Ethanol calculated azeotropic distillation

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. 

The methods used here to give the phase equilibria are reviewed, and the Azeotropic Distillation Program ADP/ADPLLE is described. Application of the program to calculate an azeotropic distillation problem is shown and discussed, and a sample computer output is given and is briefly discussed. Finally, calculated azeotropic distillation results are compared for dehydrating aqueous ethanol for the three entrainers, n-pentane, benzene, and diethyl ether. 

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).

The use of digital computers to carry out complete calculations in the design of separation processes has been the goal of many. To do this effectively, suitable methods for phase equilibria and tray-to-tray distillation calculations are required. Results calculated by the application of such methods to dehydrate aqueous ethanol mixtures using ethylene glycol as the extractive distillation solvent is discussed below. A brief review of the methods used for phase equilibria and enthalpies is followed by a discussion of the results from distillation calculations. These are compared for extractive distillation with corresponding results obtained by azeotropic distillation with n-pentane. 

The results of calculations for the two separation methods are summarized in Table VIII. Fewer trays are required in the azeotropic distillation column than the extractive distillation column. The heat loads are also smaller. The quality of the ethanol product is also slightly better for the azeotropic distillation method. Including the stripper for processing the aqueous phase, the total heat load for reboilers for the azeotropic distillation method is less than half that for the extractive distillation method. The total condenser load is roughly two-thirds that for the extractive distillation method. 

Although the azeotropic distillation scheme, using n-pentane, operates at a higher pressure, comparative calculations indicate this to be better than the extractive distillation scheme using diethylene glycol to dehydrate aqueous ethanol. 

A sample calculation is given showing the use of the program to calculate the azeotropic distillation of aqueous ethanol mixtures using the entrainer n-pentane (KNTRL 2, Option 7). 

The catalytic esterification of ethanol and acetic acid to ethyl acetate and water has been taken as a representative example to emphasize the potential advantages of the application of membrane technology compared with conventional distillation [48], see Fig. 13.6. From the McCabe-Thiele diagram for the separation of ethanol-water mixtures it follows that pervaporation can reach high water selectivities at the azeotropic point in contrast to the distillation process. Considering the economic evaluation of membrane-assisted esterifications compared with the conventional distillation technique, a decrease of 75% in energy input and 50% lower investment and operation costs can be calculated. The characteristics of the membrane and the module design mainly determine the investment costs of membrane processes, whereas the operational costs are influenced by the hfetime of the membranes. 

The following material balance calculations are carried out to determine the stream flow rates consistent with the required compositions. Designate the feed stream as F, the first column (high pressure) bottoms and distillate as B, and D and those of the second column (low pressure) as Bj and Dj/ respectively. Stream Dj is recycled and combined with F. The first column distillate composition will be taken at 44.4 mole% ethanol (slightly below the azeotrope) and that of the second column at 36.4 mole% ethanol (slightly above the azeotrope). The specifications are as follows ... 

G2. [Note This problem is quite extensive.] Biorefineries producing ethanol by fermentation have several distillation columns to separate the ethanol from the water. The first column, the beer still, is a stripping column that takes the dilute liquid fermenter product containing up to 15% solids and produces a clean vapor product that is sent to the main distillation column. The main column produces a distillate product between about 65 mole % and the ethanol azeotrope, and a bottoms product with very litde ethanol. The calculated diameter of the main distillation column is much greater at the top than elsewhere. To reduce the size and hence the cost of the main column, one can use a two-enthalpy feed system split the vapor feed into two parts and condense one part, then feed both parts to the main column at their optimum feed locations. This method reduces the vapor velocity in the top of the column, which reduces the calculated diameter however, a few additional stages may be required to obtain the desired purity. 

Calculate the percentage yield of alcohol. At the option of the instructor, determine the boiling point of the distillate using a microboiling-point method (Technique 13, Section 13.2). The boiling point of the azeotrope is 78.1°C. Submit the ethanol to the instructor in a labeled vial. ... 

Alcoholysis of a-monoalkylacetoacetic esters may be accomplished in excellent yields by heating with catalytic amounts of alkoxide in excess abs. ethanol. The startg. m. must be dry and free of acetoacetic ester.— E Ethyl a-allylaceto-acetate added to a soln. of some Na in 2.5 moles of abs. ethanol, refluxed under a column packed with glass helices and fitted with an automatic liquid-divider head, distillation started when the temp, at the head has dropped from 78 to 72 , the b. p. of ethanol-ethyl acetate azeotrope, the take off ratio adjusted to maintain the vapor temp, at 72-73 , and excess ethanol removed on a water bath when after 22 hrs. the calculated amount of azeotrope has been collected ethyl 4-pentenoate. Y 88%. F. e. s. J. J. Ritter and T. J. Kaniecki, J. Org. Chem. 27, 622 (1962). 

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