Membrane Gas Separation

14.3.1
Current Applications of Membranes in GS
As early as in 1950, Weller and Steiner [32] considered membrane processes as feasible 
for the separation of hydrogen from hydrogenation tail gas, enrichment of refi nery gas 
and air separation. However, commercial membranes capable of performing these separa-
tions economically only became available in 1970. In 1980, Permea, with its hydrogen 
separating Prism - membrane, launched the fi rst large industrial application of gas separa-
tion membranes [9,33] . Since then, membrane - based operations, substituting or integrated 
with the traditional ones, had a rapid growth, with many companies, such as Cynara -
Natco, Separex - UOP, GMS, Generon, Praxair, AirProducts, UBE are involved in this 
fi eld [34,35] . 
A list of the main industrial applications of membrane technology in GS is given in 
Table 14.1 , along with the membrane materials used and the status of the membrane 
technology.
14.3.2
Hydrogen Recovery 
As previously mentioned, the fi rst widespread commercial application of membranes in 
GS was the separation of hydrogen in the ammonia purge stream, by using Permea 
Prism ™ systems. Hydrogen recovery is applicable to several processes, divided into three 
main categories: 
• hydrogen recovery from ammonia purge streams
• syngas ratio adjustment
• hydrogen recovery in refi neries.
In the ammonia process, the purge stream, almost clean and free of condensable vapours, 
consists of a mixture of hydrogen, nitrogen, methane and argon, delivered at a high 
pressure (136 bar). It is, thus, the ideal application for the membrane technology, since 
1
10
100
1000
10000
Selecti
vity
, -
0.01
1
100
10000
Permeability, 
barrers
O
2
/ N
2
CO
2
/ CH
4
H
2
/ N
2
H
2
/ CH
4
CO
2
/ N
2
N
2
/ CH
4
Figure 14.3 Trade - off of Robeson ’ s plot for different GS applications. Data elaborated 
from [30]

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288
Membrane Gas Separation
Table 14.1  Main industrial applications of membrane technology for GS
Separation
Process
Traditional 
technology
Membrane 
material
Status of 
membrane 
technology 
application
H 
2
/N 
2
Ammonia 
purge gas
PSA
Polysulfone
Plant installed 
(Prism by 
Permea)
Pd - based
Lab scale
H 
2
/CO
Adjustment of 
H 
2
/CO 
ratio in 
syngas 
plant
PSA
Silicon rubber
Lab scale
Polyimide
Plant installed 
(Separex)
Pd - based
Lab scale
H 
2
/
hydrocarbons
Hydrogen 
recovery in 
refi neries
PSA
Silicon rubber 
Polyimide
Plant installed 
(Sinopec 
Zhenhai 
(china) 
Prism by 
Permea 
Du Pont)
Pd - based
Lab scale
H 
2
/light 
hydrocarbons
Ethylene 
cracker old 
trains
Cryogenic 
distillation
PTMSP; PMP
Pilot plant
Pd - based
Lab scale
O 
2
/N 
2
Air separation
Cryogenic 
distillation
Silicon rubber
Plant installed 
(Cynara; 
Separex; 
GMS; Air 
Products)
Polysulfone
Plant installed 
(Permea)
Polyimide
Plant installed 
(Medal; 
Dow -
Generan; 
UBE)
Polyphenilene
Plant installed 
(Aquilo)
Ethyl 
Cellulose
Plant installed 
(Air Liquide)
Ion transport 
(perovskite)
Lab scale
Pd - based
Lab scale
N 
2
/CH 
4
Nitrogen 
removal
Cryogenic 
distillation
Silicon rubber 
PMP 
Parel
Pilot plant
PEBAX
Lab scale
H 
2
O/CH 
4
Dehydration
Glycol 
absorption
Cellulose 
acetate 
Polyimide 
polyaramide
Plant installed 
[51]

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Membrane Engineering: Progress and Potentialities in Gas Separations
289
Separation
Process
Traditional 
technology
Membrane 
material
Status of 
membrane 
technology 
application
CO 
2
/CH 
4
Sweetening of 
natural gas
Amine 
absorption
Cellulose 
acetate
Offshore 
platforms 
In Thailand gulf. 
(Cynara -
NATCO); 
Grace -
Separex
Polyaramide
Plant installed 
(Medal)
Polyimide
Plant installed 
(Medal)
Perfl uoro -
polymers
Plant installed 
(MTR)
CO 
2
/
Hydrocarbons
Natural gas 
liquid 
removal
Glycol 
absorption 
and cooling 
inn a 
propane 
refrigeration 
plant 
( 
−
20 ° C)
Cellulose 
acetate 
Polyimide 
polyaramide
Early 
commercial 
stage. A 
demonstration 
system 
installed
CO 
2
/N 
2
CO 
2
capture 
from fl ue 
gas streams
Amine 
absorption
Polyimide
Pilot plant
FSC; 
PEEKWC; 
Zeolite, 
silica 
based; 
carbon
Lab scale
VOCs/gas
Polyolefi n 
plant resin 
degassing; 
Ethylene 
recovery
Adsorption, 
Refrigeration 
and turbo -
expander 
plants
Silicone 
rubber 
PTMSP
Plant installed 
(MTR; OPW 
Vaporsaver ™ )
Table 14.1 (continued)
hydrogen is highly permeable with respect to the other gases and the stream already 
provides the necessary driving force for promoting the permeation. Actually, Permea, Inc. 
(now owned by Air Products and Chemicals, Inc.) designed a two - step membrane process 
for this separation (Figure 14.4 ).
Similar membrane systems are also already applied for syngas ratio adjustment (H 
2
/
CO ratio) of the streams coming out by reformers. The process specifi cations are strictly 
related to the use of the stream. Generally, membranes are employed for stripping hydro-
gen out of the syngas in order to reduce the H 
2
/CO ratio. The stream is clean and already 
at a high pressure, thus ideal for use of a membrane. At the moment, several hundred 
hydrogen separation plants have been installed [36] . 

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290
Membrane Gas Separation
The demand of hydrogen recovery in refi neries is rapidly increasing also because of 
environmental regulations. The hydrogen content in the various refi nery purges and off -
gases ranges between 30 – 80%, mixed with light hydrocarbons (C 
1
– C 
5
); 90 – 95% hydro-
gen purity is required to recycle it to a process unit. A typical refi nery operation is the 
separation of the hydrogen contained in the stream coming out from the hydrocracker. 
Actually, this process has the disadvantage of removing 4 mol of hydrogen for every mol 
of hydrocarbon removed. The membranes can be used alone or together with an absorber 
system, at a reduced capital cost and better process effi ciency [32] . At the moment, the 
Prism ™ system (using polysulfone hollow fi bres with a thin silicone fi lm on it) is domi-
nant on the market for this kind of separation, showing interesting selectivities [37] . 
The main problem, related to an actual limited application of membrane technology, 
is membrane plasticization by strongly adsorbing components and the condensation of 
the light hydrocarbons on the membrane surface, which strongly affect the membrane 
performance and durability. The development of new membrane materials more resistant 
to high hydrocarbon partial pressure and the introduction of more effi cient pre - treatment 
stage able to reduce pollutant content, will rapidly expand this technology [7] .