Membrane Gas Separation

One stage
1.2×10
6
Nm
3
/h
10%CO
2
55 bar
2% CO
2
2% CO
2
44% CO
2
1.7 bar
Use for fuel or flare
Two stages
1.2×10
6
Nm
3
/h
10%CO
2
55 bar
86% CO
2
400 KW 
compressor
10% CO
2
Methane loss:1.5%
44% CO
2
Figure 14.6 Flow scheme of one - stage and two - stage membrane separation systems 
to remove CO 
2
from natural gas. Reprinted with permission from Ind. Eng. Chem. 
Res., Natural gas processing with membranes: an overview, by R. W. Baker and K. 
Lokhandwala, 47, 7, 2109 – 2121, Copyright (2008) American Chemical Society

---

294
Membrane Gas Separation
Water is a common impurity in natural gas that must be removed to prevent hydrate 
formation. This is a good opportunity for the application of membrane technology, but 
to be competitive, the membrane system must minimize the loss of methane with the 
permeate water [5] . This loss can be reduced, on the one hand, by choosing membranes 
with desired selectivity, but, on the other hand, by the process parameters, because this 
separation is pressure ratio - limited [5] . 
Currently, PRISM
®
membranes provide an attractive alternative to traditional glycol 
dehydration systems (Figure 14.8 ) based on simple process designs, lower costs, and the 
other benefi ts listed below. These benefi ts become even more pronounced as the industry 
produces natural gas from very remote locations [55] . An offshore membrane system for 
Shell Nigeria was designed to dry 600 000 Nm 
3
/h of natural gas from an inlet dew - point 
of 41 ° C to an outlet dew - point of 0 ° C at 38 bar. It was designed and built by Petreco, an 
Air Products PRISM Membranes licensed partner. Other plants were installed in Italy and 
Holland.
Several natural gas reserves are considered sub - quality because of the high nitrogen 
content. The gas pipeline specifi cations for inert gases, in fact, fi xes the nitrogen content 
to a 4% limit [56] . Currently, cryogenic distillation is used for this separation; however, 
b
a
Figure 14.7 Photographs of UOP membrane plants using spiral - wound modules of 
cellulose acetate membranes for CO 
2
separation. (a) Plant compact enough to be moved 
when the gas fi eld is exhausted after a few years of operation; capacity of 10 000 Nm 
3
 /h 
membrane designed to reduce CO 
2
gas from 6% to 2%. (b) A 600 000 sdNm 
3
 /h unit 
designed to reduce CO 
2
gas from 5.7% to 2%
Figure 14.8 Membrane process fl ow schematics of a natural gas dehydration plant of 
PRISM

---

Membrane Engineering: Progress and Potentialities in Gas Separations
295
Feed gas
15% N
2
Methane permeable
membrane
6% N
2
30% N
2
N
2
Cryogenic
plant
Methane
(ca. 1%N
2
)
Pipeline gas
(< 4% N
2
)
Figure 14.9 Scheme of a hybrid membrane/cryogenic distillation plant for removal 
of nitrogen from natural gas. Reprinted with permission from Industrial & Engineering 
Chemistry Research, Future directions of membrane gas separation technology, by 
R. W. Baker, 41, 1393 – 1404, Copyright (2002) American Chemical Society
membrane technology could be used here. The only challenge is the reduction of the 
methane loss in the permeate. However, methane - permeable membranes can be used 
conveniently in combination with a cryogenic plant (Figure 14.9 ). The feed gas, contain-
ing 15% nitrogen, is separated by a membrane into two streams: a residue stream (reten-
tate) containing 30% nitrogen to be sent to the cryogenic plant and a permeate stream 
containing 6% nitrogen to be sent to the product pipeline gas. The membrane unit reduces 
the volume of the gas to be treated by the cryogenic unit by more than half. Simultaneously, 
the concentrations of water, C 
3+
hydrocarbons, and carbon dioxide are brought to very 
low levels, because these components also preferentially permeate the membrane. 
Removal of these components prior to cryogenic condensation is required to avoid freez-
ing in the plant. The savings produced by using a smaller, simpler cryogenic plant more 
than offset the cost of the membrane unit [57] .