Page 117 - Design of Simple and Robust Process Plants

102 Chapter 4 Process Synthesis and Design Optimization
Gas separations These are mostly limited to:

. pressure distillations (including partial condensation);
. cryogenic distillations;
. absorption;
. adsorption/chemisorption;
. membrane; and
. reaction.
Pre-selection guidelines are:
. To remove selectively one component with absorption/desorption. Examples
are: (i) the absorption of ethylene oxide from the reaction gas into water, or of
methanol from syn-gas; (ii) chemical absorption/desorption for application
without available selective solvents, such as CO 2 and H 2 S removal from refin-
ery and synthesis gases; (iii) NO x and SO 3 conversion in nitric acid and sulfu-
ric acid processes, respectively.
. Concentrate selectively a component through adsorption/membrane, e.g.,
hydrogen from methane. This is a standard application for a pressure swing
adsorption (PSA) unit.
. Separate components with dew points of gases around ambient conditions
not lower than ±40 C through pressure distillation, e.g., distillation separa-
tions of C3 and C4 streams.
. Separate components with low boiling points £40 C, e.g., cryogenic distilla-
tion, such as C1 and C2 separations, air separation. Hydrogen is an exception
due to its very low boiling point.
. Remove low concentrations of components with absorption/adsorption/che-
misorption, e.g., hydrocarbons (odor, toxic, environmental components)
removal from vent gases.
. Remove components without the availability of a selective medium through
reaction, e.g., hydrogenation of acetylenes or di-olefins from olefins, the
removal of NO x from flue gasses, catalytic incineration of hydrocarbons.
The above guidelines are to be used as pre-selection for gas separations, though ulti-
mately the final selection must be carried out after the evaluation of alternatives on
the basis of NPV.

Liquid separations The most commonly used liquid separations for homogeneous
systems are:
. distillation, including evaporation/condensation;
. extractive and azeotropic distillation;
. extraction;
. adsorption/chemisorption;
. crystallization;
. membrane; and
. reaction.