Evaluation of coal reactivity for liquefaction based on kinetic characteristics

522 Energy & Fuels 1988,2,522-528

products formed during coal pyrolysis. Our current investigations provide new information on the probable pathways for the formation of these gaseous products in the absence of added dihydrogen. An array of free-radical processes involving hydrogen atom abstraction and 8- scission must be invoked to rationalize the formation of high yield of methane. Ipso dealkylation occurs to only a limited extent.

Acknowledgment. We wish to thank Dr. Bruce Solka and Gerald Koncar for performing the detailed mass

spectral analyses of the pyrolysates and Dr. S. R. Mahasay for preparing the propyl derivatives. This work was gen- erously supported by the Gas Research Institute.

74-98-6; C3HB, 115-07-1; [3-(methyl-d3)phenyl]methyl bromide, 114563-28-9; [4-(methyl-d3)pheny1]methyl bromide, 114563-29-0; [4-(ethyl-2,2,2-d3)pheny1]methyl bromide, 114563-30-3; [4-(propyl-3,3,3-d&phenyl]methyl bromide, 114563-31-4; [4-(methyl- d3)naphthyl]methyl bromide, 114563-32-5; [4-(ethyl-l,l-dz)- naphthyllmethyl bromide, 114563-33-6; [4-(propyl-l,l-d2)- naphthyllmethyl bromide, 114563-34-7.

Rsgistry NO. CHI, 74-82-8; C&, 7485-1; CzH4,74851; C3H8,

Evaluation of Coal Reactivity for Liquefaction Based on Kinetic Characteristics

Hiroshi Nagaishi, Hiroshi Moritomi, Yuzo Sanada, and Tadatoshi Chiba* Coal Research Institute, Hokkaido University, N13 W8 Kita-Ku, Sapporo, Japan 060

Received August 3, 1987. Revised Manuscript Received January 14, 1988

A new kinetic model is developed for evaluation of coal reactivity for liquefaction. The model assumes a coal to consist of three different componentx &,, inherently inert for liquefaction; C1, forming consecutively oils and gases via preasphaltenes and asphaltenes; C2, forming oils and gases directly without consuming external hydrogen from the solvent and the gas phase. These components have been separated for 10 different coals by analyzing experimental data taken under extreme liquefaction conditions. By use of the weight fraction of each component in the original coal together with the rate constant for individual reaction steps, the present model is shown to explain reasonably well the liquefaction yields observed not only in the present experiments but also in other investigations in the literature.

Introduction Direct liquefaction is one of the coal conversion proceases

that depends on coal properties to a large extent. Many attempts have so far been made to correlate the conversion of coal and the yields of liquefaction product to rank parameters of coal such as ultimate and proximate compo- sitions, aromaticity, maceral composition, and so on.' However, no one parameter seems to be fully satisfactory in correlating the data observed in a wide range. Some of the rank parameters themselves are a function of operating variables. For example, the yield of volatile matter depends not only on the kind of coal but also on the heating rate, temperature, and gas atmosphere.2 In other words, the yield is not a property of coal but a process-dependent variable in a sense that it depends on the operating variables, which are different more or less in different processes. Nonetheless, the volatile matter yield is analyzed as a rank parameter a t a fixed condition different from those of liquefaction. Such process-dependent rank parameters should be determined at conditions close to those where coal is processed. In designing a liquefaction reactor for various coals, evaluation of liquefaction reactivity of the coals is needed on the basis of the kinetic model. I t is necessary not only to express the rate constants as a function of rank parameters but also to define coal as

chemical reactants in liquefaction because coal is still an undefined mixture of chemical species.

A dozen kinetic models have already been proposed to explain observed changes in unreacted coal and product yields with time during coal liquefacti0n.l In most of those models, coal and liquefied products are lumped on the basis of their solubilities in different solvents because of the complex molecular structure of coal itself and a wide distribution of products as chemical species. Coal (pyridine insolubles) is generally considered to be liquefied to oils (n-hexane solubles) through a sequence of reactions via preasphaltenes (pyridine-soluble benzene insolubles) and asphaltenes (benzene-soluble n-hexane insolubles). Even with this lumping, the reaction mechanism itself appears too complicated to be a n a l y ~ e d . ~ When heated with solvent under pressure, coal may directly produce all of the above products together with gases and semicoke. Preasphaltenes may also convert to asphaltenes and oils consecutively or competitively. Experimental data point to nothing but more and more complicated reaction net- works. Shalabi et alS4 assumed all possible reaction paths among coal, preasphaltenes, asphaltenes, and oils.

