Journal of Scientific & Industrial ResearchVol. 62, September 2003, pp 876-882

Meso-phase Pitches from the Residue of Oil Refinery'R K Maheshwari* and N K S Pundhir

Central Road Research Institute (CSIR), Mathura Road, New Delhi 110020

Received:27 September 2002; rev. recd:06 June2003; accepted: 26 June 2003

The meso-phase pitches were prepared after heating bitumen (residue of oil refinery) at 370-500"C for preparation ofdifferent types of industrial products such as, carbon fibres, high density graphite, needle coke and many other value addedproducts. The variety of pitches are prepared by polymerization and condensation reactions during heat treatment, dependingupon several factors like, rate of heating, duration of heat treatment, atmosphere, rate of flowing gas, pressure and type ofvessel. Rheological behaviour indicates that polymerization takes place at higher temperature. The kinetic parameters such as,order of reaction (n) and activation energy (E,) were calculated from thermograms and the value was found to be about 34 kcal/mol. Some pitches like, binder pitches, impregnating pitches of very high industrial importance are obtained from meso-phase pitches with due selection of different properties.

Keywords: Meso-phase pitches, Oil refinery, Bitumen

IntroductionBitumen which is a residual product of oil

refinery, can be used to prepare pitches, meso-phasepitches, binder pitches and impregnating pitches.These pitches are used to prepare carbon electrode,graphite, needle coke and carbon fibre. Bitumen isnon-polar and consists of a smaller condensedaromatic ring system linked by aliphatic groups.Bitumen, when subjected to prolong heating above400°C, polymerisation and condensation reactionsproceed rapidly within the liquid phase until theproducts of the reaction remain no longer soluble inthe pitch 1,2. The insoluble portion is known as meso-phase pitch. Several workers''" have carried outthermal treatment of bitumen under differentexperimental conditions and studied the propertiessuch as, change in softening point, viscosity, kineticbehaviour, ESR and NMR. Meso-phase formationduring the heat treatment of bitumen between 370-500°C results in increase in the concentration of highmolecular weight species. The meso-phase formationis important in the manufacturing of needle coke,artificial graphite, and high modulus carbon fibre. Thephysicochemical characteristics of meso-phasepitches including their rheological behaviour play a

* Author for correspondencet The views expressed in this paper are of the authors and not

necessarily of the institution

significant role 10 the manufacturing of carbonproducts".

Thermal polymerisation during prolong heatingof bitumen causes the orientation of micelles to formthe harder type of material with change in internalstructure of the material. The study of kinetics formeso- phase formation of petroleum pitches wascarried out by using Arrhenius equation but thecomparisons with the other methods were carried out.

The paper deals with the characterization ofthermal treated bitumen by X-ray diffraction (XRD)studies, rheological behaviour, and thermal study. Thekinetic parameters from thermo grams of thermaltreated bitumen were also determined using differentcomputation methods.

Materials and Methods

MaterialsThe solvents used were of AR grade. The

nitrogen gas was used to create the nitrogenenvironment in glass reactor. The paving gradebitumen with penetration value 94 d mm, softeningpoint 40SC, ductility 100+, asphaltene content 12.0per cent, coke value 12.77 per cent was used for thepreparation of pitches with thermal treatment.

Heat Treatment ProcedureAbout 50 g bitumen was taken in a glass test

tube of 100 mL capacity and heated in a special glass

MAHESHWARI & PUNDHIR: MESO-PHASE PITCHES FROM THE RESIDUE OF OIL REFINERY 877

reactor with 50 cm in height and 10 cm in diameter having inlet and outlet for passing a stream of nitrogen at 350, 380, 400, 410, 430, 440, 460, 470, 500, and 530°C for 5 h . The nitrogen gas was passed into the reactor through the inlet at the flow rate of 200 rnL gas/min . The out-going gases were condensed/collected in a container and finally into a container fi lled with water. The heating was carried out at 200oC/h up to 300°C followed by the rate of 100oC/h till the experimental temperature was reached. The temperature was kept constant for the fu ll residence period . The test tube was cooled to room temperature to get the thermal treated bitumen (ITB) or meso-phase pitch.

