Heterogeneous Kinetics and Residue Curve Map (RCM

Ind. Eng. Chem. Res. 2007, 46, 3157-3169

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Heterogeneous Kinetics and Residue Curve Map (RCM) Determination for Synthesis of n-Hexyl Acetate Using Ion-Exchange Resins as Catalysts D. Patel and B. Saha* Department of Chemical Engineering, Loughborough UniVersity, Loughborough, Leicestershire, LE11 3TU, United Kingdom

Esterification of dilute acetic acid with n-hexanol was studied with cation-exchange resins (macroporous and gelular) in a jacketed stirred batch reactor to synthesize a value-added ester, namely, n-hexyl acetate. The effect of various parameters such as speed of agitation, catalyst particle size, molar ratio of n-hexanol to acetic acid, reaction temperature, catalyst loading, and reusability of catalysts was studied for optimization of the reaction condition. The nonideality of each component in the reacting mixture was accounted for by using the activity coefficient via use of the UNIFAC group contribution method. The kinetic data were correlated with both pseudo-homogeneous (PH) and adsorption-based reaction rate models, e.g., Eley-Rideal (ER), Langmuir-Hinshelwood-Hougen-Watson (LHHW), and the modified LHHW (ML). Residue curve maps (RCM) were experimentally generated under different conditions to elucidate the feasibility of n-hexyl acetate synthesis in a reactive distillation column (RDC). Introduction Acetic acid is one of the most important industrial chemicals with the production of about 7.83 billion pounds in the United States alone in 2002.1 As one of the most widely used carboxylic acids, it is used as a raw material for various petrochemical intermediates and fine chemical industries including acetate esters (e.g., methyl acetate, ethyl acetate, n-butyl acetate, amyl acetate, isoamyl acetate etc.), cellulose esters, acetic anhydride, monochloroacetic acid, vinyl acetate monomer, and purified terephthalic acid (PTA). The annual production of ethyl acetate by British Petroleum (BP) Amoco, UK is about 250 000 tonnes. The value-added esters of acetic acid are manufactured using either homogeneous or heterogeneous catalysts. The kinetics of esterification of acetic acid for numerous systems are reported in the literature. Synthesis of n-butyl acetate in the presence of ion-exchange resins was studied by Gangadwala et al.2 by reacting n-butanol and concentrated acetic acid. They reported that Amberlyst-15 is the best-performed catalyst for this esterification reaction compared to Indion-130, Amberlite IR-120, and Tulsion TX-66MP catalysts. Synthesis of isobutyl acetate with (Amberlite IR-120) or without catalyst was studied by Altiokka and Citak.3 They reported that the experiments performed with Amberlite IR-120 catalyst reduce the activation energy required for acceleration of the esterification reaction. Methyl acetate synthesis in the presence of Amberlyst 15 ion-exchange resin catalyst was studied by Song et al.4 Synthesis of amyl acetate was studied in the presence of ionexchange resins by Lee et al.5 They reported that Amberlyst 15 performs better than Dowex 50Wx8-100 for this reaction. Synthesis of isoamyl acetate in the presence of Purolite CT175 ion-exchange resin was studied by Teo and Saha.6 They highlighted that the equilibrium conversion of acetic acid increased with an increase in temperature and using excess isoamyl alcohol in the reacting mixture. The batch kinetic modeling of heterogeneously catalyzed esterification reactions using various models is reported in the literature. Batch kinetic data for isoamyl acetate synthesis6 and methyl acetate4 were * To whom correspondence should be addressed. Tel.: +44-1509222505. Fax: +44-1509-223923. E-mail: [email protected].

very well correlated by the Langmuir-Hinshelwood-HougenWatson (LHHW) model. A modified LHHW (ML) model predicts the kinetic data very well for amyl acetate synthesis.5 Gangadwala et al.2 used pseudo-homogeneous (PH) and ML models to predict the batch kinetic data for n-butyl acetate synthesis. The ester of n-hexanol, namely, n-hexyl acetate, is used in a wide range of industrial applications. n-Hexyl acetate is a fruity smelling fluid used as a flavoring agent and in perfumes. It is synthesized through esterification of acetic acid with n-hexanol. The self-catalyzed reaction is very slow. Hence, it is catalyzed using strong inorganic acids, like sulfuric acid, or strong acidic ion-exchange resins. Application of ion-exchange resin catalysts for the esterification process was reported by Harmer and Sun7 as well as Gelbard.8 The main advantages of ion-exchange catalysts are that the catalyst particles are easily eliminated from the reaction mixture, corrosion is reduced significantly, and both selectivity and yields are improved. Production of MTBE (methyl tert-butyl ether),9 TAME (tert-amyl methyl ether),10 ETBE (ethyl tert-butyl ether),11 etc., are a few of the many industrial applications where ion-exchange resins are used. Very recently, Schmitt and Hasse12 used batch reactors to study chemical equilibrium and autocatalyzed reaction kinetics and plug flow reactors to study heterogeneously catalyzed reaction using Amberlyst CSP2 for the synthesis of n-hexyl acetate using concentrated acetic acid. They also reported formation of two byproducts, namely, dihexyl ether and 1-hexene. Acetic acid is produced in large quantities as a byproduct from various industrial processes such as petrochemical and fine chemical industries.6,13 It is therefore important to study the kinetics of dilute acetic acid with different aliphatic alcohols to recover the acetic acid. A reactive distillation column (RDC) could be used for recovery of dilute acetic acid.14 Recovery of acetic acid by esterification with n-butanol is reported in the literature.13,15,16 The heterogeneous-catalyzed esterification of concentrated (99.85% w/w) acetic acid and n-hexanol in the presence of Amberlyst CSP2 catalyst was studied in a reactive distillation column.17 However, the reaction kinetics with dilute acetic acid and technofeasibility evaluation through residue curve map (RCM) determination for reactive distillation have not been published in the literature for n-hexyl acetate synthesis.

