US2629753A - Making ethylene by catalytic pyrolysis - Google Patents

Description

Patented Feb. 24, 1953 UNITED STATES -ATENT OFFICE MAKING ETHYLENE BY CATALYTIC PYROLYSIS ware Application August 26, 1948, Serial N0. @6328 10 Claims. 1

This invention relates to an improved method for making ethylene by the catalytic pyrolysis of ,parafiin hydrocarbons.

In the making of ethylene by the pyrolysis of ethane and heavier paraffin hydrocarbons at temperatures above 759 C. as heretofore carried out, there are produced, in addition to the desired ethylene, significant proportions of acetylene and of non-gaseous hydrocarbons, such as gasolines, tars, and normally solid cyclic hydrocarbons. Not only are these latter formed in part at the expense of the ethylene yield, but their presence also raises serious operating difiiculties. Thus, the tars and solid hydrocarbons always tend to foul the cracking equipment and even plug it, while the acetylene appears a contaminant of the ethylene and can be separated only by a troublesome purification.

These problems are especially acute when the raw material for making ethylene is the ethane present in most natural gas. When such gas is pyrolyzed above 750 C., the cracked product contains, besides ethylene, several per cent of acetylene and also a heavy brown mist or fog which is I centrating treatment and then to pyrolyze only this fraction. While effective, this process introduces the expensive complication of another separatory step and hence has found but limited use.

With these considerations in mind, the present invention has for an object the provision of an improved process for making ethylene by the pyrolysis of paraifin hydrocarbons in which the simultaneous production of acetylene and non "gaseous hydrocarbons is largely prevented. A related object is to provide a simple inexpensive method of making ethylene by the direct pyrolysis of ethane-containing natural gas. A further object is to provide a high-temperature regenerative catalytic cracking process in which there is an efiicient utilization of heat.

The invention depends on the discovery that there is a small group of catalytic materials lhlCh at temperatures of 800 to 1000" C. act

selectively to accelerate the cracking of ethane all consist essentially of refractories which are infusible and irreducible by hydrocarbons at 1000 C. and have a surface area of less than one square meter per gram and which have disposed on their surface a metal of the class consisting of manganese, iron, copper and cobalt in a proportion of from 0.001 to 1.0 per cent by weight of the refractory.

In making ethylene according to the invention, then, ethane or a higher paraffin hydrocarbon is simply passed as a stream over or through a bed of such a catalyst at a temperature of 800 to 1000" C. During contact with the catalyst, the hydrocarbon is pyrolyzed largely to ethylene, with little if any accompanying formation of acetylene and non gaseous hydro= carbons. The resulting cracked gas is then cooled and may be used as a source of ethylene for chemical reactions or may be treated to separate the ethylene in purified form. Carbon produced during the pyrolysis remains on the catalyst and is later removed by burning it off.

Since the catalysts in the process are refractories, they are well adapted to use as heatstoring media, thus permitting the pyrolysis to be carried out by a cyclic regenerative procedure or high heat economy. In such a case, other catalysts may be employed at other stages of the cycle further to treat the pyrolyzed gas as will be explained in the following detailed description of the invention.

The catalysts of the invention may be regarded as consisting essentially of two components, a refractory base and a metal disposed on it. The refractory appears to be more than a mere support, seeming to exert its own catalytic effect, which is markedly promoted by the metal. The refractory must, of course, be stable under the most extreme conditions of the pyrolysis, and to this end should not fuse at temperatures up to 1000 C. and should likewise not be reduced by hydrocarbons at that temperature.

Aside from these limitations, however, the physical state of the refractory appears to be of more significance than its chemical identity. In particular, it should be relatively dense, i. e. of limited porosity, so that, even when in the form of small pieces, it does not have a catalytic surface area of more than one square meter per gram. The area referred to is not simply the geometric areadefined by the outer surfaces of the pieces, but is the total area exposed to the molecules of the gas being pyrolyzed, and thus includes the inner surface of whatever pores there may be in the refractory. This total area may be determined by any of several known methods, summarized, for example, in the monograph The Adsorption of Gases and Vapors, vol. I, p. 271-316, by S. Brunauer, Princeton University Press, 1945. Because of its general applicability, the nitrogen adsorption or B. E. T. method (Brunauer, Emmett, Teller, J. Am. Chem. Soc. 6", 309 (1938) is preferred and was the one employed in arriving at the limiting area value given hereinbefore.

