US3424554A - Co-production of coke and ammonia synthesis gases - Google Patents

Description

Jan. 28, 1969 c. E. JAHNIG ET AL 3,424,554

(PO -PRODUCTION OF COKE AND AMMONIA SYNTHESIS GASES Filed Oct. 26, 1966 BY ilk/Qua (LIP/112v PATENT ATTORNEY United States Patent 3,424,554 CO-PRODUCTION 0F COKE AND AMMONIA SYNTHESIS GASES Charles E. Jahnig, Rumson, and Warren K. Lewis, Jr.,

Westfield, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Oct. 26, 1966, Ser. No. 589,746

U.S. Cl. 23199 6 Claims Int. Cl. Ctllc 1/04; C10g 37/04 ABSTRACT OF THE DISCLOSURE This invention relates to a process combination for the co-production of coke and by-product gases suitable for use in the synthesis of ammonia. Hydrocarbons can be continuously cracked by contact within a bed of hot particulate fluidized coke solids, while oxygen-enriched gas is simultaneously injected into the bed in quantity suflicient to provide process heat and to maintain the required operating temperatures while simultaneously forming a gaseous effluent which is suitable for ammonia synthesis. Oxygen-enriched air in concentration providing an air:oxygen ratio ranging from about 0.3:1 to about 5:1 is suitable to maintain an operating temperature ranging from about 1400 F. to about 3000" F., and preferably from about 1800 F. to about 2600 F., while generating an eflluent which includes hydrogen and nitrogen in hydrogenznitrogen ratio ranging from about 2.821 to about 4.5:1. In a preferred combination, the gas is further reacted in a water-gas shift conversion zone, and then reacted at suitable temperature and pressure to synthesize ammonia.

Thermal coking techniques or processes are known to the art. In the more conventional low temperature thermal coking processes, a bed, or beds, of particulate fluidized coke solids is maintained at relatively low temperatures ranging generally from about 900 F. to about 1400 F. Other processes, known to the art as high temperature coking processes, are conducted at considerably more elevated temperatures. The reactions involved in all coking processes are endothermic, and heat must be supplied from an external source.

The hot coke particles of a bed are contacted by generally preheated hydrocarbon feeds and cracked to produce deposition of coke on the solids as well as moderate quantities of lower molecular weight materials. The temperature, time of contact, and pressure determine to a large extent the type of products that are obtained. Low temperature coking processes produce from high molecular weight, high boiling feedstocks primarily lower molecular weight, low boiling materials and gas oils. Topped crude oils, viscous hydrocarbons, pitch and other residua of low API gravity are thus contacted with hot coke to produce lower molecular weight hydrocarbon gases containing both saturated and unsaturated compounds. In the cracking reactions, coke deposits upon the hot solids coke particles.

High temperature coking processes are conducted at temperatures ranging above about 1800 F., and generally at temperatures ranging from about 1800 F. to about 3000 F. In such processes it is essential to operate a transfer line heater or burner to supply ample heat to the process. Pursuant to such technique, coke is withdrawn from a bed of the reactor and passed, in dilute phase, to an external vessel wherein the temperature of the coke is elevated by combustion of fuel and oxygen. Upon return of the coke to the bed the heat is imparted thereto.

Very small amounts of oxygen are sometimes added to the fluidized bed of high temperature coking reactors to produce some oxidation of the cracked products, which "ice are characterized by the formation of a gaseous eflluent consisting largely of hydrogen, as well as coke. At temperatures above about 1400 F., the cracking is thus generally more or less complete and essentially complete pyrolysis of hydrocarbons occurs to liberate coke, hydrogen, and some methane.

In such processes it is necessary to burn large portions of the off-gas, or eflluent gas, inasmuch as the gas is of little value, if any. Even in locations Where potentially useful, however, the value of such gas is often reduced because of the necessity to further process the gas prior to any intended use.

In view of this and other deficiencies, however, it is the primary objective of the present invention to provide a new and improved process, and particularly a process combination which fully utilizes to advantage the off-gases generated in certain coking operations. A particular object is to provide an integrated operation, particularly a high temperature coking operation in combination with other operations, wherein suitable gases can be generated and fully utilized in synthesis of ammonia. More particularly, it is an object to provide integrated high temperature coking operations wherein the off-gases of particular quality can be conveniently produced in desired proportions, and suitably conditioned for formation of ammonia.

A further object is to provide a high temperature coking process wherein adequate heat can be supplied without the necessity of providing a transfer line heater.

These and other objects are achieved in accordance with the present invention which contemplates the injection of oxygen-enriched air with hydrocarbon feed into a relatively high temperature reaction zone, or zones, containing a fluidized bed of particulate coke solids. The hydrocarbon feedstock is cracked in the fluidized bed of a reaction zone and preferably is added at the bottom of the bed to maximize cracking. The eflluent gas includes hydrogen and methane, as well as carbon monoxide and carbon dioxide from the air and oxygen added.

