Coal Science - American Chemical Society

27 Kinetics and Mechanism of Solution of High Volatile Coal

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 31, 2017 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch027

GEORGE R. HILL, HASSAN HARIRI, R. I. REED, and LARRY L. ANDERSON Department

of Fuels Engineering,

University of Utah, Salt Lake City,

Utah

A kinetic study of the dissolution of a Utah high volatile bituminous coal in tetralin has been conducted. Equipment for obtaining rate data during the early stages of the reaction has been developed. The data are interpreted in terms of a pseudo second-order rate constant, average heats of activation, and apparent entropies of activation, the numerical values of these functions depending on the degree of extraction. The rate of solution in tetralin of this coal is not an equilibrium phenomenon but is a kinetically controlled reaction in which the average activation energy increases as the reaction proceeds.

y y h i l e solvent extraction is used to convert coal to valuable l i q u i d a n d gaseous products, it also may be used to study coal structure a n d composition. Thermal dissolution of coal (solvent extraction w i t h a reaction temperature above the boiling point of the solvent) with various solvents has been studied extensively for a number of years (5, 9, 10, 12, 13). In fact, a voluminous literature has been presented and reviewed which deals directly w i t h the extraction of coal and extraction conditions. This paper describes experimental result of kinetic studies together with a new approach to the theory a n d mechanism of solvent extraction. Experimental

Procedure

T h e coal used for this study was taken from a working face of U t a h Spring C a n y o n C o a l M i n e . T h e analysis as given by Commercial Testing a n d Engineering C o . for the sample (calculated on a dry basis) is shown below. T h e coal was ground i n the laboratory a n d sized to pass 200 mesh. A s h determinations on the ground coal sample were made, a n d an average value of 5 . 2 % was used to correct for the ash content of the extracted coals. 427

Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

COAL SCIENCE

428 Ultimate Analysis Carbon Hydrogen Nitrogen Oxygen Sulfur Chlorine

Proximate Analysis

72.88 5.58 1.51 10.82 0.65 0.19

Water Ash Volatile Matter Fixed Carbon

— 8.37 45.71 45.92


Heating value—13,237 B.t.u./lb.

Ol

I

.05

0.1

I

0.15

I

I

I

0.2

0.25

0.3

C/S Cool, grrt/Solvent, ml. Figure

I.

Determination optimum

of coal/solvent extraction

ratio

for

T o find the optimum ratio of coal/solvent ( C / S ) , a series of solubility experiments were performed at constant temperature and time but w i t h differ ent C / S . It was found that at ratios smaller than 1:8 the quantity of coal dissolved d i d not increase (Figure 1). A l l kinetic studies were made at ratios of 1:10 to ensure that excess solvent was always present. F o r each r u n , coal samples of approximately 50 grams were dried, at 1 0 0 ° C . for 4 hours and weighed after cooling i n a desiccator for % hour. In the early experiments, coal and solvent were mixed i n the autoclave, and runs were performed. It was found that the time necessary for the autoclave and mixture to be heated from room temperature to reaction temperature was 1 % - 2 hours. W h e n extraction fraction vs. time was plotted, it showed that at higher temperatures more than 8 0 % of the total possible extraction of coal dissolved before the system reached the reaction temperature. Consequently, the data obtained i n the first 2 hours were incorrect. T o overcome this problem a coal injector was designed and constructed. T h e coal injector was a stainless steel cylinder, 8 inches long, 3 inches outside diameter, and 1.5 inches inside diameter. One side of the cylinder was open, and the other was provided w i t h a 0.25-inch stainless steel female fitting (Figures 2 a n d 3 ) . A piston, provided w i t h an ' Ό " ring gasket, was used for die open side of the cylinder and could move back and forth b y a mechanical


27.

H I U ET A l .

Kinotit* of Solution

14

5'V

l ^ i e — * — ι ιβ-


5 /

429

V

7 "

—Jf

—-·,

Τ

— L -

τ 1

ï

-M THREAD CAP 3/165.

PRESSURE

P A D —

"0*RING RETAINER SQUARE

REFLUX'O" RING

,

3 7"*·

18 N . R T .

CYLINDER Figure 2.

