Process Puzzler: Sidestep a Scale-up Scare

Readers grapple with a reactor production change.

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John C. Wincek, process safety manager
Croda, Inc., Mill Hall, Pa.

GO CONTINUOUS
You make no mention of molecular weight desired in the polymer. Normally, with the thermoplastics I’ve worked with, the higher the temperature, the lower the molecular weight. And I don’t see anything about mixing or blending. Most polymers achieve a narrow molecular range when a somewhat uniform temperature is achieved in the reactor. If one has nominally high production rates, maybe a continuous reactor is the best choice.
Tom Murphy, CEO
Puritrol, Inc., Centerville, Mass.

ASSESS SIX ISSUES
Your production manager should be concerned for the following reasons:
1. Changing the way you add your ingredients is extremely likely to change your final polymer. The average molecular weight and the distribution of molecular weights will change. Both of these will affect the quality of your product.

2. In your current reactor configuration, I presume that if you lost cooling in either the jacket or overhead condenser you would be able to stop adding monomer — which you won’t be able to do if you add all your monomer at once as in the new configuration.

3. Assuming that the vessel is closed, which seems likely, the pressure in the vessel will go up by a minimum of 29% just with the decrease in headspace from 45% to 35%. With the same cooling as you have now, the pressure will go up significantly if not catastrophically, as you will be giving off more heat in a shorter period of time. This, in turn, will drive the reaction, causing it to accelerate until something gives. Even if the vessel is vented much of the cooling is probably via the overhead condenser. The higher temperature anticipated will reduce the effectiveness of the condenser and more solvent will be lost.

4. Besides being a potential environmental concern, evaporation of the solvent caused by the heating will further increase the concentration of polymer. The viscosity will increase and reduce heat transfer from the jacket.

5. At some point the pressure relief system will relieve. The system will depressure, quickly causing intense foaming. Even with an open vent, which loses solvent, the higher liquid level and viscosity will tend to cause more foam. Foaming could reach and foul the condenser, reducing still further the ability to take heat out of the system. If either of these scenarios occurs, it could seal off your pressure relief system as the polymer comes out of solution due to solvent vaporization.

6. If the system does not blow up, be prepared for a giant molecule of polymer to form, which may require vessel replacement or jack hammering out polymer. In either event downtime would be significant.

You should start by exploring the thermodynamics. Find out exactly how large the exotherm is and then define the reaction rate at various temperatures. This is the only safe way to increase production; a logical progression vs. pushing all the yield parameters all at once. You will probably find that you can safely increase production to some extent. Beyond that you will need to make improvements to the process to increase your production.
Barry Bershad, consultant
Mansfield, Mass.

DO THINGS GRADUALLY
As a process engineer, I would be very concerned with scaling-up a solvent process from a bench scale to production scale with no in-between checks to verify the scale-up. Safety should be a primary concern.

Follow a gradual plan for scale-up. Scale-up first from the bench scale incrementally to 5 gallons and measure the temperature of the jacket-cooled batch to verify that there are no safety concerns or flashing. If the process stays within a safe temperature range you should move to the next step, which is a 20-gallon or 30-gallon batch. After verifying that the process is safe at that scale, next scale up to 50 gallons, then 100 gallons, then 250 gallons, then 500 gallons, and then 1,000 gallons. There are also other concerns that may arise with rapid addition of one raw material.

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