Pervaporation membrane reactors (PVMRs) for esterification

19

Pervaporation membrane reactors (PVMRs) for esterification

G. Genduso1, P. Luis2, B. Van der Bruggen1 1 KU Leuven, Leuven, Belgium; 2Université Catholique de Louvain, Louvain-la-Neuve, Belgium

19.1

Introduction

Esterification reactions have been described extensively in the literature. Otera (2003), in his study “Esterification Methods, Reactions and Applications,” has collected approximately 5000 bibliographic references. Despite this huge collection of information, the section regarding industrial applications of esterification (and transesterification) reaction is not complete because some of the industrial processes are not fully disclosed. In the following chapter, ester compounds will be presented together with esterification methodologies and their main industrial applications. After this concise overview, the focus will be on traditional reactors and membrane reactors for esterification reactions. The final part of this chapter is left to the description of the state-of-the-art of pervaporation membrane reactors (PVMRs) for esterification, which certainly reside in the group of the most promising process-intensification technologies to be implemented in industry, obtaining cost-saving and reduction of the environmental impact of esterification reactions.

19.2

Physicochemical properties of esters

An ester is a chemical compound made of an alkoxy group (eOR) bonded with a carbonyl group (Figure 19.1(a)). Two general formulas of this compound are presented in Figure 19.1(b). When an alcohol and a carboxylic acid react in an esterification reaction, the name of the obtained ester derives from the name of the two reagents. The first part of the name is formed by adding -yl to the root of the name of the alcohol and the second Carbonyl oxygen

O A

(a)

C

O B

R

(b)

or O

R'

RCO 2R'

Alkoxy oxygen

Figure 19.1 (a) A carbonyl group and (b) the general formula of an ester. Membrane Reactors for Energy Applications and Basic Chemical Production. http://dx.doi.org/10.1016/B978-1-78242-223-5.00019-4 Copyright © 2015 Elsevier Ltd. All rights reserved.

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O

CH3 CH2 CH2 OH CH3 BP = 97.4 °C

O OCH3

BP = 57.5 °C

CH3

O CH3

CH3 CH2

BP = 56 °C

H

BP = 49 °C

O CH3 CH2 OCH3 BP = 10.8 °C

CH3 CH2

OH

BP = 141 °C

Figure 19.2 Examples of boiling temperatures of carbonyl-based substances (Bruice, 2004).

part comes from the root of the carboxylic acid name with the ending -ate or -oate (in the IUPAC nomenclature). For example, a reaction between butanoic acid and ethanol gives ethyl butanoate. Cyclic esters are called lactones. Similar to other molecules presenting a carbonyl group, esters are polar molecules (e.g., ketones and aldehydes) and therefore create dipoleedipole interactions as well as van der Waals dispersion forces. Some properties are important to understand esterification reactions and reactors. Concerning boiling points, carbonyl compounds can be listed as follows: carboxylic acid > > ester w acyl chloride > aldehyde > ketone. Because esters, acyl chlorides, ketones, and aldehydes (carbonyl compounds) are unable to form hydrogen bonds with each other, the boiling points of these molecules are lower than the boiling points of alcohols with comparable molecular weight. An example of this can be found in Figure 19.2. As can be seen, in the group of compounds chosen for this comparison, carboxylic acid has the highest boiling point; this is due to its ability to form more than one set of hydrogen bonds. Molecules derived from carboxylic acid are soluble in solvents (e.g., chlorinated alkanes and aromatic hydrocarbons). Moreover, similar to alcohols, carbonyl compounds with fewer than four carbons are soluble in water (Bruice, 2004). However, the solubility of esters in water decreases together with the increase of the chain length. As a result, none of the fats and oils (triesters of glycerol) are water soluble. In nature, esters are very important; in fact, vegetable and animal fats and oils are just large esters (the difference between a fat and an oil is the melting point).

19.3

Esterification reactions

The most relevant methodologies of reaction for the formation of esters, which are also used in the context of PVMRs for esterification, are hereafter proposed. For the purpose of this work, information about Lewis acid-activators, esterification baseactivators, carbodiimide activators, the Mitsunobu reaction, induced activation of the condensation reaction using template compounds, and other reaction methodologies between carboxylic acids and alcohols used in ester synthesis will not be proposed. For this information, the reader is referred to the literature (Otera, 2003).

Pervaporation membrane reactors (PVMRs) for esterification

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19.3.1 Esterification reaction between carboxylic acids and alcohols By supplying heat and an alcohol to a carboxyl acid, an ester can be formed. This catalyst-free reaction is desirable because there is no catalyst to be separated from the reaction medium and products. To boost the reaction and avoid the use of thermal energy, these reactions are commonly catalyzed by acid or base compounds.

19.3.1.1 Acid catalysts (Fischer esterification) The most commonly used way to form esters was invented by Emil Fischer (1852e1919) and was named Fischer esterification (also called acid-catalyzed esterification). This reaction can be conducted by using heterogeneous or homogeneous catalysis (this is done not only to catalyze the reaction but also to keep the carboxylic acid in its acid form so that it will react with the nucleophile (Bruice, 2004)). Inorganic Brønsted acids, such as H2SO4, HI (R€onnback et al., 1997), ClSO3OH (M€aki-Arvela et al., 1999), or ArSO3, or organic acids, such as p-toluenesulfonic acid (Xu, Afacan, & Chuang, 1999), have been widely used. A particular case is the two-step esterification of tartaric acid with ethanol, which is catalyzed by methanesulfonic acid, CH3SO3H (an example of the use of this catalyst in a PVMR can be found elsewhere (Lopez-Zamora, Fontalvo, & Gomez-Garcia, 2013)). Despite their lower cost, the interest for homogeneous catalysts in the chemical industry has decreased because of their miscibility with reaction media and problems of corrosion of the reaction devices (thus catalyst separation can be either expensive or technically challenging). As a result, increasing attention has traditionally been paid to heterogeneous catalysts (more information can be found further in this text). Figure 19.3 shows a general Fisher esterification reaction. Differently, Figure 19.4 shows a detailed reaction mechanism. The backward reaction that produces a carboxyl alcohol from an ester by reaction with water is a hydrolysis-type reaction. To drive the reaction toward products, an excess alcohol and/or a direct separation methodology, to drag product out from the reaction environment, is required (see next sections). An alcoholysis reaction, in which an ester reacts with an alcohol to form a new ester and a new alcohol, is called a transesterification reaction. An example of this alcoholysis reaction is given in Figure 19.5. Transesterification reactions are slow reactions because alcohols are poor nucleophiles and esters have very basic leaving groups; hence, they occur under support of acid catalysis (Bruice, 2004). Similar to esterifications, transesterification reactions are equilibrium reactions (Hoydonckx, Vos, Chavan, & Jacobs, 2004); hence, an excess of alcohol and a direct extraction of products is needed to shift the equilibrium. O R

OH

+

R' – OH

O

HA

Figure 19.3 Fischer esterification general equation.

R

OR'

+

H2O

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Membrane Reactors for Energy Applications and Basic Chemical Production

H

+ O

H

H

O

H

C 6H 5

O H

C6H5

The strong acid catalyst protonizes the carboxylic acid

– CH3OH

O+ C

– CH3OH

H O C6H5 C O CH3 O+ H H

A proton changes its position to give a more stable intermediate

C 6H 5

O H

H

The hydroxyl group of the alcohol attacks the protonated carbonyl group (a tetrahedral intermediate is formed)

H O H + C6H5 C O CH3 O H

+O

CH3 O

H

O H

A protonated ester results from the loss of a water molecule

H

O +

– H 3O

O CH3

+ H3O+

C6H 5

O CH3

The catalyst is regenerated by accepting a proton

Figure 19.4 A mechanism of the Fischer esterification (also called an acid-catalyzed reaction). O

+ OCH3

CH3 CH2 CH2 OH

Methyl benzoate

HA

O

+ OCH2 CH2 CH3

CH3 OH

Propyl benzoate

Figure 19.5 An example of a transesterification reaction using an acid catalyst.

19.3.1.2 Solid acids Nowadays, heterogeneous catalysts made of support and activated with strong acid sites are widely used. The main advantage is that the obtained products can be purified from the catalyst by simple filtration procedures. The oldest solid acid used is Nafion-H (Olah, Keumi, & Meidar, 1978). In this case, the reaction between carboxylic acid and alcohol is run at T > 95
C (w5 s contact time), obtaining a high reaction yield. Zeolite-type solid catalysts are also used in this field (Bergk, 1978; Mizukami, 1999; Cordeiro, Da Silva, Wypych, & Ramos, 2011; Diban, Aguayo, Bilbao, Urtiaga, & Ortiz, 2013). The most used zeolites for esterification reactions are H-ZSM-5,

Pervaporation membrane reactors (PVMRs) for esterification

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–(CHCH2)n–

SO3H

Figure 19.6 A scanning electron microscopy picture of Amberlyst 15 solid catalyst particles and their chemical composition (Pal et al., 2012; Russbueldt & Hoelderich, 2009). Reprinted with permission.

zeolite b, and zeolite Y (De La Iglesia, Mallada, Menendez, & Coronas, 2007). When high temperatures are also used, an elevated yield of the ester can be obtained (Gumaste, Deshmukh, & Bhawal, 1996). Another class of solid acid used in esterification is composed of ion-exchange resins. Two examples are Amberlites IR120, used in the esterification on sugars with shikimic and quinic acids (Delfourne, Despeyroux, Gorrichon, & Veronique, 1991; Lichtenthaler, Klimesch, M€ uller, & Kunz, 1993), and the resin Wolfatit KSP200, which demonstrated a successful esterification of a-hydroxy-carboxylic acid with ethanol or methanol/chloroform without occurring in racemization (Effenberger, Hopf, Ziegler, & Hudelmayer, 1991). Above all, one of the more used solid acid catalysts is Amberlyst 15 (Chakrabarti & Sharma, 1993; Pal, Sarkar, & Khasnobis, 2012; Diban et al., 2013). Figure 19.6 shows a scanning electron microscopy picture of an Amberlyst 15 resin together with the molecular structure of the active catalytic sites (in particular, strong acid sulfonic groups are used to activate a polyester-based resin). This solid acid catalyst is safe to use and can be easily removed from the reaction medium at the end of the reaction. The maximal temperature allowed from this type of catalytic resin is approximately 120
C. Other interesting types of solid acid catalysts are Nb2O5$nH2O, FeCl3 supported on salicylic resin, ZrO2$nH2O, Mo-ZrO2, and graphite bisulfate. Detailed information about the performance of these catalysts can be found elsewhere (Otera, 2003).