The validity of a liquefaction kinetic model has usually been evaluated from its applicability to conversion and yield data. However, agreement between predicted and observed results, even if obtained for a wide range of op-

(1) Shah, Y . T.; Krishnamurthy, S.; Ruberto, R. G. Reaction Engi- neering in Direct Coal Liquefaction; Shah, Y. T., Ed.; Addison-Wesley: Reading, MA, 1981; Chapter 4.

(2) Wen, C. Y.; Tone, S. ACS Symp. Ser. 1978, No. 72, 56.

0887-0624 1881 25Q2-Q522$Q1.5Q/Q

(3) Squires, A. M. Appl. Energy 1978, 4 , 161. (4) Shalabi, M. A.; Baldwin, R. M.; Bain, R. L.; Gary, J. H.; Golden,

J. 0. Ind. Eng. Chem. Process Des. Deu. 1979, 18, 414.

0 1988 American Chemical Society

Evaluation of Coal Reactivity for Liquefaction Energy & Fuels, Vol. 2, No. 4, 1988 523

0.2

0 -

Table I. Properties of Coals Used ultimate anal. proximate anal.

wt % of mf coal wt % of daf coal coal C H N S 0" VMb FC' ash TId

Reactlon time : 30 mln

- Solvent

o e tetralin ~ r 7 g tetralin (I naphthalene

I I 1

Y allourn 65.8 4.7 0.6 0.3 28.6 52.8 46.0 1.2 1.0 Sovakoishi 74.7 5.7 1.3 0.2 18.1 39.8 42.0 18.2 0.0 Hiolinhe Wandoan Taiheiyo Illinois No. 6 Datong Zaozhuang Akabira Yubarishinko

74.6 5.2 1.3 0.1 76.8 6.7 1.1 0.3 77.2 6.7 1.1 0.2 77.3 5.6 1.6 4.4 82.3 4.6 0.9 0.6 83.2 5.1 1.5 0.6 83.2 6.2 2.1 0.6 88.1 6.2 1.8 0.0

18.8 15.1 14.8 11.1 11.6 9.6 7.9 3.9

44.3 43.8 40.6 42.2 35.3 34.0 40.6 35.1

47.4 8.3 3.6 $7.8 8.4 5.1 52.3 7.1 3.3 46.6 11.2 8.0 59.2 5.5 73.4 57.7 8.3 52.3 7.1 3.0 57.4 7.5 0.0

" Oxygen by difference. Volatile matter. e Fixed carbon. Mineral-matter-free total inert defined as 1 - (vitrinite + exinite + semifusinite)/3.

7 1 0 4 T

o5

1 Autoclave (55cm3) 5 Pres8ure transducer

2 Infrared image turnace

3 Butter veesel (25Qcm3)

4 Pressure gauge

6 Gas cyllnder

7 Thermocouple

8 Pen recorder

Figure 1. Schematic diagram of experimental apparatus.

erating conditions, do not immediately mean that the model is adequate as an elementary reaction scheme. The more adjustable parameters in a model, assuming more reaction paths, the more successfully it can be made to fit the data. In this sense, existing kinetic models of coal liquefaction still seem to be empirical. Most of them have been developed for a single type of coal, and the individual reaction paths have seldom been examined independently of the others. Thus, the present study aims to deduce a kinetic model

of coal liquefaction as simple as possible to interpret log- i c d y the observed data for different conditions and Coals. Some experiments are designed with rather extreme com- binations of coal with solvent and gas in order ta characterize coal as well as individual reaction paths. Lique- faction reactivities of 10 different coals are evaluated on the basis of the kinetic model for cases where retrogressive semicoke formation can be ignored as described elsewhere.6

