Process intensification for the green solvent ethyl lactate production based on simulated moving bed and pervaporation membrane reactors

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Process Intensification for the Green Solvent Ethyl Lactate

Production based on Simulated Moving Bed and

Pervaporation Membrane Reactors

A Dissertation Presented to the Faculdade de Engenharia da Universidade do Porto for the degree of PhD in Chemical and Biological Engineering

by

Carla Sofia Marques Pereira

Supervised by Professor Alírio Egídio Rodrigues and Dr. Viviana Manuela Tenedório Matos da Silva

Laboratory of Separation and Reaction Engineering, Associate Laboratory LSRE/LCM Department of Chemical Engineering, Faculty of Engineering, University of Porto

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Acknowledgements

First of all, I want to thank my supervisors, Professor Alírio Rodrigues and Dr Viviana Silva. Professor Alírio, thank you for all the friendship, constant support and for always challenging me to reach higher goals within my work. Dr. Viviana, I want to thank you for all the encouragement, motivation, constant support, for all the long discussions and great ideas that make me go further and further within my work, and, also, for being a truly and special friend.

I am very grateful to Professor Simão Pinho, for the friendship and all the support in the framework of the project “POCI/EQU/61580/2004” and to Professor Madalena Dias for the support whenever needed.

To all my LSRE colleagues, especially Israel Pedruzzi, Pedro Sá Gomes, Michael Zabka, João Santos, Miguel Granato, João Pedro Lopes, Alexandre Ferreira and Nuno Lourenço for the friendship, collaboration, and support whenever I needed.

To my primary school teacher, Professor João Aveiro, for always believing in me and keeping me motivated along the years.

To Fundação para a Ciência e Tecnologia, for the financial support (Research Fellowship: SFRH / BD / 23724 / 2005).

To Sofia Rodrigues, Marta Abrantes, Fátima Mota, Miguel Teixeira and Nuno Garrido for all the great moments spent after work!!!

Last, but not least, I would deeply like to thank my family and friends, for all giving love, support and trust, especially to my grandmother, Dulcelina, that will always stay in my heart.

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“Green chemistry represents the pillars that hold up our sustainable future. It is

imperative to teach the value of green chemistry to tomorrow’s chemists.”

Daryle Busch, President of the American Chemical Society (June 26, 2000)

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Resumo

O principal objectivo deste trabalho foi o desenvolvimento de um novo processo eficiente para a produção do solvente verde, lactato de etilo, através da reacção de esterificação entre etanol e ácido láctico, utilizando tecnologias híbridas de reacção/separação baseadas em reactores de leito móvel simulado e processos de membranas por pervaporação. De forma a atingir a meta proposta, foram abordados os seguintes temas:

Aquisição de dados fundamentais: A resina de permuta iónica, Amberlyst 15-wet, foi avaliada tanto como catalisador para a reacção de esterificação, como adsorvente selectivo para a água. Os dados cinéticos e de equilíbrio da reacção foram medidos, na gama de temperaturas 50ºC-90ºC, e usados para a determinação da constante de equilíbrio e da lei cinética da reacção como função da temperatura, baseadas em actividades descritas pelo modelo UNIQUAC. Os dados de adsorção foram também medidos, a 20ºC e a 50ºC, e ajustados a uma isotérmica de Langmuir multicomponente, tendo-se assumido uma capacidade volumétrica da monocamada igual para todas as espécies, reduzindo o número de parâmetros de ajuste de 8 para 5, para cada temperatura. Membranas comerciais hidrófilas da Pervatech foram avaliadas para a desidratação do etanol, ácido láctico e lactato de etilo, por pervaporação. As permeâncias de todas as espécies foram determinadas em função da composição e temperatura na gama 48ºC-72ºC.

Intensificação de processo: Modelos matemáticos, considerando resistências internas e externas à transferência de massa e velocidade variável devido à mudança das propriedades da mistura multicomponente, foram desenvolvidos para reactores cromatográficos — reactores de leito fixo e de leito móvel simulado, SMBR — e validados pelos dados experimentais. O modelo matemático do reactor de membranas por pervaporação, PVMR, considera, adicionalmente, a permeação através da membrana, os efeitos de polarização por concentração e temperatura e operação não isotérmica. A avaliação teórica do comportamento da unidade SMBR foi realizada para analisar o efeito da configuração, da composição da alimentação e tempo de comutação nas regiões de separação/reacção e/ou no desempenho do processo nos pontos operacionais óptimos. O desempenho do PVMR foi avaliado para operação isotérmica e não isotérmica, e foram determinadas condições apropriadas para a maximização quer da conversão do ácido láctico quer da pureza do lactato de etilo. Finalmente, uma nova tecnologia foi desenvolvida e submetida a registo de patente, o reactor de membranas de leito móvel simulado, PermSMBR, o qual integra membranas selectivas dentro das colunas do SMBR.

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Abstract

The main objective of this work was the development of a new efficient process to produce the green solvent ethyl lactate from the esterification reaction between ethanol and lactic acid by using hybrid reaction/separation technologies based on simulated moving bed reactors and pervaporation membrane processes. To accomplish this target, the following topics were addressed:

Basic data acquisition: The acidic ion exchange resin Amberlyst 15-wet was evaluated as both catalyst for esterification and selective adsorbent for water. Equilibrium and kinetic data were measured in the temperature range 50-90ºC, and used to obtain the equilibrium constant and kinetic law as function of temperature, which are based on liquid activities described by the UNIQUAC model. Adsorption data was also obtained and fitted to a multi-component Langmuir isotherm assuming a constant monolayer capacity in terms of volume for all species, reducing the adjustable parameters from 8 to 5, for each temperature. Pervatech hydrophilic commercial membranes were evaluated for the dehydration of ethanol, lactic acid and ethyl lactate, by pervaporation. The permeances of all species were determined as function of composition and temperature in the range 48-72ºC.

Process intensification: Mathematical models, considering external and internal mass-transfer resistances and velocity variations due to the change of multi-component mixture properties, were developed for chromatographic reactors — fixed bed and simulated moving bed reactor, SMBR — and validated by experimental data. The pervaporation membrane reactor, PVMR, model also takes into account, the permeation through the membrane, concentration and temperature polarization effects, and non-isothermal operation. The theoretical assessment of the SMBR unit behaviour was performed to analyse the effect of SMBR configuration, feed composition and switching time on the reactive/separation regions and/or on the process performance at the optimal operating points. The performance of the PVMR was evaluated for isothermal and non-isothermal operation, and suitable conditions for maximization of both lactic acid conversions and ethyl lactate purity were examined. Finally, a new technology was developed and submitted to patent registration, the simulated moving bed membrane reactor, PermSMBR, which integrates perm-selective membranes inside the SMBR columns.

