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KINETICS OF PERCHLOROETHYLENE EXTRACTIONDESULFURIZATION OF COALSSunggyu Lee a , Padmakar Vishnubhatt a & Conrad J. Kulik ba Process Research Center Department of Chemical Engineering , The University of Akron ,Akron, OH, 44325-3906b Fuel Science Program , Electric Power Research Institute , Palo Alto, CA, 94303Published online: 24 Oct 2007.

To cite this article: Sunggyu Lee , Padmakar Vishnubhatt & Conrad J. Kulik (1993) KINETICS OF PERCHLOROETHYLENEEXTRACTION DESULFURIZATION OF COALS, Fuel Science and Technology International, 11:10, 1345-1365, DOI:10.1080/08843759308916136

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FUEL SCIENCE AND TECHNOLOGY INT'L.. 11(10), 1345-1365 (1993)

KINETICS OF PERCHLOROETHPLENE EXTRACTION

DESUL.FURIIAT1ON OF COALS

Sunggyu ~ee', Padmakar Vishnubhatt and Conrad J. ~ulik'

Process Research Center Department of Chemical Engineering

The University of Akron Akron, OH 44325-3906

' Fuel Science Proaram Electric Power Research institute

Palo Alto, CA 94303

ABSTRACT

The perchloroethylene coal cleaning process selectively removes the organic sulfur from coal via a hybrid mechanism of chemical reaction .and physical solvation. It was found that the chemical reaction was catalyzed by the inorganic species present in the coal. In this paper, a kinetic study was experimentally carried out to determine rate constants of the reaction. It was confirmed that the extent of organosulfur extraction depended strongly on the type of coal, and also that there is a critical extraction time which is required as the minimum time for each type of coal. Isothermal batch kinetic studies were done for various tvoes of coal. A relation was established between the typ;-of coal and its kinetics and hence the minimum time for extraction.

if: To whom all correspondence should be directed.

1345

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1346 LEE, VISHNUBHATT, AND KULIK

INTRODUCTION

The perchloroethylene (PCE) coal cleaning process

employs perchloroethylene as the solvent to extract the

organic sulfur from coal. The organosulfur extraction

efficiency strongly depends on the type of coal. Some

coals with special ingredients promote organosulfur

extraction. It was established that the extraction

process is a hybrid system of chemical reaction and

physical solvation (Lee and Vishnubhatt, 1992a). The

chemical reaction in this process is catalyzed by the

naturally available inorganic species in coal (Lee et

al., 1992a).

Process engineering studies have proven that the

minimum time required to achieve best possible

organosulfur extraction is different from coal to coal

(Lee et al., 1991a). The extent of organic sulfur

reduction depends upon the mineral matter content of the

coal, its type and distribution. Also, more importantly,

it is dependent on the type of organic sulfur existent in

the coal.

The organic sulfur in coal is present in several

forms. Qualitative studies have identified sulfur in the

form of thiols, sulfides, and heterocyclic compounds

(Stock et al., 1989). These studies have also

established that coals, in general, contain alkylated

thiophenes, benzothiophenes, naphthobenzothiophenes,

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PCE EXTRACTION DESULFURIZATION OF COALS 1347

phenanthrothiophenes, dibenzothiophenes among the array

of heterocyclic substances (White et al., 1987). It has

been found experimentally that the aliphatic form of

organic sulfur is easier to extract than the thiophenic

form. XAFS analysis on the raw and PCE extracted coal

samples provide some evidences that the perchloroethylene

extraction process predominantly removesthe sulfidic and

sulfonic forms of organic sulfur (Huggins et al., 1991).

The composition of the various forms of organic sulfur

differs from coal to coal. Coals from different regions

have significantly different macromolecular structure.

Therefore, the structures of organosulfur species and

their relative proportions are also different from coal

to coal.

It is very difficult to characterize as well as

quantify the various forms of organic sulfur in coal.

Efforts have been made to identify and quantify the

various types of organic sulfur in coal (Huggins et al.,

1991; Duran et al., 1986; Nishioka, 1988). Stock et al.

