US1959030A - Method of separating gaseous mixtures - Google Patents

Description

May 15, 1934. l. H. LEVIN METHOD OF SEPARATING GASEOUS MIXTURES Filed July 24, 1930 Patented .May 15, 1934 UNITED STATES PATENT OFF/ICE 1,959.030 METHOD OF SEPARATING GASEOUS This invention relates to the separation of air and other gaseous mixtures into their constituents by liquefaction and rectification. While my improved method may be advantageously employed for the separation of gases generally from their mixtures, the method will be described with reference to the separation of air into its constitutents.

The present invention, like other prior processes of separating air into its constituents, contemplates the compressing and cooling of air; also, the passage of the treated air to the rectifying column. r

In processes of this character, the heat inflow into the system must be counteracted by a sufficient production of cold, and a portion of this cold must be in the form of liquefied air or the like, to supply the liquid that is evaporated in the rectification column through the heat inflow into the column.

In the practice of my invention, the air is divided into two or more circuits. One circuit is compressed to approximately the pressure .required for the first stage of rectification, about 5 atmospheres pressure. Another circuit may be compressed to the pressure required by a later stage of rectification, namely, 1 atmosphere or less. Another circuit is compressed to an appropriate pressure suitable for the production of cold, and is pre-cooled from room temperature by ammonia andthe like. It is this latter circuit that produces practically all the cold that is needed.

One of the advantages of my invention is that the cost of producing the necessary cold is lowered. A further advantage is that the cost of the equipment is very much reduced.

Plants built to operate along the lines of my invention, make it possible to produce oxygen cheaply enough to be used as, for example, in the metallurgy of iron where it is not yet extensively employed because of the high present cost of oxygen.

usual manner before reaching 5. In interchanger 'In the high pressure portion 21 of the rectify- Further advantages of my invention and means for obtaining these advantages will be clearly set forth in the description of an apparatus that can be employed as a matter of example to carry out the invention. The accompanying drawing shows schematically an apparatus in which the process may be practiced.

Referring to said drawing, air or other gaseous mixtures is compressed in a compressor 6 and is supplied to an interchanger 4 as through a pipe 5. This air is compressed and purified in the '4, the air is cooled by heat transfer contact with .a refrigerant such as ammonia or the like.

The refrigerating system is operated, for ex-, ample, by the apparatus represented in the drawing, in which a compressor 1 is used for compressing the ammonia. densed in 2 and expanded in valve 3. The expanded ammonia enters into interchanger 4, and comes in heat transfer contact preferably by counter current with the air to be cooled and is then again compressed and the cycle is repeated.

'The above refrigerating cycle is given only as 'a matter of example, and can be varied in many ways, so long as the desired refrigerating effects a are produced upon the compressed air.

A valve is provided at 7 to drain any moisture that is collected in the interchanger.

The air leaving the interchanger may be expanded into expansion valve 8, or in the expansion engines 9 and/or 10, into the interchanger place through the engine without being previously expanded in' the valve. Furthermore, the work done by both engines may be combined in one unit or but a single stage engine need be used. The expansion can thus take place in either of the units shown or in the combination of all of them.

In addition to the above refrigerating circuit, another stream of air, either at the pressure of the high pressure column, or somewhat higher pressure, iscompressed in compressor 34, enters interchanger 13, as through pipe 14, where it is cooled in counter current by the cold products of separation delivered by the column, as, for example, the nitrogen through pipes 18 and the oxygen through pipes 19. A valve can be used as at 17 to withdraw moisture from interchanger 13. The cooled air leaves. interchanger 13 through pipe 15 and passes through valve 16. If the air in interchanger 13 is approximately at the pressure of expanded air going into interchanger 11, valve 16 is wide open, but if the air is at a higher pressure, the valve 16 can be used to reduce the pressure. After the air leaves valve 16, it joins the air in 12 already described.

The air leaving interchanger 11 enters the high pressure column 21, first passing through valve 20, which may be opened full or used to expand the air in the high pressure column as the case might be.

The ammonia is con-" ing column, the vapors and condensate pass through perforated plates 22 of the column, or

high pressure section 21 and engages a condenser 23 therein. Such condensed nitrogen or nitrogen liquid drops back partly into a collecting trough or chamber 24 and partly back into the high 'pressure' section 21, trickling along the plates or 'baflles 22 to eifect the separation. 30

designates a pipe leading from the condenser 23,

and is to carry off the noncondensed gases from said condenser, said pipe 30 being provided with a valve 31.

