Study of carbonization using a tube bomb: evaluation of lump needle coke, carbonization mechanism and optimization

Study of carbonization using a tube bomb: evaluation of lump needle coke, carbonization mechanism and optimization lsao Mochida,

Takashi

Oyama”,

Yozo Korai and You Qing

Fei

Institute of Advanced Material Study, Department of Molecular Technology, University 86, Kasuga, Fukuoka 876, Japan * Marifu Refinery, KOA Oil Co., Ltd, Waki-cho, Yamaguchi 740, Japan (Received 12 October 1987; revised 15 January 1988)

Kyushu

The carbonization of three representative feedstqks derived from petroleum and coal tar was studied using a tube bomb, where the carbonization temperatur; and pressure are freely set up and carbonization progress is pursued at the desired stages. The coke lumps produced in the bomb are evaluated by optical anisotropy defined through a montage technique and ccefficient of thermal expansion, which are semi-quantitatively correlated. The carbonization into needle coke follows the three major stages: the formation and growth of liquid crystal spheres; their coalescence into bulk mesophase; and the uni-axial rearrangement of mesophase molecules into a solid coke. The nature of bulk mesophase and gas evolution for the rearrangement define the quality of the needle coke. These factors are sharply influenced by the reactivity of feedstocks, carbonization temperature and pressure. Chemical analyses of the feedstocks and the carbonization intermediates describe the chemistry of the carbonization reaction, providing a structural basis for the reactivity. Based on such a mechanistic understanding of the carbonization, guiding principles to optimize the carbonization conditions of respective feedstocks into their best needle coke are proposed according to their structure and reactivity. (Keywords: coke; carbonization; tube bomb)

Needle coke, which is an essential filler for principal graphite artefacts as well as graphite electrodes used in the steel manufacturing industry, has been produced in a delayed coker of large commercial scalerP5. Critical information on its small CTE (coefficient of thermal expansion), high density, high electric conductivity and low puffing trend is required to produce better characteristics that electrodes6-’ ‘. Such qualifying strongly reflect the coke structue are effectively determined by the natures of the feedstocks and carbonization conditions. However, logical relations between them are far from established because of the difficulty in examining a wide range of reaction conditions in the commercial coker. A laboratory preparation of real needle coke that is similar to the commercial product is required. The present authors”s12 have reported that carbonization in a tube bomb provided a lump of needle coke comparable with the commercial product from the same feedstock by adjusting the carbonization pressure, temperature and heating rate. Summarizing the properties of the coke lumps produced in the tube bomb under a variety of conditions from several feedstocks may highlight which conditions should be optimized. Thus, the carbonization reactivity of the feedstock should be properly evaluated to establish logical principles. This paper reviews a preparative procedure of needle coke lump in a tube bomb, basis for the evaluation of the coke lump, properties of cokes in relation to the carbonization conditions, the detailed description of carbonization progress, chemical characterization and carbonization reactivities of feedstock, their relation to 0016-2361/88/091171-lG3.00 c) 1988 Butterworth & Co. (Publishers)

Ltd.

the carbonization scheme, and the mechanism of needle coke formation. Based on this review, guiding principles to optimize the carbonization conditions of the particular feedstock into needle coke can be proposed. LABORATORY PREPARATION OF NEEDLE COKE LUMP AND ANALYTICAL PROCEDURES OF FEEDSTOCKS AND INTERMEDIATES Feedstock (z40 g) in an aluminium foil tube is carbonized in a stainless steel tube bomb (22 mm diameter, 150 mm height) that is heated in a sand bath at the prescribed temperature. The amount of gas evolved during the carbonization is quantified by measuring the volume of purged gas and is then analysed by gas chromatography. The carbonized products, at various times, are quenched in cold water and are recovered in the foil from the tube. The coke lumps and intermediates are sectioned in a direction parallel to the tube axis to evaluate the anisotropic development of the whole area by a reflected polarized microscope, after conventional mounting and polishing. The intermediate products are extracted with hexane (H), benzene (B) and quinoline (Q) to estimate the extent of carbonization on a chemical basis. ‘H-n.m.r. of the benzene soluble fraction (BS) in the intermediate products can be measured by FT-n.m.r., and carbon aromaticity is calculated from this data using the BrownLadner method’ 3. BS is analysed by t.l.c.-FID, g.c. and g.p.c. Starting feedstocks are also characterized by a similar procedure. Details of experimental procedures are given elsewhere11~12~14.

