Direct post-cracking of volatiles from coal hydropyrolysis

Direct post-cracking hydropyrolysis 2. Influence Samuel

Furfari*

of volatiles

from

coal

of pressure and Ren6 Cyprb

Universitb Libre de Bruxelles, Facult.6 des Sciences Appliqkes, Service de Chimie G&&ale et Carbochimie, 50 av. F. D. Roosevelt, 1050 Brussels, Belgium (Received 10 June 1983; revised 37 January 1984)

Direct post-cracking of volatile material produced by hydropyrolysis of bituminous coal at 580°C under hydrogen pressure 1-5 MPa has been investigated at 700°C under constant hydrogen pressure with 0.1 and 1 s residence times. Results show that pressure promotes the formation of benzene, toluene and xylenes (BTX) and naphthalenesduring post-cracking, while phenol, cresols and xylenols (PCX) are not affected. The transformation of heavy Ohenols into PCX is not influenced by the hydrogen pressure. During post-cracking the BTX yield can be more than doyble that reached in simple hydropyrolysis. Post-cracking applied to high oil yield hydropyrolysis processes will be a valuable BTX source. (Keywords:

coal; hydropyrolysis;

cracking)

In an earlier publication’ the influence of the direct postcracking temperature (under hydrogen at 1 MPa) on the volatiles produced in fixed-bed semibatch coal pyrolysis has been described. Here, the influence of pressure on gas and oil yields and compositions is studied. The apparatus, analytical methods and material used have been described previously. With the apparatus used, the pressure is the same in the hydropyrolysis and postcracking reactor. Hence, contrary to the experiments reported previously, for each experiment two parameters are modified; the hydrogen pressure in the coal devolatilization reactor and thus the pressure in the postcracking reactor. The analysis of results had, therefore, to be related to the influence of the pressure in fixed-bed hydropyrolysi?. The experimental conditions used are shown in Table I.

(CN) and in phenol, cresols and xylenols (PCX). Figure 1 I represents the evolution of the total hydroxyl functions in the oil and the PCX hydroxyl functions in the oil, both results being expressed as wt%, daf of coal. The yields in wt’?, daf of coal of BTX, CN and PCX are shown in Figure I?

1‘.

Table 1

Experimental

conditions

Hydropyrolysis 580 a60 See post-cracking

Temperature (“C) Residence time (s) Hydrogen pressure Postcracking 1.d. reactor (mm) Residence time (s) Temperature (“C)

3.6 mo.1 700

Pressure (MPa)

9.0 =O.l 700

1.2,3

1,2.

3,5

RESULTS For both residence times studied, oil yield increases slowly with pressure (Figure I) for the same hydropyrolysis temperature (58OC) and the same post-cracking temperature (700°C). Figure 2 shows that the gas yield is strongly dependent on the residence time in the post-cracking reactor. For a residence time of 0.1 s, the total gas yield is not dependent on the pressure, but is 1.2 times higher than that obtained without post-cracking2. Under 1 MPa, the total gas yield is independent of residence time. For the 1 s residence time, the pressure exerts a strong influence on total gas yield at higher pressures between 1 and 2 MPa. Figures 3 and 4 give the yields of the different gases produced, while Figures 5-10 show the oil composition in terms of benzene, toluene and xylenes (BTX), in naphthalene, 1-methylnaphthalene and 2-methylnaphthalene

10

c

41

0016-2361/85/01004’XO5%3.00 @ 1985 Butterworth & Co. (Publishers)

40

FUEL,

1985,

Ltd

Vol 64, January

2

3

4

5

PH2 (Mb)

* Present

address: Commission of the European Communities, Directorate-General for Energy, Coal Directorate, 200 rue de la Loi, 1049 Brussels, Belgium

1

Figure

1

on oil yield.

