Petroleum-Derived Carbons - American Chemical Society

16 Carbonization and Coke Characterization Harald Tillmanns

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Sigri ElektrographitGmbH,Werk Griesheim, 6230 Frankfurt (M) 83, Federal Republic of Germany

When the carbonization process is divided into its dis tinct physical and chemical parts and both are considered according to their contributions to the over a l l process, only then is a description of the mechanism possible. Carbon precursors and the products of their carbonization are characterized by various test methods whose objectives can be the control of coking, a description of the carbon or the determination of its suitability for further application. This paper considers the significance of selected common charac terization procedures. The carbonization of natural or i n d u s t r i a l hydrocarbon mixtures annually produces about 2.2 b i l l i o n tons of s o l i d carbon which, depending on the f i n a l heat treatment temperature, i s called coke or graphite. A wide range of premium carbons that are not based d i r e c t l y on coal, produced by p l a s t i c phase pyrolysis of fusible or l i q u i d isotropic hydrocarbons, constitute about one percent (22 m i l l i o n t/a) of the t o t a l s o l i d carbon material produced; included are: % of Total Production Highly orientated carbons (e.g., needle coke)

5

Mosaic structured carbons (e.g., regular coke)

75

Nearly isotropic structured carbon (e.g., binder coke)

13

Other carbon products (e.g., isotropic coke, glassy carbon and carbon fiber)

7

0097-6156/86/0303-0215S08.25/0 © 1986 American Chemical Society

Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

PETROLEUM-DERIVED CARBONS

216

Three i n d u s t r i e s produce carbon p r o d u c t s based on the c a r b o n i zation process:


• • •

crude o i l and c o a l t a r r e f i n i n g , carbon m a n u f a c t u r i n g , and Fe and A l m e t a l l u r g i c a l i n d u s t r i e s .

I n d u s t r i e s concerned w i t h the c a r b o n i z a t i o n p r o c e s s , the feed s t o c k s , or the p r o d u c t s t h e r e o f are i d e n t i f i e d i n F i g u r e 1. The areas of a c t i v i t y of the d i f f e r e n t i n d u s t r i e s I n t e r a c t , as shown by the o v e r l a p p i n g of the carbon products of common I n t e r e s t . Basically the same t e s t i n g methods are used f o r either studying the carbonization process or characterizing carbon materials. I n s t u d y i n g c a r b o n i z a t i o n , however, the t e s t i n g p r o cedures a r e a p p l i e d under n o n - s t e a d y s t a t e c o n d i t i o n s (so c a l l e d " i n s i t u " measurements) or a r e a p p l i e d under steady s t a t e c o n d i t i o n s w i t h the r e a c t i o n s p l i t i n t o s i n g l e s t e p s by b a t c h type r e a c t i o n systems. There are two b a s i c t e s t i n g s t r a t e g i e s f o r the s e v e r a l o b j e c t i v e s of s t u d y i n g carbon m a t e r i a l s o r the c a r b o n i zation process: First:

Measure p h y s i c a l and c h e m i c a l p r o p e r t i e s of the m a t e r i a l ( f e e d s t o c k , i n t e r m e d i a t e p r o d u c t , or f i n a l product).

Second:

Measure " e m p i r i c a l d a t a " to d e s c r i b e these are strongly dependent on procedure.

Objectives:

D e s c r i b e carbon m a t e r i a l p r o p e r t i e s o r the changes properties during manufacturing. P r e d i c t manufacturing baking, g r a p h i t i z i n g ) .

behavior

(coking,

Predict properties of the product a r t i f a c t s , graphite a r t i f a c t s ) . Predict applications

the m a t e r i a l ; the testing of

calcining,

(coke,

carbon

behavior.

