Evaluation of Fundamental Characteristics of D-Threitol As

Evaluation of fundamental characteristics of D-Threitol as phase change material at high temperature On Line Number 790 Hideto Hidaka,1 Masanori Yamazaki,1 Masayoshi Yabe,1 Hiroyuki Kakiuchi,1 Erwin P. Ona,2 Yoshihiro Kojima3 and Hitoki Matsuda2

1 Mitsubishi Chemical Group Science and Technology Research Center, Toho-Cho 1, Yokkaichi, Mie 540-8530, Japan, [email protected] 2 Department of Chemical Engineering, Nagoya University, Furo-Cho, Chikusa, Aichi 464-8630, Japan 3 Research Center for Advanced Waste and Emission Management, Nagoya University, Furo-Cho, Chikusa, Aichi 464-8630, Japan

ABSTRACT This study focused on polyalcohols as phase change material, which stores a large amount of latent heat at high temperature. D-Threitol, which is an isomer of meso-Erythritol, was studied to obtain its phase change characteristics by means of DSC analysis and a lab-scale heating and cooling apparatus. Usability of D-Threitol as phase change material was evaluated by comparison with other polyalocohols. It was found that D-Threitol started to melt at around 90 degrees C with a relatively large latent heat of 225 kJ/kg. On the other hand, D-Threitol started solidification when the temperature was cooled between at 40 degrees C and 46 degrees C, indicated by a rapid rise to 89 degrees C in a lab-scale heating and cooling apparatus. It was then considered that D-Threitol was applicable as an environmental-friendly PCM for a hot water supply.

KEYWORDS Phase change materials, Polyalcohols, Threitol

INTRODUCTION Latent heat storage by Phase Change Materials (PCMs) has the advantage to store and release a relatively large quantity of heat in a constant narrow temperature range during phase change. A lot of organic and inorganic PCMs have so far been investigated and developed to meet heat storage requirements: heat storage temperature and heat storage density, etc. Recently, there has been a large demand for effective heat recovery from various thermal processes and devices such as cogeneration system, fuel cells, automobile engines, waste incinerators, etc. From the viewpoint of development of environmentally friendly energy system, utilization of renewable energy sources such as solar energy, geothermic energy, etc. are also of great concern. Inorganic salt hydrates such as magnesium nitrate hexahydrate, sodium acetate trihydrate, barium hydroxide octahydrate and magnesium chloride hexahydrate have a large amount of latent heat, but they are not completely applicable for a safe heat storage system, due to their toxicity and metal corrosiveness (Zalba et al., 2003). Hence, for utilizing

1 these salt hydrates, it is necessary to consider, for instance, encapsulation of the PCM in a container resistant to corrosion. On the contrary, organic PCMs such as alkanes, fatty acids and polyalcohols do not have the above drawbacks (Zalba et al., 2003). A wide selection of alkanes and fatty acids has been studied toward various heat storage applications. Bo et al. (1998) demonstrated the potential application of paraffin waxes and their binary mixtures for cool heat storage and air conditioning. Stearic and lauric acids, and their binary mixtures have been reported as suitable PCMs for passive solar space heating applications (Sari and Kaygusuz, 2002a). For heat storage at 61°C, palmitic acid has also been evaluated by Sari and Kaygusuz (2002b) in terms of its thermal performance as a PCM. However, only a limited number of these PCMs are suitable for high temperature applications at around 80-90°C, such as domestic hot water supply. On the other hand, polyalcohols may be promising as PCMs because of their high latent heats and relatively higher melting points compared with other organic PCMs. Hoermansdoerfer (1989) reported that polyalcohols such as meso-Erythritol, mannitol and galactitol, of which respective phase change temperatures are 119, 330 and 350°C, are useful as high-temperature PCMs. Besides Hoermansdoerfer (1989), Guex (1981) proposed xylitol as a high-temperature PCM utilizing its supercooling. Among these polyalcohols, meso-Erythritol is characterized as a high-temperature PCM having a melting temperature of around 120°C with a relatively large latent heat of 340kJ/kg, which is almost equivalent to that of the phase change of ice to water (Hoermansdoerfer, 1989). In addition, meso-Erythritol is an environmentally friendly material, which is non-toxic, non-flammable and non-corrosive. Meso-Erythritol may undergo severe supercooling similar to many salt hydrates, but we have reported that ultrasound was effective to relax supercooling (Ona et al., 2002). Though Yabe et al.(2000) studied on meso-Erythritol for a hot water supply system, it seems difficult to apply meso-Erythritol for a hot water supply system because of its relatively high melting point around at 120°C. Therefore, in the present study, we made an experimental survey of D-Threitol. Basic properties as PCM, in particular, its latent heat, melting point, thermal conductivity and density, were measured. D-Threitol, an isomer of meso-Erythritol, is known as a raw material for medicine. Hara et al. discovered the way to produce D-Threitol from meso-Erythritol biologically.

