Research Note pubs.acs.org/IECR
Effects of Various Factors on Biogas Purification and Nano-CaCO3 Synthesis in a Membrane Reactor Jun Zhou,†,‡,§ Xun Cao,†,‡,§ Xiao-yu Yong,†,‡ Shu-ya Wang,†,‡ Xi Liu,†,‡ Yi-lu Chen,† Tao Zheng,*,†,‡ and Ping-kai Ouyang† †
College of Biotechnology and Pharmaceutical Engineering, and ‡Bioenergy Research Institute, Nanjing University of Technology, Nanjing 211816, China ABSTRACT: Reducing CO2 content will significantly improve biogas quality. In this study, a CO2/Ca(OH)2 precipitation reaction was used to purify biogas and prepare CaCO3 nanoparticles simultaneously in a membrane reactor. Results showed that the rate of biogas purification was greater than 99%, and CaCO3 particles were successfully formed in the membrane reactor, which was detected with an average particle size between 300 and 550 nm with chemical additives. Further experiments revealed that the flow rate of biogas, temperature, and different chemical additives showed negligible effects on the biogas purification efficiency, while CaCO3 particle size decreased with the increasing flow rate and increased with the increasing temperature. The average particle sizes of the CaCO3 particles were 334, 537, 437, and 867 nm, corresponding to the addition of EDTA, ZnSO4, Na6O18P6, and the control, respectively. This method was proven to be an effective technique of simultaneously purifying biogas and synthesizing nano-CaCO3.
1. INTRODUCTION The development of biogas energy, which is considered as an important energy resource for the future,1 is a fitting option to solve global environmental and energy issues in a sustainable manner.2−4 Typical biogas is known to contain 50% to 65% methane (CH4), 35% to 50% carbon dioxide (CO2), moisture, and traces of hydrogen sulfide (H2S). However the presence of CO2 not only reduces the heat value of biogas but also corrodes gas production equipment.5−7 Therefore, biogas must meet pipeline requirements to increase its commercial value (CO2 concentration lower than 2%).8,9 Nowadays, several technologies such as high pressure water scrubbing, absorption,10 membrane separation,6 and pressure swing adsorption11 are used to remove CO2. However, these processes are expensive and can lead to high CH4 losses.12 Moreover, these methods cannot utilize CO2, which is released to the atmosphere after separation. As a result of the greenhouse effect, climate change induced by global warming gives rise to an excess of energy at the earth’s surface. Therefore, a new technology that can purify biogas to obtain highly purified methane and recycle CO2 with high added value simultaneously is needed urgently. Nanometer-sized CaCO3 is widely used among all nanoparticles because it can be used as a well pigment or functional filler in plastics, rubber, paper, paint, and so on.13,14 Furthermore, the CO2/Ca(OH)2 precipitation reaction can be utilized to purify biogas and prepare CaCO3 particles in numerous special morphologies, such as thin films,15 flower-like nanostructures,16 hollow microsphere shells,17 microscopic spherules,18 tubular structure agglomerates,19 twinned aragonite, spherical crystals,20 and so on. Thus far, several approaches, including precipitation or wet carbonation21 and microreactor and high-gravity reactive precipitation,22 have been developed to remove CO2 from virgin gas and generate nanometer-sized CaCO3 particles, among which precipitation is one of the best methods to meet industrial demands and reduce CO2 emission © 2014 American Chemical Society
because of the simplicity of its operation and the easy control of process variables.23 However, the study of the dispersion, mixing, and mass transfer problems in the synthesis of nanometer CaCO3 and biogas purification are still insufficient.24 Many researches have focused on the special characteristics of nanoparticles such as surface effect and quantum effect.13 Particle characteristics, such as morphology and particle size, can be adjusted by controlling the factors in the nano-CaCO3 synthesis.25 However, little information is available wherein the effect of biogas purification efficiency and nano-CaCO3 synthesis effect are simultaneously investigated. In the present study, membranous equipment was designed and utilized for simultaneous biogas purification and nano-CaCO3 synthesis. The operating factors (biogas flow rate, temperature, and chemical additives) of the reactor were set to investigate the biogas purification effect, as well as the particle size and morphological character of the resulting nano-CaCO3 under different operating conditions.
