US2567461A - Separation of gaseous mixtures at low temperatures - Google Patents

Description

%p&, H, 1951 A. AICHER 2,567,461

' SEPARATION OF GASEOUS MIXTURES AT LOW-TEMPERATURES Filed Feb. 12. 1948 Patented Sept. 11, 1951 SEPARATION OF GASEOUS MIXTURES AT LOW TEMPERATURES Alfred August Aicher, London, England, assignor to Petrocarbon Limited, London, England Application February 12, 1948, Serial No. 7,747 In Great Britain February 19, 1947 7 Claims.

This invention relates to the separation of gases by fractionation at low temperatures and more particularly to the supply of cold for the process.

The object of the invention is to provide an improved process and apparatus for the supply of cold for the separation of gases by fractionation at low temperatures, especially in such cases in which one or several refrigerants are used and in which different portions of these refrigerants are allowed to evaporate under different pres- :ures and at correspondingly different temperaures.

A known method of separating gaseous mixtures is first to cool and partially condense the mixtures and thereafter to pass the mixtures or portions thereof through fractionating towers or columns, each of which is equipped with a condenser at the top and a re-boiler at the bottom. The temperature in the condenser is lower than that in the re-boiler of the same column and heat is extracted from the condenser and supplied to the re-boiler. The extraction of heat at a low temperature requires a source of refrigeration. This source of refrigeration is frequently provided in the form of a separate refrigerating cycle circulating a known refrigerant.

It is frequently desirable to employ a number of fractionating columns such that the temperatures obtaining in the several condensers and reboilers are all different. It may then be useful to employ one or several refrigerating cycles in which one part of the refrigerant evaporates at one temperature and pressure and another part at another temperature and pressure.

Now in cases when parts of a single refrigerant are allowed to evaporate at different temperatures and pressures, it is customary to compress the whole of the refrigerant to a certain pressure, to condense the whole of said refrigerant in cooling water or in thermal contact with an evaporating second refrigerant, to sub-cool the so condenser refrigerant to a suitable temperature by known means, to expand the whole or a part of the refrigerant to an intermediate pressure, .to evaporate a part of the refrigerant, thereby performing a certain duty, and thereupon to return the evaporated refrigerant to the suction of a compressor or to an intermediate stage of a compressor. The other part of the refrigerant, which is not evaporated at the intermediate pressure, is expanded to a lower pressure and there made to evaporate, thus performing a duty at a lower temperature, whereupon it is returned through suitable heat exchangers to the suction of a compressor. 1

The invention consists in a process for the separation of gaseous mixtures by fractionation at low temperatures in which cold vapour of a refrigerant which has been condensed at an elevated pressure and expanded to a lower pressure fit which is preferably also higher than 1 atmosphere and which has performed a refrigerating duty by evaporating at this pressure is at least in part reliquefied in the re-boiler of a fractionating column and in which the liquid so formed is thereupon expanded to a still lower pressure and made to perform a further refrigerating duty by evaporating at said still lower presssure.

The invention also consists in a process in accordance with the preceding paragraph in which the reliquefied refrigerant, after expansion to the said still lower pressure, is mixed with a portion of the same refrigerant which has been expanded from the intermediate pressure to the same said still lower pressure without having as yet performed a refrigerating duty.

The invention also consists in a process in accordance with the next but one preceding paragraph in which the reliquefled refrigerant, after expansion to the said still lower pressure, is mixed with a portion of the same refrigerant which has been expanded directly to this pressure from the initial elevated pressure at which it was liquefied.

The invention also consists in a process in accordance with any of the preceding three paragraphs in which the said still lower pressure is not below 1 atmosphere.

The invention also consists in a process in accordance with any of the preceding four paragraphs applied to the removal of ethylene or ethane or both from a mixture of hydrocarbon gases containing the same. i

The invention also consists in a process and in apparatus for the separation of gaseous mixtures, substantially as described below,

The accompanying diagrammatic drawing represents an apparatus or plant, which may be used according to one form of the present invention.

