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A scaling and numerical analysis of the effects of natural convection when Sal'nikov's reaction: P → A → B occurs, together with diffusion and heat transfer in a spherical batch reactor

Campbell, AN, Cardoso, SSS and Hayhurst, AN (2007) A scaling and numerical analysis of the effects of natural convection when Sal'nikov's reaction: P → A → B occurs, together with diffusion and heat transfer in a spherical batch reactor In: Nonisothermal phenomena and processes, 2006-11-27 - 2006-12-01, Yerevan, Armenia.

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Abstract

Sal'nikov's reaction: P→A→B involves a precursor, P, in two consecutive, first-order chemical reactions, yielding a final product B via an intermediate A. Partly as an academic exercise, but partly because of its relationship with cool flames, the situation is considered where the second step is faster than the first one, which is taken to be thermoneutral without an activation energy. The second step is assumed to have a significant activation energy, although it is exothermic. The reaction proceeds batchwise inside a spherical reactor, whose walls are held at a constant temperature, but do not participate chemically. Natural convection becomes important, once the temperature is high enough for the Rayleigh number (Ra) to reach ~ 103. The subsequent behaviour of the system depends on the interaction between convection, diffusion of heat and mass, and chemical kinetics. By examining the governing equations, we develop and evaluate scales for the characteristic velocity, the concentration of the intermediate A and the temperature rise during the progress of the reaction, for the two extreme cases when transport is dominated, in turn, by diffusion and then by natural convection. These scales depend on the characteristic timescales for the interacting phenomena of chemical reaction, diffusion and natural convection. Typically, the characteristic velocity in a relatively small reactor of radius 0.27 m is as large as 0.3 m s-1, when the temperature rise is 100 K near the centre of the vessel. These theoretical predictions from scaling are verified by full numerical simulations. Oscillations of both the temperature and the concentration of the intermediate, A, can occur and the conditions for their appearance are identified. Any accompanying flow field proves to be toroidal, with the fluid ascending close to the reactor's axis, but descending adjacent to its walls. In addition, the effects of variables, such as the initial temperature of the batch reactor and its contents, the pressure and also the size of the reactor are all assessed, together with a consideration of what happens when the reaction proceeds in the liquid phase. In this case, because of the different physical properties of a liquid and a gas, natural convection is more intense than in the gas-phase and is quite likely to lead to turbulence and good mixing.

Item Type: Conference or Workshop Item (UNSPECIFIED)
Authors :
NameEmailORCID
Campbell, ANa.n.campbell@surrey.ac.ukUNSPECIFIED
Cardoso, SSSUNSPECIFIEDUNSPECIFIED
Hayhurst, ANUNSPECIFIEDUNSPECIFIED
Date : June 2007
Depositing User : Symplectic Elements
Date Deposited : 17 May 2017 12:28
Last Modified : 17 May 2017 15:04
URI: http://epubs.surrey.ac.uk/id/eprint/835493

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