GB2093059A - Coke production - Google Patents

Abstract

In the delayed coking of a coking feedstock wherein the feedstock is continuously heated in a coking heater 35 and introduced into a coke drum 36, the heater is operated to provide a coke drum temperature of from 415 DEG to 455 DEG C, and, after filling of the drum to the desired level, feedstock introduction is discontinued and the coke-drum placed off-steam (36') and the coke drum contents are heated at a temperature of 450 DEG C to 500 DEG C, which temperature is at least 10 DEG C greater than the coking temperature, by passing a non-coking vapour 101 through the contents of the drum to reduce the volatile combustible matter content to a value of 4% to 10% by weight and thereby reduce the CTE of the formed coke. The process is applicable to the production of needle coke in particular from a petroleum feedstock wherein the feedstock is initially treated by thermal soaking in the presence of sulphur, followed by thermal cracking to increase the aromaticity. Noncrystalline substances may be removed prior to coking to further improve the process. <IMAGE>

Description

SPECIFICATION Coke production This invention relates to a process for producing coke, and more particularly tb a process for producing needle coke.

It is known in the art to produce coke from a wide variety of feedstocks by a delayed coking procedure. Such procedures have been directed to the production of both anode grade coke, and to a premium quality coke, often referred to as needle coke.

U.S. Patent No. 4,108,798 describes an improved process for producing a needle coke (a highly crystalline petroleum coke) wherein a petroleum feedstock is initially heat soaked in the presence of sulphur, followed by controlled thermal cracking and separation of non-crystalline substances from the cracked feedstock. The feedstock thus treated is then subjected to delayed coking to produce highly crystalline petroleum coke.

The present invention relates to the production of coke by a delayed coking procedure.

In accordance with the present invention, there is provided a process for the coking of a coking feedstock wherein a coking feedstock is heated in a coking heater and introduced into a coke drum, and, after filling of the coke drum to the desired level, the coke drum is taken off-stream by discontinuing introduction of coking feedstock, the feedstock being coked in the coke drum at a temperature of from 41 50C to 4550C prior to taking the coke drum off-stream, and wherein, subsequent to taking the coke drum off-stream, the contents of the off-stream coke drum are heated at a temperature which is at least 1000 greater than the prior coking temperature and which is from 4500C to 5000C, said heating being effected for a time sufficient to provide a coke having a volatile combustible matter content of at least 4% by weight and no greater than 10% by weight.

The heating of the contents of the drum at a temperature which is higher than the temperature employed during the coking step or stage, serves to remove volatile combustible matter from said contents and thereby produces a coke having a reduced coefficient of thermal expansion (CTE).

Although the present invention has applicability to a wide variety of delayed coking procedures for producing coke, the present invention has particular applicability to the production of a needle coke and most particularly a high quality crystalline coke from a feedstock which has been pretreated by thermal soaking, and/or cracking and/or removing of non-crystalline substances. Treatment of a feedstock by thermal soaking and cracking and removal of non-crystalline substances is described in U.S. Patent No.

4,108,798.

In a preferred embodiment of the present invention, delayed coking is accomplished by a process which is similar to the general procedure known in the art, in which known procedure coking feedstock is continuously heated in a coking heater and introduced into a coke drum until the contents reaches a desired fill level, after which the coke drum is taken off-stream for removal of coke.However, in the preferred embodiment of the present invention, the coke drum is operated at lower temperatures as compared with the known procedure, of from 41 5"C to 4550C, preferably of from 4200C to 4500C, and after the coke drum is taken off-stream, i.e. after coking feedstock is no longer introduced into the coke drum, the contents of the off-stream coke drum are heated at a temperature which is at least 1000 greater than the coking temperature and is at least 45000, for a time to produce a coke having a volatile combustible matter content of at least 4% by weight and no greater than 10% by weight.The temperature at which the contents of the off-stream coke drum are heated is preferably at least 1 50C greater and most preferably at least 200C greater than the coking temperature. The last noted temperature is preferably at least 4600C, but preferably no greater than 48000. The heating of the contents of the off-stream coke drum is preferably effected by passing a heated non-coking vapour through the contents of the drum. The coke produced preferably has a volatile combustible matter content which is from 5% to 8% by weight. The time required for producing the coke with such volatile combustible matter content after taking the drum "off-stream" will vary with the coke produced, the feedstock employed for producing such coke and the heating temperature.However, in most cases such reduction can be accomplished by "off-stream" heating for a time period in the order of from four hours to twenty four hours.

