Reaction Characteristics of CaSO 4 /CaSO 4 1/2H 2 O Reversible Reaction for Chemical Heat Pump

Transcription

1 Journal of Chemical Engineering of Japan, Vol. 40, No. 13, pp , 2007 Research Paper Reaction Characteristics of CaSO 4 /CaSO 4 1/2H 2 O Reversible Reaction for Chemical Heat Pump Hironao OGURA, Masahiro HAGURO, Yasuhiro SHIBATA and Yasufumi OTSUBO Division of Architecture and Urban Science, Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba-shi, Chiba , Japan Keywords: Chemical Heat Storage, Reactant, Thermogravimetry, Waste Energy Utilization, Energy Saving We had proposed chemical heat pumps using a reversible calcium sulphate/water system for effective waste energy utilization. However, more kinetics studies are necessary for this system, especially for different atmosphere condition and different materials. As a result, the equilibrium line of CaSO 4 in the closed system like CHP system was found to be close to that in the open system. It means that the CHP using CaSO 4 /CaSO 4 1/2H 2 O reversible stores under 373 K level thermal energy, and releases the same level hot heat and 273 K level cold heat. Furthermore, the effects of material size on the rates could be calculated by the proposed equations, which enable the effective CHP reactor design. Introduction Received on July 23, 2007; accepted on September 7, Correspondence concerning this article should be addressed to H. Ogura ( address: hiro_ogura@faculty.chiba-u.jp). Presented at International Symposium on Innovative Materials for Processes in Energy Systems, IMPRES, Kyoto, October, From the viewpoints of energy savings and environmental impacts, we have been studying on chemical heat pumps (CHPs). We had proposed and developed CHPs using a reversible calcium oxide/water system for effective thermal energy utilization in domestic use (Ogura et al., 1993) and industrial use such as electric power use (Ogura et al., 1998) or drying (Ogura et al., 2000). Furthermore, for lower temperature heat source, we had proposed CHPs using a reversible calcium sulphate/water system for effective waste energy utilization (Ogura et al., 2004). Calcium sulphate has possibility of storing under 373 K level thermal energy and releasing the same level hot heat and 273 K level cold heat. Also Calcium sulphate is easy to get, inexpensive and safe. However, more kinetics studies are necessary for this system, especially in a closed system like CHP system for different materials. Although the kinetics of CaSO 4 has been done by some researchers (Badens et al., 1998; Hudson-Lamb et al., 1996; Strydom et al., 1995), those were done in an open system. In this study, we examine the equilibrium lines of the hydration of calcium sulphate and the dehydration of calcium sulphate hemihydrate for different kinds of material in an open system, a semi-closed system and in a closed system by considering the heat pump operations. Semi-closed system is assumed to be a closed system with forced convection such as a fan unit for mass transfer enhancement. These different kinds of condition are necessary to consider the CHP conditions. We used an improved thermogravimetry for the vacuumed condition experiments. Furthermore, the effects of material size on the rates of the hydration of calcium sulphate/the dehydration of calcium sulphate hemihydrate are examined in an open system. 1. Operating Principle of CHP Figure 1 shows a typical gas solid CHP configuration. The CHP is a closed system of coupled low and high temperature reactors connected to each other. The heat storage and release s occur at different pressure levels. The low temperature side reactor has a higher equilibrium pressure line. An evaporation/condensation also can be used for the low temperature side. The CHP, as studied earlier, operates as a batch system with a heat storing step and a heat releasing step. In the heat storing step, the heat Q H is stored in the form of thermochemical energy by decomposition of the reactant AC(s) in the high temperature side reactor. The released gas C(g) flows into the low temperature side reactor due to a pressure difference between the two reactors. The gas C(g) reacts with the reactant B(s) releasing low temperature heat Q L. As long as the reactant A(s) is separated from the gas C(g), 1252 Copyright 2007 The Society of Chemical Engineers, Japan