The refractories which most satisfactorily meet the criteria set forth above are in general of the class consisting of metal oxides and metal silicates. Thus, for example, there may b used silica alone, and alumina, beryllia, thoria, magnesia, lime, titania, zirconia, chromia, and hafnia, each of the later alone or in chemical combination with silica. In general, non-basic refractories are preferred because of their greater resistance to oxides of carbon which may be present in the pyrolysis gases. With certain refractories, such as magnesia and titania, which may in some forms exhibit surface areas over one square meter per gram, care must be taken to secure a non-porous form, as by selecting a deadburned or even a fused grade. As a practical matter, aluminum silicate refractories, particularly ceramics consisting essentially of sillimanite or mullite, being readily available, are most often used.

The refractory portion of the catalyst may be in any suitable size and shape for contact with the pyrolysis gases, a bed of small pieces being preferred. Sillimanite or mullite pebbles of 0.2 to 1.0 inch diameter are highly satisfactory, as are the corresponding sizes of the porcelain tower packing known as Berl saddles (U. S. Pat. 1,795,501).

The promoter portion of the catalyst is, as stated, made up of on or more of the metals manganese, iron, copper, and cobalt. It should be dispersed over the surface of the refractory in a proportion of from 0.001 to 1.0 per cent by weight. Lesser proportions are too slight to produce a significant effect whereas higher ones may cause serious loss of ethylene. Of the promoters, iron is active in lower concentrations than the others, though a proportion of at least 0.1 per cent is in general preferred for each of the metals. Since copper tends to exert a fiuxing action on some silicates, it is essential, with copper as a promoter, to choose the refractory so as to minimize this effect; aluminum silicates are satisfactory. However, maximum activity of copper seems to be realized on a non-silicate refractory, such as alumina.

Inasmuch as the conditions in the pyrolysis zone are highly reducing, the promoter need not be applied to the refractory as the metal itself. It is equally effective to distribute over the refractory any reducible compound of the metal, the latter being then decomposed to free metal in the first minutes of pyrolysis. Consequently, the promoter is most conveniently applied by soaking pieces of the refractory in an aqueous solution of a reducible compound of manganese, iron, copper or cobalt and then drying them. Since the concentration of promoter to be deposited is low, comparatively dilute solutions, e. g. 1 to per cent by weight, of the reducible compound can be used. In general, nitrates or acetates are preferred because of their solubility and ready reducibility.

While depositon of the promoter from aqueous solution is most convenient othe methods of application may be used, as by condensation of the metal on the refractory from the vapor phase. Good results have also been realized simply by passing the gas to be pyrolyzed over unprornoted refractory heated at 800 to 1000" C. and injecting a finely-divided powder of the promoter, either in the metallic state or in the form of an oxide, into the gas stream in the initial stages of pyrolysis.

The catalysts prepared as described are, so far as known, generally effective in accelerating the conversion of paraffin hydrocarbons to ethylone with minimum formation of acetylene and non-gaseous hydrocarbons. They are most useful, however, in the pyrolysis of normally gaseous paraffins, and particularly of ethane. In cracking ethane, best results are obtained when the partial pressure of the ethane in the gas stream undergoing pyrolysis is less than 0.2 atmosphere, either by operating at reduced pressure or in the presence of an inert diluent, such as nitrogen, hydrogen, carbon monoxide, or methane. Steam is also an operable diluent but since its presence makes necessary added equipment and extra heat for vaporization and condensation, it is preferably not used. Methane is perhaps the most practical diluent and has the extreme eco nomic advantage that mixtures of methane with 3 to 15 per cent of ethane are available as natural gas at many fields. In such gas, hydrocarbons other than methane and ethane may also be present, though usually in low concentrations, so that the ethane is ordinarily the major hydrocarbon component having more than one carbon atom per molecule.

