Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-01-14
2001-11-13
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S023000, C095S105000
Reexamination Certificate
active
06315818
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the purification of a gas containing preferentially adsorbable impurities, particularly hydrogen, by pressure swing adsorption (PSA), of the type using several adsorbers with, for each adsorber, a cycle comprising the following operative phases, which are offset from one adsorber to another with an nth duration of the duration of the cycle, in which n designates the number of adsorbers: substantially isobaric adsorption at a high pressure of the cycle, this adsorption phase comprising the admission of impure gas to be treated into the adsorber at a first end, so-called entry end, of the adsorber, with circulation of this gas through the adsorber and simultaneous withdrawal from the other end of the adsorber of purified gas of which at least a portion constitutes the gas product, the direction of circulation in the adsorber in the course of this producing phase being called co-current and the reverse direction of circulation being called counter-current; regeneration of the adsorber ending in at least one final step in which a residual gas is withdrawn counter-current from the adsorber; and increase in pressure to the high pressure of the cycle.
The invention is applicable in particular to the production of gases available in limited quantities such as hydrogen, helium and neon, the production of hydrogen being the most important application.
BACKGROUND OF THE INVENTION
For the production of such gases, the extraction output is generally the preponderant parameter. To maximize performance, the durations of the different steps are chosen to be sufficiently long that the adsorbents operate at equilibrium, which is to say in which there is no kinetic limitation. The duration of adsorption for an adsorber is thus typically of the order of 2 to 4 minutes for total cycle durations generally greater than 10 minutes.
So as to increase the output, numerous PSA adsorption cycles comprising, in the course of the regeneration phase, several successive pressure balancings, have been proposed.
However, this latter technique is costly of capital, because it requires installing a large number of adsorbers each having an increased size. Moreover, this increased investment is particularly penalizing when the maximum demand for the production gas is only temporary.
SUMMARY OF THE INVENTION
The invention has for its object to permit achieving in a particularly economical fashion, high extraction outputs, not only for impure gases already very rich (which is to say, in the case of hydrogen, containing typically at least 90%, and generally at least 98 to 99%, of hydrogen) but also for impure gases substantially purer, containing typically, in the case of hydrogen, up to about 30% of impurities.
To this end, the invention has for its object a process of the mentioned type, characterized in that:
a variable portion of said residual gas is recycled, representing from 0 to 30% of the purified gas product flow rate, into the impure gas to be treated, the rest of this residual gas constituting a purge; and
the duration of the cycle is decreased when said recycling is carried out, the more so as the recycled flow rate is the greater.
The process according to the invention can comprise one or several of the following characteristics, taken alone or in any possible technical combinations:
the flow rate of recycled residual gas is temporarily zero and temporarily comprised between 10 and 30% of the flow rate of the purified gaseous product;
residual gas is recycled when and only when the flow rate of the gaseous product is greater than a predetermined value;
the flow rate of impure feed gas is maintained constant, and the flow rate of the recycled residual gas is varied as a function of the flow rate of the purified gaseous product, the variations of these flow rates taking place in the same sense;
the decrease of the cycle duration is comprised between about 1 and 30% of the duration of the cycle without recycling;
said regeneration begins with a single co-current PSI depressurization with another adsorber in pressure increase phase;
the regeneration comprises two so-called final steps, namely
(a) a countercurrent depressurization to a low pressure of the cycle; and
(b) a countercurrent elution;
said single step of co-current depressurization by PSA is followed with a second co-current depressurization, the gas from the adsorber during this step being used as elution gas;
an adsorption unit is used with three adsorbers, and the gas from the adsorber in the course of the second co-current depressurization step is sent to an auxiliary elution storage; and
an adsorption unit with four adsorbers is used, and the gas from the adsorber in the course of the second co-current depressurization step is directly sent to another adsorber to carry out its counter-current elution.
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L'Air Liquide, Societe Anonyme pour l'Etude et l'
Spitzer Robert H.
Young & Thompson
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