US3236615A - Production of heating gas - Google Patents

Description

United States Patent 3,236,615 PRODUCTION OF HEATING GAS Moses Robert Lipkin, Havertown, Pa., assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey No Drawing. Continuation of application Ser. No.

84,767, Jan. 25, 1961. This application Nov. 5,

1964, Ser. No. 409,285

8 Claims. (Cl. 48-211) This application is a continuation of application Serial No, 84,767, filed January 25, 1961, now abandoned, which was a continuation-in-part of application Serial No. 8,525, filed February 15, 1960, and now abandoned.

The cracking of normally liquid hydrocarbons to produce natural gas substitutes has previously been practiced using various petroleum oils as the source of the liquid hydrocarbons. Straight run naphtha, distillate gas oil and residual fuels have been employed, in carbureted water gas sets, oil gas sets, catalytic gas making sets, Hasche reformer sets, and in other known equipment. A problem has been the formation of tar and light oil, requiring the provision of tar separators and light oil scrubbers, and introducing handling difiiculties that have increased the capital and operating costs. In some types of equipment, excessive coke formation also has caused difiieulty.

Since the production of heating gas from liquid hydrocarbons by cracking is to a large extent used for the supply of gas only during relatively short periods of peak demand, the problem presented by formation of undesired products is an important factor in determining the economic feasibility of the installation of equipment for such production.

The present invention provides a liquid hydrocarbon cracking process which produces heating gas of highly satisfactory quality in good yield while substantially eliminating problems of tar and other undesired products. This is accomplished by employing as charge oil a hydrocarbon mixture having composition as specified subsequently and typified by a paraflinic concentrate from the reforming of petroleum naphtha to produce gasoline, and by employing certain cracking conditions in the gas making as set forth subsequently.

The process of the invention involves in a typical embodiment the cracking, to produce heating gas, of a normally liquid hydrocarbon material previously obtained by reforming petroleum naphtha to convert cycloparafiinic hydrocarbons and some of the paraffinic hydrocarbons to aromatic hydrocarbons to produce gasoline containing aromatics, unconverted paraflins and paraflinic conversion products, and subjecting the gasoline reformate to a separation process to obtain a paraffinic concentrate containing a relatively small amount of aromatics and naphthenes as compared with the original reformate; this paraffinic concentrate is the liquid hydrocarbon employed in the cracking to produce heating gas.

Reforming, as contemplated in this embodiment, involves the conversion of hydrocarbons boiling for enample in the range from 100 to 600 F., by reactions such as cyclization of paraffins to cycloparaflins and alkyl cycloparaffins, dehydrogenation of cycloparaffins to aromatics and alkyl aromatics, and other reactions, to produce hydrocarbons boiling in the range from 100 to 600 F. Some cracking occurs, but the major product is nevertheless a gasoline or higher fraction.

Typically, reforming is performed at a temperature in the range from 850 to 1000 F. and a pressure in the range from 250 to 750 p.s.i.g. in the presence of a hydrogenation-dehydrogenation catalyst. Suitable catalysts include the known platinutn alumina-combined halogen catalyst, the known catalysts containing Group VI metal,

3,2365% Patented Feb. 22, 11966 ice e.g. chromia-a'lumina and molybdena alumina catalysts, the known bauxite and silica-alumina catalysts, etc. Other known reforming catalysts and conditions can be employed. The reforming is preferably performed in the presence of 0.2 to 10 moles of hydrogen per mole of charge, though other amounts can be used in some cases.

The normally liquid reformate obtained as product of the reforming is then subjected in this embodiment to a separation process to obtain as one product a parafiinic concentrate suit-able for gas making according to the invention, and as another product an aromatic fraction, which may be benzene, toluene, xylenes, a motor gasoline mixture, etc. The separation may be by any suitable procedure, e.g. distillation, solvent extraction, selective adsorption, etc. or by such treatments in series.

