Melkon Tatlier

Effect of zeolite particle size on the performance of polymer–zeolite mixed matrix membranes

The effect of zeolite particle size on the performance of silicalite–PDMS mixed matrix membranes is investigated at two different zeolite loadings. The separation properties of the membranes prepared are characterized by permeability measurements for O2, N2 and CO2 gases. The permeabilities of the silicalite–PDMS mixed matrix membranes are determined to increase with increasing particle size. The variations occurring in the permeability values with changes made in the particle size are much more pronounced at the higher zeolite loading. The ideal selectivity values corresponding to the mixed matrix membranes, on the other hand, generally seem to be less affected by the changes made in the particle size. The permeability values corresponding to the mixed matrix membranes exceed those pertaining to the original polymer membrane only at relatively higher zeolite loadings and/or for relatively larger particle sizes. The variations occurring in the permeabilities and selectivities with changes made in the zeolite particle size may be responsible for the different values of these parameters reported in the literature for the same types of zeolite filled polymeric membranes.

A novel approach to enhance heat and mass transfer in adsorption heat pumps using the zeolite–water pair

A mathematical model is developed for a novel arrangement proposed in order to cope with the drawbacks originating from the inefficient heat and mass transfer in adsorption heat pumps. The zeolite, used as the adsorbent, is envisioned to be synthesized on the surfaces of heat exchanger tubes as continuous coatings, which was also achieved experimentally. Simplifications regarding the boundary conditions are made by using the scaling method and a finite difference scheme is employed for the solution of the resulting differential equations to determine the variation of the temperature of the adsorbent as well as the concentration of the adsorbate within the adsorber. The typical restrictions of adsorption heat pumps are observed to be removed to a great extent and the duration of a single cycle is shortened significantly.

Preparation of zeolite coatings by direct heating of the substrates

A novel method is proposed for the preparation of zeolite coatings on substrates, which suppresses the reaction in the bulk and promotes that on the substrate by applying a temperature difference between the reaction mixture and the substrate. The substrate is heated directly, with a soldering resistance in this study, while the reaction mixture is kept at a lower temperature by means of a water bath. The method is tested for the case of zeolite 4A synthesis on stainless steel plates from a clear aluminosilicate solution. As a result of suppressed activity, the composition of the solution remains nearly constant and the solution side of the coating always experiences a lower temperature. As a result, the phase transformations of the metastable zeolites can be delayed for extremely long periods of time, depending on the volume of the reaction mixture. The deposition of crystals from the solution onto the surface of the coating is also avoided. Crystalline coatings of zeolite 4A could be prepared by the appropriate selection of the temperatures of the water bath and the substrate. A final treatment, which involves an increase in the temperature of the water bath, may be applied to the coatings at the end of their preparation, in order to remove the sparse amount of impurities observed at certain synthesis times and to increase the crystallinities when necessary. The mass of the zeolite coated on the substrate is shown to increase significantly with respect to that obtained by conventional synthesis when the proposed method is utilized. The method can thus be useful for some applications where the employment of relatively thicker zeolite coatings of metastable phases may be desired.

Effects of low-temperature gel aging on the synthesis of zeolite Y at different alkalinities

The effects of low-temperature gel aging on the synthesis of zeolite Y were investigated. The variations occurring in the crystal sizes and size distribution as well as in the crystallinity and Si/Al ratio of the solid zeolite samples with aging time were determined for two different aging temperatures and two gel compositions in which the alkalinity was varied. It was determined that both the crystal sizes and the span of the size distribution may be decreased further by aging the gel at temperatures below room temperature. The alkalinity of the gel composition seemed to play a significant role in determining the magnitude of the effects of lowering the aging temperature on crystal sizes and the size distributions of the aged zeolite samples. The Si/Al ratio of zeolite Y was enhanced when the lower aging temperature and the less alkaline gel composition were used.

Lower temperatures for the preparation of thinner zeolite A coatings

The possibility of preparing thin zeolite 4A coatings on quartz and stainless steel substrates by direct synthesis from a clear aluminosilicate solution has been investigated. In this context, the effects that originate from the synthesis temperature on the thickness of the zeolite coatings have been determined. The nucleation curves and the crystal growth rates pertaining to the coatings prepared at different temperatures have been obtained by taking into account the variation of the numbers and sizes of the zeolite particles and the sizes of the largest crystals on the substrates, respectively, with respect to time. The apparent activation energies corresponding to nucleation and crystal growth have been estimated from these data. It has been determined that the thickness of closed zeolite 4A layers may be decreased at relatively lower synthesis temperatures in case relatively longer synthesis times are employed. The reduction obtained in the thickness of the zeolite coatings with decreasing temperature is due to the rate of nucleation being less temperature sensitive than the rate of crystal growth. The crystal growth activation energies corresponding to the synthesis of zeolite 4A coatings on stainless steel plates are noticeably higher than those obtained when quartz plates are employed while the apparent activation energies for nucleation on these two substrates are quite close. Zeolite 4A coatings of 1 and 0.7 μm thickness have been prepared on quartz and stainless steel substrates, respectively, at 45°C.

