US3375674A - Prepurification of gas mixtures before separation thereof by low temperature rectification - Google Patents

Description

A nl 2, 1968 R. BECKER 3,375,674

PREPURIFICATION' OP GAS MIXTURES BEFORE SEPARATION THEREOF BY LOW TEMPERATURE RECTIFICATION Filed Aug. 18, 1966 2 Sheets-Sheet 1 I I I I I .L

RUDOLF BECKER TTORNEY April 2, 1968 R. BECKER 3,375,674

- PHEPURIFICATIQN GAS MIXTURES BEFORE SEPARATION THEREOF TEMP BY 0 ERATURE RECTIFICATION Filed Aug. 18, 1966 2 Sheets-Sheet INVENTOR LF BECKER ATTO NEY 3,375,674 Patented Apr. 2 1968 ice L ,4 12 Claims. or. 6213) This invention relates to a process for the low temperature separation of gaseous mixtures, and in particular to a process wherein CO is removed from raw gas by two simultaneous but separate treatments, one by low temperature congelation, and the other by adsorption, and, more particularly, to the subsequent removal of hydrocarbon impurities from the resultant Co -free gas.

Processes for the separation of gaseous mixtures are known wherein gas mixture to be separated is warmed to a temperature above the regenerator exit temperature before being introduced into a turbine wherein it is engine expanded (the term engine expansion meaning expansion with the production of external work), in order to provide the refrigeration requirements of the process. This is done, according to German Patent No. 1,145,649, for example, by branching off a partial stream from the gas mixture exiting from the cold end of the regenerators, warming this partial stream in the heat exchange coils provided within the regenerators, and thereafter admixing this partial stream to the cold main stream.

In another process of this type, described in German Patent No. 833,051, the warming of the gas mixture prior to engine expansion is accomplished by branching off a partial stream of raw gas incoming from the regenerators at a warm point (above the CO freezing point) and eventually returning same to the main stream exiting from the cooler end. The CO present in the branched-off partial stream is removed in CO adsorbers inserted between the point of withdrawal from the regenerator and the point of admixing with the main stream.

These processes have the disadvantage that the hydrocarbons contained in the gas mixture enter the expansion engine and the rectification column. The hydrocarbons passing into the sump of the rectification column have a higher boiling point than the oxygen. Therefore, they remain in the sump, are enriched therein, and form an explosive hydrocarbon-oxygen mixture, constituting a constant danger to the entire gas separation plant.

Whereas hydrocarbon adsorbers in gas separation plants are already known, they are usually inserted in the system in such a manner, for example in the device described in German Patent No. 1,145,649, that they do not prevent the entrance of hydrocarbons, for example, acetylene, into the rectification column. Besides, in most cases, excessive energy is necessary for desorbing these adsorbers, resulting in a disruption of the energy balance of the entire plant. Additionally, special conduits are required for feeding the desorption agent to the adsorber. From the standpoint of hazardous operation, a particular disadvantage of the conventional plant is that air can condense in the gas phase adsorber and dissolve the hydrocarbons from the adsorption medium, and that droplets of high concentration of hydrocarbons may be swept into the turbine or rectification column.

It is, therefore, an object of this invention to provide a less hazardous process for the adsorption of hydrocarbon impurities from gas mixtures, particularly air.

Another object is to provide such a process which is also economically attractive from the standpoint of direct operating costs and plant investment costs.

Upon further study of the specification and claims,

other objects and advantages of the present invention will become apparent.

For the attainment of the above objects, this invention is briefly described as an improvement in a process of separating gaseous mixtures by low temperature fractionation, which process comprises passin an impure gas mixture containing carbon dioxide and hydrocarbons as impurities into a cyclically interchangeable heat exchanger and cooling said gas mixture therein, a major portion of said gas mixture being cooled to a sufiiciently low temperature to condense out carbon dioxide, and a minor portion of said gas mixture being removed from said heat exchanger at a point wherein the temperature is above the condensation temperature of CO passing said removed minor portion through a first adsorption zone to remove carbon dioxide and produce CO -free minor portion, combining at least part of resultant CO -free minor portion with CO -free major portion, and fractionating resultant combined mixture, the improvement comprising the intermediate step of passing the combined CO -free minor and major portions through a second adsorption zone to remove hydrocarbon impurities from said combined portions, said intermediate step being conducted prior to the step of fractionating resultant combined mixture.

