US2471602A - Treatment of hydrocarbons - Google Patents

Description

May 31, 1949! P. M. ARNoLD TREATMENT 0F HYDROCARONS y Filed lay 1, 1944 Patented May 31, 1949 UNITED STATES PATENT ori-ICE 2,471,602 l TREATMENT F HYDROCARBON S Philip M. Arnold, Bartlesville, Okla.. assignor to Phillips Petroleum Company, a corporation of Delaware Application May 1, 1944, Serial No. 533,614

(Cl. 62--175L5) 6 Claims.

This invention relates to a. process f-or removing ethylene from a mixture of gases containing the same. More particularly, it relates to a process of removing ethylene from a mixture of gases containing, in addition to ethylene, hydrogen and methane, usually ethane, and very frequenly C3 hydrocarbons, namely, propylene or propane or both. v

For many purposes it is often desirable to have available substantially pure ethylene. When the stream at hand contains minor proportions of ethylene, processes whereby substantially pure ethylene may be recovered therefrom arev extremely valuable. I

The -principal object of the present invention is to provide an improved process for recovering ethylene from a feed containing the same. Another object is to recover ethylene from a feed containing the same in admixture with hydrogen,

and methane. Another object is to provide an improved method of recovering substantially pure ethylene from a feed stream containing the same in admixture with hydrogen, methane and ethane, C3 hydrocarbons being present or absent. Another object is to provide such a process which is simple and economical to operate and which produces a maximum of substantialpresence or absence of ethane or C3 hydrocarbons,

by passing such a feed to preliminary fractionator zone, there removing an -overhead product comprising most of the hydrogen contained in the feed, some methane and practically no ethylene, passing the liquid bottoms product, with or Without subjection to an intermediate flashing step t0 eiect further removal of hydrogen, to a demetham'zing zone and there fractionating the same, removing from the demethanizing zone an overhead of methane, usually containing more than 85 mol per cent of methane, condensing this overhead of methane, reiiuxing the demethanizing zone with part of the liquid condensate, and expanding the balance of the liquid methane condensate in heat exchange with theoverhead vapors from the preliminary fractionation .zone to provide reflux therefor and minimize the amount of ethylene leaving in the overhead -product therefrom.

Where a flashing step is interposed between the preliminary fractionation and the demeth- -anizing steps, it is preferred to recycle the flashed-off vapors into admixture with the incoming raw ethylene-containing feed. This is desirable because these flashed-ofi? vapors usually contain a substantial amount of ethylene which is too great to vbe thrown away or. passed to fuel.

The preliminary fractionation step may be conducted in a fractionating column which is provided with only a few trays in the upper Dortion of the column, the lower portion of the column being free from trays or other contacting means -if desired. Generally the preliminary fraction-ation is conducted at pressures ranging from 700 to 900 pounds per square inch and with the incoming feed at temperatures of from to 140 F. Preferably this operation is conducted at 800 pounds pressure with a top temperature of 160 F. and a bottom temperature which is the same as the feedtemperature which is preferably F. The overhead product from the preliminary fractionation step generally comprises at least 85 mol per cent of the hydrogen 4contained in the feed, some methane and not over 0.5 mol per cent of ethylene.

The preliminary fractionation zone is reiluxed by indirect heat exchange with a portion of the expanded liquid methane overhead derived from the subsequent demethanizing operation. This provides the necessary reflux for the preliminary fractionation operati-on and prevents ethylene from leaving in the overhead product therefrom.

The bottoms product from the preliminary fractionation step is desirably flashed to a lower pressure of from 250 to 350 pounds per square inch and to a temperature ranging from -130 to 150 F. This .serves to further remove hydrogen. Desirably where, 'the preliminary fractionation is operated at 800 pounds .per square inch pressure and with a feed temperature of 130 F.and a top temperature of 160 F., the

.ashing is conducted to a pressure of 300 pounds per square inch and a temperature of F. Since the dashed-off vapors usually contain at least 5 mol per cent of ethylene, it is usuallyl desirable to recompress them to the pressure of the incoming feed and recycle them therewith.

The purpose of the preliminary fractionation step and, where used, the flashing operation, is-

change with the preliminary fractionator overhead.

