CA2228392A1 - Method for the separation of gas mixtures by pressure-swing adsorption in a two-bed adsorber system - Google Patents

Abstract

The present invention relates to an improved and simplified method, which is also favourable as regards energy, for the adsorptive separation of gas mixtures, in particular of air, by means of inorganic adsorbents, in particular by means of molecular-sieve zeolites, by pressure-swing adsorption in a two-bed adsorber system.

Description

' CA 02228392 1998-01-30 Le A 31 439-Forei~n Countries / WA/ngb/S-P
~ -- 1 --Metha~d for the seParation of ~as mixtures by pressure-swin~ adsorption in a two-bed adsorber s~vstem 5 The present invention relates to an improved and simplified method, which is also favourable as regards energy, for the adsorptive separation of gas mixtures, in particu.lar of air, by means of inorganic adsorbents, in particular by means of molecular-sieve zeolites, by pressure-swing adsorption in a two-bed adsorber system.

10 The adsorptive separation of gas mixtures with the aid of pressure-swing adsorption has been known for over 20 years, in connection with which a variety of teclmical separation processes have been developed. However, all methods are based on the principle that the gas portion of the gas mixture (crude gas) that has the higher affinity to the adsorbent is retained in a so-called adsorption step on the 15 surface of the adsorbent in an adsorber and the less strongly adsorbed component can be withdrawn from the adsorber that is charged with adsorbent. Desorption ofthe adsorbed phase is then achieved by lowering the pressure after the adsorption step and optionally in addition by flushing the adsorbent with a portion of the less strongly adsorbed gas. If the pressure is lowered to approximately ambient 20 pressure or slightly above ambient pressure, one speaks of PSA systems (Pressure Swing Adsorption). In the case where the pressure is lowered to a pressure belowambient pressure by means of a vacuum pump, one speaks of VSA systems (_acuum Swing Adsorption). Also in this case the adsorbent is optionally flushedwith a portion of the less strongly adsorbed gas. But there are also cases in which 25 flushing is dispensed with, in particular for example in the case of oxygen enrichment of air with molecular-sieve zeolites. The processes with excess-pressu:re adsorption and vacuum desorption are designated as PVSA systems (Pressure Vacuum Swing Adsorption).

In the following explanatory details the data in "bar" are to be understood as being 30 excess pressure in relation to the ambient pressure, the evacuation pressures"mbar" are to be understood as being absolute values and Nm3 is to be understoodas m3 at 0~C and 1.013 bar abs.

If the adsorption is effected at approximately ambient pressure - that is to say~ at -0.05 to 0.1 bar for example, and the desorption is effected at a low pressure of, for example, 100 to 400 mbar (abs), then one speaks of a VSA process. After the Le A 31 439-Forei~n Countries desorption step, charging of the adsorbent with gas is always effected to the pressure of the adsorption step - in the cases of PSA adsorption, with the less strongly adsorbed gas portion or crude gas or with both simultaneously. In the case of the VSA technique, charging is effected with the less strongly adsorbed gas portion.

The aforementioned separation processes are therefore divided up into three steps:
adsorption (separation), desorption (with lowering of pressure) and recharging (with l)uild-up of pressure), on account of which three adsorbers are necessary for a PSA or VSA process that operates in totally continuous manner.

With a view to reducing the investment costs, VSA systems with 2 adsorbers and a product buffer have been customary for some time. Since also in this case the adsorption process consists of 3 stages - adsorption, evacuation, charging to adsorption pressure - but only two adsorbers are available, some stages flow smoothly into one another or are kept very short. Adsorption is mostly effected at 0.1 to 1 bar, the desired product being produced already in the course of build-up of pressure above ambient pressure with air.

The temporal processing sequence of two-bed adsorbers in the case of a PVSA
proces:s for the ~2 enrichment of air is frequently (see also Fig. 5 and Fig. 6) the following:

20 a) end of the adsorption in adsorber A at, for example, 0.4 bar (pressure =
PAd max) excess pressure b) decanting step (so-called BFP step; time tl) from adsorber A into the second adsorber B down to approximately ambient pressure or somewhat lower, for example to 700 mbar (pressure = PDeS O)7 whereby adsorber B is simultaneously evacuated and the pressure in B rises from its lowest pressure (pressure = PDes min) to a higher pressure, for example from 200 to 400 mbar (pressure = PBFP) c) evacuation of adsorber A (times t2, t3 and t4; N2 desorption with or without flush gas by means of vacuum pump "V" to between 600 and 200 mbar (abs) (pressure = PDes,min) Le A 31 439-Forei~n Countries d) decanting step (see under b), as a result an increase in pressure in adsorber A from, for example, 200 to 400 mbar (time t5) e) residual charging step to approximately ambient pressure in adsorber A and to adsorption pressure with air via blower "G", optionally in addition while circumventing "G", partly in the first charging period also with ~2 product from the buffer "R" (time t6); product gas is continll~lly withdrawn from the reservoir "R" with the aid of compressor "K".

Since the ~2 yield - that is to say, the ratio of the quantity of ~2 in the product gas to the quantity of ~2 in the air that is used - has a direct influence on the energy that has to be provided for the air blower and for the vacuum pump, attempts have been made by means of suitable measures to conclude the process prior to the evacuation in such a way that the ~2 concentration in the adsorption exit zone is virtually equal to the ~2 concentration in the entry zone. This is the case with methods in which gas from the exit zone is recycled into the entry zone.