Experimental Section All liquefaction experiments were conducted in a 55 cm3

magnetically stirred autoclave reaction (Figure 1). The reactor was heated and cooled at respective rates at 150 and 100 K/min in an infrared image furnace. Connecting the reactor with a 250 cm3 buffer vessel kept the pressure almost constant through each run." Normally, 3 g of coal and 7 g of solvent were loaded in the reactor. The reaction temperature and pressure were 723 K and 10.1 MPa, respectively. Coals used so far are listed in Table I with their properties from ultimate and proximate analyses. Each coal sample was passed through a Tyler 200 mesh screen and dried

(5) Moritomi, H.; Deng, C.-R.; Nagahhi, H.; Simomwa, S.; Sanada, Y.;

(6) M~ritomi, H.; Kurouji, A.; Sanada, Y.; Chiba, T. Liq. Fuels Tech. Chiba, T. Energy Fuels, following paper in this issue.

1984, 2, 1.

524 Energy & Fuels, Vol. 2, No. 4, 1988

Products

Extractlon by 200cm3 Benzene at 353K for 3hr

Filtration & Washings by 50cm3 Benzene at Room T.nperature

v I

Nagaishi et al.

+ Vacuum Evaporation at 353K for more than 3hr A Benzene Solubles

t Extraction by 1 00cm3 Pyridlne at 388K for 3hr

Fiitratlon & Washings by 40cm3 Pyrldlne at Room temperaturb

t

i Extraction by 200 cm3 n-Hexane at 342K for 3hr

Flitration & Washings by 20cm3 n-Hexam at Room T v a t t w e

t

at 353K for more than 3hr at 353K for more than 3hr

(Preasphritened (Asphalt mea)

Figure 3. Separation scheme for liquefaction products by solvent extraction.

1.0 I I I I 1 0 0.850

0.8 YPS / /

,/” I 30hr Soxhlet extraction 1

0.2 1 ,*------I Akabira coal J 0 I -_- __ 1.- .. .-I-----

5 0 100 150 2 0 0 250 0

extraction time , min

Figure 4. Comparison of boiling point extraction with 30-h Soxhlet extraction.

Soxhlet extraction for about 1800 min (30 h) of a product from Akabira coal liquefaction. It is seen that the yields by the boiling-point extraction increases very rapidly within 30 min and that the extraction times required for yps and yBS to reach those by the Soxhlet extraction are only 60 and 180 min, respectively. Hence, the extraction times in the present experiments were fixed as 180 min. The weight of the insolubles were determined after drying at 353 K for 3 h in the vacuum oven. The yields of the PS, BS, and HS fractions were calculated on the basis of the weight of dry ash-free (daf) coal, assuming an invariable weight of ash during liquefaction

YPS = 1 - [PII/[Cl YBS = 1 - [BII / [Cl

YHS = 1 - ([BII + [BS-HIl)/[Cl where [C] is the weight of daf coal, [PI] is the daf weight of the PI fraction, [BI] is the daf weight of the BI fraction, and [BS-HI] is the daf weight of the BS-HI fraction. From these the yields of preasphaltenes, P, asphaltenes, A, and oils, water, and gases, O&G, are defined here as

YP = YPS - Y B S

YA = YBS - YHS

YO&G = YHS

1 .o

w- I

Talhoiyo Coal Totralln H p $0 .1 MPa

0 7 2 3 K 6 7 3 K

t 0.0 -4

0 30 60 90 600

tlmo , min

Figure 5. Change with time of pyridine-soluble yield for Taiheiyo coal.

The error bound on each of the yield data was less than 0.05.

Results and Discussion Reaction Model. Figure 5 shows yields of the pyri-

dine-soluble fraction, yps, for liquefaction of Taiheiyo coal a t 673 and 723 K with an “excess amount of tetralinn7 under a 10.1 MPa of hydrogen gas atmosphere. The liquefaction condition at 723 K was of the highest severity for hydrogenation in these experiments. At both temperatures, the yields increase rapidly with time and level off before complete conversion of the coal to the PS fraction. The level-off yields at 673 and 723 K were about 0.82 and 0.92, respectively. There are two possible ex- planations for these yields. They could be either the result of the disappearance of the reactive component of coal that produced the PS fraction or the result of a reversible reaction between the unreacted coal and the PS fractionas If they resulted from the latter, the residual PI fraction would be the unreacted portion of the coal and could be further liquefied by removing the PS fraction. Thus, the PI fraction separated from the products after the level off at 673 K was liquefied at 723 K for 30 min with the ex- pectation of an overall yield of 0.99 (=0.82 + 0.18 X 0.92). However, the observed yield for the case was 0.92,5 which corresponds to the yield of only 0.56 based on the PI fraction.

