The thesis is devoted to the study of the regularities of the process of concentration of aqueous solutions of some low–volatile liquids in a flowing diaper by evaporation into a cross flow of neutral gas and the use of the results to create a new kind of the concentrator. Such kind of a concentrator makes possible vacuum–free concentration of solutions at temperatures below their boiling point, which, due to the high boiling points of volatile liquids, helps to reduce energy consumption and to avoid the scarce types of fuel used while heating the solution.
The thesis presents the results of modeling the hydrodynamics and heat and mass transferring of a flowing film of an individual (of one component) liquid evaporating into a cross flow of a neutral gas using the created mathematical model. The results demonstrate the regularities of changes in the thickness, flow rate and temperature of the film along its length, as well as the temperature of the gas above the film. It was found that the intensity of evaporation of liquid from the film during its cross–interaction with the gas flow is higher than during direct–flow interaction and can exceed the rate of evaporation during counter current interaction. The temperature of a liquid film evaporating into a cross flow of a neutral gas, with distance from its initial cross section, reaches an equilibrium value, and with a further increasing the distance from this section, the liquid evaporates completely. There has been developed an algorithm for calculating the distances at which these phenomena appear. The temperature of the gas above the film along the length of the film changes insignificantly, but by its natural characteristics this change is associated with the temperature of the film and reaches an equilibrium value, in addition.
On the basis of an experimental study of heat and mass transfer in a flowing film of water evaporating into a cross flow of air, the values of the heat transfer coefficient from the surface to the film flowing along it, as well as the coefficients of heat and mass transfer from the film to the air flow, have been established. The dependence of these coefficients on the parameters of the heat and mass transfer process showed the presence of a transition from the laminar mode of film flow to the turbulent one at the value of the number Re_ж≈500. Mathematical processing of the experimental data made it possible to obtain empirical equations for calculating the heat and mass transfer coefficients.
The obtained results of mathematical modeling and experimental study of the evaporation of an individual liquid from its flowing down film became the basis for the development of a mathematical model and study of the process of concentration of aqueous solutions of some low–volatile liquids in a flowing film by evaporation into a cross flow of neutral gas. Using this model, the regularities of changes in the composition of the solution along the length of the film and the intensity of the concentration process were established. It is shown that the concentration of the solution at the beginning of the film changes linearly, then there is a abrupt rise, continuing until the complete evaporation of water. This kind of a change in the concentration of the solution along the length of the film is associated with the temperature distribution of the film. The abrupt rise of concentration corresponds to the state temperature value.
An experimental study of the process of concentration of aqueous solutions of glycerin in their flowing film in the conditions of evaporation into an air stream was carried out, as a result of which the heat and mass transfer coefficients were determined, which are the parameters of the developed mathematical model. It was found that, in the investigated range of variation of the process parameters, for low–concentration solutions of glycerol, as well as for water, it is common to observe the transition from a laminar mode of film flow to a turbulent one, for solutions with an increased concentration of glycerol, such a transition is absent and the film flows down only in a laminar mode.
The thesis presents experimental data on a decrease in the width of a liquid film flowing down a heated flat surface, associated with capillary effects. For water and aqueous solutions of glycerin, such a change in the width of the film can reach 50-80%. The natural characteristic of the change in the width of the film along its length and the obtained empirical dependences for its assessment are established.
On the basis of the results obtained in the thesis, the study of the process of concentrating aqueous solutions of some low–volatile liquids, a new design and method for calculating a film concentrator have been developed. The design is based on the use of a plane–parallel (lamellar) packing, which makes possible a cross–flow interaction.
