RU2270803C2 - Method of electrochemical treatment of the water-salt solutions - Google Patents

Abstract

FIELD: treatment of water-salt solutions; methods of electrochemical treatment of water-salt solutions.

SUBSTANCE: the invention is pertaining to the method of the electrochemical treatment of water-salt solutions of the alkaline metals sodium chlorides for the purpose of production of the disinfectants and detergents, and also for regulation of the acid-base parameters of water. The total stream of the initial solution at the inlets into the cathode and anodic chambers is uniformly divided into the system of parallel streams. The streams are passing over the surfaces of the electrodes in the laminar flow mode through hydrodinamically separated channels formed by the surfaces of the diaphragm and the equidistant surfaces of the electrodes having a configuration of a plane wave. The treatment of the solution is conducted at the given density of the electrical current and the selected speed of the flow maintaining the ratio of the mode parameters and the geometrical parameters of the channels within the limits defined by the criterion of the hydrodynamic stability of the laminar flow of the solution to the disturbances from the concentration-gravitation convection and the condition of the bubble mode of evacuation of the electrolysis of gases in the channels. The technical result of the invention is an increased hydrodynamic stability of the processes of the electrochemical treatment of the laminar flows of the lightly compensated water-salt solutions in the large planar-slot electrode channels of the electrochemical components, an increase of intensity and power efficiency of the processes of the electrochemical treatment of solutions and increased productivity of the running electrochemical components.

EFFECT: the invention ensures an increased hydrodynamic stability of the processes of the electrochemical treatment of the lightly compensated water-salt solutions in the large planar-slot electrode channels of the electrochemical components; an increased intensity and power efficiency of the processes of the electrochemical treatment of solutions; an increased productivity of the running electrochemical components.

15 dwg

Description

The invention relates to the field of processes of applied electrochemistry and, in particular, to methods of electrochemical treatment of water-salt solutions of alkali metal chlorides in order to obtain disinfectant (acid) and detergent (alkaline) solutions and substances, as well as the regulation of acid-base water parameters [1] , [5], [13].

The invention can also be used in electrochemical plants for the synthesis of chloroxide compounds of active chlorine and gases - chlorine and hydrogen.

The essence of the known methods [2], [3], [4] of the electrochemical treatment of solutions in an electrolysis cell (FIG. 1) includes passing an initial solution 1 (usually weakly concentrated - 0.5 ... 5 g / l - sodium chloride solution NaCl) by two parallel flows through vertical planar channels formed respectively between the flat surfaces of smooth cathode 2 (cathode chamber), smooth anode 3 (anode chamber) and separation diaphragm 4 (permeable to electrolyte solution and impermeable to gases). Direct current transfer through the electrolyte layer between the cathode and the anode is carried out mainly by chlorine ions Cl ^- and sodium Na ⁺ and partially by hydroxyl ions OH ^- and hydroxonium Н ₃ O ⁺ water; at the same time, at the exit of the cathode chamber 5, the product of the interaction of Na ⁺ ions with water at the cathode is mainly obtained: Na ⁺ + Н ₂ O + е → NaOH + ½ Н ₂ , i.e., a solution with an alkaline reaction, and at the exit of the anode chamber 6, compounds of active chlorine (depending on the process conditions and the acidity of the solution - hypochlorous acid HClO, oxides Cl ₂ O, ClO ₂ , etc.), which has biocidal disinfecting properties, or gaseous chlorine and its compounds with oxygen [4].

The flat design of the chambers and electrodes of electrolysis cells in the above methods is the basic basis for the creation of industrial multi-cell electrolysis batteries with biopolar electrodes [5].

However, the drawback of existing methods of processing solutions in planar channels, and in particular in the case of weakly concentrated solutions, is the observed irregularities and instabilities in the distribution of the flow rate of the electrolyte solution along the channel width with the formation of stagnant zones, uneven distribution of electric current over the surface of the electrodes with the formation of zones of "spotted" conductivity and local overheating of the electrolyte, which generally reduces the efficiency of the solution processing process [1], [13].

