Production of isotropic coke in industrial trials

ISSN 1068�364X, Coke and Chemistry, 2014, Vol. 57, No. 5, pp. 202–207. © Allerton Press, Inc., 2014.Original Russian Text © I.V. Moskalev, D.M. Kiselkov, V.N. Strelnikov, V.A. Valtsifer, A.P. Petrovykh, A.V. Petrov, N.Yu. Beilina, 2014, published in Koks i Khimiya, 2014, No. 5,pp. 20–26.

202

Carbon�based structural materials are producedfrom coke of isotropic microstructure so as to ensurean optimal combination of thermoelectrophysicalproperties and strength. Until the mid�1990s, specialpyrolytic petroleum coke was used for the productionof carbon�based structural materials. Such petroleumcoke is produced by the pyrolysis of kerosene–gaso�line fractions, to obtain hydraulic tar, which is thencoked [1]. Deficiencies of this method include the useof scarce and expensive distilled petroleum fractions;the low coke yield; and the downtime due to rapidclogging of the reaction equipment. For economicreasons, special pyrolytic petroleum coke is not pro�duced at present.

An alternative raw material is shale oil. AtOAO NIIgrafit, tests have shown that isotropic cokeproduced from shale oil is suitable for the manufactureof high�quality carbon�based structural materials [2].To produce isotropic coke, the shale oil is coked in avat with preliminary air�blowing [3]. The main down�side of this method is that the shale oil is deficiencyraw material.

Pitch coke produced industrially was tested as asource of carbon�based structural materials in [4–11].The physical�mechanical properties of the productwere poor on account of factors such as the nonuni�form properties of the pitch coke produced in cokeoven batteries, the structural inhomogeneity, and therelatively high content of anisotropic structural ele�ments.

Experimental batches of coke with isotropicmicrostructure and an elevated yield of volatiles wereobtained in [2, 12, 13]. The raw material employedwas industrial ABP. However, its properties vary widelywith change in its production conditions: the batchcomposition, the coke conditions, the coal tar distilla�tion conditions, and the conditions of air�blowing.Instability is noted in the mesogenic properties of thepitch, which determine its mesophase transformationson carbonization and the microstructure of the cokeproduced.

For example, coke was produced from ABPobtained at OAO Severstal in [14]. The properties ofthe pitch are as follows: TSP = 197.3°C; V daf = 39.0%;

Production of Isotropic Coke in Industrial TrialsI. V. Moskaleva, D. M. Kiselkova, V. N. Strelnikova, V. A. Valtsifera, A. P. Petrovykha,

A. V. Petrovb, and N. Yu. Beilinab

aInstitute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, Perm, Russiae�mail: [email protected], [email protected], [email protected],

[email protected], [email protected] The Research Institute of Graphite�based Srtuctural Materials (OAO NIIgrafit), Moscow, Russia

e�mail: [email protected], [email protected] April 9, 2014

Abstract—The influence of the production conditions on the properties of coke is considered. Coke is pro�duced in a pilot plant at the Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, ina 200�L reactor equipped with an air�supply valve and heating elements. The first stage in coke production isair�blowing of the batch with constant temperature rise at 10–12°C/h from 290–310°C; the air flow rate is45–55 L/kg h. At this stage, the final air�blowing temperature and the batch composition are varied. The sec�ond stage is coking, with temperature rise at 25°C/h to 550–600°C. The batch consists of industrial air�blownpitch (ABP), modified by pitch tar (PT). Oxidation of the ABP, even with a very high final temperature(434°C), does not permit the production of isotropic coke. An analogous result is obtained on adding smallportions of PT to the batch (15%). On adding >50% PT, totally isotropic coke may be produced. To obtaincoke of isotropic microstructure, the optimal content of PT is 36–41%, and the final air�blowing temperatureshould be high (>390°C). The influence of PT on the structural parameters of the coke is associated with theformation of nonmesogenic structures on air�blowing. On coking, these structures suppress the growth oflarge mesophase. The isotropic coke produced has the following characteristics: limited expansion in therange 1300–2400°C; high structural strength; and optimal density. Graphite based on such coke is consider�ably superior to graphite based on industrial pitch coke in terms of its compressive strength, density, and elec�trical resistivity.

