WO1998001215A1

WO1998001215A1 - Method for the adsorptive separation of air

Info

Publication number: WO1998001215A1
Application number: PCT/EP1997/003434
Authority: WO
Inventors: Gerhard Reiss; Heinrich Amlinger
Original assignee: Bayer Aktiengesellschaft; Sgi-Prozesstechnik Gmbh
Priority date: 1996-07-08
Filing date: 1997-07-01
Publication date: 1998-01-15
Also published as: CZ4099A3; CA2259660A1; BR9710225A; ZA976025B; JP2000513997A; EP0910457A1; KR20000023604A; PL331017A1; TW380118B; DE19627422A1

Abstract

The invention concerns an improved method for the separation of oxygen or nitrogen from air according to the vacuum swing adsorption (VSA) process or pressure vacuum swing adsorption (PVSA) process using a vacuum pump system optimized for the desorption of the adsorbed air component. This system comprises a radial flow compressor and a vacuum pump operating according to the displacement principle, in particular a rotary piston compressor.

Description

Process for the adsorptive separation of air

The present invention relates to an improved method for separating oxygen or nitrogen from air in the sense of vacuum swing adsorption

(VSA) or pressure-vacuum swing adsorption (PVSA) using a vacuum pump system optimized for the desorption of the adsorbed air component

The direct generation of oxygen from air at ambient temperatures is already carried out extensively industrially with molecular sieve zeolites

Here, the preferred adsorption of nitrogen over oxygen is used, ie nitrogen in the air is adsorbed on the zeolite, the less strongly adsorbed components such as oxygen and argon are collected as a product when air flows through a zeolite bed at the outlet of this bed. The desorption of the adsorbed nitrogen can e.g. by evacuating the fill. In this case one speaks of the VSA process

(Vacuum swing adsorption), in contrast to the PSA process (= Pressure Swing Adsorption), which is also known. A continuous process can be achieved in the VSA process by the following procedure

a) Passing air through a bed of zeolite into the inlet of an adsorber in e.g. Ambient pressure, at the outlet of the adsorber

0., - enriched gas withdrawn,

b) evacuating the fill at the inlet with a vacuum pump to a pressure of about 100 to 400 hPa in counterflow to the air flow, with a portion of the product possibly being coiled at the same time,

c) filling the bed with O ₂ product to about ambient pressure in the

Counterflow to the air flow. In the PSA process, step a) is carried out at a pressure of 200 to 600 kPa and step b) is carried out at about 100 kPa with purging with part of the O ₂ product (the pressures always relate to absolute values)

Due to these three steps, you usually work with three separate ones

Zeolite beds, called adsorbers for short, which are operated alternately in cycles However, separation processes with vacuum regeneration are also described, in which two adsorbers are operated cyclically with one product store (US 397 696) or only one adsorber alternates with one product store

The economic viability of these systems is influenced by the investment, such as the amount of adsorbent, the size of the vacuum pump, and especially by the operating costs, such as the power consumption of the vacuum pump. The aim of every development is therefore to optimize the molecular sieve volume, vacuum pump size and energy consumption of the vacuum pump. Vacuum pumps previously used for vacuum desorption are two- or three-stage rotary lobe blowers with displacement function (see EP 158 262) or water ring pumps, also based on a

Verdangerwi rkung

Other compressors that can also be used are centrifugal compressors that are used as vacuum pumps (see, for example, EP 575 591). These compressors, known as radial blowers, have the property that they are operated up to a pressure ratio of back pressure to suction pressure of about 2.6 can, but for their optimal use, that is, to achieve the lowest possible energy requirement, a certain ratio between suction pressure and discharge pressure is required.This is also known as the optimal pressure ratio π. This pressure ratio π is around 1 6 to 1 7 with conventional radial blowers a radial blower is to be optimally used as a vacuum pump and the back pressure including the pressure loss of the downstream S chal 1 steam is equal to 1000 hPa, then a constant pressure of 625 or 588 hPa had to be set on the suction side. However, with VSA systems the evacuation pressure within about a minute e from a highest level (P _Des-) ), typically 950 hPa to a lowest value (Pn _{c -mιn} ), e.g. 300 hPa, the use of only one radial blower as a single stage is not possible considering the optimal low energy input