FUEL,

1988,

Vol 67, September

1171

Tube bomb

carbonization

Evaluation

study:

I. Mochida

et al. tend to have long chains. It should be noted that QIF contains a considerable amount of oxygen. FCC-DO shows intermediate values for the feedstocks. Their structural models are illustrated in Figure 2 to provide some ideas of their structural images, although they are never strictly correct, since the feedstocks consist of complex mixtures of hydrocarbons19-22. Thus, clarification of their structural distribution is required. Figures 3 and 4 illustrate structural distribution profiles of the feedstocks obtained by t.1.c. (functional composition) and g.p.c. (molecular weight), respectively. LSVR consists of 50% saturate, 30% aromatic, 4% resin and 16 % asphaltene. The former two fractions are isolated by column chromatography. The saturate fraction of the feed is essentially paraffnic and g.c. shows that this fraction consists principally of straight chain paraffins, ranging approximately from Cl3 to C35. Table 2 shows some properties of the aromatic fraction in LSVR. The aromatic fraction of LSVR carries very long alkyl chains, its fa and c appearing very small and large, respectively. Although the asphaltene fraction is difficult to characterize, it contains some condensed aromatic rings that are connected by methylene bonds into polymeric compounds. The fraction is believed to contain a number of alkyl substitutes and to form micelles, dispersing in the paraffinic and aromatic matrix as proposed by Yenz3. Sulphur is concentrated in the asphaltene. G.p.c. indicates its widely dispersed molecular weight, ranging from 800 to 7000. Such dispersed compositions may strongly influence the carbonization properties of the

of coke lump

A montage of coke lump is prepared from 100 sections of micrographs, to evaluate its anisotropic texture. Anisotropic units are evaluated by 1,” and f,, (average length of anisotropic unit vectors parallel to the CTE axis and average length of axial components) using the point count methodi2*i4. These indices are defined by the following equations, (Figure 1).

4,=(1/n) i

(1)

lj

j=l

A= (l/m) f fi i=l

A=
1,cos

2 i=l

(3)

fj

8,

(4)

where lj, length of each anisotropic unit vector (j); n, number of unit vectors in i column; m, number of columns in the coke;J, averagedfj it! i column parallel to the axis; fj, the axial component of an anisotropic unit vector in i column; tI,, angle of an anisotropic vector (j) to the CTE axis. The coke lumps are calcined at 1000°C for 1 h to measure their CTE values (from room temperature to SOOC). An intimate relation between CTE and optical anisotropy has been reported’ 4. FEEDSTOCKS FOR DELAYED PRODUCT NEEDLE COKES

COKING

TO

At present premium needle cokes have been produced from both petroleum and coal tar residues’ 5-1 ‘. These feedstocks include distilled residues of low sulphur crudes (LS), FCC-decant oil (FCCDO), naphtha cracked tar pitch (NTP) and their blends. Some of these feedstocks are modified or up-graded through catalytic or thermal processes. The latter feedstock is a coal tar pitch, free from quinoline insolubles (QIF). QI have been removed by anti-solvent precipitation, supercritical solvent extraction or filtration. In the present article, three representative feedstocks, the vacuum residues of LS (LSVR), FCCDO and QIF are reviewed. Structural

characteristics

Properties

QIF

ANISOTROPIC

Figure 1

Definition

UNITS

of anisotropic

unit vector

*

6

cHCH 0 0 0 CH3CH$H3

00

FCC

LS VR Figure

2

Structure

m 000 0

DO

0

0

CH3

QIF

model of feedstocks

of feedstocks analyses

(wt %)

Solubility

(wt %)

C

H

N

S

0 (diff.)

C/H

.K

fsb

HS

HI-BS

BI-QS

QI

86.2 88.9 91.6

12.5 9.5 5.1

0.4 0.1 1.3

0.2 0.4 0.7

0.1 1.1 1.3

0.6 0.8 1.51

0.18 0.54 0.88

0.52 0.32 0.13’

92 100 48

8 0 40

0 0 4

0 0 0

a Carbon aromaticity according to the Brown-Ladner ‘Degree of substitution of aromatic nucleus according ’ Hexane insoluble but benzene soluble

1172

AXI

of feedstocks

Elemental

LSVR FCCDO

BI3.B

kount)

The feedstocks have very different characteristics and their elemental composition, solubility and n.m.r. characteristics are summarized in Table 1. LSVR exhibits the lowest C/H ratio and aromaticity (fJ, while QIF has the highest upon analysis of its BS. The number of alkyl substituents and size of structural unit, according to Brown-Ladner method, are largest in LSVR, while QIF has the largest aromatic units. The alkyl groups in LSVR Table 1

z

n

TUBE

FUEL, 1988, Vol 67, September

method to the BrownLadner

method

Tube bomb carbonization study: I. Mochida et al. Table 2

Properties

of aromatic

fractions

Elemental

LSVR FCCDO “Carbon bDegree

of feedstocks

analysis

(wt %)

‘H n.m.r.