Measurement of p h y s i c a l o r c h e m i c a l p r o p e r t i e s r e s u l t i n r e p r o d u c i b l e d a t a which depend o n l y on the p r o p e r t i e s of the sample i f proper t e s t i n g methods are u s e d . I n c o m p a r i s o n , measurement of " e m p i r i c a l d a t a " i s determined m a i n l y by sample p r e p a r a t i o n or t e s t i n g parameters and no p u r e l y m a t e r i a l - d e p e n d e n t p r o p e r t i e s can be e v a l u a t e d . These p r i n c i p l e s must be c o n s i d e r e d i f r e s u l t s of d i f f e r e n t t e s t i n g methods a r e compared; r e l i a b i l i t y of r e s u l t s and c o n c l u s i o n s a r e c o n s t r a i n e d by the v a l i d i t y of the t e s t i n g method. A g e n e r a l d e s c r i p t i o n of the c a r b o n i z a t i o n p r o c e s s e s u s i n g the c h a r a c t e r i z a t i o n methods f o r w i d e l y used coke p r o d u c t s l i k e b i n d e r c o k e , r e g u l a r c o k e , and needle coke i s g i v e n i n t h i s r e v i e w . The


16.

TILLMANNS

217

Carbonization and Coke Characterization


p r i m a r y aim o f the a u t h o r i s to p r e s e n t the f o l l o w i n g aspects of carbonization, based on the schematic mesophase- and c o k e - f o r m a t i o n g i v e n i n F i g u r e 2:

t h r e e main model of

1.

d i f f e r e n t i a t i n g c h e m i c a l and p h y s i c a l p r o c e s s e s ;

2.

determining the i n f l u e n c e of q u i n o l i n e s t i t u e n t s i n the c o k i n g f e e d s t o c k s ; and

3.

measuring the c a r b o n i z a t i o n p r o c e s s k i n e t i c s .

insoluble

con

The d i f f e r e n t m a n u f a c t u r i n g t e c h n i q u e s f o r h i g h l y s p e c i a l i z e d carbon p r o d u c t s ( i . e . , i s o t r o p i c c o k e , g l a s s y c a r b o n , and carbon f i b e r ) a r e not covered by t h i s p a p e r . Moreover, a d i s c u s s i o n of the m u l t i p l i c i t y o f t e s t i n g methods used to study the c a r b o n i z a t i o n b e h a v i o r o f d i f f e r e n t f e e d s t o c k s and to c h a r a c t e r i z e d i f f e r e n t coke q u a l i t i e s i s a l s o beyond the scope o f t h i s p r e s e n t a t i o n . P h y s i c a l and Chemical P y r o l y s i s

Processes

Two a s p e c t s o f the p y r o l y s i s p r o c e s s must be d i s t i n g u i s h e d : •

d i s t i l l a t i o n as the p h y s i c a l a s p e c t and

•

t h e r m a l d e c o m p o s i t i o n as the c h e m i c a l

aspect.

The o b j e c t i v e s o f t h e t e s t i n g methods used a r e to s e p a r a t e the p h y s i c a l and c h e m i c a l a s p e c t s and t o d e f i n e p a r t i c u l a r s t a t e s during p y r o l y s i s . A number o f assumptions have to be taken i n t o c o n s i d e r a t i o n i n e v a l u a t i n g the proposed model and u n d e r s t a n d i n g i t s l i m i t a t i o n s and range o f i n t e r p r e t a t i o n s . These a r e : •

The i n i t i a l m a t e r i a l i s a pure hydrocarbon c o n t a i n i n g o n l y hydrogen and carbon o r the heteroatom c o n t e n t i s so s m a l l t h a t i t has no e f f e c t on the d a t a w i t h i n the p r e c i s i o n o f the t e s t i n g method.

•

P y r o l y s i s i s a combination decomposition.

•

C a r b o n i z a t i o n i s pure t h e r m a l d e c o m p o s i t i o n forming a v o l a t i l e , low m o l e c u l a r weight f r a c t i o n and a r e s i d u a l high molecular weight f r a c t i o n by d i s p r o p o r t i o n a t i o n reactions.

•

Thermal d e c o m p o s i t i o n

•

I n c r e a s e d temperature d e c r e a s e s the hydrocarbons d i s t i l l e d i n p y r o l y s i s .

of

distillation

and t h e r m a l

i s dominated by r a d i c a l r e a c t i o n s . H/C r a t i o

of

the

Pure decarbonization is not relevant for hydrocarbon p y r o l y s i s , but may be o f i n t e r e s t f o r h i g h temperature treatment o f s o l i d carbon m a t e r i a l l i k e c o k e .



REFINERY COKE,PITCH CARBON

IND

ELECTRODE STEEL


F i g u r e 1.

