(90g) Performances of Coatings of Various Zeolites for Heating/Cooling Applications

Heat and mass transfer limitations present in adsorption heat pumps, which may be used in heating and cooling applications, may be eliminated to a great extent by using zeolite coatings on metal surfaces. The stability, thickness and texture of coatings are significant parameters affecting the performances of these materials. An optimum coating thickness, maximizing the power obtained for these devices, has been shown to exist [1]. One research direction to improve the performances of sorption devices is to increase the sorption capacity of the adsorbent. For this aim, new materials, such as mesoporous adsorbents impregnated by salts and metal organic frameworks (MOFs) have been prepared. It is also lately reported that some ionic forms of zeolites have notably higher water adsorption capacities than the sodium form, in which they are originally prepared. Improvements may also be obtained by using adsorbents that may use relatively low regeneration/desorption temperatures, such as zeolite Y and SAPO-34. Direct crystallization of zeolites, A, X, Y and SAPO-34 on various supports has actually been accomplished [2-4]. LiNaA and LiNaX coatings have been prepared by the ion exchange of NaA and NaX zeolite coatings in LiCl solutions [5].

In this study, a mathematical model developed previously [1] was used to determine the performances of coatings of zeolites NaX, LiX, NaA, NaY and SAPO-34. Different operating conditions and various coating thicknesses were taken into consideration. Calculations were made to determine the power of a refrigerating system. This was performed by determining the duration of a single cycle of the device, including the four distinct periods of heating, desorption, cooling and adsorption. The cycle durations were calculated from the simulations carried out for the operation of the adsorber during its consecutive heating and cooling. The mass and energy balances written for both the metal wall of heat exchanger tubes and the adsorbent layer, together with appropriate initial and boundary conditions, allowed the determination of the temperature and concentration distributions in the adsorbent as well as the temperature gradient across the metal wall in the radial direction. A computer program utilizing implicit finite difference scheme and an iterative solution procedure was developed in MATLAB language and was employed to obtain the numerical solutions. Six subintervals were utilized in the radial direction to represent each of the two sections in the system investigated, namely, the metal wall of the heat exchanger tube and the adsorbent layer on the wall of the tube. The temperatures of the cold and hot heat exchange fluids in the stainless steel tubes (adsorption and desorption temperatures) were taken as 25 °C/50 °C and 100 °C/150 °C, respectively. The evaporator and condenser temperatures were equal to 2 °C and 25 °C, respectively.

All the zeolites investigated had noteworthy water sorption capacities but different regeneration temperatures and water diffusivities. It was observed that the optimum coating thicknesses were generally higher for the materials with higher water diffusivity. SAPO-34 generally provided relatively high maximum cooling power, owing to its high water sorption capacity coupled with the relatively low regeneration temperature. However, mass transfer resistances became quite significant at relatively high coating thicknesses, originating from the rather slow water diffusion in this zeolite. Utilizing a relatively low desorption temperature of 100 °C, instead of 150 °C, favored the relative performance of SAPO-34 coatings further. The strong temperature dependence of water diffusion in zeolites X and Y reduced the performances of these materials under these conditions. LiX and NaY coatings performed slightly better than NaX coatings while the power provided by NaA was closest to the power obtained by using SAPO-34, especially at relatively high coating thicknesses. The enhancement of the adsorption temperature from 25 °C to 50 °C resulted in improved performances for A, X and Y type coatings. NaA coating exhibited the highest performance in this case, for coating thicknesses above 50 mm. When enhanced diffusivity values were used in the calculations, to represent coatings with more open texture, instead of compact nature, the cooling power increased notably for all the zeolites investigated. The results indicated the significance of selecting the most proper temperature range and coating thickness for each zeolite to obtain the maximum profit from these materials in adsorption heat pump/cooling applications.

References

1. M. Tatlier, A. Erdem-Senatalar, Micropor. Mesopor. Mater. 28, 195-203 (1999).

2. L. Schnabel, M. Tatlier, F. Schmidt, A. Erdem-Senatalar, Appl. Therm. Eng. 30, 1409-1416 (2010).

3. A. Freni, L. Bonaccorsi, L. Calabrese, A. Capri, A. Frazzica, A. Sapienza, Appl. Therm. Eng. 82, 1-7 (2015).

4. M. Tatlier, L. Rustam, G. Munz, Chem. Eng. Comm. 206, 953-966 (2019).

5. M. Tatlier, C. Atalay-Oral, J. Sol-Gel Sci. Tech. 91, 117-126 (2019).