US2555060A - Precooling and purification of gaseous mixtures prior to liquefaction - Google Patents

Description

May 29, l951 P. M. scHUl-TAN 2,555,060

PREcooLING AND PURIFICATION oF GAsEoUs MIxTuREs PRIOR To LIQUEFACTION Filed May 14, 1947 25 Ay A x/L/Am 1% r/ eEFK/GEKANT Z' m OUT N/ Tgo GEN FEA CTM/v7 m I TGA 5501/6 SEPARAT/Na [D C0 oLAN-r I I l UNIT I our MAME-UP /2 FLUID /N PZECOOLED All? 007' INVENTQRV 24 f/ BAUL NAuff/cfSf//UFTAN ATTORNEY Patented May 29, 195i UNITED S TAT-'ES PATENT OFFICE' PREGOOI-INGA AND PUEIFICATION OF GAS- EOUS MIXTURES l.PRIOR T0 LIQUEFAC- TION The present invention relates to the 'precooling and purification f gaseous mixtures ,p'r'ir to liquefaction.

In what follows the Word impurities vn'i'ustbe understood to embracealso an impurity in' the singular.

The term fprecooling'properly refers" to the ,precooling of the gaseous mixture by means of an` lauxiliary refrigerant, whereby -the overall thermodynamic efliciencyof a liquefaction cycle employing .Y Joule-Thompson expansion is Vmuch improved. Infpractice, however the term""p'r'e cooling conveniently refers to the whole of the cooling of the gas performed before it proceeds to the liquefaction unit proper, which' usually includes also some cooling obtained from one or more of the cold gaseous products from 'the 'sequent liquefactionprocess. Precoolingiin `this sense istherefore usually performed in two'steps,

4 butmay of course in princple'be performed solely .sufficiently to ensureV prolonged operation of the liquefaction un-it proper before its construction byvsoliddeposits of these impurities. AExamples of impurities of the type referred to are waterin air,4 and benzeneand/-or naphthalene in carbonisation gases. Constituents not originally present in the gaseous mixture, such as oil vapours from a compressor-lubricant, which would -also obstructthe liquefaction unit, and in an air separation unit might cause a danger of explosion, will alsobe removed to a considerable extent in the precoolingprocess.

Precooling as. carried out. hitherto has suffered fromsseveral disadvantages. In normal practice agaseousmixture is precooled by bringing it into direct/heat Vexchange through `the walls fof a thermal conductor with -gaseous products from -the sequentliquefaction process and/or with an auxiliary refrigerant. Owing to the depositionv of impuritiesvinthe solid state, theiprecoolers become .obstructed in -a relatively short time and are ytherefore usually provided in duplicate so thatone unit maybe thawed Aout whilst the other unit-.is.performingitsV function as a cooler. In view ofthe necessity .of securing a reasonable time` of operation before complete obstruction of the gas-passages, itis-necessary for these 'pas- Y sages to be of relatively large cross-sectional ar'a `from being undesirable in*-themselvesfcausa larger amountVV of the impurities V'with' relatively high vapourpressureand relatively high freezing .point to 'reach the 'separation unit thanwouldbe the case if the temperatures and pressuresfirema'ined steady.

Afurther disadvantage'is the: necessity of'providing several'changeover valves whichiare 'prone to leakage. Theleak'age of even a small amount of warm 'gas through avalve in theclosed position intoa gas stream which hasbeen precoo'led causes formation of very finely-divided solid particles Whichmay be Vcarried far into the liquefaction `unit and cause early obstructions" therein.

It is an bject of this invention '1:0 ensure a-'continuous precooling operation Tin a 'simpledev'ice in which pressure losses and coldlossesar'e' both uniformly Vlow whereby a savinginpovv'erc'onsumption together with improved 4sileatliness 'and ease of operation may be'achieve'd.

According to this'inventionv there is provided a `method of pre'cooling a Agaseous mixture having an impurity lcontentl of relatively high vapour `pressure and vrelatively 4high Yfreezing' pointes a preliminary to liquefying atl leastv ajpartof Lthe Amixture wherein the precooling is effeeted'bycontacting the' mixture in` counter-curre'ntA flow 'with a cold liquid'me'diimof low freezing pointadapted to cool the gas 'mixture arid to retain'suchfa proportion of theiimpurity content that thejvapour pressu're'of the residual impurities inl'th'e 'cooled gaseous mixture is sof low as toV ensure extended voperationof the plant in which the seq'uentlliquefaction `is effected. ,Dependingy upon'the natureof the impurities to be removed vand AtheI temperature, the`mass` transfer may involve solutionin-the liquid'medium, or liq'uefaction `and 'thef-for'mation of' an `emulsion in the liquid medium, -orfsolidi-eation and the formation of a susperfsinbf fsolid particles'in'v the' liquid f mediuiri of thefl impurities. Usually," however one impuritywill predominate '(e. g; watervapour' in"-air)-=and in sucha caisefthe liquid medium will be chosen so as to dissolve this impurity. It is an advantage of this invention that the vapour pressure of a soluble impurity will be reduced by the presence of the liquid medium, quite apart from the effect of the low temperature.