Physical Measurements

The physical properties of bitumen were determined as per IS: 1201-20. X-ray diffraction (XRD) patterns were recorded on Philips Diffractometer, using CuKa radiation . The properties of meso-phase pitches like softening point (SP), benzene insoluble (BI), coke values and weight loss were determined as per ASTM and BIS standards. Thermal study by thermo-gravimetric analysis (TGA) of pitches was carried out at the heating rate of 10°C/min using about 4-5 mg sample on thermo­couple balance. The viscosity of pitches at different temperatures was determined usmg Haake Rotoviscometer model MY - 12.

Results & Discussion

Characterization

The softening point of meso - phase pitches changes from 40.5 to 400°C when obtained after heating at 530°C for 5 h. Similarly, coke value changes from 13.97 to 86.29 per cent. The weight loss of pitch varies from 12.77 to 69 .60 per cent. The physicochemical characterist ics of the pitches/thermal treated bitumen are given in Figure I .

X-ray Diff;·actioll Study

X -ray diffraction pattern of thermal treated bitumen have been obtained with intensi ty vs angle of deflection. The ·d' spacings of thermal treated bitumen were calcu lated from X-ray patterns shown in Figure 2 using the following Debrogle equati on :

2d Sin e = nA .

X-ray diffraction study of thermal treated bitumen indi cates that the 'd' spac ing ca lcul ated for therma l treated bitumen (TTB) at 440-500°C varies

gO -- COKE VALUE » 450

Q-O WEIGHT LOSS, PERCENT

80 ~ .soFTENING POINT C 400

~ cr--o "-HEPT ANE INSOLUeLES UJ ~ 70 350

3 9----0 ----0 /

/ \II , '" 60 ; JOO UJ ; Z I .u ... ;0 0-

~ 50 / 250 Z I :r (5

Q.

\II C> 9 40 200 ~ W

0- 0-r ... <:> 150 ~ w JO

~ --UJ

~ 20 100 > .... '" 8 10 50

340 380 420 460 500 540

TEM PERATURE ·C

Figure 1- Physical properties o f differen t thermal treated bitumens

I I I I I iii I i I I I iii I t I I I ' I i I I r i

3" 32' 30' 28' 26' 2" 22" 20' 18' 16' ". 12" 10' 8·

Figure 2 - X-ray pattern ofthermallreated bitumens

from 3.551 A of TTB 440 to 3.499 A of TTB 500. It is observed that the value of 'd' spaci ng decreases as the temperature of treatment increases. This indicates that there is a deve lopment of structure and d-spacing consequently affected. The fall in 'd' spa~ing is more at 500°C. These va lues although fall in the amorphous range, still gives an indication of

878 J SCI IND RES VOL 62 SEPTEMBER 2003

the polymerised molecules. The difference in 'd'spacing values at 460 to 480°C is more or less sameto the values between 440 to 460°C. The resultsshow that the range 460 to 470 °C is important forgetting more uniform distribution of the orientedmolecules. The fact that'd' spacing falls from avalue of 3.551 to 3.499 A within a span of 60°Cwhich indicates that bitumen on thermal treatmentdoes contain graphite forming polymerisedmolecules while the raw bitumen has no suchmolecules. Thus, it is possible to generate graphiteforming component in the bitumen.

Rheological BehaviourThe relationship between viscosity vs

temperature was plotted for different heat treatedbitumens (Figure 3) and a little parabolic nature wasobserved by achieving the curve for lowertemperature treated bitumen. There was substantialchange in viscosity between 70 to 105°C for TTB-350and TTB- 400. Viscosity fall below 120°C was steepfor bitumen, while it was steady at highertemperature. Shear stress-strain relationship of heatedbitumen at 350, 380 and 400°C for 5h are plotted inFigure 4. It is observed that stress-strain curves givealmost a straight line at particular temperature. Thisindicates a Newtonian behaviour of thermal treatedbitumen. The slope of the curve falls steeply with therise of temperature initially followed by a slowchange at higher temperature (160°C). As thetemperature of treatment is raised, the value of theslope increases. This indicates that there is a smallchange in the structure of bitumen with the heattreatment for 5 h. It is further supported by viscosity-temperature relationship that shows higher viscosityfor higher treatment temperature.