Influence of pressure during post-cracking Residence time: 0, 0.1 s; 0, 1 s

at 7Oo’C

Post-cracking

I

I

1

2

I

I

4

5

I

3 PH2( MPa)

of volatiles

Figure 2 Influence of pressure during post-cracking at 700°C on gas yield. Residence time: 0, 0.1 s; 0, 1 s

0

6

from

coal hydropyrolysis:

S. Furfari

and R. Qpr&s

This is primarily due to the methane yield (Figure 3). As has been observed in coal hydropyrolysis2 ethane production is improved by increase in pressure. Postcracking also improves this production. Consequently, under 5 MPa at 700°C post-cracking with 1 s residence time allows the production of 3.3 wt% daf ethane; namely double that produced in hydropyrolysis without postcracking. The carbon monoxide and carbon dioxide yields are not influenced by pressure (Figure 4) and residence time in the post-cracking zone. The same results have been obtained without post-cracking2. B7X

yields

With both residence times investigated, at 700°C the BTX concentration in the oil increases with pressure (Figures 5 and 6). It has been reported’ that during hydropyrolysis, pressure also increases these concentrations in the oil, but this increase was less marked as the pressure continued to increase and at 5 MPa an asymptotic value was reached. In the oil produced by postcracking, this saturation effect of pressure is not observed. The BTX concentrations increase rectilinearly at the 1 s residence time (Figure 6). These results are not in line with those of Virk et ~1.~ who have shown that the hydrogen 28

24-

4

1

2

3 P+,? (MPa)

4

5

Figure 3 Influence of pressure during post-cracking at 7Oo’C on: 0. CH,; Cl, C,He; and A, C,H, yields. Residence time: open symbols, 0.1 s; closed symbols, 1 s

12 0

0

A

I

I

10

”

Q_---_~__-_---_

I

0 J

I

I

1

2

I

3 PHr (MPa)

I

I

4

5

Figure 4 Influence of pressure during post-cracking at 700°C on COP (0) and CO (0) yields. Residence time: open symbols, 0.1 s; closed symbols, 1 s

DISCUSSION The oil yields (Figure I) are lower than the ones obtained without post-cracking in the same conditions2. At 700°C the oil yield decreases, but this fall is less under high pressure. Gas yields

The gas yield obtained with direct post-cracking is 1.4 to 1.5 times that observed without post-cracking’ (Figure 2).

ol 1

2 PHI ( MPa)

3

Figure 5 Influence of pressure during the postcracking at 7OOT on: A, benzene; A, toluene; l , xylenes; 0, BTX; and n , the sum of the monoaromatic compounds concentration in the oil. Residence time 0.1 s

FUEL, 1985, Vol 64, January

41

Post-cracking

of volatiles from coal hydropyrolysis: S. Furfari and R. Cypr&s

40

36 6 32

28

12

8

I

I

I

0

1

2

I

3 PHZ ( MPa)

I

I

4

5

1

Figure 6 Influence of pressure during post-cracking at 700°C on A, benzene; A. toluene; 0, xylenes; 0, BTX; and n , the sum of the monoaromatic compounds conceqtration in the oil. Residence

time 1 s

Table 2 Ratio of the yields (wt%, daf) obtained by postcracking to yields without post-cracking’. Hydropyrolysis conditions: Temperature, 58VC; pressure, same as postcracking; residence time, 60 s. Temperature

Residence time is)

Figure 7 Influence of pressure during the post-cracking at 700°C on: 0, naphthalene; 0, 1 -methylnaphthalene; W, 2methylnaphthalene; and A. the sum of these three naphthalenes concentration in the oil. Residence time 0.1 s

8-

0.1

1

Pressure (MPa)