One possibility is to artificially create a pressure drop using a throttle in front of the radial blower.At π of 1 6, however, the minimum optimal evacuation pressure is only 625 hPa.However, considerable energy losses will result within the evacuation time through the throttle due to the reduced suction power therefore with a radial blower a lower evacuation pressure than the pressure ratio fl prescribes, then the radial blower must be installed in the suction line and behind the radial blower a vacuum pump with approximately constant suction capacity must be connected.Vacuum pumps with almost constant suction capacity are, for example, pumps with displacement action such as water pumps or oil-driven rotary vane pumps Another displacement pump is the rotary lobe blower

It was an object of the present invention to find an energy-efficient method for enriching O ₂ with the aid of a vacuum pump stand with the lowest possible power consumption

It was found that in the case of enrichment of air by means of the VSA / PVSA process, a combination of radial blower and positive displacement pump is an arrangement which works in parallel operation and subsequent temporal operation or during the entire evacuation time only works in series operation in a wide range Pressure range brings considerable advantages over conventional two-stage rotary lobe blowers

The invention relates to a method for separating oxygen or

Nitrogen from air using an adsorption system with one or more adsorbers containing adsorbents for nitrogen or oxygen, preferably for nitrogen, and with a vacuum pumping station connected to the adsorption system, which consists of a radial fan and a vacuum pump working according to the displacer, especially a rotary lobe blower, the air in an adsorption phase at ambient pressure or slightly reduced pressure of up to -100 hPa compared to ambient pressure, or an overpressure of up to 500 hPa zB, for example, through the inlet of the adsorber into the adsorber and at the outlet of the adsorber with oxygen or with stick - substance-enriched gas is drawn off, the pressure in the adsorber after a certain adsorption time, preferably after a time of 20 to 120 seconds, in a relaxation phase to a pressure P _Des.! corresponding to the ambient pressure or to a pressure P _De. ι is brought to at least 0.6 times the ambient pressure, then in a desorption phase the adsorber containing the nitrogen or oxygen-coated adsorbent within a certain desorption time, in particular from 20 to 120 seconds, for the desorption of the adsorbed nitrogen or oxygen by means of the vacuum pump level of the higher pressure P _{l:) es.} is brought to a lower pressure Pr _Cb.mιn corresponding to at least 0.05 times the ambient pressure, and is brought back to the pressure of the adsorption phase in a covering phase, characterized in that at the beginning of the desorption phase the possibly throttled radial fan and the displacement pump of the vacuum pumping station connect the adsorber in parallel or in series, in particular connect in parallel, pump and at a lower desorption pressure that Radial blowers and the

The positive displacement pump connected in series pump the adsorber, the positive displacement pump being connected to the pressure side of the radial blower, and that during the series operation of the radial fan and the positive displacement pump, the positive displacement pump running to the pressure side is set or dimensioned such that the radial blower is on average during the evacuation phase optimal

Pressure ratio reached π

The possible changeover of the pump arrangement from parallel to row is preferably carried out at an evacuation pressure P _{Dc 0} upstream of the radial fan, in particular when the evacuation pressure P _{Dcs 0 is} at least the value formed from the pressure P ₍₎ at the outlet of the pressure-side displacement pump divided by 0 65 * reached π

Radial blowers and positive displacement pumps are preferably operated in series at the beginning of the evacuation phase

A preferred variant of the method according to the invention is characterized in that the pressure P _cs . ₀ in which the switch is made from parallel operation to series operation, is at least equal to the pressure P ₍₎ at the outlet of the pressure-side displacement pump divided by 1.15 times the pressure ratio π of the radial fan

A particularly advantageous variant of the invention is characterized in that for a given initial evacuation pressure P _Des . _] at the beginning of the desorption phase, the minimum evacuation pressure P _{Des m | n} lies within a pressure range that is

P _De - _m , _π = Po ¹⁰³ ° hPa * (0 25 * P _Des , - 100 hPa)

and

P _De , _u X l ° ³ ° hPa * (0 5 * P _{I) es} ,) is formed

The changeover of the pump arrangement from parallel operation to series operation can be controlled, for example, via a control system of the adsorption system according to a time specification or a pressure specification

In the special case that the downstream vacuum pump has to operate according to the displacement principle at a pumping pressure below 0 25 of the ambient pressure, it can consist of two or three displacement pumps connected in series

Further special embodiments of the invention can be found in the dependent claims

The VSA process according to the invention will be explained in more detail below with reference to the figures

