Microsoft Word - CET--006.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 59, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Zhuo Yang, Junjie Ba, Jing Pan Copyright © 2017, AIDIC Servizi S.r.l. ISBN 978-88-95608- 49-5; ISSN 2283-9216 The Effect of Different Mineral Materials on Preparation of CH4 from Sodium Acetate Keke Chen College of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang 453003, China lisa820928@126.com As a renewable energy source, methane can not only improve ecological benefits and save energy, but also bring good economic benefits. In order to enhance the efficiency of methane preparation, the author used sodium acetate as the base material to study the effect of various mineral materials on microbial methane preparation. The results showed that mineral materials with good conductivity play a positive role in anaerobic microbial methane preparation, which is of referential meanings to highly-effective CH4 preparation. 1. Introduction With the rapid development of various industries and the economy, the resultant problems of environmental pollution and energy shortage are increasingly serious. As a countermeasure, renewable energy sources are strongly promoted. Among them, CH4 acts as a kind of renewable energy that plays an important role (Bardi et al., 2016). The main components of biogas are methane and hydrogen, and the former one accounts for about 60% to 70%. Since biogas can replace natural gas as the fuel needed for people's life, biogas promotion can not only improve ecological benefits and save energy, but also bring good economic benefits (Iorio, 2016; Carotenuto et al., 2016; Rongwang et al., 2017; Hoo et al., 2017). CH4 can be prepared by the metabolism of known anaerobic microorganisms of over 200 species (Ettwig et al., 2008). These microorganisms survive in an anaerobic environment and eventually produce CH4 by decomposition of organic matter (Wang et al., 2009). In synthesizing CH4, acetic acid decomposition is one of the main accesses to methane (Murray and Berg, 2010). At present, microbial methane production has been applied to real life, albeit low in production efficiency and utilization rate (Thauer and Shima, 2008). Documents (Leloup Et al., 2007; Thauer, 2010) show that the efficiency of microbial methane production can be improved with such additives as sodium bicarbonate (Ağdağ and Sponza, 2005), iron and other trace elements (Zhang and Jahng, 2012), goethite (Tan et al., 2015), and enzymes (Quiñones Et al., 2012). In order to enhance the efficiency of methane preparation, the author used sodium acetate as the base material to study the effect of various mineral materials on microbial methane preparation and to analyse its working principle. 2. Experimental design 2.1 Experimental materials The anaerobic microbial fermentation experiment was carried out in a 250 ml volumetric flask in which there is sludge of a concentration of 0.165 gVS/L, sodium acetate of a concentration of 1.65 g/L, and 1 mL/L vitamin solution. The PH value is controlled at 7.0. After exhausting air from the flask, we sealed the flask and placed it into a 35C incubator. The experiment had 1 control group and 7 test groups with mineral materials of goethite, hematite, magnetite, ferrihydrite, dolomite, activated carbon, and graphite, respectively, at the ratio of 1: 1. Every test was repeated for 3 times. The related properties of each mineral material are shown in Table 1: DOI: 10.3303/CET1759002 Please cite this article as: Keke Chen, 2017, The effect of different mineral materials on preparation of ch4 from sodium acetate, Chemical Engineering Transactions, 59, 7-12 DOI:10.3303/CET1759002 7 mailto:lisa820928@126.com Table 1: The property of mineral materials Mineral Material Goethite Hematite Magnetite Ferrihy-drite Dolomite Activated carbon Graphite Specific surface area (m2/g) 14.5 92.6 19.921 198.77 5.851 20.368 6.54 Resistivity (Ω·cm) 103-106 10-3-102 10-2-10-1 1011-1014 10-2-101 10-6-10-2 Density (g/cm-3) 5.02~5.31 5.15~5.18 3.00~3.20 1.80 2.26 Grain size 60~100 60~100 60~100 60~100 200 8~20 100 2.2 Experimental measurement Gas chromatography FID was used to gauge the contents of methane and acetic acid. Here are some parameters: 25m0.25mm capillary columns, detector temperature 300 ° C, vaporization temperature 250 C, nitrogen gas as the carrier gas. The total carbon, total organic