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Low-temperature methanol synthesis in a circulating slurry bubble reactor
Fuel 82 (2003) 233–239 www.fuelfirst.com
Low-temperature methanol synthesis in a circulating slurry bubble reactorq Kai Zhanga,*, Huisen Songa, Dongkai Suna, Shunfen Lib, Xiangui Yangb, Yulong Zhaoa, Zhe Huanga, Yutang Wub a
Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, People’s Republic of China Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, People’s Republic of China
b
Received 31 January 2001; revised 26 June 2002; accepted 27 June 2002; available online 8 August 2002
Abstract A circulating slurry bubble reactor was developed to synthesise methanol via methyl formate from the gas mixture of carbon monoxide and hydrogen at low temperature. The strategy for designing and scaling up the bubble reactor involved a preliminary understanding of fluid dynamics in a cold model, continuous operations under industrial conditions and a parallel experiment in an autoclave. Per-pass syngas conversion was investigated during 100-h operations. The axial profile of solid catalyst concentration was measured just before the shutdown and the composition of liquid product was analysed after the shutdown. These results show that the circulating slurry bubble column will become a potential reactor for the commercial process of low-temperature methanol synthesis after the catalyst system has been improved. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Methanol synthesis; Low-temperature; Circulating slurry bubble reactor; Methyl formate; Syngas
1. Introduction
HCOOCH3 þ 2H2 ¼ 2CH3 OH
Methanol (MeOH) is potentially a cleaner alternative fuel for the future. The gas-phase MeOH synthesis process from syngas (CO þ H2) is regarded as a technically very well-developed industrial process [1,2]. However, there are some limitations, such as high temperature (470 – 570 K for the ICI low-pressure process), low per-pass conversion of the gaseous feed (10 – 20%), and high recycle of the unconverted gas. MeOH synthesis from syngas is highly 0 exothermic ðDH298 ¼ 290:97 kJ mol21 Þ and thermodynamically favourable at lower temperatures. The development of methanol synthesis includes catalyst improvement and reactor improvement. In recent years, the methanol synthesis via methyl formate (MeF) in a liquid phase at low temperature of around 373 K has been proposed and has achieved great success [3–12]. This process consists of MeOH carbonylation to MeF and MeF hydrogenolysis to MeOH as shown below:
Net
CH3 OH þ CO ¼ HCOOCH3 * Corresponding author. Present address: Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK. Tel.: þ 44-20-7679-3839; fax: þ44-20-7383-2348. E-mail address: [email protected] (K. Zhang). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
2H2 þ CO ¼ CH3 OH
The method can efficiently synthesise MeOH because of the potential synergy of the process coupling of reaction–reaction. The slurry bubble column is a competitive reactor for the commercial process of low-temperature methanol synthesis because it has advantages such as high heat and mass transfer rates, isothermal conditions, plug-free operation, and on-line catalyst addition and withdrawal [13 – 16]. However, the difficulty in scaling-up the results from small laboratory equipment to large industrially significant unit restricts its commercial application. Often results from small-scale test units predict reactant conversion and spacetime yield which cannot be achieved when the equipment size is increased. This problem is mostly due to bed fluid dynamics [15]. Accordingly, the understanding of fluid dynamics is helpful to eliminate the impact of scale-up processes. For gas – solid systems, much research has been successfully performed to solve this problem. Glicksman et al. [17] presented a comprehensive review, which outlined the development and application of scaling laws. For gas – liquid – solid systems, the present design practice is still closer to an art than science [15]. As mentioned earlier, the process development of methanol synthesis is composed of a catalyst system
0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 2 2 1 - 1
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K. Zhang et al. / Fuel 82 (2003) 233–239
solution, xylene and Op-10. Here, Op is short for polyethylene oxide alkyl phenol. The bed fluid dynamics, including the gas velocity for complete solid suspension, flow regime, phase mixing, and bubble movement behaviour, were qualitatively investigated in gas –liquid and gas – liquid –solid systems. The results are summarised as follows.
improvement and a reactor improvement. It is disappointing that all of the research [1,3 –12] concerning catalyst systems have so far been carried out in autoclaves. Therefore, the major objective of this study is to develop a 2T=a circulating slurry bubble reactor for the concurrent synthesis of MeOH via MeF from syngas. The preliminary fluid dynamics in a cold bubble column and the parallel experiment in an autoclave have been addressed.
1. The critical gas velocity for complete catalyst suspension is very low and the axial solid concentration profile becomes more uniform with an increase in the superficial gas velocity. 2. Bubbly flow, turbulent flow, and slugging regimes occur in the order of increasing superficial gas velocity according to the upward movement of bubble swarms in the bed. 3. At low gas velocity, bubbles are small and uniform in size, which vertically rise without remarkable interaction. 4. The gas holdup depends mainly on the superficial gas velocity and the slurry concentration at the same flow regime. Generally, overall gas holdup increases with increasing superficial gas velocity and decreasing slurry concentration.
2. Preliminary experiment in a cold model
The experimental procedure and results can be found in detail from the literature [16].
Fig. 1. An experimental set-up for cold model. (A) Column, (B) distributor, (C) rotameter and (D) valve.