10.1021/ie060725x CCC: $37.00 © 2007 American Chemical Society Published on Web 02/03/2007

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Table 1. Physical and Chemical Properties of Purolite CT-124, Purolite CT-175, Purolite CT-151, and Purolite CT-275 Catalysts properties physical form matrix moisture retention, (H%)a total H+ capacity particle size (µm) median pore diameter, d50 (Å) a specific surface area (m2/g) temp. limit (H+ form) (K) a pore vol., (cm3/g) measured BET surface, (m2/g) true density (g/cm3) a

Purolite CT-124

Purolite CT-151

Purolite CT-175

Purolite CT-275

golden-colored spherical beads gelular 56-61 1.2 +1000 < 5% -350 < 6% b b 418 b b 1.45

dark spherical beads macroporous 56-58 1.7 +1200 < 2% -425 < 1.8% 250-450 15-25 418 0.17 25.2 1.40

dark spherical beads macroporous 52-57 1.8 +1200 < 2% -425 < 2% 600-700 20-40 418 0.31 21.3 1.95

dark spherical beads macroporous 54-57 1.7 +1200 < 2% -425 < 2% 600-750 20-35 418 0.24 20.5 1.65

Manufacturers data. b Data not available.

For preliminary design and sequencing of an RDC, RCM could be useful to elucidate the feasibility of a particular operation in an RDC.18 Song et al.4 studied both reactive and nonreactive RCM measurement for a methyl acetate system. Recently, Saha et al.19 published RCM for isoamyl acetate synthesis using dilute acetic acid and isoamyl alcohol and predicted the optimum operating feed mole ratio of isoamyl alcohol to acetic acid as 1:1.9 in an RDC. The objective of this research is to study the kinetic behavior of the heterogeneously catalyzed esterification reaction of dilute acetic acid with n-hexanol in a jacketed stirred batch reactor using Purolite CT-124, Purolite CT-151, Purolite CT-175, and Purolite CT-275 catalysts to produce a value-added ester, n-hexyl acetate. The present work involves detailed catalyst characterization to elucidate the catalytic performance of the ion-exchange resin catalysts used for this reaction. The batch kinetic data obtained from this work were correlated with PH and heterogeneous kinetic models (i.e., ER, LHHW, and ML). The nonideal mixing behavior of the bulk liquid phase was taken into consideration using the activity coefficients, which were calculated using the UNIFAC group contribution method. RCM was experimentally generated using Purolite CT-124 catalyst to predict the feasibility of this reaction in an RDC. RCM will subsequently aid in the possible column configurations toward achieving a desired duty in an RDC for n-hexyl acetate synthesis. Experimental Methods Materials and Catalysts. Acetic acid (99.85%), n-hexanol (98%), and methanol (99.9+%) were purchased from Acros Organics, U.K.; n-hexyl acetate (99%) was supplied by Aldrich Chemical Co., Inc., and n-butanol (99+%) was purchased from Fisher Scientific, U.K. The purity of all chemicals was verified by gas chromatography (GC) analysis. These chemicals were used without further purification. Sulfonated cation-exchange resins, Purolite CT-124, Purolite CT-151, Purolite CT-175, and Purolite CT-275 (supplied courtesy of Purolite International Limited, U.K.), were used for the present work. The physical and chemical properties of Purolite CT-124, Purolite CT-151, Purolite CT-175, and Purolite CT-275 catalysts are listed in Table 1. The Purolite catalysts used in this work were washed with methanol and dried in a vacuum oven at 373 K for 6 h to remove any water sorbed on the catalyst before carrying out the experiments. Catalyst Characterization. The characteristics of ionexchange resin catalysts provide the basis to elucidate the catalytic activity for heterogeneous catalytic processes. Characterization of the catalysts (Purolite CT-124, Purolite CT-151, Purolite CT-175, and Purolite CT-275) was performed using

scanning electron microscopy (SEM), surface area measurement, pore size distribution using a density functional theory (DFT) model based on nitrogen adsorption, elemental analysis, true density determination, and particle size distribution. Scanning Electron Microscopy (SEM). Scanning electron micrographs were taken on a Carl Zeiss 1430 microscope at room temperature to visualize the surface morphology of the ion-exchange resin catalysts. The normal second electron mode (i.e., not backscattering) was used, and the accelerating voltage was set to 5 kV. Prior to analysis, the catalyst sample was dried in a vacuum oven at room temperature and then mounted using PVA glue on an aluminum platform and gold coated. Surface Area, Pore Volume, and Pore Size Distribution Measurements. Surface area and pore size distribution of the ion-exchange resins were measured by the nitrogen adsorption and desorption method using a Micromeritics Accelerated Surface Area and Porosimetry (ASAP) 2010 surface analyzer. The weighed catalyst was prepared by being outgassed at 373.15 K for a minimum period of 24 h on the degas port of the analyzer. Surface area was measured for linear relative pressure range between 0.05 and 0.15. Surface area and pore size distribution analysis for all samples were carried out by the N2 adsorption/desorption method at 77 K. The density functional theory (DFT) model was used to analyze the pore size distribution results. Elemental Analysis. Elemental analysis of ion-exchange resin catalysts was performed in the Department of Pure and Applied Chemistry, University of Strathclyde, U.K. The analysis involved weighing the catalyst samples accurately on aluminum foil and then inserting the samples into the Perkin-Elmer 2400 Elemental Analyzer (Series 2) instrument. Prior to the flash combustion process, the system was purged with helium carrier gas. Flash combustion was then performed at 2073 K, and the gaseous combustion products were quantified using a thermal conductivity detector. The results were obtained as weight percentages of carbon, hydrogen, and sulfur; the oxygen content was measured by difference. True Density Determination and Particle Size Distribution. The true density of the catalyst was measured using a density bottle. The density bottle of known volume was first weighed with methanol in it. After that a known quantity of catalyst sample was added into the density bottle and reweighed. Hence, knowing the volume of methanol displaced by the catalyst, the true density of the catalyst was determined. Density measurements were made for about 4 g of catalyst samples, the measurements were made in duplicate, and average values are reported. The particle size distributions of the resin catalysts were determined using standard sized sieves. Prior to sieving the catalyst, every individual sieve was cleaned, weighed, and

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Figure 1. Experimental setup for residue curve map (RCM) determination.

