US3695002A

US3695002A - Process for production of pure ethylene

Info

Publication number: US3695002A
Application number: US68672A
Authority: US
Inventors: Friedrich Rottmayr; Hans Reimann
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 1969-09-02
Filing date: 1970-09-01
Publication date: 1972-10-03
Anticipated expiration: 1989-10-03
Also published as: NL167671B; BE755590A; NL7012983A; ZA706015B; FR2060862A5; AU1949970A; DE1944505A1; SU459885A3

Abstract

A process for the production of acetylene-free ethylene from a mixture of C2-hydrocarbons by selective absorption of the acetylene and separation of the ethane by rectification in which the acetylene is selectively absorbed at a pressure of more than 5 atmospheres absolute, preferably within a pressure range of 840 atmospheres absolute with a polar aprotic solvent boiling higher than 120* C.

Description

United States Patent Rottmayr et al.

[ 51 Oct. 3, 1972 [54] PROCESS FOR PRODUCTION OF PURE ETHYLENE [72] Inventors: Friedrich Rottmayr; Hans Reimann, both of Pullach, Germany [731 Assignee: Llnde Aktiengesellsehal't Zentrale Patentabtellung, Munich, Germany [22] Filed: Sept. 1,1970

[21 I Appl. No.: 68,672

130 Foreign Application Priorlty om Sept. 2, 1971 Germany ..P 19 44 505.7

[52] US. Cl ..55/64 [51] Int. Cl. ..B0ld 19/00 [58] Field of Search ..55/63-65; 260/677,

[56] References Cited UNITED STATES PATENTS 3,272,885 9/ I 966 Davison ..55/65 3,530,199 9/1970 Lowrance ..55/64 2,849,514 8/1958 Nevitt ..260/677 A 3,002,586 10/1961 Rabourn ..260/679 A Primary Examiner-Charles N. Hart Attorney-Millen, Raptes 8:. White ABSTRACT 16 Claims, 5 Drawing Figures HII I IHHH W" PA'IENTEnum 1912 3.695.002

SHEET 1 BF 4 DMF DimefhyI/ormamide NMP N-Mefhylpyrrolidone Acetone 1 0 HMP TA Hexcrnelhylphosphoric acid them/a e P(afa) Fig. 7

NMP

Methanol HMPTA

0,5

P (afa) INVENTORS FRIEDRICH ROTTMAYR HANS REIMANN ye z AT IUHM Y5 PATENTEDBBI 3 I972 sum 2 [1F 4 Fig. 3

l- Meth onol INVENTORS FRIEDRICH ROTTMAYR HANS REIMANN PATENTEI J I972 3.695.002

sum 3 OF 4 INVENTOILS FRIEDRICH ROTTMAYR HANS REIMANN PNENTEM 3 I972 3.695.002

saw u 0r 4 INVENTORS FRIEDRICH ROTTMAYR HANS REIMANN m; 4,4011? ZMJU PROCESS FOR PRODUCTION OF PURE ETHYLENE BACKGROUND OF THE INVENTION This invention relates to a process for obtaining acetylene-free ethylene from a mixture of C -hydrocarbons by selective absorption of the acetylene and separation of the ethane by rectification.

The gaseous mixtures produced during the cracking of hydrocarbons, for example by partial combustion or thermal cracking, contain acetylene in amounts, depending on the type of the starting material and the selection of the cracking method, which can range from about 0.1 to percent and higher. In cracking processes which produce primarily ethylene and higher olefins, cracking temperatures of between about 750 and 900 C. are employed. The thus-obtained gaseous mixtures contain about 25-40 percent of ethylene and, in addition thereto, acetylene in amounts of from a few tenths of one percent up to about 2 percent. These gaseous mixtures are normally compressed, afier removing therefrom the content of heavy oil and cracked gasoline components, and cooled countercurrently to cold fractionation products. During this latter procedure, the C;,- and higher hydrocarbons are liquefied in a first separating stage; the C hydrocarbons in the remaining gaseous phase are separated in the liquid phase in a second separating stage operated at a lower temperature, the lower boiling components, such as methane, hydrogen, and nitrogen, being fed therefrom to further processing stages; and, a third separating stage, the ethylene in the condensed C,- hydrocarbons is then obtained in the pure form. Ethylene intended for the production of polyethylene, in particular, must be as completely free of acetylene as possible.

