Article pubs.acs.org/EF
Characteristics of Coal Liquefaction in an Impinging Stream Reactor Junqing Cai,* Junkai Zhang, and Wensheng Song Chemical Engineering and Pharmaceutics College, Henan University of Science and Technology, Number 48 Xiyuan Road, Luoyang, Henan 471003, People’s Republic of China ABSTRACT: Liquefaction of coal is studied in a laboratory-scale impinging stream reactor. The effects of the temperature (430−520 °C), solvent/coal ratio (1−5), slurry/gas flow ratio (0.5−0.9), and nozzle distance (12−4 cm) on coal total conversion and product distribution are investigated. To determine the role of hydrogen gas, reactions are also carried out under a nitrogen gas atmosphere at the same reaction conditions. On the basis of the above studies, the determining step of coal liquefaction is the gas/liquid mass-transfer step. The use of this reactor system reduces the impact of slow diffusion of gas into the reaction medium. Hence, rather high conversion (80.21 wt %) is achieved.
1. INTRODUCTION The efficiency of direct coal liquefaction (DCL) depends upon properties and petrographical characteristics of coal samples, the function of the catalyst and solvent, and the technical conditions, including temperature, pressure, time, coal/solvent ratio, etc. To achieve tolerable conversion and oil yield, it is desirable to understand the mechanism of DCL and the main factors affecting the liquefaction process.1 The general mechanism of DCL is described as a free-radical process, in which coal is thermally decomposed into free radicals, which are then stabilized by abstraction of a hydrogen atom from the donor molecule. Balance of these two reactions governs the performance of the DCL process.1 Therefore, lots of studies have been performed to make the rate match of hydrogenation and free-radical formation.2−8 For example, in the two-stage liquefaction process,2,3 the free-radical generation step occurs in two consecutive reactors, first at a low temperature and then at a high temperature. This method reduced peak rates of free-radical fragment generation and increased hydrogenation efficiency under milder conditions. Hydrogen-donation solvents were also used. Some researchers considered that the essential approach for promoting DCL is to provide enough hydrogen atom to stabilize free radicals produced in coal pyrolysis and result in strong bond cleavage in coal.4,5 Then, the effects of hydrogen-donation capability of solvents on DCL have been widely studied.6,7 DCL with different hydrogenation catalysts was also investigated.8 The results showed that the presence of hydrogen and catalyst improves the efficiency of DCL. Although hydrogenation reaction rates are governed by specific factors, when the slurry concentration is high, it seems that the determining step is the gas/liquid mass-transfer step.9 In an impinging stream apparatus, two or more feed streams flowing in parallel or countercurrent collide with each other and generate a zone with violent turbulence. Violent turbulence of the impinging zone can enhance the mixing efficiency in the reactor, thereby improve the mass and heat transfer.10 Tamir,11 who did a lot of the studies on impinging streams, considered that almost any process in chemical engineering can be carried out by applying this approach. Because of this reason, it is expected that impinging stream will exhibit an increase in mass© 2012 American Chemical Society
transfer rates and, hence, an increase in the overall reaction rates compared to the other DCL processes. In this paper, DCL in an impinging stream reactor is studied and the effects of the temperature, solvent/coal ratio, slurry/gas flow ratio, and nozzle distance on coal total conversion and product distribution are investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. Shenhua brown coal samples were pulverized to a particle size of less than 100 mesh and dried under vacuum at 100 °C for 10 h before use. Ultimate and proximate analyses of coal determined according to American Society for Testing and Materials (ASTM) standards were given in Table 1. All solvents used are commercial pure chemical reagent (purity higher than 99.5%) without further purification. The purity of hydrogen and nitrogen gas used in this study is 99.9%. 2.2. Apparatus and Liquefaction Procedure. A schematic of the overall system is shown in Figure 1. The system consists of a slurry preparation and feed section, a gas feed section, a liquefaction section, and a product separation section. The impinging stream reactor has two parts: the top portion of the reactor is the reaction part, where the hydrogen gas and coal slurry contact and take the reaction in this part, and the other is called the cooling section, in which the reaction is stopped. The impinging stream reactor, which is made of stainless steel with dimensions of 5 cm inside diameter and 20 cm length, is used in this experiment. The reactor has two dual-channel pressure-flow nozzles, which are put horizontally at the same plane. The diagram of a nozzle is given in Figure 2. The reactor is rated for a maximum allowable working pressure of 1.0 MPa at 600 °C. The liquid and solid products from the reactor were separated into oil (n-hexane soluble), asphaltene (toluene soluble but n-hexane insoluble, A), pre-asphaltene [tetrahydrofuran (THF) soluble but toluene insoluble, PA], and THFI (THF insoluble) by Soxhlet extraction with n-hexane, toluene, and THF, respectively. The mean value of the data was obtained in at least two equivalent experiments, and therefore, the estimated error of such results is 1 wt %.
