- Email: [email protected]
Coal conversion in co-processing with heavy petroleum residues
Fuel Processing Technology, 24 (1990) 225-230 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
225
COAL CONVERSION IN CO-PROCESSING WITH HEAVY PETROLEUM RESIDUES S. Wallace 1, K.D. Bartle 1, W. Kemp2, W. Steedman2, T. Flynn2, M.P. Burke 1, C.J. Jones 1 and N. Taylor1. 1 Department of Physical Chemistry, University of Leeds, Leeds, LS2 9JT. 2 Department of Chemistry, Heriot-Watt University, Edinburgh, EH14 4AS.
ABSTRACT
Point-of-Ayr coal was co-processed with a series of petroleum residues under both mild and severe reaction conditions. Hydrocracking conditions were chosen to optimise coal conversion, whilst the lower severity reactions were used to investigate the structural effects of processing. Under hydrocracking conditions, co-processing conversions varied with coal/oil ratio, with the optimum near 50/50. Bimetallic catalysts were found to enhance coal conversion. In a second series of experiments coal was co-processed with a petroleum enriched by the addition of petroleum asphaltenes: enhanced solubilisation was associated with increased asphaltene content. In the same series the fates of nitrogen and sulphur were also dependent on asphaltene loadings.
INTRODUCTION Coal liquefaction processes typically involve thermal degradation of the macromolecular coal structure followed by hydrogenation to stabilise the degraded material and increase the H/C ratio of the products. Usually a coal derived recycle solvent is used as a vehicle to transfer the hydrogen. Since the recycle solvent is obtained from the coal derived product the extent of hydrocracking (and hence yield) has to be limited. In principle, this problem could be alleviated by supplementing or replacing the recycle solvent stream with heavy petroleum fractions which are cheap and have a higher H/C ratio than coal derived solvents. If this is to be effective, however, it is necessary that the coal and petroleum be compatible and also that the products conform to environmental safety standards. A U.K. bituminous coal was co-processed with a series of petroleum derived solvents of varied composition under conditions simulating the extraction stage of two stage liquefaction. Small scale coal extractions were performed in tubing bombs; larger scale simulations of the liquid solvent extraction (LSE) process (ref. 1) were performed in a 250 ml Baskerville stirred autoclave. Feedstocks and liquid products were initially fractionated into pentane insoluble, saturate, aromatic and polar compound classes, (ref.2) which were then analysed by NMR. The type and distribution of sulphur and nitrogen functional groups in the raw coal (and petroleum) feeds and their fate during liquefaction have obvious implications on the subsequent environmental impact of the products. The sulphur and nitrogen contents in the products were determined and their fate under different processing conditions investigated. 0378-3820/90/$03.50
© 1990Elsevier Science Publishers B.V.
226 METHODS Materials The coal used in this work was a British bituminous coal from Point-of-Ayr. This particular coal was selected as it provides the feedstock for the British Coal LSE process. Its analysis, along with those of the petroleum residues studied, is shown in Table 1. In addition four other British coals ranging in carbon contents from 82.4 to 95.2% were also studied. TABLE 1 Ultimate analysis data for Point-of-Ayr coal and the series of petroleum residues
Analysis, wt% (dmmf) C
H
N
S (org)
H/C
Car a
Point of Ayr Coal (POA)
84.1
5.2
1.8
1.0
0.74
Marguerite Lake Atmos. Residue (AR-ML)
83.3
10.0
0.8
6.4
1.44
28.6
Vacuum Residue 'O' (VR-O)
85.5
10.5
nil
2.9
1.47
27.2
Forties Vacuum Residue (VR-FF)
87.4
11.1
0.5
0.8
1.52
24.4
Arabian Heavy Atmos. Residue (AR-AH)
84.4
11.2
0.3
4.0
1.60
19.8
Vacuum Residue 'M' (VR-M)
86.6
12.5
0.4
