Research PaperEffect of relative humidity on thermal conductivity of zeolite-based adsorbents: Theory and experiments
Introduction
Worldwide energy demand has been increased due to growth in population and industrialization. Most of the energy production is associated with fossil fuels which results in environmental hazardous emissions. In this regard, Kyoto protocol emphasized on utilization of renewable energy resources and precise use of heat energy i.e. reuse of exhaust and waste heat, storage of energy, etc. [1], [2]. It emphasizes the researchers to investigate the low-cost and energy-efficient cooling, heating and air-conditioning systems which can be operated on waste heat. From this point of view, importance of adsorption-based systems is obvious, consequently, adsorbent-adsorbate science and interactions have been extensively studied in last 2–3 decades in order to develop low-cost and sustainable thermally-driven adsorption cooling [3], heat storage [1], air-conditioning [4] and desalination systems. In this regard, thermal conductivity of the adsorbents is an important thermodynamic property by which the heat/mass transfer phenomena can be optimized [5]. Therefore, studies on effective thermal conductivity (ETC) of adsorbents gradually seek precedence [6] as it is considered one of the important parameter to enhance the performance of the adsorption heat pump (AHP) and adsorption cooling systems (ACS) [7], [8], [9].
Thermal conductivity is one of the important characteristics of the adsorbent material which describes its ability to transmit the heat. Adsorbent materials with low thermal conductivity and low mass diffusivity affect the coefficient of performance (COP) and specific cooling effect of the AHP and ACS system. Thermal conductivity of adsorbent can be increased by adding thermally conductive particles [10]. The development of new adsorbents possessing optimum thermal conductivity is also required to overcome such limitations [11], [12], [13]. In addition, most of the studies available in the literature use the fixed value of ETC for the designing of adsorption-based systems. However, it changes with the adsorption/desorption concentration because of swelling and agglomeration phenomena [14], [15]. Thereby, present work deal with the estimation of uptake effect on the effective thermal conductivity of adsorbent material at different relative humidity [16].
There are numbers of techniques applied to measure the thermal conductivity of the material include a steady-state method, transient hot-wire method, laser flash diffusivity method, and transient plane source [12], [13], [17]. However, transient plane source method is based on simplicity and instant measurement of temperature. In present study, thermal conductivity has been experimentally measured using C-Therm TCi system [12], [13] which can also work on transient plane source method. According to the available literature, thermal conductivity of porous adsorbents shows that ETC is not a constant quantity [14], [15], [18], [19], [20]. It depends on the interfacial contact between different constituent phases in a packed bed. In addition, due to coupling between fluid and solid particles ETC is considered as the function of porosity, pore structure of the media, water contents, saturation degree, phase change of water, and the temperature [16]. In adsorbent material, generally solid particles have higher thermal conductivity as compared to fluid [21]. Heat transfer through the porous material in a packed bed is considered a combination of three mechanisms i.e. gas phase conductive and radiative heat transfer, solid and gas phase conductive and radioactive heat transfer and within solid contact surface conductive heat transfer [22]. In case of saturated adsorbents, all the pores are filled with fluid therefore, heat transfer is mainly due to conduction mechanism [23]. However, in some cases, convection and radiation type heat transfer may not be neglected. In addition, ETC depends on the solid and fluid property and porosity of the material; therefore, adsorbents from the same manufacturer could also result in a change in inherent material property [24]. Thermal conductivity becomes more sensitive when the porosity of materials reaches at saturation condition. It is because the porosity will be replaced by the adsorbate [21], [25].
Effective thermal conductivity is an important transport property of the adsorbent material which has received incessant attention [26]. Many empirical and theoretical models have been presented to estimate the effective thermal conductivity for various materials. These models characterized by a single value and synchronous dependence of ETC on temperature, moisture and porosity are not considered in case of fully saturated or dry adsorbents [27]. The operating conditions of adsorption-based open and close cycle cooling systems need to be optimized instantly in order to ensure the higher system COP. Consequently, the effect of a change in ETC due to change RH must be consider for the precise estimation of system COP. Its importance has been proven through many studies by means of sorption isotherm of adsorbent i.e. amount of moisture uptake is different at different RH or relative pressure [12], [13], [28]. Similarly, the ETC of the adsorbents is supposed to be different at different RH or relative pressure.
The significance of this research is to determine the effect of relative humidity on the ETC of study materials. Heat transfer mechanism through the porous material is very complex and the combination of conduction, convection, and radiation between fluid and solid phase. Increase in the porosity causes the decrease in effective thermal conductivity [29]. The knowledge of ETC is perquisite for the heat/mass transfer through the packed bed adsorbent. The objective of the present study is to measure the thermal conductivity of adsorbents experimentally at different RH and compare the theoretical and modeled values of ETC. In addition, change in ETC is also determined at different moisture and uptake levels, consequently, empirical expressions are developed for the studies adsorbents.
Section snippets
Materials
In present study, two kinds of zeolite-based adsorbent are used which are commercially named as AQSOA-Z02 (zeolite-1) [30], [28], [31], [32] and AQSOA-Z05 (zeolite-2) [28], [31]. Zeolite-1 has structure type CHA and made of silico aluminophosphate gel with a pore size of 3.8E−10m [31] and crystal density of 1.43 g m L−1 [33]. Zeolite-2 has a AFI structure type and synthesized from aluminophosphate gel with a pore size of 7.4E−10m [31] and crystal density of 1.75 g m L−1 determined from
Data reduction
Initially, adsorbents zeolite-1 and zeolite-2 are dried in oven for 24 h at 80 °C. Samples are loaded on test rig of the C-Therm TCi sensor in the control cell which is attached with C-Therm TCi controller and data logger. The accuracy of the sensor is better than ±5% and precision is better than 1%.Control cell is maintained air tight and allowed N2 gas to pass through the control cell via evaporator according to the required experimental conditions i.e. temperature and humidity. Once the
Results and discussion
Effective thermal conductivities (ETC) of zeolite-1 and zeolite-2 has been measured at different levels of relative humidity (RH) using C-Therm TCi system shown in Fig. 1. Different levels of humidity of the control cell was maintained by passing the N2 gas via evaporator and temperature was maintained by circulating the hot water around the control cell. The experimental results of ETC are presented in Fig. 3(a and b) for both adsorbents at 25 °C dry-bulb temperature. ETC of the adsorbent
Conclusions
Present study experimentally investigates the effect of relative humidity (RH) on thermal conductivity of two kinds of zeolite-based adsorbents AQSOA-Z02 (zeolite-1) and AQSOA-Z05 (zeolite-2). In case of zeolite-1, effective thermal conductivity was found 0.060 W m−1 K−1 at oven dried conditions (i.e. RH = 0%) which increases with the increase in RH and reaches up to 0.090 W m−1 K−1 at saturation conditions (i.e. RH = 100%). Similarly, the effective thermal conductivity of zeolite-2 is found
Acknowledgements
The authors would like to acknowledge the support from Professor Shigeru Koyama (PhD), who, although no longer with us, continues to inspire by his example of hardworking and dedication to the students he served over the course of his career.
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