Therefore, a possible reaction model to explain consistently these changes of yps is one that considers that the coal inherently has an inert component, Io, and that its residual reactive component, C, reversibly forms the PS fraction. i.e.

The PI fraction, after the level off a t 723 K, can be practically approximated as the inherent inert Io in the above model, since the PS yield appears invariable to 600 min and coincides within the present experimental error with the overall yield for the subsequent liquefaction at 723 K of the PI fraction obtained a t 673 K.

When an excess amount of tetralin, i.e., 7 g for 3 g of coal, is used, changes of the PS yield are almost inde- pendent of both gas atmosphere and the presence of a catalyst.’ In Figure 6, typical changes in the yields of preasphaltenes, P, asphaltenes, A, and oils and gases, O&G,

(7) Moritomi, H.; Naruse, M.; Nagaishi, H.; Sanada, Y.; Chiba, T. h o c .

(8) Han, K. W.; Dixit, V. B.; Wen, C. Y. Ind. Eng. Chem. Process Des. Int. conf. Coal Sci., 1983, 1983, 134.

Deo. 1978, 17, 16.

Evaluation of Coal Reactivity for Liquefaction Energy & Fuels, Vol. 2, No. 4, 1988 525

1.0 6 6 - I n

Taiheiyo Coal P A 0 6 G Tetralln

7 2 3 K

0.0 L I 0 2 0 40 60 80 1 0 0

time , min

Figure 6. Typical change with time in the yields of preasphaltenes, asphaltenes, and oils and gases in Taiheiyo coal.

r Taiheiyo coal

Naphthalene 10.1 YPa-Nz 7 2 3 K e PS

Om2 r 01 I 1 I I I

0 2 0 40 60 80 1 0 0

time , mln

Figwe 7. Change with time in yields of pyridine solubles, benzene solubles, and n-hexane solubles in Taiheiyo coal without external hydrogen supply.

with time are shown for Taiheiyo coal liquefied at 723 K under such conditions. As usual, the maximum yield for P and A suggests that they are reaction intermediates, and the longer time required for asphaltenes to reach the maximum reveals that asphaltenes are formed from preasphaltenes. The monotonic increase of the O&G yield indicates that they are the final product. In addition, as shown in Figure 7 for the change of YBS and ym with time under extreme conditions where hydrogen is supplied neither from the solvent nor from the gas phase, yBs and yHs level off after about 20 min. Hence, the retrogressive reaction paths from O&G to A and from A to P can be ignored and the model given in (1) is revised to

IO PS ..................... ............................ kc kP k A i (2)

c, i c P - A -0SG i kC' i ............................................... i

It should be noted that this model (eq 2) can describe complete conversion of the reactive coal component, C, to the PS fraction with the rate constant, kp, for the formation of A from P that is much higher than the rate constants of other steps. On the other hand, when Itp is low, an appreciable amount of preasphaltenes remains in the PS fraction and, consequently, the apparent level off of the PS yield can be observed as has been shown in Figure 5 for the case at 673 K.

However, Figure 6 indicates that the increase of yHS has no apparent induction period and YHS is fairly high even in the initial stage. This rapid O&G formation cannot be described by the model given in (21, which assumes con-

Totralin H 2 10.1 MPa

I/ 0 Illlnols N0.6 A Tathotyo

Wandoan A Akablra 0 Daton

0.0 ' I 0 2 0 40 60 80 1 0 0

time , mln

Figure 8. Pyridine-soluble formation for different coals.

secutive O&G formation via P and A. Moreover, as has been shown in Figure 7, the O&G form rapidly in the initial stage even without external hydrogen and the O&G yield levels off within about 20 min. The conversion of coal also terminates at a conversion level of around 0.4 due to a lack of hydrogen from other parts of the coal because of sh~ t t l i ng . '~~J~ This rapid O&G formation in the initial stage with and without the external hydrogen can be ex- plained by assuming a component in the original coal that converts directly to the O&G without consuming any external hydrogen.ll Thus, the model given in (2) is further modified to

IO

The above model assumes that coal generally consists of three different components I,,, C1, and Cz, all of which can be quantified experimentally. Rate expressions based on the above model are given as

-dCci/dt = kclCc1- kcl'Cpz

-dCcz/dt = kczCc2 dCp/dt = kclCc1- kC1'Cp2 - kpCp

dCA/dt = kpCp - kACA

d C ~ & ~ / d t = ~ C Z C C Z + ~ A C A assuming that the reverse reaction from P to C1 is the second-order one, i.e., bimolecular in nature.

