Method and apparatus for making ultra-pure hydrogen

Ultra-high purity hydrogen is produced from a hydrogen stream containing impurities including carbon monoxide and carbon dioxide, by first depleting the stream of carbon monoxide and carbon dioxide, than passing it through a semipermeable membrane such as a palladium-silver membrane, to remove methane, water and other impurities. Preferably, a methanation catalyst is used in the first step to convert the carbon monoxide and carbon dioxide to methane and water. This stream, free of carbon monoxide and carbon dioxide, is then passed through the semipermeable membrane to separate the remaining impurities.

FIELD OF THE INVENTION 
This invention relates generally to a process for the purification of 
hydrogen gas, and more specifically to an improved process and apparatus 
for making ultra-pure hydrogen by a modified membrane separation 
technique. 
BACKGROUND OF THE INVENTION 
Hydrogen is used in a variety of industrial processes. Because of this 
demand for hydrogen and particularly for hydrogen free of impurities, an 
equally wide variety of techniques have been developed for separating 
impurities from hydrogen. 
An impetus for continued improvement of hydrogen purification to levels of 
ultra-high purity stems from development in the manufacture of integrated 
circuits where ever increasing line densities require high purity 
processing materials. Because hydrogen is a key component in many of these 
semiconductor manufacturing processes, ultra-high purity hydrogen is of 
particular importance in this field. Commercially available hydrogen 
typically contains impurities, including: carbon monoxide, carbon dioxide, 
oxygen, nitrogen, water, and methane, among others. These components must 
be separated from the hydrogen to achieve appropriate levels of purity for 
many industrial applications. 
One method for purifying hydrogen, as described in U.S. Pat. No. 5,492,682, 
involves a two-step process. The first step of the two part process 
involves the removal of carbon monoxide by contacting the stream 
containing carbon monoxide with a nickel catalyst to form nickel-carbonyl. 
This carbon monoxide-free stream is then passed through a second reaction 
zone wherein it is contacted with a titanium nickel catalyst in order to 
further purify the stream by removing methane and carbon dioxide. 
Another purification method, as described in U.S. Pat. No. 4,056,373, 
involves the use of a selectively permeable noble-metal membrane. In this 
method, a membrane is selected such that only hydrogen will pass through. 
An example is the use of a palladium-alloy filter coil. There, the 
purification filter separates hydrogen from the impurities present in the 
hydrogen stream by limiting passage to hydrogen. 
Another purification method, as described in U.S. Pat. No. 4,654,047, uses 
a two-step membrane/cryogenic process. The process involves a first step 
of separating a stream of gases into a hydrogen-rich component and a 
hydrogen-lean component by selective permeation through a cellulose 
acetate, polysulfone, or polyimide type membrane or hollow filter. The 
hydrogen-lean stream is subsequently treated by a cryogenic process to 
remove some of the remaining impurities to produce a more enriched 
hydrogen stream. 
Still another technique for the purification of hydrogen, as described in 
U.S. Pat. No. 3,251,652, begins with a stream comprised of hydrogen and 
hydrocarbons. This stream is contacted with steam and air to convert the 
hydrocarbons to carbon monoxide. That stream is then treated with a 
gaseous diffusion process (utilizing a palladium-silver alloy membrane) to 
separate the hydrogen from the carbon monoxide. The stream from that 
process containing mostly carbon monoxide (and some hydrogen) is then 
contacted with steam to produce carbon dioxide and hydrogen through a 
shift reaction. This mixture is then passed through a second 
palladium-silver alloy membrane to separate the hydrogen and carbon 
dioxide. 
Notwithstanding these various processes, there remains a need for an 
improved method for producing ultra-pure hydrogen, adequate for modern 
semiconductor manufacturing. 
