Comparison of the liquefaction behaviour of a bituminous coal in two different batch autoclaves

Comparison of the liquefaction behaviour of a bituminous coal in two different batch autoclaves Bernard B. Majchrowicz, Jan J. de Vlieger*

Dirk V. France,

Jan M. Gelan

and

Department SBNI. Limburg University, B-3610 Diepenbeek, Belgium *Laboratory of Organic Chemistry, Delft University of Technology, Delft, (Received 4 July 7988; revised 70 October 7988)

The Netherlands

A Belgian coal was subjected to non-catalytic liquefaction with tetralin as a hydrogen donor solvent in a magnetically stirred 500cm3 autoclave (no. I) and a 70cm3 autoclave which was shaken horizontally (no. II). At 425”C, coal conversion in both autoclaves increased when the tetralin/coal ratio was increased from 1 to 6, and was always smaller in autoclave no. II. At 4OO”C,an improvement in conversion was no longer found in autoclave no. I for tetralin/coal ratios greater than 2, indicating the presence of sufficient hydrogen donor solvent. When the tetralin/coal ratio was decreased in either autoclave, increased aromaticity of the coal products was observed. Under similar conditions, both the conversion and the amount of hydrogen transferred from the solvent to the coal were lower in autoclave no. II than in no. I. The different results obtained were found to be due to the degree of filling of the autoclaves, and the mode of mixing. The latter also explains the presence in autoclave no. I of coal fragments with a relatively higher average molecular weight, and an increased formation of tetralin dimers. (Keywords: liquefaction;

The mechanism

of hydrogen

bituminous coal; tetralin)

transfer from the commonly

used hydrogen donor solvent tetralinip5, to the coal matrix during coal liquefaction, is not yet fully understood, despite extensive investigation. The role of tetralin is not restricted to stabilization of free radicals generated by thermolysis’; it also acts by a solvent radical-mediated hydrogenolysis6. The hydrogen transferred can also originate from more hydroaromatic parts in the coal itself’, from the gas phase7 and from coal-derived liquids. Hydrogen from the gas phase is very mobile and enters the structural groups in coal randomly*. The extent of coal conversion is related to the hydrogen consumed in the process: at high hydrogen pressure, and in the absence of a catalyst, the extent of dehydrogenation of tetralin to naphthalene can be used as a measure of hydrogen consumption’-5. On the other hand, isomerization, thermal degradation and dimerization of tetralin can result in a lower hydrogen donor activitys,9. Although the effects of process variables on yields and conversion in coal liquefaction in batch autoclaves have been investigated7, less attention has been given to elucidation of changes in the molecular structure of coal hydrogenation products caused by variation in different reaction parameters’ O-l 2. The behaviour of coal during liquefaction will also depend on the type of reactor system

used: different results are obtained from stirred, rocking and rotating autoclaves10~‘3-‘s, and by varying stirring speedi6. In addition, the time for an autoclave to reach the desired reaction temperature13 and the catalytic effect of the autoclave wa1117 also contribute to variable performance. A critical mixing level for a continuous coal liquefaction reactor has been described, below which 0016-2361/89/06069GO6$3.00 ‘c, 1989 Butterworth & Co. (Publishers)

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FUEL, 1989, Vol 68, June

significant solid deposition and a significant reduction in the liquid yield were observedr8. In this paper, the results of a comparison of the liquefaction behaviour of a bituminous coal in two batch autoclave systems are reported. Two parallel series of autoclave experiments were carried out at 400 and 425°C with tetralimcoal ratios of 6, 2 and 1. In all parallel experiments, reaction parameters such as heating-up time (70min), reaction time (60min) and hydrogen pressure (10 MPa) were kept the same. The only differences between the parallel experiments were the mode of mixing, i.e. stirring and shaking, and the degree of filling of the autoclaves. In addition, the influences of direct coal injection (i.e. after the heating-up time) and of different reaction times were investigated.