Major parts of coal injector with their

il

measurements

screw system. T h e outside body of the coal injector's cylinder was graduated and calibrated in cubic centimeters injected per linear inch drive of the cylinder. T o use the coal injector, equal amounts of coal a n d tetralin (approxi mately 50 grams of coal which was prepared by the above mentioned pro cedure a n d 50 cc. of tetralin ) were used i n each run a n d mixed i n the coal injector. W h i l e preparing coal paste i n the coal injector, 450 cc. of tetralin were being heated i n the autoclave to reaction temperature. W h e n the auto clave containing tetralin reached the reaction temperature, the coal paste was injected into the autoclave through a fitting on the autoclave. T h e temperature was carefully observed during this operation, a n d it was found that after 1 or 2 minutes the system returned to the reaction temperature ( F i g u r e 4 ) .


COAL SCIENCE


430

Figure 3.

Coal injector used for solvent

extraction

The autoclave used for this study was a 1-liter capacity Magne D r i v e auto clave made by Autoclave Engineers. It is provided w i t h a l i q u i d sample line, a gas sample line, quenching tubes, Magne Drive stirrer, pressure gauge, heat ing jacket, and two extra connections for special uses. One of these connections was used for injecting the coal paste into the autoclave. T h e autoclave design pressure of 5000 p.s.i.g. p r o v e d adequate for all runs. E a c h run was continued for about 2 5 - 3 0 hours, and about 25 liquid samples were taken during this time. T h e volume of each sample was between 15 and 30 cc. Samples taken i n each run were treated similarly. T h e y were transferred into Soxhlet thimbles which had already been dried for Vz hour. T h e thimbles were placed i n the Soxhlet extraction unit using benzene as the solvent. T h e extraction was continued until the l i q u i d circulating i n the unit became clear and colorless (indicating pure benzene). T h e thimbles were then removed from Soxhlet extraction tubes and placed i n the oven at 1 0 0 C . for % hour and weighed. T h e difference between the weight of the thimbles before and after Soxhlet extraction was the weight of residues. F o r each r u n , the ratio of coal i n grams to the mixture (coal plus tetralin) i n cubic centimeters used i n the experiment was carefully measured; to obtain the initial amount of coal i n each sample taken, that ratio was multiplied by e

ο

δ

50

ÏOO

150

200

250

300

350

400

Time in Minutes Figure 4.

Variation

of temperature during process of solvent at 440*C.

extraction


27.

HILL ET AL.

Klnotics of Solution

431


the volume of the sample. This procedure is valid if the following assumptions are accepted: (1) T h e mixture in the autoclave is homogeneous. This was insured by keeping the Magne D r i v e stirrer always at a steady 1500 r.p.m. It was experimentally proved that even distribution of coal and tetralin i n the autoclave is a fair assumption because the fraction extracted from the last portion of mixture, w h i c h remained i n the autoclave and was taken after the system cooled to room temperature and the autoclave was opened, was found to be very close to the yield obtained from the last sample taken from the system through the sample lines at reaction temperature. (2) The volume of the coal before and after extraction does not change very m u c h . This also was found to be a fair assumption because i n a l l the runs the initial and final volumes were accurately measured and gave C/S ratios of approximately 1/10. Generally, the total volume after extraction was about 20 cc. less than the initial volume of the mixture. Since some of the coal converts to gas and some adheres to the wall of the autoclave, it is concluded that the density of the mixture at the start and end of the run d i d not change significantly. However, even the ratio of coal (grams)/total volume ( c c . ) , ( C / V ) , for this small change was corrected.

0.1 0 Figure 5.

Theory and

200 Time-yield

400 600 800 t,Time in Minutes curve for thermal dissolution in tetralin

1000 of bituminous

1200 coal

Mechanism

T h e conventional mechanism and mathematical treatment for solvent extraction kinetics was proposed by Oele et al in 1951 (11) and has since been accepted and used by others. Oele assumed that the coal w i l l enter the l i q u i d solvent in accordance with a zero-order rate law u p to a certain time;


432

COAL SCIENCE


then not only is the zero-order reaction maintained, but now a reverse reaction occurs, which was assumed to be unimolecular. W h i l e interpreting our experimental data (Figures 5, 6, 7) by this and other mathematical expressions, we concluded that there are other mechanisms w h i c h represent the data better than O e l e s model. T h e tetralin solutions were found not to be saturated with dissolved coal i n the experiments performed by us. W h e n In ^ 1 —