SUMMARY OF THE INVENTION 
The present invention provides a novel process and apparatus for producing 
ultra-pure hydrogen from a hydrogen stream containing impurities such as 
carbon monoxide, carbon dioxide, methane, water, and nitrogen, and 
particularly to such a stream including carbon monoxide and/or carbon 
dioxide. The process comprises a first step of depleting the hydrogen 
stream of carbon monoxide and carbon dioxide and then separating the 
remaining impurities from the hydrogen stream by passing the stream 
through a selectively permeable membrane. Preferably, the first step of 
the two part process involves a methanation catalyst wherein the carbon 
monoxide and carbon dioxide, present as impurities, are reacted to form 
methane and water. The carbon monoxide-free and carbon dioxide-free stream 
is then passed through a selectively permeable membrane, preferably a 
palladium-based membrane and most preferably a palladium-silver alloy 
membrane, to filter out the methane, water, and nitrogen, thus producing 
an ultra-high purity hydrogen product stream.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides an efficient and effective process and 
apparatus for recovering a hydrogen-rich stream from a stream of hydrogen 
containing impurities, particularly, carbon monoxide, carbon dioxide, 
methane, water, and nitrogen. The first step is to deplete the stream of 
carbon monoxide and carbon dioxide, preferably by converting those 
impurities into methane and water. The subsequent stream, free of carbon 
monoxide and carbon dioxide, is then passed through a selectively 
permeable membrane to separate the remaining impurities, namely water, 
nitrogen, and methane, from the hydrogen. The result is an ultra-pure 
hydrogen stream. 
In FIGURE, there is shown a schematic view of the apparatus and process of 
this invention in which a feed stream 101, containing hydrogen and 
impurities, including CO and CO.sub.2, possibly also including one or more 
of nitrogen (N.sub.2), methane (CH.sub.4), and water (H.sub.2 O), is fed 
into methanation unit 110, in which is located a methanation catalyst (not 
shown) such as a static packed bed of ruthenium on alumina spheres. Other 
catalysts could be used, however, including a monolithic type, wherein the 
active catalyst agent is coated onto a honeycomb structure. Additionally, 
various materials could be used as the active catalyst agent, including 
nickel. In the methanation unit 110, the carbon monoxide (CO) and carbon 
dioxide (CO.sub.2) are reacted with hydrogen (H.sub.2) to form methane 
(CH.sub.4) and water (H.sub.2 O). The stream exiting the methanation 
catalyst 110, intermediate stream 120, contains hydrogen (H.sub.2), along 
with some methane (CH.sub.4), water (H.sub.2 O), and nitrogen (N.sub.2). 
This intermediate stream 120 is then passed across a palladium or 
palladium alloy membrane 130A in palladium membrane unit 130. A common 
membrane material is a palladium-silver alloy. Other membrane structures 
and materials are discussed below. Inside the hydrogen membrane unit 130 
is allowed to permeate the palladium membrane 130A and exit palladium 
membrane unit 130 as ultra-pure hydrogen product stream 150. The separated 
impurities exit palladium membrane unit 130 as bleed gas stream 140, 
typically containing methane (CH.sub.4), water (H.sub.2 O), nitrogen 
(N.sub.2), and some hydrogen (H.sub.2). 
In a process and apparatus of the type illustrated in the FIGURE, hydrogen 
with less than 3 parts-per-billion (ppb) methane can be achieved from an 
impure hydrogen stream containing tens of parts-per-million (ppm) levels 
of carbon monoxide and carbon dioxide. 
The inventors believe that when carbon monoxide and carbon dioxide are not 
removed from the stream prior to contact with the selectively permeable 
membrane, but are allowed to reach the membrane, the carbon monoxide and 
carbon dioxide dissociate into their constituents (carbon and oxygen) on 
the surface of that membrane, and some of the carbon atoms pass through 
the membrane. Once on the other side, those carbon atoms react with the 
hydrogen to form methane, and the desired purity of hydrogen is therefore 
not achieved. Especially in the case of semiconductor manufacturing, even 
trace amounts of methane (on the ppb level) can be detrimental. 
Data collected in several experiments combine to support the above theory. 
Table 1 is a summary of experiments which demonstrate the effectiveness of 
the present invention. 
TABLE I 
__________________________________________________________________________ 
Hydrogen Feed Methane (CH.sub.4) 
Stream Composition 
Methanation Catalyst 
Level in 
Experiment # 
(Impurities, ppm) 
Upstream? Product Stream 
__________________________________________________________________________ 
1 20 CO No 50 ppb 
20 CO.sub.2 (at 40 minutes) 
20 CH.sub.4 
400 N.sub.2 
2 25 CH.sub.4 
No &lt;10 ppb 
(up to about 4 hours) 
3 25 CO.sub.2 
No &gt;10 ppb 
(within 10 minutes) 
4 20 CO.sub.2 
Yes &lt;10 ppb 
20 CH.sub.4 
400 N.sub.2 
__________________________________________________________________________ 
In experiment number 1, a high purity hydrogen stream with other impurities 
below 10 ppb was contaminated with 20 ppm CO, 20 ppm CO.sub.2, 20 ppm 
CH.sub.4, and 400 ppm N.sub.2 to simulate a typical industrial gas 
hydrogen supply. Such a simulated hydrogen stream was then passed through 
a palladium (Pd) silver (Ag) alloy membrane hydrogen purifier operated at 
375.degree. C. The product hydrogen was monitored using a highly sensitive 
Atmospheric Pressure Ionization Mass Spectrometer (APIMS). Within 40 
minutes after the introduction of the contaminated stream, the methane in 
the product increased to about 50 ppb. 
