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Effect of solvent hydrotreatment on product yield in the coal liquefaction process
Fuel Processing Technology 68 Ž2000. 237–254 www.elsevier.comrlocaterfuproc
Effect of solvent hydrotreatment on product yield in the coal liquefaction process Masato Kouzu a,) , Hitoshi Saegusa a , Takashi Hayashi a , Takahiro Nishibayashi a , Masatoshi Kobayashi a , Hironori Itoh b, Hideshi Hattori c a
Technical Department, Nippon Coal Oil Co., Ltd., 2, Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan b Research Group of Materials Science and Engineering, Graduate School of Engineering, Hokkaido UniÕersity, N13W8, Kita-ku, Sapporo 060-8628, Japan c Center for AdÕanced Research of Energy Technology, Hokkaido UniÕersity, N13W8, Kita-ku, Sapporo 060-8628, Japan Accepted 1 July 2000
Abstract Effects of solvent hydrotreatment on product yields in the NEDOL coal liquefaction process were examined based on the data obtained by a 150 trd pilot plant operation. When the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf Žwt.% on the basis of dry and ash-free coal., the yield of the oil fraction increased from 51.0 to 54.9 wt.%-daf with an increase in the hydrogen gas consumption from 0.7 to 1.3 wt.%-daf in the hydrotreatment stage. In the hydrotreatment stage, about 30% of the hydrogen consumed was utilized for enrichment of naphthenic hydrogen content in the solvent, and the rest was used for both the removal of nitrogen from the solvent and the production of the light oil fraction. In the liquefaction stage, the hydrogen donated from the solvent was more efficiently utilized than the gaseous hydrogen for the production of the oil fraction. Under the conditions that the total hydrogen consumption in the hydrotreatment and liquefaction stages was kept constant, the yield of oil fraction and the nitrogen content in oil fraction were higher when the hydrogen consumption in the liquefaction stage
) Corresponding author. Present address: 1352-1-3-202, Hirai, Kashima Ibaragi-ken 314-0012, Japan. Tel.: q81-299-82-6885; fax: q81-299-82-6885. E-mail address: [email protected] ŽM. Kouzu..
0378-3820r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 0 0 . 0 0 1 2 4 - 7
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increased with concomitant decrease in the hydrotreatment stage. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Coal liquefaction; Solvent hydrotreatment; Hydrogen gas consumption
1. Introduction The NEDOL process is a coal liquefaction process developed by the New Energy and Industrial Technology Development Organization ŽNEDO. in Japan. A 150 trd pilot plant was operated from March 1997 to September 1998. In the NEDOL process, coals are directly converted into oils in the presence of hydrogen donor solvent and iron catalyst under hydrogen pressure. The produced oils are divided into light, middle, and heavy oil fractions. The light and middle oil fractions are target products to be used as liquid fuels. The heavy oil fraction is recycled as the hydrogen donor solvent after hydrotreatment. The hydrogen donor solvent promotes the coal liquefaction reaction since the solvent stabilizes the coal fragment radicals by donating hydrogen w1x. The hydrogen-donating reaction is accompanied by the dehydrogenation of hydro-aromatics such as tetra-hydronaphthalene, di-hydrophenanthrene, and di-hydropyrene w2x. The heavy oil fraction utilized as the hydrogen donor solvent contains a large amount of aromatics such as naphthalene, phenanthrene, and pyrene. Since these aromatics do not possess hydrogendonating ability, they should be hydrogenated into the hydro-aromatics, which possess hydrogen-donating ability. The extent of the aromatic hydrogenation in the solvent hydrotreatment should influence the oil yield in the coal liquefaction process. The effects of the hydrogen donor solvent on coal liquefaction have been investigated in several previous studies. Sugimoto and Miki w3x carried out liquefaction of Wandoan coal with different hydrogen donor solvents, and found that the yield of the hexane soluble fraction increased with an increase in the ratio of the hydro-aromatic content to the aromatic content in the solvent. Hirano et al. w4x conducted coal liquefaction with several hydrotreated solvents derived from a heavy oil fraction produced in the PSU. They reported that the yield of oil fraction increased with an increase in the amount of naphthenic hydrogen in the solvent. In the solvent hydrotreatment in which Ni–Morg-Al 2 O 3 catalyst was used, the compounds corresponding to the light oil fraction were produced, and hydrodenitrogenation occurred w5x. Therefore, in the hydrotreatment, hydrogen gas is consumed not only for hydrogenation of aromatic compounds but also for the removal of nitrogen from the solvent and the production of the light oil fraction. The purpose of the pilot plant operation is to select the optimum conditions for economically efficient production of the oils. Since the hydrogen gas consumption in the coal liquefaction process influences the cost of the oil production w6x, efficient utilization of hydrogen is important. In the present paper, the variation of the product yield as a function of the hydrogen gas consumption in the hydrotreatment stage was examined based on the data obtained by the pilot plant operation to clarify the effects of the hydrogen-donating ability of the solvent on the product yield. In addition, the effects of
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hydrogen consumption on the product yield in the hydrotreatment stage are compared with those in the liquefaction stage for efficient utilization of hydrogen.
2. Experimental methods 2.1. Process flow of the 150 t r d pilot plant A schematic flow diagram of the 150 trd pilot plant is shown in Fig. 1. The pilot plant consists mainly of four sections, i.e., coal slurry preparation, liquefaction, distillation and solvent hydrotreatment. The slurry, a mixture of pulverized coal, liquefaction catalyst, and hydrogen donor solvent, was fed to the liquefaction reactor with pressurized hydrogen. The effluent from the reactor was separated in the distillation section into four fractions, i.e., the light oil fraction with a boiling point below 2208C Žliquefied light oil., the middle oil fraction with a boiling point ranging from 2208C to 3508C Žliquefied middle oil., the heavy oil fraction with a boiling point ranging from 3508C to 5388C Žliquefied heavy oil., and the residue with a boiling point above 5388C Žliquefaction residue.. Whole liquefied heavy oil was used as the solvent, and a part of the liquefied middle oil was mixed with the liquefaction heavy oil to keep the flow rate of the solvent constant in every operation. The prepared solvent was fed into the hydrotreating reactor in the solvent hydrotreatment section. In the reactor, the compounds whose boiling points correspond to those of the light oil fraction were also produced, and they are called hydrotreated light oil. The hydrotreated light oil was stripped from the hydrotreated solvent with a steam. 2.2. Coal and solÕent A sub-bituminous coal from the Tanito–Harum seams in Indonesia was processed in the pilot plant. Table 1 shows the composition of the coal. The coal contained 75.9 wt.% carbon and 5.8 wt.% hydrogen on a dry and ash-free Ždaf. coal basis. Table 2 shows the composition of the prepared solvent fed to the hydrotreating reactor. The composition of each solvent was not different significantly. The solvents were composed mostly of aromatic compounds, naphthalene-type aromatics being most abundant. The content of paraffin-type compounds was about 20%, naphthene-type being scarcely contained. 2.3. Operating procedures Solvent hydrotreatment was carried out in the fixed bed reactor over Ni–Morg-Al 2 O 3 catalyst developed by NEDO. The catalyst was an extruded cylindrical grain, 3 mm in length and 1.5 mm in diameter. It contained 3.0 wt.% NiO and 15.0 wt.% MoO 3 . The surface area and pore volume were 200 m2rg and 0.6 mlrg, respectively. The catalyst was presulfided at 2508C by feeding a mixture of anthracene oil, creosote oil derived from coal-tar, and 1 wt.% dimethyldisulfide under a hydrogen pressure of 10 MPa.