A rough approach to simulate fluid dynamics in hightemperature, high-pressure industrial units was carried out at atmospheric pressure and room temperature using a plexiglas column with an inner diameter of 0.05 m and a height of 2.0 m. The experimental set-up is schematically shown in Fig. 1. A three-phase system was composed of air, liquid medium, and Cu-based catalyst powder. The liquid medium was a mixture of sodium methoxide (CH3ONa)
3. Experimental 3.1. Apparatus Concurrent MeOH synthesis was carried out in a 2T=a circulating slurry bubble reactor, which is conceptually illustrated in Fig. 2. This installation, made of stainless steel,
Fig. 2. A sketch of the circulating slurry reactor. (1) Mass flow meter, (2) N2 cylinder, (3) water removal, (4) CO2 removal, (5) preheaters, (6) magnetic liquid level indicators, (7) bubble reactor, (8) separator, (9) slurry recycle pump, (10) filter, (11) water condensers, (12) chilled condenser, (13) expansion vessels, (14) product receivers and (15) wet-test meters.
K. Zhang et al. / Fuel 82 (2003) 233–239
consists mainly of a bubble reactor and a separator. The bubble reactor is divided into a cylindrical section of 45 £ 3 mm diameter with a height of 4.8 m and an enlarged section of 159 £ 6 mm diameter with a height of 1.2 m. A sintered plate is located at the bottom of the cylindrical section. Three electric heating coils outside the cylindrical section, monitored by a temperature-programmed controller, are used to maintain the temperature within ^ 5 8C inside the bubble reactor. The separator has a diameter of 159 £ 6 mm and a height of 1.64 m. A slurry pump circulates and measures the slurry from the separator bottom to above the distributor in the bubble reactor. To control the slurry volume in the system and adjust the circulating volume of the slurry, two magnetic liquid level indicators are, respectively, installed into the enlarged section of the bubble reactor and the separator. The main advantage of the circulating bubble reactor is the synergy effect of reaction– separation. 3.2. Gas –liquid –solid system MeOH carbonylation is a homogeneously catalysed reaction using alkali metal methoxides as catalyst in the liquid phase, while hydrogenolysis of MeF is carried out in both gas and liquid phases using a heterogeneous copperbased catalyst. CH3ONa was employed as carbonylation catalyst. Hydrogenolysis catalyst was a copper-based catalyst, which developed by Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences [6]. Zhao et al. [10] obtained the maximum CO conversion in xylene medium after they investigated CO conversion in an autoclave using three kinds of liquid media (xylene, liquid paraffin, and methanol). Since xylene and methanol are insoluble without strong agitation, the third liquid medium Op-10 must be added in order that xylene and methanol are mixed uniformly in the system. Therefore, the liquid medium used was a mixture containing sodium methoxide, xylene, and Op-10. The syngas with an approximate stoichiometric ratio for methanol synthesis (H2/CO ¼ 2) was used as the gaseous reactant. The experimental gas – liquid – solid system was syngas/liquid-phase medium/Cubased catalyst powders. 3.3. Procedure When syngas is fed into the bubble reactor, CO is firstly reacted with MeOH in the liquid medium through carbonisation to form MeF, and the intermediate MeF is then hydrogenised to produce MeOH. The slurry, entraining product MeOH, intermediate MeF, and unconverted syngas, enters the top of the separator from the bubble reactor via a reducing pressure valve. The slurry is then pumped back into the reactor from the bottom of the separator. Before the operation run, the trapped air in the system was purged with N2 or syngas and vented to the desired purity. Then both the liquid-phase medium and the reduced
235
Table 1 Main temperatures and pressures in the system
Bubble reactor
Cylindrical section Enlarged section
Separator
Temperature (8C)
Pressure (MPa)
80– 120 80– 100 70– 90
4.2 –4.6 0.3 –0.5
Cu-based catalyst were added into the system. The syngas with (CO þ H2) $ 99%, H2O , 380 mg/m3 and CO2 , 100 ppm from high pressure cylinders was controlled and metered by an on-line mass flow meter. It was fed into the bottom of the bubble reactor after water and CO2 was removed less than 5 and 10 ppm, respectively. We managed the feed flow rate from 0.52 to 0.56 Nm3/h. The system was pressurised at ambient temperature. Two backpressure regulators were installed to maintain the pressure in both the bubble reactor and the separator. Then the slurry pump was started at the recycle flow rate of 0.05 m3/h with a fluctuation of less than 10%. Reactant gas, recycle slurry, and the reaction section in the bubble reactor were heated up to the required temperatures by two preheaters and three electric heating coils. When the reaction section reached the desired temperature, it was taken as the start time of the operation. The main temperatures and pressures in the system are listed in Table 1. The effluent gas from the top of the separator was defoamed, cooled by water, and further cooled by ice, while the effluent gas from the top of the bubble reactor was only cooled by water. The condensed liquid was separated in the expansion vessels and was collected into product receivers. After being depressurised, the non-condensable gas from the expansion vessels was metered by an on-line wet-test meter and vented. The composition of feed gas and tail gas was analysed using a gas chromatograph with a thermal conductivity detector. The composition of liquid samples was determined by the same chromatograph but the analysis condition was different (Table 2).
4. Results and discussion Initial development of the demonstration installation was conducted in late 1990s. Two continuous 100-h runs accomplished were introduced in this investigation. Table 2 GC analysis condition For gas composition For liquid composition Column dimension Detector Separation column Column temperature Carrier gas
3 mm (diameter) £ 3000 mm (length) TCD FID ˚ PEC 20000/SH-101 silanised Molecular sieve 5 A Ambient 130 8C Ar N2
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Table 3 Composition of feeds Run no.