Parameters CIo and Cczo for Different Coals. On the basis of the model given in (3), the data on the change of the PS and HS yields with time were analyzed to de- termine the daf weight fractions, CIo, Cclo and Cc20, for different original coals. As described above, CIo of the inherently inert component can be determined from the level-off yield of the PS fraction at 723 K and 10.1 MPa of Hz with an excess amount of tetralin. Figure 8 shows changes of yps for different coals. The level off of the yield was seen for all coals within 60 min, and the values of CIo were easily determined. In Figure 9 changes of YHS are shown for Taiheiyo coal liquefaction at 673 and 723 K under a 10.1 MPa of nitrogen gas atmosphere with naphthalene. Although O&G formation at 673 K is slower than

(9) Neavel, R. C. Fuel 1976, 55, 237. (IO) Whitehurst, D. D.; Mitchell, T. 0.; Farcasiu, M. Coal Liquefac-

tion, The Chemistry and Technology of Thermal Processes; Academic: New York, 1980; p 316.

(11) Chiba, T.; Sanada, Y. J. Fuel SOC. Jpn. 1978, 57, 259.


0

' J Z

Nagaishi et al.

O W

0 0 0

I I

Taiheiyo Coal Naphthalene

1 I 1

*= '-I o - 6 1 I

0.4 1 0.2 -

8 " 0

0 7 2 3 K 6 7 3 K

0 .0 O oooo O

* O @ 0 o o re 0

0 0 - 0

0 . 0 * , 0.0 -4

0 2 0 40 60 90

time , min

Figure 9. Change with time of oil and gas yield in Taiheiyo coal without external hydrogen supply.

o.4

0.2

I I I " ,

- Naphthalene N2 10.1 MPa

- 7 2 3 K Illlnol8 No.6 - A

A " 6-

0

Wandoan

I

\ 0.0 *= '

0.4

Daton - 0.0 1 I , " I

,#

L

-

0 2 0 40 60 90

time , min

Figure 10. Oil and gas formation for different coals without external hydrogen supply.

Table 11. Weight Fraction of Io, C1, and C2 in Original Coals

daf wt fraction coal G o ccroa CCZO

~~

Yallourn (Y) 0.10 0.78 0.12 Soyakoishi (S) 0.06 , 0.71 0.23 Huolinhe (H) 0.07 0.71 0.22 Wandoan (W) 0.09 0.68 0.23 Taiheiyo (T) 0.08 0.61 0.31 Illinois No. 6 (I) 0.02 0.77 0.21 Datong (D) 0.29 0.66 0.05 Zaozhuang (Z) 0.18 0.72 0.10 Akabira (A) 0.12 0.69 0.19 Yubarishinko (Yu) 0.07 0.78 0.15

By difference.

that at 723 K, the level-off yields at both temperatures are nearly identical, suggesting that they merely depend ofi the nature of the coal. Therefore, it is assumed here that the weight fraction of C2 in the original coal, Cc20, is given as the level-off yield of O&G at 723 K. Figure 10 shows similar changes of y m for other coals, from which Ccm were determined.

In Table I1 the values of CIo, Cc20, and Cclo (= 1 - CIo - Cc20) are summarized for 10 coals employed in the present experiment. Coals with a lower CIo have a higher ultimate conversion, and coals having a higher Ccm can be liquefied with less supplied hydrogen. Among the coals, Illinois No. 6 coal has the highest ultimate reactivity with the lowest CIo of 0.02 while Datong coal is the least reactive with a CIo as great as 0.29. Furthermore, a relatively great

I I

0.4 C P c c I I lo * I * I O

0 t e o t o 0 1

60 7 0 80 '0.0 0.1 0.2 0.3'0.6 0.8 1.0 C% o/c H/C

Figure 11. Correlation of CIo and Czo with conventional coal characterization pafameters.

