US3268615A

US3268615A - High pressure cracking

Info

Publication number: US3268615A
Application number: US162921A
Authority: US
Inventors: Iii Walter F Keenan; Roger S Hovey
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1961-12-28
Filing date: 1961-12-28
Publication date: 1966-08-23
Anticipated expiration: 1983-08-23

Description

3,268,6 l5 Patented August 23, 1966 3,268,615 HEGH FRESURE CRACKING Walter F. Keenan Ill, and Roger S. Hovey, Charleston, W. Va., assignors to Union Carbide Corporation, a corporation of New York No Drawing. Filed Dec. 28, 1961, Ser. No. 162,921 9 Claims. (Cl. 260-683) This invention relates to a process for cracking hydrocarbons at high pressure. More particularly, this invention relates to the cracking of the hydrocarbon components heavier than methane which are present in natural gas by subjecting natural gas in its original state to a thermal cracking process.

One of the major methods for producing ethylene is the pyrolysis or cracking of ethane and propane and heavier fractions which have been recovered from natural gas. The ethane, propane, and heavier fractions usually are separated from the natural gas prior to cracking by cooling the natural gas at well-head or pipe-line pressures (about 600 p.s.i.g.) to a temperature of about -l C. The gas is further cooled by a Joule-Thompson expansion from 600 p.s.i.g. to a pressure of about 100 p.s.i.g. As a result of this cooling, substantially all of the components of the natural gas heavier than methane are liquified and these components after separation from the methane bylow temperature distillation procedures, are cracked at low pressure. Often steam is added to the hydrocarbon feed as a diluent to reduce the partial pressure of the hydrocarbons and thereby facilitate the cracking. After cracking, the efiluent is dried, compressed, and fractionated to recover the product ethylene and other olefins produced thereby.

It has now been discovered that the above-described preliminary fractionation, low pressure cracking, and recompression procedures are no longer necessary and that natural gas at well head or pipe line pressures and in its original state can be subjected to a direct cracking process, whereby ethylene is obtained in high yields. The methane present in the natural gas serves as a diluent, thereby eliminating the need for the addition of steam or other inert material as a diluent. Thus, by the process of this invention, the production of ethylene from natural gas is made more economical, for the apparatus required for the initial distillation and the recompression of the cracked efiiuent are eliminated and steam is eliminated as a diluent.

Although the foregoing discussion has been limited to the cracking of natural gas at well-head or pipe line pressures, it is Within the scope of this invention that any gas having a composition similar to natural gas, that is, at least about 55 volume percent methane, and preferably from about 80 to about 90 volume percent methane, the balance consisting primarily of hydrocarbon components having from 2 to about 8 carbon atoms, with minor quantities of inert material, such as hydrogen, nitrogen, and the like, can be employed in this process provided it is under pressure at its source. Accordingly, by the term natural gas as employed herein is meant any gas comprising from about 55 to 95 volume percent methane, from about 5 to about 45 volume percent hydrocarbons having from 2 to about 8 carbon atoms, and up to about 10 volume percent inert materials such as hydrogen, nitrogen, carbon dioxide, helium, and the like. By the term source pressure is meant the pressure of the gas at a point either mediately or immediately prior to the cracking process of this invention. Thus, the phrase natural gas at its source pressure encompasses gases as described above which are initially recovered under pressure, such as natural gas or refinery gas streams resulting from petroleum refining processes conducted at elevated pressure, and gases which, although initially not at elevated pressure, are pressurized for purposes of transportation by pipe line, tank cars, and the like, or for storage.

In general, the cracking can be conducted at pressures greater than p.s.i.g. The maximum pressure employed is normally about 1500 p.s.i.g. Pressures of from about 400 p.s.i.g. to about 600 p.s.i.g. are preferred. The temperature at which the cracking is conducted can be varied from about 900 C., or lower, to about 1200 C., or higher, with temperatures of from about 950 C. to about 1100 C. preferred. Cracking times of from 0.05 second or less to 0.5 second or more can be employed, with cracking times of from 0.1 to 0.3 second preferred.

After cracking, the product gas is separated into its components such as ethylene and propylene according to methods known to those skilled in the art. Uncracked components can be recycled as desired.

The process of this invention can be conducted in any apparatus capable of operating at the above-recited pressures and temperatures, such as tubular furnaces, moving bed furnaces, regenerative furnaces, and the like.

