(176c) Computer-Aided Graphical Tool for Complex Column Design

Keywords: Column section, column profile maps, novel short-cut technique, coupled columns, computer-aided graphical tool

Abstract

Distillation is the most important separation process in the chemical and other related industries. Due to extensive theoretical and experimental research in the 20th century much is known about the distillation process in a one-feed two-product column. Based on this knowledge powerful rigorous process simulators such as ASPEN PLUS have been developed and are now available allowing the simulation of complex columns. For many people this is reason enough to declare the field of distillation as very well understood and therefore require no further research.

However complex columns, such as the divided wall column, are extremely difficult (if not impossible in a lot of cases) to design successfully using the process simulators available. This is due to the fact that these simulators give little insight into the behavior of the system and are generally not suitable for conceptual distillation process design. Therefore very few complex column designs exist in industry. Although complex columns seem difficult to operate, they may provide significant cost savings e.g. by substituting two or more simple columns by a single complex column. In other words the major problem in the development of new separation systems for industry is the lack of rapid and inexpensive screening and synthesis tools.

Until recently, no simple technique to model complex column configurations such as coupled columns (e.g. Petlyuk column), divided wall columns or columns with multiple feeds and product withdrawals was available.

We have developed a computer-aided graphical tool for complex column design. This approach is based on the works of Tapp [1] and Holland [2] who used the difference point equation and column profile maps (CPM's) to determine the composition profile in a column section. A column section is the generalized version of a stripping and/or rectifying section in a distillation column and is defined as a length of a column between points of addition or removal of material and/or energy. The advantage of this approach over the traditional rectifying and stripping profile method is that any column configuration can be modeled and entire maps (CPM's) can be generated as opposed to single profiles. As CPM's are transformed RCM's insight in finite reflux behaviour can be gleaned.

Using this technique we are currently able to design any complex column configuration and network and compare different configurations to determine the optimal one for a particular separation for ideal and selected non-ideal three component systems. This technique enables us to also determine feasible regions of complex column configurations. It provides insight into the dynamics of the Petlyuk column, which would enable the successful initialization of rigorous iterative design packages. We can show that the results obtained using our technique agree with a simulation done with ASPEN however only a fraction of the time is needed for the successful determination of the parameters.

The objective of this paper is to demonstrate this powerful technique by looking at a ternary separation problem using the divided wall column set-up. We will show that by having the product composition (top and bottom) and the feed composition we are able to determine i.e. the minimum reflux/reboil ratio, the range of feasible split ratios for the minimum reflux/reboil ratio, feed stage, side-draw stage and composition and the optimal split ratio for the minimum number of stages.

References

[1] Tapp, M., Holland, S.T., Hildebrandt, D., Glasser, D., ?Column Profile Maps. 1. Derivation and Interpretation?, Ind. Eng. Chem. Res., 2004, 43(2), pp364-374.

[2] Holland, S.T., Tapp, M., Hildebrandt, D., Glasser, D., Hausberger, B., ?Novel Separation System Design Using Moving Triangles?, Comp. & Chem. Eng., 2004, 29, pp 181-189.