Column distillation passing streams

To produce a high purity product two distillation columns are operated in series. The overhead stream from the first column is the feed to the second column. The overhead from the second column is the purified product. Both columns are conventional distillation columns fitted with reboilers and total condensers. The bottom products are passed to other processing units, which do not form part of this problem. The feed to the first column passes through a preheater. The condensate from the second column is passed through a product cooler. The duty for each stream is summarised below ... 

The overhead stream from the column is a vapor that passes though a condenser. The condensed liquid is divided into two equal streams one is taken off as the overhead product (distillate) and the other (the reflux) is returned to the column. The bottom stream from the column is a liquid that is partially vaporized in a reboiler. The liquid stream emerging from the reboiler is taken off as the bottoms product, and the vapor is returned to the column as boilup. Negligible heat is lost from the column, so that the only places in the system where external heal transfer takes place are the condenser and the reboiler. 

The concentration of solute (A) in the outlet extract stream is limited to a relatively low value because the outlet extract stream is a passing stream with feed On+i. So the maximum concentration of A in Vn will occur when the column is operated at the minimum solvent flowrate. As with distillation, this limitation can be overcome by using... 

For certain types of distillation, an inert hot gas is introduced directly into the base of the column. Open steam, for example, can be used if one of the components in the mixture is water, or if the water can form a second phase thereby reducing the boiling point, as in the steam distillation of fats where heat is supplied by live superheated steam, no reboiler being used. In this application, Qb of Fig. 8.9 is a stream of composition y = 0, which with x=Xb becomes a point on the operating line, since the passing streams at this point actually exist at the end of the column. The use of open steam rather than a reboiler for the operating condition F, = F2 = L, = 0 is represented graphically in Fig. 8.20c. 

Description Hydrocarbon feed is preheated with hot circulating solvent and fed at a midpoint into the extractive distillation column (EDC). Lean solvent is fed at an upper point to selectively extract the aromatics into the column bottoms in a vapor/liquid distillation operation. The nonaromatic hydrocarbons exit the top of the column and pass through a condenser. A portion of the overhead stream is returned to the top of the column as reflux to wash out any entrained solvent. The balance of the overhead stream is the raffinate product, which does not require further treatment. 

Complete Fractionation Columns. A complete fractionation column, as shown in Fig. 6.11, may also be analyzed using the Ponchon-Savarit technique. In the McCabe-Thiele analysis, equations for two operating lines were found. These correspond to the A points in the Ponchon-Savarit analysis, where one A point represents the difference between passing streams in the column above the feed plate and the other A point represents the difference between passing streams below the feed plate. Assume that the reflux ratio, along with the composition and enthalpy of the feed, overhead distillate, and bottom product are known. Then points F, B, D, and A i may be located on... 

The fatty acids that emerge from the top of the column contain entrained water, partially hydroly2ed fat, and the Zn—soap catalyst. This product stream is passed into a vacuum dryer stage where the water is removed through vapori2ation and the fatty acid cooled as a result of this vapori2ation process. The dried product stream is then passed to a distillation system. 

Several descriptions have been pubUshed of the continuous tar stills used in the CIS (9—11). These appear to be of the single-pass, atmospheric-pressure type, but are noteworthy in three respects the stills do not employ heat exchange and they incorporate a column having a bubble-cap fractionating section and a baffled enrichment section instead of the simple baffled-pitch flash chamber used in other designs. Both this column and the fractionation column, from which light oil and water overhead distillates, carboHc and naphthalene oil side streams, and a wash oil-base product are taken, are equipped with reboilers. 

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