The stress and strain analysis shows that thegraph in shear stress vs shear strain is a straight line atall temperatures for all TTBs with the variation ofslope value. This led to the conclusion that the pitcheshave the Newtonian flow at low as well as at hightemperature. The results indicate that viscosity ofpitches increases with increase in softening point. Theviscosity was found high at low temperature and itdecreased with the increase in temperature. Theviscosity-temperature relationship of all pitches hadsimilar trend and sudden drop in viscosity for TTB-380 and 400 implied the temperature susceptibility ofthe materials.

107

lr/TTS AT o'cC>--O

10S - TT B AT 35O'C~:.-- ff. 1lT 380'C.§ ..--. TTe AT'OO·C/: 10'

in0l>!

103;:

102

60 75 90 1 S 1 0 13S 150 165TEMPERATURE f'C)

185 20S'C

Figure 3 - Viscosity-temperature relationship of bitumen andthermal treated bitumen

90'C 105 'c

-;;;d.

III24

0z«III

20~0:r•..~ 16V!V!UJ

12'"l-V!

'"«'":x:III

o 20 40 60 140 200

SHEAR STRAIN (5-1)

300

Figure 4 - Shear stress-strain relationship bitumen at differenttemperatures

Thermal Studies

Bitumen

The thermo-gravimetric analysis (TGA) showsthat bitumen decomposes with the onset temperatureof 220°C and continues up to 450°C for the observedweight loss 25.8 per cent due to the removal of lowmolecular weight species. The decomposition ofbitumen increases with rise in temperature. Thissuffers a weight loss of 100 per cent at 685°C. The'pitch (TTB-380) shows a weight loss of 13.5 per centup to 450°C, which indicates conclusively that theportion of medium molecular weight in pitch isenriched which is supported by the increase in

MAHESHWARI & PUNDHIR: MESO-PHASE PITCHES FROM THE RESIDUE OF OIL REFINERY 879

benzene insoluble (BI) content. This portion contains higher molecular weight loss in pitch (TTB-380) is reduced to 82.7 per cent as compared to 100 per cent of bitumen up to 7500 e which indicates that the remaining high molecular wei ght species decompose at 900°C.

Thermal Treated Bitumen

Thermal treated bitumen TTB-390 undergoes the decomposition with onset temperature 3400 e and continues up to 4500 e for the observed weight loss 20.5 per cent as compared to the weight loss of 25.8 per cent in bitumen. Thi s shows the increase of medium molecul ar weight spec ies in thi s temperature ran ge. The weight loss of this pitch is 59.3 per cent up to 6000e which is more or less comparable to the weight loss of 61 .5 per cent of TTB-380 . Between 450-600oe, the weight loss has decreased to 38 .8 per cent as aga in st the weight loss of 48.0 pe r cent for TTB-380 . Thi s impli es that higher molecul ar weight fracti on in bitumen is increased with the inc rease in BI values and these are driven off partl y in eac h te mperature range of 600-750oe and 750-900oe. TTB-400 suffered a weight loss of about 2 1.0 pe r cent due to the decompos iti on of partl y lower molecul ar weight fraction and partly medium molecul ar weight fraction . The we ight loss of thi s pitch decreases to 32.5 per cent inbetween 450-600oe but increases to 37.7 per cent between 600-750oe, showing the enric hment by hi gher molecular weight fraction whi ch ultimate ly is dri ven off between 750-900°C.

The results indi cate that there is huge variation in the di stribu tion of various molecul ar we ight fractions in TTB -430. The lower and higher fraction s suffer decomposition up to 4500 e which is minimi zed between 750-900°C. The medi um and hi gh mol ecul ar weight frac ti ons which decompose between 450-750°C constitute the major benzene insoluble (BI) portion. The initi al decomposition temperature (IDT) for bitumen was found to be 245°C. which increases with increase in heat treatment temperature. The IDT for TTB-380 is fou nd to be 346°e which steadil y increases up to 597°e for TTB-530 . There is a slight decrease in IDT from 395 to 3800 e when heat trea tment temperature is rai sed from 400 to 4 10oe. Thi s may be due to the decomposition of medium molecular weight fraction into low molecular weight frac ti on. The results a lso indi cate th at stabi lity of the pitch increases wi th the rise in heat treatment temperature.

