Page 194 - Design of Simple and Robust Process Plants

5.6 Overall Process Simplification 179
5.6.1.2 Additives and supplies
It is possible that minor amount of additives might be used, but with major conse-
quences. A careful review of all process additions and their impact might provide
surprising results. Let us consider the inhibitor and its solvent used as an additive
for polymer prevention. Current designs apply extensive recycling in order to reduce
environmental losses. Questions about additives include:

. Is it understood what happens to the inhibitor and its solvent during proces-
sing?
. Are they degraded during processing?
. Do these components or their degradation products leave the process, and if
so, at what point?
. If accumulation occurs, where might this happen, and what will be the con-
sequences?
Several examples exist of additives and their solvents which require process ªadd-
onsº to overcome their effects. It is not uncommon to find a washer or vent-gas
scrubber (or even an oxidizer) as an add-on provision. One might consider what type
of provisions are required if these chemicals were toxic with regard to biological
treatment. For example, the chemicals might contain nitrogen, and this would
require an additional step in a waste treatment plant, or it might add to the NO x
load. These provisions might be avoided by using other inhibitors or solvents.
A good choice for the selection of a solvent for additives is a pre-existing process
component.
The type of neutralizing agent might also be challenged. A neutralizer such as caus-
tic soda might result in the need for an expensive step to remove the sodium salt
produced. Replacement with NH 4 OH might be beneficial if the ammonium salt
formed were to remain in the product, but if it were-to appear in the wastewater, it
might lead to a high nitrogen load, or its degradation might impact on the NO x load.
Another option might be to use an organic base that would remain in the product.
A seawater cooling system was badly affected by mussels growing in the tubes of
heat exchangers. Periodic chlorine ªpulsesº to prevent mussel growth were effective,
but they introduced a very toxic and dangerous chemical at the chemical complex.
Replacement of chlorine by hypochlorite solutions was safer and avoided the need
for many on-site provisions to cope with a potential emergency situation (Paping et
al., 1999).
Chain transfer agents are quite commonly applied in polymerization processes,
and have also been the subject of critical reviews. The removal of these components
can lead to a need for expensive recovery systems. One process which was brought
to the author's attention used a chlorinated hydrocarbon that required not only an
extensive recovery system but also an incinerator with Cl destruction and HCl recov-
ery. Time spent in developing a less harmful transfer agent that was able to remain
in the product was successful, and implementation of this scheme resulted in a
much simpler overall process.