Method of preparing high purity light gas by multiple-step gas separation

A purified light gas consisting of hydrogen or helium and having a high purity of, for example, 99.99% or more is prepared from a feed gas containing at least 90 molar % or more of H.sub.2 or He and substantially no CO.sub.2 by gas-separating the feed gas through a plurality of gas-separating membrane modules each comprising at least one polymeric gas-separating membrane and having a gas-permeating ratio P.sub.A /P.sub.CH4 of 100 or more, wherein P.sub.A is a permeating rate of the light gas and P.sub.CH4 is a permeating rate of methane gas, in such a manner that each fraction of the feed gas permeated through and delivered from a preceding membrane module is fed to a next membrane module under the pressure, per se, of the delivered fraction of the feed gas without pressurizing the delivered fraction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for preparing a high purity light 
gas consisting of a member selected from the group consisting of hydrogen 
or helium. More particularly, the present invention relates to a method 
for preparing a light gas consisting of hydrogen or helium and having a 
high purity of 99.99 molar % or more when a content of water is omitted 
from the calculation of the purity, by a multiple step gas-separating 
procedure in which a feed gas to be purified is selectively permeated 
successively through a plurality of gas-separating polymeric membrane 
modules. 
The term "light gas" used herein refers to a gas consisting essentially of 
hydrogen or helium. 
Also, the term "gas-separating membrane module" used herein refers to a 
module comprising at least one polymeric membrane which selectively allows 
a light gas to permeate therethrough to concentrate the light gas and to 
separate impurities other than water therefrom. 
2. Description of the Related Arts 
In a known method used industrially a hydrogen or helium gas is 
concentrated or refined from a mixture gas containing hydrogen or helium 
by a low temperature processing method or an absorption method, and 
recently, it has become known that the concentration or refining of the 
hydrogen or helium gas can be conducted by using a gas-separating 
polymeric membrane. 
But this polymeric membrane gas-separating method has the following 
advantages and disadvantages. 
Advantages 
(A) The gas separating apparatus is compact and cheap, and requires little 
maintenance therefor. 
(B) The gas-separating procedure is simple and can be stably operated under 
a wide range of operating conditions. 
(C) When the feed gas is supplied under a sufficiently high pressure, the 
gas-separating operation can be effected without an additional supply of 
energy to the operation system. 
(D) A fraction of the feed gas, which has not permeated through the 
gas-separating membrane, can maintain a pressure at a level substantially 
the same as the original level. 
Disadvantages 
(A) A fraction of the feed gas, which has permeated through the 
gas-separating membrane, exhibits a significantly reduced pressure in 
comparison with the original pressure of the feed gas. 
(B) To obtain a high purity hydrogen or helium gas having a purity of 99.99 
molar % or more in a dry condition, the gas-separating operation must be 
repeatedly carried out in a large number of gas-separating membranes. 
(C) When a feed gas is subjected to a plurality of gas-separating 
operations through a plurality of gas-separating membranes, the permeation 
of the feed gas through each gas-separating membrane reduces the pressure 
of the feed gas. Accordingly, a fraction of the feed gas, which has 
permeated through a gas-separating membrane and thus has a reduced 
pressure, must be compressed to elevate the pressure thereof before 
feeding it to a next gas-separating operation: This compression requires a 
large amount of energy. 
In view of the above-mentioned advantages and disadvantages, it is 
conventionally believed that the polymeric membrane gas-separating method 
is useful for briefly separating a hydrogen or helium gas from a feed gas, 
at a low cost, but is not beneficial for producing a high purity hydrogen 
or helium gas having a purity of 99.99 molar % or more. 
Japanese Examined Patent Publication (Kokoku) No. 44-5526 and Japanese 
Unexamined Patent Publication (Kokai) No. 54-72778 disclose a method for 
purifying and concentrating a hydrogen or helium gas by using a plurality 
of gas-separating polymeric membranes arranged in multiple steps; the 
hydrogen or helium gas being able to easily permeate through the 
membranes. 
Nevertheless, the gas-separating method disclosed in these publications is 
disadvantageous in that a pressure-raising operation, for example, a 
compressing operation, must be applied to a fraction of a feed gas which 
has permeated through a preceding gas-separating membrane, before the 
fraction is fed to a next gas-separating membrane. 
Namely, the necessity for a pressure-raising operation in the 
above-mentioned conventional method increases the cost thereof and thus 
brings a decreased cost-efficiency in the production of the high purity 
hydrogen or helium gas. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method for preparing a 
high purity light gas consisting of a member selected from the group 
consisting of hydrogen and helium from a feed gas containing the light 
gas, by using a plurality of gas-separating polymeric membrane modules, at 
a high efficiency and a low cost. 
Another object of the present invention is to provide a method for 
preparing a light gas consisting of a member selected from the group 
consisting of hydrogen and helium, and having a high purity of 99.99 molar 
% or more when a content of water is omitted from the calculation of the 
purity, by removing therefrom organic substances, especially hydrocarbon 
compounds, for example, methane and ethane and inorganic substances, for 
example, nitrogen and oxygen, through a plurality of gas-separating 
polymeric membrane modules which can be simply and easily operated, and 
with a reduced energy consumption. 
