Gas separation: processes – Solid sorption
Reexamination Certificate
2000-07-03
2002-07-30
Smith, Duane (Department: 1724)
Gas separation: processes
Solid sorption
C095S095000, C095S096000, C095S104000, C095S117000, C095S129000, C095S139000, C095S143000, C095S144000, C095S148000, C095S902000
Reexamination Certificate
active
06425937
ABSTRACT:
FILED OF THE INVENTION
The object of the present invention is to provide a process for pretreating or purifying a gas stream consisting of atmospheric air prior to the cryogenic separation of the air, particularly by cryogenic distillation.
BACKGROUND OF THE INVENTION
It is known that atmospheric air contains compounds that have to be removed before the air is introduced into the heat exchangers of the cold box of an air separation unit, especially the compounds carbon dioxide (CO
2
), water vapour (H
2
O) and/or hydrocarbons (CnHm) for example.
This is because, in the absence of such an air pretreatment for removing its CO
2
and water vapour impurities, these impurities condense and solidify as ice when the air is cooled to cryogenic temperature, which may result in problems of the equipment, especially the heat exchangers, distillation columns, etc., becoming blocked.
Furthermore, it is also common practice to remove the hydrocarbon impurities liable to be present in the air so as to avoid any risk of deterioration of the equipment, particularly of the distillation column or columns located downstream of the cold box.
At the present time, this air pretreatment is carried out, depending on the case, by a TSA (Temperature Swing Adsorption) process or by a PSA (Pressure Swing Adsorption) process; the expression “PSA process” should be understood to mean actual PSA processes, VSA (Vacuum Swing Adsorption) processes, VPSA (Vacuum Pressure Swing Adsorption) processes and similar processes.
Conventionally, a TSA process cycle for purifying air comprises the following steps:
a) purification of the air by adsorption of the impurities at a superatmospheric pressure and at ambient temperature;
b) depressurization of the adsorber down to atmospheric pressure or below atmospheric pressure;
c) regeneration of the adsorbent at atmospheric pressure, especially by residual gases or waste gases, typically impure nitrogen coming from an air separation unit and heated to a temperature above +80° C. by means of one or more heat exchangers;
d) cooling of the adsorbent to ambient or subambient temperature, especially by continuing to introduce therein to the waste gas coming from the air separation unit, but the gas not being heated;
e) repressurization of the adsorber with purified air coming, for example, from another adsorber which is in production phase.
As regards a PSA process cycle for purifying air, this usually comprises substantially the same steps a), b) and e), but differs from a TSA process by the absence of a step for heating the waste gas or gases during the regeneration step (step c)), and therefore the absence of step d), and, in general, a shorter cycle time than in the TSA process.
In general, air pretreatment devices comprise two adsorbers, operating alternately, that is to say one of the adsorbers is in production phase while the other is in regeneration phase.
Such TSA air purification processes are especially described in U.S. Pat. No. 3,738,084 and FR-A-77/25845.
In general, the removal of the CO
2
and the water vapour is carried out over one or more beds of adsorbents, preferably several beds of adsorbents, namely generally a first adsorbent designed to preferentially stop the water, for example a bed of activated alumina, of silica gel or of zeolites, and a second bed of adsorbent for preferentially stopping the CO
2
, for example a zeolite. In this regard, mention may especially be made of documents U.S. Pat. No. 5,531,808, U.S. Pat. No. 5,587,003 and U.S. Pat. No. 4,233,038.
However, it is not an easy matter to achieve effective removal of the CO
2
and water vapour which are contained in the air over one and the same bed of adsorbent since water has an affinity for the adsorbents which is markedly greater than that of CO
2
, and it is therefore customary to use at least two beds or layers of adsorbents of different types.
Thus, it is common practice to use a zeolite of the 13X type for removing the CO
2
since the 13X zeolite is reputed to be particularly effective for stopping small amounts of CO
2
and possibly of water, as it has a strong affinity and selectivity for these polar molecules. In addition, the X zeolite has among the largest micropore diameters, enabling it to adsorb, with favourable kinetics, molecules having a kinetic diameter up to 0.8 nm, as mentioned by D.W. Breck's document “Zeolite molecular sieves”, Krieger Publishing Company, 1984, p. 612.
