Contact time distributions

The yield of a gas-solid heterogeneous reaction depends not on the total time that molecules spend in the reactor but on the time that they spend on the catalyst surface. The contact time distribution provides a standardized measure of times spent in the absorbed state. A functional definition is provided by the following equation applicable to a first-order, heterogeneous reaction in an isothermal reactor  

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Shinnar, R., Use of Residence and Contact Time Distributions in Reactor Design, Chapter 2, pp. 63-149 of Chemical Reaction and Reactor Engineering, Carberry, J. J. and Varma, A., Eds., Marcel Dekker, New York, 1987. 

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

Literature references and the measurement of the contact time distribution in a large, cold-flow model of a gas-fluidized bed are reported in... 

Contact Time Distribution Models. To overcome this difficulty and still use the information given by the RTD, models were proposed which assumed that faster gas stayed mainly in the bubble phase, the slower in the emulsion. Gilliland and Knudsen (1971) used this approach and proposed that the effective rate constant depends on the length of stay of the element of gas in the bed, thus... 

Glasser, D., Katz, S., and Shinnar, R., the measurement and interpretation of contact time distributions for catalytic reactor characterization./nd. Eng. Chem. Fundament. 12,165 (1973). 

E t) contact time distribution function in penetration theory (s )... 

Porter,K. "The effect of contact time distribution on gas absorption with chemical reaction".Trans.Instn.Chem.Enqrs. 44 (1966) T25. 

Contact Time Distribution Model Concurrent Countercurrent... 

The above equation describes the conversion and is referred to as the contact time distribution (CTD) model. Although improvement was made over the RTD models, the problem with this approach involves obtaining a meaningful E function to use in Eq. (3)... 

Fluidized-bed reactors. Tracers have been used to evaluate backmixing of the gas, gas-solid contacting, solids backmixing, and the contact time distribution, i.e. the probability density function of sojourn times in the reaction environment. The literature in this area is rather abundant and unfortunately to some extent confused. No attempt is made here at a comprehensive review. Rather, we want to point out some highlights and indicate that the interpretation of all reported results is straightforward in view of the theory reviewed in Section 6.1.1. 

Nauman and Collinge (159,160) expanded upon the concept of contact time distributions introduced by Orcutt et. al. (157) and showed how to obtain it by use of an adsorbing and a nonadsorbing tracer or of a pair of adsorbing tracers with different adsorption rate constants. The tracers must adsorb linearly and reversibly. 

Nauman, E. B. and C. N. Collinge. Measurement of Contact Time Distribution in Gas Fluidized Beds. Chem. Eng. Science 23 (1968) 1317. 

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