GB2025254A - Separating gas mixtures - Google Patents

Abstract

A gas mixture to be separated is passed under pressure over a carbon molecular sieve which preferentially adsorbs one or more components of the mixture, gas rich in the unadsorbed component or components of the mixture being withdrawn from the adsorber, the adsorber is regenerated by reducing the pressure to release a gas enriched in the adsorbed component or components, the adsorber is repressurised and the cycle repeated, the pressure during regeneration not being lowered below atmospheric pressure, which reduces energy requirements. The invention is particularly applicable to obtaining nitrogen-enriched or oxygen-enriched fractions from air and does not require the use of special measures for the preliminary removal of CO2 and water.

Description

SPECIFICATION Improvements in or relating to the separation of gas mixtures The present invention relates to the separation of gas mixtures and more particularly to a process for separating a gas mixture under pressure by passing it over a carbon molecular sieve which is capable of adsorbing at least one component of the gas mixture in preference to the other component or components, drawing off a stream of gas enriched with one or more other components, regenerating the carbon molecular sieve by lowering the pressure above it, whereby a stream of gas enriched in the at least one component is released therefrom, raising the pressure above the carbon molecular sieve to its former value and again passing the gas mixture over it.

German Patent Specification No. 2,441,447 discloses a process for obtaining a gas rich in nitrogen from a gas containing oxygen at least, as well as nitrogen, such as air, for example, in which process the air is passed over a carbon molecular sieve at increased pressure when oxygen is preferentially adsorbed, so that during the adsorption a gas enriched in nitrogen at first leaves the adsorber. A switchover to desorption takes place as soon as the level of oxygen in the nitrogen product reaches a predetermined critical value. In this known process, the pressure above the carbon molecular sieve is lowered to below 100 Torr for desorption purposes.

However, this known process has the disadvantage that it is extraordinarily costly in terms of energy, since the adsorption is carried out in the region of 2 to 5 bars and the desoprtion at below 100 Torr, preferably at 20 to 70 torr, so that not only must a considerable amount of work be expended in producing the vacuum, but also a considerable amount of work in compressión has to be done to overcome the pressure difference, since the ratio between the adsorption and desorption pressures is something like 200:1 in extreme cases. In addition, with this known process the investment costs are high owing to the need to provide a vacuum pump system and the chance of the incidence of trouble and the associated risk of lost production are high.

It is accordingly an object of the present invention to provide a process for separating gas mixtures using a carbon molecular sieve by means of which the separation is effected considerably more advantageously as regards energy.

According to the invention, there is provided a process for separating a gas mixture under pressure by passing it over a carbon molecular sieve that can adsorb at least one component of the gas mixture in preference to the other component or components, withdrawing a stream of gas enriched in said other component or components, regenerating the carbon molecular sieve by reducing the pressure above it, whereby a stream of gas enriched with said at least one component is released, again raising the pressure over the carbon molecular sieve and passing the gas mixture over it again; wherein during regeneration the pressure above the carbon molecular sieve is not lowered below atmospheric pressure.

Carbon molecular sieves have the advantage over other molecular sieves, such as zeolites, in that other adsorption agents, such as silica gel or active carbon, do not have to be located upstream of them to protect them from the effects of water and CO2, i.e. impurities which are found in most synthetic and naturally occurring gas mixtures, since the carbon molecular sieves can adsorb these materials in addition to the desired components of the gas mixture without any trouble. During desorption of the previously adsorbed components, these basically undesirable components are readily desorbed also.

However, it has hitherto been the opinion of experts in the field that such co-adsorption, in particular of water vapour, entailed desorption at a pressure that had to be far below atmospheric pressure and at most 100 Torr, since otherwise there was a danger that with continuous use, the carbon molecular sieve would gradually become so permeated with H20 molecules that its capacity to separate the other components would decline.

It has now been found, however, that these fears were based on preconceptions and that separating processes using carbon molecular sieves could also be carried out if the pressure during desorption was not lowered below atmospheric pressure, i.e. if desorption was effected without the aid of a vacuum pump.

It has been confirmed experimentally, even after more than 1000 hours in service the ability of a carbon molecular'sieve to adsorb the desired components from a moist gas mixture does not decline if desorption with such moist gas mixtures is carried out at pressures not below atmospheric pressure, even if the gas mixtures enter the carbon molecular sieve bed saturated with water vapour.

It has been found to be expedient to effect the adsorption process of the invention using adsorption pressures of not less than 4 bar, not because the process would be impossible to carry out at lower pressures but because with the latter, the yield leaves something to be desired.

An even better yield can be obtained, however, at higher pressures, for example, 12 bar, but the compression required is naturally also greater.

Another advantage when working at higher adsorption pressures lies in the reduction in the adsorber dimensions that are required for a given nominal output owing to the fact that the amount of product increases sharply with the pressure.

In general, in the process in accordance with the invention a pressure ratio between the adsorption and desorption pressures 4:1 to 14:1 is desirable, one of 4:1 to 6:1 being preferred.