The liquid bottoms product from the preliminary fractionation step.. whether orV not the intermediate ashing step has been employed, is next fed to the demethanizing column which is conveniently operated at a pressure ranging from 550 to 650 pounds per square inch, a` gure of 600 pounds per square inch being preferred. It is preferable that the feed to the demethanizing step be at a fairly low temperature, preferably about 55 F. It has been found preferable to operate the demethanizing column with a top temperature of 85.5 F. and a bottoms temperature of 67 F. Under these conditions the demethanizer overhead contains over 85 mol per cent methane. This overhead product is condensed, usually by ethylene refrigeration, and the resulting condensate. is employed in part to reflux the demethanizingcolumn and in part to provide reflux for the preliminary fractionation column as described above. l

The bottoms product from the demethanizing column is cooled to a temperature suitable for the feed to the ethylene column and is fed thereinto.

In the ethylene column the feed which has been practically freed from methane and completely freed from hydrogen, is separated into `an overhead fraction of substantially pure ethylene and a bottoms fraction containing the ethane and any heavier components of the feed such as propane and propylene. 1t has been found convenient to operate the ethylene column with a feed temperature of 6 F., a bottoms temperature of 65 F., a top temperature of 16 F., and at a pressure of 325 pounds per square inch. The overhead from the ethylene column is condensed, usually by propane refrigeration, and the resulting liquid at 25 F. is employed to supply reflux for the column.

Referring to the drawing, the raw ethylenecontainlng feed enters the system via line l and after being cooled down by heat exchange inheat exchanger 2 passes via line 3 through cooler 4 Where it is brought to a temperature at which essentially all the ethylene present is in the liquid phase. The stream is then passed via line 5 topreliminary fractionator 6 which is provided with two trays in its upper section and no trays in its lower section. The overhead from the preliminary-fractionator is condensed in condenser l intowhich a portion of the demethanizer overhead is expanded in indirect heat exchange. The re'sultingcondensate passes via line 8 into reflux accumulator@ whencea portion is returned as reuxvi'a'line l0 and the balance consisting essentially lof hydrogen and methane passes via lines Il and l2 toheat exchanger 2 and thence to fuel.

The bottoms frompreliminary fractionator 6 leavesvia line i3 and is expanded through valve I4 into flash'tank I5 where a gaseous fraction of hydrogen and methane and some ethylene leaves via line I8 and is recycled through compressor I1 and line I8.

The flashed liquid bottoms product leaves flash tank I5 via line I9 whence it is brought up to the demethanizing pressure and passed throughheat exchanger 20, where it exchanges heat with the bottoms from demethanizer 2l, and thence vialine 22 to demethanizer 2l. there is taken an overhead fraction which is condensed incondenser 23, supplied with ethylene refrigeration. The condensate passes vialine 24 intoreflux accumulator 25 whence a portion of liquid condensate is returned as reux vialine 23. The balance of the condensate, which includes any gaseous phase which may have separated inreflux accumulator 25, Vis passed vialine 21 throughexpansion valve 28 whence the expansion products pass in indirect heat exchange with the overhead frompreliminary fractionator 6. The

`expansion products leaving condenser 1 merge with the preliminary fractionator overhead product by means ofline 30 and pass to fuel.

The liquid bottoms product from demethanizer 2l iswithdrawn via. line 3| and after being cooled by heat exchange inunit 20 passes vialine 32 to theethylene column 33 where it is resolved into an overhead product of substantially pure ethylene leaving vialine 34 and a bottoms product of the ethane and heavier leaving vialine 35. The overhead is condensed by propane refrigeration in condenser 36 and passed vialine 31 toreflux accumulator 38 from which the necessary reflux is passed via line 39 -to the top of ethylene column 33'.

Example A crude ethylene-containing feed to be purified had the following analysis:

Mol per cent Hydrogen 15.67 Methane 34.80 Ethylene 26.87 Ethane 11.39 Propylene 7.81 Propane 3.46

This feed was brought by propane refrigeration to 25 F. and then by ethylene refrigeration to 130 F., while at 800 pounds per square inch. It Was then passed to a preliminary fractionator as shown in the drawing 4operated at 800 pounds pressure with atop temperature of 160 F. and a bottom temperature. of 130 F. The overhead v product withdrawn to fuel had the following analysis:

Mol per cent Hydrogen 70.29 Methane 28.57 Ethylene 0.46 Ethane 0.05

The bottom product from the preliminary fractionator was then flashed to 300 pounds pressure and a temperature of F. There was taken off a gaseous stream having the following analysis:

