GB1594310A - Coking process - Google Patents

Description

(54) COKING PROCESS (71) We, SIGRI ELEKTROGRAPHIT Gesellschaft mit beschränkter Haftung and BERGWERKSVERBAND GmbH., both German Companies, respectively of 8901 Meitingen bei Augsburg, Germany, and Brentanostrasse 2, D-6233 Kelkheim, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a continuous coking process for production of cokes suitable for the production of carbon and graphite shaped bodies in which process a hydrocarbon mixture containing aromatic compounds is pyrolysed.

A number of coking or pyrolysis processes adapted to the particular starting material used are available for the production of cokes, the starting materials including petroleum tars, coal tar pitches, pyrolysis tars and other hydrocarbon mixtures containing relatively large fractions of polynuclear aromatic compounds. Examples of these coking processes are the retort or chamber coking process used for the production of pitch cokes, fluidised bed coking processes and the delayed coking process which is used for the production of high quality cokes.In the last of these processes, selected hydrocarbon mixtures are heated to around 500"C in a tubular heater, premature coking of the starting material in the heater being prevented by subjecting it to turbulent movements and high rates of travel of the starting material or by adjusting certain temperature and pressure profiles. To convert the higher aromatic fractions into green coke, the preheated starting material is then introduced into coke drums which are operated under pressures of up to about 10 bars and at overhead temperatures of from 450 to 4800C. After a certain filling level has been reached in one coke drum, the supply of coke to that drum is interrupted and the preheated material is introduced into another coke drum during whose filling the green coke is removed from the first drum.The supply of green coke is then switched back to the first drum and so on. The residence time or coking time of the green coke in the coke drums thus fluctuates considerably, for example between 24 hours at the base of the drum and one hour at the head of the drum. The quality of the green coke varies considerably as a function of the residence time; for example the amount of volatile constitutents therein may amount to less than 5% at the bottom of the drum and to more than 20% at the head of the drum.

Since only some of these differences are eliminated by the subsequent heating or calcining of the green coke at a temperature of from about 1100 to 1400"C, most of the calcined cokes also show variations in important quality parameters, particularly in porosity, pore size distribution, particle strength and coefficient of thermal expansion. If cokes of this type are used for example for the production of carbon and graphite shaped bodies, the products obtained will generally not be of uniform quality and will not satisfy all the demands made of them on account of the variations in the quality of the starting coke.

Further disadvantages of the delayed coking process include the undesirably high cost of the twin coke drums and the poor energy utilisation factor of the process. In general, the green coke is removed hydraulically from the coke drums, size-reduced, graded and subsequently delivered to a calcining apparatus which takes the form, for example, of a revolving cylindrical furnace or disc furnace. The heat content of the coke is lost without being utilised.

The retort or chamber coking process is likewise characterised by poor utilisation of energy, in addition to which cokes produced by this process are not suitable for the manufacture of high quality graphite products for the same reasons as coke produced by the fluidised bed process. The inadequate properties of the cokes for this purpose are primarily attributable to the structure of the coke particles which contain only a small fraction of acicular elements.

According to the present invention, there is provided a process for the continuous production of coke, which comprises the steps of a) introducing a cokable hydrocarbon mixture into a preheater together with recycled condensible fractions of overheads, the preheater being operated at a head temperature of from 280 to 320"C and at a sump temperature of from 380 to 440"C, passing the mixture of hydrocarbon mixture and recycled condensible fractions of overheads through the preheater at such a rate that the mesophase content of the mixture issuing from the preheater amounts to from 30 to 60% by weight, b) recycling to the preheater at least part of the condensible fractions of the overhead product from the preheater, c) passing the mesophase-containing mixture issuing from the preheater through a coking zone comprising a plurality of heating tubes and heating said mixture in the tubes to a temperature of from 420 to 480 C, moving the mesophase-containing mixture through the coking zone at such a rate that a green coke having a mesophase content of from 70 to 100% by weight is formed, d) obtaining an overhead product from the coking zone and recycling at least part of the condensible fractions of this product to the preheater, and e) continuously removing the green coke from the coking zone, introducing it into a calcining apparatus and calcining it by heating to a temperature of from 1100 to 14000C.

