Isothermal Stage Kinetics of Direct Coal Liquefaction for Shenhua

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Isothermal stage kinetics of direct coal liquefaction for Shenhua Shendong bituminous coal Hongbo Jiang, Xiuhui Wang, Xiangen Shan, Kejian Li, Xuwen Zhang, Xueping Cao, and Huixin Weng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01484 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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Isothermal stage kinetics of direct coal liquefaction for Shenhua Shendong bituminous coal Hongbo Jiang,* , † Xiuhui Wang,† Xiangen Shan,† , ‡ Kejian Li,‡ Xuwen Zhang,‡ Xueping Cao‡ and Huixin Weng† †

Research Institute of Petroleum Processing, East China University of Science and

Technology, Shanghai 200237, China ‡ National Engineering Laboratory for Direct Coal Liquefaction, Shanghai 201108, China Abstract: In order to study the direct coal liquefaction in the isothermal stage of Shenhua Shendong bituminous coal, the direct coal liquefaction with iron-based catalyst was carried out in a 0.01t/d continuous tubular facility in the temperature range of 445℃ to 465℃, with hydrogenated anthracene and wash oil as solvent. A 8-lump kinetic model of the isothermal stage was proposed, and the kinetic parameters were estimated. The result showed that in the isothermal stage the oil was mainly obtained from PAA rather than from coal directly. The model was valid for the isothermal stage of direct coal liquefaction.

Introduction In China, coal is the most abundant fossil fuel, but oil and gas resources are relatively poor. Today, coal combustion, at a lower energy efficiency, is the main consumption way which leads to many serious environmental pollution. Direct coal liquefaction (DCL) as one of the clean coal technologies, has attracted a continuous 1

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interest for producing the substitutable transportation fuels, especially in China. Direct coal liquefaction, a complex combination of physical and chemical process, refers to the direct reaction of coal with high pressure hydrogen to form liquids in a solvent under high temperature. The studies on the mechanism and kinetics of direct coal liquefaction are beneficial to reactor design and process optimization. Francesco and Fabio1 studied the hydro-liquefaction of coal in a stirred batch reactor under constant temperature of 430℃, with tetralin/naphthalene mixtures as the solvent. The results showed that the simulated data were in good agreement with the experimental ones. Hu2 divided the DCL processes into heating-up and isothermal stages, used two lumped models to investigate Shenhua coal liquefaction kinetics, and concluded that the reaction of preasphaltene+asphaltene to oil+gas is the rate-controlled step. Li et al.3 used a 5-lump model to investigate the kinetics of both heating-up and isothermal stages in batch reactor, which concluded that the rate-controlled step is the reaction of preasphaltene to oil and gas. Shan et al.4 studied the liquefaction kinetics of Shenhua Shendong coal in the heating-up stage in a 0.01t/d continuous facility, and developed a 8-lump kinetic model. In this paper a kinetic model was proposed for isothermal stage in which the reaction pathways were the same as that for heating-up stage4. For each experiment, the reaction result of heating-up stage was simulated by the kinetic model for heating-up stage4, and the simulated reaction result of heating-up stage was the input data for the simulation of isothermal stage. The reaction rate constants of isothermal stage were estimated through comparison between simulation results and experiment 2


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data. A “gas shut-down method” 4 was adopted to measure the gas holdup of the reactor. The simulation results of the liquefaction performance were in good agreement with the experiments that the average absolute deviation and the relative deviation for all components of the isothermal stage were 1.20wt% and 6.61%. The kinetic model can well describe the isothermal stage of direct liquefaction of Shenhua Shengdong coal.

Experimental Section 2.1 Materials Bituminous coal was used in the experiment which was ground to particles less than 150 um, and the proximate and ultimate analysis data are shown in Table 1. The hydrotreated mixture of anthracene oil and wash oil was used as solvent with the density of 0.9902 kg/L (20℃), and its boiling range data is shown in Table 2. The purity of hydrogen was greater than 99.99%. Components of catalyst are shown in Table 3. Table 1.

Proximate and ultimate analysis of Shenhua coal.

Proximate Analysis (wt%) Mad 8.96 Table 2.

Ad 13.10

Vdaf 38.55

Ultimate Analysis (wt%, daf basis) C 80.32

H 4.5

N 1.01

O 13.74

S 0.43

Boiling range data of the initial solvent.