0 2 0.8 i

t O D 0.6 1-1

0 0.1 0.2 0.3 0.4

cc20 I - Figure 12. Correlation between values of CI0 and Cm The key is given in Table 11.

a m o u t of external hydrogen is required for the latter coal because of its lower value of CC2@

In Figure 11, Ccm and CIo are plotted against the weight fractions of total inerts, TI, fixed carbon, FC, volatile matter, VM, and the carbon, % C, and the atomic ratios of O/C and H/C. Only a slight correlation can be recog- nized in the plots against the H/C ratio. The values of CIo seem to decrease with the ratio, whereas Cc20 values seem to increase. However, these parameters cannot be correlated explicitly with other conventionally used rank parameters from the ultimate and proximate analyses. This implies that Ccl0, Cc20 and CIo are new rank parameters different from the conventional ones. On the other hand, there is a clear correlation between CCz0 and CIo, which is shown in Figure 12. It can be seen that (1 - CIo) increases monotonously with CC2@ In addition, Yallourn, Wandoan, and Taiheiyo coals have almost the same value of CIo but different values Cc20. This means that the hydrogen consumption values would differ for them to attain the same PS yield.

Rate Constants and Model Applicability. The rate constant of each reaction path in the present model was obtained by fitting the model prediction with the coal characterization parameters in Table 11 to the experimental conversion and yield data by means of a nonlinear re- gression analysis.12 A preliminary computation showed that the contribution of the retrogressive reaction path from P to C1 to the whole reaction was small under conditions with a sufficient amount of tetralin or with naphthalene, hydrogen gas, and catalyst. Hence, the rate

(12) Nakagawa, T.; Oyanagi, Y. Recent Developments in Statistical Inference and Data Analysis; Matsushita, K., Ed.; North-Holland Am- sterdam, 1980; p 221.

Evaluation of Coal Reactivity for Liquefaction

n

10.1 UP. 729 K

kC1 0.2s m1n-l 1 kcz 0.18

P - Naphthalena/NZ Y ns

Energy & Fuels, Vol. 2, No. 4, 1988 527

0.8

I 0.6

P '= 0.4

1.0, 1.0 I

1.0

0.8

I 0.6

m 1 a 's 0.4

-

0.8

' 0.8

m P '% 0.4

" " I_-

Tatralin/H2 Yps

- C.IG.

10.1 MPa 723 K

-

7 ) kC1 0.30 mid' kcz 0.22

'

Naphthalena/N2 YHS 1 --+

- 0.k.

10.1 MPa

k a 0.19 mid' kcz 0.22

Naphthalena/W2 YHS

I (b) wandoan

0 20 40 60 80 100 0 20 40 60 80 100

time , mln tlme , min

0.8

- C.IC.

10.1 MPa

k c l 0.15 min-1 kcz 0.21

Naphthalene/Np YHS 1 -

0 20 40 60 80 100

tlme , min

0.2 1 NaphthalenaIN2 YHS - -

0.0 1 I I

0 20 40 60 80 100

t h e , mln

Figure 13. Comparison of observed pyridine-soluble and oil and gas yields with those predicted for different coals: (a) Taiheiyo coal; (b) Wandoan coal; (c) Illinois No. 6 coal; (d) Akabira coal; (e) Datong coal.

constant kcl' can be assumed t~ be 0. In addition, the rate constant kcz could be determined from the observed change of y H S under conditions without an external hydrogen supply, as shown in Figures 9 and 10. The value of kcz thus obtained for each coal was consistently used in the analysis.

The model gives good agreement with the observed yields of PS and HS fractions under conditions for ob- taining CIo and Ccm This is shown in Figure 13a-e for five different coals. Due to a lack of data, the values of kcz for Illinois No. 6 and Datong coals could not be confidently determined by the method described above. However, since the predicted results were much less sensitive to kcz than to Ccz0, the values of kcz for Wandoan and Akabira coals were respectively used for Illinois No. 6 and Datong coals because their values of Cczo are similar. As can be

seen, the model fits the yps data for all coals within 5% error.