The removal of acetylene from ethylene is accomplished according to the process of German Pat. No. 953,700, by scrubbing the acetylene from the C -mixture with a solvent which selectively absorbs acetylene, especially acetone, at or closely above the liquefaction temperature of the C -mixture. The separation of the ethane by rectification can be conducted either prior to or after this process step.

In cracking processes employed for the production of acetylene, cracking temperatures of more than l,200 C. are employed. From the crude gas produced in this manner, after it has been cooled and subjected to an oil washing step for the removal of tar and aromatics, the thusobtained acetylene is washed out of the gaseous hydrocarbon products at ordinary temperatures and normal pressures using high-boiling solvents, such as dimethylformamide or N-methylpyrrolidone.

The number of solvents having a high dissolving power for acetylene is large. In the following list, several are set forth in the order of ascending boiling points:

Ethyl acetate CH COOCJh 77 82.4 Methyl ethyl ketone CH,COC H, 80 86.4 Methyl propionate cHpmcoocH, 80 87.5 Acetonitrile CH,CN 82 43.5 lA-Dioxane CH H 102 l2.5 Morpholine g), 128 4.9 Propionic acid CH,CH;COOH l4l 27 Dimethylformarnide HCON(CH,), I53 6i Dimethyl acetamide CH,CON(CH;), 165.5 20 Tetrarnethyl urea OC[N(CH,),]. 177 Dirnethyl sulfoxide (CH,).SO 189 18.5 Acetonylacetone CH;CO(CH, ),COCH, 193 9 N-Methylpyrrolidone (CH ),NCOCH, 202 24 y-Butyrolactone CH, ),C00 206 42 Hexarnethylphosphor amide 0PlN(Cl-l,,],: 233 7 If it is intended to selectively absorb acetylene from a mixture with ethylene and perhaps also ethane at an elevated pressure and at a temperature below ambient temperature, heretofore the low-boiling solvents, especially acetone, have been preferred. When the acetylene is absorbed at a relatively low pressure, the higher-boiling solvents have been employed, dimethylformamide being generally preferred. See Chemical Engineering Progress, Vol. 56, No. 1, January 1960, p. 54, right-hand column, and p. 55, left-hand column. The low-boiling solvents appeared to be more suitable when operating with high pressures and low temperatures, primarily because they generally also have low melting points, so that there is no danger of solid deposits. Also, under high pressure, low temperature conditions, the amount of scrubbing solvent entrained by the scrubbed gas is low. In such low temperature processes, the high vapor pressure of the solvent thus does not represent a significant disadvantage. Moreover, the low-boiling solvents can be regenerated with low energy requirements and are thermally stable at their boiling temperature.

In contrast thereto, in low pressure processes for scrubbing acetylene from ethylene, high-boiling solvents are conventionally employed so as to avoid excessive solvent losses. Such processes must then contend with the difi'iculties which arise during the solvent regeneration stage.

Dimethylformamide has been employed for scrubbing out acetylene from a C mixture at an elevated pressure. See, for example, U.S. Pat. No. 2,805,733, according to which the absorption of the acetylene takes place at 27 atmospheres and -l2 C. Solvents which are set forth as being equally suitable for the same purpose are acetone and methyl ethyl ketone, the acetals thereof, the acetals of aliphatic aldehydes or aliphatic ethers, especially those exhibiting, in addition to the ether oxygen atom, a hydrophilic group. Among such compounds are, for example, the glycol monoalkyl ethers with their free hydroxy group. Carboxylic acid esters containing a further hydrophilic group, either in the acid residue or in the alcohol residue, e.g., lactic acid esters or the glycol monoesters of lower carboxylic acids, can be employed as well as acid amides, alkylated urea compounds, bis-dialkylamides of dicarboxylic acids, lactarns, and lactones.

