Process Puzzler: Kayo a Cascade Control Complication

Readers suggest how to properly regulate an exothermic reaction.

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THIS MONTH’S PUZZLER
We react two organic chemicals in a stirred tank. The reaction is exothermic and highly sensitive to temperature. We control the temperature by adjusting the feed flows, particularly reactant A, which makes up 75% of the flow. Reactant B is ratioed off reactant A. Our new control engineer thinks we need to program some lag in the control valve for A. His first idea was to install an electric valve positioner on control valve A. The product quality has declined. Is he right about the lag? What other improvements should we consider?

REDESIGN THE PROCESS
Since no information was given on the volume of the tank and flow rates or whether this is batch or a continuous process, it is assumed that this is a continuous process and the flow is controlled based on the level transmitter. It is also assumed that the reaction is zero order and the reactor has sufficient residence time for reaction completion.

                     Click Image To Enlarge
redesigning the process
Figure 1. Alternative arrangement: This continuous processing
setup avoids the need for a reactor.


Based on the problem description and the diagram presented, the jacketed reactor with its mixing is not able to keep the reactor at or below the desired temperature. If the reaction is zero order, it will be complete very fast with its exotherm. This exotherm can be easily contained by having a heat exchanger in the line going to the tank after the pump. Both reactants would be added in the line just before the heat exchanger. Exotherm can be used to accelerate the zero order reaction and the heat exchanger can be used to control the temperature. Out flow from the heat exchanger can go to the tank for any further cooling if that is required. If no additional cooling is required, a partial stream from the heat exchanger could be recirculated to the inlet of the pump and the reactor might not be necessary. This will convert the exiting batch/continuous process to a highly productive continuous process.

An alternative is shown in the schematic diagram (Figure 1). The pipe length could be sufficient to give the necessary residence time. This reactor could be eliminated, as it is just a wide spot in the line.
Girish Malhotra, president
EPCOT International, Pepper Pike, Ohio

USE A VARIABLE SPEED MOTOR
While I am not familiar with the process you describe, you can maintain a constant level in your vessel by installing a variable speed motor on the pump. Use a level sensor in the tank to automatically control pump speed.
Glen Oswald, engineer advisor
Mosaic Phosphates, Plymouth, Minn.

GET RID OF THE POSITIONER
I bet that the operators run this cascade loop in manual. What’s that old saying, “If you can’t control a loop in manual, how can you ever hope to control it in automatic?” No truer statement was ever made. Operators are amazing aren’t they? A good one can even line up a tank at a pH of 7 better than the best system ever devised by a control engineer. Now, that’s demeaning! The problem at present is a conglomerate of old and new problems.

An electronic positioner on control valve A may not be a good idea since this builds in gain where there is already significant lag caused by the temperature transmitter. Perhaps the transmitter could be moved where mixing is better; this could cut down the temperature lag. One idea would be the bottom of the tank, away from the baffles and where the axial flow from the impellers causes the flow toward the tank bottom.

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