Azeotropic distillation ethanol/water/benzene process

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In azeotropic distillation, the third component forms an azeotrope with the system that becomes either the top or bottom product. The azeotrope is then separated into the agent and component. Sometimes such separation must be done using another process such as liquid extraction. Some typical systems (the azeotroping agent in parentheses) are acetic acid-water (butyl acetate), and ethanol-water (benzene). 

The first successful appHcation of heterogeneous azeotropic distillation was in 1902 (87) and involved using benzene to produce absolute alcohol from a binary mixture of ethanol and water. This batch process was patented in 1903 (88) and later converted to a continuous process (89). Good reviews of the early development and widespread appHcation of continuous azeotropic distillation in the prewar chemical industry are available (90). 

Water and ethanol form a low boiling point azeotrope. So, water cannot be completely separated from ethanol by straight distillation. To produce absolute (100 per cent) ethanol it is necessary to add an entraining agent to break the azeotrope. Benzene is an effective entrainer and is used where the product is not required for food products. Three columns are used in the benzene process. Column 1. This column separates the ethanol from the water. The bottom product is essentially pure ethanol. The water in the feed is carried overhead as the ternary azeotrope of ethanol, benzene and water (24 per cent ethanol, 54 per cent benzene, 22 per cent water). The overhead vapour is condensed and the condensate separated in a decanter into, a benzene-rich phase (22 per cent ethanol, 74 per cent benzene, 4 per cent water) and a water-rich phase (35 per cent ethanol, 4 per cent benzene, 61 per cent water). The benzene-rich phase is recycled to the column as reflux. A benzene make-up stream is added to the reflux to make good any loss of benzene from the process. The water-rich phase is fed to the second column. 

Keyes A process for separating water from ethanol, using azeotropic distillation with benzene. Invented in 1922 by D. B. Keyes. 

Figure 13.29. Composition profiles and flowsketches of two azeotropic distillation processes (adapted by King, 1980). (a) Separation of ethanol and water with benzene as entrainer. Data of the composition profiles in the first column were calculated by Robinson and Gilliland, (1950) the flowsketch is after Zdonik and Woodfield (in Chemical Engineers Handbook, McGraw-Hill, New York, 1950, p. 652). (b) Separation of n-heptane and toluene with methylethylketone entrainer which is introduced in this case at two points in the column (data calculated by Smith, 1963).

Water and ethanol form a low boiling point azeotrope hence, water cannot be completely separated from ethanol by conventional distillation. To produce absolute (100%) ethanol, it is necessary to add an entraining agent to break the azeotrope. Benzene is an effective entrainer and is used where the product is not required for food products. Three columns are used in the benzene process. 

Azeotropic distillation involves either an embedded azeotrope, present in the feed mixture, or a contrived azeotrope, formed by the addition of an extraneous component called an entrainer. Benzene-water may be separated into high-purity benzene and the benzene-water azeotrope this is frequently practiced to remove water from benzene when very dry benzene is needed for chemical processing. More commonly encountered are distillation separations that are enhanced through the addition of an entrainer to form an azeotrope. Perhaps the best known separation of this type is the production of anhydrous ethanol from the ethanol-water azeotrope. Here, benzene is added as the entrainer, with the result that a low-boiling ternary azeotrope is formed between benzene, ethanol, and water. This permits the higher-boiling ethanol to be taken from the bottom of the column. The distillate condenses to a heterogeneous mixture of benzene and alcohol-water phases. 

The older method to raise the azeotrope containing 95 per cent weight to 99.9 per cent ethanol consists in absorbing the water on lime (CaO). This costly process has been abandoned. Azeotropic distillation in the presence of benzene is employed today. In principle, simple low pressure distillation should allow the separation of alcohol and water, because the azeotrope disappears below 2kPa. For ethanol concentrations between 95 and 100 per cent, however, the liquid am vapor phase compositions are substantially the same, implying extremely high reflux rates and a large number of trays. 

An early application of azeotropic distillation was proposed by Guinot and Clark11 for the separation of ethanol and water by the use of benzene as the solvent. This process is based on the fact that benzene forms a ternary azeotrope with ethanol and water, which has a higher ratio of water to ethanol than does the ethanol-water azeotrope. In the first column, shown in Fig. 6-3, an azeotropic distillation is carried out. A two-phase liquid separation at 20°C in the decanter is used to concentrate the benzene in the reflux to the first column. The solvent benzene is recovered in the second column and water is removed in the third column. 

It is desired to locate a solvent for the liquid-liquid extraction of ethanol from its azeotrope with water. This dehydration has been carried out principally by heterogeneous azeotropic distillation using benzene, now known to be a carcinogen, as an entrainer. If such a solvent can be located, liquid-liquid extraction could become the preferred processing technique. 

For our second nonideal system, we look at a process that has extremely nonideal VLB behavior and has a complex flowsheet. The components involved are ethanol, water, and benzene. Ethanol and water at atmospheric pressure form a minimum-boiling homogeneous azeotrope at 351K of composition 90mol% ethanol. Much more complexity is introduced by the benzene/water system, which forms two liquid phases with partial miscibility. The flowsheet contains two distillation columns and a decanter. There are two recycle streams, which create very difficult convergence problems as we will see. So this complex example is a challenging simulation case. 

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. 

Ethanol isolation proceeds via distUlative processes. Owing to the formation of an ethanol/water azeotrope (95.5 mass% ethanol at 1 bar), the production of water-free ethanol requires the application of extractive distillation (typical entrainer benzene or cyclohexane, see Section 3.3.2.3). The isolation of water-free ethanol from fermentation is a relatively energy intense step. Even with a clever combination of several distillation columns working at different pressures, the energy input to produce 1 kg of bioethanol by fermentation is about 10 MJ (Baerns et al, 2006). In a few countries, where the production of bioethanol is particularly cheap (e.g., Brazil), there have also been attempts to convert bioethanol into chemicals in commercial scenarios. The production of ethylene from bioethanol is a potential option in this context. 

Ethanol is a monohydric primary alcohol. It melts at -117.3°C and boils at 78.5°C. It is miscible (i.e., mixes without separation) with water in all proportions and is separated from water only with difficulty ethanol that is completely free of water is called absolute ethanol. Ethanol forms a constant-boiling mixture, or azeotrope, with water that contains 95% ethanol and 5% water and that boils at 78.15°C since the boiling point of this binary azeotrope is below that of pure ethanol, absolute ethanol caimot be obtained by simple distillation. However, if benzene is added to 95% ethanol, a ternary azeotrope of benzene, ethanol, and water, with boiling point 64.9°C, can form since the proportion of water to ethanol in this azeotrope is greater than that in 95% ethanol, the water can be removed from 95% ethanol by adding benzene and distilling off this azeotrope. Because small amounts of benzene may remain, absolute ethanol prepared by this process is poisonous. 

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