Life's a batch!

By Gregory K. McMillan

SOME SAY ANY PROCESS with a distinct beginning and end is a batch process. According to this definition, continuous processes are just long batches. It is true that sequence expertise developed for batch operation can be applied to automate the startup, transition in grade, and shutdown of continuous processes to provide a faster, safer, and more efficient change in operating states.

In a conventional cascade temperature control system, the primary reactor temperature controller output is the setpoint of a secondary temperature control loop. In Figure 1 (Click the Download Now button at the end of this article to view a PowerPoint document with the figure mentioned here) the secondary loop controlled variable (CV) is the jacket or coil outlet temperature to pick up on changes in the heat transfer coefficient. This reactor also has a byproduct vapor stream and uses a condenser to reflux reactants or products trying to escape overhead.

A second common scheme uses the jacket or coil inlet temperature as the secondary loop’s controlled variable to correct upsets to the inlet faster or enforce limits on the inlet temperature associated with heat transfer surfaces, particularly important for biological reactors. A third scheme uses the outlet temperature of a heat exchanger in the recirculation line of a reactor as the secondary loop’s controlled variable.

If the secondary loop uses the difference between the inlet and outlet coil or jacket temperature for heat transfer or enthalpy control, changes in inlet temperature should be synchronized with the consequential changes in the outlet temperature by a time delay of the inlet temperature used in the calculation of the heat transfer. A time delay of the inlet temperature that does not match the actual transportation delay through the coil or jacket will cause an irregular response that can confuse the controller.

In all the schemes, the flow of coolant, steam, tempered water, oil or special heat transfer fluid such as Therminol to the jacket, coil or exchanger is manipulated by the secondary loop via the throttling of single or split-ranged valves. The addition of a flow controller to make a triple cascade of primary temperature to secondary temperature to flow to digital valve controller can be beneficial in terms of the reduction in the effect of pressure upsets and a nonlinear installed characteristic of the valve. The benefit may be questionable when the flow controller is detuned or when an equal percentage trim compensates for the nonlinearity of the secondary loop’s process gain.

The addition of a fast secondary loop makes the primary loop linear and corrects for any coil, jacket or heat exchanger upsets before they affect the reactor temperature. However, the true value of cascade control is achieved only when certain criteria is adhered to during the design and tuning of the secondary loop. The importance of the suggestions increases as the tendency of the reactor temperature response to accelerate increases.

Too Small
Probably the biggest mistake is to have the secondary temperature controller throttle a liquid flow through the coil or jacket. For these systems the combination of high process gain and high process dead time for a small liquid flow at low loads will cause a limit cycle. The secondary controller should throttle a makeup flow and control the mixture of hot and cold fluids with the total flow through the jacket or coil constant. There should be a displacement of return flow out of the coil or jacket recirculation system equal to the makeup flow to increase the self-regulation and reduce ramping in the secondary loop.

A common mistake is to tune the primary and secondary temperature controllers with too small of a controller gain, reset time and rate time. We will see in the tuning discussion how being too small in these settings can intensify the oscillations from valve deadband and in some cases lead to a runaway condition.

The overall heat transfer coefficient for the coils can be too small. In general, heat transfer coefficients are proportional to the flow rate next to the surface to approximately the 0.6 power and are tremendously degraded by coatings, whose formation rate greatly increases at low coil or jacket flow and reactor agitation rates. The integrated error for reactor control is proportional to the square of the heat transfer lag and an exothermic reactor can become unstable, regardless of tuning, if it becomes larger than the positive feedback time constant of an accelerating response.

Too Big
Large control valves will increase the amplitude of the limit cycle from valve deadband (backlash) caused by loose shaft/stem connections and gaps in linkages and resolution (stick-slip) from packing and seating friction, since both are a percent of valve stroke and hence valve capacity 2. Sliding stem (globe) valves with digital positioners are a must for tight temperature control of batch reactors. 

Controllers tend to cycle around the split range point because the stick-slip is generally larger as a valve is trying to break free from its seat. A switch from steam to coolant introduces a dead zone, discontinuity and change in coil and jacket dynamics. For minimization of the severe upset associated with a transition between heating and cooling, trim valves should be used. To avoid additional split-ranged points, a valve position control strategy can be used where small changes are handled by the trim valve and only large changes are passed on to the large valve. There is also a significant opportunity for deadband and stick-slip compensators to be inserted into the output of the secondary loop controller, and a feedforward signal of the secondary set point to be added to the output of the secondary controller with the appropriate feedforward gain and action if the secondary controller gain is low.