KR900006080B1 - Safe adsorption apparatus for separation of co2 from gas containing o2 - Google Patents

Abstract

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Description

Safe Adsorption Process for Separation of Hydrocarbons from Oxygen-Containing Gases

1 is an apparatus diagram for flooding the process of the present invention,

2 is a time-planned flow chart for the operation of the plant according to FIG.

The present invention relates to a pressure swing adsorption process for obtaining hydrocarbons from a gas stream containing hydrocarbons and small amounts of oxygen.

German Patent Publication No. 3,035,255 describes a process for obtaining or recovering hydrocarbons from a gaseous stream containing a carrier gas and a hydrocarbon which, in particular, is separated from a methanol methane-air mixture in a pressure vibration adsorption apparatus. In this system essentially methanol-free air is discharged from the outlet end of the adsorber while enriched methane is obtained as a stream of desorption during the regeneration of the adsorber. This method aims at the complete separation of all hydrocarbons, in particular the complete separation of methane from the methane-air mixture.

Adsorption processes proposed for this purpose include an important step, from the adsorption step to the air discharge with methane obtained as a readily adsorbable material, in particular as a product. Adsorption pressures, ie those discharge steps that produce maximum pressure in the process, are not beneficial because for this purpose the methane has to be recompressed to the adsorption pressure.

For applications developed by DE 3,035,255, the concentration of hydrocarbons is relatively low; The concentration of methane in crude gas is 1-40% by volume. Since the remaining components are air, the concentration of oxygen in the crude gas is about 12-20% by volume.

However, the operation of such plants is dangerous because of the high risk of explosion of the mixture due to such high oxygen concentrations. There is also a risk that the hydrocarbon-oxygen mixture in the adsorption plant will be at the local concentration of oxygen in the explosive zone when operating with crude oxygen with low oxygen content.

It is an object of the present invention to provide a process of the type described above which safely removes each hydrocarbon from a carbon hydrogen mixture contaminated with a small amount of oxygen, for example up to about 5% by volume of oxygen, typically 1-10% oxygen. .

Further objects and advantages of the invention will be apparent to those skilled in the art according to the description in the specification and the appended claims.

The process of the present invention is characterized by the following steps:

Selective adsorption of hydrocarbons obtained during the adsorption step under pressure; Removing from the outlet end of the adsorber the strip released from the hydrocarbon produced during the adsorption step and at least one cocurrent expansion step; In the subsequent reflux expansion step, the strip of desorbed material concentrated in the resulting hydrocarbon is removed at the inlet end of the adsorber; After desorption, the compressor is recompressed to the adsorption pressure with a gas of low oxygen content.

In the process of the invention, gas mixtures of various strips appear; And in all such strips the oxygen content of these mixtures is kept low so that there is no risk of explosion. The limit of oxygen concentration required for this purpose is influenced not only by the conditions of the process in which the gas is produced and operated in each case but also by the components of the gas. For example, for gases containing methane and ethylene as their main components, the explosion limit is about 19% oxygen at 10 bar pressure, whereas the oxygen concentration is about 16% at 20 bar pressure. In general, the oxygen content should be kept below 15% for safe operation, preferably below 12%, in particular below 10%. In any case, the concentration of oxygen should not exceed 90% of the calculated explosion limit for each stream, preferably 80% or less.

During the pressure oscillation process, each component must be maintained in the adsorber to cause a spatial change as well as a temporal change of the gas component. In this regard, the maximum permissible oxygen concentration should not be exceeded at any time and at any point for safe operation of the process.

To date, it has been found to be particularly advantageous for a process in which the gas removed from the adsorber is at least partly used during the expansion step, following the adsorption step, to pressurize the regenerated absorber to perform the pressure vibration adsorption process. This is always the expansion gas removed from the cocurrent adsorber, such as the adsorption direction and the components of the expansion gas substantially correspond to the components of the unadsorbed gas stream removed during the adsorption process. Process operations of this type can utilize the pressure of the inflation gas for efficient energy use, further increasing the yield of the components obtained, which are always components of the stream which are not adsorbed. Processes of this type are described, for example, in German Patent No. 1,769,936; German Patent Publication No. 2,840,357; It is shown in German Patent Publication No. 2,916,585.