However, the 13X zeolite is unable to stop all undesirable molecules liable to be present in a gas stream.
This is because the gas molecules adsorbed by the 13X zeolite are essentially, and in increasing affinity: methane, ethane, propane, nitrogen protoxide, ethylene, carbon dioxide, butane, propylene (C
3
H
6
), acetylene (C
2
H
2
), toluene and methylcyclohexane.
In this regard, reference may be made to the following documents: E. Alpay, “Adsorption parameters for strongly adsorbed hydrocarbon vapours on some commercial adsorbents”, Gas Sep. & Purif., Vol. 10, No. 1, pp 25 (1996); G. Calleja, “Multicomponent adsorption equilibrium of ethylene, propane, propylene and CO
2
on 13X zeolite”, Gas Sep. & Purif., Vol. 8, No. 4, p. 247 (1994); V. R. Choudhary, “Sorption isotherms of methane, ethane, ethene and carbon dioxide on NaX, NaY and Na-mordenite Zeolites”, J. Chem. Soc. Faraday Trans., 91(17), p. 2935 (1995); and A. Cointot, P. Cartaud, C. Clavaud, “Etude de l'adsorption du protoxyde d'azote par différents tamis moléculaires [Study of the adsorption of nitrogen protoxide by various molecular sieves]”, Journal de Chimie Physique, Vol. 71, No. 5, p. 765-770 (1974).
It therefore follows that an industrial air-prepurification unit strictly dimensioned for stopping carbon dioxide with a standard zeolite, typically a 13X or 5A zeolite, only partially stops ethylene, propane and nitrogen protoxide, as indicated by Dr J. Reyhing's document “Removing hydrocarbons from the process air of air separation plants using molecular-sieve adsorbers”, Linde Reports on Science and Technology, 36/1983.
Similarly, this situation for hydrocarbons is also described by Dr J. Reyhing in the above document.
Likewise, with regard to nitrogen protoxide, the ineffectiveness of the 5A zeolite for stopping N
2
O compared with CO
2
has been demonstrated by U. Wenning in “Nitrous oxide in air separation plants”, MUST'96, Munich Meeting on Air Separation Technology, Oct. 10-11, 1996.
Moreover, there are also similar problems with ethylene, which is an unsaturated hydrocarbon unstable in the presence of oxygen, soluble in liquid oxygen to a level of 30,000 ppm with a low solute-gas equilibrium coefficient, its freezing point being −169° C., whereas that of liquid oxygen at 1.2 bar is −181° C.
It may therefore be readily understood that, if the prepurification plant does not completely stop the ethylene, it may easily be found downstream of this plant and damage to the cryogenic distillation plant, particularly the distillation columns and/or reboilers, may then result therefrom, something which is unacceptable.
Similar problems may also arise with ethane and propane which may be found in the liquid state at the liquid oxygen temperature at a pressure of 1.2 bar.
Although certain documents provide more or less effective solutions allowing some of the impurities that may be found in a stream of atmospheric air to be removed, the problem of effective removal, that is to say complete stoppage of propane, ethylene and/or nitrogen protoxide which are contained in a gas stream, particularly an air stream, has not yet been solved.
Indeed, document EP-A-847,792 provides a process for adsorbing acetylene impurities and C
3
-C
8
hydrocarbons, in a PSA cycle, no matter whether a CaX, CaA, 5A, 13X or Na-mordenite zeolite is used. However, it should be emphasized that this type of adsorbent gives, overall, results that are less satisfactory than an activated-alumina/NaY double bed.
Moreover, documents EP-A-766,991 and EP-A-453,202 propose the use of standard activated alumina or activated alumina w
Gabillard Jean-Pierre
Kraus Georges
Millet Cyrille
Moreau Serge
Greene Jason M.
L'Air Liquide, Societe Anonyme a Directoire et Conseil de S
Smith Duane
Young & Thompson
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