0.9
1.73
13.0
a Calculated from the Brown-Ladner equation (ref.5). Batch Autoclave Experiments POA coal was co-processed with the petroleum residues in a stirred Baskerville autoclave (250ml). The baseline reaction conditions for the system were as follows : 25%POA /75% oil / 120atm. H 2 / 450°C / 90min. /10%NiMo The products from the reactor were fractionated into dichloromethane soluble (DCMS) and dichloromethane insoluble (DCMI) fractions. Coal conversion was calculated from the DCMI on a dry ash free (daf) basis. Tubin~ Bomb Exoeriments The tubing bombs were heated in a fluidised sand bath with agitation provided by an adapted flask shaker. Constant reactor loadings of 3.5g were used. Coal loading was kept at a constant
227 25%, and the solvent was made up of either whole petroleum residue or petroleum residue with added asphaltene. All experiments were performed at 410°C for 30 minutes under nitrogen. Coal extract yields were calculated on a quinoline insoluble or dichloromethane insoluble basis. Analytical Methods The liquid products from the autoclave and tubing bombs were fractionated into n-pentane solubles and n-pentane insolubles (ref.2). 1H NMR spectra of the n-pentane insolubles were recorded on a 90 MHz Jeol FX90Q Fourier Transform NMR Spectrometer. Chloroform-d was used as a solvent and tetramethylsilane (TMS) as an internal standard. Approximately 15rag of sample was required to obtain good quality spectra in a reasonable time.
RESULTS AND DISCUSSION Influence of Coal/Petroleum Ratio on Hvdro-coorocessin~ In order to assess the effect of coal/oil ratio on hydrocracking yields, POA coal was co-processed with the Marguerite Lake Residue using the baseline reaction conditions and five different coal/oil ratios. The results are shown in Figure la. As the POA loading in the reaction mixture increased there was an increase in coal conversion to a maximum conversion of 76% at a ratio of 1:1. The variations in coal conversions were reflected in the analysis of the DCM soluble products which showed an increase in aromaticity at high conversions. It is thought that the increase in conversion results from the solvent becoming more coal compatible as the processing proceeds. At high coal loadings this is particularly so, since coal derived fragments make up a greater proportion of the solvent. In a further series of experiments (all with 25% coal loading) the DCM soluble fractions from each run were used as the solvent for the subsequent run. In this way, the solvent contained a progressively greater proportion of coal-derived material in each cycle. Coal conversion was found to increase over successive passes, further supporting the above conclusion. Influence of Catalyst on Hvdro-coorocessin~ In order to determine the effect of catalyst type on coal conversion, a range of catalysts was investigated.
Again the baseline reaction system was used, with the Marguerite Lake
Atmospheric Residue. The coal conversions obtained with each type of catalyst are shown in Figure lb. In the absence of catalyst no significant conversion of coal occurred, indicating little non-catalysed hydrogen transfer between molecular hydrogen, coal and petroleum residue, and therefore little inhibition of retrogressive reactions.The two iron-containing catalysts investigated -
FeSO 4 and FeS - gave conversions of 60% and 43% respectively. The greater conversion
obtained with the iron sulphate catalyst is most likely due to its more intimate contact with the coal. Catalyst application was by impregnation from an aqueous solution, in contrast to a simple mixing technique used in all other cases. Both bimetallic catalysts - NiMo and MoCo gave similar results with conversions above 70%. There was little difference between using pellet or crushed NiMo, suggesting that the catalyst loading was in excess.
228
1 O0 o~
10 0
a
..i.-,
r-"
o ~ 60
~ 60
g4o 2o
0
(..)