In GB-A 1,559,325 (in particular Figs. 7 and 8; 3-adsorber adsorption method) atthe end of the adsorption, for example by adsorber A, the gas at adsorption pressu:re that is low in ~2 in comparison with the ~2 product gas is fed back into the already charged adsorber B at adsorption pressure into the entry zone and the product gas is withdrawn from the exit zone of adsorber B. The time for the recirculation is relatively long, since the ~2 product rate determines the recirculated quantity and the time. This method has the crucial disadvantage that the tirne for product recirculation reduces either the time for pressurising theadsorbers or the pumping-out time. In addition, dry gas is conducted through thedrying zone upstream of the zeolite feedstock, as a result of which moisture gets into the N2-O2 separation zone of the zeolite feedstock.

In GB-A 2,154,465 (in particular Fig. 3), in a 3-adsorber system at the end of the adsorption, for example by adsorber A, a pressure compensation is carried out across the exit zone of adsorber A into the entry zone of the evacuated adsorberB, whereby the pressure drops in adsorber A and increases in adsorber B. The same Imethod is described in DE-A 30 30 081 (in particular Fig. 1 and column 4, lines 18-47). A disadvantage of this method is that the quantity to be decanted is predetermined and limited by the possible pressure difference of the pressure relief -that is to say, it ceases to apply in the case of adsorption at ambient pressure.

Le A 31 439-Forei~n Countries In EP-A 0 334 495 a pressure-compensation system in a 2-adsorber system is likewise described. At the start of evacuation, evacuation is effected, for example, from a~dsorber A in downward flow, at the exit end of adsorber A gas that is lowin ~2 iS simultaneously withdrawn in upward flow and charged into the already 5 evacua,ted adsorber B into the entry zone thereof in upward flow. The quantity of recirculated gas is also limited in this case, namely within the pressure difference in the course of pumping-out, which is a disadvantage.

The known methods described above have the disadvantage that the so-called "exitgas" is dry and in the course of recirculation is conducted through the drying zone 10 and that, as a result, the water front is displaced into the zeolite zone. In addition, the quantity of gas involved in the recirculation is limited by the predetermined pressure differences of the pressure relief in co-current flow or the recirculation time is too long as a result of the predetermined parameters such as product quantil~y, for example. As a result, in the case of ~2 enrichment of air the entire possible quantity of the gas that is low in ~2 (21 to 90%) cannot be recycled.

The object was therefore to make available an improved method for the separationof gas mixtures, in particular air, by pressure-swing adsorption in a two-bed adsorber system that makes it possible to carry out the adsorptive separation in a manner that is simple and favourable as regards energy, to avoid conducting dry 20 gas through the so-called damp drying zone and to avoid the disadvantages of recirculating the product gas that is low in ~2 (residual product gas), such as loss of tim~e or limitation of the quantity of recirculated gas.

It has been possible to achieve this object with the method according to the mvention.

25 The subject of the invention is a method for the separation of air by adsorption with molecular-sieve zeolites in a two-bed adsorber system with product buffer for the purpose of m~int:~ining a continuous, constant stream of oxygen product, whereby the two adsorber beds with molecular-sieve zeolites are present in a homogeneous feedstock or in several different feedstocks of various types of 30 zeolite and optionally a layer of an adsorbent that is selective with respect to water is located upstream of the feedstocks, wherein, at approximately ambient pressure or up to an excess pressure of 0.5 bar during the adsorption, air is conducted through the first adsorber A and nitrogen is adsorbed on the molecular-Le A 31 439-Forei~n Countries sieve z:eolite preferentially in relation to oxygen, whereby product gas that is high in oxygen having an ~2 concentration amounting to between 60 and 96 vol-% is obtained at the exit end of adsorber A, after the adsorption the desorption of the nitrogen and optionally of the moisture is carried out by evacuation in counterflow to the adsorption at a lowest pressure of 100 mbar to 600 mbar (absolute), characlerised in that a) after the adsorption in adsorber A, the pressure of which drops from its final value after the adsorption step to A) ambient pressure, to B) low pressure down to 600 mbar or to C) excess pressure up to 0.2 bar, and the inlet end of which in the process is a) closed, is b) open in relation to the environment or c) an air compressor remains connected, residual product gas flows out of the outlet end of adsorber A into the outlet end of adsorber B, whereby at the start of this step adsorber B is at its lowest pressure level of 100 to 600 mbar, and optionally adsorber B is simultaneously evacuated via its inlet end, whereby the pressure in adsorber B a) rises, b) remains constant or c) drops, b) ambient air without additional compression is introduced into adsorber A
via the inlet end thereof, in the process the pressure in adsorber A remains at approximately ambient pressure or adsorber A is charged up to ambient pressure, gas that is rich in O2is taken off at the outlet end of adsorber A
and is introduced into adsorber B via the inlet end thereof until approximately ambient pressure is reached and optionally additional ambient air is introduced without additional compression directly into the inlet end of adsorber B, 25 c) ambient air with compression is conducted into adsorber B via the inlet end thereof and product gas is simultaneously supplied to the product buffer, optionally product gas from the product buffer is supplied to the outlet end of adsorber B and adsorber A is simultaneously evacuated via the inlet end thereof, d) adsorber A is evacuated to its lowest pressure of 100 to 600 mbar and is simultaneously adsorbed in adsorber B, by ambient air with compression being conducted via the entry end of adsorber B and product gas being conducted via the outlet end of adsorber B into the product buffer and Le A 31 439-Forei~n Countries optionally adsorber A being simultaneously flushed via the exit end thereof with product gas.