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Zusammenfassung

Ziel dieser Arbeit war es einen völlig neuartigen und effizienten Prozess für die Produktion von “Grünem Lösungsmittel” und Ethyl-Lakton, mittels der Verästerung aus Ethanol und Milchsäure über die Hybrid-Technologie, zu entwickeln. Die Reaktion/Trennung basiert auf simulierten Fliessbettreaktoren und Membranprozessen mittels Pervaporization. Um diese Zielsetzung zu erreichen, wurden folgende Themen diskutiert:

Folgende wichtige Daten wurden gesammelt: Für den Ionen-Austausch wurde das Harz, Amberlyst

15-wet, genutzt. Es wurde sowohl als Katalysator für die Reaktivverästherung, wie auch als selektiver Adsorbent für Wasser ausgewertet. Die Kinetischen- und chemischen Gleichgewichtsdaten wurden zwischen 50ºC und 90ºC gemessen. Als Vorlage für die Bestimmung der chemischen Gleichgewichtskonstante wie auch der Kinetik als Funktion der Temperatur, wurde das UNIQUAC Model genutzt. Die Adsorptionsdaten wurden zwischen 20ºC und 50ºC gemessen, und entsprechend einer Langmuir-Isotherme für Multikomponenten angeglichen, wobei eine gleich grosse Volumenkapazität der Monoschicht für alle Spezies angenommen wurde. Die Anzahl der zu justierenden Parameter wurde hierbei für jede Temperatur von 8 auf 5 reduziert. Kommerzielle hydrophile Membranen der Firma Pervatech wurden für die Dehydratisierung von Ethanol, Milchsäure und Ethyllakton mittels Permeation ausgewertet. Die Permeation aller Spezies wurde als Funktion der Zusammensetzung und Temperatur, zwischen 48ºC und 72ºC ermittelt.

Intensivierung des Prozesses: Es wurden mathematische Modelle für chromatographische Reaktoren

entwickelt (Modelierte Festbett- und Fliessbettreaktoren SMBR) und experimentell ausgewärtet, unter Berücksichtigung des internen und externen Massenaustausches, sowie der variablen Geschwindigkeiten. Es herrschen unterschiedlichen Vermischungseigenschaften der Multikomponenten. Das mathematische Modell der Pervaporationsmembrane (PVMR) berücksichtigt gleichfalls die folgenden Effekte: Polarization aufgrund von Temperatur- und Konzentrationsgradienten sowie nicht-isotherme Reaktionsführung. Das theoretische Verhalten der SMBR Einheit wurde unter Berücksichting der Konfiguration, der Mischungszusammensetzung beim Eintritt und der Komutationszeit in den Trennungs- und Reaktionszonen und/oder für die Prozessleistung an den operationallen Optima ausgewertet. Der Wirkungsgrad der PVMR wurde für den isothermen und nicht-isothermen Betrieb ermittelt, und es wurden entsprechende Bedingungen für den maximale Umsatz der Milchsäure und der maximalen Reinheit für Ethyl-Lakton bestimmt. Der lezte Schritt war die Entwicklung einer neuen Technolgie, ein Membranreaktor mit simuliertem Fliessbett (PermSMBR), welche als Patent angemeldet wurde. Dieser Reaktor integriert selektive Membranen innerhalb der SMBR Säule.

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1. Introduction

1.1 Relevance and Motivation

Petroleum (“black gold”) is at the heart of today’s economics and politics problems. The traditional petroleum reserves are in decline; moreover, the environmental regulation is every day more severe, being, therefore, a great challenge to the design and implementation of green products and processes.

Green solvents, which are produced from the processing of agricultural crops, were developed as a more environmentally friendly alternative to petrochemical solvents. Lactate esters solvents are 100% biodegradable, easy to recycle, non-corrosive, non-carcinogenic and non-ozone depleting. Lactate esters have found industrial applications in specialty coatings, inks, cleaners and straight cleaning use.

Ethyl lactate is a green solvent derived from nature-based feedstocks and it is so benign that the U.S. Food and Drug Administration approved its use in food products. Ethyl lactate could replace a range of environment-damaging halogenated and toxic solvents, including ozone-depleting chlorofluorocarbons, carcinogenic methylene chloride, and toxic ethylene glycol ethers and chloroform. In Figure 1.1 the ethyl lactate life-cycle is shown.

Ethyl lactate is produced from the esterification of lactic acid with ethanol through a reversible reaction, having water as a by-product. Traditionally, ethyl lactate is synthesized in a reactor followed by separation units in order to recover it, to remove the by-product (water) and to recycle the unconverted reactants to the reactor; however, this represents high costs. The objective of this work is to study equipments and techniques that are more compact,

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2 CHAPTER 1.Introduction

energy efficient, and environment-friendly sustainable processes for the ethyl lactate production. From Nature To Nature From Nature To Nature

Figure 1.1- Ethyl Lactate life-cycle.

Process intensification, regarding the integration of reaction and separation processes into a single device, provides the most feasible engineering solution to the sustainable synthesis of ethyl lactate, since at least one of the products is being removed from the reaction medium to lead to depletion of the limiting reactant. In this perspective, continuous chromatographic reactor and membrane reactor will be considered for ethyl lactate production, namely the Simulated Moving Bed Reactor (SMBR) and the Pervaporation Membrane Reactor (PVMR), respectively. The SMBR is a competitive technology for systems involving equilibrium controlled reactions catalysed by ion exchange resins, which are also selective adsorbent for water (the by-product formed in the ethyl lactate synthesis), given that the products are formed and simultaneously separated and removed from the reaction medium. The PVMR technology is a clean and economic alternative to conventional processes, since equilibrium could be shifted by continuously removing water through a selective membrane, allowing costs reduction and higher product purity. Combining the advantages of both technologies, finally, a new hybrid technology will be developed, the Simulated Moving Bed Membrane Reactor (SMBMembR or PermSMBR) that combines a reactor with two different separation techniques into a single device: continuous counter-current chromatography (SMB) with a selective permeable membrane (Pervaporation or Permeation).

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PROCESSES INTENSIFICATION FOR THE GREEN SOLVENT ETHYL LACTATE PRODUCTION 3

1.2 Objectives and Outline

The main goal of this work is the development of a process for simultaneous reaction and separation in a single device for ethyl lactate production with a high purity, yield and complete reactants conversion. Three technologies will be studied: the Simulated Moving Bed Reactor (SMBR) using a catalyst that is also a selective adsorbent for water/ethyl lactate separation, the Pervaporation Membrane Reactor (PVMR) using a hydrophilic water permselective membrane for continuous removal of the by-product water and the Simulated Moving Bed Membrane Reactor (PermSMBR), where the SMBR is integrated with the PVMR by using selective permeable membranes inside the columns of the SMBR.

