A Kinetic Study of Super Critical

8/14/2019 A Kinetic Study of Super Critical

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A KINETIC STUDY OF SUPERCRITICAL-FLUID EXTRACTIONOF COAL

Institute of Chemistry and Technology of Petroleum and Coal

Technical Uniuersity of Wroclaw (Poland)

ABSTRACTA high-volatile bituminous coal was extracted with toluene in a high-pressure system attemperatures ranging from 473 K to 713 K. At each temperature studied, a second orderequation, with respect to undissolved but potentially extractable coal, closely fits theexperimental time dependence of the extraction yield. The proposed kinetic model providesvalues for the extraction rate constant and the extractable fraction of coal, both calculable bylinear regression. Analysis of the extraction kinetic data indicates a low temperature (T < 593 K)physically controlled dissolution of coal and a high temperature (623 K--683 K) process wherechemical reactions become rate-controlling with an energy of activation of 101 kJ/mole.

INTRODUCTIONOne of the most important pieces of information characterising the process of supercriticalsolvent extraction of coal is the yield of extract. Although a number of experimental data onextract yields obtained from coals of various properties are available, most of the resultsusually found in the literature are not directly comparable, since they refer to differentextraction times and various experimental conditions. As so far, no research appears to bedirected specifically to the kinetics of this process. There is very little or no reportedexperimental data consistent enough to give an insight into time or temperature dependence ofthe extraction yield. This paper reports a kinetic study of the supercritical toluene extraction ofa high-volatile bituminous coal over a wide range of temperature at a constant solvent pressure.

EXPERIMENTALCoal studiedFor this kinetic study, a high-volatile bituminous C coal from the Upper Silesian Coal. Basinwas selected from among ten coals of various rank. This coal showed considerably higherextraction yield than the rest of coals studied at 623 K under toluene pressure of 9.8 MPa [1].The characteristics of the selected coal is given in Table 1 and refers to the size fraction, between0.5 and 1.2 mm, which was used in these experiments. As determined by the standard Gieselermethod, the coal plasticity ranged from 638 to 717 K with the maximum plasticity at 693 K.Analytical grade toluene was used as the solvent.

Table 1. Analysis of coal.

Proximate Analysis

Moisture Ash Content Volatile Matter

[Wt %, dry] [Wt %, dry] [Wt %, m.a.f.]


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1.1 3.5 41.4

Elemental Analysis [Wt % d.a.f.]

C H N S O(diff)

81.3 5.3 1.4 0.9 10.5

Petrographic Analysis [Wt %]

Vitrinite Exinite FusiniteMicrinite

PyriteClayMinerals

52.2 16.4 27.7 1.9 0.5 1.3

Apparatus and procedureA schematic diagram of the apparatus is shown in Figure 1. A stream of toluene was pumped bya high-pressure pump (2) at a rate of 0.9 1/60 min. into an electrically heated coil preheater (3).Then the solvent, heated up to the temperature assumed for the experiment, was introduced intothe extraction vessel (4) through a steel thimble containing a fixed bed of about 42 grams of coal.To avoid loss of coal dust from the thimble its lower closure was equipped with two fine-meshwire nets and a layer of quartz wool in between. After the solvent had passed the bed of coal thegaseous solution formed thereby was expanded with two throttling valves (5) into the coldcollector (6) operated at the atmospheric pressure, so that the total pressure in the extractionvessel was maintained at the assumed level of 9.8 MPa. The liquid product of extraction wasremoved from the collector periodically at 60 min intervals.


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Fig. 1. Apparatus for extraction of bituminous coal with toluene (subcriticaland supercritical).

Components include: 1-. Feed reservoir, 2. Solvent pump, 3. Electricallyheated preheater, 4. Extraction vessel, 5. Throttling valves, 6. Productcollector.

Table 2. Yield of extract as a function of extraction time.

Run

no.

T

[K]

Weight Percent, d.a.f.

