Esterification of Fusel Oil Using Reactive Distillation. Part II: Process

Article pubs.acs.org/IECR

Esterification of Fusel Oil Using Reactive Distillation. Part II: Process Alternatives Prafull Patidar and Sanjay M. Mahajani* Department of Chemical Engineering, Indian Institute of Technology, Powai, Mumbai, 400076, India S Supporting Information *

ABSTRACT: In this work, we have studied the potential of reactive distillation for the valorization of fusel oil, i.e., a mixture of alcohols obtained as a distillery waste. It is proposed to react these alcohols in the mixture with acetic acid and separate the esters in pure form. Experiments are performed in a laboratory scale reactive distillation column, and a simulation model is validated. Different process alternatives exist, and we have compared them based on the total energy consumption as the criterion. The potentially cost-effective alternatives are identified. It is shown that reactive distillation, which is a proven intensification strategy for esterification of individual alcohols, can be a promising option for simultaneous esterification of mixture of alcohols, as well.

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INTRODUCTION Esterification has been the most widely studied reaction in reactive distillation columns. In most of these cases, either pure alcohols and acids or their aqueous solutions are considered as a feedstock with the objective of making or recovering a particular ester of interest.1−3 To the best of our knowledge there has been no systematic work that deals with a mixture of alcohols as a raw material. Fusel oil is one such feedstock commonly encountered as a byproduct of ethanol manufacturing process through the fermentation route. Ethanol, obtained from the fermentation of sugar-containing substrates by yeast in the presence of nitrogenous diet (e.g., proteins), is associated with different impurities collectively termed as fusel oil. Major ones among these impurities are aliphatic alcohols that are higher in molecular weight than ethanol such as n-propanol, isobutanol, and iso-amyl alcohol.4−6 The raw fusel oil is a relatively viscous liquid with a dark-reddish color and a very unpleasant odor. As a result of these properties, direct utilization of fusel oil as a solvent has been very limited. In some countries, it is burned and used as a fuel source for processing plants.5 All the major alcohol components of fusel oil have individual economic importance as chemicals, which can be further esterified with acetic acid (see eq 1) to produce their esters, useful as solvents in the production of flavors and fragrances. CH3COOH + Cx HyOH ⇔ CH3COOCx Hy + H 2O

Since esterification of fusel oil is a multireactant and multiproduct process, one would encounter many independent processing options based on the sequence in which reactions and separations are performed for each product. It is thus a challenging task to identify and further select the best process alternative. The present work addresses this problem through experiments and simulation. The experiments in a laboratory reactive distillation column are performed to validate the steady state simulator. The simulations need information on reaction kinetics and vapor−liquid equilibrium. An activity based Langmuir−Hinshelwood−Hougen−Watson (LHHW) kinetic model that is applicable to the esterification of mixture of alcohols is already developed and presented in the first part of this work.7 The VLE data is partly taken from literature and partly generated in this work. In the present article, this model is further used to simulate feasible RD configurations. The different process schemes are identified and simulated using a commercial simulator, Aspen Plus, in order to arrive at the most promising RD configurations giving all the esters in pure form as products. The article is organized as follows: First we review the information available in literature on reactive distillation for esterification of acetic acid with individual alcohols from C1 to C5 as well as with mixtures of alcohols. The vapor−liquid equilibrium (VLE) data required to analyze the RD process is then presented. While the VLE data for most of the binary pairs is available in literature, the experiments are performed to generate the data for the remaining binary pairs. The associated binary interaction parameters are estimated. Further, we describe the experimental work on a laboratory-scale continuous RD unit. The results from the experiments are compared with the independent predictions by steady state simulations performed in Aspen Plus. The experimentally validated simulator is thus used to carry out parametric studies

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However, esterification of such a mixture and its subsequent separation is made difficult by the presence of a large number of azeotropes. There is not much information available in the literature on the processing of fusel oil to bring in value addition, and hence, in the present work we study different alternatives to obtain pure esters from fusel oil using reactive distillation. Reactive distillation combines reaction and separation so as to separate the products during the course of the reaction and, hence, drives the reaction in the forward direction. The simultaneous separation reduces the cost of downstream processing to bring cost-effectiveness and compactness to the chemical plant. © 2013 American Chemical Society

Received: Revised: Accepted: Published: 16637

May 15, October October October

2013 12, 2013 29, 2013 29, 2013

dx.doi.org/10.1021/ie401553z | Ind. Eng. Chem. Res. 2013, 52, 16637−16647

Industrial & Engineering Chemistry Research

Article

on different feasible sequences with a view to minimize total energy consumption. Finally, the most promising options are identified.

