US2560478A - Process for the mild oxidation of carbonaceous solids - Google Patents

Description

July 10, 1951 B. E. ROETHELI PROCESS FOR THE MILD OXIDATION OF CARBONACEOUS SOLIDS Filed Sept. 20, 1946 EMT Am.

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Erwz E. Zoeth-elz' Urn/eater Patented July 1951 PROCESS FOR THE IVIILD OXIDATION 01 CARBONACEOUS SOLIDS Bruno E. Roetheli, Cranford, N. Ja-assignor to Standard Oil Development Company, a corporation of Delaware Application September 20, 1946, Serial No. 698,389

4 Claims.

The present invention relates to the treatment of carbonaceous solids. More particularly, the invention is concerned with the limited oxidation of carbonaceous solids such as all types of coal,

brown coal, lignite, oil shale, tar sands, asphalts, cellulosic materials including lignin, etc.

It is known in the art that carbonaceous materials must be subjected to a controlled oxidation in order to become useful starting materials for certain processes. For example, in the preparation of various tar acids or similar organic chemicals or oil soluble coal substance from coal by an extraction with suitable solvents, the coal substance is mildly oxidized to form the desired oxygenated compounds which are thereafter recovered by extraction. Heretofore, relatively expensive inorganic oxidizing agents such as nitric acid, potassium permanganate, sodium dichromate, etc. have been used for this purpose involving a cumbersome and time-consuming wet-treatment of the coal.

Another well known mild oxidative pretreatment of carbonaceous solids has for its purpose the preparation of the charge for carbo'nization and/or gasification processes by controlling its agglutinating and/or plasticizingproperties. In this process, air is passed at temperatures of about 400-600 F. and pressures ranging from atmos pheric to several atmospheres gauge over a fixed bed or rotary-tumbling mass of coal having a. sufliciently large oxidizing surface to permit the desired limited oxidation of the agglutinating constituents within a time of about 15 to 45 minutes or more. This oxidation reaction is strongly exothermic and simultaneously requires exact temperature control to prevent the temvention to provide means for a controlled limited oxidation of solid or semi-solid carbonaceous materials.

Another object of my invention is to provide improved means for pretreating carbonaceous materials with oxidizing gases to render the carbonaceous materials more suitable for subsequent processing.

perature from rising to the ignition temperature of. the combustibles present or to the distillation temperature of the coal with resulting losses of desired volatile materials. Heretofore it has been .impossible to accomplish a preoxidation of coal .with air to the desired degree without considerable losses of desirable products because local overheating is unavoidable .in fixed bed as wellv as in rotary-tumbling bed operation as a result of irregular air distribution and. heat transfer throughout the coal be The present invention overcomes the aforementioned difficulties and affords various additional.

It is, therefore, the principal object of my in- A still further object of my invention is to provide improved means for an exact control of the reaction rate and temperature in the limited oxidation of solid carbonaceous materials.

Other and more specific objects of my invention will appear hereinafter.

In accordance with my invention, finely divided carbonaceous solids of a fluidizable particle size of say about to 400 mesh or larger sizes of up to /4" diameter are, contacted with an oxidizing gas, in the form of a dense turbulent mass of solids fluidized by an upwardly flowing gas to resemble a boiling liquid forming a well defined upper level. The oxidizing gas is preferably air and/or oxygen, although other oxidizing gases such as nitrogen oxides or other gaseous oxygen donors may be used. The fluidizing gas may be the oxidizing gas alone or admixed with an inert gas such as steam, residual air, or the like. The linear velocity of the fluidizing gas depends on the density and particle size of the solids charge and may vary within the wide limits of 0.3-10 ft. per second, velocities of about 0.3-3 ft. per second being preferred for particle sizes of about 20 to 200 mesh.

The dense turbulent fluidized mass of finely divided solids affords perfect and uniform gassolids contact and heat distribution over its entire height and cross section. Consequently, the oxidation rate and temperature may be easily controlled at any desired constant level throughout the mass of carbonaceous, solids by a proper adjustment of the oxygen-supply controlling both the degree of oxidation and the amount of exothermic heat produced. A highly important advantage resides in the ease with which the exothermic heat may be removed continuously to maintain exact temperature levels and in the prevention of an adiabatic heating of coal particles to ignition or decomposition temperature.

- This means that the drawbacks of batch or fixed bed oxidation are completely eliminated. Any

desired pretreatment of the carbonaceous material may be accomplished at optimum conditions of temperature and contact time to produce continuously a charge of the exact oxygen content desired for subsequent processing without any losses by overheating and true combustion and without requiring any moving equipment parts.

