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Energy storage through adsorption and desorption of water vapour in raw Saudi bentonite
⁎Tel.: +966 10501963869; fax: +966 14772245. nourallha333@hotmail.com (W.K. Mekhamer)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Adsorption/desorption of water vapour on raw Saudi bentonite (RB) is proposed as a heat energy storage. This is most readily achieved by adsorption and desorption of water vapour on RB at different temperatures as a function of time. The RB subjected to preheating temperature of 200 °C, before subjecting to the adsorption process carried out. The IR spectra of RB before adsorption of water vapour at 298 and 313 K were studied. The adsorbed and desorbed water vapour from bentonite surfaces at 298 and 313 K was determined at different time. The adsorptive capacities of RB sample at 298 and 313 K were 0.0097 and 0.0141 mol/g of dry RB, respectively, after 72 h. The desorbed amounts are 0.0085 and 0.01 mol H2O/g of RB at 298 and 313 K, respectively after 72 h. A kinetic models of second order of the adsorption and desorption of water vapour fitted well the experimental data. Application of Van’t Hoff’s law at two temperatures (298 and 313 K) yields the adsorption and desorption enthalpy. The adsorption enthalpy (stored energy) of RB increased with increasing contact time up to 5 h. At this time the maximum enthalpy was about 30 kJ/g dry bentonite, at which the clay has lost all the energy that could be released due to adsorption of water vapours. Then it shows a decrease in sorption energy when the time increases. On the other hand, the desorption enthalpy increases gradually with the increase of the time up to 72 h then become constant, maximum enthalpy was 14.99 kJ/g. The rate of water vapour adsorption was found to be very high so that the extracted energy from the bentonite surface would not be a problem in any practical utilization of this system.
Keywords
Energy storage
Raw Saudi bentonite
Enthalpy of adsorption: kinetic, adsorption and desorption
1 Introduction
Progress in the use of renewable energy, waste heat, and other ecologically beneficial energy sources indicates the need for long-term storage. Most systems require a seasonal storage of low or moderate temperature. Solar heating of buildings and food drying, takes advantage of the accumulation and storage of solar energy during sunny days to provide heating during nights and cloudy days. Many workers have developed systems where solar energy is stored thermally in the form of sensible heat, latent heat and heat associated with chemical reactions (Torres et al., 2008; Canbazoğlu et al., 2005; Gil et al., 2009), and adsorption process (Singh et al., 2010). The adsorption on solid material is the most investigated one (Jänchen et al., 2004). Adsorption means of the binding of gaseous or liquid phase of a component on the inner surface of a porous material. During the desorption step, the energetic charging step-heat is put into the sample. The adsorbed component, in this example water molecules, are removed from the inner surface. As soon as the reverse reaction is started by adding water molecules to the sample, the molecules will be adsorbed and the heat brought into the system during desorption will be released. The adsorption of this step represents the discharging process (Hauer, 2007).
In the case where thermal energy is stored via adsorption, the adsorbent material must have high surface area. The most commonly investigated systems which involve water vapour as adsorbate, are zeolites and silica gel (Dong et al., 2007; Fang et al., 2010). Another promising material for these systems is clay; which is chemically inert, resistant to deterioration and is commercially available in large quantities. Clay can be used for short-term heat storage. All of these materials have a high specific heat capacity, meaning they can hold a lot of heat within themselves. Bentonite clay candidates are one of the main requirements for energy storage, due their ability to absorb large quantities of water (Bulut et al., 2009). The hygroscopic property of bentonite coupled with rapid, intense exothermic reaction when taken from dehydrated to hydrated form (heat of adsorption), makes bentonite effective in the storage of solar and waste heat energy.
The most practical method to hydrate clay surfaces is by using water vapour. This vapour is a result or could be produced throughout many industrial activities, solar energy, or by any sources of waste heat energy.
