The Effect of Maldistribution on Separation in Packed Distillation Columns

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 ethylbenzenestyrene 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. INTRODUCTION Liquid or vapor maldistribution in packed distillation columns reduces the separation that is attained. Over the years, many workers have studied the problem in an attempt to predict the extent of maldistribution that occurs and to estimate its effect on separation. In a recent paper [1], the present authors introduced the concept of fmax to characterize the sensitivity of a packed bed to maldistribution. To understand the significance of fmax, consider the parallel column model shown in Figure 1 that represents liquid maldistribution in a packed bed. One side of the packed bed, represented by the left column in Figure 1, has a liquid flow rate of (1+f)L and the other (1-f)L. Hence, f is a measure of the fractional liquid maldistribution. Because of the different operating line slopes in the two columns, the overall separation obtained from the two column system is less than is obtained if the liquid is equally distributed. A typical calculated result is shown in Figure 2 where the effective number of stages from the combined two-column system decreases as the maldistribution fraction f increases. Also shown on Figure 2 is the limiting case labeled fmax. It represents the maximum maldistribution that could possibly be allowed while still being able to achieve the required separation. For a fixed arbitrary value of the maldistribution fraction f (say 0.04), whether or not a given bed is sensitive to maldistribution depends only on the value of fmax for the bed. Thus, in Figure 2, a bed having 10 theoretical stages in each parallel column corresponds to the line that cuts the ordinate at N=10 when f=0. For this bed, fmax >0.1 from Figure 2 and if f=0.04, the combined bed actually delivers about 9.5 effective theoretical stages, so that the effect of maldistribution is negligible. On the other hand, a bed having 40 theoretical stages in each parallel column (for which fmax=0.038) provides only 24 effective stages if f=0.04. Thus, the designer need only determine the value of fmax for the desired separation that is required to characterize the sensitivity of the overall bed to maldistribution. V yo Column 1 Column 2 NA NA-1