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Robust MFA Controller

In complex control applications, the following challenges may occur. (1) There is a big change in the process dynamics so that a prompt control action is required to meet the control performance criteria; (2) The dominant disturbance cannot be economically measured so that feedforward compensation cannot be easily implemented; (3) A controller purposely de-tuned to minimize the variations in its manipulated variable may lose control when there is a large disturbance or significant dynamic behavior change; or (4) Changes in dynamic behavior or load do not provide triggering information to allow the control system to switch operating modes.

For instance, controlling the reaction temperature for a batch reactor is always a challenge due to the complex nature of the process, large potential disturbances, interactions between key variables, and multiple operating conditions. A large percentage of batch reactors running today cannot keep the reactor temperature in automatic control throughout the entire operating period thus resulting in lower efficiency, wasted manpower and materials, and inconsistent product quality.

An exothermal batch reactor process typically has 4 operating stages: (1) Startup Stage: ramps up the reactor temperature by use of steam to a pre-defined reaction temperature; (2) Reaction and Holding Stage: holds the temperature by use of cooling water while chemical reaction is taking place and heat is being generated; (3) No-reaction and Holding Stage: holds the temperature by use of steam after main chemical reaction is complete and heat is not being generated; and (4) Ending Stage: ramps down the reactor temperature for discharging the products.

During the transition period from Stage 2 to Stage 3, the reactor can change its nature rapidly from a heat-generation process to a heat-consumption process. This change happens without any triggering signal because the chemical reaction can end at any time depending on the types of chemicals, their concentration, catalyst, and reaction temperature. Within a very short period of time, the reactor temperature can drop significantly. The control system must react quickly to cut-off the cooling water and inject a proper amount of steam to drive the reactor temperature back to normal. A regular feedback controller is not able to automatically control a batch reactor during this transition. In practice, batch reactors are usually switched to manual control and rely on well-trained operators during critical transitions. It is a tedious and nerve-wracking job that can result in low product quality and yield.

The Robust MFA controller is able to control the problematic processes described. There is no need to re-design the controller, use feedforward compensation, or re-tune the controller parameters. The Robust MFA controller is able to keep the system in automatic control through normal and extreme operating conditions when there are significant disturbances or process dynamic changes.

Robust MFA Controller Configuration

As shown in the following graph, the Robust MFA controller can be configured with these parameters.

Robust MFA controller configuration menu

(1) Upper and Lower Bound - The bounds for process variable (PV) being controlled. These are "intelligent" upper and lower boundaries that are typically the marginal values the PV should not exceed. PV is unlike controller output (OP) where a hard limit or constraint can be set. PV is a process variable that can only be varied by manipulating the OP. Thus, the Upper and Lower Bounds for PV are very different from the OP constraints. (2) Gain Ratio - The coefficient to increase or decrease the MFA control action based on the PV. For instance, a Gain Ratio of 3 is the default setting. As PV gets closer to reaching its bounds, MFA will react like it has a gain that is 3 times higher than its setting. Notice that this is not a gain scheduling approach, although it appears to be. Gain scheduling will not be able to resolve the complex problems described.



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