The main reason for such effects is the appearance of a gradient dc / dx of concentrations 7 and density dρ / dx of the electrolyte over the thickness δ of the channel during charge (current) and substance transfer between the electrodes (cathode 2 and anode 3) due to limited migration rates (v _M ~ 10 ^-2 cm / s) and diffusion (v _g ~ 10 ^-4 cm / s) of substance ions and, as a result, the occurrence of hydrodynamic instability and the known phenomenon of gravitational convection in the form of Rayleigh-Benard vortex cells in the channel [6], [7] (see Fig. .2, where 8, 9 are the oncoming motions of the vortex between the cathode 2 and the anode 3).

The conditions for the occurrence of gravitational convection in both horizontal and vertical channels at different liquid densities near the walls are determined during heat exchange [6], [7] by the critical value of the Rayleigh criterion Ra = Gr · Pr; in the process of mass transfer under consideration by the diffusion analogue of the criterion Ra = Ar · Pr _D ; these conditions arise when the Rayleigh criterion reaches a critical value of Ra ^* :

Here: Gr - Grashof thermal criterion; Ar - Archimedes criterion; Pr _D is the diffusion analog of the Prandtl number of the liquid, ν is the kinematic viscosity coefficient of the liquid (electrolyte solution), D is the electrolyte diffusion coefficient, Δρ is the difference in electrolyte densities at the channel walls, ρ _о is the average electrolyte density, g = 981 cm / s ² ) , Ra ^* is the critical value of the Rayleigh criterion, depending on the geometry of the channel, boundary conditions, as well as the hydrodynamic situation in the channel.

The general structure of convection can be a wave system of parallel paired vortex shafts 10 (Fig. 3) with a relative width of the vortex cell λ = I / δ, which increases with an increase in the Rayleigh criterion Ra, for Ra> 2 · 10 ⁴ a larger-scale toroidal hexagonal mode is established vortex cells 11; the longitudinal forced flow of the solution along the channel exerts a stabilizing effect, increasing the critical value of the Rayleigh criterion [7].

In a plane-slot vertical channel, the minimum critical value of the Rayleigh criterion Ra ^* _o , at which convection can occur in the initially stationary liquid, can be depending on the type of boundary conditions on the channel walls [7] (p. 82):

The negative effect of vortex convection on the uniformity of electric current transfer in the electrolyte layer is evident from figure 2, in particular:

- in section 8 of the vortex, convection accelerates the migration movement of the main current carrier in the electrolyte - anion Cl ^- ,

- in section 9 - prevents migration of the Cl ^- anion.

This causes a corresponding non-uniformity in the distribution of current density over the electrode surface and the volume of the electrolyte, local overheating, additional thermal convection perturbations in the electrolyte and generally reduce the efficiency of the process of electrochemical treatment of the solution [8], [9], [13].

Moreover, due to the sufficiently low transfer rates of the reacting substances in the electrolyte by the migration processes j _m = v · c and diffusion j _B = D · Δс / δ, even small convection speeds w (j _k = w · c) have a significant effect on the overall process transfer, respectively: w = v = 10 ^-2 cm / s and w = D / δ = 10 ^-5 cm ² / s / 10 ^-1 cm = 10 ^-4 cm / s.

2. Known methods for the electrochemical treatment of water-salt solutions [8], in which to eliminate the indicated disadvantages of the process in planar channels, the solution is processed by passing it in the channels of the circular section in devices [9], [10] having a cylindrical cathode 2, a coaxial cylindrical anode 3 and a cylindrical separation diaphragm 4 (figure 4).

These elements together form annular channels — the anode 12 and the cathode 13, through which flows of the treated solution 1 are passed.

The well-known stabilizing effect of the curvature of the channel walls [7, p. 92] makes it possible to increase the critical value of the Rayleigh number in the annular channel Ra ^* _(k) relative to Ra ^* _{(pl) of a} flat channel of the same height δ in proportion to the square of the relative curvature of the wall:

where r _n , r _in - the outer and inner radii of each channel (figure 5);

δ is the channel height, usually limited by the condition of minimizing ohmic losses: δ≈0.1 ... 0.15 cm;

k _R is a constant.

When choosing sufficiently small values of radius r _in , comparable with a small value of δ, the curvature of the cylindrical walls of the annular channels has a constraining and, due to viscous friction on the walls, inhibit the development of vortex flows in the channel, thereby contributing to an increase in the minimum value of the critical Rayleigh number in comparison with formula (2).