C

H

N

0 (diff.)

C/H

H,

H,

H,

H,

f,

ub

87.9 91.8

10.3 7.1

0.6 0.1

1.2 1.0

0.7 1.1

10 42

17 41

57 13

16 4

0.40 0.74

0.51 0.34

aromaticity measured by Brown-Ladner method of substitution of aromatic nucleus according to Brown-Ladner

saturate

aromatic

method

resin asphaltene I

uw 8%

i

3;oo

5zo 7600

A

I

13

12

11 RETENTION

10

9 TIME

5

7

6

5

(min)

Figure 4 Gel-permeation chromatograms LSVR; -.-.-, FCCDO; -----, QIF

of feedstocks:

p,

vacuum residue through their mutual interactions. Both FCCDO and QIF have much narrower molecular distributions as indicated by g.p.c. (800 to 2500), although the latter feed consists of BS (96 %) and BS-QS (4%) fractions. The former feed consists essentially of saturate (28 %) and aromatic (70 %) fractions as indicated by t.1.c. G.c. shows that this saturate fraction also consists of straight paraffins ranging from C,, to C,,. The aromatic fraction appears to carry more or less alkyl chains of variable length as shown in Table 2. Thef, of the fraction is 0.74, much higher than that of the same fraction in LSVR. It should be noted that the aromatic fraction contains a significant proportion of C=O groups as revealed by FT-i.r. QIF consists of aromatic, resin, asphaltene and BI-QS fractions as shown in Figure 2. They are all essentially polycondensed aromatic (3-10 rings) hydrocarbons of variable sizes with least alkyl and naphthenic groups, the f, being around 0.9 regardless of the fractions. It should be noted that this feedstock carries a considerable amount of phenolic group as indicated by FT-i.r. The oxygen content is also a major influence on the solubility, higher oxygen content leading to lower solubility. The aromatic rings are bonded through arylaryl (major) and methylene (minor) bonds, defining molecular weights and also solubilities of the fractions. whole

~XANE

/I

K BENZENE

>I

DICHWTHANE F&we 3 T.l.c.-FID FCCDO; C, QIF

chromatograms

of feedstocks:

A, LSVR;

PROPERTIES OF NEEDLE IN THE TUBE BOMB

COKES

PRODUCED

B,

Influences of two carbonization variables, temperature and pressure on the carbonization of feedstocks in the

FUEL, 1988, Vol 67, September

1173

Tube bomb

carbonization

Figure 5

Montage QIF at 500°C

Table 3

study: I. Mochida

micrographs

Anisotropic

of coke lump produced

development

Temperature

et al.

at the optimum

of coke lump produced

440

at various

temperatures

temperatures

460

at 480°C; c,

15 kgcm-2 500

520

.f”a”

&wb

f a”

&I> -

h\

I,,

f a”

1a”

f a”

1a”

Cf.4

Cw)

Olm)

Olm)

Olm) ___~

Olm)

Olm)

WI

Olm)

bm)

15.2 _

18.4 _

16.0 20.4

20.4 22.0

15.8 26.2

16.8 26.2

13.8 19.6

16.0 20.5

_ _

_

_

_

18.4

23.2

19.8

22.0

15.6

QIF y Average b Average

under

15 kg cm -‘: a, LSVR at 460°C; b, FCCDO

480 __-

(“C)

LSVR FCCDO

under

length length

18.2

of vectors parallel to the CTE axis of anisotropic unit vectors

Temperuture 3.0

2.0

i ” w IU 1 .o

0

UO

&60

480

CARBONIZATION

500

520

540

560

TEMPEFfATLRE(‘c 1

Figure 6 CTE value of coke lumps produced at various temperatures under 15 kgcm-’ from feedstocks: 0, LSVR; A, FCCDO; 0, QIF

bomb are anisotropy14,

1174

FUEL,

summarized by describing the optical and CTE of the resultant coke lumps.