Effects of metal mass on the performance of adsorption heat pumps utilizing zeolite 4A coatings synthesized on heat exchanger tubes

Abstract The effects of the wall thickness of stainless steel heat exchanger tubes on the performance of adsorption machines, employing zeolite 4A coatings synthesized on metal heat exchanger tubes, are investigated. A recently developed mathematical model is used to determine the cycle durations when various wall thicknesses of the heat exchanger tubes as well as different zeolite layer thicknesses are utilized. For each case, the power and the COP cycle values of the system are estimated. In general, very high power and quite low COP cycle values are obtained when the proposed arrangement is utilized in the adsorption heat pumps. The zeolite layer thicknesses that may result in obtaining high COP cycle values are generally much higher than the optimum layer thickness value that maximizes the power and the utilization of layers thicker than the optimum value may lead to significant extensions in the cycle durations and hence to a decrease in the power obtained from the system. Decreasing the wall thickness of the heat exchanger tubes increases both the power and the COP cycle values when the optimum zeolite layer thickness for each wall thickness is taken into account. The possibility of such an enhancement will most probably be limited by the minimum wall thickness value that can actually be obtained by the available technology. The COP values of adsorption heat pumps may also be increased by using regenerative processes. Due to the generally low COP values obtained, the proposed arrangement seems especially suitable to be employed in adsorption machines utilizing energy sources of low economical value, such as waste heat. An optimum compromise between the COP value, which is closely related to the operating costs, and the power of the system should be provided, in case more valuable energy sources are utilized.

Artificial neural network methods for the estimation of zeolite molar compositions that form from different reaction mixtures

The possibility of using artificial neural network (ANNs) methods for the estimation of the zeolite molar composition and hence the zeolite phase that may be obtained from a certain initial reaction mixture composition was investigated. Three different artificial neural network methods, namely feed forward back propagation (FFBP), radial basis function-based neural networks (RBF) and generalized regression neural networks (GRNN), were tested for this purpose. A data set obtained from the literature was used in the training of the neural networks. The results obtained for a second data set were compared to experimental findings as well as to estimations made by using multilinear and non-linear regression. It was determined that the neural networks learn quite efficiently from experimental zeolite synthesis data. The predictions made by using artificial neural network methods were, in general, more reliable than those performed by regression. The best prediction of the Si contents of the zeolites investigated were made by the GRNN and FFBP methods while the H2O content was predicted better by the RBF method. The results indicate that using artificial neural network methods may decrease significantly the number of experiments that have to be performed to discover new synthesis compositions.

When do thin zeolite layers and a large void volume in the adsorber limit the performance of adsorption heat pumps

Abstract In order to allow for the required pressure change in the adsorption heat pumps during the isosteric heating and cooling periods, a sufficiently high ratio between the zeolite mass and the void volume should be provided in the system. In this study, the effect of the percentage of void volume in the adsorber on the p – T relationship of the zeolite 4A-water pair during the heating period was investigated for various thicknesses of zeolite coatings and radii of heat exchanger tubes. As a result, it was determined that a significant reduction in the performances of the adsorption heat pumps does not occur for the parameter values of practical concern. Even very thin zeolite coatings (5 μm) may be used with a high void volume (e.g. 80%) in the adsorber. Calculations in a recent study, leading to the claim that coatings of thicknesses

The effects of thermal gradients in a solar adsorption heat pump utilizing the zeolite–water pair

Abstract An arrangement involving zeolite synthesized on metal wire gauzes was recently proposed to eliminate the limitations caused by insufficient heat transfer within the solar collector and a related mathematical model was presented. According to such an arrangement, zeolite 4A is envisioned to be synthesized as continuous coatings on stainless steel wire gauzes which are assumed to be placed in the collector vertically in a manner assuring a firm contact between the absorber plate and the wire gauzes. In this study, the same mathematical model is employed to determine the efficiency of the solar adsorption heat pumps under various conditions and the results are compared with those obtained by assuming the employment of a conventional arrangement leading to various amounts of thermal gradients in the solar collector. The effects of the thermal gradients on the performance of the system are investigated by taking into account the employment of zeolite 4A and zeolite 13X, and by varying the weather conditions as well as the difference between the condenser and evaporator temperatures. As a result, the presence of the thermal gradients in a solar collector is observed to decrease the COPsolar values, indicators of the efficiency of the system, to a great extent, especially when a large amount of zeolite is to be utilized. In the case of the employment of zeolite 13X, the adverse effects of thermal gradients on COPsolar are more pronounced.

THE PERFORMANCE ANALYSIS OF A SOLAR ADSORPTION HEAT PUMP UTILIZING ZEOLITE COATINGS ON METAL SUPPORTS

The presence of temperature and concentration gradients inside a solar collector may decrease the efficiency of a solar adsorption heat pump. In this study, an arrangement involving zeolite synthesized on metal wire gauzes is proposed.in order to remove any possible limitations originating from the inefficient heat and mass transfer inside the solar collector. Accordingly, zeolite 4A is envisioned to be synthesized as continuous coatings on stainless steel wire gauzes which are assumed to be placed in the collector vertically in a manner assuring a firm contact between the absorber plate and the wire gauzes. A related mathematical model is presented which is solved by using the Euler-explicit, finite difference scheme and the effects of such an arrangement on the COP values, indicators of the efficiency of the solar adsorption heat pumps, are investigated. The mass of the zeolite and the inert materials, as well as the condenser and evaporator temperatures are taken into consideration. As a result, no possible drawback concerning the design of such an arrangement is anticipated and a zeolite layer thickness around 50-100μm; is determined to be sufficient to neglect the adverse effect of the mass of the metal wire gauzes on the efficiency of the solar adsorption heat pumps