With respect to the attached drawings, FIGURE 1 is a schematic representation of a preferred embodiment of this invention, FIGURE 2 in particular illustrating the combination of two adsorption zones in one adsorber.

By means of the improved process of the present invention, the entire amount of the gas mixture can be freed of hydrocarbons in a gas phase adsorption zone before it enters the rectification column. Furthermore, though branched-off minor portion is first passed through a separate CO adsorption zone, the hydrocarbon adsorption zone and the CO adsorption zone can be arranged so that the same desorption medium stream can be used to desorb both zones. For example, an impure separation product can be passed successively through both adsorption zones to desorb same, thereby resulting in the avoidance of two separate conduit systems for the desorption cycle.

Upon recombining the warm CO -free minor portion which amounts to about 5 to 20%, preferably 5 to 10% of the original raw gas, with the CO -free major portion exiting from the cyclically interchangeable heat exchanger, the temperature of the combined portions is increased to such an extent that any entrained liquefied gases present in the major portion is evaporated with certainty before reaching the hydrocarbon adsorber. In this way, the hazard associated with the hydrocarbon dissolution action of entrained droplets of liquefied gas is completely eliminated.

In the case of air separation in particular, the temperature of the major portion leaving the cyclically interchangeable heat exchangers is about to K., preferably 105 to 107 K., the temperature of the branchedoff minor portion about to 150 K., preferably to K., and the combined CO -free portions prior to hydrocarbon adsorption about 107 to 115 K., preferably about 110 K., if the heat exchangers and the adsorbers are operated at a pressure of 8 atmospheres absolute. Other operation pressures would require other temperatures.

According to a preferred embodiment of this invention, the two adsorptive zones can also be superimposed in a single shell, so that equipment costs are reduced.

When it is desired to subject the gas mixture to an engine expansion, this can be done by means of an expansion turbine or the like which is placed between the hydrocarbon adsorber and the rectification column. A hydrocarbon adsorber provided upstream of the expansion turbine can be more readily desorbed with expanded separation products than an adsorber operating under a lower pressure, i.e., arranged downstream of the expansion turbine. This also results in more favorable economics for the process.

As a further extension of the invention, a part, about 4 to 10%, preferably 6 to 9% of the starting mixture is branched off from the minor portion after the latter has passed through the CO adsorber, but before being combined with the major portion coming from the regenerator outlet. This part of the minor portion is expanded to the pressure of the low pressure section of a two-stage rectification column, and then passed to the low pressure column.

Referring again to FIGURE 1, the preferred embodiment relating to an air separation plant will now be described in detail. It is to be understood, however, that the invention can be employed for the separation of other gaseous mixtures, particularly where H O or CD and hydrocarbon impurities, are present in the raw gas.

The air to be separated, amounting to about 50,000 Nm. h. is compressed in compressor 1 to about 8 atmospheres absolute and passed to the periodically reversible regenerators, designated 2, 3 and 14, 15. A minor portion of this amount of air, about 4,000 Nmfi/h. having a temperature of 135 to 145 K., is branched off from the regenerators at 4 and 5, passed through a C adsorber 6 containing, for example silica gel, aluminum oxide, or a zeolite.

The CO -free minor portion is rejoined at junction 7 with the major portion of the cooled gas mixture to be separated, exiting from the regenerators 2 and 3. The entire quantity of air then flows through the hydrocarbon adsorber 8 which can contain adsorption agents selective for hydrocarbons, for example, silica gel, aluminum oxide gel, or silicious zeolite. From there, the air is passed through expansion turbine 9 wherein the gas is engine expanded to about 5.6 atm. abs., and the resultant expanded gas is passed into the high pressure section of a two-stage rectification column 10.