^

was plotted vs. t, a straight line was obtained by

the method of least squares; it d i d not pass the second boundary condition of O e l e s equation—i.e., at time zero, χ was not zero. Chariot ( I ) used this model, and his graph of In ^ 1 —

χ" )

v s

' **

m e

R i + L i + G i Jfci' R i ?± X i * -> R + L + G 2

2

2

fc ' R ?± X * -> R + L a + Ga kn 2

2

R» ^± X**

2

3

R«+i + L«+i + Gn+i

W h e r e X*'s are activated complexes for the reactions. These reactions should not be confused w i t h consecutive reactions since i n this case R i , R , and R* are the remaining unreacted coal rather than a new product. 2

Table IV.

Γ, °K. fci/Ti *2/T

2

k /T< 4

ke/Te k /T k /Ts 7

H

7

623 648 673 683 693 703 713 723

Variation of Enthalpy and

1/T(°K.)-' 1.605 1.542 1.486 1.464 1.443 1.422 1.4025 1.383

1010101010101010-

χ= 0 0.0947 0.1850 0.5527 0.7935 1.0907 1.5647 1.9915 3.0570

ΔΗί (kcaL/mole) AS* (cal./mole °K.) apparent


X X X X X X X X

io-< io-< ίοίο10 10 1010-

37.2 —19.5

1

4

H

H


27.

HILL ET A l .

Figure

441

Kinetic* of Solution

11.

Flot of k/T vs. l/T for activation enthalpies

evaluating

The value of — 1 9 . 5 e.u. for the apparent entropy of activation obtained at the beginning of the extraction includes a term for the limited number of sites where dissolution could occur (Table I V a n d Figure 1 1 ) . It is considered probable that the dissolution of material from the pores (process Rr) occurring simultaneously can account for part of the large negative value. Chariot ( I ) Entropy of Activation with Fraction Extracted χ = 0.5 0.0290 0.07514 0.2459 0.3579 0.5003 0.7312 0.9327 1.4661

X Χ Χ Χ Χ Χ Χ Χ

40.0 —16.8

χ = 0.8

10" ΙΟ" ΙΟ" ΙΟ" ΙΟ" 10" 10" 10"

χ = 0.9

4

4

4

4

4

4

4

4

0.0091 0.0618 0.0966 0.1426 0.2310 0.2973 0.51170

Χ Χ Χ X Χ Χ Χ

51.8 —2.1

10" 10" 10" 10' 10" 10" 10"

4 4 4

4 4 4

4

0.0095 0.0234 0.0640 0.08555 0.1936

Χ Χ Χ Χ X

10" 10" 10" 10" 10" 4

4

4

4

85.5 41.0



442

COAL SCIENCE

found that a two-stage mechanism fits the lower temperature range of dissolution well. To determine whether the low temperature "physical" dissolution could be separated from the processes with higher activation energies, the following experiments were conducted. Coal and tetralin with a ratio of about 1 gram of coal to 10 cc. solvent were mixed in the autoclave, and the temperature was raised to 2 5 0 ° C . This was the temperature at which transition of a diffusion process to a surface reaction process takes place as postulated by Chariot (I). The contents of the autoclave were divided into three parts and placed in three thimbles, which were already treated and weighed according to the procedure previously described (Figures 12 and 13).

100

200

900

400

500

600

t,Time in Minutes Figure 12. Variation of fraction extraction with time for the second stage of two-stage solvent extraction of coal; first stage at 250°C. for 8 hours not shown The thimbles were treated in the Soxhlet extraction apparatus using benzene. The residue left in the thimbles was dried and carefully weighed. A total of 12% coal was extracted in this manner. These three coal samples were extracted with fresh tetralin at reaction temperatures of 4 0 0 ° , 4 2 0 ° , and 440 C. The experimental procedure was the same as previously described—i.e. part of the tetralin was heated in the autoclave to trie selected reaction terne



27.

HILL ET AL.

Kinetics of Solution

100 200 300

443

400 500 600

700

t Time in Minutes 9

Ο One stage at T= 440 C. #

• Two stages (first at 250*0, second:440 C) #

Ο Chariot s data for 250 C #

Figure 13.