In one embodiment, the separation is by distillation, to obtain a parafiinic concentrate as a relatively low-boiling distillate having end-point for example in the range from to 210 F. The aromatics in reformate are generally of higher boiling point than the lowest boiling par-aifins in the reform-ate, and a suitably low boiling distillate therefore contains such parafiins in concentrated form. The aromatic product obtained in the distillation can be removed as a motor gasoline or other product. Alternatively, the aromatic product may be thermally cracked, e.g. at 1100 to 1300 F., according to known procedure, to obtain improved motor gasoline. Again, alternatively, it may be subjected to solvent extraction or selective adsorption to separate aromatics from higher boiling paraffins in the reformate. The higher boiling parafilns obtained in such separation constitute an additional paraffi ni-c concentrate suit-able for gas making. The separated aromatics can be removed as a motor gasoline or other product, or thermally cracked as described above.

In another embodiment, the normally liquid reformate product is subjected, without intervening distillation, to solvent extraction or selective adsorption to obtain a parafiinic concentrate suitable for gas making and an aromatic product suitable for removal as a motor gasoline or other product, or for thermal cracking as described above.

A paraffinic concentrate recovered from catalytic reformate has differences in composition from a straight run gasoline of the same boiling range, which differences produce superior results in thermal cracking for the prep aration of heating gas. The parafiinic concentrate from reformate has considerably lower naphthene content than the corresponding boiling range fraction of the straight run gasoline from which the reformate is produced, and has a higher ratio of branched chain paraffins to straight chain parafiins. The low naphthene content results from the dehydrogenation conditions which prevail in the reforming, and the higher chain branching results from isomerization reactions occurring in the reforming. The paraffinic concentrate can also be reduced in aromatic content, in the separation of the reformate into aromatic and paraffinic constituents, to an aromatic content below that of the corresponding fraction of the reforming charge stock, but this is not essential. The structural differences result in the paraffinic concentrate being a superior material to the corresponding fraction of the reformer charge stock, from the standpoints of reduced tar formation, ease of cracking in the gas making, and other advantages. Because of the greater ease of cracking, greater amounts of charge can be cracked in a given length of time, thereby increasing the capacity of given equipment for the production of gas. These advantages are obtained simultaneously with the obtaining of a heating gas which is essentially equivalent to, in composition and heating value, or better than that obtainable from the straight run gasoline.

The following table gives a comparison of typical compositions, as determined by :gas liquid phase chromatography and mass spectrometer analysis, of a .parafiinic concentrate from reformate, for use according to one embodiment of the invention, and of a corresponding boiling range fraction of a straight run naphtha:

Frequently, higher naphthene contents are encountered in straight run naphtha, e.g. up to 50 percent or more.

The greater branched chain paraffin content of the paralfinic concentrate provides superior ease of cracking to desired gases, as compared with the straight run naphtha. The lower content of cyclic hydrocarbons, and particularly of naphthenes, in the paraffinic concentrate, results in reduced production of tars and liquid products, as compared with the straight run naphtha.

The previously described paraffinic concentrate from catalytic petroleum reformate is a preferred starting material for the process of the invention. Other hydrocarbon mixtures containing 75 to 100 volume percent of parafiinic hydrocarbons including at least 25 volume percent and preferably at least 40 volume percent of branched chain paraffinic hydrocarbons, less than 15 volume percent and preferably less than 12 volume percent of naphthene hydrocarbons, less than 15 volume percent and preferably less than 10 volume percent of aromatic hydrocarbons can be employed. Preferably, the mixture contains less than 10 volume percent, and more preferably less than 5 volume percent of olefin hydrocarbons. Examples of suitable materials are acyclic hydrocarbon concentrates from thermally or catalytically cracked or reformed hydrocarbons, hydrogenated olefin polymers, alkylation products, etc., which have the indicated composition. Preferably, the mixture boils in the range from 150 to 400 F., more preferably 150 to 300 F.

The hydrocarbon mixtures having the indicated composition provide the aforementioned advantages over various normally liquid charge stocks of the prior art. They also provide advantages, with respect to convenience of storage and handling, and with respect to product composition, as compared with normally gaseous starting materials such as propane.