In particularly preferred manner the method according to the invention is carried out in such a way that 5 a) after the adsorption in adsorber A residual product gas is conducted out of the outlet end of adsorber A, the pressure of which drops in the process from between 0.2 and O.S bar excess pressure to approximately ambient pressure, into adsorber B, which is at its lowest pressure level of 100 to 600 mbar, via the outlet end thereof and optionally adsorber B is simultaneously evacuated via the inlet end, whereby the pressure in adsorber B rises to max. 90% of the ambient pressure or remains constant, b) ambient air without additional compression is introduced into adsorber A
via the inlet end thereof, gas that is rich in O2is taken off at the outlet end of adsorber A and is introduced into adsorber B via the inlet end thereof until approximately ambient pressure is reached and optionally additional ambient air without additional compression is introduced directly into adsorber B, c) ambient air with compression is conducted into adsorber B via the inlet end thereof and optionally product gas is introduced simultaneously from the product buffer into adsorber B via the outlet end thereof and adsorber A is simultaneously evacu~tecl7 d) adsorber A is evacuated to its lowest pressure of 100 to 600 mbar and is simultaneously adsorbed into adsorber B, by ambient air with compression being conducted through via the entry end of adsorber B and product gas being conducted via the exit end of adsorber B into the product buffer and optionally adsorber A being flushed simultaneously via the exit end thereof with product gas.

An adsorption method for ~2 enrichment of air in a two-bed adsorber system with molecular-sieve zeolites has been discovered, with the aid of which the energy demand for the purpose of generating the oxygen can be considerably reduced in comparison with conventional methods, wherein after the adsorption step of air - Le A 31 439-Forei~n Countries separation in particular at 0.05 to 0.5 bar (max. 1 bar, excess pressure) optionally a lowering of pressure in the co-current flow direction in relation to the adsorption to, for example, ambient pressure is effected, this pressure-relieved gas is passed in known manner into an already evacuated adsorber, then ambient air is passed 5 through the relieved adsorber in the co-current flow direction in relation to the adsorption, at the adsorber exit thereof gas containing ~2 iS withdrawn and thisexit gas is introduced into the entry zone of the evacuated and optionally partly repressurised adsorber, and this charging is concluded at a charging pressure ofapprox.imately ambient pressure or with an ~2 concentration of the charge gas 10 amounting to between 15 and 90%, whereby optionally during the charging ambient air flows simultaneously into the entry zone of the adsorber to be pressurised, then this adsorber is charged further with ambient air and simult;meously with ~2 from the buffer in counterflow to the adsorption to a final presswre of 0.03 to 0.5 bar, max. 1 bar (excess pressure), prior to reaçhing its final 15 presswre ~2 product is emitted into a reservoir, namely at a time when the reservoir pressure is slightly below the adsorber pressure, during this time thesecond adsorber is evacuated in counterflow and during the entire evacuation period or at the end of the evacuation step is flushed with ~2 product gas in counterflow to the separation of air.

20 The nnethod according to the invention avoids the disadvantages of the recirculation and introduction of dry gas into the entry side of the drying zone.
No loss of time arises as a result of the recirculation, and no limitation of the quantil:y of recirculated gas.

The recirculation of the product gas that is low in ~2 (residual product gas) at the 25 end of ~2 production (adsorption) and optionally the decanting step may be .
carrlecl out In vanous ways.

In Figures 4a to 4d the individual process stages of one half-cycle are represented schematically by way of example. In addition, the pressure profile has been sketched in schematically in the adsorbers.

30 Embodiment of Figure 4a: at time tl adsorber "A" has concluded its ~2 production step at approximately 1 atm, the air continues to pass at approximately 1 atm inco-current flow in relation to the adsorption by adsorber "A", gas cont~ining O2is withdrawn at the adsorber exit, said gas being conducted in counterflow to the Le A 31 439-Forei~n Countries adsorption through adsorber "B", the vacuum pump at adsorber "B" is still connected, the "quantity of decant gas" is such that a sufficient flushing action arises, as a result of which the pressure in the adsorber may possibly not drop any further but remains constant or rises. In the next step (time t2) air continues to be 5 conducted in co-current flow through, for example, adsorber "A", gas that is low in ~2 iS withdrawn at the adsorber exit and this gas is sent in co-current flow in relatio:n to the adsorption into adsorber "B" which is to be brought to adsorption pressure, whereby simultaneously, as needed, ambient air is sent into adsorber "B"
in co-c,urrent flow in relation to the adsorption. The pressure in adsorber "A"
10 which emits gas that is low in ~2 amounts to approximately ambient pressure.
The amount of gas conducted away that is low in ~2 iS not limited in its total quantily, since sufficient ambient air is available for the subsequent separation.
Since the gas that is low in O2is generated at ambient pressure, as a result of the loss oi.' pressure of the system a low pressure in the adsorber emitting gas that is 15 low in ~2 may arise (see Fig. 4a, time t2). Then in time t3 adsorber "A" is evacuated in counterflow to the adsorption, air at approximately ambient pressure is passed through adsorber "B" and ~2 product is introduced into a product reservoir. At the end of the adsorption step and the evacuation step, adsorber "A"
may optionally be flushed with a partial current of the product (time t4).

