Liquid phase esterification, equilibrium

One of the most important characteristics of IL is its wide temperature range for the liquid phase with no vapor pressure, so next we tested the lipase-catalyzed reaction under reduced pressure. It is known that usual methyl esters are not suitable for lipase-catalyzed transesterification as acyl donors because reverse reaction with produced methanol takes place. However, we can avoid such difficulty when the reaction is carried out under reduced pressure even if methyl esters are used as the acyl donor, because the produced methanol is removed immediately from the reaction mixture and thus the reaction equilibrium goes through to produce the desired product. To realize this idea, proper choice of the acyl donor ester was very important. The desired reaction was accomplished using methyl phenylth-ioacetate as acyl donor. Various methyl esters can also be used as acyl donor for these reactions methyl nonanoate was also recommended and efficient optical resolution was accomplished. Using our system, we demonstrated the completely recyclable use of lipase. The transesterification took place smoothly under reduced pressure at 10 Torr at 40°C when 0.5 equivalent of methyl phenylthioacetate was used as acyl donor, and we were able to obtain this compound in optically pure form. Five repetitions of this process showed no drop in the reaction rate (Fig. 4). Recently Kato reported nice additional examples of lipase-catalyzed reaction based on the same idea that CAL-B-catalyzed esterification or amidation of carboxylic acid was accomplished under reduced pressure conditions. ... 

Esterification is the first step in PET synthesis but also occurs during melt-phase polycondensation, SSP, and extrusion processes due to the significant formation of carboxyl end groups by polymer degradation. As an equilibrium reaction, esterification is always accompanied by the reverse reaction being hydrolysis. In industrial esterification reactors, esterification and transesterification proceed simultaneously, and thus a complex reaction scheme with parallel and serial equilibrium reactions has to be considered. In addition, the esterification process involves three phases, i.e. solid TPA, a homogeneous liquid phase and the gas phase. The respective phase equilibria will be discussed below in Section 3.1. 

Transesterification is the main reaction of PET polycondensation in both the melt phase and the solid state. It is the dominant reaction in the second and subsequent stages of PET production, but also occurs to a significant extent during esterification. As mentioned above, polycondensation is an equilibrium reaction and the reverse reaction is glycolysis. The temperature-dependent equilibrium constant of transesterification has already been discussed in Section 2.1. The polycondensation process in the melt phase involves a gas phase and a homogeneous liquid phase, while the SSP process involves a gas phase and two solid phases. The respective phase equilibria, which have to be considered for process modelling, will be discussed below in Section 3.1. 

When lipases are used for enzymatic conversions, the enzyme is mainly active at a phase boundary, which can effectively be provided by a membrane. Additionally, for conversions requiring two phases (e.g. fat splitting [84—86] and esterifications [87]), the membrane also keeps the two liquid phases (an oil and an aqueous phase, respectively) separated. This is schematically depicted in Fig. 13.11. The equilibrium reactions involved are... 

The law of mass action, the laws of kinetics, and the laws ol distillation all operate simultaneously in a process of this type. Esterification can occur only when the concentrations of the acid and alcohol are in excess of equilibrium values otherwise, hydrolysis must occur The equations governing the rate of the reaction and the variation of the rale constant (as a function of such variables as temperature, catalyst strength, and proponion of reactants) describe Ihe kinetics of the liquid-phase reaction. The usual distillation laws must he modified, since must esterifications arc somewhat exothermic and reaction is occurring on each plate. Since these kinetic considerations are superimposed on distillation operations, each plate must be treated separately by successive calculations after Ihe extent of conversion has been determined. See also Distillation. 

Therefore, it seems that tridireotional zeolites HY and HB are the most promi sing ones for liquid phase reactions. In the case of HB zeolites, two items deserve special comments. Firstly, the yield (82%) of ethyl phenylacetate for the equimolar esterification of phenylacetic acid and ethanol in the presence of the fl-10 sample is substantially higher than that of the equilibrium (69%) at the same temperature and solvent (Table 3). Analogous results have been already observed with dealuminated acidic Y faujasites and can be due to zeolite water adsorption and/or to the hydrophobicity of the in surfaces (ref. 2). The hydrophobic character of high silica... 

As mentioned, esterification is reversible, and with ethanol and ethanoic acid the equilibrium constant for the liquid phase is about 4 (AG° = —0.8 kcal) at room temperature, which corresponds to 66% conversion to ester ... 

All of these hydrolysis reactions are energetically favorable (AC0 < 0), but they do not occur directly because ATP reacts slowly with water. However, hydrolysis of ATP is the indirect result of other reactions in which it participates. For example, as we showed in Section 15-4D, equilibrium for the direct formation of an ester from a carboxylic acid and an alcohol in the liquid phase is not very favorable (Equation 15-2). However, if esterification can be coupled with ATP hydrolysis (Equation 15-3), the overall reaction (Equation 15-4) becomes much more favorable thermodynamically than is direct esterification. 