The cracking of the hydrocarbon mixture to make heating gas is performed according to the invention at a temperature in the range from 1200 to 2000 F., preferably 1450 to 1600 F. The residence time in the cracking is at least 0.1 second, and preferably in the range from 1.5 seconds to or seconds, though longer times can be used in some cases. Such temperature and residence time provide optimum production of desired heating gas. The cracking severity is preferably equivalent to that provided at 1500 F. with a residence time of 1.5 to 5 seconds. The pressure is not critical. Typical operation is at about one atmosphere, though elevated pressure up to 5 atmopheres or higher can be employed.

The cracking may be performed either in the presence or absence of steam. Preferably, the heat for the cracking is provided indirectly, by passage of the paraffinic concentrate through an externally heated tube. In this embodiment, the parafiinic concentrate is passed through an elongated confined cracking zone in the absence of added oxygen and is indirectly heated during its passage through the zone. Other methods of supplying heat can be employed, e.g. combustion of a portion of the hydrocarbon charge, or of carbon formed in a previous cycle, etc., but the procedure noted above is superior from the standpoints of efiiciency, economy and quality of heating gas produced.

The product gas obtained in the process of the invention contains at least 15 volume percent, and preferably at least 25 volume percent, of methane. Preferably, it contains at least 10 volume percent, more preferably at least 20 volume percent, of hydrogen.

The olefin content of the product gas is minimized, since the illuminating properties of the olefins are usually undesirable in heating gas. The olefin content is less than 40 volume percent, preferably less than 30 volume percent. Preferably, the ethylene content is less than 30 volume percent, more preferably less than 25 volume percent. The mole ratio of methane to ethylene is at least 1:1 and preferably at least 1.25:1. Olefin content is minimized by maintaining relatively high temperature or relatively long residence time or both. Selection of proper combinations of temperature and residence time within the previously specified ranges results in the production of the desired heating gas. The composition of the product gas is typically, but not necessarily, in the following range, as determined by mass spectrometer analysis:

Volume percent Hydrogen 10 to 30 Methane 15 to 45 Ethylene 15 to 30 Ethane 1 to 5 Higher hydrocarbons 5 to 15 Other components Less than 20 Such other components include oxygen, nitrogen, carbon dioxide, and carbon monoxide. Such components are preferably minimized in order to avoid excessively high specific gravity in the product gas. These components can be largely eliminated by processes wherein indirect heating is employed, rather than combustion reactions wherein the products of combustion are permitted to mingle with the products of cracking of the reformed hydrocarbons.

The following is a typical composition of Waterscrubbed product gas from the conversion of a parafiinic concentrate from reformate in a carbureted water gas set at about 1520 F. and residence time of 2 seconds:

Vol. percent Oxygen 0.5 Nitrogen 6.6 Carbon dioxide 1.2 Carbon monoxide 6.1 Hydrogen 23.9 Methane 29.1 Ethylene 22.1 Ethane 3.2

Propylene 4.9 Propane 0.2 Butylenes 0.9 Butanes 0.9

Isopentane 0.1 Benzene 0.3

Because of the avoidance of tar formation in the process of the invention, it is frequently possible to eliminate tar separating equipment The products from the cracking equipment can in such cases be taken directly to a water scrubber or an oil scrubber and from there to storage or to distributing conduits. The following examples illustrate the invention:

Example 1 Straight run gasoline boiling in the range from to 245 F. is catalytically reformed at a temperature of about 930 F. and pressure of about 300 p.s.i.g., employing a catalyst composed of alumina, about 0.5 Wt. percent platinum, and about 0.3 wt. percent fluorine. The liquid hourly space velocity is about 3 volumes of gasoline per volume of catalyst bed per hour, and the hydrogen amount about 5 moles per mole of charge.

The normally liquid reform-ate product is separated from hydrogen and other gaseous materials, and then contacted with diethylene glycol containing about 2% Water at 300 F. and 100 p.s.i.g., the amount of glycol being 5 volumes per volume of liquid product. Diethylene glycol containing dissolved aromatic hydrocarbons, principally benzene and toluene, is separated from undissolved material constituting a paraffin-ic hydrocarbon concentrate. The paraffinic concentrate has the following properties: API gravity of 71, boiling range of 160 to 280 F., and Reid vapor pressure of 4 pounds per square inch.