However, none of the conventional acetylene-absorption processes are completely satisfactory, since they do not offer any assurance, with respect to the selection of the solvent, that the acetylene would be removed from the C,-mixture with sufiicient selectivity, even under pressures of more than 5 atmospheres.

OBJECTS OF THE INVENTION It is the object of this invention to provide a process in which acetylene is absorbed from a C.,-mixture at elevated pressures with optimum selectivity.

SUMMARY OF THE INVENTION According to this invention, the above-described problems are solved by selectively absorbing acetylene from ethylene under a pressure of more than 5 atmospheres absolute with a polar aprotic solvent having a boiling point higher than 120 C.

DETAILED DISCUSSION The present invention is based on the discovery that the selectivity of the conventional acetylene solvents, i.e., the ratio of the solubility coefficient A (Nm of dissolved gas per ton of solvent and per atmosphere of partial pressure) of the acetylene to that of ethylene or ethane, decreases with increasing pressure to a very different degree, depending on the particular solvent. It was found that the high-boiling polar aprotic solvents exhibit, in addition to a high selectivity at low pressures, only a minor decrease in selectivity with increasing pressure. The term aprotic solvent" means a solvent having no I-I-atom capable of forming a hydrogen bridge, in contrast to proton-active solvents, e.g., water, alcohols, and carboxylic acids.

The characteristic value for the selectivity is the ratio of the solubility coefficients (C H )t (CJL). The larger this ratio, the more favorably the absorption process can be designed. If the value of this ratio is l, both components are dissolved in equal amounts, Le, a separation is impossible, even when employing the theoretical amount of scrubbing agent and an infinite number of plates.

The invention will hereinafter be described with reference to the drawings, in which:

FIGS. 1 and 2 are graphs showing the effect of pres sure on the selectivity of several solvents at C. and -20 C. respectively;

FIG. 3 is a graph showing the effect of methanol on the melting point of N-methyl-pyrrolidone; and

FIGS. 4 and 5 are schematic representations of apparatus which can be employed to practice the process of this invention.

In order to demonstrate the course of the selectivity for the solvents investigated, the quotient (F, F,)/( F l is plotted in FIGS. 1 and 2 as a measure of the decrease in selectivity against the total pressure P (in case of p C l-I, p C,I-I l 100). Namely, F is the ratio of A (C l-I A (C l-I at the total pressure of l atmosphere absolute, and F,, is the same ratio at x atmospheres absolute. Thus, the quotient indicates the difference with respect to the initial selectivity in multiples of the initial selectivity reduced by I. At F, l, i.e., when selectivity is lost, the quotient becomes 1. In FIG. 1, the quotient at 0 C., and in FIG. 2, the quotient at C. is given.

When comparing the acetone, dimethylformamide, N-methylpyrrolidone, and hexamethylphosphorarnide as solvents, it can clearly be seen that the higher-boiling solvents exhibit better selectivity characteristics with increasing pressure than acetone; the higher the boiling point of the respective solvent, the better the selectivity characteristics thereof. At lower temperatures, where the selectivity of the acetone rapidly vanishes with increasing pressure, the higher-boiling solvents remain selective. Thus, it can be seen from FIG. 2 that the quotient (F, F ,,)/(F l) of acetone, at 20 C. and about 21 atmospheres absolute, is 1, Le, the solvent exhibits no selectivity, whereas N-methylpyrrolidone under these conditions still possesses about 60 percent of its initial selectivity. The decrease in selectivity is still smaller in case of hexamethylphosphoramide. The selectivity of the proton-active methanol assumes a median position, since the quotient (F, F,,)/(F l) first decreases more strongly at low pressures and decreases less strongly at high pressures, than in the case of acetone. However, the initial selectivity at l atmosphere absolute and the absolute dissolving power for acetylene are lower, compared to the aprotic solvents.