R e l a t i o n o f carbon h a n d l i n g

A) ISOTROPIC RAW MATERIAL B)

IND

ISOTROPIC RAW MATERIAL

>CROSSLINKAGE WITHOUT ORIENTATION CONDENSATION AND CROSSLINKAGE WITH ORIENTATION

fc

industries.

ISOTROPIC MICRO AND MACRO STRUCTURE OF THE COKE .ANISOTROPIC MICRO AND MACRO STRUCTURE OF THE COKE

RAW MATERIAL DISTILLATION OIL

i

CONDENSATION

PRIM.CROSSLINKAGE ψ

CRISTALLOID PITCH (MESOPHASE)

REACTIVITY OF FEEDSTOCK| STRUCTURE OF FEEDSTOCK COKING CONDITIONS MICRO AND MACRO ISOTROP. COKE (GLASSY CARBON)

_ / SEC. OR I ENTAT I ON COAGULATI?N WITH SEC.ORIENTAT ION

AGGLOMERATION AND PART.COAGULAT ION

AGGLOMERATION WITHOUT SEC.OR I ENTAT ION

FIBER STRUCTURE NEEDLE COKE

MOSAIC STRUCTURE REGULAR COKE

MICRO MOSAIC STRUCTURE ISOTROPIC COKE

F i g u r e 2.

Model o f coke f o r m a t i o n .



16.

TILLMANNS


219

Different processes contributing to pyrolysis are l i s t e d In Figure 3. There Is a basic difference between the f i r s t and the subsequent three processes. Distillation removes molecules unchanged i n atomic composition, whereas the other three processes cause a degradation of the molecules. Molecular degradation results i n lower molecular weight fractions (which are v o l a t i l e ) and i n a residual f r a c t i o n consisting of non-volatile constituents and recombined r a d i c a l fragments which form high molecular weight components. A s i g n i f i c a n t difference Is observed i n the composition of the v o l a t i l e products. As i l l u s t r a t e d i n Figure 4, the hydrogen content of the t o t a l v o l a t i l e s i s reduced with increasing d i s t i l l a t i o n temperatures. In contrast, carbonization produces an increase of the hydrogen content of the t o t a l v o l a t i l e products as carbonization progresses. Experimental data shown i n Figure 5 demonstrate how spin concentration and the formation of methane and hydrogen indicate the beginning of carbonization and the resultant s i g n i f i c a n t change i n the H/C r a t i o of the v o l a t i l e matter. A carbonization model results i f i t i s assumed that, apart from d i s t i l l a t i o n , the carbonization processes of the pyrolysis can be described by three parameters: 1.

r e l a t i v e weight loss or residue,

2.

r e l a t i v e carbon content, and

3.

r e l a t i v e hydrogen content.

Using these three parameters the carbonizing materials or the carbonizing part of the i n i t i a l material can be characterized i n any reaction state by a point laying on the triangle area A-B-C i n Figure 6. The t h e o r e t i c a l amount of mesophase (characterized by a hydrogen content of 3.5%) formed i s described by the area of D-H-F for decarbonizing materials and the area of H-F-G-C f o r dehydrogenating materials given i n Figure 7. The Intersection of area A-B-C with area D-H-F and the area H-F-G-C represents the state of 100% mesophase defined by the hydrogen content of 3.5%. The state Is given In Figure 8 by the dotted l i n e between E-F and F-C. Pyrolysis follows a reaction path described schematically i n Figure 8. The starting material ( I ) Is characterized by i t s hydrogen and carbon content and i t s mass i s related to the mass of the i n i t i a l material at the start of carbonization ( I I ) . Distill a t i o n reduces the mass from I to the 100% point ( I I ) ; the reduced carbon and hydrogen content are depicted between I and I I . Pure d i s t i l l a t i o n , which i s not part of the carbonization, i s followed by the thermal degradation II to I I I ; thus weight loss and changes in the hydrogen and carbon content are depicted. At point I I I , theoretical 100% mesophase i s formed, which f i n a l l y i s converted to pure residual carbon (IV). 1

1



Distillation c

n m

^" n r r /

H

c

H

Dehvdroqe nation C H n

» . n C • m-Hf

m

Decor bonisation C H n

^ C

m


^

n

.

x

H +xCf m

n-Cf • m-H/

Carbonisation c

n m

*

H

c

n-x m-y* H

c

x y' H

^"C _ + m-y-Hf • C H f n

x

x

y

F i g u r e 3. P o s s i b l e r e a c t i o n s d u r i n g the c a r b o n i z a t i o n .