In a preferred procedure embodying this invention, the green coke is passed through a secondary coking zone which is arranged between the coking zone and the calcining apparatus; the volatile products formed in this secondary coking zone are removed and at least part of the condensible fractions of this product are recycled.

In the context of this invention, what is meant by the mesophase is the optionally anisotropic phase of a pitch containing polynuclear aromatic compounds which is dispersed in an isotropic matrix and which is insoluble in quinoline or pyridine. The quantity and structure of the mesophase largely determine the structure and properties of the coke product. Moreover, what is meant by geeen coke is an infusible mass consisting mainly of carbon and which softens at elevated temperature.

The process of this invention yields a particularly uniform coke which is eminently suitable for use in the manufacture of high quality carbon or graphite products. By carrying out the process, it is possible to improve the energy utilisation of conventional coking processes and to reduce outlay, particularly on apparatus.

The difference between the process according to this invention and conventional coking processes lies in the continuous, uniform travel of the starting material and of the intermediate products formed during the process through the various reaction zones, including the calcining apparatus. The constant residence times in the individual zones enable cokes having uniform properties to be produced and hence leads, in turn, to improvement of carbon and graphite products produced from these cokes. In addition, by altering the throughput rate, while keeping within the process of this invention, it is possible readily to adapt the individual residence times and their ratios with respect to one another in accordance with the particular starting material used, so that the process may be employed with a wide range of hydrocarbon mixtures containing aromatic compounds.

Finally, the variability of the process parameters enables different cokes having particular properties of value to particular uses to be obtained.

The liquid hydrocarbon mixtures which are employed in the process according to this invention will generally be petroleum tars, including residual oils and decanted oils, coal tar pitches or pyrolysis tars. Mixtures of two or more substances containing aromatic compounds, for example mixtures of two coal tar pitches having different softening points, may also be employed. The softening point of the starting material, as determined by the Kramer-Sarnow method, is preferably at least 500C whilst its boiling point at the ambient atmospheric pressure is preferably at least 2750C. If necessary, the hydrocarbon mixtures employed may be subjected to flash evaporation to separate off troublesome light oil fractions before introduction into the preheater.

The starting material is delivered together with recycled condensate, possibly after preheating in one or more heat exchangers, to a preheater which is operated at a head temperature of from 280 to 3200C, at a sump temperature of from 380 to 440 C and under a pressure preferably of from 1 to 10 bars. The readily volatile components of the starting material which are given off are delivered to a fractionating unit. Mesophase is formed in the heavy fraction. Its content on leaving the preheater should amount to from 30 to 60% by weight. In order that the mesophase content. which increases with the temperature of the preheater and the residence time of the starting mixture, should be controlled, the temperatures and throughput rates are best determined by preliminary tests for the particular starting material used.Minor fluctuations in the quality of the starting material can be compensated by altering the througnput rate without any change in the temperature of the preheater. Any increase in the mesophase content beyond 60% by weight complicates the advance of the mixture through the subsequent coking zone or zones. A mesophase content of less than 30% by weight gives an inadequate coke yield.

The mesophase-containing mixture is then passed continuously through a coking zone containing a plurality of tubes and is heated to a temperature of from 420 to 480"C. The throughput rate has to be selected in such a way that a green coke having a mesophase content of from 70 to 100% by weight is formed. Green cokes having a mesophase content of less than 70% by weight give a relatively poor coke yield and the coke obtained is not of very good quality. The products formed in the coking zone are continuously removed.The volatile substances are delivered to a fractionating unit, whilst the green coke having a mesophase content of from 70% to 100% by weight is introduced into a calcining apparatus and calcined by heating to a temperature of from 1100 to 1400"C. Calcining may be carried out in, for example, a revolving cylindrical furnace or a revolving disc furnace.

When a secondary coking zone is employed between the coking zone and the calcining apparatus, its operating temperature is preferably substantially equal to the maximum temperature of the first coking zone. The secondary coking zone will usually be provided with a mechanical transport arrangement, for example a screw or a chain conveyor. The main function of the secondary coking zone is to degas the green coke, the gaseous products being removed and delivered to a fractionating unit. The secondary coking zone also serves to compensate for minor fluctuations in the quality of the coke. Transport means are preferably used for carrying the largely solidified green coke at a uniform rate from the exit of the coking zone to the entrance of the calcining apparatus.The overhead products removed from the preheater, the coking zone and the secondary coking zone are split up into fractions in one or more fractionating columns and at least part of the condensible products obtained are recycled and added to the hydrocarbon mixture containing aromatic compounds which is to be used for coking preferably in such a quantity that the starting material contains from 0.1 to 0.4 parts by weight of overhead products per part of starting hydrocarbon mixture.