Keywords: isotropic coke, pitch tar, air�blowing, coking, pilot plant, carbon�based structural materials

DOI: 10.3103/S1068364X1405007X

COKE

COKE AND CHEMISTRY Vol. 57 No. 5 2014

PRODUCTION OF ISOTROPIC COKE IN INDUSTRIAL TRIALS 203

Ad = 0.1%; content of the toluene insolubles (TI)59.9%; content of the quinoline insolubles (QI)44.3%; content of the isooctane insolubles 27.6%; andcontent of the isooctane solubles 12.5%. Of all thepitch produced industrially in Russia, this has thehighest softening point and the greatest content of theTI and QI. That favors the formation of isotropicmicrostructure of the coke. ABP is coked in differentconditions: at atmospheric pressure, with heating ratesof 0.1–10°C/min; at atmospheric pressure, with 10�hholding at 290–490°C; and with excess pressure up to5 MPa. The results show that variation of the cokingconditions does not lead to the formation of isotropiccoke suitable for the production of structural graphite.On average, the microstructure score is 3.5–4.5.

Isotropic coke may be produced by processing theanthracene fraction under its own vapor pressure (upto 5 MPa) at 500°C, with a heating rate of around10°C/min [15]. The coke obtained in such conditionshas uniform isotropic microstructure (with a score of 2)and a fine�pore macrostructure. It does not containash impurities and its content of heteroatoms is lessthan for regular pitch coke. However, the need for hightemperatures and pressures limits the applicability ofthis method.

Thus, the production of isotropic coke with therequired properties from available materials, at mini�mum costs, remains an unresolved challenge.

In the present work, we investigate the influence ofthe air�blowing parameters and the composition of theraw materials on the properties of the coke—in partic�ular, on the microstructure. The raw material consid�ered is ABP, with the coking technology in [14]. Asalready noted, the coke formed from such pitch is notisotropic. In the present work, the ABP is modifiedwith PT and undergoes air�blowing, prior to coking.

A pilot plant is used for air�blowing and coking ofthe initial mixture; its operating principle was outlinedin [16]. Mixture portions (100 kg) are charged in areactor with an air�supply valve. The heater is switchedon and the temperature is raised to 290–310°C; thenair is admitted. The air consumption in reaction is 45–55 L/kg h. In air�blowing, the temperature rises at aconstant rate of 10–12°C/h. At the end of air�blow�ing, the air supply is switched off, and heating contin�ues to 550–600°C, at 25°C/h.

The initial material is modified by PT, produced bypreliminary coking of the ABP in the following condi�tions: 150�kg charge; heating rate 3°C/min; final cok�ing temperature 600°C. The PT contains 90.48% car�bon, 5.02% hydrogen, 0.51% sulfur, and 1.87% nitro�gen; it is completely dissolved in toluene.

The microstructure of the coke is analyzed inaccordance with State Standard GOST 26132 bymeans of an Olympus BX51 microscope, in polarizedreflected light (100× magnification). The yield of vol�atiles and coke density are determined in accordancewith State Standard GOST 22898; the ash content is

established on the basis of State Standard GOST22692. The content of carbon, hydrogen, nitrogen,and sulfur is determined by means of a LECO CHNS�932 instrument. Representative samples (2 mg) ofpitch and coke (particle size ≤200 μm) are analyzed.

The structural strength of the coke is determined bythe OAO NIIgrafit method (Methodological Instruc�tions MI 4807�310–2005) and characterized by thework consumed in the formation of a new surface, onaccount of the disintegration of particles of the mate�rial.

The change in volume of the coke samples in high�temperature treatment is investigated at OAO NIIgrafit,on a Tamman furnace with an LOP�72 pyrometer,R321 1�Ω resistance coils, and V7�23 universal digitalvoltmeter. The samples take the form of cubes (side25 mm).

Laboratory (model) blanks are made in the stan�dard method for laboratory testing of new materials. Ina heated mixer, coke�pitch composite is formed; pitchacts as the binder. After cooling, the composite iscrushed, and then the resulting powder is passedthrough a 315�μm screen. The samples are shaped inan unheated dead�end matrix.

The apparent density is determined as themass/volume ratio of the blank.

The compressive strength of samples is determinedat room temperature by the OAO NIIgrafit method(MI 00200851�142–2007) on a Zwick/Roell�Z250universal test machine. The electrical resistivity isdetermined by a four�probe method (MI 002851�339–2010).

The influence of the coking conditions of thecoke’s microstructure is determined by varyingthe following parameters: the content of PT in the ini�tial material; the air flow rate; and the final air�blow�ing temperature (Table 1). The distribution of thecoke’s structural components is shown in Fig. 1.