In many cases, it is now well known that even such highly proven methods of separating individual hydrocarbons from oxygen-containing hydrocarbon mixtures are not possible, since excessively high oxygen concentrations occur in this method. As a result, in the process of the present invention, at least one type of hydrocarbon is selectively adsorbed, and other hydrocarbons, oxygen and other components which may be contained in the gas flow through the adsorber and exit from the adsorber in a concentrated state. In the course of the cocurrent expansion step following the adsorption step, similarly similar gases are withdrawn from the adsorber. Oxygen content below the explosive limit However, it must still be maintained in concentrated gases, from which the total oxygen content of the crude gas is recovered. Moreover, an important point of the present invention is that no gas should be used for pressure raising of the regenerated adsorber. The reason is that the concentration of local oxygen increases when it is injected into the adsorber to pressurize this gas again, and this concentration easily exceeds the maximum allowable oxygen concentration. This is because there is no uniform oxygen distribution in the pressurized adsorber; Rather, increasing oxygen concentration appears towards the adsorber outlet end.

For this reason, the pressure build up of the regenerated adsorber consists of a low oxygen containing gas in the process of the invention. Suitable for such a gas stream is in many cases a separate gas stream, and the oxygen content is lower than the oxygen content of the gas exiting the adsorber end. However, pressurization can be influenced by other suitable and useful gas streams, such as, for example, purified hydrocarbon gases or mixtures of hydrocarbons that may be compressed if necessary. It is particularly advantageous to use an oxygen free gas such as methane when it contains a relatively high concentration of oxygen in the initial gas stream. This is because it can safely prevent unacceptable high concentrations of oxygen during the repressurization of the regenerated adsorber.

Pressurizing the adsorber with a low concentration of oxygen gas such as that represented by the separated gas mixture results in an uneven distribution of oxygen in the adsorber. From the crude gas used for pressurization, the hydrocarbons obtained are selectively separated from the adsorption front that moves to the outlet end, while the unadsorbed components immediately rise to the end of the blocked outlet of the adsorber. However, the retention of each component causes the oxygen concentration to increase toward the outlet end of the adsorber. Among them, the increase in oxygen concentration also depends on the selectivity of the adsorbent for the hydrocarbon obtained. For example, when separating gas streams whose main components are methane and ethylene using activated carbon as adsorbent, in addition to the desired adsorption of ethylene, the coadsorption of methane is relatively strong, e.g. when silica gel is used as the adsorbent. Higher concentrations of oxygen are produced at the outlet end of the adsorber.

In a large development of the present invention, gas must be removed through the end of the adsorber outlet prior to the end of the pressure intensification step to avoid unwanted or dangerous high concentrations of oxygen at the outlet end of the adsorber during the pressurization step.

This may be influenced by generally well known control means such as for example a flow controller.

This step prevents the formation of dangerous oxygen peak concentrations. This kind of operation is particularly convenient when using activated carbon as an adsorbent when ethylene is obtained from a gas stream comprising methane and ethyl. When silica gel is used as the adsorbent, it becomes unnecessary in this process during the process, which occurs for safety reasons.

During the process of the present invention, cocurrent expansion is carried out after the adsorption step, during which cocurrent expansion gas without components adsorbed is discharged, and the offgas is discharged through the empty space in the adsorbent charge during the cocurrent expansion process. . When the adsorption advance point does not reach the end of the adsorber outlet, the components obtained contained in the gas mixture filling the void space of the packing during the cocurrent expansion stage can be separated while the adsorption advance point further advances to the end of the adsorber outlet. So that the adsorption step is stopped.

Although a gas mixture is obtained during the co-expansion expansion step and the oxygen content is higher than that of the crude gas and cannot be used to pressurize the regenerative adsorber, it can be used for cleaning the sorbent prior to the pressurization step due to the low concentration of the components obtained with such mixture.

Such scavenging step follows the reflux expansion of the adsorber, in which part of the actual product is removed, thereby optionally reducing the residual load of the adsorber. However, in all cases, the adsorption of the adsorber is not necessary.

The desorbent stream produced during the reflux expansion process contains hydrocarbons obtained at concentrations that depend on the selectivity of the adsorbent as well as the concentration in the crude gas. If the desired product component concentration in this gas stream is not suitable, this desorbent stream may be separated into a highly concentrated stream of residual hydrocarbons and a residual gas stream via another separator.

Continuous separation suitable for doing so takes place, for example, in other pressure vibration adsorption devices.

In some cases it is possible that the crude gas stream to be separated contains components which are more easily adsorbed than the hydrocarbons to be recovered. In such a case, the more easily adsorbed components according to the process of the present invention may be separated in an adsorber installed in an upstream stream, optionally using another adsorbent. Adsorbers in such upstream streams can be arranged in a separate adsorber department or can be designed as a bed in relation to the actual adsorption bed and can be arranged in a normal housing together with the pre adsorber. Particularly suitable scavenging gas for regenerating the adsorbers of such upstream stream is the gas stream which is exhausted from the outlet end of the first adsorber.