arranged in decreasing sieve diameters. A known amount of the ion-exchange resin catalyst was accurately weighed and placed in the top sieve. The catalyst was then sieved down through the tower of sieves with the aid of a mechanical shaker. The weight of each sieve was subsequently measured, after which the weight of the ion-exchange resin catalyst collected on each sieve was determined. This procedure was repeated several times to ensure reproducibility of the results. The results of the sieve test were then counter checked using a MalVern Mastersizer. This equipment is used based on the principle of laser diffraction. A representative amount of catalyst sample was wetted with demineralized water. The sample was agitated with a built-in stirrer and recirculated around a sample loop. The measured values were noted. Apparatus and Procedure. Batch Kinetic. The kinetic experiments were conducted in a jacketed stirred (five necked) batch reactor of 0.5 × 10-3 m3. One of the necks of the reactor was connected to the condenser, and the other neck was used for withdrawing samples. Stirring of the reacting mixture was provided by a stirrer motor (IKA-WERKE). A digital thermocouple (Digitron Instrumentation) was inserted through one of the necks of the batch reactor to monitor the temperature of the reacting mixture. Known quantities of acetic acid [30% (w/w); except only one experiment that was carried out to study the effect of acetic acid concentration using 20% (w/w) acetic acid] and n-hexanol were charged into the jacketed stirred batch reactor. The temperature of the reacting mixture was allowed to reach the desired value. A known amount of catalyst was added to the jacketed stirred batch reactor when the reacting mixture attained the desired temperature. The time at which the catalyst was added was taken as zero time (t ) 0), and sample was withdrawn at t ) 0. The samples were withdrawn at regular time intervals and analyzed by a PYE UNICAM 104 Series gas chromatograph (GC). During the experiment the temperature of the reacting mixture was maintained in the range of (0.5 K.

The effect of various parameters such as speed of agitation, catalyst particle size, mole ratio of n-hexanol to acetic acid, concentration of acetic acid (w/w), reaction temperature, catalyst loading (w/w), and reusability of the catalysts were studied for optimization of the reaction conditions. Residue Curve Map. Figure 1 is a schematic of the experimental setup used for the residue curve map determination. A three-necked glass flask of 1.0 × 10-3 m3 capacity was used for the experiment. Two condensers were used for the total reflux of the reaction mixture and condensation of the vapor. Two thermocouples were inserted through the glass flask to measure the vapor and liquid temperatures. The reacting mixture in the flask was stirred by a magnetic stirrer. A heating element was used to heat the reacting mixture in the flask. The reaction flask and distillation head were insulated with glass wool to minimize heat loss to the surroundings. About 10% (w/w) catalyst loading and about 0.21 × 10-3-0.23 × 10-3 m3 of the liquid mixture of desired initial composition was charged to the reaction flask. Valve A of the distillation head was kept open, and valve B was closed. Constant heat input was supplied to the flask, and electrical supplies to the magnetic stirrer were switched on so that the liquid was totally refluxed and the temperatures of both liquid and vapor phases of the mixture reaches the steady state. At steady state, valve A was closed and valve B was opened, so that the vapor was condensed continuously into a collection flask. During the distillation process liquid samples were taken at regular intervals and analyzed by a PYE UNICAM 104 Series gas chromatograph (GC) for their composition. The RCM determination experimental run was stopped when the amount of liquid residue in the reaction flask was small enough to preclude sampling. Subsequent experimental runs were carried out at compositions close to the measured end point of the previous experimental run so as to obtain a significant portion of each residue curve. For generation of one particular curve at a specific feed mole

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Figure 2. Mechanism of the heterogeneous catalyzed n-hexyl acetate synthesis.

Figure 3. Scanning electron micrographs of Purolite catalysts.

ratio of n-hexanol to acetic acid, the experiment was terminated when the boiling point of the residue reaches the maximum (i.e., the reactant mixture in the still reaches either the maximum boiling azeotrope or the highest boiling pure component composition). During the experimental run the vapor was in equilibrium with the liquid remaining in the still. Since the vapor was always richer in the more volatile components than the liquid, the liquid mixture composition changed continuously with time, becoming more concentrated in the least volatile species. Each experimental run took about 0.75-1.5 h to reach the steady state and was continued for about 1.0-3.0 h depending upon the initial reactant feed composition before the still ran dry for collecting the equilibrium RCM data. Analysis. The samples withdrawn from both the batch kinetic experiments and RCM determination experiments were analyzed using a PYE UNICAM gas chromatograph (GC). The GC was fitted with a thermal conductivity detector (TCD). A packed stainless steel PORAPAK-Q column (supplied by Supelco) with dimensions (d ) 3.175 × 10-3 m and L ) 1.829 m) was used to separate all the components present in the reaction mixture. High-purity (99.9%) helium was used as the carrier gas. The flow rate of the helium gas was maintained at 7.1667 × 10-10 m3 s-1. Both the detector and the injector temperatures were maintained isothermally at 503.15 K. Internal standard method was used for analysis of samples, using n-butanol as an internal standard.

Mechanism of Reaction. Acetic acid reacts with n-hexanol in the presence of Purolite CT-124/Purolite CT-151/Purolite CT175 or Purolite CT-275 cation-exchange resins to produce a value-added ester, n-hexyl acetate. It was proposed that the mechanism of this heterogeneous-catalyzed esterification reaction is similar to a homogeneous-catalyzed esterification reaction.20 The proposed mechanism is shown in Figure 2. Acetic acid accepts a proton from strong acid cation exchanger, i.e., Purolite catalysts. The n-hexanol molecule then attacks the protonated carbonyl group to form an intermediate. A proton is lost at one oxygen atom and gained at another to form another intermediate. The intermediate formed in the above step then loses a molecule of water to give a protonated ester. A proton is then transferred to a water molecule to give the desired ester, n-hexyl acetate. All the above steps are reversible. It is to be noted that no byproducts (such as dihexyl ether or 1-hexene) were formed during the course of the reaction. Experimental Results Catalyst Characterization. Scanning Electron Microscopy (SEM) Results. Figure 3 presents the microscopic examination of the surface morphology of the Purolite ion-exchange resin catalysts employed in the present work. Figure 3a shows a very smooth and gel-like surface of Purolite CT-124 that corresponds well with its gelular polymeric matrix structure (as shown in


Figure 5. Pore size distribution of Purolite catalysts. Figure 4. BET surface to pore volume ratio of ion-exchange resin catalysts.