The process of this invention is conducted at pressures of more than 5 atmospheres absolute, preferably about 8-40 atmospheres absolute, e.g., about 15-35 atmospheres absolute. As will be apparent to those skilled in the art, different pressures within this range can be employed at different stages of the process.

The selective absorption can be conducted at any temperature conventionally employed in low temperature, high pressure ethylene purification processes, e.g., between 55 C at 8 ata and 0 C at 40 ata, preferably between -50 C at 10 ata and 30 C at 20 ata.

It is especially advantageous to conduct the selective absorption at or slightly above the liquefaction temperature of the C -mixture because the selectivity of the solvent is highest in this temperature range. In this manner, ethylene of particularly high purity can be obtained with simple devices with solvent and energy low requirements. The energy required for maintaining the liquefaction temperature is even less significant when considering that the selective absorption takes place in the course of a low-temperature separation process which must be conducted in any event.

The polar aprotic solvents employed in the process of this invention have a boiling point, at atmospheric pressure, of above C. Preferred are those having a boiling point of at least C., especially those having a boiling point above C. Obviously, these solvents must have a melting point above the absorption temperature employed in the process. Those having a melting point below 0 C., especially those having a melting point below 50 C., are preferred.

A wide variety of solvents can be employed, so long as they are polar, aprotic and have a boiling point, at atmospheric pressure, of above 120 C. Classes of compounds which fall within this class are tertiary amines, especially the N-lower-alkyl-heterocyclic amines, e.g., of the pyrrolidine, piperidine, homopiperidine, morpholine and thiomorpholine series; N,N-dialkyltertiary amides and N-alkyl-heterocyclic amides, e.g., of lower-fatty acids and phosphoric acid; dialkyl sulfoxides; dialkyl sulfones; loctones, especially of lower-fatty acids.

Among the solvents which can be employed in the process of this invention are dimethylformamide, dimethylacetamide, tetramethyl urea, dimethyl sulfoxide, acetonylacetone, N-methylpyrrolidone, butyrolactone, and hexamethylphosphoramide.

The solvents employed in the present invention have the disadvantage that the melting point of those solvents which have the highest selectivity have the highest melting points. Thus, the scrubbing temperature cannot be lowered to the optimum extent. For this reason, in a preferred embodiment of this invention, the absorption is conducted with a mixture of the highboiling polar aprotic solvent and a low-boiling solvent. The low-boiling solvent is preferably one which lessens the selectivity of the high-boiling solvent to an only minor degree. Especially preferred are acetone, methanol and low-boiling esters or ethers. As shown in FIG. 3, the temperature at which solids are deposited from pure N-methylpyrrolidone is reduced from 24 to 40 C. by the addition of by weight of methanol. In so doing, selectivity is reduced only by about 10 percent.

The feature of conducting the process with a mixture of a high-boiling solvent and a low-boiling solvent offers a further advantage in the regeneration of the solvent. In order to drive off the dissolved components from the spent solvent, stripping vapor must be produced, i.e., solvent must be evaporated. if the absorption liquid consists of such a mixture, the stripping vapor can be produced at the boiling temperature of this mixture, i.e., the sump temperature of the regenerating column is lower then the boiling point of the high-boiling component. The rising solvent vapor is condensed as reflux and the thus-liberated acetylene escapes via the head. The regenerated solvent mixture is withdrawn from the sump of the column and recycled to the absorption stage. The sump of the column is maintained at the boiling temperature of the mixture so that the regenerated absorption solvent retains its composition. By conducting the process in this manner, it is possible to employ a solvent which is not entirely stable at its boiling point and which decomposes under the constant effect of higher temperatures and would otherwise cause decomposition products to enter into the acetylene and, particularly, into the ethylene which has been freed of acetylene in the absorption column and is discharged as product.