Table 1. Different Possible Reaction and Separation Sequences for Esterification of Fusel Oil Using RD

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sequence no.

PREVIOUS STUDIES Esterification of acetic acid with individual alcohols is a wellstudied reaction in RD. Some of the earliest investigations report successful implementation RD for the synthesis of methyl acetate.8,9 Many authors have studied reaction of acetic acid with individual alcohols in RD through experiments. Table S1, given in the Supporting Information, summarizes the selected studies on esterification with alcohols of interest that range between C1 to C5. Huss et al.21 have described a hierarchy of methods and models for the design and simulation of an RD column for the production of methyl acetate. Tang et al.22 have studied the esterification of acetic acid with five different alcohols, ranging from C1 to C5, using RD. They have classified process flowsheets based on thermodynamic phase behavior of these alcohols. Hu et al.23 have proposed an entrainer enhanced RD process for ethyl acetate, involving sidedraw to RD column, for removal of water. Their simulations indicate that this process is more energy efficient than the one without side-draw. Lee et al.24 have studied esterification of acetic acid with a mixture of amyl alcohol and n-butanol and examined two alternative methods for using the alcohol mixtures as feed for RD. The first method separates the mixture into pure alcohols (separation-first) followed by the esterification using the RD column, while the second method uses direct esterification of the alcohol mixture in an RD column (reaction-first) followed by separation of the mixedester products. The authors conclude that the reaction-first scheme is the more economical of the two. All in all, the literature suggests that, for all the individual alcohols, reactive distillation when designed appropriately enhances the performance of the process significantly. However, barring few patents25,26 and only one paper24 that considers a binary mixture of two similar alcohols, there is no work reported on the analysis of a multialcohol feed mixture, especially when alcohols vary in their volatility significantly. The simultaneous trans-esterification reactions and formation of cross-azeotropes are the unique features of this system that make it different from the esterification of individual alcohols. It would be interesting to examine the influences, if any, of these factors on the process design.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

reaction and separation in sequence C2 C3 C4 C5 (react), C2Ac C3Ac C4Ac/C5Ac, C2Ac C3Ac/ C4Ac, C2Ac/C3Ac C2 C3 C4 C5(react), C2Ac C3Ac C4Ac/C5Ac, C2Ac/C3Ac C4Ac, C3Ac/C4Ac C2 C3 C4 C5(react), C2Ac C3Ac/C4Ac C5Ac, C2Ac/C3Ac, C4Ac/C5Ac C2 C3 C4 C5(react), C2Ac/C3Ac C4Ac C5Ac, C3Ac C4Ac/ C5Ac, C3Ac/C4Ac C2 C3 C4 C5(react), C2Ac/C3Ac C4Ac C5Ac, C3Ac/C4Ac C5Ac, C4Ac/C5Ac C2 C3 C4 C5, C2(react)/C3 C4 C5(react), C3Ac/C4Ac C5Ac, C4Ac/C5Ac C2 C3 C4 C5, C2(react)/C3 C4 C5(react), C3Ac C4Ac/C5Ac, C3Ac/C4Ac C2 C3 C4 C5, C2 C3 C4(react)/C5(react), C2Ac C3Ac/C4Ac, C2Ac/C3Ac C2 C3 C4 C5, C2 C3 C4(react)/C5(react), C2Ac/C3Ac C4Ac, C3Ac/C4Ac C2 C3 C4 C5, C2(react)/C3 C4 C5, C3(react)/C4 C5(react), C4Ac/C5Ac C2 C3 C4 C5, C2(react)/C3 C4 C5, C3 C4(react)/C5(react), C3Ac/C4Ac C2 C3 C4 C5, C2 C3(react)/C4 C5(react), C2Ac/C3Ac, C4Ac/ C5Ac C2 C3 C4 C5, C2 C3(react)/C4 C5, C2Ac/C3Ac, C4(react)/ C5(react) C2 C3 C4 C5, C2 C3/C4 C5(react), C2(react)/C3(react), C4Ac/C5Ac C2 C3 C4 C5, C2 C3 C4/C5(react), C2 C3(react)/C4(react), C2Ac/C3Ac C2 C3 C4 C5, C2 C3 C4/C5(react), C2(react)/C3 C4(react), C3Ac/C4Ac C2 C3 C4 C5, C2 C3 C4/C5(react), C2 C3/C4(react), C2(react)/C3(react) C2 C3 C4 C5, C2 C3 C4/C5(react), C2(react)/C3 C4, C3(react)/C4(react) C2 C3 C4 C5, C2(react)/C3 C4 C5, C3(react)/C4 C5, C4(react)/C5(react) C2 C3 C4 C5, C2(react)/C3 C4 C5, C3 C4/C5(react), C3(react)/C4(react) C2 C3 C4 C5, C2 C3/C4 C5, C2(react)/C3(react), C4(react)/ C5(react)