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Fig. 1. Schematic flow diagram of the NEDOL 150 trd pilot plant.
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Table 1 Composition of the processed coal C Žwt.%-daf.
H Žwt.%-daf.
N Žwt.%-daf.
S Žwt.%-daf.
Odiff Žwt.%-daf.
Ash Žwt.%-dry.
75.9
5.8
1.8
0.2
16.3
5.5
Coal liquefaction was carried out in a bubble column reactor. Natural pyrite imported from Finland was used as liquefaction catalyst. The natural pyrite contained 48 wt.% Fe and 51 wt.% S, and was pulverized to a median diameter of 0.7 mm. The pulverized pyrite of 3 wt.%-dry coal was made an addition to the feed coal slurry. Operating conditions and hydrogen gas consumption are shown in Table 3 for four runs. Run-A, Run-B, and Run-C, were performed to examine the variations in the product yields as a function of the hydrogen gas consumption in the hydrotreatment stage. In Run-A, the solvent was hydrotreated at 2958C under a hydrogen pressure of 10 MPa with a liquid hourly space velocity ŽLHSV. of 1.0 hy1 . In Run-B, LHSV was 0.8 hy1 and the temperature was 3008C. In Run-C, the hydrotreating temperature was raised to 3308C with an LHSV of 0.8 hy1 . Hydrogen gas consumptions in the hydrotreatment stage of Run-A, Run-B, and Run-C were 0.7, 0.9, and 1.3 wt.%-daf, respectively. Coal liquefaction was conducted by feeding the slurry composed of a coal and hydrotreated solvent at a coal concentration of 40–50 wt.% under the temperature of 450–4608C, nominal residence time of 0.8–1.0 h, and total pressure of 17 MPa. Hydrogen gas consumption in the coal liquefaction stage was kept constant at 4.7 wt.%-daf for Run-A, Run-B, and Run-C.
Table 2 Composition of the solvents fed to hydrotreating reactor Elemental composition (wt.%) C H N S Aromaticity, fa Žy. a Compound type (%)b Paraffin Naphthene Benzene Naphthalene Phenanthrene Anthracene Pyrene Other aromatics c a
Run-A
Run-B
Run-C
Run-D
87.4 9.6 1.0 - 0.1 0.50
87.7 9.5 0.9 - 0.1 0.50
87.7 9.4 0.9 - 0.1 0.52
87.3 9.4 1.0 - 0.1 0.52
20.0 0.4 5.1 16.8 3.3 2.7 3.9 26.4
19.5 0.4 4.9 14.6 3.8 3.1 4.7 27.7
19.5 0.4 5.1 12.1 3.3 3.5 6.6 26.0
17.1 0.3 4.9 15.6 3.6 3.0 4.8 28.4
Calculated on 1 H NMR spectra and elemental composition. Measured by GC-MS. c Other aromatics include compounds of phenol, biphenyl, fluorene, carbazole and fluoranthene type. b
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Run-A Run-B Run-C Run-D a
Solvent hydrotreatment stage Operating conditiona DH 2 Žwt.%-daf. Temperature LHSV Ž8C. Žhy1 .
Liquefaction stage Operating condition Temperature Pressure Ž8C. ŽMPa.
Coal concentrationb Žwt.%.
NRT c Žh.
295 300 330 295
455 450 460 462
40 40 50 40
1.0 0.8 0.8 1.0
1.0 0.8 0.8 1.0
0.7 0.9 1.3 0.7
17 17 17 19
The solvent hydrotreatment stage was operated under a hydrogen pressure of 10 MPa in all runs. Coal concentration in feed slurry to liquefaction reactor. c Nominal residence time of liquefaction reactor. b
DH 2 Žwt.%-daf. 4.7 4.7 4.7 5.3
Total DH 2 Žwt.%-daf.
5.4 5.6 6.0 6.0
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Table 3 Operating conditions and hydrogen gas consumption Ž DH 2 . of 150 trd pilot plant
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Run-D was performed to examine the product yields at different levels of hydrogen gas consumption in the liquefaction stage. Coal liquefaction was conducted at 4628C under 19 MPa. The hydrogen gas consumption was 5.3 wt.%-daf in the liquefaction stage. The operating conditions and hydrogen gas consumption in the hydrotreatment stage for Run-D were the same as those for Run-A. Total hydrogen gas consumption of Run-D was equal to that of Run-C for which the hydrogen gas consumption in the hydrotreatment stage was the highest. 2.4. Analysis Composition of the gaseous products was measured by GC. Elemental compositions of the produced oils and the liquefaction residue were measured with a CHN and S analyzers. The naphthenic hydrogen of the hydro-aromatics contained in the solvent Žnaphthenic hydrogen. was defined as the hydrogen which shows the chemical shift ranging from 1.5 to 2.0 ppm in the 1 H NMR spectrum w7x. The residue was extracted with n-hexane, toluene, and tetrahydrofuran. The n-hexane soluble fraction, the toluene-soluble and n-hexane insoluble fraction, the tetrahydrofuran-soluble and toluene insoluble fraction, and the tetrahydrofuran-insoluble fraction were defined as oil, asphaltene, preasphaltene, and unreacted coal, respectively. The product yields and hydrogen gas consumption were calculated on the daf coal basis. 3. Results and discussion 3.1. Enhancement of the hydrogen-donating ability of the solÕent in the hydrotreatment stage Fig. 2 shows the variation of the naphthenic hydrogen contents in the prepared solvent and the hydrotreated solvent as a function of the hydrogen gas consumption in the hydrotreatment stage. The naphthenic hydrogen content was obtained by multiplying the hydrogen content of the solvent by the ratio of the naphthenic hydrogen of the hydro-aromatics to the total hydrogen determined from the 1 H NMR spectrum. Although the liquefaction conditions were different, the naphthenic hydrogen content of the prepared solvent was almost the same for each run. The naphthenic hydrogen content of the hydrotreated solvent increased with an increase in the hydrogen gas consumption. This indicates that aromatic hydrogenation occurred in the solvent hydrotreatment. Since the naphthenic hydrogen content in the prepared solvent kept constant against time on stream, the same amount of hydrogen as that of the naphthenic hydrogen increasing in the hydrotreatment stage should be consumed in the liquefaction stage. Therefore, as the content of naphthenic hydrogen increases in the hydrotreatment stage, the amount of hydrogen utilized in the liquefaction stage increases. If the differences in the naphthenic hydrogen content between hydrotreated solvent and prepared solvent were converted into those on wt.%-daf base, and plotted against the hydrogen consumption in solvent hydrotreatment, the slope gives the fraction of the hydrogen utilized for the formation of the naphthenic hydrogen. The slope was 0.33, indicating that 33% of the consumed
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Fig. 2. Variation of naphthenic hydrogen contents in prepared Žfeed. solvent Ž`. and hydrotreated solvent Žv . as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment.
