US2878163A - Purification process - Google Patents

Description

March 17, 1959 LE ROI E. HUTCHINGS 2,878,163

PURIFICATION PROCESS Filed Aug. 9, 1956 COKE GRINDER a HEATER I8 CONDENSER EVAPORATOR 6 IO II 14 I3 DIGESTER HEATER WASHER DRIER 22 2s 27 2d Pa INVENTOR.

LE 'Rol E. HUTCHINGS ATTORNEY United States Patent PURIFICATION PROCESS Le Roi E. Hutchings, Crystal Lake, 111., assignor to The Pure Oil Company, Chicago, 111., a corporation of Ohio Application August 9, 1956, Serial No. 603,115

16 Claims. (Cl. 202-31) This invention relates to the purification of petroleum coke and, more particularly, to a method for the removal of sulfur and nitrogen compounds from petroleum coke by treatment with an alkali metal hydroxide at temperatures of at least about 700 F. and under conditions whereby the alkali metal hydroxide is in a molten condition.

Naturally occurring sulfur and nitrogen compounds found in petroleum coke are known to have a deleterious efiect when the coke is used in certain applications, such as metallurgic electrodes. For this reason it is desirable that the sulfur compounds and nitrogen compounds be removed. However, little is known so to the form in which sulfur exists in petroleum coke, but there is evidence to show that compounds such as mercaptans, sulfides, thiophanes, thiophenes and disulfides are the products of the decomposition of different high molecular weight sulfur compounds originally present in the crude oil. The amount and kinds of sulfur compounds present in petroleum coke depend upon the amount of total sulfur in the asphalt from which the coke is prepared, and the manner of its preparation. When asphalt is converted to coke, there is a general degradation of the majority of sulfur compounds with the evolution of hydrogen sulfide, leaving a residual quantity of sulfur and nitrogen compounds which is difficult to remove and is highly undesirable as far as the end characteristics of the coke are concerned. Numerous methods have been proposed for the production of low-sulfur, solid, carbonaceous fuels. In general, many of these methods relate to coal desulfurization and the principal problem is sulfur removal without excessive conversion of the coal into gas or vapor. Furthermore, the pressures and temperatures are not adequate for the treatment of petroleum coke, and these processes are concerned with the preparation of a different product, not coke. In addition, the processes directed to the desulfurization of coal do not effect a sufficient reduction in the sulfur and nitrogen content to be applicable in petroleum coke desulfurization.

According to the prior art, coke from various sources may be desulfurized by treatment under wide temperature ranges, up to l600 F., using oxidizing, reducing or inert atmospheres. The art teaches that various molten salts have been used to treat petroleum coke for the removal of sulfur, but the percentages of sulfur reduction obtained have not been sufiicient to warrant commerical adaptation of the processes. Such materials as sodium hydroxide, sodium carbonate, naphthalene, phenol and zinc have been tried and found to lack sufiicient desulfurizing ability to be of interest. The art has also clearly recognized-a distinction between desulfurizing coal and the preparation of low-sulfur petroleum coke, especially if the latter is to be used for metallurgical purposes. As a result, recent developments in this art show a trend to hydrogenation procedures which involve expensive apparatus, even catalysts, and rather involved processing steps. The degree of desulfurization obtained even then is not always commensurate with the outlay for equip ment, and an efiicient practical method. of treatment has not as yet been evolved. Some investigators of the use of molten salts have disparaged this avenue of approach because the extent of desulfurization was found to be minor, with some doubt as to whether any desulfurization was obtained at all because of the possible contamination of the coke samples with the treating agent. Also, the prior art teaches that such notably good desulfurizing agents as zinc are ineffective when applied in this direct method of desulfurization. The results obtained by these prior art methods vary with the method of sulfur determination, that is, whether by the combustion method or gravimetric method, and the processes could not be reproduced in toto. In addition, the products could not be used for preparing metallurgical coke because of the high sulfur content. The method as practiced with salts also was not economical because of the high cost of recovery of the treating agent.