The above-mentioned objects can be attained by the method of the present 
invention, which comprises subjecting a feed gas containing at least 90 
molar % of a light gas consisting of a member selected from the group 
consisting of hydrogen and helium and substantially no carbon dioxide to a 
gas separating procedure in a plurality of steps, through a plurality of 
gas-separating membrane modules, each gas-separating membrane module 
comprising at least one gas-separating membrane and having a 
gas-permeating rate ratio P.sub.A /P.sub.CH.sbsb.4 of 100 or more, wherein 
P.sub.A represents a permeating rate of the light gas and P.sub.CH.sbsb.4 
represents a permeating rate of methane gas, in such a manner that a 
fraction of the feed gas, which has permeated through and has been 
delivered from a preceding gas-separating membrane module, is fed to a 
next gas-separating membrane module without an increase of the pressure of 
the delivered fraction of the feed gas.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a conventional method for preparing a high purity light gas consisting 
of hydrogen or helium, a feed gas is successively permeated through a 
plurality of gas-separating polymeric membrane modules. For example, 
referring to FIG. 14, a feed gas is fed to a first gas-separating membrane 
module 1 through a feeding path 1a and is permeated through the membrane 
module 1. A pressure loss of the feed gas is inevitable in the membrane 
module 1, i.e., the pressure of a first fraction of the feed gas which has 
passed through and has been delivered from the membrane module 1 become 
lower than the original pressure of the feed gas. The pressure of the 
first fraction of the feed gas is increased by a first pressure-increasing 
apparatus X.sub.1, for example, a compressor, and then fed to a second 
gas-separating membrane module 2. A second fraction of the feed gas which 
has passed through and has been delivered from the second membrane modules 
2 is fed to a second pressure-increasing apparatus X.sub.2. A first 
residual fraction of the feed gas which has not passed through the second 
membrane modules 2 is discharged or returned to the feeding path 1a and 
mixed with a fresh feed gas to be fed to the first membrane module 1. 
The pressure-increased second fraction of the feed gas is fed to a third 
gas-separating membrane module 3. The resultant third fraction of the feed 
gas, which has passed through the third membrane module, is collected as a 
high purity gas. A second residual fraction of the feed gas, which has not 
passed through the third membrane module 3, is discharged or returned to a 
second membrane module 2. 
In the above-mentioned conventional gas-separating method and apparatus, 
the necessity for the provision of the pressure-increasing apparatuses 
X.sub.1 and X.sub.2 increases the cost of the method and apparatus, and 
thus brings a low cost-efficiency. 
The inventors of the present invention found that, when a specific 
gas-separating polymeric membrane is used, a high purity hydrogen or 
helium gas having a purity of, for example, 99.99 molar % or more, when a 
content of water is omitted in the calculation of the purity, can be 
prepared by successively permeating a feed gas through a plurality of 
gas-separating membrane modules, without increasing a pressure of a 
fraction of the feed gas which has permeated through a preceding membrane 
module and has been delivered therefrom. That is, in the method of the 
present invention, a feed gas containing at least 90 molar % of a light 
gas consisting of a member selected from the group consisting of hydrogen 
and helium and substantially no carbon dioxide is subjected to a 
gas-separating procedure in a plurality of steps. 
The feed gas usable for the method of the present invention may contain 
water (moisture), but preferably, the feed gas is substantially free from 
water. In this gas-separating procedure, the feed gas is successively 
permeated through a plurality of gas-separating membrane modules, each 
comprising at least one polymeric gas-separating membrane and having a 
gas-permeating rate ratio P.sub.A /P.sub.CH4 of 100 or more, wherein 
P.sub.A represents a permeating rate of the light gas and P.sub.CH4 
represents a permeating rate of methane gas, in a manner such that a 
fraction of the feed gas which has permeated through and has been 
delivered from a preceding gas-separating membrane module is directly fed 
to a next gas-separating membrane module, without increasing the pressure 
of the delivered fraction of the feed gas. 
The gas-separating polymeric membrane module usable for the method of the 
present invention has a gas-permeating rate ratio P.sub.A /P.sub.CH4, that 
is, P.sub.H2 /P.sub.CH4 or P.sub.He /P.sub.CH4, of 100 or more, preferably 
150 or more, more preferably 200 or more. This type of membrane module can 
be prepared from aromatic imide polymer membranes as disclosed in Japanese 
Unexamined Patent Publication (Kokai) No. 61-19815 and U.S. Pat. No. 
4,718,921. 
The gas-permeating rate of a gas-separating membrane module was determined 
in the following manner. 
A gas-separating polymeric membrane module to be tested was fixed to a cell 
made of stainless steel to provide a gas-separating apparatus. 
A feed gas to be tested was fed to the gas-separating apparatus under a 
pressure of 2 kg/cm.sup.2 and at a temperature of 45.degree. C., and an 
amount of a fraction of the feed gas permeated through the membrane module 
was measured by a flowmeter. The permeation rate of the permeated gas was 
calculated in accordance with the following equation: 
##EQU1## 
wherein P represents a permeation rate of the permeated gas in N-cm.sup.3 
/cm.sup.2 .multidot.sec.multidot.cmHg, Ga represents a total amount the 
permeated gas in N-cm.sup.3, A represents a gas-permeating area of the 
gas-separating membrane module in cm.sup.2, T represents a gas-permeating 
time in seconds, and DP represents a differential partial pressure of the 
permeated gas in cmHg. 
The polymeric membrane module usable for the present invention is 
preferably composed of a number of gas-permeable hollow fibers which have 
a large gas-permeating area, but the membrane module may be composed of a 
wound gas-permeable membrane, a flat gas-permeable membrane or a flat 
sheet consisting of two or more superposed gas-permeable membranes. 
The hollow fibers usable as a gas-permeable membrane module preferably have 
an outside diameter of 50 to 2000 .mu.m, more preferably 200 to 1000 .mu.m 
and a wall thickness of 10 to 200 .mu.m, more preferably 50 to 150 .mu.m. 
When the outside diameter is too small, the resultant hollow fiber causes 
an excessively large pressure loss of a gas flowing through the hollow of 
the fiber, and when the outside diameter is too large, the resultant 
membrane module has an excessively decreased gas-permeating area per unit 
volume of the module. 
