Concluding Remarks

Distillation as such is an energy intensive unit operation. Dividing wall columns have gained credibility in refineries and petrochemical complexes as an attractive alternative to conventional distillation sequences. The dividing wall helps to prevent remixing of components thereby improving the efficiency of the multi-component separation and lowering the energy requirements. DWCs have been proven to provide energy and capital cost savings as compared to conventional multi-component distillation.

Plant geographical location plays a key role in deciding the DWC configuration since utility costs and CAPEX vary by geography. The article looks at the various configurations and establishes a baseline for choosing a DWC configuration based on the client’s utility arrangements and CAPEX requirements. The configurations that has been studied for this purpose include – a Middle DWC, a Top DWC and a Bottom DWC. An example of BTX separation is presented for each configuration to discuss the different technical solutions required in different parts of the world.

Middle DWCs: These are the most common and energy efficient configuration. However, the column typically operates under the highest level of utility for reboiling duty and the lowest level of utility for condensing duty. For example, the heating duty shifts from low pressure (LP) steam to medium pressure (MP) steam for a BT separation. For geographical locations where there is a significant difference in the price for LP and MP steam, a Middle DWC will not produce the necessary cost savings.

Top DWCs: These columns typically provide lower overall utility savings as compared to a middle DWC. The column will however have two separate condensing temperatures, each requiring a separate condenser. The middle product overhead vapors can be used for heat integration purposes, which offsets the higher equipment costs.

Bottom DWCs: Similar in design to a Top DWC, the ‘dividing wall’ is located at the bottom. These columns hence accommodate the use of two separate reboiling utilities. This configuration provides significant energy savings in geographical regions with high utility costs.