Oct-2000

Divided-wall columns — a novel distillation concept

The technology of divided-wall columns using a single distillation tower, described in this article, is claimed to have investment and operating advantages over conventional two-column systems for hydrocarbon fractionation

Frank Ennenbach, Baerbel Kolbe & Uwe Ranke
Krupp Uhde

Article Summary

Although distillation is often costly and energy intensive, it remains the most commonly used separation technology in the petrochemical and refining industry. Besides the optimisation of conventional fractionation technologies, specific fractionation systems, such as extractive distillation for hydrocarbons, which are boiling in the same range and/or forming azeotropes, have been developed by Krupp Uhde.

These extractive distillation technologies are being applied in various applications around the world, known as Morphylane, Octenar and Butenex. Another of the company’s fractionation activities is the “divided-wall column technology”, used to separate a hydrocarbon mixture into three high-purity fractions by one distillation tower only. Compared with conventional fractionation, where two-column systems are required, the divided wall column entails much lower investment and operating cost. It therefore offers cost effective solutions for a wide range of fractionation tasks, especially in the refining and petrochemical industry.

Whenever three high-purity fractions have to be separated from a hydrocarbon mixture by conventional fractionation, two distillation columns are required as conventional fractionation by one column with a side-draw often does not fulfil the purity requirements for the side-draw fraction. In general, there are two possibilities for the separation of a hydrocarbon mixture into three high-purity fractions: fractionation by separate columns and fractionation by thermally coupled columns.

State of the art separation of hydrocarbon mixtures into three different fractions is mostly realised by a separate column arrangement. In general, two conventional options for this arrangement are commonly applied .

In the direct sequence (Figure 1) the hydrocarbon mixture is introduced to the first column where the low boiling fraction A is recovered as the top product of the column. The bottom product is fed to the second column where the medium boiling fraction B is distilled off as the top product and the high boiling fraction C remains as bottom product.

In the indirect sequence (not shown in Figure 1) the fractions A and B are recovered as top product in the first column while the high boiling fraction C is gained as bottom product. In the second column fraction A and fraction B are separated.

The selection of the optimum sequence is made considering all required aspects, such as quantity of individual fractions, relative volatilities between the components of the desired fractions, thermal stability of the components, available utilities – eg steam temperature levels, etc – in order to have the lowest possible investment and operating cost.

Compared with the previously mentioned options, the thermally coupled column does not require any additional heating or cooling source for the second column, shown in Figure 1. The medium boiling fraction B accumulates in the side column and is used to strip out the light components of fraction A at the top of the side column and to wash down the heavy components of fraction C to the bottom of the side column. Virtually any purity of medium boiling fraction B can be achieved in the side draw of the side column provided that no azeotropes are existing.

A further step in development of the thermally coupled column is realised by integration of the separation task within one column shell only, thus offering considerably lower investment and also saving operating costs. If thermally coupled columns are integrated into one single shell, it is referred to as a divided wall column (DWC) or partitioned wall column, as shown in the figure.

A partition wall in the middle section of the column separates the main column and the side column. In this fully thermally coupled column realised as a DWC the multicomponent feed enters the main column, where a cut between low- and high-boiling components takes place. The middle-boiling components of fraction B distribute to the top of the partition wall together with the low boiling components of fraction A (AB) as well as to the bottom of the partition wall, but along with the high-boiling components of fraction C (BC). Thus, as a major advantage of this configuration, the components of fraction C do not enter the side column at the top and the components of fraction A do not enter the side column at the bottom.

The mixture of low and middle boiling components (AB) is separated in the upper column section of the main column and the same applies to the high boiling and middle boiling components (BC) which are separated in the lower section of the main column.   
 
With regard to the middle boiling components (B) it is obvious, that the composition of B at the top and the bottom of the side column match the composition of B in the main column. There is a peak in the composition of B in the middle of the side column where fraction B is withdrawn. It can be seen that neither low boiling components of fraction A can pass to the bottom part of the side column, nor high boiling components of fraction C can pass to the upper part of the side column. Thus, contamination of middle boiling fraction B can be avoided.     

Although the principle of the DWC technology has been known for a long time, latest developments have enabled calculation techniques to be used, and the establishment of generic guidelines for thermodynamic simulations. Based on its pilot plant facilities and experience in fractionation design, Krupp Uhde established engineering tools to allow proper application of the technology and to make full use of the thermodynamic advantages.

To explain the energetic advantages of a divided-wall column a conventional fractionation with two columns is compared to a DWC for the recovery of a benzene cut from a typical reformate feedstock. Figures 2 and 3 show the corresponding profiles of the middle boiling components for the conventional fractionation and the DWC fractionation.