Narrow cut (℃)

Accumulative

Narrow cut (℃)

yield (wt%)

Accumulative yield (wt%)

IBP-200

0.65

280-300

69.08

200-220

9.32

300-320

82.54

220-240

27.83

320-340

87.73

240-260

47.29

≥340

99.13

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260-280 Table 3.

59.03

Composition analysis of catalyst (wt%)

water

coal

Fe

OOH-

NH4

sulfate

other

4.37

85.29

5.61

3.310

0.325

0.985

0.094

2.2 Experimental procedure and product fractionation The 0.01t/d continuous experimental apparatus is shown in Figure 1. Slurry mixed with hydrogen was introduced into the first preheater and second preheater at a certain mass flow rate, reaching 410℃ before entering the reactor. The preheaters and reactor were pressurized to 18MPa with hydrogen. r e g n a h c x e t a e H s a g t n e V r e t e m t e W r o t Ta Lr a Pp He s

r o t Ta Hr a Pp He s

H2 r e dt na oe c h ee r Sp

y r r u l s l a o C

r e t e m t e W

r e t se s am w Mo l f

M

r e t e m t e W

r o t c a e R

r e t t a se r i h Fe r p

p lm au op c y r Pr Hl u s

t c ur de oi v r pe tc he gr i L

t c ur de o ri v pe yc v e ar e H

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Figure 1. Flow of 0.01t/d direct liquefaction process of Shenhua coal (HPHT = high pressure and high temperature; HPLT = high pressure and low temperature; HP = high pressure). In this work, the concentration of coal and catalyst in slurry was 40wt%, where Fe in the catalyst was at a concentration of 1wt % based on the mass of dry ash free (daf) coal and the S/Fe molar ratio of 2 is maintained. The experiment steps were set as follows:

4


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(1) The wash oil was used as transitional phase which was first pumped into the system and generally lasted ~3 h before the coal slurry entered the system; (2) The heavy and light liquefied product receiving tanks were drained off and all wet flow meters were set to zero; (3) The heating rate was adjusted to make sure the reactor and preheaters reach the set values of temperature, then the coal-slurry was pumped into the system; (4) The preheater was kept under constant temperature of 410 ℃ for examining the influence of reactor temperature and reaction time on the process; (5) After the liquefaction, samples of heavy products and light products were collected, and exhaust gases were measured using a wet flow meter and analyzed by gas chromatography. After the liquefaction, liquid products were separated by Soxlet solvent extraction with n-hexane, toluene and tetrahydrofuran (THF) in turn. The n-hexane soluble fraction, the n-hexane insoluble but toluene soluble fraction, the toluene insoluble but THF soluble fraction, the THF insoluble fraction were defined as oil, asphaltene, preasphaltene and residue, respectively, as shown in Figure 2. The conversion was defined as the percentage of coal transformed into preasphaltene and asphaltene (hereafter referenced as PAA), oil, gas and H2O during the liquefaction.

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Products from reactor Extraction with n-hexane (48h)

Soluble (oil)

Insoluble Extraction with toluene (48h)

Soluble (asphaltene)

Insoluble Extraction with THF

Soluble (preasphaltene)

Insoluble (residue)

Figure 2. Separation procedure of products.

Results and discussion 3.1 Effect of residence time on product yields Isothermal liquefaction of Shenhua Shengdong coal at 455 ℃ with different residence time were carried out. Table 4 shows the yield of products obtained in the liquefaction reactions. Within 10 min after the start of reaction, about 80wt% of coal was converted into PAA, oil, gas, and H2O. The weight fraction of unreacted coal decreases gradually, reaching a level of less than 10wt% within 70 min at 455℃. The yield of PAA decreases gradually as the reaction time increases. On the other hand, the yield of oil increases rapidly in the isothermal stage of liquefaction reaction, which is higher and quicker than those of H2O and gas. The results are shown in Figure 3, which can show the influence of reaction time on conversion and products yields.

