Process for Separating a Gas Mixture by Pressure Swing Adsorption

The present invention relates to processes for separating gas mixtures by pressure swing adsorption in adsorption devices comprising at least one adsorber having an inlet that can be connected to a rotary machine and an outlet that can be connected to a production gas vessel and a cycle vessel, comprising, successively in a cycle, a production phase, a regeneration phase and a repressurization phase.

A process of this type is described in the document EP-A-0 743 087 in the name of the Assignee.

The object of the present invention is to provide an improved process allowing a gain in specific energy without reducing productivity.

For this purpose, according to one characteristic of the invention, the regeneration phase comprises an initial depressurization step in which the gas is extracted in co-current from the adsorber and sent to the cycle vessel and which comprises a first sub-step in which the gas mixture is sent concomitantly in co-current to the adsorber.

According to another characteristic of the invention, the repressurization phase includes an initial step of repressurizing the adsorber in counter-current with some of the gas taken from one of the vessels, comprising a first sub-step in which gas is extracted concomitantly in counter-current from the adsorber.

According to one aspect of the invention, the end of the first sub-steps corresponds to a reversal of the direction of rotation of the rotary machine, that is to say the rotational speed of the rotary machine passes through zero as it changes from a compression mode to a suction mode, or vice versa.

Other characteristics and advantages of the present invention will emerge from the following description, which is given by way of illustration but without implying any limitation, and is made with reference to the appended drawings, in which:

FIG. 1 is a schematic view of an adsorption device for implementing a process according to the invention;

FIG. 2 schematically represents a cycle of a process according to the invention;

FIG. 3 is a diagram, correlated with the one in FIG. 2, showing the variations in the rotational speed of the rotary machine during the phases of the cycle in FIG. 2.

FIG. 1 shows a device for separating gases by adsorption, of the so-called VSA type, as described in the document EP-A-0 743 087 mentioned above. In the embodiment represented by way of example, the device essentially comprises an adsorber 1, of vertical general configuration, having a return permanently connected to a reversible rotary machine 3, typically a Roots machine or a multi-lobed rotor machine.

The adsorber 1 has an outlet 4 which selectively communicates, via a valve 5, with a cycle vessel 6 and, via a valve 7, with the inlet 8 of a production gas vessel 9 which has an outlet 10 that can be connected to a user circuit.

As represented in FIG. 2, a device according to FIG. 1 is employed using a cycle which includes the following successive steps:

a) (t.sub.0 -t.sub.1): a step of pure repressurization by supplying the inlet 2 (in co-current) with gas mixture compressed by the machine 3 which is operating as a compressor without extracting gas through the outlet 4 (valves 5 and 7 closed)

b) (t.sub.1 -t.sub.3): a phase of production to the cycle high pressure, in which the gas mixture to be separated continues to be introduced in co-current into the adsorber while the gas fraction not retained in the adsorber is extracted through the outlet 4 and sent to the vessel 9, the valve 7 being open

c+d) (t.sub.3 -t.sub.5) an initial depressurization step, in which the gas available in the upper part of the adsorber is extracted in co-current through the outlet 4 and sent to the vessel 6, the valve 5 being open (the valve 7 being closed)

e) (t.sub.5 -t.sub.7): a step of pure counter-current depressurization via the inlet 2, the rotary machine 3 operating as a pump (the valves 5 and 7 being closed)

f) (t.sub.7 -t.sub.9): an elution step continuing the previous step e) of counter-current discharge, with extra flushing of the adsorber in counter-current with gas taken from the vessel 6, the valve 5 being open

g) (t.sub.9 -t.sub.12): an initial repressurization step with counter-current application through the inlet 4 of production gas taken from the vessel 9, the valve 7 being open.

According to one aspect of the invention, as can be seen in FIG. 3, the speed of the rotary machine 3, which is operating as a compressor, is constant (nominal speed+R.sub.n) during step a) and approximately a first half of step b).

At a time t.sub.2 between t.sub.1 and t.sub.3, the rotary machine ceases to be driven and its rotational speed decreases until a time t.sub.4, between the times t.sub.3 and t.sub.5, that is to say during the initial depressurization step, at which the speed becomes zero. At time t.sub.4, the rotary machine which is at rest is driven in the other direction so as to operate as a pump. Its rotational speed therefore increases until a time t.sub.6, following the time t.sub.5, that is to say at the start of the pure depressurization phase e). From this time t.sub.5 until a time t.sub.8, while between the times t.sub.7 and t.sub.9, that is to say after a first part of the elution step f), it is kept constant (nominal speed -R.sub.n).

At time t.sub.8, the rotary machine ceases to be driven and its rotational speed decreases to zero at a time t.sub.10, between the times t.sub.9 and t.sub.12, that is to say during a first part of the first recompression step.

At time t.sub.10, the rotary machine starts to be driven in the other direction, to operate again as a compressor, and its speed increases rapidly until reaching the nominal speed R.sub.n, at time t.sub.11 which still lies in the interval t.sub.9 -t.sub.12, after which the rotational speed of the rotary machine is maintained until time t.sub.2 as seen above.

As can be seen in FIG. 2, this sequence of driving, relaxing and re-driving the rotary machine has the effect that the first depressurization step c+d) is broken down into a first sub-step c) (t.sub.3 -t.sub.4) in which, concomitantly with the co-current extraction of the unretained gas fraction to the cycle vessel 6, the gas mixture to be separated continues to be introduced in co-current, with a flow rate decreasing to zero at time t.sub.4, and into a second sub-step d) (t.sub.4 -t.sub.5) in which concomitantly with the co-current extraction, and with the rotary machine having started to rotate as a pump, a counter-current extraction takes place which increases progressively until time t.sub.5, after which the valve 5 is closed until time t.sub.7.

Similarly, the first repressurization step g-h) (t.sub.9 -t.sub.12) is broken down into a first sub-step g) (t.sub.9 -t.sub.10) in which, concomitantly with the counter-current introduction of production gas taken from the vessel 9, counter-current discharge takes place through the inlet 2, with a flow rate which is reduced to zero at time t.sub.10 when, with the rotary machine being reversed and starting to operate as a compressor, the pressurization continues in a second sub-step with application, through the two ends, in counter-current of production gas taken from the vessel 9 and in co-current of gas mixture to be separated, up to full-speed operation of the rotary machine operating as a compressor until time t.sub.2.

The present invention finds a particular application for the production of medium-purity oxygen from air on an adsorbent of the zeolite type. The cycle in FIG. 2 corresponds typically to a cycle lasting less than 90 seconds, typically of the order of 70 to 80 seconds, with a cycle high pressure that does not exceed 1.5.times.10.sup.5 Pa and a cycle low pressure that does not exceed 0.5.times.10.sup.5 Pa.