(1989), have discussed in detail various methods to

determine the different forms of sulfur associated with

coal. However, in spite of the tremendous efforts by

many researchers, only a small fraction of the organic

sulfur constituents of coal have been identified (Stock,

1992). Due to the idiosyncratic characteristics of

certain samples as well as the inconsistent and

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1348 LEE, VISHNUBHAlT, AND KULIK

insufficient data available in literature, it is

difficult to explain the differences in organosulfur

extractability between coals, on the basis of its

organosulfur constituents.

The objective of this paper is to pin-point these

differences on the basis of kinetic study., Various types

of coals were studied to explain the variations in the

minimum time for extraction as well as extent of

organosulfur removal from coal to coal. Since there is

no kinetic or mechanistic information, it is assumed that

the organosulfur extraction-reaction follows a pseudo-

first order reaction (Lee et al., 1989). Rate constants

for several types of coals were determined accordingly.

Such global kinetic information is very useful in the

evaluation of process chemistry as well as in the full-

scale investigation of chemicalmechanisms. Furthermore,

such information is essential for detailed design of

process plants of various scales.

EXPERIMENTAL

Coal SamDles

Three types of coal samples from various regions

were chosen for this experiment viz. , (1) Ohio 5/6 (1:l

mixture of Ohio 5 and Ohio 6), (2) Illinois 6, and (3)

Indiana 5 (Petersburg). These samples were standardized

to obtain a homogeneous composition throughout the entire

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PCE EXTRACTION DESULFURlZAnON OF COALS

Table I

Coal Analyses (Dry Basis)

Coal Types

Illinois 6 Ohio 5/6 Indiana 5 ........................................................ Volatile Matter 33.14 37.49 35.07

Fixed Carbon 38.93 54.39 42.70

Ash Content 27.88 8.12 22.23

Carbon 52.26 69.83 53.70

Hydrogen 4.18 5.10 4.27

Nitrogen 0.92 2.18 1.05

Note: All numbers are in mass percentages.

Table I1

Forms of Sulfur Analyses of Coal Samples

Coal Sample Total Pyritic Sulfatic Organic Sulfur Sulfur Sulfur Sulfur

-

Illinois 6 3.75 1.40 0.25 2.10

Ohio 5/6 3.07 1.27 0.21 1.58

Indiana 5 7.76 3.67 1.67 2.42

Note: All numbers are in mass percentages.

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1350 LEE, VISHNUBHATT, AND KULIK

investigation (Lee et al., 1991a). Upon standardization,

the coal samples were stored in polyethylene bags in an

inert atmosphere to prevent thermal and compositional

degradation. The ultimate analyses and proximate

analyses (dry basis) of these samples are presented in

Table I. The forms of sulfur analyses of the coal

samples chosen for this investigation are illustrated in

Table 11.

Oraanosulfur Removal Usina the PCE Drocess

In order to obtain reproducible results, a standard

benchmarked procedure for organosulfur extraction was

followed (Lee et al., l99la). The same procedure was

employed to remove organosulfur from all the coal samples

to ensure a fair assessment of organosulfur removal.

In the benchmarked procedure, coal and

perchloroethylene are well contacted in a batch reactor.

Immediately after the extraction, coal and

perchloroethylene are separated by "hot filtration".

"Hot filtration" involves a fast and isothermal

separation of coal and perchloroethylene (Lee and

Vishnubhatt, 1992b). The sulfur-rich perchloroethylene

is sent for analysis and distilled for re-use. The coal

is sampled for analysis and then sent for mineral matter

removal. In order to obtain a fair and unconfounded

assessment of the extent of organosulfur removal only,

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PCE EXTRACTION DESULFURIZATTON OF COALS 1351

the extracted coal was not demineralized for this

specific study.

Due to the specific objective of this research, the

time of extraction was varied. Three sets of experiments

were performed, on each of the three different types of

coals. In each set, a series of batch experiments were

performed by varying the extraction time. In this way,

isothermal batch kinetic data were obtained for the three

types of coal. The solubility of sulfur in

perchloroethylene is a very strong function of

temperature. With the decrease in process temperature,

below the boiling point of the solvent, the sulfur

solubility in perchloroethylene reduces drastically.

Moreover, the melting point of sulfur (amorphous form,

120°C at atmospheric pressure) and the normal boiling

point of perchloroethylene (121.2a°C at atmospheric

pressure) are also close to the process temperature which

in turn affects the process efficiency. Thus, the

temperature dependence of the kinetics was not

investigated in this study.