19.3.1.3 Enzymatic catalysis The use of enzymes in esterification reactions to produce industrially important products, such as emulsifiers, surfactants, wax esters, chiral molecules, biopolymers, modified fats and oils, structured lipids, and flavor esters, is well documented. The use of lipases in aqueous and nonaqueous media has found applications in organic synthesis, chiral synthesis or resolution, modification of fats and oils, and in many other fields. Moreover, lipases are highly stable even under adverse conditions such as organic solvents and high temperatures (Gandhi et al., 2000; Hari Krishna & Karanth, 2002).

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Membrane Reactors for Energy Applications and Basic Chemical Production

In the past decade or so, lipase-catalyzed esterifications and transesterifications in anhydrous media (e.g., organic solvents and supercritical fluids) have been an area of intensive research. In particular, the use of organic solvents, which normally allow a higher stability of enzymes than in water (Bock, Jimoh, & Wozny, 1997), has been demonstrated. Reviews of the applications have been made by Hari Krishna and Karanth (2002) and Gandhi et al. (2000), dealing with fundamental and practical aspects of lipase catalysis. In particular, they concentrated on various immobilization strategies and factors (e.g., temperature, reaction medium, water activity) as well as the methods of preparation (which affect and influence the stability of the lipases).

19.4

Industrial relevance of esterification reactions

One of the most important applications of esters is the fragrances and flavors industry. For example, combining esters with other organic compounds allows for obtaining fruit flavors (Greever, 1995). Esters are also very important in the synthesis of polymers (e.g., the DacronÒ polymer, shown in Figure 19.7), medicines (Uragami, Kishimoto, & Miyata, 2012), and biodiesels (Mazubert, Poux, & Aubin, 2013). Biodiesel consists mainly of fatty acid methyl esters, and it is fed in diesel engines mostly in blends ranging between 5 and 20 vol% with petrodiesel (Tremblay, Cao, & Dube, 2008). The transesterification of trialkyl glycerides with basic catalysis is the first way to produce fatty acid methyl esters and the second one is the acid-catalyzed esterification of free fatty acids with alcohols (Eqn (19.1)) in which water is obtained as a byproduct (Kiss & Bildea, 2012). FFA þ Alcohol ðe:g: MeOHÞ5FAME þ H2 O

(19.1)

Lactones also have a great relevance in industry (as cyclic esters they are common in natural sources; e.g., vitamin C, napetalactone) (Drioli & Giorno, 2010). These compounds are also a polymer source; for example, caprolactone can give polycaprolactone (IUPAC: (1,7)-polyoxepan-2-one) or, after conversion into caprolactam, it is used to synthesize Nylon 6 (IUPAC: poly(hexano-6-lactam)). A list of esters, their precursors, and their industrial application can be found in Table 19.1. The potential uses of lipase technology are not presented in this context; however, some information can be found elsewhere (Gandhi et al., 2000).

O O

C

C

O O n

Figure 19.7 General formula of the polyethylene terephthalate (DraconÒ) polymer.

A review of esters, their precursors, and their application

Ester

Carboxylic acid

Alcohol

Application

References

Butyl acetate

Acetic acid

Butanol

Solvent (e.g., lacquer and coatings industry)

Tian, Zheng, Huang, Qiu, and Wu (2012)

Ethyl acetate

Acetic acid

Ethanol

Solvent (e.g., lacquer and coatings industry)

Tian et al. (2012)

Ethyl lactate

Lactic acid

Ethanol

Food, perfumery, flavor chemicals, and solvents

Delgado et al. (2010)

Ethyl oleate

Free fatty oleic acid

Ethanol

Biodiesel industry

Figueiredo et al. (2010); Yin et al. (2012)

Geranyl acetate

Acetic acid

Geraniol

Flavor and perfumery industry

Kang et al. (2005)

i-Amyl acetate

Acetic acid

i-Amylol

Food flavors and cosmetic industry

Lan, Lan, Xie, and Liao (2012)

i-Butyl acetate

Acetic acid

i-Butanol

Cosmetic and paint industry

Korkmaz et al. (2011)

i-Propyl acetate

Acetic acid

i-Propanol

Solvent for printer inks industry; compounds of flavors and perfumes used in food industry

Bernal et al. (2002)

Propyl propanoate

Propionic acid

n-Propanol

Solvent for printer inks industry; compounds of flavors and perfumes used in food industry

Cruz-Diaz et al. (2012)

Sugar (glucose) fatty acid esters

Fatty acids

Carbohydrates

Pharmaceutical, cosmetic, and food industry

Ganske and Bornscheuer (2005) 571

Adapted from Diban et al. (2013).

Pervaporation membrane reactors (PVMRs) for esterification

Table 19.1

572

19.5 19.5.1

Membrane Reactors for Energy Applications and Basic Chemical Production

Reaction-separation coupled methodology A traditional approach: reactive distillation

Water is a byproduct of many different catalytic reactions. Hence, its removal is fundamental to enhance the thermodynamic equilibrium of those reactions and prevent poisoning of the catalysts used (Diban et al., 2013). The concept is simple: during the reaction of two reagents, a product and a byproduct (water) are formed. The continuous removal of the products (or in the batch mode, of one of the two products) allows for a high yield of reaction (ideally the reaction can be driven to completion), avoiding the use of a large excess of reactants. Many technologies have been proposed to remove water from the reaction medium (a review of them can be found elsewhere (Zhang, Yang, Hu, Liang, & Chen, 2013)): azeotropic distillation (Gubicza, Kabiri-Badr, Keoves, & Belafi-Bako, 2000), the use of inert dry gas to be blown through the reaction medium (Jeong & Lee, 1997), and the application of salt hydrates (Kvittingen, Sjursnes, Anthonsen, & Halling, 1992; Wehtje, Kaur, Adlercreutz, Chand, & Mattiasson, 1997). However, the most promising and explored technologies are reactive distillation (Bock et al., 1997; Sundmacher & Kienle, 2006), sorption (Trusek-Holownia & Noworyta, 2007; Iliuta, Iliuta, & Larachi, 2011; Cruz-Diaz et al., 2012), and pervaporation/vapor permeation membrane reactors (Okamoto et al., 1994; Parulekar, 2007; Rohde, Schaub, Khajavi, Jansen, & Kapteijn, 2008). Drawbacks of adsorption technology are related to the dimension of the required fixed-bed reactor and the possible generation of new waste when the adsorbent cannot be regenerated (Gubicza et al., 2000). Reactive distillation is a powerful but still limited technology in which separation is performed by high-energy consumption distillation methodology. This means that in case the reaction water/alcohol mixture forms an azeotrope, reactive distillation can no longer be used, except when another component is fed into the distillation column (extractive-reactive distillation column (Zhang et al., 2013)). The first patent about the esterification reaction conducted in a distillation column was published by Backhaus (1921). However, it was only in 1945 that empiric observations were published (Leyes and Othmer), dealing with the esterification of acetic acid with an excess of n-butanol to produce n-butyl acetate and water (the catalyst was sulfuric acid). The concept of esterification conducted in reactive distillation columns, defined by Backhaus, differs from the classical approach, in which a third reactive component, E, reacts with the highest boiling point component of an A/B mixture (Terrill, Sylvestre, & Doherty, 1985) and is extracted from the bottom. In particular, the difference is that the two initial A and B components are reagents of an esterification reaction (catalyzed by acids), which gives a C product and a D byproduct. In general, the chemical reaction occurs in the liquid phase (in the presence of a homogeneous catalyst) or on the surface of the solid catalyst (which is in contact with the liquid phase) present on the tray of the distillation column.

