Business Considerations - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 81

own identity. In a continuous process acetic acid, ethanol, and catalyst are
continuously fed to a heated reactor. At the same rate, crude reaction mixture
is continuously removed from the reactor and continuously fed to a distillation
column. At the same rate, the fractions are continuously removed from differ-
ent sections of the column, with more volatile materials continuously exiting
the top of the column and less volatile chemicals continuously exiting the
bottom of the column. Ideally, the streams are also analyzed on a continuous
basis.

H + catalyst

+ CH 3 CO 2 CH 2 CH 3

CH 3 CO 2 H CH 3 CH 2 OH

Acetic acid Ethanol Ethyl acetate

Each method has its advantages. It is easier to start up and shut down a
batch process. Batch processes are desirable if many different materials are
to be made in the same equipment. If the reaction causes a coating or scale
on the reactor walls, a batch process is more desirable because reactors can
be cleaned between batches. Batch processes are also best if the reaction is
not performed on a 24 hours per day and seven days per week schedule. If
something goes wrong in a batch reaction, you can stop and investigate the
problem prior to starting the next batch.
If the same or a very similar material is to be made in the same equipment
for an extended period of time, often a continuous process is better. There
are not the long lags of time lost as a batch process is begun or ended. Once
the process is on stream, product is continuously being made. There is a
better opportunity for consistent quality. Once the parameters of the process
are set, the reaction can run for extended periods of time making the same
very uniform material. Typically, more material can be made in the same
size equipment when a continuous versus batch process is used. Usually,
less manpower is needed to keep a continuous process running versus the
demands of starting and finishing batch-wise reactions. In one example, a
drug firm, Dr. Reddy's, replaced a batch process with a continuous process
and achieved an annual savings of $1.1 million [12].
Even if property differences are needed for the marketplace, they can
often be accommodated in a continuous process by making adjustments
to the operating parameters without stopping the process. For example,
polyethylene is made by a continuous process yet the same plant will make
different grades that vary in density. This is done by adjusting the input ratio
of comonomer, such as 1-hexene, to monomer ethylene. There will be a time
during the transition when intermediate density polyethylene is made. With