Depending on the use to which the pretreated carbonaceous material is to be put, the oxidation may be conducted at temperatures within the approximate range of 150-600 E, which should be lower than the temperature at which the combustible constituents of the charge burn to a substantial degree to. their ultimate combustion products such as CO, CO; and H20; contact times of about 1 minute'to several hours may be used to obtain a degree of oxidation equivalent to the absorption of about 05-40% of oxygen by the carbonaceous solids. I

For example, when it is desired to enrich extraction coal with oxygenated extractable substances relatively low temperatures of aboutl50 to 450 F. and relatively long contact times of about 30 minutes to several hours, that is up to about hours, preferably 30 minutes to about 2 hours are preferred to effect an absorption of about -40% by weight of oxygen by the coal while minimizing thermal decomposition of the extractable oxidation products. When a strongly coking coal is tobe prepared for subsequent carbonization or gasification by reducing its agglutinating and/or plasticizing tendencies, higher temperatures of about 400-650 F. and shorter contact times of about 1 to 30 minutes resulting in the absorption of less than 15%, preferably about 0.5-5% by weight of oxygen by the coal are generally adequate to remove or render harmless the agglutinating constituents. Simultaneously the tar acid content of the coal distillate may be substantially increased. If desired, oxidation catalysts such as sulfides of the iron group, -pyrites,'etc. maybe added to the material to be oxidized in small amounts of about 0.1 to 5%, preferably less than 2% in order to reduce the residence time particularly when high oxidation levels, that is high levels of oxygen absorption, are desired.

The present invention may be combined to greatest advantage with any of the systems described in my copending application, Serial No. 487,187, filed May 15. 1943, and its'continuation application, Serial No. 609,662, filed August 8,

1945, relating to the carbonization and/or gasiflcation of carbonaceous solids in the form of a dense turbulent mass of solids.

The agglutinating and/or plasticizing tendencies of most of the coals suitable for carbonization and/or gasification seriously interfere with I proper fluldization of the charge at the treating temperatures which lie considerably above the plasticizing temperature of the coal. This difficulty may be alleviated by maintaining a relatively large mass of absorbent dry material at high temperatures in the reaction zone and feeda ing thereto the plasticizing or agglutinating charge at a relatively low rate, which results in an ineflicient utilization of available reaction space. In accordance with my present invention, the coal is rendered non-agglutinating by my oxidizing pretreatment and may be fed to the processing zone proper at any desired rate depending merely on the desired degree of processing in the processing zone.

Other possible uses of the invention include the pretreatment of coals for hydrogenation. It is known that lignites are more easily hydroincreases the potential extractable material of high solvency of the coal, which may later be used for high pressure coal extractions. Intermediate to high oxidation levels of say 5-25% and temperatures of about 300-600 F. are suitable for this purpose.

Coal oxidized in accordance with the invention to the higheroxidation levels is also of interest for destructive hydrogenation. The oils produced in this manner are of relatively low aromaticity since oxidized compounds revert to naphthene and sometimes even paraffin during the hydrogenation process. It is, therefore, possible to produce by this procedure from bituminous coal hydrocarbon fractions suitable for use as Diesel fuels, burning oils, jet propulsion fuel and under special circumstances lubricating oils, which are not normally obtained when hydrogenating bituminous coals.

.Having set forth the general nature and objects, the invention will be best understood from the more detailed description hereinafter in which reference will be made to the accompanying drawing which illustrates semidiagrammatically a; system suitable for carrying out a. preferred modification of the present invention. Referring now in detail to the drawing, the solid carbonaceous charge such as a bituminous high volatility coal is supplied through line I to a conventional pulverizing mill 3 wherein it is ground to a particle size of say about and preferably 90% passing through a 50 mesh sieve and not more than about 10% smaller than about 200 mesh. Instead of a bituminous coal, other carbonaceous solids such as lignite, oil shale, tar sands, asphalt, cellulosic materials, etc. may be subjected to oxidation. The ground material may be passed to a screening device 5 from which coarse material may be returned through line I to pulverizer 3 in any conventional manner.

Coal of the proper particle size passes into a suitable feeding device such as a 'conventional standpipe 9 aerated through one or more taps static pressure of standpipe 9 into oxidation genated than bituminous coals. The preoxidachamber l5 wherein it is converted into a dense turbulent mass of solids fluidized by an upwardly flowing gas to form a well defined upper level L15, as will appear more clearly hereinafter.