The decomposition of bitumen occurs in three steps with T max . at 310, 503 and 635°C. On increasing the temperature of heat treatment to 41 ooe, the first step di sappears with steep fa ll in T max. to 498°e for second stage. There is irregular trend in ri se of T max. and weight loss in different steps for bitumen and other pitches. On increasing the heat treatment temperature, the three steps decomposition is changed into two-steps between 410 - 4800 e and at 500-530oe, there is single step decomposition . The final decomposition temperature (FDT) of bitumen is 685°C. FDT of pitches obtained at 380, 390, 400 and 410 is 820°C. The FDT of higher temperature treated bitumen varies from 852 to 1000oe, thereby confirming the presence of polymerisation reacti ons.

The results show th at on thermal treatment of bitumen, the benzene in soluble (BI) content increases and consequently the hi gher molecul ar weight species increase. The hi gher molecul ar we ight spec ies are converted to lower molecular weight species on decompos ition at higher temperature. The di stribution of lower, medium and hi gher molecular weight species in thermally treated bitumen does not foll ow any regular pattern due to the compl exity of chemical constituti on of bitumen .

Activation Ener~y

The activation energy was ca lcul ated from thermograms by the fo ll owi ng methods :

(i) Arrhenius Method

Vant hoff' s equation In kldT = qlRf2 re lates the temperature vari ation of equilibrium constant k whi ch is measured at constant volume or pressure with the heat q, Arrhenius proposed to take the ve locity coeffi c ient k in to the fo ll ow ing equation :

dIn k IdT = E,.IRT 2,

where Eo is a measure of the difference in energies possessed by ac ti ve and pass ive molecul es of the reactants and known as ac tivat ion energy. By integrat ion, the fo llowing equation is obtained .

In k = constant - E" I RT.

k =:: A eE"/RT .

If a graph of log k vs l iT is plotted , a straight line is obtained (Figure 5) and s lope of which will give the ac ti vation energy Ea. From the thermograms of bitumen and pitches, a graph is pl otted in log k vs I IT x 10" The re lation of log k vs I IT comes out to be a straight line whose slope is the measure of the

880 J SCI IND RES VOL 62 SEPTEMBER 2003

activation energy . It IS calculated by the following expression.

Eo = 4 .6 x slope.

(ii) Coat and Redfern Method

The activation energy of various steps of thermograms of bitumen and pitches was also calculated with the help of equation suggested by Coat and Redfern, which is given below:

log I-(I- atn AR J = log

t(l-n) a E"

2RT Eo (1--)---

Ea 2.3 RT

where a is the thermal decomposition rate of the compound calculated from the formula a = (wo-w) I (W-Wk) where wo, Wk and ware initial weight, final weight and weight of compound at temperature T, respectively. 'A' is pre-exponential factor and n is order of reaction .

The thermo-grams of bitumen and pitches were used to determine the value of log PT/W, log k and log [- 111( 1- a)l r]. The log [- In(1 - a)/r] values were plotted against IITX 10' for each stage of decomposit ion. The straight line was obtained for each step indicating the order of reaction is one

(Figure 5).

(iii) Doyle Method

The activation energy was also calculated by the equation proposed by Doyle:

log PTIW = Eo I 4.6 T + log Z (RH),

where PT = -dw/dt and (RH) = heating rate and Z is frequency. When log PT IW is plotted against liT (Figure 6), the value of Eo is calculated by the relationship:

E" = 4.6 x slope.

A graph of first order reaction of thermograms from bitumen and thermal treated bitumen between log PT IW vs liT x lO' was plotted and the activation energy was calculated.

The activation energy En was calcu lated6-9 from

the slope of the line and the values thus calculated are

given in Table I .