20 In another embodiment, according to Figure 4b the entire process proceeds similarly to the process of Figure 4a, only the air separation is carried out at a pressure above ambient pressure, for example 0.1 to 0.6 bar. Then, after the adsorption step, adsorbers "A" and "B" are connected at the outlet end, a so-called BFP step takes place - that is to say, adsorber "A" drops in pressure to ambient25 pressure in adsorber "B", whereby adsorber "B" is still connected to the vacuum pump and the pressure in adsorber "B" can rise. The relief of adsorber "A" may also be effected in two stages, firstly the BFP step from, for example, 0.5 bar to 0.2 bar and continuation of the pressure compensation (PB step) to ambient pressu:re in "A", whereby the vacuum pump in "B" is not connected.

30 In another embodiment according to Figure 4c, the recirculation of gas that is low in ~2 into adsorber A at time t2 proceeds at decreasing excess pressure from, for example, 0.1 bar down to approximately ambient pressure, since the decanting steps "BFP" and/or "PB" into adsorber "A" have been concluded at 0.1 bar.

Le A 31 439-Forei~n Countries g In another embodiment, according to Figure 4d the recirculation of gas that is low in ~2 into adsorber A proceeds at time t2 at increasing pressure from, for example, 800 mbar (abs) - that is to say, beginning at low pressure, rising to approximately ambient pressure - since the decanting steps "BFP" and/or "PB" into adsorber "A"5 have been concluded at 800 mbar (abs).

With reference to Figs. 1, 2a, 2b and 3 an implementation of the method according to the invention will be elucidated in more detail.

In Fig. 3 the temporal pressure profile of adsorbers A and B and of the ~2 buffer is shown, in Figs. 2a and 2b the process stages at the respective times are 1 0 represented.

Time tl: Via valve A4/B4 a partial pressure compensation is effected.
Valves A1/A21A31B l/B3/AB 1 are closed. The air blower "G"
operates in the circuit. Valve B2 can be closed, in this case a "pure" pressure compensation (PB) arises, the vacuum pump "V"
lS then operates in the circuit. But it is also possible to accelerate the charging in the PB step by valve B1 additionally being opened and, as a result of this, air from blower "G" flowing in from the entry side. In another variant, valve B2 is open (BFP process), as a result of which adsorber B continues to be evacuated. During time tl the pressure in the adsorber rises from PDes,min to PPB or PBFP
respectively.
A combination of the BFP and PB steps is also possible - that is to say, firstly the BFP step is effected (ie, inclusive of evacuation), lowering of the pressure of adsorber A to above ambient pressure, then the PB step (ie, partial pressure compensation) with a relief of adsorber A to approximately ambient pressure (or slightly lower).
The decanting step (PB) may also be effected via valve A4/B3 and lines Lp and LR, whereby optionally adsorber B is also simultaneously topped up with air via valve B1.
During time t1, product that is high in ~2 iS withdrawn from reservoir "R" to the consumer.

Time ~2: Valves A4, B3 and A1, B1 are open, rem~ining valves are closed.
The air compressor is operated at about ambient pressure, air flows Le A 31 439-Foreign Countries via valve A1 into adsorber A, gas that is low in ~2 leaves adsorber A via valve A4, flows into a recirculation line LR and flows via valve B3 into adsorber B, whereby optionally in addition air flows via the air line LL into adsorber B (valve Bl) (= RBF step). In this case the vacuum pump operates in the circuit. As a result of the loss of pressure of the feedstock of the adsorber, at time t2 the pressure in adsorber A can easily fall below ambient pressure. The quantity of gas that is low in ~2 which reaches adsorber B via line LR may be adjusted by regulation of the quantity of air at valve B1 - for example by delayed opening of B1. This quantity of air depends on the minimum extraction pressure, the difference of the relief pressure in adsorber "A" and the quantity of the recycled gas that is high in ~2 The recirculation of the gas that is low in O2iS
concluded when the charging pressure amounts to approximately ambient pressure or the ~2 concentration thereof reaches approximately 15-90 vol-%. If the quantity of charge gas is limited on account of too small a difference in the charging pressure (between ambient pressure and evacuation pressure), the final ~2 concentration of the charge gas may also be far above 21%. In this case an additional charging of "B" with air is unnecessary.

Time t3: Adsorber A is exhausted via valve A2. Adsorber B is pressurised via valve B1 with the aid of the air compressor, whereby valves B2/B3 are closed or in another embodiment is simultaneously charged with ~2 product from the reservoir "R" via valves B4 and AB1. During the evacuation of adsorber A, adsorber A can be flushed with product via valve AB1/A4. The end of time t3 is reached when the reservoir pressure has dropped from maximum PAd maX to its lowest pressure PR m Time t4: Adsorber A is exhausted via valve A2 to its lowest pressure PDeS min and is simultaneously flushed with product gas from the product-gas line LP via valve A4. The flush gas, which reaches adsorber A via valve A4, must accordingly be optimised in quantity. In the case of a BFP process (see time t1), this quantity of flush gas is very small or can be omitted. Adsorber B continues to be brought to its final ~ Le A ~1 439-Forei~n Countries pressure via valve B1, the emission of ~2 product into the reservoir "R" is effected within time t4.

The process is continued in a manner analogous to these process steps (times tl-t4) by exch~ngin~ the adsorbers A and B.

5 Suitable by way of adsorbents for the method according to the invention for the ~2 enrichment of air are molecular-sieve zeolites exhibiting preferential adsorption of N2 in relation to ~2~ such as zeolite A and zeolite X in the Na form or in the form substituted with divalent alkaline(-earth metal) ions (such as Ca, Mg, Sr or mixtures thereof) or in the form substituted with monovalent ions such as lithium 10 with substitution rates above 85%, or natural zeolites or their synthetically produced forms such as mordenite or chabazite.