EXPERIMENTAL Materials

The Beringen coal sample, obtained from the Dutch Centre for Coal Specimens, was ground ( < 70 pm) before use; analysis (as received) gave C. 68.8; H, 4.5; N, 1.2; ash, 14.0; moisture, 4.2 and V.M., 30.3%. Tetralin was distilled before use; tetrahydrofuran and hexane were reagent grade and used without further purification. Apparatus

The batch reactor system (no. I) used for one set of coal liquefaction experiments consisted of a 500cm3 stainless steel autoclave equipped with a magnetically driven stirrer, as described elsewhere’. The other batch reactor (no. II) had a volume of 70cm3 and was shaken

Liquefaction

behaviour

of a bituminous

horizontally. With autoclave no. I, the injection of a coal sample at the required reaction temperature was possible. Procedure

Both autoclaves, containing solvent and coal, were pressurized with hydrogen and heated to the desired reaction temperature in exactly 70min. In experiments with various solvent/coal ratios, the degree of filling of each autoclave at room temperature was kept constant; autoclave no. I was tilled to one-third capacity and autoclave no. II was half filled. When coal injection was applied with autoclave no. I, a suspension of 25 g coal in 25 g solvent was injected. The fall in reaction temperature was restored within 5 min. The stirring speed in autoclave no. I was kept at 800 r.p.m., while autoclave no. II was shaken horizontally at 200 strokes per minute during all of the experiments. After 60min, the autoclaves were rapidly cooled with air. The contents of each autoclave were removed with tetrahydrofuran (700ml); the suspension obtained was filtered and the solids were extracted with tetrahydrofuran (600ml) in a Soxhlet apparatus for 24 h. The insoluble residue was dried and weighed. The conversion was defined by: wt coal (d.a.f.)- wt residue (d.a.f.) x looo, cl Conversion = -- ---~ wt coal (d.a.f.) After evaporation of tetrahydrofuran from the combined tetrahydrofuran extracts, 1 ml of the remaining solution (A) was diluted with 15 ml hexane, filtered and analysed for tetralin and naphthalene. A Varian Model 3700 gas chromatograph was used with a capillary CP Sil5 column (25 m x 0.23 mm) using temperature programming (60-l 30°C at 5°C min- ‘) and flame ionization detection (FID). The rest of solution A was concentrated in a rotary evaporator (120°C 0.5 mm Hg). The last traces of solvent and solvent-derived material were then quantitatively removed from the coal products by the following procedure, which gave less than 15% loss ofcoal material. The coal product concentrate (1 g) was dissolved in tetrahydrofuran (5 ml), and hexane (200 ml) was slowly added at room temperature. The tetrahydrofuransolubleehexane-insoluble product (denoted as solvent relined coal) was then filtered off and dried (lOO”C, under N& The filtrate was analysed by capillary g.c. using a Carlo Erba instrument equipped with an on-column injector and FID. (Capillary column: SE 52; 25 m x 0.32mm; temperature programme: 80-320°C at 4°C min- ’ ; helium as carrier gas). In this way, information was obtained about the tetralin dimer concentration’. Gel permeation chromatography (g.p.c.)

Gel permeation chromatography was carried out with a Waters Associates instrument using 100 A and 500 A micro-Styragel steric exclusion columns (Waters Associates) in series, with tetrahydrofuran as eluant and refractive index detection. Polystyrene standards (Waters Associates) were used to calibrate the system. 13C and ‘H n.m.r. spectroscopy

The solvent refined coal (SRC, 0.5g) was trimethylsilylated with hexamethyldisilazane (4 ml) in dry pyridine (10 ml) by refluxing for 12 h. The solvent and byproducts were distilled off in vacua and the residue was dried

coal in two batch autoclaves:

B. B. Majchrowicz

et al.

