Addressing Difficult Dynamics, Process Interaction

Single-loop and multi-loop control may be used to address a wide variety of control requirements. However, if a process is characterized by extremely long process delays or by a high degree of process interaction, then other control techniques may be required. In Chapter 16 of Control Loop Foundation – Batch and Continuous Processes, pulp bleaching and primary reformer temperature control are used to illustrate processes that are better addressed using other control techniques.

In the bleaching area of a paper pulp mill, as illustrated below, bleaching chemicals are used to achieve a desired level of pulp brightness. Since the reaction after chemical addition is very slow, the bleach tower is designed for long residence times. Because of the transport delay associated with the bleach tower, a brightness measurement made after the bleach tower may not reflect a change in bleaching chemical addition for 30–60 minutes depending on the pulp feedstock flow rate. To account for changes in the feedstock consistency, the bleaching
chemical addition rate may be set by ratio control of chemical/dry stock. However, to effectively adjust this ratio based on deviations of brightness from setpoint, it is necessary to use a control technique such as MPC that accounts for the very long process delay.

In an ammonia plant, the primary reformer is made up of multiple tube bundles, known as harps, which contain a catalyst. Gas burners are located between each harp at the bottom of the reformer. To drive the reforming process to completion, it is important to maintain a high gas
temperature leaving the harp. The outlet temperature of each harp is measured. However, any adjustment of one burner will impact the outlet temperature of adjacent harps. Such interactions are best addressed using MPC as illustrated below.

An introduction to MPC is provided in Chapter 14 of Control Loop Foundation – Batch and Continuous Processes. Also, By accessing the book’s web site, you may complete MPC control workshop using your web browser. More in-depth information on MPC and other advanced control techniques is provided in the book Advanced Control Unleashed – Plant Performance Management for Optimum Benefit. This book contains a CD that includes videos and discussions of MPC applications.