Show in the figures

1 shows the suction power characteristics of a known single-stage rotary blower

2 shows the suction power characteristics and power consumption of a known two-stage rotary blower

3 shows the characteristic curve and the pressure-dependent power consumption of a pumping station made up of radial blowers and rotary lobe blowers connected in series

4 shows the characteristic curve and the pressure-dependent power consumption of a pump stand made up of radial and rotary piston blowers when changing from parallel to

Row operation

5 shows the diagram of a VSA system for carrying out the method according to the invention

6 shows the measured pressure curve at the adsorber during the pumping-out phase a) a combination of radial blowers and rotary lobe blowers in series operation

b) a two-stage rotary blower

c) a combination of radial blower and rotary blower with switch from parallel to series operation

at a starting pressure of 950 hPa,

Fig. 7 pressure curve as in Fig. 6 at a starting pressure of 800 hPa

The diagram of the VSA system used for the implementation of the following examples can be seen in FIG. 5

_ _? _

Examples

The VSA system has the following components

Inlet valves 1 1 A, 12Λ, M B, 12B, 1 I C, 12 C

Exhaust valves 13A, 14A, 15A, 13B, 14B, 15B, 13C, 14C, 14C

Control valves 17ABC, 18ABC

16ΛBC valve

Air blown C I O

H 10 heating

Product blown G10

Vacuum pumping station VI 0

The following abbreviations are used in the description of the method below

P ₀ = pressure at the outlet of the pump station 4 ambient pressure plus

Back pressure of the sound absorber at the end of the pumping station PD _C - I ⁼ pressure in front of the pumping station at the beginning of the evacuation phase

Po _es - ⁼ pressure in front of the pump stand, in the case of parallel operation

Series operation is switched Pö _es -om _m ⁼ lowest pressure upstream of the pump level, in which the system is switched from parallel operation to series operation π _e - _mm ⁼ lowest evacuation pressure upstream of the pump level

The adsorbers A, B and C are filled with Ca zeolite A granules with a grain size of 1 to 2.5 mm, which is produced according to Example 2 from the published patent application EP-A 0 170 026. The nitrogen adsorption on these granules is 1000 hPa and 25 ° C 14 Nl / kg the oxygen adsorption 4.3 Nl / kg The nominal diameter of the adsorbent was 1 000 mm, the height of the rubble of the total fill was 2200 mm. A 20 cm layer of silica gel was attached to the inlet of the adsorber. The height of the rubble of the zeolite granules was 200 cm, the weight of the zeolite was 1 000 kg

The adsorbers A, B, C are operated in cycles. The representation of the process sequence begins at time t = 0, at which the adsorption in adsorber A has ended

The following occurs in the period up to t = 8 sec (also called BFP time)

Only valve 15 A on adsorber A is open. Only valves I2C and 13C are open on adsorber C. Sucked in by pumping station V10, O - rich gas flows from adsorber A via valve 15A, open control valve 17 ABC and valve 13C in adsorber C The pressure in adsorber A thus drops from the adsorption pressure to a lower pressure P _{D s} , (relaxation phase) In adsorber C, the evacuation is ended, the pressure in adsorber C falling from the final pressure

increases to a higher pressure

Adsorber B begins with the air separation (adsorption phase), that is, ambient air passes through valve I IB into adsorber B, 0 -, - real product gas leaves the adsorber via valve 14B and is discharged with compressor G10 to the product supply (not shown)

The following happens every 8 to 60 seconds

Valve 15A on adsorber A is closed again and only valve 12A is open. In the desorption phase, adsorber A becomes pressurized by P _es. , sucked to the pressure Po _e - _mm with the vacuum pump VI 0. The adsorber B is in the adsorption phase, ie the valves I IB and 15B are open. At the same time, the valves 18 ABC, 16ABC and 13C open the adsorber C with O ₂ -reιchem Gas filled

Only valve 13C is open on adsorber C The full quantity is dimensioned so that at the end of this period the pressure in adsorber C almost reaches the adsorption pressure (covering phase) In the next cycle of the cycle, adsorber C separates the air (adsorption phase), in the third cycle of the cycle adsorber A, ie the two cycle times of 0-8 sec and 8-60 sec are repeated accordingly

To evaluate the following test examples, the O quantity produced at a concentration of 93% and the time course of the evacuation pressure upstream of the pump stand, the evacuated gas quantity and the volume output of the pump stand at 300 hPa were used

