ARTICLE pubs.acs.org/EF
Detailed Description of Coal Tar Hydrogenation Process Using the Kinetic Lumping Approach Fei Dai,†,‡ Mingjie Gao,† Chunshan Li,*,† Shuguang Xiang,*,‡ and Suojiang Zhang‡ †
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China ‡ Hi-Tech Institute for Petroleum, Chemical Industry, Qingdao University of Science, Technology, Qingdao Shandong 266042, China ABSTRACT: A new eight-lump kinetic model containing 19 kinetic constants is proposed to describe coal tar hydrogenation. The model contains lump 1 (>300 °C), lump 2 (250300 °C), lump 3 (200250 °C), lump 4 (20200 °C), diesel, gasoline, gas, and coke as lumps. The kinetic parameters were determined using least-squares regression analysis of the experimental data, obtained in two-stage fixed beds filled with the laboratory-made catalysts at various operating conditions. The proposed model was also validated. Comparisons between the experimental data and predictions using the lumping kinetic model showed good agreement. The variation in product yields and product distribution with operating conditions and feed properties was predicted. The effects of space velocity, hydrogen/oil ratio, temperature, initial hydrogen pressure, and other reaction conditions on hydrogenation performance were also investigated.
1. INTRODUCTION Major efforts are channeled toward the development of various usable energy sources to ensure energy security. These have been driven mainly by growing concerns on the petroleum depletion crisis and by rising fuel prices. China is one of the largest coal producers in the world and extensive studies have been focused on its production of fuel from coal.14 Abundant coal tar is produced from coal carbonization and gasification every year.5 Therefore, the hydrogenation of coal tar has received substantial attention in recent years as one of the most viable process alternatives for the conversion of low-value coal tar into a valuable high-clear fuel (e.g., gasoline and diesel). Coal tar hydrogenation has been examined extensively in the literature;611 however, most studies have focused mainly on the laboratory research stage. The calculation of a kinetic model is necessary to acquire a good understanding of the hydrogenation of coal tar, particularly to design and simulate a reactor, to predict reaction behavior, and to optimize operating conditions. The lumping kinetic approach has been studied widely in the modeling of hydrogenation.1217 For example, Mosby18 proposed a seven-lump kinetic model to describe residual oil hydrotreatment. The model is based on the difficulty of cracking, in which residues are classified as lumps that are either “easy” or “difficult” to crack, reflecting basically the residual hydrocracking reaction. Huang19 proposed the hydrotreatment of coal hydroliquefaction residue and its kinetics, in which a four-lumped kinetic model was established using solubility-based lumped fractions. The hydro-conversion behavior of CHLR was studied under moderate hydrotreatment conditions to determine the possibilities and advantages of using recycled CHLR as a coreactant in CHL. A eight-lump kinetic model for the catalytic pyrolysis of heavy oils was recently developed by Meng,20 this article also presents a new catalyst deactivation model, as a function of feed properties and operating conditions, in which the deactivation constant does not vary with reaction temperature, r 2011 American Chemical Society
because the influence of temperature has already included in the deactivation function. In addition, there are other lumped kinetics of hydrogenation2125 that have also achieved good results. However, most lumping models for hydrogenation, including those mentioned previously, are applied mainly for heavy oil and other residues. On the contrary, the lumping models of coal tar hydrogenation have received less research attention. A few basic studies focused on the types of hydrogenation, catalysis, and effects of feed, catalyst acidity, pore diffusion, and catalyst poisons on hydrogenation reaction. The current paper aims to develop an eight-lump kinetic model for coal tar hydrogenation using the catalyst NiMo/Al2O3. Kinetic parameters are estimated using a specially compiled program based on the least-squares algorithm. The variation in product yields is predicted with operating conditions. The present study also focuses on the effect of temperature, space velocity, hydrogen/oil ratio, initial hydrogen pressure, and other reaction conditions on hydrogenation performance.