hydrogen was utilized for the formation of the naphthenic hydrogen, and the rest Ž67%. was utilized for the other reactions such as hydrodenitrogenation and hydrocracking. Fig. 3 shows the variation of the nitrogen contents in the prepared solvent, the hydrotreated solvent, and the hydrotreated light oil as a function of the hydrogen gas consumption in the hydrotreatment stage. The nitrogen contents in the hydrotreated solvent and hydrotreated light oil decreased with an increase in the hydrogen consumption. It is obvious that hydrodenitrogenation occurred in the hydrotreatment stage. Hydrogen was consumed in the hydrotreatment stage not only for the formation of naphthenic hydrogen by hydrogenation of aromatics, but also for the removal of nitrogen by hydrodenitrogenation. Fig. 4 shows variation of the yields of the organic gases and the light oil fraction produced in the hydrotreatment stage as a function of the hydrogen gas consumption. The organic gases include methane, ethane, and propane. The yield of the organic gases was less than 0.2 wt.%-daf at all levels of the hydrogen gas consumption. The yield of the light oil increased with an increase in the hydrogen consumption. It is not certain as to whether the light oil fraction was formed by hydrodenitrogenation, hydrocracking, or some other reactions. However, it is certain that the formation of the light oil fraction was accompanied by the consumption of hydrogen. 3.2. Effects of the solÕent hydrotreatment on coal liquefaction The variation of the product yields as a function of the hydrogen gas consumption in the hydrotreatment stage was examined to elucidate the effect of the hydrogen-donating ability of the solvent on coal liquefaction.
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Fig. 3. Variation of nitrogen contents in prepared Žfeed. solvent Ž`., hydrotreated solvent Žv ., and hydrotreated light oil Žlight oil fraction produced in solvent hydrotreatment stage: '. as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment.
Fig. 5 shows the variation of the yields of the oils, the liquefaction residue, and the organic gases as a function of the hydrogen gas consumption in the hydrotreatment stage
Fig. 4. Variation of yields of organic gases Ž`. and light oil fraction Žv . produced in the hydrotreatment as a function of hydrogen gas consumption Ž DH 2 ..
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Fig. 5. Variation of yields of oils Žv ., liquefaction residue ŽB., and organic gases Ž'. produced in total coal liquefaction system as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf.
while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf. The oil yield is a sum of the light and middle oil fractions produced in the liquefaction stage and the light oil fraction produced in the hydrotreatment stage. The organic gases include methane, ethane, and propane produced in both the liquefaction and the hydrotreatment stages. The yield of the oils increased from 51.0 to 54.9 wt.%-daf with an increase in the hydrogen gas consumption from 0.7 to 1.3 wt.%-daf in the hydrotreatment stage. The yield of the liquefaction residue decreased from 25.1 to 18.8 wt.%-daf and that of the organic gases increased from 10.4 to 13.0 wt.%-daf with an increase in the hydrogen gas consumption. It is clear that an increase in the hydrogen consumption in the solvent hydrotreatment stage resulted in the promotion of liquefaction reaction for the production of oils. Fig. 6 shows the variation of the yields of the fractions contained in the liquefaction residue, i.e., oil, asphaltene, preasphaltene, and unreacted coal, as a function of the hydrogen gas consumption in the hydrotreatment stage. All of the liquefaction residues included oil equivalent to about 1.0 wt.%-daf. The yields of the unreacted coal and preasphaltene were less than 2.0 wt.%-daf at all levels of the hydrogen gas consumption. The yield of the asphaltene decreased from 21.0 to 15.6 wt.%-daf with an increase in the hydrogen gas consumption. In Fig. 6, as compared with Fig. 5, it is suggested that the decrease in the residue resulted mainly from the decrease in the asphaltene. It is reasoned that the hydrogen donated from the solvent combines with the radical
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Fig. 6. Variation of yields of fractions in liquefaction residue; oil Ž`., asphaltene Žv ., preasphaltene Ž'., and unreacted coal ŽB. as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf.
fragments from coal thermal dissociation and so reducing the asphaltene by repolymerization of the radical fragments. This is consistent with the results reported by Nagaishi et al. w8x. Fig. 7 shows the variation of the yields of the liquefied light oil and the liquefied middle oil as a function of the hydrogen gas consumption in the hydrotreatment stage. The yield of the liquefied light oil increased with the increase in the hydrogen gas consumption while that of the liquefied middle oil slightly decreased. The conversion of middle oil fraction to light oil fraction occurred in addition to the conversion of asphaltene to the oil fraction as mentioned above. Fig. 8 shows the variations of the nitrogen content of the liquefied light oil fraction and middle oil fraction as a function of the hydrogen gas consumption in the hydrotreatment stage. The nitrogen content decreased with an increase in the hydrogen gas consumption for both the liquefied light oil and middle oil fractions. Two possibilities are to be taken into consideration to explain the decrease in the nitrogen content. One possibility is as follows. A considerable fraction of light and middle oils originates from the solvent. The solvent contains less amount of nitrogen as the hydrogen consumption in the hydrotreatment stage increases. A part of the solvent decomposes to form light and middle oil fractions. The other possibility is that hydrodenitrogenation proceeds to a larger extent to form oil fraction in the liquefaction stage when the solvent contains a larger amount of hydrogen. It is reported that a part of the liquefied light and middle oils originate from the solvent w9x. However, it is not certain which case contributes much to
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Fig. 7. Variation of yields of light Žv . and middle oil Ž`. fractions produced in liquefaction stage as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf.
Fig. 8. Variations of nitrogen content in liquefied light oil Žv . and liquefied middle oil Ž`. as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf.