The off-stream heating of the contents of the coke drum is, as noted above, preferably accomplished by the use of a non-coking vapour, Any one of a wide variety of non-coking materials may be employed for such heating and as representative examples of suitable materials, there may be mentioned light coker distillate, coker gases (C,C4 hydrocarbons), steam, nitrogen and other noncoking gases, (except gases which are oxidizing gases).The selection of a suitable gas for accomplishing the heating is deemed to be within the scope of those skilled in the art from the teachings herein.It is to be understood that in some cases it may be possible to accomplish such heating by means other than passing a heated gas through the coke drum contents; however, for most commercial operations, the preferred and most practical manner of heating the coke drum contents is by use of superheated vapours.

It is to be understood that in some cases the heating of the coke drum contents may be accomplished with a material which is capable of forming coke, such as a heated coker recycle or coker recycle plus relatively low concentrations of coker feedstock; however, in most cases the quality of the coke produced by the use of such materials is lower than the quality produced by the use of a noncoking material.

In the preferred process embodying the present invention, the delayed coking is conducted at a temperature lower than normally used in the art until the coke drum is filled to the desired level, and after taking the drum off-stream the contents thereof are heated at a higher temperature to effect a reduction in the volatile combustible matter content thereof. The combination produces a final product which when calcined has a reduced CTE. The specific temperatures employed in each of the coking steps and subsequent off-stream heating will vary with the particular feedstock as well as the desired CTE for the final product.In general, it has been found that the use of lower temperatures within the hereinabove described general range of coking temperatures produces a final product with a lower CTE; however, with some feeds higher temperatures within the hereinabove described general range of working temperatures are required to achieve a suitable rate of reaction.

As hereinabove noted, the present invention is particularly applicable to the production of a highly crystalline coke (needle coke), and most particularly to the production of a highly crystalline petroleum coke from a petroleum feedstock which has been pretreated in accordance with the teachings of U.S.

Patent No. 4,108,798 or U.S. Patent No. 4,199,434.

In the preferred embodiment of the present invention, the feedstock is initially heat-soaked in the presence of sulphur, followed by heating the heat-soaked feedstock to a higher temperature to effect controlled thermal cracking thereof, which enhances the aromaticity of the feedstock. The material from the thermal cracking is then treated to separate non-crystalline substances therefrom, followed by heating of the coking feedstock, free of non-crystalline substances, in a coking heater to provide coke drum temperatures in the order of from 41 SOC to 455 or, and after filling of the coke drum, which is taken off stream, the contents thereof are heated at a temperature which is at least 1 OOC greater than the coking temperature and which temperature is from 4500C to 500 C, for a time sufficient to reduce the volatile combustible matter content thereof to a value within the range specified above.

The feedstocks which are generally suitable for the production of coke by processes embodying the invention are heavy petroleum feedstocks, such as a distillation residue derived from a crude oil, lube oil extract and hydrodesulphurized lube extract, a cracking residue or a hydrodesulphurized product of a residue from the distillation or cracking of petroleum. Preferred feedstocks are the so-called pyrolysis fuel oils or black oils which are the residual heavy black oils boiling above pyrolysis gasoline; i.e. boiling above 1 870 to 21800 which are produced together with olefins in the pyrolysis of liquid hydrocarbon feeds, catalytic cracker decant oils, thermally cracked tars, lube oil extract and its hydrodesulphurized product coal tar or pitch distillates and the like.In general, such feedstocks have low sulphur content; i.e. sulphur content of 1.5 wt.% or less, preferably of 0.8 wt.% or less. Blends of such feeds may be employed.