The contacting of the gaseous mixture with the cold liquid medium may be eifected in any suitable contacting device such as a scrubber or a packed or plate column. The liquid medium at the required low temperature is introduced at or near the top of the device and as it travels down the column in counter-current to the upwardly ovving gas stream, both heatand mass-transfer from the gaseous mixture to the liquid medium occur. As a result of the heat transfer, under eflicient operating conditions the liquid will leave the contacting device at a temperature only slightly below the inlet temperature of the gaseous mixture.

After contacting the gaseous mixture, the liquid medium may be recooled for re-use and prior to re-use may be subjected to a purification treat- Y ment for a reduction of the impurity content.

Where the impurities are in solution they may be removed by distillation, crystallisation or solvent extraction; where they are in the immiscible liquid state they may be removed by decantation, and where they are in the solid state they may be removed by ltration.

Where one or more gaseous products is or are available from the sequent lquefaction, such gaseous product or products or part thereof (hereinafter termed "the gaseous coolant, which term is intended to cover both gaseous products which are to be conserved as valuable products of the liquefaction process and those which are to be rejected as waste products) may be used to recool the liquid medium, additional cold being supplied if necessary from an auxiliary refrigerating cycle. The liquid medium may, for instance, be recooled by bringing it into indirect heat exchange with the gaseous coolant through the wall of a .thermal conductor.

Alternatively, the liquid medium may be recooled by direct counter-current contact with the gaseous coolant, additional cold being supplied if necessary from an auxiliary refrigerating cycle.

When such direct contact recooling is adopted it is of advantage to retain in the liquid medium up to the stage when it is contacted with the gaseous coolant at least a part of those impurities having a vapour pressure higher than that of any other constituent or constituents of the liquid medium whereby the cooling produced by heat transfer between the gaseous coolant and the liquid medium is augmented by cold produced by mass A transfer of impurity to the gaseous coolant.

Where the impurity to be removed is a substance which is present as an initial constituent of the liquid medium and has a vapour pressure those cases where the vaporisation of the im-V purity into the gaseous coolant is not objectionable (as, for instance, When the impurity is of no value and when there is no objection to contamination of the gaseous coolant by it), for firstly the necessity for a separate apparatus to remove the impurity from the liquid medium is avoided and secondly additional cooling of the liquid medium is produced by the evaporation of the impurity. This Will result in a reduction in the cold production required and therefore in the power consumption of the plant. For instance, in an air separation process where water vapour is an impurity to be removed and a glycol-water solution is used as the liquid medium, the latter may be at least partly recooled by bringing it into counter-current contact with the nitrogen fraction as the gaseous coolant whereby an appreciable amount of extra cooling of the liquid medium will normally be obtained as a result of water evaporation into the nitrogen stream. This extra cooling will reduce the power consumption of the separation plant or alternatively that of the auxiliary refrigeration cycle if such is used to provide iinal recooling of the liquid medium.

Where the amount of the substance evaporated from the liquid medium exceeds that removed as impurity from the gaseous mixture, it is necessary, as stated above, to add to the liquid medium a quantity of the substance equal to the excess evaporated so as to restore the liquid medium to its original composition. The addition may conveniently be made by bringing the substance to be added into intimate contact with the eiuent gaseous coolant so as to strip therefrom any valuable component of the liquid medium which may have been vaporised by the gaseous product. For instance, when a glycol-water solution is used as the liquid medium to remove water vapour from compressed air, the water added to compensate for the excess evaporated over that removed from the compressed air can be used to strip glycol vapour from the emerging gaseous coolant.

The choice of the liquid medium will depend on the precooling temperature to be achieved and on the nature of the impurities to be removed from the gaseous mixture and will also be inuenced by particular conditions obtaining in the plant under consideration. Depending on the nature of the impurities to be removed, liquid media Which could be used are, for example, hydrocarbons, glycerol and other polyhydric alcohols, both aliphatic and aromatic, monohydric alcohols, acetones, aldehydes, organic acids, or liquids of 10W freezing point composed of inorganic and/or organic substances dissolved in wa ter.