The hollow fiber preferably has a ratio Th/Od of from 0.1 to 0.3, wherein 
Od represents an outside diameter of the hollow fiber and Th represents a 
thickness of wall of the hollow fiber, that is 
##EQU2## 
wherein Id represents an inside diameter of the hollow fiber. 
When the wall thickness Th is too small, the resultant hollow fiber 
exhibits an excessively small resistance to pressure and is easily crushed 
flat, and when the wall thickness Th is too large, the resultant hollow 
fiber sometimes exhibits a small gas-permeating rate. 
In the method of the present invention, the feed gas to be purified 
contains at least 90 molar % of the light gas consisting of hydrogen or 
helium and is substantially completely free from carbon dioxide. 
In the method of the present invention, the feed gas is permeated through a 
plurality of gas-separating polymeric membrane modules in a manner such 
that a fraction of the feed gas which has been permeated through and 
delivered from a preceding membrane module is directly fed to a next 
membrane module without increasing the pressure of the delivered fraction. 
Referring to FIG. 1 showing a three step gas-separating procedure, a feed 
gas supplied through a feeding line 11 is fed to a first gas-separating 
polymeric membrane module 1 through a flow rate-regulating valve V.sub.1, 
a guard filter F and a temperature-regulator H, at a predetermined flow 
rate and temperature and under a predetermined pressure, which can be 
detected by a flowmeter FI, a thermometer TI, and a pressure meter PI. The 
guard filter is used to eliminate dust and/or mist from the feed gas and 
to protect the gas-separating polymeric membrane module from contamination 
by impurities. Accordingly, when the feed gas is clean and is free from 
dust and mist, the guard filter is omitted from the gas-separating 
apparatus. 
The temperature regulator is used for two purposes, as follows. 
(1) Since the gas-separating efficiency of the polymeric membrane is 
variable to a large extent, depending on the temperature of the 
gas-separating system, the temperature of the feed gas must be adjusted to 
a predetermined level so that the gas-separating membrane is constantly 
maintained at a predetermined temperature and stably exhibits a constant 
gas-separating efficiency. 
(2) The gas-separating polymeric membrane must be maintained in a dry 
condition throughout the gas-separating procedure. If the polymeric 
membrane is wetted with a liquid contained in the feed gas, the 
gas-separating efficiency of the polymeric membrane varies to a large 
extent and sometimes is damaged by the liquid. Accordingly, it is 
important that the feed gas is dried before coming into contact with the 
polymeric membrane. For this purpose, the feed gas is heated to a 
predetermined temperature at a position upstream of the polymeric membrane 
module. 
If the feed gas has a predetermined temperature, and is completely dry and 
thus does not cause the polymeric membrane to be wetted, the temperature 
regulator can be omitted from the gas-separating apparatus. 
Generally, the feed gas is preferably fed to the first gas-separating 
polymeric membrane module at a pressure high enough to cause a pressure of 
a fraction of the feed gas, which has permeated through and has been 
delivered from a last membrane module for a final gas-separating 
operation, to be 30 mmHg Ab or more, preferably an ambient atmospheric 
pressure or more. For example, when the gas-separating procedure is 
carried out through three gas-separating membrane modules as shown in FIG. 
1, the original pressure of the feed gas to be fed to the first membrane 
module is preferably adjusted to a level of 10 to 150 kg/cm.sup.2 
.multidot.G. Also, the temperature of the feed gas is preferably regulated 
to a level of 0.degree. C. to 100.degree. C. by the temperature-regulator. 
In the first membrane module 1 of FIG. 1, the feed gas is separated into a 
first fraction which has permeated through the first membrane module 1 and 
a first residual fraction which has not permeated through the first 
membrane module 1. 
The first permeated fraction of the feed gas is fed to a second 
gas-separating polymeric membrane module 2 through a flow line 12 without 
increasing the pressure thereof. The first non-permeated fraction of the 
feed gas is discharged from the first membrane module 1 through a flow 
line 15 and a flow rate-regulating valve V.sub.2. 
In the second membrane module 2 of FIG. 1, the first permeated fraction of 
the feed gas is divided into a second permeated fraction and a second 
non-permeated fraction of the feed gas. 
The second permeated fraction of the feed gas is fed to a third 
gas-separating polymeric membrane module 3 through a flow line 13 and the 
second non-permeated fraction of the feed gas is discharged from the 
second membrane module 2 through a flow rate-regulating valve V.sub.3 and 
a flow line 16. 
In the third membrane module 3 of FIG. 1, the second permeated fraction of 
the feed gas is separated into a third permeated fraction and a third 
non-permeated fraction of the feed gas. The third permeated fraction of 
the feed gas is collected as a high purity gas through a flow line 14. The 
third non-permeated fraction of the feed gas is discharged through a flow 
rate-regulating valve V.sub.4 and a flow line 17. 
Where the feed gas to be fed to the gas-separating apparatus has a pressure 
which is not high enough to accomplish the method of the present 
invention, the pressure of the feed gas is increased by using a 
pressure-increasing apparatus, for example, a compressor, as shown by X in 
FIG. 2. 
Referring to FIG. 2, the pressure-increased feed gas to be fed to the 
gas-separating apparatus is filtered by a guard filter F and heated by a 
temperature regulator H, if necessary, and then is successively 
gas-separated by the first, second, and third gas-separating polymeric 
membrane modules 1, 2, and 3, in the same manner as described above. 
Referring to FIG. 3, a feed gas supplied through a feed line 11 is fed to 
two separate first gas-separating polymeric membrane modules 1a and 1b 
through flow lines 11a and 11b, respectively, and is separated into two 
first permeated fractions and two first non-permeated fractions of the 
feed gas in the two separate first membrane modules 1a and 1b. 
