International Journal of Green Energy
ISSN: 1543-5075 (Print) 1543-5083 (Online) Journal homepage: http://www.tandfonline.com/loi/ljge20
Effects of temperature and relative humidity on the methane permeability rate of biogas storage membranes Fubin Yin, Zifu Li, Xiaoqin Zhou, Xiaofeng Bai & Jing Lian To cite this article: Fubin Yin, Zifu Li, Xiaoqin Zhou, Xiaofeng Bai & Jing Lian (2016) Effects of temperature and relative humidity on the methane permeability rate of biogas storage membranes, International Journal of Green Energy, 13:9, 951-956, DOI: 10.1080/15435075.2015.1088446 To link to this article: http://dx.doi.org/10.1080/15435075.2015.1088446
Published online: 09 Aug 2016.
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Date: 09 August 2016, At: 21:21
INTERNATIONAL JOURNAL OF GREEN ENERGY 2016, VOL. 13, NO. 9, 951–956 http://dx.doi.org/10.1080/15435075.2015.1088446
Effects of temperature and relative humidity on the methane permeability rate of biogas storage membranes Fubin Yina,b, Zifu Lib, Xiaoqin Zhoub, Xiaofeng Baib, and Jing Lianb a Chinese Academy of Agricultural Sciences, Institute of Environment and Sustainable Development in Agriculture, Beijing, China; bDepartment of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing, P. R. China
ABSTRACT
KEYWORDS
Selecting a material for biogas storage membranes is becoming increasingly vital because of the wide applications of biogas storage membranes in biogas plants. Material selection has numerous influencing factors, including gas permeability, strength, density, and so on. Among these, gas permeability has a vital role in biogas storage membranes. In this study, three kinds of biogas storage membranes with the same thickness were selected to investigate the effects of temperature (10, 20, 30, and 40°C) and relative humidity (RH; 0%, 50%, and 100%) on the permeability rate of biogas storage membranes. Results demonstrated that when various membrane samples with the same RH values were tested, temperature exhibited a strong effect on permeability rate. Kinetic analysis showed that the relationship between permeability and temperature agrees with the Arrhenius equation. However, no remarkable variation in methane permeability was observed for membranes with the same temperature but different RH values, thus suggesting that RH nearly has no obvious direct influence on the permeability rate of membranes.
Kinetic; methane permeability rate; relative humidity; storage membrane; temperature
Introduction Energy and environmental problems have become increasingly prominent in recent years. Large and medium-sized biogas plants have attracted increasing attention worldwide in terms of waste disposal and energy recovery engineering technology. Biogas, an emerging product in the renewable energy technology field, is obtained by using bacteria to degrade organic matter under anaerobic conditions (Weiland 2010). Typically, a large or medium-sized biogas plant comprises processes such as collection and pretreatment of raw materials, digestion, expected post-processing, biogas purification, and storage. The main objective of such operation is to produce and use gas, and biogas storage is an integral part of the process (Li et al. 2010). In addition, biogas storage is also an important part of the collection process to produce heat and energy in thermoelectric plants. As a new structural material, membrane structures do not only have extensive and extraordinary applications in large buildings, but they also exhibit tremendous values in biogas storage for biogas plants (Zhang 2010). Biogas storage membrane materials are generally composed of several layers, including the surface layer, bottom coating, and base fabric. The base fabric is a thick and wide polymer produced via a specific process in which composite materials are bound together (Figure 1). The fiber is woven from various kinds of layers or bases and must possess the required mechanical properties of membranes
that can provide adequate strength. The coating and surface layers, which can protect the base cloth, are used to ensure the density of membranes, as well as their self-cleaning and antipollution capabilities, UV-resistance, and durability, among others. Biogas storage membranes have a vital role in biogas storage because of the extensive applications of biogas storage membranes in biogas plants. Biogas is typically composed of methane (50% to 70%, v/v), hydrogen sulfide (approximately 3,000 ppm), or some ammonia (in ppm), as well as carbon dioxide to ensure balance (Makaruk, Miltner, and Harasek 2010). If the rigidity of the biogas storage membrane is inadequate, then a large amount of methane and carbon dioxide escape into the atmosphere, thus resulting in global warming and climate change (Rosso and Stenstrom 2008). Analyzing factors (e.g., temperature and relative humidity (RH)) that influence the permeability rate is necessary because the permeability rate is an important parameter in characterizing membrane quality (Liu and Pawliszyn 2006). Some studies have focused on the permeability rate of food and drug packaging films. Chen et al. (2009b) determined the effect of RH on the oxygen transmission rate of polyvinyl alcohol films. The results of this previous study indicate that RH has an obvious influence on the oxygen transmission rate of polyvinyl alcohol films, and that the increasing trends of oxygen transmission rates agree with the exponential function. Wu et al. (2013) analyzed the influences of temperature on the gas permeability of three types of food packaging films (polyamide/ethylene/vinyl alcohol copolymer, polypropylene
