Method for the separation of gases at low temperatures

A method for separating gases with enhanced selectivity comprises causing a mixture of gases to be separated to flow through a molecular sieving membrane (MSM), at cryogenic temperatures.

FIELD OF THE INVENTION 
The present invention relates to the separation of gases at low 
temperatures. More particularly, the invention relates to the use of 
carbon molecular sieve membranes for the separation of gases at 
temperatures approaching those at which gases liquefy. 
BACKGROUND OF THE INVENTION 
Carbon molecular sieve membranes are an example of microporous membranes 
(pore size less than 20 Angstroms), which separate gas molecules on the 
basis of their different diameters (molecular sieving). The mechanism for 
such membrane transport is based on a combination of adsorption within and 
activated diffusion along the pore length. Studies of gas permeation 
through microporous membranes show selectivities much greater than would 
be expected from Knudsen diffusion--the predominant bulk process in 
micropores. Therefore, the process can be seen to occur predominantly 
through hopping between adsorption sites. The permeation rate will be 
proportional to the product of the local gradient of the concentration of 
gas molecules adsorbed on the pore and the effective diffusion coefficient 
(D) at that point in the pore. 
The temperature dependence of the permeability in membranes will depend on 
the temperature dependence of the activated diffusion coefficient D, and 
that of the extent of adsorption. While adsorption isotherms show that the 
extent of adsorption for most gases on most materials will increase with a 
drop in temperature, the activated diffusion coefficient often drops much 
more drastically, leading to a net drop in permeability with the drop in 
temperature. This is particularly the case for glassy polymers where the 
process for activated transport involves some movement of polymer segments 
to allow movement of gas molecules through interconnected free volumes. 
Even though some polymer membranes are known to show an increase in 
selectivity with decrease in temperatures, they suffer a disastrous loss 
of permeability even for the faster member of a gas pair as the membrane 
approaches the temperature where all polymer motion is frozen out. Such 
polymer membranes are therefore of limited applications at low 
temperatures. This is illustrated by FIG. 1 which shows that for 
polytrimethylsilyl propyne (PTMSP), one of the more state of the art 
membranes, the permeability drops drastically below -20.degree. C. 
THE PRIOR ART 
The art has provided a number of solutions for the separation of 
difficultly separable gases, which are expensive and complicated to build 
and to operate. One representative example of such a process is the 
separation of close boiling gases, such as oxygen from argon, to produce 
pure argon. According to the known art this is done with the use of 
cyrogenic distillation columns in which the energy efficiency is very low 
because of the closeness of the boiling points of oxygen and argon 
(.DELTA.T.sub.Bp =3.degree. K) requiring a high reflux ratio. While a 
prior art membrane or adsorption process could be theoretically considered 
as an alternative, this would be enormously inefficient energetically, 
because according to known processes it is required to heat the gas 
mixture to near ambient to effect the separation and then to cool again to 
reliquefy the products. U.S. Pat. No. 4,398,926 teaches the separation of 
hydrogen from a high-pressure stream, using a permeable membrane. U.S. 
Pat. No. 4,681,612 deals with the separation of landfill gas, and provides 
for the removal of impurities and carbon dioxide in a cryogenic column. 
Methane is then separated by a membrane process. The temperature in the 
membrane is 80.degree. F. U.S. Pat. No. 4,595,405, again, combines a 
cryogenic separation unit and a membrane separation unit. The membrane 
unit is operated with gas at or near ambient temperature. 
SUMMARY OF THE INVENTION 
It has now been surprisingly found, and this is an object of the present 
invention, that contrary to the expectation that lowering of the 
temperature will lead to loss of permeability, as is known in the art with 
a number of membranes, working at subambient temperatures with molecular 
sieve membranes (MSM) leads to tremendous increases in selectivity with 
little or no loss in permeability. This fact opens the door to a number of 
novel applications of these membranes, as will be more fully detailed 
hereinafter. 
Accordingly, it is an object to the present invention to provide an 
efficient process, utilizing molecular sieve membranes, by means of which 
such separations can be carried out at low temperatures. 
It is another object of the invention to provide membranes which can be 
used for such low-temperature separations. 
Without wishing to be bound by any particular theory, the inventors believe 
that the reason for this unexpected behavior resides in the structure of 
the molecular sieve membrane and in the different transport mechanism 
therein as compared, e.g., with glassy polymer membranes. 
The molecular sieve membrane can be of any suitable type, e.g., carbon 
molecular sieve membranes (MSM) or glass molecular sieve membranes (GMSM). 
For the sake of brevity, reference will be made hereinafter mostly to CMSM 
as the representative membrane, it being understood that the same 
description applies, mutatis mutatandis, to other molecular sieve 
membranes as well. 
Other objectives and advantages of the invention will become apparent as 
the description proceeds. 
The invention also encompasses the use of microporous molecular sieving 
membranes to separate between gaseous molecules of different diameter at 
sub-ambient temperatures, resulting in much greater selectivities than can 
be achieved at ambient temperature. Because molecular sieve membranes such 
as GMSM and CMSM can operate at or near the temperatures of air 
liquefaction, they will be referred to hereinafter as a "cryomembrane".