Finally, it is of advantage when carrying out this invention for the mesophase-containing mixture from the preheater to pass vertically downwards through the coking zone. The vertical movement utilises the natural sedimentation of the mesophase and enables a stable concentration gradient favourable to the transport of the mixture to be achieved.

The uniform movement of the hydrocarbon mixture through the preheater and the following coking zone may be effected by provision of a gas cushion under pressure in the head of the preheater, the pressure being gauged in such a way that a residence time in the preheater and coking zone corresponding to a particular temperature gradient is obtained.

The gas pressure in the preheater preferably amounts to from 1 to 10 bars. In order to suppress undesirable premature coking of the hydrocarbon mixture in the coking zone, it is preferred that the mesophase-containing mixture be passed through the coking zone at a higher rate of travel than through the preheater. The ratio between the rates of travel preferably amounts to from 3:1 to 10:1. In addition, the relatively high rates of travel through the tubes of the coking zone enable greater velocity gradients to be obtained in the mesophase-containing mixture and, as a result, lead to deformation of the originally substantially spherical mesophase particles; this is desirable insofar as the structure of the coke is concerned. Linear temperature gradients are preferably maintained between entry and exit in the preheater and/or the coking zone.Since the degree of deformation can be kept at a certain value by adjusting the rate of advance and can be maintained with only minor fluctuations, coke having a predetermined structure can readily be produced in a uniform quality.

Calcination of the green coke is preferably carried out in calcining apparatus comprising several heating zones which enables the green coke to be slowly heated at low temperatures, particularly at temperatures in the range from 500 to 800"C, and to be rapidly heated above these temperatures. Cokes treated in this way have a particularly favourable porosity and pore structure.

Reference has been made herein to condensible fractions of overheads. In the context of this invention, these are components of the overhead product which are liquid or solid at ambient temperature and normal pressure, and include tar oils. If necessary, they may be subjected to an additional flash treatment before use. Recycled condensible fractions enable pitches having a high softening point to be used, improve the mobility of the mesophase-containing mixture in the coking zone and give a higher coke residue. Finally, the recycling of condensible overhead products is of particular value in cases where pitches, in particular are purified, for example by filtration before coking. The additions provide for a considerable increase in the efficiency of separation and for better selectivity.

Insofar as the starting materials for use in the process of this invention are concerned, it is preferred to use a hydrocarbon mixture which contains less than 0.5% by weight of quinoline-insoluble fractions. The starting material used, especially when a coal tar pitch, is preferably filtered before use for separating off quinoline-insoluble fractions. Cokes produced from purified starting pitch have an acicular texture and are particularly suitable for use in the production of highly anisotropic graphite products. The invention is by no means limited however to the production of acicular cokes and it is, in fact, of general applicability to the production of cokes having uniform properties over a wide range. For example, it is applicable to the production of isotropic cokes.

For a better understanding of this invention and to show how the same can be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, wherein: Figure 1 is a flow chart of the process according to the invention; and Figure 2 illustrates schematically the preheater, the coking zone, the secondary coking zone and the calcining plant for use in the process of this invention.

In the interests of clarity, the flow chart of Figure 1 has been simplified, i.e. the auxiliary apparatus normally employed, such as pumps, valves, measuring and control systems, have not been shown in detail.

Referring to Figure 1, a starting material which is usually petroleum tar, coal tar pitch or pyrolysis tar, is introduced through a pipe 10 to a heated mixer in which it is mixed with recycled pyrolysis condensate fed through a pipe 32. The contents of the mixer are heated in the mixer to a temperature about 100 to 2000C above the softening point of the mixture being produced. Alternatively, a pipe 12 leads off from pipe 10 to a flash tower 13 which is operated under such temperature and pressure conditions that the overhead fraction obtained contains components having a boiling point of up to 5000C. The temperature in the flash tower amounts for example to from 400 and 480"C and the pressure to from 0.02 to 0.1 bar.An overhead fraction is removed from the flash tower 13 through a pipe 14, compressed to atmospheric pressure and cooled to a temperature, for example 200 to 250"C, at which the vaporous product condenses and the condensate is delivered to a fractionating unit 15.