Pervaporation membrane reactors (PVMRs) for esterification

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Many studies have proposed this methodology. Some relevant and recent examples are esterification of acetic acid with methanol (Agreda & Partin, 1984; Chandrakar, Agarwal, Chand, & Wasewar, 2007), esterification of succinic acid and ethanol (Kolah, Asthana, Vu, Lira, & Miller, 2008a; Orjuela, Kolah, Lira, & Miller, 2011; Orjuela, Kolah, Hong, Lira, & Miller, 2012), reaction of citric acid (CA) with ethanol to give triethyl citrate (Kolah, Asthana, Vu, Lira, & Miller, 2008b) and CA (Kolah, Asthana, Vu, Lira, & Miller, 2007), esterification of 1-propanol and propionic acid (Kotora, Buchaly, Kreis, G orak, & Markos, 2008; Altman et al., 2010; Cruz-Diaz et al., 2012; Buchaly, Kreis, & Gorak, 2012), esterification of acetic acid and ethanol (Yagyu et al., 2013) (which is the most studied reaction concerning reactive distillation for esterification (Drioli & Giorno, 2010)), esterification of acetic acid with n-butanol (Arpornwichanop, Koomsup, & Assabumrungrat, 2008), esterification of acryl acid with 1,4-butanediol to obtain 4-hydroxybutyl acrylate (Yang, Cho, Park, & Lee, 2007), reaction of lactic acid with n-butanol (Kumar & Mahajani, 2007), esterification of acrylic acid and n-butanol (Niesbach, Fuhrmeister, Keller, Lutze, & Gorak, 2012), synthesis of an n-butyl acetate system using a reactive distillation from an agroindustrial waste (i.e., starting from oil of Fusel) (Altomare, Labrador, Rodriguez, Morales, & Fernandez, 2012), and esterification of butyric acid and n-butanol (Ju et al., 2011). Moreover, kinetic studies and the applicability of reactive distillation technology have been recently assessed on the esterification reaction of n-butanol with acetic acid catalyzed by noncorrosive Brønsted acidic ionic liquids, which are prospective substitutes for the establishment of liquid catalyst-based reactive distillation processes (Tao et al., 2011). Despite these many studies, reactive distillation is not the best solution to couple reaction and separation, mainly for three reasons: (1) in most of the cases, the esterification medium (reagents plus products) is a nonideal system (not really fit for vaporequilibrium-based technology such as reactive distillation); (2) pure water cannot be selectively removed from the top or the bottom of the column; and (3) reactive distillation is a high-energy consumption technology (because separation is based on conventional distillation) (Lim, Park, Hung, Sahimi, & Tsotsis, 2002; Drioli & Giorno, 2010). Thus, theoretically, membrane technology may be the best candidate to couple reaction and separation. In the literature, it is possible to find some hybrid units coupling the reliability of the reactive distillation system with the pervaporative technology when an esterification reaction is run (Riemenschneider & Bolt, 2000; Buchaly et al., 2012; Bida, Gongping, Xueliang, Wang, & Wanqin, 2012). This allows for a reduction of energy consumption and an enhancement of process quality compared with stand-alone reactive distillation.

19.5.2 Pervaporation membrane reactors Many different reviews dealing with membrane reactors and covering all technical and application aspects of this technology (reactor configuration, specific application, and system modeling) can be found (Sirkar, 1997; Lipnizki, Field, & Ten, 1999; Sirkar,

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Shanbhag, & Kovvali, 1999; Sanchez & Tsotsis, 2002; Paturzo, Basile, & Drioli, 2002; Drioli & Giorno, 2010; Drioli, Stankiewicz, & Macedonio, 2011; Gallucci, Basile, & Hai, 2011; Carstensen, Apel, & Wessling, 2012; Diban et al., 2013). PVMRs for equilibrium-limited esterifications are an attractive alternative to distillation-based reactive technologies because (1) the vapour liquid equilibrium (VLE) of the involved compounds does not rule the separation; (2) only permeate experiences a phase change, therefore the energy consumption is lower than for distillation; and (3) when a suitable membrane is defined, a particular PVMR can be run at the reaction temperature. Pervaporation-based hybrid processes can be divided into two categories: separation-based hybrid processes and reaction-based hybrid processes. Both categories were described by Lipnizki et al. (1999), in particular the latter were denoted as R-type processes: R1 and R2. The difference between the two R-type technologies is the pervaporative step of separation. According to the R1 methodology (Figure 19.8(a)), the main product of interest is extracted as permeate. Otherwise, in the R2 type (Figure 19.8(b)), the byproduct is removed from the reaction medium. In both cases it is possible to move a reaction equilibrium to the right, increasing reagents conversion and the yield of reaction. The classification introduced by Lipnizki et al. is further used in this chapter. Going more into detail of a PVMR, it is possible to consider another classification. In the case in which the membrane separation system and the reaction section are two physically distinct unit operations (coupled in the same unit), the system can be denoted as an external separation unit (ESU); for an in situ separation unit (ISU), the reaction and separation occur in the same place (Sanchez & Tsotsis, 2002). The ISU configuration (Figure 19.9(a)) generally has more disadvantages than the ESU configuration (Figure 19.9(b)) because the membrane cell is inside of the reactor unit. This means that the reactor needs to be opened in case of maintenance or replacement of the membrane. A design in which the membrane is in a separate cell makes the

Type R1

(a) Product

Reactor

PV

Type R2

(b) Reactor

By-product

PV

Figure 19.8 Reaction-type hybrid pervaporation configurations in (a) type R1 (the main product permeates through the membrane) and (b) type R2 (the byproduct is selectively removed from the reaction medium).

Pervaporation membrane reactors (PVMRs) for esterification

575

Reaction zone Catalyst filter

Retentate

Membrane

(Vacuum)

Permeate

Permeate

(Vacuum)

Catalyst filter

Membrane

(a)

Dead-end configuration (internal pervaporation unit)

(b)

Two pieces configuration (internal pervaporation unit)

Figure 19.9 Representation of the two configurations: (a) internal separation (pervaporation) unit (ISU) and (b) external separation unit (ESU).

(a)

(b)

Retentate a1

Permeate Feed

Tank reactor

Permeate

Feed Retentate

Permeate Permeate b1 a2

Permeate

Retentate b2

a3

(d)

(c) Tubular reactor

Retentate Permeate Feed

Feed

Retentate

Permeate Permeate

Figure 19.10 A matrix of membrane reactor configurations: (a1) semibatch tank reactor with ESU (SBR or BR-ESU); (a2 and a3) batch reactor with flat and tubular ISU (BR-ISU); (b1 and b2) continuously stirred reactor with flat and tubular ISU (CSTR-ISU); (c) plug-flow reactor with ESU (PFR-ESU); and (d) PFR with ISU (PFR-ISU). Reprinted (adapted) with permission from Diban et al. (2013). Copyright 2013, American Chemical Society.

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Membrane Reactors for Energy Applications and Basic Chemical Production

(a)

(b)

(c)

Retentates Membrane

Retentates Catalytic membrane

Permeate Membrane

Catalyst

Permeate product

Permeate product

Catalyst (fixed bed)

Feed reactives

Permeate product

Feed reactives

Figure 19.11 Catalytic pervaporation membrane reactor configurations: (a) suspended catalyst membrane reactor (SC-MR), (b) catalytic-packed bed membrane reactor (CB-MR), and (c) a membrane reactor using a membrane with catalytic properties (CMR). Reprinted with permission from Diban et al. (2013). Copyright 2013, American Chemical Society.

pervaporation reactor for esterification more flexible. In fact, in the case of membrane replacement, the unit does not need to be stopped because the flux coming from the reaction part of the pervaporation reactor can be deviated to another pervaporation cell just by switching the service valves. Moreover, in a two-part integrated system, the shapes, dimensions, and materials of the membrane can be easily varied, and it is possible to think of a module, instead of a single membrane, attempting the separation. On the other hand, the ISU system, in which pervaporation (PV) and reactor flow patterns are directly coupled, has been extensively used in biotechnology and bioprocesses in which products may degrade rapidly to undesirable compounds so that separation should occur directly on the reaction site (thus internal to the reactor) (Drioli & Giorno, 2010). These proposed classifications can be further extended in two ways: considering the reactor design (how it is clearly shown in the matrix of Figure 19.10) and considering the possibility of using inert membranes (inert pervaporation membrane reactors (I-PVMRs)) or catalytic membranes (catalytic pervaporation membrane reactors (C-PVMRs)), as shown in Figure 19.11.

19.6

R2-type pervaporation reactors for esterification reaction

The first publication describing a membrane reactor to improve esterification reactions was a patent dated 1960 (Binning). Another patent was published in 1987 (Pearce). An image of this latter work is shown in Figure 19.12. In 1988, Kita et al. published an experimental study of the esterification of oleic acid/acetic acid with ethanol using, among others, hydrophilic asymmetric

Pervaporation membrane reactors (PVMRs) for esterification

577

Vapour permeate, water rich 5–25m. bar A (trapped out in cardice / IPA bath) Porous metal sinter

Heating tape

Silicone rubber 0 ring

Feed

Retentate product ethyl acetate rich

2:1 Ethanol : acetic acid Membrane area = 12 cm2

Reactor chamber P = 4 bar A Ion exchange resin catalyst T = 50–90°C. Rohm and Haas Vol = 13 ml. Amberlyst A15 Vol = 6 ml

Figure 19.12 The R2-type esterification membrane reactor as presented in the second historical patent (Pearce, 1987).

polyetherimide (PEI) membranes. In 1991, Bitterlich et al. introduced the use of ionexchange resin instead of liquid sulfuric acid (this avoids the need of any further neutralization step and corrosion of the structure linked to the use of liquid strong acid catalysts). In the same year, Dams and Krug proposed three possible designs, to couple the reactor (for a general reaction, ethanol þ acid 4 water þ ester) to distillation aided pervaporation. In the first layout, shown in Figure 19.13(a), the vapor

(a)

(b)

87 wt% ethanol 13 wt% water

Ester-alcohol-acid

98 wt% ethanol 2 wt% water

Water Reaction

Reaction Water

(c)

Alcohol–water

Reaction

Water

Figure 19.13 (aec) Three pervaporative-reactive hybrid configurations for batch esterification, see text for explanation. Reprinted with permission from Dams and Krug (1991). Copyright 1999 Elsevier.