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Table 4. Experimental results of Shenhua coal liquefaction in the isothermal stage T (℃)

t1 (min)

t2 (min)

455 455 455 455 465 455 455 455 455 455 455 455 455 445 455

3.4 4.2 5.0 5.8 4.4 3.2 3.0 3.7 4.9 6.3 5.1 4.9 5.8 4.9 4.0

7.4 9.1 10.8 12.5 9.3 32.2 29.8 37.2 51.6 70.0 52.5 51.2 60.5 56.0 88.0

Conversion (wt%, daf) 79.49 78.54 82.26 81.23 79.65 77.17 76.18 84.59 84.66 92.24 87.98 84.82 88.23 87.34 91.41

PAA (wt%, daf) 28.93 27.02 24.20 21.92 25.43 20.08 20.59 15.54 11.73 11.04 16.17 16.64 12.29 14.51 6.99

Oil (wt%, daf) 38.45 40.33 43.64 46.15 39.74 42.89 43.83 49.54 53.01 61.19 58.94 53.28 56.78 55.32 64.02

Gas (wt%, daf) 8.84 8.62 10.08 8.10 10.11 10.15 9.16 14.14 13.87 15.45 10.03 11.26 14.42 13.14 16.18

H2O (wt%, daf) 6.45 6.24 8.04 8.45 8.06 5.60 5.91 7.75 9.05 8.21 7.01 7.90 9.79 9.40 9.81

Note: t1 means the residence time of preheater; t2 means the residence time of reactor

90 80 70

Yield (wt%, daf)

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conversion PAA oil Gas H2O

60 50 40 30 20 10 0

10

20

30

40

50

60

70

80

90

Time(min)

Figure 3. Influence of reaction time on conversion and product yield 3.2 Liquefaction kinetic model Up to now, many works have been carried out for the kinetics study of direct coal liquefaction, and various kinetic models have been reported4-9. Several reaction 7


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pathways were tried and the best one was chosen as shown in Figure 4. The model included serial and parallel reactions as shown in the figure. H2O

k4

Gas

k8

k6

k7

k9

PAA

Oil

H2 k2

k3

k5

k1 C1

C2

C3

Figure 4. Kinetic model for the isothermal stage of direct coal liquefaction In order to solve the model, the following assumptions were made: (1) First-order irreversible reactions were assumed for all reaction paths5-9. (2) All the rate constants fitted the Arrhenius law. (3) The length and inner diameter of the reactor were 12.5m and 15mm, respectively; due to the high ratio of length to diameter (L/D), the reactor was assumed as plug-flow reactor. (4) The liquefied products were divided into preasphaltene and asphaltene (PAA), oil (Oil), gas (Gas), H2O. (5) Based on experimental results and related published work10-12, the original coal was classified into three parts, which were easy reactive part (C1), hard reactive part (C2) and unreactive part (C3). The easy reactive part (C1) can be converted into PAA, oil (Oil), and gas (Gas); the hard reactive part (C2) can only be converted into PAA; the unreactive part (C3) does not participate in any reaction. 8


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(6) PAA can be converted into oil (Oil), gas (Gas) and H2O. (7) Itoh et al.13 concluded that hydrogen is largely consumed in the hydrogenation of PAA and Oil in reactors, so the hydrogen consumption was taken into account. The influences of the particle size, as well as heat transfer and mass transfer, were ignored. The model equations of direct coal liquefaction reactions were established as follows: dM C1 dt

= - ( k1 + k 2 + k3 + k4 ) × M C1

dM C2

（ 1）

（ 2）

= - k5 × M C 2

dt

dM PAA = - ( k6 + k7 + k8 ) × M PAA + k1 × M C1 + k5 × M C2 + k9 × M PAA dt dM Oil = k2 × M C1 + k6 × M PAA dt dM Gas = k3 × M C + k7 × M PAA dt 1

dM H 2 O dt dM H 2 dt

（3）（4）（5）

（6）

= k4 × M C1 + k8 × M PAA

（7）

= −k9 × M PAA

where M C , M C , M PAA , M Oil , M Gas , M H 2O and M H 2 represent mass fractions of 1

2

easy reactive part, hard reactive part, PAA, liquefied oil, gas, water and hydrogen, respectively, using the dry ash-free (daf) basis of feed coal as a benchmark. t is the reaction time (min). ki is the reaction rate constant (min−1), with a formula defined as ki = ki0•exp(−Eai/(RT)), where ki0 (min−1) is the pre-exponential factor, Eai is the activation energy (kJ/mol), T is the reaction temperature (K), and R is the gas constant (R = 8.3145 J/(mol·K)). 9


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Kinetic parameters of the model were obtained on the premise that the heating-up and isothermal stages were a continuous whole, which meant the coal slurry entered the preheaters and reactor in turn. The data of M C1 , M C2 , M C2 for Shenhua Shendong bituminous coal before liquefaction reaction were 0.6278, 0.2914 and 0.0808 respectively, which were decided in the study of heating-up stage model4. The products obtained from the preheaters were used as the inlet constituents of reactor. The weight percentage of coal conversion, PAA, oil, gas, and H2O in the heating-up stage can be obtained with heating-up stage kinetic model4 according to the condition of preheaters in the experiments. Based on the DCL kinetic model mentioned above, the calculation of DCL reaction was carried out by using Fortran programs, where the mathematical model equations were solved by Treanor numerical integral method14, and kinetic parameters were optimized by BFGS variable metric method15. The optimized kinetic parameters are shown in Table 5.