DETAILED DESCRIPTION OF THE INVENTION 
A setup for operating according to one embodiment of the invention is 
illustrated in FIG. 2. Of course, this setup is provided only by way of 
example and is not intended to limit the invention in any way. In 
particular, one could devise different module geometry and different ways 
of maintaining the low temperature conditions of the module, other than 
those shown in the present example. For example, modular devices of 
various geometries (e.g., hollow fiber, plate and frame, spiral wound) can 
be used, placed inside a commercial cryogenic cold box with local heat 
exchanger and heater networks to maintain a local temperature within the 
module. 
Referring now to FIG. 2A, the following elements are shown: 
T--Trap for liquid chemical vapor deposition material carried by a gas; 
CV--Calibrated volume; 
FC--Flow controller; 
FM--Flow meter; 
DPT--Differential pressure transducer; 
Pg--Pressure gauge; 
Pr--Pressure regulator 
BPR--Back pressure regulator; 
C--Cryostat; 
MC--Membrane cell. 
The membrane cell (M) is placed inside an oven during manufacture, or in a 
cryostat (C) during low temperature permeability measurements. The gases 
used for tailoring the pore size and for subsequent permeation 
measurements are fed from a manifold on which a flow controller (FC) and 
pressure gauge allow metering of the absolute amounts fed to the membrane 
cell. If the material used for chemical deposition of vapor (CVD) on the 
membrane is a liquid, its vapor is picked up from a liquid trap (T) 
through which a carrier gas (usually N.sub.2) is bubbled. The permeability 
of the membrane is measured by monitoring the change with time in the 
feed-side pressure volume product (dPV/dt) as given by the pressure gauge 
(Pg) and the calibrated volume (CV), while simultaneously monitoring the 
transmembrane pressure drop with the transducer DPT. 
In FIG. 2B the gas flow within the membrane cell is illustrated. Numeral 1 
indicated the shell entrance, numeral 2 the shell exit, and numeral 3 the 
bore entrance/exit. 
FIG. 2C shows a cryostat which can be used in the system of FIG. 2A, which 
is described in detail below, with reference to Example 1. 
In the examples given below the membrane module was made by potting hollow 
fiber carbon molecular sieve membranes in a U-shaped glass tube. The 
module has connections for introducing a feed gas, and removing separate a 
reject streams (nonpermeate) and permeate streams. 
The membranes were prepared according to the methods described in U.S. Pat. 
No. 4,685,940 and GB 2,207,666. This membrane preparation involved the 
following steps: 
1. Carbonization of a precursor hollow fiber 
2. Activation of the fiber with oxygen at 100.degree.-400.degree. C. 
3. Removal of excess adsorbed oxygen with hydrogen at T&gt;500.degree. C. 
4. Chemical vapor deposition (CVD) 
5. Several further steps of activation followed by inert gas or hydrogen 
treatment and/or CVD. 
Details of the manufacturing process are given in the examples below, and 
with reference to the tables. Unless otherwise indicated, the membrane 
referred to below was prepared according to the CVD and activation steps 
detailed in Table 1. 
Such membranes can be made to provide permeabilities greater than 100 
L/(M.sup.2 H-atm) and selectivities greater than 4.0 at room temperature 
conditions. It is found that such room temperature selectivities are large 
enough to generate enhanced selectivities at subzero temperatures. Thus, 
in one embodiment of the invention, the method comprises producing a CMSM 
having a permeability greater than 100 L/M.sup.2 H-atm and a selectivity 
greater than 4.0 at room temperature, and then cooling the CMSM to 
cryogenic temperatures and separating a mixture of gases therewith. 
The gas mixture may be fed from the shell or bore side. If the gas mixture 
is fed in the shell side, a preferred method is to feed it down the center 
tube, where impurities can be precondensed in a packed bed contained in 
the center tube, and the feed gas splits and feeds both arms of the U tube 
simultaneously. The reject is recombined in a reject manifold and led away 
from the module. 
The module can be cooled by placing it in any controlled subzero 
environment, such as a commercial cold box. The pressure difference to 
drive the permeation process can be applied by providing a pressurized 
feed while the permeate is kept at ambient pressures. Alternatively, the 
gas can be provided at ambient pressures while drawing a vacuum or using a 
sweep gas on the permeate side. Or any combination of these methods of 
effecting a pressure difference between the two sides of the membrane can 
be used. One particular embodiment would be to use as feed the vapor in 
equilibrium with a low-boiling gas such as oxygen, or an oxygen/argon 
mixture, such that at the temperature of operation, the vapor pressure of 
the low-boiling gas would be very high. For example, FIG. 1 illustrates 
how the vapor pressure of oxygen varies with temperature. In such an 
instance the permeate could be maintained at a high enough pressure to 
allow recompression and liquefaction of the permeate with a moderate 
outlay of energy, if the permeate stream were of value. 
All the above and other characteristics and advantages of the invention 
will be better understood through the following illustrative and 
non-limitative description of examples relative to preferred embodiments. 