578

Membrane Reactors for Energy Applications and Basic Chemical Production

phase of the reactor is sent to a distillation column; at the same time, a PV unit treats the condensed distillate, recycling its retentate (rich in ethanol) to the reactor (water streams are obtained from the PV-permeate and from the bottom of the column). In this case, the authors proved that a high conversion rate and low energy consumption can be ensured. It was also considered that the membrane needs to be highly effective to accelerate the reaction process and that the presence of the water-ethanol azeotrope does not affect the process. With the second layout (Figure 19.13(b)), an external R2-type pervaporation unit is proposed (extensively discussed in this chapter). The third layout, shown in Figure 19.13(c), represents an interesting alternative because the vapor phase of the pervaporation unit is condensed and treated in the PV section, recycling the retentate to the reactor. In this case, any possible problem of corrosion, due to the presence of the acid catalyst, is solved. Even if the technological impact of these three configurations is impressive, industrially this is not enough: cost optimization is needed and can lead to more complex configurations, as emphasized by Br€ uschke, Ellinghorst, and Schneider (1995). They found that when using a batch system and a continuous pervaporation-reactor cascade with recycle (this last one already shown in Figure 19.13(b)), it would lead to high investments costs. Thus, they proposed a cost-optimized configuration (Figure 19.14) able to give conversion rates up to 97% and water content lower than 0.5 wt%. This configuration was then applied for a medium-sized esterification plant, operated in batch mode, for the production of various esters (Br€uschke, 1995). Another interesting contribution in the field of PVMR came from David, Gref, Nguyen, and Neel (1991). They studied the selective pervaporation of water produced by esterification of 1-propanol and 2-propanol with propionic acid. In parallel, they developed a model able to describe the kinetics of the PVMR system (reaction rate constants were determined experimentally). More in detail, an ESU system was set using commercial polyvinyl alcohol (PVA) membrane from Sulzer Chemtech/GTF. From the experimental and modeling work, an almost complete conversion of the reactants was obtained. In a following paper (David, Nguyen, & Neel, 1991), the influence of the operating parameters was assessed. In particular, the temperature, initial molar ratio, membrane area to reaction volume, and catalyst concentration (here listed in order of relevance) were identified as the most important parameters in ruling the process. Another parameter assessment was done by Feng and Huang

97% conversion

Reaction mixture (equilibrium)

97% conversion

Reactor 1

Reactor 2

Water

Water

Product purification

Figure 19.14 The cost-optimized configuration (esterification hybrid process) proposed by Br€ uschke et al. (1995).

Pervaporation membrane reactors (PVMRs) for esterification

579

(1996). They discovered that when an excess of one of the two reactants is applied, a complete conversion is achievable (a general esterification reaction was considered). By means of modeling, Lim et al. (2002) and Parulekar (2007) evaluated how a configuration can affect the water removal performance of a PVMR. Plug-flow reactor (PFR) and continuously stirred reactor (CSTR) designs, working in continuous or in batch mode, were assessed with the separation taking place at the same time (in this case retentate is recycled to the reactor) or in two different steps. Above all, it was concluded that an in situ removal of water (via ISU) yields superior conversion values to an ESU. In 1995, Kwon, Song, Hong, and Rhee (1995) demonstrated that pervaporation is potentially applicable to the removal of the water produced from various enzymatic processes for the synthesis of several esters. Therefore, they studied the lipasecatalyzed (Lipozyme(R)) esterification reaction of oleic acid with n-butanol in isooctane to produce n-butyl oleate. An R2-type ISU-PVMR, using a dense polymeric membrane of cellulose acetate to remove water, was set. Oleic acid conversion was improved from the equilibrium value of 61.1e91%. An overview of interesting studies about PVMRs used for esterification reactions is presented in Table 19.2 (most were published during the last 10 years). This table can be used to find some supplementary information not further included in this chapter. It can be noticed that PVA-based membranes are commonly the most often used membrane types. There are many commercial PVA-based membranes that can be used for water removal in PVMRs for esterification reactors (the most relevant are also listed in Table 19.2). However, when the esterification of the alcoholic groups of this type of membrane is possible (with a loss of separative properties (Vankelecom, 2002)), this is the case in which the membrane is in contact with low PH reaction mediums (e.g., homogenous acid catalysis), PVA-based membranes have to be cross-linked. Another solution is to replace PVA-based membranes by low-pH-resistant inorganic membranes. In addition, in this last case, in literature it is possible to find several publications. For example, the T-type, mordenite (MOR) and merlionite (MER), phillipsite (PHI) and (CHA) zeolites membranes belong to this group (Tanaka, Yoshikawa, Ying, Kita, & Okamoto, 2001; De La Iglesia et al., 2007; Li, Kita, Zhu, Zhang, & Tanaka, 2009; Xiansen, Kita, Hua, Zhenjia, & Tanaka, 2009; Hasegawa, Abe, Mizukami, Kowata, & Hanaoka, 2012). However, as already mentioned, liquid acid catalysts for homogeneous catalytic reactions have some important drawbacks, including corrosion, instability of the used membrane, and the need of a neutralization unit. Therefore, the introduction of ion-exchange resins was a step forward (above all, Amberlyst 15 is widely used for these applications) (Benedict, Parulekar, & Tsai, 2003; Sanz & Gmehling, 2006; Benedict, Parulekar, & Tsai, 2006; De La Iglesia et al., 2007; Parulekar, 2007; Figueiredo et al., 2008; Roth, Lauterbach, Hoffmann, & Kreis, 2011). Novel configurations and approaches have been also assessed in the literature to improve reaction and PV separation performances of the PVMR, including the use of adsorbents at the permeate side of the PV unit (to reduce the partial pressure of water and to increase the driving force) (Park & Tsotsis, 2004), the employment of ionic liquids, hybrid bifunctional membranes, and microwave heating (De La Hoz, Moreno,

A list of I-PVMRs and C-PVMRs for water separation from esterification reaction mediums

Esterification product

Membrane

Membrane configuration

T (
C)

Catalyst

Membrane reactor configuration

References

PEI, Chitosan (kyowayushi), Nafion 117 (du Pont)

Flat

60/75

p-Toluene sulfonic acid

BR þ ISU

Kita et al. (1988)

n-Butyl oleate

Cellulose acetate

Flat

25

Lipase-catalyzed (Lipozyme(R))

BR þ ISU

Kwon et al. (1995)

Propyl propanoate

Cross-linked blend PVA/ poly(styrene sulfonic acid) 33/67

Flat

50

Bifunctional membrane

BR þ ISU

Nguyen et al. (2003)

Ethyl acetate

PVA

Tubular

nd

Sulfuric acid

PFR or CSTR þ ISU

Lim et al. (2002)

i-Amyl acetate

PVA cross-linked with sulfonic acid groups

Flat

80

Bifunctional membrane

SBR

Khajavi et al. (2010)

Ethyl lactate and ethyl succinate

GFT-1005 and T1-b (PVA-based)

Flat

8095

Amberlyst XN-1010 and Nafion NR50

SBR

Benedict et al. (2006)

i-Propyl acetate

Pervap 2201

Flat

5070

Amberlyst 15

SBR

Sanz and Gmehling (2006)

Membrane Reactors for Energy Applications and Basic Chemical Production

Ethyl oleate/ ethyl acetate

580

Table 19.2

Pervap 2201D

Flat stack module

8595

Amberlyst 46

SBR

Roth et al. (2011)

Ethyl acetate

PVA

Tubular

nd

nd

nd

Waldburger and Widmer (1996)

Ethyl acetate

PEI þ ceramic support

Tubular

30e80

Sulfuric acid (96%)

PFR þ ISU

Zhu et al. (1996)

Ethyl acetate

Zeolite T þ alumina support

Tubular

70

Amberlyst 15

BR þ ISU

Tanaka et al. (2001)

Ethyl acetate or methyl acetate

MER, PHI, CHA

Tubular

40

Scandium trifluoromethanesulfonate or Ambersyt 15

SBR

Inoue et al. (2007)

Ethyl acetate

Zeolite A þ PVA

Flat

20e50

Sulfonated cation exchange resin or sodium hydroxide

BR þ ISU

Gao et al. (1996)

Ethyl acetate

MOR and zeolite A

Tubular

85

Amberlyst 15

PFR þ ISU

De La Iglesia et al. (2007)

Ethyl acetate

Two-layered H-ZSM5-MOR/ a-alumina composite

Tubular

6090

Bifunctional membrane

PFR þ ISU

De la Iglesia et al. (2006)

Ethyl/butyl acetate

H-SOD

Tubular

7090

Amberlyst 15

SBR

Khajavi et al. (2010)

Ethyl acetate

Pervap 1000 and 1001 (PVA/PAN support)

nd

60

Amberlyst 15

PFR þ ESU or SBR

Dos Reis Gonçalves et al. (2005) 581

Continued

Pervaporation membrane reactors (PVMRs) for esterification

n-Propyl propanoate

Continued

Esterification product

Membrane

582

Table 19.2

Membrane configuration

T (
C)

Catalyst

Membrane reactor configuration

References

PFSA-PVA-SiO2/ PVA/PAN

Hollow fibers

40

Bifunctional membrane

SBR

Ma et al. (2010)