The feedstock is preferably initially heat-soaked in the presence of at least 30 parts per million of sulphur, with such sulphur preferably being provided by adding sulphur (generally in the form of at least one member selected from the group consisting of elemental sulphur, mercaptan and carbon disulphide). In most cases, the added sulphur does not exceed 200 ppm. The soaking preferably effected for at least 5 minutes, and most preferably from 5 to 1 20 minutes. The appropriate soaking temperature is generally in the order of from 2300C to 31500. It is to be understood that if the requisite sulphur is present in the feed, sulphur need not be added thereto.In some cases, it may be possible to achieve the desired results by soaking at a temperature of from 2300 to 31 50C without the use of sulphur. It is believed that the soaking step improves the overall operation by polymerizing polymerizable components.

The soaked feedstock, in the preferred embodiment, is then heat treated to effect controlled thermal cracking thereof and to thereby increase the aromaticity (reduced API gravity). The heat treatment to effect cracking, which follows the initial thermal soaking, is performed, in the preferred embodiment, by heating the feedstock, generally in a tubular heater, under a pressure in the order of from 4 to 50 kg/cm2G to an outlet temperature in the order of from 4500 to 5950C. The cracking is effected for a period of time to increase the aromaticity, with such cracking generally being in the order of from 1 5 to 120 seconds. In general, the API gravity is decreased by at least 1 O (based on the material boiling above 2600C).

Subsequent to the heat treating, the feedstock may be processed to remove non-crystalline substances, and non-distillable heavy components, with such separation generally being easily accomplished by the use of high temperature flashing, with such flashing generally being at a temperature of 3800 to 51 OOC under a pressure of from 0.1 kg/cm2(A) to 2 kg/cm2G. In the flashing, the non-crystalline substances can be selectively removed as a pitch bottoms. The material recovered as the coking feedstock generally boils within the range of from 2600C to 5380C. Lighter components from the feedstock, such as gas, gasoline and gas oil may be separately recovered.

The optimum coking temperature varies with each feedstock. The coking pressure is generally in the order of from 2 to 10 kg/cm2G.

It is to be understood that one or more of the steps referred to above as being used in the preferred embodiment for pretreating the feed may be eliminated, although in most cases the combination of the three pre-treating steps mentioned, in combination with the controlled coking heater outlet temperatures and subsequent off-stream heating to reduce the volatile combustible matter content produces the highest quality coke. Thus, for example, the initial soaking may be eliminated, and/or the cracking of the feedstock may be eliminated and/or the separation of non-crystalline components may be eliminated without departing from the invention as defined by the claims.

U.S. Patent 4,199,434 for example, discloses pre-treating a coking feedstock by the combination of soaking at a first temperature in the presence of sulphur, followed by heating at a higher temperature to reduce API gravity. U.S. Patent 3,687,840 discloses pretreating a coking feed in the presence of sulphur.

An embodiment of the invention is described below, by way of example, with respect to the accompanying drawing which is a simplified flow diagram for a process embodying the present invention.

Referring now to the drawing, a feed, in line 10, is introduced into a soaking drum, schematically generally indicated as 12, with sulphur, if required, being introduced into the drum 12 through line 1 3.

in drum 12, the feed is thermally soaked, as hereinabove described, with such soaking effecting polymerization of highly unsaturated compounds.

The soaked feedstock is withdrawn from drum 12 through line 14 and introduced into a coil 1 5 in a thermal cracking heater 1 6 wherein the feed is subjected to thermal cracking conditions, as hereiriabove described, in order to increase the overall aromaticity thereof (reduce API gravity). The cracked feedstock is withdrawn from coil 1 5 in line 17, quenched with a light gas oil, obtained as hereinabove described, in line 18, and the combined stream passed through a pressure reduction valve 19 into a vacuum flash tower, schematically generally indicated as 21. The vacuum flash tower is operated at temperatures and pressures to separate from the feed non-crystalline substances, and other heavy components.In general, the flash tower is operated at a temperature in the order of from 3800 to 51 OOC, and at a pressure in the order of from 0.1 kg/cm2A to 2 kg/cm2G.

