US3635038A

US3635038A - Joint separation of acetylene and ethylene from cracked gases

Info

Publication number: US3635038A
Application number: US825323A
Authority: US
Inventors: Otto Nagel; Rolf Platz; Kurt Taglieber; Kurt Weinfurter; Dieter Wolf
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1968-05-16
Filing date: 1969-05-16
Publication date: 1972-01-18
Anticipated expiration: 1989-01-18
Also published as: BE733158A; NL6907467A; GB1257372A; DE1768460C2; FR2008664A7; CH525697A; DE1768460B1; AT284070B; SE339070B; ES367286A1

Abstract

A gas mixture (obtained by thermal cracking of hydrocarbons followed by quenching of the cracked gas and freed from carbon dioxide and water) is separated into a gas mixture containing hydrogen, carbon monoxide and methane and another gas mixture containing ethylene, acetylene and higher hydrocarbons by (a) cooling the gas mixture which is at superatmospheric pressure in at least one condensation stage to a temperature which is above the solidification temperature of acetylene mixed with the other condensed constituents, (b) separating the condensate thus obtained and subdividing it in a rectifying column into a first mixture containing C2 hydrocarbons and a second mixture containing C3 and higher hydrocarbons, and (c) freeing the residual gas remaining from (a) from residual acetylene and ethylene in a scrubber by treatment with some of the mixture consisting of C3 and higher hydrocarbons from (b).

Description

Nagel et al.

154] J QIN T SEPARATION OF ACETYLENE AND ETHYLENE FROM CRACKED GASES Inventors: Otto Nagel, Hambach; Rolf Platz, Mannheim; Kurt Taglieber; Kurt Weinfurter; Dieter Wolf, all of Ludwigshafen, all of Germany Badische Anilinit Soda-Fabrlk Aktiengesellschatt, Ludwigshafen/Rhein, Germany Filed: May 16, 1969 App]. No.: 825,323

Assignee:

Foreign Application Priority Data May 16, 1968 Germany ..P 17 68 460.5

us. Cl. ..62/17, 62/23, 62/28,

* 55/64, 260/679 A Int. Cl ..F25j 1/00, F253 3/00, F253 3/02 Field of Search ..62/23, 24, 27,28, 29, 39,

References Cited UNITED STATES PATENTS 6/1943 Felbeck .62/ l 7 11/1962 Cost 12/1933 Wullf 1 H1938 Brewster ..62/39 8lillllillll Primary Examiner-Norman Yudkoff Assistant Examiner-Arthur F. Purcell Attorney-Johnston, Root, OKeefe, Keil, Thompson & Shurtleff 1 [57] ABSTRACT A gas mixture (obtained by thermal cracking of hydrocarbons followed by quenching of the cracked gas and freed from carbon dioxide and water) is separated into a gas mixture containing hydrogen, carbon monoxide and methane and another gas mixture containing ethylene, acetylene and higher hydrocarbons by (a) cooling the gas mixture which is at superatmospheric pressure in at least one condensation stage to a temperature which is above the solidification temperature of acetylene mixed with the other condensed constituents, (b) separating the condensate thus obtained and subdividing it in a rectifying column into a first mixture containing C hydrocarbons and a second mixture containing C and higher hydrocarbons, and (c) freeing the residual gas remaining from (a) from residual acetylene and ethylene in a scrubber by treatment with some of the mixture consisting of C and higher hydrocarbons from (b).

6 Claims, 3 Drawing Figures lllllilllili H PATENIEDJmwEm 3.635038 SHEET 1 BF 2 FIG. I

a b c He, CO,CH

-80-IZOC V e f C FIG. 2

G) llllllllllll INVENTORS: OTTO NAGEL 28 ROLF PLATZ KURT TAGLIEBER KURT WEINFURTER DIETER WOLF ATT'YS PATENTEDJANIB-IBTZ- 3.635.038

SHEET 2 0F 2 mllllllllllll INVENTORSI OT O NAGEL ROLF PLATZ KURT TAGLIEBER KURT WEINFURTER DIE ER WOLF JOINT SEPARATION OF ACETYLENE AND ETIIYLENE FROM CRACKED GASES This invention relates to a process for separating gas mixtures which contain hydrogen, carbon monoxide, methane, ethylene, acetylene and higher hydrocarbons, said mixtures having been obtained by thermal cracking of hydrocarbons followed by quenching of the cracked gas.

It is known from US. Pat. application Ser. No. 659,394 filed Aug. 9, 1967, by Rolf Platz et al., now U.S. Pat. No. 3,471,584, that acetylene and ethylene can be separated together from such gas mixtures by first removing carbon dioxide and hydrogen sulfide from the cracked gas, then drying the gas, separating some of the methane and all the other hydrocarbons by condensation and then separating the acetylene and ethylene together with methane and ethane by pressure distillation from a liquid fraction which contains the whole of the C to C, hydrocarbons and any carbon oxysulfide and carbon disfulfide.