The composition of slurry CH3ONa solutiona (l)
1 2
3.50 4.08
Xylene (l)
Op-10 (l)
Total liquid (l)
Cu-based cat (g)
14.02 15.58
2.48 2.51
20.00 22.17
600 700
Run no.
The mean composition of syngas (vol%)
1 2
H2 68.81 68.94 a
CO 30.14 30.64
CH4 1.05 0.42
Note: solvent, methanol; CH3ONa concentration, 26.1%; NaOH concentration, 0.4%.
Table 3 provides the load of liquid and catalyst. The composition of syngas is plotted in Fig. 3 and its mean value can be found in Table 3. 4.1. Per-pass conversion The per-pass conversions of CO can be estimated by two methods as below. Method A: V C 2 V1 C1 X¼ 0 0 £ 100% V0 C0 Method B: V C 2 ðV1 C1 þ V2 C2 Þ X¼ 0 0 £ 100% V0 C0
where X, V and C are the conversion of CO, the volumetric flow rate of syngas, and the volumetric concentration of CO, respectively. Subscript 0 stands for the feed gas, 1 for the effluent gas from the top of the bubble reactor and 2 for the effluent gas from the top of the separator. The calculation methods are abstractly drawn in Fig. 4. Figs. 5 and 6 indicate the per-pass conversions of CO and H2 at different reaction periods. As shown in Fig. 5, CO conversion keeps almost constant within 45 h in Run no. 1 and 50 h in Run no. 2, then it decreases gradually with increasing reaction time. These results show that the satiability of the catalyst system is poor although its initial activity is quite high. 4.2. Effects of temperature and pressure The effects of temperature and pressure on per-pass conversion were tested during the steady operation. Per-pass syngas conversion increased with an increase in temperature in the range from 80 to 120 8C. The temperature affects reaction rate, syngas solubility, and solution vapour pressure. According to thermodynamic theory, a decrease in temperature results in the equilibrium composition towards the formation of MeF for MeOH carbonylation reaction. However, the hydrogenolysis rate is too slow at low temperature. From the viewpoint of kinetics, an increase in temperature favours the rate of a chemical reaction. Therefore, it is a consequence of two competing effects
Fig. 3. The composition of syngas during operation. (A) Run no. 1 and (B) Run no. 2.
Fig. 4. A schematic diagram for the calculation.
K. Zhang et al. / Fuel 82 (2003) 233–239
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stated above that syngas conversion increases with an increase of temperature in the range 80– 120 8C. Zhao et al. [10] also reached the same conclusion in an autoclave. As expected, an increase in pressure is advantageous to a reaction of volume contraction such as low-temperature MeOH synthesis. The pressure inside the bubble reactor affects the partial pressure of CO or H2 and the residence time. A higher CO or H2 partial pressure results in a higher CO or H2 mass transfer from the bulk gas phase to the bulk liquid phase through the gas – liquid interface, which in turn increases CO or H2 concentration in the slurry. Residence time rises as pressure increases because of decreasing volumetric flow rate. The increase in either partial pressure or residence time leads to an increase of per-pass conversion. 4.3. Axial catalyst concentration profile in the bubble reactor
Fig. 5. CO per-pass conversion during the operation. (A) Run no. 1 and (B) Run no. 2.
Just before the shutdown, the axial catalyst concentration profile in the cylindrical section of the bubble reactor was measured by a sampling technique. Theoretically, the axial profile of solid concentration in the bubble columns, described by the sedimentation –dispersion model [13,16], decreases exponentially from the bottom to the top. The axial profile of catalyst concentration in the system was nearly uniform as shown in Fig. 7. This result comes from the small diameter of catalyst, a high density ratio of liquid to solid and a high slurry circulating velocity. The relatively uniform catalyst concentration in the slurry bubble reactor is helpful to remove the reaction heat, especially for strong exothermal reactions such as liquid-phase methanol synthesis and Fischer – Tropsch synthesis. 4.4. The composition of liquid product and slurry remaining After the shutdown, the condensed liquid product and the slurry remaining in the circulating slurry bubble reactor were analysed using the aforementioned gas chromatograph. As listed in Table 4, the composition of liquid product consisted of MeOH, MeF, and DME (dimethyl
Fig. 6. H2 per-pass conversion during the operation. (A) Run no. 1 and (B) Run no. 2.
Fig. 7. Axial catalyst concentration profile just before shutdown.
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K. Zhang et al. / Fuel 82 (2003) 233–239
Table 4 Composition of the condensed liquid product (wt%) Run no.
