Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-02-14
2001-06-05
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S408000
Reexamination Certificate
active
06240745
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes for the separation of atmospheric air by cryogenic distillation which are improved by removing the impurities by adsorption, prior to the distillation.
BACKGROUND OF THE INVENTION
It is known that atmospheric air contains the compounds or impurities that have to be removed before any cryogenic separation of the air, that is to say prior to introducing the air into the heat exchangers of the cold box of an air separation unit.
In particular, mention may be made of the compounds of the carbon dioxide (CO
2
) and/or water vapour (H
2
O) type, but also of other impurities.
This is because, in the absence of such a pretreatment of the air, these impurities, particularly CO
2
and/or water vapour, would inevitably condense and solidify as ice while the air is being cooled to a cryogenic temperature, something which would cause problems of blocking in the cryogenic separation equipment or unit, especially the heat exchangers, distillation columns, etc., and thereby cause the equipment or unit to be damaged.
To avoid these problems, it is common practice to pretreat the air that has to be cryogenically separated before this cryogenic separation.
This pretreatment of the air is usually called “front end” scrubbing or purification since it is carried out upstream of the cryogenic separation unit.
Currently, the air is pretreated by a TSA (Temperature Swing Adsorption) process or by a PSA (Pressure Swing Adsorption) process depending on the case.
Conventionally, a TSA process cycle comprises the following steps:
a) purification of the air by adsorption of the impurities at superatmospheric pressure;
b) depressurization of the adsorber down to atmospheric pressure or below atmospheric pressure;
c) complete regeneration of the adsorbent at atmospheric pressure using a hot gas, especially the residual gases or waste gases, typically impure nitrogen coming from an air separation unit and heated by means of one or more heat exchangers;
d) cooling of the adsorbent, especially by continuing to introduce into it the residual gas coming from the air separation unit, but not heated;
e) repressurization of the adsorber using the purified air, coming, for example, from another adsorber which is in production phase.
Moreover, as regards a PSA process cycle, this comprises substantially the same steps a), b) and e), but is distinguished from a TSA process by the residual gas or gases not being heated during the regeneration step (step c)), and therefore by the absence of step d) and, in general, a shorter cycle time than in a TSA process.
Preferably, the air pretreatment devices comprise at least two adsorbers, operating in parallel, that is to say alternately, one of the adsorbers being in production phase while the other is in regeneration phase.
Such TSA or PSA air purification processes are described, for instance, in documents U.S. Pat. No. 3,738,084, 5,531,808, 5,587,003 and 4,233,038.
However, it is known that, when particles of adsorbent are used to prepurify the air before it is separated by cryogenic distillation it is common practice to adjust (by water cooling) the temperature of the compressed air from a temperature usually of at least 80° C., or even higher, down to room temperature and then subsequently to precool the air before it is introduced into the adsorber or adsorbers, this usually being carried out by a refrigeration unit which cools the air from room temperature down to a temperature below room temperature.
This is, moreover, clearly explained in the document “Industrial Gases & Cryogenics Today, IOMA Broadcaster, Air Purification for cryogenic air separation units, January-February 1984, p. 15 et seq.” or by the document EP-A-438,282.
In fact it is particularly recommended to precool the air before subjecting it to an adsorption separation step since, as is known by those skilled in the art, the lower the adsorption temperature the more effective the adsorption of the impurities.
In other words, the effectiveness of the air prepurification is markedly improved at low temperatures, that is to say temperatures close to 5° C., or improved even more at markedly lower temperatures.
Next, after its passage through the purification zone, that is to say in the adsorber or adsorbers, the air stripped of all or some of its deleterious impurities, especially of the CO
2
and H
2
O type, is then conventionally cooled to a cryogenic temperature, that is to say generally to a temperature of less than approximately −120° C., before being sent into the cryogenic distillation unit and introduced into one or more distillation columns for the purpose of being separated therein in order to recover the nitrogen, oxygen and/or argon.
However, the fact of having to employ an air precooling step before the air is introduced into the adsorber or adsorbers has several drawbacks which have a negative impact on the industrial advantage of the overall process.
This step of precooling the air has the effect of appreciably increasing the overall investment, of complicating the process, of possibly causing reliability problems and therefore of resulting in an additional cost for the plant, given that it is then necessary to provide cooling means, such as heat exchangers or the like, that is to say a refrigeration unit.
Mention may be made of the documents U.S. Pat. No. 4,249,915, EP-A-733393, EP-A-718576, U.S. Pat. No. 5,463,869 and JP-A-54103778 describing various processes for the treatment of the air before it is cryogenically separated for the purpose of producing nitrogen and oxygen.
SUMMARY OF THE INVENTION
Thus, the object of the present invention is to alleviate the abovementioned problems and drawbacks by providing a process for the cryogenic separation of air which does not require the air to be precooled by a refrigeration unit before it is introduced into the adsorber or adsorbers of the front-end scrubbing unit, that is to say a cryogenic air separation process which is simplified compared with the currently existing processes.
The present invention therefore relates to a process for the cryogenic separation of air containing impurities, which is carried out according to the steps of:
(a) compressing the air to be separated to a pressure of at least 11.4×10
5
Pa;
(b) introducing the air at a temperature greater than or equal to +15° C. and at a pressure of at least 11.4×10
5
Pa in at least one adsorption vessel containing particles of at least one adsorbent;
(c) adsorbing at least some of the impurities contained in the air on the particles of adsorbent at a pressure of at least 11.4×10
5
Pa;
(d) cooling the air purified in step (c) down to a cryogenic temperature of less than −120° C., preferably down to a temperature of less than −170° C.;
(e) cryogenically distilling the air cooled in step (d).
As a variant, the invention relates to a process for the cryogenic separation of air containing impurities, which is carried out according to the steps of:
(a) compressing the air to a pressure of at least 11.4×10
5
Pa;
(b) introducing the compressed air into at least one adsorption vessel containing particles of at least one adsorbent;
(c) adsorbing at least some of the impurities contained in the air on the particles of adsorbent at a pressure of at least 11.4×10
5
Pa and at a temperature of at least +15° C.;
(d) cooling the air purified in step (c) down to a cryogenic temperature of less than −120° C., preferably down to a temperature of less than −170° C.;
(e) cryogenically distilling the air cooled in step (d).
Depending on the case, the process according to the invention may comprise one or more of the following characteristics:
after cryogenic distillation, at least one compound chosen from nitrogen, oxygen, argon or their mixtures is recovered. The compound or compounds thus produced may be of variable purity and at least one of these compounds is preferably recovered in liquid form in order to utilize the energy provided by the increase in the pressure
Fraysse Philippe
Guillard Alain
Kraus Georges
Le Bot Patrick
Doerrler William
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Young & Thompson
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