US2805199A - Electrodes from fluid coke - Google Patents

Description

Unite ELECTRODES FROM FLUlD COKE Fred W. Barres, Westfield, and James H. McAteer, Cranford, N. 3., assignors to Esso Research and Engmeermg Company, a corporation of Delaware No Drawing. Application Qctober 22, 1954,

Serial No. 464,158

2 Claims. (Cl. 204-294) nature which can be utilized for the obtaining of aluminum from its ores.

In the manufacture of aluminum by electrolytic reduction of alumina in a suitable fused bath, the necessary carbon electrodes have usually been manufactured from so-called petroleum coke, of relatively high purity.

This petroleum coke had been obtained largely from coking processes such as delayed coking which provide particles of relatively large diameter and densities. It was thus thought that a particle size distribution of less than 200 mesh to up to and high true or particle densities, i. e. 2 or higher, were required for satisfactory electrodes. The principal criteria of these finished electrodes are a minimum compression strength of 4400 pounds per square inch, a minimum real density of about 1.45 and a maximum resistivity of 3 l0- ohrn-inch. As stated it was believed that an aggregate containing large particles of calcined coke were necessary to obtain these characteristics.

There has recently been developed an improved process known as the fluid coking process for the production of fluid coke and the thermal conversion of heavy hydrocarbon oils to lighter fractions. The fluid coking unit consists basically of a reaction vessel or coker and a heater or burner vessel. In a typical operation the heavy oil to be processed is injected into the reaction vessel containing a dense turbulent fluidized bed of hot inert solid particles, preferably coke particles. Uniform temperature exists in the coking bed. Uniform mixing in the bed results in virtually isothermal conditions and effects instantaneous distribution of the feed stock. In the reaction zone the feed stock is partially vaporized-and partially cracked. Product vapors are removed from the coking vessel and sent to a fractionator for the recovery of gas and light distillates therefrom. Any heavy bottoms is usually returned to the coking vessel. The coke produced in the process remains in the bed coated on the solid particles. Stripping steam is injected into the stripper to remove oil from the coke particles prior to the passage of the coke to the burner.

The heat for carrying out the endothermic coking reaction is generated in the burner vessel. A stream of coke is transferred from the reactor to the burner vessel employing a standpipe and riser system; air being supplied, to the riser for conveying the solids to the burner. Suflicient coke or carbonaceous matter is burned in the burning vessel to bring the solids therein up to a temperature suflicient to maintain the system in heat balance. The burner solids are maintained at a higher temperature than the solids in the reactor. About 5 %l5 of coke, based on the feed, is burned for this purpose. The unburned portion of the coke represents the net coke formed in the process and is withdrawn.

ea-ivy hydrocarbon oil feeds suitable for the coking process are heavy or reduced crudes, vacuum bottoms,

ates Patent ice pitch, asphalt, other heavy hydrocarbon petroleum residua or mixtures thereof. Typically, such feeds can have an initial boiling point of about 700 F. or higher, an A; P. I. gravity of about 0 to and a Conradson carbon residue content of about 5 to wt. percent. (As to Conradson carbon residue see ASTM Test D-1O-52.) It is preferred to operate with solids having a particle size ranging between 100 and 1000 microns in diameter with a preferred average particle size range between 150 and 400 microns. Preferably not more than 5% has a particle size below about 75' microns, since small particles tend to agglomerate or are swept out of the system with the gases.

The method of fluid solids circulation described above is well known in the prior art. Solids handling technique is described broadly in Packie Patent 2,589,124, issued March 11, 1952.

The fluid coke product is laminar in structure and may comprise some 30 to 100 superposed layers of coke. The size distribution is such that a predominant portion, i. e., about 90 weight percent has a diameter smaller than 400 microns with a range of about75 to 850 microns. The real density of these coke particles after the required calcining is in the range of 1.83 to 1.93, preferably 1.87 to 1.92.

It had consequently been thought that this fluid coke because of the small size and low densities was not adapted for fabrication into the desired carbon electrodes. The laminar and largely spherical structure was also considered a possible disadvantage. It is impractical on the other hand to modify the fluid coking process to obtain larger particles as these would be inconsistent with proper fluidization of the coker bed.

It has now been found that this fluid coke can be utilized in the'preparation of the indicated carbon electrodes. This is accomplished by calcining the fluid coke to a real density in the range of 1.83 to 1.93, preferably 1.87 to 1.92 and a resistivity of 2030 10* ohm-inch. This resistivity is determined at 500 p. s. i. on a sample of coke, 1 sq.-inch cross-sectional area by 1 inch length, comprising material of 210 to 420 microns diameter. The coke is ground so that 20 to weight percent of the total fluid coke charge to the electrode manufacturing step has a diameter of less than microns. The calcining preferably precedes the grinding but can follow the latter. The fluid coke charge is thereafter processed into electrodes in the conventional manner. The finished electrodes have a minimum real density of about 1.45 and a resistivity of. below.3 10-' ohm-inch.