2. EIGHT-LUMP KINETIC MODEL 2.1. Lump and the Model. A number of complex reactions occur during hydrogenation, and the product consists of a mixture of many compounds. The description of complex mixtures by lumping large numbers of chemical compounds into smaller groups of pseudocomponents has been employed widely by researchers to provide a tractable number of kinetic equations. Generally, the more lumps a model includes, the more kinetic parameters need to be estimated intrinsically. Consequently, the more experimental data are required.26 Thus, the establishment Received: July 26, 2011 Revised: October 6, 2011 Published: October 11, 2011 4878
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(1) The reactions are far from chemical equilibrium, so they are considered first-order reactions. (2) The reaction among the various lumps is not reversible, that is, the back of a lump can only be generated from the previous one and not vice versa. (3) The reaction is conducted under the condition that the internal and external effects of diffusion have been eliminated. Therefore, the reaction is controlled by dynamics. (4) The coke deposited on the catalyst is normally less than 0.30 wt % after the reactions. To simplify the model calculation process, catalyst decay is excluded in the present work. On the basis of the above assumptions, the reaction rate equations of the proposed model are expressed as follows: Figure 1. Eight-lump reaction network for coal tar hydrogenation.
of an appropriate model that can give all the key kinetic data is necessary. Generally, the lumping division for heavy oil and residue feeds is performed based on composition and structure. This process is similar to that done on the feed lumps of catalytic cracking models reported by Meng,27 in which the feed is lumped into paraffinic carbons, naphthenic carbons, and aromatic carbons. However, coal tar, a particulate material, is often divided based on the boiling point. Therefore, to predict coal tar hydrogenation and product distribution adequately, the feed in the model can be lumped into four groups (lump 1 (>300 °C), lump 2 (250300 °C), lump 3 (200250 °C), lump 4 (20200 °C)). The products obtained from coal tar hydrogenation are lumped into four groups, considering that the primarily desired products from coal tar hydrogenation are diesel and gasoline. Therefore, diesel and gasoline can be considered as two separate lumps. In addition, alkanes and other low-carbon pyrolysis gases that are also byproduct of coal tar hydrogenation can be considered as one lump. Coke is another significant byproduct because coke yield is an important parameter for the design of a regenerator, and its formation mechanism is different from that of other products. Therefore, coke alone should be considered as a lump. Based on the above-mentioned division, the simplified reaction network among lumps is shown in Figure 1. According to experience, lump 1 is hardly cracked into lump 7 (cracked gas), the reaction between the lumps was neglected in this study, because lump 1 is constituted mainly by a mixture of aromatic and aliphatic hydrocarbons. Ring-opening reactions do not occur easily given this composition and the operating conditions of hydrogenation, similar to what was reported by Wang.28 To simplify the calculation, the reaction from lump 4 to cracked gas (lump 8) and to diesel (lump 5) can also be excluded because only a very small amount of the yields is converted from these reactions. Research results show that the gasoline and diesel produced in primary reactions catalyze secondary reactions, including cracking, hydrogen transfer, isomeration, aromatization, alkylation, and condensation, among others. 2.2. Kinetic Models. For each reaction, a kinetic expression (ri) was formulated as a function of lump mass fraction (yi) and kinetic constants (ki). The reaction among the eight lumps is a highly coupled, complicated, parallel-order response. To simplify the model, a practical calculating process should have the following assumptions:
dy1 ¼ ðk12 þ k15 þ k16 þ k18 Þy1 dt
ð1Þ
dy2 ¼ k12 y1 ð þ k23 þ k25 þ k26 þ k27 þ k28 Þy2 dt
ð2Þ
dy3 ¼ k23 y2 ðk34 þ k35 þ k36 þ k37 þ k38 Þy3 dt
ð3Þ
dy4 ¼ k34 y3 ðk46 þ k47 Þy4 dt
ð4Þ
dy5 ¼ k15 y1 þ k25 y2 þ k35 y3 ðk56 þ k58 Þy5 dt
ð5Þ
dy6 ¼ k16 y1 þ k26 y2 þ k36 y3 þ k46 y4 þ k56 y5 k67 y6 dt ð6Þ dy7 ¼ k27 y2 þ k37 y3 þ k47 y4 þ k67 y6 dt
ð7Þ
dy8 ¼ k18 y1 þ k28 y2 þ k38 y3 þ k58 y5 dt
ð8Þ
where yi represents the mass fraction of each lump (i = 18), ki,j refers to the kinetic constant for the reaction of lump i to lump j, and t is the residence time. 2.3. Solution to the Model. A program utilizing MATLAB language and based on the nonlinear least-squares method was used to estimate the kinetic parameters in the proposed lump model. The numerical solutions for eqs 18 were obtained using a fourth-order RungeKutta method. The objective function to be minimized was the sum of the squares of the difference between the experimental and the calculated values. The parameter estimation problem was formulated mathematically using the following nonlinear optimization program described in eq 9 min FðtÞ ¼
n
n
cal 2 ∑ fi 2 ðtÞ ¼ i∑¼ 1½ðyexp i Þ ðyi Þ i¼1
ð9Þ
where yiexp and yical refer to the experimental value at a given operating condition and the corresponding value calculated using eqs 18, respectively. In the current paper, a reasonable initial value was first attached to the kinetic constant. Then the differential equations 4879
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Table 1. Quality of Coal Tar properties
Table 2. Catalyst Characterization value
composition (wt. %)
elemental analysis (wt. %) C
84.86
H
8.39
N
1.69
S
0.96
O
4.10
H/C molar ratio
1.19
distillation range (°C) IBP
118
10%
196
50%
261
90%
306
density (20 °C) (g 3 mL1)
1.0078
were solved using a fourth-order RungeKutta method, and the integral results obtained were taken into the objective function. Finally, an assessment on whether the error square between the integral results and the experimental data reached the minimum was made. If the minimum is not reached, the rate constant is changed based on relevant rules of the simplex method. The process continues until the objective function reaches a minimum.