Experiment Samples D-Threitol (Mitsubishi Chemical Co. Ltd.) and meso-Erythritol (Mitsubishi Food Chemical Co. Ltd.) were employed as PCM samples. Chemical structures of D-Threitol and meso-Erythritol are shown in Figure 1.

Evaluation of physical and thermal properties D-Threitol(a) meso-Erythritol(b) An experimental survey was performed by evaluating the melting characteristics of D-Threitol. The Figure 1. Chemical Structures of melting point and latent heat were measured by Differential D-Threitol(a) and meso-Erythritol(b)

2 Scanning Calorimetry (DSC220C, Seiko Instruments Inc.). The calibration of the DSC data was performed using indium (purity, >99.99%) and tin (purity, >99.99%) as standard materials for the determination of heat of melting. About 7-9 mg sample was sealed in a DSC aluminum pan. After a thermal equilibrium state of the sample was attained, it was heated at a rate of 20, 10, 5 or 2°C/min from 0°C to 150°C, was kept 150°C for 10 min and was cooled at a rate of 20, 10, 5 or 2°C/min from 150°C to 0°C in purge air with a flow rate of 200ml/min. The latent heat was calculated from the area of the DSC curve, while the peak temperature was taken as the melting point of D-Threitol. Basic properties such as specific heat, density and thermal conductivity were evaluated. The specific heat of a 10g sample was measured using a heat insulation type calorimeter (SH3000, ULVAC- RIKO Inc.) with a 1.4W heater. The density of a 150 cm3 sample was measured by Archimedes method. The thermal conductivity was measured using a hot wire type apparatus (ARC-TC-1000, Agne Gijutsu Center Inc.). For comparison with meso-Erythritol, melting temperature, latent heat, specific heat and thermal conductivity of meso-Erythritol were similarly measured. To describe more precisely the melting and solidification behaviors of D-Threitol, heating and cooling cycle tests of about 30g PCM sample were performed. The D-Threitol sample was placed in a 50ml cylindrical glass bottle with a thermocouple at its center to continuously monitor its temperature. The bottle was heated from 25 to 120°C at a rate of 2°C/min and kept at 120°C for 3 hours. Afterwards, it was cooled by the air.

RESULTS AND DISCUSSIONS Latent heat, melting point, thermal conductivity and density The DSC patterns (Time v.s. Heat flow) of D-Threitol are shown in Figure 2. Latent heat (Δ H) of D-Threitol was calculated from the peak area of DSC chart. At a heating rate of 2°C/min, the latent heat of D-Threitol was 228 kJ/kg. The latent heat was found independent of heating rate in these DSC patterns. Figure 3 shows DSC patterns (Temperature v.s. Heat flow). From the figure, the melting temperatures, the beginnings of the endothermic peaks, were in the range of 84 to 86°C, independent of heating rate. On the other hand, the solidifying temperatures varied in the range of 15 to 22°C. D-Threitol exhibit a large degree of supercooling.

40 40 20℃/min 5℃/min Cooling rate (solidification 30 30 temperature) ℃ 20℃/min (15℃) 10 /min 2℃/min 20 20 10℃/min (17℃) 5℃/min (22℃) 10 10 2℃/min (22℃)

0 0 DSC[mW] -10 DSC[mW] -10 Heating rate (melting point) 2℃/min 219 J/g（86-90-93℃） 2℃/min 219 J/g（86-90-93℃） -20 5℃/min 219 J/g（85-92-97℃） -20 5℃/min 219 J/g（85-92-97℃） 10℃/min 213 J/g（85-94-101℃） Endo.10℃/min 213 J/g（85-94-101 Exo. ℃） -30 Endo Exo. -30 20℃/min 228 J/g（84-95-120℃） 20℃/min 228 J/g（84-95-120℃） -40 -40 0 50 100 150 200 0 20 40 60 80 100 120 140 160 Time[min] Temperature[℃] Figure 2. DSC chart of D-Threitol Figure 3. DSC chart of D-Threitol (Time v.s. Heat flow) (Temperature v.s. Heat flow) 3 In Table 1, the basic thermal properties such as density, thermal conductivity and specific heat of D-Threitol and meso-Erythritol are shown. In this table, the melting points and latent heats are values obtained at a rate of 2°C/min.