In experiment number 2, conditions were maintained similar to experiment 
number 1, with one exception. In this second experiment, the high purity 
hydrogen stream was contaminated only with 25 ppm methane (CH.sub.4). This 
time, the methane (CH.sub.4) level in the product remained below 10 ppb 
for four (4) hours. 
In experiment number 3, again otherwise similar to experiment number 1, the 
high purity hydrogen stream was contaminated only with 25 ppm carbon 
dioxide (CO.sub.2). The methane (CH.sub.4) level in the product stream 
increased within 10 minutes after the introduction of the contaminated 
stream, and the level continued to climb well above the acceptable 10 ppb 
level. 
In experiment number 4, the same conditions were used as in experiment 
number 1, except that a methanation unit (as illustrated in the FIGURE) 
was installed upstream of the PdAg alloy membrane. High purity hydrogen 
was produced with each tested impurity present at levels lower than 10 
ppb, and this ultra-pure product was produced for up to four hours. 
The inventors believe that the decreased methane (CH.sub.4) presence in the 
final product stream of experiment 4 was due to the fact that the CO and 
CO.sub.2 present in the feed stream were reacted with hydrogen to form 
CH.sub.4 and H.sub.2 O in the methanation catalyst. In experiment 3, 
however, the inventors believe that the CO.sub.2 dissociated into carbon 
and oxygen on the surface of the Pd-membrane and some of that carbon (C) 
passed through the membrane and reacted with hydrogen on the other side to 
form CH.sub.4. This hypothesis is further supported by experiment 2, where 
the CH.sub.4 content in the product stream remained constant and low where 
no CO or CO.sub.2 was present in the feed stream. Thus, in experiment 2, 
there was no free carbon (C) available to pass through the membrane and 
form CH.sub.4 on the pure side. The inventors further believe that 
CH.sub.4 does not dissociate on the surface of palladium as does the CO 
and CO.sub.2. Such a belief is supported by the empirical data obtained 
and discussed above. 
An apparatus utilizing this method may comprise, for example, a first 
vessel wherein the feed stream is allowed to contact the catalyst under 
conditions determined to be adequate for desired conversion of the carbon 
monoxide and carbon dioxide into methane and water. Such a device could 
be, for example, a packed bed or a monolithic type catalyst reactor. A 
monolithic type catalyst reactor is a reactor wherein the active catalyst 
agent is coated onto a honeycomb structure and the reactants are allowed 
to contact the surface of the honeycomb. 
An apparatus utilizing this invention would also comprise a second 
vessel--a membrane device. The membrane material could be of several 
types, including, as discussed above, a palladium based alloy. A palladium 
based alloy is one constructed of predominately palladium, and also some 
other noble-metal, such as gold, silver, mercury, platinum, iridium, 
rhodium, ruthenium, and osmium. A palladium based membrane is a membrane 
constructed of a palladium based alloy. Additional elements could also be 
utilized as membrane material, including a tungsten/tantalum combination. 
The membrane device itself could be a tube type, wherein tubes constructed 
of the membrane material are coiled within a vessel, and the stream is 
passed through the coils such that the components which can pass through 
the membrane pass through and enter into the area within the vessel 
outside of the coiled tube. Additionally, several tubes may be placed 
within a single vessel. Alternatively, the membrane could be constructed 
as a plate or foil, through which the stream is passed. The membrane could 
also be a micron-thin, thin-film coating on a ceramic substrate, the 
coating comprising Pd or some other effective membrane material. 
Although this invention has been described with reference to specific 
examples thereof, it is understood that variations of the invention as 
described and exemplified may be made by those skilled in the art without 
departing from the true spirit of the invention. It is intended that the 
appended claims be construed to include all such variations.