0
percent
2o '6 .I m 0
POA
Figure 1. Variation of coal conversion with (a) amount of coal and (b) type of catalyst. Influence of Asphaltene Content on Co-processing (i) Process Yields In order to investigate the effect of structure on co-processing, POA coal was processed with a suite of oils, under relatively mild conditions, in small tubing bombs. Of the oils studied, significant solvent-coal interactions were only observed with the three most aromatic petroleum feedstocks. In contrast to the hydrotreatment experiments, there was no significant conversion to DCM soluble products, conversion was only to quinoline solubles. Further, processing with the n-pentane soluble fractions from the oils was found to completely eliminate the interaction, whilst processing using asphaltenes as the solvent gave an enhanced interaction (ref.3). In another series of experiments, AR-ML was processed with a suite of coals ranging in carbon contents from 82.4 to 95.2%. Again, no significant conversion to DCM soluble products was observed. For the highest rank coal a negative conversion was recorded, i.e. the DCM insoluble fraction represented a greater proportion of the sample than the original coal. To further study the effect of asphaltenes on processing yield, various quantities of asphaltene were added to one of the petroleum residues (Arabian Heavy), before processing, to increase artificially the asphaltene content. Although total aromatic hydrogen (on a wt % basis) did not correlate well with extraction, it was found that the amount of aromatic hydrogen in the asphaltenes, expressed as a fraction of the total sample, did. Figure 2 shows the variation of coal conversion with the aromatic asphaltene content of the solvent for the whole range of samples studied (including those from the other residues). These results indicate that a minimum aromatic asphaltene content is required for a significant interaction. This is probably due to the more condensed asphaltene species being more efficient hydrogen shuttlers and therefore better able to stabilise the transient coal radicals (ref.4). The maximum yield attainable after this point, however, appears to be dependent on other factors (e.g. hydrogen-donor ability).
229
60
t--
~40 e-
L~ 20
•
0
,
0
2 Pentane
.
4 insoluble
6 Har,Wt %
8
Figure 2. Relationship between aromatic asphaltene content and coal conversion. (&) are derived from hydrotreatment of the Marguerite Lake Atmospheric Residue.
(ii)Heteroatom Content If co-processing is to be used as a viable alternative to conventional processing technologies it is important that the products conform to environmental safety standards with the minimum of upgrading. Of particular importance, therefore, is the heteroatom content of the products. Figures 3a and 3b show the variation of sulphur and nitrogen contents of the DCM soluble products with asphaltene loading in the feed solvent mixture. The theoretical points were calculated on the assumption that there is no elemental fractionation of the products and are calculated from a simple mixing of the starting materials. The experimental values for sulphur content all lie below the calculated values, indicating that the gas (or residue) was relatively rich in sulphur. This would be expected since H2S is a known product of processing. In the case of nitrogen, however, at low asphaltene loading the DCM soluble product was relatively rich in nitrogen, whilst, at high asphaltene loading the nitrogen content was significantlyreduced. CONCLUSIONS 1) Good coal conversions were obtained under hydrotreatment conditions, optimum conversion being at a coal to oil ratio of 1:1. 2) The highest conversions were observed with bimetallic catalysts. 3) Co-processing yield under less severe conditions was dependent on the content of aromatic asphaltene in the solvent. 4) At high asphaltene loadings sulphur and particularly nitrogen were depleted in the products.
230
6
1-4
• S - c a Ic o S-expt t--
tO. 4 -I
Or)
@
•
O
o o
1.2
b • N-calc o N-expt @
O
O)
P 1.o
O o
O
Z
~0-8'
2
0-6 2 .0 . 4 0 . 6. 0 80 A s p h a l t e n e , w t % in f e e d
2b 4'0 6'0 Asphaltene, wt%
8'0 in f e e d
Figure 3. Variation of (a) sulphur content and (b) nitrogen content with asphaltene loading.
REFERENCES 1. G.O. Davies, Chem. and Ind. 15 (1978) 560. 2. K.D. Bartle, W.R. Ladner, T.G. Martin, C.E. Snape and D.F. Wiliams, Fuel, 58 (1979) 413. 3. S. Wallace, K.D. Bartle, M.P. Burke, B. Egia, S. Lu, N. Taylor, T. Flynn, W. Kemp and W. Steedman, Accepted for publication in Fuel. 4. A. Grint, S. Mehani, M. Trewhella and M.J. Crook, Fuel, 64 (1985) 1355. 5. J.K. Brown and W.R. Ladner, Fuel, 39 (1960) 87.