Ethyl acetate

PVA-MA and PVA-CA

Flat

30e90

Sulfuric acid

SBR

Das et al. (2007)

Ethyl acetate

Pervap 2255-50 coupled with Pervap 2201

Flat

30e60

Novozym 435 (immobilized CALB)

CSTR þ ESU

Gubicza et al. (2008)

Ethyl acetate

H-ZSM5

Tubular

6090

Bifunctional membrane

PFR þ ISU

Bernal et al. (2002)

Ethyl acetate

Pervap 1000 and PVA þ Amberlyst 35 particles

Flat

80

Bifunctional membrane

Modified CSTR þ ISU

Figueiredo et al. (2008)

Ethyl acetate

PFSASiO2/PVA/ PAN

Flat

50e70

Bifunctional membrane

SBR

Lu et al. (2013)

Ethyl acetate

PEI/g-alumina composite

Tubular

3070

Sulfuric acid

PFR þ ISU

Park and Tsotsis (2004)

Ethyl lactate

GFT-1005 (PVA) þ T1-b (PVA)

Flat

7595

Amberlyst XN-1010 and Nafion NR50

SBR

Benedict et al. (2003)

Membrane Reactors for Energy Applications and Basic Chemical Production

Ethyl acetate

Flat

80

p-Toluene sulfonic acid

SBR

Domingues et al. (1999)

Methyl acetate

Pervap 2210

Flat

60

Amberlyst 15

BR þ ISU, model of PFR and CSTR

Assabumrungrat et al. (2003)

Methyl acetate

Pervap 2210

Flat

50

Amberlyst 15

CSTR model over various experimental data

Lopez-Zamora et al. (2013)

Butyl acetate

PVA-ceramic composite

Flat

60e90

Zr(SO4)2$4H2O

BR þ ISU

Liu et al. (2001)

Butyl acetate

H-USY zeolite þ amorphous silica layer

Hollow fibers

75

Bifunctional membrane

PFR þ ISU

Peters et al. (2004)

Isobutyl acetate

PVA, PDMS house-made membranes, Pervap 1201, Pervap 2216, Nafion 117

Flat

60e70

Dowex 50W-X8

BR þ ISU

Korkmaz et al. (2011)

Geranic acid

Cellulose acetate þ ceramic substrate composite

Flat

30

Immobilized CALB

SBR

Kang et al. (2005)

Ethyl lactate

Pervap 2201

Flat

6585

Amberlyst 15

SBR

Delgado et al. (2010) Continued

583

GTF-Pervap 1005

Pervaporation membrane reactors (PVMRs) for esterification

Benzyl acetate

584

Table 19.2

Continued

Membrane

References

Amberlyst 15

SBR and CSTR þ ESU

Pereira et al. (2010)

Amberlyst 15

PFR þ ISU

Collazos et al. (2012)

Amberlyst 15

PFR þ ESU (parallel, series) and PFR þ ISU

Nemec and Van Gemert (2005)

T (
C)

Catalyst

48e72

Ethyl lactate

Methyl silica (Pervatech)

Tubular

Ethyl lactate

Methyl silica (Pervatech)

Tubular

Diethyl tartarate

Pervatech silica

Hollow fibers

Adapted and modified from Diban et al. (2013).

Membrane reactor configuration

Membrane configuration

85

Membrane Reactors for Energy Applications and Basic Chemical Production

Esterification product

Pervaporation membrane reactors (PVMRs) for esterification

585

& Vazquez, 1999; Kabza, Chapados, Gestwicki, & Mcgrath, 2000; Uragami et al., 2012). In particular, attention has been paid to bifunctional membranes. These are membranes combining the catalytic and separator functions. The material for these membranes can be very different, ranging from zeolitic membranes without further modification (the acid groups from the zeolite material act as catalyst) (Bernal, Coronas, Menendez, & Santamaria, 2002; Peters, Fontalvo, Vorstman, & Keurentjes, 2004; Peters et al., 2005; Peters, Benes, & Keurentjes, 2005; De La Iglesia et al. (2006)) to polymeric membranes modified by cross-linking with acid groups (such as PVA membranes cross-linked with sulfonic acid groups (Castanheiro, Ramos, Fonseca, & Vital, 2006)) or blended and cross-linked with ionic polymers, such as poly(styrene sulfonic acid) (Nguyen, M’bareck, David, Métayer, & Alexandre, 2003).

19.6.1 R2-type PVMRs for the esterification of acetic acid A typical example of equilibrium-limited reactions is the esterification of carboxylic acids with alcohols. Historically, two main methods have been performed for the equilibrium shift. The first has been the employment of an excess of alcohol (because the attack of the carboxylic group by the alcohol is the limiting step of the reaction) (Riemenschneider & Bolt, 2000). However, in this case, it is necessary to separate the excess of alcohol, causing an increase of the operational costs (Lilja et al., 2002). The second possibility is to separate the formed water from the reaction medium by means of a coupled separation-reaction system (e.g., PVMRs or reactive distillation). In the following sections, a review of studies concerning the esterification reaction of acetic acid with various alcohols in PVMRs is given. In particular, attention is paid to describing the reaction involving acetic acid with ethanol, which is the most widely assessed esterification reaction to date (Drioli & Giorno, 2010).

19.6.1.1 PVMRs for the esterification between acetic acid and ethanol Apart from the work of Kita, Sasaki, Tanaka, Okamoto, and Yamamoto (1988), the first patent about the esterification reaction between acetic acid and ethanol to ethyl acetate was published by the University of Chicago in 1995 (Datta & Tsai). In 1996, Waldburger and Widmer published a study about a continuous membrane tubular reactor using an inert PVA membrane and a solid acid catalyst. Compared with the distillation process, a decrease of the energy input of over 75% and of the investment and operating costs of over 50% each was calculated when using this type of PVMR. Also in 1996, Zhu et al. studied the same reaction in a continuous PVMR using a polymeric-ceramic composite membrane. These membranes were synthesized by dipcoating, as described by Kita, Tanaka, Okamoto, and Yamamoto (1987), of PEI on top of a ceramic support tube. The membrane, secured inside of the reactor, divided the reactor into a tube-side and a shell-side chamber (sulfuric acid was used as homogeneous catalyst). The two liquid reactants are fed at the tube side. Reactants and

586

Membrane Reactors for Energy Applications and Basic Chemical Production

products permeating across the membrane to the reactor shell side are withdrawn from the shell-side outlet. It is important to stress that PVA membranes are the most explored polymeric membranes. They are cheap and of rather good quality. Two interesting examples are the work of Dos Reis Gonçalves, Pizarro-Borges, and Piacsek-Borges (2005), who studied the synthesis of ethyl acetate using simple PVA commercial membranes PERVAP 1000 and 1001 by Le Carbone Lorraine (more information can be found in Table 19.2), and the work of Das, Banthia, and Adhikari (2007), who prepared crosslinked PVA membranes using maleic acid (MA) and CA as cross-linking agents. In this case, PVA-CA membranes were found to have a higher separation efficiency than PVA-MA membranes, leading to a 98% conversion to ethyl acetate from ethanol and acetic acid when working at 90
C. Another group of membranes used in this field is composed of zeolite-based membranes. These membranes are capable of very specific interactions with target molecules (e.g., selective adsorption, molecular sieving) because they have pores in the range of molecular sizes. This makes them ideal candidates to integrate reaction and separation (Zhu, Minet, & Tsotsis, 1996). However, references about acid zeolite membrane reactors, involving esterification reactions, are still scarce (in general, the focus has been paid in assessing batch technology and the number of studies dealing with a continuous approach is still rather limited, as can be clearly seen in Table 19.2) (De La Iglesia et al., 2007). Tanaka et al. (2001) used zeolite T membranes in an ISU membrane reactor (at 343 K). Almost complete conversion was reached. The reaction time courses were well described by a model based on two assumptions: (1) the reaction obeyed to a second-order kinetic and (2) the permeation flux of each component was proportional to its concentration. Various hydrophilic zeolite membranes tolerant against acidic environment were also tested by Inoue et al. (2006, 2007) in the following configuration: ISU batch membrane reactor þ Amberlyst 15 þ tubular membrane reactor. In particular, when merlinoite (MER, Si/Al ¼ 2.34) membranes were used, the reaction was accelerated. The ethyl acetate yield of reaction reached 90% after 56 h of operation. Gao, Yue, and Li (1996) studied the same reaction using a zeolite A-PVA composite membrane (at temperatures ranging from 20 to 50
C). In this work, together with the pervaporation-aided catalytic esterification of acetic acid with ethanol, the reaction between salicylic acid with methanol was also treated. Among other results, it showed that the continuous removal of water from the system displaced the equilibrium limit (79%), making possible a 95% conversion, when using PVA, PVA þ KA, and PVA þ CaA membranes for 20.0, 11.3, and 10.0 h, respectively. Khajavi, Jansen, and Kapteijn (2010) used an R2-type ESU-PVMR for the esterification of acetic acid with ethanol and acetic acid with 1-butanol. In these two cases, a tubular hydroxy sodalite (SOD) membrane was used. The reactions were performed using equimolar mixtures of acetic acid and organic alcohol at 363 K. Amberlyst 15 was chosen as solid acid catalyst. The special feature of the SOD membrane was the absolute selectivity toward water and the relative high stability under the reaction conditions. In fact, the water selectivity in these experiments was above 1,000,000 and