2 Fig. 2 Experimental unit for open system Fig. 1 the heat can be stored for any length of time in the form of chemical energy. In the heat releasing step, the gas C(g) flows from the low temperature side reactor to the high temperature side reactor by opening a valve due to the pressure difference between the reactors. The exothermic of the reactant A(s) at a high temperature level with the gas C(g) takes place in the high temperature side reactor. The low temperature side reactor stores the low temperature heat Q L or is cooled down and release cold heat by its decomposition/evaporation heat. 2. Experiments Operating principle of chemical heat pump 2.1 Experimental unit Figure 2 shows an example of the experimental unit. This figure shows the open system unit. A thermogravimetry (TGA-50H, Shimadzu Corp.) is connected with an evaporator/condenser controlled by a temperature control bath. For the open system experiments, the evaporated water vapor is introduced to the thermogravimetry by N 2 gas flow. For the semi-closed system experiments, it is a forced convection type. The water is evaporated and introduced to the thermogravimetry by vacuuming from the thermogravimetry side. For the closed system experiments, the thermogravimetry and the evaporator/condenser are connected in a vacuumed and closed condition. In every system, water vapor is introduced to the thermogravimetry via the port close to the reactant instead of the rear port. It means the water vapor flow to balance part of the thermogravimetry is reduced. 2.2 Working materials For the experiments, the calcium sulphate hemihydrate: CaSO 4 1/2H 2 O (Calcium Sulfate, Calcined, Wako Corp.) and the chemical gypsym: CaSO 4 2H 2 O (Calcium Sulfate Dihydrate, Wako Corp.) were used as chemical materials. Also, the natural gypsums CaSO 4 2H 2 O (Produced in Australia, mean diameter: 40 µm under, µm and µm) were used as start materials. 2.3 Experimental procedure For the thermogravimetry experiments, 10 mg of the CaSO 4 1/2H 2 O and natural gypsum are installed in the cell of the thermogravimetry. For the open system experiments, the evaporated water vapor is introduced to the thermogravimetry by N 2 gas flow (100 ml/min). For the semi-closed system experiments, the water is evaporated and introduced to the thermogravimetry by vacuuming from the thermogravimetry side. For the closed system experiments, the reactor and the evaporator/condenser are vacuumed to the appropriate pressures. The reactor is heated up with 5 10 K/min and kept at constant temperature for 10 min. The joint valve connecting the reactor and the evaporator/condenser is opened, and CaSO 4 1/2H 2 O is dehydrated in this heating-up stage as a heat-storing step. After that, the hydration of CaSO 4 occurs as a heat-releasing step with the reactor temperature cooling down with 5 10 K/min. The hydration and the dehydration operations were repeated. The conversion changes and the heat were calculated from the weight changes of the reactants. This occurs between β-caso 4 1/2H 2 O and γ-caso 4 in our experimental temperature range (Badens et al., 1998). 3. Results and Discussion 3.1 Reaction equilibrium Figure 3 shows the equilibrium line of CaSO 4 /CaSO 4 1/2H 2 O reversible obtained by the reference (Lee et al., 1986) and the 1253

3 Fig. 5 F1 h changes for natural gypsums Fig. 3 Fig. 4 Reaction equilibrium line of CaSO 4 /CaSO 4 1/2H 2 O Reaction equilibrium line of CaSO 4 /CaSO 4 1/2H 2 O for natural gypsums equilibrium plots by our experimental data in different systems. All of the data reproduce well the reference equilibrium line. However, for the same pressure, the open system data needs slightly higher temperature than the others. It is probably because that the water vapor reacts with CaSO 4 effectively in the semi-closed and closed system. From these results, it can be seen that the CHP using CaSO 4 /CaSO 4 1/2H 2 O reversible stores under 373 K level thermal energy, and releases the same level hot heat and 273 K level cold heat. Figure 4 shows the CaSO 4 /CaSO 4 1/2H 2 O equilibrium line of four kinds of natural materials in a open-system. The natural gypsum should be more eco-friendly and more flexible in size than chemical gypsum. All of the four kinds of our experimental data reproduce well the reference equilibrium line (Lee et al., 1986). The large mean diameter material reacts at higher temperature than the small diameter material. It can be seen from these results that the chemical and the natural material less than 1000 µm mean diameter can use the reference equilibrium line although the rates need to be considered in different ways. 3.2 Reaction rate At this stage, it is not easy to get many data on rate in the closed system. So, we examined rates in the open system especially focused on the effects of the reactant diameter. In the similar system using CaO/H 2 O, it was found that the rate in the open system is not so different from that of the semi-closed system (Shokrat et al., 2005). We believe that even the open system data can be important data for CHP reactor design. Equations (1) and (2) are reference equations (Lee et al., 1986). (hydration ) 1 (1 X h ) 1/3 = k h P h t, P h = (P P e )/[P s (P P e )] (1) (dehydration ) 1 (1 X d ) 1/3 = k d P d t, P d = (P e P)/[P s (P e P)] (2) In order to use Eqs. (1) and (2) for rate calculation, we need to prove that our experiment data follow the grain model concept. Figures 5 and 6 show that the F1 h (=1 (1 X h ) 1/3 ) changes and the F1 d (=1 (1 X d ) 1/3 ] changes for natural gypsums. These figure show that all materials have almost linear relationships between F1 h /F1 d and time. So, all natural materials can be thought to follow the grain model concept JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

