Membrane Gas Separation

Membrane Engineering: Progress and Potentialities in Gas Separations
305
Hydrogen product purity and the levels of specifi c product impurities are, of course, 
critical to process selection. Cryogenic and membrane processes normally produce hydro-
gen at 90 – 98 vol%, whereas the PSA process normally produces hydrogen at 99+ vol%. 
However, depending on the fi nal destination of the upgraded hydrogen, it can be more 
conveniently applied to other operations (Table 14.3 ). If the upgraded hydrogen is used 
as a primary source of make - up hydrogen to a high pressure hydrotreater or hydrocracker, 
a high purity hydrogen is required and the PSA is often the best choice. If the upgraded 
hydrogen is only an incremental portion of the make - up hydrogen to a hydroprocessor, 
lower product purity is usually required and membrane systems are preferable, owing to 
the lower capital costs. If the feedstock to be upgraded contains only hydrogen and 
hydrocarbons, then the principal impurity in the hydrogen product will be methane. When 
3 – 10 vol% methane can be tolerated in the hydrogen product, the membrane and cryo-
genic systems are the most economical. If the feed to the upgrading process contains 
substantial quantities of CO and CO 
2
, then PSA and the membrane units are most often 
selected, whereas cryogenic is not suitable.
Apart from the economic considerations, the choice among the three separation proc-
esses is generally made taking into account the specifi c process from which the hydrogen 
is produced and the specifi c requirements that the upgraded H 
2
must have, also consider-
ing the fi nal destination of the stream. Table 14.4 reports some selection guidelines of the 
most important processes of hydrogen production in refi neries [77] .
The largest source of easily recovered hydrogen in a refi nery is the off - gas from cata-
lytic reforming. This off - gas typically contains from 70 – 90+ vol% hydrogen with the 
balance being C 
1
to C 
6+
hydrocarbons. If large quantities of reformer off - gas are to be 
upgraded for use as a primary source of make - up hydrogen for a hydrocracker or hydrot-
reater, the PSA process is normally used. The high hydrogen content makes the adsorbent 
requirements low, and the hydrocarbons in the tail gas sent to fuel are relatively small 
with respect to the feed gas. The aromatic and HCl content of the feed do not require 
special pre - treatment. In some cases, there is a requirement for relatively small quantities 
of hydrogen at 98+ vol% purity for uses such as catalyst regeneration. Membrane systems 
using feed compression may be most economical for these applications requiring less than 
about 1000 m 
3
/h of hydrogen product, owing to low capital costs. For large fl ows, cryo-
genic upgrading has sometimes been used. However, the feed hydrogen purity is often 
too high, and external refrigeration is required. Aromatic removal in the pre - treatment 
section is also an important design consideration. Unless recovered hydrocarbons are of 
high value, the cryogenic process is not normally used to process reformer off - gas alone. 
The high pressure and low pressure purge gases from high pressure hydrocrackers and 
hydrotreaters are good candidates for hydrogen upgrading. The recovered hydrogen is 
Table 14.3  Comparison among the three units as function of feed and product conditions 
Membrane system
PSA
Cryogenic
Feed composition, (H 
2
%)
30 – 90%
75 – 90%
30 – 75%
Feed pressure and 
product fl ow
>
10 bar 
<
30 000 m 
3
/h
15 – 30 bar 
1000 – 10 
5
m 
3
/h
>
20 bar 
>
10 
5
m 
3
/h
Product purity
90 – 98%
>
99%
90 – 98%

---