Properly Control Multivariable Processes

An effective configuration requires a real understanding of the process

By Cecil L. Smith, Cecil L. Smith, Inc.

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Most processes are multivariable and, thus, need a control configuration with two or more loops. The typical starting point is a single-loop configuration that relies on proportional-integral-derivative (PID) controllers, the justification being “keep it simple.” These usually deliver adequate performance but occasionally interaction between the loops renders one or more loops “untunable.”

Model predictive control (MPC) performs very effectively in applications plagued by interaction. Process optimization normally is required to provide the economic benefits to justify installing and supporting an MPC application. For applications within the main process, benefits of optimization often suffice. However, this is far less likely in auxiliary parts of the process.

The alternative is to insert simple elements such as characterization functions and summers into the control configuration to reduce the degree of interaction to the point where the loops can be tuned successfully [1]. The result is advanced regulatory control; its objective is to maintain the key process variables at or near their targets. The control configuration provides no optimization. However, by reducing the variance in the key process variables, it can enable operating the process closer to the constraints, with ensuing benefits.

For such configurations to be accepted in a production facility, they must comply with the following requirements [2]:

1. Process operators must be able to take control of any final control element and specify its opening. When a summer block or the like is inserted between the output of the PID and the final control element, the customary approach of switching the PID controller to manual and specifying its output isn’t satisfactory.

2. All transitions between the manual and automatic modes of operation must be smooth or “bumpless.” Achieving this requires proper configuration of output tracking.

3. One or more final control elements may be driven to a limit. So, the configuration must prevent reset windup in a PID controller.

These issues are the primary focus of this article. However, first, let’s start by describing the process, illustrating the tuning problem for the single-loop configuration, and formulating a revised configuration.

Off-Gas Process

Figure 1 depicts a process to remove contaminants from an off-gas stream that contains particulate matter, sulfur dioxide (SO2), and sufficient water vapor that the dew point is above the ambient temperature. The bag house eliminates the particulate matter and then scrubbers remove the SO2. Because condensate would be acidic, temperatures must remain above the dew point until the off-gas stream arrives at the scrubbers.

A furnace containing a heat exchanger produces a hot air stream. (The products of combustion don’t enter the process.) Sufficient hot air is added upstream of the bag house to keep the off-gas stream above its dew point through the bag house. Additional hot air is added downstream of the bag house so the off-gas stream remains above its dew point until it arrives at the scrubbers.

In 1976, Trevathan used such an off-gas process in an article on process control as practiced in the chemical industry [3]. That paper included a simple flowsheet along with a very brief description of the process but provided no values for flow rates, temperatures, equipment sizing, etc. The parameters we’ll use have no relationship to Trevathan’s process but, like Trevathan, our results will come from a simulation of the process.

Single-Loop Control Configuration

The control configuration shown in Figure 1 reflects the customary approach of “control every variable with the nearest valve.” The appropriate order for tuning the controllers is:

Hot air pressure controller (PC). This loop can be tuned without the furnace in operation. The loop responds very rapidly, the dynamics being dominated by the 10-sec travel time for all dampers.

Hot air temperature controller (TC). The thermal capacity of the furnace makes this loop significantly slower.

Bag house inlet TC. This temperature results from mixing two gas streams. Mixing processes respond quickly.

Scrubber duct inlet TC. This temperature also results from mixing of two gas streams.

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