Table 1. Basic thermal properties of D-Threitol and meso-Erythritol D-Threitol meso-Erythritol a Melting Point [℃] 88.7 119 a Heat of Fusion [kJ/kg] 225 340 Specific Heat [kJ/kg・K] b 1.28 at 20℃ 1.38 at 20℃ 1.61 at 60℃ 2.77 at 140℃ 2.40 at 100℃ Density [kg/dm3] c 1.386 at 20℃ 1.48 at 20℃ 1.357 at 60℃ 1.30 at 140℃ 1.299 at 100℃ d Heat Conductivity [W/m・K] 0.73 at 20℃ 0.73 at 20℃ 0.66 at 60℃ 0.33 at 140℃ 0.48 at 100℃ a measured by DSC (DSC220C) of Seiko Instruments Inc. b measured by the Heat Insulation Type Specific Heat Measurement Apparatus (SH3000) of ULBAC-RICO, Inc. c measured by Archimedes Method d measured by Hot Wire Type Heat Conductivity Measurement Apparatus (ARC-TC-1000) of Agne Gijutu Center Inc.

From the results, D-Threitol has a large amount of latent heat, 305 kJ/dm3 in solid state at 60°C, which is almost same as latent heat of ice, 305kJ/dm3 in solid state at 0°C. And thermal conductivity of D-Threitol is 0.73Wm-1K-1 at 20°C and 0.48Wm-1K-1 at 100 °C, which is larger than thermal conductivity of paraffin. For example, the thermal 1000 100 -1 -1 conductivity of Octadecane, C18, is 0.148Wm K Enthalpy -1 -1 800 80

Specific heat K]

at 20°C and 0.358Wm K at 25°C (Sasaguchi and ・

Viskanta, 1989). Therefore, D-Threitol is applicable 600 60 for use in a heat storage system. Figure 4 show the temperature variations 400 40 Enthalpy[kJ/kg]

of the specific heats of D-Threitol. The enthalpy 200 20 heat[kJ/kg Specific curve of D-Threitol is also indicated in Figure 4. In the figure, a steep rise in enthalpy with melting can 0 0 0 50 100 150 200 be observed at 84-92°C. From the enthalpy curve, Temperature[℃] we can store 313 kJ/kg of latent heat from 80 to Figure 4. Specific heat and 100°C. enthalpy curve of D-Threitol

Melting and solidifying characteristic of D-Threitol Figure 5 shows the temperature changes in the sample bottle and outside of the sample bottle at the second cycle of the heating and cooling cycle tests of about 30g of D-Threitol. Accompanied with heating, the temperature of sample rose. The D-Thritol began to melt with

4 endothermic curve at 80°C and melted completely at 90°C, which accorded with the results of DSC. Then, the sample was cooled and began to solidify at 43°C. The temperature of the sample rose suddenly up to 85°C as it solidified. This temperature rise means the crystal growth rate of D-Threitol is fast enough to supply heat for sudden heat load increasing. Figure 6 shows the result of the heating and cooling cycle test. D-Threitol induces large degree of supercooling. The method to relax supercooling has to be found, considering similar methods employed to meso-Erythritol (Ona et al. 2002).

140 120

Oil Bath ] 120 Sample in the glass bottle ℃ 100

] 100

℃ 80 80 60 60 40

Temperature[ 40 20 20 Solidification temperature[ Solidification 0 0 0 2 4 6 8 10 12 14 12345678910 Time [h] Cycle[-]

Figure 5. Melting and solidifying Figure 6. Solidification temperature of

characteristics of D-Threitol D-Threitol against heating and cooling

cycle characteristics of D-Threitol

Mixture of D-Threitol and meso-Erythritol The DSC pattern (Temperature v.s. Heat flow) of mixture of D-Threitol and meso-Erythritol is shown in Figure 7. The mixture of 50 wt% D-Threitol and 50wt% meso-Erythritol melted with 243 kJ/kg of latent heat at 80.7-86.7-130.6°C. Comparing with D-Threitol, endothermic peak of mixture shifts to a lower temperature and is broad. The addition of meso-Erythritol demonstrates that the melting point of D-Threitol can be adjusted for use in a wide range of applications.

243 mJ/mg (80.7-86.8-130.6℃) 0

[mW] DSC -5

-10 0 50 100 150 200 Temperature [℃]

Figure 7. DSC chart of mixture of D-Threitol and meso-Erythritol (Temperature v.s. Heat flow)

CONCLUSIONS It was found that D-Threitol, isomer of meso-Erythritol, started to melt at around 90°C with a relatively large latent heat of 225 kJ/kg. And latent heat per unit volume of D-Threitol is almost equal to that of ice, 305kJ/dm3. D-Threitol started solidification when the temperature was cooled between at 40 degrees C and 46 degrees C, accompanied by a rapid rise to 89 degrees C in a lab-scale heating and cooling apparatus. It was then considered that D-Threitol was applicable as an environmental-friendly PCM for a hot water supply.

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