In carrying the invention into effect in one form by way of example as applied to the production of ethylene gas from a mixture of hydrocarbon gases and hydrogen, the mixture, which contains hydrogen, methane, ethylene, ethane, propylene, propane, butylene, butane and butadiene, is compressed to a pressure between 4 and 10 atmospheres and cooled and partially condensed in thermal contact with several refrigerants. After the methane has been removed in a fractionating tower, the remaining liquid, now containing ethylene, ethane, propylene, propane, butylene, butane and butadiene, is treated for the removal of ethylene and ethane, both s'i'ibstantially pure, in the manner indicated below.

Referring to the accompanying diagrammatic drawing the remaining liquid referred to above is fed at intermediate point a to the suction side of a fractionating'column a which operates at 4 atm. pressure, and is here separated into a gaseous overhead containing substantially only ethylene and ethane, and a liquid residue containing all components with boiling points higher than that of ethane. The gaseous overhead leaves at the top of the column and is thereupon condensed in a system of tubes b in the re-boiler of a second column c, expanded in a valve d to approximately 1.5 atm. abs. and fed into the middle point e of the second column c. From this column c, gaseous ethylene is taken as overhead at the top and gaseous ethane as residue from the lower end of the column.

The refrigerant, which is ethylene, is compressed from 1 to 4 atmospheres in a low pressure compressor e and from 4 to 13 atm. in a high pressure compressor 1'. It is then pre-cocled in a heat exchanger g with cold ethylene gas returning partly at 4 and partly at 1 atm. pressure. The pro-cooled compressed ethylene is liquefied in a condenser h in thermal contact with evaporating liquid ammonia from a separate refrigerating cycle.

The liquid ethylene then separates into two streams. One stream passes through a sub-cooler 1' and expansion valve 7' to a vessel k which is at 4 atm. pressure; the other stream passes through another sub-cooler l and expansion valve m to a vessel n which is at approximately 1 atm. pressure.

The liquid ethylene from the vessel k is evaporated in a reflux condenser o of the first fractionating column a at 4 atm. pressure. The resulting vapour is returned to the vessel It and a part is taken back to the high pressure compressor I through the sub-cooler i and heat exchanger 9. The remainder of the vapour is conducted according to this invention to a system of tubes b in the re-boiler of the fractionating column 0, where it is reliquefled. The liquid is further cooled in a sub-cooler q with cold gas from another part of the plant, expanded to 1 atm. pressure in an expansion valve r and returned to the vessel n. The liquid ethylene from the vessel n, a part of which has entered from the expansion valve m, and another part after being reliquefled has entered through the expansion valve 1' is evaporated in a reflux condener s of the second fractionating column 0, whence the vapours return to the vessel n and are passed back to the suction side of the low pressure compressor e through the sub-cooler l and heat exchanger g. The bottom t of column a is heated by the reboiler t to the temperature required to boil off the ethylene and ethane while leaving higher boiling components to be withdrawn as residue from the bottom of the tower.

General According to this invention three pressures are employed in the process: an elevated pressure, an intermediate pressure and a low pressure. It is not essential for the intermediate pressure to be higher than 1 atmosphere, though it is preferred to operate the plant so that the pressure is never at any point below 1 atmosphere in order to avoid any possibility of air leaking into the plant. Not the whole of the portion of the refrigerant which is evaporated at the intermediate pressure is returned to the suction or to the intermediate stage of the compressor, but instead a part or the whole of this vapour is condensed in thermal contact with the liquid boiling in the re-boiler of a fractionating tower, the temperature of which is lower than the condensing temperature of the refrigerant at the obtaining pressure.

According to a further feature of the invention, the liquid refrigerant, which has now persuitable heat exchangers to the suction of the compressor.

According to a further variant of the same invention, the reliquefled refrigerant, on being expanded from the intermediate to the'lower pressure, mixes with another portion of the same refrigerant which has been expanded from the intermediate to the lower pressure without having performed a duty at this intermediate pressure and without having been reliquefled. Alterantively, the reliqueiled refrigerant. after being expanded from the intermediate to the lower pressure may be mixed with a portion of the same refrigerant which has been expanded directly from the initial higher pressure.