In accordance with the present invention, a method has been discovered to apply molten alkali metal hydroxides,

particularly molten sodium, potassium and lithium hydroxides and their mixtures, at particular temperatures whereby these problems are overcome, i. e., consistent results are obtainable, and the product meets the requirements for metallurgical coke for the preparation of electrodes in aluminum manufacture and similar purposes. This discovery is based primarily on the unexpected finding that if the alkali metal hydroxide is used in an amount sufficient to form a melt or semi-fluid mixture with the pulverized coke, it becomes possible after the completion of the desulfurization heat treatment to vaporize off the alkali metal hydroxide by increasing the temperature and adding a diluent gas such as steam, or decreasing the pressure, and obtain substantial separation thereof and very eifective desulfurization. The recovery of alkali metal hydroxide is, thus, greatly simplified and made more complete. Also, the alkali metal hydroxides so recovered may be recycled or re-used without further treatment. The alkali metal hydroxides have been found to vaporize from the melt or semi-fluid mass at temperatures and pressure conditions well below their normal boiling points.

The attached drawing shows a simplified flow diagram of one form of the process of this invention.

In order to demonstrate the invention, a number of experiments were carried out in which samples of a petroleum coke were treated with various agents under different conditions. The petroleum coke used in these experiments was obtained from a sour Texas crude (Yates) by treatment in a delayed coking unit for residues from crude distillations plus small amounts of cycle stocks. The by-product coke was of the following composition: water, 8.23%; ash, 0.19%; volatiles, 10.21%; fixed carbon, 81.37%. It had a total sulfur content of 2.88 wt. percent (dry basis) as determined by the Shell combustion method, and the nitrogen content was, 1.28% wt. After calcining to remove volatiles, the sulfur content was unchanged, indicating the volatiles had about the same sulfur content as the coke.

Example 1 A 10 gm. sample of this crude petroleum coke, containing 2.88 wt. percent sulfur, was mixed with 200 cc. of 20% aqueous sodium hydroxide, and the mixture was heated to the boiling point and held at this temperature for minutes. The mixture was then cooled, the solid coke phase was separated and water washed 6 times with fresh water, and then was dried at 230 F. for 6 hours in an oven. The sulfur content of the dried product was determined to be 2.83 wt. percent. This indicated a very low sulfur reduction and the possibility that the reduc- 3 7 tion shown was due to the contamination of the product with sodium hydroxide, which is very diflicult' to wash from petroleum coke.

Example 2 extreme oxidizing conditions are inefiective techniques for desulfurizing petroleum coke.

Example 3 p In another experiment, 17 gms. of the same petroleum coke used in the preceding examples were ground to 40-60 mesh and mixed with 17 gms. of powdered sodium hydroxide. The mixture was placed in a muffle furnace and held at 750-780 F. for two hours. .The mixture was cooled, washed 6 times with 200 cc. portions of water at about 200 F., and dried at 230 F, for 6 hours. The sulfur content of the purified cokeproduct was found to be 1.26 wt. percent. Under these conditions, using equal parts of coke and sodium hydroxide, the mass retained a semi-solid state and although the degree of sulfur reduction was substantial, the condition of the mass indicated that intimate contact was not achieved.

Example 4 Experimenting further, 16 gms. of this same petroleum coke, ground to 100 mesh size, were mixed with 40 gms. of powdered sodium hydroxide and placed in a steel bomb fitted with an inlet for the introduction of hydrogen and pressure-indicating means. Hydrogen gas was admitted to the bomb and, after sealing, the bomb was heated to 755 F. At this temperature, the pressure was determined to be 3900 p. s. i. g. The reaction mass was held at these conditions for 70 minutes, after which it was cooled, washed with hot water and dried as in Exampie 3. The sulfur content of the product was determined to be 1.42 wt. percent, and miscellaneous, unidentified lay-products were produced.