H

Cn m

C H H s

r

2

Distillation

dT

ϋ

2

d H/C d2 * 0 T

Figure 4.

D i s t i l l a t i o n and c a r b o n i z a t i o n .




TILLMANNS

F i g u r e 6-

Model of

carbonization.



Max. mesophase yield % residue

!'\ '

•χ.

\

'

Ν N

ι

1

\


— *

'G

y F i g u r e 7.

Model of

carbonization.

Max. coke yield/residue

F i g u r e 8.

Model o f

carbonization.


16.

TILLMANNS


223

Thus the pyrolysis of hydrocarbons can be generally described by the following information provided by the proposed model: • • • • •

H/C r a t i o of the starting material ( I ) , amount of evaporable or d i s t i l l a b l e material (Ι', I ) , H/C r a t i o of the i n i t i a l material at the start of carbon ization (II), t h e o r e t i c a l y i e l d of mesophase (III) i n comparison to residue with hydrogen content of 3.5 %, and carbon y i e l d (IV) based on the i n i t i a l material at the start of carbonization.


This model characterizes the reaction path of the hydrocarbon pyrolysis by four reaction states (I-II-III-IV), with each reaction state being described by three parameters: • •

r e l a t i v e hydrogen content, r e l a t i v e carbon content, and

•

residue related to the start of carbonization.

Influence of Quinoline Insolubles The model described i n the preceding section did not take into account any factor which might influence the structure of the coke formed. As can be seen i n Figure 9, quinoline insolubles are formed above the temperature where weight loss has begun. In addition to the r e a c t i v i t y of the i s o t r o p i c - l i q u i d phase, quinoline Insoluble p a r t i c l e s within hydrocarbon mixtures are of s i g n i f i c a n t importance i n Influencing the structure of the coke formed during carbonization. Generally, two d i f f e r e n t types of quinoline i n s o l u bles can be distinguished: •

primary or o r i g i n a l type of quinoline insolubles which are only poorly defined o p t i c a l l y and have a low hydrogen content of 1.5%; and

•

secondary or mesophase type of quinoline insolubles which are s p h e r i c a l l y shaped, are characterized by well-known o p t i c a l structure, and have a hydrogen content of 3.5%.

The f i r s t question that arises with respect to the carboni zation reaction i s whether the d i f f e r e n t types of quinoline insolubles cause differences i n the formation rate of quinoline insoluble mesophase. Figure 10 indicates there i s no difference i n the formation rate of primary and secondary types of quinoline insolubles. The formation rate depends not s i g n i f i c a n t l y on the type but strongly on the amount of quinoline insolubles present during carbonization. The data In Figure 10 show that the f o r mation rate at 400 °C Is Increased from 2% Ql/h up to 10% Ql/h i f the quinoline insolubles content i n the s t a r t i n g material i s increased from 5 to 25%. The formation rate of quinoline i n s o l u bles i s related nearly l i n e a r l y to the quinoline insolubles content of the s t a r t i n g material, being (at 400°C i n t h i s experiment) 0.4% per hour per % quinoline insolubles of the starting material.


224


vo

100 total

initial material

50

0

200

1000 °C

£ 100 Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch016

material

ι ι Ql of the residue

Ql of the initial material

0 F i g u r e 9.

200

600

1000 °C

Weight l o s s and t he Q l f o r m a t i o n .

%QI/h SOAKING AT 400 "C

oORIGINALLY NO Ql A PRIMARY TYPE OF Ql xSECONDARY TYPE OF Ql

12 Pu Ο

Ο

ο tu

0

5 15 25 % ORIGINAL CONTENT OF Q l

F i g u r e 1 0 . F o r m a t i o n r a t e of Q l as a f u n c t i o n o f t h e o r i g i n a l Ql c o n t e n t .


16.