The oxidation of pure ABP (sample C�1) with afinal temperature of 434°C does not lead to the pro�duction of isotropic coke.

Table 1. Conditions for coke production in pilot plant

SampleContent of PTin initial mate�

rial, %

Final air�blowing tem�perature, °C

Microstruc�ture, score

C�1 0.0 434 3.1

C�2 15.0 430 4.0

C�3 52.7 341 1.0

C�4 36.0 341 2.7

C�5 36.4 394 2.5

C�6 36.2 413 2.4

C�7 40.7 410 2.1

204


MOSKALEV et al.

For sample C�2, with 15% PT, coke with micro�structural elements corresponding to scores of 4, 5,and 6 is produced (Fig. 1). Obviously, adding PT toABP reduces the viscosity of the reaction mass, whichpromotes the growth of meso phase.

On adding more than 50% PT to ABP (sample C�3),totally isotropic coke is formed. To understand theinfluence of the PT on the properties of the coke pro�duced, we need to understand why isotropic coke isnot formed on coking ABP in the absence of additives.In addition, the oxidation of ABP (sample C�1) athigh temperatures does not much reduce the micro�structure score.

In air�blowing, most of the polymerization involvesthe polycondensation of aromatic rings, by the

removal of aromatic hydrogen with oxygen, accordingto [17]. Planar molecules with high molecular massare formed. On carbonization, they participateactively in the formation of mesophase. Consequently,ABP and its oxidized derivative form anisotropic coke.

In comparison with pitch, the components of PTare characterized by lower molecular mass and feweraromatic rings. On air�blowing, such componentsform compounds with biphenyl, methylene, and oxy�gen bridges or more complex cross�linked structures.In pyrolysis, such structures form nonplanar radicals,which cannot form mesophase. Thus, in the produc�tion of sample C�3, a considerable quantity of the PTin the initial material produces nonmesogenic struc�tures, which fully suppress the growth of mesophase.

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Fig. 1. Distribution of the structural elements in coke.



This finding is somewhat inconsistent with the viewthat the presence of relatively nonvolatile hydrocar�bons in the PT increases the content of anisotropicstructures in the product [18]. However, the authorsdid not mention the quantity of PT is added to thepitch. In producing the subsequent samples, the con�tent of PT is reduced to level 35–45%.

Reducing the content of PT in the initial materialto 36% (sample C�4) proves effective. In this case, theproduct has a microstructure resembling that of iso�tropic coke, with a score of 2.7. The distribution ofstructural components is bimodal, with a large quantityof isotropic elements (with scores of 1 and 2) and also ofanisotropic elements (with scores of 4 and 5). This isprobably due to instability of pitch as a disperse systemin the course of pyrolysis. The mesophase and car�boids are partially stratified and form fully isotropicand anisotropic regions in the coke produced. Furtherdecrease in microstructure scores is possible byincreasing the final temperature in air�blowing.

In terms of its microstructure score, sample C�5 isisotropic. However, it includes a high content of bothanisotropic structural elements (more than 25% of thetotal) and totally isotropic structural elements (with ascore of 1). Sample C�6 meets the requirements on iso�

tropic coke and contains more than 80% of the mostvaluable structural elements (with a score of 2).

In producing sample C�7, the conditions are simi�lar to the previous case, but the content of PT in theinitial material is increased by 4.5%. This furtherreduces the microstructure score to 2.1 and the con�tent of totally isotropic structural elements is 48.3%.However, the structural elements with a score of 1 arecharacterized by limiting dimensions of 2.0–3.5 μm(Fig. 2). Accordingly, they are close to a score of 2 andmay be regarded as improving the coke properties.

In the production of these experimental batches,the microstructure score is reduced by two means:adding a low�molecular compounds (PT) to the initialmaterial; and increasing the air�blowing temperature(>400°C). The use of only one of these approachesdoes not ensure the production of isotropic coke.

Coke samples C�6 and C�7 differ in appearancefrom regular pitch coke in that they have thick inter�pore walls, with no macropores. Table 2 summarizesthe characteristics of such coke.

We conclude from Table 2 that samples C�6 andC�7 are comparable in density with isotropic coke andare characterized by high structural strength. The rel�atively low VM content of sample C�6 is evidentlyassociated with the high final coking temperature

50 μm

Fig. 2. Microphotograph of isotropic structural elements in sample C�7 (magnification ×500).