The first permeated fractions delivered from the first membrane modules 1a 
and 1b through flow lines 12a and 12b are mixed together and directly fed 
into a second membrane module 2 through a single flow line 12 without 
increasing the pressure thereof. The first non-permeated fractions of the 
feed gas are separately discharged from the first membrane modules 1a and 
1b through flow lines 15a and 15b, respectively. The second and third 
gas-separating operations are carried out in the same manner as mentioned 
above. 
In an embodiment of the method of the present invention as shown in FIG. 4, 
a feed gas is divided into two portions thereof, each divided portion of 
the feed gas is gas-separated in two sub-steps in a first gas-separating 
step, and the first permeated fractions of the feed gas are mixed 
altogether and then further gas-separated in two sub-steps in a second 
gas-separating step. 
Referring to FIG. 4, a feed gas supplied through a feed line 11 is divided 
into two portions thereof, and the two separate flows of the feed gas ar 
fed to two separate first gas-separating polymeric membrane modules 1a and 
1b and are separated therein into two permeated fractions and two 
non-permeated fractions. 
The non-permeated fractions discharged from the first membrane modules 1a 
and 1b are fed into additional first membrane modules 1aa and 1bb through 
flow lines 18a and 18b, respectively. A non-permeated fraction in the 
additional first membrane module 1aa is discharged through a flow line 
20a, and another non-permeated fraction in the additional first membrane 
module 1bb is discharged through a flow line 20b. 
The first permeated fractions of the feed gas delivered from the first 
membrane modules 1a and 1b through flow lines 12a and 12b, respectively, 
are mixed with the additional first permeated fractions of the feed gas 
delivered from the additional first membrane modules 1aa and 1bb through 
the flow lines 19a and 19b, respectively, and the mixed first permeated 
fraction of the feed gas is directly fed into a second membrane module 2 
through a flow line 12 without increasing the pressure thereof. 
The mixed first permeated fraction of the feed gas is separated into a 
second permeated fraction and a second non-permeated fraction of the feed 
gas by the second membrane module 2. The second non-permeated fraction is 
fed into an additional second membrane module 2a through a flow line 21 
and is separated into an additional second permeated fraction and an 
additional second non-permeated fraction thereof. The additional 
non-permeated fraction of the feed gas is discharged from the additional 
membrane module 2a through a flow line 22. 
The second permeated fraction delivered from the second membrane module 2 
through a flow line 13a is mixed with the additional second permeated 
fraction of the feed gas delivered from the additional membrane module 2a 
through a flow line 13b and the mixed second permeated fraction of the 
feed gas is fed into a third gas-separating polymeric membrane module 3 
through a flow line 13. The gas-separating operation by the third membrane 
module 3 is carried out in the same manner as mentioned above. 
The additional first and second membrane modules 1aa, 1bb and 2a 
effectively recover the light gas (hydrogen or helium) from the 
non-permeated fractions of the feed gas. 
In the gas-separating method shown in FIG. 5, a feed gas is purified 
successively by first to fourth gas-separating operations. The second 
operation is carried out by using two gas-separating polymeric membrane 
modules arranged in series. 
Referring to FIG. 5, a feed gas supplied through a feed line 11 is 
permeated through a first membrane module 1. A non-permeated fraction of 
the feed gas is discharged through a valve V and a discharge line 15. A 
first permeated fraction is fed to a second membrane module 2 through a 
flow line 12, and the resultant second permeated fraction of the feed gas 
is collected as a purified light gas through a flow line 23. 
The resultant second non-permeated fraction of the feed gas is fed to an 
additional second membrane module 2a through a flow line 24, and the 
resultant additional second permeated fraction of the feed gas is 
collected as a purified light gas through a flow line 25. The resultant 
additional second non-permeated fraction of the feed gas is fed to a third 
membrane module 3 through a flow line 26, the resultant third 
non-permeated fraction of the feed gas is discharged through a flow line 
28, and the resultant third permeated fraction of the feed gas is fed to a 
fourth membrane module 4 through a flow line 27. 
The resultant fourth non-permeated fraction of the feed gas is discharged 
through a flow-line 30, and the resultant permeated fraction of the feed 
gas is collected as a purified light gas through a flow line 29. 
In the gas-separating procedures as shown in FIGS. 6 and 7, the pressure of 
a non-permeated fraction of the feed gas discharged from the second or 
downstream gas-separating polymeric membrane module is increased and the 
fraction is returned to a preceding membrane module, to recover the 
residual light gas in the non-permeated fraction. 
Referring to FIG. 6, a feed gas supplied through a feed line 11 is fed into 
a first membrane module 1 and is separated into a first permeated fraction 
and a first non-permeated fraction of the feed gas. The first 
non-permeated fraction of the feed gas is discharged through a valve 
V.sub.1 and a flow line 15. The first permeated fraction of the feed gas 
is fed into a second membrane module 2 and is separated into a second 
non-permeated fraction and a second permeated fraction of the feed gas. 
The second non-permeated fraction of the feed gas is withdrawn through a 
valve V.sub.2 and a flow line 31, is pressurized to a similar pressure to 
that of the feed gas in the flow line 11 by a pressure-increasing 
apparatus X.sub.1 and then is returned to a supply line 11 in which the 
pressure-increased second non-permeated fraction is mixed with a fresh 
feed gas. The mixed feed gas is fed into the first membrane module 1. 
The second permeated fraction of the feed gas is fed into a third membrane 
module 3 and is separated into a third non-permeated fraction and a third 
permeated fraction of the feed gas. The third non-permeated fraction of 
the feed gas is withdrawn through a valve V.sub.3 and a flow line 32, is 
pressurized to a similar pressure to that of the first permeated fraction 
in the flow line 12 by a pressure-increasing apparatus X.sub.2, and then 
is returned into the flow line 12 and mixed with the first permeated 
fraction. 