The results reveal that the activation energies calculated by different methods are comparable in most cases. The activation energies of bitumen are 11 .76, 17.25 and 20.96 k cal/mol for step I, II, and Ill,

7.0 4.5

6.8 4.3

6.6 4.1

6.4 19

6.2 /' 3.7

Lo >< 35~

-,....... N -; ... ;:: 5.8 33 g'

1 ~

c .,

.L5.6 3.1 go ~

I

5.4 2.9

5.2 2.7

5.0 +--,--,--,--,--,---,---+-2.5 0.9 1.0 1.1 1.4 1.5 1.6

Figure 5 - Arrhenius plOl between log k vIs I IT x 10 .1 and Coat

and Redfern plot between -log [- In (l- u)IT 2] vIs IITX 101

1.4

1. 2

1.0

0.8

d~ '" .3 0.6 I

0.4

0.2

0 .0+--,--,--.--.--1.0 1. 1 1.2 1.3 1.4

tx 10J

I 1.5

I 1.6

I 1.7

Figure 6 - Doyle plot of lhermo-gravemetric analysis for thermal treated bitumens

respectively. This shows that the activation energy for thermal decomposition of lower molecular weight fraction is low, while the activation energy for higher molecular weight fraction decomposition is high . The activation energy for thermal decomposit ion of TTB-530 is 34.83 k cal/mol, thereby showing the higher

MAHESHWARI & PUNDHIR: MESO-PHASE PITCHES FROM THE RESIDUE OF OIL REFINERY 881

Table I - Activation energy of thermal treated bitumen by different methods

Name of Stages

sample

Bitumen

II

III

TTB-380

II

III

TTB-390

II

III

TTB-400

II

III

TTB-410

II

TTB-430

II

TTB-460

II

TTB-480

II

TTB-500

II

TTB-530

II

Activation energy, k cal/mol

Arrhenius Redfern

11 .76 9.52

17.25 12.27

20.96 18 .89

12.46

15.33

15.22

15.76

10.12

30.36

16.25

4.29

20.52

12.19

12.40

28 .57

19.82

15 .33

13 .58

14.07

11.50

16.62

21.16

34.83

15.64

12.45

11 .20

11.42

14.80

9.20

24.53

15.60

3.37

19.01

10.9 1

13.80

24.53

16.10

14.85

13.32

11.30

10.45

17.43

20.24

42.03

14.65

Doyle

·11.32

16.80

19.65

12.38

11.50

8.05

14.40

9.81

29.20

16.47

4.12

20.80

12.08

11.73

26.22

17.80

14.24

12.63

13.21

10.82

18.20

20.22

35.20

13.60

value of decomposition for high molecular weight fraction. This leads to the conclusion that polymerized oriented species have higher activation energy.

Applications

The meso-phase pitches prepared under different environment conditions at higher varing temperature can be used for the preparation of industrial

electrodes, needle coke, carbon fibre and high density graphite. The carbon products are also prepared using binder pitches and can be densified usi ng impregnating pitches. The carbon fibre can be used for the preparation of automobi le parts and body, light aircraft, sports goods due to its light weight and high tensile strength with negligible expansion coefficient. The steel can be replaced with the .carbon fibre in the construction industry due to its light weight and chemical inertness

Conclusions

The following conclusions are drawn from the study:

(i) The softening point and benzene insoluble progressively IOcrease with the IOcrease of thermal treatment temperature.

(ii) The coke value and weight loss increases with the heat treatment temperature .

(iii) X-ray studies show the development of crystal structure with the rise 10 heat treatment temperature.

(iv) A judicious combination of time and temperature of treatment can help in formation of the desired polymeric re-structure in bitumen .

(v) The thermal studies show that three stages decomposition of bitumen changes to two­stages and finally in single stage decomposition of thermal treated bitumen with IOcrease 10

initial decomposition temperature.

(vi) The activation energy of thermally treated bitumen is comparable in all the three methods and increases with the ri se in temperature. The order of reaction was found to be one.

Acknowledgement

The authors are grateful to Prof. P K Sikdar, Director, Central Road Research Institute, New Delhi for his kind permission to publish this paper.

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882 J SCIIND RES VOL 62 SEP1EMBER 2003

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