3. EXPERIMENTAL SECTION Experimental data were obtained from a bench scale plant equipped with a two-stage fixed bed reactor. The detailed descriptions of the experimental setup and procedure were given in another study.29 The distillate (under 360 °C) of the middle-temperature coal tar was used as feedstock in the current study. Some properties of the feedstock are listed in Table 1. A laboratory-made NiMo supported on an Al2O3 catalyst was employed. The properties of the catalyst are shown in Table 2. The experiments were conducted at temperatures of 360, 370, 380, and 390 °C, and four levels of pressure in the range of 612 MPa. The space velocity was varied from 0.4 to 1.2 h1, and the H2/oil ratio was varied from 1400 to 1800. The true boiling point distillation of the feedstock and products was analyzed using the ASTM D-5307 simulated distillation method.
4. RESULTS AND DISCUSSION 4.1. Reaction Conditions. On the basis of the results of the experiment on coal tar hydrogenation, the current article focused on the effect of temperature, space velocity, hydrogen/oil ratio, initial hydrogen pressure, and other reaction conditions on hydrogenation performance. 4.1.1. Space Velocity. Table 3 mainly shows the variation in product yields (diesel and gasoline) with space velocity while maintaining the reaction temperature, initial hydrogen pressure, and hydrogen/oil ratio at 380 °C, 6 MPa, and 1600, respectively. As the space velocity increases, the desired product yields decrease, all product yields tend to a maximum value at space velocity of 0.4 h1. A lower space velocity (a higher contact time) is observed to result in a higher level of hydrogenation. Space velocity reflects the length of time the tar is in contact with the catalysts. For the same catalyst bed, a higher space velocity value results in a shorter contact time. During the hydroprocessing of coal tar, the hydrogenation reaction involving three
catalyst
Mo (%)
Ni (%)
BET area (m2 3 g1)
pore volume (mL 3 g1)
bulk density (g 3 mL1)
MoNi/
12.32
2.56
209
0.56
0.78
γ-Al2O3
phases proceeds at a very slow rate. For this multistep hydrogenation reaction, an adequate reaction time is necessary. At a higher space velocity, an inadequate amount of time is provided to ensure the occurrence of certain reactions. In addition, at a lower space velocity, the desired product yield can be obtained at a lower reaction temperature, and the life cycle of the catalyst can be extended. 4.1.2. Hydrogen/Oil Ratio. Table 4 shows the effect of hydrogen/oil ratio on the hydroprocessing performance at a stepwise hydrogen/oil ratio ranging from 1400 to 1800 while maintaining the reaction temperature, initial hydrogen pressure, and space velocity at 380 °C, 8 MPa, and 0.4 h1, respectively. The diesel and gasoline conversion reached a very high level under a hydrogen/oil ratio of 1600. With the increase of hydrogen/oil ratio continuously, the products yields slightly increase or even remain unchanged. Therefore, the percent conversions of the desired products will be optimal at hydrogen/oil ratio = 1600, while other experimental conditions are kept constant. However, the conversions of diesel and gasoline were not affected evidently by the hydrogen/oil ratio. More hydrogen is needed in hydrogenation because the primary composition of coal tar is aromatic, which can sufficiently generate small molecules of hydrocarbons such as alkanes, aliphatic, and some aromatic saturated hydrocarbons. The aromatic saturation hydrogenation reaction is also a strong exothermic reaction process which needs plenty of hydrogen to remove heat from the reactor. However, an extremely high hydrogen/oil ratio will affect directly the economics of the device. 