0 min 60 min 120 min 180 min 240 min 300 min 360 min

1 473 0.51 1.94 2.77 3.53 4.00 4.35 4.55

2 503 1.00 4.24 5.67 6.50 7.48 8.20 8.40

3 533 1.71 7.50 8.49 9.28 10.1 11.0 11.7

4 563 1.82 8.30 10.5 11.4 12.4 13.2 14.0

5 593 3.30 11.3 13.2 14.2 15.4 16.0 16.5


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6 623 4.50 13.5 17.2 19.5 21.7 23.4 24.9

7 638 7,28 19.1 24.2 26.7 28.3 29.5 30.5

8 653 8.14 23.6 27.4 29.7 31.0 32.0 33.1

9 668 11.4 26.0 30.2 32.2 33.3 34.2 34.910 683 12.7 26.5 28.8 29.9 30.6 31.3 31.8

11 713 11.3 23.6 25.2 26.2 27.3 28.0 28.6

The extraction runs were continued up to 360 min. From these liquid products the solvent wasdistilled off in a rotary vacuum evaporator (420 K, 7.7 kPa). The residual non-distillable extractswere dried in vacuum at 313 K and weighed to determine the time dependence of the extractionyield calculated in percent by mass on d.a.f. basis. The results are presented in Table 2.Extraction time was measured starting from the moment when the assumed values oftemperature and pressure had been reached. The pre-heating time varied from 25 to 30 mindepending on the extraction temperature. During the preheating period the coal sample was

subjected to an atmosphere of solvent under pressure which progressively increased to 9.8 MPa.The extraction temperature range studied in this work i.e. from 473 K to 713 K covers bothsupercritical and subcritial regions of toluene. The critical temperature for toluene is T = 593 K.

RESULTS AND DISCUSSIONThe kinetic treatment of the extraction data is supported on the general assumption that only apart of the organic coal substance can be extracted after any finite time regardless of how longthe extraction is continued. The a priori unknown potentially extractable fraction of coal (i.e.extraction yield at infinite time) cannot be determined directly, being rather a matter ofcalculation from extraction kinetic data. In the case when only the increase in extract yield is

measured, the kinetic equation should involve a constant ultimate yield term X for t , and

also allow for the pre-heating period. Relevant first-order and second-order equations withrespect to potentially soluble but not dissolved coal have been tested to fit the experimental data.The Guggenheim's formula [2] for first order mechanism has the form:

(1)

where:

t = extraction time, min,

X = extract yield at time t, % d.a.f.,

t = constant extraction time increment (here equal to 60 min),

X = extract yield increment corresponding to the time increment, % d.a.f.,

kf = first-order extraction rate constant, min-1,

X = extraction yield at infinite time, % d.a.f.,


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tp = time correction for preheating, min.

If the first-order mechanism is true, a plot of 1nX versus t should give a straight line havingslope -kf.However, this was not the case. The Guggenheim's method, although successfully usedin the previous work[1], does not provide satisfactory agreement with the present resultscovering a wide range of temperature and much longer extraction times. For the second-order

mechanism an analogous relation may be derived by combination of the following equations:

(2a)

(2b)

where:

X = extract yield at time t, % d.a.f.,

k = second order extraction rate constant, (min % d.a.f.)-1.

Multiplying eqn. (2a) by X [X- (X + X)] and eqn. (2b) by X (X -X) followed bysubtraction and further formal rearrangement leads to the result:

(3)

Equation (3), when plotted as X/X + t/t versus 1 /X, reasonably approximates to most of thedata by a set of straight lines. Accepting the linear determination coefficient as a criterion,analysis of the toluene extraction data indicates that this process is second-order rather than first-order with respect to undissolved but potentially soluble coal. Whereas some indirectconfirmation for the second-order kinetics of this process at high temperatures may be found in

literature [3-5], the early work of Landau and Asbury [6] supports the validity of the second-order extraction of coals in the region of lower temperatures, say below 593 K. These authorsused for estimating the ultimate yield of extraction the following equation, considered by them tobe purely empirical one:

(4)


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where: b = constant value interpreted by above mentioned authors as the ultimate yield ofextraction, a = "empirical" term (which may be interpreted in terms of the second-order kinetic

theory as 1/Xk).

