Page 227 - Design of Simple and Robust Process Plants

212 Chapter 5 Process Simplification and Intensification Techniques
. Reactors, using homogeneous gas and liquid systems, and heterogeneous sys-
tems (L/L, G/S and L/S, G/L , G/L/S). Although different systems have differ-
ent simple designs, some overall conclusions can be drawn:
± Adiabatic designs are favorable, and can be enforced on the system in differ-
ent ways; these include higher reactant or diluent (solvent) flow, lower inlet
temperature, and cold feed injection.
± Non-adiabatic designs have an energy household which is preferably based
on evaporative cooling followed by indirect heat transfer.
± Heterogeneous systems are often limited by mass transfer; in such cases agi-
tated stirred tanks are a poor choice. (Most developments on intensification
concentrate on improvements on mass transfer limited systems; see Section
5.5.2.)
± Multi-stage reactor systems and tubular reactors have a higher level of com-
plexity.
. Distillation trains can be significantly reduced in the number of columns, by
taking maximum benefit of dephlegmators, the application of side draws,
and divided wall columns.
. Extraction operations are more frequently practiced as a multi-stage separa-
tion, which provides the opportunity to combine more extraction/wash func-
tions within the same apparatus. Other advantages of multi-stage separations
include increased extraction efficiency, with less solvent recovery. Thus, dif-
ferent types of columns are applied, from packed column, tray columns and
mechanically agitated columns up to centrifugal extraction
. Adsorption is most commonly performed in fixed beds; indeed, the applica-
tion of PSA at high-frequency switching for the gas adsorption desorption
cycle is being increasingly utilized as a first choice.
. Heat transfer units are moving towards plate (finned) exchangers, next to the
conventional types. This trend is based largely on the compactness and cost
effectiveness of the plate systems.
. Fluid transport requires further emphasis on the application of gravity flow or
pressure differences as driving forces. In this respect, the elimination of
pumps and compressors should be the primary target.
. Piping may be a major contributor to complexity, and several guidelines have
been developed to make piping systems more robust, and less complex.
. Instruments are the eyes, ears and hands of the process. The requirements on
instruments concentrate on the robustness of the installed measurement.
The avoidance of superfluous instruments is an option that is exploited by
companies with remotely operated process in order to avoid call-out visits by
engineers.