This kind of operation is particularly advantageous if the gas which is not adsorbed is obtained, for example, as a low quality residual gas used only for heating purposes.

Adsorption devices having at least three adsorbers, preferably four adsorbers, are particularly suitable for the process of the invention.

The pressure vibration adsorption apparatus shown in FIG. 1 is comprised of four adsorbers (1) (2) (3) (4).

The adsorber is connected to the crude gas feed pipe 5 through valves 11, 21, 31 and 41 at the inlet and through the valves 12, 22, 32 and 42 at the outlet. It is connected to the residual gas discharge pipe 6 from which unadsorbed components are removed. The pressure is reduced in the expansion valve 7 which is regulated prior to the discharge of residual gas. Furthermore, the outlet ends of the adsorbers are respectively connected to the tube 8 via valves 13, 23, 33, 43 and to the residual gas discharge line via the valve 9. Unadsorbed constituent expansion gas generated in the first expansion stage is discharged into the residual gas through this discharge pipe.

The valves 14, 24, 34, 44 are respectively at the inlet side of the adsorber and connect the inlet ends of the adsorber to a pipe 51 connected to the buffer or surge tank 50. The tank 50 is connected to the product discharge pipe 53 through the control valve 52. The desorption gas and the scavenging gas generated in the scavenging stage as well as the product gas generated in the reflux expansion stage are injected into the tank 50 through the pipe 51.

Finally, the valves 15, 25, 35, 45 are arranged at the inlet side of the adsorber and these valves are connected to the crude gas feed line 5 via a tube 54 and a control valve 55. The pressure build up of the gas scavenged adsorber takes place through this tube.

2 shows a time schedule for the operation of the device. The four adsorbers operate at equal intervals in time to each other so that the adsorbers ensure continuous operation. After the adsorption step ADS operated at constant pressure, the first cocurrent expansion E1 is carried out. The expansion gas thus generated is discharged into the residual gas. In the second cocurrent expansion stage, more expanded gas is removed through the outlet end of the adsorber. This gas is passed through the valves 44 and pipes 51 to the outlet end of the adsorber 4 via open valves 13 and 43 before going to the buffer tank 50 and at the scavenging stage S. You will enter (4).

Following the second cocurrent expansion, it is carried out in countercurrent expansion stage E3 while going to buffer tank 50 via open valve 14 and tube 51 of the desorption stream. After the reflux expansion E3 is finished, the adsorber goes through the scavenging stage S. For this purpose the inflation gas from the cocurrent expansion stage E2 of the adsorber 2 is directed in the adsorption direction through the adsorber 1 countercurrently through the open valves 23 and 13. The scavenging gas still discharges the adsorption product present at the outlet end of the adsorber at a high concentration. The scavenging step is kept relatively short in order to prevent unnecessary dilution of the product gas which goes to the buffer tank 50 with the scavenging gas. In other variations of the process, it is also possible to perform desorption at subatmospheric pressure instead of the scavenging step S. However, improvements in the quality of the product thus obtained can be achieved at an increased energy cost for the vacuum pump.

After the scavenging step S is finished, the adsorber can be repressurized to the adsorption pressure. This process takes place during the pressure dropping stage B, during which the crude gas is injected into the adsorber 1 through open valves 55 and 15. After the pressure of the crude gas in the adsorber 1 is increased, the adsorber 1 ends the entire cycle. The remaining adsorbers are subjected to the same period only by the transitioned time schedule as described in FIG.

The process of the invention is for example like waste gas separation in an ethylene oxide production plant, which is particularly suitable; This waste gas may in some cases contain inert gases such as argon, nitrogen or carbon dioxide and other light hydrocarbons, as well as especially expensive ethylene, less expensive methane and a small amount of oxygen. The amount of ethylene contained in such gas streams depends on the method of production of ethylene oxide, usually in the range of 10-75%, for example when the ethylene oxide is produced with about 2-8% oxygen, the oxygen content is generally about It contains about 25 mol% ethylene compared to 1-8 mol%.

Those skilled in the art will readily appreciate that the present invention may be useful to the fullest extent with the above teachings.

Therefore, the following good special cases should only be assumed as explanations and in any case should not be assumed to be the remaining constraints of the presentation. In the following examples all temperatures are expressed in uncorrected degrees Celsius and unless otherwise indicated all percentages and percentages are relative to volume.