For a higher accuracy of the isotherm, the activities in the liquid phase instead of the concentrations can be used. If a thermodynamic model for the computation of the solid-phase activities exists, the equilibrium concentration q and qi can also be calculated from the condition of equal activity, as was done for example for the esterification of acetic acid on Amberlyst 15 using the Flory-Huggins activity model for the solid phase and a UNI FAC model for the liquid phase [17]. 

Lehtonen et al. (1998) considered polyesterification of maleic acid with propylene glycol in an experimental batch reactive distillation system. There were two side reactions in addition to the main esterification reaction. The equipment consists of a 4000 ml batch reactor with a one theoretical plate distillation column and a condenser. The reactions took place in the liquid phase of the reactor. By removing the water by distillation, the reaction equilibrium was shifted to the production of more esters. The reaction temperatures were 150-190° C and the catalyst concentrations were varied between 0.01 and 0.1 mol%. The kinetic and mass transfer parameters were estimated via the experiments. These were then used to develop a full-scale dynamic process model for the system. 

Process Applications The prodnction of esters from alcohols and carboxylic acids illustrates many of the principles of reactive distillation as applied to equilibrium-limited systems. The true thermodynamic equilibrium constants for esterification reactions are nsnally in the range of 5 to 20. Large excesses of alcohols mnst be nsed to obtain acceptable yields, resulting in large recycle flow rates. In a reactive distillation scheme, the reaction is driven to completion by removal of the water of esterification. The method used for removal of the water depends on the boiling points, compositions, and liquid-phase behavior of any azeotropes formed between the prodncts and reactants and largely dictates the structure of the reactive distillation flow sheet. 

Surprisingly, there is limited nonproprietary experimental data on methanol esterification with acetic acid (29). Studies have been confined to liquid-phase systems distant from equilibrium (30), in regions where hydrolysis is unimportant. A physical study of the ternary methanol—methyl acetate—water system is useful for design work (31). Methyl acetate and methanol form an azeotrope which boils at 53.8°C and contains 18.7% alcohol An apparent methanol—water azeotrope exists, boiling at 64.4°C and containing about 2.9% water. These azeotropes seriously complicate methyl acetate recovery. Methyl acetate is quite soluble in water, and very soluble in water—methanol mixtures, hence two liquid phases suitable for decanting are seldom found. 

Esterification is finally an equilibrium reaction (35 per cent methyl methacrylate), which can be continued to completion by removing one or both of the products obtained as soon as they are formed. It takes place preferably in the liquid phase, in the presence of sulfuric acid or cation exchange resins as a catalyst, with a slight excess of methanol (1.2/1 in mol), at temperatures (110 to 115°Q apd pressures (30 to 50 kPa absolute) designed to limit polymerization reactions. The addition of an inhibitor (such as hydro-quinone) is also practised. With residence time of about 1 h. once-through conversion is total and the molar yield is close to 99 per cent. 

As an example of a more complicated case than the two step sequence, we will discuss the esterification of a carboxylic acid with an alcohol. This is a very old and well-known category of homogeneous liquid-phase reactions. The esters of carboxylic acids are of an enormous practical importance for example, millions of tons of polyesters are produced via the reaction of dicarboxylic acids with diols and a wide variety of mono- and di-esters are used in the production of fine and specialty chemicals, such as pharmaceuticals, herbicides, pesticides and fragrances. The esterification reaction is a homogenous liquid-phase process where the limiting conversion of the reactants is determined by equilibrium. Typically the equilibrium constants of esterification reactions have values of 1-10, which implies that considerable amounts of reactants exist in the equilibrium mixture. 

The question of whether it is possible to predict the concentration dependence of from information on Ky via equation (4.22) is discussed here using the esterification of 1-butanol with acetic acid at 80 °C as an example. The experimental data given in Fig. 4.9 show that is a strong function of the composition for that liquid-phase reaction. The numbers for are in the range 2-8 and systematically vary with composition. In the studied system, there is a liquid-liquid miscibility gap intersecting with the chemical equilibrium surface. No measurements with liquid-liquid phase split were carried out in the present work. 

Table 11.10 Equilibrium compositions for esterification of ethanol (11.3.5) at 355 K and 1.0133 bar the system is in vapor-liquid equilibrium with liquid-phase x, vapor phase y, and overall composition z. V = 90 mol%...

NaA/polyelectrolyte multilayer-pervaporation membrane showing a greater stability under acidic conditions in comparison with a pure zeolite A membrane and maintaining a high selectivity for water over alcohols. For the same purpose, Kita et al. [181-183] proposed a zeolite T membrane, prepared by ex situ crystallization, for the per-vaporation-aided or vapor-permeation-aided esterification of acetic acid with ethanol. This membrane has a higher acid resistance and can be directly immersed in the liquid-phase reaction mixture. The conversions achieved exceed the equilibrium limit and reached to almost 100% after a stabilization period of 8 h. 

The activity coefficients Yi consider the deviation from an ideal liquid phase, and the fugacity coefficients (p sat the deviation of the pure vapor from an ideal gas. In Example 4.2.6, the equilibrium of a liquid phase reaction (esterification) of such a... 

Table 4.2.4 Calculation ofthe equilibrium of a liquid-phase reaction based on the Gibb s enthalpy of the gas-phase reaction and thermodynamic data for the vapor and the liquid phase example esterification of ethanol at 100 C. Data from Cmehlingand...

