METHOD FOR APPROXIMATE CALCULATION OF MULTISTAGE LIQUID EXTRACTION

An engineering method is proposed for calculation of the number of theoretical and actual extraction stages. The extraction of phenol from waste water, and acetic acid from an aqueous solution is calculated on the basis of this method, and the data obtained are compared with results of other authors. Counter-current liquid extraction [1] is employed for separation of near-boiling and azeotropic mixtures and substances that are sensitive to elevated temperatures, and removal of components from highly dilute solutions ‐ in these cases, separation of components by distillation or evaporation is impossible. Multistage extraction of mutually insoluble or partially mutually insoluble phases is calculated by an analytical method based on the from-stage-to-stage scheme [2], or by a graphical method (with the use of triangular or rectangular equilibrium diagrams) [1, 3, 4]. The concentration of the extract at the outlet from a multistage unit during counter-current extraction cannot be above the equilibrium value corresponding to the concentration of the initial solution. Irrigation of the unit with reflux is employed to increase the degree of separation of the initial solution [1]. In the case when a portion of the extract is returned in the form reflux, separation is improved, but consumption of extraction agent, and the dimensions and cost of the extraction unit are increased. Calculation of this process with the use of an z ‐ x, y diagram [4] is ineffective (graphical-plotting errors are possible). Moreover, the amount of extraction agent is selected within the range of from minimum to maximum values corresponding to the maximum and minimum number of extraction stages. Capital outlays for building of the unit are determined by the number of separation stages. Optimal consumptions of extraction agent and reflux and the number of separation stages are therefore determined in the phase of preliminary-design development of the equipment as a result of feasibility analysis. The relative degree of extraction of a component from the solution in a stage (the ratio of the amount of component removed to the amount of the component delivered) can be used (as for determination of the effectiveness of the separation stages during distillation [5]) to determine the number of extraction stages. Formulas for calculation of the relative degree of extraction of a component in the ith stage, which is expressed in the concentrations of the solution being extracted ϕ xi and the extraction agent ϕ yi , assume the form: where L i‐1 , G i‐1 , L i , and G i are the flow rates of the solution and the extraction agent at the inlet to and outlet from the ith stage; and x i‐1 , y i‐1 and x i and y i are the corresponding concentrations of the solution and the extraction agent at the inlet to and outlet from the stage. ϕϕ xi i i i i i i yi i i i i i i Lx L x L x G y G y G y i