4.1.3. Reaction Temperature. In Figure 2, the effect of reaction temperature on the yield of desired products, at space velocity of 0.4 and 0.6 h1, is investigated. A change in the temperature from 360 to 380 °C was observed to exhibit a great increase for desired products, whereas when the temperature was higher than 380 °C, the yields show a downward trend slightly. This finding means that at temperatures lower than 380 °C, the reaction is mainly dominated by hydrogenation. yet the yield of diesel and gasoline decreases slightly when the temperature is higher than 380 °C, which can be attributed to the secondary cracking of products. In addition, it is observed that the desired products yield also increases when the space velocity increases, but in contrast to space velocity, the effect of temperature is more noticeable in the conversion of the products because the rate of hydrocracking reaction and hydrogenation is controlled mainly through temperature. However, the temperature range of more than 420 °C will result in a reduction in the depth of aromatic hydrogenation saturation and in the shortening of the life of the catalyst, which can be attributed to the condensation of polycyclic compounds into coke. 4.1.4. Initial Hydrogen Pressure. The effect of initial hydrogen pressure on the yield of the products (gas and coke) at different space velocity is shown in Figure 3. All percentage conversions of 4880
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Table 3. Effect of Space Velocity on Properties of Gasoline and Diesel Productsa
product properties yield (vol. %)
0.4 (h1) 19.9
effect of space
effect of space
velocity on gasoline
velocity on diesel
0.8 (h1)
1.2 (h1)
19.5
18.3
0.4 (h1) 75.8
0.8 (h1)
1.2 (h1)
71.6
69.2
S&N analysis (wt. %) S (ppm)
75
82
172
57
69
89
N (ppm)
17
22
148
10
56
233
H/C molar ratio
1.85
1.84
1.77
1.76
1.75
1.64
95 121
96 120
96 128
50%
275
275
273
90%
343
340
342
distillation range (°C) IBP 10%
FBP
286
290
279
360
358
365
density (20 °C) (g 3 mL1)
0.8057
0.8064
0.8082
0.8861
0.8894
0.8938
RON
93.0
93.1
95.0
AKI
88.4
88.5
90.1 56.0 +4.2
56.1 +5.2
53.8 +5.2
cetane value solidifying point (°C) a
Other experimental conditions: at 6 MPa, 380 °C, and hydrogen/oil ratio = 1600.
Table 4. Effect of Hydrogen/Oil Ratio on Properties of Gasoline and Diesel Productsa effect of hydrogen/oil ratio on gasoline product properties yield (vol. %)
1400 18.9
1600
1800
19.8
20.0
effect of hydrogen/oil ratio on diesel 1400 70.2
1600 75.8
1800 76.2
S&N analysis (wt. %) S (ppm)
174
82
75
86
70
61
N (ppm)
148
22
16
233
59
14
H/C molar ratio
1.69
1.78
1.82
1.58
1.65
1.70
IBP
96
96
95
10%
128
120
121
50%
272
274
273
90% FBP
284
340 363
338 356
341 358
0.8935
0.8892
0.8860
distillation range (°C)
a
277
289
density (20 °C) (g mL1)
0.8084
0.8063
0.8055
RON
94.9
93.0
92.8
AKI
90.1
88.5
88.0
cetane value
53.4
55.8
56.0
solidifying point (°C)
+5.0
+5.0
+4.2
Other experimental conditions: at 6 MPa, 380 °C, and space velocity = 0.4 h1.