Table 1). Figure 3b shows hairline cracks on the surface of Purolite CT-151 catalyst. Micrographs 3(c) and 3(d) show the surface morphology of Purolite CT-175 and Purolite CT-275 ion-exchange resin catalysts, respectively. They show agglomerates of microspheres which look like cauliflowers and smaller nuclei (10-30) nm more or less fused together that are observed within each microsphere. Intermediate pores (20-50 nm), known as mesopores, are accountable for the moderate surface areas. Large pores (50-1000 nm), otherwise known as macropores, are responsible for the pore volume of the catalyst. These large pores allow the reactants to permeate easily into the catalyst structure, regardless of whether the microspheres are swollen by the reactants. Similar observations were reported for Purolite CT-175 and Purolite CT-275 catalysts by Teo and Saha.6 Surface Area, Pore Volume, and Pore Size Distribution. Table 1 shows that Purolite CT-151 has the largest BrunauerEmmett-Teller (BET) surface area (25.2 m2 g-1) among the ion-exchange resin catalysts analyzed, while both Purolite CT175 and Purolite CT-275 show almost similar surface area values, with Purolite CT-175 having a BET surface area of 21.3 m2 g-1 while the BET surface area of Purolite CT-275 sample is 20.5 m2 g-1. It can also be seen from Table 1 that Purolite CT-175 has the biggest pore volume (0.31 cm3 g-1) among the ion-exchange resins analyzed. Purolite CT-275 exhibited a pore volume of 0.24 cm3 g-1, while Purolite CT-151 showed a pore volume of 0.17 cm3 g-1. BET surface area to pore volume ratio results of ion-exchange resin catalysts used in this study are shown in Figure 4. Purolite CT-151 showed the highest (1.48 × 108 m2 m-3) BET surface area to pore volume ratio, whereas Purolite CT-175 represented the lowest value (6.87 × 107 m2 m-3). It is to be noted that it was not possible to obtain the surface area and pore size distribution data for Purolite CT124 catalyst as the gelular matrix structure of the polymeric resin collapsed during the course of the analysis in the Micromeritics Accelerated Surface Area and Porosimetry (ASAP) 2010 analyzer. The pore size distributions of the ion-exchange resins were measured through the density functional theory (DFT) model based on nitrogen adsorption assuming slit pore geometry. The DFT model has been recognized as a powerful tool for the study of inhomogeneous fluids.6,21,22 Purolite CT-151, Purolite CT175, and Purolite CT-275 are macroporous sulfonated styrene divinylbenzene polymeric resins. Figure 5 shows a comparison of the pore size distribution results for different ion-exchange resin catalysts and confirms the macroporous nature of all the analyzed catalysts. All the polymeric resins exhibited significant pore volume in the macroporous regions. However, more prominent access pores were observed in Purolite CT-175 as

Table 2. Elemental Analysis Results of Purolite Catalysts used in this Work weight (%) catalysts used

C

H

S

O

Purolite CT-124 Purolite CT-151 Purolite CT-175 Purolite CT-275

37.13 41.04 44.57 49.89

6.59 5.96 5.64 4.47

12.46 13.78 15.07 17.50

43.82 39.22 34.72 28.14

compared to Purolite CT-151 and Purolite CT-275, which exhibited much less prominent catalyst access pores. Elemental Analysis. The elemental analysis results are presented in Table 2. It is to be noted that nitrogen was not detected in any of the analyzed ion-exchange resin catalysts. The oxygen content presented in Table 2 was not analyzed directly from the elemental analysis, however, but determined by the difference from the weight percentage compositions of other elements (i.e., C, H, and S). It can be seen from Table 2 that Purolite CT-124 catalyst contains the highest proportion of oxygen (43.82%), whereas Purolite CT-275 catalyst has the highest percentage of sulfur content (17.5%). Elemental analysis also confirms that all the catalysts used are basically sulfonated styrene divinylbenzene polymeric resins. True Density Determination and Particle Size Distribution. The measured true densities of Purolite CT-124, Purolite CT-151, Purolite CT-175, and Purolite CT-275 catalyst are 1.45, 1.40, 1.95, and 1.65 g/cm3, respectively. The particle size distribution results of different ion-exchange resin catalysts are presented in Figure 6. Figure 6a shows that Purolite CT-124 has a wide range of size distribution with the majority of the catalyst particles (∼77%) laying within the size range of 300600 µm. Figure 6b confirms that approximately 86% (w/w) of Purolite CT-151 catalyst particles lie within the size range of 500-600 µm. Purolite CT-124 exhibits ∼22% of catalyst particles in the size range of 850-1400 µm. Figure 6c shows that about 94% (w/w) of the Purolite CT-175 catalyst particles fall within the size range of 600-850 µm, while the remaining 6% (w/w) of the catalyst particles are placed within the size range of 355-500 µm. Figure 6d illustrates that Purolite CT275 catalyst has a wider particle size distribution as compared to Purolite CT-175 catalyst. It is to be noted that ∼89% (w/w) of Purolite CT-275 catalyst particles lie within the size range of 420-710 µm and 10% (w/w) of the catalyst particles fall within the size range of 180-355 µm. Similar results were also reported elsewhere.6,23 Batch Kinetic Studies. The effect of agitation speed, catalyst particle size, catalyst types, catalyst loading, acetic acid concentration, feed mole ratio of n-hexanol to acetic acid on the rate of reaction, and reusability of the catalysts were investigated for the synthesis of n-hexyl acetate in a jacketed stirred batch reactor.

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Figure 6. Particle size distribution of various Purolite catalysts.

Figure 7. Effect of stirrer speed on conversion of acetic acid at catalyst loading: 5% (w/w); temperature, 368.15 K; feed mole ratio (n-hexanol to acetic acid), 2:1; acetic acid concentration, 30% (w/w); catalyst, CT-175.

