Plug flow reactor fast mixing

There is one further point of comparison. Interpretation of results from a stirred-tank reactor depends on the assumption that the contents of the tank are well mixed. Interpretation of results from a tubular reactor rests on the assumption of plug flow unless the flow is laminar and is treated as such. Which of these two assumptions can be met most satisfactorily in practical experiments Unless the viscosity of the reaction mixture is high or the reaction extremely fast, a high speed stirrer is very effective in maintaining the contents of a stirred tank uniform. On the other hand, a tubular reactor may have to be very carefully designed if back-mixing is to be completely eliminated, and in most practical situations there is an element of uncertainty about whether the plug flow assumption is valid. 

Most of the models assume that neutral-species transport can be represented with either a well-mixed model or a plug flow model. The major drawback to these assumptions is that important inelastic rate processes such as molecular dissociation are usually localized in space in the reactor and are often fast compared with rates of diffusion or convection. As a result, the spatial variation of fluid flow in the reactor must be accounted for. This variation introduces a major complication in the model, because the solution of the nonisothermal Navier-Stokes equations in multidimensional geometries is expensive and difficult. 

The plug-flow model indicates that the fluid velocity profile is plug shaped, that is, is uniform at all radial positions, fact which normally involves turbulent flow conditions, such that the fluid constituents are well-mixed [99], Additionally, it is considered that the fixed-bed adsorption reactor is packed randomly with adsorbent particles that are fresh or have just been regenerated [103], Moreover, in this adsorption separation process, a rate process and a thermodynamic equilibrium take place, where individual parts of the system react so fast that for practical purposes local equilibrium can be assumed [99], Clearly, the adsorption process is supposed to be very fast relative to the convection and diffusion effects consequently, local equilibrium will exist close to the adsorbent beads [2,103], Further assumptions are that no chemical reactions takes place in the column and that only mass transfer by convection is important. 

The plane front is the most favourable type for fast chemical reactions with low MW compounds. This mode is intermediate between the torch and drift, due to the strongly developed turbulence in reactant mixing zone 1 (coaxial supply) and zone 2 (radial supply). The plane front is characterised by the homogeneity of the reaction flow composition along the reactor radius, which is typical of the quasi-plug flow mode in turbulent flows. 

The torch front and plane front are also formed during the simple mixing of coloured flows without any chemical reactions [8-11]. Thus, these fronts of liquid flow mixing can appear both during fast chemical reactions and without any reactions. This fact led to further investigation into the influence of the reaction rate constant on the conditions of quasi-plug flow mode formation in turbulent flows (plane front of reaction). Solutions of reactants were prepared which interacted with each other, at different rate constants, with the formation of coloured products these solutions were introduced into the tubular reactor for the formation of reaction front macrostructures. The second-order reactions, which occur at rate constants in the range of = 10 -10 1/mol-s, were studied ... 

Thus, Equations 4.1-4.3, in the case of the homogeneous mixing of liquid flows, characterised by density and viscosity, provide an efficient commercial application of tubular turbulent reactors at almost any stage limited by mass exchange. The optimal operation conditions of the tubular turbulent reactors in the quasi-plug flow mode can be calculated due to the changes of the physical characteristics of the liquid flow, the kinetic parameters of the fast chemical reactions and the reaction construction parameters. 

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