US2283232A - Cooling of gaseous mixtures - Google Patents

Description

May 19,` 1942- P. M. scHUFTAN COOLINGOF G'AsEoUs MIXTURES l Filed Maren 28, 1940 N M F w a m E M M L U m Patented May 19, 1942 UNITI-:p4 STATES PATENT oFF'lcE COOLIG F yGASEOUS MIXTURES Pani Maurice Schuftan, Richmond Hill, England,

assignor to The British Oxygen Company Limited, London, England Application March 28, 1940, Serial No. 326,466 i Y In Great Britain Match 24, 1939 9 claims (chez-122) Thismventin relates to the cooling of gaseous mixtures, and is particularly applicable to a process and apparatus for precoling and prepurif'ying gaseous mixtures in periodically alternating regenerators or cold storers.

During the cooling of the gaseous mixture inone of such regenerators, condensation will occur on the surface of the storage mass. In the erato'rs bythe linflux of heat from an exterior source is prevented. This is accomplished by absorbing heat from the insulating material in an amount sumcient to compensate for the amount of-.het passing through the insulating material from' the exterior to the reg'enerators.

case of coke-oven gas, for example, water vapour,

vcarbon dioxide, hydrogen sulphide and. also propylene, propane and other relatively high boil' ing point hydrocarbons will be condensed and retained in the regenerator. It isessential that such condensates retained in the regenerator be l spondingly lower temperature than that whichv would have been required in the next reversal period for cooling the warm 'gas to the desired temperature, if no influx of heat had occurred. This practice necessarily resulted in an increase of the temperature diiIerence at the coldend oiV the regenerators, and this extra temperature difference made the revaporization or resublimation of condensates deposited in the regenerators It was also necessary to maintain high pressure ratios for the purpose of insuring the complete removal of the condensates during the cooling down period and of avoiding an accumulation of By providing a cold shield in accordance with the invention it is possible to obtain a lower temperature difference at the cold end of 'the-regenerators and to operate with a lower pressure ratio between the gas in the warming up and cooling down periods with a consequent increase in the eiliciency of the plant.

Alternatively, if the pressure ratio be maintained higher than the minimum necessary for revaporisation, the provision of the cold shield will result in a substantial decrease in the content.ot impurities in the pre-cooled gases. This is o! particular advantage in cases such as the treatment of coke-oven gas for extracting ethylene, where the condensation=` range of the impurity-carbon dioxide-partly' overlaps that of ethylene.

In both cases owing to the reduced temperature difference at the respective cold ends of the regenerators, the gas can now be precooled to a lower temperature than would be possible if no cold shield were provided. In the former case, the amount of impurities left in the precooled gas will not be decreased but there is the advantage of operating with a reduced working pressure with a considerable Saving in power; in the latter case the amount of impurities is reduced in profportion to the increased working pressure.

The cold required for establishingl the cold shield may be derived fromV any convenient source of cold. A part of the fluids from a later cooling stage may be diverted and circulated around the peripheral portions of the storage material. For

said condensates with consequent danger of ob-y struction in subsequent steps. Thus, higher pressure ratios between the warm and the cold gar/lv were employed than would have been necessary if no heat could have entered the regenerators.-

that is to say, if the regenerators could be operated` as an ideal adiabatic system. It will be appreciated that the higher pressure ratios employed resulted in high power consumption for the separation process. i'

According tothe present invention, in a process for precooling and 4prepuriiying gaseous mixtures by means of periodically alternating regenerators surrounded by heat insulating material, interference with the thermal conditions in the regenexample, where a refrigerating cycle is used to' obtain additional cold at a later stage, a part of the reirigerating medium in the cycle may be diverted for this purpose in liquid or gaseous form. By using a liquid a larger amount of cold is available at the cold end of the regenerators and as a result of the vaporisation of the liquid the available cold progressively decreases towards the-warm end of the regenerators. As thecold losses through the lagging also progressively decrease towards the warm end, the use of a liquid in the cold shield permits of a ready adjustment of the supply of cold to the demand.