A heavy pitch-like bottoms is recovered from tower 21 through line 22. A light gas oil is recovered from tower 21 through line 23, with a portion thereof being employed in line 18 as a quench oil. Naptha and lighter gases are recovered from the flash tower through line 24.

The pretreated coking feedstock, which is recovered through line 25 is generally those components which are within a boiling temperature range in the order of from 2680 to 5380C and such components are introduced into a coker combination fractionator tower, schematically generally indicated as 27.

The coker combination fractionator tower 27 is operated, as known in the art, to recover the coking feedstock bottoms, and to also recover lighter components, which are generally not employed in the coking feedstock, such as a heavy coker gas oil in line 28, a light coker gas oil in line 29 and coker naphtha and gases in line 31. The coker combination fractionating tower 27, as known in the art, is also provided with coke drum overhead vapours through line 32.

Bottoms withdrawn from the tower 27 through line 34 is passed through a coking heater, of a type known in the art, and schematically generally indicated as 35, and the heated material is introduced into a coke drum, schematically generally indicated as 36. The coke drum is operated at the temperatures and pressures hereinabove described. Overhead vapours are withdrawn from coke drum 36 through line 38, and after quenching by a portion of the light gas oil in line 39, such overhead vapours are introduced into the coker fractionator 27 through line 32.

After the coke drum is filled, as known in the art, the coke drum is taken "off-line" i.e. the drum is no longer provided with coking feedstock. The off-line drum is indicated as 36' in the drawing.

The off-line drum 36' is heated to a higher temperature to reduce the volatile combustible matter content thereof and reduce the CTE. As shown in the drawing, superheated gas such as light coker distillate, naphtha, coker gas, etc. is introduced into the coking drum 36' through line 101, with such gas generally being at a temperature and pressure sufficient to maintain the off-line drum 36' at the temperatures hereinabove described for reducing the volatile combustible matter content. In general, the vapour is introduced at a temperature in the order of from 450 to 5250C and at a pressure in the order of from 2 to 10 Kg/cm2G.The vapour introduced through line 101, as well as volatile matter driven off from the drum contents is withdrawn from off-line coke drum 36' through line 102, and introduced into a quench tower, schematically generally indicated as 103, designed and operated to recover the non-coking vapour to be employed in off-line drum 36'. In quench tower 103, lighter components are recovered through line 104, as a gas; the material to be used as the drying gas is recovered through line 105, as a liquid, the heavier components are recovered through line 106. A portion of the material in line 105 is passed through line 107, including a cooler 108 for introduction into tower 103 through line 109, as reflux. The remaining portion in line 111 is heated in heater 112 to effect vapourization thereof for use as a drying gas.The heated material from heater 112 is introduced into a separator 11 5 to separate unvapourized material which is withdrawn through line 11 6.

Superheated vapour is withdrawn from separator 11 5 through line 101 for introduction into the "offline" drum 36' to provide a coke having a volatile combustible matter content, within the range 4 to 10% by weight.

In one modification, the vapour for the drying step could be recovered from the coker combination tower and the material withdrawn from the off-line drum is returned to the coker combination tower.

Thus, in such an operation, the coker combination tower is employed for both the "on-line" and "offline" coke drums.

Similarly, although the preferred embodiment has been described with reference to pretreating the feed (1) a low temperature soak to polymerize unsaturates; (2) thermal cracking to increase aromatic content (reduce API gravity): and (3) separation of pitch, the invention is also applicable to coke production without said pretreatment and to coke production which employs one or more of such pretreatme.nt steps.