Table 5. Reaction kinetic parameters of the isothermal Stage -1

Eai (kJ•mol )

ki (min-1)

-1

k0i (min )

k1 k2 k3 k4 k5 k6 k7 k8

91.51 91.47 91.51 90.53 92.89 81.01 82.19 84.16

2.01 × 10 7.64 × 105 4.47 × 104 7.14 × 104 1.31 × 106 1.35 × 105 3.99 × 104 9.00 × 102

438℃ 0.0382 0.1462 0.0008 0.0160 0.1965 0.1510 0.0367 0.0006

k9

100.61

1.27 × 105

0.0052

5

445℃ 0.0444 0.1700 0.001 0.0186 0.2290 0.1726 0.0420 0.0007

455℃ 0.0548 0.20100 0.0122 0.0229 0.2836 0.2079 0.0507 0.0008

465℃ 0.0673 0.2574 0.0150 0.028 0.3492 0.2492 0.0610 0.0010

0.0061

0.0077

0.0097

Table 5 shows that the reaction rate of easy reactive part C1 (k1+k2+k3+k4) is faster 10


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than that of hard reactive part C2 (k5), which is consistent with the model assumption. Meanwhile, the rate of reaction from C1 to Oil, Gas, and H2O (k2+k3+k4) is greater than that from easy reactive part C1 to PAA (k1), which means the easy reactive part is partly converted to the PAA in the isothermal stage. The rate from C1 to PAA (k1) is smaller than that from C2 to PAA (k5), which means the increase of PAA is mostly from the hard reactive part in the isothermal stage. Meanwhile, the reaction rate from PAA to Oil, Gas, and H2O (k6+k7+k8) is greater than that from easy reactive part C1 (k2+k3+k4), which means in the isothermal stage PAA is the main intermediate, and the oil is mainly obtained from PAA rather than from coal directly. The absolute deviations between calculation and experiment results of the coal conversion, PAA, Oil, Gas, and H2O were 1.40wt%, 1.34wt%, 2.10wt%, 0.87wt%, and 0.03wt%, respectively; and the corresponding relative deviations between calculation and experiment results were 8.08%, 7.41%, 4.53%, 9.13%, and 3.91%, respectively. The average absolute deviation and the relative deviation for all components were 1.20wt% and 6.61%, respectively. Detailed comparisons are shown in Figure 5~9.

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92

Conversoin

Calculated values (wt%)

90 88 86 84 82 80 78 76 76

78

80

82

84

86

88

90

92

Experimental values (wt%)

Figure 5. Comparison between calculated values and experimental values of coal conversion.


40

PAA

35

30

25

20

15

10

5 10

15

20

25

30

35

40


Figure 6. Comparison between calculated values and experimental values of PAA.

Oil

65


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60

55

50

45

40

35 35

40

45

50

55

60


12


65

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Figure7. Comparison between calculated values and experimental values of Oil. 20

Gas


18

16

14

12

10

8

6 6

8

10

12

14

16

18

20


Figure 8. Comparison between calculated values and experimental values of Gas.

H2O

10


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9

8

7

6

5 5

6

7

8

9

10


Figure 9. Comparison between calculated values and experimental values of H2O. 3.3 Prediction value of products With the parameters estimated above, the coal conversion and the yield of products can be predicted by the kinetic model. The results are shown in Table 6 under conditions of t1 = 6.3 min, t2 = 60~100 min, and T = 455℃ or 465℃.

