Process Puzzler: Kayo a Cascade Control Complication

April 24, 2009
Readers suggest how to properly regulate an exothermic reaction.

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?

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
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

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.

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.

Moving the temperature transmitter to the pump discharge is tempting but you want a little lag in the system afforded by the tank volume. Otherwise, the secondary loop will wind up and overshoot.

Another issue may be the orifice plate. The response by the secondary flow loop will be to the flow squared. Ideally, you want the response to be linear. Then, there is the startup state. Orifice plates typically are accurate to only ±1% at full scale and become more inaccurate at low flow. The operators probably start this process in manual and never put it in automatic.
Dirk Willard, senior process engineer
International Steel Services, Inc., Pittsburgh

We’re in the midst of investigating a fatal accident caused by a fire that occurred at a petrochemical storage facility. A farmer living nearby reportedly smelled gasoline for several days before the accident. He says he called the refinery several times but wasn’t taken seriously. The Fluid Catalytic Unit (FCU) that produces much of the refinery gasoline was having an emergency turnaround during this period, so staff were stretched thin. They didn’t see much urgency because of the tank farm’s safety system, which relies on redundant controls with an automatic gauging system and high/high alarms on each tank. Finally, two operators drove out to the tank farm and died in the fire. How should we approach this investigation? Do you have any suggestions for improvements in the tank farm? What do you think killed the two operators?

Send us your comments, suggestions or solutions for this question by June 12. We’ll include as many of them as possible in the July 2009 issue and all on Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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