Gas separation: processes – With control responsive to sensed condition – Temperature sensed
Reexamination Certificate
2000-08-31
2002-06-11
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Temperature sensed
C095S015000, C095S018000, C095S021000, C095S099000, C095S105000, C095S106000, C095S120000, C095S123000, C095S139000, C096S112000, C096S113000, C096S130000, C096S144000
Reexamination Certificate
active
06402809
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the purification of air intended to be cyrogenically fractionated, of the type in which the air is dried, decarbonated and at least partially stripped of the secondary atmospheric contaminates, such as hydrocarbons, nitrogen oxides, etc., by passing it through one or more bodies of adsorbent.
BACKGROUND OF THE INVENTION
Conventionally, the body of adsorbent is cyclically regenerated by heating and/or flushing before it is used again in the adsorption phase.
At the present time, two types of processes are more particularly used for this purpose, namely:
PSA processes in which most of the regeneration power is provided by a pressure effect; and
TSA processes in which most of this same regeneration power is provided by a temperature effect.
Less conventionally, hybrid solutions have also been proposed for this purpose, especially in document U.S. Pat. No. 5,614,000.
Usually, a TSA (Temperature Swing Adsorption) process for purifying air comprises the following steps:
a) purification of the air by adsorption of the impurities at superatmospheric pressure and at ambient temperatures;
b) depressurization of the adsorber down to atmospheric pressure;
c) regeneration of the adsorbent at atmospheric pressure, especially by the residual or waste gases, typically impure nitrogen coming from an air separation unit and heated to a temperature above ambient by means of one or more heat exchangers;
d) cooling the adsorbent down to ambient or subambient temperature, especially by continuing to introduce air into the waste gas coming from the air separation unit, but this air not being heated;
e) repressurization of the adsorber with purified air coming, for example, from another adsorber in production phase.
Less conventionally, the regeneration may be carried out at a pressure substantially different from atmospheric pressure, either greater or even less than the latter by using, in this case, suitable vacuum pumping means.
As regards a PSA (Pressure Swing Adsorption) process cycle for purifying air, this comprises substantially the same steps a), b) and e) but is distinguished from a TSA process by the absence of heating of 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, the 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 described for instance in document U.S. Pat. No. 3,738,084.
The process, whether it is a PSA or TSA process, may involve one or more adsorbent beds, that is to say a multi-bed process.
The adsorbents used are, without being limiting, zeolites, activated aluminas, silica gels, exchanged zeolites, doped aluminas, etc.
Furthermore, depending on the case, the adsorbers may have their axis vertical or horizontal, or else they may be of the radial type, etc.
However, in all cases, the objective of the purification is to stop the H
2
O and CO
2
impurities, and the other possible contaminants likely to be present in the gas stream, down to contents compatible with the proper operation of the cryogenic unit, whatever the performance or safety level of the equipment.
This is because, in the absence of such an air purification treatment for removing its CO
2
and water impurities, there will be condensation and/or solidification of these impurities while cooling the air to cryogenic temperatures, which as a result may cause blockage problems in the equipment, especially the heat exchangers, distillation columns etc.
Furthermore, it is also common practice to remove, at least partially, the hydrocarbons and nitrogen oxide impurities liable to be present in the air so as to avoid any risk of damaging the equipment, particularly the distillation column or columns located downstream of the cold box.
Typically, the maximum values of impurity contents permitted are, at peak, less than 1 vpm and, on average, substantially less than this value of 1 vpm.
Usually, the various operating conditions that may be encountered on a site are taken into account when designing the unit so as to ensure that the cryogenic distillation unit located downstream of the air purifiers, this unit generally being called the ASU (Air Separation Unit), operates properly under all circumstances.
The improvements made hitherto to this type of process relate:
either to the selection of the adsorbents: nature, multi-bed, particle size etc.;
or to the processes themselves: regeneration temperature, choice of number of adsorbers, etc.;
or to reducing the energy in a given process.
With regard to the improvements made to the adsorbents or to the processes themselves, mention may especially be made of the following documents: U.S. Pat. No. 5,531,808, U.S. Pat. No. 5,587,003, U.S. Pat. No. 4,233,038, U.S. Pat. No. 5,232,474, EP-A-744,205, EP-A-590,946, EP-A909,823, EP-A-909,824, EP-A-909,825, EP-A-862,937, EP-A-862,936, EP-A-766,991, EP-A-766,989 and EP-A-862,938.
Moreover, with regard to the improvements made to the reduction in energy consumed, mention may be made of the documents: U.S. Pat. No. 4,472,178 relating to the recovery of the adsorption heat in a regenerator; U.S. Pat. No. 4,627,856 relating to a process allowing the necessary energy to be reduced by separately regenerating the adsorbent for stopping the water and the adsorbent for stopping the CO
2
; and U.S. Pat. No. 5,766,311 relating to a regeneration process using multiple thermopulses.
The use of adsorbers in which the gas stream flows radially, allowing the head losses to be reduced, may also fall within this category, the reduction in energy then relating to the compression of the air.
All these methods therefore consist in using additional items of equipment: regenerator, internal heater, connecting pipework, etc.
Consequently, it will be immediately understood that these processes increase the complexity of the equipment.
It follows that the resulting increase in the costs of the plant must therefore be more than compensated for by energy saving so that these processes are useful from an industrial standpoint.
OBJECTS OF THE INVENTION
In contrast, the present invention aims to improve the known gas purification processes, particularly air purification processes, by appreciably reducing the amount of energy consumed.
In other words, the present invention aims to reduce the amount of energy consumed in the known gas purification processes no longer by adding expensive items of equipment but by optimizing the regeneration conditions at each cycle.
Put another way, the object of the present invention is to modify the operating conditions of a TSA-type gas purification process according to certain external parameters, such as the environmental conditions for example, so as to save energy by profiting from operating conditions which are more favourable than those considered during the design work, as conventionally done in the prior art, that is to say by using new calculation and regulation methods rather than new equipment.
Indeed, although a number of documents relating to processes for regulating gas purification units using TSA processes are known, none of them makes it possible to achieve performance levels as high as those obtained by the present invention.
By way of example, mention may be made of the documents:
U.S. Pat. No. 3,808,773 which describes a process comprising a step of regenerating the adsorbent, beginning as soon as the CO
2
impurities break through, that is to say as soon as the bed of adsorbent is saturated by these CO
2
impurities. The regeneration conditions undergo no modification or variation during this regeneration step;
U.S. Pat. No. 4,472,178 which specifies that the adsorption step terminates when the concentration of CO
2
impurities in the purified air reaches 1 vpm. Here again, the regeneration conditions undergo no modification or adjustment during the regeneration step;
U.S. Pat. No
Combier Alain
Miniscloux Didier
Monereau Christian
L'Air Liquide, Societe Anonyme a Directoire et Conseil de S
Spitzer Robert H.
Young & Thompson
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