MeOH
MeF
DME
1 2
79.52 82.16
19.25 17.04
1.23 0.80
ether). The result showed that the selectivity of product, including MeOH and MeF, was greater than 98%. Moreover, about 5 (wt) MeF was found in the slurry remains. This value was less than in the liquid product. 4.5. Parallel experiment in a stirred reactor In order to evaluate the circulating slurry reactor, a parallel experiment was conducted in a 1-l magnetically stirred autoclave under the same operating conditions. A schematic drawing of the experimental set-up is shown in Fig. 8. After the autoclave was cleaned and dried, the desired amount of slurry was charged. The composition of slurry was the same as that used in Run no. 2 listed in Table 3. The autoclave was then vacuumed, flushed with syngas or N2, and heated to the required temperature. The syngas from a high-pressure cylinder was controlled and metered by an on-line mass flow controller and was fed into the autoclave. A back-pressure regulator was used to maintain the pressure inside the stirred reactor. Effluent gas was withdrawn from the top of the autoclave and cooled in a high pressure condenser, while non-condensable effluent gas was depressurised and then measured with a wet-test meter before it was vented. The stirrer speed was 1000 rpm to eliminate the effect of diffusion. Full details of the procedure can be found in the paper by Zhao et al. [10]. Fig. 9 compares per-pass syngas conversions in the circulating bubble reactor to the autoclave. It was surprising that there was no effluent gas from either the bubble reactor of the circulating system or the autoclave at the beginning reaction period. The reason was that the initial activity of
Fig. 9. Comparison of per-pass conversion of CO and H2 in different reactors. (A) CO conversion vs dimensionless time and (B) H2 conversion vs dimensionless time.
catalyst was so high that the system pressure decreased sharply and was lower than the designed back-pressure value. It was found that the effluent gas from the separator was strongly affected by the recycle flow rate of slurry. The unreacted syngas was dissolved into the bulk slurry in the bubble reactor under high pressure and then degassed, together with the gaseous products, from the slurry in the separator under low pressure. We adopted the recycle flow rate equal to about one-tenth of the volume of the circulating bubble reactor, which was much higher than the design
Fig. 8. A sketch of the autoclave. (1) Syngas cylinder, (2) N2 cylinder, (3) pressure reducing regulator, (4) mass flow meter, (5) water removal, (6) CO2 removal, (7) stirred reactor, (8) liquid sampling tube, (9) chilled condenser, (10) back pressure regulator, (11) vacuum pump, (12) gas chromatograph and (13) wet-test meter.
K. Zhang et al. / Fuel 82 (2003) 233–239
parameter of a commercial process (between one-150th and one-100th) because of the limitation of the slurry pump used. The per-pass conversion estimated by Method A increases with decreasing the recycle flow rate of slurry. The per-pass conversion calculated from Method B stands for the behaviour of the circulating slurry bubble reactor rather than the ability of catalyst. The reasonable description for the actual activity of catalyst system is between Method A and Method B. Method A provides the upper limitation, while Method B provides the lower limitation. Based upon Method B, the scale-up efficiency of the circulating slurry bubble reactor was about 80% of that achieved in the autoclave.
5. Conclusion and perspectives A 2T/a circulating slurry bubble reactor was developed to synthesise methanol using the catalyst system of MeONa/Cu-based powder at low-temperatures. The design strategy for scaling up this bubble reactor included a preliminary investigation on fluid dynamics in a cold model, two continuous 100-h operations in the hot installation and a parallel experiment in an autoclave. Although the equilibrium conditions favour lowtemperatures, methanol converters must be operated between 80 and 1208C to ensure the catalyst system keep in an active state. The efficiency of the scale-up can reach about 80% of the autoclave, which indicates that the circulating slurry bubble reactor is a promising choice for the future commercial process of low-temperature methanol synthesis. The catalysts were deactivated in a short period in both the circulating slurry bubble reactor and the autoclave, which leads to a serious problem in the application of the catalytic system to the commercial process. To develop a kind of catalyst system with stable activity will become the main work in the near future.
Acknowledgements This project was funded by Chinese Academy of Sciences. The authors would like to thank Liang Bai, Yunqing Hu, Zhaohui Lu, Peng Liang, Chongzhi Meng, Guoping Song, Hualian Su, Xiaoli Su, Jinfeng Wang, Shoushu Wang, Xiufeng Wang, Jianing Zhang, Lingxian
239
Zhang, Liangfu Zhao, and Yulei Zhu for their hard work during the hot experimental operation. Dr Kai Zhang would also like to acknowledge the kind hospitality of Professors. Rex Thorpe and John Davidson whilst he wrote this paper at the Department of Chemical Engineering, University of Cambridge supported by Royal Society Royal Fellowship.