At the bottom of the rectification column, an oxygenrich mixture is withdrawn, further cooled countercurrently against the pure nitrogen gas leaving the low pressure section of the column in the countercurrent supercooler 11, and, after being expanded to the pressure of about 1.2 atm. abs, the pressure maintained in the low pressure section of the column, the resultant liquid-vapormixture is passed into said section. The nitrogen rising from the bottom of the high pressure section condenses in the condenser-vaporizer 13, is withdrawn at that point, passed, for further cooling, through the countercurrent supercooler 12 in indirect heat transfer relationship with pure nitrogen gas, and expanded into the head of the low pressure section. At the top of the low pressure section, about 40,000 Nm. /h. of pure nitrogen is obtained and being passed through supercoolers 12 and 11, and then employed for cooling the regenerator 14.

Pure oxygen, in an amount of about 10,000 Nm. /h. can be withdrawn in the gaseous phase from the sump section of the low pressure section, and is then passed through regenerator 15 in order to cool same.

The dashed lines in the drawing represent the conduit system for the case wherein a part of the CO -free minor portion exiting from adsorber 6 is branched off at junction 16 before being combined with the major portion. This branched-off part, amounting to about preferably 6-9% of the air to be separated, is expanded to the pressure of the low pressure column in the expansion turbine 17, and passed to the low pressure column.

This embodiment of the invention is preferably used in large plants, eg for the production of 30,000 Nm. /h. 0 In smaller plants (capacity e.g. 5,000 Nm. /h. 0 only one expansion turbine will be used, provided upstream of the high pressure section of the two stage rectification column. Of course it would be possible to obtain the necessary refrigeration from one turbine also for large plants, but this would require a markedly increased starting pressure and the energy costs for the higher pressure of great quantities of air surmount those for the installation of a, second turbine.

Referring to FIGURE 2, a preferred embodiment of this invention having the two adsorption stages superimposed in a single shell, will now be described in detail.

There is an outer shell 18, preferably provided with a heat insulation, which shell has inlet openings 7 and 21 and outlet openings 16 and 22. If this apparatus were incorporated in the embodiment of'FIGURE 1, the openings 7, 16, 21 and 22 would be connected to the cold ends of regenerators 2 and 3, to the expansion machine 17, to the branch points 4 and 5. and to the expansion machine 9, respectively. The shell 18 comprises an upper portion 8 and a lower portion 6, both filled with an adsorbing agent, such as silica gel, aluminum oxide gel, or molecular sieve. The rooms filled with the adsorbing agent are bordered by sieves 19. In order to prevent any direct flowing of gas from the inlet opening 7 to the outlet opening 16 there are provided screens 20. With the apparatus shown it is possible to adsorb such substances as water, carbon dioxide and hydrocarbons from two gaseous mixtures which are introduced into the apparatus via conduits 7 and 21, the first mixture passing only the upper section and the second one passing both sections from the bottom to the top, provisions being made for branching off a part of the second mixture via conduit 16.

This apparatus has the definite advantage that its regeneration can be performed in one step, whereas, if two separate adsorbers were used, two regeneration steps would be necessary which would desire providing all the equipment (shell, conduits, etc.) twofold.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are prop erly, equitably, and intended to be within the full range of equivalence of the following claims.

What is claimed is:

1. In a process of separating gaseous mixtures by low temperature fractionating, which process comprises passing an impure gas mixture containing carbon dioxide and hydrocarbons as impurities into a cyclically interchangeable heat exchanger and cooling said gas mixture therein, a major portion of said gas mixture being cooled to a sufficiently low temperature to condense out carbon dioxide, and a minor portion of said gas mixture being removed from said heat exchanger at a point wherein the temperature is above the condensation temperature of CO passing said removed minor portion through a first adsorption zone to remove carbon dioxide and produce CO -free minor portion, combining at least part of resultant CO -free minor portion with CO -free major portion, and fractionating resultant combined mixture, the improvement which comprises the intermediate step of passing the combined CO -free minor and major portions through a second adsorption zone to remove hydrocarbon impurities from said combined portions, said intermediate step being conducted prior to the step of fractionating resultant combined mixture.

2. A process as defined by claim 1 wherein said heat exchanger is a regenerator.

3. .A process as defined by claim 1 wherein impure gas mixture is air.

4. A process as defined by claim 3 wherein said fractionating comprises rectification in a double column having a high pressure section and a low pressure section.