However, significant disadvantages of the known method [9] and the devices implementing it [10] are:

- the fundamental limitation of the size of the diameters of the electrodes and, consequently, the working surface of a single electrolysis cell by a condition of type (3), since with increasing diameter the Rayleigh resistance decreases hyperbolically,

- the fundamental impossibility of using the method with cylindrical elements in modern filter press structures of industrial multi-cell electrochemical batteries based on flat bipolar electrodes,

- the method does not contain essential features that determine the process conditions necessary to suppress vortex convection in functional connection with the main process parameters - electric current density, specific flow rate of the treated solution and its physicochemical parameters that determine the initial cause of convection - the density difference in the solution Δρ.

The rationale for the functional relationship between the density difference Δρ and the operating parameters i, w, necessary to determine ways to eliminate the considered disadvantages of the prototype [9], [10], is given in clause 3 based on an analysis of the processes of electric current transfer, mass transfer, and hydrodynamics during water saline in the electrode channel.

3. The concentration field in the stationary mode of current transfer with a density of i, a / cm ² through a layer of a stationary electrolyte - NaCl solution - with a thickness of 2δ (4a) is determined by the system of equations of charge and substance transfer [11]:

- for the current of anions Cl ^- - (index “a”) taking into account diffusion and migration

- for the current of cations Na ⁺ (index "k")

- for total current

- the equation of mass balance for Na in the electrolyte, taking into account the opposite directions of Na ⁺ migration and diffusion of the Na ion discharged in the cathode layer of water in the composition of the NaOH molecule:

- as well as the condition of electroneutrality for this binary NaCl electrolyte with a concentration of:

- the Nernst-Einstein equation for the relationship between the mobility u and the diffusion coefficient of ions D:

- the boundary condition specified in one of the options:

where c _A, C _to C _of - electrolyte concentration, respectively, at the surface of the anode, cathode and average by volume, the electrolyte concentration (Figure 6).

The solution of the system of equations (4) ... (7) gives a linear distribution of concentrations along the height of the channels in a stationary electrolyte and the value of the difference in concentrations at the channel walls with a height of δ (Fig.6):

Here: D _i - diffusion coefficient;

i - component in solution, cm ² / s;

u _i - mobility of i - ion, cm ² / s · V; u _i = λ _i / F;

λ _i - molar ionic conductivity, cm ² / Ohm · mol;

F - 96.5 · 10 ³ C / mol - the Faraday number;

φ is the potential, V;

z = 1 is the charge of NaCl electrolyte ions;

R = 1.98 cal / mol · ° C - universal gas constant;

T is the absolute temperature, K;

λ = λ _a + λ _k - molar conductivity of the electrolyte, cm ² / Ohm · mol;

t _a is the anion transfer number, t = λ _a / λ.

For the case of forced transmission of electrolyte solution in the channels with the velocity orders w ~ 10 cm / s and channel heights δ ~ 10 ^-1 cm adopted in similar devices [1], the hydrodynamic and mass transfer conditions in the channels of the electrolysis cell are characterized by rather low Reynolds criterion Re = w · 2δ / ν = 10 · 10 ^-1 / 10 ^-2 = 100 and high values of the Prandtl number Pr = ν / D = 10 ^-2 / 10 ^-5 = 10 ³ (here ν = 0.8 · 10 ^-2 cm ² / s is the kinematic viscosity coefficient of a weak aqueous solution of NaCl; D- (1 ... 2) · 10 ^-5 cm ² / s is the diffusion coefficient of the electrolyte).

This means that already in the initial section of the channel L / 2δ = 0.08 Re = 0.08 · 10 ² = 8 (i.e., L = 1.6 cm), a hydrodynamically stabilized laminar flow regime is established, while the thickness the diffusion boundary layer δ _D on the surfaces of the electrodes remains much smaller than the channel height δ up to the relative channel length L / 2δ = 0.014Re · Pr = 0.014 · l0 ² · 10 ³ = 1400 due to the low values of the diffusion coefficient of the electrolyte compared to the kinematic viscosity [ 12].

In this mode, over the entire length L (in particular, of the cathode channel — see FIG. 7), a significant part of the channel height is occupied by the laminar flow of electrolyte 13 with a constant electrolyte concentration c _о along the channel height δ and part 14 diffusion boundary layer of thickness δ _D <δ (4 - diaphragm, 2 - electrode); the intensity of the diffusion mass transfer of ions i in the electrolyte solution is determined by the mass transfer coefficient β _i , determined through the diffusion analogue of the Nusselt criterion Nu _D = β _i · δ _e · D _i ^-1 .