We inspect the esterification of ethanol with acetic acid as an illustrative example for the equilibrium of a liquid phase reaction ... 

Industrially important esterifications are the reactions of alcohols with saturated and unsaturated aliphatic c2u-boxylic acids, e.g., acetic acid, fatty acids and acrylic acid, and aromatic dicarboxylic acids such as terephth llic acid. These reactions may be carried out in both liquid and vapor phases. Esterification reactions are usually limited by equilibrium particularly in liquid phase. Continuous removal of water produced and/or operation with an excess of one of the reactants are necessary to obtain higher yields of ester. Vapor-phase esterification is favored from this standpoint, and numerous studies have been directed toward the use of solid acid catalysts. " ... 

The kinetics and equilibrium of autocatalyzed and ion exchange resin (Amberlyst-15) catalyzed esterification of acetic acid with methanol and hydrolysis of methyl acetate were studied by Popken et. al. (2000) in a temperature range of 303 - 343 K. The homogeneous reaction has been described with a simple power-law model. To compare pseudo-homogeneous and adsorption-based kinetic models for the heterogeneously catalyzed reaction, independent binary liquid phase adsorption experiments were used to estimate the adsorption equilibrium constants to keep the number of adjustable parameters the same for each model. 

Another reactive separation processes studied for ethyl lactate production is the catalytic extractive reaction (Figure 20.4.7). In this case, the esterification is performed in a biphasic liquid solvent system composed by a reactive polar liquid phase which contains the esterification constituents lactic acid, eflianol and catalyst, and an extractive organic solvent selective of the ester. Therefore, ethyl lactate should preferably be dissolved in the extractive organic phase shifting, in this way, the reaction equilibrium to ester formation. The immiscible extractive solvent is an aromatic or other solvent like toluene, benzene or diethyl ether, among others. Nevertheless, it has also been used an immiscible solvent based on fatty acid methyl ester, but in this case, the procedure represents a method to produce an organic biosolvent and not just ethyl lactate as solvent. 

Table 6.6 reports on the molar conversions of the esterifications of geraniol with hexanoic acid and of cinnamyl alcohol with butyric acid obtained using these two biocatalysts under different conditions. In order to exclude possible adsorption of reagents and/or products on the biocatalysts, a procedure of extensive rinsing of the biocatalysts was applied and the hquid phases were analyzed by high-performance liquid chromatography. No appreciable presence of products nor reagents was observed. The effective achievement of the equilibrium was confirmed in all cases by the addition of further fresh biocatalyst after the reactions stopped. 

Mainly PV aided conversions have been studied and more in particular esterifications, a typical example of an equilibrium limited reaction with industrial relevance and well-known reaction mechanisms. " This hybrid process has already made it to several industrial applications. The thermodynamic equilibrium in such a reaction can be easily shifted and obtained in a shorter reaction time by removing one of the products. Pervaporation is especially interesting because it is not limited by relative volatility or azeotropes and energy consumption is generally low, because only the fraction that permeates undergoes the liquid/vapor phase change. It can also be operated at lower temperatures, which can better match the optimal conditions for reaction. 

Phases gas-liquid, gas-liquid catalytic solid, gas-liquid plus catalytic solid minimizes catalyst poisoning, lower pressure than fixed bed. Used for hydrogenation reactions and MTBE and acrylamide production. For example, 90% conversion via reactive distillation contrasted with 70% conversion in fixed-bed option. Liquid with homogeneous catalyst etherification, esterification. Liquid-liquid HIGEE for fast, very fast, and highly exothermic liquid-liquid reactions such as nitrations, sulfonations, and polymerizations. Equilibrium conversion <90%. Use a separate prereactor when the reaction rate at 80% conversion is >0.5 initial rate. The products should boil in a convenient temperature range. The pressure and temperature for distillation and reaction should be compatible. 

We consider esterification of ethanol with acetic acid to form ethyl acetate and water. This reaction has been much used for testing algorithms that perform simultaneous phase and reaction-equilibrium calculations. At ambient pressures, we assume the reaction occurs in a vapor phase but depending on the exact values for T and P, the mixture may exist as one-phase vapor, one-phase liquid, or a two-phase vapor-hquid system. The feed contains equimolar amounts of ethanol and acetic acid. The problem is to determine the equilibrium state the phases present and their compositions at 1.0133 bar and temperatures near 355 K. 

A general esterification reaction consists of reacting an alcohol with an acid in the presence of a catalyst (such as sulfonic acid) to produce the ester and water. This is an equilibrium reaction and leads to low conversion. The catalyst is usually neutralized with inorganic base after the completion of the reaction. If carried out in a countercurrent reactor under two-phase conditions, this reaction has many benefits. For example, conversion of maleic anhydride to dialkyl maleates or fatty acids to fatty acid esters is performed in a column packed with a solid catalyst. Liquid (acid) flows down the column from the top. Alcohol vapor flows upward from the bottom and absorbs water that is formed and carries it up (see Fig. 6.30). The removal of water by the alcohol drives the... 

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