EXAMPLE 1 
Preparation of the Module with Greatly Enhanced Selectivity at Subzero 
Temperatures 
A module was prepared by taking hollow carbon membrane potted in a U-shaped 
glass housing, as shown in FIG. 2B. It was placed in an apparatus (FIG. 
2A) for measuring permeability in which activation steps could be 
effected, as described in GB 2,207,666 and summarized in Table 1. The 
permeabilities were measured on the same apparatus with the gases fed on 
the shell side of the membrane through the center of the U tube. A plug of 
quartz wool was inserted in the center tube to serve as a prefilter on 
which to adsorb the impurities. 
This module was enclosed in a cryostat designed to maintain the module at 
constant temperatures ranging from 77.degree. K to RT. The cryostat is 
illustrated in FIG. 2C. The cryostat consists of a metal block, 4, 
suspended in a Dewar flask 5 in which liquid nitrogen 6 has been 
introduced to a level near but not touching the bottom surface of the 
metal block. A heater 7 is immersed in the liquid nitrogen for the purpose 
of boiling nitrogen to generate a cold nitrogen vapor which can rise and 
cool the metal block. A heating element is wrapped around the metal block 
to allow local heating of the block for purposes of moderating the cooling 
effects of the nitrogen vapor or for quickly moving to a new operating 
temperature higher than the previous one. A thermocouple 8 is inserted 
into a hole in the block and its signal is fed as the input into a 
temperature controller. The temperature controller controls the current to 
the cooling heater to maintain a particular setpoint. This arrangement 
allows operation of the membrane at temperatures between ambient and the 
boiling point of nitrogen (77.degree. K). In addition, an arrangement is 
made for adsorbing low vapor pressure impurities in the feed gas (e.g., 
water vapor and CO.sub.2), prior to introducing the feed gas into the 
membrane itself. 
The membrane was inserted into a slot 9 in the center of the metal block of 
the cryostat and its temperature systematically was varied between ambient 
and -176.degree. C. The results are plotted in FIG. 1, where the 
permeability is shown on the left axis and the selectivity is shown on the 
right axis. For the sake of comparison the permselectivity of the PTMSP 
membrane is plotted on the same axis as a function of the temperature. The 
retention of the permeability at low temperatures for the CMSM membrane as 
compared to the PTMSP membrane is striking. 
EXAMPLE 2 
Membrane of Intermediate Pore Size with Enhanced Permeability at Subzero 
Temperatures 
A membrane was prepared as described in Example 1 but with a somewhat 
different schedule of activation and CVD, as detailed in Table 2. The gas 
feed was fed into the center tube where impurities were adsorbed as the 
gas cooled, at the bottom the gas flow split to travel up to two legs of 
the U tube and was conducted away through a reject manifold at the exit of 
the U tube at the top. 
On measuring the permeabilities of the membrane to pure oxygen and nitrogen 
at room temperature, the membrane was found to have a permeability to 
oxygen of P.sub.o2 =270, and a selectivity .alpha.(=P.sub.o2 
/P.sub.N2)=3.6. 
The permeability and selectivity was then measured over a range of 
temperatures down to -170.degree. C. The results are given in FIG. 3. As 
can be seen, the oxygen permeability actually increases with the 
temperature drop, contrary to what would have been expected by the man of 
the art. At a certain temperature, this permeability goes through a 
maximum as the temperature effect on the activated transport coefficient 
finally begins to outstrip the effect of increased adsorption at low 
temperatures. 
As will be apparent to the skilled person, the invention leads to 
unexpected results, enabling substantially improved separation results, in 
a manner which was not possible according to prior art methods and 
devices. Many modifications can of course be made in the various 
membranes, devices utilizing them and processes which incorporate the 
devices, and many different separation processes can be carried out, using 
different gases, all without exceeding the scope of the invention. 
TABLE I 
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Time Temperature 
P 
Step # Description 
(minutes) 
(.degree.C.) 
(In Torr) 
______________________________________ 
1 Activation 
45 280 1216 
2 CVD 5 700 720 
3 Activation 
20 280 760 
4 Activation 
30 280 760 
5 Activation 
20 250 760 
6 Activation 
30 280 760 
7 Activation 
30 250 760 
F (02) final: 
370 L/M2-H-ATM 
.alpha. (02/Ar) 
5.6 
8 Regeneration (H2) 
15 620 760 
F (02) Final 
494 L/M2-H-ATM 
.alpha. (02/Ar) 
5.6 
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TABLE II 
______________________________________ 
Temperature 
Time Gas P 
Step # Type (.degree.C.) 
(minutes) 
(In Torr) 
______________________________________ 
1 Activation 
250 30 800 
2 CVD 700 5 800 
3 Activation 
265 60 800 
4 Activation 
265 60 800 
5 Activation 
250 20 800 
6 CVD 700 5 400 
7 CVD 710 5 800 
8 Activation 
270 54 800 
9 Activation 
260 25 800 
10 Activation 
280 20 800 
11 Activation 
280 15 800 
12 Activation 
280 25 800 
13 Activation 
280 30 800 
14 Activation 
250 15 800 
F (02) final: 
276 L/M2-HR-ATM 
.alpha. (02/Ar): 
3.1 
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