It is surprising to find that the fluid coke can be so utilized especially in view of the fact that it requires additional grinding of particles all of which are in a size range which ordinarily would represent only a very small fraction of the coke aggregate. As a matter of fact data establish that in the absence of this grinding, unsatisfactory electrodes are prepared.

The calcining of the fluid coke is performed in the conventional manner, i. e., a calcination at a temperature in the range of 2000- to 2800 F. or higher. This can be done in a fluid moving or fixed bed in the presence of an atmosphere such as air,- nitrogen, carbon dioxide, hydrogen, etc. The calcination is conducted until a real density in the range of 1.83-1.93 preferably 1.87- 1.92 is obtained. Thetime necessary is thus in the range of 0.5 to '10 hours. Longer calcining times may be used, especially in the lower temperature range, without deleterious effects.

The grinding of the coke particles can be accomplished in the conventional manner. It is to be understood that 0 the term grinding as used herein connotes generically any means of mechanically diminishing the particle sizes and includes high velocity attrition, ball and mill grinding etc.

In the manufacture of the electrode itself the coke is admixed with and charged together with a carbonaceous binder to the fabrication system. The binders utilized are conventional and include materials such as the aromatic coal tar pitch binders e. g. see U. S. Patent No. 2,683,107. Such binders generally have melting points lying within the range of 70-120 C. They contain small amounts of hydrogen (about or less). The concentration of benzene and nitrobenzne insoluble portions represent preferably about 20-35% and 5-15%, respectively, of the binder. The binder is utilized in an amount of about 18 to 45 parts by weight per 100 parts of fluid coke.

In general, two types of electrodes are employed by the industry (a) prebaked and (b) Soderberg self-baking electrode. In the former, a mixture comprising about 78- 82% of calcined coke aggregate and 18-22% of coal tar pitch is molded at pressures of about 3000-5000 p. s. i.

or extruded and then baked for periods up to 30 days at 1800-2400 F. These preformed electrodes are then used in electrolytic cells, being slowly lowered into the molten alumina as they are consumed, Butts of the unconsumed electrodes are reground and used in subsequent electrode preparations.

The Soderberg process involves the continuous or intermittent addition of a coke-coal tar pitch paste to the top of the cell as the electrode components in the lower part of the cell are consumed. In this operation the paste represents a blend of about 70-72% coke aggregate and 28-30% of pitch. The cells operate usually at temperatures of 1700l900 F. and electrodes are consumed at the rate of about 0.5 to 1.0 inch per day. The paste is baked into an electrode by the hot cell gases in the period between the time it is added at the top and time it is used. The net consumptionof coke represents 0.4 to 0.7 lb. per pound of aluminum metal produced. It can be seen that the actual manner of fabricating the el'ec trodes is not the essence of this invention. Both methods have in common the baking of the mixed fluid coke and binder at a temperature inthe range of l700 to 2400 F. 7

This invention and its advantages will be better illustrated by the following examples of electrodes prepared in the manner taught.

EXAMPLE 1 A sample of fluid coke was calcined in air at about 2000 F. until a real density of about 1.9 was obtained. The particle size distribution after'grinding was approximately 50% in the 75-850 micron. rangeand 50 wt. percent smaller than .75 microns. Seventy parts of this aggregate was mixed with about 30 parts lay-weight of coal tar pitch and .the paste baked in the mold for 4 days. The temperature was increased regularly up to 1800" F. over the first three days and was held at 1832 F. for the remaining time. The baked electrode had a compression strength of 4600 pounds per square inch and an electrical resistivity of 2.6 to 2.8Xl0- ohm-inch. In these and other characteristics the products more than meetthe specifications for a satisfactory product.

EXALIPLEZ' crons, and the third con-taining60 wt; percent of particles smaller than 75 microns. Thus, only the second sample was within the process taught by this invention. All three samples. were then molded at 300 p'. s. i'. ,g. in-a charge of f8 wt. percent coal tar pitch and 82 wt. percent coke aggregates, all other conditions being kept the same. The results are presented below:

TABLE Baked carbon electrode properties Wt. percent of 75 microns and smaller particles in coke aggregate 0 30 60 Density, g./om.= 1. 36 1. 48 1. 40 Resistivity, ohm-imXlO-L- 3. 59 2. 57 3. 02 Compression Strength, p. s. l 6, 200 6, 200 7, 600

It should be noted from this example how only the sample prepared with 30 wt. percent of the smaller particles, or within the 20 to 50% range taught by this invention, satisfactorily meets the requirements in terms of density and resistivity of the finished product.

In order to give more details on the preparation of fluid coke, the following conditions of operation of the fluid coker are set forth below.

i These electrodes find greatest utility in their use as anodes for the obtaining of aluminum from its ores by the electrolytic process. The principles involved can be utilized however in the preparation of other electrodes. It is to be understood that this invention is not limited to the specific examples which have been oiiered merely as illustrations and that modification may be made without departing from the spirit of the invention.

