US2666303A - Apparatus for the separation of gas mixtures by liquefaction and rectification - Google Patents

Description

Jan. 19, 1954 P. M. SCHUFTAN 2,666,303

APPARATUS FOR THE SEPARATION OF GAS MIXTURES BY LIQUEFACTION AND RECTFICATION Filed June 8, 1950 INVENTOR Bwz. Mnumcz ficfluFrAxv BY a? W STZQL,

ATTORNEY Patented Jan. 19, 1954 APPARATUS FOR THE SEPARATION OF GAS MIXTURES BY LIQUEFACTION AND REC- TIFICATION Paul Maurice Schuftan, Richmond Hill, England,

assignor to The British Oxygen Company Limited, London, England, a British company Application June 8, 1950, SerialNo.166,975

zolaims. (01. 62-123) The present invention relates to the liquefaction or separation of gas mixtures.

For the liquefaction or separation of gas mix--' tures, a method of providing at least a part of the necessary cold requirements is to subject a part or the whole of the initial gas mixture or of at least one of its separation products to isentropic expansion. Attempts have been made to transform the energy of expansion into useful work but it has proved difiicult to utilise this work in the liquefaction or separation process itself.

In the case where the expansion is performed in a reciprocating expansion engine, the usual method is to couple the engine by belt drive with the compressor used for the initial compression of the gas mixture to be liquefied or separated. A disadvantage of such an arrangement is that the compressor and the expansion engine have to be positioned near to each other, and this usually imposes limitations in the arrangement of the plant. Further, a belt drive is not very efilcient and not feasible in plants handling explosive gases. Moreover, the speed of the expansion engine is fixed which imposes a limitation with regard to the flexibility of operation.

It has also been proposed to provide the expansion engine with an extra cylinder in which the final compression of the gas mixture to be liquefied or separated is accomplished;- while this arrangement has the advantage that the positions of compressor and expansion engine are independent and also avoids undesirable andinefiicient belt drive, it does not provide freedom of speed for the expansion engine. In fact, any variation of the speed would reflect bacl; on the stage pressures of the 'main' compressor.

In rotating expansion engines suchasi turbines, hydraulic or electric brakes are usually preferred. While hydraulic brakes, for example pumps, could in most cases be coupled directly to the expansion engine, even in the case of high speed turbines, the characteristics of turbine and pump are generally difierent sothatadjustments of thefb ralie are required whenever the speed of the turbine fluctuates. In addition, there is seldom an eihcient use for the water or other liquid under pressure provided by such an hydraulic brake.

Electric brakes, on the other hand, require a somewhat large and costly reducing gear and if the current produced is to be utilised to supplement that in the supply mains, the electric brake will have to be run at a synchronous speed, thereby losing the freedom of speed in the turbine.

An object of the present invention is to provide a method of and means for converting the energy of expansion into useful work which obviate the difficulties described above.

According to this invention in a process for liquefying or separating a gas mixture in which at least'a part of the necessary cold requirements is derived by isentropic expansion, the energy of expansion is applied to reduce to a value below atmospheric the pressure at which the non-liquefied part of the gas mixture or one of the separation products thereof, leave the liquefaction or separating device. By utilising the energy of expansion to effect direct pressure reduction in this manner a self compensating system can be established since any fluctuation in the amount of gas to be expanded will, as a rule, correspond to similar fluctuations of the product from the liquefaction or separation unit/ The means whereby the energy of expansion is applied to reduce the pressure comprises a pneumatic brake such as a vacuum pump directly coupled to the expansion engine.

The characteristics of expansion engine and brake can be made very similar as they are both pneumatic devices, so that the whole unit will be,

' to a large extent, self-adjusting. The work suppliedby the expansion engine can be utilised with high thermodynamic and mechanical eificiency in' the liquefaction or separation process as will be clear from a consideration of a specific case, namely, the separation of air with the production of commercially pure oxygen and a waste nitrogen fraction.

As is well known in the art the separation of air can conveniently be effected in a double column, the upper column of which operates at a pressure only slightly in excess of atmospheric. In such case the invention may be carried into effect by passing the waste nitrogen fraction through the pneumatic brake and the pressure at the outlet of the'separation unit lowered thereby to a value below; atmospheric pressure. This will in turn lower thepressure of the upper column and the temperatureof the oxygen boiling in the condenserr As a result; the pressure of the lower column will drop, which in turn will reduce the pressure to which the air to be separated is to be compressed, and thus reduce the power consumption of the plant. If the oxygen is to be produced as liquid, the flash loss at its withdrawal from the condenser will be reduced.

