(139c) Dividing Wall Column - Design for a Retrofit

A market opportunity arose for a higher value market for an existing product line if key impurities could be reduced an order of magnitude below standard product specifications. This would require more theoretical stages than are available in the existing product tower.  Because of structural limitations, the product tower would have to be replaced with a new column and foundation, involving substantial capital investment. 

It was noted that an isomer separation column upstream of the product column could be de-bottlenecked by replacing trays with structured packing.  This prompted the question whether the resulting extra hydraulic capacity could be used to facilitate the production of high-purity products by converting the tower into a dividing wall column (DWC), effectively combining the purification tower functionality with the isomer separator tower, obviating the need for a new product tower and foundation, reducing the capital investment.

Process simulation studies and indicated this was feasible.  A further benefit was ~ 20% less energy requirement, compared to the conventional separation sequence.  Consultations with column vendors with experience in DWC confirmed the project was an excellent candidate for a DWC retrofit.  However, because the tower was small diameter, it was more cost-effective and required less down-time to replace the column shell with a new vessel, rather than modifying the internals of the original column.  

This case study illustrates the advantages and trade-offs to be considered when contemplating retrofitting an existing unit as a DWC.