TILLMANNS

225


However, as shown i n F i g u r e 1 1 , the type o f q u i n o l i n e i n s o l u b l e s s t r o n g l y i n f l u e n c e the s t r u c t u r e o f the coke formed. Primary q u i n o l i n e i n s o l u b l e s l e a d to an i s o t r o p i c coke w i t h a v e r y h i g h coefficient o f t h e r m a l e x p a n s i o n (16x10 ~ 6 / K ) . i contrast, secondary q u i n o l i n e i n s o l u b l e s a l s o produce n e a r l y i s o t r o p i c c o k e , but they h a r d l y a f f e c t the c o e f f i c i e n t of thermal expansion n

(0% s e c . QI = 3.5x10 ~ 6 / K ; 20% s e c . QI = 4.1x10 ~6/κ).

There e x i s t s a wide range o f o t h e r v a r i a b l e s which a f f e c t coke s t r u c t u r e ( e . g . , s o l u b l e r e a c t i v i t y , c o k i n g c o n d i t i o n s , and a d d i t i v e s ) which a r e not d i s c u s s e d h e r e . However, f o r a l l t e c h n i c a l h y d r o c a r b o n s , and e s p e c i a l l y f o r the c o k i n g of c o a l based t a r s and p i t c h e s , q u i n o l i n e i n s o l u b l e s have the dominant i n f l u e n c e on t h e structure.


K i n e t i c s of C a r b o n i z a t i o n A number of methods a r e used to e v a l u a t e a d a t a s e t to d e s c r i b e the k i n e t i c s o f c a r b o n i z a t i o n , such as d e t e r m i n i n g the t e m p e r a t u r e / t i m e dependence o f v i s c o s i t y , q u i n o l i n e i n s o l u b l e s f o r m a t i o n , o r weight loss. T h e r m o g r a v i m e t r i c a n a l y s i s can f o l l o w weight l o s s d u r i n g hydrocarbon p y r o l y s i s . A l a b o r a t o r y thermobalance w i t h a sample s i z e o f 100 mg to 1 g g i v e s v e r y s i m i l a r r e s u l t s f o r d i f f e r e n t types o f p i t c h e s as shown i n F i g u r e 12 (KS s o f t e n i n g p o i n t : 50° and 90°C). But k i n e t i c d e t e r m i n a t i o n s based on u s i n g the A r r h e n i u s e q u a t i o n t o meet the weight l o s s c u r v e must r e c o g n i z e t h a t one b a s i c assumption of t h i s type of c a l c u l a t i o n i s not met. That i s the a s s u m p t i o n o f a homogeneous r e a c t i o n o f the c a r b o n i z a t i o n p r o c e s s from room temperature up to the f i n a l s t a t e . In r e a l i t y , I t i s w e l l known t h a t t h e r e I s a d r a s t i c change i n the c o m p o s i t i o n o f t h e r e s i d u a l m a t e r i a l which p r o b a b l y m o d i f i e s the r e a c t i o n o r d e r , the frequency f a c t o r , and the a c t i v a t i o n e n e r g y . I n c o n t r a s t to t h i s , the technique r e p o r t e d h e r e i n uses the weight l o s s o f p i t c h d u r i n g heat treatment to e v a l u a t e a s e t of apparent k i n e t i c d a t a ( r e a c t i o n o r d e r , frequency f a c t o r , and a c t i v a t i o n e n e r g y ) , t a k i n g i n t o account t h a t the c o m p o s i t i o n and n a t u r e of the r e a c t a n t s a r e v a r y i n g . The e x p e r i m e n t a l way to s o l v e the a n a l y t i c a l problem i s to heat up the sample to a d e f i n e d weight l o s s a t d i f f e r e n t h e a t i n g r a t e s (assuming t h a t the u l t i m a t e c o m p o s i t i o n o f the r e s i d u e i s the same) and observe t h a t the p o i n t o f e q u i v a l e n t weight l o s s I s reached a t d i f f e r e n t temperature l e v e l s . At t h i s p o i n t the temper a t u r e treatment i s changed to i s o t h e r m a l c o n d i t i o n s and one f o l l o w s the f u r t h e r weight l o s s . T h i s t e c h n i q u e p e r m i t s measuring weight l o s s r a t e at d i f f e r e n t r e a c t i o n temperatures. The e x p e r i m e n t a l procedure I s i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 1 3 . The r e s u l t i n g weight l o s s curves u s i n g t h i s e x p e r i m e n t a l t e c h nique a r e p l o t t e d f o r d i f f e r e n t i s o t h e r m a l treatment temperatures i n F i g u r e 1 4 . The r a t e of weight l o s s v e r s u s the r e s i d u e produces a l i n e a r r e l a t i o n . The l i n e a r l i n e s f o r the i s o t h e r m a l weight l o s s c u r v e s a r e s h i f t e d a l o n g t h e o v e r a l l weight l o s s c u r v e with i n c r e a s i n g temperature. I n the b e g i n n i n g the s l o p e s of the l i n e s are v e r y s i m i l a r ; above 400°C the s l o p e s of the l i n e s d e c r e a s e .