In a preferred embodiment of the process of this invention the apparatus employed is a regenerative reactor consisting essentially of two beds of refractory material connected by a conduit which serves as a cracking zone. Natural gas is passed through one of the refractory beds where the gas is heated to approximately the cracking temperature by heat retained by the refractory material from a previous cycle. The heated gas is then cracked in the cracking zone and the cracked eflluent is passed to the second refractory bed where it is quenched to below cracking temperatures and further cooled to about ambient temperatures. When the temperature of the first refractory bed falls too low to sufficiently preheat the feed gas, the gas flow is reversed and the cycle: is repeated, the second refractory bed serving as a preheater and the first refractory bed serving as a quencher and cooler for the cracked efi luent.

The refractory material employed can be any of those known to the art that is capable of withstanding the conditions employed. As examples of suitable refractory materials one can mention silica brick, sillimanite, mullite, kaolin, magnesite, fused alumina, and the like. The physical form of the packing can be any form known to the art, such as brick, pebbles, checkerwork, and the like.

The refractory bed which serves as the preheater has an initial temperature gradient ranging from about the temperature of the feed gas or above to from about 700 C. to about 1200 C. or higher. The feed gas is thus heated to from about 700 C. to about 950 C. before introduction to the cracking zone.

As previously indicated, the cracking zone is a conduit, the internal temperature of which is maintained at about 900 C. to about 1200 C., and preferably at about 950 C. to about 1100 C. Heat for cracking; the natural gas and to make up for heat losses is preferably supplied to the cracking zone by a high pressure burner wherein methane or other fuels are burned in admixture with oxygen or air. The heat alternatively may be supplied by external heating means, such as by an external heating jacket, heating coils, and the like.

The feed rate of the natural gas to the furnace is such that the residence time in the cracking zone is preferably from about 0.1 to about 0.3 second.

The cracked effiuent then passes from the cracking zone to the second refractory bed which has an initial temperature gradient from a maximum of about 700 C. to a minimum of about ambient temperature, wherein the cracking eflluent is rapidly quenched to about 700 C. and then cooled to about ambient temperature, preferably 3 to the temperature of the feed gas to the first refractory bed.

When the temperature of the gas leaving the first or preheater bed falls below about 700 C. to about 950 C., the cycle is reversed and feed gas is passed through the second bed, which now serves as a preheater, the cracking zone, and then the first refractory bed which now serves as a quencher.

The following example is illustrative:

The apparatus employed consisted of a 3 mm. ID. x mm. O.D. fused silica tube passing through a combustion analysis type electric furnace. The heated section of the tube, or cracking zone, was about 6 inches long. The temperature of the cracking zone was measured by a thermocouple aflixed to the outside of the tube and was 960 C. Natural gas at a pressure of 250 p.s.i.g. was fed from a gas cylinder through the tube at a rate such that the residence time of the gas in the cracking zone was 0.107 second. The cracked effluent was sampled periodically and analyzed with a mass spectrometer, whereby the ethylene yield, based upon the hydrocarbon components heavier than methane in the feed gas, was found to be 52 percent.

Employing apparatus and procedures similar to those employed above, two additional runs were conducted in which natural gas was cracked at 400 p.s.i.g. and 984 C., and 600 p.s.i.g. and 1015 C., respectively, employing residence times of from 0.1 to 0.2 seconds. The cracked efiluent from each of these runs was quenched in a watercooled condenser and 50-liter samples of the gas were collected in a liquid nitrogen-cooled Dewar flask over a period of minutes. The samples thus obtained were fractionated in a low-temperature fractionating column to obtain four fractions:

(1) A small initial fraction containing some methane and all the hydrogen and nitrogen in the effluent gas;

(2) A large fraction of essentially pure methane;

(3) A fraction containing hydrocarbons having two carbon atoms;

(4) A fraction containing hydrocarbons having three or more carbon atoms.

Each fraction was then subjected to a mass spectrometer analysis and an Orsat analysis.

The analysis of the inlet and outlet gases and the cracking conditions for all three runs are set forth in tabular form below, together with the yield of ethylene, based upon C components in the feed gas.