the products increase with an increase in the initial hydrogen pressure, which is similar to the results obtained with temperature. However, temperature affects the conversion of products to a greater extent. In addition, coal tar hydrogenation should be conducted at a high hydrogen pressure and low space velocity. Considering that the initial hydrogen pressure affects slightly the diesel yield from 8 to 12 MPa compared with the range from 6 to 8 MPa, the optimal initial hydrogen pressure should be approximately 8 MPa. The enhancement of initial hydrogen pressure is not only beneficial for the removal of sulfur, nitrogen, and some other
impurity atoms present in coal tar, which promotes the hydrogenation saturation of aromatic compounds. It also improves the output of relevant products, reduces the coke rate, and extends the life of the catalyst. However, an extremely high initial hydrogen pressure will increase the investment needed for plant construction and operating costs. 4.2. Parameter Estimation. The reaction rate of the hydrogenation of coal tar is related mainly to temperature and initial hydrogen pressure. Therefore, multiple sets of experimental data for the yields of all feeds and products were obtained at various 4881
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Table 5. Results of Reaction Total Kinetic Constants Estimated temperature (°C) 1
Figure 2. Effect of space velocity on the yield of the aimed products (diesel (9 and2) and gasoline (b and 1)) vs temperature (at 8 MPa and hydrogen/oil ratio = 1600).
Figure 3. Effect of space velocity on the yield of the products (gas (9 and 1) and coke (2 and 1)) vs initial hydrogen pressure (at 8 MPa and hydrogen/oil ratio = 1600).
reaction rate constant (h )
360
370
380
k12
0.0003
0.0008
0.0011
k15
6.4908
7.8386
9.3211
k16 k18
0.3539 0.0014
0.4458 0.0027
0.5577 0.0041
0.6028 0.0053
k23
1.4676
1.6071
1.7251
1.8068
k25
4.4522
4.7275
5.069
5.224
k26
0.7754
0.8343
0.9422
0.9982
k27
0.0182
0.0357
0.0578
0.0695
k28
0.00012
0.00022
0.00029
0.00034
k34
0.1677
0.2141
0.3243
0.3903
k35 k36
7.3458 0.9208
7.7034 0.9862
8.792 1.685
9.322 2.105
k37
0.0195
0.0727
0.1768
0.2318
k38
0.0032
0.0045
0.0061
0.0072
k47
0.5211
0.7965
0.9123
0.9776
k46
6.7586
7.6893
9.022
9.885
k56
0.0061
0.0069
0.0077
0.0112
k58
0.007
0.0147
0.0232
0.0298
k67
0.0509
0.0638
0.0816
0.0943
390 0.0013 10.226
Table 6. Results of Reaction Total Kinetic Constants Estimated initial pressure of hydrogen (MPa)
levels of temperature and initial hydrogen pressure. A program was compiled in MATLAB language to determine the kinetic constants based on the experimental data. The kinetic constants of the eight-lump model are estimated and listed in Tables 5 and 6. The general expression of kinetic constants can be formulated as a function of the reaction temperature (T) and the initial hydrogen pressure (P) as follows: ki ¼ Ai eEi=RT Pai
ð10Þ
where Ai represents the pre-exponential factor of each reaction lump (h1 3 MPaai), Ei refers to the apparent activation energies (kJ/mol), and ai is the initial hydrogen pressure index. Pressure Index. On the basis the data in Table 6, the logarithm is applied to both sides of eq 10, and the following equation is obtained: ln ki ¼ ai ln P þ ln Ai eEi=RT
ð11Þ
On the basis of the above equation, a linear relationship is expressed between ln ki and ln P. A coefficient term obtained by conducting linear regression to eq 10 is the initial hydrogen pressure (ai) of each reaction lump, as shown in Table 7. Apparent Activation Energy. On the basis of the data in Table 5, the logarithm is applied to both sides of eq 10, and the following equation is obtained: ln ki ¼
Ei þ ln Ai Pai RT
ð12Þ
On the basis of the above equation, a linear relationship is established between ln ki and 1/T. A coefficient term obtained
reaction rate constant (h1)
6 MPa
8 MPa
10 MPa
12 MPa
k12
0.0003
0.00056
0.00082
0.00106
k15
6.4908
7.524
8.446
9.655
k16 k18
0.3539 0.0014
0.4277 0.0022
0.4808 0.0030
0.5569 0.0042
k23
1.4676
1.5421
1.6108
1.7024
k25
4.4522
4.6431
4.7986
4.8967
k26
0.7754
0.8434
0.9028
0.9882
k27
0.0182
0.03123
0.04186
0.05429
k28
0.00012
0.00019
0.00025
0.00033
k34
0.1677
0.2145
0.2558
0.3225
k35 k36
7.3458 0.9208
7.8227 1.246
8.226 1.488
8.848 1.802
k37
0.0195
0.0682
0.114
0.1805
k38
0.0032
0.0041
0.0049
0.0062
k47
0.5211
0.6554
0.7522
0.8943
k46
6.7586
7.339
8.048
8.892
k56
0.0061
0.0069
0.0075
0.0083
k58
0.007
0.0124
0.0168
0.0242
k67
0.0509
0.0632
0.0711
0.0825
