Adsorption cycle control

Cycle Control Valves are the heart of cycle control for cyclic adsorption systems. These on/off valves switch flows among beds so... 

Adsorption-Control Equipment If a gas stream must be treated for a short period, nsnally only one adsorption unit is necessary, provided, of course, that a sufficient time interval is available between adsorption cycles to permit regeneration. However, this is usually not the case. Since an nninternipted flow of treated gas is often required, it is necessary to employ one or more units capable of operating in this fashion. The units are designed to handle gas flows without interruption and are charac terized by their mode of contact, either staged or continuous. By far the most common type of adsorption system used to remove an objectionable pollutant from a gas stream consists of a number of fixed-bed units operating in such a sequence that the gas flow remains nninternipted. A two- or three-bed system is nsn ly... 

Table V lists the independent variables and their respective ranges which were included in this study. Dependent variables can be defined in numerous ways depending upon the objectives of the particular experimental series being conducted. Typically, a pressure swing adsorption cycle program is input to the apparatus s computer control program, e.g., per Figure 8, and the cycle repeated under computer control until the experimental objective is achieved. For this work, 6-24 cycles were normally used with total run times up to 8 hours.

A well-established stepwise procedure was followed [16, 23, 25, 56]. Small successive doses of the adsorptive were admitted and left in contact with the adsorbent until the thermal equilibrium was attained. The 1st run of adsorption performed on the activated sample (pretreated in high vacuum conditions and/or in controlled atmosphere) will be hereafter referred to as ads. I. At any individual dose of gas introduced in the system, the evolved heat AQ " was measured within the calorimetric cells, while the adsorbed amount Anads was measured by volumetry. Ads. I was followed by a desorption run (des. I), performed by simple evacuation of the cell. In such a way the reversibly adsorbed phase was desorbed and either the pristine surface was restored, in case of an entirely reversible adsorption, or the pristine surface was not recovered, in case of a (partially) irreversible adsorption. Ads. II was subsequently performed in order to assess which fraction (if any) of the pristine surface sites was irreversibly occupied by the adsorbed phase (in the adopted conditions). By subtracting the ads. II curve from the ads. I one, the adsorbed fraction not removed by evacuation is evaluated. The ads. n component will be hereafter referred to as the reversible adsorbed phase, whereas the (ads. I - ads. n) component will be referred to as the irreversible phase (in the adopted conditions). Subsequent runs of adsorption (ads. IE, IV etc.) are performed in some cases, if the irreversible modification of the surface is expected/suspected not to be extinguished during the ads. I [21, 23, 26]. Adsorption measurements are usually performed at least twice on a virgin portion of the same batch of the material, activated in the same conditions, to check the experiments reproducibility. The routinely run protocol of adsorption-desorption-adsorption cycles is schematically illustrated in Fig. 1.8. 

Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

Rapid Adsorption-Desorption Cycles For rapid cycles with particle diffusion controlling, when the cycle time is much smaller than the time constant for intraparticle transport, the LDF approximation becomes inaccurate. The generalized expression... 

Adsorption causes few technical difficulties. The kerosene is vaporized and fed either undiluted or diluted with a carrier gas into a fixed bed reactor. In contrast to adsorption, desorption is considerably more difficult, proceeds slower, and is therefore the rate-controlling step of the cycle. Particular attention was paid to this step during the technical development of the process. In order to be able to process continually, several reactors are operated at the same time and the adsorption and desorption carried out alternately. The adsorbate can be desorbed in various ways [16] ... 

Examples of multi-disciplinary innovation can also be found in the field of environmental catalysis such as a newly developed catalyst system for exhaust emission control in lean burn automobiles. Japanese workers [17] have successfully merged the disciplines of catalysis, adsorption and process control to develop a so-called NOx-Storage-Reduction (NSR) lean burn emission control system. This NSR catalyst employs barium oxide as an adsorbent which stores NOx as a nitrate under lean burn conditions. The adsorbent is regenerated in a very short fuel rich cycle during which the released NOx is reduced to nitrogen over a conventional three-way catalyst. A process control system ensures for the correct cycle times and minimizes the effect on motor performance. 

However, the principle idea in the studies of that time dealt with assessment of possible changes of inherent properties of a semiconductor caused by its interaction with gaseous phase. In other words, this question was directly linked with problems of quickly developing in that time semiconductor physics. The well known gas cycle of Bardeen-Brattain [81] provides a typical example of the situation of those days. This cycle deals with a opportunity to control the potential of the surface of a semiconductor by adsorption means. 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

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