The method according to the invention will be elucidated in more detail in the following Examples.

Examl~les 15 For all the Examples the following data remain constant:

Reservoir volume 1.5 m3 Inside diameter of adsorber 550 mm Packing height of adsorber 1,800 mm Ambient pressure about 1045 mbar 20 Adsorbent charge per adsorber:

56 dm3 medium-pore silica gel at the lower end of the adsorber, residual feedstock in each case 240 kg molecular-sieve Ca zeolite A, produced in accordance with EP-A 0 170 026, Example 2. Calcination was effected in a stream of nitrogen at 500 to 600~C. The molar CaO/AI2O3 ratio amounted to 0.72. The granulation 25 amounted to 1 - 2.5 mm in diameter, spherical form.

The crude gas supplied had a temperature of +30~C and was always 75~/0 saturatedwith water at ambient pressure and 30~C. By way of vacuum pump, use was made of a 2-stage rotary-piston blower ("Roots") that was capable of being Le A 31 439-Forei~n Countries adjusted via a mech~nism The crude gas was compressed with the aid of a rotary-piston blower. The pressure was measured in each case at the lower end (air-entry side) of the adsorber.

The energy demand of the vacuum pump was calculated from the pressure profile 5 durin~; pumping-out upstream of the feedstock, use being made of the characteristic (= energy demand as a function of the extraction pressure) of a known Roots blower having an extraction capacity of 20,000 m3/h at 1.03 bar, abs..

The energy demand of the air blower was calculated in accordance with the followïng formula:

Pm = 1045 mbar (0.306 x Pm - 286) x Vo Vo = amount of air at 1.03 bar, abs. (m3/h) 10,306 x ~
,u = efficiency = 0.95 Example 1 (Comparison) Use was made of an installation corresponding to Figure 5. The process sequence and the pressure profile are reproduced in Figure 6.

15 Time tl: 0 to 4 seconds An air compressor was located in the circuit. The pressure in adsorber B rose from I'DeSmin = 250 mbar to PBFP = 450 mbar7 simultaneously in adsorber A the maxim.um pressure PAd maX = 0 3 bar dropped, whereby at the upper end gas was conducted out of adsorber A via valve A03 into adsorber B via valve B03, at the 20 lower end adsorber B was evacuated via valve B02 with a vacuum pump. The pressure in adsorber A dropped to PDeS O = 990 mbar. Reservoir R supplied produc;t gas at a pressure of about 0.3 bar; valve AB01 was closed.

Le A 31 439-Forei~n Countries Time t2: 4 to 9 seconds Adsorber A was evacuated via valve A02 to approximately 650 mbar, the upper end of the adsorber was closed. Adsorber B was pressurised to 1 atm at the lowerend vi.a valve B01 with air from the air compressor "G", simultaneously a 5 charging with gas from the reservoir "R" was effected via a volume-controlled valve AB01 and valve B03, whereby the pressure in the reservoir dropped from about 0.3 bar to approximately 0.2 bar. Product gas continued to be withdrawn from reservoir R.

Time t3: 9 to 13 seconds 10 Adsorber A was evacuated further, adsorber B was charged with air from air compressor "G" and product from the reservoir "R" as in time t2. Reservoir pressure and pressure in adsorber B reached 0.05 bar.

Time t4: 13 to 30 seconds Adsorber A was evacuated further, whereby a final pressure of 250 mbar was 15 reached. Via the air compressor "G" and valve B01 air flowed into adsorber B,product gas was fed into reservoir R via valves B03 and AB01, the pressure in adsorber A and in the reservoir R reached a final pressure of 0.3 bar.

The process proceeded further in a manner analogous to times t1/t2lt3/t4, only adsorber A was exchanged for adsorber B.

20 A quantity of product gas from reservoir R of 19 Nm3/h with an ~2 concentration of 93~/'o was obtained. The ~2 yield amounted to 46%, the specific energy value of the vacuum pump was calculated as 0.378 kWh/Nm302, the total energy value derived from pump and air compressor amounted to 0.472 kWh/Nm3O2.

Example 2 (Comparison) 25 Use was made of an installation corresponding to Figure 5. The process sequence and the pressure profile are reproduced in Figure 6. During time tl, however, the vacuum pump was disconnected from the adsorber with valves A02 and B02.

Le A 31 439-Forei~n Countries Time tl: 0 to 4 seconds Valve B01 was open. Ambient air flowed into the evacuated adsorber B. Valve B02 was closed. The pressure in aclsorber B rose from PDeSmjn = 250 mbar to PPB-I = 650 mbar, simultaneously the pressure in adsorber A dropped from its 5 maximum value PAd maX = 0 3 bar to PDeS o = 990 mbar, whereby at the upper endgas flowed out of adsorber A via valve A03 into adsorber B via valve B03.
Reservoir R supplied product gas at a pressure of about 0.3 bar; valve AB01 was closed.

Time t2: 4 to 9 seconds Adsorber A was evacuated via valve A02 to approximately 650 mbar, the upper end of the adsorber was closed. Adsorber B was pressurised to ambient pressure at the lower end via valve B01 with air from the air compressor, simultaneously a charging with gas from the reservoir R was effected via a volume-controlled valve AB01 and valve B03, whereby the pressure in the reservoir dropped from about 15 0.3 bar to approximately 0.2 bar. Product gas continued to be withdrawn from reservoir R.

Time t3: 9 to 13 seconds Adsorber A was evacuated further. Adsorber B was charged with air and product gas from the reservoir, as in time t2. Reservoir pressure and pressure in adsorber 20 B reached approximately ambient pressure.