The present study is designed in a manner analogous to what actually happens in nature. In our previous work (Sadek and Mekhamer, 2000, 2001), the behavior of Ca and Na-saturated montmorillonites during the hydration–dehydration reactions, to evaluate their performance for thermal energy storage has been studied. As an extension of this work, this paper aims to investigate the kinetics of the adsorption and desorption of raw Saudi bentonite by measuring weight loss at a given temperature (298 and 313 K) and time. Weight losses are measured as a function of time by means of a thermo balance. The weight losses thus measured are analyzed using pseudo second order models. Also adsorption and desorption enthalpy are estimated by using Van’t Hoff’s law. Results are compared to the heat of sorption reported in the literature.
2 Materials and methods
A raw bentonite (RB) was collected from Kholais region in Saudi Arabia and grinded in our laboratory. The bentonite sample was used for the experiments without any preliminary treatment. The particle size distribution of bentonite was determined by Zetasizer (Zetasizer Nano ZS, Malvern).
The particle size distribution of bentonite was 4.848 μm (33.8%), 1.757 μm (61%), 3.226 μm (5.2%).
The mineralogical and the elemental analysis of RB were determined in our pervious work (Mekhamer, 2010).
The specific surface area (SSA) and adsorption average diameter (AAD) of this sample was determined by BET equation. The SSA and AAD are 69 m2/g and 78 Å, respectively.
2.1 Methods
2.1.1 Water adsorption on bentonite
Adsorption of water vapour on bentonite was studied at 298 and 313 K. To carry out this experiment, the RB was first dried at 200 °C for 24 h in a temperature controlled oven. After the drying process, 2 g of RB was weighed out into a (50 cm3) beaker which suspended in 1000 cm3 beaker half filled with water attached to it (500 ml). The two coincided beakers were covered with aluminum foil. This system was put in an insulator system (calorimeter). The water uptake was determined by weighing the beaker containing the clay at different time. This system was used to determine the adsorbed amount of water vapour onto bentonite.
The amount of adsorbed water in a sample was calculated as a function of time by using the following formula:
2.1.2 Water desorption from bentonite
Desorption experiments were performed in order to estimate kinetic of released water from bentonite surface. After the bentonite sample was fully hydrated with water vapour, the two coincided beakers were recovered to allow the desorption process of water to take place from bentonite surfaces at 298 and 313 K. The weight loss due to water desorption was determined at different time.
2.2 Infrared analysis
The bentonite samples before and after water adsorption were pressed with potassium bromide to form a disk. The infrared spectra of these samples were recorded using FT-IR spectrometer 1000, Perkin Elmer (frequency range of 4000–600 cm−1).
3 Results and discussion
3.1 Adsorption/desorption of water vapour
The adsorption and desorption of water vapour onto RB were represented in Figs. 1 and 2 as a function of contact time from 1 h to 72 h, under equilibrium conditions. The adsorption or desorption of water was expressed as the maximum amount of water adsorbed or desorbed on or from bentonite, respectively (mol H2O/g dry clay). For comparative reasons, the adsorptive capacity after 72 h was taken, as a reference point, where this is the time when the preheated RB sample has reached saturation. It was observed that with increasing time, the adsorbed amount of water vapour increased rapidly in the first 40 min and then, slowed down as equilibrium was approached (Figs. 1 and 2). The increase was not significant after 180 min at temperature, 298 K and 360 min at 313 K. The increasing temperature has resulted in a higher uptake of water at the same interaction time. The high initial uptake rate may be attributed to availability of a large number of adsorption sites. As the sites are gradually filled up, adsorption becomes slow and the kinetics becomes more dependent on the rate at which the adsorptive is transported from the bulk phase to the actual adsorption sites. Berend et al. (1995) stated that the amount of water adsorbed as one-layer hydrated form for all montmorillonites corresponded to an amount of adsorbed water 6–7 mmol/g. The two-layer hydrate state seemed to be formed only for Li- and Na-montmorillonite (13–14 mmol). In our study, the adsorbed amount of water vapour by preheated RB is 0.00917 and 0.0134 mol/g of RB at 298 and 313 K, respectively. These values were very close to the amounts of water in one and two monolayers of water adsorbed at 298 and 313 K, respectively (Cases et al., 1997). The water vapour uptake of RB increased with increasing temperature due to increasing relative humidity above bentonite as a result of increasing rate of water vapourization (Hernandez, 2005).