The maximum adsorption pressure was always 1 100 hPa, the minimum evacuation pressure P _{]) CS-mm was} always 300 hPa. In addition to the type of pump stand, the pressure P _{cs was} compared, at the beginning of the evacuation step. In the first variant, this starting pressure was 950 hPa and compared to it 800 hPa was the pressure at the outlet of the pump stand (P ₀ & ambient pressure including the strau pressure of the muffler behind the pump stand) was on average 1050 hPa

The operation of the following pump levels was examined in connection with the separation process

D) A two-stage rotary lobe blower, the capacity of the test at 300 hPa being approximately 1,000 m / h

E) A combination consisting of radial blowers and rotary lobe blowers, i.e. both blowers were always connected in series, with the

Capacity at 300 hPa was about 1,000 m ³ / h

F) A combination consisting of radial blower and rotary lobe blower with performance data according to Fig. 4, whereby up to an evacuation pressure of 650 hPa in front of the pump stand radial blowers and rotary lobe blowers pumped in parallel and pumped in series at a pressure below 650 hPa, i.e. the radial blower in the suction side and that Rotary lobe blower on the pressure side, the delivery rate was 300 hPa at approx. 1,000 m ³ / h

In Figure 1, the characteristic of a single-stage rotary blower is shown. It can be seen that at a suction pressure of less than 400 hPa Suction power is already considerably restricted compared to the suction power at 1000 hPa

The characteristic curve of a two-stage rotary blower is shown in FIG. 2. The second stage, connected in series, has a 40% lower suction power at ambient pressure than the first stage on the suction side, depending on the gradation ratio. Between 1,000 hPa and 200 hPa, the suction power of the overall characteristic curve drops by about 10%. from

The characteristic curve of a pump station with a radial blower on the suction side and a rotary blower blower on the pressure side (series operation) is shown in FIG

The same pump parts for determining the characteristic curve according to FIG. 3 were used for the measurement of the characteristic curves in FIG. 4. However, in the range from 650 to 1000 hPa, the radial blower and the rotary lobe blower suck in parallel, the radial blower being regulated by a throttle to 650 hPa in the suction side In the range below 650 hPa, radial blowers and rotary lobe blowers are connected in series as in the characteristic curve measurement in FIG. 3

In another example of the process control, the evacuation pressure P _Dc is achieved in that the valve 12C is closed in the above-mentioned time cycle of "0-8 sec", ie a pressure equalization or a partial pressure equalization between adsorber A and C takes place during this time evacuates the vacuum pump

VI 0 not adsorber C and works in "idle mode"

In a further example, the evacuation _pressure P _Dcs- ι is achieved in that in the above-mentioned time cycle of "0 - 8 sec" on adsorber C only valve 12C is open, thereby adsorber C is evacuated to its final pressure. Adsorber B is only Valve I IB opened, causing the air blower C10 to

Adsorber B with air fills On adsorber A, only valve 14A is open, whereby the product compressor sucks G10 O ₂ -reιches gas in adsorber A and the pressure to the desired hvakuierungsdruck P _dLS, Trash

In a further process example, the optimum starting pressure P _es] for evacuation with a connected vacuum pump level is achieved relatively quickly by only using adsorber C in the time interval of "0 - 8 sec." 98/01215. ,,.

til 13C is open Only valves 1 IB and 14B are open on adsorber B, whereby the air blower CIO fills the adsorber B with air and already produces O -, - rich gas. On adsorber A, the valves 12A and 15A are open fills the adsorber C via valves 15A, 17ABC and 13C. The pressure in adsorber A drops rapidly to the desired optimal starting pressure P _D - _I ^re ' ^at ' ^v due to the vacuum pump VI 0 connected to valve 12A

With the help of the "ADSIM" calculation program from ASPEN TECH / Cambridge, the evacuation process for an O ₂ enrichment system with 5000 Nm ³ / h oxygen in the product and an O ₇ concentration of 93 vol%

Use of the above three pump levels D), E), F) in the two evacuation areas 950 hPa (= PJ _J ^.,) To 300 hPa (= P _Dcs . _Πιιn ) and 800 to 300 hPa and by determining the oxygen _yields (ratio of the _amount of oxygen in the product for atmospheric oxygen) the pump sizes were calculated. Here the characteristic data, the quantity requirement and the