Time t4: 13 to 30 seconds Adsorber A was evacuated further, whereby a final pressure of 250 mbar was reached. Via valve B01 air flowed into adsorber B, product gas was fed into reservoir R via valves B03 and ABOI, the pressure in adsorber A and in the 25 reservoir R reached a f1nal pressure of 0.3 bar. During this time adsorber A was flushed with ~2 product gas, namely via the volume-controlled valve B03, whereby the quantity of flush gas was adjusted on the basis of a maximum ~2 concentration of 18 vol-% in the residual gas downstream of the vacuum pump.

Le A 31 439-Forei~n Countries The process proceeded further in a manner analogous to times tllt2lt31t4, only adsorber A was exchanged for adsorber B.

A quantity of product gas from reservoir R of 17.68 Nm3/h with an ~2 concentration of 93% was obtained. The ~2 yield amounted to 44%, the specific energy value of the vacuum pump W;1S calculated as 0.379 kWh/Nm302, the total energy value derived from pump and air compressor amounted to 0.46~ kWh/Nm302.

Example 3 (Comparison; according to EP-A 0 334 495 (Figs. 3d/4d)) Use was made of an inst~ tion corresponding to Figure 1. The process sequence and the pressure profile are evident from Figure 6.

Time tl: 0 to 4 seconds An air compressor was located in the circuit. The pressure in adsorber B rose from PDeS min = 250 mbar to PBF-I = 450 mbar, simultaneously in adsorber A the pressure dropped from its maximum pressure PAd maX = 0 3 bar, whereby at the upper end gas flowed out of adsorber A via valve A4 into adsorber B via valve B4 and at the lower end adsorber B was evacuated via valve B3 with a vacuum pump. The pressure in adsorber A dropped to PDeSo = 990 mbar. Reservoir R
supplied product gas at a pressure of about 0.3 bar; valve AB1 was closed.

Time t2: 4 to 6 seconds Adsorber A was evacuated via valve A2 to approximately 650-700 mbar, the upper end of adsorber A was opened via valve A4. Adsorber B was charged with gas that was low in ~2 from adsorber A to approximately 650 mbar, namely via valve A4, lines Lp and LR and inlet valve B3. Product gas was withdrawn from reservoir R.

Time t2b: 6 to 9 seconds Adsorber A was evacuated via valve A2 to approximately 450 mbar, the upper end of adsorber A was closed. Adsorber B was pressurised to about ambient pressure with air from the air compressor at the lower end via valve B1 and with ' CA 02228392 1998-01-30 Le A 31 439-Forei~n Countries gas from the reservoir R (valves A131 and B4). Product gas continued to be withdrawn from reservoir R, the pressure of the reservoir R dropped to 0.15 bar.
Time t3: 9 to 13 seconds Adsorber A was evacuated further, adsorber B was charged with air and product S gas from the reservoir, as in time t2b; reservoir and pressure in adsorber B reached 0.05 bar.

Time t1: 13 to 30 seconds Adsorber A continued to be evacuated, via valve A4 ~2 gas was admitted as flush gas, whereby a final pressure of 250 rnbar was reached. Via valve B1 air flowed 10 into adsorber "B", product gas flowed via valves B4 and AB1 into the reservoir R;
the pressure in adsorber A and in the reservoir R reached a final pressure of 0.3 bar. The quantity ~f ~2 flush gas was adjusted with valve A4.

The process proceeded further in a manner analogous to times tllt2a/t2blt31t4, only adsorber A was exchanged for adsorber B.

A quantity of product gas from reservoir R of 19.8 Nm3/h with an ~2 concentration of 93% was obtained. The ~2 yield amounted to 47.5%, the specific energy value of the vacuum pump was calculated as 0.375 kWh/Nm3O2, the total energy value derived from pump and air compressor amounted to 0.466 kWh/Nm3O2.

20 Example 4 (according to the invention) Use was made of an installation corresponding to Figure 1. The process sequence is reproduced in Figures 2a/2b and the pressure profile is reproduced in Figure 3.
During time t1 the vacuum pump was disconnected from the adsorber with valves A2 and B2.

25 Time tl: 0 to 4 seconds All valves of adsorbers A and B except A4 and B4 were closed. As a result, gas that was high in ~2 flowed out of adsorber A into the previously evacuated Le A 31 439-Forei~n Countries adsorber B. The pressure in adsorber B rose from PDes min = 250 mbar to PPB1 =
650 mbar, simultaneously in adsorber A the pressure PAd maX = 0 3 bar (excess pressure) dropped to PDeS O = ambient pressure. Reservoir R supplied product gasat a pressure of about 0.3 to 0.25 bar.

5 Time t2: 9 to 13 seconds Adsorber B was charged to approxirxlately ambient pressure, by air flowing via valve A1 into adsorber A, gas that was low in ~2 being withdrawn via valve A4 and flowing into adsorber B via lines Lp and LR and valve B3. Simultaneously adsorber B was topped up with air via the volume-controlled valve B1. The 10 charging with gas that was low in ~2 via line LR was concluded at an ~2 concentration of 20-40 vol-%. Product gas continued to be withdrawn from reservoir R.

Time t3: 9 to 13 seconds Adsorber A was evacuated via valve A2 from ambient pressure to approximately 15 650 mbar, adsorber B was charged with air via valve B1 and with product gas via valves AB1 and B4 from the reservoir until reservoir pressure and pressure in adsorber B reached approximately the same level. Product gas continued to be withdrawn from reservoir R.

