Simple overhead distillation

A simple overhead topping distillation process, without fractionation, is illustrated in Fig. 3.57. 

For a simple binary distillation, the component balance equation becomes 

The relative volatility, a, is usually related to the compound having the higher boiling point, which in this ca,se is B and hence 

Assuming that the liquid and vapour compositions in the still are in equilibrium, i.e., that the still acts as a theoretical stage 

The combination of the two mass balance equations, together with an explicit form of equilibrium relationship gives a system that is very easily solvable by direct numerical integration, as demonstrated in the simulation example BSTILL. 

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Figure 3.57. Model representation of a simple overhead distillation.

Figure 3.50. Model representation of a simple overhead distillation. The total material balance is given by...

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. 

FIG. 13-1 Schematic diagram and nomenclature for a simple continuous distillation column with one feed, a total overhead condenser, and a partial reboiler. 

Figure G.4 illustrates a simple generic plant, an isothermal reactor coupled with a distillation column. A mixture of two species, mainly A but also some B, is fed to a reactor where the reaction A B takes place isothermally. The binary distillation column has 20 stages and produces two product streams an overhead (distillate) stream rich in A and a bottoms stream rich in the desired product B. The A-rich distillate is recycled to the reactor to increase the conversion of A to B.

Pot still A simple batch distillation unit used for the production of potable alcoholic beverages. It consists of a vessel that is heated by an internal steam coil or by an external fire, and an overhead vapor pipe leading to a condenser. 

The pressure used in producing gas wells often ranges from 690— 10,300 kPa (100—1500 psi). The temperature of the inlet gas is reduced by heat-exchange cooling with the gas after the expansion. As a result of the cooling, a liquid phase of natural gas liquids that contains some of the LPG components is formed. The liquid is passed to a set of simple distillation columns in which the most volatile components are removed overhead and the residue is natural gasoline. The gas phase from the condensate flash tank is compressed and recycled to the gas producing formation. 

An azeotrope limits the separation that can be obtained between components by simple distillation. For the system described by cui ve B, the maximum overhead-product concentration that could be obtained from a feed with X = 0.25 is the azeotropic composition. Similarly, a feed with X = 0.9 could produce a bottom-product composition no lower than the azeotrope. 

For a batch differential distillation where no reflux is used, there is only boilup of a mixture of the desired lighter component, which leaves the kettle, and a desired residual bottoms composition is left in the kettle. This type of distillation follows the Raleigh equation to express the material balance. However, while simple, not having tower packing or trays or reflux does not offer many industrial applications due to the low purities and low yields involved. Repeated charges of the distillate back to the kettle and redistilling w411 improve overhead purity. 

Consider the binary batch distillation column, represented in Fig. 3.58, and based on that of Luyben (1973, 1990). The still contains Mb moles with liquid mole fraction composition xg. The liquid holdup on each plate n of the column is M with liquid composition x and a corresponding vapour phase composition y,. The liquid flow from plate to plate varies along the column with consequent variations in M . Overhead vapours are condensed in a total condenser and the condensate collected in a reflux drum with a liquid holdup volume Mg and liquid composition xq. From here part of the condensate is returned to the top plate of the column as reflux at the rate Lq and composition xq. Product is removed from the reflux drum at a composition xd and rate D which is controlled by a simple proportional controller acting on the reflux drum level and is proportional to Md-... 

Ester manufacture is a relatively simple process in which the alcohol and an acid are heated together in the presence of a sulfuric acid catalyst, and the reaction is driven to completion by removing the products as formed (usually by distillation) and employing an excess of one of the reagents. In the case of ethyl acetate, esterification takes place in a column that takes a ternary azeotrope. Alcohol can be added to the condensed overhead liquid to wash out the alcohol, which is then purified by distillation and returned to the column to react. 

Intermediate Reboilers and Condensers A distillation column of the type shown in Fig. 13-2a, operating with an interreboiler and an intercondenser in addition to a reboiler and a condenser, is diagramed with the solid lines in Fig. 13-38. The dashed lines correspond to simple distillation with only a bottoms reboiler and an overhead condenser. Total boiling and condensing heat loads are the same for both columns. As shown by K ihan [Am. Inst. Chem. Eng. J. Symp. Ser. 76, 192, 1 (1980)], the adclition of interreboilers and intercondensers increases thermodynamic efficiency but requires additional stages, as is clear from the positions of the operating lines in Fig. 13-38. 

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