1

2

3

1

2

3

5

7-

BTX PCX Naphthalenes

1.05 0.62 1.46

1.18 0.84 1.30

1.44 0.88 2.01

1.57 0.62 1.67

1.96 1.04 2.16

2.27 0.86 2.30

2.70 0.80 2.06

6-

residence time, the BTX yield is increased by a factor of 2.7. On the contrary, Finn et aL4 also report a benzene yield increase by post-cracking at 800°C. Hydrogen prevents the polymerization or condensation of the radicals formed by the tars’ degradation. It has been observed4 that the initial step of these reactions is always a thermal degradation, and it is only when the molecule is destabilized that hydrogen is able to modify the reaction path. In Table 2 the ratios of the yields (in wt%, daf) obtained in this work and those obtained without post-cracking’ are shown. This allows the effect of pressure on hydropyrolysis and the effect of pressure on post-cracking to be separated. Hydrogen pressure during post-cracking definitely promotes the BTX yield. The longer the residence time in the post-cracking zone, the higher the tars’ degradation to form BTX. Under 5 MPa and with 1 s

42

FUEL, 1985, Vol64,

3

9-

700

(“C)

2 PH, (MPa)

January

I

0

I

1

4

:MPa PHz

Figure 8 Influence of on: 0, naphthalene; 0, methylnaphthalene; and concentration in the oil.

I

I

I

2

I

5

1

pressure during post-cracking at 7Oo’C 1 -methylnaphthalene; n , 2A, the sum of these three naphthalenes Residence time 1 s

Post-cracking

of volatiles from coal hydropyrolysis:

23- 21 -

19-

2

17-

r ‘; 15s i? E 2

13-

s ll-

g-

i

-

zzY@

S. Furfari and R. Cypr&

The results presented here show a modification in behaviour of PCX as a function of hydrogen pressure. It is possible that the difference in the conclusions drawn is due to the different scale of pressure investigated. In previous work the present authors have observed that an increase from 0.1 to 1 MPas of hydrogen partial pressure promotes the formation of PCX. Table 2 indicates that with post-cracking at 700°C with 0.1 s residence time, the higher the pressure the lower the PCX degradation in the pressure range studied. With 1 s residence time, 2 MPa seems to be the optimum pressure to limit PCX degradation; at this pressure the PCX production is almost the same with or without postcracking. In Figure I1 the curves for total hydroxyl functions and the PCX hydroxyl functions are almost parallel at each residence time investigated. Hence, in the range of pressure investigated, and at a temperature of post-cracking of 700°C the pressure increases do not promote the transformation of heavy phenols into PCX. This transformation is, therefore, only dependent on the post-cracking temperature’. The wt% yields (Figure 12) show that the BTX production reaches 2.4% with 1 s residence time under 5 MPa at a post-cracking temperature of 700°C. The PCX maximum yield reached is 2.1 wt%.

PH2 ( MPa) Figure 9 Influence of pressure during past-cracking at 7OOT on: 0, phenol; x, cresols; W, xylenols; and A, PCX concentration in the oil. Residence time 0.1 s

pressure between 0 and 10 MPa does not modify the kinetic rate of the non-alkylated aromatic compounds.

26

Naphthalene yields

The increase in naphthalene, 1-methylnaphthalene and 2-methylnaphthalene concentrations with pressure cannot be attributed to the post-cracking effect (Figures 7and 8) since, without post-cracking, a similar phenomenon has been observed2. But Table 2 shows that naphthalene production is strongly increased by a relatively high postcracking pressure. Beyond 3 MPa, although the naphthalenes yield still increases (Figure 8), the role of the postcracking is less important. PCX yields It has been previously reported2, that without postcracking, the PCX concentrations in the hydropyrolysis oil increase slowly but rectilinearly with hydrogen pressure. With post-cracking at 700°C the xylenols concentration in the oil decreases with the increase in pressure for both residence times investigated (Figure 9 and 10). For 0.1 s residence time, the phenol and the cresols concentrations increase between 1 and 3 MPa (Figure 9). For 1 s residence time the phenol concentration is at a maximum at 3 MPa, whereas the cresols concentration reaches a maximum at 2 2 MPa. Wells and Long’ report that between 1 and 3 MPa, the hydrogen pressure effect on the o-cresol cracking is negligible. Fillo et a1.6 and Fillo and Massey’ have also shown that the decomposition of phenol is not dependent upon hydrogen pressure. With hydrogen partial pressure of 0.01, 0.02 and 0.05 MPa these authors show that this does not influence the phenol decomposition rate but it determines the reaction path6. The influence of the hydrogen pressure has not been investigated for the other phenols’.

