Energy-Efficient Reactive Dividing Wall Column for Simultaneous

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Energy-Efficient Reactive Dividing Wall Column for Simultaneous Esterification of n-Amyl alcohol and n-Hexanol WONJOON JANG, Heecheon Lee, Jong-In Han, and Jae W Lee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00324 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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Industrial & Engineering Chemistry Research

Energy-Efficient Reactive Dividing Wall Column for Simultaneous Esterification of n-Amyl Alcohol and nHexanol Wonjoon Jang,†, ‡ Heecheon Lee,† Jong-in Han, ‡ and Jae W. Lee*,† †

Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of

Science and Technology (KAIST), 291 Daehak-ro, Daejeon 34141, Republic of Korea ‡

Department of Civil and Environmental Engineering, Korea Advanced Institute of

Science and Technology (KAIST), 291 Daehak-ro, Daejeon 34141, Republic of Korea KEYWORDS: Reactive dividing wall column (RDWC), Vapor recompression heat pump (VRHP), Simultaneous esterification.

ABSTRACT This study reports significant energy savings of combined reaction and distillation in a single reactive dividing wall column (RDWC) for the concurrent production of pure n-amyl and n-hexyl esters, which are the stable node (SN) in each phase diagram of the quaternary reaction system. The continuous removal of the byproduct, water, from the unstable node (UN) azeotrope by L-L decantation drives the two esterifications in the forward direction and enables the production of the pure ester products. A vapor recompression heat pump (VRHP) is introduced to the RDWC and the annual operating cost is reduced to 62.5 % of the case without the 1

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VRHP. Implementing the multiple reactions with V-L-L separation in a single RDWC and its heat integration through the VRHP may provide chemical industries with enhanced economic feasibility.

1. INTRODUCTION Distillation is widely used in chemical processes for the separation of multicomponent mixtures. To explore the synergistic effect of reaction and distillation, the idea of combining distillation and reaction (so-called reactive distillation) was introduced in the early twentieth century.1,

2

Both industry and academia have

focused on developing a design methodology for this combined reaction and separation system since the appearance of methyl tertiary-butyl ether (MTBE) and methyl acetate production systems in the early 1980s.3-5 Early design methods were based on mathematical and geometrical interpretations using transformed coordinates6, 7 or difference point concepts.8-11

The main benefits of reactive distillation (RD) are dramatic economic savings; for example, a reduction to 16 % of the capital and operation costs of the conventional reactor-separation column sequence was reported for a methyl acetate production system.12 Another advantage is that RD provides high product yields by overcoming the limitation of phase and reaction equilibrium.13-16 After RD was practically 2


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implemented, chemical industries showed increasing interest in developing a more integrated process combining reactive distillation and the Petlyuk column configuration in a single unit. The single unit is called the reactive dividing wall column (RDWC) and its potential application has been investigated.17-20

The

RDWC has attractive advantages such as energy savings with reduced investment costs and increased efficiency. In the RDWC, there are two configurations according to the position of the separating wall; one is an upper RDWC (Figure 1a) and the other is a lower RDWC (Figure 1b).21 In general, the upper RDWC derived from the direct sequence has two condensers and one reboiler to load different condenser duties for each divided section at the top of the column. The lower RDWC derived from the in-direct sequence consists of two reboilers and one condenser to load different reboiler duties for each divided section near the bottom of the column.

It was reported that the upper RDWC can reduce energy consumption by 12 % and total annual costs (TAC) by 16 %, respectively, when compared with the conventional two column processes such as indirect and direct RD sequences.22 Regarding the lower RDWC to synthesize diethyl carbonate, it can reduce energy consumption by 19 % and TAC by 14 %.23 A novel dimethyl ether process was studied for methanol dehydration in a RDWC and it showed that the RDWC process

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can reduce energy consumption by 58 %, CO2 emission reduction by 30 %, and investment costs by 20 % when compared with the conventional RD process.19

The energy requirement in the distillation column is one of the most important factors in reducing operating costs, and several studies have reported an integrated process combining a vapor recompression heat pump (VRHP) and RDWC with a single reaction system.21 The main principle of the VRHP is using a high temperature vapor stream pressurized by a compressor for providing heat to the reboiler or a low temperature vapor stream depressurized by a control valve for removing the heat from the condenser. The addition of the VRHP to the RDWC was reported to reduce the total energy consumption by up to 50 % when compared with the bare RDWC.21

Semiconductor and pharmaceutical plants produce a waste mixture including namyl and n-hexyl alcohols and the coproduction of their valuable esters is more beneficial way to reuse the waste alcohol mixture without the separation of the alcohols.24 Even though a two or three conventional RD column sequence was proposed to produce n-hexyl acetate and n-amyl acetate,24 the design of a single RDWC having different reaction regions with the two esterification reactions has never been explored using kinetically controlled reactions. Because the simultaneous 4


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esterifications occur, it is not trivial to realize the concurrent production of the two pure products in a single RDWC. By utilizing the feasibility criteria of the single reaction in the batch RD columns,25-27 this study proposes feasible design alternatives of a RDWC with a lower reaction zone, a RDWC with an upper reaction zone, and a VRHP-integrated RDWC for further energy savings. We then evaluate the economic feasibility of the design alternatives of RDWC in terms of energy consumption and total annual costs.