-An independent supply of cold equivalent 'to `the ingress of heat from the exterior into theregenerators oiers a further advantage affecting partly or wholly produced by compressed gas of 5 'which a part passes through a liqueiier cooled by cold expanded gas, while the balance is adiabatically expanded in an engine. and if a part of the liquid obtained is used in the cold shield, the

amount of cold gas returning into the liqueiler is diminished, and therefore a greater proportion of the compressed gas than before can be expanded in the engine. This results in an increased cold production. Alternatively, if the amount of gas expanded in the engine is kept the same as bel5 fore, the temperature difference in the liquefler increases and its suriace can therefore be dlminished. 4

, One embodiment of the invention is shown by way of example in the accompanying drawing in which:

'Fig.'1 shows in elevation and somewhat diagrammatically a pair ofregenerators encased in the usual lagging;

Fig. 2' is a plan view of Fig. l, the lagging over- 25 lying the regenerators being removed in order to expose the cold shield;

Figs. 3 and 4 are sectional drawings of lparts of the apparatus on an enlarged scale, showing the preferred shape and method of mounting o`i the cold shield; and

Fig. 5 depicts a diagrammatic representation of a process according to the invention.

Referring to the drawing, two regenerators S and l, each containing a storage mass and aras indicated by the arrows 'A and B.' the coldest 40 parts of the. regenerators being at the upper ends.

In order to compensateq for the ingress oi heat to the storage mass, the regenerators are, in accordance with the invention.. provided with a cold shield, which in the example illustrated is 4 shown to be a helical tube l through which a liqueiied gas is circulated, the inlet being at the upper end of regenerator l and the outlet at the lower end of regenerator l, the direction of cold rows C and D. The liquid will be vaporlsed after traversing'some of the turns, say ilve turns, of the helical tube 9.

As shown, the pitch of the helical tube in- 1 A Vcreases progressively towards the lower end: the 255 pitch is determined by the temperature gradient in the regenerators and the heat. transfer conditions.

wnue it is generauy convenient u use metal both for the cold shield and for the walls ot the regenerator, it may, in view of the high heat `conductivity of the metal, and the high heat transfer coemclent ofboiling liquid, be n to reduce the rate of heat conduction between the wall of the regenerator and those turns of the helical tube in which the liquid vaporlses, in

, ,order to avoid the regenerator walls being undercooled locally. As a matter of fact, such an under-cooling of the regenerator wall and subsequently of the gas traversing the regenerator 70 reduction in neat conductivity may be achieved 7:5

by inserting b etween the regenerator wall I l and the helical tube I a vlayer of' heat insulating material II. A plurality of brackets I2, spaced at intervals and secured to the wall I0 may be provided tohold the tube l in position. Atthe lower or warmer parts of the regenerators, where the liquid has become vaporised, the

, tube 9 may be secured directly to the wall I0, as,

for example, by solder I3 (Fig. 4).

The helical tube may, in cross section, be circular, rectangular, or other convenient shape, but it is preferred that the side adjacent to the regenerator wall be substantially tlat, as is shown in the drawing, in order to provide good heat transfer conditions betweenV the tube and the regenerator.

In order to minimise cold losses by convection. it is usually the practice to mount in a common casing the regeneators and other units, such as tubular heat interchangers and rectification co1- umns in which further cooling and separation of the gases which are 'pre-cooled in the regenerators takes place, the whole being packed in heat insulating material. l'

In such cases, cold loses in the regenerators occur only on the' side remote from the other units and, as shown4 in Fig. 2, the cold shield need then extend only over those parts of the regenerator walls from which cold losses occur. An arrangement of apparatus wherein a part of a gas to be separated is comp to'a slight pressureV and cooled in regenerators, while another part is compressed to a high pressurel and cooledby expansion, is illustrated atically at Fig. 5. 'Ihe apparatus shown comprises two regenerators l, C arranged for alternate operation as a regenerative system in a manner well known to the art. When separating air, for example, the major portion of the-air is compressed to a slight pressure and passes through regenerator l, as shown by the arrows in the` gure relative to a single reversalperiod, it being assumed that regenerator I has been precooled in the Aprevius period as described infra. 'I'he air' then passes into a separating device I, whichmay be of any appropriate form, such as a rectiilcation column with the usual condenser and vaporiser, as those skilled in the art 'will understand. The minor iluid through the tube being indicated by the arportion of the air is compressed to a high pressure, is cooled in a tubular heat'interchanger 2,

is expanded through, the throttle valve I,v and thence passes into the separator I. In the Aseparator the air is separated into an oxygen fraction and a nitrogen rinaction in a manner wellr known to the art. The nitrogen fraction is returned into regenerator l to cool the same for,

the succeeding reversal period. 'I'he oxygen fraction through the tubular intere er 2 in 00 heat exchange with the highpressure a Before entering the heat interchanger 2, a small portion of the cold oxygenJraction-from the separator I is branched oi! and supplied toa cold shield I which. as shown in detail in Figs. 1 and 2, extends wholly or partially around the regenerators.

