Divided Wall Column MPC Control with Wireless Instrumentation

Distillation is an important process from crude oil refining to fine sipping whiskey production. Managing this process safely, efficiently and reliably is the challenge.

Emerson's Terry Blevins

In a recent Hydrocarbon Processing article, Use model predictive control to achieve real-time management of a DWC, Emerson’s Terry Blevins teamed up with Eastman Chemical Company and the University of Texas here in Austin to study optimum control for a dividing wall column (DWC) application.

Wireless MPC-based control on a Divided Wall ColumnThe authors open citing the advantage of a DWC in energy usage and capital costs. It is:

…a distillation column with a vertical partition that divides the column into two parts—pre-fractionate and mainfractionate. This configuration reduces capital costs by utilizing only one column, and it reduces thermodynamic losses by portioning between the feed and side product.

The study addressed:

…the control design to provide the flexibility needed to test different types of control. Also, it was desirable to address closed-loop control using wireless measurements provided by WirelessHART flow and temperature transmitters.

The authors highlight differences from the established Ling and Luyben method of control using an alternate approach:

…to regulate reflux to maintain accumulator level, and to regulate distillate flow to maintain distillate composition.

All temperature measurements were taken with wireless temperature transmitters on an 8-second update rate to in order to provide 5 to 7 year battery life. The DeltaV distributed control system used included the PIDPlus algorithm designed to provide control with these slower wireless update rates. Emerson licensed royalty free patent rights to the PIDPlus technology to the FieldComm Group that developed the WirelessHART international standard (IEC 62591).

Due to the interactive nature of the DWC process and loop interactions, model predictive control provided a better solution that single-loop PID control. Initially:

…a module was created in which the MPC was configured to only address composition control and energy consumption based on column temperature. Therefore, the MPC block only addresses control of the four temperature measurements (defined as controlled parameters), the column feed (defined as a disturbance parameter), and the four manipulated parameters (PID setpoint for reflux and bottoms flow control, and the ratio setpoints for liquid split and side takeoff)…

Read the article to see the P&IDs, trend displays and operator screens and for a deeper description of the control strategy and commissioning process.

The authors concluded the article with these observations:

The control design implemented on the DWC at the University of Texas has proven to be effective in providing stable column operation. Experience with the column operation over a variety of operating conditions has shown the following:

  • Closed-loop control using wireless measurements and the PIDPlus algorithm effectively addresses fast processes, such as liquid flow and steam flow, as well as slower processes, such as temperature control, using an 8-sec. periodic communication update rate.
  • MPC satisfies process control requirements using wireless instrumentation. For the DWC control, MPC has been shown to outperform single-loop control.

You can connect and interact with other DeltaV process control experts in the DeltaV group in the Emerson Exchange 365 community.

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