by performing linear regression to eq 12 is Ei/R (i = 1, 219). The constant term is ln AiPai. The pre-exponential factor (Ai) and the apparent activation energies (Ei) were calculated by taking the value of R and ai into Ei/R and the constant term, respectively, as shown in Table 7. 4882
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Table 7. Results of Reaction Kinetic Parameters Estimated reaction rate
activation energy
pre-exponential factor
pressure
constant (h1)
(Ei)/(kJ 3 mol1)
(Ai)/(h1MPaai)
index (ai)
k12
165.64
1.832 107
1.8278
k15
53.783
59270
0.5633
k16
63.78
19024.4
0.6393
k18
154.5
2.945 107
1.5618
k23
24.311
101.85
0.2096
k25
19.215
115.643
0.1390
k26 k27
30.73 157.76
218.72 6.48 1011
0.3414 1.5630
k28
119.26
1664.42
1.4409
k34
103.07
9.3 107
0.9207
k35
29.577
1866.3
0.2609
k36
105.160
8.776 107
0.9534
k37
291.5
2.453 108
3.1702
k38
95.811
30635.9
0.9317
k47 k46
71.043 45.456
112892.76 29647.5
0.7616 0.3917
k56
67.17
1365.5
0.4352
k58
168.27
4.6125 1011
1.7517
k67
73.421
17667.57
0.6806
Some values of the activation energies in Table 7 are lower than 63.78 kJ/mol. One characteristic of these reactions is the difficulty in generating light fuel by the high-boiling lump. This condition is attributed to the composition of the lump, which is mainly aromatic and aliphatic hydrocarbon. Hydrocracking reactions cannot occur given the operating conditions of hydrogenation. Most of the apparent activation energies determined in the current research are near or above 100 kJ/mol, which explain why thermal hydrocracking plays a significant role in coal tar hydrogenation. This finding shows good agreement with the reaction mechanistic pathway for the catalytic pyrolysis of heavy oil.27 In addition, a low-boiling lump is composed of alkanes in which the CC or CH bond is broken easily. Therefore, these reactions have lower activation energies than other reactions in coal tar hydrogenation. 4.3. Validation of the Model. Once the parameters of the eight-lump kinetic model were estimated, some experimental data should be used to verify whether the model meets the requirements. Figure 4 shows a comparison between the experimental and predicted yields for diesel and gasoline at constant operating conditions using the kinetic model proposed in the present work. Figure 5 shows the experimental and calculated yields of diesel at operating temperatures of 360, 370, 380, and 390 °C. Figure 6 shows the experimental and calculated yields of gasoline at operating initial hydrogen pressure levels of 6, 8, 10, and 12 M Pa. The proposed kinetic model gives accurate predictions of product yields in coal tar hydrogenation process with average deviations is 4%, indicating that the eight-lump kinetic model predicts the experimental data well and that the predicted values are reliable. 4.4. Application of the Model. 4.4.1. Predicting the Distribution of Products. The prediction of the distribution of products is a vital application for the proposed lump model. This model can predict the yields of the products for various process operating conditions and feed compositions. Further experiments and
Figure 4. Experiment yields (plots) vs predicted yields (line) at 380 °C, 8 MPa, space velocity = 0.4 h1, and hydrogen/oil ratio = 1600.
Figure 5. Experiment yields (plots) vs predicted yields (line) for gasoline (at 8 MPa, space velocity = 0.4 h1, and hydrogen/oil ratio = 1600).
Figure 6. Experiment yields (plots) vs predicted yields (line) for diesel (at 380 °C, space velocity = 0.4 h1, and hydrogen/oil ratio = 1600).
analysis are unnecessary with the kinetic model. Figure 7 shows the predicted product distribution of coal tar hydrogenation with the residence time at 380 °C, while maintaining the hydrogen pressure, hydrogen/oil ratio, and space velocity at 8 MPa, 1600, and 0.4 h1, respectively. 4.4.2. Optimization of Operating Conditions. For coal tar hydrogenation, the variation in product yields with every operating condition can be predicted using the proposed eight-lump kinetic model. The optimal operating conditions can then be obtained. Figure 8 shows the effect of the residence time of oil gas on the yields of diesel at 360, 370, 380, and 390 °C. As the residence time is prolonged, the percentage conversion of diesel increases rapidly until the maxima are reached at about 0.5 h. The con4883
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shown from 6 to 8 MPa. Considering various factors, the optimal initial hydrogen pressure is 8 MPa, whereas the other conditions are kept constant.
