Sorbents zeolites

The prerequisites of the evaluation of data characteristic of intracrystalline processes in the case of zeolite sorbents are discussed, along with the conditions under which diffusion can be compared to self-diffusion. Selected results of investigations carried out in the author s laboratory are given in order to demonstrate the consistency of sorption kinetic data with intracrystalline mobility data of single components on molecular sieves (HS). Various types of surface barrier which may influence the uptake rate are also described. 

The shape of a zeolite sorption uptake isotherm, a quantitation of the amount of a given sorbate taken up as a function of its partial pressure in the gas phase in equilibiitun with the zeolite sorbent, depends both on the zeolite sorbate interaction and on the sorbate - sorbate interactions. Simulation of such isotherms for one or more sorbates is accomplished by the Grand Canonical Monte Carlo method. Additional to the molecular reorientation and movement attempts is a particle creation or annihilation, the probability of which scales with the partial pressure [100,101]. This procedure thus simulates the eqmlibrium between the sorbed phase in the zeolite and an infinite gas / vapor bath. Reasonable reproduction of uptake isotherms for simple gases has been achieved for a small number of systems (e.g. [100,101]), and the molecular simulations have, for example, explained at a molecular level the discontinuity observed in the Ar - VPI-5 isotherm. 

The form of the equilibrium isotherms and the dependence of the heat of sorption on the degree of saturation indicate that most zeolite sorbents contain more than one type of sites or cells to accommodate sorbate molecules. In this general case there will be several different frequencies vn (or jumping probabilities) for the migration from a site of type i to a neighboring site of type j. If the degrees of occupancy... 

A customized Cu(I)Y zeolite is employed as a sorbent to actively collect CO in air samples to measure the concentration of CO in ambient air.The interaction is selective to CO only, but not to N2, O2, and CO2. The sorption process is facilitated by formation of Cu(l)-CO complexes, while CO can be desorbed at 300°C under helium flow for 2 min. Before the gas chromatographic analysis, a methanizer is used to reduce CO to CH4, which can then be quantified by FID. Detection limit of methane by this method is approximately 0.2 ppm. The laboratory data shows the capacity of the Cu(I)Y zeolite sorbent as 2.74 mg CO/g of sorbent. For a typical sorbent tube containing 0.5 g of treated zeolite sampling at the PEF of 50 ppm with a nominal flow rate of 100 ml/min, sampling can last as long as approximately 4 h before a breakthrough point is reached. Furthermore,... 

Heats of sorption for the rare gases on zeolitic sorbents may be calculated in essentially the same way as for sorption on graphite but the calculation is... 

PSA Reactor. The idea of a PSA reactor was first suggested by Vaporciyan aud Kadlec (1987 1989). The basic idea is to combine sorption and catalytic reaction in order to shift the thermodynamic equilibrium of the reaction. The sorbent selectively adsorbs one of the products and is regenerated during the low pressure half-cycle. By doing so, the conversion is increased and simultaneous separation is also accomphshed. Vaporciyan and Kadlec (1989) demonstrated the idea for CO oxidation reaction by mixing a 5A zeolite sorbent and a Pt/alumina catalyst in the adsorber. This idea was pursued by Sircar and co-workers (Carvill... 

The description in the literature of early gas desulphurization processes that utilize zeolites does not mention duly the formation of COS during the removal of H2S, if CO2 is present in the feed gas, except in a few cases, e.g., ref. [20,28,65]. Since modern desulphurization plants work in accordance with the same principles and utilize identical zeolite types, the COS formation reaction may have strong implications for the currently employed processes for desulphurization of gases by means of those sorbents. Therefore, it is necessary to investigate (i) the COS formation as dependence on the zeolite type, the type and content of cations in the sorbent, the concentration and contact time of reactants with the sorbent, the temperature and the conditions of co-adsorption (ii) the mechanism of that reaction on the sorbent with specific emphasis on its sorption and catalytic properties and (iii) to develop a mathematical model to simulate dynamic processes that proceed in adsorbers/reactors of technical dimension. This investigation should lead to novel formulations of modified zeolite sorbents and to alternatives with regard to operating conditions of sorption plants with the purpose of either minimization or maximization of the formation of COS. 

Auxiliary equipment like vacuum system, thermostat, a data acquisition system including a PC and safety installations also have to be provided. Depending on the specific surface and the density of the material used for examination an amount between 0.5 g and 2g has to be filled into the adsorption vessel. At the beginning of an experiment the sample material is activated by simultaneous evacuation and heating up of the adsorption vessel outside the thermostat. For activated carbon sorbent materials temperatures about (100°C - 150°C) are recommended. For zeolite sorbent materials often activation temperatures about (400°C) and even higher may be needed. 

If the two sorptive gas components (1,2) are mixed with a carrier gas (0) which is not adsorbed on the sorbent material considered, equations (4.15) have to be modified. This situation may occur for example in purification processes of air or natural gas including polar components which are strongly adsorbed on zeolitic sorbent materials compared to non-polar components like (N2, O2, CH4, etc.). Then the basic equations (4.2 - 4.5) should be substituted by... 

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