Dividing Wall Column Control

Advanced Control of a Dividing Wall Distillation Column

• Sponsors: Eastman Chemical Company, Emerson Process Management, and Sulzer Chemtech
• Principal Investigator: Dr. Bruce Eldridge
• Graduate Research Assistant: Melissa Donahue
• Status: Complete

Introduction

Although distillation remains the backbone of separations technologies and is used throughout the chemical engineering industry, distillation remains inefficient, requiring immense energy usage and oftentimes multiple columns to achieve multiple high purity product streams. A dividing wall column (DWC) is a multicomponent separation technology that incorporates process intensification and advanced process integration concepts to improve upon the inefficiencies of traditional distillation columns. A dividing wall column is a distillation column containing a vertical partition which essentially serves to separate the feed and product side of the column (Figure 1). The integrated nature of DWCs leads to lower capital investment and less energy usage when compared to conventional distillation sequences.

Their intensified nature and additional degrees of freedom when compared to single traditional distillation columns make DWCs a unique control challenge. Overall, the DWC control literature is centered on the separation of a small number of prototype mixtures yet reports on a surprisingly broad array of control structures and strategies. The formulation of a transparent framework for connecting DWC design and operational objectives to control structure selection remains an open research question.

Modeling
A dynamic model has been built to match the design of the pilot plant. This model was used to test candidate chemical systems and to determine steady state operating objectives. In addition, the model was used to design control pairings using conventional controller design techniques such as singular value decomposition and relative gain array. If DWCs are to be widely adopted, it must be determined if conventional techniques can be used to design control structures or if more complex techniques are required due to the intensified nature of dividing wall columns.

Experimental Set-up
The pilot scale experimentation was conducted using the six inch diameter dividing wall distillation column at the James R. Fair Process Science Technology Center (PSTC) Pilot Plant at The University of Texas at Austin (Figure 2).