Distillation column design stages

Knowledge about the distillation process regularities under minimum reflux is the background of the distillation theory, and this mode analyzing is the most important stage of the distillation column design. 

The design of a distillation column is based on information derived from the VLE diagram describing the mixtures to be separated. The vapor-liquid equilibrium characteristics are indicated by the characteristic shapes of the equilibrium curves. This is what determines the number of stages, and hence the number of trays needed for a separation. Although column designs are often proprietary, the classical method of McCabe-Thiele for binary columns is instructive on the principles of design. 

Figure 7-4. The Scientific Design Co. process for producing ethylene glycols from ethylene oxide (1) feed tank, (2) reactor, (3,4,5) multiple stage evaporators, 4 operates at lower pressure than 3, while 5 operates under vacuum, evaporated water is recycled to feed tank, (6) light ends stripper, (7,8) vacuum distillation columns.

It is important to understand whether there will be two-liquid phases present in the column. If two-liquid phases form in a large part of the column, it can make the column difficult to operate. The formation of two-liquid phases also affects the hydraulic design and mass transfer in the distillation (and hence stage efficiency). If it is possible to avoid the formation of two-liquid phases inside the column, then such behavior should be avoided. Unfortunately, there will be many instances when two-liquid phases on some plates cannot be avoided. The formation of two-liquid phases can also be sensitive to changes in the reflux ratio. 

EXAMPLE 12.1 OPTIMAL DESIGN AND OPERATION OF A CONVENTIONAL STAGED-DISTILLATION COLUMN... 

Example 12.1 Optimal Design and Operation of a Conventional Staged-Distillation Column... 

From a thermodynamics basis, the transesterification reaction favors the formation of methylphenyl carbonate (Equation 7.4), whilst its further disproportionation in a second-stage continuous reactive distillation column affords DPC with selectivity >99%. Although both reactions occur at a relatively high temperature ( 473 K), optimization of the reaction conditions and engineering design would allow a productivity that fitted with the economics [17, 27]. 

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