Instead of increasing the thermodynamic effi ciency of the process as above, the extra pressure potential provided by the pneumatic brake may be utilised for improving heat exchange conditions. In this case, a higher pressure drop can be tolerated in the heat exchanger for the waste nitrogen stream and in consequence the exchanger could be made either more efiicient or less expansive.

For coupling to expansion turbines, an axial flow blower is preferred as a pneumatic brake. This can run at the same speed as the turbine and although this type or blower is normally rather susceptible to dust, it can be used without difficulty in the present case as the waste nitrogen is dust free.

In the case of reciprocating expansion engines, the pneumatic brake will be in the form of an extra cylinder operable to bring about the desired pressure reduction in accordance with the invention the arrangement being similar to those cases in which the expansion work is used for additional compression, with the important difference that freedom of speed is obtained.

The accompanying drawing is a diagrammatic illustration of an arrangement by which the process of this invention is practiced.

Referring to the drawing, which relates to the application of this invention to a plant of known type for the production of gaseous oxygen using a reciprocating expansion engine, feed air after compression in compressor I passes through heat exchanger 2 and is cooled by indirect heat exchange with the two product gas streams. Part of the air after passing through the whole length of the exchanger 2 is expanded in an expansion valve 3, and the resulting mixture of liquid and vapour is fed to the bottom of column 4. The remainder of the air is taken out of the exchanger 2 at an intermediate point and passes to the expansion engine 9, where it is expanded to the pressure of column 4, then passing to the bottom of column 4.

The re-combined air stream passes up through trays in column 4 to the condenser 5 where it is condensed. Part of the liquid thus formed returns down the column in contact with the surrounding vapour, becoming enriched in oxygen. The vapour entering into condenser 5 is correspondingly enriched in nitrogen. A proportion L of the liquid nitrogen formed in the condenser 5 passes through an expansion valve 8 and oes to the top of column 6. The enriched liquid in the bottom of column 4 leaves by way of expansion valve i and passes to an intermediate point in column 6. This column has the usual trays over which the liquid fiows in contact with the vapour rising to produce a further enrichment of the liquid in oxygen. At the bottom of column 6 this oxygen enriched liquid is evaporated in condenser 5 thereby producing liquid reflux in column 4. A fraction of the oxygen enriched vapour leaves the bath surrounding the condenser 5, leaves the column and passes to the heat exchanger 2 Where it is warmed by indirect heat exchange with the incoming air stream. The nitrogen enriched vapour reaching the top of column 6 also passes to the heat exchanger 2 and is warmed by indirect heat exchange with the incoming air stream. After exchanger 2 the nitrogen fraction passes through an axial flow blower l l which is directly coupled through the shaft 10 to the air expansion turbine 9. The pressure of the nitrogen at the outlet of the column 6 is lowered and this in turns lowers the temperature of the oxygen boiling in the condenser 5 and consequently the pressure of column 4 will drop. Thus the pressure to which the air must be compressed in compressor I and. hence the power consumption of the plant is reduced. Alternatively in a large scale gas plant the expansion machine 9 and vacuum pump H may be of the rotary type.

Correspondingly in a liquid oxygen plant the arrangement will be similar to that shown in the figure with the exception that the oxygen product, instead of being withdrawn as vapour from above condenser 5 and warmed up in exchanger 2, will be withdrawn as liquid direct from condenser E. The remaining unliquefied portion of the air will leave the plant as waste nitrogen and will, as before, be compressed in vacuum pump II.

I claim:

1. Apparatus for the separation of a gas mixture by liquefaction and rectification into a higher-boiling fraction and a lower-boiling fraction including a rectification zone wherein said gas mixture is separated into said product fractions, an expansion turbine for subjecting at least a part of said gas mixture to isentropic expansion to provide at least a part of the cold requirements of said separation apparatus, and an axial flow blower directly coupled to said turbine and adapted to rotate at the same speed as said turbine, for reducing to a value below atmospheric the pressure at which one of the said product fractions leaves said rectification.

2. Apparatus for the separation of air by liquefaction and rectification into an oxygen fraction and a gaseous nitrogen fraction, including a rectification zone wherein the air is separated into said product fractions, an expansion turbine for subjecting at least a part of the air to isentropic expansion to provide at least a part of the cold requirements of said separation apparatus, and an axial flow blower, directly coupled to said turbine and adapted to rotate at the same speed as said turbine, for reducing to a value below atmospheric the pressure at which the said gaseous nitrogen fraction leaves said rectification zone.

PAUL MAURICE SCHUFTAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,571,461 Van Nuys et al Feb. 2, 1926 2,000,992 Schlitt May 14, 1935 2,097,434 De Baufre Nov. 2, 1937 2,116,191 De Baufre May 3, 1938 2,355,167 Keith Aug. 8, 1944.-

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