Method for capturing nitrogen from air using gas separation membrane

This invention comprises an improved process for generating Nitrogen from air in which a vacuum is placed on the permeate side of a gas separation membrane, usually of the polysulfone type, resulting in highly enhanced flow rates and nitrogen purity sufficient for oil and gas pipeline repair use and for use in grain silos.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improved method useful for separating 
one permanent gas from another or from a mixture of gases. More 
particularly, the present invention relates to a method for effectively 
and efficiently separating nitrogen of sufficient purity from air using a 
membrane separator for use in pipelines and other equipment undergoing 
repair or for use in grain silos and the like. 
2. Prior Art 
Membranes have been employed that use the principle of selective permeation 
to separate a mixture of gases into various components. Each gas has a 
characteristic permeation rate that is a function of its ability to 
dissolve into and diffuse through the membrane. For selective separation 
of one or more of the gases in a mixture to be commercially attractive the 
membrane not only must withstand the conditions of use but also provide an 
adequately selective separation of the gases at a sufficiently high flux, 
i.e. the permeation rate of the permeate per unit surface area, so that 
the use of the membrane separation procedure is on an economically 
attractive basis. A given membrane has a particular permeability constant 
for a given gas. The rate of permeation, i.e. the flux, is related to the 
permeability constant and influenced by such variables as the membrane 
thickness, the chemical and physical nature of the membrane, the glass 
transition of the polymeric membrane material, the partial pressure 
differential of the permeate gas, the molecular weights of the gases to be 
separated, and the like. 
In the repair of pipelines, tanks, and related equipment in the petroleum 
and chemical industries it is known to use nitrogen purging and blanketing 
or inerting in order to effect cutting, welding, brazing and like 
operations without danger of combustion or explosion. The general practice 
is to remove as much combustible material as possible from the confined 
space, blanket or purge with nitrogen, and effect the repair. For a 
pipeline the practice is to force a purging device, called a pig, through 
the section of pipeline requiring repair such that the liquids, e.g. crude 
oil, diesel, gasoline, etc. are forced out of the pipeline section ahead 
of the pig. The motive force to move the pig through the pipeline section 
is nitrogen pressure using pressures up to 150 psi to 500 psi or even 1200 
psi (pounds per square inch ) and beyond, and after passage of the pig, 
nitrogen remains in the pipeline section thereby affording a safe 
atmosphere for the aforementioned repair methods, such as welding, 
cutting, etc. Commonly the nitrogen is supplied for these operations by 
cryogenic liquid nitrogen, delivered to the site by liquid nitrogen 
transport trucks. In order for cryogenic nitrogen to be used it must be 
not only transported to the site of use but it also must be heated to 
effect vaporization prior to compression to the required use pressure. The 
cost of transport and heating of the cryogenic nitrogen are as much as 50% 
of the repair cost for repair of pipelines in remote locations, e.g. 
Alaska, Rocky Mountains, offshore platforms, etc . . . A portable membrane 
separation unit for nitrogen from air with an improved high rate of 
nitrogen flow would eliminate the need and cost of liquid nitrogen 
transport and the cost of heating facilities and provide operational 
flexibility for remote site repairs. Present membrane separators would be 
improved for practical use in pipeline repair by improved flow rate. 
Present membrane separators are capable of about 25000 SCF/Hr (standard 
cubic feet per hour) which is not enough for pipeline repair and work-over 
where a capacity of about two to four times that rate, or even a higher 
flow rate, is required. 
In another application it is known that nitrogen blanketing of grain silos 
is an effective means of reducing or eliminating the danger of fire and 
explosion and, simultaneously, of eliminating rodents, fungi, and other 
deleterious pests from the grain. It is impractical and costly to use 
liquid nitrogen for this application because of the need to heat the 
liquid nitrogen prior to compression and use. 
The purity of the nitrogen for the uses of pipeline and equipment repair 
and for grain silo treatment is such that the oxygen content should be 8% 
or less and nitrogen content should be in the range of 92% or greater, 
nitrogen content of 95% to 99% is preferred. 
There remains a need for an economically attractive method or means of 
rapid nitrogen generation using a selective membrane to produce nitrogen 
in the range of about 92% to about 97% or 99% purity for use in petroleum, 
petroleum products, chemical or other materials pipeline, tanks, and 
related equipment while undergoing repair and for use in grain silos for 
combustion and pest control. 
For the most part, with exceptions noted below for the special case of 
pervaporation, research has focussed on chemical modification of the 
polymeric membrane to achieve effective separation of gases. For example, 
multicomponent membranes are described in U.S. Pat. No. 4,230,463, 
polyimide membranes are described in U.S. Pat. No. 4,705,540, U.S. Pat. 