Analvsis

The raw and organosulfur extracted coals were

subjected to several analyses to determine the extent of

organosulfur removal. The total sulfur analysis was done

in LECO SC-132 Sulfur Determinator as well as by wet

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1352 LEE, VISHNUBHATT, A N D KULlK

chemical analysis (ASTM D-3177 standard). In order to

eliminate any influence on the measured sulfur content by

residual perchloroethylene in coal, both vacuum dried (8

hours at 90°C) and stem stripped (Lee et al., 1992b)

samples were analyzed for sulfur content determination.

The forms of sulfur analysis was carried out using ASTM

D-2492 standard. The perchloroethylene was analyzed on

a gas chromatograph / mass spectrometer (GC/MS).

RESULTS

piscussion

The batch kinetic data obtained for Illinois 6 coal

are summarized in Table 111. Figure 1 graphically

illustrates the same data. It should be noted from

Figure 1 that the extraction efficiency increases rapidly

with time till it reaches a maximum of 36 % at 30

minutes. Then, the extraction efficiency remains

constant irrespective of the time.

Table IV shows the kinetic data obtained for Ohio

5/6 coal. Figure 2 shows the same data graphically. It

can be noticed that the organosulfur extraction of this

coal is similar in nature to the Illinois 6 coal.

However, the extraction efficiency on an absolute scale

is higher. This coal also requires 30 minutes of

extraction to reach its maximum organosulfur

extractability.

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PCE EXTRACIION DESULFURlZATION OF COALS

Table I11

Batch Kinetic data on Illinois 6 Coal

Time Extraction (minutes) Efficiency

( % conversion)

E x t r o c t i ~ Time (Mlrutes)

Figure I. Kinetic Onto on Illinois 6 Cool

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LEE, VISHNUBHAlT. AND KULIK

Table IV

Batch Kinetic data on Ohio 5/6 Coal

Time Extraction (minutes) Efficiency

( % conversion)

01 0 10 20 30 40 50 BO A

Extraction Time (Minutes)

Flgure 2. K inet ic Data on Ohio 5/6 Cool

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PCE EXTRACTION DESULFURJZATION OF COALS 1355

Table V shows the kinetic data obtained for Indiana

5 coal. Figure 3 shows that the kinetic behavior of this

coal is quite different from the previous two coals. The

time required to reach its maximum organosulfur

extractability is only 15 minutes. And on reaching this

peak value, the organosulfur extraction efficiency drops

down and continues so, with time.

These results, as well as previous studies of the

Process show that the reaction under consideration may be

postulated as:

The species A represents the organosulfur compound

present in the macromolecule of coal. The reaction step

A to B represents the C-S bond cleavage reactions leading

to the formation of an intermediate B. The active

species B immediately decomposes into labile sulfur S and

an active organic species P which may be regarded as a

part of the parent organosulfur compound without the

sulfur in it. The reaction S to P represents an

undesired reactioninvolvingthe liberated sulfur species

re-entering the coal organic matrix viainter-penetrating

polymeric network ( I P N ) or cross-linking.

According to the proposed mechanistic model, the

desirable reaction is from Ato S. Perchloroethylene has

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LEE, VISHNUBHAIT, AND KULlK

Table V

Batch Kinetic data on Indiana 5 Coal

Time Extraction (minutes) Efficiency

( % conversibn)

Figure 3. K ine t i c Oato on Indiano 5 Cool

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PCE EXTRACTION DESULFURlZATlON OF COALS 1357

the capability of reaching the remote sites in the coal

matrix to dissolve and extract this labile sulfur in its

elemental or oligomeric form (S6+). As represented in

the model, the intermediate species B decomposes into an

active organic compound, which seeks to be stabilized.

1f the reaction mixture is kept in the extraction

environment longer than necessary, the labile sulfur

species S would react with the active organic species to

form cross-linked sulfur compounds (Lee et al., 1991b).

These inter-penetrating polymer networks are very

difficult to break. Thus it becomes almost impossible to

recover and extract such cross-linked sulfur. Therefore,

formation of such inter-penetrating polymeric network

(IPN) is undesirable. Depending upon the time and

temperature, the rate constants and hence the rates of

each mechanistic step vary. Therefore, it becomes

necessary to analyze the kinetics of the process to

manipulate the reaction network to proceed in the desired

fashion, i.e., the liberation of labile sulfur.