The thesis comprises 8 chapters dealing with different aspects of ethyl lactate production, in addition to the present one.

In Chapter 2, the state of the art of production process aspects as patented processes for esters production; the advantages of heterogeneous catalyst, such as ion-exchange resins; the methods used to displace equilibrium towards ester formation are addressed. An overview in some reactive separations as reactive distillation, chromatographic reactors and membrane reactors applied to the production of oxygenates is reviewed in order to improve the overall efficiency of the process of ethyl lactate synthesis.

Chapter 3 addresses the kinetic studies for ethyl lactate production by heterogeneous catalysis; the influence of catalyst loading, temperature and initial molar ratio of reactants are analysed. A methodology based in the UNIQUAC model for determination of the thermodynamic equilibrium constant is developed.

Experimental and simulated results for the ethyl lactate production in a fixed bed adsorptive reactor are shown in Chapter 4. Dynamic adsorption experiments of binary non-reactive mixtures were performed in order to obtain multicomponent adsorption equilibrium isotherms of Langmuir type. The reaction kinetics and adsorption data were used in the mathematical model of the adsorptive reactor, which also included axial dispersion, velocity variations and external and internal mass-transfer resistances.

In Chapter 5, the simulated moving bed reactor technology for the ethyl lactate production is evaluated by experiments as well as by simulations. In order to describe the dynamic behaviour of this unit, a mathematical model considering external and internal mass-transfer resistances and variable velocities is developed. The influence of operational parameters, as

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4 CHAPTER 1.Introduction

feed composition, SMBR configuration and switching time, on the SMBR performance is presented.

Pervaporation processes using hydrophilic silica membranes are evaluated in Chapter 6 for the ethyl lactate system. The effects of feed composition and operating temperature on the membrane performance are analyzed. Mathematical models, considering concentration and temperature polarization and non-isothermal effects, are developed and applied to analyze the performance of batch pervaporation and continuous pervaporation membrane reactor, in both isothermal and non-isothermal conditions.

In Chapter 7, a new technology, the simulated moving bed membrane reactor, is presented and applied for the ethyl lactate synthesis. The potential of this new equipment is demonstrated by comparing its performance to other reactive separation processes.

Finally, the general conclusions drawn from this work and the suggestions for future work will be presented in Chapter 8.

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2. State of the art on Green Solvent Ethyl Lactate

In this chapter, a review on the green chemistry principles is made and a literature survey on applications of ethyl lactate and production processes (renewable resources, patents, reactive separations) is presented.

2.1 Green Chemistry

Green chemistry is the best use of chemistry for pollution prevention. More specifically, green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It is a highly effective approach to pollution prevention because it applies innovative scientific solutions to real-world environmental situations.

Currently, a great challenge is the design and implementation of completely green products and processes. There is not a systematic and reliable method for ensuring that the chemistry being implemented is green, since the number of chemicals synthesis pathways is enormous. Indeed, it is more correct to verify if a proposed manufacturing process is “greener” than other alternatives. Anastas and Warner have developed the “Twelve Principles of Green Chemistry” to aid one in assessing how green is a product or a process (Anastas and Warner, 1998), which are:

1. It is better to prevent waste than to treat or clean up waste after it is formed.

2. Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.

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6 CHAPTER 2. State of the art on Green Solvent Ethyl Lactate

3. Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

4. Chemical products should be designed to preserve efficacy of function while reducing toxicity.

5. The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary whenever possible and, innocuous when used.

6. Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperature and pressure.

7. A raw material or feedstock should be renewable rather than depleting whenever technically and economically practical.

8. Unnecessary derivatization (blocking group, protection/deprotection, and temporary modification of physical/chemical processes) should be avoided whenever possible. 9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

10. Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products.

11. Analytical methodologies need to be further developed to allow for real-time in-process monitoring and control prior to the formation of hazardous substances.

12. Substances and the form of a substance used in a chemical process should be chosen so as to minimize the potential for chemical accidents, including releases, explosions, and fires.

Based on these twelve principles, this thesis focuses:

1) The synthesis of the green solvent Ethyl lactate produced from renewable raw material that is a more environmentally friendly alternative to petrochemical solvent: 7th_principle.

2) Ethyl lactate is 100% biodegradable, easy to recycle, corrosive, non-carcinogenic and non-ozone depleting: 3rd_{, 4}th_{and 10}th_principles.

3) The use of solid acid catalysts to improve the reaction kinetics without using increase the stoichiometric of reactants, and it is more advantageous then

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PROCESS INTENSIFICATION FOR THE GREEN SOLVENT ETHYL LACTATE PRODUCTION 7

homogenous catalysts, since these are more corrosive and require a further step of neutralization: 1st and 9th principles.

4) The process intensification by using hybrid technologies where reaction and separation of at least one product take place in a single unity (SMBR, PVMR and PermSMBR) will reduce/eliminate the use of solvents and requires less energy consumption: 5th_{and 6}th_principles.

2.2 Ethyl lactate applications

The reaction between an alcohol and a carboxylic acid to form an ester and water is of considerable industrial interest (Dhanuka et al., 1977). Organic esters are a very important class of chemicals having applications in a variety of areas in the chemical industries such as perfumes, flavours, pharmaceuticals, plasticizers, solvents and intermediates (Weissermel and Arpe, 1997).

Ethyl lactate is an important organic ester, which is biodegradable and can be used as food additive, in perfumery, as flavour chemicals and solvent, which can dissolve acetic acid cellulose and many resins (Tanaka et al., 2002). It is a particularly attractive solvent for the coatings industry as a result of its high solvency power, high boiling point, low vapour pressure and low surface tension. Ethyl lactate is a desirable coating for wood, polystyrene and metals and also acts as a very effective paint stripper and graffiti remover. It has replaced solvents including N-methyl Pyrrolidone (NMP) (Reisch, 2008), toluene, acetone and xylene, which has resulted in the workplace being made a great deal safer. In Table 2.1 solvating properties of ethyl lactate and NMP are presented.

Table 2.1 Solvating properties of ethyl lactate and N-methyl Pyrrolidone. Ethyl Lactate N-methyl pyrrolidone

Kauri Butanol(KB) Value >1000 350

Hildebrand 21.3 23.1

Disperse 7.8 8.8

Polar 3.7 6.0

Hydrogen 6.1 3.5

Solubility Miscible in Water and

Hydrocarbons

Miscible in Water and Hydrocarbons

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Other applications of ethyl lactate include being an excellent cleaner for the polyurethane industry and for metal surfaces, efficiently removing greases, oils, adhesives and solid fuels. Beyond all these applications, ethyl lactate can also be used in the pharmaceutical industry as a dissolving/dispersing excipient for various biologically active compounds without destroying the pharmacological activity of the active ingredient. It proves to be a very effective agent for solubilising biologically active compounds that are difficult to solubilise in usual excipients (Muse and Colvin, 2005).