Pervaporation membrane reactors (PVMRs) for esterification

587

in solutions with pH  2.9 the membrane was stable for long operation times (below this pH value, the membrane disintegrated after a certain time). In both reactions, conversion was increased by 30%, driving the esterification reactions almost to completion. Moreover, a reactor model was developed to predict conversion and concentration profiles of the reactions (good agreement with the experimental results was claimed). As can be seen in Table 19.2, and with some more details in Table 19.3 (this last table shows a short list of some relevant membranes used in the ethyl acetate production research together with separation factors and fluxes obtained), zeolite-based membranes (mordenite and zeolite A) were also tested by De La Iglesia et al. (2007) in an ISU-type continuous membrane reactor packed with Amberlyst 15. Both membranes were capable of shifting the equilibrium (in <1 day) and, in particular, mordenite membranes allowed conversions of approximately 90% and high separation factors of H2O/ethanol and H2O/acetic acid (>170). Moreover, because of the lower content in aluminum, under acid conditions, mordenite membranes were more stable than zeolite A. Hence, mordenite was also used by De La Iglesia et al. (2006), in another work, to prepare two-layered mordenite-ZSM-5 composite membranes, as shown in Figure 19.15. A tubular alumina tube was used as support. As a result, the feasibility of coupling the separation characteristics of the mordenite layer with the catalytic behavior of the H-ZSM-5 layer was demonstrated. With regard to C-PVMRs using bifunctional membranes, an interesting contribution was also made by Bernal et al. (2002). In particular, the H-ZSM-5 membrane used in this work had sufficient catalytic activity to perform the esterification reaction, and at the same time it was selective for water permeation. The conversion obtained at Table 19.3

Collection of membranes published in the literature tested

for H2O Membrane

Separation factor H2O/EtOH

Fluxes kg/m2 h

References

PVA (cross-linked)

41.9

36.2

Das et al. (2007)

Zeolite A

92

0.15

De La Iglesia et al. (2007)

NaA

1000

0.104

Jafar, Budd, and Hughes (2002)

MOR

164

0.075

De la Iglesia et al. (2007)

Zeolite T

830

0.81

Tanaka et al. (2001)

Hydroxy sodalite (SOD)

1,000,000

8.5 (tw0 h)

Khajavi et al. (2010)

Adapted and modified from Khajavi et al. (2010).

588

Membrane Reactors for Energy Applications and Basic Chemical Production

Permeate Mordenite H-ZSM-5

Support

H-ZSM-5

Reaction medium

Figure 19.15 The two-layer membrane designed by De La Iglesia and co-workers. Reprinted with permission from De La Iglesia et al. (2006). Copyright 2006 Elsevier.

the same feed rate and catalyst loading was larger than in conventional fixed-bed reactors or in reactors in which a zeolite membrane is kept separated from the catalyst. Another interesting example can be found in the work of Figueiredo et al. (2008). In this case, a suspension of Amberlyst 35 particles was cast in a layer and crosslinked to the PVA polymeric material of the membrane. It was considered that because the mass of catalyst incorporated in the PVA membrane was lower than in experiments using a conventional membrane reactor configuration with an independent catalyst (Amberlyst 15 and 35), this leads to a lower ester yield. However, higher conversions were observed when the same unit used this catalytic membrane instead of just a noncatalytic Pervap 1000 (i.e., a PVA-based commercial membrane). For the same comparison the catalytic membrane also gave a higher water permeance. This effect was explained noting that water production in the catalytically active membrane increases the water flux driving force and consequently the flux. Ma, Xu, Liu, and Sun (2010) used perfluorosulfonic acid-poly(vinyl alcohol)-SiO2/ poly(vinyl alcohol)/polyacrylonitrile (PFSA-PVA-SiO2/PVA/PAN) bifunctional hollow-fiber composite membranes. The catalytic and the selective layer of the membrane were independently optimized. These membranes were synthesized by dipcoating. The performance of these bifunctional membranes was evaluated by dehydrating the ternary azeotropic composed of a water, ethanol, and ethyl acetate system (top product of a reactive distillation process of esterification of acetic acid with ethanol), obtaining separation factors of water/ethanol up to 379. An extensive assessment on the esterification reaction of ethanol-acetic acid was later published (Lu, Xu, Ma, & Cao, 2013). In this case, the reaction equilibrium was broken in less than 5 h, and a 90% conversion of acetic acid was achieved after 55 h. Enzyme-type catalyst (Novozym 435, i.e., immobilized Candida antarctica lipase B (CALB)) was used by Gubicza, Belafi-Bako, Feher, and Frater (2008) within an ESU configuration PVMR. The 1-butyl-3 methylimidazolium hexafluorophosphate,

Pervaporation membrane reactors (PVMRs) for esterification

589

also known as BMIM-PF6 ionic liquid, was used during the manufacturing of the ethyl acetate. The membranes Pervap 2201 (hydrophilic membrane) and Pervap 2255e50 (hydrophobic membrane) were set in series, and acetic acid and ethanol were added as they were consumed; thus, steady-state concentrations in the reactor were maintained and continuous operation for 72 h was ensured without any activity loss of the enzyme, as shown in Figure 19.16. The initial water content was varied in the range 0.5e5.0% (the initial water content of the reaction mixture also has a strong effect on the progress of the reaction). Increasing the initial water content, the initial reaction rate became higher and higher, up to 3%. However, a considerable decrease at 5% was observed. In this range, approximately 90% ester yield was reported by the authors. An early model describing the pervaporative reaction of acetic acid with ethanol was presented by Krupiczka and Koszorz (1999). It was a simple, three-parameter model describing the concentration profiles in the process (a kinetic approach was considered) in the form of three differential equations. The activity coefficients were calculated using the UNIFAC property method. A hydrophilic membrane PERVAP 1005 GFT was used. Differently, Tanna and Mayadevi (2007) developed a two-step series model to study the performance of the membrane reactor. In this case, the goal was the assessment of the parameters that drive the process. As a conclusion of this work, the authors suggested using a low-flux membrane with a sufficient surface area when designing a PVMR. A more detailed simulation model was developed by Lim et al. (2002) using activities and transport relations through the membrane to be able to describe any nonideal effect. Using this model, the authors simulated the following configurations: PFR-ISU, CSTR-ISU, batch reactor (BR)-ISU, PFR-ESU, CSTR-ESU, and the batch version of this last configuration (i.e., semibatch reactor (SBR)). These configurations are schematically shown in Figure 19.10.

Hydrophobic membrane unit

Reagent tanks

Hydrophilic membrane unit

Permeate condensers

Vacuum pump Reactor

Figure 19.16 The CSTR-ESU system for the continuous enzymatic production of ethyl acetate employed by Gubicza et al. (2008).

590

Membrane Reactors for Energy Applications and Basic Chemical Production

Finally, it is also interesting to report that in the literature it is possible to find studies dealing with esterification membrane reactor units coupled with other than distillation-based separation technologies. An example is the work of Park and Tsotsis (2004), who linked to the permeate side of the membrane reactor an adsorption step to increase byproduct extraction. Adsorption is a very powerful technology and leads to higher conversion in comparison with a conventional reactor (up to 10%). However, the cost of the adsorbent and of related equipment makes this design economically disadvantageous.

19.6.1.2 PVMRs for the esterification reaction between acetic acid and other alcohols Similar to the esterification of acetic acid with ethanol, the esterification of acetic acid with other alcohols has followed the same path of investigation: mainly discontinuous operational mode, mostly of the time using PVA-based membranes (as can be noted from Table 19.2). For example, a commercial PVA membrane (GTF-Pervap 1005) was tested by Domingues, Recasens, and Larrayoz (1999) for the reaction of acetic acid and benzyl alcohol. In this study, the kinetic parameters of the esterification were also determined. Moreover, under the conditions of 80
C and a membrane surface area of 170 cm2, a 99% conversion was obtained. Assabumrungrat, Phongpatthanapanich, Praserthdam, Tagawa, and Goto (2003) used their in-house-derived kinetic parameters of the esterification reaction of methanol with acetic acid over Amberlyst 15 and the permeation parameters for a commercial PVA membrane (Pervap 2210) to model three PVMR configurations: batch, plugflow, and continuous stirred tank. The authors report how the membrane allows an unwanted permeation of methanol, which compromises the performance of the PVMR. Hence, they suggest the use of more selective membranes. Moreover, they demonstrate the superiority of the PFR configurations (in general) and CSTR (in certain operational conditions). Using the same reaction kinetic data obtained by Assabumrungrat et al. (2003), Lopez-Zamora et al. (2013) published detailed design guidelines when using a CSTR-PVMR configuration. It was found that the equilibrium conversion decreases with temperature. Moreover, the authors showed that conversions higher that 90% can be obtained when Da > 150 and 0.01 < Pe < 100 (where Da and Pe are the Damk€ ohler and the Peclet number, respectively). For the esterification with n-butanol, Liu, Zhang, and Chen (2001) used a crosslinked PVA-ceramic composite membrane (in the temperature range of 60e90
C). This paper is a rare case of use of the Zr(SO4)2$4H2O catalyst. On the basis of a simple Fickian model, the influence of several esterification process variables, such as process temperature, initial mole ratio of acetic acid to n-butanol, the ratio of the effective membrane area to the volume of reacting mixture, and catalyst content, were discussed. A more rigorous model was defined by Liu and Chen (using a kinetic approach) comparing model outcomes with experimental outcomes (Inoue et al., 2007). An interesting study on commercial and in-house synthesized membranes was proposed by Korkmaz, Salt, and Dincer (2011) for the reaction between isobutanol and