References [1] Wender I. Reactions of synthesis gas. Fuel Process Technol 1996;48: 189–297. [2] Tijm PJA, Waller FJ, Brown DM. Methanol technology developments for the new millennium. Appl Catal A: Gen 2001;221:275 –82. [3] Sapienza RS, Slegeir WA, O’Hare TE, Mahajan D. US Patent 4,614, 749; 1986. [4] Liu Z, Tierney JW, Shah YT, Wender I. Methanol synthesis via methyl formate in a slurry reactor. Fuel Process Technol 1989;23: 149–67. [5] Marchionna M, Lami M, Galletti AMR. Synthesizing methanol at lower temperatures. Chem Technol 1997;27(4):27–32. [6] Liu XQ, Wu YT, Chen WK, Yu ZL. Concurrent synthesis of methanol and methyl formate catalyzed by copper-based catalysts in a slurry phase. Stud Surf Sci Catal 1998;119:557–60. [7] Marchionna M, Girolamo M, Tagliabue L, Spangler MJ, Fleisch TH. A review of low temperature methanol synthesis. Stud Surf Sci Catal 1998;119:539 –44. [8] Ohyama S. Low-temperature methanol synthesis in catalytic systems composed of nickel compounds and alkali alkoxides in liquid phases. Appl Catal A: Gen 1999;180(1/2):217–25. [9] Lee ES, Aika K. Low-temperature methanol synthesis in liquid-phase with a Raney Nickel-alkoxide system: effect of Raney Nickel pretreatment and reaction conditions. J Mol Catal A:CHEM 1999; 141(1–3):241–8. [10] Zhao YL, Bai L, Hu YQ, Zhong B, Peng SY. Catalytic performance of CuCr/CH3ONa used for low temperature methanol synthesis in slurry phase. J Nat Gas Chem 1999;8(3):181–7. [11] Li K, Jiang D. Methanol synthesis from syngas in the homogeneous system. J Mol Catal A:CHEM 1999;147(1/2):125–30. [12] Tsubaki N, Ito M, Fujimoto K. A new method of low-temperature methanol synthesis. J Catal 2001;197(1):224–7. [13] Fan LS. Gas–liquid–solid fluidization engineering. Stoneham, MA: Butterworth; 1989. [14] Dudukovic MP, Larachi F, Mills PL. Multiphase reactors—revisited. Chem Engng Sci 1999;54(13/14):1975–95. [15] Joshi JB. Computational flow modelling and design of bubble column reactors. Chem Engng Sci 2001; 56(21/22):5893–933. [16] Zhang K. Axial solid concentration distribution in tapered and cylindrical bubble columns. Chem Engng J 2002;86(3):299– 307. [17] Glicksman LR, Hyre MR, Farell PA. Dynamic similarity in fluidization. Int J Multiphase Flow 1994;20S:331 –86.
Low-temperature methanol synthesis in a circulating slurry bubble reactorq Kai Zhanga,*, Huisen Songa, Dongkai Suna, Shunfen Lib, Xiangui Yangb, Yulong Zhaoa, Zhe Huanga, Yutang Wub a
Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, People’s Republic of China Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, People’s Republic of China
b
Received 31 January 2001; revised 26 June 2002; accepted 27 June 2002; available online 8 August 2002
Abstract A circulating slurry bubble reactor was developed to synthesise methanol via methyl formate from the gas mixture of carbon monoxide and hydrogen at low temperature. The strategy for designing and scaling up the bubble reactor involved a preliminary understanding of fluid dynamics in a cold model, continuous operations under industrial conditions and a parallel experiment in an autoclave. Per-pass syngas conversion was investigated during 100-h operations. The axial profile of solid catalyst concentration was measured just before the shutdown and the composition of liquid product was analysed after the shutdown. These results show that the circulating slurry bubble column will become a potential reactor for the commercial process of low-temperature methanol synthesis after the catalyst system has been improved. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Methanol synthesis; Low-temperature; Circulating slurry bubble reactor; Methyl formate; Syngas
1. Introduction
HCOOCH3 þ 2H2 ¼ 2CH3 OH
Methanol (MeOH) is potentially a cleaner alternative fuel for the future. The gas-phase MeOH synthesis process from syngas (CO þ H2) is regarded as a technically very well-developed industrial process [1,2]. However, there are some limitations, such as high temperature (470 – 570 K for the ICI low-pressure process), low per-pass conversion of the gaseous feed (10 – 20%), and high recycle of the unconverted gas. MeOH synthesis from syngas is highly 0 exothermic ðDH298 ¼ 290:97 kJ mol21 Þ and thermodynamically favourable at lower temperatures. The development of methanol synthesis includes catalyst improvement and reactor improvement. In recent years, the methanol synthesis via methyl formate (MeF) in a liquid phase at low temperature of around 373 K has been proposed and has achieved great success [3–12]. This process consists of MeOH carbonylation to MeF and MeF hydrogenolysis to MeOH as shown below:
Net
CH3 OH þ CO ¼ HCOOCH3 * Corresponding author. Present address: Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK. Tel.: þ 44-20-7679-3839; fax: þ44-20-7383-2348. E-mail address: [email protected] (K. Zhang). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
2H2 þ CO ¼ CH3 OH
The method can efficiently synthesise MeOH because of the potential synergy of the process coupling of reaction–reaction. The slurry bubble column is a competitive reactor for the commercial process of low-temperature methanol synthesis because it has advantages such as high heat and mass transfer rates, isothermal conditions, plug-free operation, and on-line catalyst addition and withdrawal [13 – 16]. However, the difficulty in scaling-up the results from small laboratory equipment to large industrially significant unit restricts its commercial application. Often results from small-scale test units predict reactant conversion and spacetime yield which cannot be achieved when the equipment size is increased. This problem is mostly due to bed fluid dynamics [15]. Accordingly, the understanding of fluid dynamics is helpful to eliminate the impact of scale-up processes. For gas – solid systems, much research has been successfully performed to solve this problem. Glicksman et al. [17] presented a comprehensive review, which outlined the development and application of scaling laws. For gas – liquid – solid systems, the present design practice is still closer to an art than science [15]. As mentioned earlier, the process development of methanol synthesis is composed of a catalyst system
0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 2 2 1 - 1
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K. Zhang et al. / Fuel 82 (2003) 233–239
solution, xylene and Op-10. Here, Op is short for polyethylene oxide alkyl phenol. The bed fluid dynamics, including the gas velocity for complete solid suspension, flow regime, phase mixing, and bubble movement behaviour, were qualitatively investigated in gas –liquid and gas – liquid –solid systems. The results are summarised as follows.
improvement and a reactor improvement. It is disappointing that all of the research [1,3 –12] concerning catalyst systems have so far been carried out in autoclaves. Therefore, the major objective of this study is to develop a 2T=a circulating slurry bubble reactor for the concurrent synthesis of MeOH via MeF from syngas. The preliminary fluid dynamics in a cold bubble column and the parallel experiment in an autoclave have been addressed.