1988,

Vol 67, September

Montage microphotographs of cokes produced from feedstocks at temperatures under 15 kg cmP2 pressure are illustrated in Figure 5. Each coke lump exhibits a needle gathering appearance, carrying pores in a direction parallel to the bomb axis, as observed in the commercial coke. Tub/e 3 and Figure 6 summarize the anisotropic evaluation according to the procedure described above and CTE values of coke lumps produced at various temperatures under 15 kgcm-’ pressure in the tube bomb, respectively. The values of_&\ and l,,, both of which are indices for the degree of uni-axial orientation of the anisotropic units, are strongly dependent upon the carbonization temperature. The CTE values of the cokes also vary, depending upon the carbonization temperature in the same manner as optical anisotropy. Each feedstock has its own optimum carbonization temperature, at which the highest values of both anisotropic indices and the lowest CTE are observed, while higher or lower temperatures deteriorate the coke quality. The optimum temperatures of the QIF, FCCDO and LSVR are found to be 500, 480 and 46O”C, respectively. It should be noted that the lowest CTE values, 0.3 x 10e6/“C, obtained by carbonization of FCCDO at 480°C is beyond that of commercial coke at present. The carbonization completion times are dependent upon both the temperature and the feedstock. LSVR and FCCDO take 2 and 3 h, respectively, for complete carbonization under 15kgcm-’ at 480°C. QIF takes more than 10 h to complete the carbonization under at 45O”C, while it takes only 1 h at 550°C. 15kgcm-’

Tube bomb

Pressure

CTE values of coke lumps produced under a series of pressures from LSVR and QIF23*24. Higher carbonization pressures tend to provide lower CTE values of coke lumps from both feeds at 500°C. It is often reported that the mesophase development is better under higher pressure3*“. However, QIF shows optimum carbonization pressure, giving the lowest CTE in carbonization at 550 and 470°C. At 55O”C, 15 kg cme2 is the optimum pressure while 0 kg cm - ’ is the optimum at 470°C. The cooperation of temperature and pressure, both of which influence the carbonization reaction, through the extent of thermal activation and volatiles held in the liquid phase, may define the optimum conditions for needle coke production.

The mesophase at the bottom appears to become slightly viscous at this stage because a few bubble pores are found. The anisotropic area grows rapidly after 65 min (Figure 7.5.1). The bulk mesophase regions spreads throughout the carbon to surround the spherecontaining isotropic regions. Flow textures in the bulk mesophase are almost completely parallel to the bomb axis (Figure 7.6.11). The carbonization was completed at 70 min. The carbonization schemes for other feedstocks or under different conditions follow the same steps as described above, although the rates of carbonization

Anisotropic texture of mesophase development Figure 7 shows a series of micrographs of the carbons

produced from FCCDO in a carbonization time course under 15 kgcm-’ at 500°C (Ref. 27). A number of small mesophase spheres are found in the major isotropic matrix 30min after the start of carbonization (Figure 7.2.1).

Feedstock LSVR

QIF QIF QIF

from LSVR and QIF under

Carbonization temperature (“C)

Measured temperature (RT) (“C)

500 550 500 410

100 500 500 500

volatlll2ationOgeneration of

Carbonization

TJppyrolysls

tion phase

of

of

meso-

mosaic

(kg cme2)

7

15

25

2.3 1.5 0.5

1.2 1.4 _ 1.0

0.8 1.3 0.5 1.2

0.8 2.0

@coalescenceaqrowth of

of

texture

1.9 0.1 -

_ _

of

@Increase

sphere @formation

bulka

of

ascending of

mesophase

mesophase

bulk

in bottom

40

_-__-

meso-

sphere

@formation

sphere @sedimenta-

33 convection

phase

pressure

0

-_-__

@growth

mesophase

pressure

range

e-e--

3

et al.

(Figure 7.4.11).

ANISOTROPIC TEXTURE AND CHEMICAL STRUCTURE OF CARBONIZATION INTERMEDIATES BEFORE SOLIDIFICATION

CTE values of coke lumps produced

study: I. Mochida

Although the middle part of the product still shows many large single spheres and associated coalesced spheres, with a number of small spheres after 45 min (Figure 7.34, a bulk mesophase layer where a broad anisotropic belt spreads parallel to the bottom surface is observed (Figure 7.3.11). The viscosity of the bulk mesophase appears rather low because no trace of bubble pore is found in the layer. After 60 min, anisotropic regions prevailed in the lump. Very small spheres were still dominant at the upper part of the lump, while pillars of bulk mesophase grow irregularly upwards from the lower layers of bulk mesophase (Figure 7.4.1). A high magnification reveals that flow textures grow in pillars parallel to the bomb axis from the layers of bulk mesophase belt near the bottom

Table 4 summarizes

Table 4

carbonization

of

viscosity

volatile

matter to

bub_its

arrange

mesophase

mesophase

sphere

Into

un-axial

flow

texture

@formation of

cigar-like

needle Figure 7 Series ofmontage are given in brackets

and microphotographs

ofcarbons

produced

in a time course under

15 kg cm _ * from FCCDO.