определяется критериями Рейнольдса и Прандтля [12]:The average length L of the flat channel

determined by the Reynolds and Prandtl criteria [12]:

where A = 1.85

δ _e - hydrodynamic equivalent linear channel size; for a flat channel δ _e = 2δ; for a closed channel profile δ _e = 4S / p; S is the cross-sectional area, p is the perimeter;

w is the average flow rate over the channel cross section.

Equations (1), (4) of system (1) ... (7) in the case of a mobile electrolyte are written for the boundary surfaces (electrodes) of the channel:

where by definition:

with _e - concentration at the wall of the electrode.

The solution of system (10a) ... (10c) gives the difference in electrolyte concentrations on the boundary surface of the channel and in the volume of the stream:

For weak aqueous solutions of alkali metal salts (NaCl) up to ~ 1 mol / L, the relationship between concentration and density is almost linear

4. In order to eliminate the disadvantages of clause 2 of the known method, it is proposed:

The method of electrochemical treatment of water-salt solutions to obtain disinfectants and detergents and control the acid-base state of solutions, which includes passing electrolyte solutions in separate streams through the chambers between the surfaces of the cathode and anode of the electrolysis cell, respectively, and the surface of the diaphragm separating them, permeable to electrolyte, characterized the fact that the total flow of the initial solution at the entrance to the cathode and anode chambers is evenly divided into a system of parallel flows ; flows are passed over the surfaces of the electrodes in a laminar flow regime at a velocity w through a system of parallel, hydrodynamically separated channels that form in the chambers the surface of the separation diaphragm and the equidistant surfaces of the cathode and anode having a plane wave configuration with parameters of length λ, amplitude h and surface curvature radii r ; processing is carried out at current density i, maintaining the ratio of parameters within the limits determined by the criterion of hydrodynamic stability of the laminar flow of the solution to disturbances from concentration-gravity convection and by the condition of the bubble regime of evacuation of electrolysis gases:

K ₁ + K ₂ · 0,0208Θ (σ / ρ _o) ^1/2 ≡δ _emin <δ _e <δ _{emax ^{≡K o · Ra o * · w}} 1,75 / (i 0,75 L 0,25)

and conditions for the equidistance of electrode surfaces:

r _max -r _min ≡b = (h + δ _m ) · [1+ (2h / λ ² ) ^{- (1/2)} ]

0 <r _min <(1/2) · [λ (1 + λ ² / 4h ² ) ^-1/2 -b]

where δ _e = 2h [1+ (1 + 4h ² / λ ² ) ^1/2 ] ^-1 is the equivalent hydrodynamic diameter of the channel;

r _max , r _min - the maximum and minimum radii of curvature of the surface profile of the electrodes;

b is the normal distance between the equidistant surfaces of the anode and cathode in the channels;

L is the channel length downstream;

δ _m - the thickness of the diaphragm;

Ra _o ^* = π ⁴ [1 + 4 (2h / λ) ² ] ² - the minimum critical value of the diffusion analogue of the Rayleigh criterion in a channel with a stationary fluid;

To _o = [(4,5 · 10 ^-3 · ρ _o · R · T · (1 + t _a ) · λ _el · D ^2/3 ) · (g · F · ν · M) ^-1 ] ^{0, 75} - the constant of the physico-chemical parameters of the solution;

K ₁ = r _max [1- (1 + 4h ² / λ ² ) ^1,2 ] · [1+ (1 + 4h ² / λ ² ) ^-1/2 ] ^-l

K ₂ = [1+ (1 + 4h ² / λ ² ) ^{l / 2} ] ^-l

ρ _{o the} density of the solution, g / cm ³ ;

R is the universal constant;

T is the temperature;

t _a is the anion transfer number in the electrolyte;

λ _e - mole equivalent conductivity solution ^cm2 / Ohm · mol;

D is the diffusion coefficient of the salt solution in the electrolyte, cm ² / s;

g is the acceleration of gravity, cm / s ² ;

F is the Faraday number, A · h / mol;