No. 4,717,393, and U.S. Pat. No. 4,717,394, brominated polycarbonates are 
described in U.S. Pat. No. 4,840,646 and treatment of membranes with acids 
and bases are described in U.S. Pat. No. 4,472,175 and U.S. Pat. No. 
4,634,055 respectively. Siloxane coating of gas separation membranes is 
described in U.S. Pat. No. 4,484,935. The coating may also be of rubber. 
In each case the objective is to provide improved separation. The activity 
in this field suggests that improvement in gas selectivity and permeation 
rate continue to be objectives of continued research. 
The use of physical means also has been used to improve separation in the 
special case of pervaporation in which membranes are used to separate 
organic, condensable liquids. In U.S. Pat. No. 5,032,148, U.S. Pat. No. 
4,218,312, U.S. Pat. No. 4,311,594 and U.S. Pat. No. 4,962,270 and 
references cited therein a vacuum is applied to the permeate side of a 
membrane separator in order to improve separation of condensable vapors of 
organic liquids. The need to operate gas separation membranes at high 
pressure differentials is noted in U.S. Pat. No. 4,472,175, U.S. Pat. No. 
4,486,202 and U.S. Pat. No. 4,654,055. The utility of improved performance 
by means of permeate side vacuum in the case of permanent gases is not 
disclosed, however. 
The theory of membrane gas separation as presently understood is given in 
Hwang and Kammermeyer, Techniques of Chemistry, Vol. VII, Membranes in 
Separation, John Wiley & Sons, 1975 (herein incorporated by reference), 
pages 9, 56, 57, and 72. The method of determination of the permeability 
constant is given on pages 297-300. In Hwang, the flow rate, F, is given 
by F=QS (P1-P2)/I 
where Q is the permeability constant, S is the area, I is the thickness, 
and P1 and P2 are the pressures on the feed and permeate sides of the 
membrane, respectively. Thus for a membrane of fixed area and thickness 
the flow rate is a simple function of the pressure differential. Also, 
according to Friedlander and Rickles, Anal. Chem. 37, 27A (1965) cited in 
Hwang on Page 56, the permeability constant, Q, is independent of pressure 
for the permanent gases such as nitrogen and oxygen. For example, if the 
feed pressure is 150 psig ( pounds per square inch gauge) and the permeate 
pressure is 15 psig then lowering the permeate pressure to about zero psig 
is a 10% pressure differential change and would be expected to change the 
flow rate, F, by 10%. Feed pressures of 200 psig, or even higher, may be 
encountered, and a vacuum on the permeate side up to 3.4 psia (pounds per 
square inch absolute) would result in a prediction of flow rate 
improvement of about 10%, based on the theory given by Hwang. 
SUMMARY OF THE INVENTION 
In accordance with the present invention an economical and practical 
process is provided for producing from air an inert gas having an oxygen 
content of about 1% to about 8% at a flow rate such that a practical means 
nitrogen blanketing is accomplished. This is done by providing a vacuum on 
the permeate side of a gas separation membrane. The flow rate attained by 
this method is at least twofold, and may be fourfold or even higher, than 
the flow rate without applied vacuum on the permeate side of the membrane. 
The economic advantage of the flow rate improvement attained is obvious 
since for a given flow rate and desired purity only half or less of 
membrane surface area is required with the assistance of vacuum on the 
permeate side. The membrane unit is composed of a plurality of elongated, 
parallel, hollow fibers juxtaposed within an elongated shell or container. 
The process of the present invention provides for capturing nitrogen of 
about 92% to about 97% or 99% purity at a substantially enhanced flow 
rate, substantially and surprisingly above the flow rate improvement 
predicted by theory for a permanent gas. First, air is compressed from a 
source. The feed air is passed through an in line rotameter and into and 
through the bundle of hollow fiber membranes. A first pressure gauge and 
first control valve are placed on the non-permeate stream and a second 
pressure gauge and second control valve are placed on the permeate stream. 
A vacuum gauge on the permeate stream is used to verify vacuum levels 
being applied by a vacuum pump. The permeate gases exiting from the 
membrane are vented for disposal or may be collected for other uses. In 
particular, the permeate gases, rich in oxygen, may be used to improve the 
combustion efficiency of the diesel or gasoline engines used to provide 
power to the compressor and the vacuum pump. By this means a hotter, 
cleaner burn of fuel is obtained and a portion of the vacuum can be 
obtained from the engine intake. The result of using the permeate gases in 
this way is a further improvement in economy of operation. The entire 
apparatus thus constitutes a system wherein nitrogen suitable for pipeline 
and grain silo uses is produced while the permeate gases, mostly oxygen, 
are simutaneously utilised in the energy generation used to oprate the 
system. The non-permeate gas is composed substantial entirely of nitrogen. 