(lOO’C, N,, 30min). The samples thus obtained were completely soluble in CDC13. Solution 13C and ‘H n.m.r. spectra were recorded in the pulse Fourier transform mode with broad-band decoupling, using a Varian XL200 spectrometer. The 13C spectra were accumulated with a sweep width of 10000 Hz, a pulse width of 10~s (90” pulse), acquisition time of0.95 s and a delay of 5 s. A total of 8000 transients were collected and averaged for each spectrum. The carbon aromaticity v,“) was calculated from the integral values Of Caiiphatic(1&60ppm) and Caromatic(lo&180 ppm) in the following way: f,” = cafe. CalOmalic aromatic

+

Caliphatic

Similarly, the hydrogen aromaticity f,“) was calculated from the integral values of Haliphatic (0%5ppm) and H aromatic(6.7-9.2 ppm). The 13C aromaticities were only used to compare SRCs in a relative way.

RESULTS Coal conversion Table 1 gives details of the experiments performed in autoclaves no. I and no. II, and also shows coal conversion. Figures 1 and 2 show coal conversion for three different tetralimcoal ratios at liquefaction temperatures of 400 and 425°C respectively. The conversion was always higher in autoclave no. I than in no. II, except for experiment K with a tetralin/coal ratio of 1 at 400°C. Using tetralimcoal ratios greater than 2 in autoclave no. I at 4OO”C,does not result in a higher conversion, indicating the presence of sufficient hydrogendonating solvent. At 425”C, however, even tetralimcoal ratios of 6 do not seem to be sufficient to control the fast radical reactions. Hydrogen transfer and coal conversion

For each experiment, the hydrogen (H,,,,,) transferred from the tetralin to the coal, calculated from the naphthalene concentration in the products, is given in Table I. The following observations can be made: 1. A higher coal conversion is associated with an increase in hydrogen transfer (Figure 3). 2. Under similar conditions, H franSwith autoclave no. II is lower than with no. I (experiments A and M, B and N, G and 0, H and P, K and R, and L and S). 3. Liquefaction at 425°C results in a greater hydrogen transfer than at 400°C. 4. Normally, higher H,,,,, values are found with autoclave no. I than with autoclave no. II at the same level of coal conversion (Figure 3). G.p.c.

The molecular weight distributions of the SRCs from 8 autoclave experiments, 4 (A, B, K and L) obtained in autoclave no. I and 4 (M, N, R, and S) obtained in autoclave no. II, are depicted in Figure4. Absolute average molecular weights (mw) of liquefaction products cannot be obtained by g.p.c.“, but this technique can be used to indicate relative changes. In experiments under similar conditions, autoclave no. II gave SRC with a lower average mw than that from autoclave no. I. As expected from the higher degradation rates at higher

FUEL, 1989, Vol 68, June

697

Liquefaction Table 1

behaviour

Experimental

of a bituminous

conditions

coal in two

batch

autoclaves:

et al.

B. B. Majchrowicz

and results”

Experiment

Autoclave no.

Tetralin/coal wt. ratio

A

I

6

B

I

6

C

I

D Eb

Wt. of coal

Tdh

x 100 (%)”

Temperature (“C)

Reaction time (min)

Conversion (%)

H franse

25.0

400

60

13

0.86

25.0

425

60

80

2.68

0.46

6

25.0

400

30

70

0.81

0.52

I

6

25.0

400

0

64

0.70

0.55

I

6

25.0

400

60

63

1.27

0.59

Fb

I

6

25.0

425

60

69

1.46

0.86

G

I

2

60.0

400

60

73

1.36

2.96

H

I

2

60.0

425

60

77

1.54

2.13

I

I

2

100.0

400

60

72

0.96

0.53

J

I

2

25.0

400

60

69

1.20

0.53

K

I

1

90.0

400

60

66

0.73

4.09

L

I

1

90.0

425

60

73

1.65

2.70

M

II

6

4.7

400

60

68

0.77

0.57

N

II

6

4.7

425

60

76

1.29

0

II

2

12.0

400

60

67

0.54

n.d.’ 2.67 1.92 1.67 3.50 2.17

(9)