2. MATERIALS AND METHODS 2.1. Experimental Equipment and the Biogas. The process flow diagram of the reactor is shown in Figure 1. The biogas used in this experiment was obtained from the biogas plant of Nanjing University of Technology after desulfuration and dehydration. The biogas was composed of 48.8% CO2 and 51.2% CH4. Approximately 120 L of the calcium hydroxide suspension was initially placed in the reactor. The biogas was purified in the membranous reactor, where the biogas was continuously fed from the top of the column, and the Ca(OH)2 Received: Revised: Accepted: Published: 1702
October 16, 2013 January 7, 2014 January 13, 2014 January 13, 2014 dx.doi.org/10.1021/ie4034939 | Ind. Eng. Chem. Res. 2014, 53, 1702−1706
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Research Note
that all the Ca(OH)2 has been totally carbonated. The typical pH behavior during carbonation is shown in Figure 2.
Figure 1. The process flow diagram of the reactor for biogas purification. (1, bin; 2, flange; 3, liquid pump; 4, fluid flowmeter; 5, pressure gauge; 6, membranous reactor; 7, dehydrating tower; 8, desulfurizing tower; 9, gasholder; 10, gas flowmeter; 11, liquid−gas separator).
Figure 2. The behavior of pH value during carbonation (temperature, 22 °C; the flow rate of biogas, 40 L/min; additive, EDTA).
2.3. Experimental Parameters. The operating factors on the particle size of precipitated CaCO3 formed in the wet carbonation process include the biogas bubble size,24 CO2 concentration,25 biogas flow rate,26 reaction temperature,27 and chemical additives.28 In this experiment, biogas bubble size was controlled at 500 nm through the membranous tubes, and CO2 concentration was 48.8%. The parameters investigated in the present study included biogas flow rate, temperature, and chemical additives. The biogas flow rate was controlled by a gas flowmeter. The temperature of the solution was controlled by a condensate system and hot water. The additives used were EDTA, zinc sulfate (ZnSO4), and sodium hexametaphosphate (Na6O18P6). The parameters and their values are shown in Table 1.
suspension was fed from the left margin of column. The membranous reactor contained seven membranous tubes that were 1.16 m long, and the pore diameter on the surface of each membranous tube was 500 nm. Biogas, which entered each membranous tube through the pores found on the surface, was in the form of very small bubbles under the external pressure. After the reaction between CO2 and calcium hydroxide suspension, CH4 was separated from the mixture of liquid and gas as it flowed through the liquid−gas separator. The Ca(OH)2 suspension was circulated between a membranous reactor and a liquid vessel through the pump. The biogas flow rate was regulated using a gas flowmeter. The CO 2 concentrations in the biogas entering and leaving the column were constantly monitored by a gas chromatograph. The pH and temperature of the reactants were measured online by a computer. The concentrations of CH4 were analyzed using a gas chromatograph (HP7890) equipped with a thermal conductivity detector and a PLOT-Q column (30 m × 0.53 mm × 15 μm), and helium was used as the carrier gas. All measurements were repeated three times, and data obtained were analyzed statistically using the SPSS 19.0 program. 2.2. CO2 Capture and CaCO3 Synthesis. First, 5 kg of calcium oxide was added to 40 kg of water at 70 °C, after which hydration was immediately carried out in the solution with stirring for 15 min. The suspension was filtered through a 325 μm sieve after standing for 24 h. The filtering liquid medium was then transferred to the reactor. CO2 dissolved in solution provided the carbonate ions that reacted with Ca2+ ions to form CaCO3. Then CaCO3 will precipitate out because of its lower solubility in water (∼0.0012 g/100 g of water) than Ca(OH)2. At the initial stage of the reaction, the pH level of the solution remains alkaline for a long time as the consumed Ca2+ ions are replenished by suspended Ca(OH)2. As the reaction continues, Ca(OH)2 becomes depleted and the concentration of Ca2+ ions could no longer be maintained at its solubility limit. Meanwhile, as CO2 is continuously dissolved in the solution, the accumulation of H+ ions leads to the acidification of the solution. Eventually, the pH stabilizes at about 6.5, indicating
Table 1. Experimental Conditions parameters
values
biogas flow rate temperature additive
10 L/min, 20 L/min, 30 L/min, 40 L/min, 50 L/min 10 °C, 16 °C, 22 °C, 40 °C EDTA 0.1 wt %, ZnSO4 0.1 wt %, Na6O18P6 0.1 wt %
2.4. Characterization of CaCO3 Samples. The crystal structure of the prepared precipitates was examined by X-ray powder diffraction (XRD, Rigaku Rotaflex D/max, Japan) using CuKα radiation (40 kV, 40 mA), and transmission electron microscopy (TEM, JEM-200CX, Japan) with the 200 nm scale. The XRD pattern can indicate the purity and the crystal form of CaCO3 samples, and TEM images can show the morphology and structure of the CaCO3 samples. The average diameter of the CaCO3 particles was determined by measuring at least 300 particles in the image using an image analysis software.