Energy requirements from Figures 2, 3 and 4 used

Figure 6 gives the measured pressure curve at a starting pressure P _{I c;>. ]} from 950 hPa again The pumping station E) (series connection of radial blowers with rotary lobe compressors) with its low suction capacity at higher pressures has a relatively high pressure level in the suction process compared to type D (two-stage rotary lobe blowers)

The pumping station F (start with parallel operation of the radial blower and rotary lobe compressor) with its higher suction capacity at higher pressures has a relatively low pressure level in the extraction process compared to type D (two-stage rotary lobe blower), which suggests an unfavorable energy requirement

FIG. 7 shows the measured pressure curve at a starting pressure P _Dcs.) Of 800 mbar. Compared to FIG. 6, the evacuation characteristics are not so far apart

The following pump sizes were determined Table 1

Evacuation pressure 950 to 300 hPa

Table 2

Evacuation pressure 800 to 300 hPa

Since the evacuation pressure in the range of 600 to 700 hPa, in which a changeover of the radial blower / rotary blower system from parallel plumbing to series operation should take place, is carried out relatively quickly, this change can be carried out via the control system of the O ₂ -VSA / PVSA system This can be done by switching accordingly when a certain evacuation pressure is reached, or by specifying an elapsed evacuation time As Figure 1 shows, a single-stage rotary blower already drops relatively strongly at a pressure of 400 hPa in order to achieve lower evacuation pressures below 25% or below 15% of the ambient pressure with a series combination of radial compressor and rotary blower, without the radial blower due to If the compression ratio is too high, it is suggested that the rotary lobe blower be designed in two or more stages if necessary (series operation)

From the course of the evacuation and the characteristic data of the vacuum pump levels D), E) and F) and the calculated pump sizes, oxygen for 5000 Nm ³ / h in the product and an O ₂ concentration of 93 vol% became the energy requirement of the three

Pump levels D), E), F) for the two pressure ranges 950 to 300 hPa and 800 to 300 hPa were calculated. Here, the characteristic data for the power consumption according to FIGS. 2, 3 or 4 were used proportionally converted to other pump sizes. The energy requirement relates here the amount of oxygen generated

The following specific power consumptions were determined with an efficiency of 4%

Table 3

It is surprising that the pump type E) with continuous series operation (of the radial blower with the rotary lobe blower), despite the restricted demand at higher pressures, even when the evacuation pressure begins, for example 800 hPa, thus far above the theoretically most favorable starting pressure (P _{cs 0} = P ₀ / π of about 1050 / 1.6 = 650 hPa) compared to the combination D) (- two-stage rotary blower) and even compared to the pump type F) with the beginning of parallel operation and series operation at 650 hPa shows the lowest energy requirement (see table 3, second column, 800 hPa start pressure)

It is surprising that energy savings with pump type E) compared to a two-stage rotary blower (type D)) are achieved even at higher starting pressures, e.g. 950 hPa (table 3, first column)

In an O ₂ VSA process with a vacuum pump set consisting of a radial blower and rotary lever blower, the combination of radial blower and rotary lobe blower can be achieved well before the optimal initial evacuation pressure P _{e 0} ( ⁼ ambient pressure including dynamic pressure of the muffler and optimum pressure ratio U of the radial blower) is reached be switched to row operation

Normally the ambient pressure would be 1000 hPa and 50 hPa dynamic pressure

Silencer and a pressure ratio IT of 1.6 the favorable starting pressure of the series operation (P _{I) ι; s.} <)) about 650 hPa In the case of O2-trapping of air with VSA technology, however, evacuation can already be carried out optimally with a row of the radial blower / rotary lobe blower at relatively high pressures. This means that even at pressures far above the optimal starting pressure P _Dcs.0 the pump _combination can be switched from parallel operation to series operation, or that the series operation of the pump combination is started at the beginning of the evacuation

Claims

claims

1 Process for separating oxygen or nitrogen from air below

Use of an adsorption system with one or more adsorbers containing adsorbents for nitrogen or oxygen, preferably for nitrogen, and with a vacuum pumping station which consists of a radial fan and a vacuum pump working according to the displacer principle, in particular a rotary piston blower, the air being in an adsorption phase at ambient pressure , slightly reduced pressure compared to ambient pressure of -0.1 10 ⁵ Pa, or an overpressure of up to 0.5 10 ^"1 Pa into the adsorber and withdrawn at the outlet of the adsorber with oxygen or nitrogen-enriched gas, the pressure in the adsorber after a certain adsorption time, preferably from 20 to 120 seconds, in a relaxation phase to a pressure