By Material Balance:

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1358 LEE, VISHNUBHAlT, AND KULlK

Case 1 <Illinois 6 1 :

By quasi-steady state assumption, it can be assumed

that B is a fleeting species, therefore Equation (2)

reduces to

dCs - d t

- O :. k2CBm -k4Cs, = O ( 6 )

Also at X = X,;

dCB - d t

- 0 :. k 3 C B s + k , C s ~ = 0 ( 7 )

The subscript "en in the equations denotes the

concentrations or fractions at equilibrium.

From Equations ( 6 ) and (7), k, and k, are negligible, in

order for the desulfurization reaction to be significant.

Then, Equation (5) becomes,

k ,C , -k,CB = O ( 8 )

As a result,

Thus, a simple first-order equation is justified.

where k is the first order rate constant, and C, is

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PCE EXTRACTION DESULFURIZATION OF COALS

Figure 4 . Regression flnolysis to Obtoin

Rote Constant fo r I l i ino is 6 Cool

By integral zing between the time limits, 0 and t (t < te 1

i.e., any time before the time required to achieve

equilibrium), we get,

By making a plot of -ln(l-x) versus t, the first order

rate constant can be obtained by regression. Figure 4

shows this plot for Illinois 6 coal. For this Illinois

6 coal, the rate constant was found to be 0.0745 sec",

at 120°c.

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1360 LEE, VISHNUBHAIT. AND KULIK

t (Minutes)

Figure 5. Regression Rnolysis to Obtain

Rote Constmt for Ohio 5/6 Cool

Case 2 (Ohio 5/61:

This coal is very much similar to Illinois 6 coal in

terms of its kinetic behavior. Hence the mathematical

treatment is the same. Figure 5 shows the linear plot

between -ln(l-X) and t. The first order rate constant for

Ohio 5/6 was found to be 0.0695 sec" at 120°C.

Case 3 (Indiana 51:

This coal is different in the sense that, after the

steady state time is reached, the side reactions become

dominating and thus rate controlling. Again, by quasi-

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PCE EXTRACllON DESULFURIZATION OF COALS 1361

Figure 6. Regresssion Rnoiysis to Obtoin

Rate Constant for Indiono 5.

steady state assumption, B is taken as the fleeting

species. By a similar mathematical treatment, it can be

established that the organosulfur reaction is a pseudo-

first order reaction system. Using Figure 6, the first

order rate constant was found to be 0.115 sec-' at 120°C.

The first order rate constant is evaluated using Figure

6. However, after the equilibrium time is reached, the

side reactions become more significant. As a result of

these competing and consecutive reactions, the reaction

direction is diverted towards P rather than S.

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1362 LEE, VISHNUBHATT. AND KULlK

CONCLUSIONS

The results obtained in this investigation have a

significant effect on the economics as well as process

engineering. In case of Ohio 5/6 and Illinois 6 coals,

during the first 30 minutes of the reaction, the desired,

progressive reaction leading to the generation of labile

sulfur via C-S bond cleavage reactions is most

predominant. However, after 30 minutes, the undesired

reactions leading to the re-absorption of the active,

labile sulfur into the coal matrix become equally

dominant. Therefore, it would be uneconomical and

wasteful to extract Ohio 5/6 and Illinois 6 more than 30

minutes. In these coals, the rates of desulfurization

and the rates of formation of inter-penetrating polymer

network achieve a state of dynamic equilibrium.

However, in case of Indiana 5, the desulfurization

time should not exceed more than 15 minutes. After 15

minutes, the magnitude of the rate of formation of inter-

penetrating network becomes significantly higher than the

desulfurization rate. The rates of these undesired

competing reactions increase with time. Thus, any

desulfurization after 1 s minutes is detrimental to the

cpal, because the organic sulfur which could have been

extracted and separated is lost in the coal matrix. Such

a cross-linked sulfur in the coal matrix is very hard to

extract by an additional process treatment.

