The Effect of Maldistribution on Separation in Packed Distillation Columns

Abstract

For a packed bed in a distillation column, fmax. is the maximum fractional liquid maldistribution that can be tolerated in a parallel column model whilst still being able to achieve the design separation. It was previously shown that fmax can be easily calculated from a conventional column simulation output and it is a measure of the sensitivity of a packed bed to maldistribution. In this paper, fmax is applied to examples taken from air separation and ethylbenzene-styrene distillation. Using air separation plant data, it is shown that design separation shortfalls can be correlated against fmax When fmax <0.05, it is extremely difficult to achieve the design separation. A case study is given where fmax was increased from a very low value by splitting the bed, thereby achieving the design separation. Application of fmax to ethylbenzene-styrene distillation leads to the conclusion that two and four packed beds should be used in the rectifying and stripping sections, respectively. By varying the number of stages in the beds in the stripping section to equalize the sensitivity to maldistribution, it is shown that it may be possible to use only three beds in future designs.