US2765637A - Frigorific separation process of gas mixture by liquefying and rectifying - Google Patents

Description

Oct. 9, 1956 Filed Oct. 10, 1951 A. ETIENNE FRIGORIFIC SEPARATION PROCESS OF GAS MIXTURE BY LIQUEFYING AND RECTIF'YING 3 Sheets-Sheet 1 @ct. 9, 1956 ETIENNE 2,765,637

A. FRIGORIF'IC SEPARATION PROCESS OF GAS MIXTURE BY LIQUEFYING AND RECTIFYING Filed Oct. 10, 1951 5 Sheets-Sheet 2 Oct. 9, 1956 ET|ENNE 2,765,637

FRIGORIFIC SEPARATION PROCESS OF GAS MIXTURE BY LIQUEF'YING AND RECTIFYING Filed Oct. 10, 1851 3 Sheets-Sheet 3 Fig.3.

2,75,37 Patented Oct. 9, 1956 ice FRIGORIFIC SEPARATION PROCESS OF GAS MIXG TU RE BY LIQUEFYING AND RECTIFY- IN Alfred Etienne, Paris, France, assignor to LAir Liquide, Societe Anonyme pour lEtude et lExploitation des Procedes Georges Claude, Paris, France Application October 10, 1951, Serial No. 250,630 Claims priority, application France October 30, 1950 10 Claims. (Cl. 62-175.5)

The present invention relates to improvements in the cold fractionation of gaseous mixtures. More particularly it relates to the case where the sole fractionation or each of the successive fractionations delivers two fractions and is operated in two successive rectification columns Working at different pressures and separated by a common heat transfer unit functioning as a condenser for the vapors in the higher pressure column and as a boiler for the liquid in the lower pressure column.

A first object of this invention is to minimize the operating power supply in the cases where the separation of the constituents is made uneasy, either by the narrow ditference of their boiling points, or by a puzzling ratio between the respective amounts of the constituents, especially when the more volatile one makes only a low percentage of the whole mixture.

A further object is to effect economically a sharp fractionation, that is to say to obtain pure constituents not or little contaminated by each other.

Another object is to avoid the use of auxiliary refrigeration cycles using pure separated constituents expanded with or without performance of external work.

In the following, the two rectification columns will be termed as high pressure column" and low pressure column disregarding the absolute operation pressures. The two fractions between which the incoming mixture is separated in these two columns will be termed as low boiling fraction and high boiling fraction being understood that the first mentioned is the one, the boiling point of which is the least expressed in Kelvin degrees.

The improvements comprising this invention reside in the following features:

A liquid resulting from the partial condensation of a compressed gaseous mixture is expanded and so partly vaporized. The resulting mixture of gas and liquid enters the low pressure column from the bottom of which the high boiling fraction is withdrawn either in a liquid condition or in a gaseous one. The gases issuing from the top of the low pressure column are compressed and led to the bottom of the high pressure column wherein they are rectified. From the top of the high pressure column issues the low boiling fraction in a gaseous state. The upper part of the high pressure column is cooled, as it is conventional, by the boiling of the liquid in the bottom of the low pressure column contained in a common heat transfer unit. The liquid issuing from the bottom of the high pressure column is expanded and used as scrubbing liquid at the top of the low pressure column.

From the above summary of the invention appear the following differences with the prior art:

(a) The feed is only on the low pressure column instead of being on the high pressure or of being divided between both columns.

(b) The two fractions resulting from the separation are Withdrawn, one from each column, instead of being both withdrawn from the low pressure column.

() An intermediate product is led from the low pressure column to the high pressure column, being therefore compressed.

It appears that this process overcomes the difiiculty that is raised when a mixture to be fractionated holds such a low percentage of its low boiling component that the amount of washing liquid is not suflicient to perform with the conventional devices a separation of this component in a good purity condition. A similar difiiculty appears when the two components to be separated have closely boiling points, and is eliminated by the present invention. It is to be noted that it is known to increase in such cases the amount of washing liquid by supplying an extra amount of cold through the compression, and further expansion, preferably with performance of external work, of one of the two separated components. The present invention fulfills the same purpose by compressing an intermediate mixed product, the previous purification of which is consequently not needed.

It is to be emphasized that a less power supply than in the conventional devices is needed, owing to the fact that the feeding is performed on the low pressure column Where from one of the fraction is directly withdrawn, and that consequently only a part of the incoming mixture has to be compressed at the high pressure.

complex gaseous mixture, this separation may have occurred by the conventional rectification or partial condensation.