Ethyl lactate can also be applied as a more environment friend alternative route to produce 1,2-propanediol, which is normally produced by the hydration of propylene oxide derived from petrochemical resource (Huang et al., 2008). In Table 2.2 the major benefits of the ethyl lactate are presented.

Table 2.2 Ethyl lactate major benefits.

Ethyl Lactate Benefits

100% Biodegradable Renewable - made from corn and other carbohydrates FDA approved as a flavour additive EPA approved SNAP solvent

Non carcinogenic Non corrosive

Great penetration characteristics Stable in solvent formulations until exposed to water Rinses easily with water High solvency power for resins, polymers and dyes High boiling point Easy and inexpensive to recycle

Low VOC Not a Ozone Depleting Chemical

Low Vapor Pressure Not a Hazardous Air Pollutant

2.2.1 Solvent Market Analysis

Almost all manufacturing and processing industries depend on the use of solvents (see Figure 2.1). The world solvent market is estimated at 30 million pounds per year at prices from $0.90 to $1.70 per pound. The ethyl lactate green solvent has the potential to displace 80 % of these solvents (Energetics Incorporated, 2003).

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Figure 2.1 Solvent demand (AAE Chemie, 2009).

Selling prices for ethyl lactate have ranged from $1.50 to $2.00 per pound, but processing advances could drive the price as low as $1.00 to $0.85 per pound (Argonne, 2006), enabling ethyl lactate to compete directly with the petroleum-derived toxic solvents currently used. Moreover, the crude prices have risen sharply, making of ethyl lactate green solvent more commercially attractive. Among this, due to an environmental consciousness, some consumers are willing to pay more for products that are less detrimental to the environment.

2.3 Synthesis of ethyl lactate

The conventional way to produce ethyl lactate is the esterification of lactic acid with ethanol catalyzed by an acid catalyst, according to the reaction:

) ( ) ( ) ( )

(Eth LacticAcid La EthylLactate EL Water W

Ethanol ₊ _←_{⎯ →}_⎯H+ ₊

The use of these reactants (ethanol and lactic acid) has the advantage of both being produced from renewable resources (by glucose or sugar fermentation processes).

Esterifications are self-catalyzed reactions, since the H+_{cation released from the partial} dissociation of the carboxylic acid used as reactant catalyses the reaction. However, the use of catalyst is favourable for the reaction rate as the kinetics of the self-catalyzed reaction is extremely slow, since its rate depends on the autoprotolysis of the carboxylic acid. For

Agricultural chemicals 2% Dry Cleaning 1% Others 8% Paints 46% Pharmaceuticals 9% Adhesives 6% Printing Inks 6% Personal Care 6% House/Car 6% Metal/Industrial Cleaning 4% Rubber/Polymer Manufacture 4% Oil Seed Extract 2%

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example, the lactic acid acidity constant is pKa=3.86 @ 25 ºC, and therefore an aqueous solution with 85 % of lactic acid (about 10.8 M) has a pH=1.4. Typically, the catalytic production of lactates is performed with homogeneous catalysts using acids, such as sulphuric acid, phosphoric acid and anhydrous hydrogen chloride. However, the use of heterogeneous catalyst (as for example, zeolites, ion-echange resins like Amberlyst 15-wet, Nafion NR50, among others) has clear advantages:

- easy to separate from the reaction medium; - long life time;

- higher purity of products (side reactions can be eliminated or are less significant); - elimination of the corrosive environment caused by the discharge of acid

containing waste.

As previously mentioned, the esterification is a reversible reaction and, in order to obtain acceptable ester yields, the equilibrium must be displaced towards the ester production, which might be accomplished by different methods, such as:

1. to use a large excess of one of the reactants, in general the alcohol; however, this results in a relatively inefficient use of reactor space and in very diluted products, which will require an efficient separation afterwards;

2. to eliminate the water by azeotropic distillation between a solvent and water – the solvent and water must be partially miscible and the boiling points of the different components in the reaction medium must be compatible with that azeotrope;

3. to use reactive separations (as reactive distillation, simulated moving bed reactor, pervaporation reactor, etc.) in order to remove the products from the reaction medium.

In reactions limited by chemical equilibrium where more than one product is formed conversion can be enhanced in multifunctional reactor where the products are separated as they are formed. Novel reactor configurations and choice of operating conditions can be used to maximise the conversion of reactants and improve selectivity of desired product, thereby reducing the costs associated with the separation step. Recently, reactive distillation,

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chromatographic reactors and membrane reactors have been intensively applied to esterifications processes, as it will be discussed latter within this chapter.

2.3.1 Renewable Resources

In recent years, an increasing demand on using biorenewable materials instead of petroleum based feedstocks for producing chemicals, driven by environmental concerns and by the concept of sustainability, has been noticed. Biobased products are one of the main pillars of a sustainable economy. Nature produces 170 billion tons of biomass per year by photosynthesis, 75 % of which belong to the class of carbohydrates; however, just 3-4 % of these compounds are used by humans for food and non-food purposes (Röper, 2002). Carbohydrates are very abundant renewable resources and they are currently considered as an important feedstock for the Green Chemistry of the future (Lichtenthaler, 1998; Lichtenthaler, 2002; Lichtenthaler and Peters, 2004). Industrial plants, named as biorefineries, have been created where biomass is converted economically and ecologically, in chemicals, materials, fuels and energy (see Figure 2.2). The biorefineries could be the basis of the new bioindustry and its concept is similar to the petroleum refinery; the difference is that the biorefinery is based on conversion of biomass feedstocks instead of crude oil.