FUEL, 1988, Vol67,

Carbonization

coke times (min)

September

1175

Tube bomb carbonization

study: I. Mochida

et al. FCCDO exhibits a much reduced carbonization rate in terms of increased QI and decrease of the soluble fractions. The solidification may occur between 2.5 and 3.0 h after the carbonization started, while QI increases steadily from 22 to 36%. The amount of HS is much smaller than that from LSVR immediately before solidification, indicating a homogeneous phase during the carbonization. QIF reveals a slower carbonization as described before, producing maximum QI at 6.0 h. The increase in QI is steadier and more gradual in comparison with those from the former two feedstocks. Thus, the solidification may take place between 4.0 and 6.0 h, while QI increases from 38 to 67%. The amount of HS remaining just before solidification is very small (2%). The amounts of intermediate and its solubility in a carbonization time course for LSVR and QIF, at their respective optimum carbonization temperatures under 15kgcm-’ are shown in Figure 9. The optimum temperature (460°C) for LSVR reduces the rate of carbonization inducing a more gradual increase of QI, and the solidification is delayed to 2.5-3.011. In contrast, the optimum temperature of 500°C for QIF increased the rate of carbonization in terms of increase of QI and decrease of soluble fractions, the solidification taking place between 2.0 and 3.0 h, although the QI increase remained steady. It is suggested that the optimum conditions for the respective feedstock can provide a particular rate which results in completion of carbonization in 2-3 h.

Figure 8 Solubility of intermediates produced at 480°C under 15 kg cm-‘: unshaded area, ellhient; shaded area, HS; diagonal hatched area,HI-BS; vertical hatched area, BI-QS; black area,QI; A, LSVR; B, FCCDO; C, QIF

differ. Thus, the carbonization into needle coke follows two, rather independent stages: growth of anisotropy (formation of liquid crystal) and the rearrangement of its components parallel to the bomb axis at solidification. Compositional changes of the feeds during the carbonization Figure 8 shows the amount of intermediate and its solubility during the carbonization of the feedstocks at 480°C under 15 kgcm-‘. The temperature is optimum for FCCDO, but is too high for LSVR and too low for QIF. The LSVR, initially totally soluble in hexane, is decreased to 49 % of the starting quantity within 1 h. The intermediate remaining in the tube at this time consists of 26 % HS, 7 % HI-BS, 10% BI-QS and 6 % QI on a feedstock basis. A longer carbonization time further decreases the amount remaining in the tube and increases the content of QI. The carbonization is completed at ~2 h because the weight of QI at 2 h is the same as that at 6 h, although 27 % of QS remains in the tube. The period of solidification, which determines the final anisotropic texture, may be located between 1.5 and 2.0 h after the carbonization started, while QI increased from 7 to 17 o/o, dominating the intermediate. The increase of QI for the initial 1.5 h is rather slow and increases rapidly on solidification. It should be noted that 20% of HS remains just before solidification, since HS and QS may form separate phases.

1176

FUEL, 1988, Vol 67, September

Chemical structure of intermediates prior to solidification Table 5 summarizes analytical data and structural indices of the HI-BS fractions in the intermediate products just before the solidification of the feedstocks at 480°C. The fractions may constitute the matrix, influencing the viscosity of the carbonizing system. At the same time, their structural characteristics may indicate the chemical changes during the carbonization. The carbon aromaticity (f,) of the intermediate from QIF (QIF-I) is largest, whereas that of LSVR-I is smallest. In

Figure 9 Solubility of intermediates produced at the respective optimum temperature under 15 kgcm-‘: unshaded area, emuent; shaded area, HS; diagonal hatched area, HI-BS; vertical hatched area, BI--QS; black area, QI; A, LSVR; B, QIF

Tube bomb Table 5

Properties

of HI-BS

fraction

analyses

et al.

study: I. Mochida -2

in the intermediates produced just before the solidification at 480°C under 15 kg cm Elemental

LSVR-I FCCDO-I QIF-I LSVR-I-A’ FCCDO-I-A’

carbonization

(wt %)

Carbonization time (min)

C

H

N

0 (diff.)

fa”

2

C/H

90 150 240 90 150

91.2 92.6 92.8 91.6 92.8

5.8 5.8 5.0 7.0 6.0

1.0 0.1 1.0 0.3 0.1

2.0 1.5 1.2 1.1 1.2

0.72 0.89 0.96 0.74 0.88

0.37 0.17 0.08 0.30 0.19

1.3 1.3 1.5 1.1 1.3

“Carbon aromaticity measured by Brown-Ladner method bDegree of substitution of aromatic nucleus according to Brown-Ladner c Aromatic fraction separated by column chromatography

saturate

method

promat is resin * ,

I

I

1

MW

14

13

800

12

11

LSVR; -.-.-,

x00

40

3700

RETENTION Figure 11

t

,

G

IO

8

9

TIME

Gel-permeation chromatograms FCCDO; ---, QIF

J 6

7

(min) of feedstocks:

p,

the number of substituted groups per unit structure of QIF-I is smallest, and that of LSVR-I is largest. FCCDO-I shows intermediate values in terms of these structural indices. Thus, the intermediates of the feedstock strongly reflects its initial structure. Figure 10 illustrates t.1.c. compositions of BS fractions in LSVR-I and FCCDO-I, respectively. The fraction LSVR-I consists of 9 % saturate, 67 % aromatic, 9 % resin and 15% asphaltene. G.c. shows that the saturate consists of straight paraffins, ranging from C, 3 to C,,. It should be noted that these straight paraffins remained in the intermediate, although their content is much lower than that in the starting feedstock. The aromatic hydrocarbons still carry a considerable amount of long side chains. The fraction FCCDO-I essentially consists of aromatic hydrocarbons, comprising mainly short alkyl chains. The saturate in FCCDO disappears completely during the carbonization, suggesting its high reactivity during pyrolysis. Figure 12 shows gel permeation chromatograms of HIBS fractions of the intermediates. Molecules in the LSVRI fraction are largest and most dispersed in spite of the short carbonization time and its complete solubility in benzene, while those of FCCDO are smallest and their distribution is narrowest. Molecules in the QIF-I fraction have a size and distribution intermediate to the other two feedstocks. Such structural characteristics may suggest structural continuity of the carbonization intermediates contrast,

B

>r

K HEXANE K BENZENE

h

DICHLOR&iii?4NE Figure 10 T.l.c.-FID A, LSVR; B, FCCDO

chromatograms

of BS fraction

of intermediates:

FUEL, 1988, Vol 67, September

1177

Tube bomb Table 6

carbonization

Analysis

VR FCC

QIF

et al.

study: 1. Mochida

of evolved gases

(mol%)

CH,

CA

CA

C,H,

46.3 88.0 16.9

19.4 9.3 15.2

3.2 0.1 5.4

12.0 1.6 0.1

C& 4.3 0.2 0.2 ___-

C,H,o

C&

C,H,o

c,+c*

5.8 0.6 0.7

3.8 0.1 0.3

5.2 0.1 1.2

68.9 91.4 91.5

Gas evolution and its composition

0

30

60

90

120

150

160

210

240

270

300

330

360

Figure 12 Profile of gas evolution during carbonization at 480-T under 15 kg cm-* pressare: A, LSVR; 0, FCCDO; 0, QIF. Region of solidification for: (1) LSVR; (2) FCCDO; (3) QIF

Figure 12 illustrates profiles of gas evolution during the carbonization of the respective feedstocks at 480°C and 15 kg cm-*. The regions of the solidification are also indicated. The amount of gas evolved from LSVR increases sharply to a maximum at 40 min, and then decreases to zero at 150 min. The rate of evolution at 1.52.0 h, which is the region of solidification, decreases from 35 to 8 ml/10 min. The quantities of gas evolved from FCCDO increases to reach a maximum rate at 50 min, which is much lower than that from LSVR and then decreases gradually. The rate of evolution in the solidification range decreased from 14 to 6ml/lOmin. In marked contrast to these feedstocks, the amounts of gas evolved from QIF is very small, and the evolution rate on solidification is as little as 2 ml/ 10 min. The profiles of gas evolution from the three feedstocks at their respective optimum carbonization temperatures are shown in Figure 13. The evolution of gas from LSVR at 460°C is more delayed than that at 480°C. The rate of gas evolution at solidification decreases from 13 to 3 ml/10 min, which is similar to that observed in FCCDO at 480°C. The amount of gas evolution from QIF at 5OO”C, its optimum temperature, is a little more upon solidification than at 48O”C, although at 500°C it is still <6 ml/10 min, which is much smaller than that from FCCDO at 480°C. The carbonization pressure hardly influences the rate of gas evolution, however it varies the solidification time so that the amount of the gas evolution at the solidification is effected by the pressure. Table 6 shows the components of evolved gas from the respective feedstocks at the solidification stage. The gases evolved from FCCDO and QIF consist almost exclusively of C,-C, hydrocarbons, although that of LSVR contains a considerable amount of C,-C, hydrocarbons. The composition and quantity of evolved gas should reflect the structure, especially alkyl side chains, of the intermediates described in the previous section. CARBONIZATION COKE

TIME(min)

Figure 13 Profile of gas evolution during carbonization at optimum A, LSVR at 460°C; 0. temperatures under 15 kg cm-* pressure: FCCDO at 480°C; 0, QIF at 500°C. Region of solidification for (1) LSVR; (2) FCCDO; (3) QIF

from FCCDO and QIF, while that from LSVR still carries paraffinic natures of straight paraffins and long side chains, the heavy asphaltene and mesophase fractions being anxious to precipitate.