The third permeated fraction of the feed gas is collected as a purified 
light gas through a flow line 14. 
In the gas-separating procedure shown in FIG. 7, the same first, second and 
third gas-separating steps as those shown in FIG. 6 are carried out except 
that the third non-permeated fraction of the feed gas withdrawn from the 
third membrane module 3 through a valve V and a flow line 32 is introduced 
into a receiver tank 31a and mixed with the second non-permeated fraction 
of the feed gas withdrawn from the second membrane module 2 through a 
valve V.sub.2 and the flow line 31 in the receiver tank 31a, the resultant 
non-permeated fraction mixture of the feed gas is pressurized to a similar 
pressure to that of the fresh feed gas in the feed line 11 by a 
pressure-increasing apparatus X and then the pressurized non-permeated 
fraction mixture is introduced into the feed line 11 and mixed with the 
fresh feed gas therein. The residual light gas in the second and third 
non-permeated fractions can be recovered. 
In the gas-separating procedures as shown in FIGS. 8 and 9, the temperature 
of the feed gas and/or a permeated fraction of the feed gas is adjusted to 
a predetermined level by using a temperature regulator, to control the 
gas-separating temperature in each membrane module to a predetermined 
level. 
In the gas-separating procedure shown in FIG. 8, a feed gas supplied 
through a feed line 11 is filtered by a guard filter F, is 
temperature-regulated to a predetermined level by a first 
temperature-regulator H.sub.1 and then is fed into a first gas-separating 
polymeric membrane module 1. In the membrane module 1, the feed gas is 
separated at the regulated temperature into a first permeated fraction and 
a first non-permeated fraction of the feed gas. The first non-permeated 
fraction of the feed gas is discharged through a valve V.sub.1 and a flow 
line 15. The first permeated fraction of the feed gas delivered from the 
first membrane module 1 through a flow line 12 is temperature-regulated to 
a predetermined level by a second temperature regulator H.sub.2. The 
temperature-regulated first permeated fraction is fed into a second 
membrane module 2. The resultant second non-permeated fraction of the feed 
gas is discharged through a valve V.sub.2 and a flow line 16. The 
resultant second permeated fraction of the feed gas is fed into a third 
membrane module 3. The resultant third non-permeated fraction is 
discharged through a valve V.sub.3 and a flow line 17. The resultant third 
permeated fraction is collected as a purified light gas through a flow 
line 14. 
In the gas-separating procedure indicated in FIG. 9, the same operations as 
those shown in FIG. 8 are carried out except that the second temperature 
regulator H.sub.2 is arranged between the second membrane module 2 and the 
third membrane module 3, and a fourth membrane module 4 is connected to 
the flow line 14. That is, the first permeated fraction of the feed gas 
delivered from the first membrane module is directly fed into the second 
membrane module 2 without temperature regulation, and the second permeated 
fraction of the feed gas delivered from the second membrane module 2 is 
temperature-regulated to a predetermined level by the 
temperature-regulator H.sub.2 and then fed into a third membrane module 3. 
Further, the third permeated fraction of the feed gas delivered from the 
third membrane module 3 is fed into a fourth membrane module 4 through the 
flow line 14. The resultant fourth non-permeated fraction of the feed gas 
is discharged through a valve V.sub.4 and a flow line 35, and the 
resultant fourth permeated fraction of the feed gas is collected as a 
purified light gas to the outside of the apparatus through a flow line 34. 
In the method of the present invention, each gas-separating operation in 
each membrane module is preferably carried out at a temperature of from 
-100.degree. C. to 150.degree. C., more preferably from -70.degree. C. to 
120.degree. C., still more preferably from 0.degree. C. to 100.degree. C. 
All or some of the membrane modules may have at least one 
temperature-regulator (steam or electric heater and/or cooler) arranged 
directly before the modules. Also, all or some of the flow lines of the 
feed gas may be heat-insulated by covering the flow lines with a 
heat-insulating material. 
In the method of the present invention, it is preferable that the 
gas-permeating areas of the membrane modules be substantially equal to 
each other. For this purpose, it is preferable that each membrane module 
cause a pressure loss of the feed gas in an amount of 30% to 80% of the 
preceding pressure of the feed gas to be fed into the membrane module. 
For example, referring to FIG. 1 or 2, the pressure of the feed gas to be 
fed into the first membrane module 1 is controlled to a predetermined 
level by controlling the first valve V.sub.1 or the pressure increasing 
apparatus X, for the feed gas and the second valve V.sub.2 for the first 
non-permeated fraction of the feed gas. Also, the pressure of the first 
permeated fraction of the feed gas to be fed into the second membrane 
module 2 corresponds to 30% to 80% of the pressure of the feed gas fed 
into the first membrane module 1 and is controlled by the third valve 
V.sub.3 for discharging the second non-permeated fraction of the feed gas. 
Further, the pressure of the second permeated fraction of the feed gas to 
be fed into the third membrane module usually corresponds to from 30% to 
80% of the pressure of the first permeated fraction of the feed gas and is 
controlled by the fourth valve V.sub.4. 
Where the feed gas or the purified light gas contains water (moisture or 
water vapor), the water is preferably removed from the feed gas or the 
purified light gas by a conventional method, for example, a freeze-drying 
method, absorption method or adsorption method. 
When a purified light gas substantially free from water is required, the 
feed gas is preferably subjected to an absorption treatment with silica 
gel or a molecular sieve. In this absorption treatment, the water in the 
feed gas can be substantially completely removed and the resultant feed 
gas exhibits a dew point of -60.degree. C. or less. 