The paratfinic concentrate is vaporized in a preheater and the vapors are passed through a tube furnace wherein they are heated to 1500 F. for a residence time of 2 seconds. The cracked products are passed into a wash box wherein they are contacted with water. The product gas has a heating value of about 1000 B.t.u. per standard cubic foot, the thermal recovery being over 100,000 B.t.u. per gallon of parafiinic concentrate charged.

In the cracking of other hydrocarbon materials, e.g. diesel fuel, straight run naphtha, etc., in place of the paraffinic concentrate from reformate, under similar conditions, inferior results are obtained with respect to ease of cracking to desired heating gas subs-tituents, and with respect to light oil, coke and tar formation. The parafiinic concentrate provides greater throughput capacity and thermal recovery while avoiding undesired products.

Example 2 Straight run gasoline having boiling range of about 210 to 280 F. is catalytically reformed under conditions generally similar to those described in Example 1, except that the pressure is about 500 p.s.i.g. A paraflinic concentrate is separated, in a manner generally similar to that described in Example 1, from diethylene glycol containing aromatics, in this case primarily xylenes. The parajnic concentrate is cracked in the manner described in Example 1, to obtain heating gas. Generally similar results are obtained in the cracking.

Example 3 Stabilized motor reformate having boiling range of about 120 to 430 F., and obtained by catalytic reforming of straight run naphtha having boiling range of about 270 to 410 F., is distilled to obtain a liquid fraction boiling up to 205 F. The higher boiling material (205 to 450 F.) is thermally cracked at a temperature of about 1100 F. in order to produce a motor fuel having high octane number. The 205 F. end-point fraction has quite low aromatic content because benzene production in the reforming is quite low. This fraction therefore constitutes a paraffinic concentrate, and it is thermally cracked at 1500 F. and atmospheric pressure, with residence time of 2 seconds, to produce a heating gas. The performance of the paraffinic concentrate in the cracking is generally similar to that of the parafiinic concentrates in the preceding examples Generally similar results to those in the above examples are obtained using other charge stocks and conditions such as those described previously.

The hydrocarbon mixture for cracking according to the invention preferably has sulfur content less than 0.5 weight percent, though this is not essential. Parafiinic concentrates from petroleum reformates, for example, are generally substantially free of sulfur, and the gases pro duced therefrom are consequently much less corrosive than gases produced from materials containing more sulfur. The carbcn-to-hydrogen atomic ratio of the hydrocarbon mixture is preferably but not necessarily in the approximate range from 4 to 6, and its API gravity is preferably in the range from 50 to The yield of gaseous products in the cracking of the hydrocarbon mixture is usually at least 50 weight percent based on the hydro carbon mixture charged.

The gas produced according to the invention can be employed as heating gas, either alone or in admixture with other gas, e.g. natural gas.

The process of the invention is particularly beneficial in permitting the use of relatively high cracking temperatures Without forming excessive amounts of lampblack. For example, a cracking temperature of 1500 to 1600 F. can be employed in oil gas sets, using a hydrocarbon mixture according to the invention as charge stock, with entirely satisfactory results from the standpoint of avoiding lampblack formation, whereas with prior art charge stocks such as straight run or cracked gas oils the deposition of lampblack in conduits and auxiliary equipment is so great at relatively high temperatures as to make operation at the higher temperatures difficult or impossible.

It is beneficial to be able to use such relatively high temperatures, since the latter result in the obtaining of a gas product having specific gravity, e.g. in the range from 0.6 to 0.8, which is lower than that, e.g. about 0.85, of a gas product obtained at lower temperatures. The lower specific gravity product is closer in gravity to natural gas and therefore more interchangeable with natural gas in equipment designed for gas having the specific gravity of natural gas. This permits the use of greater proportions of gas obtained according to the invention in mixtures withmatural gas while remaining within required limits of specific gravity. This factor is important in the pro vision of heating gas to meet peak demands.

