(168a) Balancing Distillation Column Diameter with Vapor Feeds

Methods for balancing distillation vapor loads to achieve smaller, more uniform calculated column diameters are developed for columns with vapor feeds and a large calculated column diameter in the enriching section. The column diameter for distillation is usually designed to be a set percentage of flooding, typically in the range of 60 to 80 %. The diameter calculated at different locations usually varies. Typically, the column is designed at the location with the largest column diameter and built with a constant diameter unless the calculated diameters at different locations differ by a large amount. When this design procedure is followed, the percentage of flooding will be less, often significantly less, than the nominal design percent of flooding everywhere in the column except for the location with the maximum calculated diameter. In cases with extreme variations in calculated diameter, the column is built in two sections with different diameters. In this research we consider a large difference in the calculated diameters as potential to either reduce the column volume or as potential to process more material in a retrofitted column. If the calculated diameter is high in the enriching section, we reduce the vapor flow rate in this section. To do this we reduce the reflux ratio from the condenser and do additional condensing either with an intermediate condenser or with a condenser on all or part of the feed.

The calculated diameter can be reduced by: 1. cooling the entire feed to produce a two phase feed or a liquid, 2. condensing vapor inside the column with an intermediate condenser or 3. cooling or condensing a part of the feed while leaving the other part vapor with a two-enthalpy feed system. The first two methods are well-known in the literature while the third is not. To retain the same product purities it may be necessary to increase the energy loads to the column or increase the number of stages.

First, applications are illustrated for the main distillation column in an ethanol biorefinery that receives the vapor from the beer still as feed. In this case the use of the two-enthalpy feed system has an advantage since a considerable amount of vapor can be condensed without changing the minimum boilup rate. For example, for distillation of a saturated vapor feed of 10 mole % ethanol and 90 mole % water the use of two-enthalpy feed resulted in a 53 % reduction in column volume with the same purity and no increase in energy use if 11 % more stages were added. Reductions in volume as large as 76 % are predicted but with a significant increase in reboiler duty. This method can also be used for debottlenecking existing columns that were constructed with a large diameter.

Additional calculations are illustrated for the separation of vapor feeds of methanol and water and for vapor feeds of propane and n-butane. Since the diameter balancing method is a processing technique for distillation, it is generally applicable to any type of equipment used for distillation. Thus, the methods developed in this paper can be applied to different types of trays and different random and structured packings.