T E M P E R A T U R E °C

Figure 12.

Thermogram o f p i t c h .



TILLMAN Ν S


RELTIME % Figure 13.

'

Scheme o f e v a l u a t i o n .

I

ι

logdOO-R) F i g u r e 14.

Rate o f weight l o s s v s . r e s i d u e .




228

These thermobalance d a t a were used to c a l c u l a t e a s e t o f apparent k i n e t i c d a t a f o r each s t a t e o f weight l o s s . The A r r h e n i u s p l o t s ( F i g u r e 15) show an i n c r e a s e o f the s l o p e w i t h i n c r e a s i n g r e l a t i v e weight l o s s up t o 76.7%; a t 91.4% r e l a t i v e weight l o s s t h e slope i s decreased. The r e s u l t i n g apparent k i n e t i c d a t a a r e g i v e n I n F i g u r e 16. The f i r s t c o n c l u s i o n i s , as e x p e c t e d , t h a t c a r b o n i z a t i o n i s not a homogeneous p r o c e s s d e s c r i b e d by one s e t o f k i n e t i c d a t a . The r e a c t i o n o r d e r i s n e a r l y c o n s t a n t a t about 3 up to 400°C and I s determined by the e x p e r i m e n t a l p r o c e d u r e . The a c t i v a t i o n energy i n c r e a s e s l i n e a r l y w i t h weight l o s s up t o 75%. A c t i v a t i o n e n e r g i e s range from 30 t o 600 K G / M o l . The maximum a c t i v a t i o n energy i s 2 t o 2.5 times t h e e v a p o r a t i o n energy o f a r o m a t i c compounds w i t h a m o l e c u l a r weight o f 700-800. The h i g h l e v e l o f a c t i v a t i o n energy i s c e r t a i n l y caused p a r t l y by the s t r o n g I n t e r a c t i o n o f the aromatic molecules i n the i s o t r o p i c phase which u l t i m a t e l y o r i e n t a t e and form t h e mesophase. The frequency f a c t o r also I n c r e a s e s w i t h weight l o s s , r e a c h i n g a peak a t about 75% weight l o s s ; the subsequent d e c r e a s e may be r e l a t e d to s o l i d i f i c a t i o n o f the r e s i d u e . T h i s d r a s t i c change i n the phase o f the r e s i d u e must i n f l u e n c e t h e apparent k i n e t i c d a t a s i g n i f i c a n t l y . T h i s m a t h e m a t i c a l model d e s c r i b i n g the inhomogeneous p y r o l y s i s r e a c t i o n s by a s e t o f apparent k i n e t i c d a t a (which a r e changing w i t h the p r o g r e s s o f the p y r o l y s i s ) s h o u l d be understood as a f i r s t attempt to s e t up a mechanism to p r e d i c t p y r o l y s i s . The t a r g e t o f the a p p l i c a t i o n o f t h i s model would be to e v a l u a t e the i n f l u e n c e o f temperature programs on b a k i n g b e h a v i o r . Coke C h a r a c t e r i z a t i o n A l l the many methods used to c h a r a c t e r i z e coke samples cannot be d i s c u s s e d w i t h i n the scope o f t h i s p a p e r . F o r b r e v i t y and s i m plicity, this discussion on coke q u a l i t y focuses on t h r e e parameters : • • •

hydrogen c o n t e n t o f c a l c i n e d coke to c h a r a c t e r i z e the calcination severity, m e c h a n i c a l p r o p e r t i e s o f coke g r a i n s , and I r r e v e r s i b l e d i l a t a t i o n o f carbon a r t i f a c t s caused by coke puf f i n g .