In making ethylene according to the invention, maximum temperature in the cracking zone should be in the range 800 to 1000 C. At temperatures much below 800 the yield of ethylene is reduced whereas above 1000 acetylene formation may be encountered. Values of 900 to 950 are most advantageous. Pyrolysis should be rapid, with contact time at the cracking temperature not exceeding 2 seconds, even at 800 C., and preferably being much shorter, e. g. 0.1 second or less, especially at temperatures approaching 1000 C. In operating under these conditions carbon is gradually deposited in the catalyst, reducing its activity. The catalyst may be regenerated and the carbon easily removed by burning it off in air or other oxygen-containing gas.

The cracked gas produced according to the process described ordinarily contains too little acetylene to interfere with most uses to which the gas is put. However, if desired, last traces of acetylene may be removed by passing the gas through a bed of small pieces of a refractory having deposited thereon an oxide of at least one of metals copper and iron, e. g. copper oxide in a proportion of 0.5 to 2.0 per cent by weight, such bed being at a temperature of to 375 C. at its outlet end. Details of this latter treatment are given in U. S. Patent 2,398,301. The supporting refractory need not meet the surface area limitation which is essential in the cracking catalyst of the present invention.

The new process may be further described with reference to the accompanying drawing, in which Fig. 1 is a schematic elevation, partly in section, showing one arrangement of apparatus in which the pyrolysis may be carried out in a regenerative cyclic procedure;

Fig. 2 is a flow diagram representative of the cracking period in'the apparatus-of Fig. 1, showing the temperatures at the end of the period; and

Fig. 3 is a similar diagram for the regeneration period, showing the temperatures at the end.

In the apparatus of Fig. l, pyrolysis is carried out in a vertical cylindrical reactor formed of a steel shell 5 lined with courses of insulating brick l5 and refractory silica brick I and lagged with heat insulation 8. The top of the reactor is closed by an insulated flanged head 8, from which opens a transfer line I0 provided with a heat exchanger H. The bottom !2 is similarly flanged to connect to a transfer line 13 also provided with a heat exchanger 10. Part way up the reactor, the wall is penetrated by a number of gas inlets 15 all at the same level and supplied from a common header It by a fuel supply line H.

Fixed transversely within the reactor just above the bottom is a steel grating It serving as a support for the packing which fills the tower. The portion of the reactor extending from the grating It to just below the inlets it functions as a quench zone and is filled with half-inch diameter mullite pebbles having 1.0 per cent by weight of copper oxide on the surface thereof prepared according to U. S. Patent 2,398,301. The central part of the reactor from the fuel inlet level to a level somewhat higher constitutes the cracking zone 20 and is filled with catalyst according to the present invention, e. g. halfinch mullite pebbles having 01 per cent by weight of manganese and 0.1 per cent of iron distributed on the surface thereof. The upper part of the reactor forms a preheat zone 2! filled with halfinch untreated mullite pebbles.

The movement of gases through the lines i0, I3, and I? serving the reactor is controlled by a bank of shut-oii valves 22 to 30. These valves are in part interconnected by headers, as shown in the drawing, and can be operated simultaneously by automatic timing mechanism not shown to open or close predetermined gas flow paths through the reactor.

The ethane-containing natural gas used in making ethylene is derived from a source 3! at somewhat above atmospheric pressure. During the cracking period, the natural gas flows in through the valves 23 and 20 and the line i0, passes downwardly through the reactor 4, and leaves through the line it, and the valve 23, and the product line 32. From the latter it passes to use or to ethylene separation apparatus not shown.

During the regeneration or burn-off cycle, air from a source 33 enters through the valves 24 and 29, passing through the line It, the reactor 4, the line l0, and the valve 21 to a vent 34. At the same time, fuel gas from a source 35 enters through the valve 30, line H, and inlets l5, merging with the air stream. Between the crack ing and regeneration cycles, the reactor and piping may be purged with inert gas, e. g. steam or nitrogen, from a source 36.

In explaining the operation of the apparatus, it will be assumed that it has been in use for some time and is entering a cracking period. Under these conditions, the valves 23, 26, and 23 are open and natural gas from the source 3! is beginning to flow downwardly through the reactor and out the product line 32. As a result of the regeneration period just completed, the refractory bed in the preheat zone 2| is at a temperature of 400 to 500 C. at the top and nearly 800 C. at the bottom. In the cracking zone '20, the refractory is :at 950 to i000 C. and in the quench zone l9 it is at 400 to 500 C. at the top and 100 to 200 C. at the bottom. As the natural gas, entering at ordinary temperature, flows down through the refractory bed, it is first preheated nearly to 800 C., and then in the cracking zone is raised very rapidly to nearly 950 C. On leav ing the latter zone, the cracked gas is quickly chilled in the quench zone, leaving at not over 300 C. It then passes through the heat exchanger Id, surrendering further heat, and flows into the product system 32 at about 100 C.