Elimination of Mass-Transfer Resistances. The esterification reaction was studied with different stirrer speeds using Purolite CT-175 catalyst. There are two types of mass transfer resistances associated with this reaction: One across the solidliquid interface, i.e., the influence of external mass transfer resistances, and other in the intraparticle space, i.e., internal mass transfer resistance which is associated with different catalyst particle size. In order to confirm the influence of external mass transfer resistances, experiments were carried out using different stirrer speeds using Purolite CT-175 catalyst. The results are shown in Figure 7. Figure 7 shows that conversion of acetic acid after about 6 h for stirrer speeds of 300 and 500 rpm was about 41% under otherwise identical conditions. It was therefore confirmed that there was little effect of stirrer speed on the conversion of acetic acid in the range of stirrer speeds of 300

and 500 rpm, and hence, there was no external mass transfer resistances for this esterification reaction. Thus, all further experiments were performed at a stirrer speed of 500 rpm with Purolite CT-175 catalyst. On the basis of above discussion, it was concluded that there was no influence of mass transfer resistances on the rate of the esterification reaction at a stirrer speed of 500 rpm, and hence, all the further experiments were conducted at 500 rpm with Purolite CT-124, Purolite CT-151, Purolite CT-175, and Purolite CT-275 catalysts. The nonexistence of intraparticle resistances was confirmed using catalyst particles of different size distribution. Also, it was reported in the literature for esterification of acetic acid with isoamyl alcohol6 and n-butanol2 that no evidence of intraparticle resistances was encountered when the experiments were performed using different catalyst particle diameters. Teo


Figure 8. Effect of different types of catalyst on conversion of acetic acid at catalyst loading: 10% (w/w); temperature, 368.15 K; stirrer speed, 500 rpm; acetic acid concentration, 30% (w/w); feed mole ratio (n-hexanol to acetic acid), 2:1.

Figure 9. Effect of catalyst loading on conversion of acetic acid at feed mole ratio (n-hexanol to acetic acid) 2:1: temperature, 368.15 K; catalyst, CT-124; acetic acid concentration, 30% (w/w); stirrer speed, 500 rpm.

and Saha6 used Purolite CT-175 catalyst particle diameters of 200-400, 500-710, and 800-900 µm, while Gangadwala et al.2 used Amberlyst 15 catalyst particle diameters of 25, 72, and 100 mesh, respectively. Effect of Catalyst. Synthesis of n-hexyl acetate was studied with different types of ion-exchange resin catalysts, such as Purolite CT-124, Purolite CT-151, Purolite CT-175, and Purolite CT-275 catalysts. The results are shown in Figure 8. It was observed that the performance of Purolite CT-124 catalyst was the best for this reaction. Conversion of acetic acid after 6 h using Purolite CT-124 catalyst was about 78%, while that for Purolite CT-151, Purolite CT-175, and Purolite CT-275 (all macroporous) catalysts was about 56%, 60%, and 35%, respectively, and hence, all subsequent experiments were conducted with Purolite CT-124 catalyst. It is to be noted that for the present system the reaction mixture consists of a substantial amount of water. This is because of using dilute acetic acid as one of the reactants [containing 30% (w/w) acetic acid and 70% (w/w) water] and also due to formation of water during the course of the reaction. It appears that the gelular matrix of Purolite CT-124 resin catalyst swells in the presence of this reacting mixture that also consists of a substantial amount of water, which subsequently increases its catalytic activity.2 On the other hand, four sulfonic groups of the macroporous resins could be attached to one water molecule, and since the water is present in abundance, it may hinder the rate of reaction for the macroporous resin catalysts.24,25 Effect of Catalyst Loading. The esterification reaction was studied at different catalyst loading using Purolite CT-124 catalyst. Catalyst loading is defined as the ratio of mass of

catalyst to the mass of reactants fed. The results are shown in Figure 9. It was observed that the increase in catalyst loading increases the rate of reaction and hence improves the conversion of acetic acid. The reason for this is that the higher the catalyst loading, the higher the total number of available active catalytic sites for the esterification reaction. As a result the rate at which the esterification reaction reaches equilibrium is faster. Figure 9 illustrates that conversion of acetic acid after 5 h for a catalyst loading of 5% (w/w) was 37% and that for a 10% (w/w) catalyst loading was 74%. For all further experiments a catalyst loading of 10% (w/w) was chosen. It should be noted that the equilibrium conversion, however, is independent of the catalyst loading. Similarly, the esterification reaction was also studied at different catalyst loading using Purolite CT-175 catalyst. The results are shown in Figure 10. A similar trend was also observed for Purolite CT-175 catalyst. From Figure 10 it can be seen that conversion of acetic acid after 6 h for a catalyst loading of 5% (w/w) was 41% and that conversion of acetic acid after 6 h for a catalyst loading of 10% (w/w) was 60%. Effect of Acetic Acid Concentration. Synthesis of n-hexyl acetate was studied at different concentrations of acetic acid using Purolite CT-124 catalyst. The results are shown in Figure 11. It was observed that the increase in acetic acid concentration increases the rate of conversion. Figure 11 shows that the conversion of acetic acid after 6 h for 20% (w/w) acetic acid concentration was 73% and that for 30% (w/w) acetic acid concentration was 78%. All further experiments were carried out at an acetic acid concentration of 30% (w/w). Typical values of acetic acid from many chemical and petrochemical processes

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Figure 10. Effect of catalyst loading on conversion of acetic acid at feed mole ratio (n-hexanol to acetic acid) 2:1: temperature, 368.15 K; catalyst, CT-175; acetic acid concentration, 30% (w/w); stirrer speed, 500 rpm.

Figure 11. Effect of acetic acid concentration on conversion of acetic acid at catalyst loading: 10% (w/w); feed mole ratio (n-hexanol to acetic acid), 2:1; temperature, 368.15 K; catalyst, CT-124; stirrer speed, 500 rpm.