3. RESULTS AND DISCUSSION 3.1. Effect of the Temperature. It is widely accepted that the molecular structure of most coals is elementary units Received: January 30, 2012 Revised: May 30, 2012 Published: May 30, 2012 3510
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Table 1. Proximate and Ultimate Analyses of the Coal Sample proximate analysis (wt %, db)
a
ultimate analysis (wt %, daf)
sample
ash
volatile matter
fixed carbon
C
H
N
Oa
S
raw coal
8.56
41.36
50.08
72.13
4.61
1.02
21.82
0.42
By difference.
Figure 1. Scheme of the experimental system.
connected by chemical bonds. Pyrolysis is the first stage of liquefaction of coal. Different bonds will be cleaved at different temperatures. First, the weak chemical bonds are broken at lower temperatures, and then the stronger bonds will be cleaved at relatively higher temperatures. Thereby, radicals are generated. The radicals are terminated by active hydrogen to form liquid products. Thus, the temperature is an important factor during liquefaction. Several temperature points from 430 to 520 °C were selected in this work. The results are shown in Figure 3. First, the total conversion and oil yield sensitively increase with the rise of the temperature. The total conversion and oil yield reach the maximum values of 80.21 and 71.79% at 490 °C, respectively.
The reason may be that the rates of free-radical fragment generation increase with the rise of the temperature, which can combine exactly with the matched amount of active hydrogen before 490 °C, being stabilized and converted into smallmolecule liquid products. However, a further increase to 520 °C did not seem beneficial. It can also be seen that the yield of PA + A decreases with the rise of the temperature, which means that the formation rate of PA + A is slower than that of its decomposition to oil in all ranges of the temperature. 3.2. Effect of the Solvent/Coal Ratio and Atmosphere. Tetralin as the solvent has strong penetration, hydrogen donation, and hydrogen transportation ability, which improves the hydrogenation and hydrocracking process with the inhibition of the condensation reaction. Figure 4 shows the variation of product distribution with the solvent/coal ratio (w/ w). The total conversion and oil and PA + A yields increase with solvent/coal ratios between 1 and 2 and then change little. Because of its strong hydrogen-donating and transporting
Figure 3. Effects of the temperature on the conversion and product distribution. Solvent/coal, 2; slurry/gas flow ratio, 0.8 (m3 m−3); and nozzle distance, 8 cm.
Figure 4. Effects of the solvent/coal ratio on the conversion and product distribution under a hydrogen atmosphere. Temperature, 490 °C; slurry/gas flow ratio, 0.8 (m3 m−3); and nozzle distance, 8 cm.
Figure 2. Diagram of the nozzle.