configurations nos. 1 to 5, the reactant alcohol mixture is reacted first and subsequently the esters are separated. In RD configurations 17 to 21, the reactant alcohols are separated one by one and subsequently reacted to produce esters. It may be noted that we have restricted ourselves to simple distillation and RD columns. The heat integration, side draw options, and thermal coupling of the columns are not explored.

RD SEQUENCES

The production of the esters from fusel oil, using RD, can be accomplished in many possible ways. A simple sequence can be to separate the mixture of alcohols and then make them react individually with acetic acid. On the other extreme, we can carry out reactive distillation to react all the alcohols from the mixture completely and then separate the mixture of esters. In the first case wherein alcohols are being separated first, the separation can be easily accomplished as there are no azeotropes. In the second case, there would be a mixture of esters, water, and unreacted alcohols forming several azeotropes. Apart from these two options, many alternate sequences evolve as we introduce reactive distillation at different separation steps, starting with the first case. In all, 21 independent simple column sequences may be identified to accomplish this task. Table 1 describes the reaction and separation sequences for these different configurations. In RD

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MATERIALS AND ANALYSIS Chemicals and Catalyst. Ethanol, n-propanol, iso-butanol, iso-amyl alcohol, 2-ethylhexanol, n-propyl acetate (each >99 wt %), and acetic acid (99.9 wt %) are supplied by s.d. Fine− Chem Ltd., India. iso-Propyl alcohol (AR grade, moisture 99 wt %) are supplied by Merck Ltd., India. Amberlyst15, a strong-acid ion-exchange resin, used as a catalyst in the experiments is obtained from Rohm and Hass, India. Prior to its use, the fresh catalyst is washed with distilled water, followed by isopropyl alcohol and dilute hydrochloric acid, and again with distilled water. The washed resin is then dried under vacuum at 70 °C for about 14 h to remove the moisture present 16638



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Figure 1. Comparison of the T−x−y plots generated through experiments, at 1 atm, and model predictions for the binaries (a) ethanol−isobutyl acetate, (b) ethyl acetate−iso-amyl alcohol, (c) iso-butyl acetate−iso-amyl alcohol, (d) iso-butanol−iso-amyl acetate, (e) n-propyl acetate−iso-butanol, (f) n-propyl acetate−iso-amyl alcohol, (g) n-propanol−iso-amyl acetate, (h) iso-amyl alcohol−iso amyl acetate, and (i) n-propanol−iso-butyl acetate.

in it. The resin is filled in the KATAPACK-S structure used in the reactive distillation experiments. Analysis. The collected samples are analyzed using a gas chromatograph (Chemito, GC 8610), equipped with flame ionization detector (FID). A 30 m long capillary column (BP5) with 0.53 mm ID is used to separate the components of interest. Nitrogen is used as a carrier gas with a flow rate of 0.5 mL/min in order to establish elution. 2-Ethylhexanol is used as an external standard. The oven temperature is varied from 70 to 190 °C. Water is analyzed separately using another gas chromatograph (GC-911; MakAnalytica India, Ltd.) equipped with a thermal conductivity detector (TCD). For this analysis, Porapack-Q column is used with hydrogen as a carrier gas at a flow rate of 30 mL/min. Independent titrations with standard

(0.25 N) alcoholic sodium hydroxide (NaOH) solution, using phenolphthalein as an indicator, are performed to verify the results obtained by GC.