P _{1) c} ., According to the ambient pressure or to a pressure below ambient pressure (up to at least 0.6 times the ambient pressure), then in a desorption phase the adsorber, which contains the nitrogen or oxygen-enriched adsorbent, within a certain desorption time , in particular from 20 to 120 seconds, for the desorption of the adsorbed nitrogen or oxygen, if necessary by means of the vacuum pump from the higher pressure P _cs- ι to a lower pressure Pr _jcs-mιn corresponding to at least 0.05 times the ambient pressure, and in a stringing phase again on the

Pressure of the adsorption phase is brought, characterized in that at the beginning of the desorption phase the possibly throttled radial fan and the positive displacement pump of the vacuum pumping station connect the adsorber in parallel or in series, in particular in parallel, pump and pump the radial fan and the positive displacement pump in series at a lower desorption pressure Pump the adsorber, the positive displacement pump being connected to the pressure side of the radial blower, so that during the series operation of the radial fan and positive displacement pump, the positive displacement pump running to the pressure side is set or It is dimensioned that the radial blower attains its optimum pressure ratio π during the pumping-out phase, and that the possible changeover of the pump arrangement from parallel valve to series operation at an evacuation pressure P _{Dc 0 in} front of the radial blower takes place especially when the evacuation pressure

P _{Dci. () Has} at least reached the value formed from the pressure P "at the outlet of the pressure-side displacement pump divided by 0 65 * π

A method according to claim 1, characterized in that at the beginning of the evacuation phase, the radial blower and positive displacement pump are operated in series

A method according to claim 1, characterized in that the pressure Po _e - _Omm ^ ^eι ^ ^{em is switched from} parallel operation to series operation, at least equal to the pressure P ₀ at the outlet of the pressure-side displacement pump divided by 1.15 times the pressure ratio π of the radial is blown

Process according to claims 1 to 3, characterized in that, for a given initial evacuation pressure P _{] ^,} the minimum evacuation pressure P _{[) ei, -mιn is} within a pressure range that is

Pß _es mn ^{= P} _Ü / ¹⁰³ ° ^hPa * (0 25 * P _I. _B. , - 100 hPa)

and

_{D s} - _m „χ Po / 10 0 hPa * (0 5 * P _{Dcs l} )

is formed

Method according to claims 1 to 4, characterized in that the changeover of the pump arrangement from parallel operation to series operation takes place via a control system of the adsorption system in accordance with a time specification

Method according to claims 1 to 4, characterized in that the changeover of the pump arrangement from parallel operation to series operation via a control system of the adsorption system according to a pressure specification.

Process according to claims 1 to 6, characterized in that the downstream vacuum pump is formed according to the displacement principle at a pumping pressure below 0.25 times the ambient pressure from two or three displacement pumps connected in series

Process according to Claims 1 to 7, characterized in that one to three adsorbers (A), (B), (C), preferably two or three adsorbers, are operated intermittently in the alternation of adsorption phase, relaxation phase and desorption phase and covering phase

Method according to Claim 8, characterized in that the adsorber (A) to be evacuated is connected at the end of its adsorption phase at its outlet to the outlet of an absorber (B) which is in the evacuation phase, and absorber (A) is connected to the outlet at the inlet of the absorber (B ) connected vacuum pump level up to a maximum of one pressure

P- _eS -o. _{m is} evacuated

Method according to Claim 8, characterized in that the adsorber (A) to be evacuated is connected at the end of its adsorption phase at its outlet to the outlet or inlet of an already evacuated adsorber (B) and adsorber (A) via pressure equalization or partial pressure equalization with adsorber (B) is relaxed to a maximum of a negative pressure ^p - _ües -o _{, mm}

A method according to claim 8, characterized in that the adsorber (A) to be evacuated remains connected to a product compressor at its outlet at the end of its adsorption phase or at the end of its relaxation phase, the inlet end of the adsorber (A) being closed, and the pressure in the adsorber (A) drops below ambient pressure, maximum to a pressure P- _{Dci.0 m}

Method according to Claim 8, characterized in that the adsorber (A) to be evacuated at the end of its adsorption phase, or on

End of its relaxation phase at ambient pressure at its inlet a pumping station is evacuated and at the same time is connected at its outlet to the outlet of an already evacuated adsorber (B) and is thereby covered with adsorber (B)