Conversions of Low Molecular Weight Hydrocarbons to Higher Molecular Weight Hydrocarbons Using a Boron Compound Containing Catalyst

It is the business of many refineries and chemical plants to obtain, process and upgrade relatively low value hydrocarbons to more valuable feeds, or chemical raw materials. For example, methane, the simplest of the saturated hydrocarbons, is often available in rather large quantities either as an undesirable by product in admixture with other more valuable higher molecular weight hydrocarbons, or as a component of an off gas from a process unit, or units. Though methane is useful in some chemical reactions, e.g., as a reactant in the commercial production of methanol and formaldehyde, it is not as useful a chemical raw material as most of the higher molecular weight hydrocarbons. For this reason process streams which contain methane are usually burned as fuel.

Methane is also the principal component of natural gas, which is composed of an admixture of normally gaseous hydrocarbons ranging C.sub.4 and lighter and consists principally of methane admixed with ethane, propane, butane and other saturated, and some unsaturated hydrocarbons. Natural gas is produced in considerable quantities in oil and gas fields, often at remote locations and in difficult terrains, e.g., offshore sites, arctic sites, swamps, deserts and the like. Under such circumstances the natural gas is often flared while the oil is recovered, or the gas is shut in, if the field is too remote for the gas to be recovered on a commercial basis. The construction of pipelines to carry the gas is often not economical, due particularly to the costs of connecting numerous well sites with a main line. Transport of natural gas under such circumstances is also uneconomical because methane at atmospheric pressure boils at -258.degree. F. and transportation economics dictate that the gas be liquefiable at substantially atmospheric pressures to reduce its volume. Even though natural gas contains components higher boiling than methane, and such mixtures can be liquefied at somewhat higher temperatures than pure methane, the temperatures required for condensation of the admixture is nonetheless too low for natural gas to be liquefied and shipped economically. Under these circumstances the natural gas, or methane, is not even of sufficient value for use as fuel, and it is wasted.

The thought of utilizing methane from these sources, particularly avoiding the tremendous and absolute waste of a natural resource in this manner, has challenged many minds; but has produced few solutions. It is highly desirable to convert methane to hydrocarbons of higher molecular weight (hereinafter, C.sub.2 +) than methane, particularly admixtures of C.sub.2 + hydrocarbon products which can be economically liquefied at remote sites; especially admixtures of C.sub.2 + hydrocarbons rich in ethylene or benzene, or both. Ethylene and benzene are known to be particularly valuable chemical raw materials for use in the petroleum, petrochemical, pharmaceutical, plastics and heavy chemicals industries. Ethylene is thus useful for the production of ethyl and ethylene compounds including ethyl alcohol, ethyl ethers, ethylbenzene, styrene, polyethylbenzenes ethylene oxide, ethylene dichloride, ethylene dibromide, acetic acid, oligomers and polymers and the like. Benzene is useful in the production of ethylbenzene, styrene, and numerous other alkyl aromatics which are suitable as chemical and pharmaceutical intermediates, or suitable in themselves as end products, e.g., as solvents or high octane gasoline components.

It has been long known that methane, and natural gas could be pyrolytically converted to C.sub.2 + hydrocarbons. For example, methane or natural gas passed through a porcelain tube at moderate red heat will produce ethylene and its more condensed homologues such as propylene, as well as small amounts of acetylene and ethane. Methane and natural gas have also been pyrolytically converted to benzene, the benzene usually appearing in measurable quantities at temperatures above about 1650.degree. F. (899.degree. C.), and perhaps in quantities as high as 6-10 wt. % at 2200.degree. F. to 2375.degree. F., (1204.degree. to 1302.degree. C.) or higher. Acetylene and benzene in admixture with other hydrocarbons, have been produced from methane and natural gas in arc processes, cracking processes, or partial combustion processes at temperatures ranging above about 2775.degree. F. (1524.degree. C.). Heat for such reactions has been supplied from various sources including electrically heated tubes, electric resistance elements, and spark or arc electric discharges. These processes characteristically require considerable heat energy which, most often, is obtained from combustion of the by-product gases. The extreme temperatures coupled with the low yields of higher molecular weight hydrocarbons have made the operation of such processes uneconomical. Numerous attempts have been made to catalyze these reactions at lower temperatures, but such attempts have met with failure.