By injection of oxygen-enriched air or suflicient air and oxygen, heat is supplied to the process, and a total gas mixture can be produced which is suitable for further processing to provide a final hydrogenmitrogen ratio ranging from about 2.8 to about 4.5, and preferably in ratio of about 3. From such mixtures, very suitable ammonia synthesis gases can be obtained. The reaction is conducted at temperatures ranging from about 1400 F. to about 3000 F., and preferably from about 1800 F. to about 2600 F. In fact, temperatures ranging from about 2000 F. to about 2400 F. are found to provide a coke product of high quality, as well as a suitable gaseous mixture which can be further utilized in ammonia synthesis. The reactions can be conducted at atmospheric pressure, but preferably are conducted at supra-atmos pheric pressures. Pressures can range as high as about 1000 atmospheres, and higher. Preferably, however, pressures ranging from about 5 to about 50 atmospheres are employed.

It is essential to add or inject both oxygen and air into or following the fluidized bed of the reactor, and in certain critical proportions. The oxygen and air can be injected separately, or can be combusted with fuel in a separate precombustion reaction and the flue gases then injected into the fluidized bed of coke. Added oxygen is essential to supply heat for the reactions. Air alone cannot be used for this purpose because of the dilution effect of the inert gases which lessens the fuel value. Nor can it be used alone, as a practical matter because its use would necessitate increasing the size of the reactor, which houses the process. On the other hand, air is desired to furnish the nitrogen which is essential to the overall process.

Air and oxygen are injected into the fluidized bed, or fluid cracking zone, pursuant to these conditions in proportions of air-to-oxygen ranging from a ratio of about 0.3:1 to about 5:1. In other words, ambient air can be enriched with added oxygen in concentration sufficient to provide up to about 85 percent by volume oxygen. It is highly preferable that the oxygen or air, or both, be added to the reactor at a point very near the top of the fluid bed, so as to minimize reaction with coke and give more emcient use of the oxygen. Often it is desirable to add some of the oxygen or air, or both, separately at a point well above the bed level so as to raise the gas temperature above that of the fluid bed, and thereby facilitate thermal reforming of the methane and soot.

In accordance with these conditions, gases of relatively high hydrogen content can be generated. Though carbon monoxide is present in the gas in concentrations ranging from about 5 to about 20 volume percent, the presence of liquid reaction products of relatively higher molecular weight, e.g., tars and polymeric substances, are minmized. The carbon monoxide can be removed by shifting to hydrogen in a water-gas shift conversion reaction. In ac cordance therewith, steam (water) is injected into the carbon monoxide gas-containing mixture (which in some instances must be desulfurized) and the carbon monoxide is converted to carbon dioxide, which can be readily removed, and water is converted to hydrogen. The watergas shift conversion reaction is conducted in the p esence of a suitable catalyst, at temperatures ranging up to about 1000 F., and preferably at temperatures of from about 400 F. to about 900 F. Atmospheric or supraatmospheric pressures can be employed, but preferably the pressure ranges from about 5 to about 50 atmospheres. The resultant reaction product mixture thus contains high concentrations of carbon dioxide and hydrogen, as well as nitrogen.

The eifluent from the shift conversion reaction, consisting essentially of carbon dioxide, hydrogen, and nitrogen is then treated for removal of undesired constituents, including particularly carbon dioxide and water. The carbon dioxide, which is an acid gas can be readily removed ;by scrubbing with suitable basic compounds, e.g., alkali metal carbonates, alkaline earth metal carbonates, and certain basic amines. Water can be readily removed, if desired, by condensation upon cooling.

Following the water-gas shift conversion reaction and removal of carbon dioxide, the treated gas generally contains hydrogen and nitrogen in ratio ranging from about 2.811 to about 3.5: 1, and preferably in ratio ranging from about 3.0:1 to about 3.2:1. To produce ammonia from nitrogen and hydrogen, the gas is subjected to temperatures ranging from about 750 F. to about 1200 F. and to pressures ranging from about 100 to about 1000 atmospheres, preferably from about 200 to about 500 atmospheres.

The foregoing will be more readily understood by reference to the following detailed description and to the attached flow sheet or diagram to which reference is made in the description.

Referring to the diagram is shown a reactor which contains a fluidized bed 11 of coke solids particles maintained at a temperature of, e.g., 2000 F. The temperature of the bed 11 is maintained by burning oxygen in the bed.

Recycle gas can be introduced via line 14 to reactor 10 to fluidize the particulate coke particles of the bed, and a hydrocarbon feedstock sprayed into the bed of reactor 10 through a plurality of nozzles represented by line 15. Oxygen-enriched air is injected into reactor 10 via inlet 16. Coke is withdrawn from reactor 10 via line 17.

