Theory of scaling up and hydrodynamic modelling of industrial mass transfer equipment

Abstract

The hydrodynamic nature of the scaling effect (i.e. the decrease in efficiency of mass transfer columns with increasing dimensions) is discussed. It is shown that this effect results either from non-uniformity of the distribution of hydrodynamic flows over the cross section of an industrial column or from an increase (due to scaling up) in the characteristic dimensions responsible for longitudinal mixing. Columns with countercurrent and cross-current flow of phases are considered. The simplest models for transverse non-uniformity of the flow distribution are discussed and calculated. It is shown that the available experimental data on industrial columns confirm the theoretical predictions. The hydrodynamic nature of the scaling effect allows industrial equipment to be designed by means of hydrodynamic model experiments, without using pilot plant tests. The main aim of such experiments is to evaluate the optimum flow pattern in largescale mass transfer apparatus. The similarity criteria for hydrodynamic modelling of industrial equipment of different types (plate columns, packed columns, mixer-settlers etc.) with cross-current and counter-current contacting of phases in liquid-gas, liquid-liquid and liquid-solid systems are discussed. Constructive measures for eliminating transverse non-uniformity are considered; these measures follow from the theory presented. Examples of the use of the hydrodynamic modelling method are given. The substitution of hydrodynamic modelling for pilot plant tests leads to a speeding up of the design procedure for industrial mass transfer columns and chemical reactors.