Example 5 Ten gms. of petroleum coke, as above identified and ground to 100 mesh, were mixed with gms. of powdered sodium hydroxide and heated in a furnace to 750- 865 F. for a period of 7% hours during which time the mass was agitated. After cooling, water-washing and drying in accordance with the foregoing examples, the product was found to have a sulfur content of only 0.07 wt. percent and a nitrogen content of 0.98%.

Example 6 temperature schedule:

Time mm) Tern Remarks 480 9% stirred-mixture fluid. s00 stirred. 910 Most of N aOH distilled ofi.

Thereafter, the coke was allowed to cool and was rethis specification.

moved from the crucible by washing with boiling water. The coke was then washed 4 times with hot water and dried. Results of the run were as follows:

Recovery4.5 gms. Sulfur, wt. percent0.08 (originally 2.88) NaOH, wt. percent0.02

From these experiments it is apparent that substantial reductions of the sulfur content of petroleum coke are made possible and, by raising the reaction temperature, after about 2 hours contact time at an average temperature 'of about 750-800 F., to around 910 F., there is accomplished a good separation of the sodium hydroxide in vapor form. This vaporization carries off most of the sodium hydroxide and apparently a large portion of the deleterious sulfur compounds. It is considered that the vaporization phenomenon is unusual because the vaporization point of pure sodium hydroxide is considerably higher than 910 F., namely above about 1300 F., and it exhibits a boiling point of about 2534 F.

The process of the invention may be demonstrated by reference to the attached drawing forming a part of Fresh petroleum coke is ground in grinder 1 to about mesh and is preferably screened to remove larger particles which are returned to the grinder by a mechanism not shown. The powdered coke is then conveyed through conduit or conveyor 2 to kiln 3 wherein it is calcined at 1000 F., to remove any volatile materials which pass ofi by means of line 4. The calcined petroleum coke is passed by conduit 5 to digester 6 fitted with suitable agitation means (not shown), and is mixedwith melted sodium hydroxide from heater 7, conveyed by means of line 8. Make-up sodium hydroxide may be added to heater 7 or to digester 6 through line 9. Sodium hydroxide leaving heater 7 is maintained at about 900 F.

The relation of the volume of digester 6 to the petroleum coke and caustic feed rates is such that a contact time of about one hour results. The fluid sodium hydroxide-petroleum coke mixture is then pumped or fed by gravity through line 10 and sprayed into evaporator 11 by means of spray-head 12. Evaporator 11 is heated by means of hot inert gases, such as nitrogen or superheated steam supplied by line 14 and line 15 from superheater 13. Evaporator 11 is provided with elongated condensing section 16 fitted with one or more collecting rings 17. Within condensing section 16, which may be externally cooled by air or other means, the rising steam-sodium hydroxide vapors are cooled to slightly above the melting point of sodium hydroxide, about 604 F., so that the caustic condenses, collects on the internal walls thereof, and runs down to collecting ring 17, from which it is conveyed by line 18 to heater 7.

Steam passing overhead from condensing section 16 contains traces of sodium hydroxide and is fed to total condenser 19 by line 20, from which a dilute caustic solution is removed at line 21. Coke fines carried over from evaporator 11 and condenser section 16 are separated (by means not shown) from the dilute sodium hydroxide steam in line 21 and conveyed to water washer 22, which receives the main charge of desulfurized coke from evaporator 11 by means of conduit or conveyer 23. The petroleum coke in line 23 contains some sodium hydroxide and sodium sulfide which are removed in water washer 22 by countercurrent contact with hot water, at a temperature of about to 212. F., flowing therein through line 24. The dilute sodium hydroxide solution produced in water washer 22 is removed by line 25 and the washed petroleum coke proceeds through line 26 to drier 27. Drier 27 operates by hot-air flow or other heating means, sufiicient to evaporate olf the water contained therein. Desulfurized petroleum coke is removed by line 28. The dilute sodium hydroxide streams may be dried to obtain the sodium hydroxide for re-use in the process, or may be used as such in other processes as desired.