The method of the present invention is very effective for preparing a 
purified light gas consisting of hydrogen or helium having a purity of 
99.99 molar % or more when a content of water is omitted in the 
calculation of the purity, at a low cost with a high efficiency, by using 
a plurality of specific polymeric membrane modules. 
EXAMPLES 
The present invention will be further explained by way of specific 
examples, which are representative and do not restrict the scope of the 
present invention in any way. 
Referential Example 1 (Preparation of Gas-Separating Polymeric Membrane 
Modules A, B, C, and D) 
In accordance with the method described in Japanese Unexamined Patent 
Publication (Kokai) No. 61-19813, a number of aromatic imide polymer 
hollow filaments each having an outside diameter of about 380 .mu.m and an 
inside diameter of about 210 .mu.m were produced from an aromatic imide 
polymer which was a polymerization product of 100 parts by weight of 
3,3',4,4'-biphenyltetracarboxylic dianhydride, 80 parts of weight of a 
diaminodimethyldiphenylenesulfone isomer mixture and 20 parts by weight of 
2,6-diaminopyridine, and then the hollow fibers were connected to 
polymeric membrane modules A, B, C and D each consisting of about 840 
hollow fibers, having an effective gas-separating area (a total outside 
peripheral surface are of the hollow fibers) of about 0.20 m.sup.2, an 
effective fiber length of 200 mm, and exhibiting the gas permeating rates 
shown in Table 1. 
Referential Example 2 (Preparation of Gas-Separating Polymeric Membrane 
Modules E, F, G and H) 
The same hollow filaments as those described in Referential Example 1 were 
coated with a polymer comprising as a main component, a polysiloxane in 
accordance with the method disclosed in Example 2 of Japanese Unexamined 
Patent Publication (Kokai) No. 58-8514, to provide composite hollow fibers 
having an outside diameter of about 375 .mu.m and an inside diameter of 
about 210 .mu.m. 
The composite hollow filaments were converted to gas-separating polymeric 
membrane modules E, F, G and H each consisting of about 850 hollow fibers, 
having an effective gas-separating area of about 0.20 m.sup.2 (a total 
outside peripheral surface area of the hollow fibers) and an effective 
fiber length of 200 mm and exhibiting the gas-permeating rates shown in 
Table 1. 
The gas-permeating rates of the membrane module A to H were measured by 
permeating pure hydrogen, helium, methane, nitrogen and oxygen gases 
through each module at a temperature of 45.degree. C. under a pressure of 
2 kg/cm.sup.2 G. The resultant gas-permeating rates are indicated in 
cm.sup.3 /cm.sup.2 .multidot.sec.multidot.cmHg at a temperature of 
0.degree. C. under a pressure of one atmosphere. 
TABLE 1 
______________________________________ 
Type of Gas-permeating rate at .degree.C. 
membrane (N-cm.sup.3 /cm.sup.2 sec cmHg) .times. 10.sup.-5 
module H.sub.2 He CH.sub.4 
N.sub.2 
O.sub.2 
______________________________________ 
A 6.1 8.0 0.020 0.036 
0.19 
B 6.2 8.1 0.021 0.039 
0.20 
C 6.0 7.9 0.019 0.035 
0.18 
D 6.1 8.0 0.020 0.037 
0.19 
E 7.3 7.2 0.16 0.21 0.44 
F 7.5 7.3 0.18 0.24 0.48 
G 7.2 7.1 0.16 0.20 0.43 
H 7.3 7.2 0.17 0.22 0.45 
______________________________________ 
EXAMPLE 1 
The polymeric membrane modules A, B and C mentioned in Referential Example 
1 were arranged in series as shown in FIG. 10. 
A feed gas consisting of 99 molar % of hydrogen, 0.5 molar % of methane, 
and 0.5 molar % of nitrogen was fed through a pressure-regulating valve 
V.sub.1, a guard filter F, a temperature-regulator H.sub.1 and a feed line 
101 into the first membrane module A under the pressure and at the 
temperature and flow rate shown in Table 2. A first non-permeated fraction 
of the feed gas was discharged from the first membrane module A through a 
valve V.sub.2 and a flow line 103, and then a flow line 108. A first 
permeated fraction of the feed gas delivered from the first membrane 
module A was fed into the second membrane module B through a flow line 
102. A second non-permeating fraction of the feed gas was discharged from 
the second membrane module B through a valve V.sub.2 and a flow line 105 
and then a flow line 108. A second permeated fraction of the feed gas 
delivered from the second membrane module B was fed into the third 
membrane module C through a flow line 104. Then a third permeated fraction 
of the feed gas delivered from the third membrane module C was collected 
as a pure hydrogen gas through a flow line 106, a temperature regulator 
H.sub.2 and a valve V.sub.5. A third non-permeated fraction of the feed 
gas has discharged from the third membrane module C through a flow line 
107 and then a flow line 108. 
Each of the permeated fractions of the feed gas in the flow lines 102, 104, 
and 116 and the non-permeated fractions of the feed gas in the flow lines 
103, 105, 107 and 108 had the pressure, temperature, flow rate, and 
composition shown in Table 2. 
Also, Table 2 shows a percent recovery of hydrogen from the feed gas. 
TABLE 2 
__________________________________________________________________________ 
Flow line Flow line 
Po- 
101 Flow line 
Flow line 
Flow line 
Flow line 
106 Flow 
Flow line 
Item sition 
(Feed gas) 
102 103 104 105 (Final product) 
107 108 
__________________________________________________________________________ 
Pressure 17.5 11.7 17.5 7.0 11.7 3.0 7.0 0.5 
(kg/cm.sup.2 .multidot. G) 
Temperature (.degree.C.) 
