Timur Doǧu

Deactivation model for textural effects of kinetics of gas-solid noncatalytic reactions “char gasification with CO2”

Abstract Variation of the reactivity of a solid reactant due to changes in number of working active sites and pore structure was represented by a deactivation model. The model was shown to give good aggreement with the experimental data obtained for the gasification of chars of different lignites. All the kinetic data obtained at different temperatures (800°–950°C) and also for different types of coals were represented with a single generalized relation. The increase of the surface area of the char at the initial stages of gasification did not cause a corresponding increase in the reaction rate. Results indicated that the surface was composed of highly reactive and less reactive sites. It was shown that the reactivity of unit surface area of the solid reactant first showed a rapid decrease due to decrease of number of more reactive sites as the initial stages and then reached to about one fourth of its initial value.

Single pellet reactor for the dynamic analysis of gas-solid reactions “reaction of SO2 with activated soda”

Single-pellet moment technique which was initially proposed for the measurement of effective diffusivities in porous catalysts and adsorption rate and equilibrium constants was modified for the analysis of gas-solid reactions. In this technique dynamic version of the Wicke-Kallenbach single-pellet cell was used. A pulse of a tracer gas was introduced into stream flowing over one end face of the pellet. In the modified pulse-double response procedure, experimental moments of the response peaks of both reactant and product gases measured in both top and bottom outlet streams of the single-pellet reactor were used for the evaluation of intrapellet rate and equilibrium parameters. Application of the technique to the reaction of SO 2 with activated soda indicated that the value of Thiele modulus decreased from 6.74 to 0.33 by the increase of fractional conversion of Na 2 CO 3 to Na 2 SO 3 from zero to 0.63. In this range of conversion values a significant variation in pore structure and also a decrease of effective diffusivity of SO 2 from 0.045 cm 2 /s to 0.025 cm 2 /s was observed. A cell model which considered the changes in pore length and radius with reaction extent and variations in product layer diffusion resistance was shown to be successfully used in the analysis of experimental observations. A review of single-pellet dynamic reactor studies for catalytic and noncatalytic gas-solid reactions was also given.

Dynamic analysis of a trickle bed reactor by moment technique

The performance of a trickle bed reactor is investigated by the moment technique. Residence time distributions of SO2 tracer in both gas (Helium) and liquid (distilled water) effluents are used to predict zero reduced and first absolute moments and these values are compared with the derived theoretical expressions. Correlations are suggested for gas-liquid mass transfer coefficient, liquid hold up, and extent of axial mixing in liquid phase. True adsorption equilibrium constant of the system is estimated as 0.378 from liquid full bed experiments and contacting efficiency of the trickle bed reactor is found as 0.987. Effect of axial dispersion is not significant on gas-liquid mass transfer coefficient since absorption factor is small, but is found to be quite important on the true estimation of adsorption factor.

A moment method for the analysis of a two-phase model for fluidized beds

Abstract The parameters of a two-phase model for fluidized beds are determined by the application of a moment method. The experimental results obtained for four different fluidized beds with different diameters indicate that the dependences on radial position in the bed of the number of eddy diffusion units, of the number of transfer units and of the fractional flow through the bubble phase are negligible. The variation of these parameters with bed height is found to be small, especially in columns with diameters of less than 4 cm. The volume fraction of the bubble phase is found to be correlated with the superficial gas velocity.

The effect of axial dispersion on mass transfer between gases and liquids in trickle bed reactors

Abstract The cocurrent flow of gas and liquid through a packed bed is an extensively used operation in chemical industries where mass transfer and fluid dynamics affect the design equations. Reported overall mass transfer coefficients between gases and liquids are considered usually with the assumptions of plug flow for both phase [1]. That may be valid for gas phase, however liquid backmixing, where axially dispersed plug flow model is an adequate representation, are expected especially for short trickle bed reactors [2]. The effect of axial dispersion on mass transfer coefficients should be minimized and the true overall mass transfer coefficients should be used for design purposes. According to the model transient mass conservation equations for gas and liquid phases are where C is concentration, u is superficial velocity, K L a is overall gas liquid mass transfer coefficient, D L is axial dispersion coefficient, ϵ is hold up volume fraction, H is reciprocal of Henrys law constant. The subscripts G and L stands for gas and liquid respectively. Initial and boundary conditions can be stated as; at t = 0, C G = C L = O for all z; at z = 0, C G = Mδ(t), and −D L ∂C L /∂z + u L C L = 0; at z = Z, ∂C L /∂z = 0 at any time t. Simultaneous solution of equations 1 and 2 with boundary conditions resulted the following expression for m * OL that is the fraction of species transferred to liquid phase in infinite time at column height z. where When axial dispersion is neglected, A = 1 and B is a function of K La only [3]. The model may consider adsorption b including a similar mass conservation equation written for the species in the pores of catalyst particles and mass transfer term from liquid to solid eqn. ( 2 ). Experimental studies are done with nitrogen flowing concurrently with water at 20 °C and 1 atmosphere in a laboratory size trickle bed reactor packed with active carbon pellets. Impulse of sulfur dioxide is given to gas phase.