- Atthe bottom of said high pressure portion 21, a mixture of liquid nitrogen and oxygen collects at 25, and the same passes from section 21 through a pipe 26 having a regulating and expansion valve 27 therein. Pipe 26 introduces the mixture ofnitrogen and oxygen into the low pressure portion of the rectifying column 32, which is designated 33. The nitrogen liquid from trough or compartment 24 passes'through -a pipe 28 having a valve 29 therein, to the top of the low pressure section 33 of the rectifying column. From said low pressure portion 33 of the rectifying column, the products of separation, namely, nitrogen and oxygen, pass respectively through pipe 18 and 19, the former conducting the nitrogen from the coldest point or zone of the low pressure section 33 of the rectifying column. I

While I have referred to the heat transfer means employed at 11 and 13 as interchangers, this term is used broadly, since equivalents may be substituted. as, for instance, recuperators, or instead of interchangers, regenerators may be employed.

The cold products of separation pass into interchanger 11 where they cool the combined portions or circuits of air, where they may effect a condensation and the condensate and the saturated vapors pass into the high pressure portion of the rectification column. The cold products of separation are warmed but are still considerably colder than room temperature. These cold products then pass into interchanger 13- where they heat transfer-with another portion of the air and are herein brought to approximately room temperature.

One advantage of my invention is that all the' air treated is rectified and thus separated into oxygen and nitrogen.

A further advantage is that the high pressure air can be expanded to the pressure of the column instead of to only 40 atmospheres, that is, about the critical pressure, thereby getting a much greater amount of refrigerating effect from the expansion. s

In the present invention, I take advantage of the property of air or other gaseous mixtures at low temperatures and relatively low pressures to abosorb large quantities of frigories by heat transfer.

This takes place in interchanger 11,- wherein other useful purposes are accomplished, namely, in methods of separating air into its constituents by liquefaction and rectification, it is highly advantageous that the air be delivered tothe column at the pressure of the column, that is to say, to either the high pressure section of the column, or the low pressure section thereof. The delivery of air. at such pressures, however, to the rectificationcolumn entails dimculties, such, forexample, as arise from the fact that the expanded air is not fully conditioned before entering the column, to provide the necessary cold to counter act heat losses of the same.

In order to properly condition the air before its introduction into the rectifying column, I find it to be extremely important to subject the expanded air to a product or products of separation having a very low temperature.' This may be accomplished by taking ofl, for example, the

nitrogen at the top of the column and then subjecting the expanded air to heat transfer with this -very cold nitrogen. The product of separation is brought into heat transfer relation with the air at 'a temperature below the temperature and pressure of the expanded air.

This has a further advantage in that air is furnish@ to thecolumn in the form of a saturated vapor. This results in a great regularity of operation, since the changes which tend to upset 'the equilibrium in the column are reduced to a minimum, the air enters at practically a constant temperature with a more or less greater proportion of. condensate.

It is one of the advantages of my 'invention that the cold producing unit is flexible and can be adapted to conform with what is the most appropriate and economical way to bring about the abstraction of heat in the .air. Compare the above method with the process where the air to be expanded is branched off from another circuit of air under pressure, where the pressure may be fixed and cannot be modiiied to conform to the pressure best required for. the circuit that is to be expanded. No other stream needbe carried at high pressure in order to balance out properly the circuits and permit the cold products of separation -to leave the interchanger at room temperature.

It can be shown that, although one may start with the air at two different pressures, say 200 atmospheres and 100 atmospheres, one can, nevertheless, condition the air to the same end point, for example, 175C. at 5 atmospheres, precooling the air in each instance to the same temperature, say -50 C. In the case of 200 atmospheres, air is expanded into the expansion 'valve to 50 atmospheres and then in the engine to 5 /atmospheres. In the case of the 100 atmospheres, the air is expanded directly in an engine from 100 atmospheres to 5 atmospheres.

The above is but an example of the combination of pressure, cooling temperature and methods of expansion, that is, with or without external work that could be employed for producing the desired cooling effect.

It will also be noted that only a portion of the air is compressed to high pressures. In view of the fact that at the high pressure, the energy required to produce a unit of cold is much lower than at the lower pressure, only a comparatively small portion of the air need be compressed to the high pressure. It is one of the advantages of my invention that only about 5% of the total amount of air handled need be so compressed in order that the proper amount of liquid necessary to carry out the rectification be produced.

In order'that both streams of air be at the same'temperature when they join to enter interchanger l1, and especially that the cold products of separation leave the interchanger at practically the room temperature, the streams of air must be properly proportioned as to amounts. The quantity ineach stream, in'turn, will depend upon the initial pressure of the air in each stream and the treatment the air will undergo. No matter what amount of interchanger surfaces is used, if the proper proportioning and conditioning are not provided for, the cold products of separation will not leave the interchanger at practically room temperature, and will thus make it necessary for extra cold to be produced by the refrigerating circuit or by the circuit that enters interchanger 13.. r -In designing equipment cordance with my invention, one may consider that a circuit of' air, (called the first circuit),

after a preliminary cooling by ammonia and the like, and expanded, enters interchanger 11, and that cold products of separation leave interchangerll at considerably below room temperature, and that another circuit of air (called the second circuit), is used as in interchanger 13 to warm these cold-products of separation to room temperature. After this preliminary cooling of the second circuit, both circuits join and are further cooled in inerchanger 11.