CONTACT Zifu Li
[email protected] Department of Civil and Environmental Engineering, University of Science and Technology Beijing, Xueyuan Road 30, Beijing 100083, China. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ljge. © 2016 Taylor & Francis Group, LLC
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method were observed in container oxygen permeability; however, this method is not as effective as the differential pressure method when testing the commonality of gases (Su et al. 2007; Zhao 2005a). The differential pressure method has many advantages in terms of detection because it does not exhibit selectivity in test gases, it is low cost, and it has a high success rate. However, gas purity requirements in test environments are the most significant disadvantage of the differential pressure method. Considering the characteristics of biogas storage membranes and the advantages of differential pressure and equal pressure methods, the former was found to be more effective than the latter in determining the methane permeability of biogas storage membranes.
Figure 1. Composition of membrane materials.
film, and biaxially oriented polypropylene). The results of their study showed that the relationship between permeability coefficient and temperature is consistent with the Arrhenius equation for all three films. However, studies on biogas storage membrane permeability are limited. Li et al. (2010) reported that no standard for characteristic indices and testing methods to measure the methane permeability of biogas storage membranes is available; however, some characteristic indices were mentioned. Li et al. (2013b) identified methane permeability differences between specialized biogas storage membranes and common polyvinyl chloride membranes. However, the influences of temperature and RH on permeability have not yet been explored. In the present study, the effects of temperature and RH on the methane permeability rate of biogas storage membranes were investigated systematically and comprehensively.
Materials and methods
Materials Biogas storage membranes for all experiments were provided by Hangzhou & Environmental Engineering Co. Ltd., Shanghai Shenda–Kobond New Material Co. Ltd., and Beijing Yingherui Environmental Engineering Co. Ltd. The brands of these membranes include German Mehler, German AgriKomp, German Heytex, and Shanghai Shenda–Kobond. The parameters of these membranes are listed in Table 1.
Permeability test principle Among membrane permeability test methods, different pressure and equal pressure methods are frequently used in related standards, such as GB/T1038-2000, ASTM D1434, ISO 2556, ISO 15105, JIS K7126, YBB00082003, and DIN53380. The outstanding advantages of the equal pressure Table 1. Parameters of the membranes. No. Material 1# PVC 2# PVC 3# PVC
Thickness of membrane/mm 2 2 2
Weight (g/m2) 1,500 1,200 1,100
Temperature resistance −30°C to 70°C −30°C to 70°C −30°C to 70°C
Surface treatment PVDF PVDF PVDF
Permeability test methods This research adopted the differential pressure method to determine the methane permeability of biogas storage membranes (Li et al. 2013). To reduce the influences of gas phase and membrane resistance, pure nitrogen was used to feed and run the system, and methane was used as the test gas. Figure 2 shows the experimental apparatus and the membrane performance test. Two homemade steel chambers were used to calculate gas permeability. The diffusion cell was divided into upper and lower chambers by a membrane. The upper chamber contained feed gas, whereas the lower chamber was filled with permeating gas. Two microprocessor-controlled manometers were used to measure accurately the pressure of the feed and permeate chambers during the permeability tests. The results indicate that the membrane surface with an area of 38.48 cm2 was in direct contact with the permeating gas. Prior to the measurements, a stream of pure nitrogen was introduced into the testing system to remove residual gases from previous experiments. The entire system was maintained in an isothermal condition, which was monitored by a thermocouple (with an accuracy of ±0.1°C) that was in direct contact with the chamber surface. After injecting the feed gas, at least 21 hr was required to ensure that the entire system was maintained in a steady state in a thermally stable pure environment. The entire system was vacuumed for 10 hr to decrease the pressure to less than 27 KPa to satisfy national standard requirements. The feed gas was introduced into the upstream side of the cell, which was maintained at a constant pressure of 0.1 MPa. Gas permeability was calculated with respect to the rate of pressure increase in the permeate chamber. Various permeability parameters of the specimen were obtained by measuring the pressure in the lower chamber. For each experiment, membrane permeability rate is calculated as follows (Su et al. 2007; Barghi, Adibi, and Rashtchian 2010): Qg ¼
dp dt
VS PT00T ðP124 P2 Þ ;
(1)
where Qg is the permeability rate (cm3/m2∙day∙Pa); V is the volume of gas passing through the permeate side (cm3); S is the area of the membrane surface exposed to the gas (m2); p0
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Figure 2. Experimental setup for the gas permeability test.
