DK148997B - PROCEDURE FOR FRACTIONING A GAS MIXTURE - Google Patents

Description

148997

The present invention relates to a process for fractionating a gas mixture using at least two beds of a material capable of selectively adsorbing at one of the components of the gas mixture selectively adsorbed and alternately bearing the beds at a relatively high pressure. pressure, respectively, is relieved from this pressure, whereby the pressure application and relief for each bearing are carried out by a number of sub-operations, in which in sub-operations (I) and (II) an adsorption of the adsorbable component and a formation of product gas takes place in sub-operation (III ) a pressure drop occurs by pressure equalization between the two bearings, in sub-operation (IV) a pressure drop is applied to the desorption pressure, in sub-operations (V) and (VI), the bearings are flushed with product gas, and in sub-operations (VII, VIII, IX and X) there is a pressure increase in the bearings until the adsorption pressure is reached.

U.S. Patent No. 3,788,036 discloses a process for fractionating a multi-component gas mixture, which method is carried out in one of its embodiments in interaction between two adsorption columns containing a material capable of selectively adsorbing one of the components of the gas mixture. and wherein the adsorption columns are alternately subjected to and maintained at a relatively high pressure, respectively, for this pressure, the pressure application or relief for each column being carried out in a number of steps in which for each of the columns an adsorption of the adsorbable is carried out in the following order. component and a product gas formation, a pressure drop by pressure equalization between the two adsorption columns, an additional pressure drop to the desadsorption pressure, a flushing of the adsorption column, optionally with product gas, and a pressure increase in the adsorption column until adsorption pressure is reached. In this method, however, some contamination of the outlet end of the second adsorption bed with adsorbable component occurs at the pressure equalization, with pressure lowering in an adsorption bed always releasing a portion of the adsorbed gas, thereby deteriorating the purity of the product gas in subsequent production steps.

In the process according to the invention, in relation to the above-mentioned process, firstly, a product gas (e.g. oxygen) with a higher purity is obtained, keeping the outlet end of the adsorbent free of adsorbable component (e.g. nitrogen), and secondly, better utilization of the gas, feedstock is reduced.

This is achieved by the method according to the invention being characterized in that the pressure equalization between the bearings in part operation (III) is carried out by connecting the outlet end (bottom) of the first bed with the inlet end (top) of the second bed, that the rinsing of the bearings in the sub operations (V ) and (VI) are carried out with product gas from one product gas tank and the pressure increase in sub-operation (VII) is carried out by adding the product gas to the outlet end (bottom) of the bearings and in sub-operation (VIII) by connecting the outlet end (bottom) of the second bearing with the first bearing is inlet end (top).

An embodiment of the method according to the invention is characterized in that the product gas is supplied to a product gas container via a product pipeline and that the product gas in the first part operation (VII). of the pressure application of a bearing (eg 1) 3 148997 is taken from the product container via a pipeline which is different from the product pipeline.

In the following, the invention will be described in more detail in connection with an embodiment with two bearings and with reference to the drawing, in which fig. 1 shows a wiring diagram of the system; FIG. 2 shows the time sequence of the different operating phases of the two bearings included in the system; FIG. 3 shows the gas streams that occur in the plant in various phases of operation, and FIG. 4 shows the concentration profile in a bed at different times over an entire operating cycle.

The FIG. Figure 1 schematically illustrates a two-bed system which can be used to fractionate any gas mixture, but in the present case it is assumed that it is intended for fractionation of air, so that as a product gas oxygen is obtained which is practically free of nitrogen. The plant comprises two beds 1 and 2. Both contain a zeolitic molecular sieve with the ability to adsorb nitrogen to a substantially higher degree than oxygen at relatively high pressure. The raw gas, which in this case is compressed air, is supplied to a connection point 3. The supplied air 2 can be compressed to a pressure of approx. 4.1 kg / cm. From the connection point 3, the raw gas passes through a pressure regulator 4 and an adjustable throttle valve 5 to a pair of controllable valves 6 and 7.

Through these, the raw gas can be directed either to the bearing 1 or to the bearing 2. The product gas leaving the bearing 1 at its outlet side reaches via a controllable valve 8 and an adjustable throttle valve 9 to a product tank 10, which is intended to collect the resulting product gas. . The product gas produced in the bearing 2 reaches analogously via a controllable valve 11 and the throttle valve 9 to the product tank 10. From this, the product gas can be extracted via an outlet pipeline 12. The throttle valve 9 is part of a product pipeline 13.