carbon, total inorganic carbon, carbon dioxide gas concentration, and carbon content in the solid were measured by a JenaC/N 2100 TOC analyser. The modified Gompertz equation was used to simulate methane preparation in this paper and has been widely applied to the simulation of the production of similar products to methane (Adam et al., 2011; Roy et al., 2012). The equation is: 103-106  max max maxexp exp[ / ( ) 1]P P R e P t    (1) Where P is the amount of methane prepared, Pmax is the maximum methane produced during the anaerobic reaction, Rmax is the highest rate of methane production in the anaerobic reaction process,  is the time lag in reaction, e is a constant equal to 2.71828, t is the accumulated time of anaerobic reaction. 3. Experimental results and analysis 3.1 Methane and carbon dioxide production The contents of methane and carbon dioxide were measured in each group in a daily basis, and the data results are shown in Figure 1 and Figure 2: -5 0 5 10 15 20 25 30 35 0 5 10 15 20 25 T o ta l c o n te n t o f C H 4 /m M Time/d Graphite Activated Carbon Magnetite Hematite Goethite Dolomite Ferrihydrite Blank Control Figure 1: The change of total content of CH4 8 -5 0 5 10 15 20 25 30 35 0.0 0.2 0.4 0.6 0.8 1.0 1.2 T o ta l c o n te n t o f C O 2 / m M Time/d Graphite Activated Carbon Magnetite Hematite Goethite Dolomite Ferrihydrite Blank Control Figure 2: The change of total content of CO2 It can be seen from Figure 1 that the CH4 content in each group increases over time. In terms of the CH4 increment, the graphite group ranks the first, followed by the activated carbon group and the magnetite group. The CH4 content in these groups is much higher than that of the control group. The CH4 contents in the hematite group and the goethite group are slightly higher than that of the control group. The CH4 contents in the dolomite group and the ferrihydrite group are lower than that of the control group, reflecting the inhibitory effect on CH4 preparation. If we compare Figure 1 with Figure 2, we will find that CO2 content is much smaller than CH4 content, and the reason is that some of the CO2 gas dissolves in water. Through comparison, the content of CO2 produced in the dolomite group and the hydrothermal group is also smaller than that of the control group due to the same reason. For further analysis, we fitted the CH4 production data in each group according to the formula (1), and the fitting data are listed as follows: Table 2: Fitting data of Gompertz equation Mineral Material Pmax Rmax  R2 Graphite 24.69352 2.09251 6.14872 0.98389 Activated Carbon 23.47294 2.00017 6.69032 0.99252 Magnetite 20.89245 1.78895 6.41096 0.98973 Hematite 21.06163 1.31874 5.86149 0.98721 Goethite 20.89371 1.40973 6.07730 0.98913 Dolomite 19.52407 1.48037 8.99358 0.98995 Ferrihydrite 17.73129 1.10258 11.35999 0.97986 Blank Control 20.09785 1.51204 8.97783 0.99504 It can be seen from Table 2 that the experimental data of each group are well fitted and the fitting coefficients are above 0.97. The maximum CH4 content Pmax and the maximum production rate Rmax are the largest in the graphite group but the smallest in the ferrihydrite group. Considering the mineral property parameters shown in Figure 1 and Table 1, that phenomenon is linked to the conductivity of mineral materials. The lower the resistivity is, the better the conductivity is, and the higher value P and R have. Graphite is such an example. Minerals with stronger conductivity are better at electron storage in the process of anaerobic fermentation reaction, and thus stimulate electron transfer between micro-organisms. In terms of the time lag data listed in Table 2, the  value of minerals with better conductivity is around 6 days, and the  value in the control group is about 9 days, indicating that minerals with better conductivity can have CH4 production accelerated to reach the maximum methane amount as fast as possible. 