Figure 2.2 Schematic diagram of a biorefinery for precursor-contained biomass. (Kamm and Kamm, 2004a; Kamm and Kamm, 2004b)

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2.3.1.1 Ethanol Platform

Ethanol is an important raw material in the chemical industry and can also be used as transportation fuel. It can be produced from a variety of biomass crops, including sugar crops (e.g., sugarcane and sugar beet), starch crops (e.g., corn and cassava), or cellulosic feedstocks (e.g., wood, grasses and agricultural residues).The production of ethanol from starch crops involves as main steps: liquefaction and saccharification (conversion to sugar), milling, pressing, fermentation and distillation. The production from cellulosic feedstocks is similar, however it is significantly more difficult and costly to convert cellulose and hemicellulose into their component sugars (glucose and xylose, respectively) than is the case for starches (Sagar and Kartha, 2007). Currently, more than 37 billion litters of ethanol are produced worldwide per year from starch and sugar crops (Rass-Hansen et al., 2007; Tilman et al., 2006). In 2008, cellulosic ethanol industry developed some new commercial-scale plants. In the United States, plants with 12 million liters capacity per year were operational, and an additional 80 million liters per year of capacity (26 new plants) was under construction. In Canada, capacity of 6 million liters per year was operational. In Europe, several plants were operational in Germany, Spain, and Sweden, and capacity of 10 million liters per year was under construction (REN21, 2009). Ethanol derived from cellulosic crops is appealing since it broadens the scope of potential feedstocks beyond starch and sugar-based food crops. Moreover, cellulosic ethanol can be more effective and promising as an alternative renewable biofuel than corn ethanol because its use reduces even more the net greenhouse gas (GHG) emissions when compared with the petroleum fuel (Wang et al., 2008)(see Figure 2.3).

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2.3.1.2 Lactic acid Platform

Lactic acid (2-hydroxypropionic acid) is an important platform chemical for the biorenewable economy. It is an α-hydroxy acid containing a hydroxyl group adjacent to the carboxylic acid functional group; a review on the lactic acid chemistry can be found in literature (Holten, 1971). Lactic acid can be produced through chemical synthesis or through the fermentation of different carbohydrates, such as, glucose (from starch), maltose (produced by specific enzymatic starch conversion), sucrose (from syrups, juices, and molasses), or lactose (from whey) (Corma Canos et al., 2007). Nowadays, it is commercially produced by fermentation of glucose. One of the most important steps in the lactic acid production is the recovery from fermentation broths. The separation and purification stages represent about 50 % of the total production cost. However, current advances in membrane-based separation and purification technologies, particularly in microfiltration, ultrafiltration and electrodialysis, have originated new processes which should reduce the lactic acid cost production (Wasewar et al., 2004).

The lactic acid production is around 350,000 tons per year and it is defended by some observers that the worldwide growth per year is of 12-15 % (Wasewar et al., 2004). A lot of products are derived from lactic acid; some of them are new chemical products and others, which represent biobased routes to chemicals, currently produced from petroleum.The most important ones are shown in Figure 2.4.

Figure 2.4 Some potential derivatives of lactic acid (Corma Canos et al., 2007).

2.3.2 Patented Processes Overview

There are a great number of patents related to esters production. A summary of those patented processes is presented in Table 2.3.

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Table 2.3 Patented processes for esters production. Commercial Solvents

Corporation

(Bannister, 1936)

Lactic acid is dehydrated and mixed with ethanol and concentrated sulphuric acid (catalyst). This mixture is refluxed for one hour and the esterification takes place to a determined extent. Afterwards, distillation is started to separate the components of the reaction mixture. Temperature range of the process 135ºC-145ºC.

USA-Secretary of Agriculture

(Filachione and Fisher, 1951)

Process to produce esters from the reaction between the basic nitrogen salt of the carboxylic acid with an alcohol. It is a batch process and distillation is applied to separate the final mixture components. The process achieved from 61 to 92% ammonia removal and from 49 to 67% conversion to butyl lactate. Temperature range 89ºC-195ºC.

The American Oil Company

(Jennings and Binning, 1960)

Process, where the esterification extent is enhanced, using pervaporation membranes in order to selective remove water from the reaction zone. It is a continuous method, which integrates reaction and separation in the same unit and uses as catalyst acid ion exchange resins. A temperature of 100ºC to 200ºC is maintained in the feed zone and the pressure is kept in order to maintain the water in liquid phase.

BASF Aktiengesellschaft

(Bott et al., 1986) Process to produce optically pure alkyl D- or L-lactates by reaction of calcium lactate with an alcohol in the presence of a strong acid; the water present in the reaction mixture or formed during the esterification is separated off by azeotropic distillation with the aid of an entraining agent.

Battelle Memorial Institute

(Walkup et al., 1991; Walkup

et al., 1993)

Batch process for the preparation of esters of lactic acid directly from ammonium lactate and an alcohol. In this method the use of CO2 as a

catalyst is required and the preferred range for the reaction mixture temperature is from 100ºC to 200ºC. A yield of lactate of about 75% is reported.

E. I. Du Pont de Nemours and Company

(Cockrem and Johnson, 1993)

Recovery of high purity lactate ester from fermentation broth containing ammonium lactate or other basic salt of lactic acid; acidifying in the presence of an alcohol using continues addition of sulphuric acid or other strong acid and crystallizing to precipitate out some or all of the basic salt of the strong acid; simultaneously or sequentially removing water while also esterifying the lactic acid with the alcohol to form impure lactate ester; removing the crystals formed; distilling the lactate ester to remove impurities.

Musashino Chemical Laboratory Ltd.

(Akira et al., 1994)

Method for producing a lactic ester by microorganic fermentation of lactic acid with a simple apparatus. A pressure in the range of 100 to 760 mmHg and a temperature of about 130ºC are recommended for this process.

BASF Aktiengesellschaft

(Sterzel et al., 1995) Process for the synthesis of lactates by fermentation of sugars mixtures, conversion of the lactic acid obtained during the fermentation to its salts, followed by esterification.

DAICEL CHEM IND LTD

(Yukio, 1996)

In this process lactic acid is esterified with ethanol in the presence of a catalyst such as p-toluenesulfonic acid. The catalyst, water and unreacted ethanol are removed to give a solution (A). The solution A is neutralized with (B) a solution of an alkali metal salt in an alcohol and distilled to give the ethyl lactate.

(Feng et al., 1996)

Rectification process to produce ethyl lactate from lactic acid and ethanol. This patented process includes technological steps, such as, determination of lactic acid, ethanol, sulphuric acid and benzene amounts, catalytic reaction, rectification for dewatering, neutralization and reduced distillation. Yield rate up to over 90% is reported.

Argonne National Laboratory

(Rathin and Shih-Perng, 1998)

Process for the synthesis of high purity ethyl lactate and other lactate esters from carbohydrate feedstock. This process consists in a reactor coupled with a pervaporation membrane unit for water removal and followed by separation of the reaction mixture in two consecutive distillation columns. Alternatively, the reactor is followed by a plurality of pervaporation steps. It is reported a conversion greater than 99 %, for an initial ethanol/lactic acid molar ratio of 2:1, a reaction mixture temperature of 95ºC, a permeate-side vacuum pressure less than 0.5 mbar and as catalyst an ion-exchange resin, Amberlyst XN-1010, at 10% of lactic acid weight.

Mitsubishi Gas Chemical Company, Inc.