In all such processes of converting methane to C.sub.2 + hydrocarbons a partial oxidation mechanism is involved, because hydrogen must be removed either as water, molecular hydrogen or other hydrogen-containing species. Likewise, any other polymerization mechanism wherein methane is converted to C.sub.2 + hydrocarbon products requires a tremendous amount of energy, most often supplied as heat, to provide the driving force for the reactions. In the past the molecular hydrogen liberated by the reaction has often been separated and burned to provide the necessary process heat. This route has proven an abomination to the production of C.sub.2 + hydrocarbons, but alternate reaction pathways have appeared little better, if any, for these have resulted in the production of large quantities of the higher, less useful hydrogen deficient polymeric materials such as coke, and highly oxidized products such as carbon dioxide and water.

Typical of low temperature prior art processes are those disclosed in U.S. Pat. Nos. 4,239,658, 4,205,194 and 4.172,180 which use a regenerable catalyst-reagent. U.S. Pat. No. 4,239,658, for example, teaches a process for the conversion of methane to higher molecular weight hydrocarbons. In the process, a three component catalyst-reagent is utilized which comprises a mixture of various metals and metal oxides, particularly a Group VIII noble metal, nickel or a Group VI-B noble metal, a Group VI-B metal oxide and a Group II-A metal. The patent teaches process temperatures from about 1150.degree. to 1600.degree. F. (621.degree. to 871.degree. C.), preferably 1250.degree. F. to about 1350.degree. F. (677.degree. to 732.degree. C.).

It has also been reported in Science 153, 1393, (1966), "High Temperature Synthesis of Aromatic Hydrocarbons From Methane", that aromatic hydrocarbons can be prepared from methane by contact with silica at 1000.degree. C. (1832.degree. F.). The yield of hydrocarbons was in the range of 4.8 to 7.2 percent based on the methane used in a single pass at a space velocity of 1224 hr.sup.-1.

In the J. Chinese Chem. Soc., Volume 29, pages 263-273 (1981), it is reported that methane can be converted to C.sub.2 + hydrocarbons at temperatures of 800.degree. to 1130.degree. C. and space velocities of 3100 hr.sup.-1 or less using a metal oxide catalyst. However, the total conversion of methane, at best, is 7.5 mole percent using a thorium oxide catalyst.

Franz Fischer, reports in an article entitled: "The Synthesis of Benzol Hydrocarbons From Methane At Ordinary Pressure and Without Catalyst" (Brennstoff Chemie, Vol. 9, pp 309-316, 1928) that methane is converted to benzol hydrocarbons by passing methane through a hot tube. In carrying out this work Fischer tested many substances for catalytic activity at temperatures ranging from 650.degree. to 1150.degree. C. and at high flow rates and concluded that the substances tested were not catalytic and not necessary. Among the substances tested were elemental iron, copper, tungsten, molybdenum, tin and carbon; and the compounds potassium hydroxide and silica gel.

A process for the production of higher molecular weight hydrocarbons from lower molecular hydrocarbons comprising the steps of:

(a) introducing into a reaction zone a lower molecular weight hydrocarbon-containing gas and contacting said gas in said zone with a boron compound containing catalyst under C.sub.2 + hydrocarbon synthesis conditions such that at least 15 mole percent of the lower molecular weight hydrocarbons in said gas are converted to higher molecular weight hydrocarbons, said conditions including a temperature of greater than 1000.degree. C. and a gas hourly space velocity of greater than 3200 hr.sup.-1 ;

(b) withdrawing from said reaction zone a higher molecular weight hydrocarbon-containing stream.

The process of the present invention affords high conversions of 19 mole percent or more of the lower molecular weight hydrocarbons with high selectivity, that is, 80 mole percent or more of the reaction products comprise higher molecular weight hydrocarbons.

It is a primary object of the present invention to provide an improved process for the conversion of low molecular weight hydrocarbons to higher molecular weight hydrocarbons with a high conversion of the lower molecular hydrocarbons and high selectivity in the conversion to higher molecular weight hydrocarbons.

It is an essential feature and critical to obtaining the above objects that the process of the present invention is carried out under critical reaction conditions. These critical conditions include the use of a boron compound containing catalyst, a temperature of at least 1000.degree. C. and a gas hourly space velocity of at least 3200 hr.sup.-1.

It has been surprisingly found that by using the high contact temperatures of the present invention coupled with the high space velocity, the boron compound containing catalyst used in the present invention do not rapidly foul and the yield of less valuable coke is low while the yield of higher molecular weight hydrocarbons is high.

As used in the present invention the word "metal" refers to all those elements of the periodic table which are not non-metals. "Non-metals" for the purpose of the present invention refers to those elements having atomic numbers 2, 5 through 10, 14 through 18, 33 through 36, 52 through 54, 85 and 86.