2. REACTION AND FEASIBILITY STUDY Kinetic and phase equilibrium. The kinetic data of n-amyl acetate (AmAC) and nhexyl acetate (HexAC) production are taken from prior studies2, 28, as shown below. Both esterification reactions are activated with an acid ion exchange resin as a catalyst. The reaction rate of n-amyl esterification is based on the concentration and that of n-hexyl esterification is expressed by the activity. Calculating the rate of two parallel reactions with each kinetic constant, the consumption of n-hexyl alcohol (HexOH) is much faster than that of n-amyl alcohol (AmOH). In other words, the reaction equilibrium constant in esterification (2) is approximately 15 times higher than that in esterification (1) around 400 K.

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Acetic acid (AC) + Amyl alcohol (AmOH) ↔ Amyl acetate (AmAC) + H2O (1) 𝑟 = 𝑚𝑐𝑎𝑡(𝑘1𝐶𝐴𝐶𝐶𝐴𝑚𝑂𝐻 ― 𝑘1 ―1𝐶𝐴𝑚𝐴𝐶𝐶𝐻2𝑂)

𝑘1 = 31.1667𝑒

―

𝑘1 ―1 = 2.2533𝑒

51,740 𝑅𝑇

―

45,280 𝑅𝑇

Acetic acid (AC)+Hexyl alcohol(HexOH)↔Hexyl acetate(HexAC)+H2O (2) 𝑟 = 𝑚𝑐𝑎𝑡(𝑘2𝑎𝐴𝐶𝑎𝐻𝑒𝑥𝑂𝐻 ― 𝑘2 ―1𝑎𝐻𝑒𝑥𝐴𝐶𝑎𝐻2𝑂)

𝑘2 = [4883.18 ― 4769.06𝑥𝐻𝑒𝑥𝑂𝐻]𝑒

𝑘2 ―1 = [71.92 ― 70.24𝑥𝐻𝑒𝑥𝑂𝐻]𝑒

―

―

49,000 𝑅𝑇

46,215 𝑅𝑇

Aspen Custom ModelerTM is used to simulate the kinetic equation in a RD column and the NRTL-NTH property method is used in Aspen Plus simulations to calculate the phase equilibrium. NRTL (non-random two liquid) is for vapor-liquid-liquid equilibrium (VLLE) and the NTH model is incorporated for the chemical theory of dimerization to consider solvation effects and strong association of acetic acid. The NRTL model parameter24 is summarized in Table 1. The amount of catalyst is determined by the weir height and column diameter with a bulk density of the ion 6


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exchange resin (AmberlystTM CSP2, 600g/L).29 For each simulation run, the liquid holdup and the amount of catalyst are iteratively obtained to agree with the column internal sizing.

2.2 Feasibility study and visualization for conventional RD processes. The feasibility problem of RD can be defined as whether the integration of reaction and separation can lead to complete conversion and production of desired products.25-27, 30

The feasibility of batch RD was studied using a single esterification with a

quaternary mixture of acetic acid, isopropyl alcohol, isopropyl acetate, and water.25 Isopropyl acetate and water are produced by the reaction of acetic acid with isopropyl alcohol. Acetic acid does not form azeotropes with any of the other three components, and is a stable node (SN) in the residue curve map of the four component system. The reacting mixture has three minimum-boiling binary azeotropes and a minimum-boiling heterogeneous ternary azeotrope. From the heterogeneous ternary azeotrope, water can be decanted from the condensed liquid whose composition is close to the azeotropic point and removed at the top. The removal of water from the reaction mixture drives the reaction forward and eventually the desired ester product is produced at the bottom. Similarly, even if multiple esterification reactions are employed in RD, the system is feasible to

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produce pure esters as long as the pure water removal is available at the top decanter and the two ester products do not form an azeotrope. 25, 26, 28, 29

Based on the above feasibility criterion, we identify every singular point to verify the feasibility of our two-esterification system. Azeotropic singular point information about each quaternary system of AmOH-AC-Water-AmAC and HexOH-AC-Water-HexAC is shown in Figure 2. Both systems have nearly the same positions of the singular points in the tetrahedron composition space. Two minimum-boiling binary azeotropes exist between alcohol and water and between water and acetate while one minimum-boiling ternary azeotrope is present in the mixture of water, alcohol, and acetate. Note that acetic acid does not form any azeotrope with the other components in the total six component system, as shown in Table 2. Both LLE regions and boundaries in Figure 2 show almost identical shape for each quaternary mixture of water, alcohol, acetate, and acetic acid. The most important aspect about separating products from both systems is that removing water from decantation at the top induces the forward reaction and consequently the two esters that have no azeotrope can be recovered at the bottom of the columns.