Fluidizing gas containing the proportion of oxygen or other suitable oxidizing gas required for the desired reaction and preheated, particularly during the starting period, to a temperature sufficiently high to initiate and maintain the desired oxidation reaction is supplied through line I I from line i9 by means of blowers 2| and/or 23 and enters chamber l5 through its lower conical section 25 and a perforated distributing plate such as grid 21. The linear velocity of the fluidizing gas within chamber I5 is preferably maintained at about 0.3-1.5 ft. per second to establish the desired degree of fluidization at the particle size mentioned above. Bed densities of about 10 to 25 lbs. per cu. ft. have been found suitable for all practical purposes. The temperature of the turbulent solids mass is perfectly uniform and once oxidation has been initiated may be controlled exclusively by the oxygen content of the fluidizing gas.

Oxidized coal is removed downwardly from a point above grid 21 through a withdrawal well 30 leading into a standpipe 32 aerated and stripped by a; suitable gas admitted through one or more taps 34 and controlled by a slide valve 36. It will be understood that the rate of withdrawal of solids through standpipe 32 controls the average residence" time of the charge withinchamber I and thus the degree of oxidation.

- If desired, vertical back mixing of solids may be limited by the arrangement of one or more perforated overflow plates 38 permitting a countercurrent motion of solids and gases and the withdrawal of a solid product of highest, rather than average, degree of oxidation through well 30 from the lowest fluidized bed in chamber l5. Vertical back mixing may also be efiiciently avoided by using the packed column type of fluidized bed containing a packing of bodies of nonfluidizable size, such as Raschig rings or the like. Another method accomplishing the same purpose resides in arranging vertical baflles within the bed, which prevent eddying of the partly oxidized solid particles to a zone of lower average oxygen content. In this case, the solids withdrawal may take place by means of overflow above the baflles or as entrainment in the spent gases having a conventional upflow type of reaction.

The oxidized coal withdrawn through standpipe 32 may be passed, if desired, substantially at the temperature of chamber IE to any further processing stage such as an extraction zone or a fluid carbonization or gasification zone schematically indicated at 50.

In certain cases it may be desirable to provide for a relatively high oxygen concentration of the fluidizing gas combined with relatively low oxidation temperatures requiring withdrawal of exothermic' heat. For this purpose, a side stream of fluidized solids may be branched off well 30 and passed through an aerated standpipe 40, similar to standpipe 32, into line 42 wherein it is picked up by fluidizing gas supplied from line I! through line 44 to form a dilute suspension of solids in fluidizing gas. The suspension formed is passed under the pseudo-hydrostatic pressure of standpipe 40 through a cooler 45 and returned through line 41 to chamber IE to reduce the temperature of the fluidized solids mass therein by the desired degree. Simultaneously the overall level of oxidation of the solids withdrawn through well 30 may be raised in this manner.

Residual oxidizing and fluidizing gas carrying small amounts of suspended solids fines is taken overhead from level L15 and passed through line 52 to a conventional dust separator 54 provided with solids withdrawal line 56. Spent oxidizing gas may be either vented through line 58 or, when dilute oxygen is desired in chamber 15, wholly or in part returned to chamber l5 through lines l9 and I! by means of blower 2|. Fresh oxidizing gas such as air and/or oxygen is supplied through line 22 by way of blower 23. The gas in line l9 retains some of the heat of oxidation from chamber 15 and may, in most cases, be readily maintained at the desired gas feed temperature without the addition of extraneous heat. If desired, a heater 60 may be provided to add extraneous heat. In cases in which heat must be withdrawn 7 from chamber 15, a suitable cooler may take the place of heater 60.

In certain cases it may be desirable to humidify or saturate the fluidizing and oxidizing gas with water vapor or steam. For this purpose, the gas may be bubbled through a layer of water con tained in a humidifier 62.

It will be understood that instead of aerated standpipes 9, 32 and/or 40 any other conventional means for conveying finely divided solids such as screw or other mechanical conveyors may be used.

My invention will be further illustrated by the following specific examples.

Example I In order to prepare coal for extraction a fiuid ized bed of coal having a particle size of 90% passing through a 50 mesh sieve and about 2% smaller than 200 mesh is dried with natural gas and slowly heated to 400 F. by blowing preheated flue gas through the bed. Thereupon air is admitted to the bed and the treatment is continued at the conditions specified below.

Rate of coal charge, 2,000 lbs/hr. Temperature of coal charged, 80 F.

Rateof air charge, 0.45 c. f. m./lb. of coal charged/hr.

Temperature of air charged, 150 F.

Pressure, 2 atm. gauge Linear gas velocity, 1.5 ft./sec.

Bed density, 15 lbs/cu. ft.

Oxidation rate, '75 cc. of O2 absorbed/gram of coal/hr.

Coal residence time, 1.4 hrs.

Coal recycle, 3.? lbs. of 300 F./lb. of coal charged/hr.

Reaction temperature, 450 F.