In the operation of the process, the petroleum coke is ground to about 100 mesh size in grinder 1 and charged to digester 6, after volatiles are removed in kiln 3, at a rate of about 100 lbs/hr. along with about 400 lbs/hr. of anhydrous, molten sodium hydroxide. The volatile materials amount to about by wt. of the petroleum coke. The temperature in digester 6 is maintained at about 750 to 910 F. and preferably about 900 F., wtih a low positive pressure of about 1 to 20 lbs. per square inch gauge. The residence time at 900 F. is about 1 hour, and for temperatures below 900 F., that is, at 750 to 800 F.,' residence time is 2 hours or more. Agitation may be provided in digester 6 by means of rotating impellers to insure intimate contact and shorten the time involved.

The mixture of coke and sodium hydroxide is sprayed into evaporator 11 at a pressure of about 10 to 20 lbs. p. s. i. g. wherein it contacts the rising, hot, inert gases at about 925 to 975 F., preferably at 950 F. The inert gas flow rate, which may vary from 10 to 100 cf./hour, is controlled so that the carbon particles fall to the bottom of the chamber, and substantially all of the sodium hydroxide is vaporized. The petroleum coke, after washing with hot water or other solvent for the sodium hydroxide, has a sulfur content of 0.1% by weight or lower, qualifying as a metallurgical coke. Condenser section 16 is maintained at about 620 F. to condense the sodium hydroxide for recycle to heater 7. Sodium sulfide and other impurities are maintained at a low level in the recovered sodium hydroxide by replacing a portion of the latter with fresh sodium hydroxide at heater 7 or line 9. The major portion of such impurities which have been transformed to watersoluble form remain in the petroleum coke and are removed in the water washer 22.

From this description, it is apparent that the process of this invention may be carried out by subjecting the petroleum coke containing undesirable impurities, particularly sulfur and nitrogen compounds, to the action of molten alkali metal hydroxide, which may be sodium hydroxide, potassium hydroxide or lithium hydroxide and mixtures thereof. The amount of alkali metal hydroxide used is sufficient to form a semi-fluid mass when mixed and heated with the petroleum coke. In general, the quantities used for this purpose have been found to be at least equal quantities by weight of the alkali metal hydroxide with the petroleum coke. Excessive amounts of the alkali metal hydroxide, that is weight ratios of 55-45, 60-50 and 70-30 of alkali to coke, may be used without deleterious efir'ect upon the reaction. The temperature in the heating or digestion zone is maintained at about 750 F. (850 with lithium hydroxide) as a lower limit and may be as high as 800 to 950 F., preferably about 900 F. The semi-fluid mass in the digestion zone is maintained for a time sufficient to cause the alkali metal hydroxide to react with the impurities present. Following this, the purified petroleum coke is recovered from the reaction mass.

Recovery of the petroleum coke and the alkali metal hydroxide is carried out by evaporating the alkali metal hydroxides by raising the temperature of the semi-fluid mass after completion of the reaction in the digestion zone. This evaporation may be conducted by merely raising the temperature of the mass to at least about 925 to 975 F., preferably about 950 F., for a time sufficient to evaporate the alkali metal hydroxide. Evaporation is facilitated by spraying the semi-fluid petroleum coke-alkali metal hydroxide reaction mass into the evaporation zone. The time required for evaporation of these alkali metal hydroxides is considerably shortened by contacting the sprayed, semi-fluid, reaction mass with a stream of hot, superheated inert diluent, such as steam or nitrogen. The use of a diluent such as steam facilitates evaporation of the alkali metal-hydroxides and, in the case of lithium hydroxide, suppresses decomposition to the oxide, reducing the temperature and time necessary for evaporation due to the partial pressure effect. The vaporous mixture of the alkali metal hydroxide and diluent is subjected to condensation at temperatures above the melting point of the alkali metal hydroxide, and well above the condensation temperature of the water or other diluent and gases formed in the digester, whereby the semi-quanti tative separation of the alkali metal hydroxides is brought about. The hydroxide so separated may be recycled directly to the initial digestion zone or sent through a preheater before going into the digestion zone.