Figure 7. Predicted product distribution with the residence time (at 380 °C, 8 MPa, space velocity = 0.4 h1, and hydrogen/oil ratio = 1600).
5. CONCLUSION On the basis of the analysis of coal tar hydrogenation over a laboratory-made NiMo/Al2O3 catalyst, an eight-lump (lump 1 (>300 °C), lump 2 (250300 °C), lump 3 (200250 °C), lump 4 (20200 °C), diesel, gasoline, gas, and coke) kinetic model that includes 19 total kinetic constants for coal tar hydrogenation is proposed. The results show that the model is more suitable to the calculation of coal tar hydrogenation. The effect of temperature, space velocity, hydrogen/oil ratio, and initial hydrogen pressure on hydrogenation performance is also discussed. The results indicate that the hydrogenation of coal tar should be conducted at a low space velocity of 0.4 h1, hydrogen/oil ratio of 1600, initial hydrogen pressure of 8 MPa, and temperature of 380 °C to reach the optimal yields of products. In addition, comparisons between the experimental data and predictions using the lumping kinetic model showed good agreement, with an average absolute error lower than 5%. Therefore, the model can serve as an effective guide for coal tar hydrogenation. ’ AUTHOR INFORMATION Corresponding Author
*Tel./Fax: +86-10-82544800 (C.L.); 0532-84022699 (S.X.). E-mail:
[email protected] (C.L.);
[email protected] (S.X.). Figure 8. Variation in the yield of diesel with the residence time (at 8 MPa, space velocity = 0.4 h1, and hydrogen/oil ratio = 1600).
Figure 9. Variation in the yield of lump 2 with the residence time (at 380 °C, space velocity = 0.4 h1, and hydrogen/oil ratio = 1600).
version of diesel then shows a downward trend attributed to the secondary cracking of products. Considering the extent of coal tar hydrogenation, when the temperature is increased to 380 °C, the yield is increased significantly. The yield then increases slightly, which shows a trend similar to that obtained in the coal tar hydrogenation experiment. Therefore, the reaction temperature should be approximately 380 °C, whereas the other conditions are kept constant. Figure 9 shows the predicted variation in lump 2 yield with the residence time at 6, 8, 10, and 12 MPa. When the residence time is increased, the yield of lump 2 decreases rapidly. The predicted lump 2 yields at a high initial hydrogen pressure are lower than those at a low initial hydrogen pressure, but the fastest decrease is
’ ACKNOWLEDGMENT This research is supported by the National Natural Science Foundation of China (No. 21006113) and National Basic Research Program of China (973 Program No. 2009CB219900). ’ NOMENCLATURE A = m, pre-exponential factor, (h1MPaai) E = apparent activation energy, kJ 3 mol1 k = rate constant, h1 R = gas constant, 8.314 J 3 mol1 3 K1 t = residence time, h T = reaction temperature, K P = initial hydrogen pressure, MPa y = mass fraction, wt% a = Pressure index i = each lump IBP = initial boiling point FBP = final boiling point RON = research octane number AKI = antiexplosion index ’ REFERENCES (1) Liu, Z.; Shi, S.; Li, Y. Chem. Eng. Sci. 2010, 65, 12–7. (2) Cui, H.; Yang, J.; Liu, Z.; Bi, J. Fuel 2002, 81, 1525–31. (3) Yang, J.; Zhu, J.; Xu, L.; Liu, Z.; Li, Y. Fuel 2002, 81, 1485–9. (4) Zhang, L.; Yang, J.; Zhu, J.; Liu, Z.; Li, B.; Hu, T.; Dong, B. Fuel 2002, 81, 951–8. (5) Li, C.; Suzuki, K. Resour., Conserv. Recycl. 2010, 54, 905–15. (6) Wailesa, P. C.; Bella, A. P.; Alfred, C. K. Fuel 1980, 59, 128–132. (7) Froment, G. F.; Castaneda-Lopez, L. C.; Marin-Rosas, C. Catal. Today 2008, 130, 446–54. 4884
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