Oxidation level, 15% 02 Example II For the preparation of coal for carbonization and/or gasiflcation processes the coal is dried and preheated as outlined in Example I whereupon air is admitted and the process is carried out at the conditions given below.

While the foregoing description and exemplary operations have served to illustrate specific applications and results of the invention, other modifications obvious to those skilled in the art are within the scope of the invention. Only such limitations should be imposed on the invention as are indicated in the appended claims.

I claim:

1. In the mild oxidation of carbonaceous solids with an oxidizing gas at temperatures lower than improvement which comprises maintaining a series of dense turbulent beds of said solids fluidized the temperature at which the combustible constituents of said solids burn to a substantial degree to their ultimate combustion products, the

by a gas, said beds being positioned at successively lower levels, contacting the solids in each bed with the oxidizing gas at said lower temperature, introducing fresh solids into the uppermost bed, removing oxidized solids from thefirst bed at a point substantially above the bottom of the bed and flowing them downwardly into each lower bed in succession, removing oxidized solids from ,7 the lowermost bed cooling a portion of said withdrawn solids below said lower temperature and returning solids so cooled to one of said beds positioned above said lowermost bed.

2. The method of claim 1 wherein said process comprises the recovery of oxygenated compounds from coal by extraction, said oxidizing temperature is about 150-450 F. and said contact time is about 30 minutes to several hours.

3. The method of claim 1 wherein said process comprises the conversion of carbonaceous solids into volatile fuels, said oxidizing temperature is about 400-650 F., said contact time about 1-30 minutes and said oxidized carbonaceous solids are supplied to said process substantially at the temperature of said oxidizingzone.

4. The method of claim 1 wherein said process comprises the hydrogenation of carbonaceous solids, said oxidation temperature is about 300- 600 F. and said contact time is adapted to cause the absorption of about 5-25% by weight of oxygen by said carbonaceous solids.

BRUNO E. ROETHELI.

Name Date I Wisner Apr. 29, 1930 Number 8 Number Name Date 1,775,323 Runge Sept. 9, 1930 1,796,100 Heller Mar. 10, 1931 1,932,296 Trent Oct. 24, 1933 1,983,943 Odell Dec. 11, 1934 1,984,380 Odell Dec. 18, 1934 2,111,579 Winkler Mar. 22, 1938 2,270,903 Rudbach Jan. 27, 1942 2,331,433 Simpson et a1 Oct. 12, 1943 2,337,684 Scheineman Dec. 28, 1943 2,363,274 Wolk Nov. 21, 1944 2,414,586 Eglofi' Jan. 21, 1947 2,432,135 Barr Dec. 9, 1947 2,436,225 Ogorzaly et a1 Feb. 17, 1948 2,443,673 Atwell June 22, 1940 2,444,990 Hemminger July 13, 1948 FOREIGN PATENTS Number Country Date 253,498 Great Britain Oct. 21, 1926 301,975 Great Britain Dec. 13, 1928 335,740 Great Britain Oct. 2, 1930 578,711 Great Britain Jul 9, 1946 OTHER REFERENCES Lowry; "Chemistry of Coal Utilization; chapt. 12, pages 425-449, vol. I, John Wiley and Sons, 1945.

Claims (1)

1. IN THE MILD OXIDATION OF CARBONACEOUS SOLIDS WITH AN OXIDIZING GAS AT TEMPERATURES LOWER THAN THE TEMPERATURES AT WHICH THE COMBUSTIBLE CONSTITUENTS OF SAID SOLIDS BURN TO A SUBSTANTIAL DEGREE TO THEIR ULTIMATE COMBUSTION PRODUCTS, THE IMPROVEMENT WHICH COMPRISES MAINTAINING A SERIES OF DENSE TURBULENT BEDS OF SAID SOLIDS FLUIDIZED BY A GAS, SAID BEDS BEING POSITIONED AT SUCCESSIVELY LOWER LEVELS, CONTACTING THE SOLIDS IN EACH BED WITH THE OXIDIZING GAS AT SAID LOWER TEMPERATURE, INTRODUCING FRESH SOLIDS INTO THE UPPERMOST BED, REMOVING OXIDIZED SOLIDS FROM THE FIRST BED AT A POINT SUBSTANTIALLY ABOVE THE BOTTOM OF THE BED AND FLOWING THEM DOWNWARDLY INTO EACH LOWER BED IN SUCCESSION, REMOVING OXIDIZED SOLIDS FROM THE LOWERMOST BED COOLING A PORTION OF SAID WITHDRAWN SOLIDS BELOW SAID LOWER TEMPERATURE AND RETURNING SOLIDS SO COOLED TO ONE OF SAID BEDS POSITIONED ABOVE SAID LOWERMOST BED.

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