In order to further facilitate the reaction, the petroleum coke is subjected to grinding and any particles larger than mesh size are removed. Volatile materials are removed from the petroleum coke prior to digestion by calcining at temperatures of at least about 1000 F.

After the evaporation step has been completed, the powdered, desulfurized petroleum coke is sent to a washing zone wherein it is washed with a hot solvent such as water or alcohol for the purpose of removing any remaining alkali metal hydroxide or other impurities that may be present.

The invention has been described in connection with various examples wherein the digestion reaction is carried out using porcelain or nickel containers. Under the conditions imposed in the examples, there was a substantial disintegration of porcelain where used, which in some instances added considerably to the ash content of petroleum coke. With a nickel crucible, this disadvantage .was overcome. In conducting the digestion process on a commercial scale, various metal and metal alloy containers may be used in various units thereof which would minimize the corroding and eroding effect of the alkali metal hydroxides and powdered coke. Examples of such materials are stainless steel, Admiralty-phosphorized alloy, Berylco alloys, cast iron, Buflokast gray iron, nickel and type 321 stainless steel. Other metals and metal alloys are known and are available which are resistant to alkali metal hydroxides and sulfur compounds under the conditions of those described herein. 1 Although the invention has been described in relationto a number of specific examples, the only limitations applying thereto are recited in the appended claims.

What is claimed is:

1. The process for the removal of undesirable impurities from petroleum coke comprising heating a mixture of a petroleum coke and a solid alkali metal hydroxide selected from the group consisting of sodium hydroxide, potassium hydroxide and lithium hydroxide and mixtures thereof, the amount of said alkali metal hydroxide being sufficient to form a semi-fluid mass when the resulting mixture is heated to a temperature of about 25 F. above the melting point of said alkali metal hydroxide, maintaining the mixture at a temperature of at least about 700 F. for a time sufiicient to cause said alkali metal hydroxide to react with said impurities, raising the temperature of the resulting semifluid mass to a distilling temperature of about 900 F. to about 1000 F., but below the boiling point of said alkali metal hydroxide when heated in the absence of said petroleum coke, recovering said alkali metal hydroxide as a distillate from said heated mass and recovering a purified petroleum coke from the residue of said distillation.

2. The process in accordance with claim 1 in which the amount of said alkali metal hydroxide used is at least equivalent to the amount of said petroleum coke to form said semi-fluid mass on the application of heat.

3. A process in accordance with claim 1 in which the alkali metal hydroxide is sodium hydroxide.

4. The process in accordance with claim 1 in which the alkali metal hydroxide is potassium hydroxide.

5. The process in accordance with claim 1 in which the alkali, metal hydroxide is lithium hydroxide.

6. The process in accordance with claim 1 in which the semi-fluid'mixture is initially heated to a temperature of about 700 to 865 F. for at least about 2 hours before the mass is raised to a distilling temperature.

7. The process in accordance with claim l'in which the heated semi-fluid mass of alkali metal hydroxide and petroleum coke is dispersed under conditions while at said distilling temperatures to form a vapor phase of said alkali metal hydroxide and a powdered solid phase comprising said petroleum coke, condensing and separating said vapor phase to recover molten alkali metal hydroxide for recycle to said reaction, leaching said powdered solid phase of petroleum coke to remove any remaining alkali metal hydroxide and lay-products therefrom and drying the resulting purified petroleum coke.

8. The process in accordance with claim 9 in which an inert diluent in superheated condition selected from the group consisting of steam and nitrogen is dispersed into said evaporating mixture to form a vapor phase comprising a mixture of said diluent and said alkali metal hydroxide, and partially condensing said mixed vapor phase to recover said molten alkali metal hydroxide for recycle.

. 9.-The process in accordance with claim 8 in which said inert diluent is dispersed as an upwardly flowing stream tosupply the heat for said evaporation.