Time t4: 13 to 30 seconds 20 Adsorber A continued to be ev~c~l~terl, whereby a final pressure of 250 mbar was reached. In this connection, with a view to assisting the desorption of N2, gas that was high in ~2 was introduced into adsorber A via valve A4, namely in such a quantity that the ~2 concentration of the exhaust gas of the vacuum pump did notexceed a value of 18 vol-%. With the aid of the air blower adsorber B was 25 pressurised to its final pressure of 0.3 bar. Product gas flowed into reservoir R via valves B4 and AB1.

The process proceeded further in a manner analogous to times tllt2lt3lt4, only adsorber A was exchanged for adsorber B.

Le A 31 439-Forei~n Countries A quantity of product gas from reservoir R of 22 Nm3/h with an ~2 concentration of 93% was obtained. The ~2 yield amounted to 48%, the specific energy value of the vacuum pump was calculated as 0.341 kWh/Nm302, the total energy value derived from pump and air blower amounted to 0.437 kWh/Nm302.

S Example 5 (according to the invention) Use was made of an installation corresponding to Figure 1. The pressure profile has been reproduced in Figure 3. The process sequence can be seen from Figures 2a/2b.

Time tl: 0 to 4 seconds 10 Only valves B2, B4 and A4 were open; adsorber B was connected to the vacuum pump. As a result, gas that was high in ~2 flowed out of adsorber A into the previously evacuated adsorber B. The pressure in adsorber B rose from PDesmin =
250 mbar to PBFP ~ = 450 mbar, simultaneously in adsorber A the pressure PAd max= 0.3 bar dropped to PDeS O = ambienl pressure. Reservoir R supplied product gas at a pressure of about 0.3 to 0.25 bar.

Time t2: 4 to 9 seconds Adsorber B was charged to approximately ambient pressure, by air flowing into adsorber A via valve A1, gas that was low in ~2 being withdrawn via valve A4, being pressurised into adsorber ]3 via lines Lp and LR and valve B3.
20 Simultaneously adsorber B was topped up with air via the volume-controlled valve B1. The charging with gas that was low in ~2 via line LR was concluded at an ~2 concentration of 20-40 vol-%. Product gas continued to be withdrawn from reservoir R.

Time t3: 9 to 13 seconds 25 Adsorber A was evacuated via valve A2 from ambient pressure to approximately 650 mbar, adsorber B was charged with air via valve B1 and with product gas from the reservoir via valves AB1 and B4 until reservoir pressure and pressure in adsorber B reached about the same level. Product gas continued to be withdrawn from reservoir R.

Le A 31 439-Forei~n Countries Time t4: 13 to 30 seconds Adsorber A continued to be evacuated, whereby a final pressure of 250 mbar was reached. With the aid of the air blower adsorber B was pressurised to a final pressure of 0.3 bar. Product gas flowed into reservoir R via valves B4 and AB1.
5 Adsorber A was not flushed with ~2 product gas.

The process proceeded further in a manner analogous to times tllt2/t31t4, only adsorber A was exchanged for adsorber B.

A quantity of product gas from reservoir R of 22 Nm3/h with an ~2 concentration of 93% was obtained. The ~2 yield amounted to 51%, the specific energy value of the vacuum pump was calculated as 0.342 kWh/Nm302, the total energy value derived from pump and air blower amounted to 0.438 kWh/Nm302.

Example 6 (according to the invention) Use was made of an installation corresponding to Figure 1. The process sequence and the pressure profile are reproduced in Figure 3. Process sequence and pressure profile over time can be seen from Figures 2al2b and 3.

Time tl: 0 to 4 seconds Only valves B2, B4 and A4 were open; adsorber B was connected to the vacuum pump. As a result, gas that was high in ~2 flowed out of adsorber A into the previously evacuated adsorber B. The pressure in adsorber B rose from PDesmin =
250 mbar to PBFP-I = 450 mbar, simultaneously in adsorber A the pressure PAd maX= 0.3 bar dropped to PDeSo = 1 atrn. Reservoir R supplied product gas at a pressure of about 0.3 to 0.25 bar.

Time t2: 4 to 9 seconds Adsorber B was charged to approximately ambient pressure, by air flowing into adsorber A via valve A1, gas that was low in ~2 being withdrawn via valve A4.
Via lines Lp and LR and valve :B3 adsorber B was thereby pressurised.
Simultaneously adsorber B was toppecl up with air via the volume-controlled valve B1. The charging with gas that was low in ~2 via line LR was concluded at an ' CA 02228392 1998-01-30 Le A 31 439-Forei~n Countries ~2 concentration of 20-40 vol-%. P:roduct gas continued to be withdrawn from reservoir R.

Time t3: 9 to 13 seconds Adsorber A was evacuated via valve A2 from ambient pressure to approximately 650 mbar, adsorber B was charged with air via valve Bl and with product gas from the reservoir via valves AB1 and B4 until reservoir pressure and pressure in adsorber B reached about the same level. Product gas continued to be withdrawn from reservoir R.

Time t4: 13 to 30 seconds Adsorber A continued to be evac l~terl, whereby a final pressure of 250 mbar wasreached. With the aid of the air blower adsorber B was pressurised to its final pressure of 0.3 bar. Product gas flowed into reservoir R via valves B4 and AB l .
Adsorber A was flushed with ~2 product gas by product gas flowing out of line Lp into adsorber A via valve A4. lhe quantity of flush gas was adjusted with valve A4.

