Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1999-04-07
2001-01-09
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S100000, C095S103000, C095S130000, C096S130000, C096S144000
Reexamination Certificate
active
06171371
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing oxygen from air by transatmospheric pressure swing adsorption, of the type in which, using a unit having at least two adsorbers, a cycle of duration T is implemented in each adsorber, comprising successively and cyclically:
a phase (a) of pressurization to the high pressure of the cycle using production oxygen introduced in countercurrent and/or air introduced in cocurrent, this phase ending with a cocurrent production step in which the oxygen produced is sent to a buffer volume;
a first cocurrent decompression phase (b);
a countercurrent purge phase (c) comprising pumping to the low pressure Pm of the cycle; and
a purge/elution phase (d) in which gas output by another adsorber in the first cocurrent decompression phase (b) is introduced in countercurrent and, simultaneously, the countercurrent pumping is continued.
BACKGROUND OF THE INVENTION
PSA processes (Pressure Swing Adsorption) are being used more and more for producing oxygen with limited purity from atmospheric air. There is, of course, a constant preoccupation with reducing the cost of the oxygen to be produced. In order to do this, use may be made of adsorbents which are particularly specific and effective for O
2
/N
2
separation, such as lithium-exchanged zeolites. However, the cost of these adsorbents is very much greater than that of conventional adsorbents such as 5A or 13X molecular sieves, so that one condition for their acceptance is that a small amount of adsorbents is necessary.
In order to minimize the volumes of adsorbents to be utilized, it has been proposed to shorten the cycle, in particular overlapping steps which have normally been sequential. Thus, the air may be introduced into the adsorber as soon as possible after the end of the purge/elution step, or immediately following it. However, all other things being equal, this introduction of air at low pressure has a detrimental effect on the propagation of the adsorption fronts, which advance commensurately faster if the pressure in the adsorbent is low.
This is true both for the retention of nitrogen and for that of atmospheric impurities (water, CO
2
, traces of hydrocarbons, etc.), for which use is made either of a specific bed such as an alumina or silica gel bed, or an adsorbent used for O
2
/N
2
separation. In all cases, the introduction of air at low pressure actually requires greater volumes of adsorbents, which is contrary to the intended result.
Furthermore, enlarging the zone allocated to retaining the impurities increases the volume of air which is unproductively compressed then pumped, with a negative effect on the specific energy.
One way of limiting these drawbacks is to minimize the quantity of air introduced at the lowest pressures by arranging for the pressure in the adsorber to rise very rapidly.
Thus, according to the teaching of EP 758 625 A1,
FIGS. 4 and 5
, after the purge/elution step the following steps are employed in a cycle of the aforementioned type, the duration of which is 120 seconds:
first countercurrent repressurization using countercurrent decompression oxygen from the other adsorber, lasting 2 seconds;
second repressurization, simultaneously in cocurrent by introducing air and in countercurrent using cocurrent decompression oxygen from the other adsorber, lasting 4 seconds;
third repressurization, solely in cocurrent by introducing air, the outlet of the adsorber being closed, lasting 2 seconds; and
a step of final repressurization and cocurrent production, lasting 44 seconds.
This leads to sequencing, in spite of the relatively long duration of the cycle, steps which have very short durations—from one second to a few seconds at most—with multiple valve-opening/closing operations in the short corresponding lapse of time.
SUMMARY OF THE INVENTION
The object of the invention is to make it possible, simply and reliably, to produce particularly short cycles, less than 120 seconds, typically less than 90 seconds, which consume little energy.
To this end, the invention relates to a process of the aforementioned type, characterized in that the buffer volume is subdivided into a buffer tank and into an auxiliary volume comprising an auxiliary pressurization tank, in that during an initial part of the production step, the oxygen produced is sent only into the auxiliary pressurization tank, and in that the gas contained in the auxiliary tank is, with regard to the adsorber, used solely during a fraction of the pressurization phase (a) of the adsorber.
The process according to the invention may have one or more of the following characteristics, taken alone or in any of their technically feasible combinations:
the fraction has a duration at most equal to T/10, and preferably, for a cycle with a duration advantageously less than 120 seconds, at most equal to 10 seconds;
during at least a part of the fraction, air is simultaneously introduced into the adsorber in cocurrent;
during at least a part, in particular an initial part, of the fraction, gas output by another adsorber in the first cocurrent decompression phase (b) is simultaneously sent into the adsorber in countercurrent;
during the fraction, the auxiliary tank is isolated from the buffer tank and this auxiliary tank is set in communication with the adsorber until the pressures are balanced between them;
the isolation is carried out by means of a one-way valve which allows oxygen to flow from the auxiliary tank to the buffer tank, but prevents any reverse flow;
the auxiliary tank is left in communication with the adsorber until the end of the pressurization phase (a), so that this auxiliary tank is returned to the high pressure of the cycle.
The invention also relates to a unit for producing oxygen from air by transatmospheric pressure swing adsorption, intended for the implementation of the process defined above. This unit, of the type comprising at least two adsorbers in parallel which are selectively connected to an air compressor, to a vacuum pump and to a buffer production tank, the outlets of the adsorbers being connected in pairs by a duct provided with a flow control valve, is characterized in that it comprises at least one auxiliary pressurization tank, means for selectively connecting this auxiliary tank to the outlet of each adsorber, and means which are intended to connect the auxiliary tank to the buffer tank and are designed to set these two tanks in communication when, and only when, the pressure of the auxiliary tank is at least equal to that of the buffer tank.
In particular embodiments of this unit:
the means for connecting the auxiliary tank to the buffer tank comprise a one-way valve;
the volume of the or each auxiliary tank is less than that of the buffer tank.
REFERENCES:
patent: 3636679 (1972-01-01), Batta
patent: 4561865 (1985-12-01), McCombs et al.
patent: 5032150 (1991-07-01), Knaebel
patent: 5163978 (1992-11-01), Leavitt et al.
patent: 5370728 (1994-12-01), LaSala et al.
patent: 5505765 (1996-04-01), Kaji et al.
patent: 5518526 (1996-05-01), Baksh et al.
patent: 5565018 (1996-10-01), Baksh et al.
patent: 5658371 (1997-08-01), Smolarek et al.
patent: 5679134 (1997-10-01), Brugerolle et al.
patent: 5702504 (1997-12-01), Schaub et al.
patent: 5735938 (1998-04-01), Baksh et al.
patent: 5871565 (1999-02-01), Leavitt
patent: 5882380 (1999-03-01), Sircar
patent: 5928407 (1999-07-01), Amlinger
patent: 5935297 (1999-08-01), Amlinger
patent: 5961694 (1999-10-01), Monereau et al.
patent: 6010556 (2000-01-01), Petit
patent: 0 663 229 (1995-07-01), None
patent: 0 743 087 (1996-11-01), None
patent: 0 758 625 (1997-02-01), None
Derive Nathalie
Dubois Anne
Monereau Christian
L'Air Liquide, Societe Anonyme pour l'etude et l'
Spitzer Robert H.
Young & Thompson
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