10. The process in accordance with claim 1 in which the pressure of said reaction is above atmospheric and the pressure is released during said distillation.

11. The process for removing sulfur and nitrogen compounds from petroleum coke comprising heating petroleum coke with at least an equal amount of an alkali metal hydroxide selected from the group consisting of sodium hydroxide, potassium hydroxide and lithium hydroxide to form a semi-fluid mass at a temperature of about 750 F. to about 900 F. for about 2 hours, dispersing the semi-fluid reaction mass under evaporation conditions in the presence of superheated steam whereby the temperature is raised to about 950 F. but below the boiling point of said alkali metal hydroxide in the absence of said petroleum coke, removing said alkali metal hydroxide as a condensate, removing the petroleum coke as a powdered solid phase from said evaporation, washing said petroleum coke with hot water to remove the remaining alkali metal hydroxide, and drying the washed petroleum coke to obtain. a refined product.

12. The processin accordance with claim 10 in which the alkali metal hydroxide'is sodium hydroxide.

. 13. The process in accordance with claim 11 in which the alkali metal hydroxideis-potassium hydroxide.

14. The process in accordance with claim 10 in which the alkali metal hydroxide is lithium hydroxide and the temperature of said evaporation is 850* to 900 F.

, 15. The process in accordance .with claim 11 in which said petroleum coke is reduced to about mesh size, and calcined to a temperature of at least about 1000 F. prior to heating'with said alkali- 16. The process for, removing sulfur and nitrogen compounds frompetroleum coke which comprises grinding said .coke into particles of at least about 100 mesh size, heating said finely ground petroleum coke to a temperature of at least about 1000" F. to vaporize therefrom any volatile material, digesting said heated coke in the presence of at least an equal amount by weight of solid sodium hydroxide to form a semi-fluid mass at a temperature of about 750 to 900 F. for about 2 hours, spraying the semi-fluid reaction mass into an evaporation zone in the presence of superheated steam whereby the temperature is raised to about 950 F. and held for a time sufficient to vaporize a substantial portion of said sodium hydroxide therefrom, subjecting the vaporized sodiumv hydroxide to condensation to remove same as a liquid phase for recycle to said digestion step, removing the petroleum coke as a powdered solid phase from said evaporation zone, washing the petroleumcoke with hot water to remove the remaining sodium hydroxide and impurities and drying the washed petroleum coke to obtain a sulfur-free coke suitable for the preparation of metallurgical electrodes.

References Cited in the fileof this patent UNITED STATES PATENTS Italy Nov. 20,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Non 2,878,163 March 17, 1959 Le Roi E, Hutuhings Column 1, line 27, for known so" read known as column '7, line 20 for the claim reference numeral "9" read '7 Signed and sealeci this 11th day of August 1959,

(SEAL) Aitest:

KARL H, AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents

Claims (1)

1. THE PROCESS FOR THE REMOVAL OF UNDESIRABLE IMPURITIES FROM PETROLEUM COKE COMPRISING HEATING A MIXTURE OF A PETROLEUM COKE AND A SOLID ALKALI METAL HYDROXIDE SELECTED FROM THE GROUP CONSISTING OF SODIUM HYDROXIDE, POTASSIUM HYDROXIDE AND LITHIUM HYDROXIDE AND MIXTURES THEREOF, THE AMOUNT OF SAID ALKALI METAL HYDROXIDE BEING SUFFICIENT TO FORM A SEMI-FLUID MASS WHEN THE RESULTING MIXTURE IS HEATED TO A TEMPERTURE OF ABOUT 25* F. ABOVE THE MELTING POINT OF SAID ALKALI METAL HYDROXIDE, MAINTAINING THE MIXTURE AT A TEMPERATURE OF AT LEAST ABOUT 700* F. FOR A TIME SUFFICIENT TO CAUSE SAID ALKALI METAL HYDROXIDE TO REACT WITH SAID

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