is the pressure under ambient conditions (1.0133 × 105 Pa); and (p1 – p2) is the pressure difference across the membrane (Pa), where p1 and p2 are the pressures of the upstream and downstream sides, respectively. T0 is the temperature under ambient conditions (273.15 K); T is the absolute temperature (K); and dp/dt is the increase in the rate of the downstream pressure (Pa/hr), where dp is the amount of the analyzed permeate (cm3) and dt is the duration of the experiment (hr). This study mainly focused on the performance and influences of various temperature and RH values. The main operating conditions during the experiments are shown in Table 2.
Error analysis The standard deviation comparison method was used for systematic error analysis. Tests for the permeability of methane (T = 298 K, RH = 0%, p = 1.0133 × 105 Pa) were conducted six times for each sample. The results are shown in rP ffiffiffiffiffiffiffiffiffiffiffiffiffi! n x2 i¼1 i and Peters ¼ Table 3, and the Bessel formula σ 1 n1 Pn j x j i i¼1 ffi were used to calculate the formula σ 2 ¼ 1:253 pffiffiffiffiffiffiffiffiffiffi nðn1Þ
standard deviation of the measurement data. μ values could 2 be obtained based on σσ 21 ¼ 1 þ μ. If jμj pffiffiffiffiffiffi , then systeman1 tic errors exist in the measurement data; otherwise, no systematic error exists (BIPM-IEC-IFCC-ISO-IUPAP-OIML 1995; Yang 2009; Fei 2010). Table 2. Main operating conditions during the experiments. Item Temperature Relative humidity (RH) Thickness of membrane Test area
Operating conditions 10°C/20°C/30°C/40°C 0% /50%/100% 2 mm 38.48 cm2
Table 3 shows that for all three kinds of biogas storage 2 membranes, the values of σσ 21 , μ, and pffiffiffiffiffiffi are 1.253, 0.253, n1 2 , we concluded and 0.894, respectively. Given that μ< pffiffiffiffiffiffi n1 that no systematic error existed. The Grubbs method (G ¼ jxi σxj ) was used for random P error n
x
analysis, in which the arithmetic mean value is x ¼ ni¼1 and ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffi rP rP Pxi 2 ffi n x v2i 2ð Þ =n i¼1 i . If G standard deviation is σ ¼ n1 ¼ n1 G0 ðα; nÞ (α is the significant level, n is the measurement time, G0 is the critical value to measure n times at significant level α), then the result implies that the measurement data are not credible (BIPM-IEC-IFCC-ISO-IUPAP-OIML 1995; NIST/ SEMATECH 2003; Fei 2010). According to the critical values of the Grubbs method, G0 (0.05, 6) and G0 (0.01, 6) are 1.832 and 1.944, respectively. Then, G0 (0.05, 6) and G0 (0.01, 6) were compared with G(i) (Table 3). For all G(i) values, GðiÞ