This method of separation gives good results but has the disadvantage that the gas mixture, after separation by condensation of the hydrocarbons with the exception of methane, still contains small amounts of ethylene and acetylene so that there is a risk that solid acetylene may separate upon further cooling, for example by flashing.

It is therefore the object of this invention to provide a process which does not have this disadvantage.

We have now found that gas mixtures containing hydrogen, carbon monoxide, methane, ethylene, acetylene and higher hydrocarbons and devoid of carbon dioxide and water, which have been obtained by thermal cracking of hydrocarbons followed by quenching of the cracked gas can be separated into (i) a gas mixture containing hydrogen, carbon monoxide and methane which is practically devoid of other hydrocarbons and (ii) a mixture containing ethylene, acetylene and higher hydrocarbons which may be separated in further stages into the individual components, by a process wherein a. the initial gas mixture which is under superatmospheric pressure, preferably from 5 to 35 atmospheres gauge, is cooled in one or more condensation stages to a temperature which is above the solidification temperature of acetylene in admixture with other condensed constituents, preferably to -82 to -l C.;

b. the condensate thus obtained is separated and subdivided in a rectification column into a mixture containing the C hydrocarbons and a mixture containing C and higher hydrocarbons; and I c. the residual gas mixture which still contains small amounts of acetylene and ethylene is freed from acetylene and ethylene in a scrubber by treatment with some of the mixture containing C and higher hydrocarbons obtained in stage (b).

The term thermal cracking includes the conventional cracking methods in which a gas mixture containing ethylene and acetylene is obtained. It includes in particular the cracking of crude oil or hydrocarbon fractions in a flame burning beneath the surface of the liquid (submerged flame). This method is described in detail in Chem. lng. Technik 26, No. 5, page 253, and British Pat. No. 834,419, to which reference is made.

Preliminary separation of carbon dioxide and hydrogen sulfide may be carried out in known manner by scrubbing, for example with an aqueous solution of an alkali metal salt of an amino acid. This scrubbing, in which most of the carbon dioxide together with most of the hydrogen sulfide is separated, may be followed by afterpurification from carbon dioxide. This afterpurification is advantageously carried out by scrubbing with a dilute solution of an alkali metal hydroxide.

The cracked gas freed from carbon dioxide and hydrogen sulfide is dried before it is subjected to condensation. This drying may be carried out for example by scrubbing or saturation with methanol. The cracked gas freed from carbon dioxide and hydrogen sulfide and treated in this way is cooled in one or more condensation stages to a temperature above the point at which acetylene separates as a solid, for example to from about to l20 C., the condensate containing C and higher hydrocarbons is separated and the residual gas which contains acetylene and ethylene is freed from acetylene and ethylene in a scrubber at superatmospheric pressure, advantageously at the pressure at which the cracked gas is formed, by scrubbing with the mixture of C and higher hydrocarbons obtained as described below. This is shown in the simplified FIG. 1 (in which a denotes cracked gas, b cooler, c residual gas, d scrubber, e condensate, f laden scrubbing liquid, g distillation unit, and h scrubbing liquid) of the drawings which does not take into account countercurrent heat exchange. The gas mixture leaving the scrubbing column is advantageously supplied for the production of cold to an expansion machine for performing external work, in which it is expanded to practically atmospheric pressure, thereby cooled and then after cooling the scrubbing liquid for the scrubbing column is used for cooling the incoming cracked gas countercurrently. The condensate and the bottoms product of the scrubber are released from pressure and also passed countercurrent to the incoming cracked gas through the condensation stages for cooling and condensation of the incoming cracked gas. The mixture of vapor and liquid formed from the condensate by partial evaporation is'compressed to a pressure of from 10 to 20 atmospheres and separated in a rectifying column at pressures of for example 5 to 40 atmospheres gauge, preferably from 10 to 20 atmospheres gauge, into an overhead product containing C hydrocarbons and constituents of lower boiling point, and a bottoms product containing C hydrocarbons and constituents of higher boiling point, and according to one embodiment of the invention (see FIG. 2) liquid ethylene obtained in the subsequent separation of the overhead product into acetylene and ethylene is supplied as a reflux. For further separation the overhead product is subjected for example to scrubbing with acetone. Some of the mixture containing C hydrocarbons and constituents having a higher boiling point is withdrawn and serves as scrubbing liquid for the above-mentioned scrubbing of the residual gas.