Table 2. Characteristics of coke samples

Sample Volatile matter (VM), %

Ash content, %

True density dtr, g/cm3

Structural strength σstr, MPa

Sulfur content, %

Nitrogen content, %

C�6 2.0 0.34 2.054 187.8 0.35 2.11

C�7 4.0 0.27 2.043 – 0.35 2.12

OAO Severstal pitch coke 1.51 0.40 2.100 150.0 0.31 1.49

206


MOSKALEV et al.

(600°C). Pitch coke from OAO Severstal is consider�ably denser, with a higher microstructure score (3.1).The lower nitrogen content is evidently due to the hightemperature of coke production (around 1000°C).Roasting samples C�6 and C�7 at 1300°C for 5 hreduces their nitrogen content to 1.17 and 1.09%,respectively. For all the samples, regardless of thedegree of roasting, the sulfur content is about thesame: 0.30–0.35%.

Table 3 presents test data regarding the volumechange on heat treatment.

The coke produced in the pilot plant is character�ized by only slight increase in volume at temperaturesof 1300–2400°C; the change is several times than forOAO Severstal pitch coke.

Table 4 presents the test results for blanks ofMPG�7 graphite produced on the basis of samples C�6and C�7 and OAO Severstal pitch coke.

The strength of the graphite from sample C�6 is49% greater than that of samples from industrial pitchcoke. In addition, its electrical resistivity is lower(by 35%), while its density is higher. Obviously, thebetter characteristics of the graphite produced from

sample C�6 are due to its isotropic microstructure.The graphite based on sample C�6 may be classified asdense and strong.

The graphite based on sample C�7 has considerablybetter characteristics, probably because the higher VMcontent results in greater shrinkage. In addition, thesize of the isotropic structural elements may play a rolehere. They are in the range 2–5 μm for sample C�7and 4–10 μm for sample C�6. Smaller particles shouldbe deposited more uniformly and ensure higher den�sity. In turn, higher density implies more contactsbetween the filler grains and hence lower electricalresistivity. The graphite based on sample C�7 may beclassified as very dense and very strong.

Thus, an effective means of controlling the cokestructure is to add PT to the ABP employed, with sub�sequent air�blowing of the mixture. The influence ofPT on the structural parameters of the coke is associ�ated with the formation of nonmesogenic structuresduring air�blowing. On coking, these structures sup�press the growth of large mesophase.

Obviously, the PT employed may be replaced withother materials such as pitch distillates or anthracene

Table 3. Change in mass and volume of coke samples in heat treatment

Sample Test20–1300°C 1300–2400°C

ΔV, % Δm, % ΔV, % Δm, %

C�6

1 –18.22 –6.19 +1.17 –2.06

2 –24.67 –6.01 +0.44 –2.22

3 –24.10 –6.06 –0.11 –2.21

Mean –22.33 –6.09 +0.50 –2.16

C�7

1 –27.46 –5.94 +1.33 –2.06

2 –27.95 –5.74 +0.33 –2.10

3 –25.84 –6.16 +1.21 –2.12

Mean –27.09 –5.95 +0.96 –2.09

OAO Severstal pitch coke

1 –23.68 –5.76 +5.32 –2.19

2 –21.64 –5.68 +2.76 –2.16

3 –22.18 –5.69 +2.78 –2.10

4 –21.95 –5.76 +2.68 –2.16

Mean –22.36 –5.72 +3.39 –2.15

Table 4. Characteristics of graphitized samples

Coke sample

Bulk density dco, g/cm3 Shrinkage ΔV/V, % Mass loss Δm/m, % Compressive strength σstr, MPa

Resistivity, μΩ munroasted roasted graphitized roasted graphi�

tized roasted graphi�tized

OAO Severstal pitch coke 1.10 1.32 1.61 – – – – 65.0 38.0

C�6 1.09 1.39 1.70 31.4 23.1 12.2 5.9 97.1 24.8

C�7 – – 1.82 – – – – 141.0 17.9



fractions whose degree of concentration is similar.Together with the high air�blowing temperature(>400°C), the addition of PT permits the formation ofpitch suitable for the production of high�quality iso�tropic coke characterized by lower expansion in therange 1300–2400°C and high structural strength.Graphite based on such coke is considerably superiorto graphite based on industrial pitch coke in terms ofcompressive strength, density, and electrical resistivity.The positive outcome of trials on the pilot plant dem�onstrates the possibility, in principle, of adopting thisapproach on an industrial scale.

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Translated by Bernard Gilbert

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Production of isotropic coke in industrial trials