA, and MA is essentially included in the residue stream as an oxidation by-product in the TPA unit; therefore, its impurity level does not need to be carefully controlled in the recovered HAc. These features of the PX+MA mixed solvent allow a simpler and safer process configuration. The proposed hybrid process provides a promising option for efficient HAc recovery in the TPA production process.

ASSOCIATED CONTENT The supporting information is available free of charge on the ACS Publications website. VLE binary parameters for both systems; distillation and extraction column correlations; capital, operating, and total annual costs evaluation.

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AUTHOR INFORMATION Corresponding author: School of Chemical Engineering, Yeungnam University, Dae-dong 214-1, Gyeongsan 38541, Republic of Korea; Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the Engineering Development Research Center (EDRC) funded by the Ministry of Trade, Industry & Energy (MOTIE) (No. N0000990). This work was also supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A6A1031189).

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Process Design and Optimization of an Acetic Acid Recovery System

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