The mixture obtained in the mixer 11 is removed through a pipe 16 and introduced into a separator 17 usually, but not essentially a filter. The sump product of the flash tower 13 is introduced through a pipe 18 into the filter 17 or alternatively is passed through a by-pass 19 by-passing the filter into a pipe 20. There is no need for the starting material to be specifically purified when the hydrocarbon mixtures used are completely soluble in quinoline or pyridine or where it is desired to produce cokes having a relatively high coefficient of thermal expansion. In cases such as these, the mixture from mixer 11 is introduced into the pipe 20 through a pipe 21, thereby by-passing the filter 17. The filter 17, operated at a temperature of about 100 to 200"C above the softening point of the mixture, consists of several filters connected in parallel.Part of each filter can then be periodically disconnected for removing the filter cake formed thereon. The selectivity of filtration can be improved by adding filtering aids such as kieselguhr, to the mixture in the mixer 11. It is also possible to use centrifuges or sedimentation units instead of filters for separating off the quinoline-insoluble fractions of the starting material. The residue obtained is discharged at 22.

The filtrate or other liquor obtained from the separator 17 or the mixture, which is not subjected to any special purifying treatment, is introduced through a pipe 20 into a preheater 23 which is operated at a head temperature of from 280 to 320"C and at a sump temperature of from 380 to 440"C. The preheater preferably has a linear temperature and pressure profile, the pressure in the head of the heater amounting to from 1 to 10 bars.

Under these conditions, partial evaporation of the charge occurs and is intensified in the coking zone 24 which forms a unit with the preheater, the two communicating with one another through pipes or openings 25, 26. The ascending vapour bubbles keep the charge in turbulent motion and deform the mesophase formed in the preheater and coking zone to form, in accordance with the feedstock, acicular to filament-like structural elements. The coking zone 24 adjoining the heater contains a plurality of tubes whose entire cross-section is smaller than the free cross-section of the heater. Because of this cross-sectional ratio, the rate of flow of the material through the coking zone is higher than through the preheater.

The charge is preferably passed through the coking zone at a speed higher by a factor of 3 to 10 than the feed rate through the preheater. The mixture is heated in the coking zone 24 to a temperature of from 420 to 4800C, a linear temperature profile preferably being provided over the length of the zone. The residence times in the preheater 23 and in the coking zone 24 are gauged in such a way that the mesophase content increases to 30 to 60% by weight in the heater and to 70 to 100% by weight in the coking zone. The material containing 70 to 100% by weight of mesophase is continuously removed from the coking zone 24 and introduced into a calining apparatus 29 either directly by a conveyor 27 or through a secondary coking zone 28.

The vaporous products formed in the preheater 23 and the coking zone 24 are removed through a pipe 30 and the volatile products are removed from the secondary coking zone 28 through a pipe 31 and are introduced into a fractionating unit 15 which is operated under such temperature and pressure conditions that a sump fraction boiling at temperatures above 260"C and also a light oil fraction and a gas fraction are formed. The fractionating unit 15 also receives the overhead product of the flash tower 13 which is introduced through a pipe 14. A heavy oil fraction having the above mentioned boiling range is removed through a pipe 32 and at least partly delivered to the mixer 11, the recycle ratio amounting to from 0.1:1 to 0.4:1.In general, higher recycle ratios reduce the overal capacity of the installation, whilst lower ratios reduce the efficiency of the filter 17.

The green coke introduced into the calcining apparatus 29 still contains approximately 10% to 13% by weight of volatile products. The proportion of these products is reduced usually to form 0.1 to 0.5% by weight by heating to a temperature of from 1100 to 1400"C.