Pervaporation membrane reactors (PVMRs) for esterification

591

acetic acid. They used an R2-type (using in-house-made PVA, commercial Pervap 1201 and Pervap 2216, PVA-based membranes) and an R1-type unit (using in-house-made polydimethylsiloxane (PDMS) membranes) in an SBR-ISU configuration. The perfluorinated membrane Nafion 117 was also tested. The reaction was catalyzed by Dowex 50W-X8 (ion-exchange resin). The effects of membrane type, the ratio of membrane area to reaction volume, and temperature were investigated. An increase in temperature and ratio of membrane area to reaction volume increased the conversion. It was concluded that the selective removal of water by using hydrophilic membranes shifts the equilibrium to higher yields of isobutyl acetate, which can later be recovered using a hydrophobic PDMS membrane. The highest performances were obtained using the hydrophilic Pervap 1201 membrane and the hydrophobic in-housemade PDMS membrane. Again for the same reaction, zeolite-based composite membranes were used by Peters, Benes et al. (2005). These membranes were prepared by dip-coating technique (the thickness of the catalytic layers can be varied by adjusting the number of dipcoating steps), applying catalytic zeolite H-USY on top of hollow-fiber silica membranes using TEOS as binder material. The advantage of such a membrane is that the selective layer and the catalytic layer can be independently optimized (in this way coating failure due to drying and calcination steps can be avoided). The ISU-PVMR used behaved as an isothermal plug-flow reactor in which permeate consists mainly of water (acid, alcohol, and ester losses through the membrane were negligible). On the basis of this consideration, a model was developed and solved by Matlab and the Runge-Kutta integration method. A computational reactor evaluation was also performed to show all critical process parameters that evaluate the PVMR performance (parameters such as catalyst position, layer thickness, reaction kinetics, and membrane permeability were considered). It was found that the ratio of water removal to water produced (which is in turn directly influenced by the ratio of feed flow rate to membrane area and by the amount of catalyst load into the reactor) is the key parameter affecting reactor performance. It was also found that when increasing the catalytic layer thickness, the conversion becomes no longer limited by the amount of the catalyst present inside of the reactor. The outlet conversion of the C-PVMR was calculated to be 85%, whereas a conventional I-PVMR with the same catalyst loading shows an outlet conversion of 79%. Among others cases (Kwon et al., 1995; Drioli & Giorno, 2010; Diban et al., 2013), it is interesting to report a case of an enzymatic catalyst. Immobilized CALB was used by Kang, Pfromm, and Rezac (2005) for the esterification of geraniol with acetic acid. Hexane was used as solvent. The PV reactor was an SBR type. Reaction kinetics were investigated at 30
C at a water activity (aw) ranging between 0.02 and 1.0. Water activity was actively controlled throughout the reaction by using a highly waterselective pervaporation membrane (avoiding any changes in aw during the reaction). For this purpose, a defect-free cellulose acetate/highly porosity ceramic substrate composite membrane was manufactured, as described in detail elsewhere (Delgado, Sanz, Beltran, & N u~ nez, 2010). The authors propose that an optimized batch operation strategy would include shifting aw during the reaction so that the reaction rate is initially emphasized (a maximum in the initial reaction rate was found approximately

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Membrane Reactors for Energy Applications and Basic Chemical Production

at aw ¼ 0.1) whereas complete conversion is targeted toward the end of the batch by equilibrium shift achieved at low aw. This would lead to complete conversion of reagents. For continuous reactors, interstage aw adjustment would achieve the same goal.

19.6.2

Other relevant cases

In 1992, Nijhuis et al. studied the production of hexadecyl eruciate from eruic acid and cetyl alcohol using an ESU configuration and hydrophilic hollow-fiber membranes. They obtained an increase of conversion rate from 53% to 90% with low energy consumption and low investment costs in comparison with other systems. In 1993, Okamoto et al. studied the reactive pervaporation process for the production of ethyl oleate starting from oleic acid and ethanol. An ESU configuration was used together with a noncatalytic asymmetric PEI/4,4-oxydiphenylene pyromellitimide and p-toluene sulfonic acid as catalyst. A model was set by the authors (coupling reaction kinetics with the pervaporation permeate flux relation) to evaluate all critical parameters to tune PVMR parameters and obtain a conversion of 98%. In 1995, Ni et al. studied the PVMR involving the reaction between valeric acid and ethanol to give ethyl valerate. A hydrophilic modified aromatic polyimide membrane and p-toluene sulfonic acid as catalyst were used. The integration in a membrane reactor of reaction and pervaporation allowed for obtaining a conversion rate of 95.2%. More recently, Delgado et al. (2010) studied the synthesis of ethyl lactate from ethanol and lactic acid in an SBR configuration. The Amberlyst 15 catalyst and the hydrophilic membrane Pervap 2201 were used. The results proved that when the process is performed for a sufficiently long time, a close to 100% conversion of lactic acid could be reached. Moreover, performing kinetic experiments, water production and removal rates were acquired independently and then used to simulate the behavior of the coupled system. The influence on kinetics of the initial reactant molar ratio, the amount of catalyst, the temperature, and the ratio of membrane area to the initial volume of reaction was analyzed in terms of ethyl lactate yield. Simulation outcomes yielded a very good approximation of the experimental results. It was also noticed that (1) the initial reactant molar ratio determines the final ethyl lactate concentration in the reactor; (2) when the ratio of membrane area to initial reacting mixture volume increases, higher ester conversions can be obtained; and (3) the catalyst concentration has no significant influence on the esterification process in the case of dilute lactic acid aqueous solutions. Finally, the initial water concentration in the reaction medium has also been assessed because lactic acid is purchased as aqueous solutions (thus, the effect of lactic acid concentration is an important parameter to be understood). In this case, the authors showed that total permeation is proportional to water concentration in the reactor, sensibly affecting the performance of the process. The commercial PVA membranes GFT-1005 and T1-b (PVA-based) were used by Bendict et al. (2003, 2006) in a stirred batch reactor coupled with an ESU pervaporation cell for the esterification of lactic acid/succinic acid and ethanol. In these two studies, two solid catalysts Amberlyst XN-1010 and Nafion NR50 were used. The kinetics of pervaporation were studied to obtain a correlation for the flux of water

Pervaporation membrane reactors (PVMRs) for esterification

593

in terms of temperature and water concentration on the feed side of the pervaporation cell. They proposed to use conventional multistage distillation as an adequate way to separate and recover ethyl lactate and diethyl succinate from pervaporation retentates. Some of the outcomes of these papers (in particular concerning the reaction between lactic acid and ethanol) were used by Wasewar, Patidar, and Agarwal (2009) to prove the ability of their own developed model to predict the experimental data of a PVMR. The authors claimed an excellent agreement with the experimental data and that the model could be used for various other esterification reactions in a PV reactor. Parameters such as temperature and initial molar ration were investigated, defining optimal values to be used to run this particular reaction. Other publications dealing with the same reaction are those of Pereira, Silva, Pinho, and Rodrigues (2010) and Collazos, Fontalvo, and Gomez-García (2012). The latter presented a methodology for the systematic analysis and design of a PVMR treating the production of ethyl lactate using three different tools: thermodynamic analysis, residue curves, and design charts (in particular a PFR-ISU, Amberlyst 15 as catalyst, and methyl silica-type membranes were used). Nemec and Van Gemert (2005) made a comparison of the performance of an esterification reaction to produce diethyl tartarate in various configurations by considering a process of two separated units or a coupled separation-reaction unit in a multifunctional module. Moreover, regarding the configuration of the two separated steps (reaction and separation), a comparison between operating the system in parallel (catalytic esterification reaction and PV in two different loops) or in series (both stages belonging to the same loop) was also assessed. The results of this analysis are that the coupled separation-reaction configuration is to be preferred in the case of fast reactions, when high selectivity is a major concern for removing the product in situ (on the other hand, the main drawbacks are the low flexibility in the design and operation and the difficult maintenance); moreover, the in-series configuration showed a slightly better performance than the parallel configuration, which, on the contrary, allowed the system to work without PV in the initial reaction time in which the medium is highly acidic and may damage the membranes.