Hydrogen c o n t e n t o f c a l c i n e d coke i s o f t e n used to c o n t r o l commercial coke c a l c i n i n g s e v e r i t y . I t I s w e l l known t h a t o v e r a l l the hydrogen c o n t e n t I s r e l a t e d to f i n a l c a l c i n i n g t e m p e r a t u r e . But d e t a i l e d a n a l y s e s show t h a t the hydrogen c o n t e n t a l o n e cannot be used to deduce the f i n a l heat treatment f o r cokes o f d i f f e r e n t quality. B e s i d e s the f i n a l t e m p e r a t u r e , t h e r e i s a l s o an i n f l u e n c e o f the r e s i d e n c e time and the s u l f u r c o n t e n t o f the green c o k e . Residence time i s o f l e s s e r s i g n i f i c a n c e f o r t e c h n i c a l c a l c i n i n g p r o c e s s e s because a s t a t e c l o s e to e q u i l i b r i u m i s r e a c h e d . The i n f l u e n c e o f green coke s u l f u r content i s o f much g r e a t e r s i g n i f i cance; i t can cause misjudgments w i t h r e s p e c t t o the r e q u i r e d c a l c i n i n g temperature i f d i f f e r e n t q u a l i t y cokes a r e compared.




TILLMANNS

0

1 2 10~ TEMPERATURE, 1/T Κ 3

Figure 15.

0

I

1

300 F i g u r e 16.

Arrhenius

1

plot.

1

400 500 TEMPERATURE °C Kinetic data.



230


As shown i n Figure 17, coke with a 1.5% sulfur l e v e l has the highest rate of hydrogen release, whereas the coke samples with 0.6% and 0.2% sulfur have a much smaller rate of hydrogen release. Thus, the sulfur present i n green coke can result i n higher hydrogen release during calcination; i f the hydrogen content i s solely used as the controlling parameter, over calcination of the coke could occur. Another important property of commercial coke i s the crushing strength of coke grains as an indicator of their handling sensit i v i t y (but not the strength of the f i n a l carbon a r t i f a c t ) . There i s a good c o r r e l a t i o n between the c o e f f i c i e n t of thermal expansion and the crushing strength (Figure 18). A second factor influencing the crushing strength i s the bulk density of the coke grains. The importance of the crushing strength i n respect to the c o e f f i c i e n t of thermal expansion w i l l increase i n the future. S e n s i t i v i t y of cokes of different quality to handling during their manufacture, their transportation and manufacture of the carbon products therefore may be a c r i t e r i o n for selection.

Figure 17.

Hydrogen release as function of temperature



16.

231


TILLMANNS

One of the most d i s c u s s e d items i n r e c e n t times i s the p u f f i n g b e h a v i o r of c o k e , e s p e c i a l l y o f c o a l t a r based needle c o k e . The d i l a t a t i o n b e h a v i o r o f cokes o f d i f f e r e n t q u a l i t y , as measured d u r i n g g r a p h i t i z a t i o n o f e l e c t r o d e s , a r e shown i n F i g u r e 1 9 . O v e r all, t h e c u r v e s c o r r e s p o n d to the p u f f i n g c u r v e s measured i n l a b o r a t o r y equipment. One f a c e t r e l a t e d to p u f f i n g t h a t i s seldom d i s c u s s e d i s the f a c t t h a t b e s i d e s the s t r o n g e l o n g a t i o n c a l l e d p u f f i n g , a v e r y s t r o n g s h r i n k a g e i n the f i n a l s t a t e o f g r a p h i t i z a t i o n a l s o takes p l a c e . I n s o f a r as the i n t e r n a l s t r e s s / s t r a i n s i t u a t i o n i s c o n c e r n e d , t h i s l a t t e r phenomenon may cause problems s i m i l a r to ( o r worse than) those caused by p u f f i n g . W h i l e t h e r e a r e many a p p l i c a b l e methods r e p o r t e d i n the l i t e r a t u r e , t h i s paper was r e s t r i c t e d t o a s u b j e c t i v e s e l e c t i o n o f t e s t i n g procedures i n the hope t h a t i t would s t i m u l a t e the d e v e l o p ment o f b e t t e r a n a l y t i c a l t e c h n i q u e s , r e c o g n i z i n g t h a t t h e r e i s always a l i m i t a t i o n of v a l i d i t y f o r each method and e v a l u a t i o n o f results. A n a l y s i s o f carbon m a t e r i a l s and e s p e c i a l l y o f the c a r b o n i z a t i o n p r o c e s s w i l l never l o s e i t s a t t r a c t i o n as a f a s c i n a t i n g a p p l i c a t i o n of the a r t o f b l a c k m a g i c .