Calcining apparatus particularly suitable for this purpose are calcining apparatus comprising several heating zones, in which at least one zone is used for slowly heating the green coke to a temperature of from about 700 to 750"C and a further zone for rapid heating to a final temperature of 1100 to 14000C. The average temperature gradient amounts to approximately 100"C per hour in the first zone(s) and to approximately 500"C per hour in the further zone. To achieve such heating conditions, it is possible to use several calcining apparatuses arranged one behind the other, the calcining apparatus used being revolving cylindrical furnaces, disc furnaces or furnaces which are provided with chicanes in certain zones for regulating the residence time.

Referring next to Figure 2, the preheater 23 into which liquid feedstock is introduced and from which the volatile products are removed is so operated that mixture 33 therein is uniformly moved therethrough so that its mesophase content increases to from 30 to 60%.

The coking zone 24 is connected to the preheater, lying below it. It has a smaller free cross-section than the preheater so that the rate of flow of the material is increased accordingly. At the output end of the coking zone, the mesophase content amounts to at least 70%. The heated secondary coking zone 28 lying beyond the coking zone 24 contains, as transport means, a screw 34 which uniformly delivers the green coke formed into the calcining apparatus 29 which is a revolving cylindrical furnace.

The following examples illustrate this invention: Examples Using the apparatus of Figures 1 and 2, two experiments were carried out using different starting materials. Details thereof and of the products obtained as well as details of operating conditions are tabulated as follows, percentage contents of components of materials being expressed on a weight basis.

Example 1 Example 2 Starting material coal tar pitch low-temperature tar softening point (KS) DIN 1995 99"C quinoline-insoluble fraction 6.9% toluene-insoluble fractionless ss-resins (quinoline insoluble fraction) 18.1% coke residue DIN 51720 42.0% recycle ratio 0.15:1 0.2::1 filtration temperature 1800C 1600C Example 1 Example 2 Starting material coal tar pitch low-temperature tar Filtrate softening point 90"C 94"C quinoline-insoluble fraction 0.3% 0.1% ss-resins 21.5% 7.4% coke residue 38.5% 25.6% Preheater pressure 2 bars sump temperature 440"C residence time 5 h mesophase content 50% Coking zone sump temperature 480"C residence time 2h mesophase content 85 - 90% Secondary coking zone sump temperature 550"C residence time 0.1 h mesophase content 100% Calcining apparatus max. temperature 1300"C Coke yield 61% 49% density 2.13 g/cm3 2.09 g/cm2 thermal volume expansion coefficient 4.3 x 10-6/K 7.6 x 10-6/K degree of anisotropy 1.8 1.3 WHAT WE CLAIM IS: 1. A process for the continuous production of coke, which comprises the steps of a) introducing a cokable hydrocarbon mixture into a preheater together with recycled condensible fractions of overheads, the preheater being operated at a head temperature of from 280 to 3200C and at a sump temperature of from 380 to 440"C, passing the mixture of hydrocarbon mixture and recycled condensible fractions of overheads through the preheater at such a rate that the mesophase content of the mixture issuing from the preheater amounts to from 30 to 60% by weight,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (23)

**WARNING** start of CLMS field may overlap end of DESC **. Example 1 Example 2 Starting material coal tar pitch low-temperature tar Filtrate softening point 90"C 94"C quinoline-insoluble fraction 0.3% 0.1% ss-resins 21.5% 7.4% coke residue 38.5% 25.6% Preheater pressure 2 bars sump temperature 440"C residence time 5 h mesophase content 50% Coking zone sump temperature 480"C residence time 2h mesophase content 85 - 90% Secondary coking zone sump temperature 550"C residence time 0.1 h mesophase content 100% Calcining apparatus max. temperature 1300"C Coke yield 61% 49% density 2.13 g/cm3 2.09 g/cm2 thermal volume expansion coefficient 4.3 x 10-6/K 7.6 x 10-6/K degree of anisotropy 1.8 1.3 WHAT WE CLAIM IS:

1. A process for the continuous production of coke, which comprises the steps of a) introducing a cokable hydrocarbon mixture into a preheater together with recycled condensible fractions of overheads, the preheater being operated at a head temperature of from 280 to 3200C and at a sump temperature of from 380 to 440"C, passing the mixture of hydrocarbon mixture and recycled condensible fractions of overheads through the preheater at such a rate that the mesophase content of the mixture issuing from the preheater amounts to from 30 to 60% by weight,

b) recycling to the preheater at least part of the condensible fractions of the overhead product from the preheater, c) passing the mesophase-containing mixture issuing from the preheater through a coking zone comprising a plurality of heating tubes and heating said mixture in the tubes to a temperature of from 420 to 4800C, moving the mesophase-containing mixture through the coking zone at such a rate that a green coke having a mesophase content of from 70 to 100% by weight is formed, d) obtaining an overhead product from the coking zone and recycling at least part of the condensible fractions of this product to the preheater; and e) continuously removing the green coke from the coking zone, introducing it into a calcining apparatus and calcining it by heating to a temperature of from 1100 to 14000C.

2. A process as claimed in claim 1, wherein said hydrocarbon mixture is a petroleum tar, a coal tar pitch or a pyrolysis tar.

3. A process as claimed in claim 2, wherein the hydrocarbon mixture contains less than 0.5% by weight of quinoline-insoluble fractions.

4. A process as claimed in claim 3, wherein quinoline-insoluble fractions are removed from the hydrocarbon mixture by filtration, centrifugation or sedimentation.

5. A process as claimed in claim 4, wherein the hydrocarbon mixture is a coal tar pitch which is subjected to filtration to remove quinoline-insoluble fractions therefrom.

6. A process as claimed in any one of the preceding claims, wherein the hydrocarbon mixture has a softening point, as determined by the Kramer-Sarnow method of at least 50"C.

7. A process as claimed in any one of the preceding claims, wherein the hydrocarbon mixture has a boiling point at the ambient atmospheric pressure of at least 275"C.

8. A process as claimed in any one of the preceding claims, in which the hydrocarbon mixture is subjected claims, in which the hydrocarbon mixture is subjected to flash evaporation of light oil fractions therefrom prior to feeding thereof to the preheater.

9. A process as claimed in any one of the preceding claims, wherein a mixture of one part by weight of starting cokable hydrocarbon mixture and from 0.1 to 0.4 part by weight of recycled condensible fractions of overheads is fed to the preheater.

10. A process as claimed in any one of the preceding claims, wherein the preheater is operated under a gas pressure of from 1 to 10 bars.

11. A process as claimed in any one of the preceding claims, wherein the mesophasecontaining mixture from the preheater is passed vertically down through the coking zone.

12. A process as claimed in any one of the preceding claims, wherein feedstock is passed through the coking zone at a rate of from 3:1 to 10:1 the rate of passage of feedstock through the preheater.

13. A process as claimed in any one of the preceding claims, wherein linear temperature gradients between entry and exit are provided in the preheater and/or the coking zone.

14. A process as claimed in any one of the preceding claims, wherein the calcination apparatus comprises a plurality of temperature zones for heating of the green coke up to a temperature of from 1100 to 14000

15. A process as claimed in claim 14, wherein the green coke is heated to a temperature of from 500 to 800"C in a first heating zone or zones under a first temperature gradient and wherein the green coke is further heated in a second heating zone or zones to a temperature of from 1100 to 1400"C under a steeper temperature gradient.

16. A process as claimed in claim 15, wherein the calcining apparatus is operated with at least one temperature zone in which green coke is heated at a temperature gradient of about 1000hour to from 700 to 750"C and a further temperature zone in which the green coke is heated at a temperature of about 5000hour to said temperature of 1100 to 1400"C.

17. A process as claimed in any one of the preceding claims, wherein the calcining apparatus is a revolving cylindrical furnace or a revolving disc furnace.

18. A process as claimed in any one of the preceding claims, wherein green coke is fed from the coking zone to the calcining apparatus on mechanical conveying apparatus at constant rate.

19. A process as claimed in claim 18, wherein the mechanical conveying apparatus is a screw or chain conveyor.

20. A process as claimed in any one of the preceding claims, wherein the green coke is passed through a secondary coking zone between said coking zone and the calcining apparatus, at least part of the condensible fractions of the overhead product produced being recycled to the preheater.

21. A process for the continuous production of coke, substantially as hereinbefore described with reference to the accompanying drawings.

22. A process as claimed in claim 21, substantially as described in either of the foregoing Examples.

23. Coke which has been produced by the process claimed in any one of the preceding claims.