Adsorption and desorption kinetics of water vapour at 298 K by RB.
Adsorption and desorption kinetics of water vapour at 313 K by RB.
By comparing the adsorption and desorption of water vapour by RB, the desorption of water did not follow the adsorption path and showed quite significant hysteresis. The hysteresis was small at temperature 298 K and more significant at 313 K, which means, the adsorption of water vapour was largely dependent on the pore structure of the samples. The maximum desorption percentage of H2O at 298 and 313 K were 0.93% and 0.73%, respectively. In general, incomplete desorption was observed in our results.
Although the bentonite in this work was used as a raw material, its capacity for water adsorption in this work was close to the adsorbed amount by Na/Ca-bentonite (Montes and Geraud, 2004), which confirmed the compatibility of the RB as water absorber material.
3.2 Infrared analysis
The IR spectra of RB before and after adsorption of water vapour at 298 and 313 K were shown in Fig. 3. A weak band at about 3630 cm−1 was due to lattice hydroxyls OH stretching mode, which arose from the vibration of firmly bounded H2O. Also strong broad band at 3420 cm−1 can be attributed to the H–O–H stretching vibration of H2O. This band reflected the free H2O adsorbed onto the structure and/or inter-laminar water OH stretch. The band near 1630 cm−1 was due to the water of crystallization bending vibration. The Si–O–Si stretching vibration appeared near 1030 cm−1 as a strong band. The 920 and 850 cm−1 bands were due to Al–OH and Mg–Al–OH, respectively.
IR spectra of raw bentonite (RB) before adsorption, after adsorption of water vapour at 298 and 313 K from up to down, respectively.
The relative amount of the adsorbed water vapour to the clay surface can be elucidated through the IR spectroscopy data. This can be done by assuming that the absorption of radiant energy by water and the clay obeys the Beer–Lambert equation. The ratio of absorbance, log I0/I, of the stretching vibration band at 3420 cm−1 to the bending vibration band at 1030 cm−1 can be taken as an indication of the water content or moisture content adsorbed on clay surface. This ratio was increased by increasing the temperature as indicated in Table 1.
| Bentonite sample | Relative absorbance of (3420/1030) |
|---|---|
| RB before adsorption | 1.24 |
| 298 K | 2.37 |
| 313 K | 7.9 |
The results of IR indicated the same observation of increasing the water content with increasing temperature. This was comparable to the adsorption isotherm trend observed at 298 K and 313 K, where the amount of water adsorbed by RB increased with increasing temperature.
3.3 Kinetic study
The study of sorption kinetics described the water vapour uptake rate and evidently this rate controls the residence time of water vapour at bentonite surface (Ho, 2006). A pseudo-second-order reaction rate equation was used to study the kinetics of adsorption. The Ho et al. pseudo-second-order equation is given by
The second model was used to evaluate the rate of adsorption and desorption of water vapour on RB from the experimental data. The kinetics parameters as well as fitting parameters were experimentally determined by plotting (t/qt) vs (t) (Figs. 4 and 5) and the results were shown in Table 2. The plot showed a linear relationship. The correlation coefficients, R2 proved that the pseudo-second order model, fits with the experimental data of adsorption and desorption, Table 2 (R2 = 0.998 and 0.997 for adsorption at 298 and 313, respectively, 0.98 and 0.99 for desorption at 298 and 313 K, respectively). As shown from Table 1, the rate constant, K for adsorption (K = 11.8 and 12.79 h mol/g at 298 and 313 K, respectively were higher than those for desorption (K = 5.1 and 5.9 at 298 K and 313 K, respectively). Moreover, the initial adsorption rate, h (mol/g h) was higher than the initial desorption rate at both the two temperature. These results meant that the rate of water vapour adsorption by RB was always inferior to the rate of water vapour desorption. Gonzalez et al. (2001) observed that the kinetics of desorption in sepiolite/water vapour system was faster than that of adsorption. The reverse trend was observed in our result, may be due to specific interactions between active adsorption centers such as oxygen ions, where water molecules coordinated to deferent cations and silanol groups on RB surface and the adsorbed water molecules. This first water molecule is acting as nuclei for other water molecules which are hydrogen-bonded, thus producing an important increase in the retention of water by RB. Therefore the hysteresis was observed as the desorption path was not coincident with the adsorption path.
