Page 148 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 133
± What is the effect on the operability of the process?
± What is the effect on the controllability?
± What is the impact on the safety and environmental aspects?
± Constraints can often been neutralized by adequate provisions. Are these pro-
visions included in the economics?
. Integration of raw material streams at chemical complexes (direct connection
between processes) receives growing attention, as economic savings can be
considerable.
. Integration of raw materials receives specific value in cases when:
± lower-purity (below market quality) products can be processed. This often
leads to less separation effort at the supplier side; for example, an ethyl ben-
zene process can effectively separate ethylene from ethane during reaction.
± a product can be delivered in a condition where it effectively can be utilized
by the consumer. Why are process streams concentrated or condensed, while
the consumer dilutes or evaporates these before usage? The do, undo, redo
statement is also applicable between processes.
± the storage of highly toxic materials can be avoided or minimized, e.g., for
chlorine.
. Integration of utility streams. Utility streams can be rated in quality terms.
For thermal energy, the quality term is its capability to generate work also
expressed as exergy, next to temperature and steam level. Other streams such
as water and hydrogen are often ranked in purity terms. The quality rating
was introduced to enable reuse of a stream by cascading it over several pro-
cess functions, as is widely applied for integration.
. The technique to minimize usage of utility streams is called ªpinch technol-
ogy/analysisº. This was originally developed for energy reduction, after the
energy crisis in the 1970s. The method was based on the identification of the
minimum amount (target) required for the flowsheet under study, and
taught how to approach this target. Pinch technology, meanwhile, has been
extended with other service streams such as water and hydrogen.
. Pinch analysis has resulted in the development of design guidelines which
have been listed.
. Pinch analysis should not be limited to the process, but also applied to site/
complex integration.
. A well-known technique to identify opportunities for improvements is based
on exergy analysis. This technique has been described and illustrated with
exergy flow diagrams also called. Grassmann diagrams clearly show where
exergy losses occur.
. The order of priorities to achieve simple and low-cost (energy-wise) solutions is:
Apply process integration in the following order of: priority; unit, section, process,
site (complex).
± Another order of priority is:
Apply heat cross-exchange between flows of similar heat capacity (mass flow times
specific heat).
These two orders of priority do not need to be in conflict.