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the decrease in the nitrogen content in the oil fraction. It should be noted that the removal of nitrogen from the solvent is favorable for the reduction of the nitrogen content of the oil produced in the liquefaction stage. 3.3. Hydrogen utilization efficiency in coal liquefaction The product yields at different levels of hydrogen gas consumption in the liquefaction stage were examined to discuss the hydrogen utilization efficiency in the total coal liquefaction process. Fig. 9 shows the yields of the oils, liquefaction residue, and organic gases for Run-A, Run-C, and Run-D. The hydrogen consumptions in hydrotreatment stage were 0.7, 1.3, and 0.7 wt.%-daf for Run-A, Run-C, and Run-D, respectively. The hydrogen consumptions in the liquefaction stage were 4.7, 4.7, and 5.3 wt.%-daf for Run-A, Run-C, and Run-D, respectively. The total hydrogen consumptions were the same for Run-C and Run-D, and less for Run-A. The yields of the oils and organic gases were in the order, Run-D) Run-C) Run-A. The yield of the liquefaction residue was in the order Run-A) Run-C) Run-D. The differences between Run-A and Run-C are described earlier. The differences between Run-C and Run-D reflect the difference in the share of the same amount of hydrogen consumption into the hydrotreatment stage and the liquefaction stage. Under the same total hydrogen consumption, the yields of oil fractions and organic gases were higher, and the yield of residue was lower when the hydrogen consumed in the liquefaction stage was increased with concomitant decrease in the hydrotreatment stage. This is because only 33% of hydrogen consumed in the hydrotreatment stage was utilized for hydrogenation of aromatics to form compounds acting as hydrogen donor in the liquefaction stage. Therefore, the results in Fig. 9 do not necessarily mean that the gaseous hydrogen is more efficient than the naphthenic hydrogen in the solvent for coal liquefaction. To find out if there are any difference in the efficiency of hydrogen between gaseous hydrogen and the naphthenic hydrogen in the solvent for coal liquefaction, the yields of oil fractions and asphaltene are plotted against Anet hydrogen consumption in the liquefaction stageB in Fig. 10. The Anet hydrogen consumption in the liquefaction stageB is defined as the sum of the hydrogen gas consumption in the liquefaction stage and the naphthenic hydrogen formed in the hydrotreatment stage. The latter is equivalent to 33% of the hydrogen gas consumption in the hydrotreatment stage. For Run-A Žv ., Run-B Ž'., and Run-C ŽB., the hydrogen gas consumption in the liquefaction stage was the same. Therefore, the slope of solid line is an index of the efficiency of naphthenic hydrogen for the liquefaction. On the other hand, for Run-A Žv . and Run-D Ž`., the hydrogen gas consumption in the hydrotreatment stage was the same, but the hydrogen gas consumption in the liquefaction stage was higher for Run-D than for Run-A. Therefore, the slope of dotted line is an index of the efficiency of gaseous hydrogen for the liquefaction. The slope of the solid line is steeper than that of the dotted line, indicating that the naphthenic hydrogen in the solvent acts more efficiency than the gaseous hydrogen for the formation of oil fraction in the liquefaction stage. Likewise, the naphthenic hydrogen in the solvent acts more efficiently than the
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Fig. 9. Yields of oil fraction, liquefaction residue, and organic gases for Run-A, Run-C, and Run-D.
gaseous hydrogen for the reduction of asphaltene in the liquefaction stage Žlower figure.. A higher efficiency of naphthenic hydrogen as compared with gaseous hydrogen is consistent with the results reported with a model compound w10x as well as with a coal w4x. Fig. 11 shows the yields of the light oil fractions produced in both the liquefaction and hydrotreatment stages, and the middle oil fraction. As already shown in Fig. 9, the yield of oil fraction was in the order, Run-D) Run-C) Run-A. As to the yield of the light oil fractions, no obvious difference was observed between Run-C and Run-D, but distinct low yield for Run-A as compared with Run-C and Run-D. As to the yield of the
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Fig. 10. Variation of yields of oils and asphaltene as a function of Anet hydrogen consumption in liquefaction stageB: Run-A Žv ., Run-B Ž'., Run-C ŽB., and Run-D Ž`..
middle oil fraction, almost the same yield was obtained for Run-A and Run-D, but lower for Run-C. Under the same consumption of total hydrogen, the yield of the oil fraction increased as the share of the hydrogen consumption in the liquefaction stage increased ŽRun-C vs.
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Fig. 11. Yields of light oil fractions Žproduced in liquefaction stage, B, and produced in solvent hydrotreatment stage, I. and middle oil fraction for Run-A, Run-C, and Run-D.
Run-D.. The increase in the oil fraction is due to the increase in the middle oil fraction. The total light oil fraction produced in the liquefaction and hydrotreatment stages was almost the same if the total hydrogen consumption was the same regardless of the share of the hydrogen consumption in the liquefaction and hydrotreatment stages. Therefore, it is suggested that the share of hydrogen consumption more in the liquefaction stage results in an increase in the yield of total oil fraction, and that the share of hydrogen consumption more in the hydrotreatment stage results in an increase in the ratio of the light oil fraction to the middle oil fraction. Fig. 12 shows the nitrogen contents in the liquefied light oil, the hydrotreated light oil, liquefied middle oil, and the hydrotreated solvent for Run-A, Run-C, and Run-D. For all, the nitrogen contents were the lowest for Run-C. It is well known that the catalyst used in the hydrotreatment stage possesses hydrodenitrogenation activity. It is reasonable that the lowest nitrogen content in both the hydrotreated solvent and the hydrotreated light oil were observed for Run-C. A lower nitrogen content in the liquefied light and middle oil fractions for Run-C as compared with those for Run-A was discussed earlier in relation to Fig. 8. It is
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Fig. 12. Nitrogen contents in liquefied light oil, hydrotreated light oil, liquefied middle oil, and hydrotreated solvent for Run-A, Run-C, and Run-D.
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noteworthy that the nitrogen contents in the liquefied light and middle oil fractions were essentially the same for Run-A and Run-D, though the hydrogen gas consumption in the liquefaction stage was much higher for Run-D than for Run-A. This suggests that the removal of nitrogen from the light and middle oil fractions scarcely take place in the liquefaction stage. This may be reasonable because the pyrite catalyst used in the liquefaction stage is not sufficiently active for hydrodenitrogenation. It is also noted that Run-D in which the share of hydrogen consumption was higher in the liquefaction stage than in Run-C required higher hydrogen pressure and reaction temperature.
4. Summary Based on the data obtained from the operation of a 150 trd pilot plant, solvent hydrotreatment was concluded to have the following effects on the product yields in the NEDOL coal liquefaction process. 1. Hydrogenation of the solvent promotes the production of oil fraction, especially light oil fraction, in the liquefaction stage. 2. The share of hydrogen consumption more in the liquefaction stage results in a higher yield of oil fraction, especially the middle oil fraction. 3. The share of hydrogen consumption more in the hydrotreatment stage results in a smaller nitrogen content in the oil fraction. 4. The consumption of hydrogen more in the liquefaction stage requires higher hydrogen pressure and temperature in the liquefaction stage.
Acknowledgements The authors are grateful to NEDO for supporting this work.
