GB2093061A - Process for calcining and desulfurizing petroleum coke - Google Patents

Abstract

Low sulfur calcined coke having an adequate density value for industrial consumers is produced from high sulfur raw coke by treating the coke in three consecutive heating stages under controlled conditions, the first stage being carried out in an oxidizing atmosphere and the second stage being in the presence of added hydrogen.

Description

SPECIFICATION Process for calcining and desulfurizing petroleum coke The invention relates generally to a process for improving the properties of raw or "green" cokes obtained by known processes from materials of petroleum origin and particularly to a process for calcining and desulfurizing such cokes to provide a product having acceptable sulfur content with satisfactory density characteristics.

Industrial petroleum coke is manufactured by methods well known in the art, the major method being delayed coking. Unfortunately, many petroleum cokes produced by this and other known methods contain appreciable amounts of sulfur, and cannot be directly utilized in the fabrication of some carbon products due to this impurity. Aluminum producers, for example, the largest consumer in total quantitv of calcined petroleum coke, require low sulfur coke to satisfy environmental regulations. These producers currently specify that the sulfur content of these cokes must be at a level of no more than about 2.5 wt.% to be acceptable for use in the fabrication of anodes for aluminum reduction cells.

Raw petroleum coke for industrial purposes is conventionally calcined at temperatures in the range of about 1 500--13000C. by methods well known in the art to remove substantially all of the volatile matter content of the coke and to provide increased density and conductivity therefor. It is known that the customary methods utilized for petroleum coke calcination are, in and of themselves, not adequate to bring about desulfurization of the coke without deterioration of other important coke properties.

A physical property of calcined petroleum coke recently recognized by those skilled in the art as being useful in predicting the apparent density, strength, and consumption rates of baked carbon anodes made from that coke in aluminum (Hall) cells is vibrated bulk density (VBD). A method for determining this property generally comprises placing a 100.0 gram sample of the calcined coke particles sized between 300 and 850 microns (-20/+48 mesh Tyler Screen Scale) in a 250 cc graduated cylinder mounted in a jogger (shaker) unit and vibrating the cylinder for 5 minutes at a predetermined jogging rate at which maximum particle compaction occurs.The volume of the compacted coke particles is recorded and the VBD, expressed in g/l 00 cc, is calculated as follows: VBD=(Ai8)x 100 where: A = sample weight in grams B = compacted volume in cubic centimeters. The particle size of the coke sample used in the VBD determination is approximately midpoint in the conventional anode aggregate particle size distribution.

It has been found that a VBD value for calcined coke of at least about 78 g/l 00 cc is necessary to provide acceptable quality for use in anode production.

It is known in the art that the temperatures at which calcination of high sulfur raw petroleum coke is conventionally carried out are not sufficient to reduce the coke's sulfur level to a value acceptable to consumers.

One method known for desulfurizing raw coke comprises directly heating the coke in a single stage to a temperature above about 1 5000 C. in a rotary kiln or the like. Experience has taught that while this procedure effectively reduces the coke's sulfur content, the VBD and other physical properties are substantially deteriorated during the heat treatment process, as compared to coke properties after calcination at conventional temperatures.

U.S. Pat. No. 4,160,814 to Hardin et al. provides a two stage process for calcining and thermally desulfurizing raw petroleum coke without lowering its bulk density (BD), as defined below, comprising heating the coke at 4900C. to 8500 C. for 30 to 60 minutes while retaining at least 30 wt.% of the coke's volatile content, then heating the partially devolatilized coke at a temperature of at least 1 5000 C.

for 30 to 70 minutes to calcine and desulfurize the coke. The BD value referred to in the patent is the weight per unit volume of the coke particles, and is determined by transferring a weighed sample of the coke, having a particle size either in a range of 3.36 to 4.76 mm (-41+6 mesh Tyler Screen Scale) or Run of Kiln (ROK) size, into a graduated container and calculating the BD from the displaced volume and sample weight. While the process provided in this patent advanced the art of coke desulfurization over known processes by providing retention of normal bulk density values, it was learned that the coke product exhibited lowered VBD properties compared to conventionally calcined coke.The lowered VBD value indicates a decreased strength and increased consumption of anodes made from coke produced according to this patent, compared to coke calcined by conventional methods without desulfurization.