Figure 14 shows the applicability of the model to the data for Taiheiyo coal liquefaction with tetralin at 723 K and 10.1 MPa of Np Here, the curves represent the predicted results. Although there are some discrepancies between the predicted and observed yp and yA, the model can be seen to fit the data reasonably well. The optimal set of rate constants shown in the figure indicates that the rate constant for O&G formation from asphaltenes is smallest, roughly a tenth of those for other reaction paths.

Figure 15 shows the tetrahydrofuran-soluble yield, ym, of Illinois No. 6 coal observed by Mohan and Silla.13 In

(13) Mohan, G.; Silla, H. Znd. Eng. Chem. Process Des. Deu. 1981,20, 349.


1 .O Telheiyo Cod N2 10.lMPa

Nagaishi et al.

and gases without consuming any external hydrogen, and C1, which forms oils and gases consecutively through preasphaltenes and asphaltenes. The weight fractions of these in the original coal have been determined experimentally for 10 different coals as new parameters to characterize coal reactivity for liquefaction.

Close agreement has been obtained between the observed and predicted yields of pyridine- and n-hexane- soluble fractions (conversion of coal and yield of oils and gases, respectively), where the predictions are much more sensitive to the weight fractions of I,, and C2 in the original coal than to the rate constants. However, the model is not successful enough in describing the change of preas- phaltene and asphaltene yields with time. In the present work most attention has been directed to model reaction paths in which coal takes part, whereas those among preasphaltenes, asphaltenes, and oils have been examined rather qualitatively. Therefore, a further experimental examination of the latter reaction paths will be necessary. The presence of retrogressive reaction paths and their effect on the overall reaction will be discussed in a subsequent paper.

Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research, Special Project Research on Energy (Energy(1)-No. 58040001 and No. 59040002), from the Ministry of Education, Science and Culture, Japan.

Nomenclature Ci = weight fraction of component i Cclo = weight fraction of component C1 in original daf coal CczO = weight fraction of component C2 in original daf coal CIo = weight fraction of component Io in original daf coal E = weight fraction of exinite FC = weight fraction of fixed carbon k = apparent rate constant, m i d Sf = weight fraction of semifusinite TI = weight fraction of total inerts (= 1 - V - E - Sf/3) V = weight fraction of vitrinite VM = weight fraction of volatile matter y = product yield

- : 0.0 v

0 10 20 30 40

tlmb , mln

Figure 14. Comparison of observed yields with those predicted for Taiheiyo coal under nitrogen gas atmosphere: (0) unreacted coal; (A) preasphaltenes; (0) asphaltenes; (A) oils and gases. The curves give calculated yields.

I .o 663 - 723 K

Ccx) 0.67

c c p 0.21 CK) 0.12

k c l k h kcz o 0.90 - 0.22 mln-'

0.10 0.006 0.03

0.0 L I 1

0 20 40 00 80

tlmo , mln

Figure 15. Applicability of proposed model to data by Mohan et al. for Illinois No. 6 coal, The curves give calculated yields.

their case the unreacted coal was defined as tetrahydrofuran insolubles. It is shown in the figure that a t 723 K YTHF levels off as yps. The level off again indicates that the coal has a component Io that is inherently inert for liquefaction. Since the level off of yTHF at 723 K is about 0.88, an alteration was made in the prediction for the value of CIo for the coal from 0.02 in Table I1 to 0.12 without changing Ccm It can be seen that the model fits the data well for both temperatures.

Conclusions A kinetic model has been developed to interpret logically

the characteristic changes in the conversion of coal and the yields of produds lumped conventionally on the basis of their solubilities in different solvents.

In the model, a coal is assumed to consist of three different pyridine-insoluble portions, I,,, which is inherently inert for liquefaction, Cp, which converts directly to oils

A BI BS C C1

c2

HI HS b O&G P PI PS THF

Components asphaltenes (BS-HI) benzene-insoluble fraction benzene-soluble fraction dry ash-free (daf) coal portion of coal forming O&G consecutively via P

and A portion of coal forming directly O&G without

consuming external hydrogen n-hexane-insoluble fraction n-hexane-soluble fraction portion of coal inherently inert for liquefaction oils and gases (HS) preasphaltenes (PS-BI) pyridine-insoluble fraction pyridine-soluble fraction tetrahydrofuran-soluble fraction

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Evaluation of coal reactivity for liquefaction based on kinetic characteristics