Another aspect of the process of this invention resides in admixing a low-boiling solvent with the acetylene-laden, high-boiling solvent immediately prior to or during the regeneration thereof, vaporizing the low-boiling solvent therefrom in the sump of the regenerating column to a residue, condensing the vaporized low-boiling solvent at the head of the regenerating column, and then again admixing the condensed low-boiling solvent with the high-boiling solvent to be regenerated. The amount of the low-boiling solvent remaining in the sump liquid during the regeneration is regulated so that the boiling temperature of the solvent mixture does not exceed the temperature at which the high-boiling solvent is thermally stable.

With the above-described regenerating process, it is possible to utilize the advantage provide by the addition of the low-boiling solvent during the regeneration stage even when the absorption step is conducted with high-boiling solvent alone, i.e., when the melting point of the high-boiling solvent with respect to the desired scrubbing temperature is sufficiently low, that a solvent mixture is not required in order to avoid precipitation of the high-boiling solvent. Also, the disadvantages resulting from conducting the absorption with a solvent mixture, viz., the reduction in selectivity and the possibility of contamination of the scrubbed gas with the vapors of the low-boiling solvent, is eliminated or substantially diminished.

Another embodiment of the invention resides in conducting the selective absorption with a mixture of two polar aprotic solvents boiling at above 120 C. Examples are mixtures of N-methylpyrrolidone with 'y-butyrolactone or dimethylformamide. The eutectic of the N-methylpyrrolidone 'y-butyrolactone system is approximately 65 percent by weight of 'y-butyrolactone and -65 C. The eutectic of the N-methylpyrrolidone dimethyl-formamide system is about percent by weight of dimethylformamide and -7l C. With such systems, the solid deposit point of the absorbent solvent is towered so that the scrubbing temperature can be lowered without having to add a low-boiling solvent which would result in a marked decrease in selectivity.

If only a mixture of high-boiling solvents is employed in the absorption stage, special measures should be taken in order to protect the high-boiling solvent mixture during the solvent regeneration stage. This is suitably done by lowering the boiling point thereof by evacuation or, along the lines of the above-described regenerating process, by adding a low-boiling solvent to the high-boiling solvent mixture immediately prior to or during the regeneration step. The acetylene can be obtained in this manner free of foreign gases. However, if the acetylene is to be combusted as waste by-product, the high-boiling solvent mixture can also be regenerated by stripping with a foreign gas, e.g., ethane, methane or nitrogen.

Finally, another embodiment of the invention resides in that the absorption is conducted with an absorbent saturated with ethylene, preferably liquid ethylene. During the absorption step proper, it is then only necessary to remove the heat of solution of the acetylene. The use of liquid ethylene offers the additional advantage that because only a minor amount of mixing heat occurs as heat of reaction during the saturation of the solvent, the heat of solution of the acetylene is compensated for by vaporization of part of the dissolved ethylene.

When ethane is also present in the ethyleneacetylene mixture, it is preferably separated by rectification from the ethylene obtained after the selective absorption of acetylene therefrom. However, the above sequence of steps can be reversed, i.e., the ethane can first be separated by rectification from the mixture with ethylene and acetylene, and the acetylene subsequently absorbed therefrom.

The process of this invention will now be explained with reference to the two schematic representations illustrated in FIGS. 4 and 5 by way of example. Identical parts are denoted by identical reference numerals in both figures.