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KINETICS AND VAPOR−LIQUID EQUILIBRIUM Reaction Kinetics. A unified activity based LHHW kinetic model developed for esterification of fusel oil with acetic acid, in the presence of cationic resin Amberlyst-15, is reported in our earlier work.7 This kinetic model has been developed by incorporating data for various esterification and transesterification reactions that take place simultaneously during the reaction of mixtures of alcohols under consideration with acetic acid. The model works equally well for the esterification of individual alcohols. Although homogeneous catalysts such as 16639



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Figure 2. Comparison of composition profiles obtained by experiments and simulation with an equimolar mixture of C2−C3−C4 alcohols: (a) ethanol, (b) ethyl acetate, (c) n-propanol, (d) n-propyl acetate, (e) iso-butanol, (f) iso-butyl acetate, (g) acetic acid, and (h) water.

sulfuric acid and pTSA can also be used for some of the esterification reactions using individual alcohols, we prefer to use solid ion-exchange resin as a catalyst, owing to the flexibility it provides in the placement of the reactive section within the RD column. Furthermore, catalyst recovery and material corrosion problems are obviated with the use of solid catalyst. Vapor−liquid Equilibria. In order to reliably represent the nonideality involved in the vapor−liquid and possible vapor− liquid−liquid phase equilibria, activity coefficient models such as UNIQUAC or NRTL are most suited. Moreover, in order to account for the nonideality introduced by dimerization of acetic acid in the vapor phase, the Hayden−O’Connell equation of

state should also be used. Thus, in the present work, we have chosen UNIQUAC-HOC as the thermodynamic property model for calculating phase equilibria in the simulations. The reacting mixture of interest has 10 components with 45 component pairs. The corresponding binary interaction parameters are taken from ASPEN Databank. There are 15 pairs for which the interaction parameters are not available in the literature. The experiments are performed to generate VLE data at atmospheric pressure to estimate the values for nine of these pairs which are expected to exhibit nonideal behavior, while the rest (involving ester−ester pairs) are estimated using UNIFAC method. The VLE data is generated at atmospheric 16640



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Figure 3. Comparison of composition profiles obtained by experiments and simulation for esterification of acetic acid with an equimolar mixture of C4−C5 alcohols: (a) iso-amyl alcohol, (b) iso-amyl acetate, (c) iso-butanol, (d) iso-butyl acetate, (e) acetic acid, and (f) water.

pressure using a modified Othmer still.27 In the still, a liquid mixture is continuously boiled by heating it. The vapor−liquid mixture is sent to the equilibrium cell wherein sufficient residence time is provided for equilibration. The liquid exiting from the cell and the associated vapors upon condensation are recycled back to the still. Once the equilibrium is attained, which is ascertained by constant temperature (T) and compositions of liquid (x) and vapor condensate (y), the samples can be collected to note the equilibrium liquid and vapor compositions (x − y*) data at the observed temperature. The experiments are repeated with different liquid mixture

compositions to generate vapor−liquid equilibrium data over the entire range of binary mixture compositions (0 ≤ x ≤ 1). More details of the setup and its working principle may be found elsewhere.27 This experimentally generated VLE data is used to estimate the UNIQUAC interaction parameters with the help of the Property Estimation module in Aspen Properties. The regression involved solving a maximum-likelihood objective function with the help of a Britt-Luecke algorithm using Deming initialization method. The values of interaction parameters for all the component pairs used are listed in Table S2 provided as Supporting Information. Figure 1 shows the comparison of T−x−y data predicted using regressed values of 16641