It the expansion' of the high pressure air through the throttle valve I results in the production of an insumciently low temperature in said air, a further amount of heat can be absorbed therefrom by passing a portion of the high pressure air through an expansion engine l. with recovery of external work.k 'I'he portiorl of the air which is thus expanded is then ref united with'the air expanded through the throttle valve 3.

It will be appreciated that, in certain cases, especially where the cold for the cold shield is derived from a cold gas, the amount of cold gasV passed through the cold shield may diminish progressively or in stages towards the warm end of the regenerators. Further, instead of using a helical tube for the cold shield, the storage mass of the regenerator may be housed within the -inner vessel of a double walled container, the

space between the two walls being adapted for circulating a cold fluid and thereby constituting the cold shield.

For the cold shield, any convenient cooling medium may be used, but in the case of a liquid A its boiling point should be at a temperature somewhat lower than the lowest temperature to which the gas traversing the regenerators has to be cooled. In the case of the separation of air, either liquid nitrogen or liquid oxygen may be uid from at least one tubular heat exchanger.,

and absorbing sufiicient heatfrom the heat insulating material in said heat absorbing uid to compensate for heat'passing through `said material from external sources to the regenerators.

2. In a process for separating a gaseous mixture into at least two components which includes compressing a portion of said gaseous mix.

ture to a slight pressure, passing said compressed portion through a precooled regenerator surrounded by heat insulating material to cool said portion, compressing the remainder of said gaseous mixture to a high pressure, dividing said remainder into two highly compressed portions,

which cold lossesl occur for conveying a heat absorbing fluid therethrough `whereby heat from external sources passing through said heat insuy used, while in the case i the extraction of ethyllating ma rial can be absorbed.'

4. An improved heat transfer device for precooling and prepurifying a gaseous mixture which comprises a regenerator surrounded by heat insulating material, and a helical conduit extending around the regenerator for conveying a heat absorbing uid whereby heat passing through the heat insulating material from external sources' to the regenerator can be absorbed. l

5. The heat transfer device set forth in claim 4 wherein the pitch of said helical conduit increases from the, cold end of the regenerator toward the` warm end of the regenerator.

v6. A process for precooling and prepurifying a gaseous mixture, which comprises passing a gaseous mixture through a precooled regenerator chamber whilst passing a fraction of cold gas Y through a second chamber of a pair of alternating regenerator chambers surrounded by heat insulating material, and absorbing heat from the portions of said insulating material surrounding the chambers, from which cold losses occur in an amount suicient to compensate for heat absorbed by said insulating material from external sources.

'7. A process for precooling and prepurifying a gaseous mixture, which comprises passing a gaseous mixture through a precooled regenera-` tor chamber whilst passing a fraction ot cold gas through a second chamber of a pair of alternating regenerator chambers surrounded by heat insulating material, and passing through a restricted path in the portions of said heat insulating material surrounding the chambers kfrom passing one .of said highly compressed portionsinto a tubular heat exchanger having a heat absorbing medium, expanding said highly compressed portion through an expansion valve,

adiabatically expanding the. other highly com'v pressed portion, /passing both the expanded highly compressed gas and gas emerging from the regenerator into a separation device, and recovering at least one component thereof, the improvement comprising passing at least a part of the component recovered into the heatin-v sulating material surrounding the regenerator, and absorbing therein sumcient heat tov compensate the amount oi' heat passing through said heat insulating material fromvexterfnal sources to the regenerator. l

3. An improved heat transfer device for pre-u cooling and prepurifying a gaseous mixture which comprises a pair of alternating regenerators surrounded by heat insulating material, and a conduit in said heat insulating material forming a cold shield over said regenerator walls from which cold losses occur, a huid at a temperature sufliciently low to absorb heat passing through the insulating material from outside to the heat transfer chambers, whereby heat from external sources is compensated.

8. A process for precooling andvprepurifying a gaseous mixture, which comprisespassing a gaseous mixture through a precooled regenerator chamber whilst passing a fractionvof cold sas through a second chamber o! a pair of alteinating regenerator chambers surrounded by heat insulating material, and passing back and forth through a restricted path in said heat insulating material surrounding the vchambers from which cold losses occur, a fluid at a temperature sumciently low to absorb heat passing through theV insulating material from outside -to the heat transfer chambers, whereby heat from externa sources is compensated.

'9. An improved heat transfer device for precoolingfand prepurifying agaseous mixture which comprises alpair of alternating regenerators surrounded by heat insulating material, and a conduit passing back and forth in said heat"insu lating `materialforming a cold shield overl said regenerator walls from which' cold losses occur for conveying a heat absorbing duid therethrough whereby heat from external sources passing through said heat materlal' can be absorbed.

PAUL MAURICE senor-rsu.

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