Page 93 - Design of Simple and Robust Process Plants

78 Chapter 4 Process Synthesis and Design Optimization
. Destroy without recovery
. Controlled waste disposal ( to be avoided, but like in the mining industry it
isn't avoidable)
Process designs often apply internal recycling as a practical solution. There are
often unrecognized constraints to recycling, and the build-up of impurities in
recycle streams might lead to considerable problems. During process synthesis the
separations are often designed based on the main components, while impurities are
supposed to leave the process via vents, tar or product streams. The real answer
must be quantified by determining all impurities in the feed and the reactor outlet.
A mass balance which includes all impurities must be prepared, together with a
clear identification of where they leave the process and where accumulation takes
place. The installation of drag streams might be unavoidable. These impurities can
become the key components for separations, and can have a major impact on the
optimal process train.

4.1.2.5 Decomposition reactions
These may play an important role; if they are exothermal they are already considered
under reactive chemical constraints. Product degradation without heat release can
lead to an undesired situation for product purity and reasons of selectivity. They
may also place constraints on the processing temperatures.

4.1.2.6 Other constraints
Fouling constraints might be reactive chemical constraints with very low reaction
rates; they are very difficult to predict without long-duration testing. The effects are
often experienced during operations such as reboiler fouling where the wall temper-
ature might have an effect . Catalyst deactivation or aging are often measured in the
laboratory or in the production plant, and can often be reduced by selecting other
operational conditions. The root causes may be physical, and include the precipita-
tion of solids as a result of temperature and concentration effects. The fouling of
seawater exchangers, air compressors, and air coolers is subject to external sources.
As the latter can be overcome by specific actions, they are ± strictly speaking ± not
constraints.
Corrosion constraints may determine part of the separation sequence. The removal
of highly corrosive material often leads to a need for very expensive and specific con-
structions. Consequently, such components are removed early in the process. An
example is the nitration reaction which is processed in glass-lined or tantalum
equipment, and where neutralization of these streams before further processing is
standard practice.
Utility constraints in theory do not exist, as any utility level can be generated in the
process, though in practice this might lead to high cost. Problems associated with
low temperature cooling, or a need for temperatures above available steam tempera-
ture, can be solved but require high levels of investment ± making the process less
attractive.