(Abe et al., 1998)

Process to produce lactates from acetaldehyde and formate; The method described is characterized by the fact that there is no formation of ammonium salts as by-products as in the case of the conventional techniques.

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A.E. Staley Manufacturing Co.

(Cockrem, 2001)

Process for the simultaneously production of an organic acid and of an ester of the organic acid. A mixture of ammonium salt of an organic acid with alcohol is rapidly heating in order to produce a liquid stream containing acid, ester, and unreacted ammonium salt.

Eastman Chemical Company

(Arumugam et al., 2003)

Process where a solution of carboxylic acid in a solvent and an alcohol are fed to a simulated moving bed reactor (SMBR), that contains a solid(s) (adsorber/catalyst) to produce two streams, one comprising a solution of the ester of the carboxylic acid and the alcohol and other comprising the solvent. The process is particularly valuable for the preparation of an alkanol solution of an alkyl 2-keto-L-gulonate ester (AKLG). The simulated moving bed reactor is maintained at a temperature of 30 to 60ºC and a pressure of 3.5 to 20 bars.

Cargill, Incorporated

(Eyal et al., 2003; Eyal et al., 2006)

Techniques for processing lactic acid/lactate salt mixtures obtained from fermentation broths. These techniques generally concern the provision of separated lactic acid and lactate streams from the mixtures. In this patent preferred methods of separation and processing of each of the streams are provided.

A.E. Staley Manufacturing Co.

(Cockrem, 2003)

Process to produce an ester that comprises the following steps: (a) feeding to a first vessel a mixture of organic acid, alcohol, and water, where the organic acid and alcohol react to form monomeric ester and water (temperature of 150 to 220ºC) (b) feeding the mixture obtained in to a second vessel (temperature range 30 to 100ºC), where are produced a vapour stream, that comprises alcohol, ester and water, and a liquid one, that can be recycled to the first vessel.

La Chemical SpA

(Ruggieri et al., 2003)

Process to produce esters in a chromatographic reactor in which the heterogeneous solid phase acts both as catalyst and as a means exhibiting preferential adsorption towards one of the reaction products (typically water). This process is particularly improved compared with the conventional technology since for regenerating the catalyst, it is used a desorbent mixed with a second compound, normally the anhydride of the acid used in the esterification reaction, which, by chemical reaction, completes the removal of the water adsorbed.

(Xueming and Jing, 2003)

Disclose a technique that uses ammonium lactate as raw material to make ethyl lactate by rectifying. The adopted equipment includes rectifying tower, condenser on the top of tower and oil-water separator. It uses metal halide as catalyst, an initial molar ratio of lactic acid and ethanol being 1:1 and benzene being 30%-50% of lactic acid ammonium weight.

Arkema (FR)

(Tretjak et al., 2006; Tretjak and Teissier, 2004)

Process that relates to a continuous method to produce ethyl lactate from the esterification between lactic acid and ethanol in the presence of a catalyst (H2SO4 98%) ; this method consists in continuously extracting a mixture comprising ethyl lactate, ethanol, water and different heavy

products from the reaction medium at partial lactic acid conversion rate and, then, fed the mixture to a reduced-pressure flash separation, producing an overhead stream containing a mixture of ethyl lactate, ethanol and water, that is subjected to a fractional distillation column. A purity higher than 94.6 % of ethyl lactate is reported for an initial ethanol/lactic acid molar ratio equal to 2.5; esterification carried out at 80ºC; flash separation at 85ºC and 50 mbar, and fractional distillation at a column bottom temperature of 155°C and top temperature of 77.2ºC.

Arkema (FR)

(Martino-Gauchi and Teissier, 2004; Martino-Gauchi and

Teissier, 2007)

Continuous method for preparing ethyl lactate which consists in reacting lactic acid with ethanol (ethanol/lactic acid molar ratio higher than 2.5) in the presence of a catalyst (H2SO498%) at a reflux of the reaction medium of about 100ºC under pressure ranging between 1.5 to 3 bars.

This method is characterized by the continuous extraction of a near-azeotropic water/ethanol gas mixture from the esterification reaction medium, followed by dehydration of this mixture using molecular sieves and recuperation from the dehydration mixture an ethanol gas stream capable of being recycled to the esterification reaction medium and a flow consisting of water and ethanol which is fed to a distillation column.

Board of Trustees of Michigan State

(Miller et al., 2006)

Lactic acid esterification by continuous countercurrent reactive distillation with alcohols, especially ethanol (to produce ethyl lactate). In this invention recycle of dimmers and trimmers and other oligomers of lactic acid are provided in order to improve yields. For absolute ethanol fed near to the bottom of the column at 82ºC and lactic acid solution (85 wt % in water) fed near to the top of column at 25ºC (molar ratio of ethanol to lactic acid of 3.3) it is reported a lactic acid conversion of 83% and a ethyl lactate yield of 82%.

Roquette Freres

(Fuertes et al., 2008)

Method for preparing a lactic acid ester composition based on a lactic acid composition involving two steps: (a) transforming of the composition into a lactic acid oligomeric composition; (b) mixing and reacting the oligomeric composition with an alcohol, in the presence of a transesterification catalyst, to esterify all or part of the lactic acid contained in the oligomeric composition. This invention also discloses the use of ethyl lactate as solvent for preparing gelified compositions.

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The esterification reaction is usually catalyzed by a strong acid, being most common sulphuric acid. However, the use of solid acid catalysts, as ion exchange resins, is also mentioned. In order to overcome equilibrium limitations excess of one reactant is commonly applied, normally the alcohol. Another technique is the use of a solvent, as benzene, substantially immiscible with water in order to extract the ester. Reaction and separation are, in almost all patented processes, separated steps, being distillation the most used separation technology.

2.3.3 Reactive Separations

In the last years, chemicals, petrochemicals and pharmaceuticals industries have been gone through a permanently increasing interest in the development of hybrid processes combining reaction and separation mechanisms into a single, integrated operation known as ‘reactive separation’. The combination of the two stages into a single unit brings important advantages, such as energy and capital cost reductions, increased yield and removal of some thermodynamic restrictions, e. g. azeotropes. A variety of separation principles and concepts can be incorporated into a reactor, see Figure 2.5.

Figure 2.5 Separation functions integrated into a reactor.

Important examples of reactive separations are reactive distillation, reactive absorption, reactive extraction or reactive membrane separation. Until now, such processes have had industrial application, mainly in areas like the homogeneously catalysed synthesis of acetates and the heterogeneously catalysed production of fuel additives. The potential is much wider; however, optimal functioning depends on careful process design, with appropriately selected

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column internals, feed locations and catalyst placement. Greater understanding of the general and particular features of the process behaviour is equally essential.