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a reactive stage as reboiler. The second RD setup (Supporting Information Figure S2b) is used to perform the esterification of the C4−C5 alcohol mixture. It has an internal diameter of 50 mm and a total height of 3.18 m, consisting of a reactive section (1.38 m) along with nonreactive rectifying and stripping sections (0.9 m each). The reactive section is packed with Sulzer Katapak-S (NTSM = 3) filled with cation exchange resin Amberlyst-15 and the nonreactive sections are packed with HYFLUX packings. The column thus provides 19 equilibrium stages out of which nonreactive rectifying and stripping sections (including reboiler) have 7 and 8 stages, respectively. The reactive zone is equivalent to 4 equilibrium stages. A proper insulation is provided to reduce the heat losses to the surroundings. The state of the run is monitored by observing the temperatures and concentration profiles. The temperature sensors (Pt-100) and sampling ports are provided along the height of the column at several locations. The reboiler, electrically heated at a desired rate, is initially charged with a mixture at a composition close to the one obtained by simulation in order to reduce the time required to attain the steady state. Once the reflux starts, the mixture of alcohols and acetic acid, in stoichiometric proportions, are fed at suitable locations with respect to the reactive section. Condensate is collected in a decanter (350 mL) which is used to separate the two immiscible liquid phases. Water formed during the reaction is removed continuously from the decanter after phase separation. A part of the organic phase is also removed continuously as distillate stream while the other is returned to the column as reflux. The run is continued until the steady state is achieved. The attainment of steady state is ensured by constancy in temperatures and concentrations with respect to time. Typically the steady state is achieved in approximately 8− 12 h. Esterification of acetic acid with different equimolar mixtures of fusel oil constituent alcohols, i.e., C2−C3, C2−C3−C4, and C2−C3−C4−C5 alcohols, as feed, are performed in the first RD setup (Supporting Information Figure S2a). Both acetic acid and alcohol are introduced directly to the reboiler, which holds the catalyst within reactive packings. The reaction takes place in the reboiler and the product esters along with water formed and unreacted reactants, if any, rise to the top as vapors. These vapors are condensed, and after phase separation in the decanter, organic phase rich in esters is partly collected as distillate with the rest sent back as reflux. The RD setup used to perform esterification of C4−C5 alcohols has a reactive zone placed between nonreactive rectifying and stripping sections (Supporting Information Figure S2b). The feed streams, i.e., acetic acid as well as C4−C5 alcohols mixtures, are fed at the middle of the reactive section. The product esters are collected from the bottom stream. Water formed in the reaction is removed continuously in the form of an aqueous stream leaving the decanter, while all the organic phase from the decanter is returned back as reflux to the column. The experimental results are compared with the results of steady state simulations, performed using RADFRAC module of ASPEN Plus simulator, with identical values of input parameters as the ones used in the experiments. Effective reboiler duty, taken as input for simulation, was calculated by taking into account the heat lost to the surroundings in the existing experimental setups which was about 50 to 55% of the heat input, as determined through separate experiments. About half of this heat loss was observed to take place from the reboiler and the rest through other parts of the column.

interaction parameters with those observed through the experiments. The agreement is satisfactory in almost all the cases.

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CONTINUOUS REACTIVE DISTILLATION Continuous RD experiments were performed on two different setups (see Figure S2a and S2b in the Supporting Information

Figure 4. Comparison of temperature profiles obtained by experiments and simulation for esterification of acetic acid with (a) an eqimolar mixture of C2−C3−C4 alcohols and (b) an eqimolar mixture of C4−C5 alcohols.

for the schematics), which differ in terms of the placement of the reactive section relative to the nonreactive sections. For the feed mixture involving C2 and/or C3 alcohols, it is suggested that the reaction be performed in the reboiler and the products are removed as distillate followed by further downstream separation. This is because C2 and C3 esters are more volatile than acetic acid. On the other hand, for RD feed containing only C4 or C5 alcohols or their mixture, it is imperative to have a reactive section in the middle of column and a nonreactive stripping section just below it. Again, this is because of the lower volatility of C4 and C5 esters compared to acetic acid. The stripping section serves the purpose of separating acetic acid from esters and sending it back to the reactive zone; esters are removed as bottom products from the column. The first RD setup (Supporting Information Figure S2a) is used to perform the esterification of mixture of C2−C3, C2−C3−C4, and C2− C3−C4−C5 alcohols with acetic acid. It is a glass column with internal diameter 50 mm and a total height of 2 m, comprising of a reactive reboiler (1 lit) along with nonreactive rectifying section (2 m). The reactive section which is limited to reboiler alone is packed with Sulzer Katapak-S filled with cation exchange resin Amberlyst-15. The nonreactive section is packed with HYFLUX low pressure-drop structured, wire mesh packing composed of fine metallic wires (NTSM = 7 to 8) from Evergreen India Ltd. For the ease of representing the experimental results and comparing them with simulations, we treat the packed column as equivalent to a staged column. In all, the column provides 14 nonreactive equilibrium stages and 16642



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Figure 5. Comparison of total vapor flow obtained through simulations of different RD schemes for esterification of fusel oil, for a feed of 100 kmol/ h acetic acid and 100 kmol/h equimolar mixture of ethanol, n-propanol, iso-butanol, and iso-amyl alcohol.