Mol per cent Hydrogen 19.74 Methane 68.54 Ethylene 8.72 Ethane 2.06 C3 hydrocarbons 0.94

The flashed liquid bottoms which wasvthe feed to the demethanizing step had the following analysis:

The liquid bottoms from theash tank was raised to a pressure of 600 pounds and cooled by heat exchange to 55 F. whereupon it Was fed into the demethanizing column which was operated with a bottom temperature of 67 F. and a lowing analysis:

top temperature of 85.5 F. The overhead was cooled by ethylene refrigeration and passed to a reflux accumulator maintained at 600 pounds pressure and 123 F. A portion of the condensate was employed as reiiux in a ratio of from 1.4:1

based on the overhead product. The balance of the overhead was withdrawn from the reflux accumulator and had the following analysis:

Mol per cent I This stream was expanded to a pressure of 75 pounds and a temperature of 210 F. and passed in heat exchange with the preliminary fractionator overhead. The resulting expansion products were merged with the gaseous overhead product from the preliminary fractionator and passed to fuel.

The demethanizer bottoms product had the fol- Mol per cent Hydrogen 0.00 Methane 1.02 Ethylene 53.17 Ethane 22.84 Propylene 15.92 Propane 7.05

Hydrogen 0 Methane 1.86 Ethylene 95.43 Ethane 2.71

The bottoms product from the ethylene column had the following analysis:

Mol per cent Hydrogen Methane 0 Ethylene 1.67 Ethane 47.37

Propylene 35.33 Propane 15.63

In the foregoing example and in the specific discussion of the drawing, the demethanizing column had 25 trays and a diameter of 2 ft. The ethylene column was a 60 tray column and had a diameter of 2 ft. 6 inches. The ethylene column was providedv with reflux in the ratio of 5.5:1 based on the overhead product.

From the foregoing, it will be seen that the present invention provides a, simple and economical method of recovering substantially pure ethylene from crude feed. The refrigeration requirements of the process are at a minimum and the equipment required is simple and easy to operate. The amount of equipment is likewise kept at a minimum. The processl gives a very pure ethylene product with a minimum of loss of ethylene in the gaseous hydrogen and methane fraction and in the ethane and heavier fraction. Numerous other advantages of the process of the present' invention will be apparent to those skilled in the art.

I claim:

1. The process of recovering ethylene from a feed stock containing the same in admixture with hydrogen and methane which comprises subjecting said feed stock to a fractionation operation and removing therefrom an overhead product comprising most of the hydrogen, some methane and practically no ethylene, passing the liquid bottoms from the fractionation operation to a demethanizing operation and therein fractionating same, removing an overhead product of methane from said demethanizing operation, and condensing this overhead product, refluxing said demethanizing operation with a portion of this condensed liquid methane, and flashing the balance lof the condensed liquid methane and passing same in heat exchange with the overhead vapors from the first fractionation operation whereby a portion of said overhead vapors is condensed to a liquid, and refiuxing said first fractionation operation with said condensed liquid, and removing bottoms of ethylene from said demethanizing 0peration.

2. The process of recovering ethylene from a feed stock containing same in admixture with hydrogen and methane which comprises subjecting said feed stock to a fractionation operation and therefrom removing an overhead product comprising most of the hydrogen, some methane and practically no ethylene, and a liquid bottoms product, flashing this bottoms product and removing therefrom additionalhydrogen, passing the resulting flashed liquid bottoms product to a demethanizing operation and therein fractionating same, removing an overhead product of methane from said demethanizing operation, and condensing this overhead methane product, reiluxing said demethanizing operation with a portion of this condensed liquid methane, and flashing the balance of the condensed liquid methane and passing same in heat exchange relation with the overhead vapors from the first mentioned fractionation operation whereby a portion of the last mentioned overhead vapors is condensed to a l liquid, and reiiuxing said rst mentioned fractionation operation with said condensed liquid .to

prevent ethylene from entering the overhead` product therefrom, and removing bottoms of ethylene from saidl demethanizing operation.