Table V. T, °C. 440 420 400

Comparison

extraction

Final Results for Second Stage of Two-Stage Solvent Extraction of Coal" l/Xn,

0.84 0.80 0.79

of one-stage and two-stage of coal

( « - £ )

1.19 1.25 1.265

ΔΗ* in kcal. AS* (for * = 1.0)

1.1914 1.276 1.266

k' for χ = 0

k* for χ = 0.7

l/T

0.1914 0.0506 0.0140

0.0362 0.0060 0.0014

0.0014025 0.0014430 0.0014a59

60.0 -1-6.2 e.u.

75.0 4-25.5 e.u.

• First stage—250*C. for 8 hours; 12% was extracted and is not shown here..

perature a n d the coal paste which was prepared by mixing the coal sample with the remaining solvent i n the coal injector was then injected into the reactor. A series of samples was taken to determine the amount of residue left. The data obtained were treated exactly as those obtained i n former experi ments, a n d a series of average enthalpies of activation a n d apparent entropies of activation was obtained for different x's (Table V , Figures 14 a n d 15). It was found that removing only 1 2 % of coal during the first stage of extraction increased the initial apparent entropy of activation from a negative to a positive value. It further shows that this value increases as χ increases. This is expected if the surface area available for reaction increases during the course of the reaction.


444

COAL SCIENCE


T h e experimental results confirm the idea that at the initial stage of the experiment, the reaction is under diffusion control. This process has a very l o w activation energy, and when combined w i t h thermal disintegration of the coal at higher temperatures, it lowers the average free energy of activation for the reaction.

05

0.6 0.7 OA Fraction Extracted, x « - J J —

Figure 14. Variation of rate constant with χ for the second stage of two-stage extraction of coal; first stage at 250°C. for 8 hours not shown Conclusion C o a l is a complicated material, and more than one simple chemical process takes place during solvent extraction. The activation energy necessary for dissolving coal increases with the extent of the process up to a point; at a certain temperature insufficient energy is available for more extraction. W h e n the activation energy necessary for extraction becomes more than maximum energy supplied, additional coal w i l l not dissolve. The rate constants, average heats of activation, and apparent activation entropies predict the dissolution


27. HILL ET AL.

4 3 2


T c C

445

Klnotics of Solution

i

0 1

08 ftfî

Cl Λ

M

i ο

.02

*

A .001 ° .008 f .006

y .

* .004 .002 .001

1.4

1.45 (I/T)I03 inCK)*

1.5 1

Figure 15. Arrhenius plot for the second stage of a two-stage extraction of coal for two values of x; first stage at 250° not shown rate of this coal i n tetralin. T h e heat of activation increases, and the apparent entropy of activation becomes increasingly negative during the course of the reaction. Acknowledgment Appreciation is expressed to Yacob Shifai, Norbert Kertamus, a n d L a r r y Chariot for their contribution to this paper. T h e research reported here is supported by the Office of C o a l Research, Department of the Interior under Contract N o . 14-01-0001-271 and by the University of U t a h Research F u n d . Literature Cited (1) Charlot, L. Α., Master's thesis, University of Utah, 1963. (2) Dryden, I. G. C., "Chemistry of Coal Utilization," H . H. Lowry, Ed., p. 248, Wiley and Sons, New York, 1963. (3) Dryden, I. G. C., Fuel 37, 444 (1958). (4) D'yakova, M . K., Davtyan, Ν. Α., Bull. Acad. Sci. URSS, Classe Sci. Tech., 1945, 203; J. Appl. Chem. 21, 113 (1948).



446

COAL SCIENCE

(5) D'yakova, M . K., "Production of Synthetic Liquid Fuels and Chemical Products by Thermal Dissolution of Solid Fuels," p. 86, Academy of Science of the U.S.S.R., 1957. (6) Glasstone, S., Laidler, K. J., Eyring, H., "The Theory of Rate Processes," McGraw-Hill Book Co., 1941. (7) Hill, G. R., Lyon, L. B., Intl. Eng. Chem. 54, 36 (1962). (8) Huck, Kartweil, Brennstoff-Chem. 34, 97, 129 (1953). (9) Kiebler, M . W., "The Chemistry of Coal Utilization," H . H . Lowry, Ed., Vol. I, p. 724, Wiley and Sons, New York, 1945 (10) Lowry, H . H., Rose, H. J., Bur. Mines Inform. Circ. 7420 (1947). (11) Oele, A. P., Waterman, H. I., Goedkoop, M. L., Van Krevelen, D. W., Fuel 30, 169 (1951). (12) Pilipetz, M. G., Kuhn, E., Friedman, S., Storch, H. H., U.S. Bur. Mines, Rept. Invest. 4546 (1949). (13) Storch, H. H., Chem. Rev. 29, 483 (1941). (14) Van Krevelen, D. W., "Coal," p. 178, American Elsevier Publishing Co., New York, 1961. RECEIVED October 5, 1964.