What is claimed is: r

1. A method of making a carbon electrode having a minimum real density of about 1.45, a maximum resistivity of 3 10- ohm-inch and a minimum compression strength of 4400 p. s. i. from a charge of fluid coke particles having a diameter distribution in the range of about -850 microns with about weight percent having a diameter smaller than 400 microns said fluid coke particles having been produced by contacting a heavy petroleum oil coking charge stock at a coking temperature with a body of fluidized coke particles in a reaction zone wherein the oil is converted to product vapors and carbonaceous solids are continuously deposited on the coke particles, removing product vapors from the coking zone, heating a portion of the coke particles from the coking zone in a heating zone to increase the temperature of said fluidized particles, returning a portion of the heated coke particles from the heating zone'to the coking zone and withdrdawing coke product particles, which comprises the steps of calcining the fluid coke particles to areal density of 1.87-1.92; grinding the calcined fluid coke particles so that 20-50 weight percent ofthe fluid coke charge has a diameter of less than 75 microns; admixing the ground fluid coke with a carbonaceous binder in the ratio of approximately 18-45 parts by wt. per parts of fluid coke; and baking the mixture at a temperature in the range of 1700-2400 F.

2. A method of making a carbon electrode having a minimum real density of about 1.45, a maximum resist'ivity of 3x10 ohm-inch and a minimum compression strength of 4400 p. s. i. from a charge of fluid coke particles having a diameter distribution in the range of about 75-850 microns with about 90 weight percent having a diameter smaller than 400 microns'said fluid coke particles having been produced by contacting a heavy petroleum oil coking charge stock at a coking temperature with a body of fluidized coke particles in a reaction;

zone wherein the oil is converted to product vapors and carbonaceous solids are continuously deposited on the coke particles, removing product vapors from the coking zone, heating a portion of the coke particles from the coking zone in a heating zone to increase the temperature of said fluidized particles, returning a portion of the heated coke particles from the heating zone to the coking zone and withdrawing coke product particles, which comprises the steps of grinding the fluid coke particles so that 20-50 weight percent of the fluid coke charge has a diameter of less than 75 microns; calcining the fluid coke particles to a real density of 1.87-

1.92; admixing the ground fluid coke with a carbonaceous binder in the ratio of approximately 18-45 parts by weight per 100 parts of fluid coke; and baking the mixture at a temperature in the range of 1700-2400 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,600,078 Schutte June 10, 1952 10 2,700,642 -Mattox Jan. 25, 1955 FOREIGN PATENTS 491,522 Canada Mar. 24, 1953

Claims (1)

1. A METHOD OF MAKING A CARBON ELECTRODE HAVING A MINIMUM REAL DENSITY OF ABOUT 1.45, A MAXIMUM RESISTIVITY OF 3X10-3 OHMI-INCH AND A MINIMUM COMPRESSION STRENGTH OF 4400 P. S. I. FROM A CHARGE OF FLUID COKE PARTICLES HAVING A DIAMETER DISTRIBUTING IN THE RANGE OF ABOUT 75-850 MICRONS WITH ABOUT 90 WEIGHT PRECENT HAVING A DIAMETER SMALLER THAN 400 MICRONS SAID FLUID COKE PARTICLES HAVING BEEN PRODUCED BY CONTACTING A HEAVY PETROLEUM AOIL COKING CHARGE STOCK AT A COKING TEMPERATURE WITH A BODY OF FLUIDED COKE PARTICLES IN A REACTION ZONE WHEREIN THE OIL IS CONVERTED TO PRODUCT VAPORS AND CARBONACEOUS SOLIDS ARE CONTINUOUSLY DEPOSITED ON THE COKE PARTICLES, REMOVING PRODUCT VAPORS FROM THE COKING ZONE, HEATING A PORTION OF THE COKE PARTICLES FROM THE COKING ZONE IN A HEATING ZONE TO INCREASE THE TEMPERATURE OF SAID FLUIDIZED PARTICLES, RETURNING A PORTION OF THE HEATED COKE PARTICLES FROM THE HEATING ZONE TO THE COKING ZONE AND WIRHDRDAWING COKE PRODUCT PARTICLES, WHICH COMPRISES THE STEPS OF CALCINING THE FLUID COKE PARTICLES TO A REAL DENSITY OF 1.87-1.92; GRINDING THE CALCINED FLUID COKE PARTICLES SO THAT 20-50 WEIGHT PERCENT OF THE FLUID COKE CHARGE HAS A DIAMETER OF LESS THAN 75 MICRONS; ADMIXING THE GROUND FLUID COKE WITH A CARBONACEOUS BINDER IN THE RATIO OF APPROXIMATELY 18-45 PARTS BY WT. PER 100 PARTS OF FLUID COKE; AND BAKUNG THE MUXTURE AT A TEMPERATURE IN THE RANGE OF 1700-2400*F.