The effectiveness of a process embodying the invention is illustrated further below by reference to the following examples: EXAMPLE I Decant oils having the properties summarized in Table 1 were added with 50 parts per million of sulphur and heat soaked at a temperature of 2600C. The feedstock so treated was introduced into SUS tube of 6 mm inner diameter and thermally cracked at a temperature of 5000C under a pressure of 20 kg/cm2G. (The residence time was 78 seconds on the cold oil basis.) The feedstock was then introduced into a flash tower maintained at 4800C under normal pressure and non-volatile substances were removed from the bottom of the flash tower as pitch. The oil obtained by cooling the overhead effluent was used as the coking feedstock.

TABLE 1 Specific gravity, 1 50/40C 1.0187 API gravity 7.4 Asphaltenes (C7 insolubles) 1.6 wt % Conradson carbon 5.71 wt % Sulphur content 0.75 wt % Ash 0.01 wt % Delayed coking was carried out under the conditions shown in Table 2, using the oil obtained under the above-mentioned conditions. The coke drum which was about 30 cm in inner diameter and about 50 cm in height was placed in a molten salt bath and was so designed as to permit external heating.

After the drum was filled, the drum was taken off-stream and heated, as tabulated.

TABLE 2 Run No. A B C D E Temp (OC) 425 435 440 430 460 Press. 5 5 5 5 5 (Kg/cm2G) On- Time (hr) 24 24 24 24 24 Stream Delayed Coking Recycle 1.0 1.0 1.0 1.0 1.0 Ratio Temp (OC) 460 460 480 430 480 Time(hr) 7 7 4 7 4 Off Stream Heating Heated Coker Coker Coker No No Vapour Light Light Light Gas Gas Gas Oil Oil Oil *External *External *External *External *External Green Coke 8.8 8.2 5.3 26.3 4.2 VCM (%) *External: The heating temperature was maintained by use of a molten salt bath without having recourse to heated vapour.

Runs A through C shown in Table 2 were performed by the process according to the present invention, while runs D through E were performed under different conditions: the off-stream temperature in run D was lower; and the on-stream coking temperature in run E was higher than those used in the process of this invention. Green coke obtained under these conditions was calcined at 1 ,4000C by the ordinary method, and calcined coke was pulverized. Each sample of calcined coke was blended with coal tar pitch as a binder and the mix was extruded into rods to make electrodes. The electrodes were baked at 1 ,0000C and graphitized at 3,0000C. The coefficient of thermal expansion (CTE) in the direction parallel to the extrusion was measured. The measurements obtained are shown in Table 3.

TABLE 3 A B C D E CTE (x 1 0-6/0C) in the direction parallel 0.79 0.74 0.89 1.31 1.21 to the extrusion (100 to 4000C The green coke which was obtained in run D contained a lot of pitchy substance in the upper portion. It melted and foamed during calcining and had a very poor appearance. The green coke obtained in run E was a spongy one having a lot of foam.

As is clear from Table 3, the cokes obtained by the process embodying the invention had very high quality.

EXAMPLE II Decant oils having the properties shown in Table 4 were pretreated under the conditions summarised in Table 5 to obtain a coking feedstock.

TABLE 4 Specific gravity, 1 50/40C 1.0192 Asphaltenes (C7 insolubles) 3.7 wt % Conradson carbon 6.4 wt % Sulphur content 0.64 wt % Ash 0.01 wt% TABLE 5 Soaking Amount of sulphur added 50 ppm Temperature 2700C Residence time 1 5 min Cracking Tube inner diameter 6 mm Outlet temperature 4900C Pressure 22 kg/cm2G Residence time 78 sec Flashing Temperature 480"C Pressure Atm.

The material balance in the pretreatment was shown in Table 6.

TABLE 6 Pitch 11.1wt% Coker feedstock (2900C+) 84.3 wt % Distillate (2900 C-) 2.4 wt % Cracked gas and loss 2.2 wt % Coking was performed on the feedstock so obtained under the conditions summarized in Table 7, and the VCM content of green coke so derived is also shown in the same table.