45 45 45 45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.2 0.171 0.029 0.1458 
0.0255 
0.1269 0.0189 
0.0731 
Composition 
H.sub.2 
99.0 99.96463 
93.25026 
99.99894 
99.76893 
99.99998 
99.99206 
97.26406 
(Vol %) CH.sub.4 
0.5 0.01254 
3.40556 
0.00024 
0.08263 
0.00000 0.00183 
1.36799 
N.sub.2 
0.5 0.02283 
3.34418 
0.00082 
0.14844 
0.00002 0.00611 
1.36795 
Percent recovery of 
64.1 
hydrogen (%) 
__________________________________________________________________________ 
EXAMPLE 2 
The same procedures as those described in Example 1 were carried out except 
that the feed gas had the pressure, temperature, flow rate and composition 
shown in Table 2. 
The results are shown in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Flow line Flow line 
Po- 
101 Flow line 
Flow line 
Flow line 
Flow line 
106 Flow 
Flow line 
Item sition 
(Feed gas) 
102 103 104 105 (Final product) 
107 108 
__________________________________________________________________________ 
Pressure 18.2 11.6 18.2 6.9 11.6 3.0 6.9 0.5 
(kg/cm.sup.2 .multidot. G) 
Temperature (.degree.C.) 
45 45 45 45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.2 0.169 0.031 0.148 0.021 0.124 0.024 0.076 
Composition 
H.sub.2 
97.0 99.89740 
81.45341 
99.99711 
99.16889 
99.99993 
99.98284 
92.10538 
(Vol %) CH.sub.4 
2.0 0.05372 
12.44316 
0.00109 
0.43829 
0.00002 0.00651 
5.26313 
N.sub.2 
1.0 0.04888 
6.10343 
0.00180 
0.39282 
0.00005 0.01065 
2.63149 
Percent recovery of 
63.9 
hydrogen (%) 
__________________________________________________________________________ 
EXAMPLE 3 
The same procedures as those described in Example 1 were carried out except 
that the second membrane module consisted of the membrane module C, the 
third membrane module consisted of the membrane module D described in 
Referential Example 1, and the feed gas had the pressure temperature, flow 
rate and composition indicated in Table 4. 
The results are shown in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Flow line Flow line 
Po- 
101 Flow line 
Flow line 
Flow line 
Flow line 
106 Flow 
Flow line 
Item sition 
(Feed gas) 
102 103 104 105 (Final product) 
107 108 
__________________________________________________________________________ 
Pressure 18.8 11.4 18.8 6.8 11.4 3.0 6.8 0.5 
(kg/cm.sup.2 .multidot. G) 
Temperature (.degree.C.) 
45 45 45 45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.2 0.164 0.036 0.144 0.020 0.120 0.024 0.080 
Composition 
H.sub.2 
95.0 99.81143 
72.68037 
99.99446 
98.49687 
99.99988 
99.96704 
87.50018 
(Vol %) CH.sub.4 
3.0 0.08525 
16.52115 
0.00172 
0.68516 
0.00002 0.01030 
7.49997 
N.sub.2 
2.0 0.10332 
10.79840 
0.00382 
0.81797 
0.00010 0.02266 
4.99985 
Percent recovery of 
63.2 
hydrogen (%) 
__________________________________________________________________________ 
EXAMPLE 4 
The same procedures as those described in Example 1 were carried out with 
the following exception. 
The membrane modules A, B, C and D were arranged in series as indicated in 
FIG. 11. 
The feed gas had the pressure, temperature, flow rate and composition as 
indicated in Table 5. 
The results are shown in Table 5. 
TABLE 5 
__________________________________________________________________________ 
Flow line 
101 Flow line 
Flow line 
Flow line 
Flow line 
Item Position 
(Feed gas) 
102 103 104 105 
__________________________________________________________________________ 
Pressure (kg/cm.sup.2 .multidot. G) 
18.4 12.2 18.4 8.6 12.2 
Temperature (.degree.C.) 
45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.2 0.163 0.037 0.140 0.023 
Composition H.sub.2 
95.0 99.53811 
75.01763 
99.94295 
97.06721 
(Vol %) CH.sub.4 
4.0 0.21891 
20.64903 
0.00930 
1.49821 
N.sub.2 
1.0 0.24298 
4.33334 
0.04775 
1.43458 
Percent recovery of 
54.7 
hydrogen (%) 
__________________________________________________________________________ 
Flow line 
Flow line 
Flow line 
110 Flow line 
Flow line 
Item Position 
106 107 (Final product) 
109 108 
__________________________________________________________________________ 
Pressure (kg/cm.sup.2 .multidot. G) 
5.6 8.6 3.0 5.6 0.5 
Temperature (.degree.C.) 
45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.120 0.020 0.104 0.016 0.096 
Composition 
H.sub.2 
99.99134 
99.65139 
99.99872 
99.94431 
89.58472 
(Vol %) CH.sub.4 
0.00034 
0.06327 
0.00001 0.00244 
8.33332 
N.sub.2 
0.00832 
0.28534 
0.00127 0.05325 
2.08196 
Percent recovery of 
hydrogen (%) 
__________________________________________________________________________ 
COMATIVE EXAMPLE 1 
The same procedures as those described in Example 1 were carried out except 
that the first, second and third membrane modules respectively consisted 
of the membrane modules E, F and G mentioned in Referential Example 2 and 
the feed gas had the pressure, temperature, flow rate and composition as 
indicated in Table 6. 
The results are shown in Table 6. 
TABLE 6 
__________________________________________________________________________ 
Flow line 
101 Flow line 
Flow line 
Flow line 
Flow line 
Flow line 
Flow 
Flow line 
Item Position 
(Feed gas) 
102 103 104 105 106 107 108 
__________________________________________________________________________ 
Pressure 16.4 10.4 16.4 6.4 10.4 3.0 6.4 0.5 
(kg/cm.sup.2 .multidot. G) 
Temperature (.degree.C.) 