3. RESULTS AND DISCUSSION 3.1. Effect of Different Operating Conditions on Biogas Purification. 3.1.1. Effect of Different Flow Rate on Biogas Purification. For the capture of CO2, the effects of the operating conditions on the purification level were investigated with the experimental biogas contained 51.2% CH4 and 48.8% CO2 after desulfuration and dehydration. Table 1703
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rate and then reduce the reaction time. Therefore, EDTA is the most suitable chemical additive for biogas purification and was selected in the following experiments. The reactor was able to simultaneously remove large amounts of CO2 (over 99% removal efficiency), resulting in CH4-enriched biogas. However, the process was time dependent as the gas concentrations were found to decrease with time. Initially, the Ca(OH)2 suspension was reacted rapidly and almost completely absorbed CO2. The gas concentration at the outlet stream were considerably small compared with the original values. As the absorption process proceeded with time, CO 2 was continuously accumulated in the Ca(OH) 2 suspension. As more CO2 was generated, CO2 started to evolve in the outlet stream. The end of each run was determined when the reactant became completely saturated or neutralized (pH < 7). Attempts were made to generate high-quality CH4-enriched gas and nano-CaCO3. CH4 was collected until its content was below 97%, during which the outlet stream was placed in the gas storage holder for the next purification. 3.2. Effect of Different Operating Conditions on CaCO3 Particle Size. 3.2.1. Effect of Biogas Flow Rate on CaCO3 Particle Size. For the controllable synthesis of CaCO3 particles, the effects of operating conditions on the particle size were investigated. Figure 3 showed the particle size of the
2 showed the composition of the biogas downstream of the experimental setup after 20 min at different flow rates, which Table 2. Compositions of Biogas after Treatment at Different Flow Rates (L/min)a CH4 (%) CO2 (%) a
10 L/min
20 L/min
30 L/min
40 L/min
50 L/min
100 0
99.8 0.2
99.7 0.3
99.7 0.3
87.2 12.8
Temperature, 22 °C; chemical additive, EDTA.
showed the rate of purification was maintained above 99% when the flow rate of the biogas ranged from 10 L/min to 40 L/min, but the rate of purification was reduced to 87.2% when the flow rate reached 50 L/min. It could be calculated that the reaction time for biogas purification could be reduced at high flow rates. Thus, the handing capacity of 40 L/min can meet the requirement at the present stage in this membrane reactor. 3.1.2. Effect of Different Temperature on Biogas Purification. Table 3 shows the biogas purification rate at Table 3. Compositions of Biogas after Treatment at Different Temperature (°C)a CH4 (%) CO2 (%) a
10 °C
16 °C
22 °C
40 °C
99.8 0.2
99.6 0.4
99.7 0.3
99.7 0.3
Flow rate, 40 L/min; chemical additive, EDTA.
different temperature of the Ca(OH) 2 suspension. A condensate system and hot water were employed to control the temperature of the Ca(OH)2 solution. There was no significant difference between the rate of purification, which was maintained above 99% when the temperature changed from 10 to 40 °C. Therefore, a room temperature of 22 °C was the most suitable for biogas purification in consideration of the energy consumption and economy. 3.1.3. Effect of Different Chemical Additive on Biogas Purification. Several studies have found that chemical additives could accelerate carbonation rate, thereby reducing the reaction time for biogas purification.25,29,32 Table 4 shows the influence Table 4. Compositions of Biogas after Treatment with Different Chemical Additivesa CH4 (%) CO2 (%) a
EDTA
ZnSO4
Na6O18P6
control
99.7 0.3
99.6 0.4
99.7 0.3
99.5 0.5
Figure 3. Effect of biogas flow rate on particle size CaCO3 particles (temperature, 22 °C; additive, EDTA).
CaCO3 particles decreased with the increase of biogas flow rate from 10 to 40 L/min. In addition to the increased amount of CO2, the mass transfer rate was accelerated by increasing the concentration of the two phases with the increase in biogas flow rate.24 However, the increased flow rate caused an increase in production, thereby reducing the time of purification.30 The reduced time would limit the further growth of the CaCO3 particles. The handling capacity of 40 L/min was chosen because it can meet the requirements at the present stage. 3.2.2. Effect of Temperature on Particle Size. Temperature is known to be one of the most important factors that affect the reaction. Several properties will be changed with the variation of temperature, especially the diffusion of CO2 and the solubility of Ca(OH)2 in this process.24 Figure 4 shows the influence of temperature on the average size of the CaCO3 particles. The diffusion of CO2 is increased with increasing
Temperature, 22 °C; flow rate, 40 L/min.
