The energetic and/or economic optimisation or heterogeneous reactors

Abstract

The first part of this paper presents the bases for the technico-economic optimisation of reactors. The optimum design of a reactor is by minimising an objective function which, depending on the choice of the decision-maker and for a given production rate, could be : - the cost in the national currency - the cost in foreign exchange - the consumption of primary energy - the consumption of exergy. In addition the decision-maker could in each case also aim to minimise : - the cost of equipment (investment) - the operating cost - the total cost, that is the weighted sum of the capital equipment cost and the operating cost for a chosen period of amortizement. These various optima are compared one with another, notably those which minimise money cost and those which minimise the consumption of primary energy. It is shown that between each there exist invariant ratios : for each optimum the contribution of each of the terms (equipment, operating…) to the total cost is a constant and is independent of economic factors. It is only a function of technical factors such as the flow regime (laminar or turbulent) transport process (diffusion, convection…). In the part two, the general concepts developped in part one are applied to the design of heterogeneous catalytic reactors (with fixed, fluidised or transported particles). A “simplissime” model is proposed which depends on only one parameter representing the hydrodynamic structure of the flow in the reactor. This model includes various types of flow structure : short circuit, recycle, total or partial macro-mixing, anisotropic jet hitting the active surface, etc… A general expression is established for the “Specific Operating Consumption of Mechanical Energy” (or SOCME) in joules per kg of useful product. The way the SOCME varies with the process parameters, especially the Reynolds number is examined for the main operating regimes of the reactor. - Volume effect regime : transport by convection - macro-mixing - short circuiting. - Surface effect regime : heterogeneous chemical reaction or transport by diffusion through the boundary layer. Operating conditions for which the limitation due to diffusion in the boundary layer is negligible are defined for the general case of any reactor. Illustration is given of how to apply these concepts to the optimisation of a fixed bed reactor where the requirement is to simultaneously : - maximise the productivity - minimise the energy consumption - minimise the construction cost of the reactor. Thus in any optimized reactor, the consumption of mechanical operating energy is a non-negligible fraction of the total optimisable expenditure (from 10 to 30 %) and this is valid even and especially in the chemical reaction regime. It may be concluded that in the future, reactors which will be required to consume less and less energy, should no longer be designed to operate in the chemical reaction regime but should be allowed to have certain physical rate limitations whether diffusional or convective.