As an example, this complex gaseous mixture is a natural gas; its higher boiling component gases are condensed by compression followed by cooling, and it is this condensate which is fractionated according to the present invention, the gaseous residue of the cooling being then itself either fractionated or used as a fuel.

An another example, the initial mixture is air which embodiment of my invention.

Fig. 2 is a diagrammatic view of another embodiment of my invention, applied to the fractionation of a mineral oil gas.

Fig. 3 is a diagrammatic View of an apparatus separ ating atmospheric air into nitrogen, oxygen and an argonr1ch fraction operating according to my invention.

According to Fig. l, the compressed mixture to be equal to, or higher than that at Which occurs the partial liquefaction in the exchanger E2. At the base of column Cl, is removed, in a liquid condition, through line F and regulation valve B5, a fraction containing the less volatile constituents of the initial mixture; this fraction comes out of the apparatus at U, after having, by its vaporization and its warming to room temperature, supplied part of the necessary cold to exchangers E2 and E1. From the top of column C1 issues at G a gas mixture. This mixture is reheated first in an exchanger H1 then in another exchanger 11, before being compressed at 11. The compressed mixture is then cooled by exchanger 11, then enters, at K2, the high pressure column C2, after having possibly undergone a partial expansion through a gate or valve B2. The column C2 is cooled at its upper portion by the above mentioned condenser-vaporizer. A gaseous fraction consisting of the most volatile portions of the mixture fed to column C1 issues from the top of column C2 through the tube L. The reflux in the column is obtained by the partial condensation of the ascending gases in the condenser-vaporizer.

The liquid issuing from the bottom of column C2, through the tube M2, is cooled at H1 by a heat exchange with the gases issuing from the top of the column C1. It is expanded by the valve N and is used as a scrubbing liquid in the column C1.

The gases issuing from the head of the column C2 through the tube L are united with the gases which escaped the liquefaction in the exchanger E2 and arriving through the pipe P. The whole then undergoes a treatment similar to the one just undergone by the initial mixture. It is cooled and partly liquefied in the exchanger E3. The liquid portion feeds, after being partially vaporized in a valve B3, a column C3, the bottom of which is heated by a condenser-vaporizer toppng a rectification column C4 operated at a higher pressure.

A liquid fraction is removed at the bottom of column Cs and issues from the apparatus at V, after being successively vaporized and warmed up in the exchangers E3, E2, E1. From the top of column C2, issues at Q, a gas mixture which coolsfirstly an exchanger H2, then another exchanger I2 before being compressed at J2. The compressed mixture is cooled in exchanger 12, then enters the column Ct at K4. Column C4 is cooled, at its upper portion, by the condenser-vaporizer already mentioned. The gases issuing from the top of the column C4 are united with the gases which escaped liquefaction in the exchanger E3. The whole enters at R an exchanger S, through which it passes three times, undergoing two intermediate expansions with external Work, in accordance with U. S. Patent No. 2,534,903 granted to the applicant. In exchanger S, due to the cooling caused by the expansion, there is a partial liquefaction of the gas under pressure. The liquid so formed returns to the exchanger E3 where it joins the liquid formed by the cooling, in said exchanger, of the gas from the first stage of separation, and feeding the column C3. The expanded gas cools in succession, firstly the exchanger S, then the exchangers E3, E2, E1, and finally issues at W.

Figure 2 shows a modified embodiment of the invention as applied to a gas of mineral oil origin, with approximately the following composition:

Percent Propane 6.5 Propylene 8 Ethane 21 Ethylene 14.5 H2, CO, N2, CH4 -50 The references of the parts common with the diagram of Figure l are the same as in said figure, to which reference will be made for the details not mentioned hereinafter.

The gas mixture, compressed to 25 atmospheres is cooled to about C. in the exchanger E1, then to about C. in the exchanger E2 wherein condenses a liquid rich in propane and propylene, which is treated in the column C1 after expansion at B1, as indicated above in connection with Figure l. in contrast with the latter, the portion not condensed in the exchanger E2 is rectified in a column C5, the top of which is cooled by the vaporization in a bundle of pipes T of the liquid fraction removed through line F and regulation valve B5, at the base of the column C1 and consisting here in a propanepropylene mixture substantially free of the most volatile constituents. The liquid fraction resulting from the rectification in the column C5 flows therefrom through the tube M5 back to the exchanger E2, While the gas fraction from this same rectification, is sent through the pipe X to the exchanger E3, undergoing therein a partial condensation as in the case of Figure l.