In a manner known per se, the outlet side of one bearing can be connected to the inlet side of the other bearing. This is done via controllable valves 14 and 15 at the outlet end of the bearings and controllable valves 16 and 17 at the inlet end of the bearings and an adjustable throttle valve coupled between these valve pairs. Furthermore, the inlet end of the bearing 1 can be via a controllable valve 19 and the inlet end of the bed 2 via a controllable valve. 20 is connected directly to the ambient atmosphere or to a vacuum pump via a relief pipeline 21.

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In order to enable the flushing and pressure application of a bearing with product gas independently of the operating state of the second bearing, the device is designed so that product gas from the tank 10 can be supplied to the outlet side of the respective bearings via a pipeline 22 and each of the the controllable valves 14 and 15. For reasons which will become apparent from the following, the pipeline is divided into two parallel branches, each containing a controllable valve 23 and 24, respectively, and an adjustable throttle valve 25 and 26, respectively.

Conveniently, the various controllable valves are automatically controlled so that during the sub-operations designated I____ X, which together constitute an entire operating cycle, the gas flows seen in FIG. 3. Corresponding partial operations as a function of time are shown in FIG. 2, where the top line shows the sub operations for the bearing 1 and the bottom line shows the sub operations for the bearing 2. Thus, for the bearing 1, adsorption of adsorbable component and the emergence of product gas occurs during phases I and II, during phase III pressure reduction. by pressure equalization between bearings 1 and 2 (Tsl) and during phase IV pressure lowering to the relatively low desorption pressure (Ts2). during phases V and VI, there is a flushing with product gas in the bed 1, after which a pressure increase is carried out in the bed 1, namely during the phase VII with the product gas (Tø2), during the phase VIII by pressure equalization between the bearings (Töl) and finally during phases IX and X using raw gas (Tø3). Then this rental pouch is ready to produce product gas. In the embodiment selected here, an entire cycle time is 208 seconds, and of FIG. 2 shows the times when the plant moves from one sub operation to the next. Figures 2 and 3 show that the operating cycle of the bearing 2 is time-shifted with respect to the operating cycle of the bearing 1 in such a way that during pressure equalization between the bearings a pressure decrease in one bearing and a pressure increase in the other bearing takes place.

FIG. 4 shows an example of the appearance of the concentration profile, i.e. the oxygen content, in different parts of the bed 1 from the inlet side at the top to the outlet side, ie at the bottom of the respective part figure. Each part figure illustrates the concentration profile at a time specified in the respective part figure during the transition from one part operation to the next.

The processes that are carried out in the bearing 1, below, will now be explained in more detail with reference to the drawing.

5 148997

At time 0, the adsorption pressure is obtained in the bed i by supplying air during sub operations IX and X. At the inlet end of the bed, the molecular sieve is saturated with air of the prevailing pressure. At the outlet end there is "pure" product gas, that is, a gas consisting approximately of 95% oxygen and 5% argon. In the course of sub-operation I, product gas is withdrawn from the bearing at a rate controlled by the throttle valve 9, and the pressure regulator 4 at the inlet end causes air to be supplied in such a way that the pressure is kept constant at all times. As the gas passes through the bed, a fractionation occurs as nitrogen is adsorbed to a greater extent than oxygen. As the adsorption progresses, the bed becomes increasingly saturated with air. By maintaining the gas velocity below the highest critical value, a nitrogen-free zone is maintained near the outlet end throughout the period in which product gas is withdrawn from the bearing.

During sub-operation II, production of nitrogen-free product gas from the bearing continues 1. This production is interrupted on time 66 (sec), immediately before the content of nitrogen increases in the outgoing product gas.

During part operation III, the outlet end of the bearing 1 has been connected to the inlet end of the bearing 2 at the same time as the supply of air to the bed 1 has been interrupted, and consequently a pressure drop in the bed 1. The partial pressure reduction for the gas components which is the result, causing desorption of these into the bearing. The first gas fraction leaving the bed 1 contains approx. 95% oxygen, while the latter has a content of 40-60%. Thus, the average oxygen content in the gas leaving the bed 1 is significantly above the oxygen content in air.

During sub-operation IV, a pressure drop in the bearing 1 occurs by connecting its inlet end to the atmosphere or to a vacuum pump. The lowering of the partial pressure then gives a further desorption and part of the desorbed material is flushed out.

During part operation V, the bed 1 is flushed from its outlet end to its inlet end, whereby nitrogen-free product gas is supplied from the product tank 10 via the controllable valve 23 and the adjustable throttle valve 25 in the line 22. The adjustable throttle valve 25 is then adjusted so that the supply of product gas takes place relatively slowly.

During sub-operation VI, the flushing of the bed 1 continues with product gas via the valve 23 and the throttle valve 25. The low partial pressure of nitrogen in the flushing gas causes the nitrogen to be desorbed from the molecular sieve and flushed from the bed.