Generally, the present invention relates to a three-stage process for producing calcined petroleum coke having a sulfur content in the range of about 1.5 wt.% to about 2.5 wt.% and a VBD of at least about 78 g/l 00 cc from raw petroleum coke having a sulfur content greater than about 2.5 wt.% and a volatile content of at least 7 wt.% comprising: (a) heating the coke at a temperature in the range of about 250"C. to about 4500C. in an oxidizing atmosphere for no less than about 0.5 hour, preferably not over two hours; (b) heating the oxidized coke at a temperature in the range of about 6000C. to about 8000 C. in an atmosphere containing added hydrogen for a period of time sufficient to reduce the sulfur content of the coke to a level such that no more than about 1.5 wt.% of sulfur, preferably about 0.3 to about 1..0 wt.%, is required to be removed in the stage (c): and (c) heating the partially desulfurized coke at a temperature in the range of about 13500C. to about 16000C. in the absence of added hydrogen, preferably in an inert or reducing atmosphere, for a period of time sufficient to reduce the sulfur content of the coke to within the range of about 1.5 to about 2.5 wt.%.

Preferably, the treatment stages of the invention are controlled to provide a final coke yield of no less than about 80 wt.%, on a dried feed coke basis. When using air to supply oxygen during the first stage, it is preferable to prevent excessive burning by utilizing nitrogen or the like as a diluent to provide an oxidizing atmosphere having about 6 to about 14 wt.% oxygen.

It is critical in this process that the desulfurization of the coke is not allowed to proceed below about 1.5 wt.%, preferably about 1.8 wt.%, as further sulfur reduction results in an unacceptably low VBD valueforthe calcined coke product The total coke processing time necessary for carrying out the process of the invention is generally not over about 8 hours and usually does not require more than about 6 hours, the elapsed time depending on the sulfur content, volatile content and particle size of the raw coke feed material.For example, a process for producing a calcined petroleum coke having a sulfur content of about 1.8 to about 2.5 wt% and a VBD of at least about 78 g/100 cc from a raw coke having a sulfur content in the range of about 3.5 to about 5.0 wt.% and a volatile matter content in the range of about 9 to about 14 wt.% generally requires an oxidation treatment period in the range of about 1 hour to about 2 hours in stage (a) of the process of the invention; about 2 hours to about 6 hours in hydrodesulfurization stage (b) to produce a coke having a sulfur content in the range of about 2.8 to about 3.3 wt.%, 6 hours being the preferred treatment period for an exceptionally low sulfur product coke with an acceptable VBD value; and about 0.5 hour to about 1.5 hours, preferably about 1.0 hour, in thermal treatment stage (c) to produce a calcined coke product having a sulfur content in the range of about 1.8 to about 2.5 wt.%.

The time required for hydrodesulfurization stage (b) can be shortened significantly by utilizing a small particle size feed coke, e.g., below 150 microns (-100 mesh Tyler Screen Scale).

The optimum conditions for each stage of the invention vary according to the characteristics of the particularcoke being treated. The individual treatment phases can be carried out using any known heating apparatus, such as rotary kilns, rotary or multipte hearth furnaces or the like. Minor modification of the selected heating unit may be necessary to provide the appropriate atmosphere required for the oxidation and hydrodesulfu rization stages.

The preferred embodiment of the invention will now be described in non-limiting Example A.

Additional examples are provided to illustrate additional embodiments. In view of the sulfur and volatile contents of the coke used in the examples, the temperatures and heating periods for the coke calcinationldesulfurization process were selected to provide a coke sulfur content of 2.8 to 3.3 wt.% after the hydrodesulfurization treatment and a final coke product having a sulfur content of 1.8 to 2.5 wt.%.

EXAMPLE A The coke employed in this example was a "regular" raw petroleum coke, also known in the art as sponge coke, produced from reduced crude feedstock by the conventional delayed coking process. This raw coke had a sulfur content of 4.4 wt.% and a volatile content of 10.5 wit%. The particle size distribution of the coke is shown below.

Sieve Size, mm Cumulative Wt.% 6.73 19.6 3.36 39.5 0.84 72.0 0.30 86.8 0.21 89.9 0.15 92.3 7.7 Wt.% of the coke had a particle size below 0.15 mm.

A 460 gram sample of the raw coke was charged into a 5.7 cm diameter tube positioned in a tube furnace. Air diluted with nitrogen to produce a gas having about 6.7% oxygen was passed through the sample at a rate of about 2 liters/minute via a perforated closure in the tube and the furnace was rapidly heated to a temperature of 3500C. The sample was treated in this manner for about 1 hour. After purging the tube with nitrogen to eliminate the oxidizing gas from the sample, a hydrogen flow was passed through the sample at approximately the same rate as the oxidizing gas.The furnace temperature was rapidly increased to 6500C. and held for about 6 hours to reduce the coke's sulfur content to 3.0 wt.%. The tube was then removed from the furnace and the coke sample was transferred to a tray which was placed in a resistance heated graphite tube furnace having a nitrogen atmosphere and preheated to 1 4000C. The sample was heated at this temperature for about 1 hour. The calcined coke product had a sulfur content of 2.1 wt.% and a VBD value of 81 g/l 00 cc, and the coke yield was 84.8 wt.% (dried feed coke basis).

