Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2001-01-04
2002-06-11
Simmons, David A. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S139000, C095S141000, C095S117000, C095S901000
Reexamination Certificate
active
06402813
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of the purification of a gas or gas mixture by adsorption of the impurities which are contained therein on a carbon adsorbent formed by a combination of several different active carbons, in particular a PSA process for purifying a gas, such as hydrogen, nitrogen, oxygen, carbon monoxide, argon, methane or gas mixtures containing them.
BACKGROUND OF THE INVENTION
A PSA (Pressure Swing Adsorption) unit for purifying a gas usually contains an adsorbent or a combination of adsorbents which has to be capable of selectively retaining the impurities contained in the gas to be treated.
PSA processes and units have proved to be highly effective for separating gas mixtures and especially for obtaining oxygen or nitrogen from air and above all for producing pure hydrogen from gas mixtures contaminated by various impurities.
Now, the production of high-purity hydrogen is of great importance industrially, it being widely used in many synthesis processes such as hydrocracking, methanol production, oxoalcohol production and isomerization processes.
In general, PSA processes benefit from the adsorption selectivity of a given adsorbent for one or more of the contaminating substances in the gas mixture to be purified.
Thus, in the case of hydrogen purification, the impurities that usually have to be removed are: water vapour; CO
2
, CO, nitrogen, saturated or unsaturated, linear, branched or cyclic hydrocarbons containing one or more carbon atoms in their hydrocarbon structure, for example C
1-C
8
compounds, such as CH
4
, C
2
H
4
, C
2
H
6
, C
3
H
8
, BTX (benzene-toluene-xylene) compounds; mercaptans; H
2
S; SO
2
, chlorine, ammonia, amines; alcohols, for example C
1
-C
3
light alcohols; other volatile organic compounds, such as esters, ethers and halogenated compounds.
These compounds are generally removed by a number of adsorbents, that is to say layers of adsorbents placed in series. Thus, it is conventional to use alumina or silica gel to retain, in particular, water vapour; activated carbon for retaining, in particular, hydrocarbons, CO
2
and water vapour; and zeolite for removing barely adsorbable impurities such as CO and nitrogen.
Usually the adsorbents are placed in a single adsorber but more usually in several adsorbers operating in alternation.
The proportion of the various adsorbents within the composite adsorbent bed depends on the composition of the gas to be treated and on the pressure, and there are therefore many possible combinations of composite adsorbents.
Usually, an H
2
PSA unit employs, within each adsorber, a pressure cycle comprising, schematically:
an approximately isobaric production phase at the high pressure of the adsorption cycle;
an adsorbent regeneration phase comprising at least one cocurrent decompression step by pressure equalization with another adsorber; a final, countercurrent depressurization step with discharge of waste gas; and generally an elution step at the low pressure of the cycle, the eluting gas generally coming from a second cocurrent decompression step of an adsorber; and
a repressurization phase comprising at least one step of pressure equalization with another adsorber and a final recompression step by means of production gas.
In general, the cycles may include several, total or partial, equalization steps, preferably from 1 to 4 equalization steps. Gas transfers can take place directly from adsorber to adsorber or via one or more gas storage tanks. The steps of recompression by equalization and of recompression by production gas may or may not be at least partially simultaneous and may optionally include a partial repressurization via a gas feed. Complementary purging steps may be introduced, particularly if it is desired to recover, for reutilization, another fraction other than hydrogen from the gas to be treated. In addition, the cycle may also include standby times during which the adsorbers are isolated.
Conventionally, the adsorption pressure is between 5 bar and 70 bar, preferably between 15 bar and 40 bar; the desorption pressure is between 0.1 bar and 10 bar, preferably between 1 and 5 bar; and the temperature of the stream of hydrogen to be purified is between −25° C. and +60° C., preferably between +5° C. and +35° C.
Moreover, this is illustrated, for example, by the documents U.S. Pat. No. 3,702,525, U.S. Pat. No. 3,986,849, U.S. Pat. No. 4,077,779, U.S. Pat. No. 4,153,428, U.S. Pat. No. 4,696,680, U.S. Pat. No. 4,813,980, U.S. Pat. No. 4,963,339, U.S. Pat. No. 3,430,418, U.S. Pat. No. 5,096,470, U.S. Pat. No. 5,133,785, U.S. Pat. No. 5,234,472, U.S. Pat. No. 5,354,346, U.S. Pat. No. 5,294,247 and U.S. Pat. No. 5,505,764, which describe PSA process operating cycles for producing hydrogen.
The impurities are removed by one or more adsorbents placed in series from the upstream end of the adsorber, that is to say the side where the gases to be treated enter the said adsorber. In general, the choice and the proportion of adsorbent(s) to be used depend on the nature or composition of the gas mixture to be treated and on the pressure.
Furthermore, mention may be made of document WO-A-97/45363 which relates to a process for the purification of hydrogen-based gas mixtures polluted by various impurities, including carbon monoxide and at least one other impurity chosen from among carbon dioxide and C
1
-C
8
saturated or unsaturated, linear, branched or cyclic hydrocarbons. The gas stream to be purified is brought into contact, in an adsorption zone, with a first adsorbent selective with respect to carbon dioxide and to C
1
-C
8
hydrocarbons and with a second adsorbent which is a faujasite-type zeolite exchanged to at least 80% with lithium and the Si/Al ratio of which is less than 1.5, in order to remove at least the carbon monoxide (CO). According to this document, the improvement made by the process is due to the use of a particularly effective zeolite, namely an X zeolite exchanged with lithium.
As regards document U.S. Pat. No. 3,150,942, this teaches the use of a zeolite containing sodium cations or sodium and calcium cations in order to purify a stream of hydrogen.
Similarly, document U.S. Pat. No. 4,477,267 describes a process for purifying hydrogen which uses an X zeolite exchanged to from 70 to 90% with calcium cations and also containing an inert binder.
Document U.S. Pat. No. 4,957,514 discloses a process for purifying hydrogen employing an X zeolite exchanged to from 60 to 80% with barium cations.
Furthermore, document U.S. Pat. No. 5,489,327 relates to the purification of gaseous hydrogen by bringing it into contact with a hydride of a zirconium alloy.
Finally, document JP-A-860146024 describes a PSA process for purifying impure gases using a mordenite-type zeolite exchanged with lithium, on the production side, and another zeolite, on the feed side.
On the other hand, certain documents point out that the adsorbent or adsorbents used in a PSA process for purifying hydrogen are of little, or even no importance.
Thus, the document by D. M. Ruthvens, S. Farooq and K. S. Knaebel <<Pressure Swing Adsorption>>, 1994 published by VCH, teaches, on page 238, that <<
since the selectivity for most impurities is high compared with that for hydrogen, any adsorbent can be used
>>to purify hydrogen.
Similarly, according to document U.S. Pat. No. 4,299,596, any conventional adsorbent can be used to produce hydrogen, for example active carbons, silica gels, molecular sieves, such as zeolites, carbon screens, etc.
Moreover, document U.S. Pat. No. 4,482,361 mentions the possibility of using whatever suitable adsorbent, such as zeolitic molecular sieves, active carbons, silica gels, activated aluminas or similar materials.
Likewise, document U.S. Pat. No. 4,834,780 teaches that the adsorption can be carried out in all cases where an adsorbent has been selected so as to be suitable for the separation process in question, for example active carbons, silica gels, aluminas or molecular sieves.
Moreover, document U.S. Pat. No. 5,275
Monereau Christian
Moreau Serge
L'Air Liquide, Societe Anonyme a Directoire et Conseil de S
Lawrence Frank M.
Simmons David A.
Young & Thompson
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