During passage through the cracking zone where it is in contact with the promoted refractory catalyst of the invention, the ethane in the natural gas is dehydrogenated to ethylene with but very little formation of acetylene or of nongaseous hydrocarbons. The carbon produced in the pyrolysis remains largely in the cracking zone in the form of luster carbon deposited on the catalyst pebbles. However, a part of the carbon is carried into the quench zone and there retained in a soft, finely-divided, almost pyrophoric form. As the cracked gas goes through the quench zone, remaining traces of acetylene in it are largely decomposed during contact with the copper oxide catalyst, forming cuprene and some additional soft carbon.

Since the refractory body in the preheat and cracking zones gradually cools off during the cracking period, the latter must be terminated when the cracking zone approaches 800 C. Ordinarily this temperature is reached long before carbon is deposited to an extent sufiicient to reduce the activity of the cracking catalyst significantly. At this time, the preheat zone will have cooled to around 200 C. at the top and 600 at the bottom, and the quench zone will have risen to nearly 700 C. at the top and 300 C. at the bottom. The cracking cycle is ended by closing the valve 23, and the system is purged by opening the valve 22 for a brief interval, after which all valves are closed.

The regeneration period is next started by opening the valves 20, 21, 28, and 30. Air at ordinary temperature flows upwardly through the reactor; fuel gas enters through the inlets iii, and the gaseous products escape, through the exchanger H where their heat is recovered, to the event 34. As the air moves upwardly through the quench zone, it encounters the fine carbon and cuprene in the refractory, very quickly burning the latter clean. The cuprene and copper oxide seem to lower the ignition temperature of the carbon, insuring adequate combustion. Then, as air flow continues, the quench zone is gradually cooled back to the temperatures required for the start of the cracking period.

The fuel gas entering the reactor ignites instantly in the steam of air flowing upwardly, producing combustion gases at high temperature which gradually heat the cracking and quench zones back to the temperatures of the cracking period. The hot air and combustion gas also burn off all carbon in the cracking zone early in the period, leaving the catalyst completely regenerated. When the desired temperatures are attained, the regeneration period is ended by closing the valves 24 and 30, and the valve 25 is opened to allow the purge gas to sweep out the system. All valves are then closed ready for another cracking period.

The ethylene-containing product gas leaving the system through the line 32 may be used di- Example 1 In a pilot-plant demonstration of the apparatus of Fig. 1, the reactor was 35 inches long and 3.5 inches inside diameter, lined with fused silica.

The quench zone was 12 inches long, the cracking zone inches, and the preheat zone 13 inches, all filled with pebbles of the size and chemical composition explained with reference to Fig. 1.

and was 11 inches long. The cracking catalyst was a bed of 0.31-inch mullite pebbles which had been coated in place with approximately 0.2 per cent by weight of copper introduced into the gas stream as a dust. The quench zone contained similar pebbles having 1.0 per cent by weight of copper oxide deposited on them by soaking the pebbles in copper nitrate solution and then heating in air to 500 C. Operation was essentially at atmospheric pressure.

The temperature in the cracking zone was 840 to 900 C. while the quench zone ranged from 250 to 590 C. Feed rate was 2.0 cubic feet per minute, corresponding to a catalyst contact time of 0.3 to 0.4 second.

Podbielniak analyses of the natural gas and cracked product were:

The cracking period was 10 minutes and the re- Component @33 aig generation period minutes, with 10 seconds nl for each purge with nitrogen. Operating tem- Pme'zb peratures were in the ranges stated, the maxivuzm w mum cracking temperature being 925 C. The Q? g g pressure was essentially atmospheric. The feed 0 10.5 stock and fuel gas were from the same source, a 5 natural gas from the Goldwater field of central 1.3 0.1 Michigan. Feed rate on cracking was 1.35 cubic 8' 28 feet per minute, corresponding to a contact time 6.2 5.0 of about 0.2 second.