TABLE 7 Run No. F G H On-Stream Temp. (OC) 435 440 447 430 Delayed Coking Press. (kg/cm2G) 5 5 5 5 Time (hr) 24 24 24 24 Recycle ratio 0.6 0.6 0.6 0.6 01 in coke drum 3 3 4 4 Off-Stream Temp. (OC) 460 460 447 430 Heating Time (hr) 6 6 6 6 Heated vapour STM STM STM STM Green coke 6.2 5.8 11.7 23.6 VCM (wt %) Runs F and G were performed by the process of this invention, while run H was performed at a higher on-stream coking temperature than the invention, and run I at a lower off-stream heating temperature, than that of the invention. Electrodes were made from coke obtained under the conditions summarized in Table 7 and graphitzed at 3,0000C. The CTE and MR of graphitized electrodes are shown in Table 8.

TABLE 8 Run No. F G H CTE (x10-6/0 C) 0.86 1.00 1.17 1.33 in the direction parallel to the extrusion (1 00-4000C) MR(%) 21.6 17.5 12.9 10.2 As is clear from Table 8, coke obtained by the process embodying this invention had a very high equality.

EXAMPLE Ill Pyrolysis tar obtained as a by-product in the thermal cracking of gas oil was pretreated under the conditions summarized in Table 9 and coke made from the coking feedstock so refined.

TABLE 9 Soaking Amount of sulphur added 100 ppm Temperature 2600C Residence time 20 min Cracking Tube inner diameter 6 mm Outlet temperature 4700C Pressure 25 kg/cm2G Residence time (based on cold oil) 62 sec Flashing Temperature 4600C Pressure Atm.

Coking was performed on this coking feedstock under the conditions summarized in Table 10.

TABLE 10 Run No. J K L M On-Stream Temp. (OC) 435 460 440 445 Delayed Coking Pressure 6.5 6.5 6.5 6.5 (kg/cm2G) Time (hr) 24 24 24 24 Recycle ratio 1.0 1.0 1.0 1.0 Off-Stream Temp. (OC) 460 460 460 460 Heating Time(hr) 8 8 8 8 Heated vapour Coker None Light Light light (external) Coker Coker Distillate Distillate Distillate Green Coke VCM (wt%) 6.3 5.7 Run J was performed by a process embodying this invention, whereas run K was performed at a higher on-stream coking temperature than in run J. Electrodes were made from coke in the same way as in Example I, and the CTE of the electrodes graphitized at 3,0000C was measured. The results of measurement are shown in Table 11.

TABLE 11 Run No. J K L M CTE (x 1 0-6/0C) in the direction parallel to the extrusion 0.73 1.38 0.87 0.99 (1 00-4000C) EXAMPLE IV Hydrodesulphurized decant oil having the properties shown in Table 12 was pretreated under the same conditions as shown in Table 5 of Example II to obtain a coking feedstock.

TABLE 12 Specific Gravity, 15/4/OC 1.0142 Asphaltenes (C7 insolubles) 0.2 wt.% Conradson Carbon 2.6 wt.% Sulphur Content 0.52 wt.% Ash 0.01 wt.% Coking was performed on this coking feedstock under the conditions summarized in Table 13, and the VCM content of green coke so derived is also shown in the same Table.

TABLE 13 Run No. N O P On-Stream Temp. (OC) 445 455 465 Delayed Coking Press. (Kg/cm2G) 6.5 6.5 6.5 Time (hr) 24 24 24 Recycle ratio 1.0 1.0 1.0 Off-Stream Temp. (OC) 465 455 465 Heating Time (hr) 6 2 6 Heated Vapour Coker Steam Coker Light Light Distillate Distillate Green Coke 7.8 12.5 5.2 VCM (PCT) Run N was performed by the process of this invention, while Run P was performed at a higher on stream coking temperature than specified in the invention. Run 0 was performed at an on-stream coking temperature of 4550C and then after the drum was filled, the contents of the drum were purged with non-heated steam for 2 hours, without the use of a temperature greater than the coking temperature. Electrodes were made from coke obtained under the conditions summarized in Table 13 and graphitized at 30000 C. The CTE values of graphitized electrodes are shown in Table 14.