Figure 12. Effect of feed mole ratio of n-hexanol to acetic acid on conversion of acetic acid at catalyst loading: 10% (w/w); temperature, 368.15 K; acetic acid concentration, 30% (w/w); catalyst, CT-124; stirrer speed, 500 rpm.

are in the range of 5-65% (w/w).6 The study of the effect of acetic acid was therefore of utmost importance so as to recover the acetic acid from these petrochemical and fine chemical industries. The present study was directed toward esterification of 30% (w/w) dilute acetic acid by esterification with n-hexanol. Effect of Feed Mole Ratio. Synthesis of n-hexyl acetate was studied using different feed mole ratios (FMR) of n-hexanol to acetic acid using Purolite CT-124 catalyst [10% (w/w) catalyst loading]. The results are shown in Figure 12. Synthesis of n-hexyl acetate is an equilibrium-limited esterification reaction. Use of an excess amount of n-hexanol will therefore increase the rate of reaction and conversion of acetic acid. The results

were very similar for feed mole ratios (n-hexanol to acetic acid) of 2:1 and 4:1. Figure 12 demonstrates that conversion of acetic acid after 4 h for both 2:1 and 4:1 feed mole ratios (n-hexanol to acetic acid) is about 71%. All experiments were therefore studied using the lower feed mole ratio (n-hexanol to acetic acid) of 2:1 to minimize loss of unreacted n-hexanol for n-hexyl acetate synthesis. Effect of Catalyst Reusability. Reusability of Purolite CT124 catalyst was studied for the synthesis of n-hexyl acetate. The catalyst was reused by washing it with methanol and dried in a vacuum oven at 373 K for about 6 h. The results are shown in Figure 13. It was observed that Purolite CT-124 catalyst gives a very similar conversion of acetic acid after being reused once


Figure 13. Effect of catalyst reusability on conversion of acetic acid at catalyst loading: 10% (w/w); temperature, 358.15 K; acetic acid concentration, 30% (w/w); feed mole ratio (n-hexanol to acetic acid), 2:1; catalyst, CT-124; stirrer speed, 500 rpm.

Figure 14. Comparison of experimental and calculated values for the effect of reaction temperature on conversion of acetic acid at catalyst loading: 10% (w/w); feed mole ratio (n-hexanol to acetic acid), 2:1; catalyst, CT-124; acetic acid concentration, 30% (w/w); stirrer speed, 500 rpm.

under otherwise identical conditions. Figure 13 shows that conversion of acetic acid after 6 h for fresh Purolite CT-124 catalyst was 62%, while the same for reused (once) catalyst was 58%. It is therefore concluded that Purolite CT-124 catalyst can be used for several runs without any appreciable decrease in the conversion of acetic acid. Teo and Saha6 also reported that the Purolite CT-175 resin catalyst can be used repeatedly without significant change in conversion of acetic acid for isoamyl acetate synthesis. Kinetic Modeling. n-Hexyl acetate was synthesized at different temperatures using the best-performed Purolite CT124 catalyst. The results are shown in Figure 14. It was observed that with an increase in temperature the rate of reaction and conversion of acetic acid increase. A number of runs at different temperatures help in modeling the reaction kinetic and hence for determination of the rate constant, activation energy, and Arrhenius pre-exponential factor. Both the PH and heterogeneous kinetic rate models, e.g., Langmuir-HinshelwoodHougen-Watson (LHHW), Eley-Rideal (ER), and modified LHHW (ML), were applied for correlating the kinetic data at different temperatures. Reaction rates were calculated by the differential methods as proposed by Cunill et al.26 The reaction rates for the heterogeneously catalyzed esterification reaction for synthesis of n-hexyl acetate can be written as

(-rA)V ) NA0

( ) dxA dt

(1)

where -rA is the reaction rate of acetic acid (limiting reactant), V is the volume of the reacting mixture, NA0 is the initial number of moles of acetic acid, xA is the conversion of acetic acid, and t is the reaction time. The equilibrium constant for the reaction was calculated from knowledge of liquid-phase mole fraction and activity coefficient of the components at equilibrium through eq 2

Keq )

( ) ( )( ) aCaD aA aB

)

eq

xCxD xAxB

eq

γCγD γ Aγ B

(2)

eq

where Keq is the equilibrium constant of the reaction, ai is the activity of the ith component, xi is the mole fraction of the ith component, and γi is the activity coefficient of the ith component. The subscripts A, B, C, and D represent acetic acid, n-hexanol, n-hexyl acetate, and water, respectively. The parameters for the different models were estimated by minimizing the sum of the residual squares (SRS) between the experimental and calculated reaction rates using eq 3

SRS )

∑

samples

(rexp - rcalcd)2

(3)

3166


Table 3. Activity Coefficient Values at 368.15 K As Calculated by UNIFAC Group Contribution Method (A ) acetic acid; B ) n-hexanol; C ) n-hexyl acetate; D ) water) mole fraction (xi)

activity coefficient (γi)

temp. (K)

xA

xB

xC

xD

γA

γB

γC

γD

368.15

0.0810 0.0724 0.0683 0.0602 0.0532 0.0510 0.0450 0.0418 0.0348 0.0321 0.0273 0.0270 0.0246 0.0238 0.0200

0.1738 0.1652 0.1611 0.1530 0.1460 0.1438 0.1378 0.1346 0.1276 0.1249 0.1201 0.1198 0.1174 0.1166 0.1128

0.0118 0.0204 0.0245 0.0326 0.0396 0.0418 0.0478 0.0510 0.0580 0.0607 0.0655 0.0658 0.0682 0.0690 0.0728

0.7334 0.7420 0.7461 0.7542 0.7612 0.7634 0.7694 0.7726 0.7796 0.7823 0.7871 0.7874 0.7898 0.7906 0.7944

0.9630 0.9550 0.9510 0.9440 0.9370 0.9350 0.9300 0.9260 0.9200 0.9170 0.9120 0.9120 0.9100 0.9090 0.9050

3.2520 3.2860 3.3020 3.3340 3.3610 3.3690 3.3930 3.4050 3.4320 3.4430 3.4610 3.4630 3.4720 3.4750 3.4900

12.9080 13.0210 13.0740 13.1800 13.2700 13.2990 13.3760 13.4170 13.5070 13.5410 13.6020 13.6060 13.6370 13.6470 13.6950