Figure 6. Comparison of total vapor flow obtained through simulations of different RD schemes for esterification of fusel oil, for a feed of 100 kmol/ h acetic acid and 100 kmol/h alcohol mixture with a composition of 10 mol % ethanol, 10 mol % n-propanol, 15 mol % iso-butanol, and 65 mol % iso-amyl alcohol.

reactive section to separate higher boiling C4 and C5 esters from the other components and obtain them as bottom products. We also explored this RD column configuration for esterification reaction with C2−C3−C4−C5 alcohols mixture looking at the possibility of getting C4 and C5 esters from the bottom and esters of C2 and C3 from the top of the RD column; however, high conversions of all the alcohols could not be achieved using such an option. Conversion observed was less than 98% in this scheme. Nevertheless, this configuration shows a reduction in reflux ratio required and thus less energy consumption in the RD column and is worth exploring further for somewhat relaxed product purity specifications. For comparison of cost associated with different schemes, it would be simple and appropriate to compare them on the basis of total vapor flow in all the columns in the scheme.28 For a given feed, a larger value of vapor flow indicates difficult separation requiring large diameter columns and higher utilities consumption in reboilers and condensers. To start with, in all the simulations an equimolar mixture of ethanol, n-propanol, iso-butanol, and iso-amyl alcohol is considered to represent the fusel oil. The end products are the esters of all the alcohols, and purity of each is targeted at 99.5 mol % or above. Initial estimates for optimum number of stages and reflux ratio for the separation columns, especially ones handling near-ideal

Figures 2 and 3 compare the typical experimental concentration profiles observed within the RD column with the results obtained from steady state simulations, and Figure 4 compares the temperature profiles. It can be seen that concentration and temperature profiles match reasonably well with those predicted through the simulations. The results of different RD experiments performed are summarized in Table S3 given in the Supporting Information. In almost all experiments high conversion of alcohol is observed.

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SIMULATION RESULTS Reaction and separation sequences for the different RD configurations described in Table 1 are simulated using the kinetics and thermodynamics inputs from sources as discussed in the previous sections. In these flowsheets, for the reaction of acetic acid with C2 and C3 alcohols or mixtures of these alcohols with other alcohols, viz., C2−C3, C2−C3−C4, C3−C4− C5, or C2−C3−C4−C−C5, the reaction is performed in the reboiler of RD column without any stripping section. All the products are drawn as distillate from the RD column with negligible bottom product. For the RD column handling the reaction of acetic acid with C4 and C5 alcohols or a mixture of C4 and C5 alcohols, reaction is performed in the middle of the RD column, wherein a stripping section is placed below the 16643



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Figure 7. Process flow diagram showing simulation details of RD sequence 12 (see Table 1) for esterification of fusel oil.

mixtures of alcohols or esters alone, are obtained from shortcut distillation design using the Winn−Underwood−Gilliland method in the DSTWU model of Aspen Plus simulator. For reactive columns, reflux ratios are optimized using simulation based parametric studies aimed at achieving the desired conversion. In the RD column used for the reaction of acetic

acid with C2 and C3 alcohols or their mixtures, it is observed that the conversions are more sensitive to reflux ratio rather than the number of stages, and the conversions typically increase with increase in reflux ratio. Hence, the reflux ratio for the RD column is set just enough to achieve the desired conversion level. In general, in each configuration the different 16644



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Figure 8. Process flow diagram showing simulation details of RD sequence 13 (see Table 1) for esterification of fusel oil.

parameters such as number of reactive and nonreactive stages, reflux ratio, and catalyst loading for different distillation columns are varied to minimize total vapor flow, while meeting the targeted product specification. For this purpose, the algorithm proposed by Tang et al.22 for flowsheet optimization that uses a sequential design procedure was implemented. We do not use the cost objective function here, as most of the

reactive packings such as KATAPAK and others are proprietary and not much information is available about their costs in the open literature. In view of this, we have restricted ourselves to the criterion of total vapor flow, knowing that it may not be the best indicator of the total cost, especially when the flowsheet includes reactive distillation. Nevertheless, it may be considered as a useful parameter for the initial screening of a relatively 16645



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acetic acid and subsequent separation of the products, proves to be the relatively less energy-intensive option. This work shows that fusel oil, which is commonly considered as an industrial waste, can be advantageously converted to useful esters of high purity through reactive distillation. Heat integration and complex sequences may be considered as the next step in process synthesis to reduce the cost further and make an appropriate choice of the sequence based on detailed cost estimation.