Run No 1 2 3 Pressure p.s.i.g 250 400 600 Temperature, o.-. 960 984 1, 015 Time, seconds 0. 107 0. 1-0. 2 0. 1-0. 2

Inlet Gas, Volume Percent 100.0 100. 0 100.0 Carbon/Hydrogen Ratio 0. 2670 0. 2681 0. 2681 Outlet Gas, (JR/100 OF. of Inlet Gas 5. 3 6. 7 6. 9 91.2 92. 9 92. 0 0.6 0. 6 0.8 5. 3 4. 9 4. 0 3. 0 3.1 3. 7 trace trace trace 1. 0 0.9 0.8 0. 2 0. 2 0.1 0. 2 0. 2 0.2 0.3 0. 3 0. 4

107.1 109. 8 109. 5 Carbon/Hydrogen Ratio 0. 2679 0. 2642 0. 2638 02114 Yield, Percent..- 52.0 46. d 37. 9 Percent of C Ht in CZS- 63. 6 60. 7 51.8

What is claimed is:

1. The process for thermally cracking the hydrocarbon components which have at least two carbon atoms present in natural gas, said natural gas containing at least 55 volume percent methane and being under elevated pressure at its source, which comprises heating said natural gas, which is maintained substantially at its source pressure, to a temperature of from about 900 C. to about 1200 C. whereby the hydrocarbon components of said natural gas which have two or more carbon atoms are thermally cracked, said process being conducted in the absence of added steam.

2. The process for thermally cracking the hydrocarbon components which have at least two carbon atoms present in natural gas, said natural gas containing at least 55 volume percent methane and being under a pressure of from 100 p.s.i.g. to 1500 p.s.i.g. at its source, which comprises heating said natural gas, which is maintained substantially at its source pressure, to a temperature of from about 900 C. to about 1200 C. whereby the hydrocarbon components of said natural gas which have two or more carbon atoms are thermally cracked, said process being conducted in the absence of added steam.

3. The process for thermally cracking the hydrocarbon components which have at least two carbon atoms present in natural gas, said natural gas containing at least 55 volume percent methane and being under elevated pressure at its source, which comprises passing said natural gas which is maintained substantially at its source pressure, through a cracking zone that is maintained at a temperature of from about 900 C. to about 1200 C., the residence time of said natural gas in said cracking zone being from 0.05 second to 0.5 second, whereby the hydrocarbon components of said natural gas which have two or more carbon atoms are thermally cracked, said process being conducted in the absence of added steam.

4. The process for thermally cracking the hydrocarbon components which have at least two carbon atoms present in natural gas, said natural gas containing at least 55 volume percent methane and being under a pressure of from 100 p.s.i.g. to 1500 p.s.i.g. at its source, which comprises passing said natural gas, which is maintained substantially at its source pressure, through a cracking zone that is maintained at a temperature of from about 900 C. to about 1200 C., the residence time of said natural gas in said cracking zone being from 0.05 second to 0.5 second, whereby the hydrocarbon components of said natural gas which have two or more carbon atoms are thermally cracked, said process being conducted in the absence of added steam.

5. The process for thermally cracking the hydrocarbons having two or more carbon atoms present in a gas containing 55 to volume percent methane, 5 to 45 volume percent hydrocarbons having from 2 to 8 carbon atoms, and up to 10 volume percent inert materials, which comprises heating said gas under a pressure of from p.s.i.g. to 1500 p.s.i.g. at a temperature of from 900 C. to 1200 C. whereby said hydrocarbons having two or more carbon atoms are thermally cracked, said process being conducted in the absence of added steam.

6. The process for thermally cracking the hydrocarbons having two or more carbon atoms present in a gas containing 55 to 95 volume percent methane, 5 to 45 volume percent hydrocarbons having from 2 to 8 carbon atoms, and up to 10 volume percent inert materials, which comprises passing said gas under a pressure of from 10.0 p.s.i.g. to 1500 p.s.i.g. through a cracking zone which is maintained at a temperature of from about 900 C. to about 1200 C. the residence time of said gas in said cracking zone being from 0.05 second to 0.5 second, whereby said hydrocarbons having two or more carbon atoms are thermally cracked, said process being conducted in the absence of added steam.

7. The process for thermally cracking the hydrocarbon components having at least two carbon atoms present in natural gas, said natural gas containing at least 55 volume percent methane and being under elevated pressure at its source, which comprises passing said natural gas, which is maintained substantially at its source pressure through a first bed of a refractory material having a maximum temperature of from 700 C. to 1000 C., whereby said natural gas is heated to a temperature of 700 C. to 950 C.; passing the heated natural gas through a heated cracking zone maintained at a temperature of from 950 C. to 1100 C. whereby the hydrocarbon components heavier than methane which are present in the natural gas are cracked; passing the cracked effluent through a second bed of refractory materials, said second bed having an initial maximum temperature of 700 C. wherein said cracked efiiuent is cooled to a temperature of less than 700 C., for a period of time sufficient to reduce the temperature of the cracked natural gas efiiuent leaving said first refractory bed to below 700 C. to 950 C., then reversing the direction of flow whereby natural gas passes first through said second refractory bed, then the cracking zone, and through said first refractory bed, and repeating the cycle, said process being conducted in the absence of added steam.