According to FIG. 4, the C and higher hydrocarbons are separated from a crude ethylene fraction consisting of 40 percent by volume of hydrogen and methane, 1 percent by volume of acetylene, 49 percent by volume of ethylene and ethane, and 10 percent by volume of C and higher hydrocarbons, by conventional means, not shown, by compressing the mixture to 35 atmospheres absolute and cooling countercurrently to separate the C and higher hydrocarbons. In a second separation stage, likewise not shown, the hydrogen and part of the methane are separated by conventional means. The c -hydrocarbon fraction containing acetylene, ethylene, ethane, and methane, pass through conduit 1 into methane column 2 in which the hydrocarbon mixture is rectified at a pressure of about 35 atmospheres absolute. The pure methane leaves the column via conduit 3. A condensate is collected in the sump of column 2 which consists of acetylene, ethylene and ethane, along with perhaps traces of c -hydrocarbons and other impurities, e.g., sulfur compounds or carbon dioxide. The acetylene concentration of this condensate has increased to about 2 percent by volume due to the separation of the higher boiling components and those boiling lower than the C -hydrocarbons. The sump product is completely evaporated in evaporator 4 and passes at or slightly above its condensation temperature, i.e., depending on the ratio of the concentration of ethylene ethane, between 8 C. and about +5 C., at a pressure of 35 atmospheres absolute. The evaporated sump product passes through conduit 5 into the lower section of scrubbing column 6. the scrubbing agent employed is N-methylpyrrolidone to which percent by weight of methanol has been added. The solid deposit point of this mixture is about -40" C. Therefore, it can still be utilized as the scrubbing agent when a lower pressure is employed in the scrubbing stage. For example, when the scrubbing column is operated, at atmospheres, the condensation temperature of the C -mixture is lowered to about 35 to 28 C.

The scrubbing agent, after having been precooled to approximately the temperature of the C -mixture to be scrubbed, is introduced via conduit 7 into column 6 several plates below the gas outlet thereof where it mixes with the mixture of ethylene and ethane condensed at coil 8 as reflux liquid. The only heat of reaction in this procedure is the minor amount of mixing heat. The scrubbing agent, thus saturated with liquid etyhlene and ethane, absorbs the acetylene and other impurities, such as propylene and organic sulfur compounds, from the rising c mixture, the heat of solution of the acetylene being compensated by the evaporation of the dissolved ethylene. ln the column section disposed above the solvent feed inlet, the solvent vapors are finally removed from the scrubbed gas by the reflux condensate. The ethylene-ethane mixture leaving the head of column 6 via conduit 9 is thereafter rectified in ethylene column 10. From the head of column 10, pure ethylene is withdrawn via conduit 11 as product containing less than 1 ppm. of acetylene. Ethane is withdrawn from the sump by way of conduit 12.

In order to supply the required amount of refrigeration, a propylene cycle is provided. The propylene is compressed in compressor 13, cooled first in water cooler 14 and then in the countercurrent heat exchanger 15 by heat exchange with cold propylene and ethylene, and then liquefied by heating evaporator 16 of column 10 and finally in countercurrent heat exchanger 17 by further heat exchange with cold propylene and ethylene. One portion is expanded via valve 18 into condenser 19 of column 10, and the other portion is expanded via valve 33 into condenser 8 of column 6, where the propylene evaporates under the condensation of reflux liquid. The cold gaseous propylene is then warmed as set forth above and recycled to the compressor.

The scrubbing agent, the temperature of which has risen somewhat during its descent over the plates due to the heat of solution of the acetylene, leaves scrubbing column 6 via conduit 20 and is warmed in heat exchanger 21. The thus-liberated ethylene is recycled into column 6 via conduit 22. The scrubbing agent is now introduced, via expansion valve 23, into regenerating column 24, which is operated at a pres sure slightly above atmospheric pressure. The sump of the column is maintained by evaporator 25 at the boiling temperature of the mixture of N-methylpyrrolidone with 10 of methanol, i.e., about 117 C. The thusproduced methanol vapors strip the acetylene and the still-dissolved ethylene and ethane from the descending scrubbing agent. The methanol vapor is cooled and condensed in water cooler 26 and in low-temperature cooler 27 and recycled into the column via separator 28. The exhaust gas, which consists of 50-90 percent by volume of acetylene and the remainder ethylene and ethane, leaves the plant via conduit 29. The regenerated scrubbing agent is withdrawn from the sump of column 24 and is introduced, via pump 30, heat exchanger 21 and conduit 7, again into column 6.

