Membrane Gas Separation

308
Membrane Gas Separation
For the process selection, general selection guidelines can be helpful, at least in elimi-
nating an inappropriate process. In order to use such guidelines, feed characteristics, 
contaminant levels, required product purity, allowable product impurities, pressure levels 
and fl ow rates must be known. These parameters can be used in conjunction with expe-
rience - based, application - specifi c guidelines to select the optimum process. The mem-
brane systems, owing to their high fl exibility, reliability, modularity and ease of control, 
compete well with the other two consolidated technologies, in particular for some specifi c 
applications typical of refi nery hydrogen upgrading. 
In order to compare the three different systems, energy and mass intensity [78,79] were 
calculated as reported below, taking into account the power required by the system 
(Equation 14.1 ) and the steam and cooling water necessary (Equation 14.2 ). Moreover, 
a new indicator was defi ned as productivity to footprint ratio (Equation 14.3 ).
energy intensity
power required
H product flow rate
=
2
(14.1)
mass intensity
steam and cooling water required
H product f
=
2
llow rate
(14.2)
productivity footprint
H product flow rate
footprint
=
2
(14.3)
Table 14.5 summarizes the data for H 
2
recovery from refi nery off - gas by means of three 
different separation technologies, as reported by Spillman [80] , considering polyimide 
membranes. A comparison among them is provided by the calculation of the relative three 
indicators. A lower investment cost than pressure swing adsorption (PSA) or cryogenic 
separation was estimated for the H 
2
recovery from refi nery off - gas by polymeric mem-
branes. This comparison is from 1989; however, since then polymeric membrane capital 
prices have dropped.
Table 14.5 H
2
recovery from refi nery off - gas (data adapted from Reference 80 ) 
Membrane 
system (80 ° C)
Membrane 
system (120 ° C)
PSA
Cryogenic
H 
2
recovery, %
87
91
73
90
H 
2
purity, %
97
96
98
96
Product fl ow rate, m 
3
/h
3257
3375
2643
3375
Power, kW
220
220
370
390
Steam, kg/h