Process Puzzler: Sidestep a Scale-up Scare

Readers grapple with a reactor production change.

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Things to look out for are two phases/phase separation/phase changes, viscosity changes, different agitation requirements, changes in consistency, etc. For agitation concerns, consult a known reliable vendor. If there are changes in the amount and or rate that the solvent evaporates, verify that the condenser above the reactor is sized accordingly. Also, measure the solvent concentration in the air on scale-up to verify the equipment has adequate venting. Battery operated portable meters are available.

Since you are also implementing a formula change by increasing the polymer concentration by 2%, you should also do a full quality-control analysis on each scale-up batch to verify that your targets are staying consistent. Other tips for improving efficiency and turnaround time are to pre-weigh and stage raw materials before addition, use a pump to discharge the product instead of discharging by gravity, also discharge into secondary containers like totes or a secondary tank if filling smaller packages. Depending on how the scale up goes you may need to upgrade your processing equipment.
Errol Williams, production engineer
AVEKA Inc., Woodbury, Minn.

Polymers are formulated for specific purposes. Changes in the process
could affect the final product. For instance, more or less monomer and
high heat and curing changes may make the polymer more or less appropriate for its intended use. Also, by forcing the reaction, will appropriate cross-linking occur for product performance and are studies needed?
Richard Ashley, associate director, technical affairs
Barr Pharmaceuticals, Pomona, N.Y.

The restraint of “slowly” adding the monomer implies that it is controlling the rate of reaction. The first step should be to increase the rate of addition in discrete steps to verify control of the reaction. Then the batch size may be increased if the head space has not been compromised by foaming or carryover as the vapor disengages. The objective may be attained in a safe manner and allay the nervous concern. The condenser will remove most of the heat of reaction as the heat transfer to the jacket will be negligible at high viscosities. Bench-scale equipment does not have the same concern for removing heat as a production-size reactor. Going directly to a bulk charge is not recommended. To go to a bulk polymerization requires a review of the heat removal capability of the condenser and the relief area of the reactor. Adjusting the free space in the reactor may be necessary if foaming is a concern or cannot be controlled by defoaming agents; if this is true, the faster reaction rate will aggravate the situation, possibly to an unacceptable level that may require smaller batch sizes rather than larger ones.
Jim Morris, senior process engineer
Flint Hills Resources, Odessa, Texas

Obviously the production increase should be delayed until it can be tested at a laboratory scale (1–2 gallons), then a bench scale (10–50 gallons) and finally a pre-production scale (100–200 gallons). But what will the results tell us? Perhaps, not enough.

I remember my first engineering assignment: to suggest the reason for an explosion caused during scale-up of a new solid rocket propellant. Tests at the laboratory scale provided few answers other than the obvious ones: mixing and temperature control decline with scale-up. For this problem it may be best to address the symptoms.

If the batch is thickened by chemical reaction and by concentration, agitation will decline and the bulk temperature will increase as surface area to volume decreases. Agitation cannot be easily improved for viscosities much above 100 cP. However, heat transfer can be increased by changing the media and temperature differential.

First, the solvent could be chilled prior to addition to the reactor. I suggest changing the cooling media for the jacket and the condenser to chilled brine; this may require a material change in the condenser and jacket from carbon steel. Use an operating minimum of about 40oF. Perhaps a maximum of 80oF would be acceptable. Changing the differential temperature might more than double the actual heat transfer.

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