The non-permeate gas is removed from the exit end of the membrane and is 
collected and confined under pressure for use. Preferably, the hollow 
fiber membranes are constructed of polysulfone polymer which has been 
coated with a suitable coating such as silicone. In practice, coated 
hollow fiber membranes are longitudinally disposed within the container 
and sealed so that the permeate gases such as oxygen, carbon dioxide, 
water vapor and the like can be collected and disposed of and the 
non-permeate gases such as nitrogen can be collected for use in inerting 
and other uses.

DETAILED DESCRIPTION OF THE INVENTION 
With reference in detail to the drawings, as shown in FIG. 1, numeral 1 
denotes a feed air compressor supplying up to 120 psig of feed air. The 
feed air is then passed through the filter (2) for removing water vapor 
and particulates in the air line. The pressure control valve (3) and gauge 
(4) are on the feed stream. The air the is passed through a rotameter (5) 
and into the hollow fiber membrane (6). Another pressure gauge (7) on the 
non-permeate stream, is used to check the pressure across the membrane 
itself. Once the system is filled with air, the pressure control valves on 
the permeate (12) and non-permeate (9) are opened. A rotameter (8) is 
installed on the non-permeate stream for determination of nitrogen flow. 
In both the permeate and non-permeate streams the oxygen levels are 
monitored by oxygen analyzers (10, 13). The vacuum gauge (11) on the 
permeate stream is used to verify vacuum levels being applied by the 
vacuum pump (14). 
Using the apparatus described in FIG. 1 reference readings were taken and 
vacuum levels at approximately 3, 5, 7, and 9 psia were taken with three 
repetitions at each feed pressure. Readings were obtained for feed 
pressures of 40, 50, 60, and 80 psig while producing 95%, 97% and 99% pure 
nitrogen. The data are illustrated in FIGS. 2, 3, 4, and 5. As a result of 
reducing the pressure on the permeate stream the volume of nitrogen 
produced increased dramatically. At 40 psig feed pressure the volume of 
95% pure nitrogen climbed from 0.45 standard cubic feet per minute (SCFM) 
to 1.81 SCFM, an increase of 302% while at 97% and 99% purity increases of 
1966% and 1033% were obtained. In general over a three to ten fold 
increase in productivity was observed for product purity of 99% and a two 
to four fold increase in productivity was obtained for product nitrogen of 
95% purity. The practical effect of this invention is that a commercial 
unit capable of 25000 SCF/Hr of 95% pure nitrogen can be improved to a 
flow rate of about 100,000 SCF/Hr., thereby making such a unit practical 
for pipeline repair use at significantly lower cost than cryogenic 
nitrogen and even lower cost than current membrane-generated nitrogen. The 
data used for FIG. 2 are shown below in tabular form, Table 1. 
TABLE I 
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Non-Permeate Flow Rate (Nitrogen) 
Vacuum at various purities (SCFM) 
psia 95% 97% 99% 
______________________________________ 
13.2 0.45 0.06 0.06 
9.5 0.75 0.34 0.23 
7.8 1.13 0.57 0.34 
5.9 1.41 0.85 0.45 
3.4 1.81 1.24 0.68 
______________________________________ 
In Table 1 psia = pounds per square inch absolute. SCFM = standard cubic 
feet per minute. 
From FIG. 3, at 50 psig feed pressure the produced volume increase for 
nitrogen of 95% purity increased from about 0.8 SCFM to 2.7 SCFM as 
permeate pressure declined from 13.2 psia to 3.41 psia, an increase of 
237%. 
From FIG. 4 for 60 psig feed pressure the nitrogen product flow rate for 
95% purity nitrogen increased from 1.4 SCFM to 3.3 SCFM as permeate 
pressure declined from 13.2 psia to 3.89 psia, an increase of 136%. 
From FIG. 5. for 80 psig feed pressure the nitrogen product flow increased 
from 2.7 SCFM to 4.6 SCFM for 95% purity nitrogen as permeate pressure 
declined from 13.2 psia to 4.88 psia, an increase in flow rate of 70%. 
As can be seen from FIGS. 2, 3, 4, and 5 the flow rate increases at each 
feed pressure with increasing permeate vacuum manyfold times faster than 
predicted by theory. This invention allows for the use of a membrane 
separator with very high nitrogen flow rate in practical pipeline and tank 
repair and in grain silo nitrogen treatment without the need for heating 
facilities required by cryogenic nitrogen. The nitrogen purity attained by 
the instant method is sufficient for its intended purposes. 
The data shown in FIGS. 2, 3, 4, and 5 are examples of the improvement 
provided by the instant invention and not limiting in the scope of the 
invention.