The process according to H0. 5 differs from the above-described embodiment in that N-methylpyrrolidone is employed as the scrubbing agent with an only minor content of methanol, which does not influence selectivity, and the methanol required for the production of the necessary stripping vapor is added thereto immediately prior to the regeneration step.

Additionally, about 3 percent by weight of methanol from separator 28 is added to the spent N-methylpyrrolidone containing about 2 percent by weight of methanol and the mixture is then warmed in heat exchanger 21 to about 40 C. and expanded in valve 23 to approximately atmospheric pressure. The N-methylpyrrolidone solution, which now contains about 5 percent by weight of methanol, is warmed in the heat exchanger 31, countercurrently to a portion of the regenerated solvent, to the solution boiling temperature of about C. and then introduced to the head of regenerating column 24. The sump of column 24 is maintained at about C., Le, at a temperature at which the N-methylpyrrolidone is still entirely stable. During this step, the methanol evaporates to a content of about 2 percent by weight, which for all practical purposes does not lessen the selectivity of the absorbent at all, and simultaneously entrains the acetylene, ethylene, and ethane from the N-methylpyrrolidone solution. The mixture of methanol vapor and the C hydrocarbons escaping overhead is first conducted through water cooler 26 and then through low-temperature cooler 27. The thus-condensed methanol is collected in separator 28, to be recycled into the N- methylpyrrolidone solution to be regenerated. The acetylene-containing waste gas is withdrawn via conduit 29. The regenerated N-methylpyrrolidone, with the low methanol content, is drawn by pump 30 through heat exchanger 31 and water cooler 32, further cooled in countercurrent heat exchanger 21, and then recycled to scrubbing column 6 via conduit 7.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

What is claimed is:

l. in a process for the production of acetylene-free ethylene from a mixture of c -hydrocarbons by selective absorption of the acetylene therefrom and separation of the ethane therefrom by rectification, which comprises selectively absorbing the acetylene at a pressure of more than atmospheres absolute with a highboiling polar aprotic solvent boiling above 120C, and regenerating the resultant loaded high-boiling solvent by stripping the acetylene therefrom, and conducting the selective absorption at or slightly above the liquefaction temperature of the C mixture,

the improvement comprising admixing a low-boiling solvent with the resultant loaded high-boiling aprotic polar solvent immediately prior to or during the regeneration thereof, evaporating the lowboiling solvent therefrom in a solvent regenerating zone, condensing the low-boiling solvent, and then admixing the condensed low-boiling solvent with further loaded high-boiling solvent prior to the regeneration thereof.

2. A process according to claim 1, wherein ethane is separated from the ethylene by rectification from the mixture of ethylene and ethane obtained during the selective absorption of the acetylene.

3. A process according to claim 1, wherein the high boiling solvent is N-methylpyrrolidone.

4. A process according to claim 1, wherein the highboiling solvent is hexamethylphosphoric acid triamide.

5. A process according to claim 1, wherein the absorption of the acetylene is conducted with a mixture of the high-boiling solvent and a low-boiling solvent.

6. A process according to claim I, wherein the selective absorption of the acetylene is conducted with a mixture of two polar aprotic solvents boiling above C.

7. A process according to claim 7, wherein the highboiling solvent is regenerated at a temperature below its boiling point by stripping lower-boiling components therein, employing a foreign gas as a stripping gas.

8. A process according to claim 1, wherein the absorption is conducted with a high-boiling solvent saturated with ethylene.

9. A process according to claim 1, wherein the acetylene is absorbed at a pressure of 8-40 atmospheres absolute.

10. A process according to claim 9, wherein the ab sorption is conducted with a high-boiling solvent saturated with ethylene.

II. A process according to claim 10, wherein the high-boiling solvent is regenerated in a solvent regenerating zone maintained below the boiling point of the high-boiling solvent by the presence of said lowboiling solvent therein.