According to another embodiment of the invention, the mixture of ethylene andacetylene obtained is not separated into its constituents but used direct as a mixture for further reactions. For this purpose, in accordance with FIG. 3, the condensate consisting of C to C hydrocarbons, after partial reevaporation, is separated in a rectifying column at a pressure of from 1.5 to 4 atmospheres absolute into a mixture of ethylene and acetylene as overhead product and a bottoms product containing C to C hydrocarbons. In other respects, what has been said concerning the first embodiment applies in this case, too.

In order that the residual gas consisting of monoxide, hydrogen and methane should be devoid of carbon disulfide, the scrubbing liquid must not contain any carbon disulfide which has not been removed from the cracked gas by the hydrogen sulfide scrubbing. For this purpose the scrubbing liquid consisting substantially of C and higher hydrocarbons is advantageously not taken from the bottoms of the rectifying column but from a lateral stream withdrawn between the bottom and point of supply of the incoming mixture. In this way the sulfur compounds having a higher boiling point than the said hydrocarbons (essentially carbon disulfide) are removed with the bottoms product. The scrubbing liquid withdrawn as a lateral stream is passed through the heat exchanger of the condensation stages and cooled to a temperature of from about 80 to l20 C. and then supplied to the scrubbing column.

The invention is illustrated by the following examples.

EXAMPLE l A plant as shown diagrammatically in FIG. 2 is used. 10,000 m. (STP)/hour of cracked gas (obtained by an oxygen flame burning beneath the surface of a pool of oil) which has been scrubbed free from carbon dioxide and hydrogen sulfide and has been dried is introduced at a pressure of 8.5 atmospheres absolute and a temperature of +20 C. through line 1 into a heat exchanger 2. The cracked gas has the following composition:

percent by volume CH,OH 0.69.

The cracked gas is cooled to C. and the condensate obtained is separated in a separator 3. The residual gas which remains is passed through line 4 through a heat exchanger 5 in which it is cooled to 1 10 C. and freed from condensate in a 20 separator 6. The residual gas has the following composition (in percent by volume):

inert gas (11,, CO, CH 95.76 ethylene 2.78 acetylene 1 46 1t passes through line 7 into a scrubber 8 where it is substantially freed from acetylene and ethylene by countercurrent treatment with a mixture consisting of 55 percent of C hydrocarbons, 39.5 percent of C hydrocarbons and 5.5 percent of C hydrocarbons introduced through line 9. The residual gas leaving the scrubber 8 through line 10 at 1 12 C.

has an acetylene content of 50 p.p.m. and an ethylene content of 95 p.p.m. it passes through line 10 to an expansion turbine 11 where it is expanded from 8 to 1.5 atmospheres absolute and thus cooled to 163 C. This cold gas is used to cool, in

the

intermediate coolers

13 and 14 of the scrubber 8, the solvent flowing down therethrough, the gas thus being heated up to l35.5 C. Through line 12 it is passed through

heat exchangers

5 and 2, giving up its cold therein and leaving the plant at a temperature of +10 C. The solvent containing acetylene and ethylene from the bottoms of the scrubber 8 is flashed together with the condensate from the separator 6 through a valve into a line 15 and passed through heat exchanger 5 to give up its sensible heat and heat of evaporation. It is then mixed with the condensate coming from separator 3 through line 16 and released from pressure. In order to give up its sensible heat of evaporation, this mixture is passed by line 15 through heat exchanger 2 which it leaves at about 5 +10 C. Gas and liquid are separated in separator 17, the pressure of the liquid is raised by a pump 18 and supplied through an ammonia-cooled heat exchanger 19 at 20 C. to a rectifying column 20. The gas leaving separator 17 through line 21 is compressed by compressor 22 to 18 atmospheres absolute, cooled to 25 C. with ammonia in heat exchanger 23 and also supplied to the column 20. Here the hydrocarbon mixture is separated into higher hydrocarbons and into a bottoms product containing sulfur compounds such as carbon disulfide. To maintain a mixture of acetylene and ethylene which cannot be ignited by a detonator, liquid ethylene is supplied through line 24 and also gaseous ethylene partly through line 25 from outside the plant into the column 20. The amount of 3,500 kg./hour of solvent required for the scrubber 8 is 5 withdrawn through line 26 from a tray lying between the bottoms and the point of supply of the C -C hydrocarbon mixture, precooled with water in heat exchanger 27, then after expansion to 8.5 atmospheres absolute passed by line 9 through

heat exchangers

2 and 5 and thus cooled to -1 10 C., and passed into scrubber 8. The mixture consisting of C hydrocarbons and higher hydrocarbons and a part of the sulfur compounds and which has been condensed out and separated from the cracked gas entering the plant is withdrawn from the plant from the bottom of the rectifying column 20 through line 28.