The hydrocarbon feedstock upon contact with the hot coke solids particles of the bed 11 cracks and vaporizes, leaving a residue upon the individual particles which forms coke. Injection of the oxygen-enriched air is sufficient to produce formation of car-bon monoxide, and also some carbon dioxide and water.

An effluent gas is removed from reactor 10 via line 18 after passage through a cyclone separator 19, or separators, which removes most of the entrained coke and returns it to bed 11. If desired, the gaseous eflluent can next be subjected to reforming to convert methane to hydrogen and carbon monoxide. This is done by passing the eflluent through, e.g., a refractory lined tubular reactor (not shown) into which steam or oxygen-containing gas is injected to give temperatures ranging generally from about 2000 F. to about 2700" F., or preferably from about 2200 F. to about 2500 F. Also, where excess sulfur is present, sulfur can be removed, e.g., by passage of the reformed efliuent through a guard chamber containing iron powder.

The gaseous efliuent is then passed via line 18 to a conversion zone 30 and steam is injected therein via line 21. The total mixture is reacted in a bed 31 of iron oxides, at temperatures ranging from about 600 F. to about 900 F. In the ensuing reaction, the carbon monoxide is oxidized to carbon dioxide and steam is converted or shifted to hydrogen. In this manner, then, carbon monoxide is shifted to hydrogen.

The effluent which evolves via line 32 is then cooled and quenched to remove water in vessel 33, and carbon dioxide is separated from the gaseous effluent by scrubbing as with monoethanolamine in scrubber 34.

The treated eflluent from the latter stages 33, .34 is then injected into a pressurized vessel 40 operated at a pressure of about 1000 atmospheres, while also subjected to a temperature on the order of about 600-900 F. In accordance therewith, the hydrogen and nitrogen are combined to produce ammonia.

The ammonia can be admixed with water and separated from the reaction products of vessel 40 by removal via line 41 or the bottom of vessel 40. After the synthesis, the ammonia can also be separated by passage to a scrubber and stripper combination (not shown) wherein ammonia is removed as a liquid while the unreacted gases are vented or returned to the process.

Having described the invention, what is claimed is:

1. In an endothermic process for the continuous production of coke by cracking hydrocarbons by contact within a bed of hot particulate fluidized coke solids at temperatures ranging from about 1400 F. to about 3000 F., the coke being deposited on the coke solids particles which grow by accretion while gases, consisting essentially of carbon monoxide and hydrogen, are evolved, the improvement comprising injecting oxygen-enriched air into the :bed to provide process heat and to form a gaseous efliuent which includes not only carbon monoxide and hydrogen, but also nitrogen in proportions providing, within the total gas mixture, a hydrogenznitrogen ratio ranging from about 2.8: 1 to about 4.5: 1.

2. The process of claim 1 wherein the hydrogenznitrogen ratio ranges to about 3.0, and the temperature of the reaction ranges from about 1800 F. to about 2600 F.

3. The process of claim 1 wherein the oxygen-enriched air is one wherein oxygen is admixed with sufficient air to provide an air-oxygen ratio ranging from about 0.3:1 to about 5: 1.

4. The process of claim 3 wherein the oxygen-enriched air is injected near the top of the fluidized bed, and the hydrocarbon is injected below the point of the oxygenenriched air injection.

5. In an endothermic process for the continuous production of coke by cracking hydrocarbons by contact within a bed of hot particulate fluidized coke solids, the coke being deposited on the coke solids particles which grow by accretion while gases, consisting essentially of carbon monoxide and hydrogen, are evolved, the improvement comprising:

injecting oxygen-enriched air into the bed to provide suflicient process heat to maintain a temperature of from about 1400 F. to about 3000 F. and to form a gaseous effluent including carbon monoxide, hydrogen, and nitrogen at a hydrogen-nitrogen ratio ranging from about 2.8:1 to about 4.5: 1.

withdrawing the efiluent, admixing steam with the effluent, and subjecting the mixture, in the presence of a catalyst, to temperatures ranging from about 600 F. to about 900 F. in a Water-gas shift conversion reaction to convert carbon monoxide to carbon dioxide and water to hydrogen,

treating the efiiuent to remove carbon dioxide and water therefrom,

and then subjecting the gases to temperatures ranging from about 750 F. to about 1200 F., and to pressures ranging from about 100 to about 1000 atmospheres, to produce the synthesis of hydrogen and nitrogen to 'form ammonia.

6. The process of claim 5 wherein the ammonia is separated by scrubbing with Water to form an ammonia solution.

References Cited EARL C. THOMAS, Primary Exal'uincr.

HOKE S. MILLER, Assistant Examiner.

US. Cl. X.R.

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