Simple analysis of this relation shows it to be an integrated form of the second-order kineticequation with respect to the ( b - X ) term:

(5)

with the lower limit set: X = 0 when t = 0 . If the unreality of the lower limit set forintegration is taken into account then the theoretical nature of eqn. (4), in fact representing aspecial case of the second-order kinetics, explains the excellent agreement of its predictions withextraction data [6] at long extraction times and also the deviations observed during early stages

of extraction. Consequently, the results of Landau and Asbury are a solid support for the second-order kinetics of solvent extraction of coal at temperatures of about 530 K. Since eqn. (3) does

not yield numeric values fork or tp it is desirable to use kinetic equations in a form permitting

simultaneous calculation ofk , X and tp. Such kinetic model should contain at least threeparameters calculable by regression analysis. Multiplying eqn. (2b) by (X-- X ) / k followed byfurther rearrangement leads to a model complying with the above requirement

( 6 )

The parameters of the model (6) were found by least squared multiple regression oft X versus tand X. Next, the model (6) was transformed to the form convenient for illustration:

( 7 )

The numeric values of X_ , k and tp are listed in Table 3. The last two columns ofTable 3comprise a curvilinear determination coefficient


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and standard deviation for each experiment

where: Xi = extraction yield at a given experimental point,

Xt = corresponding value predicted by eqn. (7), and

= arithmetic mean of experimental values X i.

Table 3. Kinetic parameters of the toluene extraction of coal at 9.8 MPa.

Run

no.

Temperature

[K]

X

[% d.a.f.]

k104

[min%d.a.f.]-1

tP

[min]R2

S

[% d.a.f.]

1 473 6.85 7.91 13.1 0.9986 0.0664

2 503 11.6 6.29 13.4 0.9965 0.190

3 533 13.9 8.90 12.0 0.9790 0.591

4 563 16.9 6.99 11.4 0.9940 0.395

5 593 18.9 9.29 12.9 0.9945 0.414

6 623 33.4 2.19 23.2 0.9957 0.562

7 638 35.9 4.13 17.3 0.9999 0.103

8 653 36.9 5.92 13.7 0.9976 0.514

9 668 38.1 7.29 15.5 0.9962 0.622

10 683 33.4 13.8 13.7 0.9979 0.377

11 713 30.5 11.8 16.8 0.9912 0.691

Figure 2 and the numeric values of Rz and S show that curves (7) closely fit the experimentaldata. Figure 3 presents the temperature dependence of the extraction rate constant. TheArrhenius interpretation of the results is readily obtained when the subcritical (T < Te) and thesupercritical (T > T,) regions of the extraction temperature are discussed separately.


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Figure 2. Time-yield curves for extraction of bituminous coal with toluene (subcritical andsupercitical).

Energy of activation of about 3 kJ/mole calculated from the subcritical portion of extraction ratedata indicates the physical nature of the rate-controlling step. The small activation energy and thelow extraction yields in the temperature range from 473 K to 593 K suggest a diffusion-controlled dissolution of interstitial substances from the coal matrix by compressed liquidtoluene.


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Figure. 3. Arrhenius plots for extraction of bituminous coal with toluene(subcritical and supercritical).

Evaluation of the energy of activation for the transient region of extraction temperature i.e.between 593 K and 623 K was not possible, based on the results-obtained in this work. However,

the observed decrease in the numeric value of k due to extraction temperature increase from 593K to 623 K (runs 5 and 6) may be recognized as substantiated in the context of the kinetic modelaccepted and the physical/chemical nature of coal. The drop of the extraction rate constant,shown in Figure 2 as a discontinuity, is accompanied by a rapid increase in extraction yield atlong extraction times as well as in the model-predicted values at t = . As it is suggested byFigures 2 and 4, it seems that some new and probably large areas of the porous structure of coalare exposed for the action of solvent. These areas are possibly rich in weakly bonded substancesforming toluene-soluble extract. Any better solvent penetration into the porous structure of coal