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Effect of solvent hydrotreatment on product yield in the coal liquefaction process Masato Kouzu a,) , Hitoshi Saegusa a , Takashi Hayashi a , Takahiro Nishibayashi a , Masatoshi Kobayashi a , Hironori Itoh b, Hideshi Hattori c a
Technical Department, Nippon Coal Oil Co., Ltd., 2, Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan b Research Group of Materials Science and Engineering, Graduate School of Engineering, Hokkaido UniÕersity, N13W8, Kita-ku, Sapporo 060-8628, Japan c Center for AdÕanced Research of Energy Technology, Hokkaido UniÕersity, N13W8, Kita-ku, Sapporo 060-8628, Japan Accepted 1 July 2000
Abstract Effects of solvent hydrotreatment on product yields in the NEDOL coal liquefaction process were examined based on the data obtained by a 150 trd pilot plant operation. When the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf Žwt.% on the basis of dry and ash-free coal., the yield of the oil fraction increased from 51.0 to 54.9 wt.%-daf with an increase in the hydrogen gas consumption from 0.7 to 1.3 wt.%-daf in the hydrotreatment stage. In the hydrotreatment stage, about 30% of the hydrogen consumed was utilized for enrichment of naphthenic hydrogen content in the solvent, and the rest was used for both the removal of nitrogen from the solvent and the production of the light oil fraction. In the liquefaction stage, the hydrogen donated from the solvent was more efficiently utilized than the gaseous hydrogen for the production of the oil fraction. Under the conditions that the total hydrogen consumption in the hydrotreatment and liquefaction stages was kept constant, the yield of oil fraction and the nitrogen content in oil fraction were higher when the hydrogen consumption in the liquefaction stage
) Corresponding author. Present address: 1352-1-3-202, Hirai, Kashima Ibaragi-ken 314-0012, Japan. Tel.: q81-299-82-6885; fax: q81-299-82-6885. E-mail address: [email protected] ŽM. Kouzu..
0378-3820r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 0 0 . 0 0 1 2 4 - 7
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increased with concomitant decrease in the hydrotreatment stage. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Coal liquefaction; Solvent hydrotreatment; Hydrogen gas consumption
1. Introduction The NEDOL process is a coal liquefaction process developed by the New Energy and Industrial Technology Development Organization ŽNEDO. in Japan. A 150 trd pilot plant was operated from March 1997 to September 1998. In the NEDOL process, coals are directly converted into oils in the presence of hydrogen donor solvent and iron catalyst under hydrogen pressure. The produced oils are divided into light, middle, and heavy oil fractions. The light and middle oil fractions are target products to be used as liquid fuels. The heavy oil fraction is recycled as the hydrogen donor solvent after hydrotreatment. The hydrogen donor solvent promotes the coal liquefaction reaction since the solvent stabilizes the coal fragment radicals by donating hydrogen w1x. The hydrogen-donating reaction is accompanied by the dehydrogenation of hydro-aromatics such as tetra-hydronaphthalene, di-hydrophenanthrene, and di-hydropyrene w2x. The heavy oil fraction utilized as the hydrogen donor solvent contains a large amount of aromatics such as naphthalene, phenanthrene, and pyrene. Since these aromatics do not possess hydrogendonating ability, they should be hydrogenated into the hydro-aromatics, which possess hydrogen-donating ability. The extent of the aromatic hydrogenation in the solvent hydrotreatment should influence the oil yield in the coal liquefaction process. The effects of the hydrogen donor solvent on coal liquefaction have been investigated in several previous studies. Sugimoto and Miki w3x carried out liquefaction of Wandoan coal with different hydrogen donor solvents, and found that the yield of the hexane soluble fraction increased with an increase in the ratio of the hydro-aromatic content to the aromatic content in the solvent. Hirano et al. w4x conducted coal liquefaction with several hydrotreated solvents derived from a heavy oil fraction produced in the PSU. They reported that the yield of oil fraction increased with an increase in the amount of naphthenic hydrogen in the solvent. In the solvent hydrotreatment in which Ni–Morg-Al 2 O 3 catalyst was used, the compounds corresponding to the light oil fraction were produced, and hydrodenitrogenation occurred w5x. Therefore, in the hydrotreatment, hydrogen gas is consumed not only for hydrogenation of aromatic compounds but also for the removal of nitrogen from the solvent and the production of the light oil fraction. The purpose of the pilot plant operation is to select the optimum conditions for economically efficient production of the oils. Since the hydrogen gas consumption in the coal liquefaction process influences the cost of the oil production w6x, efficient utilization of hydrogen is important. In the present paper, the variation of the product yield as a function of the hydrogen gas consumption in the hydrotreatment stage was examined based on the data obtained by the pilot plant operation to clarify the effects of the hydrogen-donating ability of the solvent on the product yield. In addition, the effects of
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hydrogen consumption on the product yield in the hydrotreatment stage are compared with those in the liquefaction stage for efficient utilization of hydrogen.
2. Experimental methods 2.1. Process flow of the 150 t r d pilot plant A schematic flow diagram of the 150 trd pilot plant is shown in Fig. 1. The pilot plant consists mainly of four sections, i.e., coal slurry preparation, liquefaction, distillation and solvent hydrotreatment. The slurry, a mixture of pulverized coal, liquefaction catalyst, and hydrogen donor solvent, was fed to the liquefaction reactor with pressurized hydrogen. The effluent from the reactor was separated in the distillation section into four fractions, i.e., the light oil fraction with a boiling point below 2208C Žliquefied light oil., the middle oil fraction with a boiling point ranging from 2208C to 3508C Žliquefied middle oil., the heavy oil fraction with a boiling point ranging from 3508C to 5388C Žliquefied heavy oil., and the residue with a boiling point above 5388C Žliquefaction residue.. Whole liquefied heavy oil was used as the solvent, and a part of the liquefied middle oil was mixed with the liquefaction heavy oil to keep the flow rate of the solvent constant in every operation. The prepared solvent was fed into the hydrotreating reactor in the solvent hydrotreatment section. In the reactor, the compounds whose boiling points correspond to those of the light oil fraction were also produced, and they are called hydrotreated light oil. The hydrotreated light oil was stripped from the hydrotreated solvent with a steam. 2.2. Coal and solÕent A sub-bituminous coal from the Tanito–Harum seams in Indonesia was processed in the pilot plant. Table 1 shows the composition of the coal. The coal contained 75.9 wt.% carbon and 5.8 wt.% hydrogen on a dry and ash-free Ždaf. coal basis. Table 2 shows the composition of the prepared solvent fed to the hydrotreating reactor. The composition of each solvent was not different significantly. The solvents were composed mostly of aromatic compounds, naphthalene-type aromatics being most abundant. The content of paraffin-type compounds was about 20%, naphthene-type being scarcely contained. 2.3. Operating procedures Solvent hydrotreatment was carried out in the fixed bed reactor over Ni–Morg-Al 2 O 3 catalyst developed by NEDO. The catalyst was an extruded cylindrical grain, 3 mm in length and 1.5 mm in diameter. It contained 3.0 wt.% NiO and 15.0 wt.% MoO 3 . The surface area and pore volume were 200 m2rg and 0.6 mlrg, respectively. The catalyst was presulfided at 2508C by feeding a mixture of anthracene oil, creosote oil derived from coal-tar, and 1 wt.% dimethyldisulfide under a hydrogen pressure of 10 MPa.