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Table 6. Prediction value of Shenhua coal liquefaction in the isothermal stage T (℃)

Time (min)

Conversion (wt%, daf)

PAA (wt%, daf)

Oil (wt%, daf)

Gas (wt%, daf)

H2O (wt%, daf)

60 70 80 90 95 100 60 70 80 90 95 100

86.94 87.80 88.49 89.05 89.29 89.51 88.04 88.81 89.41 89.87 90.07 90.23

11.99 10.47 9.09 7.86 7.30 6.77 10.07 8.46 7.07 5.87 5.34 4.85

58.74 60.87 62.72 64.33 65.05 65.72 61.51 63.65 65.44 66.84 67.39 67.68

11.23 12.12 12.63 12.01 12.15 12.21 12.35 12.76 13.24 13.57 13.73 13.85

8.87 8.90 8.94 9.02 9.04 9.05 8.75 8.99 9.02 9.05 9.06 9.07

455

465

According to the prediction results, the coal conversion is about 90wt% when t1 = 6.3min, t2=90~100min. The coal conversion increases as reaction time and temperature increase, so do the yields of oil and gas. On the other hand, the yield of PAA decreases gradually, reaching a level of less than 5wt% within 100 min at 465℃, which means PAA is the main intermediate in the isothermal stage. The overall trend is consistent with the analysis of experimental results.

Conclusion The direct liquefaction reaction of Shenhua Shendong bituminous coal in the isothermal stage was studied in a 0.01t/d continuous facility. A 8-lump kinetic model was proposed and the kinetic parameters were estimated with BFGS variable metric method. The coal conversion can be more than 90wt% according to the prediction results. The weight percentages of unreactive coal, PAA, oil, gas, and H2O in the isothermal 14


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stage calculated by the kinetic model are consistent with experimental values approximately. And the oil is mainly obtained from PAA rather than from coal directly in the isothermal stage.

AUTHOR INFORMATION Corresponding Author *Tel.: 86-21-64252816. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was funded by the Innovation Program approved by China Shenhua Group (SHJT-12-15). The authors are also appreciative for the support from the East China University of Science and Technology.

REFERENCES (1) Francesco, G.; Fabio, M. Kinetics of hydrogen donation in the liquefaction of coal. Ind. Eng. Chem. Res. 1988, 27, 1978-1983. (2) Hu, S. Catalyst and Mechanism for Direct Liquefaction of Shenhua Coal. Master Degree Thesis, Dalian University of Technology, 2007. (3) Li, X.; Hu, H.; Zhu, S.; etc. Kinetics of coal liquefaction during heating-up and isothermal stages. Fuel 2008, 87, 508–513. (4) Shan X.; Li, K.; Zhang, X.; etc. Reaction Kinetics Study on the Heating Stage of the Shenhua Direct Coal Liquefaction Process. Energy & Fuels, 2015, 29, 2244-2249. (5) Ikeda, K.; Sakawaki, K.; Nogami, Y; etc. Kinetic evaluation of progress in coal 15


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liquefaction in the 1 t/d PSU for the NEDOL process. Fuel 2000, 79, 373-378. (6) Pradhan, V. R.; Holder, G. D.; Wender, I.; etc. Kinetic modeling of direct liquefaction of Wyodak coal catalyzed by sulfated iron oxides. Ind. Eng. Chem. Res.

1992, 31, 2051-2056. (7) Shah, Y. T.; Cronauer, D. C.; Mcllvried, H. G.; etc. Kinetics of catalytic liquefaction of Big Horn coal in a segmented bed reactor. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 288-301. (8) Ramdoss, P. K; Tarrer, A. R. Kinetic model development for single-stage coal coprocessing with petroleum waste. Fuel Process Technol. 1997, 51, 83-100. (9) Kidoguchi, A.; Itoh, H.; Hiraide, M.; etc. Simulation of initial stage reactions in the direct coal liquefaction of sub-bituminous coals. Fuel 2001, 80, 1325-1331. (10) Wiser, W. H. Some chemical aspects of the conversion of coal to liquid and gaseous fuels. In Proceedings of the Electric Power Research Institute Conference on Coal Catalysis, Santa Monica, CA, 1973. (11) Vahrman, M. Smaller molecules derived from coal and their significance. Fuel

1970, 49, 5-16. (12) Li, W. H.; Huo, W. D.; Shu, G. P.; etc. Hydroliquefaction characteristics of Majiata coal and its macerals components. J. Fuel Chem. Technol. 2001, 29(2), 104−107. (13) Itoh, H.; Hiraide, M.; Kidoguchi, A.; etc. Simulator for coal liquefaction based on the NEDOL process. Ind. Eng. Chem. Res. 2001, 40, 210-217 (14) Xu, S. FORTRAN Algorithms Procedures Set; Tsinghua University Press: Beijing, 16


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1997; pp 317-321. (15) Jiang, J.; He, C.; Pan, S. Optimization Computation Method; South China University of Technology Press: Guangzhou, 2007; p10.

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Isothermal Stage Kinetics of Direct Coal Liquefaction for Shenhua

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