As used in the present invention the phrase "lower molecular weight hydrocarbons" means hydrocarbons containing at least one or more carbon atoms, i.e., methane, ethane, propane, etc. Also as used in the present invention, the phrase "higher molecular weight hydrocarbons" means hydrocarbons containing two or more carbon atoms and at least one carbon atom more than the lower molecular weight feedstock.

As used herein the phrase "C.sub.2 + hydrocarbon synthesis conditions" refers to the selection of feedstock, reaction temperature, space velocity and catalyst described hereafter such that higher molecular weight hydrocarbons are produced in the process with yield as described hereafter. Other process parameters necessary to maintain C.sub.2 + hydrocarbon synthesis conditions, such as the selection of particular types of reaction vessels, etc., is readily determined by any person skilled in the art.

Generally, the feedstock lower molecular weight hydrocarbon of the present invention will comprise methane or natural gas containing C.sub.1 to C.sub.4 hydrocarbons. The product higher molecular weight hydrocarbons will comprise C.sub.2 + hydrocarbons, particularly mixtures of C.sub.2 + hydrocarbons which can be economically liquefied. Preferably, the higher molecular weight hydrocarbon product streams will be rich in ethylene or aromatics such as benzene, or both.

As will be readily apparent to any person skilled in the art, the process of the present invention is extremely simple. The lower molecular weight hydrocarbons are introduced into a reaction zone containing a suitable boron compound containing catalyst. The reaction-zone catalyst system can be either of the fixed bed type or fluid bed type and the lower molecular weight hydrocarbons can be introduced into the top or bottom of the reaction zone with the product stream removed from either the top or bottom. Preferably, a fixed bed catalyst system is used and the feed stream is introduced into the top of the reaction zone and product is withdrawn from the bottom.

It is critical to the process of the present invention that a high temperature greater than 1000.degree. C. is maintained in the reaction zone along with a high gas hourly space velocity of greater than 3200 hr.sup.-1. Preferably, the temperature will be greater than 1100.degree. C. with a space velocity greater than 6000 hr.sup.-1. Still more preferably the temperature is greater than 1150.degree. C. with a space velocity greater than 9000 hr.sup.-1.

Generally, the temperature will be in the range of 1001.degree. to 1300.degree. C. while the gas hourly space velocity is in the range 3200 to 360,000 hr.sup.-1. Preferably, the temperature is in the range 1100.degree. to 1200.degree. C. with a gas hourly space velocity of 6,000 to 36,000 hr.sup.-1. More preferably the temperature is in the range 1140.degree. to 1175.degree. C. with a gas hourly space velocity in the range of 9,000 to 18,000 hr.sup.-1. Generally, high temperatures are used with high space velocities and low temperatures are used with low space velocities.

The process can be operated at sub-atmospheric, atmospheric, or supra atmospheric pressure to react and form the higher molecular weight C.sub.2 + hydrocarbons. It is preferred to operate at or near atmospheric pressure.

The process of the present invention affords high conversions of the lower molecular weight hydrocarbons with high selectivity to higher molecular weight hydrocarbons. More particularly, as measured by the disappearance of the lower molecular weight hydrocarbons, the process of the present invention affords conversions of 19 mole percent or more of the lower molecular weight hydrocarbons, and preferably, the conversions are greater than 25 mole percent and more preferably greater than 40 mole percent. The carbon-containing reaction products comprise 80 mole percent or more higher molecular weight hydrocarbons, preferably, greater than 90 mole percent, and more preferably greater than 95 mole percent. Based on the feed, at least 15 mole percent, and preferably at least 20 mole percent, and more preferably at least 40 mole percent of the lower molecular weight hydrocarbons are converted to higher molecular weight hydrocarbons, which is referred to herein as selectivity.

Boron compound containing catalysts and catalyst supports may be used in the present invention. The word "catalyst" is used in the present invention to mean a substance which strongly affects the rate of a chemical reaction but which itself undergoes no chemical change although it may be altered physically by chemically absorbed molecules of the reactants and reaction products. It is also understood that the catalyst of the present invention may be formed in situ. For example, in the present invention when an oxide, nitride, or carbide metal catalyst is initially charged to the reactor, the oxide and nitride may be converted in situ to the carbide which then functions as the catalytic species.

Boron compound containing catalysts for use in the present invention will provide conversion of the lower molecular weight hydrocarbons of at least 19% and will maintain this conversion for at least 3 hours under the temperature and space velocity conditions previously discussed. Preferred caatalysts of the present invention will provide conversions of 30% or more of the lower molecular weight feed and remain active for 3 hours or more.