8. The process for thermally cracking the hydrocarbon components having two or more carbon atoms present in natural gas, said natural gas containing at least 55 volume percent methane and being under a pressure of from 100 p.s.i.g. to 1500 p.s.i.g. at its source, which comprises passing said natural gas, which is maintained substantially at its source pressure, through a first bed of a refractory material having a maximum temperature of from 700 C. to 1000 C., whereby said natural gas is heated to a temperature of 700 C. to 950 C. passing the heated natural gas through a heated cracking zone maintained at a temperature of from 950 C. to 1100 C. whereby the hydrocarbon components heavier than methane Which are present in the natural gas are cracked; passing the cracked eflluent through a second bed of refractory materials, said second bed having an initial maximum temperature of 700 C. wherein said cracked efiiuent is cooled to a temperature of less than 700 C., for a period of time sufiicient to reduce the temperature of the cracked natural gas effluent leaving said first refractory bed to below 700 C. to 950 C.; then reversing the direction of flow whereby 0 natural gas passes first through said second refractory bed, then the cracking zone, and through said first refractory bed, and repeating the cycle, said process being conducted in the absence of added steam.

9. The process for cracking the hydrocarbons having at least two carbon atoms present in a gas containing to volume percent methane, 5 to 45 volume percent hydrocarbons having from 2 to 8 carbon atoms, and up to 10 volume percent inert materials, which comprises passing said gas under a pressure of p.s.i.g. to 1500 p.s.i.g. through a first bed of a refractory material having a maximum temperature of from 700 C. to 1000 0, whereby said natural gas is heated to a temperature of 700 C. to 950 C.; passing the heated natural gas through a heated cracking zone maintained at a temperature of from 950 C. to 1100 C. whereby the hydrocarbon components heavier than methane which are present in the natural gas are cracked; passing the cracked effluent through a second bed of refractory materials, said second bed having an initial maximum temperature of 700 C. wherein said cracked effiuent is cooled to a temperature of less than 700 C., for a period of time suflicient to reduce the temperature of the cracked natural gas effluent leaving said first refractory bed to below 700 C. to 950 C. then reversing the direction of flow whereby natural gas passes first through said second refractory bed, then the cracking zone, and through said first refractory bed, and repeating the cycle, said process being conducted in the absence of added steam.

References Cited by the Examiner UNITED STATES PATENTS 1,933,845 11/1933 Egloif 260673 2,580,002 12/1951 Carr 260683 2,672,489 3/1954 Holland 260-683 2,678,956 5/1954 Hasche 260 679 2,859,258 11/1958 Fischer et al 260-683 2,875,148 2/1959 Schofield 260-683 X 2,985,698 5/1961 Pechtold et a1. 260-683 DELBERT E. GANTZ, Primary Examiner. ALPHONSO D. SULLIVAN, Examiner. C. E. SPRESSER, Assistant Examiner.

Claims

1. THE PROCESS FOR THERMALLY CRACKING THE HYDROCARBON COMPONENTS WHICH HAVE AT LEAST TWO CARBON ATOMS PRESENT IN NUTURAL GAS, SAID NUTRAL GAS CONTAINING AT LEAST 55 VOLUME PERCENT METHANE AND BEING UNDER A PRESSURE OF FROM 100 P.S. I.G. TO 1500 P.S.I.G. AT ITS SURCE, WHICH COMPRISES HEATING SAID NUTRAL GAS, WHICH IS MAINTAINED SUBSTANTIALLY AT ITS SOURCE PRESSURE, TO A TEMPERATURE OF FROM ABOUT 900* C. TO ABOUT 1200* C. WHEREBY THE HYDROCARBON COMPONENTS OF SAID NATURAL GAS WHICH HAVE WTO OR MORE CARBON ATOMS ARE THERMALLY CRACKED, SAI PROCESS BEING CONDUCTED IN THE ASENCE OF ADDED STREAM.