1178

FUEL, 1988, Vol 67, September

SCHEME

INTO

NEEDLE

The carbonization scheme for anisotropic development observations of of microscopic using a series carbonization intermediates and products: a consecutive step including the formation of mesophase spheres, their growth and coalescense into bulk mesophase of flow texture28-35 has been proposed by many researchers. This carbonization scheme has been established to show a route for the formation of large isochromatic units of optical anisotropy observed in the needle coke. However, how the needle gathering structure of a commercial needle coke is induced in the delayed coker drum is still unknown, although the gas evolution is assumed widely as a driving force for the uni-axial arrangement of flow

Tube bomb

Figure 14

Model of needle coke

texture in the coke2,3*28*36-38. The scheme for carbonization into typical needle coke can be illustrated as shown in Figure 7 (Ref. 27) based on results described in this article. The formation of anisotropic spheres is followed by their growth, coalescence and precipitation to form the bulk mesophase, first at the bottom of bomb and then gradually to the whole region. The extent of anisotropic development is strongly dependent upon the carbonization conditions, as well as the carbonizing feedstock39-50. The bulk mesophase is rearranged into the flow texture parallel to the bomb axis by gas evolution at the point of solidification of the mesophase into a solid lump of needle coke. The mesophase components of aromatic molecules are orientated along bubble pores, which are traces of evolved gas, being solidified as shown in Figure 14. Thus, the timing and volume of gas evolution on solidification of the mesophase determine the extent of orientation of the resultant coke. This timing is strongly influenced by the temperature and pressure of carbonization, as well as the structure and reactivity of the intermediates as described above. The extent of orientation is also dependent upon the viscosity of the bulk mesophase at the start of solidification. Again, the structure and reactivity of the intermediate under the carbonization conditions are influential. It should be noted that the three major factors to determine the quality of needle coke, anisotropic development, viscosity of bulk mesophase and gas evolution, may require optimum conditions in relation to the structure of feedstocks.

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allowing sufficient growth and coalescence of the mesophase spheres to give bulk mesophase of low viscosity, since the increase of QI and disappearance of The optimum soluble fractions are moderate. carbonization temperature may also assure high mutual solubility of the components in the carbonization also favourable for the anisotropic intermediate, development through a better cocarbonization scheme. Thus, the major portion of the heaviest fraction of the the matrix, is reactivity, remaining in highest cocarbonized to follow the carbonization steps. Such may allow the development of cocarbonization homogeneous anisotropy in the resultant coke without precipitation to the bottom, reducing formation of the bottom mosaic coke. At the same time, the amount of gas evolved at solidification is sufficient to rearrange the bulk mesophase into needle-like orientation, because the carbonization under appropriate conditions is completed within an adequate period. Thus, the best needle coke is produced. At a too higher carbonization temperature, all the changes take place too rapidly, and there is little time for growth of an isochromatic area. The development of bulk mesophase and maximum evolution of turbulent gas tend to overlap, so that the textures are arranged randomly to give flaky cokes. The most reactive fraction yields its condensed product through rapid coking in an early stage of the carbonization, while medium fractions are quickly lost forcing the precipitation of the condensed fraction to the bottom, where the mosaic coke is formed. At a too low temperature, the carbonization progress is slow, however the cocarbonization of the very components may not work properly because of their different reactivity and lower mutual solubility at this temperature. The gas evolution was delayed and was very low at solidification, and thus the uni-axial arrangement of the bulk mesophase was not observed. A higher pressure is effective in delaying solidification, by maintaining the lighter fractions in the carbonization system to dissolve the heavier fractions, thus favouring the growth of mesophase. Since the pressure is not a

CORRELATION BETWEEN CARBONIZATION REACTIVITY OF FEEDSTOCK AND CONDITIONS Figure

15 illustrates schematic models of carbonization progress at three representative temperatures in terms of changes in viscosity of the carbonizing substances, its solidification range and gas evolution24’27. The viscosity increase reflects the rates of condensation (QI formation) and devolatilization of the soluble fraction, while the gas evolution shows the pyrolytic reaction of the carbonizing substances providing the gaseous products, except for the very initial stage where distillation may occur. The profiles may vary greatly, depending on both the carbonization temperature and pressure, as well as the reactivity of the feedstock. At the optimum temperature and pressure, the increase of viscosity after reaching its minimum is moderate,

:

I

K

higher temp. optimum temp.