A a yses of the feedstock and Of a Composite The cracked gas was clear and colorless. No sample of the cracked gas taken over 750 c0nliquid or tarry condensate was produced. secutive 30-minute process cycles are given below. F Z 3 Analyses were made by the Podbielniak low- Jami temperature distillation procedure. In a series of pyrolysis tests, natural gas was passed through an externally-heated 2.5 cm. in Comment g gq side diameter silica tube filled for a length of 57 4 cm. with small porcelain (quartz-sillimanite) Percent a, Bel'l l s. The efiiuent gas was immediately volume 40 quenched in a water-cooled heat-exchanger. In

i-g the first test, not in accordance with the inven- 1 tion, the saddles were untreated. In each of the g 9 9;? other tests, illustrative of the invention, the 117 51.2 saddles were coated with one of the promoter 8- metals iron, manganese, and cobalt applied by l 4,6 soaking the saddles in an aqueous solution of a nitrate of the metal and then drying. Details of The conversion of ethane to ethylene was 51.5 the tests are given in accompanying Table A. per cent and the chemical efficiency of the con- From the table, the very great reduction in nonversion was 96.9 per cent. The cracked gas was gaseous hydrocarbon products (cloudiness) and clear and colorless. No appreciable quantity of tar or liquid condensate was obtained.

the significant decrease in acetylene formation occasioned by each promoter are clearly evident.

TABLE A Promoter-pcrccnt by weight None Cracking temperature, C

Natural gas rate cclminm Appearance of cracked gas...

2,113 2, very cloudy yellow.

Gas Analysis Natural Gas Vol. percent Example 2 What is claimed a the invention is: 1. In a process of pyrolyzing ethane to produce Natural gas was cracked in a regenerative system similar to that of Fig. 1, except that the cracking zone proper had a diameter of 5 inches a high yield of ethylene with minimum formation of acetylene and non-gaseous hydrocarbons, the step which comprises passing a stream of ethane and an inert diluent wherein the proportion of ethane is such that the partial pressure thereof is les than 0.2 atmosphere through a bed of a catalyst at a temperature of 800 to 1000 C. and under reducing conditions such catalyst consisting of small pieces of a refractory of the class consisting of metal oxides and metal silicates which is infusible and irreducible by hydrocarbons at 1000 C. and is of such limited porosity as to have a surface area of less than one square meter per gram, such refractory pieces having deposited on the surface thereof a metal consisting of manganese, iron, copper, and cobalt in a proportion of from 0.001 to 1.0 per cent by weight thereof.

2. In a proces of pyrolyzing ethane to produce a high yield of ethylene with minimum formation of acetylene and non-gaseous hydrocarbons, the step which comprises passing a stream of natural gas containing ethane as the major hydrocarbon component having more than one carbon atom per molecule, the proportion of ethane being such that the partial pressure thereof is less than 0.2 atmosphere, through a bed of a catalyst at a temperature of 800 to 1000 C. at a flow rate such that the pyrolysis time is less than 2 seconds and under reducing conditions, such catalyst consisting of small pieces of an aluminum silicate refractory of such limited porosity as to have a surface area of less than one square meter per gram and a metal of the class consisting of manganese, iron, copper, and cobalt deposited on the surface of the refractory pieces in a proportion of from 0.001 to 1.0 per cent by weight thereof.

3. A process according to claim 2 wherein the metal of the catalyst is manganese.

4. A process according to claim 2 wherein the metal of the catalyst is iron.

5. A process according to claim 2 wherein the refractory consists essentially of mullite.

6. A process of making ethylene which comprises passing a stream of natural gas containing ethane as the major hydrocarbon component having more than one carbon atom per molecule, the proportion of ethane being uch that the partial pressure thereof is less than 0.2 atmosphere, through a bed of a catalyst at a temperature of 800 to 1000 C. at a flow rate such that the pyrolysis time is less than 2 seconds and under reducing conditions such catalyst consisting essentially of small pieces of a refractory of the class consisting of metal oxides and metal silicates which is infusible and irreducible by hydrocarbons at 1000 C. and is of such limited porosity as to have a surface area of less than one square meter per gram and a metal of the class consisting of manganese, iron, copper, and cobalt deposited on the surface of the refractory pieces in a proportion of from 0.001 to 1.0 per cent by weight thereof, cooling the gas stream after passage through the catalyst bed, and separating ethylene from the cooled gas.