Pseudo-second-order kinetic plots for adsorption/desorption of water vapour by RB at 298 K.
Pseudo-second-order kinetic plots for adsorption/desorption of water vapour by RB at 313 K.
| Temperature | Adsorption parameters | Desorption parameters | ||||||
|---|---|---|---|---|---|---|---|---|
| R2 | qe (mol/g) | h (mol/g h) | K (g/mol h) | R | qe (mol/g) | h (mol/g h) | K (g/mol h) | |
| 298 K | 0.99 | 0.010136 | 0.0012 | 11.8 | 0.97 | 0.010119 | 0.00052 | 5.1 |
| 313 K | 0.99 | 0.0134 | 0.00229 | 12.79 | 0.99 | 0.0115 | 0.00088 | 5.9 |
3.4 Adsorption enthalpy from Van’t Hoff’s law
The pseudo-equilibrium state corresponding to adsorption and desorption at a given time t and temperature, was calculated by using Van’t Hoff equation (Kharroubi et al., 2009):
Fig. 6 showed the relation between contact time and enthalpy of water adsorption and desorption. The ΔH of adsorption on dried bentonite increased at the beginning with increasing the time up to 5 h. At this time the maximum enthalpy was about 30 kJ/g dry bentonite, where the clay has lost most energy that could be released due to adsorption of water vapour. Then it showed a decrease in sorption energy when the moisture content increased around RB sample, which could qualitatively be explained by considering that the adsorption occurred initially in the most active available sites involving high energies upon water vapour adsorption. With increasing occupation of adsorbent sites by water vapour, adsorption took place on the less active sites which involved lower energies (Sanchez et al., 1997; Mihoubi and Bellagi, 2006). Such a maximum in sorption energy, associated with the presence of one complete layer of water between clay layers was already mentioned in the literature (Zhang and Low, 1989). At low water content, binding is mainly governed by interaction between the water molecules and the RB surface. The shape of the curve could be explained due to the differences in hydrogen bonding as moisture was adsorbed probably due to differences in attractive forces between water molecules and sorption sites and between water molecules themselves that adsorbed on RB surface.
Enthalpy of adsorption/desorption of water vapour by RB.
Comparing the ΔH value of water adsorption for raw bentonite with the ΔH value reported a previous work (Sadek and Mekhamer, 2000, 2001), it was found the ΔH for Na- and Ca-exchanged montmorillonite was 781 and 487 cal/g, respectively, which are very lower than the enthalpy of water vapour adsorption in the present study. Therefore the result of adsorption confirmed that RB can be considered as more efficient solar energy storage material than montmorillonite in pervious work (Sadek and Mekhamer, 2000, 2001). On the other hand, ΔH of water desorption from RB surface increased gradually with the increase of the time up to 72 h then became constant. Maximum enthalpy was 14.99 kJ/g dry bentonite. The ΔH for water desorption was lower than those of adsorption. This result may be due to water molecules/cation interactions are stronger resulting in fewer desorbed water molecules from RB surface. Consequently, this effect lead to lower desorption enthalpy lower than that of adsorption enthalpy.
To sum up, the water desorption investigated here appeared to be driven by the competition between attractive, i.e., water molecule/cation, and repulsive, i.e., water molecule/silicate interlayer surface and swelling, contributions of RB.
4 Conclusion
The kinetic models of second order reaction, was used to describe the adsorption and desorption of water vapour by RB. The result of adsorption confirmed that RB can be considered as an efficient solar energy storage material and it fulfills the following criteria: The stored energy 30 kJ/g when the RB was preheated to 200 °C, the handling of bentonite was very easy and did not need sophisticated technology and/or equipment and the bentonite is commercially available at low cost.
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