If compressors are already available in a plant and if the specific energy requirement does not play a significant part, it is, however, expedient to carry out the process of the invention even at much higher pressure ratios, e.g. 20:1.

The process in accordance with the invention can be applied to all gas mixtures the components of which exhibit differing adsorption characteristics towards a carbon molecular sieve.

The process can thus be applied to the separation of air, in particular with the object of obtaining gas enriched in nitrogen having a residual oxygen content of down to 1% by volume. Such impure nitrogen can be used industrially for many different purposes, such as, for example, for rendering inert, flushing, or protecting oil, petrol, tar or methane tanks. It can also be used in chemical processes to prevent explosive mixtures forming between gaseous components from the process and any air present.

In addition, the nitrogen can be used to set the calorific value of a natural gas mixture, i.e. for conditioning natural gas. Lastly, nitrogen is employed in the foodstuffs industry for preservation purposes (e.g. for filling beer containers).

When separating air to obtain nitrogen, the yield attainable with the process of the invention is about 40% with an adsorption preessure of 4 bars and a residual oxygen level of 3% by volume in the product. An even better yield can be achieved at higher pressures, of 12 bars, for example. The yield with an oxygen content of 3% by volume in the nitrogen is then almost 50%.

The invention will now be further described with reference to the drawing, which is a flow sheet of one form of plant for carrying out the invention.

The plant illustrated in the drawing comprises two adsorbers 1 and 2, each of which contains 700 g of a carbon molecular sieve. The functioning of the two adsorbers can be reversed cyclically.

Air under pressure enters the adsorber 1, which is on stream, through a line 3 and a valve 4. The CO2 content of the air is about 300 ppm, whilst the moisture content of the air is that of saturation state at ambient temperature. In the adsorber 1 , the carbon molecular sieve adsorbs oxygen in preference to nitrogen, so that a gas rich in nitrogen leaves through valves 5 and 6 and a line 7.

The oxygen content of this gas is not constant; rather it fluctuates about a certain mean value and generally increases the longer the adsorber 1 is on stream. It has also been observed, however, that the oxygen content of the gas leaving the adsorber increases relatively sharply for a short time after switching over the adsorber in question to adsorption, which is probably attributable to a pressure wave in the adsorber preceding the adsorption front when the adsorber is repressurised after the pressure has been lowered for regeneration purposes, with the result that some oxygen breaks through.

In practice, therefore, the oxygen content of the gas flowing through the line 7 is monitored. If, for example, a nitrogen is required containing 3% oxygen, the range of variation in the oxygen content may be between 2 and 4%. If the amount of oxygen in the gas leaving the plant exceeds about 4%, the adsorber is switched off by closing the valves 4 and 5, so that finally the mean oxygen concentration in the nitrogen product obtained is 3%. When nitrogen containing about 1% oxygen is to be produced, the range of variation of the oxygen content may be 0.8 to 1.2%.

While the adsorption of oxygen is taking place in adsorber 1, the adsorber 2 is desorbed by opening valves 8 and 9 to connect the adsorber to atmosphere. As a result, a gas mixture flows from the adsorber 2 through a line 10 which has an oxygen concentration higher than that of air and which also contains the carbon dioxide and water which were also adsorbed together with the oxygen. At the moment when the pressure within the adsorber 2 reaches atmospheric pressure and the desorption of the adsorbed components has progressed sufficiently, the valves 8 and 9 are closed whilst valves 5 and 1 1 are opened, all the other valves being closed or remaining closed, so that now a pressure equilibrtum is brought about between the adsorber 2 and the outlet end of adsorber 1.Since with this pressure equilibrium, the gas in the cavities of the adsorber 1 initially passes into the adsorber 2 and this gas at the outlet end of adsorber 1 is predominantly nitrogen, any residual content of oxygen that may be present at the outlet end of adsorber 2 is forced back towards the inlet end of this adsorber. This results in a smaller oxygen content in the nitrogen product in the subsequent adsorption phase of adsorber 2.

It is possible to open valves 8 and 12 in addition to valves 5 and 1 1 in order to achieve the pressure equilibrium, so that the pressure equalisation takes place through both the outlet and the inlet ends of both adsorbers. In this way, an even purer product can be obtained in the subsequent adsorption cycle in adsorber 2, since the gas in the cavities of adsorber 1 is richer in oxygen at its inlet end than at its outlet end. This oxygen-rich gas, which with the first pressure equalisation method would pass through the outlet end of the adsorber 1 into the outlet end of adsorber 2, flows in through the inlet end in the second method, so that the outlet end of adsorber 2 is contaminated with oxygen to a lesser degree.

When the pressure has been equalised, the adsorber 2 is fed with compressed air through the line 3 until the adsorption pressure is reached, by closing valve 1 1 and possibly valve 8, and by opening valve 13. When this pressure is reached, valve 1 1 is opened and nitrogen-rich product is withdrawn through the line 7 and valve 6. While the adsorber 2 is being pressurised, the adsorber 1 is desorbed by closing the valve 5 and opening valves 12 and 9, when oxygen-rich gas flows out through the line 10.