5. A process as defined by claim 4, further comprising engine expanding hydrocarbon-free combined portions prior to passing same to the high pressure section of the double column.

6. A process as defined by claim 5, further comprising the steps of engine expanding a part of said CO -free minor portion from said first adsorption zone, and passing resultant expanded part to the low pressure section of said double column.

7. A process as defined by claim 6 wherein said heat exchanger is a regenerator.

8. A process as defined by claim 4, further comprising the steps of engine expanding a part of said Co -free minor portion from said first adsorption zone, and passing resultant expanded part to the low pressure section of said double column.

9. A process as defined by claim 1 wherein said fractionating comprises rectification in a double column having a high pressure section and a low pressure section.

10. A process as defined by claim 9, further comprising engine expanding hydrocarbon-free combined portions prior to passing same to the high pressure section of the double column.

11. Apparatus for separating gaseous mixtures by low temperature fractionating which mixtures contain carbon dioxide and hydrocarbons as impurities, which apparatus comprises (a) four cyclically interchangeable regenerators or reversing heat exchangers provided with inlet and Outlet conduits at their warm and cold ends and with branch conduits positioned between the inlet and outlet conduits;

(b) two serially switched adsorption stages, each having an inlet and an outlet conduit, the inlet conduit of the first of said two stages being in communication with said branch conduits, the outlet conduit of said first stage being in communication with the inlet conduit of the second of said two stages, said inlet conduit of said second stage being in communication with said outlet conduits of said regenerators at their cold ends;

(c) an expansion turbine, having its inlet side in communication with said outlet conduit of said second adsorption stage; and

(d) a two-stage rectification column having -a high pressure section and a low pressure section, said high pressure section being in communication with the outlet side of said expansion turbine and said low pressure section being in communication with said inlet conduits of said regenerators at their cold ends.

12. Apparatus for separating gaseous mixtures by low temperature fractionating which mixtures contain carbon dioxide and hydrocarbons as impurities, which apparatus comprises (a) four cyclically interchangeable regenerators or re- Versing heat exchangers provided with inlet and outlet conduits at their warm and cold ends and with branch conduits positioned between the inlet and outlet conduits.

( b) two serially switched adsorption stages, each having an inlet and an outlet conduit, the inlet conduit of the first of said two stages being in communication with said branch conduits, the outlet conduit of said first stage being in communication with the inlet conduit of the second of said two stages, said inlet conduit of said second stage being in communication with said outlet conduits of said regenerators at their cold ends;

(c) a first expansion turbine, having its inlet side in communication with said outlet conduit of said second adsorption stage;

(d) a second expansion turbine having its inlet side in communication with said outlet conduit of said first of said two adsorption stages; and

(e) a two-stage rectification column, having a high pressure section and a low pressure section, said high pressure section being in communication with the outlet side of said first expansion turbine and said low pressure section being in communication with said inlet conduits of said regenerators at their cold ends and with the outlet side of said second expansion turbine.

References Cited UNITED STATES PATENTS NORMAN YUDKOFF, Primary Examiner. V. W. PRETKA, Assistant Examiner.

Claims (1)

1. IN A PROCESS OF SEPARATING GASEOUS MIXTURES BY LOW TEMPERATURE FRACTIONATING, WHICH PROCESS COMPRISES PASSING AN IMPURE GAS MIXTURE CONTAINING CARBON DIOXIDE AND HYDROCARBONS AS IMPURITIES INTO A CYCLICALLY INTERCHANGEABLE HEAT EXCHANGER AND COOLING SAID GAS MIXTURE THEREIN, A MAJOR PORTION OF SAID GAS MIXTURE BEING COOLED TO A SUFFICIENTLY LOW TEMPERATURE TO CONDENSE OUT CARBON DIOXIDE, AND A MINOR PORTION OF SAID GAS MIXTURE BEING REMOVED FROM SAID HEAT EXCHANGER AT A POINT WHEREIN THE TEMPERATURE IS ABOVE THE CONDENSATION TEMPERATURE OF CO2, PASSING SAID REMOVED MINOR PORTION THROUGH A FIRST

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