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At the beginning of sub-operation VII, the outlet is closed to the atmosphere and the supply of product gas to the bed 1 now continues through the controllable valve 24 and the adjustable throttle valve 26. The nitrogen-free product gas which now flows into the bed causes a pressure increase and partly a desorption of nitrogen. at the lower outlet end. The desorbed nitrogen is fed with the gas flow towards the upper inlet end of the bed and is again adsorbed in the molecular sieve when the gas partial pressure of nitrogen exceeds the value corresponding to the equilibrium ratio of the molecular sieve.

Thus, during sub-operation VII, nitrogen is moved from the outlet end of the bed to its inlet end while establishing a nitrogen-free zone at the outlet end of the bed.

At the beginning of sub-operation VIII, the supply of product gas to the bed 1 is stopped and instead the inlet end of the bed 1 is connected to the outlet end of the bed 2. Gas thus flows from the bed 2 to the bed 1 under pressure increase therein. This sub-operation is carried out until pressure balance between the bearings is achieved. For the pressure increase of the bearing 1, a relatively oxygen-rich gas is used, as stated in connection with the description of sub-operation III. Since this gas has a significantly lower oxygen content than the gas located at the outlet end of the bed 1, the pressure equalization must not be carried out so rapidly that the nitrogen-rich gas flows through the bed 1 at such a rate that the nitrogen cannot be adsorbed. The required low flow rate is obtained in this case with the adjustable throttle valve 18.

During part operation IX, compressed air flows in at the inlet end of the bed 1. This causes a slow, continued pressure increase, and during this sub operation the bed is saturated with air at the inlet end while maintaining a nitrogen-free zone at the end of the bed. .

During sub-operation X, the pressure increase in the bed 1 continues with the supply of air until the adsorption pressure is reached, after which the bed is ready for production, which means that the above-described sub-operations are cyclically repeated.

As can be seen from the above description, the pressure increase in the bearing 1 occurs up to the final pressure in three steps. During the first step, part operation VII, the pressure increase with product gas from product tank 10. Immediately prior to this part operation, flushing with product gas has been carried out, and the concentration profile in the bed is then almost straight, as can be seen from the sub-figure in fig. 4, which is designated t = 152. When, during sub-operation VII, nitrogen-free product gas is supplied via the outlet end of the bed 1 during simultaneous pressure increase, an increase in the ratio of the partial pressure of the oxygen to the nitrogen is obtained in the gas phase. The nitrogen is desorbed and introduced into the bearing towards the inlet end. After this pressure increase, the bed has approximately the concentration profile shown in FIG. 4 can be seen in the part figure designated t = 170. From this it can be seen that a zone of nitrogen-free gas is established here closest to the outlet end. In this zone, the final fractionation takes place later during the adsorption. Nitrogen-containing gas can now be released into the bed 1, but this must be done through the inlet end of the bed, as the said nitrogen-free zone would otherwise be disturbed. In this context, it may be pointed out that if nitrogen-containing gas had been introduced into the bed's inlet end directly after flushing during suboperation VI, this would have resulted in some nitrogen passing through the bed without adsorbing. Thereby, nitrogen-containing gas would have been obtained at the outlet end and, consequently, a nitrogen-containing product gas.

The second step of the pressure increase is provided by pressure equalization between the bearings. This is done for bearing 1 during sub-operation VIII. At this pressure equalization, a pressure drop takes place in the bearing 2, thereby desorbing some of the nitrogen that comes into the bed 1, which must be pressurized. For this reason, the supply to the bed 1 is effected at its inlet side so that the nitrogen-free zone can be maintained at the outlet end. The nitrogen-containing gas supplied is fractionated in the bed 1 during this sub-operation, and at the end of the pressure equalization the concentration profile obtained in FIG. 4 is shown in the sub-figure designated t = 183.

The pressure increase to the final adsorption pressure occurs by supplying air from the compressor at the inlet end of the bed 1. During this pressure increase, the inflowing air is fractionated and when the final adsorption pressure has been obtained after the sub-operations IX and X, the bed 1 exhibits a concentration profile according to the part figure of FIG. 4, which is designated t = 208. The bearing 1 is thus again ready for production.

The pressure variation sequence described herein differs from and involves improvements compared to previously known sequences in the following respects. Pressure increase with product gas free of adsorbable component occurs as the first step and the product gas is introduced via the outlet end of the bed. This creates a concentration distribution in the bed, which is favorable for gas fractionation, which is introduced later during the operation cycle. Furthermore, all gas containing the adsorbable component is always introduced at the inlet end of the bed and after product gas which is free of adsorbable component has been introduced via the outlet end.