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is due to its thermal alteration rather than to an improvement in the solvent power itself[7];likewise, in the temperature range from 593 K to 623 K at the constant pressure of 9.8 MPa, onecannot expect any stepwise changes of toluene properties. The supercritical region from 623 K to683 K provides the energy of activation of 101 kJ/mole which is markedly above the range of 0-40 kJ/mole usually accepted for processes under physical control. The numeric value of theactivation energy suggests that for the formation of the products soluble in supercritical toluene,at least in part, chemical reactions are responsible, primarily involving cleavage of weak non-covalent bonds. It is also possible that at temperatures above 623 K thermal rupture of covalentbonds throughout the coal organic substance occurs resulting in formation of free radicals [5].


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Fig. 4. Plot of ultimate yield versus temperature for extraction of bituminous coal withtoluene (subcritical and supercritical).

As free radicals are highly reactive, they may undergo secondary mutual stabilization forminghigh-molecular-weight solids insoluble even in supercritical toluene. High extractiontemperature and low density of the supercritical solvent would promote secondary

polymerization of radicals and thus limit the apparent yield of extract. Since the kinetic modelused in this work ignores secondary and backward processes, the model predictions for k and X_at 713 K (the points in parentheses in Figures 3 and 4) have been eliminated from the evaluationof activation energy and from further discussion at this stage of work.

CONCLUSIONS(1) Over the whole range of extraction temperatures studied, covering subcritical and

supercritical regions of solvent, the rate of toluene extraction of a high-volatilebituminous coal may be expressed by a second order equation with respect to undissolvedbut potentially extractable coal.

(2) At low extraction temperatures ranging from 473 K to 593 K and under the constantsolvent pressure of 9.8 MPa, a physically controlled dissolution of coal takes place. Themaximum extract yield obtained under these conditions does not exceed 20% by mass ond.a.f. coal.

(3) In the supercritical range of temperature from 623 K to 683 K the rate of extraction isdetermined by the rate of chemical reactions contributed in the thermal decomposition ofcoal. Above 623 K as much as 35% by mass of d.a.f. coal can be extracted with toluene at9.8 MPa.

(4) At high extraction temperatures, above 683 K, under the solvent pressure of 9.8 MPa thedissolution process becomes more complex. The yield of extract is likely to be limited bysecondary polymerization of extractable but unstable materials formed as a primary

product of thermal decomposition of the organic substance of coal.REFERENCES

1. Slomka, B., Rutkowski, A, and Stoiarski, M., Badania kinetyczne ekstrakcji weglikamiennych w warunkach nadkrytycznych (Kinetic studies of the supercritical extractionof coals), Koks Smola Gaz - submitted for publication.

2. Guggenheim, E.A., 1926. Philosophical Magazine, 2 (7): 538.

3. Hill, G.R., 1966. Experimental energies and entropies of activation - Their significancein reaction mechanism and rate prediction for bituminous coal dissolution. Fuel, 45:329.

4. Wiser, W.H., Hill, G.R. and Kertamus, N.J., 1967. Kinetic study of the pyrolysis of a

high-volatile bituminous coal. Industrial and Engineering Chemistry. Process Designand Development, 6(1): 133.

5. Wiser, W.H., 1968. A kinetic comparison of coal pyrolysis and coal dissolution. Fuel,47: 475.

6. Landau, H.G. and Asbury, R.S., 1938. Ultimate yield of solvent extraction of coal -Calculation from rate of extraction. Industrial and Engineering Chemistry, 30: 117.


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7. Blessing, J.E. and Ross, D.S., 1978. Supercritical solvents and the dissolution of coaland lignite. In: J.W. Larsen (Editor), American Chemical Society, Symposium Series,pp. 171-185.

[COLOR CODING KEY]:

443 K CORNFLOWERBLUE

473 K DEEPSKYBLUE

503 K SKYBLUE

533 K AQUA

563 K LIGHTCYAN

593 K ORANGERED

623 K CORAL

638 K LIGHTCORAL

653 K DARKORANGE

668 K LIGHTSALMON

683 K ORANGE

713 K DARKKHAKI

743 K DARKGRAY

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A Kinetic Study of Super Critical