9 3.2 PH value change in each experimental group The time-varying change of PH value in each group is detected and listed in Figure 3: -5 0 5 10 15 20 25 30 35 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6 P H Time/d Graphite Activated Carbon Magnetite Hematite Goethite Dolomite Ferrihydrite Blank Control Figure 3: The change of PH value in each group As can be seen from the curves of PH value change, the change trends are similar to each other. Specifically speaking, the PH value increases fast at the beginning of the reaction, slows down and remains basically unchanged at the 15th day. The increment in PH value for minerals with better conductivity is higher than that in the control group. For example, the PH value reaches 8.4 in the graphite group and 8.35 in the activated carbon group, while the PH value in the control group is 8.3; correspondingly, the PH value of the ferrihydrite group with poor conductivity is significantly lower than that of the control group (which is 8.25). With respect to the overall trend, the PH value increases over time, which means the alkalinity of the solution increases. Accordingly, CO2 solubility is enlarged, and the CO2 gas content is much smaller than methane content. The reason why the solution is more alkaline is that: on the one hand, due to the existence of microbes, the reaction in formula 2 happens in the solution, leading to the production of HCO - 3; on the other hand, CO2 gas dissolves in the solution, reacts and generates HCO - 3. The production of HCO - 3 increases the PH value of the solution. 3 2 4 3 CH COO H O CH HCO      (2) 3.3 The concentration changes of sodium acetate With microorganisms, sodium acetate is decomposed into methane and carbon dioxide. As the reaction prolongs, sodium acetate is gradually consumed, and its concentration change is shown in Figure 4: 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 40 C o n c e n tr a ti o n o f a c e ti c a c id ( C -m M ) Time/d Graphite Activated Carbon Magnetite Hematite Goethite Dolomite Ferrihydrite Blank Control Figure 4: The change of acetic acid concentration 10 It can be seen from Figure 4 that the concentration of acetic acid is decreasing and approaches zero after a month, indicating that all of the microorganisms have been decomposed. The rate of concentration incline is high in the first 15 days and then lowers down, which accords with the change law of methane content in Figure 1. In the initial stage, microbes have high activity and a high metabolic rate, and thus the acetic acid consumption is fast; what is more, minerals of good conductivity can enhance microbial activities because they provide surfaces for microbial growth. Therefore, the decrement in acetic acid concentration in groups with minerals of good conductivity is much higher than that in the control group, which also applies to methane content change. 3.4 Carbon balance in each group The carbon distribution before and after the anaerobic fermentation reaction in each group is shown in Table 3: Table 3: Distribution of carbon before and after reaction Mineral Material At the beginning (C-mM) Carbon in gas phase(C-mM) Carbon in liquid phase (C-mM) solid carbon (C-mM) Recovery (%) Proporti- on of CH4 (%) Organic Inorga- nic CH4 CO2 Orga- nic Inorga- nic Graphite 56.2 0 24.11 1.05 2.61 21.5 2.34 91.83 42.90 Activated Carbon 56.38 0 22.51 0.92 3.29 22.11 3.37 92.59 39.93 Magnetite 55.04 0 19.32 0.88 2.03 23.49 2.36 87.04 35.10 Hematite 55.61 0 19.03 0.95 2.85 22.92 2.49 86.75 34.22 Goethite 56.09 0 18.54 0.85 2.32 23.58 2.43 85.08 33.05 Dolomite 56.15 0 17.87 0.93 2.5 24.39 2.56 85.93 31.83 Ferrihydrite 55.57 0 15.28 0.85 2.41 24.51 2.54 82.04 27.50 Blank Control 55.78 0 17.75 0.96 2.48 23.62 2.2 84.28 31.82 Table 3 is a collection of carbons contained in the gas, liquid and solids in each experimental group. After the reaction is finished, the total carbon content in each group is found to be below 100%, which is inevitable as there are natural carbon losses in sampling, measurement and other operations. As can be seen from table 3, the inorganic carbon content in the solution is high, most of which higher than the carbon content in methane. There are two reasons that cause this phenomenon: 1. CO2 produced from anaerobic fermentation dissolves in the solution and is converted into CO - 3; 2. there are other unbeneficial bacteria among microbes that can decompose acetic acid and produce inorganic carbons of other forms. In the presence of methane, the proportion of carbon is the highest in the graphite group (42.9%), which is 11% higher than that of the control group. Therefore, in the descending order, graphite, activated carbon, and magnetite have significant effects on the production of methane by microbial anaerobic fermentation. 4. 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