ν is the kinematic viscosity coefficient, cm ² / s;

M is the molecular weight of the salt of the solution, g / mol;

δ is the surface tension of the solution, dyne / cm;

Θ is the contact angle of the electrode surface with the solution,

which is equivalent to the condition:

K ₁ (K ₂ + d _o ) = δ _emin <δ _e <δ _emax = K _o (Ra ^* _o / i) ^0.75 w ^1.75 L ^-0.25

and the condition of equidistant electrode surfaces:

b = r _max -r _min = d _o + δ _m + r _min (1 / sinα-1) = (h + δ _m ) sinα

0 <r _min <r ^* _min = [d _o + δ _m (1 / sinα-1)] · sin ² α / (1 + cosα-sinα)

where K ₁ = 2 / (1 / sinα + 1);

K ₂ = r _max / (1 / sinα-1);

α = arctan (λ / 2h);

d _o - the height of the formed channel; d _o ≥d _Neg;

d _otr = 0,0208Θ (σ / ρ _о ) ^1/2 - the maximum detachable diameter of the gas bubble;

σ is the surface tension of the solution, dyne / cm;

Θ is the contact angle of the electrode surface with the solution;

ρ _{about the} density of the solution, g / cm ³ ;

δ _e = 2h · [1 + 1 / sinα] ^-1 is the equivalent hydrodynamic diameter of the channel;

δ _emin is the minimum acceptable value of δ _e according to the condition of the bubble regime of gas evacuation; δ _emax - the maximum allowable value of δ _e according to the condition of hydrodynamic stability of the flow in the channel when exposed to electric current;

Ra ^* _o = π ⁴ [1 + 4 (2h / λ) ² ] ² - the minimum critical value of the diffusion analogue of the Rayleigh criterion in a channel with a stationary fluid;

To _o = [(3,7 · 10 ^-2 · ρ _o · R · T · (1 + t _a ) · λ _el · D ^2/3 ) · (g · F · ν · M) ^-1 ] ^{0, 75} - the constant of the physico-chemical parameters of the solution;

r _max , r _min - the maximum and minimum radii of curvature of the surface profile of the electrodes;

L is the channel length downstream;

b is the normal distance between the equidistant surfaces of the anode and cathode in the channels:

δ _m - the thickness of the diaphragm:

R is the universal constant;

T - temperature:

t _a is the transfer number of the anion in the electrolyte:

λ _e - mole equivalent conductivity solution ^cm2 / Ohm · mol;

D is the diffusion coefficient of the salt solution in the electrolyte, cm ² / s;

g is the acceleration of gravity, cm / s ² ;

F is the Faraday number, A · h / mol;

ν is the kinematic viscosity coefficient, cm ² / s;

M is the molecular weight of the salt of the solution, g / mol;

5. The essential features of the method and the achieved effects

5.1. The aim of the proposed method is to increase the hydrodynamic stability of laminar flows of solutions in the processes of electrochemical processing to disturbances from gravitational convection, to eliminate the fundamental limitations of the magnitude of the working surface of the electrodes and productivity inherent in the prototype, to increase the intensity and energy efficiency of the process of processing the solution in the cells of the electrolyzer.

To this end, in the proposed method, in contrast to the prototype and known analogues, the total flow of the solution at the entrance to the cathode and anode chambers is evenly divided into a system of parallel flows (Fig. 9).

Streams are passed for processing through a system of parallel, hydrodynamically separated channels in a laminar (vortex-free) flow regime, while the flow velocity, electrolysis current density, and channel geometric configuration parameters are set so that the solution concentration gradient Δρ arising over the channel thickness (see (11) ), (12) and Fig. 7) did not cause vortex disturbances of the plane-parallel laminar flow of the solution, i.e. so that the condition of hydrodynamic stability of the flow to perturbations from gravitational convection (1) is satisfied:

where Ra ^* is the critical Rayleigh number in the laminar flow of the solution in the channel.

The critical Rayleigh number Ra ^* , at which perturbations from gravitational convection begin to disrupt the plane-parallel laminar fluid flow in the channel, is approximated by the expression [7] (p. 272):

where Ra ^* _o is the critical Rayleigh number in the channel in the absence of forced fluid motion, determined, other things being equal, by the geometry of the channel;

Re is the Reynolds number in the channel;

w is the average cross-sectional velocity in the channel;

B- = 2.5 · 10 ^-3 · α · ² · is a constant;

α-w _max / w is the ratio of the maximum and average cross-sectional flow rates in the channel (for a closed section α = 2; for a slotted channel α = 1.5 [12]).