Discussion George K a p o : In your opinion what is the extent of the available internal area for solution? George R . H i l l : In the l o w temperature " p h y s i c a l " solution process the surface area would probably be that determined by B E T adsorption measurements. In the high temperature process, apparently the coal structure is opened u p , and the surface would be the total surface of a l l the "molecular" units. This occurs, as the dissolution proceeds, by a combination of chemical bond breaking a n d solvent action with hydrogen transfer to the free radicals produced. D r . K a p o : H o w does diffusion influence the kinetics of solution? D r . H i l l : In the l o w temperature solution process the activation energy value suggests that a physical process—probably diffusion—is rate controlling. T h e large ( a n d increasing) value of the heat of activation for the major portion of the dissolution reaction requires that the rate is a chemically controlled process—very likely the breaking of chemical bonds. K u l a i A . K i n i : Y o u said that there was no swelling when coal was extracted with aromatic solvents. W h a t method was used to measure swelling? D r . H i l l : N o direct measurements other than usual observation were made on the swelling of the coal. T h e bulk volume remained unchanged. Norbert Berkowitz: I think the kinetic treatment of the experimental data is of questionable validity. T h e extraction process is evidently accompanied by considerable changes i n the geometry of the coal particles (e.g., swelling a n d dispersion); there is the unresolved question of whether the extract forms a solution or dispersion; finally, there is an obvious but somewhat indefinite effect of coal decomposition. T h e latter point alone w o u l d make determining a temperature effect (and hence, calculating an "activation energy") very difficult practically, if not impossible.



27.

Hill ET A l .

Klnotlc* of Solution

447

D r . H i l l : T h e point is well made that the solution of coal i n solvents is a complicated process involving separation of macérais, breaking of relatively weak hydrogen bonds, a n d , increasingly at high temperatures, the rupture of covalent chemical bonds. T h e "activation energy" calculated from a plot of log of rate vs. l/T obviously must be an average value for a l l the processes which are occurring. The slope of the line obtained from this plot is, however, independent of the final state of the coal in solution and is meaningful in terms of the " m i n i m u m " average chemical bond strength of those bonds broken i n the dissolution reaction. Observing a low activation energy at low temperatures requires the conclusion that a primarily physical separation process is occurring below 2 8 0 ° C , but that above that temperature the breaking of chemical bonds of increasing strength becomes rate controlling. T h e data also require a conclusion that the degree of solution (and pyrolytic decomposition) is determined by a rate process and is not an equilibrium phenomenon. T h e very high (80 kcal.) average heat of activation obtained at high temperatures suggests that many carbon-carbon bonds are being severed i n the high temperature range. It is agreed that a physical interpretation of the entropy of activation is most tenuous; nevertheless, it is a useful beginning for understanding what is occurring. T h e absolute value of has no meaning unless the initial state is well defined a n d constant. However, the change i n AS* as the reaction proceeds is significant and requires an explanation like the one proposed. If a better method of utilizing the data can be suggested, we should be pleased to apply it. Leslie Reggel: C o u l d you say more about the structure of the coal "molecule" you showed us? Is enough hydroaromatic hydrogen included? D r . H i l l : T h e coal "molecule" (reproduced from Ind. Eng. Chem. 54, 36 ( 1 9 6 2 ) ) does i n fact include hydroaromatic hydrogens. A l l of the R° Ν alicyclic rings (some six or seven i n the diagram) have hydroaromatic hydro gens as does the alicyclic ring i n the lower left hand corner of the model. I n this "average structural unit" some 14.7% of the hydrogen is aromatic, 7 7 % of the hydrogen is alicyclic a n d aliphatic, a n d 8 . 3 % of the hydrogen is i n functional groups. T h e percentage of hydroaromatic hydrogen corresponds to that reported by Given (Fuel 39, 147 (1960) ) .

American Chemical Society Library 1155 16th St., N.W. Washington, OX. 20036 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

Coal Science - American Chemical Society

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