225
COAL CONVERSION IN CO-PROCESSING WITH HEAVY PETROLEUM RESIDUES S. Wallace 1, K.D. Bartle 1, W. Kemp2, W. Steedman2, T. Flynn2, M.P. Burke 1, C.J. Jones 1 and N. Taylor1. 1 Department of Physical Chemistry, University of Leeds, Leeds, LS2 9JT. 2 Department of Chemistry, Heriot-Watt University, Edinburgh, EH14 4AS.
ABSTRACT
Point-of-Ayr coal was co-processed with a series of petroleum residues under both mild and severe reaction conditions. Hydrocracking conditions were chosen to optimise coal conversion, whilst the lower severity reactions were used to investigate the structural effects of processing. Under hydrocracking conditions, co-processing conversions varied with coal/oil ratio, with the optimum near 50/50. Bimetallic catalysts were found to enhance coal conversion. In a second series of experiments coal was co-processed with a petroleum enriched by the addition of petroleum asphaltenes: enhanced solubilisation was associated with increased asphaltene content. In the same series the fates of nitrogen and sulphur were also dependent on asphaltene loadings.
INTRODUCTION Coal liquefaction processes typically involve thermal degradation of the macromolecular coal structure followed by hydrogenation to stabilise the degraded material and increase the H/C ratio of the products. Usually a coal derived recycle solvent is used as a vehicle to transfer the hydrogen. Since the recycle solvent is obtained from the coal derived product the extent of hydrocracking (and hence yield) has to be limited. In principle, this problem could be alleviated by supplementing or replacing the recycle solvent stream with heavy petroleum fractions which are cheap and have a higher H/C ratio than coal derived solvents. If this is to be effective, however, it is necessary that the coal and petroleum be compatible and also that the products conform to environmental safety standards. A U.K. bituminous coal was co-processed with a series of petroleum derived solvents of varied composition under conditions simulating the extraction stage of two stage liquefaction. Small scale coal extractions were performed in tubing bombs; larger scale simulations of the liquid solvent extraction (LSE) process (ref. 1) were performed in a 250 ml Baskerville stirred autoclave. Feedstocks and liquid products were initially fractionated into pentane insoluble, saturate, aromatic and polar compound classes, (ref.2) which were then analysed by NMR. The type and distribution of sulphur and nitrogen functional groups in the raw coal (and petroleum) feeds and their fate during liquefaction have obvious implications on the subsequent environmental impact of the products. The sulphur and nitrogen contents in the products were determined and their fate under different processing conditions investigated. 0378-3820/90/$03.50
© 1990Elsevier Science Publishers B.V.
226 METHODS Materials The coal used in this work was a British bituminous coal from Point-of-Ayr. This particular coal was selected as it provides the feedstock for the British Coal LSE process. Its analysis, along with those of the petroleum residues studied, is shown in Table 1. In addition four other British coals ranging in carbon contents from 82.4 to 95.2% were also studied. TABLE 1 Ultimate analysis data for Point-of-Ayr coal and the series of petroleum residues
Analysis, wt% (dmmf) C
H
N
S (org)
H/C
Car a
Point of Ayr Coal (POA)
84.1
5.2
1.8
1.0
0.74
Marguerite Lake Atmos. Residue (AR-ML)
83.3
10.0
0.8
6.4
1.44
28.6
Vacuum Residue 'O' (VR-O)
85.5
10.5
nil
2.9
1.47
27.2
Forties Vacuum Residue (VR-FF)
87.4
11.1
0.5
0.8
1.52
24.4
Arabian Heavy Atmos. Residue (AR-AH)
84.4
11.2
0.3
4.0
1.60
19.8
Vacuum Residue 'M' (VR-M)
86.6
12.5
0.4
0.9
1.73
13.0