Assuming that 2% of the air is to be liquefied,-

to overcome heat inflow, what is the amount of air that can be treated in interchanger 13 if the air from the first circuit enters interchanger 11 at 100 C., and what must the pressure be in interchangers 13 and .11.

The proportion of air in the two circuits, when a balance is reached (that is, the cold products of separation will leave the interchanger at the temperature of the air entering and that the first and second circuit will join at pipe 12 at practically the same temperature, assuming that the interchangers are designed with ample surface for complete heat transfer), can be shown to be such that one pound of airconditioned to 10 atmospheres and 100 C. atthe point of entering line 12 will permit of the use of nineteen pounds of air entering interchanger 13 at 10 atmospheres pressure. It will be noted that 100 C. is very easily attainable in the pre-' liminary cooling with gases such as ethylene, and the process can be used by substituting ethylene for ammonia, and dispense with expansion engine cooling. For a similar production of 2% liquefaction, if one brings the air of the first circuit to -l40 C. by the preliminary cooling, then the air entering interchanger 13 need be but -6 atmospheres.

The problem may be varied for a study of lower pressures for the air entering interchanger 13, temperatures of cooling of the first circuit, varying amounts of liquefaction required, etc. If the oxygen is to be withdrawn as liquid, about 15 to 20% of the air will have to be liquefied. The quantity of air in the various circuits and the amount of cooling of the air of the first circuit will have to be sufficient to give the required overall cooling.

The word gas as used in the claims may refer to either a gas like N11, or a gas like air,

that is, to an element or a mixture of gases. The

for operating in ac-- product of separation while said second portion is at a temperature substantially higher than the temperature of the first'portion after the first portion has been cooled, whereby 'said second portion is cooled and the product of separation is warmed to substantially the entering tempersecond portion by an external refrigerant prior to rectification.

2. The method'of separating a gas into its constituents, comprising compressing one portion of gas, cooling it by non-regenerative cooling in order to increase the refrigerating capacity of the system, expanding it to substantially the pressure required for rectification, further cooling said first portion by a product of separation, compressing a second portion of gas, cooling the second portion by a product of separation while said second portion is at a temperature substantially higher than the temperature of the first portion after the first portion has been cooled, whereby said" second portion is co led and the product of separation is warmed to substantially the entering temperature of said second portion, expanding said second portion to substantially thepressure required for rectification, and subjecting both portions to rectification without cooling said second portion by an externalrefrigerant prior to rectification.

3. The method of separating a gas into its constituents, comprising compressingone portion of gas, cooling it by non-regenerative cooling in order to increase the refrigerating capacity of the system, compressing a second portion of gas, cooling said second portion. by a product of separation while said second portion is at a temperature substantially higher than the cooled first portion, whereby said second portion is cooled and the product of separation is warmed to substantially the entering temperature of said second portion, combining the cooled first portion and. cooled second portion, further cooling the combined portions by a product of separation, and subjecting the combined portions to rectification. f

4. The method of separating a gas into its constituents, comprising compressing one'portion of gas, cooling it by non-regenerative cooling in order to increase the refrigerating capacity of the system, expanding it, compressing a second portion, cooling said second portion by a product ofseparation while said second portion is at a temperature substantially higher than the cooled first portion, whereby said second portion is cooled and the product of separation is warmed to substantially the entering temperature of said second portion, combining the cooled first portion and cooled second portion, further cooling the combined portions by a product of separation, and subjecting the combined portions to rectification.

5. The method of separating a gas into its constituents, comprising compressing one portion of gas, cooling it by non-regenerative cooling in order to increase the refrigerating capacity of the system, compressing a second portion of gas, cooling said second portion by a product of separation while said second portion is at a temperature substantially higher than the cooled first portion, whereby said second portion is cooled andthe product of separation is warmed to substantially the entering temperature of said second portion, cooling said second portion by a product ofseparation while said second portion is at a temperature substantially higher than the cooled first portion, whereby saidsecond portoin is cooled and the product 01' separation is warmed to substantially the entering temperature 01 said second portion, combining the cooled first portion and cooled second portion, further cooling the combined portions by a product of separation, expanding said combined portions, and subjecting the combined portions to rectification.

ISAA'C LEVIN.

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