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Fig. 1. Schematic flow diagram of the NEDOL 150 trd pilot plant.
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Table 1 Composition of the processed coal C Žwt.%-daf.
H Žwt.%-daf.
N Žwt.%-daf.
S Žwt.%-daf.
Odiff Žwt.%-daf.
Ash Žwt.%-dry.
75.9
5.8
1.8
0.2
16.3
5.5
Coal liquefaction was carried out in a bubble column reactor. Natural pyrite imported from Finland was used as liquefaction catalyst. The natural pyrite contained 48 wt.% Fe and 51 wt.% S, and was pulverized to a median diameter of 0.7 mm. The pulverized pyrite of 3 wt.%-dry coal was made an addition to the feed coal slurry. Operating conditions and hydrogen gas consumption are shown in Table 3 for four runs. Run-A, Run-B, and Run-C, were performed to examine the variations in the product yields as a function of the hydrogen gas consumption in the hydrotreatment stage. In Run-A, the solvent was hydrotreated at 2958C under a hydrogen pressure of 10 MPa with a liquid hourly space velocity ŽLHSV. of 1.0 hy1 . In Run-B, LHSV was 0.8 hy1 and the temperature was 3008C. In Run-C, the hydrotreating temperature was raised to 3308C with an LHSV of 0.8 hy1 . Hydrogen gas consumptions in the hydrotreatment stage of Run-A, Run-B, and Run-C were 0.7, 0.9, and 1.3 wt.%-daf, respectively. Coal liquefaction was conducted by feeding the slurry composed of a coal and hydrotreated solvent at a coal concentration of 40–50 wt.% under the temperature of 450–4608C, nominal residence time of 0.8–1.0 h, and total pressure of 17 MPa. Hydrogen gas consumption in the coal liquefaction stage was kept constant at 4.7 wt.%-daf for Run-A, Run-B, and Run-C.
Table 2 Composition of the solvents fed to hydrotreating reactor Elemental composition (wt.%) C H N S Aromaticity, fa Žy. a Compound type (%)b Paraffin Naphthene Benzene Naphthalene Phenanthrene Anthracene Pyrene Other aromatics c a
Run-A
Run-B
Run-C
Run-D
87.4 9.6 1.0 - 0.1 0.50
87.7 9.5 0.9 - 0.1 0.50
87.7 9.4 0.9 - 0.1 0.52
87.3 9.4 1.0 - 0.1 0.52
20.0 0.4 5.1 16.8 3.3 2.7 3.9 26.4
19.5 0.4 4.9 14.6 3.8 3.1 4.7 27.7
19.5 0.4 5.1 12.1 3.3 3.5 6.6 26.0
17.1 0.3 4.9 15.6 3.6 3.0 4.8 28.4
Calculated on 1 H NMR spectra and elemental composition. Measured by GC-MS. c Other aromatics include compounds of phenol, biphenyl, fluorene, carbazole and fluoranthene type. b
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Run-A Run-B Run-C Run-D a
Solvent hydrotreatment stage Operating conditiona DH 2 Žwt.%-daf. Temperature LHSV Ž8C. Žhy1 .
Liquefaction stage Operating condition Temperature Pressure Ž8C. ŽMPa.
Coal concentrationb Žwt.%.
NRT c Žh.
295 300 330 295
455 450 460 462
40 40 50 40
1.0 0.8 0.8 1.0
1.0 0.8 0.8 1.0
0.7 0.9 1.3 0.7
17 17 17 19
The solvent hydrotreatment stage was operated under a hydrogen pressure of 10 MPa in all runs. Coal concentration in feed slurry to liquefaction reactor. c Nominal residence time of liquefaction reactor. b
DH 2 Žwt.%-daf. 4.7 4.7 4.7 5.3
Total DH 2 Žwt.%-daf.
5.4 5.6 6.0 6.0
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Table 3 Operating conditions and hydrogen gas consumption Ž DH 2 . of 150 trd pilot plant
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Run-D was performed to examine the product yields at different levels of hydrogen gas consumption in the liquefaction stage. Coal liquefaction was conducted at 4628C under 19 MPa. The hydrogen gas consumption was 5.3 wt.%-daf in the liquefaction stage. The operating conditions and hydrogen gas consumption in the hydrotreatment stage for Run-D were the same as those for Run-A. Total hydrogen gas consumption of Run-D was equal to that of Run-C for which the hydrogen gas consumption in the hydrotreatment stage was the highest. 2.4. Analysis Composition of the gaseous products was measured by GC. Elemental compositions of the produced oils and the liquefaction residue were measured with a CHN and S analyzers. The naphthenic hydrogen of the hydro-aromatics contained in the solvent Žnaphthenic hydrogen. was defined as the hydrogen which shows the chemical shift ranging from 1.5 to 2.0 ppm in the 1 H NMR spectrum w7x. The residue was extracted with n-hexane, toluene, and tetrahydrofuran. The n-hexane soluble fraction, the toluene-soluble and n-hexane insoluble fraction, the tetrahydrofuran-soluble and toluene insoluble fraction, and the tetrahydrofuran-insoluble fraction were defined as oil, asphaltene, preasphaltene, and unreacted coal, respectively. The product yields and hydrogen gas consumption were calculated on the daf coal basis. 3. Results and discussion 3.1. Enhancement of the hydrogen-donating ability of the solÕent in the hydrotreatment stage Fig. 2 shows the variation of the naphthenic hydrogen contents in the prepared solvent and the hydrotreated solvent as a function of the hydrogen gas consumption in the hydrotreatment stage. The naphthenic hydrogen content was obtained by multiplying the hydrogen content of the solvent by the ratio of the naphthenic hydrogen of the hydro-aromatics to the total hydrogen determined from the 1 H NMR spectrum. Although the liquefaction conditions were different, the naphthenic hydrogen content of the prepared solvent was almost the same for each run. The naphthenic hydrogen content of the hydrotreated solvent increased with an increase in the hydrogen gas consumption. This indicates that aromatic hydrogenation occurred in the solvent hydrotreatment. Since the naphthenic hydrogen content in the prepared solvent kept constant against time on stream, the same amount of hydrogen as that of the naphthenic hydrogen increasing in the hydrotreatment stage should be consumed in the liquefaction stage. Therefore, as the content of naphthenic hydrogen increases in the hydrotreatment stage, the amount of hydrogen utilized in the liquefaction stage increases. If the differences in the naphthenic hydrogen content between hydrotreated solvent and prepared solvent were converted into those on wt.%-daf base, and plotted against the hydrogen consumption in solvent hydrotreatment, the slope gives the fraction of the hydrogen utilized for the formation of the naphthenic hydrogen. The slope was 0.33, indicating that 33% of the consumed
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Fig. 2. Variation of naphthenic hydrogen contents in prepared Žfeed. solvent Ž`. and hydrotreated solvent Žv . as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment.