45 45 45 45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.2 0.165 0.035 0.143 0.022 0.124 0.019 0.076 
Composition 
H.sub.2 
95.0 98.97886 
76.01239 
99.83129 
93.36062 
99.97843 
98.85155 
86.87732 
(Vol %) CH.sub.4 
3.0 0.55849 
14.65117 
0.08275 
3.69405 
0.00932 
0.57166 
7.87953 
N.sub.2 
2.0 0.46265 
9.33644 
0.08596 
2.94533 
0.01225 
0.57679 
5.24315 
Percent recovery of 
65.2 
hydrogen (%) 
__________________________________________________________________________ 
COMATIVE EXAMPLE 2 
The same procedures as those described in Example 4 were carried out except 
that the first, second, third and fourth membrane modules respectively 
consisted of the membrane modules E, F, G and H described in Referential 
Example 2 and the feed gas had the pressure, temperature, flow rate and 
composition as indicated in Table 7. 
The results are shown in Table 7. 
TABLE 7 
__________________________________________________________________________ 
Flow line 
101 Flow line 
Flow line 
Flow line 
Flow line 
Item Position 
(Feed gas) 
102 103 104 105 
__________________________________________________________________________ 
Pressure (kg/cm.sup.2 .multidot. G) 
20.0 13.5 20.0 9.1 13.5 
Temperature (.degree.C.) 
45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.2 0.170 0.030 0.146 0.024 
Composition 
H.sub.2 
95.0 98.45764 
75.62844 
99.59116 
91.36260 
(Vol %) CH.sub.4 
4.0 1.08831 
20.31286 
0.23692 
6.41740 
N.sub.2 
1.0 0.45405 
4.055870 
0.17192 
2.22000 
Percent recovery of 
48.9 
hydrogen (%) 
__________________________________________________________________________ 
Flow line 
Flow line 
Flow line 
Flow line 
Flow line 
Item Position 
106 107 108 109 110 
__________________________________________________________________________ 
Pressure (kg/cm.sup.2 .multidot. G) 
5.6 9.1 3.0 5.6 0.5 
Temperature (.degree.C.) 
45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.128 0.018 0.093 0.035 0.107 
Composition 
H.sub.2 
99.90110 
97.39274 
99.98119 
99.68812 
90.67056 
(Vol %) CH.sub.4 
0.04298 
1.61257 
0.00549 
0.14268 
7.47186 
N.sub.2 
0.05592 
0.99469 
0.01332 
0.16920 
1.85758 
Percent recovery of 
hydrogen (%) 
__________________________________________________________________________ 
EXAMPLE 5 
The same procedures as those described in Example 1 were carried out except 
that the first and second membrane module consisted of the membrane 
modules B and D as mentioned in Referential Example 1, the third membrane 
module was omitted as shown in FIG. 12, and the feed gas had the pressure, 
temperature, flow rate and composition as indicated in Table 8. 
TABLE 8 
__________________________________________________________________________ 
Flow line Flow line 
101 Flow line 
Flow line 
104 Flow line 
Flow line 
Item Position 
(Feed gas) 
102 103 (Final product) 
105 108 
__________________________________________________________________________ 
Pressure (kg/cm.sup.2 .multidot. G) 
13.3 7.6 13.3 3.0 7.6 0.5 
Temperature (.degree.C.) 
45 45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.2 0.168 0.032 0.146 0.022 0.054 
Composition 
H.sub.2 
98.7 99.96747 
91.93532 
99.99935 
99.76191 
95.18691 
(Vol %) CH.sub.4 
0.8 0.01518 
4.98869 
0.00022 0.11166 
2.96240 
N.sub.2 
0.5 0.01735 
3.07595 
0.00043 0.12643 
1.85069 
Percent recovery of 
74.0 
hydrogen (%) 
__________________________________________________________________________ 
EXAMPLE 6 
The same procedures as those described in Example 1 were carried out with 
the following exception. The feed gas supplied through the flow line 101 
was fed into two separate first membrane modules respectively composed of 
the membrane modules A and B through flow lines 101a and 101b. 
The first permeated fractions of the feed gas delivered from the first 
membrane modules A and B were fed into the second membrane module 
consisting of the membrane module C through flow lines 102a and 102b and 
then a flow line 102c. The first non-permeated fractions of the feed gas 
were discharged from the first membrane modules A and B through flow lines 
103a and 103b and then a flow line 103c and a flow line 108. 
The third membrane module consisted of the membrane module D. 
The results are shown in Table 9. 
TABLE 9 
__________________________________________________________________________ 
Flow line Flow line 
Po- 
101 Flow line 
Flow line 
Flow line 
Flow line 
106 Flow 
Flow line 
Item sition 
(Feed gas) 
102c 103c 104 105 (Final product) 
107 108 
__________________________________________________________________________ 
Pressure 16.2 12.0 16.2 7.2 12.0 3.0 7.2 0.5 
(kg/cm.sup.2 .multidot. G) 
Temperature (.degree.C.) 
45 45 45 45 45 45 45 45 
Flow rate (Nm.sup.3 /H) 
0.2 0.088 0.012 0.150 0.026 0.127 0.023 0.073 
Composition 
H.sub.2 
97.0 99.78078 
77.26141 
99.99403 
98.48913 
99.99986 
99.96265 
91.78105 
(Vol %) CH.sub.4 
2.0 0.11616 
15.37188 
0.00227 
0.80594 
0.00003 0.01431 
5.47940 
N.sub.2 
1.0 0.10306 
7.36668 
0.00370 
0.70493 
0.00011 0.02304 
2.73955 
Percent recovery of 
65.5 
hydrogen (%) 
__________________________________________________________________________