19.7

R1-type pervaporation membrane reactors (PVMRs) for esterification

A general review about R1-type PVMRs can be found elsewhere (Drioli & Giorno, 2010). In this section, a review of R1-type PVMRs for esterifications is given. The concept is simple: instead of water, the main product (ester) pervaporates through a hydrophobic membrane. It is clear that this technology is less developed because each type of ester produced may need a specific membrane. In any case, to define continuous processes, hydrophobic ester-selective membranes may also need to be used. Penkova (2013) studied the methyl acetate synthesis reaction by PVMRs. Membranes with a base of poly-(2,6-dimethyl-1,4-phenylene oxide) (PPO) were used in an SBR configuration; in particular, a homogeneous PPO membrane and a

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Membrane Reactors for Energy Applications and Basic Chemical Production

composite membrane (PPO thin selective layer on the surface of a fluoroplastic composite hydrophobic membrane) were synthesized. By the removal of methyl acetate, both of these membranes were able to shift the esterification reaction equilibrium in pervaporation. However, the composite membrane yielded a higher permeability and selectivity than the homogeneous membrane. Above all, a membrane type that has been successfully used for the hydrophobic permeation of esters is the PDMS-based membranes. An example is reported by Hasano glu, Salt, Keles¸er, and Dinçer (2009) (Hasanoglu & Dinçer, 2011). They used a BR-ESU PVMR, in the range of 50e70
C, for the esterification of acetic acid and ethanol to ethyl acetate. Amberlyst 15 and sulfuric acid were used as catalysts. The PDMS membranes used were in-house made (permselective to ethyl acetate). As a result, conversions were found increasing with molar ratios of reactants and temperature increase (a strong influence of temperature was found). Together with the experimental work, a model was also presented. The model satisfactorily agreed with the experimental results obtained and made a complete assessment of the process operating conditions possible, making it useful in a scale-up procedure of this technology. The authors demonstrated that beyond the 200 min of work and for temperatures higher or equal to 70
C, the rate of ethyl acetate conversion is larger than the production rate, indicating that the conversion could attain 100% (thus, the process is limited by the ethyl acetate production rate). As already reported, PDMS commercial membranes were also tested by Korkmaz et al. (2011) (see Section 19.6.1.2: PVMRs for the esterification reaction between acetic acid and other alcohols). Moreover, in a previous work (2009), Korkmaz and co-workers reported about the isobutyl acetate production by PVMRs using in-house synthesized cross-linked PDMS membranes. A BR-ISU configuration was set using homogeneous (sulfuric acid) and heterogeneous (Dowex 50W-X8) catalysts. The effects of catalyst loading, catalyst type, reaction time, membrane thickness, temperature, and initial molar ratio of reactants were investigated. In conclusion, at 70
C, the resulting conversion increased from 75% to more than 80%, proving the effectiveness of their PDMS in-house-made membranes.

19.8

Conclusions

In this chapter, a review of acid-catalyzed reactions, catalysts, and configurations was coupled with an assessment of conventional reaction-separation technologies to introduce the core theme of the work—PVMRs for esterification reactions. This chemical unit was explored in detail, considering catalysts, configurations, membrane materials, performances, and overall conversions obtained, showing that this is a young but promising technology (for certain cases (e.g., esterification of acetic acid with ethanol or n-butyl acetate), it is possible to add the adjective “mature”). Moreover, the sustainability and the flexibility of the PVMR were reviewed. In particular, when an R2-type batch configuration is used together with an ESU, hydrophilic membranes, and ionexchange resin, low investment costs and high product quality were demonstrated. These membranes are often cheap and easy to prepare. Other than PVA-based

Pervaporation membrane reactors (PVMRs) for esterification

595

polymeric membrane, more often zeolite-type membranes give high water selectivity and permeance, leading to a high yield of the ester. On the other hand, these membranes suffer from low reproducibility and (sometimes) high costs. Thus, their commercialization (for industrial applications) is still rather scarce. In any case, their performance is mostly higher than PVA-based membranes and in general higher than polymeric-based membranes. A possible compromise is represented by the multilayered membranes, also made of polymeric and inorganic materials; in fact, they couple the quality of both of these materials in a high-performance membrane technology. A step further is constituted by the bifunctional multilayered membranes, which are completed with a catalytic layer that increases the reagent conversion and, together with the water permeation driving force, water flux. Some biocatalytic (enzyme catalyst) R2-type PVMRs were also reported. In this case, an organic medium is used to stabilize the enzyme and the pervaporation hydrophilic membrane is used to control water activity (which affects the chemical equilibrium and the enzyme stability by poisoning it). A very high yield can be obtained, and in certain cases the stereoselectivity of the enzyme can lead to high-quality ester products. Finally, a concise overview of R1-type configurations for esterification PVMRs was provided. This subject is less studied in the literature because ester-selective membranes are more difficult to be designed (also considering that theoretically for each ester an optimized membrane should be defined). Nevertheless, PDMS membranes showed to be a good reference for these types of applications, leading to good conversions. Moreover, it must be stressed that when hydrophilic membranes are not extremely selective toward water, in the case of a continuous setup, a hydrophobic membrane should be considered to obtain pure ester as product (e.g., in series with the water-selective membrane). This means that the development of reference membranes for ester pervaporation is an interesting research path.

19.9

Future trends

The future trends of this technology are already indicated (e.g., the improvement of the membrane (materials, methods, and reproducibility), cost optimization (mainly for industrial implementation), engineering and assessment of hydrophobic membranes (broadly selective toward esters, to be set in series with hydrophilic membranes in continuous units), and so forth). In addition, considering the many references, this chapter suggests that the technology can already be considered successful and ready for further industrial implementation. However, the concrete success will depend on future energy source cost because the core advantage of the PVMR technology is linked to its energy economy (a lower energy consumption in comparison to conventional separation systems). Finally, it has to be remarked that, except for what concerns other equilibrium reactions that produce water (etherification reactions, Knoevenagel reactions, etc.), the real future challenge for PVMRs is to switch from water to alcohols. In fact, transesterification reactions, as an equilibrium reaction, produce alcohol as byproduct. In this

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Membrane Reactors for Energy Applications and Basic Chemical Production

case, the problematic separations involve an organiceorganic medium (the definition of suitable membranes is the starting point to go toward a new industrial breakthrough technology).

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Hari Krishna, S., & Karanth, N. (2002). Lipases and lipase-catalyzed esterification reactions in nonaqueous media. Catalysis Reviews, 44, 499e591. Hasanoglu, A., & Dinçer, S. (2011). Modelling of a pervaporation membrane reactor during esterification reaction coupled with separation to produce ethyl acetate. Desalination and Water Treatment, 35, 286e294. Hasanoglu, A., Salt, Y., Keles¸er, S., & Dinçer, S. (2009). The esterification of acetic acid with ethanol in a pervaporation membrane reactor. Desalination, 245, 662e669. Hasegawa, Y., Abe, C., Mizukami, F., Kowata, Y., & Hanaoka, T. (2012). Application of a CHA-type zeolite membrane to the esterification of adipic acid with isopropyl alcohol using sulfuric acid catalyst. Journal of Membrane Science, 415, 368e374. Hoydonckx, H. E., De Vos, D. E., Chavan, S. A., & Jacobs, P. A. (2004). Esterification and transesterification of renewable chemicals. Topics in Catalysis, 27, 83e96. Iliuta, I., Iliuta, M. C., & Larachi, F. (2011). Sorption-enhanced dimethyl ether synthesis— multiscale reactor modeling. Chemical Engineering Science, 66, 2241e2251. Inoue, T., Nagase, T., Hasegawa, Y., Kiyozumi, Y., Sato, K., Kobayashi, K., et al. (2006). Ester condensation from a stoichiometric mixture of a carboxylic acid and an alcohol at 313 K assisted by pervaporation via zeolite membranes. Chemistry Letters, 35, 76e77. Inoue, T., Nagase, T., Hasegawa, Y., Kiyozumi, Y., Sato, K., Nishioka, M., et al. (2007). Stoichiometric ester condensation reaction processes by pervaporative water removal via acidtolerant zeolite membranes. Industrial & Engineering Chemistry Research, 46, 3743e3750. Jafar, J. J., Budd, P. M., & Hughes, R. (2002). Enhancement of esterification reaction yield using zeolite A vapour permeation membrane. Journal of Membrane Science, 199, 117e123. Jeong, J. C., & Lee, S. B. (1997). Enzymatic esterification reaction in organic media with continuous water stripping: effect of water content on reactor performance and enzyme agglomeration. Biotechnology Techniques, 11, 853e858. Ju, I. B., Lim, H. W., Jeon, W., Suh, D. J., Park, M. J., & Suh, Y. W. (2011). Kinetic study of catalytic esterification of butyric acid and n-butanol over Dowex 50Wx8-400. Chemical Engineering Journal, 168, 293e302. Kabza, K. G., Chapados, B. R., Gestwicki, J. E., & Mcgrath, J. L. (2000). Microwave-induced esterification using heterogeneous acid catalyst in a low dielectric constant medium. The Journal of Organic Chemistry, 65, 1210e1214. Kang, I. J., Pfromm, P. H., & Rezac, M. E. (2005). Real time measurement and control of thermodynamic water activities for enzymatic catalysis in hexane. Journal of Biotechnology, 119, 147e154. Khajavi, S., Jansen, J. C., & Kapteijn, F. (2010). Application of a sodalite membrane reactor in esterification—coupling reaction and separation. Catalysis Today, 156, 132e139. Kiss, A. A., & Bildea, C. S. (2012). A review of biodiesel production by integrated reactive separation technologies. Journal of Chemical Technology & Biotechnology, 87, 861e879. Kita, H., Sasaki, S., Tanaka, K., Okamoto, K.-I., & Yamamoto, M. (1988). Esterification of carboxylic acid with ethanol accompanied by pervaporation. Chemistry Letters, 2025e2028. Kita, H., Tanaka, K., Okamoto, K.-I., & Yamamoto, M. (1987). The esterification of oleic acid with ethanol accompanied by membrane separation. Chemistry Letters, 2053e2056. Kolah, A. K., Asthana, N. S., Vu, D. T., Lira, C. T., & Miller, D. J. (2007). Reaction kinetics of the catalytic esterification of citric acid with ethanol. Industrial & Engineering Chemistry Research, 46, 3180e3187. Kolah, A. K., Asthana, N. S., Vu, D. T., Lira, C. T., & Miller, D. J. (2008a). Reaction kinetics for the heterogeneously catalyzed esterification of succinic acid with ethanol. Industrial & Engineering Chemistry Research, 47, 5313e5317.

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