The Role of Gravity in the Evolution of the Concentration Field in the Electrochemical Membrane Cell

Abstract

The subject of the study was the osmotic volume transport of aqueous CuSO4 and/or ethanol solutions through a selective cellulose acetate membrane (Nephrophan). The effect of concentration of solution components, concentration polarization of solutions and configuration of the membrane system on the value of the volume osmotic flux ( J v i r ) in a single-membrane system in which the polymer membrane located in the horizontal plane was examined. The investigations were carried out under mechanical stirring conditions of the solutions and after it was turned off. Based on the obtained measurement results J v i r , the effects of concentration polarization, convection polarization, asymmetry and amplification of the volume osmotic flux and the thickness of the concentration boundary layers were calculated. Osmotic entropy production was also calculated for solution homogeneity and concentration polarization conditions. Using the thickness of the concentration boundary layers, critical values of the Rayleigh concentration number ( R C r ), i.e., the switch, were estimated between two states: convective (with higher J v i r ) and non-convective (with lower J v i r ). The operation of this switch indicates the regulatory role of earthly gravity in relation to membrane transport.

Evaluation of the Global S-Entropy Production in Membrane Transport of Aqueous Solutions of Hydrochloric Acid and Ammonia

“…Unlike binary solutions, the density of ternary solutions, composed of a solvent and substances causing an increased and decreased solution density (i.e., glucose and ethanol, CuSO 4 and ethanol, KCl and ammonia or HCl and ammonia, etc.) may be lower than, equal to or greater than that of the solvent [ 4 , 8 , 31 ].…”

“…The subject of considerations, as well as several of our previous works, is transport in a membrane system illustrated schematically in Figure 1 [ 4 , 26 ]. This figure shows a model of a membrane system in which the membrane (M), situated in the horizontal plane, separates two solutions with the initial concentrations C hk and C lk ( C hk > C lk , k = 1, 2).…”

“…Membrane transport processes belong to the group of basic phenomena occurring at the level of organization of physicochemical systems, in which the membrane constitutes a selective barrier separating the interior of the system from its surroundings [ 1 , 2 , 3 ]. The driving forces of these transport phenomena are a consequence of the occurrence of various types of physical fields, such as concentration, pressure, temperature or electric potential fields, participating in shaping the field constitution of nature [ 4 ]. The flows resulting from the action of these forces, such as diffusion or osmosis, modify the physical fields, an example of which in the case of the concentration field is concentration polarization [ 5 , 6 , 7 , 8 ].…”

“…In [ 4 , 8 ] it was shown that and depend on the transport properties of the membrane, the configuration of the membrane system as well as the physicochemical properties and composition of solutions separated by the membrane. The value of these fluxes is greater under convective than in non-convective conditions.…”

The Rr Form of the Kedem–Katchalsky–Peusner Model Equations for Description of the Membrane Transport in Concentration Polarization Conditions

“…This value corresponds to the ( R C ) crit. = 1100.6, obtained for the case of the rigid membrane surface and the free liquid interior (rigid-free borders) [ 44 , 45 ]. For electrolysis occurring in a cell containing electrodes placed in parallel in horizontal planes, the critical Rayleigh number depends strongly on the distance between these electrodes and for amperostatic conditions takes the values in the range of R C = 1070 ÷ 1540 [ 46 ].…”