In the thermal cracking of parafiinic concentrates according to the invention, to produce heating gas, employing for example carbureted water gas sets, Hasche sets, etc., the production of tar is frequently eliminated, and passage of the product through a tar separator is unnecessary. Use of straight run naphtha under the same conditions in such sets, however, results in the production of substantial amounts of tar, and tar separation is necessary. In some thermal cracking processes, the paraflinic concentrate results in the production of some tar, but the amount is considerably less than in the case Where straight run naphtha is used in such processes. Production of light oil is also considerably less in the case of the paraffinic concentrate than in the case of straight run naphtha.

Example 4 A parafiinic concentrate having the following properties is prepared by the alkylation of isobutane with buytlene according to known procedure, followed by distillation of the product to obtain a motor alkylate fraction boiling from 356 F. to 516 F.:

Saybolt Universal Viscosity at 100 F 31.4 Flash point F 129 Sulfur content wt. percent less than 0.002 C/H ratio 5.65 Paraffinic hydrocarbon content percent 98 Boiling range, F.:

Initial 356 5% 361 10% 363 20% 365 30% 368 40% 371 50% 375 60% 380 70% 386 397 430 473 Endpoint 516 This parafilnic concentrate is cracked according to the procedure of Example 1, with generally similar results.

The invention claimed is:

1. Process for producing heating gas which comprises subjecting a normally liquid hydrocarbon mixture boiling in the range from 150 to 400 F. and having API gravity in the range from 50 to 75 containing 75 to 100 volume percent of paraffinic hydrocarbons including at least 25 volume percent of branched chain paraflinic hydrocarbons, less than 15 volume percent of naphthene hydrocarbons, and less than 15 volume percent of aromatic hydrocarbons, to a temperature in the range from 1200 to 2000 F. for at least 0.1 second, thereby to produce a gas containing at least 15 volume percent of methane and less than 40 volume percent of olefins and having a mole ratio of methane to ethylene of at least 1.0.

2. Process according to claim 1 wherein said mixture is a paraifinic concentrate from catalytic petroleum reformate.

3. Process according to claim 2 wherein said concentrate is obtained by solvent extraction of the reformate.

4. Process according to claim 1 wherein said subjecting involves passing the parafiinic concentrate through a confined elongated cracking zone in the absence of added element-a1 oxygen and indirectly heating the paraffinic concentrate during passage through said zone.

5. Process according to claim 1 wherein said temperature is in the range from 1450 to 1600 F.

6. Process according to claim 1 wherein the pressure is in the range from 1 atmosphere to 5 atmospheres absolute.

7. Process according to claim 1 wherein said parafiinic hydrocarbons include at least 40 volume percent of branched chain parafiinic hydrocarbons.

8. Process according to claim 1 wherein said mixture is the product of alkylation of isobutane with butylene.

References Cited by the Examiner UNITED STATES PATENTS 2,708,621 5/1955 Shapleigh 48-214 X 2,871,254 1/1959 Hoog et al. 3,060,116 10/1962 Hardin et al.

MORRIS O. WOLK, Primary Examiner.

Claims (1)

1. PROCESS FOR PRODUCTING HEATING GAS WHICH COMPRISES SUBJECTING A NORMALLY LIQUID HYDROCARBON MIXTURE BOILING IN THE RANGE FROM 150 TO 400*F. AND HAVING API GRVITY IN THE RANGE FROM 50 TO 75* CONTAINING 75 TO 100 VOLUME PERCENT OF PARAFFINIC HYDROCARBONS INCLUDING AT LEAST 25 VOLUME PERCENT OF BRANCHED CHAIN PARAFFINIC HYDROCARBONS, LESS THAN 15 VOLUME PERCENT OF AROMATIC HYDROCARBONS, TO A TEMPERATURE IN THE RANGE FROM 1200 TO 2000*F. FOR AT LEAST 0.1 SECOND, THEREBY TO PRODUCE A GAS CONTAINING AT LEAST 15 VOLUME PERCENT OF METHANE AND LESS THAN 40 VOLUME PERCENT OF OLEFINS AND HAVING A MOLE RATIO OF METHANE TO ETHYLENE OF AT LEASE 1.0.