The reflux necessary for rectification is produced with a condenser 29 cooled with ammonia. The mixture of C hydrocarbons (consisting substantially of acetylene and ethylene and containing only a few p.p.m. of higher hydrocarbons) leaving the condenser through line 30 is withdrawn from the plant, supplied to a scrubber and separated into the individual components.

EXAMPLE 2 For the production of a mixture of acetylene and ethylene according to P16. 3, 10,000 m. (STP) per hour of dried cracked gas (from which carbon dioxide and hydrogen sulfide have already been removed) is introduced through line 1 at a pressure of 8.5 atmospheres absolute and a temperature of +10 C. into a heat exchanger 2. The cracked gas has the composition given in example 1. After cooling to 20 C. in heat exchanger 2, the condensate formed is separated in a separator 3. The gas is cooled to -1 10 C. in a heat exchanger 5 and freed from condensate in a separator 6. The remaining gas has the composition given in example 1. Residual acetylene and ethylene is scrubbed out in scrubber 8 with a C C fraction. Connection of the scrubber and expansion turbine 11 is identical with the arrangement described in example 1. The condensate collected in line 15 is heated to 0 C. in

heat exchangers

5 and 2 to utilize sensible and evaporation heat. After cooling with the overhead product from column 20 in heat exchanger 31, the supply to the column 20 is separated in a separator 17 into a liquid phase and a gas phase which are supplied to different trays according to their composition.

Since according the procedure of this example the C mixture is not separated, it is not possible to supply ethylene for the rectifying section of the column 20.

To avoid decomposition of acetylene the column is therefore operated at low pressure (2 atmospheres absolute). Contrasted with the procedure of example 1, compressor 22 and pump 18 are not required here. The cold necessary for the top condenser 29 is produced by an ethylene refrigeration plant. The mixture of higher hydrocarbons obtained at the bottom of column 20 is withdrawn through line 28.

We claim:

1. A process for separating a gas mixture containing hydrogen, carbon monoxide, methane, ethylene, acetylene and higher hydrocarbons which is devoid of carbon dioxide and water and which has been obtained by thermal cracking of a hydrocarbon followed by quenching of the cracked gas, which process comprises a. cooling the initial gas mixture which is under superatmospheric pressure in at least one condensation stage to a temperature which is sufficient to condense the C and higher hydrocarbons but which is above the solidification temperature of acetylene in admixture with other condensed constituents;

b. separating and dividing the condensate thus obtained in a rectifying column into a first mixture consisting essentially of the C hydrocarbons and a second mixture containing the C and higher hydrocarbons; and

c. freeing the residual uncond'ensed gas mixture of step (a) which still contains small amounts of acetylene and ethylene from said acetylene and ethylene in a scrubber by treatment with some of said second mixture containing C and higher hydrocarbons obtained as a liquid condensate from step (b), thereby obtaining a separate third gas mixture which contains hydrogen, carbon monoxide and methane and which is practically devoid of other hydrocarbons.

2. A process as claimed in claim 1 wherein the initial gas mixture under (a) is at from 5 to 35 atmospheres gauge.

3. A process as claimed in claim 1 wherein cooling under (a) is to from 82 to l20 C.

4. A process as claimed in claim 1 wherein the residual gas freed from acetylene and ethylene and leaving the scrubber at a temperature of from to 1 20 C. is expanded in an exevaporated in countercurrent heat exchange.

6. A process as claimed in claim 1 wherein a portion of the mixture containing C and higher hydrocarbons is withdrawn at a lateral point between the bottom of said rectifying column and the point at which the condensate to be separated is supplied.

Claims

2. A process as claimed in claim 1 wherein the initial gas mixture under (a) is at from 5 to 35 atmospheres gauge.
3. A process as claimed in claim 1 wherein cooling under (a) is to from -82* to -120* C.
4. A process as claimed in claim 1 wherein the residual gas freed from acetylene and ethylene and leaving the scrubber at a temperature of from -80* to -120* C. is expanded in an expansion machine while performing external work so that the gas mixture is cooled, the gas is used for intermediate cooling of the scrubbing liquid of the scrubber in one or more intermediate coolers and then used for countercurrent cooling in the condensation stages.
5. A process as claimed in claim 1 wherein for cooling the gas mixture in the condensation stages, the condensate obtained is released from pressure and wholly or partly evaporated in countercurrent heat exchange.
6. A process as claimed in claim 1 wherein a portion of the mixture containing C3 and higher hydrocarbons is withdrawn at a lateral point between the bottom of said rectifying column and the point at which the condensate to be separated is supplied.