2.3.3.1 Reactive Distillation (RD)

Reactive distillation (RD) is an unit operation that combines chemical reaction and distillation within a single vessel, thereby reducing equipment and recycle costs. A typical RD column is shown in Figure 2.6. Other advantages offered by reactive distillation include high selectivity, reduced energy uses, and reduction or elimination of solvents (Malone and Doherty, 2000). It is an effective method that has considerable potential for carrying out equilibrium-limited reactions, such as esterification and ester hydrolysis reactions; conversion can be increased far beyond chemical equilibrium conversion due to the continuous removal of reaction products from the reactive zone.

B D Reactive Section A C

Figure 2.6 Typical Reactive Distillation Column (T < T < T < T ).b,C b,B b,A b,D

Reactive distillation has received much attention in the last years (Sundmacher and Kienle, 2002; Taylor and Krishna, 2000; Tsai et al., 2008). It has been used for the esterification of fatty acids (Dimian et al., 2008; Steinigeweg and Gmehling, 2003), as well as being devised as a new method to clean industrial water from acetic acid (Bianchi et al., 2003). The RD technology was, also, applied on the ethyl lactate synthesis, first by Asthana and collaborators (Asthana et al., 2005), where it is reported higher lactic acid conversions (>95 %) and good ethyl lactate yields (>85 %), and, more recently, by Gao and co-workers (Gao et al., 2007).

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18 CHAPTER 2. State of the art on Green Solvent Ethyl Lactate

However, applications of this technology in industry are still limited to a few reactive systems, mainly etherification (e.g. MTBE), esterification (e.g. methyl acetate), and alkylation (e.g. ethylbenzene or cumene) (Tuchlenski et al., 2001).

The production of methyl acetate is a classic example of successful RD (Agreda and Partin, 1984; Agreda et al., 1990). Conventional processes use one or more liquid-phase reactors with large excess of one reactant in order to achieve high conversions of the other. A typical flow sheet of a conventional process for the methyl acetate production is shown in Figure 2.7 in which the reaction section is followed by eight distillation columns, one liquid-liquid extractor and a decanter. This process requires a large capital investment, high energy costs and a large inventory of solvents. In the reactive distillation process for methyl acetate, the entire process is carried out in a single unit (see Figure 2.7), which represents one-fifth of the capital investment of the conventional process and consumes only one-fifth of the energy (Krishna, 2002).

Figure 2.7 Task-integrated methyl acetate column is much simpler than conventional plant (Stankiewicz and Moulijn, 2000).

In spite of all advantages of the RD technology, there are still some constraints and difficulties in its implementation, mainly due to volatility limitations. In order to maintain high concentrations of reactants and low concentrations of products in the reaction zone, the reactants and products must have suitable volatility. Also, it is necessary that both products have different boiling points to ensure the separation. The major disadvantage of RD

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technology, for exothermic reactions, is that chemical reaction has to show significant conversion at distillation temperature (Pöpken et al., 2000).

2.3.3.2 Simulated Moving Bed Reactor (SMBR)

Simulated Moving Bed (SMB) systems are used in industry for separations that are either impossible or difficult using traditional techniques. This technology uses differences in the adsorptivity of the different components involved rather than differences in their volatility, being an interesting alternative to distillation when the species involved exhibit small volatility differences, are non-volatile or are sensitive to temperature, as in the case of many fine chemical and pharmaceutical applications.

The combination of SMB and chemical reaction has been, in the last years, a subject of considerable attention in the scientific research, being this integrated reaction-separation technology called Simulated Moving Bed Reactor (SMBR). A schematic diagram of a SMBR unit is presented in Figure 2.8 where a reaction of type A+B↔C+D is considered, for the case of D being more adsorbed than C. The SMBR consists of a set of columns connected in series that are packed with a solid, which acts as both adsorbent and catalyst. Typically, there are two inlets (feed and desorbent) and two outlets (extract and raffinate). The component A is used as reactant and desorbent, therefore it is introduced in the system in the feed and desorbent streams. The other reactant B is used as feed. The products D and C are collected in the extract and the raffinate, respectively, since D is more adsorbed than C. At regular time intervals, called switching time period, all streams are switched for one bed distance in direction of the fluid flow. A cycle is completed when the number of switches is equal to a multiple of the columns number. In this way, the countercurrent motion of the solid is simulated with a velocity equal to the length of a column divided by the switching time. According to the position of the inlet and outlet stream the unit can be divided in four sections. In section I, positioned between the desorbent and extract nodes, the adsorbent is regenerated by desorption of the more strongly adsorbed product (D) from the solid. In section II (between the extract and feed node) and section III (between the feed and raffinate node) the reaction is taking place and products (C and D) are formed. The more strongly adsorbed product D is adsorbed and transported with the solid phase to the extract port. The less strongly adsorbed product C is desorbed and transported with the liquid in direction of the raffinate port. In section IV, positioned between the raffinate and desorbent node, the desorbent is regenerated before being recycled to section I.

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Desorbent (A) Raffinate (A+C)

Extract

(A+D) Direction of fluid flow

and port switching

1 2 3 4 5 6

12 11 10 9 8 7

Feed

(A+B)

A + B Ù C + D

Figure 2.8 Scheme of a Simulated Moving Bed Reactor (SMBR).

The simultaneous reaction and separation in a SMBR has shown that considerable improvements in some processes performance can be achieved, as for example, on the synthesis of acetals (Pereira et al., 2008; Rodrigues and Silva, 2005; Silva and Rodrigues, 2005), fructose (Azevedo and Rodrigues, 2001; Da Silva et al., 2005; Zhang et al., 2004), lactosucrose (Kawase et al., 2001; Pilgrim et al., 2006), methylacetate (Lode et al., 2003) and MTBE (Zhang et al., 2001). In the last years, cation exchange resins are being widely used as catalyst of esterifications and acetalizations. Moreover, those resins adsorb selectively water, a by-product of those kind of reactions. Therefore, combining these two properties of acidic resins, and knowing that esterifications are reversible reactions, chromatographic reactors appear as promising technologies; in particular the SMBR, since the products are continuously separated and removed from the reaction medium, leading to complete conversion. This has motivated several studies on esterification reactions by means of the SMBR technology; examples are the esterification of acetic acid with methanol (Lode et al., 2003; Yu et al., 2003), ethanol (Mazzotti et al., 1996) and β-phenethyl alcohol (Kawase et al., 1996), and the esterification of acrylic acid with methanol to form methyl acrylate (Ströhlein et al., 2006).

Although the SMBR technology allows 100 % of conversion with 100 % of recovery of the desired product, a further step is necessary to separate the product from the raffinate mixture, and to recover the reactant A, used as desorbent, from both extract and raffinate streams, in order to recycle it to the SMBR unit.