I claim:

1. In the fractionation at low temperatures of a mixture of gases in a plurality of fractionating columns connected in series and operating at successively lower pressures and provided with reflux condensers and reboilers; the

process of supp ying cold and heat to said condensers and reboilers which comprises passing a compressed and cooled refrigerant liquid in a closed circuit through a reflux condenser of one fractionating column whereby the refrigerant liquid is evaporated and cold supplied to the condenser, passing at least part of the resulting refrigerant vapor through the reboiler of a succeeding fractionating column whereby the refrigerant vapor is condensed and heat is supplied to the reboiler, expanding the thus reliquefled refrigerant to a lower pressure and passing it through the reflux condenser of said succeeding fractionating column, whereby the reliquefled refrigerant is evaporated and cold is supplied to the reflux condenser of said succeeding column.

2. In the separation of gases of different boiling points from mixtures thereof wherein such a mixture is compressed, cooled to low temperatures and passed into a first fractionating column wherein it is separated into an overhead aseous fraction and a bottom fraction and the overhead gaseous fraction is then passed at a reduced pressure into a second fractionating column wherein it is again separated into an overhead gaseous fraction and a bottom fraction, both of said fractionating columns being equipped with reboilers and reflux condensers, and wherein the reflux condenser of said first column is operated at about the same temperature as the reboiler of said second column; the improvement which comprises supplying cold to the reflux condenser'of said first column, heat to the reboiler and cold to the reflux condenser of said second column, by the consecutive evaporation, condensation and evaporation steps respectively of a refrigerating cycle which comprises compressing a refrigerant in vapor form to an elevated pressure, cooling and condensing the compressed refrigerant vapor, expanding the resulting refrigerant liquid to an intermediate pressure and passing it through the reflux condenser of said first column to supply cold to said condenser, whereby said refrigerant liquid is vaporized, passing at least part of the vaporized refrigerant through the reboiler of said second column to supply heat to said reboiler, whereby the vaporized refrigerant is reliquefled, then expanding the reliquefled refrigerant to a lower pressure and passing it through the reflux condenser of said second column to supply cold thereto, whereby the reliquefied refrigerant is vaporized and then passing the vapor formed in the last-mentioned step to the first compression step of said cycle.

3. The process of claim 2 wherein the gas mixture separated comprises ethylene, ethane, propylene, propane, butylene, butane and butadiene while the refrigerating fluid is ethylene, the system being operated to separate ethylene and ethane as the overhead fraction from said first fractionating column, and ethylene as the overhead fraction from said second fractionating column.

4. The process of claim 2 in which only part of the compressed refrigerant liquid at said elevated pressure is subjected to the said expansion to said intermediate pressure followed by evaporation, condensation and expansion to said lower pressure and part is expanded directly from said elevated pressure to said lower pressure and the two parts at said lower pressure are mixed and the mixture evaporated to perform the second refrigerating duty.

5. In the separation of a mixture of hydrocarbons comprising ethylene and ethane wherein the highly compressed gaseous mixture is passed into a first fractionating column equipped with a reflux condenser serving to control the temperature at the top of the column so that a mixture of ethylene and ethane leaves the column as overhead and this mixture is introduced at a reduced pressure into a second fractionating column equipped with a reboiler at its bottom. which operates at substantially the same temperature as the reflux condenser of said first column, and with a reflux condenser at its top, which operates at a lower temperature so that ethylene only leaves the second fractionating column as overhead; the process of supplying heat and cold to said reboiler and reflux condensers which comprises compressing and liquefying ethylene, passing the liquid ethylene through the reflux condenser of said first column to provide cooling therefor, whereby the liquid ethylene becomes at least partly vaporized, passing at least part of the vaporized ethylene through the reboiler of said second column to supply heat thereto, whereby at least part of the ethylene becomes condensed, reducing the pressure on the so-liquefled ethylene and passing it through the reflux condenser of said second column to supply cooling thereto, and then passing the so-vaporized ethylene back to the compression step of the process.