18

'

I

I

I

I

I

12

3

-2

10 1

2

t

o4 0

1

2

3

4

5

PHz (MPa) Influence of pressure during post-cracking at 700°C Figure 10 on: 0, phenol; 0, cresols; n , xylenols; and A, PCX concentration in the oil. Residence time 1 s

FUEL, 1985, Vol 64, January

43

of volatiles from coal h ydrop yrolysis: S. Furfari and R. Cyprb

Post-cracking

i3 0

I

I

I

I

I

1

2

13

4

5

PHr (MPa)

Figure 11 Influence of pressure during post-cracking at 700°C on the yield of total hydroxyl functions in the oil (0) and on the yield of PCX hydroxyl functions in the oil (0). Residence time:

open symbols, 0.1 s; closed symbols, 1 s

J

3

I 1

I

I

I

I

2

3

4

5

PHr (MPa) Figure 12 Influence of pressure during the post-cracking at 700% on: 0, the BTX; q, PCX; and A, naphthalenes yieids. Residence time: open symbols, 0.1 s; closed symbols, 1 s

CONCLUSIONS It is possible, with direct post-cracking of the volatiles produced by coal hydropyrolysis, to adjust the oil composition. The oil yield decreases by this operation while the gas yield is strongly increased. The additional quantities of gas are mainly due to methane and ethane formation resulting from the dealkylation of the lowtemperature hydropyrolysis oil compounds. Post-cracking influences liquid phase composition. According to the compound whose yield is to be optimized, either post~racking parameter can be modified. At 700°C under 5 MPa with a residence time of 1 s, BTX production is maximized. Under the same conditions but at 2 MPa the PCX yield is optimized. The heavy phenols are easily upgraded into PCX and BTX by mild thermal degradation under hydrogen pressure. Nevertheless, this operation occurs with a loss in total phenols as not all the heavy phenols are converted into light phenols. The maximum yield of naphthalene is reached” at 900°C under 1 MPa but probably at this temperature the yield would be higher at higher pressure. In practice, the post-cracking operation is a relatively simple technique. Oil produced in hydropyrolysis goes through a tubular reactor maintained at the correct temperature. This operation also has the advantage of drastically simplifying the oil composition. Numerous

44

FUEL, 1985, Vol 64, January

compounds present in low concentration in the lowtemperature oil are cracked. Relining is thus simplified. ACKNOWLEDGEMENTS The authors wish to thank the Commission of the European Communities, Coal Directorate, for the financial assistance it has provided in the framework of the ‘Chemical and Physical Valorisation of Coal’ programme of the ECSC. REFERENCES Cypres, R. and Furfari, S. Fuel 1985,64,33 Cypres, R. and Furfari, S. Fuel 1981,60,768 Virk, P. S., Chambers, L. E. and Woebcke, H. N. Am. Chem. Sot. Adv. Chem. Ser. 1974,131,237 Finn, M. J., Fynes, G., Ladner, W. R. and Newman, J. 0. H. Fuel 1980,59,397 Wells, G. L. and Long, R. Ind. Eng. Chem. Process. Des. Dev. 1962,1, 73 Fillo, J. P., Massey, M. J., Strakey, J. P., Nakles, D. V. and Haynes, W. P. US DOE Report, PERC/RI-7716, 1977 Fillo, J. P. and Massey, M. J. ‘Environmental Aspects of Fuel Conversion Technology’, Proceedings of 4th Symposium, US Environ. Prot. Agency,Off. Res. Dev. [Rep.], EPA 1979, EPA-600/7-79217, PB 80-134729 279 Cypres, R. and Furfari, S. Fuel 1982,61,721