3. The process of recovering ethylene from a feed stock containing the same in admixture with hydrogen` and methane which comprises subjecting said feed stock to afractionation operation and therefrom removing an overhead product comprising most of the hydrogen, some methane and practically no ethylene and a liquid bottoms product, flashing this bottoms product to a lower pressure and removing from this flashing operation additional hydrogen and methane in admixture with an amount of ethylene, and recycling this mixture into the feed stock prior to the first mentioned fractionation operation, passing the resulting ashed liquid bottoms product to a demethanizing operation and therein fractionating same, removing an overhead product of methane from said demethanizing operation, and condensing lthis overhead methane product, refluxing said demethanizing operation with a portion of this condensed liquid methane, and flashing the balance of the condensed liquid methane and passing same in heat 'exch-ange relation with the overhead vapors from the first mentioned fractionation operation whereby a portion of the last mentioned overhead vapors the overhead product therefrom, and removing bottoms of ethylene from said demethanizing operation.

4. The process of recovering ethylene from a feed stock containing the same in admixture with hydrogen, methane and ethane which comprises subjecting said feed stock to a fractionation operation and therefrom removing an overhead product comprising most of the hydrogen, some methane and practically no ethylene and a liquid bottoms product, and passing this bottoms product to a demethanizing operation and therein fractionating same, removing an overhead product of methane from said demethanizing operation, and condensing this overhead methane product, refiuxing said demethanizing operation with a portion of this condensed liquid methane, and ashing the balance of the condensed liquid methane and passing same in heat exchange relation with the overhead vapors from the rst mentioned fractionation operation whereby a portion of the last mentioned overhead vapors is condensed to a liquid and reuxing said first mentioned fractionation operation with said condensed liquid to prevent ethylene from entering the overhead product therefrom, and passing liquid bottoms separated in the demethanizing operation into an ethylene fractionation operation and therein separating an overhead product of ethylene of at least 95% purity from the ethane and any heavier components contained in the feed stock.

5. The process of recovering ethylene from a feed stock containing the same in admixture with hydrogen, methane, and material higher boiling than ethylene including ethane which comprises subjecting said feed stock at a pressure of from 700 to 900 pounds per square inch and a temperature of from -120 to 140 F. to a fractionation operation operated at said pressure and therein removing an overhead product comprisingat least 85 mol percent of the hydrogen, some methane and not over 0.5 mol per cent ethylene, and a liquid bottoms product, flashing this liquid bottoms product to a pressure of from 1.

per square inch and therein fractionating same,

removing an overhead product of methane from said demethanizing operation, and condensing this overhead methane product, refluxing said demethanizing operation with a portionof this condensed liquid methane, and flashing the balance of the condensed liquid methane and passing same in heat exchange relation with the Number overhead vapors from the first mentioned fractionation operation whereby a portion of the last mentioned overhead vapors is condensed to a liquid, and reiluxing said rst mentioned fractionation operation with said condensed liquid to prevent ethylene from entering the overhead product therefrom, and passing liquid bottoms separated in the 'demethanizing operation into an ethylene fractionation operation 'operated at from 300 to 350 pounds per square inch and therein separating an overhead product of ethylene of at least purity from the ethane and any heavier components contained in the feed stock.

6. The process recovering ethylene from a raw ethylene containing feed stock which also contains hydrogen, methane, propane and propylene in addition to ethane which comprises cooling said feed stock to a temperature of F. while maintaining it under a pressure of 800 pounds per square inch and thereby condensing the ethylene content into the liquid phase, subjecting the resulting mixture to a fractionation operation and therein removing most of the hydrogen therefrom, flashing the remaining liquid product to a pressure of 300 pounds per square inch and removing from this dashing operation essentially all of the hydrogen content thereof in admixture. with a small amount of ethylene and some methane, recompressing the flashed off gases to 800 pounds pressure per square inch and recycling same into adrnixture with the raw feed stock, demethanizing by fractional distillation the liquid resulting from the flashing step at a pressure of 600 pounds per square inch with a top fractionator temperature of --85" F. and thereby removing the methane content thereof overhead, condensing the overhead methane product and refiuxing said demethanizing operation with a portion of the condensed methane product, flashing the balance of the condensed methane product to 75 pounds pressure per square inch and a temperature of 210 F. and passing the flashed product in heat exchange with the overhead vapors from the first mentioned fractionation operation, and fractionally distilling the bottoms product from said demethanizing operation to separate an overhead fraction of ethylene from the bottoms fraction containing the ethane and heavier components of the feed file of this patent:

UNITED STATES PATENTS Name Date 1,853,743 Pollitzer Apr. 12, 1932 2,180,435 Schlitt Nov. 21, 1939 2,214,790 Greenwalt Sept. 17, 1940 2,267,761 Steward Dec. 30, 1941

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