1.4510 1.4490 1.4480 1.4450 1.4430 1.4430 1.4410 1.4400 1.4380 1.4370 1.4360 1.4360 1.4350 1.4350 1.4330

where SRS is the sum of residual squares resulting in the fitting procedure and r is the reaction rate. The subscripts exp and calcd denote experimental and calculated values, respectively. The PH model is used considering the Helfferich concept27 which considers that catalysis of liquid-phase reactions using ion-exchange resins is similar to homogeneous catalysis by dissolved electrolytes. Numerous authors have modeled the heterogeneous kinetic data using the PH model.2,12,28-32 The LHHW and ML models are used whenever the ratedetermining step is the surface reaction between the adsorbed molecules. On the other hand, the ER model is applied if in the rate-limiting step, surface reaction, takes place between one adsorbed species and one nonadsorbed reactant from the bulk liquid phase. Moreover, because of the strong affinity of Purolite CT-124 resin for water, the activity of water in the catalyst gel phase, where the reaction occurs, is distinctly different from that in the liquid phase and was also reported by Lee et al.5 The rate expressions for each model are as follows. PH Model

( )(

)

( )(

)

-rA ) Af

Ar -E0 aAaB - aCaD RT Af

(4)

LHHW Model

Af -rA )


(1 + KAaA + KBaB + KCaC + KDaD)2

(5)

ER model

Af -rA )

( )(


)

(1 + KAaA + KBaB + KCaC + KDaD)

(6)

ML model

Af -rA )

( )(

Ar -E0 aAaB - aC(aD)R RT Af

)

(1 + KAaA + KBaB + KCaC + KD(aD)R)2

(7)

where Af and Ar are the Arrhenius preexponential factors for the forward and reverse reactions, E0 is the activation energy of the reaction, R is the gas constant, T is the reaction

Table 4. Parameters of the PH Model Used To Fit the Experimental Data for Synthesis of n-Hexyl Acetate activation energy (kJ/mol) 75.6

pre-exponential factor (mol g-1 s-1) 3.5 × 108

temperature, KA, KB, KC, and KD are the adsorption equilibrium constants, and aA, aB, aC, and aD are the activities for acetic acid, n-hexanol, n-hexyl acetate, and water, respectively; R is the empirical constant that was introduced in the LHHW model to give an expression for the modified LHHW (ML) model. The kinetic data for esterification of dilute acetic acid with n-hexanol would be useful for the simulation and design of an RDC for removing acetic acid from aqueous stream and to obtain a value-added product, n-hexyl acetate. The activity of the components was taken into account instead of the concentration of the components for kinetic modeling to account for the nonideal mixing behavior of the bulk liquid phase. The UNIFAC group contribution method was used for estimation of the activity coefficients.33 Activity coefficient values as calculated by the UNIFAC group contribution method at 368.15 K are given in Table 3. The PH and heterogeneous kinetic models, e.g., ER, LHHW, and ML, were used to correlate the experimental kinetic values by minimizing the sum of residual squares (SRS) between the experimental and calculated reaction rates through eq 3. The heterogeneous kinetic models gave negative values of adsorption coefficient. Gangadwala et al.2 also reported the negative values for the LHHW model without modification (i.e., R ) 1). Schmitt and Hasse12 also concluded that the adsorption model for synthesis of n-hexyl acetate “fails to correctly describe reaction kinetics”. However, the PH model gave a better correlation between the experimental and model reaction rates as compared to adsorption rate models. The parameters of the PH model used to fit the experimental data are reported in Table 4. Residue Curve Map. Residue curve map determination experiments, also known as simple distillation experiments, were performed to elucidate the feasibility of n-hexyl acetate synthesis through recovery of dilute acetic acid in an RDC. All of the residue curves originate at the light (lowest boiling) pure component in a region. It then moves toward the intermediate boiling component. At the end it moves toward the heavy (highest boiling) pure component in the same region. The lowest temperature nodes are termed as unstable nodes, while the highest temperature points in the region are termed stable nodes, the point at which the residue curve terminates. The points at which the trajectories approach from one direction and end in


Figure 15. Representation of residue curve at different feed mole ratio (FMR) of n-hexanol to acetic acid; catalyst loading, 10% (w/w); acetic acid concentration, 30% (w/w); constant stirrer speed; constant heat input.

a different direction (as always is the point of intermediate boiling component) are termed saddle points. Residue curves that divide the composition space into different distillation regions are called distillation boundaries. The RCM gives a preliminary idea of the feed mole ratio (FMR) of n-hexanol to acetic acid, which is one of the important parameters for operation in an RDC. Therefore, for RCM determination, experiments were carried out over a wide range of feed mole ratios (FMR) of n-hexanol to acetic acid (FMR ) 4:1, FMR ) 2:1, FMR ) 1:1, FMR ) 0.5:1, and FMR ) 0.3:1) to obtain the maps. The results are shown in Figure 15. The RCM for the esterification of 30% (w/w) dilute acetic acid with n-hexanol to produce n-hexyl acetate and water was defined by a rectangular diagram as illustrated in Figure 15. The four corners of the square represent the four pure components (i.e., acetic acid, n-hexyl acetate, n-hexanol, and water), and the four edges represent the four binary nonreactive mixtures consisting of one reactant and one product (i.e., water-acetic acid, acetic acidn-hexyl acetate, n-hexyl acetate-n-hexanol, and n-hexanolwater). The interior of the square represents a four-component mixture (i.e., water-acetic acid-n-hexanol-n-hexyl acetate) in phase and reaction equilibrium. For this esterification reaction n-hexyl acetate is chosen as the reference compound and the independent transformed compositions are given by eqs 8-10

XA ) xacetic acid + xn-hexyl acetate

(8)

XB ) xn-hexanol + xn-hexyl acetate

(9)