T

P

II

2

12.0

425

60

73

0.93

Q

II

2

8.0

400

60

72

0.99

R

II

1

18.0

400

60

67

0.58

s

II

1

18.0

425

60

69

0.91

“All experiments except E and F had a heating-up time of 70min from room to reaction temperature temperature was applied bCoal samples were injected at the reaction temperature from tetralin, in g H per 1OOg coal (d.a.f.) ‘H,,,,, is hydrogen transferred “T,#,,, is the total amount (in g) of dimeric tetralin products found in the solvent after the liquefaction g) of tetralin taken for the experiment ‘nd-not determined

while a 1OMPa

experiment;

tetr

0.76

H, pressure

at the reaction

T,,,, is the original

amount

(in

80 z e .-6 ln k > 75 !

M

t

70

65

0

4

2 -

tetralin

/coal

6 ratio

Figure 1 Coal conversion versus tetralin/coal ratio for liquefaction experiments at 400°C: A, autoclave no. I; A, autoclave no. II

t

I

I

I

0

2

4

6

-

tetralinlcoal

ratlo

Figure 2 Coal conversion versus tetralin/coal ratio for liquefaction experiments at 425‘C: 0, autoclave no. I; n , autoclave no. II

temperatures, increasing the liquefaction temperature from 400 to 425”C, gave more light products.

were higher at 425 than at 400°C. When the tetralin/coal ratio was decreased, both f,” andf,” increased for SRCs from both autoclaves.

N.m.r.

Tetralin

spectroscopy

Thefz andf: values of SRCs from 10 experiments are presented in Table 2. In similar experiments, fat and f,”

698

FUEL, 1989, Vol 68, June

dimer formation

During coal liquefaction, tetralin forms dimeric species’: 2,2’-bitetralyl is the most dominant at 4OO”C,

Liquefaction

behaviour

of a bituminous

I

I

I

I

65

70 -

75

80

01

conversion

(%/.)

Figure 3 Relation between hydrogen transferred from tetralin in g H per 1OOg coal (d.a.f.) and coal conversion: A, autoclave no. I, 400°C; 0, autoclave no. I, 425°C; A, autoclave no. II, 400°C; n , autoclave no. II. 425°C

coal in two batch autoclaves:

60

I

50 40 30 20 10 0

experment no

outoc1ove

Iquefact~on cO”“erSl0” te,rQlIn

,coa,

temperature rat,0

A I

B I

M II

N II

K

L

I

I

I? II

S II

400 73

425 60

400 66

425 76

400 66

425 73

400 67

425 69

6

6

6

6

1

1

1

1

Figure 4 Molecular weight distributions of SRCs as indicated by g.p.c.. The equivalent molecular weight distribution is expressed in terms of the fractional contributions of three well-defined molecular weight areas: q, mw<600; 0, 6001800

and JB” of SRCs

G H K L M N 0 P R S

0.69 0.73 0.78 0.81 0.71 0.72 0.72 0.75 0.73 0.80

0.40 0.44 0.42 0.49 0.33 0.34 0.34 0.37 0.39 0.42

4oo"c

d

I

I

200 __)

and tetralinlcoal

ratio

The major part of the liquefaction reaction is believed to occur in the liquid phase. Therefore one would expect,

as

f,”

180

of filling

10 experiments,

ls’

I

Ejfect ofmixing, degree on coal conversion

from

Experiment

while more aromatic dimer structures are present at l,l’-, 1,2- and 425°C as depicted in Figure5. 2,2’-Bitetralyl structures are not stable at these temperatures, so unless they are dehydrogenated to aromatic products, tetralyl radicals are reformed. Dimeric structures will mostly be at an equilibrium concentration, especially at 400°C. The total amount of dimeric products found after each liquefaction experiment is presented in Table 1. With the same autoclave, higher relative amounts of dimers are found at 400°C than at 425°C (compare, for example, experiments A and B, 0 and P). The increasing relative amount of dimers found in experiments with decreasing tetralimcoal ratios at the same liquefaction temperature, is remarkable; this is an indication of relatively higher concentrations of tetralin radicals (compare experiments A, G and K in autoclave no. I and M, 0 and R in autoclave no. II, etc.). Furthermore, it can be seen that under similar experimental conditions, the relative amount of dimers was higher with autoclave no. I. DISCUSSION

et al.

