Azeotropes ethanol dehydration

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

Ethanol dehydration 231 Simulation study 50% cost reduction in comparison with conventional azeotropic distillation... 

EXAMPLE 10.5 ETHANOL DEHYDRATION BY EORMING A TERNARY AZEOTROPE... 

Following the experience gained in ethanol dehydration, in 1988 the first plant started its operation in the chemical industry for the dehydration of an ester. Soon other applications for dewatering followed, covering today a broad range of solvents and solvent mixtures, especially those forming azeotropes with water. In 1994 a first plant started its operation in which water was continuously removed from a reaction mixture, in order to shift the reaction equilibrium towards the wanted product, in this case a diester [15], and, by nearly totally converting one of the educts, to increase the yield and to facilitate the downstream purification of the product. 

One method for ethanol dehydration is heterogeneous azeotropic distillation, which has been used for many decades. A suitable light entrainer component (benzene, cyclohexane, isooctane, ethylene glycol, and so on) is added to modify the relative volatilities. The water is driven overhead with the entrainer and a high-piu ity ethanol bottoms stream is produced in the azeotropic column. The overhead vapor is condensed and fed to a decanter. The organic phase is refluxed back to the column. The aqueous phase is fed to another column that produces a bottoms product of high-purity water and a distillate that is recycled back to the azeotropic column. A third column in the front end of the process is used to preconcentrate the low-concentration stream from the fermenter up to a concentration closer to the azeotrope before feeding this into the azeotropic column. 

In addition to heterogeneous azeotropic distillation, several alternative methods are available for ethanol dehydration such as extractive distillation, adsorption, and pervapo-ration. A comprehensive review of the subject, including 302 references, has been presented by Vane. A recent paper by Kiss and Paul claims that the heterogeneous azeotropic distillation process is more economical than adsorptive drying because of the large amount of energy required to regenerate the adsorbent. 

It should be noted that the dehydration of the ethanol by the azeotropic rectification requires considerable operational and energy expenses. Ethanol dehydration technologies using the adsorption on molecular sieves and evaporation through the membrane are less power consuming ones. However, the ethanol dehydration by the evaporation through the membrane requires significant capital investment and the smooth/uninterrupted operation of the factory. 

Anhydrous ethanol means an ethyl alcohol that has a purity of >99%, exclusive of added denaturants, meeting the requirements of ASTM D4806. Hydrous (or wet, also sometimes known as azeotropic) ethanol is the most concentrated grade of ethanol that can be produced by simple distillation, without the further dehydration step necessary to produce anhydrous (or dry) ethanol. 

Besides the methods illustrated so far in this book, there are other ways for separating azeotropes. One way is to react the azeotrope away in a reactive distillation column to form other useful products. The design and control of various reactive distillations have been extensively studied in a recent book by Luyben and Yu. Another way commonly used in ethanol dehydration is to use the hybrid distillation-adsorption process. In this process, distillation is used to purify the mixture to a composition near the ethanol-water azetrope, and then an adsorption unit (e.g., molecular sieves) is used to adsorb the remaming water so that anhydrous ethanol can be obtained. The key technology in this process is the performance of the adsorbent material in removing water from the mixture and is beyond the scope of this book. 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

A selection of industrial appHcations of extractive distillation includes (/) the separation of the / -butane—butadiene azeotrope in mixed C -hydrocarbon streams using furfural [98-01-17, as the solvent (36) (2) the dehydration of ethanol using ethylene glycol [107-21-1] (37—39) (J)... 

Consider azeotropic distillation to dehydrate ethanol with benzene. Initial steady-state conditions are as shown in Fig. 13-108. The overhead vapor is condensed and cooled to 298 K to form two hquid phases that are separated in the decanter. The organic-rich phase is returned to the top tray as reflux together with a portion of the water-rich phase and makeup benzene. The other portion of the water-rich phase is sent to a stripper to recover organic compounds. Ordinarily, vapor from that stripper is condensed and recycled to the decanter, but that coupling is ignored here. 

Ethanol [64-17-5] M 46.1, b 78.3 , d 0.79360, d 0.78506, n 1.36139, pK 15.93. Usual impurities of fermentation alcohol are fusel oils (mainly higher alcohols, especially pentanols), aldehydes, esters, ketones and water. With synthetic alcohol, likely impurities are water, aldehydes, aliphatic esters, acetone and diethyl ether. Traces of benzene are present in ethanol that has been dehydrated by azeotropic distillation with benzene. Anhydrous ethanol is very hygroscopic. Water (down to 0.05%) can be detected by formation of a voluminous ppte when aluminium ethoxide in benzene is added to a test portion. Rectified... 

Volkov (1994) has given a state-of-the-art review on pervaporation. A number of industrial plants exist for dehydration of ethanol-water and (.vwpropanol-water azeotropes, dehydration of ethyl acetate, etc. There is considerable potential in removing dissolved water from benzene by pervaporation. The recovery of dis.solved organics like CH2CI2, CHCI3, CCI4, etc. from aqueous waste streams also lends itself for pervaporation and pilot plants already exist. 

Norman, W. S. Trans. Inst. Chem. Eng. 23 (1945) 66. The dehydration of ethanol by azeotropic distillation. Ibid. 89. Design calculations for azeotropic dehydration columns. 

Hydrophilic membranes with a preferential permeation of water are mainly used for the dehydration of organic solvents with an emphasis on azeotropic mixtures. Membranes for the removal of small alcohol molecules like methanol and ethanol are also of a hydrophilic nature. 

Ethanol (EtOH) [5h] Commercial products are pure enough for most purposes. The impurities are MeOH, PrOH, Ac, water, etc. In order to reduce 2000 ppm of water in absolute EtOH, about 5% of hexane or cyclohexane is added to the EtOH and the mixture is fractionated to remove the aqueous azeotrope. By this step, the water content is reduced to < 500 ppm. The dehydrated EtOH is then slowly passed through a column of dry molecular sieves (4A) to reduce the water content to 7 ppm. 

The liquid product streams are fed to a distillation system to remove the light impurities and to recover the ethanol as a 95% volume ethanol—water azeotrope. To produce anhydrous ethanol, the ethanol—water azeotrope is fed to a dehydration system. 

Several hundred plants have been installed for the dehydration of ethanol by pervaporation. This is a particularly favorable application for pervaporation because ethanol forms an azeotrope with water at 95 % and a 99.5 % pure product is needed. Because the azeotrope forms at 95 % ethanol, simple distillation does not work. A comparison of the separation of ethanol and water obtained by various pervaporation membranes and the vapor-liquid equilibrium line that controls separation obtained by distillation is shown in Figure 9.9 [40], The membranes... 

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