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ability, tetralin can promote the hydrogenation of coal. Therefore, the conversion and the yield of oil and PA + A increase with the ratio of solvent/coal. When the ratio is above 2 and increases further, the redundant solvent has no positive effect on the hydrogenation of coal, which leads to the nearly unchanged conversion and product yields. Some researchers reported that the total conversion is mainly influenced by the hydrogen-donating ability of the solvent, and contribution of hydrogen is negligible during quick DCL at high temperatures.12 To determine the role of hydrogen in the impinging stream reactor, parallel experiments are performed under nitrogen gas. The results are shown in Figure 5. The total Figure 6. Effect of the slurry/gas flow ratio on the conversion and product distribution. Temperature, 490 °C; solvent/coal, 2; and nozzle distance, 8 cm.
Figure 5. Effects of the solvent/coal ratio on the conversion and product distribution under a nitrogen atmosphere. Temperature, 490 °C; slurry/gas flow ratio, 0.8 (m3 m−3); and nozzle distance, 8 cm.
conversion reaches the maximum value at the solvent/coal ratio of 4. Nitrogen is considered as an inert gas during DCL; therefore, this conversion level is attributed to pyrolysis and coal dissolution because of the hydrogen-donating ability of the solvent. In comparison to liquefaction under a hydrogen atmosphere, this process needs more solvent to achieve maximum conversion, which indicates that hydrogen gas may provide active hydrogen atoms. The hydrogen atoms may react with coal directly or react with coal through the solvent. 3.3. Effect of the Slurry/Gas Flow Ratio. With the assumption that the determining step of DCL is the gas/liquid mass-transfer step, a large interface area during liquefaction will be favorable. Therefore, the slurry/gas flow ratio may be an important parameter and needs to be optimized. The results on this test are shown in Figure 6. To keep the atomization conditions essentially the same, the experiments are carried out at a fixed slurry flow rate, VS (0.06 m3/s−1), while the gas flow rate, VG, is controlled according to the requested VS/VG. It can be seen from Figure 6 that total conversion increases continuously with the flow ratio increasing in the range of VS/VG < 0.8. Obviously, this is because of the increase in the interface area. However, when VS/VG is over 0.8, the total conversion changes smoothly and then decreases. Note that the increase in VS/VG at a fixed VS implies a decreasing gas flow rate, which will lead to an insufficient active hydrogen atom. Thus, a further increase in this ratio becomes meaningless. 3.4. Effect of the Nozzle Distance. Figure 7 demonstrates the variations of total conversion of coal versus the nozzle distance. As may be observed from the figure, a decrease in the nozzle distance from 12 to 8 cm causes an increase in the total conversion. This behavior may be attributed to a decrease in the contacting device volume and an increase of the impinging
Figure 7. Effect of the nozzle distance on the conversion and product distribution. Temperature, 490 °C; solvent/coal, 2; and slurry/gas flow ratio, 0.8 (m3 m−3).
velocity at the impingement zone. However, as the distance changed from 8 to 4 cm, the total conversion decreases continuously. The most possible reason is that an increased density of droplets in the impingement zone at a smaller nozzle distance leads to enhanced collision and coalescence between droplets and, thus, a decreased interface area.
4. CONCLUSION A laboratory-scale impinging stream reactor is successfully built to carry out research in the DCL process at relatively higher temperatures. The optimum operating conditions were obtained as follows: temperatures at approximately 490 °C, solvent/coal ratio at 2, slurry/gas flow ratio at 0.8 (m3 m−3), and nozzle distance at 8 cm. The reactor provides a zone with violent turbulence, which can enhance the mixing effect in the reactor and improve the mass transfer. However, some other important reaction parameters should be further studied, such as the mass-transfer coefficient, impinging velocity, residence time distribution, etc. These will be performed in our next work.
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AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +86-379-64231914. E-mail: tdcoal@yahoo. com.cn. Notes
The authors declare no competing financial interest. 3512
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ACKNOWLEDGMENTS The authors express their sincere thanks and appreciation for assistance in laboratory measurements from the Henan University of Science and Technology.
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REFERENCES
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