The portion liquefied in the exchanger E2 is expanded through the valve B1 to about 3.5 atmospheres, and rectified in the set of the two columns C1 and C2, the latter being operated under a pressure of about 30 atmospheres. Contrary to the diagram of Figure l, the gas portion resulting from the rectification in column C1 is sent through the tube L in the column C4 which is operated under the same pressure. The rest of the apparatus is identical with that of Figure 1.

Upon the whole, the apparatus delivers:

At U a propane-propylene fraction.

At V an ethylene-ethane fraction.

At W the most volatile products, i. e. methane, carbon, oxide, nitrogen, and hydrogen.

Each one of these fractions may be treated by known means to be separated into its constituents.

Instead of exchangers of the continuous operating type shown in the figures, one can use, at least partly regenerators or exchangers of the circuit inversion type.

Figure 3 shows diagrammatically a separation of air into oxygen, argon and nitrogen, utilizing the present invention. The air to be separated, previously cooled and under pressure, runs through a coil W arranged in the bottom of high pressure column C'. Being so partially liquefied and providing so the necessary heating of the column the air enters the column C in A. In this column, the air is separated into a liquid fraction, holding about 45% of oxygen, which leaves column C at B and into substantially pure nitrogen which is condensed in the upper part of column C in a condenser-reboiler B. Part of this nitrogen flows back in column C as scrubbing liquid, the remainder being withdrawn by pipe T3.

Column C is topped by a second column C" operated at atmospheric pressure, these columns being separated by the heat transfer unit E. Column C" is fed firstly at F with the 45% oxygen liquid removed at B from the bottom of column C and expanded by valve V1, and then at its top with liquid nitrogen withdrawn through pipe T3 from column C and expanded by valve V3. Gaseous nitrogen issues by pipe 1 from the top of column C and gaseous oxygen by pipe S2 from the lower part of the same column. In addition, a preferably liquid stream is extracted from column C by a side pipe T at such a height that it is made with argon and oxygen. with substantially no nitrogen. This mixture enters at D in a rectification column C1, operated at the same pressure as C. Column C is bound through a conventional condenser-vaporizer with a column C2 operated at a higher pressure. The set of both columns C1 and C2 operates acwrdin g to my invention.

In column C1, the argon-oxygen mixture separates into liquid oxygen which is withdrawn by a pipe T1 from the base F1 of column C1 and is led back to column C, and into a gaseous argon-oxygen fraction issuing at G from the top of column C1. This fraction is partly reheated in an exchanger H, then in another exchanger 1 before being compressed by a compressor I, then cooled in exchanger 1 and enters the bottom of column C2 at K2. Column C2 is cooled at its top by the above mentioned condenser-vaporizer ensuring the production of used as a scrubbing liquid in column C1.

In case the side stream extracted by pipe T from column C is in the gaseous state, the oxygen return from C1 to C" should be effected in the gaseous state too, through a pipe T2.

Instead of withdrawing from column C" through T a mixture of oxygen and argon, it is also possible though in that special case not so advantageous, to withdraw from a higher level of C a mixture of argon and nitrogen and to submit it to treatment in columns C1 and C2. In such a case, argon would be removed at P1 (being of course not returned to column C), and nitrogen at L, being combined after a suitable expansion with nitrogen issuing at S1 from column C.

What I claim is:

1. A method for low temperature separation of a gaseous mixture into at least two fractions having different boiling points, by successive fractionation, in two rectification columns operating under different pressures and provided with a heat transfer unit acting as a condenser for the top of the higher pressure column and as a boiler for the bottom of the lower pressure column, which comprises, partly liquefying said gaseous mixture and entering it in the lower pressure column, removing from the bottom of said column the higher boiling fraction and from its top a gaseous efiluent, compressing the whole of said effluent and entering it in the higher pressure column, withdrawing from the top of the last said column the lower boiling fraction and withdrawing said fraction in heat exchange with the incoming gaseous mixture to cool the same and withdrawing from its bottom a liquid which is then expanded and used as a scrubbing liquid in the lower pressure column.

2. A method for low-temperature separation of a gaseous mixture into at least three fractions having different boiling points, which comprises separating said mixture into two fractions according to claim 1 and repeating the separation method according to claim 1 on either of these two fractions.

3. A method of cold separating a gaseous mixture with three components of different volatilities into said components by rectification, which method comprises, separating in a first stage said mixture into three fractions, one of which comprises substantially the whole of the component with the middle volatility, and submitting the least named fraction to the process according to claim 1.

