Chemistry Reference
In-Depth Information
addition rates, I was able to calculate the water flow needed. This process of
absorbing an effluent gas is called “scrubbing.” A gas can be scrubbed with
either a liquid or solid.
Laboratory scale reactions are often done during a normal work day. On a
larger scale, things can take longer and equipment is often in demand. There-
fore larger scale reactions are typically run in shifts and around the clock.
Everyone understood the plan for the bromination and we began working in
shifts. When I arrived for my shift, acidic gas was exiting out the roof vent of
the pilot plant. When I got inside, I was greeted with, “the reaction was going
so well, we decided to up the rate of bromine addition.” They had done this
without also upping the water flow in the scrubber. Therefore the water was
not able to absorb the HBr and was venting out of the roof. We immediately
stopped the bromine addition and got things under control.
There are two lessons to be learned from this mistake. One is that although
it is obvious to a chemist that the rate of HBr evolution is tied to the rate of
bromine addition, it is not necessarily obvious to everyone (in this example
an experienced chemical operator and a chemical engineer). I should have
done a better job in communicating this ahead of time. The second lesson
is that procedures should not be arbitrarily changed during a scale-up. This
problem was quickly remedied before any significant harm was done but it
could have been much more serious. There should be strict guidelines about
any process changes and what approvals are needed before the changes
are made.
There are many things to consider when scaling a process and many
distinctions between large-scale chemical reactions and small laboratory
scale reactions. I will discuss some of them here. The discussion is not
meant to be exhaustive but should serve to get the reader to begin thinking
about some of the differences. Each reaction will have its own extra things
to be considered. Some extra consideration should be given to raw materials,
chemicals used, heat exchange, safety, material of construction, waste,
agitation, and time. Often, there is a need to use existing equipment so the
process needs to fit the equipment and not vice-versa.
Raw materials are often overlooked but sometimes can cause unexpected
results. Often in the laboratory, a single source or even a single bottle of a
starting material is used. When scaled, larger quantities are used. The starting
materials are often purchased from different suppliers and not laboratory
supply vendors. The quality can vary from what was used in the laboratory.
Sometimes, even minor differences in quality can have an important and
unexpected effect on the process. I was involved with one product that had
been successfully made and sold in Europe. When the manufacturing needs
were so large that it also needed to be made in the U.S., there were terrible
quality issues. After months of fighting the problem, we learned the quality
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