hydrogen was utilized for the formation of the naphthenic hydrogen, and the rest Ž67%. was utilized for the other reactions such as hydrodenitrogenation and hydrocracking. Fig. 3 shows the variation of the nitrogen contents in the prepared solvent, the hydrotreated solvent, and the hydrotreated light oil as a function of the hydrogen gas consumption in the hydrotreatment stage. The nitrogen contents in the hydrotreated solvent and hydrotreated light oil decreased with an increase in the hydrogen consumption. It is obvious that hydrodenitrogenation occurred in the hydrotreatment stage. Hydrogen was consumed in the hydrotreatment stage not only for the formation of naphthenic hydrogen by hydrogenation of aromatics, but also for the removal of nitrogen by hydrodenitrogenation. Fig. 4 shows variation of the yields of the organic gases and the light oil fraction produced in the hydrotreatment stage as a function of the hydrogen gas consumption. The organic gases include methane, ethane, and propane. The yield of the organic gases was less than 0.2 wt.%-daf at all levels of the hydrogen gas consumption. The yield of the light oil increased with an increase in the hydrogen consumption. It is not certain as to whether the light oil fraction was formed by hydrodenitrogenation, hydrocracking, or some other reactions. However, it is certain that the formation of the light oil fraction was accompanied by the consumption of hydrogen. 3.2. Effects of the solÕent hydrotreatment on coal liquefaction The variation of the product yields as a function of the hydrogen gas consumption in the hydrotreatment stage was examined to elucidate the effect of the hydrogen-donating ability of the solvent on coal liquefaction.
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Fig. 3. Variation of nitrogen contents in prepared Žfeed. solvent Ž`., hydrotreated solvent Žv ., and hydrotreated light oil Žlight oil fraction produced in solvent hydrotreatment stage: '. as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment.
Fig. 5 shows the variation of the yields of the oils, the liquefaction residue, and the organic gases as a function of the hydrogen gas consumption in the hydrotreatment stage
Fig. 4. Variation of yields of organic gases Ž`. and light oil fraction Žv . produced in the hydrotreatment as a function of hydrogen gas consumption Ž DH 2 ..
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Fig. 5. Variation of yields of oils Žv ., liquefaction residue ŽB., and organic gases Ž'. produced in total coal liquefaction system as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf.
while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf. The oil yield is a sum of the light and middle oil fractions produced in the liquefaction stage and the light oil fraction produced in the hydrotreatment stage. The organic gases include methane, ethane, and propane produced in both the liquefaction and the hydrotreatment stages. The yield of the oils increased from 51.0 to 54.9 wt.%-daf with an increase in the hydrogen gas consumption from 0.7 to 1.3 wt.%-daf in the hydrotreatment stage. The yield of the liquefaction residue decreased from 25.1 to 18.8 wt.%-daf and that of the organic gases increased from 10.4 to 13.0 wt.%-daf with an increase in the hydrogen gas consumption. It is clear that an increase in the hydrogen consumption in the solvent hydrotreatment stage resulted in the promotion of liquefaction reaction for the production of oils. Fig. 6 shows the variation of the yields of the fractions contained in the liquefaction residue, i.e., oil, asphaltene, preasphaltene, and unreacted coal, as a function of the hydrogen gas consumption in the hydrotreatment stage. All of the liquefaction residues included oil equivalent to about 1.0 wt.%-daf. The yields of the unreacted coal and preasphaltene were less than 2.0 wt.%-daf at all levels of the hydrogen gas consumption. The yield of the asphaltene decreased from 21.0 to 15.6 wt.%-daf with an increase in the hydrogen gas consumption. In Fig. 6, as compared with Fig. 5, it is suggested that the decrease in the residue resulted mainly from the decrease in the asphaltene. It is reasoned that the hydrogen donated from the solvent combines with the radical
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Fig. 6. Variation of yields of fractions in liquefaction residue; oil Ž`., asphaltene Žv ., preasphaltene Ž'., and unreacted coal ŽB. as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf.
fragments from coal thermal dissociation and so reducing the asphaltene by repolymerization of the radical fragments. This is consistent with the results reported by Nagaishi et al. w8x. Fig. 7 shows the variation of the yields of the liquefied light oil and the liquefied middle oil as a function of the hydrogen gas consumption in the hydrotreatment stage. The yield of the liquefied light oil increased with the increase in the hydrogen gas consumption while that of the liquefied middle oil slightly decreased. The conversion of middle oil fraction to light oil fraction occurred in addition to the conversion of asphaltene to the oil fraction as mentioned above. Fig. 8 shows the variations of the nitrogen content of the liquefied light oil fraction and middle oil fraction as a function of the hydrogen gas consumption in the hydrotreatment stage. The nitrogen content decreased with an increase in the hydrogen gas consumption for both the liquefied light oil and middle oil fractions. Two possibilities are to be taken into consideration to explain the decrease in the nitrogen content. One possibility is as follows. A considerable fraction of light and middle oils originates from the solvent. The solvent contains less amount of nitrogen as the hydrogen consumption in the hydrotreatment stage increases. A part of the solvent decomposes to form light and middle oil fractions. The other possibility is that hydrodenitrogenation proceeds to a larger extent to form oil fraction in the liquefaction stage when the solvent contains a larger amount of hydrogen. It is reported that a part of the liquefied light and middle oils originate from the solvent w9x. However, it is not certain which case contributes much to
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Fig. 7. Variation of yields of light Žv . and middle oil Ž`. fractions produced in liquefaction stage as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf.
Fig. 8. Variations of nitrogen content in liquefied light oil Žv . and liquefied middle oil Ž`. as a function of hydrogen gas consumption Ž DH 2 . in solvent hydrotreatment while the hydrogen gas consumption in the liquefaction stage was kept constant at 4.7 wt.%-daf.