1. The critical gas velocity for complete catalyst suspension is very low and the axial solid concentration profile becomes more uniform with an increase in the superficial gas velocity. 2. Bubbly flow, turbulent flow, and slugging regimes occur in the order of increasing superficial gas velocity according to the upward movement of bubble swarms in the bed. 3. At low gas velocity, bubbles are small and uniform in size, which vertically rise without remarkable interaction. 4. The gas holdup depends mainly on the superficial gas velocity and the slurry concentration at the same flow regime. Generally, overall gas holdup increases with increasing superficial gas velocity and decreasing slurry concentration.
2. Preliminary experiment in a cold model
The experimental procedure and results can be found in detail from the literature [16].
Fig. 1. An experimental set-up for cold model. (A) Column, (B) distributor, (C) rotameter and (D) valve.
A rough approach to simulate fluid dynamics in hightemperature, high-pressure industrial units was carried out at atmospheric pressure and room temperature using a plexiglas column with an inner diameter of 0.05 m and a height of 2.0 m. The experimental set-up is schematically shown in Fig. 1. A three-phase system was composed of air, liquid medium, and Cu-based catalyst powder. The liquid medium was a mixture of sodium methoxide (CH3ONa)
3. Experimental 3.1. Apparatus Concurrent MeOH synthesis was carried out in a 2T=a circulating slurry bubble reactor, which is conceptually illustrated in Fig. 2. This installation, made of stainless steel,
Fig. 2. A sketch of the circulating slurry reactor. (1) Mass flow meter, (2) N2 cylinder, (3) water removal, (4) CO2 removal, (5) preheaters, (6) magnetic liquid level indicators, (7) bubble reactor, (8) separator, (9) slurry recycle pump, (10) filter, (11) water condensers, (12) chilled condenser, (13) expansion vessels, (14) product receivers and (15) wet-test meters.
K. Zhang et al. / Fuel 82 (2003) 233–239
consists mainly of a bubble reactor and a separator. The bubble reactor is divided into a cylindrical section of 45 £ 3 mm diameter with a height of 4.8 m and an enlarged section of 159 £ 6 mm diameter with a height of 1.2 m. A sintered plate is located at the bottom of the cylindrical section. Three electric heating coils outside the cylindrical section, monitored by a temperature-programmed controller, are used to maintain the temperature within ^ 5 8C inside the bubble reactor. The separator has a diameter of 159 £ 6 mm and a height of 1.64 m. A slurry pump circulates and measures the slurry from the separator bottom to above the distributor in the bubble reactor. To control the slurry volume in the system and adjust the circulating volume of the slurry, two magnetic liquid level indicators are, respectively, installed into the enlarged section of the bubble reactor and the separator. The main advantage of the circulating bubble reactor is the synergy effect of reaction– separation. 3.2. Gas –liquid –solid system MeOH carbonylation is a homogeneously catalysed reaction using alkali metal methoxides as catalyst in the liquid phase, while hydrogenolysis of MeF is carried out in both gas and liquid phases using a heterogeneous copperbased catalyst. CH3ONa was employed as carbonylation catalyst. Hydrogenolysis catalyst was a copper-based catalyst, which developed by Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences [6]. Zhao et al. [10] obtained the maximum CO conversion in xylene medium after they investigated CO conversion in an autoclave using three kinds of liquid media (xylene, liquid paraffin, and methanol). Since xylene and methanol are insoluble without strong agitation, the third liquid medium Op-10 must be added in order that xylene and methanol are mixed uniformly in the system. Therefore, the liquid medium used was a mixture containing sodium methoxide, xylene, and Op-10. The syngas with an approximate stoichiometric ratio for methanol synthesis (H2/CO ¼ 2) was used as the gaseous reactant. The experimental gas – liquid – solid system was syngas/liquid-phase medium/Cubased catalyst powders. 3.3. Procedure When syngas is fed into the bubble reactor, CO is firstly reacted with MeOH in the liquid medium through carbonisation to form MeF, and the intermediate MeF is then hydrogenised to produce MeOH. The slurry, entraining product MeOH, intermediate MeF, and unconverted syngas, enters the top of the separator from the bubble reactor via a reducing pressure valve. The slurry is then pumped back into the reactor from the bottom of the separator. Before the operation run, the trapped air in the system was purged with N2 or syngas and vented to the desired purity. Then both the liquid-phase medium and the reduced
235
Table 1 Main temperatures and pressures in the system
Bubble reactor
Cylindrical section Enlarged section
Separator
Temperature (8C)
Pressure (MPa)
80– 120 80– 100 70– 90
4.2 –4.6 0.3 –0.5
Cu-based catalyst were added into the system. The syngas with (CO þ H2) $ 99%, H2O , 380 mg/m3 and CO2 , 100 ppm from high pressure cylinders was controlled and metered by an on-line mass flow meter. It was fed into the bottom of the bubble reactor after water and CO2 was removed less than 5 and 10 ppm, respectively. We managed the feed flow rate from 0.52 to 0.56 Nm3/h. The system was pressurised at ambient temperature. Two backpressure regulators were installed to maintain the pressure in both the bubble reactor and the separator. Then the slurry pump was started at the recycle flow rate of 0.05 m3/h with a fluctuation of less than 10%. Reactant gas, recycle slurry, and the reaction section in the bubble reactor were heated up to the required temperatures by two preheaters and three electric heating coils. When the reaction section reached the desired temperature, it was taken as the start time of the operation. The main temperatures and pressures in the system are listed in Table 1. The effluent gas from the top of the separator was defoamed, cooled by water, and further cooled by ice, while the effluent gas from the top of the bubble reactor was only cooled by water. The condensed liquid was separated in the expansion vessels and was collected into product receivers. After being depressurised, the non-condensable gas from the expansion vessels was metered by an on-line wet-test meter and vented. The composition of feed gas and tail gas was analysed using a gas chromatograph with a thermal conductivity detector. The composition of liquid samples was determined by the same chromatograph but the analysis condition was different (Table 2).