4 Fig. 7 k h η r for natural gypsums Fig. 6 F1 d changes for natural gypsums However, the influence of material diameter is not considered in Eqs. (1) and (2). We propose Eqs. (3) and (4) that include the influence of mean diameter of material. η r is a term of the diameter effect. (hydration ) X h =1 (1 k h η r ( P h )t) 3, k h = exp( /RT) (3) (dehydration ) X d = 1 (1 k d η r ( P d )t) 3, k d = exp( /RT) (4) Fig. 8 k d η r for natural gypsums k h and k d are rate constants of reference (Lee et al., 1986). We revised k d the to reproduce the reference calculated line to the reference experimental data. Figures 7 and 8 show the k h η r, k d η r data calculated by F1 h, F1 d in Figures 5 and 6 in comparison with reference line. These figure show that the larger mean diameter of material are the smaller k h η r, k d η r. This result is expected to be caused by the heat and mass transfer existence (Shokrat et al., 2005). We assume that the k h η r, k d η r are affected mainly by the mean diameter of material. From Figures 7 and 8, we calculated the diameter effect α by Eq. (5) α = ln(k i η 10 ) ln(k i η r ) (5) By Eq. (5), we got η r for η 10 = 1. η r =1.012 e r (6) Figures 9 and 10 are example of conversion changes for natural gypsums in hydration and dehydration s. It can be seen from these figures that all of the calculated lines by Eq. (3) (6) reproduce the experimental data well. The difference between calculated data and experimental data in the mean diameter µm might be caused by using average of hydration η r and dehydration η r when we calculate η i. From these results, we can simulate the rates considering the material size by the proposed Eqs. (3) and (4) in CHP operations, and estimate the CHP power using CaSO 4 /CaSO 4 1/2H 2 O reversible. Conclusions The reactivity of the hydration of calcium sulphate and the dehydration of calcium sulphate hemihydrate were studied by an improved thermogravimetry. As a result, the equilibrium line of CaSO 4 in the closed system like CHP system was close to that in the open system. It means the CaSO 4 /CaSO 4 1/2H 2 O reversible is considered as one of the best candidates for low-temperature CHP reactant, that stores under 373 K level thermal energy and releases hot/cold heat. Furthermore, the effects of material size on the rates are calculated by our proposed equations. So, the chemical heat pump power using different size CaSO 4 /CaSO 4 1/2H 2 O can be estimated by the proposed equations, and it enables the effective CHP design. VOL. 40 NO

5 t = time [min] X = conversation [ ] P = driving pressure term [ ] η r = term of mean diameter influence [ ] <Subscript> d = dehydration g = gas h = hydration i = h, d l = liquid r = mean diameter (10, 25, 300, 750) s = solid Fig. 9 Conversion changes for natural gypsums in hydration Fig. 10 Conversion changes for natural gypsums in dehydration Nomenclature F1 i = 1 (1 X) 1/3 [ ] k i = rate constant [1/min] P = dimensionless partial pressure of water vapor[ ] P e = equilibrium pressure [ ] P w = saturated vapor pressure [ ] T = temperature [K] T w = condenser and evaporater temperature [K] Literature Cited Badens, E., P. Llewellyn, J. M. Fulconis, C. Jourdan, S. Veesler, R. Boistelle and F. Rouquerol; Study of Gypsum Dehydration by Controlled Transformation Rate Thermal Analysis (CRTA), J. Solid State Chem., 139, (1998) Hudson-Lamb, D. L., C. A. Strydom and J. H. Potgieter; The Thermal Dehydration of Natural Gypsum and Pure Calcium Sulphate Dehydrate (Gypsum), Thermochim. Acta, 282/283, (1996) Lee, S. K., H. Matsuda and M. Hasatani; Fundamental Studies of CaSO 4 (1/2)H 2 O/CaSO 4 Thermochemical Reaction Cycle for Thermal Energy Storage by Means of Chemical Reaction, Kagaku Kogaku Ronbunshu, 12, (1986) Ogura, H. and A. S. Mujumdar; Proposal for a Novel Chemical Heat Pump Dryer, Drying Technol., 18, (2000) Ogura, H., M. Kanamori, H. Matsuda and M. Hasatani; Generation of Low-Temperature Heat by Use of CaO/H 2 O/Ca(OH) 2 Reaction, Kagaku Kogaku Ronbunshu, 19, (1993) Ogura, H., S. Fujimoto, H. Iwamoto, H. Kage, Y. Matsuno, Y. Kanamaru and S. Awaya; Basic Performance of CaO/H 2 O/Ca(OH) 2 Chemical Heat Pump Unit for Night-Electric Heat-Storage and Cold/Hot Heat-Recovering, Kagaku Kogaku Ronbunshu, 24, (1998) Ogura, H., H. Hamaguchi, H. Kage and A. S. Mujumdar; Energy and Cost Estimation for Application of Chemical Heat Pump Dryer to Industrial Ceramics Drying, Drying Technol., 22, (2004) Shokrat, A., H. Ogura and H. Kage; Final Conversion and Reaction Rate of CaO Hydration in Different Experimental Systems, Kagaku Kogaku Ronbunshu, 31, (2005) Strydom, C. A., D. L. Hudson-Lamb, J. H. Potgieter and E. Dagg; The Thermal Dehydration of Synthetic Gypsum, Thermochim. Acta, 269/270, (1995) 1256