The process proceeded further in a manner analogous to times tllt21t3lt4, only adsorber A was exchanged for adsorber B.

A quantity of product gas from :reservoir R of 22.5 Nm3/h with an ~2 concentration of 93% was obtained. The ~2 yield amounted to 52%, the specific energy value of the vacuum pump was calculated as 0.339 kWhlNm3O2, the total energy value derived from pump and air compressor amounted to 0.433 kWh/Nm3O2.

The comparison of Examples 3 and l shows that the recirculation of the product gas that was low in ~2 of Example 3 produces an increase in the ~2 yield, but onaccount of a disadvantageous pressure profile in the course of pumping-out no improvement in the current demand of the vacuum pump is achieved.

On the other hand, the emission of the product gas that is low in ~2 after the adsorption step at approximately ambient pressure in Examples 4 to 6 according to Le A 31 439-Forei~n Countries the invention shows a clear improvement in the energy demand of the vacuum pump in comparison with Comparative Examples 1 to 3.

Claims (8)

1. Method for the separation of air by adsorption with molecular-sieve zeolites in a two-bed adsorber system with product buffer for the purpose of maintaining a continuous, constant stream of oxygen product, whereby the two adsorber beds with molecular-sieve zeolites are present in a homogeneous feedstock or in several different feedstocks of various types of zeolite, in which at approximately ambient pressure or up to an excess pressure of 0.5 bar during the adsorption air is conducted through the first adsorber A and nitrogen is adsorbed on the molecular-sieve zeolite preferentially in relation to oxygen, whereby product gas that is high in oxygen with an O2 concentration of 60 to 96 vol-%
is obtained at the exit end of adsorber A, after the adsorption the desorption of the nitrogen is carried out by evacuation in counterflow to the adsorption at a lowest pressure of 100 mbar to 600 mbar (absolute), characterised in that a) after the adsorption in adsorber A, the pressure of which drops from its final value after the adsorption step to A) ambient pressure, to B) low pressure down to 600 mbar or to C) excess pressure up to 0.2 bar, and the inlet end of which in the process is a) closed, is b) open in relation to the environment or c) an air compressor remains connected, residual product gas flows out of the outlet end of adsorber A
into the outlet end of adsorber B, whereby at the start of this step adsorber B is at its lowest pressure level of 100 to 600 mbar, whereby the pressure in adsorber B a) rises, b) remains constant or c) drops, b) ambient air without additional compression is introduced into adsorber A via the inlet end thereof, in the process the pressure in adsorber A remains at approximately ambient pressure or adsorber A is charged up to ambient pressure, gas that is rich in O2 is taken off at the outlet end of adsorber A and is introduced into adsorber B via the inlet end thereof until approximately ambient pressure is reached, c) ambient air with compression is conducted into adsorber B via the inlet end thereof and product gas is simultaneously supplied to the product buffer, d) adsorber A is evacuated to its lowest pressure of 100 to 600 mbar and is simultaneously adsorbed in adsorber B, by ambient air with compression being conducted via the entry end of adsorber B and product gas being conducted via the outlet end of adsorber B into the product buffer and optionally adsorber A being simultaneously flushed via the exit end thereof with product gas.

2. Method for the separation of air by adsorption with molecular-sieve zeolites in a two-bed adsorber system with product buffer for the purpose of maintaining a continuous, constant stream of oxygen product in accordance with Claim 1, characterised in that a) after the adsorption in adsorber A residual product gas is conducted out of the outlet end of adsorber A, the pressure of which drops in the process from between 0.2 and 0.5 bar excess pressure to approximately ambient pressure, into adsorber B, which is at its lowest pressure level of 100 to 600 mbar, via the outlet end thereof, whereby the pressure in adsorber B rises to max. 90% of the ambient pressure or remains constant, b) ambient air without additional compression is introduced into adsorber A via the inlet end thereof, gas that is rich in O2 is taken off at the outlet end of adsorber A and is introduced into adsorber B via the inlet end thereof until approximately ambient pressure is reached and optionally additional ambient air without additional compression is introduced directly into adsorber B, c) ambient air with compression is conducted into adsorber B via the inlet end thereof and optionally product gas is introduced simultaneously from the product buffer into adsorber B via the outlet end thereof and adsorber A is simultaneously evacuated, d) adsorber A is evacuated to its lowest pressure of 100 to 600 mbar and is simultaneously adsorbed into adsorber B, by ambient air with compression being conducted through via the entry end of adsorber B and product gas being conducted via the exit end of adsorber B into the product buffer.

3. A method according to Claim 2, wherein in step a) of Claim 2 adsorber a is simultaneously evacuated via the inlet end.

4. A method according to Claim 2 or 3, wherein in step d) of Claim 2, adsorber A is flushed simultaneously via the exit end thereof with product gas.

5. A method according to any one of Claims 1 to 4, wherein the system further comprises a layer of adsorbent which is selective with respect, to water, said layer being located upstream of the feedstock or feedstocks and wherein in the counterflow evacuation moisture is also desorbed together with the desorption of the nitrogen.

6. A method according to any one of Claims 1 to 5, wherein the step a) of Claim 1 adsorber B is simultaneously evacuated via its inlet end.

7. A method according to any one of Claims 1 to 6, wherein in step b) of Claim 1 ambient air is introduced without additional compression directly into the inlet end of adsorber B.

8. A method according to any one of Claims 1 to 7, wherein in step c) of Claim 1 product gas from the product buffer is supplied to the outlet end of adsorber B and adsorber A is simultaneously evacuated via the inlet end thereof.