K K

lower temp. >I timeof

bulk mesophase formation

TIME Figure 15 Carbonization schemeat three representative temperatures: - - - - -, higher temperature; -----,optimum temperature;-.-.-,lower temperature; 0, solidification zone; black arrow, higher pressure; white arrow, lower pressure

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major influence on the gas evolution (due to unimolecular pyrolysis), the gas evolution at high pressure tends to occur prior to solidification. No rearrangement of the bulk mesophase is achieved at the carbonization temperature when rapid gas evolution takes place. Thus, too higher carbonization pressure (over 40 kg cm- ‘) deteriorates the uni-axial arrangement of the mesophase, although the units of the mesophase grow well. Such discussion clearly indicates the existence of optimum carbonization conditions (temperature and pressure) relative to the reactivity of feedstocks, since the best coke is produced through a balance of the mesophase development and gas evolution. The most reactive LSVR, requires the lowest structure, of most diverse carbonization temperature and highest pressure for the production of better needle coke. Its asphaltene fraction tends to precipitate unless the matrix maintains the high dissolution activity. FCCDO of moderate reactivity, which appears the best feedstock for needle coke, exhibits optimum pressures at 500 and 480°C. Although its aromatic fraction is certainly a major source for the coke, their alkyl groups and the paraffin fraction may also participate in the carbonization reaction, initiating some of the condensation reaction. The least reactive feedstock, QIF, shows an optimum pressure at 55O”C, however, it is better to use a lower temperature (500°C) and higher pressure to balance the solidification and gas evolution. The viscosity of bulk mesophase from QIF may be very low on solidification, since it contains highly aromatic components, which assure high thermal stability and hence reduce the increase of viscosity at this stage. The small gas evolution observed may allow rearrangement because of the low viscosity of the intermediate. CORRELATION BETWEEN REACTIVITY STRUCTURE OF THE FEEDSTOCK

AND

The carbonization reaction consists of pyrolytic radical reactions, which proceed through radical initiation, propagation, recombination (condensation) and termination. The radicals are stabilized through hydrogen transfer or coupling to terminate the chain reaction. Thus, the reactivity of the feedstock is defined by the extent of radical initiation and termination. The aromatic hydrocarbons are thermally very stable, while the alkyl substituents or some C-O bonds are very reactive and give radicals. Hydrogen transfer is a major termination mechanism with no increase of molecular size to suppress the radical reaction, and the carbonization reaction is delayed through this reaction. Thus, the carbonization reactivity is related to the structural indices, aromaticity, number of alkyl chains and number of naphthenic rings, and these reflect the thermal stability, the reactivity for radical initiation and the hydrogen transferral stabilization, respectively. Hence, the reactivity of the feedstocks studied is in the order: CTP-QIF < FCCDO < LSVR deduced from their structural characteristics as described above, reflecting the decreasing optimum carbonization temperatures. Phase separation may accelerate the condensation reaction of heavy precipitates, since dilution of radicals may decrease the chance of radical coupling. Their solidification is also accelerated since no dissolution is expected. Such a phase separation takes place when the mutual solubility of the components is low or the

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common solvent fraction is lost. A mixture of long paraffins and heavy aromatic hydrocarbons (e.g. asphaltene) may show the greatest phase separation. The diverse structure distribution of LSVR tends to create greatest phase separation, producing more mosaic and bottom coke. The gas evolved at solidification is a product of pyrolysis. It should be noted that the distillation of a lighter fraction may not contribute at solidification. Hence, the intermediate should contain a source for gas evolution. Short chain alkyl substituents and naphthenic hydrogens may be major sources of such gaseous product in FCCDO and QIF. Long paraffins in LSVR that are fairly stable in the carbonization may decompose gradually to contribute to the gas evolution at solidification. Modi$cation of reactivity of the feedstock Modification of reactivity of the feedstock is expected to give better coke. The general approaches for modification can be classified into two categories: first, the modification of the chemical structure of the feedstocks and second, blending to modify the composition. Examples of the first approach are hydrogenation, hydrocracking, pyrolytic treatment and removal of heavy fractions. The merit of these treatments has been discussed in terms of larger isochromatic units of optical anisotropy39-46*51-54. Gas evolution upon solidification that has not been focus& should also be taken into account for the production of better needle coke. The second approach is connected to the idea of cocarbonization or blending technique29*55-60, which has been applied in a commercial delayed coker15. The mechanism is not fully understood, although the of cocarbonization to improve the mechanism anisotropic texture has almost been established. The influence of the blend ratio on the quality of the needle coke is documented. It is necessary to study in detail the roles of additives or feedstock components for the design of better needle coke, taking account of the rather independent carbonization steps, namely the growth and rearrangement of anisotropic units. There are several ideas to modify the carbonization physically. It is reported that a steam stream in the coke chamber improves the arrangement of the resultant coke61. A lower pressure during the carbonization may improve the arrangement at the solidification stage, while higher pressure can maintain the volatile matter at the initial stage6’.

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I1

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