of the different chemical additives (EDTA, ZnSO4, and Na6O18P6) on the composition of biogas downstream of the experimental setup after approximately 20 min, which were found to have little influence on the rate of purification (all over 99%). However, the reaction times of the four experimental groups (EDTA, ZnSO4, Na6O18P6, control) were 220, 320, 310, and 450 min, respectively. The addition of EDTA significantly accelerated the carbonation rate and reduced the reaction time. This result was in accord with previous literature reports,21,25,32 which showed that the additives could enhance the nucleation of CaCO3 prticles, and then reduce the reaction time. Xiang et al.29 reported that EDTA had an obvious increase of soluble Ca2+ in the solution, thus helping to accelerate the carbonation 1704
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Figure 4. Effect of temperature on particle size CaCO3 particles (flow rate of biogas, 40 L/min; additive, EDTA).
Figure 5. XRD pattern of calcium carbonate crystals (temperature, 22 °C; flow rate of biogas, 40 L/min; additive, EDTA).
temperature. On the contrary, the solubility of Ca(OH)2 is decreased as the temperature is increased, which will reduce the supersaturation for the low concentration of Ca2+. The particle size changed slightly when the temperature was increased but maintained below 22 °C (Figure 4). Considering that a temperature of 10 °C or lower favors the formation of smaller CaCO3 particles, using a condenser system to reduce the reaction temperature will consume large amounts of energy. 3.2.3. Effect of Chemical Additives on Particle Size. Table 5 showed the effect of different chemical additives on the average Table 5. The Influence of the Different Chemical Additives on Average Sizes of CaCO3 Particlesa average sizes (nm) a
EDTA
ZnSO4
Na6O18P6
control
334
537
431
867
Figure 6. TEM morphology observation of PCC particles (temperature, 22 °C; flow rate of biogas, 40 L/min; additive, EDTA).
Temperature, 22 °C; the flow rate of biogas, 40 L/min.
size of CaCO3 particles. The resulting average particle sizes of the CaCO3 particles were 334, 537, 437, and 867 nm under the addition of the EDTA, ZnSO4, Na6O18P6, and the control, respectively. The chemical additives helped to accelerate the carbonation rate, and the resulted particle sizes were considerably smaller than the control. A faster carbonation rate was deduced to be favorable for the formation of superfine CaCO3 particles.29 A similar result was also found in previous literature reports.21,25,32 Na6O18P6, ZnSO4, and EDTA were usually used as additives in the synthesis of nanometer CaCO3 particles.21,25,32 These additives created an obvious increase of soluble Ca2+ in the solution, which helped to enhance the nucleation of CaCO3 particles and then reduce the size of the particles.29 3.3. XRD and TEM Characterization. 3.3.1. XRD Results of the CaCO3 Particles. Figure 5 shows the characteristic diffraction peaks attributed to calcite at (2θ) 29.5, 47.3, and 48.6, which agree well with the literature data for calcite.31 3.3.2. TEM Results for the CaCO3 Particles. TEM was performed to analyze the detailed morphology and structure of the CaCO3 particles formed at 22 °C at a biogas flow rate of 40 L/min (Figure 6). The results also confirmed that the crystals were composed of large amounts of the nanosized particles. A special image analysis software was used to measure the sizes of the particles, which ranged from 300 to 550 nm.
The results showed that the proposed method could efficiently remove CO 2 in biogas and produce CaCO 3 nanoparticles during the purification. The process will decrease the cost of biogas purification and simultaneously produce a high-value coproduct.
4. CONCLUSION An efficient CO2/Ca(OH)2 precipitation reaction method was successfully employed to purify biogas and prepare CaCO3 nanoparticles simultaneously in a membrane reactor. It was also found that the flow rate of biogas, temperature, and different chemical additives showed negligible effects on the biogas purification efficiency. The rate of biogas purification was all greater than 99% in different conditions, while, CaCO3 particle size decreased with the increasing of the biogas flow rate and increased with increasing temperature. The average particle sizes of the CaCO3 particles were 334, 537, 437, and 867 nm, corresponding to the addition of EDTA, ZnSO4, Na6O18P6, and the control, respectively. From all these results, it could be concluded that the membrane reactor method was an appropriate alternative to traditional methods for the biogas purification and nano-CaCO3 synthesis. 1705
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-25-58139929. Fax: +86-25-58139929. E-mail:
[email protected]. Address: P.O. Box 77, Nanjing University of Technology, No. 30 South Puzhu Road, Nanjing 211816, P. R. China. Author Contributions §
Both J.Z. and X.C. contributed equally to this paper.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Basic Research Program of China (973 program, 2013CB733504), the National Natural Science Foundation of China (21307058), the Jiangsu Province Science Foundation for Youths (BK20130931, BK20130932), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (13KJB610006).
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