1 00

I Ι Ο ζ u tt h

80

L

60

L

40

L

20

L

W

(9 Ζ M

I D

(0

α ο

0 6 CTE

Figure 18.

8 CVOL.5

10

12

10E-6/K

C o r r e l a t i o n o f c r u s h s t r e n g t h and t h e r m a l e x p a n s i o n .


232



τ

0

30

60

90 REL .

Figure 19.

Γ

1 20

GRAPHIΤIΖ.

1 50

1 80

PROGRESS

D i l a t a t i o n curves of e l e c t r o d e s .

BIBLIOGRAPHY Review Papers: E. Fitzer, K. Mueller, W. Schaefer, "Chem. and Phys. of Carbon", M. Dekker, Inc. 1970, Vol. 7, p. 237. K. J . Huettinger, Habilitationsschrift, Universitaet Karlsruhe, Inst. Chem. Tech. 1973. E. Fitzer, 9th World Petroleum Congress, Tokyo 1975, review paper no. 12. E. Fitzer, H. Tillmanns, 2nd Iranian Congress on Chemical Engineering, Teheran 1975. H. Marsh, Ph. L. Walker, J r . , "Chem. and Phys. of Carbon", M. Dekker, Inc. 1980, Vol. 15. H. A. Kremer, Chemistry and Industry, London No. 18, 1982, p. 702. Carbonization, Mesophase and Quinoline Insoluble: J.

E. Zimmer, J . L . White, Proc. 12th Bienn. Conf. on Carbon, Pittsburgh 1975, p. 223. H. Tillmanns, H. Pauls, G. Pietzka, Abstr. Carbon 76, Baden-Baden 1976, p. 374. H. Tillmanns, G. Pietzka, Abstr. 13th Bienn. Conf. on Carbon, Irvine 1977, p. 332. G. R. Romovacek, Proc. AIME, 1977, p. 275. H. Pauls, G. Pietzka, Fuel, 1978, Vol. 57, p. 171. J . W. Stadelhofer, Fuel, 1980, Vol. 59, p. 360. A. J . Perotta, R. M. Henry, J . D. Bacha, E. W. Albaugh, High Temp. High Press. 13, 1981, p. 159. H. Tillmanns, Abstr. 15th Bienn. Conf. on Carbon, Philadelphia 1981, p. 142.


16. TILLMANNS


H.

J.

A. Kremer, S. Cukier, Proc. Roy. Microscop. Soc., 1981, University of York. L. White, J . F. Tellers, Jour. of Appl. Polym. S c i . , Appl. Polym. Symp. No. 33, p. 137. I . Romey, H. Glaser, R. Marrett, H. Tillmanns, 27. DGMK Haupttagung 1982, Aachen, p. 172. H. Tillmanns, Proc. 6th Intern. Carbon and Graphite Conf., London 1982, p. 362.


Thermobalance Analysis: E. C. C. A. H. C. J. K. K. K. K. T.

S. Freeman, B. Carroll, J . Phys. Chem., 1958, Vol. 62, p. 394. D. Doyle, et al., J . Appl. Polym. Sci., 1961, p. 285. D. Doyle, et al., J . Appl. Polym. Sci., 1962, p. 639. W. Coats, et al., Nature, 1964, Vol. 201, p. 68. L. Friedman, J. Appl. Polym. S c i . , 1964, p. 183. D. Doyle, Nature, 1965, Vol. 207, p. 290. Zsako, J . Phys. Chem., 1968, Vol. 72, p. 2406. J . Huettinger, Erdoel und Kohle, 1970, Bd. 9, p. 559. J . Huettinger, Bitumen, Teere und Asphalte, 1970, p. 528. J . Huettinger, Bitumen, Teere und Asphalte, 1970, p. 487. J . Huettinger, Chem. Ing. Tech., 1970, Bd. 42, p. 812. Ozawa, J. of Therm. Analysis, 1976, Vol. 9, p. 217.

RECEIVED December 9, 1985


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