4. Results and discussion Initial development of the demonstration installation was conducted in late 1990s. Two continuous 100-h runs accomplished were introduced in this investigation. Table 2 GC analysis condition For gas composition For liquid composition Column dimension Detector Separation column Column temperature Carrier gas
3 mm (diameter) £ 3000 mm (length) TCD FID ˚ PEC 20000/SH-101 silanised Molecular sieve 5 A Ambient 130 8C Ar N2
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Table 3 Composition of feeds Run no.
The composition of slurry CH3ONa solutiona (l)
1 2
3.50 4.08
Xylene (l)
Op-10 (l)
Total liquid (l)
Cu-based cat (g)
14.02 15.58
2.48 2.51
20.00 22.17
600 700
Run no.
The mean composition of syngas (vol%)
1 2
H2 68.81 68.94 a
CO 30.14 30.64
CH4 1.05 0.42
Note: solvent, methanol; CH3ONa concentration, 26.1%; NaOH concentration, 0.4%.
Table 3 provides the load of liquid and catalyst. The composition of syngas is plotted in Fig. 3 and its mean value can be found in Table 3. 4.1. Per-pass conversion The per-pass conversions of CO can be estimated by two methods as below. Method A: V C 2 V1 C1 X¼ 0 0 £ 100% V0 C0 Method B: V C 2 ðV1 C1 þ V2 C2 Þ X¼ 0 0 £ 100% V0 C0
where X, V and C are the conversion of CO, the volumetric flow rate of syngas, and the volumetric concentration of CO, respectively. Subscript 0 stands for the feed gas, 1 for the effluent gas from the top of the bubble reactor and 2 for the effluent gas from the top of the separator. The calculation methods are abstractly drawn in Fig. 4. Figs. 5 and 6 indicate the per-pass conversions of CO and H2 at different reaction periods. As shown in Fig. 5, CO conversion keeps almost constant within 45 h in Run no. 1 and 50 h in Run no. 2, then it decreases gradually with increasing reaction time. These results show that the satiability of the catalyst system is poor although its initial activity is quite high. 4.2. Effects of temperature and pressure The effects of temperature and pressure on per-pass conversion were tested during the steady operation. Per-pass syngas conversion increased with an increase in temperature in the range from 80 to 120 8C. The temperature affects reaction rate, syngas solubility, and solution vapour pressure. According to thermodynamic theory, a decrease in temperature results in the equilibrium composition towards the formation of MeF for MeOH carbonylation reaction. However, the hydrogenolysis rate is too slow at low temperature. From the viewpoint of kinetics, an increase in temperature favours the rate of a chemical reaction. Therefore, it is a consequence of two competing effects
Fig. 3. The composition of syngas during operation. (A) Run no. 1 and (B) Run no. 2.
Fig. 4. A schematic diagram for the calculation.
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stated above that syngas conversion increases with an increase of temperature in the range 80– 120 8C. Zhao et al. [10] also reached the same conclusion in an autoclave. As expected, an increase in pressure is advantageous to a reaction of volume contraction such as low-temperature MeOH synthesis. The pressure inside the bubble reactor affects the partial pressure of CO or H2 and the residence time. A higher CO or H2 partial pressure results in a higher CO or H2 mass transfer from the bulk gas phase to the bulk liquid phase through the gas – liquid interface, which in turn increases CO or H2 concentration in the slurry. Residence time rises as pressure increases because of decreasing volumetric flow rate. The increase in either partial pressure or residence time leads to an increase of per-pass conversion. 4.3. Axial catalyst concentration profile in the bubble reactor
Fig. 5. CO per-pass conversion during the operation. (A) Run no. 1 and (B) Run no. 2.