The direct sequence of two columns is shown in Figure 3 (a). The two alcohols of AmOH and HexOH are less volatile than acetic acid and thus the alcohols are fed to 8


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the upper feed stage and acetic acid is provided to the lower feed stage in a stoichiometric amount. The byproduct of water is decanted from the top decanter. The first double feed RD column can produce the acetate products in the bottom because the reactants lighter than the acetates are stripped up and consumed in the reaction section. The nonreactive rectifying section in Figure 3 (a) thus can enrich the mixture containing water, acetate, and alcohol without AC at the top. As a result, the two different quaternary reaction systems continuously remove water in the decanter and the acetate products can be produced at the bottom.

Figures 3 (b) and (c) show the reaction equilibrium curves, the LLE envelope, and the liquid composition profile of the reactive section for both AmOH-AC-H2OAmAC and HexOH-AC-H2O-HexAC systems in each projected quaternary space. AC is fed to stage 5 while the alcohols are fed into stage 2 and moved down due to their low relative volatility. The concentration and the activity of the reactants are increased from stage 2 to the bottom of the reactive section. Thus the forward reaction rate is accelerated and all liquid compositions at the reactive stage between stage 2 and 42 are within the forward reaction region because the liquid composition profile lies in the reactant rich side (under the reaction equilibrium curves in Figure 3 (b)). After passing the last reactive stage of the bottom side, the liquid composition profile approaches the acetate product by non-reactive distillation in the stripping 9



section. As a result, the double feed RD column is feasible to produce two acetates because the acetate products that do not form a binary azeotrope can be easily separated to AmAC at the top and HexAC at the bottom of the second column.

On the other hand, the in-direct sequence shown in Figure 4 (a) first produces the least volatile acetate at the bottom of the first RD column and the other acetate is produced in the second non-RD column. In order to produce the least volatile acetate first, it is necessary to feed the reactants to the lower part of column so that the least volatile component of HexAC can be enriched in the bottom part of the non-RD column. Figures 4 (b) and (c) show the reaction equilibrium curves, the LLE region, and the liquid composition profile of the reactive section for each quaternary system in the projected four component space. The reaction rate is accelerated at stage 28 where the top product of the second non-RD column is returned due to the recycle of unconverted AmOH (53 %). The entire liquid composition at the reactive stage between stage 2 and 58 is within the forward reaction region in Figure 4 (b). As a result, the heaviest HexAC can be produced at the bottom of the first RD column. The mixture from which the water is removed at the decanter enters the second nonRD column where AmAC is recovered at the bottom.

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The top stage composition (x0 in Figure 4 (c)) is close to the unstable ternary azeotropic point containing AmAC. From this top product of the RD column, water is removed by the decanter and the remaining organic rich phase containing AmAC is provided to the second non-RD column. The heaviest component of AmAC in the second non-RD column is produced at the bottom and the rest of the reactant rich composition (x0,2nd lying within the forward reaction region) is returned to stage 28 of the first RD column. After the water decanting, x0 yields xF,2nd, which is the feed composition of the second non-RD column. Therefore, pure AmAC, which is the least volatile in the non-reactive section, can be produced at the bottom of the second column.

3. DESIGN OF UPPER AND LOWER RDWC 3.1 Design of the upper RDWC. The conventional direct sequence is a two-column process including the first RD column for producing AmAC and HexAC as the bottom product followed by a non-RD column that separates the binary product mixture. Intuitively, the upper RDWC integrates these two columns performing the reaction in the upper left part and non-RD in the right part against a dividing wall that prevents the organic phase in the right top from mixing with the water-rich phase in the left top. If the dividing wall does not seal the top section and does not separate the reaction zone from the non-reactive rectifying part, the water-rich 11



mixture enters the non-reactive top section and then the pure acetates will not be separated in the non-reactive section on the right side of the dividing wall.

The upper RDWC is modeled by Aspen Custom ModelerTM and the design having the two reactions is difficult to converge in simulations due to the structural complexity of the column and a large number of design variables. The most important task is to find the proper liquid flowrate (FLR) and the vapor flowrate (FVR) exchanging between reactive and non-reactive sections at the bottom part as shown in Figure 6. In order to find FLR and FVR, we did sensitivity analyses with a flow constraint of FLR=FVR+100 and a liquid composition constraint of xH2O

Energy-Efficient Reactive Dividing Wall Column for Simultaneous

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