4. A method of cold separating a gaseous mixture in three fractions of different volatilities, using two successive rectification columns at different pressures separated from each other by a common heat transfer unit functioning as a condenser for the vapors in the higher pressure column and as a boiler for the liquid in the lower pressure column, which comprises, partly liquefying the said gaseous mixture by cooling, the portion remaining in the gaseous state after said liquefaction forming the first separated lowest boiling fraction, entering the resulting liquid after a partial expansion in a first rectification column operating under a relatively low pressure, removing from the bottom of said column the second separated highest boiling fraction and from the top a gaseous efliuent, compressing this gaseous efliuent and entering it in a second rectification column operating under a relatively high pressure and in heat exchange with the first rectification column, removing from the top of this column a gaseous efliuent making up the third separated middle boiling fraction, and from the bottom of this same column a liquid which is then expanded and used as a scrubbing liquid in the lower pressure rectification column.

5. A method of cold separating a gaseous mixture in several fractions of diiferent volatilities, which comprises separating it in three fractions according to claim 4, joining the first and third fractions and repeating on said joined fractions the method according to claim 4.

6. A method of cold separating a gaseous mixture according to claim 4, which comprises, expanding the gaseous lowest boiling fraction and using this cold expanded fraction for the cooling of the mixture to be separated.

7. A method of cold separating a gaseous mixture in several fractions of different volatilities which comprises separating it in three fractions according to claim 4, submitting the gaseous portion remaining after the partial liquefaction of the initial mixture to a rectification using indirect contact with the expanded liquid second separated fraction, the liquid resulting from said rectification being used for cooling the initial mixture and thereafter collected for treatment according to claim 4, the gaseous effluent from said rectification being also submitted to a process according to claim 4.

8. A method of cold separating a gaseous mixture in more than three fractions of different volatilities, which comprises separating it in three fractions according to claim 4 and repeating the method according to claim 4 on the separated lowest boiling fraction.

9. A method of cold separating a gaseous mixture in more than three fractions of different volatilities, which comprises separating it in three fractions according to claim 4 and repeating the method according to claim 4 on the separated middle boiling fraction.

10. A method of cold separating a gaseous mixture which comprises, cooling and partly liquefying this mix ture, expanding the liquid portion, separating in two fractions the resulting mixture of gas and liquid by successive fractionation in two rectification columns operating under different pressures and provided with a heat transfer unit acting as a condenser for the top of the higher pressure column and as a boiler for the bottom of the lower pressure column, said fractionation comprising the steps, entering said liquefied portion of the gaseous mixture into the lower-pressure column, removing from the bottom of said column the higher boiling fraction and from its top a gaseous effluent, compressing the whole of said effluent and entering it in the higher pressure column, withdrawing from the top of the last said column the gaseous lower boiling fraction and from its bottom a liquid which is then expanded and used as a scrubbing liquid in the lower pressure column, and which comprises further, expanding said gaseous lower boiling fraction resulting from this last separation and using this cold expanded fraction for the cooling of the initial mixture.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. A METHOD FOR LOW TEMPERATURE SEPARATION OF A GASEOUS MIXTURE INTO AT LEAST TWO FRACTIONS HAVING DIFFERENT BOILING POINTS, BY SUCCESSIVE FRACTIONATION, IN TWO RECTIFRICATION COLUMNS OPERATING UNDER DIFFERENT PRESSURES AND PROVIDED WITH A HEAT TRANSFER UNIT ACTING AS A CONDENSER FOR THE TOP OF THE HIGHER PRESSURE COLUMN AND AS A BOILER FOR THE BOTTOM OF THE LOW PRESSURE COLUMN, WHICH COMPRISES, PARTLY LIQUEFYING SAID GASEOUS MIXTURE AND ENTERING IT IN THE LOWER PRESSURE COLUMN, REMOVING FROM THE BOTTOM OF SAID COLUMN THE HIGHER BOILING FRACTION AND FROM ITS TOP A GASEOUS EFFLUENT, COMPRESSING THE WHOLE OF SAID EFFLUENT AND ENTERING IT IN THE HIGHER PRESSURE COLUMN, WITHDRAWING FORM THE TOP OF THE LAST SAID COLUM THE LOWER BOILING FRACTION AND WITHDRAWING SAID FRACTION IN HEAT EXCHANGE WITH THE INCOMING GASEOUS MIXTURE TO COOL THE SAME AND WITHDRAWING FORM ITS BOTTOM A LIQUID WHICH IS THEN EXPANDED AND USED AS A SCRUBBING LIQUID IN THE LOWER PRESSURE COLUMN.

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