Page 135 - Design of Simple and Robust Process Plants

120 Chapter 4 Process Synthesis and Design Optimization
. The accumulation of trace components in the receiving process; an overall
balance needs to be made to track the removal or destruction of these compo-
nents.
. The effect of trace components on the reaction of the receiving plant±catalyst
poisoning may in particular be a concern.
. Fouling caused by impurities need to be verified before final selection.
Although the constraints and potential problem areas outlined here must be exam-
ined carefully before a decision is made, the potential for the development of more
efficient processes is clear. As mentioned previously, while consideration of the con-
straints is primary in the achievement of clever integration, the integration of raw
materials evolves automatically in line with site integration (see Chapter 7).

4.2.3.3 Integration with utility streams
Utility streams are streams that serve as aids for the operation, for example power
steam nitrogen. They are quite commonly supplied by a utility plant either intern-
ally or externally, have connections between the plants over the site, and are often
rated in terms of quality (purity and temperature/pressure level). For thermal
energy these quality ratings can be expressed as a capability to generate work, or
also expressed as exergy next to temperature or pressure level for steam. For specific
material flows such as water and hydrogen, the quality is expressed in purity terms.
These streams are capable of performing a function at a certain level, and might be
reused several times at decreasing quality levels. The most exploited stream function
is heat, which cascades down by reuse in a process at lower temperature. Energy in
the form of fuel might generate power with a gas turbine, and in turn generate
high-pressure steam with a boiler unit. The heat then cascades through different
energy exchange steps (which generally includes steam turbines and process hea-
ters) until it ends up in the environment. The same applies to specific material
streams that are used at high purity, but may be reused at a lower purity level. This
technique has been in use for several years, and has resulted in the development of
higher efficiency designs. The technique to support these designs is known as
ªpinch technology/analysisº, and this ± together with exergy analysis to obtain more
efficient process designs ± will be outlined in the next section.

Pinch technology Pinch technology was first extensively described in A User Guide
on Process Integration for the Efficient Use of Energy (IChemE, 1997; originally 1982),
and has been captured in a number of different software programs. Pinch analysis
is used to address the integration of heat and other service streams.
The principle will be explained below in rudimentary form for heat integration. A
temperature±enthalpy (T±H) diagram is made for the process streams to be cooled
(hot streams), and the process streams to be heated (cold streams). The T±H curves
(known as ªcomposite curvesº) are an accumulation of the hot and cold process
streams (Figure 4.30).
By shifting the cold stream to the left, the composite curves will approach each
other until there is a defined minimum temperature difference DT min between the