The following Table gives the results of some tests made with the apparatus described above in carrying out the process of the invention.

TABLE Test No. 1 2 3 4 5 6 7 8 9 Troughput of starting gas Nl/h 100 160 200 195 265 305 370 480 560 Pressure after equalisation bar 2.5 2.5 2.5 3.5 3.5 3.5 5.5 5.5 5.5 Pressure during adsorption bar 4 4 4 5 6 6 10 10 10 Amount of product (N2) Nl/h 10 36 60 20 75 110 40 140 210 Residual O2 in product % Vol. 1 2 3 1 2 3 1 2 3 Product yield % 12.5 27.9 36.8 12.9 35.1 44.3 13.5 36.2 46.0 The process of the invention is not restricted as regards cycle duration. However, its advantages are particularly apparent with the so-calied adiabatic alternating-pressure processes that have been developed in recent times and which are distinguished by the use of very short cycles of the order of 1 to 3 minutes, so that the various phases in the individual cyclically switched adsorbers only last a few seconds.

The process of the invention also makes it possible to obtain an oxygen-enriched gas from air.

For this purpose, air is fed through the line 3 and valve 4 to the adsorber 1 at a pressure of more than 4 bars. Both the adsorber 1 and the adsorber 2 are provided with carbon molecular sieve beds. As a result of the differing adsorption rates, oxygen, water vapour and CO2 are preferentially adsorbed by the carbon molecular sieve. The stream of gas flowing out through valves 5 and 6 and line 7 has a low residual oxygen content. Whilst the adsorber 1 is switched to adsorption, the adsorber 2 charged during the preceding phase of the cycle is regenerated in counter-current by reducing the pressure. For this purpose, valves 8 and 9 are opened so that the desorption gas can flow out through the line 10. This desorption gas (the product in this case) is enriched with oxygen to a value of up to 50% by volume.

As soon as the oxygen concentration at the outlet of adsorber 1 reaches a value almost matching the oxygen concentration of the air fed in, a rapid pressure equalisation is effected between the two adsorbers through their outlet or inlet ends with the aid of valves 5 and 1 1, or valves 8 and 12.

The adsorber 1 is then regenerated by opening valves 12 and 9. The desorption gas flowing out through the line 10 is enriched in oxygen up to a value of 50% by volume. At the same time with valve 1 1 closed, the adsorber 2 is supplied with air and raised to a pressure lying between that prevailing on completion of pressure equalisation and the final adsorption pressure. The the air flows through the adsorber 2 with the valves 1 1 and 13 opened as the pressure rises further.

As soon as the oxygen concentration at the outlet of adsorber 2 approaches the oxygen concentration of the air fed in, the pressure is again equalised between the two adsorbers and the process described above is repeated.

To increase the oxygen concentration of the product produced at the time of regeneration (pressure reduction), it is expedient to separate a first fraction of the gas produced by pressure reduction as exhaust gas. The nitrogen contained in the drained part of the adsorber is then partially discharged and the product subsequently obtained is then richer in oxygen.

The use of the process of the invention to obtain an oxygen-enriched gas from air will now be further described with reference to a practical example.

In this example, two adsorbers 1 and 2 each contained 700 g of a carbon molecular sieve for the selective adsorption of oxygen. The starting gas was air at a pressure of 6 bar. There was no preliminary treatment of the air to remove small amounts of impurities. The CO2 concentration was normal (about 300 ppm). The moisture content was that at saturation point.

One cycle took 60 seconds, 1 second being allowed for pressure equalisation, 3 seconds for pressure build-up, 3 seconds for the first pressure reduction (exhaust), 56 seconds for the second pressure reduction (product) and 56 seconds for adsorption.

The measurements were taken with an adsorption pressure of 6 bars and a regeneration pressure of 0.96 bars (equal to ambient pressure).

Result: Air flow (760 Torr; 0O C): 480 I/h 1160 I/h Product flow (760 Torr; 0O C): 132 I/h 142 I/h 02 concentration in product: 44% by vol. 49.5% by vol.

02 yield: 57.6% 28.8%

Claims (5)

1. A process for separating a gas mixture under pressure by passing it over a carbon molecular sieve that can adsorb at least one component of the gas mixture in preference to the other component or components, withdrawing a stream of gas enriched in said other component or components, regenerating the carbon molecular sieve by reducing the pressure above it, whereby a stream of gas enriched with said at least one component is released, again raising the pressure over the carbon molecular sieve and passing the gas mixture over it again; wherein during regeneration the pressure above the carbon molecular sieve is not lowered below atmospheric pressure.

2. A process as claimed in Claim 1, wherein the pressure ratio between adsorption pressure and desorption pressure is in the range 4:1 to 14:1.

3. A process as claimed in Claim 2, wherein the pressure ratio between adsorption pressure and desorption pressure is 4:1 to 6:1.

4. A process as claimed in Claim 1, wherein the pressure ratio between adsorption pressure and desorption pressure is about 20:1.

5. A process for separating a gas mixture substantially as hereinbefore described with reference to the drawing.