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PCE EXTRACTION DESULFURIZATION OF COALS 1363

Thus, it is concluded that the organic sulfur

extraction in coal depends upon its type and the nature

of organosulfur species present in it. In Ohio 5 / 6 and

Illinois 6, the species promoting the formation of P are

not as predominant as they are in Indiana 5. Unlike the

Ohio 5/6 and Illinois 6 coals, in Indiana 5, there is no

dynamic equilibrium at the end of extraction time. This

observation is very important from the point of view of

process engineering. In case of Indiana 5, the coal-

perchloroethylene separation has to follow immediately

after 15 minutes of extraction. If not, the extracted

sulfur re-enters the coal matrix in the form of cross-

linked chains which are normally more difficult to break

than the original coal matrix.

At this stage it is very difficult to characterize

the cross-linked sulfur in the form of inter-penetrating

polymer network. However, the characterization of the

various forms of organic sulfur and their effect on the

organosulfur removal efficiency is beyond the scope of

this paper. Further research is being done to determine

the composition of the aliphatic and thiophenic compounds

in coal and their desulfurization.

ACKNOWLEDGEMENTS

The authors would like to thank the Electric Power

Research Institute (EPRI) for financially supportingthis

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1364 LEE, VISHNUBHATT. AND KULlK

project under the contract RP 2655-16/8003-15. The

authors express their sincere thanks to Kathy L.

Fullerton (UA) and Theodore Thome (UA) for their help and

valuable suggestions.

REFERENCES

Duran, J.E., Mahasay, S.R., and Stock, L.M., 1986. The Occurrence of Elemental Sulphur in Coals, Fuel, 1986, 65, pp 1167-1168.

Huggins, F.E., Mitra, S., Vaidya, S., Taghiei, M.M., Lu, F., Shah, N., and Huffman, G.P., 1991. The Quantitative Determination of all Major Inorganic and Organic Sulfur Forms in Coal from XAFS Spectroscopy: Method and Applications, Proceeding, 4'" International Conference on Processing and Utilization of High Sulfur Coals, Idaho Falls, ID, August 26-30, 1991.

Nishioka, M., 1988. Aromatic Sulfur Compounds other than Condensed Thiophenes in Fossil Fuels: Enrichment and Identification, Energy 6 Fuels, 2, pp 214-219.

Lee, S., Fullerton, K.L., and Culik C.J., 1991a. Process Engineering Studies of the Perchloroethylene Coal Cleaning Process, Fuel Sci. & Tech. Int'l, 9(7), 873-888.

Lee, S., Fullerton, K.L., and Vishnubhatt, P., 1991b. Updates of Perchloroethylene Coal Extraction Process Development, Proceedings: Sixteenth Annual EPRI Conference on Fuel Science and Conversion, May, 1991, in press.

Lee, S. and Vishnubhatt, P., 1992a. Perchloroethylene Extraction Desulfurization of Low Sulfate Coals, Fuel Sci. & Tech. Int'l, in press.

Lee, S. and Vishnubhatt, P., 1992b. Effect of Filtration Temperature on Organic Sulfur Removal from Coal by Perchloroethylene Coal Cleaning Process, Fuel Sci. & Tech. Int'l, in press.

Lee, S., Vishnubhatt, P., and Culik C.J., 1992a. Perchloroethylene Extract ionDesul fur izat ionof Weathered Coals, Fuel Sci. 6 Tech. Int'l, in press.

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PCE EXTRACTION DESULFURlZATlON OF COALS 136.5

Lee, S., Vishnubhatt, P., Wilinson, M. and Adamec T., 1992b. Removal of Residual Chlorine from Coal by Steam Stripping, Fuel Sci. & Tech. Int'l, in press.

Stock, L.M., Wolny, R., and Bal, B., 1989. Sulfur Distribution in American Bituminous Coals, Energy & Fuels, 3, 6, pp 651-661.

Stock, L.M., 1992. Toward Organic Desulfurization, Energeia, University of Kentucky, Center for Applied Energy Research (CAER) 3, 1.

White, C.M., Douglas, L.J., Perry, M.B., Schmidt, C.E., 1987. Characterization of Extractable Organosulfur constituents from Bevier Seam Coal, Energy 6 Fuels, 1987, 1, pp 222-226.

RECEIVED: July 27, 1992 REVISED MANUSCRIPT ACCEPTED: August 17. 1992

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