In order to avoid excessive losses of valuable substances contained in such liquid media, it will be of advantage to use ingredients having the lowest possible vapour pressure compatible with the low freezing point of the liquid.

In the specic case of the precooling of air prior to liquefaction and separation into a nitrogen fraction and an oxygen-fraction where water vapour is present in the air as an impurity liable to choke the liquefaction and separation units, the liquid medium may be a glycol-water solution. When using the eutectic composition containing about 58% by weight of glycol and 42% by weight of water, it is possible to vobtain precooling temperatures approaching minus 49 C. At such low temperatures the vapour pressure of the residual water content in the air emerging from the pre-cooler Will be so low that the liquefaction and separating units may opern ate for prolonged periods before having to be thawed out on account of choking with4 ice. The water taken up from theair by the glycol/water mixture may be removed by any known means, such as revaporisation or kcrystallisation, so as to enable the liquid medium to be re-used. It will be of' particular advantage to use for this purpose one of the productsof separation, the nitrogen rfraction or the oxygen fraction, both of which are substantially dry Vand available at low temperature, so that vcooling and water removal are effected simultaneously as already explained in more general terms.

The invention will now be described in further detail with reference to the accompanying drawing, which shows diagrammatically in Figure i one form of apparatussuitable for use in carry ing out the invention, employing indirect recooling of the precooling liquid; and-Figure 2 shows diagrammatically another form of apparatus employing direct recooling.

A compressed gaseous mixture to be precooled, and having an impurity content of relatively high Vapour pressure and relatively high freezing point, is led from a compressor (not shown) to the bottom of a counter-current contact unit IB through an inlet pipe IE. The contact unit l may comprise a packed column or a plate column or other suitable contact device. The air passes upwardly through the unit It! in counter-current contact with a descending stream of precooling fluid'consisting of a glycol-water solution, and the purified air leaves the top of the contact unit through outlet pipe I2 and passes to a separating unit I4 in which the nitrogen fraction is separated and passed through pipe I5 for use as a gaseous coolant.

The precooling liquid enters the top of the unit it through pipe I3, and after absorbing the iml purities from the gaseous mixture being treated, passes from the bottom of the unit I through pipe I8 to the top of a heat exchanger Il, in which it is brought into heat exchange relatione ship with the gaseous products of separation er1- tering the exchanger I'I through pipe I5 and pipe I8 and leaving through pipe I9. From the exchanger II, the precooling fluid is led through pipe 2) to a circulating pump 2| and thence through pipe 22 to a second heat exchanger 23, where it is passed in indirect heat exchange with an auxiliary refrigerant which enters exchanger 23 through pipe 24 and leaves it through pipe 25. From the exchanger 23, the re-cooled fluid is conveyed through pipe I3 to the top of the contact unit IIi.

The heat exchanger II may be of the type in which heat transfer takes place through the wall of a thermal conductor, for example, it may be a tubular heat exchanger, as shown in Figure 1; in this case water vapor condensed from the glycol-water solution during its passage through the exchanger II is removed through a drain pipe 26. It may on the other hand be a direct contact heat exchanger or scrubber similar to the unit EB, as shown in Figure 2. In the latter case, the volume of gaseous coolant passing through the exchanger I'I may be so great as to remove more water from the glycol-water solution than the latter has absorbed from the air. The glycol-water solution is in this case restored to its original composition by the addition of make-up water. Make-up water is supplied as indicated at 21 in Figure 2.

I claim:

1. The method of precooling air containing water vapour as a preliminary to liquefying at least part of the air and separating therefrom a nitrogen fraction, said method comprising bringing the air into direct contact and in counter-current flow with a glycol-water solution at a temperature below 0 C. adapted to cool the air and to retain such a proportion of the Water content that the partial vapour pressure of any residual water in the cooled airis so low as to ensure Vextended operation of the plant in which the sequent liquefaction is effected, and restoring the glycol-water solution to a condition suitable for re-use by a regenerationprocess which comprises bringing it into direct contact with a gaseous coolant derived from the sequent liquefaction in order simultaneously partially to re-cool the glycol-water solution and to reduce the water content1 thereof, additional cold being supplied `to the glycol-water solution to complete the re-cooling thereof from an auxiliary refrigerating cycle.