Just before the shutdown, the axial catalyst concentration profile in the cylindrical section of the bubble reactor was measured by a sampling technique. Theoretically, the axial profile of solid concentration in the bubble columns, described by the sedimentation –dispersion model [13,16], decreases exponentially from the bottom to the top. The axial profile of catalyst concentration in the system was nearly uniform as shown in Fig. 7. This result comes from the small diameter of catalyst, a high density ratio of liquid to solid and a high slurry circulating velocity. The relatively uniform catalyst concentration in the slurry bubble reactor is helpful to remove the reaction heat, especially for strong exothermal reactions such as liquid-phase methanol synthesis and Fischer – Tropsch synthesis. 4.4. The composition of liquid product and slurry remaining After the shutdown, the condensed liquid product and the slurry remaining in the circulating slurry bubble reactor were analysed using the aforementioned gas chromatograph. As listed in Table 4, the composition of liquid product consisted of MeOH, MeF, and DME (dimethyl
Fig. 6. H2 per-pass conversion during the operation. (A) Run no. 1 and (B) Run no. 2.
Fig. 7. Axial catalyst concentration profile just before shutdown.
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Table 4 Composition of the condensed liquid product (wt%) Run no.
MeOH
MeF
DME
1 2
79.52 82.16
19.25 17.04
1.23 0.80
ether). The result showed that the selectivity of product, including MeOH and MeF, was greater than 98%. Moreover, about 5 (wt) MeF was found in the slurry remains. This value was less than in the liquid product. 4.5. Parallel experiment in a stirred reactor In order to evaluate the circulating slurry reactor, a parallel experiment was conducted in a 1-l magnetically stirred autoclave under the same operating conditions. A schematic drawing of the experimental set-up is shown in Fig. 8. After the autoclave was cleaned and dried, the desired amount of slurry was charged. The composition of slurry was the same as that used in Run no. 2 listed in Table 3. The autoclave was then vacuumed, flushed with syngas or N2, and heated to the required temperature. The syngas from a high-pressure cylinder was controlled and metered by an on-line mass flow controller and was fed into the autoclave. A back-pressure regulator was used to maintain the pressure inside the stirred reactor. Effluent gas was withdrawn from the top of the autoclave and cooled in a high pressure condenser, while non-condensable effluent gas was depressurised and then measured with a wet-test meter before it was vented. The stirrer speed was 1000 rpm to eliminate the effect of diffusion. Full details of the procedure can be found in the paper by Zhao et al. [10]. Fig. 9 compares per-pass syngas conversions in the circulating bubble reactor to the autoclave. It was surprising that there was no effluent gas from either the bubble reactor of the circulating system or the autoclave at the beginning reaction period. The reason was that the initial activity of
Fig. 9. Comparison of per-pass conversion of CO and H2 in different reactors. (A) CO conversion vs dimensionless time and (B) H2 conversion vs dimensionless time.
catalyst was so high that the system pressure decreased sharply and was lower than the designed back-pressure value. It was found that the effluent gas from the separator was strongly affected by the recycle flow rate of slurry. The unreacted syngas was dissolved into the bulk slurry in the bubble reactor under high pressure and then degassed, together with the gaseous products, from the slurry in the separator under low pressure. We adopted the recycle flow rate equal to about one-tenth of the volume of the circulating bubble reactor, which was much higher than the design
Fig. 8. A sketch of the autoclave. (1) Syngas cylinder, (2) N2 cylinder, (3) pressure reducing regulator, (4) mass flow meter, (5) water removal, (6) CO2 removal, (7) stirred reactor, (8) liquid sampling tube, (9) chilled condenser, (10) back pressure regulator, (11) vacuum pump, (12) gas chromatograph and (13) wet-test meter.
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parameter of a commercial process (between one-150th and one-100th) because of the limitation of the slurry pump used. The per-pass conversion estimated by Method A increases with decreasing the recycle flow rate of slurry. The per-pass conversion calculated from Method B stands for the behaviour of the circulating slurry bubble reactor rather than the ability of catalyst. The reasonable description for the actual activity of catalyst system is between Method A and Method B. Method A provides the upper limitation, while Method B provides the lower limitation. Based upon Method B, the scale-up efficiency of the circulating slurry bubble reactor was about 80% of that achieved in the autoclave.
5. Conclusion and perspectives A 2T/a circulating slurry bubble reactor was developed to synthesise methanol using the catalyst system of MeONa/Cu-based powder at low-temperatures. The design strategy for scaling up this bubble reactor included a preliminary investigation on fluid dynamics in a cold model, two continuous 100-h operations in the hot installation and a parallel experiment in an autoclave. Although the equilibrium conditions favour lowtemperatures, methanol converters must be operated between 80 and 1208C to ensure the catalyst system keep in an active state. The efficiency of the scale-up can reach about 80% of the autoclave, which indicates that the circulating slurry bubble reactor is a promising choice for the future commercial process of low-temperature methanol synthesis. The catalysts were deactivated in a short period in both the circulating slurry bubble reactor and the autoclave, which leads to a serious problem in the application of the catalytic system to the commercial process. To develop a kind of catalyst system with stable activity will become the main work in the near future.
Acknowledgements This project was funded by Chinese Academy of Sciences. The authors would like to thank Liang Bai, Yunqing Hu, Zhaohui Lu, Peng Liang, Chongzhi Meng, Guoping Song, Hualian Su, Xiaoli Su, Jinfeng Wang, Shoushu Wang, Xiufeng Wang, Jianing Zhang, Lingxian
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Zhang, Liangfu Zhao, and Yulei Zhu for their hard work during the hot experimental operation. Dr Kai Zhang would also like to acknowledge the kind hospitality of Professors. Rex Thorpe and John Davidson whilst he wrote this paper at the Department of Chemical Engineering, University of Cambridge supported by Royal Society Royal Fellowship.
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