12. A process according to claim 11, wherein the lo -boilin solvent me anol.

l3. A p rocess aczord i rig to claim 1], wherein the high-boiling solvent is N-methylpyrrolidone.

14. A process according to claim 11, wherein the high-boiling solvent is hexamethylphosphoric acid triamide.

15. A process for the production of acetylene-free ethylene from a mixture of C hydrocarbons by selective absorption of the acetylene therefrom and separation of the ethane therefrom by rectification, which comprises absorbing the acetylene with a high-boiling solvent composed of hexamethylphosphoric acid triamide saturated with ethylene, the selective absorption being conducted at 8-40 atmospheres absolute and at or slightly above the liquefaction temperature of the C,-mixture, and regenerating resultant loaded high boiling solvent in a solvent regenerating zone maintained below the boiling point of the high-boiling solvent by the presence of a low-boiling solvent therein.

16. A process according to claim 15, wherein the low-boiling solvent is methanol.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 95,002 Dated October 3, 1972 Inv nt Friedrich Rottmayr, et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE HEADING, UNDER 'FOREIGN APPLICATION PRIORITY DATA" "Sept. 2, 1971 Germany ....P 19 44 505.7" should read Sept. 2, 1969 Germany ....P 19 44 505.7

Signed and sealed this 16th day of April 19714..

(SEAL) Attest:

IJDWAI'iD PLPMQTGHERJR. 3 MARSHALL DANN Attesting Officer Commissioner of Patents F ORM PO-105O (10-69] USCOMM-DC COSTS-P59 n us. aovenuuzn'r nmmus ornc: In! 0-30-33.

Claims

2. A process according to claim 1, wherein ethane is separated from the ethylene by rectification from the mixture of ethylene and ethane obtained during the selective absorption of the acetylene.
3. A process according to claim 1, wherein the high boiling solvent is N-methylpyrrolidone.
4. A process according to claim 1, wherein the high-boiling solvent is hexamethylphosphoric acid triamide.
5. A process according to claim 1, wherein the absorption of the acetylene is conducted with a mixture of the high-boiling solvent and a low-boiling solvent.
6. A process according to claim 1, wherein the selective absorption of the acetylene is conducted with a mixture of two polar aprotic solvents boiling above 120*C.
7. A process according to claim 7, wherein the high-boiling Solvent is regenerated at a temperature below its boiling point by stripping lower-boiling components therein, employing a foreign gas as a stripping gas.
8. A process according to claim 1, wherein the absorption is conducted with a high-boiling solvent saturated with ethylene.
9. A process according to claim 1, wherein the acetylene is absorbed at a pressure of 8-40 atmospheres absolute.
10. A process according to claim 9, wherein the absorption is conducted with a high-boiling solvent saturated with ethylene.
11. A process according to claim 10, wherein the high-boiling solvent is regenerated in a solvent regenerating zone maintained below the boiling point of the high-boiling solvent by the presence of said low-boiling solvent therein.
12. A process according to claim 11, wherein the low-boiling solvent is methanol.
13. A process according to claim 11, wherein the high-boiling solvent is N-methylpyrrolidone.
14. A process according to claim 11, wherein the high-boiling solvent is hexamethylphosphoric acid triamide.
15. A process for the production of acetylene-free ethylene from a mixture of C2-hydrocarbons by selective absorption of the acetylene therefrom and separation of the ethane therefrom by rectification, which comprises absorbing the acetylene with a high-boiling solvent composed of hexamethylphosphoric acid triamide saturated with ethylene, the selective absorption being conducted at 8-40 atmospheres absolute and at or slightly above the liquefaction temperature of the C2-mixture, and regenerating resultant loaded high boiling solvent in a solvent regenerating zone maintained below the boiling point of the high-boiling solvent by the presence of a low-boiling solvent therein.
16. A process according to claim 15, wherein the low-boiling solvent is methanol.