2. `The method of precooling airr containing water vapour as a preliminary to liquefying at least part ofthe air and separating therefrom a nitrogen fraction, said method comprising bringing the air into direct contact and in counter current ow with a glycol-water solution at a temperature below 0 C. adapted to cool the air and to retain such a proportion of the water content that the partial vapour pressure of any residual water in the cooled air is so low as to ensure extended operation of the plant in which the sequent liquefaction is effected, and restoring the glycol-water solution to a condition suitable for re-use by a regeneration process which comprises bringing it into direct contact with a gaseous coolant derived from the sequent liquefaction in order simultaneously to re-cool the glycol-water solution and to reduce the Water content thereof, the volume of gaseous coolant being such that the water removed during the regeneration process exceeds that absorbed by the glycol-water solution and restoring the glycol-water solution to its original composition by adding water thereto.

3. The method of precooling air containing water vapour as preliminary to liquefying at least part of the air and separating therefrom a nitrogen fraction, said method comprising bringing the air into direct contact and in counter current ow with a glycol-water solution at a temperature below 0o C. adapted to cool the air and to retain such a proportion' of the water content that the partial vapour pressure of any residual water in the cooled air is so low as to ensure extended operation of the plant in which the sequent liquefaction is effected, and restoring the glycol-water solution to a condition suitn able for re-use by a regeneration process which comprises bringing it into direct contact with a gaseous coolant derived from the sequent liquefaction in order simultaneously to re-cool the glycol-water solution and to reduce the water content thereof, the volume of gaseous coolant being such that the water removed during the regeneration process exceeds that absorbed by the glycol-water solution, and restoring the glycol-water solution to its original composition by adding water thereto, the added Water being brought into intimate contact with the effluent gaseous coolant so as to strip therefrom and return to the glycol-Water solution any glycol entrained by the gaseous coolant.

4. The method of precooling air containing water vapour as a preliminary to liquefying at least part of the air and separating therefrom a nitrogen fraction, said method comprising bringing the air into direct contact and in counter current flow with a glycol-water solution at a temperature below 0 C. adapted to cool the air and to retain such a proportion of the water content that the partial vapour pressure of any residual water in the cooled air is so low as to ensure extended operation of the plant in which the sequent liquefaction is effected, and restoring the glycol-water solution to a condition suitable for re-use by a regeneration process which comprises re-cooling the glycol-water solution with its augmented water content by bringing into direct contact therewith the separated nitrogen fraction in a volume exceeding that required to vapourise from the glycol-water solution that quantity of water which was removed from the air by the original glycol-water solution, and restoring the glycol-water solution to its original composition by adding thereto an amount of water equal to the excess evaporated by said nitrogen fraction, said added water being brought into intimate Contact with the effluent nitrogen fraction so as to strip therefrom any glycol entrained therein.

PAUL MAURICE SCHUFTAN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,501,415 Lafferty July 15, 1924 1,724,513 Pollitzer Aug. 13, 1929 1,791,086 Sperr Feb. 3, 1931 2,093,805 De Baufre Sept. 21. 1937 2,134,699 Brewster Nov. 1, 1938 2,141,997 Linde et a1. Dec. 27, 1938 2,198,142 Wade Apr. 23, 1940 I2,214,678 Raigorodsky Sept. 10, 1940 2,245,028 Farris June 10, 1941 2,288,461 Keith et al June 30, 1942 FOREIGN PATENTS Number Country Date 591,095 France June 27, 1925

Claims (1)

1. THE METHOD OF PRECOOLING AIR CONTAINING WATER VAPOUR AS A PRELIMINARY TO LIQUEFYING AT LEAST PART OF THE AIR AND SEPARATING THEREFROM A NITROGEN FRACTION, SAID METHOD COMPRISING BRINGING THE AIR INTO DIRECT CONTACT AND IN COUNTER-CURRENT FLOW WITH A GLYCOL-WATER SOLUTION AT A TEMPERATURE BELOW 0* C. ADAPTED TO COOL THE AIR AND TO RETAIN SUCH A PROPORTION OF THE WATER CONTENT THAT THE PARTIAL VAPOUR PRESSURE OF ANY RESIDUAL WATER IN THE COOLED AIR IS SO LOW AS TO ENSURE EXTENDED OPERATION OF THE PLANT IN WHICH THE SEQUENT LIQUEFACTION IS EFFECTED, AND RESTORING THE GLYCOL-WATER SOLUTION TO A CONDITION SUITABLE FOR RE-USE BY A REGENERATION PROCESS WHICH COMPRISES BRINGING IT INTO DIRECT CONTACT WITH A GASEOUS COOLANT DERIVED FROM THE SEQUENT LIQUEFAC-

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