XC ) xwater + xn-hexyl acetate

(10)

where XA, XB, and XC are the transformed variables while xi is the liquid-phase molar composition of the ith component. The azeotrope reacted into a four-component equilibrium mixture in the presence of Purolite CT-124 gelular resin. The remarkable feature of the RCM was formation of the four-component minimum-boiling reactive azeotrope (see Figure 15). Formation of the four-component minimum-boiling reactive azeotrope was

reported for esterification of acetic acid with isoamyl alcohol.19 The closed big square on the X axis of Figure 15 indicates the position of the reactive azeotrope as closely as it could be determined from the present work from a simple distillation experiment. The measured molar composition of the reactive azeotrope was 88.78% water, 10.23% acetic acid, 0.59% n-hexanol, and 0.4% n-hexyl acetate. The corresponding transformed compositions of the n-hexyl acetate system are XA ) 0.1063 and XB ) 0.0099 at 375.3 K. From Figure 15 it can be seen that if the synthesis of n-hexyl acetate is performed in a reactive distillation column using a feed mole ratio of n-hexanol to acetic acid of 4:1, then the top product will be a mixture of mostly n-hexanol and the desired product, n-hexyl acetate, while the bottom product will consist of mainly water, some unreacted acetic acid, and n-hexanol. For recovery of dilute acetic acid in an RDC, it is desirable to have the bottom product that contains mainly water and a minute quantity of unreacted acetic acid. Moreover, the presence of a huge amount of unreacted n-hexanol along with the desired product, n-hexyl acetate, in the top product stream of RDC is not a viable option. Thus, a feed mole ratio (n-hexanol to acetic acid) of 4:1 is not recommended for RDC experiments. On the other hand, if a very low feed mole ratio of n-hexanol to acetic acid (0.3 and 0.5) is used, then the top product will be a mixture of acetic acid and n-hexyl acetate while the bottom product will be a mixture of mostly water with some quantities of unreacted acetic acid and n-hexanol. A low feed mole ratio of n-hexanol to acetic acid indicates that the top product in the RDC will contain a substantial amount of acetic acid, which is not desirable from the acetic acid recovery point of view. This observation is also supported by the azeotropic data reported by Schmitt et al.17 and by the fact that there is a large difference in the boiling point of n-hexyl acetate (176 °C) and acetic acid (117.79 °C). Hence, the RCM results suggest that a feed mole ratio of n-hexanol to acetic acid in the range of 1.2-2.0 is the most viable option for the recovery of dilute acetic acid in continuous RDC operation. Conclusions The kinetics of n-hexyl acetate synthesis from n-hexanol and dilute 30% (w/w) acetic acid in the presence of Purolite CT124, Purolite CT-151, Purolite CT-175, and Purolite CT-275 ion-exchange resins was studied. The agitation speed and catalyst particle size had no significant effect on acetic acid conversion. Conversion of acetic acid was found to increase with an increase in temperature and catalyst loading. No evidence of formation of byproducts, such as 1-hexene or dihexyl ether, was found during the experiments. Purolite CT124, a gelular resin, was found to be the best catalyst for this heterogeneous-catalyzed esterification reaction as compared to macroporous resins (Purolite CT-151, Purolite CT-175, and Purolite CT-275). Also, out of the three macroporous catalysts used, Purolite CT-175 was found to be the best-performed catalyst for the synthesis of n-hexyl acetate. The heterogeneous kinetic models were unable to predict the experimental data very well since as all of them gave negative values of adsorption coefficients. On the other hand, the pseudo-homogeneous (PH) model gave a better representation of the kinetic behavior for this heterogeneous catalytic esterification reaction over a wide range of temperatures and feed mole ratios of alcohol to acid as compared to heterogeneous kinetic models. A residue curve map for the quaternary system was experimentally generated under different reaction conditions to elucidate the feasibility of this operation in an RDC. The most interesting feature

3168


observed from generation of the RCM was formation of a fourcomponent minimum-boiling reactive azeotrope at 375.3 K. The experimental results obtained from both the batch kinetic and RCM will now be useful for recovery of dilute acetic acid and also for synthesis of n-hexyl acetate simultaneously in a reactive distillation column (RDC). Acknowledgment The authors thank Purolite International Limited, U.K. (and also Dr. Jim Dale), for kindly supplying the catalysts for this work. Nomenclature A,B,C,D ) acetic acid, n-hexanol, n-hexyl acetate, and water, respectively ai ) activity of the ith component in the liquid phase Af ) Arrhenius pre-exponential factor for the forward reaction (mol s-1 g-1) Ar ) Arrhenius pre-exponential factor for the reverse reaction (mol s-1 g-1) d ) diameter of the column (m) E0 ) activation energy of the reaction KA ) adsorption equilibrium constant for acetic acid KB ) adsorption equilibrium constant for n-hexanol KC ) adsorption equilibrium constant for n-hexyl acetate KD ) adsorption equilibrium constant for water Keq ) equilibrium constant of the reaction L ) length of the column (m) NA0 ) initial moles of the acetic acid in reacting mixture (mol) R ) gas constant (J mol-1 K-1) r ) rate of the reaction (mol g-1 s-1) -rA ) reaction rate of acetic acid (mol g-1 s-1) T ) temperature (K) t ) time (s) V ) volume of the reacting mixture (m3) X ) transformed composition XA ) % conversion of acetic acid xi ) mole fraction of the ith component Greek Letters R ) constant in modified LHHW model γ ) activity coefficient of component List of AbbreViations ER ) Eley-Rideal model FMR ) feed mole ratio GC ) gas chromatograph LHHW ) Langmuir-Hinshelwood-Hougen-Watson model ML ) modified LHHW model PH ) pseudo-homogeneous model RCM ) residue curve map RDC ) reactive distillation column Subscripts calcd ) calculated values eq ) equilibrium exp ) experimental values f ) forward reaction i ) component r ) reverse reaction Literature Cited (1) Shi, Y. H.; Fan, M. H.; Li, N.; Brown, R. C.; Sung, S. W. The Recovery of Acetic Acid with Sulphur Dioxide. Biochem. Eng. J. 2005, 22, 207.

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ReceiVed for reView June 7, 2006 ReVised manuscript receiVed November 6, 2006 Accepted November 7, 2006 IE060725X

Heterogeneous Kinetics and Residue Curve Map (RCM

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