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Gas and liquid separations by adsorption adsorptive process development, multicomponent adsorption equilibria, kinetics and heats, physi- and chemi-sorbents, thermodynamics, diffusion in porous solids, nanoporous adsorbent membranes, hybrid sorption-reaction concepts.
Multicomponent Gas Adsorption Equilibria
One of the key topics of my research concerns the experimental and theoretical investigation of multicomponent gas adsorption equilibria, kinetics, and heats which are fundamental properties for design of adsorptive gas separation schemes. Such adsorptive processes have found numerous practical applications in the chemical, petrochemical, pharmaceutical and environmental industries in the areas of gas drying, air separation, hydrogen production, synthetic gas production, pollution abatement, land-fill gas treatment, drug purification, electronic gas purification, etc. Potential new areas of applications include hydrogen production for fuel cells, removal and recovery of global warming gases, development of nano-porous adsorbent membranes, etc.. Despite such versatile applications of adsorption technology, basic research on multicomponent gas adsorption equilibria and kinetics remains unsatisfactory. Experimental data for adsorption systems containing more than two components are rare. Theoretical models to predict multicomponent data are very system specific. They often work well when the adsorbate gases are nearly equal in size and non-polar, the adsorbent is nearly homogeneous and the lateral interactions between adsorbed molecules are absent. Unfortunately, most practical adsorption systems deal with polar as well as non-polar adsorbate molecules of unequal sizes on adsorbents which are energetically heterogeneous. Thus, the objective of my research in this area is to judiciously generate multicomponent adsorption data and to develop new theoretical models to incorporate the effects of adsorbate size differences and polarities, adsorbent heterogeneity, and lateral interactions in describing multicomponent adsorption equilibria and kinetics. .
Hydrogen Production By Steam-Methane Reforming
This research concerns the application of the thermal swing sorption enhanced reaction concept for direct production of essentially carbon oxide free hydrogen by steam-methane reforming (SMR). The concept uses an admixture of a CO2 chemisorbent and a SMR catalyst in a packed bed configuration. A mixture of steam and methane is passed through a regenerated bed at a temperature of 400 - 500?C to produce the product hydrogen. The concept, I believe, can potentially offer a compact single-unit-operation system for production of fuel-cell grade hydrogen for residential use. Simultaneous SMR reaction and removal of CO2 from the reaction zone by the chemisorbent produces the desired product by Le Chatelier�s principle. The chemisorbent is then regenerated by directly heating it with steam at ~600?C. The endothermic heat of SMR reaction is supplied by the sensible heat stored in the bed after the regeneration step. This research, then, deals with testing the feasibility of the concept and if successful, generating process performance data and process model for optimization. Specific research objectives include testing available CO2 chemisorbents, ascertaining their thermal stability under the conditions of operation of the concept, generating basic properties for ad(de) sorption of CO2 on the material under appropriate conditions, evaluating the appropriate ad(de)sorptive column dynamic properties, testing the process concept by simulating the individual sorption-reaction and regeneration steps, and finally carrying out the actual cyclic process steps.
Multi-Component Gas Adsorption Equilibria and Kinetics on Heterogeneous Adsorbents by Isotope Exchange Technique
Accurate knowledge of multicomponent gas adsorption equilibria and kinetics are critical for design and development of gas separation concepts using cyclic pressure and thermal swing adsorption, continuous nano-porous adsorbent membranes, sorption-enhanced reaction processes, ultrarapid adsorptive processes, etc. Unfortunately, multicomponent adsorption data on heterogeneous adsorbents of practical use are scarce and their estimation by existing models are often unsatisfactory. Lack of data also prevents serious testing of the available models and development of new theories.
This research is designed to alleviate this problem by systematically measuring multicomponent adsorption equilibria and kinetics using a variety of polar and non-polar gas mixtures (up to four components) of different molecular sizes on various synthetically produced heterogeneous adsorbents. The data will be used to critically evaluate existing models and develop new analytical models which explicitly account for the effects of adsorbate size difference and adsorbent heterogeneity with or without lateral interactions. The recently developed isotope exchange technique will be used for simultaneous measurement of adsorption equilibria and kinetics. Isothermal measurement of adsorption kinetics and complete control over the final equilibrium states are two key features of this method. Failure to achieve these conditions by most conventional methods creates ambiguity and complication in data analysis.
Heat of Adsorption for Multicomponent Gas Mixtures by Differential Calorimetry
All practical adsorptive gas separation processes are carried out non-isothermally in adiabatic adsorbers. Isosteric heats of adsorption determine the local temperature changes inside the adsorber, which in turn, govern the local adsorption equilibria and kinetics, and thus the overall separation performance. Unfortunately multicomponent heats of adsorption are the least studied topic in adsorption science and technology. Experimental data are practically non-existing. Practical adsorbents are energetically heterogeneous where heats decrease with surface coverages. Lateral interactions between adsorbed molecules at higher coverages increase heats with increasing loadings. These functionalities are very complex and they play a critical role in process design. They must be measured or predicted. The current design practice of assuming constant heats of adsorption for the gas components often leads to misleading analysis of process data and ambiguous scale-up.
The purpose of this research is to measure pure and multicomponent heats of adsorption using a Tian-Calvet type differential calorimeter which has recently been developed and tested for this purpose. Data will be measured systematically using a variety of binary, ternary and quaternary gas mixtures consisting of adsorbates of different sizes and polarities on homogenous and synthetically produced heterogeneous adsorbents (mixtures of homogeneous zeolites) over a large range of conditions. A patch-wise homogeneous model of heterogeneous solid will be developed to describe multicomponent heats and it will be tested using the measured data.
Gas-Solid Heat Transfer in Packed Bed Adsorbers
The knowledge of gas-solid heat transfer coefficients in a packed bed adsorber can be a critical variable for design. This is particularly true for certain steps of practical pressure swing or thermal swing adsorption processes where the gas flow rates through the adsorber are low. These steps include the critical desorption steps as well as the heating and cooling steps. It is commonly assumed that instantaneous gas-solid thermal equilibrium is achieved during these steps even though the actual heat transfer coefficients can be very low. Numerous experimental and theoretical studies of gas-solid heat transfer in packed beds have been published but they are all limited to the relatively simpler cases of heating or cooling a bed of solid particles by an inert gas. The gas-solid heat transfer in an adsorption column is accompanied by transfer of a substantial amount of gas molecules into the porous solid or vice versa with simultaneous generation or consumption of heat within the solid. The goals of the present work are to (a) experimentally measure gas-solid heat transfer under actual ad(de)sorption conditions using different gas flow rates with different initial and final adsorbate loadings, (b) test the predictability of the measured data by the existing theoretical models, and (c) propose new models for estimation (or correlation) of gas-solid heat transfer coefficients in a packed bed adsorber.