Thermal cracking is carried out in tubular reactor system with the heat supply from the reactor wall. The resistance in the radial direction to heat and mass transfer dictates the extent of reaction and product distribution obtained from the reactor. Annular reactors have a distinct advantage of lower diffusion path over the tubular reactors, which lowers the radial resistance. They also provide higher surface area which can be utilized for better heat transfer. The radial diffusional mass transfer in a fully developed laminar flow through an annular cylindrical reactor for a first order heterogeneous reaction at the wall has been studied by Houzelot and Villermox (1977). The contributions of the resistances to diffusion and to chemical reaction to the overall mass transfer resistance have been established. The annular reactors show higher mass transfer efficiency than tubular systems. Subramanian and Berhe (1976) have reported a theoretical treatment of the transient dispersion in an annular reactor with catalytic walls. A generalized model for whole cell hollow fibre reactor and annular reactor is given by Davis and Watson (1986). The annular catalytic reactors have been found to show better isothermal conditions, higher space velocities and reaction temperatures than the fixed bed reactors (Beretta et al. 1999). The annular reactor followed by the quencher having uniform and varying annular spaces have been compared in a study by Houzelot and Villermaux (1984) using plug flow model. Ferrer and Lede (1999) have developed a model for quencher with cylindrical and annular geometries, using plug flow conditions. The annular quencher is reported to give very high cooling rates. Bolton (2000) calculated the UV fluence rate distribution in an annular reactor and studied the significance of refraction and reflection.

An annular reactor-quencher assembly is considered in which the homogeneous vapour phase reversible cracking reaction of ethane is carried out. The annular reactor followed by an annular quencher of same dimensions with an intermediate gas mixer is assumed to be used for thermal cracking. The dominant homogeneous cracking reaction for ethane cracking is considered and all the other reactions are disregarded (Sundaram and Froment 1976). The annular reactor has uniform wall temperature, higher than the feed temperature. The reaction product stream from the reactor is cooled immediately in the quencher to reduce the undesirable product formation. Under quenching conditions, the reverse reaction influences the final product. The annular reactor walls i.e. the outside of inner wall and the inside of outer wall are taken to be at 800^oC. Both the walls of the quencher are maintained at 30^oC to minimize the reaction reversal. Under laminar flow conditions, the axial conduction and diffusion are neglected in comparison to the convective terms. The mass and energy balance equations constitute a set of coupled partial differential equations. The non-dimensionalized balance equations are solved numerically by the backward implicit finite difference scheme. The resulting tri-diagonal banded matrix is solved by using Srivastava's algorithm (Srivastava 1983). The quencher is found to suppress the undesired reactions effectively. A range of model parameters, which are dependant on the reaction conditions, have been studied to simulate the effect of these parameters on the reactor-quencher system output and temperature profiles.

References: 1. Beretta A., P. Baiardi, D. Prina and P. Forzatti, “Analysis of a Catalytic Annular Reactor for Very Short Contact Times”, Chem. Eng. Sci., 54, 765-773(1999)

2. Bolton J. R., “Calculation of Ultraviolet Fluence Rate Distributions in an Annular Reactor: Significance of Refraction and Reflection”, Water Research, 34, 3315-3324(2000)

3. Davis M. E. and L. T. Watson, “Mathematical Modeling of Annular Reactors”, The Chem. Eng. J., 33, 133 -142(1986)

4. Ferrer M. and Lédé J., “Modelling of the Quenching in Cylindrical and Annular Reactors”, Solar Energy, 66, 151- 163 (1999)

5. Houzelot J.L. and J. Villermaux, “Mass Transfer in Annular Cylindrical Reactors in Laminar Flow”, Chem. Eng. Sci., 32, 1465-1470(1977)

6. Houzelot J.L. and J. Villermaux, “A Novel Device for Quenching: The Cylindrical Annular Exchanger in Laminar Flow”, Chem. Eng. Sci., 39, 1409-1413(1984)

7. Srivastava V. K., “Thermal Cracking of Benzene in a Pipe Reactor”, PhD. Thesis, University of Wales, U.K.(1983)

8. Subramanian R. S. and S. Berrhe, “Unsteady Convective Diffusion in an Annular Catalytic Reactor”, Chem. Eng. Sci., 31, 1005-1017(1976)

9. Sundaram K. M. and G. F. Froment, “Modeling of Thermal Cracking Kinetics-I: Thermal Cracking of Ethane, Propane and their Mixtures”, Chem. Eng. Sci., 32, 601-608 (1977)