in experiments with similar conditions in both autoclaves, and using a higher degree of filling in autoclave no. II, that no. II would produce a higher conversion. However, both Figures 1 and 2 show that in nearly all experiments under similar conditions, autoclave no. I produced more gaseous and THF soluble products. The difference in the mode of mixing proves to be more important than any effect resulting from the greater tilling of autoclave no. II. In addition, to stress the effect of the mode of mixing, experiment M was repeated with a 25% higher shaking rate, which resulted in an increase in conversion of 3%. To investigate any effect of filling upon conversion, additional experiments were carried out at 400°C. Experiments I and Q were performed in autoclave no. I and no. II under the same conditions as experiments G and 0, but now with the same degree of filling as in autoclave no. II and no. I, respectively. The change in filling of autoclave no. I hardly had any effect upon conversion (compare experiments I and G) due to good mixing. However, in autoclave no. II, an increase in

Table 2 Values off,” determined by n.m.r. %

B. B. Majchrowicz

220 t (“C)

of the hexane-soluble Figure 5 Capillary column gas chromatograms fractions obtained from experiments at 400 and 425’C. Only the temperature region from 18@220 C. relevant to the tetralin dimers, is presented. a, I,I’-bitetralyl; b. 1.2’-bitetralyl: c. 2,2’-bitetrdlyl; d. aromatic dimer structures

FUEL,

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699

Liquefaction

behaviour

of a bituminous

coal in two

batch

conversion of 5% was found in experiment Q compared with experiment 0, despite the fact that in experiment Q less tetralin will be present in the liquid phase at reaction temperature. Most likely, the better mixing conditions in a less filled autoclave (experiment Q) are responsible for this effect. However, adequate mixing in an autoclave does not imply that the degree of filling has no effect. This was illustrated in experiment J, where compared with experiment G the degree of filling was more than halved, which resulted in a lower conversion. The reduction in conversion is explained by a decrease in the amount of tetralin, such that at reaction temperature, most of the solvent would be present in the vapour phase. This phenomenon also explains the slightly lower conversion obtained with experiment K in autoclave no. I in comparison with experiment R in autoclave no. II. In these parallel experiments, performed at 400°C with a tetralimcoal ratio of 1, in which not as much tetralin will be present in the liquid phase, a slightly lower conversion was obtained in the less filled autoclave no. I. Consequently, an increase in the tetralin/coal ratio is expected to result in an increase in conversion, as is shown in Figures 1 and 2. However, above a certain level of tetralin/coal ratio, an improvement in conversion is no longer found. While the tetralimcoal ratio is too low, the hydrogen needed for the stabilization of radicals cannot be provided by the hydrogen-donating solvent. The minimum tetralimcoal ratio is 2 for the experiments at 400°C in autoclave no. I. For the experiments at 425°C where many more radicals are formed, even a tetralimcoal ratio of 6 is not sufficient for radical quenching. Hydrogen

transfer

It is generally accepted that to obtain a higher conversion, more hydrogen has to be transferred to the coal. Comparison of similar experiments in the same autoclave performed at 400 and 425°C (Table 2) illustrates this relation between conversion and higher hydrogen transfer. In addition, an increase in the reaction time at the same reaction temperature (experiments D, C and A) also results in an increase in H,,,,,. Figure 3 indicates that autoclave no. II generally shows a lower H,,,,, than autoclave no. I at the same coal conversion. The reason for this can be found in the higher degree of filling of autoclave no. II. The presence of more tetralin in the liquid phase at the reaction temperature will result in an increased solvating and hydrogen donating capacity of the solvent, and thus in a better swelling of the coal. This also explains the difference in H ,TanSbetween experiments A and G, which were both performed in autoclave no. I with the same degree of filling at room temperature and with the same conversion, but a different tetralin/coal ratio. In experiment A, more tetralin will be present in the liquid phase at 4OO”C,which will result in a lower H,,,,,. To study the effect of coal injection at the reaction temperature, an experiment (E) with coal injection was compared with an experiment (D) with only a 70min heating-up time and no further reaction period. In the coal injection experiment, not only was the conversion slightly lower but also more hydrogen was transferred from tetralin to the coal. The instantaneous formation, after coal injection, of a large number of coal radicals