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2.3.3.3 Pervaporation Membrane Reactor (PVMR)

Pervaporation is one of the membrane processes that can be employed for the separation of liquid mixtures that are difficult or not possible to separate by conventional methods, such as distillation. One example is the application of pervaporation to break the azeotrope of the mixture ethanol-water, where it is much more economical to use pervaporation or vapour permeation than other conventional methods (see Table 2.4).

Table 2.4 Dehydration costs of ethanol from 99.4 to 99.9 vol % by different methods (Drioli and Romano, 2001).

Utilities Permeation Vapor

($/ton) Pervaporation ($/ton) Entrainer Distillation ($/ton) Molecular Sieve Adsorption ($/ton) vapor - 12.8 120.0 80.0 electricity 40.0 17.6 8.0 5.2 cooling water 4.0 4.0 15.0 10.0 Entrainer - - 9.6 - Replacement of membranes and molecular sieves 19.0 30.6 - 50.0 total costs 63.0 65.0 152.6 145.2

The pervaporation process has significant separation potential for various types of solutions, being specially suited for organic-water and organic-organic separations (Feng and Huang, 1996; Fleming and Slater, 1992; Huang and Rhim, 1991; Neel, 1991; Neel, 1995). It is used to separate a liquid mixture by partly vaporizing it through a nonporous permselective membrane, as shown in Figure 2.9. The feed liquid mixture is allowed to flow along one side of the membrane, and a fraction of it, the “permeate”, is recovered in the vapour state on the other side of the membrane, by means of vacuum or sweep gas. The mass transport through the membrane is induced by maintaining a low vapour pressure on the permeate side, eliminating thereby the effect of osmotic pressure. The permeate stream, enriched in the most

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permeating component, might be then condensed in order to recover it. The remaining feed that does not permeate through the membrane, called the “retentate”, is depleted in the permeating component (Neel, 1995).

Feed Retentate

Permeate

Pervaporation membrane Liquid

Vapor

Figure 2.9 Schematic representation of the pervaporation process.

There are a number of reviews on pervaporation processes (Dutta et al., 1996; Song et al., 2004; Van Hoof et al., 2004) and also on pervaporation membrane reactors (PVMRs) (Lim et al., 2002; Lipnizki et al., 1999; Waldburger and Widmer, 1996), since its application to equilibrium-limited reactions improves conversion by selectively removing one reaction product e.g. (Benedict et al., 2006; Castanheiro et al., 2006; David et al., 1991; Domingues et al., 1999; Lauterbach and Kreis, 2006; Peters et al., 2005a; Peters et al., 2005b; Sanz and Gmehling, 2006; Tanaka et al., 2002). PVMRs are, therefore, a type of membrane reactors that combines chemical reaction and separation by pervaporation, which is usually implemented by two different semi-batch processes: (i) the pervaporation unit (PV) is coupled to the reactor, i.e., the PV unit is an external process unit (see Figure 2.10a); and (ii) the reactor and the membrane are integrated in the same unit (see Figure 2.10b).

Although there is a recent interest in PVMRs, its discovery goes back to 1960, according to the first patent for a continuous process that integrates reaction and pervaporation in the same unit applied to an esterification reaction catalyzed by an acid ion exchange resin and using water selective membrane in order to remove it from the reaction zone and therefore enhancing the conversion (Jennings and Binning, 1960). Also, in 1986, another process was patented for the acetic acid esterification reaction with ethanol (Pearce, 1986), which reports complete conversion of the acetic acid by using a pervaporation membrane reactor consisting of two half-cells with a flat membrane disk (commercial PVA or Nafion) placed in the middle, as shown in Figure 2.11.

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Figure 2.10 Layout of a Semi-batch Pervaporation Membrane Reactor (SBPVMR): (a) external pervaporation unit;

(b) membrane and reactor in the same unit.

Figure 2.11 Experimental set-up for the pervaporation membrane reactor (Pearce, 1986).

Even though PVMRs are finding broad uses, esterifications appear to be a key application (Marcano and Tsotsis, 2002). The esterification reactions are a typical example of equilibrium-limited reaction that produces by-product water. Considering a catalytic esterification reaction scheme of the type:

H

A B

+

_←⎯⎯

⎯⎯→

+

C D

+

where C is the desired ester product and D is the by-product (water). Due to the thermodynamic equilibrium limitation of the esterification reaction, a conventional reactor

Membrane

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24 CHAPTER 2. State of the art on Green Solvent Ethyl Lactate Feed (A+B) Retentate (C) Permeate (D) Membrane

will operate at low conversion; however, if a membrane is integrated in the reactor, as shown in Figure 2.12, wherein the water is removed through the permselective membrane from the reaction zone to the other side of the membrane, the reaction will proceed in the forward direction and therefore high conversion is expected to be attained in a reasonably short time.

Figure 2.12 Schematic representation of a membrane reactor for by-product withdrawal in a reversible reaction.

Zhu and collaborators studied the esterification reaction of acetic acid with ethanol both by experiment and simulation in a continuous-flow PVMR using a polymeric/ceramic composite membrane (Zhu et al., 1996). The same reaction was studied in a continuous tube membrane reactor (Waldburger and Widmer, 1996). This reaction was also studied in a PVMR, housing the reactor and membrane in the same unit, applying a zeolite T-membrane since it is stable under acidic conditions (Tanaka et al., 2001). Nafion tubular membranes, which also act as catalyst, were applied for the esterification of acetic acid with methanol and n-butanol, where the equilibrium conversions of 73 % and 70 % were increased to 77 % and 95 %, respectively (Bagnell et al., 1993). The esterification of acetic acid with butanol was more significantly improved than with methanol, due to the higher membrane selectivity towards water in butanol/water system. This particular esterification was also studied using Zr(SO4)·4H2O as catalyst and using cross-linked polyvinyl alcohol (PVA) membranes (Liu et al., 2001; Liu and Chen, 2002). Experiments and simulations were conducted to investigate the effects of several operating parameters, such as reaction temperature, initial molar ratio of acetic acid to n-butanol, ratio of the membrane area to the reacting mixture volume and catalyst concentration.

Regarding the operating modes of PVMR, semi-batch esterification process coupled by pervaporation is the most used (Xuehui and Lefu, 2001), being applied for the synthesis of ethyl tert-butyl ether (ETBE) from tert-butyl alcohol (TBA) and ethanol (Kiatkittipong et al., 2002); and applied for the esterification of acetic acid with isopropanol leading to conversions higher than 90 % (Sanz and Gmehling, 2006). The esterification of lactic acid and succinic

Process intensification for the green solvent ethyl lactate production based on simulated moving bed and pervaporation membrane reactors