a Calculated from the Brown-Ladner equation (ref.5). Batch Autoclave Experiments POA coal was co-processed with the petroleum residues in a stirred Baskerville autoclave (250ml). The baseline reaction conditions for the system were as follows : 25%POA /75% oil / 120atm. H 2 / 450°C / 90min. /10%NiMo The products from the reactor were fractionated into dichloromethane soluble (DCMS) and dichloromethane insoluble (DCMI) fractions. Coal conversion was calculated from the DCMI on a dry ash free (daf) basis. Tubin~ Bomb Exoeriments The tubing bombs were heated in a fluidised sand bath with agitation provided by an adapted flask shaker. Constant reactor loadings of 3.5g were used. Coal loading was kept at a constant
227 25%, and the solvent was made up of either whole petroleum residue or petroleum residue with added asphaltene. All experiments were performed at 410°C for 30 minutes under nitrogen. Coal extract yields were calculated on a quinoline insoluble or dichloromethane insoluble basis. Analytical Methods The liquid products from the autoclave and tubing bombs were fractionated into n-pentane solubles and n-pentane insolubles (ref.2). 1H NMR spectra of the n-pentane insolubles were recorded on a 90 MHz Jeol FX90Q Fourier Transform NMR Spectrometer. Chloroform-d was used as a solvent and tetramethylsilane (TMS) as an internal standard. Approximately 15rag of sample was required to obtain good quality spectra in a reasonable time.
RESULTS AND DISCUSSION Influence of Coal/Petroleum Ratio on Hvdro-coorocessin~ In order to assess the effect of coal/oil ratio on hydrocracking yields, POA coal was co-processed with the Marguerite Lake Residue using the baseline reaction conditions and five different coal/oil ratios. The results are shown in Figure la. As the POA loading in the reaction mixture increased there was an increase in coal conversion to a maximum conversion of 76% at a ratio of 1:1. The variations in coal conversions were reflected in the analysis of the DCM soluble products which showed an increase in aromaticity at high conversions. It is thought that the increase in conversion results from the solvent becoming more coal compatible as the processing proceeds. At high coal loadings this is particularly so, since coal derived fragments make up a greater proportion of the solvent. In a further series of experiments (all with 25% coal loading) the DCM soluble fractions from each run were used as the solvent for the subsequent run. In this way, the solvent contained a progressively greater proportion of coal-derived material in each cycle. Coal conversion was found to increase over successive passes, further supporting the above conclusion. Influence of Catalyst on Hvdro-coorocessin~ In order to determine the effect of catalyst type on coal conversion, a range of catalysts was investigated.
Again the baseline reaction system was used, with the Marguerite Lake
Atmospheric Residue. The coal conversions obtained with each type of catalyst are shown in Figure lb. In the absence of catalyst no significant conversion of coal occurred, indicating little non-catalysed hydrogen transfer between molecular hydrogen, coal and petroleum residue, and therefore little inhibition of retrogressive reactions.The two iron-containing catalysts investigated -
FeSO 4 and FeS - gave conversions of 60% and 43% respectively. The greater conversion
obtained with the iron sulphate catalyst is most likely due to its more intimate contact with the coal. Catalyst application was by impregnation from an aqueous solution, in contrast to a simple mixing technique used in all other cases. Both bimetallic catalysts - NiMo and MoCo gave similar results with conversions above 70%. There was little difference between using pellet or crushed NiMo, suggesting that the catalyst loading was in excess.
228
1 O0 o~
10 0
a
..i.-,
r-"
o ~ 60
~ 60
g4o 2o
0
(..)