In an ethylene oxide plant, ethylene is converted to ethylene oxide products by oxygen. The purge gas is removed at the plant at 10 bar and 40 ° C. and 500 Nm ³ / h is removed. This gas has the following components:

In order to recover the ethylene component of this gas, an adsorption process is used as described in the drawing. Silica gel is used as the adsorbent and the plant is run every 16 minutes with a duration of 4 minutes for each adsorption step. From the outlet end of the adsorption process, residual gas with reduced ethylene content at 6 bar 40 ° C. is removed by an amount of 222 Nm ³ / h. The composition of the residual gas is as follows:

During the regeneration of the previously used adsorber, ethylene-rich gases are obtained in an amount of 278 Nm ³ / h at 15 bar 40 ° C. The composition of this gas is as follows:

This embodiment can be repeated with similar success by changing the reactants described generically or specifically or by changing the operating conditions of the present invention with respect to the operating conditions used in the above examples or by changing both at the same time.

Those skilled in the art from the above can easily identify the essential characteristics of the present invention, and the present invention can be variously changed or modified to suit various conditions and uses without departing from the spirit or scope of the present invention.

Claims (18)

A pressure vibration adsorption process carried out in a plurality of cycles of exchangeable adsorption beds for recovering hydrocarbons from a feed gas stream containing hydrocarbons and a small amount of oxygen, the process comprising: (a) adsorption performed under elevated pressure During the cold phase, selectively adsorb the hydrocarbon to be recovered: (b) during the adsorption step and at least one cocurrent expansion step following this step, the gas stream depleted of hydrocarbon to be recovered is discharged through the outlet end of the adsorption bed: (c) during the subsequent reflux expansion step, withdraw the hydrocarbon-rich stream to be recovered through the feed end of the adsorption bed: and (d) after desorption of step (c), the adsorption bed with a gas containing a small amount of oxygen. And re-pressurizing to the adsorption pressure to perform the pressure intensification step, and then repeating the cycle. The process according to claim 1, wherein the gas used for repressurization is a feed gas stream to be separated. The process according to claim 1, further comprising discharging a portion of the gas stream depleted of hydrocarbon to be recovered through the discharge end of the adsorption bed prior to the end of the pressure increasing step. The process according to claim 2, further comprising discharging a portion of the gas stream depleted of hydrocarbon to be recovered through the discharge end of the adsorption bed prior to the end of the pressure increasing step. The process according to claim 1, further comprising feeding the hydrocarbon enrichment stream of step (c) to another pressure vibration adsorption system, separating the stream into a stream with a very high hydrocarbon concentration and the remaining gas stream. It further comprises a preadsorption step which selectively adsorbs the components contained in the feed gas stream in the pre adsorption bed while being more adsorptive than the hydrocarbons to be recovered: and at the same time the hydrocarbon is depleted at the same time as the gas exiting from the outlet end of the adsorption bed. The process according to claim 1, wherein the preliminary adsorption layer is purified and regenerated using the prepared gas. It further comprises a preadsorption step which selectively adsorbs the components contained in the feed gas stream in a preadsorbent bed, which are more adsorptive components than the hydrocarbons to be recovered: The process according to claim 3, wherein the preliminary adsorption layer is purified and regenerated by using the prepared gas. The process of claim 1 wherein the feed gas stream contains primarily methane and methylene as hydrocarbons, wherein ethylene is the hydrocarbon to be recovered. The process according to claim 8, wherein silica gel or activated carbon is used as the adsorbent. The process according to claim 8, wherein activated carbon is used as the adsorbent. The process according to claim 10, further comprising venting a portion of the gas stream depleted of hydrocarbons to be recovered through the outlet end of the adsorption bed prior to the end of the pressure increasing step. The process according to claim 1, wherein the small amount of oxygen is 15 vol% or less. The process according to claim 1, wherein the small amount of oxygen is 12% by volume or less. The process according to claim 1, wherein the small amount of oxygen is 10% by volume or less. The process according to claim 1, wherein the small amount of oxygen is 8% by volume or less. The process according to claim 1, wherein the feed gas stream contains mainly ethylene, methane, carbon dioxide, nitrogen and argon, wherein ethylene and optionally methane are recovered. The process of claim 1, wherein the gas used for repressurization is a separated hydrocarbon or a mixture of separated hydrocarbons. 18. The process of claim 17, wherein the gas used for repressurization is compressed.

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