700

FUEL, 1989, Vol 68, June

autoclaves:

B. B. Majchrowicz

et al.

which cannot be stabilized immediately, will result in regressive reactions. These reactions will cause reduced formation of low molecular weight products and less coal solubility. At a liquefaction temperature of 425°C with coal injection (experiment F), the conversion was still less than was obtained in an experiment at 400°C with a heating-up time of 70min (experiment A). As can be seen in Table I, the same coal conversion can be reached with a different H,,,,, when other experimental conditions are varied with the same autoclave (e.g. experiments A, G and L). If a higher tetralin/coal ratio does not result in an increase in conversion (experiments A and G), it does not imply that no further improvement in conversion can be obtained when other reaction parameters are changed. To underline this, the THF insolubles of experiment I were reacted under the experimental conditions of experiment A. This resulted in a conversion of 39% and a H,,,,, of 2.05. Obviously, more hydrogen from the solvent is needed in the later stages of the liquefaction reaction. Product distribution

In parallel experiments, the presence in autoclave no. I of coal fragments with a relatively higher average molecular weight, together with the higher values of points to a higher concentration conversion and HtTBIIS, of large stabilized coal fragments in the hydrogen donor solvent. In the set of parallel experiments, only experiments K and R show practically the same conversion. Comparison of the product distributions of these two experiments still indicates the formation of relatively heavier SRC products in autoclave no. I. In this way, higher molecular weight fragments from the coal network are formed and taken up in the solvent. The product distribution of the SRCs is fairly independent of the tetralimcoal ratio (e.g. compare experiments A and K, B and L), i.e. the tetralin/coal ratio influences the conversion but not the product distribution. Aromaticity

of products

SRCs obtained at 425°C are more aromatic in nature than those produced at 400°C probably because of increased dehydrogenation of the hydroaromatic components within the coal. When the tetralimcoal ratio is decreased, an increase in aromaticity is observed, because when less tetralin is available the coal itself will be more involved in the hydrogen donor reactions. Dimers

With both autoclaves, the relative amounts of dimers formed increased with decreasing tetralin/coal ratio. Together with the fact that no bitetralyls were formed from pure tetralin alone under the same experimental conditions, this shows the catalytic effect of coal upon dimer formation. For parallel experiments in both autoclaves, the relative amounts of dimers were higher in autoclave no. I, showing a higher formation rate of tetralyl radicals probably due to the more vigorous mixing. When the liquefaction temperature was increased from 400 to 425°C more aromatic dimer species were formed (Figure5) although the relative amount decreased. At the higher temperature, the dimers formed are less stable and can dehydrogenate to give more stable species.

Liquefaction

behaviour

of a bituminous

coal in two batch autoclaves:

CONCLUSIONS The mode of stirring and the degree of filling of an autoclave influences not only coal conversion but also the distribution and the nature of the products, and the hydrogen transfer reactions from the solvent and the coal itself. Care is therefore necessary when comparing coal liquefaction results obtained with different autoclaves, even if the same coal and the same set of the experimental parameters have been used.

9

ACKNOWLEDGEMENT Discussions with Professor Dr H. van Bekkum, Professor Dr A. P. G. Kieboom and Professor Dr H. J. Martens were a great help and are appreciatively acknowledged. The authors also thank E. Wurtz and F. Miniaci for technical assistance during the various experiments. REFERENCES 1 2

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