0
percent
2o '6 .I m 0
POA
Figure 1. Variation of coal conversion with (a) amount of coal and (b) type of catalyst. Influence of Asphaltene Content on Co-processing (i) Process Yields In order to investigate the effect of structure on co-processing, POA coal was processed with a suite of oils, under relatively mild conditions, in small tubing bombs. Of the oils studied, significant solvent-coal interactions were only observed with the three most aromatic petroleum feedstocks. In contrast to the hydrotreatment experiments, there was no significant conversion to DCM soluble products, conversion was only to quinoline solubles. Further, processing with the n-pentane soluble fractions from the oils was found to completely eliminate the interaction, whilst processing using asphaltenes as the solvent gave an enhanced interaction (ref.3). In another series of experiments, AR-ML was processed with a suite of coals ranging in carbon contents from 82.4 to 95.2%. Again, no significant conversion to DCM soluble products was observed. For the highest rank coal a negative conversion was recorded, i.e. the DCM insoluble fraction represented a greater proportion of the sample than the original coal. To further study the effect of asphaltenes on processing yield, various quantities of asphaltene were added to one of the petroleum residues (Arabian Heavy), before processing, to increase artificially the asphaltene content. Although total aromatic hydrogen (on a wt % basis) did not correlate well with extraction, it was found that the amount of aromatic hydrogen in the asphaltenes, expressed as a fraction of the total sample, did. Figure 2 shows the variation of coal conversion with the aromatic asphaltene content of the solvent for the whole range of samples studied (including those from the other residues). These results indicate that a minimum aromatic asphaltene content is required for a significant interaction. This is probably due to the more condensed asphaltene species being more efficient hydrogen shuttlers and therefore better able to stabilise the transient coal radicals (ref.4). The maximum yield attainable after this point, however, appears to be dependent on other factors (e.g. hydrogen-donor ability).
229
60
t--
~40 e-
L~ 20
•
0
,
0
2 Pentane
.
4 insoluble
6 Har,Wt %
8
Figure 2. Relationship between aromatic asphaltene content and coal conversion. (&) are derived from hydrotreatment of the Marguerite Lake Atmospheric Residue.
(ii)Heteroatom Content If co-processing is to be used as a viable alternative to conventional processing technologies it is important that the products conform to environmental safety standards with the minimum of upgrading. Of particular importance, therefore, is the heteroatom content of the products. Figures 3a and 3b show the variation of sulphur and nitrogen contents of the DCM soluble products with asphaltene loading in the feed solvent mixture. The theoretical points were calculated on the assumption that there is no elemental fractionation of the products and are calculated from a simple mixing of the starting materials. The experimental values for sulphur content all lie below the calculated values, indicating that the gas (or residue) was relatively rich in sulphur. This would be expected since H2S is a known product of processing. In the case of nitrogen, however, at low asphaltene loading the DCM soluble product was relatively rich in nitrogen, whilst, at high asphaltene loading the nitrogen content was significantlyreduced. CONCLUSIONS 1) Good coal conversions were obtained under hydrotreatment conditions, optimum conversion being at a coal to oil ratio of 1:1. 2) The highest conversions were observed with bimetallic catalysts. 3) Co-processing yield under less severe conditions was dependent on the content of aromatic asphaltene in the solvent. 4) At high asphaltene loadings sulphur and particularly nitrogen were depleted in the products.
230
6
1-4
• S - c a Ic o S-expt t--
tO. 4 -I
Or)
@
•
O
o o
1.2
b • N-calc o N-expt @
O
O)
P 1.o
O o
O
Z
~0-8'
2
0-6 2 .0 . 4 0 . 6. 0 80 A s p h a l t e n e , w t % in f e e d
2b 4'0 6'0 Asphaltene, wt%
8'0 in f e e d
Figure 3. Variation of (a) sulphur content and (b) nitrogen content with asphaltene loading.
REFERENCES 1. G.O. Davies, Chem. and Ind. 15 (1978) 560. 2. K.D. Bartle, W.R. Ladner, T.G. Martin, C.E. Snape and D.F. Wiliams, Fuel, 58 (1979) 413. 3. S. Wallace, K.D. Bartle, M.P. Burke, B. Egia, S. Lu, N. Taylor, T. Flynn, W. Kemp and W. Steedman, Accepted for publication in Fuel. 4. A. Grint, S. Mehani, M. Trewhella and M.J. Crook, Fuel, 64 (1985) 1355. 5. J.K. Brown and W.R. Ladner, Fuel, 39 (1960) 87.