TABLE 14 Run No. N O P CTE in the direction parallel 0.83 1.22 1.19 to the extrusion (100 to 4000C As is clear from Table 14, coke obtained by the process embodying this invention (Run N) had a very high quality.

The present invention is particularly advantageous in that by employing the combination of delayed coking at a lower temperature than normally used in the art, followed by off-stream heating of the coke drum contents, at a higher temperature to produce a coke with a specified VCM content, the coke thus produced (after calcining and graphitizing) has a lower CTE. If the coke is produced at the lower temperatures, followed by calcination and graphitization (no off-stream heating in the coke drum at controlled temperature to provide a VCM content as hereinbefore described, prior to calcination), the CTE of the graphitized coke is higher than that provided in accordance with the invention.

The invention, as hereinabove described, is particularly applicable to the production of needle coke (CTE 135 x 10-s/cC measured at 1000--4000C), and also super needle coke (CTE 1.1 x 10-6/OC, measured at 1000--4000C).

The present invention has particular applicability to the coking of a feedstock which has been pretreated by (1) thermal soaking at 230O31 50C, generally in the presence of sulphur (although in some cases sulphur is not required) to decrease the tendency to deposit coke and/or polymer in subsequent lines or equipment and/or (2) thermal cracking at 4500 to 5950C to increase aromaticity and/or (3) separation of non-crystalline substances. Although pretreatment is not required and/or pretreatment by use of only one or two of the pretreatment steps may be employed, in general, the best results (lowest CTE) are achieved by use of the three pretreatment steps in combination with the coking at controlled temperatures, followed by off-stream heating to reduce VCM content.

Claims (13)

1. A process for the coking of a coking feedstock wherein a coking feedstock is heated in a coking heater and introduced into a coke drum, and, after filling of the coke drum to the desired level, the coke drum is taken off-stream by discontinuing introduction of coking feedstock, the feedstock being coked in the coke drum at a temperature of from 41 50 C to 455 OC prior to taking the coke drum off-stream, and wherein, subsequent to taking the coke drum off-stream, the contents of the off-stream coke drum are heated at a temperature which is at least 1 00C greater than the prior coking temperature and which is from 4500C to 5000C, said heating being effected for a time sufficient to provide a coke having a volatile combustible matter content of at least 4% by weight and no greater than 10% by weight.

2. A process according to claim 1, wherein the temperature of said heating is at least 1 50C greater than the prior coking temperature.

3. A process according to claim 2, wherein the coking temperature is from 4200C to 4500C.

4. A process according to claim 3, wherein the temperature of said heating is at least 4600C and no greater than 48O0C.

5. A process according to claim 1, wherein the off-stream coke drum is heated by passing a heated non-coking vapour through the contents thereof,

6. A process according to any preceding claim wherein the coking feedstock is pretreated by heat soaking at a temperature of from 2300C to 31 50C to polymerize unsaturates in the feedstock.

7. A process according to claim 6, wherein said heat soaking is effected in the presence of at least 30 ppm of dissolved sulphur.

8. A process according to any preceding claim, wherein the feedstock, prior to coking, is pretreated by heating to effect thermal cracking thereof at a final temperature of from 4500C to 5950C.

9. A process according to claim 6 or claim 7, wherein subsequent to the heat soaking the feedstock is pretreated by heating to effect thermal cracking thereof at a final temperature of from 45O0C to 5950C.

10. A process according to claim 9, wherein, after said heating of the heat-soaked feedstock to effect thermal cracking thereof non-crystalline substances and heavy components are separated from the remainder to produce a pitch free feed which is then supplied to said coking heater and introduced into said coking drum for said coking step effected at from 41 50C to 455 OC.

11. A process according to claim 10, in which the feedstock is a pyrolysis fuel oil, a lube oil extract, or hydrodesulphurized lube oil extract, a catalytic cracker decant oil, a thermally cracked tar or a mixture thereof.

1 2. A process for the coking of a coking feedstock, substantially as hereinbefore described with reference to, and as illustrated in the accompanying drawing.

13. Any novel feature or combination of features described herein.