Both Vapor and Liquid Distillate Streams

Distillation columns frequently produce both vapor and liquid distillate product streams from the reflux drum when the feed stream contains small amounts of light components 

In the operation of these systems, we usually want to condense as much as possible, so as to minimize compression costs of dealing with the vapor product. Therefore, the flow rate of cooling water should be maximized. This section demonstrates a realistic way to model a partial-condenser distillation system with both vapor and liquid distillate streams using Aspen simulation. 

With a reflux-drum design temperatures set at 120 °F, the required pressure depends on the composition of the overhead and whether the distillate product is removed as a liquid or as a vapor. In the former case, the reflux drum would operate at the bubblepoint pressure at 120 °F. In the latter case, the reflux drum would operate at the dewpoint pressure at 120 °F. There is no difference in these pressures if the overhead is a single pure component. If the overhead is a mixture of chemical components, the bubblepoint pressure is larger than the dewpoint pressure. If the volatilities of the components are quite different, running with a total condenser (bubblepoint pressure) can require a much higher pressure than running with a partial condenser. This is true even if some of the distillate product is taken off as liquid and some is taken off as vapor. 

The normal distillation column with either a liquid or a vapor distillate product (but not both) has two steady-state design degrees of freedom once feed conditions, column pressure, total trays, and feed location are fixed. Distillate flow rate and RR are usually manipulated to achieve two product-composition specifications (the heavy-key impurity in the distillate and the light-key impurity in the bottoms). 

A distillation column that is designed to produce both a liquid distillate stream and a vapor distillate stream from the reflux drum has an additional design degree of freedom. This is usually specified to be the reflux-drum temperature. Under these conditions, the split between the flow rates of the vapor and liquid distillates is fixed. The condenser heat duty is also fixed. Specifying a reasonable design minimum temperature differential temperature (at the either the cold or the hot end of the condenser), and a reasonable overall heat-transfer coefficient fixes the heat-transfer area. This also fixes the required flow rate of 

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Three different distillation condenser models have been compared for columns producing both vapor and liquid distillate streams. When the objective is to minimize the flow rate of the vapor, we should try to condense as much of the overhead vapor as possible. The realistic means for accomplishing this objective is to maximize the flow rate of the cooling medium. Therefore, the model that assumes a fixed flow rate of cooling water gives the most realistic predictions of performance to disturbances. 

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