For comparison, samples of the same raw coke having the same particle size distribution were calcined by known methods. The sulfur and VBD values of each product, and those of the calcined coke produced according to the process of the invention, are presented below.

Treatment Sulfur VBD Process Temperature(s) OC. wt.% g/100 cc St'd Calcination 1300 (0.5 hr) 3.9 85 One Stage High 1400(1 hr) 1.9 70 Temperature Calcination Two Stage High 650/1400 1.9 73 Temperature (1 hr/1 hr) Calcination According To The 350/650/1400 2.1 81 Invention (1 hr/6 hr/1 hr) EXAMPLES BG The samples employed in the examples shown in the table below were taken from the same raw coke described above, with the same particle size distribution. The treatment apparatus and general procedure, including the flow rates of the oxidizing gas and hydrogen, were about the same as that used in Example A. The sample weights were in the range of 400 to 465 grams.

Stage A Stage B Stage C Oxidation Hydrodesulfurization Thermal Treatment Product Properties Sample Temp., Time, O2 Temp., S, Temp., Time, S, VBD No. C. Hrs. % C Hrs. % C Hrs. % g/100cc B 400 1 6.7 650 4 3.2 1400 1 2.2 80 C 350 1 6.7 650 6 3.2 1400 1 2.3 81 D 350 1 6.7 650 6 3.0 1400 1 2.1 81 E 350 2 13.3 650 3 3.2 1400 1 2.3 79 F 350 3 8.0 650 3 3.3 1400 1 2.1 79 G 300 1 6.7 650 6 3.1 1400 1 2.0 80 The data indicate that the process of the invention is an effective method whereby raw petroleum coke of the type defined can be treated to produce a calcined coke with both sulfur content and VBD values currently acceptable to industrial consumers.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope and spirit thereof, and, therefore, the invention is not intended to be limited except as indicated in the appended claims.

Claims (6)

1. A three-stage process for producing calcined petroleum coke having a sulfur content in the range of 1.5 to 2.5 wt.% and a vibrated bulk density of at least 78 g/l 00 cc from raw petroleum coke having a sulfur content greater than 2.5 wt.% and a volatile content of at least 7 wt.% which comprises: (a) heating the coke at a temperature in the range of 2500C. to 4500C. in an oxidizing atmosphere for a period no less than 0.5 hour; (b) heating the oxidized coke at a temperature in the range of 6000 C. to 8000C. in an atmosphere containing added hydrogen for a period of time sufficient to reduce the sulfur content of said coke to a level such that no more than 1.5 wt.% of sulfur is required to be removed in the final stage; and (c) heating the partially desuifurized coke at a temperature in the range of 1 3500C. to 1600 C. in the absence of added hydrogen for a period of time sufficient to reduce the sulfur content of the coke to within the range of 1.5 to 2.5 wt.%.

2. A process according to claim 1, wherein the oxidized coke is heated in the range of 6000 C. to 800"C. in an atmosphere containing added hydrogen for a period of time sufficient to reduce the sulfur content of said coke to a level such that 0.3 to 1.0 wt.% of sulfur is required to be removed in the final stage.

3. A three-stage process for producing calcined petroleum coke having a sulfur content in the range of 1.8 to 2.5 wt.% and a vibrated bulk density of at least 78 g/l 00 cc from raw petroleum coke having a sulfur content in the range of 3.5 to 5.0 wt.% and a volatile content in the range of 9 to 14 wt.% which comprises:: (a) heating the coke at a temperature in the range of 2500C. to 4500 C. in an oxidizing atmosphere for a period in the range of 1 hour to 2 hours; (b) heating the oxidized coke at a temperature in the range of 6000 C. to 8000C. for a period of time of 2 hours to 6 hours in an atmosphere containing added hydrogen such that the sulfur content of said coke is reduced to a level in the range of 2.8 to 3.3 wt.%; and (c) heating the partially desulfurized coke at a temperature in the range of 1 3500C. to 1 6000C. in the absence of added hydrogen for a period of time of 0.5 hour to 1.5 hours such that the sulfur content of the coke is reduced to a level of 1.8 to 2.5 wt.%.

4. A process according to any one of the preceding claims, wherein the oxidizing atmosphere consists of 6 to 14 wt.% oxygen.

5. A process according to any one of the preceding claims, wherein the final coke yield is no less than 80 wt.% on a dried feed coke basis.

6. A process for producing calcined petroleum coke substantially as hereinbefore described with reference to the Examples.