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the decrease in the nitrogen content in the oil fraction. It should be noted that the removal of nitrogen from the solvent is favorable for the reduction of the nitrogen content of the oil produced in the liquefaction stage. 3.3. Hydrogen utilization efficiency in coal liquefaction The product yields at different levels of hydrogen gas consumption in the liquefaction stage were examined to discuss the hydrogen utilization efficiency in the total coal liquefaction process. Fig. 9 shows the yields of the oils, liquefaction residue, and organic gases for Run-A, Run-C, and Run-D. The hydrogen consumptions in hydrotreatment stage were 0.7, 1.3, and 0.7 wt.%-daf for Run-A, Run-C, and Run-D, respectively. The hydrogen consumptions in the liquefaction stage were 4.7, 4.7, and 5.3 wt.%-daf for Run-A, Run-C, and Run-D, respectively. The total hydrogen consumptions were the same for Run-C and Run-D, and less for Run-A. The yields of the oils and organic gases were in the order, Run-D) Run-C) Run-A. The yield of the liquefaction residue was in the order Run-A) Run-C) Run-D. The differences between Run-A and Run-C are described earlier. The differences between Run-C and Run-D reflect the difference in the share of the same amount of hydrogen consumption into the hydrotreatment stage and the liquefaction stage. Under the same total hydrogen consumption, the yields of oil fractions and organic gases were higher, and the yield of residue was lower when the hydrogen consumed in the liquefaction stage was increased with concomitant decrease in the hydrotreatment stage. This is because only 33% of hydrogen consumed in the hydrotreatment stage was utilized for hydrogenation of aromatics to form compounds acting as hydrogen donor in the liquefaction stage. Therefore, the results in Fig. 9 do not necessarily mean that the gaseous hydrogen is more efficient than the naphthenic hydrogen in the solvent for coal liquefaction. To find out if there are any difference in the efficiency of hydrogen between gaseous hydrogen and the naphthenic hydrogen in the solvent for coal liquefaction, the yields of oil fractions and asphaltene are plotted against Anet hydrogen consumption in the liquefaction stageB in Fig. 10. The Anet hydrogen consumption in the liquefaction stageB is defined as the sum of the hydrogen gas consumption in the liquefaction stage and the naphthenic hydrogen formed in the hydrotreatment stage. The latter is equivalent to 33% of the hydrogen gas consumption in the hydrotreatment stage. For Run-A Žv ., Run-B Ž'., and Run-C ŽB., the hydrogen gas consumption in the liquefaction stage was the same. Therefore, the slope of solid line is an index of the efficiency of naphthenic hydrogen for the liquefaction. On the other hand, for Run-A Žv . and Run-D Ž`., the hydrogen gas consumption in the hydrotreatment stage was the same, but the hydrogen gas consumption in the liquefaction stage was higher for Run-D than for Run-A. Therefore, the slope of dotted line is an index of the efficiency of gaseous hydrogen for the liquefaction. The slope of the solid line is steeper than that of the dotted line, indicating that the naphthenic hydrogen in the solvent acts more efficiency than the gaseous hydrogen for the formation of oil fraction in the liquefaction stage. Likewise, the naphthenic hydrogen in the solvent acts more efficiently than the
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Fig. 9. Yields of oil fraction, liquefaction residue, and organic gases for Run-A, Run-C, and Run-D.
gaseous hydrogen for the reduction of asphaltene in the liquefaction stage Žlower figure.. A higher efficiency of naphthenic hydrogen as compared with gaseous hydrogen is consistent with the results reported with a model compound w10x as well as with a coal w4x. Fig. 11 shows the yields of the light oil fractions produced in both the liquefaction and hydrotreatment stages, and the middle oil fraction. As already shown in Fig. 9, the yield of oil fraction was in the order, Run-D) Run-C) Run-A. As to the yield of the light oil fractions, no obvious difference was observed between Run-C and Run-D, but distinct low yield for Run-A as compared with Run-C and Run-D. As to the yield of the
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Fig. 10. Variation of yields of oils and asphaltene as a function of Anet hydrogen consumption in liquefaction stageB: Run-A Žv ., Run-B Ž'., Run-C ŽB., and Run-D Ž`..
middle oil fraction, almost the same yield was obtained for Run-A and Run-D, but lower for Run-C. Under the same consumption of total hydrogen, the yield of the oil fraction increased as the share of the hydrogen consumption in the liquefaction stage increased ŽRun-C vs.
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Fig. 11. Yields of light oil fractions Žproduced in liquefaction stage, B, and produced in solvent hydrotreatment stage, I. and middle oil fraction for Run-A, Run-C, and Run-D.
Run-D.. The increase in the oil fraction is due to the increase in the middle oil fraction. The total light oil fraction produced in the liquefaction and hydrotreatment stages was almost the same if the total hydrogen consumption was the same regardless of the share of the hydrogen consumption in the liquefaction and hydrotreatment stages. Therefore, it is suggested that the share of hydrogen consumption more in the liquefaction stage results in an increase in the yield of total oil fraction, and that the share of hydrogen consumption more in the hydrotreatment stage results in an increase in the ratio of the light oil fraction to the middle oil fraction. Fig. 12 shows the nitrogen contents in the liquefied light oil, the hydrotreated light oil, liquefied middle oil, and the hydrotreated solvent for Run-A, Run-C, and Run-D. For all, the nitrogen contents were the lowest for Run-C. It is well known that the catalyst used in the hydrotreatment stage possesses hydrodenitrogenation activity. It is reasonable that the lowest nitrogen content in both the hydrotreated solvent and the hydrotreated light oil were observed for Run-C. A lower nitrogen content in the liquefied light and middle oil fractions for Run-C as compared with those for Run-A was discussed earlier in relation to Fig. 8. It is
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Fig. 12. Nitrogen contents in liquefied light oil, hydrotreated light oil, liquefied middle oil, and hydrotreated solvent for Run-A, Run-C, and Run-D.
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noteworthy that the nitrogen contents in the liquefied light and middle oil fractions were essentially the same for Run-A and Run-D, though the hydrogen gas consumption in the liquefaction stage was much higher for Run-D than for Run-A. This suggests that the removal of nitrogen from the light and middle oil fractions scarcely take place in the liquefaction stage. This may be reasonable because the pyrite catalyst used in the liquefaction stage is not sufficiently active for hydrodenitrogenation. It is also noted that Run-D in which the share of hydrogen consumption was higher in the liquefaction stage than in Run-C required higher hydrogen pressure and reaction temperature.
4. Summary Based on the data obtained from the operation of a 150 trd pilot plant, solvent hydrotreatment was concluded to have the following effects on the product yields in the NEDOL coal liquefaction process. 1. Hydrogenation of the solvent promotes the production of oil fraction, especially light oil fraction, in the liquefaction stage. 2. The share of hydrogen consumption more in the liquefaction stage results in a higher yield of oil fraction, especially the middle oil fraction. 3. The share of hydrogen consumption more in the hydrotreatment stage results in a smaller nitrogen content in the oil fraction. 4. The consumption of hydrogen more in the liquefaction stage requires higher hydrogen pressure and temperature in the liquefaction stage.
Acknowledgements The authors are grateful to NEDO for supporting this work.
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