For better understanding laws managing physical mechanism of process multi-component rectification the large significance has doubtless above mentioned the rule of F.Boshnyakovich. From this rule directly follows, that with help isothermal diffusion or ïso thermal rectification" the real process cannot be directed in side of separation of mixture.
The rule of F.Boshnyakovich, determining direction of heat flow, is induces the idea, that the true mechanism of process rectification has motivated by processes of phase conversion, which run on phase boundary.
About liquid phase, heated up to boiling, it is possible to deliver only the determined quantity heat, that is strict agreed with heat capacity of phase and change of its temperatures of boiling that in its turn is varied with composition's change.
In these conditions disorder flow of heat's excess does not reach the liquid phase as it is spent wholly on equilibrium process of its phase's conversion.
The transformation of size of this flow in zero present oneself important condition, that determines alone, apart dependence from existence of common principle of the minimum speed of entropies production, the minimum speed of production of this quantity in each infinitely small volume rectifications column, working in stationary regime [1].
It is important, that in this case this conclusion was received apart dependence from existence of a common principle minimum speed of entropies production and confirms its validity.
Other condition mentioned above consist in change of liquid phase in direction of achievement of maximal üuseful work".
In turn, large speed and dynamic character of processes of phase conversions of liquid determine the stability of trajectories, on which are going the change of liquid's compositions and the monotonous increase of their temperatures of boiling followed to laws of Konovalov and Vrevsky [2].
Here the components enter in liquid phase only in such ratio, at which the trajectories of stationary processes multi-component rectification cannot essentially deviate from corresponding trajectories of quasistatic processes of opened evaporation. The latter is equivalent to imposing on relative size flows of components conditional restrictions and connections, under effect of which the liquid phase preserves for the change only one conditional degree of freedom.
With thermodynamic point of sight the coincidence of mentioned trajectories in phase space of concentration of liquid phase take place owing to that both processes run in the same potential field in each point of which all components have the same chemical potential and collectively the same potential of Gibbs that equal specific quantity free enthalpy of liquid heated up to temperature of boiling.
The very closely coincidence of points both trajectories is determined by their extreme character, by which on each unit their lengths should be made the maximal useful work, and the trajectories should be perpendicular to all intersected equipotential surfaces of field [3].
Not the small role in coincidence of trajectories plays:
- that both named processes have the same physical mechanism, connected with delivering of heat to boiling liquid and overcoming by its molecules of attractions forces, acting on phase boundary;
- that in that and in other processes equation of material and thermal balances do not impose any restrictions and connections on liquid's degree of freedom that determined view and location trajectory of process in phase space of concentration. It is fundamental difference equation of material and thermal balances, described a circulation the multi-component liquid in rectification column from another, that concern to isolated systems.
- that the laws, determining form of trajectories and their stability, are described by the same fundamental equation of phase [2]
- that on this reason heat has been perceived with the liquid phase is spent on the same thermodynamics processes: heating and useful work of separating.
- that named stationary processes are put into conformity such trajectories, which are decisions of the same autonomous systems of ordinary differential equations. If these trajectories have one common point they theoretically should coincide in phase space, as should not be intersected with self and with one another [4].
The correctness this conclusion is supported by large experience of separating multi-component (including azeotropic) mixes, as well as by analysis of experimental data are given in literature[5,6].
Enclosed in frame the physical law in rectification processes determines the opportunity of scale transition from laboratory to industrial columns having various sizes and designs.
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In processes of rectification the change of composition of liquid phase and the monotonous increase of its temperature of boiling cannot go on phase space of concentration intersecting force lines of potential field formed free enthalpy of Gibbs. |