6. In a process for the separation of a mixture of gases by compression and cooling to low temperatures followed by fractionation in a plurality of fractionating columns connected in series and operating at successively lower pressures which process involves a number of cooling operations for supplying cold for the cooling of themixture of gases and for providing reflux for the cooling of gases in the reflux condensers located in the upper ends of the fractionating columns and a number of heating operations for supplying heat for re-boiling in the re-boilers located in the bottoms of the fractionating columns; the heat transfer steps comprising passing a compressed and cooled refrigerant liquid in a closed circuit in heat exchange relation with gases undergoing liquid is evaporated and cold is supplied for the cooling operation, passing at least part of the resulting refrigerant vapor at substantially the same pressure in heat exchange relationship with liquids being boiled in a following heating operation taking place in the re-boiler of another column of said series whereby the refrigerant vapor is condensed and heat is supplied for the heating operation, reducing, the pressure on the thus reliquefled refrigerant and then passing it in heat exchange relation with gases being cooled in a further cooling operation, which is subsequent to the said heating operation and which takes place in a reflux condenser of a column following the first column of said series, whereby the re-liquefied refrigerant is evaporated and cold is supplied for said further cooling operation and then compressing and cooling the refrigerant vapors to the initial pressure and temperature to supply the liquid refrigerant of the-first step of said process.

'7. In the separation of gases of different boiling points from mixtures thereof wherein such a mixture is compressed, cooled to low temperature and then passed in a closed system in indirect heat-conducting relationship with a refrigerant fluid through a series of cooling zones, wherein the refrigerant supplies cooling, and through at least one heating zone, wherein said refrigerant supplies heat, and wherein at least one of the cooling zones and the heating zone is operated at substantially the same temperature and a second cooling zone is operated at a temperature which is lower than the first mentioned temperature; the improvement which comprises supplying cooling to the first mentioned cooling zone, heat to the heating zone and cooling to the second cooling zone, by the consecutive evaporation, condensation and evaporation steps, respectively, of a refrigerating cycle which comprises compressing a refrigerant in vapor form to anlelevated pressure, cooling and condensing the compressed refrigerant vapor, expanding the resulting refrigerant liquid to an intermediate pressure and passing it in indirect contact with the gases to be separated in said first mentioned cooling zone to supply cooling thereto, whereby said refrigerant liquid is vaporized, passing at least part of the vaporized refrigerant at substantially the same intermediate pressure in indirect contact with the gases to be separated in the said heating zone to supply heating thereto, whereby the vaporized refrigerant is reliquefied, then expanding the reliquefied refrigerant to a lower pressure and passing it in indirect contact with the gases to be separated in the second cooling zone to supply cooling thereto, whereby the re-- liquefied refrigerant is vaporized and then passing the vapor formed in the last-mentioned step I to the first compression step of the cycle.

ALFRED AUGUST AICHER.

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

UNITED STATES PATENTS Number Name Date I 1,529,625 Rafferty et al Mar. 10, 1925 1,843,043 Patart Jan. 26, 1932 2,057,804 Twomey Oct. 20, 1936 2,146,197. Twomey Feb. 7, 1930 2,424,201 Van Nuys July 15, 194'!

Claims (1)

1. IN THE FRACTIONATION AT LOW TEMPERATURES OF A MIXTURE OF GASES IN A PLURALITY OF FRACTIONATING COLUMNS CONNECTED IN SERIES AND OPERATING AT SUCCESSIVELY LOWER PRESSURES AND PROVIDED WITH REFLUX CONDENSERS AND REBOILERS; THE PROCESS OF SUPPLYING COLD AND HEAT TO SAID CONDENSERS AND REBOILERS WHICH COMPRISES PASSING A COMPRESSED AND COOLED REFRIGERANT LIQUID IN A CLOSED CIRCUIT THROUGH A RELUX CONDENSER OF ONE FRACTIONATING COLUMN WHEREBY THE REFRIGERANT LIQUID IS EVAPORATED AND COLD SUPPLIED TO THE CONDENSER, PASSING AT LEAST PART OF THE RESULTING REFRIGERANT VAPOR THROUGH THE REBOILER OF SUCCEEDING FRACTIONATING COLUMN WHEREBY THE RE-

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