Madhu Prasad

Effect of packing density and adsorption parameters on the throughput of a thermal compressor

Vapour adsorption refrigeration systems (VAdS) have the advantage of scalability over a wide range of capacities ranging from a few watts to several kilowatts. In the first instance, the design of a system requires the characteristics of the adsorbate-adsorbent pair. Invariably, the void volume in the adsorbent reduces the throughput of the thermal compressor in a manner similar to the clearance volume in a reciprocating compressor. This paper presents a study of the activated carbon +HFC-134a (1,1,1,2-tetrafluoroethane) system as a possible pair for a typical refrigeration application. The aim of this study is to unfold the nexus between the adsorption parameters, achievable packing densities of charcoal and throughput of a thermal compressor. It is shown that for a thermal compressor, the adsorbent should not only have a high surface area, but should also be able to provide a high packing density. Given the adsorption characteristics of an adsorbent-adsorbate pair and the operating conditions, this paper discloses a method for the calculation of the minimum packing density necessary for an effective throughput of a thermal compressor

A method for the calculation of the adsorbed phase volume and pseudo-saturation pressure from adsorption isotherm data on activated carbon

We propose a new method for evaluating the adsorbed phase volume during physisorption of several gases on activated carbon specimens. We treat the adsorbed phase as another equilibrium phase which satisfies the Gibbs equation and hence assume that the law of rectilinear diameters is applicable. Since invariably the bulk gas phase densities are known along measured isotherms, the constants of the adsorbed phase volume can be regressed from the experimental data. We take the Dubinin-Astakhov isotherm as the model for verifying our hypothesis since it is one of the few equations that accounts for adsorbed phase volume changes. In addition, the pseudo-saturation pressure in the supercritical region is calculated by letting the index of the temperature term in Dubinins equation to be temperature dependent. Based on over 50 combinations of activated carbons and adsorbates (nitrogen, oxygen, argon, carbon dioxide, hydrocarbons and halocarbon refrigerants) it is observed that the proposed changes fit experimental data quite well.

Modeling of thermal conductivity of charcoal–nitrogen adsorption beds

A knowledge of the effective thermal conductivity of an adsorption compressor bed is necessary for the design of thermal compressors of adsorption-based coolers. Three existing models have been adopted to compute the effective thermal conductivity considering the adsorbent particles to be porous bodies. Although the models are tested for an activated charcoal–nitrogen system, the calculation scheme can be used for any adsorbent adsorbate pair. Experimental isotherm data are used to calculate the equilibrium concentration of nitrogen in charcoal. This calculation scheme has been validated by experimental thermal conductivity measurements covering the range 300–400 K at 1 atm and 100–1000 mbar at 300 K. The calculation scheme based on the Luikov et al. model is found to be the most successful. An empirical correlation is proposed to describe the dependence of effective thermal conductivity on the packing density and temperature.

Adsorption characteristics of the charcoal-nitrogen system at 79-320 K and pressures to 5 MPa

Adsorption isotherms are obtained for the charcoal-nitrogen system in the temperature range 79-320 K and pressures to 5 MPa for two samples of activated charcoal. Correlations relating temperature dependence of adsorption potential are obtained using four methods of extrapolating pseudo-saturation state beyond the critical point. The values obtained are compared with literature data. Using the experimental results and the BET theory surface area and micropore volume are calculated. It is brought out that samples of activated charcoal need to be characterized prior to their use in the design of low temperature adsorption based cryorefigeration systems

Adsorption parameters of activated charcoal from desorption studies

A procedure of extracting the adsorption parameters (\alpha and \gamma ) of an adsorbent–adsorbate pair, from desorption studies is described. The method is validated through experiments on a powder specimen of activated charcoal with nitrogen as adsorbate and comparing the results with isotherm data from volumetric method. Adsorption isotherms for two more granular samples of activated carbon are generated with nitrogen as adsorbate. The value of \gamma is found be about 0.1 for all samples, while \alpha values ranged from about 0.4 for granular samples to 0.6 for powder specimen.

Effect of packing density on performance of charcoal-nitrogen adsorption cryocoolers

This paper presents the development and validation of a computational scheme for an analysis of activated charcoal-nitrogen adsorption cryocoolers. Using this procedure a comparative analysis of four samples of charcoal is carried out. It is shown that the specific power (SP) of the cryocooler is governed by the packing density achievable in addition to the charcoal adsorption characteristics, cycle operating pressures and temperatures. A 77 K cryocooler with an SP of the order of 400 W W-1 can be achieved with a charcoal packing density of about 1 g cm(-3) for several available charcoals. The analysis shows that Fluka and BCGI specimens can yield promising results under the present technologically realizable conditions.

Development of a laboratory model of activated charcoal–nitrogen adsorption cryocooler

The development of a laboratory model of a charcoal-nitrogen adsorption cryocooler is described. The results on characterization of various components of this cooler and performance of the ensemble are presented. Four compressor cells are operated 90° out of phase to generate 0.0364 g/s (1.75 slpm) of nitrogen at a pressure of 5.2 MPa. The cycle time per compressor was 8 min, equally apportioned between heating and cooling. The high pressure gas is pre-cooled to 193 K and expanded through a J-T/heat exchanger assembly. A cold tip temperature of 124 K is obtained at a parasitic heat load of 0.36 W.