cheaper option from a number of different alternate configurations. It is therefore recommended that when it comes to decision making, one should do a rigorous cost analysis by obtaining sufficient cost data in order to find the best configuration. The optimum values of vapor flow for nonreactive distillation, corresponding to optimum reflux ratios, are in general 1.2 to 1.5 times the minimum values. The comparison of the total vapor flow in different schemes is depicted in Figure 5. Furthermore, simulations are also performed with a different feed composition (65 mol % iAmOH, 15 mol % iBuOH, 10 mol % nPrOH, 10 mol % EtOH) that more closely resembles the fusel oil composition. The results for these simulations are shown in Figure 6. It is observed that, in both the cases, sequences 12 and 13 show reasonably low heat consumption as against the other sequences for the same product specifications. The simulation details of these flowsheets are shown in Figures 7 and 8, respectively. The simulation details for other flowsheets are provided as Supporting Information to this article. These results suggest that separating the alcohols mixture first in C2− C3 and C4−C5 pairs, followed by their reaction and separation of the resulting ester mixture, involves less energy consumption. Also it is observed that as the amount of less volatile alcohols increases in the feed composition, separating the alcohols followed by their reaction proves to be more economical rather that reaction first followed by separation of esters. In reactionfirst schemes, much of the heat consumption was observed to be associated with the RD column, which required high reflux ratio to achieve high conversion of alcohols in order to meet the targeted purity of products. Esterification of acetic acid with C2 and C3 alcohols falls into type-II RD configurations as classified by Tang et al.,22 whereas esterification with C4 and C5 alcohols falls into a type-III RD configuration.22 Esterification of acetic acid with their mixtures, i.e., C2−C3 and C4−C5 can also be performed in similar RD configurations, i.e., type-II and type-III, respectively, as with individual alcohols. The type-II RD configuration includes an RD column, a decanter, and a stripper. The water formed in the reaction is removed in the form of the aqueous stream from the decanter, and the esters are removed as bottom products from a separate stripper that receives organic phase as the feed. This configuration works well for esterification with C2 and C3 alcohols as well as their mixtures owing to higher volatility of esters formed. On the other hand type-III RD configuration includes only a single RD column with a decanter, wherein ester formed is removed as bottom product from the RD column itself and the water finds outlet in the aqueous stream from the decanter. This configuration works well for esterification with C4 and C5 alcohols as well as their mixtures owing to lower volatility of esters formed. Performing esterification reaction of alcohols and separation of the product esters in an appropriate RD configuration, according to the volatility of the components, is seen to reduce the separation difficulty involved, resulting in lower heat duty requirements and lower total vapor flow.

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ASSOCIATED CONTENT

S Supporting Information *

Flowchart showing different reaction−separation sequences; schematics of experimental RD setups; process flow diagrams giving simulation details of different configurations; UNIQUAC binary interaction parameters used in the work; and RD experiments summary. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE Nfeed feed stage location in the distillation column Nreac reactive stage no. in the distillation column NT total number of stages in the distillation column QR reboiler duty T temperature, °C xi mole fraction of species i in the liquid phase yi mole fraction of species i in the vapor phase

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ABBREVIATIONS AcH acetic acid DBE dibutyl ether EtAc ethyl acetate EtOH ethanol FID flame ionization detector GC gas chromatograph iAmAc iso-amyl acetate iAmOH iso-amyl alcohol iBuAc iso-butyl acetate iBuOH iso-butanol LHHW Langmuir−Hinshelwood−Hougen−Watson model nPrAc n-propyl acetate nPrOH n-propanol PH pseudohomogeneous model RD reactive distillation RR reflux ratio Sim simulation TCD thermal conductivity detector VLE vapor−liquid equilibrium

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CONCLUSION Different RD sequences for the production of esters from fusel oil are identified and simulated with the help of an experimentally validated simulator and compared from the viewpoint of energy consumption. RD process alternatives with low energy consumption and meeting the required product specifications are identified. In general, separating the fusel oil into C2−C3 and C4−C5 pairs, followed by their reaction with

REFERENCES

(1) Singh, A.; Hiwale, R.; Mahajani, S. M.; Gudi, R. D.; Gangadwala, J.; Kienle, A. Production of Butyl Acetate by Catalytic Distillation. Theoretical and Experimental Studies. Ind. Eng. Chem. Res. 2005, 44, 3042−3052. (2) Singh, A.; Tiwari, A.; Mahajani, S. M.; Gudi, R. D. Recovery of Acetic acid from Aqueous Solutions by Reactive Distillation. Ind. Eng. Chem. Res. 2006, 45, 2017−2025.

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Esterification of Fusel Oil Using Reactive Distillation. Part II: Process

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