7. In a process of pyrolyzing a paraflin hydrocarbon containing more than one carbon atom per molecule to produce a high yield of ethylene with minimum formation of acetylene and nongaseous hydrocarbons, the steps which comprise passing the hydrocarbon as a stream through a bed of a catalyst at a temperature of 800 to 1000 C. and under reducing conditions such catalyst consisting of small pieces of a refractory of the class consisting of metal oxides and metal ilicates which is infusible and irreducible by hydrocarbons at 1000 C. and has a surface area less than one square meter per gram and a metal of the class consisting of manganese, iron, copper and cobalt deposited on the surface of the refractory pieces in a proportion of from 0.001 to 1.0 per cent by weight thereof, and cooling the cracked gas stream leaving such catalyst bed by passing it through a bed of small pieces of a refractory having deposited thereon at least one substance of the class consisting of th oxides of copper and iron, such bed being at a temperature of M0 to 375 C. at its outlet end.

8. A process according to claim 7 wherein the refractory in the cooling bed has copper oxide deposited thereon in a proportion of from 0.5 to 2.0 per cent by weight. I

9. A process according to claim 8 wherein the feed stock is natural gas containing ethane as the major hydrocarbon component having more than one carbon atom per molecule, the proportion of ethane being such that the partial pressure thereof is less than 0.2 atmosphere.

10. A process of making ethylene which comprises passing a stream of a normally gaseous parafiin hydrocarbon containing more than one carbon atom permolecule through a bed of a catalyst at a temperature of 800 to 1000 C., such catalyst consisting of small pieces of a refractory of the class consisting of metal oxide and metal silicates which is infusible and irreducible by hydrocarbons at 1000 C. and is of such limited porosity as to have a surface area of less than one square meter per gram, such refractory piece having deposited on the surface thereof a metal consisting of manganese, iron, copper, and cobalt in a proportion of from 0.001 to 1.0 per cent by weight thereof, cooling the gas stream after passage through the catalyst bed, and separating ethylene from the cooled gas.

LUDO K. FREVEL. JOHN W. HEDELUND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,411,255 Alexander Apr. 4, 1922 1,973,851 Feller et a1 Sept. 18, 1934 2,174,196 Rogers Sept. 26, 1939 2,186,590 Martin et a1. Jan. 9, 1940 2,301,727 Klein et a1 Nov. 10, 1942 2,366,531 Ipatiefi et al Jan. 2, 1945 2,398,801 Frevel Apr. 9, 1946

Claims (1)

1. IN A PROCESS OF PYROLYZING ETHANE TO PRODUCE A HIGH YIELD OF ETHYLENE WITH MINIMUM FORMATION OF ACETYLENE AND NON-GASEOUS HYDROCARBONS, THE STEP WHICH COMPRISES PASSING A STREAM OF ETHANE AND AN INERT DILUENT WHEREIN THE PROPORTION OF ETHANE IS SUCH THAT THE PARTIAL PRESSURE THEREOF IS LESS THAN 0.2 ATMOSPHERE THROUGH A BED OF A CATALYST AT A TEMPERATURE OF 800* TO 1000* C. AND UNDER REDUCING CONDITIONS SUCH CATALYST CONSISTING OF SMALL PIECES OF A REFRACTORY OF THE CLASS CONSISTING OF METAL OXIDES AND METAL SILICATES WHICH IS INFUSIBLE AND IRREDUCIBLE BY HYDROCARBONS AT 1000* C. AND IS OF SUCH LIMITED POROSITY AS TO HAVE A SURFACE AREA OF LESS THAN ONE SQUARE METER PER GRAM, SUCH REFRACTORY PIECES HAVING DEPOSITED ON THE SURFACE THEREOF A METAL CONSISTING OF MANGANESE, IRON, COPPER, AND COBALT IN A PROPORTION OF FROM 0.001 TO 1.0 PER CENT BY WEIGHT THEREOF.

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