Representative boron compound containing catalysts are refractory materials and include boron carbide, or boron nitride.

The catalyst must be thermally stable under the operating condition in the reaction zones. Boron nitride is particularly preferred. Preferably, the catalyst can also be regenerated by the periodic burning-off of any undesirable deposits such as coke. The regeneration of catalyst by the burning off coke is well known in the catalyst and petroleum processing art.

Catalysts useful in the present invention may be used with and without catalyst supports. However, it is generally preferred to use a catalyst support such as the well known aluminas or silicas or combinations thereof.

The catalysts of the present invention may have a wide range of surface areas as measured by the BET method (Jour. Am. Chem. Soc., vol. 60, pp 309 (1938). Low surface areas are prepared but the catalyst must have at least 1 m.sup.2 /gram surface area, preferably at least 2 m.sup.2 /gram. High surface area catalysts, having less than about 300 m.sup.2 /gram, preferably less than 200 m.sup.2 /gram are satisfactory. The most preferred catalysts have surface areas in the range of 4 to 1500 m.sup.2 /gram.

A particularly preferred catalyst for use in the present invention is boron nitride.

The advantages of the present invention will be readily apparent from a consideration of the following examples.

The examples illustrating the invention were carried out as follows:

The apparatus comprises a vertical reactor tube made of high purity alumina of 3/8" O.D. and 1/4" I.D. This tube is 24" long, the central 12" of which is surrounded by a high temperature electric furnace (Marshall Model 1134). The heated section of the tube is packed with the test catalyst. A small piece of close fitting alumina honeycomb, or monolith, at the bottom of the bed supports the catalyst. An "O"-ring sealed closure at the top of the reactor tube connects it to a gas flow system, which permits either argon or methane to be passed into the reactor at a measured rate. Gas flows into the reactor are measured with pre-calibrated flowmeters. Gas exiting from the reactor is first passed through a trap packed with dry stainless steel "saddles" (distillation column packing), then through a tube fitted with a rubber septum. Gas samples are taken through the septum with a syringe. Off gas exits the system through a "U"-tube partially filled with oil. Bubbles passing through the oil provide a visual indicator of the gas flow.

In operation, the central section of the reactor tube is packed with the catalyst to be tested. The catalyst particles range in size from 8 mesh to 12 mesh. About 10 cm.sup.3 of catalyst is charged to the reactor. The reactor is then placed in the cold furnace, and the necessary input and output connections are made. A slow flow of about 15 to 20 ml/min. of argon is continuously passed through the reactor, which is then brought to the desired temperature over a period of about 150 min. Temperatures reported herein are measured in the furnace wall. Temperatures are measured by a thermocouple mounted in the furnace wall. Calibration curves, previously developed from a thermocouple in the catalyst bed and compared to the furnace wall thermocouple, are used to determine the reaction temperatures reported herein.

Once the apparatus is at the desired temperature, argon flow is stopped and methane flow is started at the predetermined flow rate. Space velocities are calculated on the basis of the temperature, pressure, methane flow rate into the reactor and on the catalyst bed dimensions. On each run, the reaction is allowed to level out for 15 to 20 minutes before the first analytic sample is withdrawn through the septum. Two samples are taken each time, using one ml gas-tight syringes. Aliquots of these samples (0.25 ml) are separately injected into a gas chromatograph packed with Poropak Q. Analysis is made for hydrogen, methane, and light hydrocarbons having less than 5 atoms of carbon. Other aliquots of the same samples are injected into another gas chromatograph column packed with Bentone 1200. This analysis is made for aromatics, including benzene, toluene, xylenes, etc.

Calculation of the below tabulated conversion values from the gas analysis data only is as follows. The reaction is assumed to be given by the general expression:

wherein "CH" represents higher hydrocarbons, C is coke and .beta. and .gamma. are the number of moles of coke and aromatics, respectively. Then, for one mole of methane fed to the reaction zone, .alpha. is the fraction that reacts according to the above equation and ##EQU1## wherein X.sub.CH.sbsb.4 is the mole fraction of methane in the product gas atream. Finally, an iterative procedure is used to calculate .beta. and .gamma. based on the gas analysis results.

Table I below gives the details of runs made in accordance with the above description. The table gives the catalyst composition, the space velocity, temperature, and results of runs made on the conversion of methane to C.sub.2 + hydrocarbons.

Examples 1-3, illustrate various preferred embodiments of the invention using various boron compound containing catalyst and reaction conditions. All of these examples illustrate that high yields of higher molecular weight hydrocarbons are obtained when using the critical process conditions of the present invention.