δ _e is the equivalent diameter of the channel;

ν is the kinematic viscosity of the solution.

Hence, substituting into condition (15) the expressions for Ra (14), for Ra * (16) and resolving it with respect to δ _e , we obtain the stability condition for the laminar flow of the solution in the channel, expressed in terms of the maximum allowable value of the characteristic linear channel size δ in the function operating parameters of the process of electrochemical processing of the solution - current density i of electrolysis, flow rate w of the solution and its physico-chemical parameters:

where L is the length of the channels;

To _about = [3,7 · 10 ^-3 · ρ _o · R · T · (1 + t _a ) · λ _el · D ^2/3 · (g · F · ν · M) ^-1 ] ^0,75 - constant of physico-chemical parameters of the solution.

The distribution of flows and the provision of the required operating conditions are realized using a system of channels, which in the method are formed by surfaces of a flat separation diaphragm and equidistant surfaces of the cathode and anode having a plane wave configuration with a wavelength λ, amplitude parameter h and curvature radii that meet the conditions for ensuring equidistance ( figure 10):

- on cylindrical sections of the surfaces of the electrodes with:

- on plane-parallel sections of the surfaces of the electrodes a:

- at the interface points of sections a and c:

where b is the normal distance between the equidistant surfaces of the electrodes;

r _max , r _min - the maximum and minimum radii of curvature of the surface of the electrodes in the channels;

δ _M is the thickness of the diaphragm;

d _o - the height of the channel formed between the surface of the electrode and the diaphragm;

h is the amplitude parameter of the generating wave;

λ is the wavelength of the wave, which coincides with the width of the channel;

ψ = λ / 2h = tgα is the wave parameter numerically equal to the tangent of the angle α of the disclosure of the profile of the formed channel (see Fig. 10);

r * _min is the upper limit value of the minimum radius of curvature according to the smoothness conditions of the mating portions of the electrode surfaces, expressed in terms of the primary values d _o and δ _M , the r * _min value in (20) is:

Conditions (18) ... (21) are the equidistance conditions for these wave surfaces necessary to maintain a constant ohmic resistance of the electrolyte layer and a constant current density over the entire surface of the electrodes.

Moreover, it follows from Figs. 10 and (20), (21) that, as r _min → r * _min, the electrode surfaces completely pass into the system of cylindrical sections, and the interelectrode channel into the system of slotted cylindrical channels of the same curvature, expanded across the entire width of the planar electrodes, regardless of their size.

A fundamentally new feature of the process in this system of surfaces forming alternating sections of limited flat-gap a and cylindrical annular channels from each other is the possibility of a significant increase in the hydrodynamic stability of the process, regardless of the overall dimensions of the electrodes and the electrolysis cell.

The equivalent hydrodynamic diameter of the channels thus formed with a sufficient approximation can be determined by the ratio:

and the critical Rayleigh number [7] (p. 89):

where S, P is the cross-sectional area and the perimeter of the channel.

The channel cross-section height d _o should be sufficient for free, without contact with the walls, evacuation of gas bubbles of electrolysis products (mainly hydrogen at the cathode). The magnitude of the maximum diameter of the tear gas bubbles generating surface by d _Neg neglecting hydrodynamic forces πc _f (Re) ρ _o w d _o ^{2 2/8} not exceeding 10 ... 20% of the lift πgρ _o d _o ^3/6, according to Fritz-End [12] is d _otr = 0,0208θ (σ / ρ _o g) ^1/2 ; therefore, the condition must be met:

or a condition for the equivalent diameter of the channel, taking into account the geometry of the channel and formula (23):

where K ₁ = 2 [(1 + 1 / ψ ² ) ^1/2 +1] ^-1 = 2 / (1 / sinα + 1)

K ₂ = r _max [(1 + 1 / ψ ² ) ^1/2 -1] = r _max (1 / sinα-1)

Thus, the set of conditions of the method (17), (22), (23), (25) is:

The hydrodynamic stability of the solution flows in the method is controlled on the basis of relations (17), (22), (23) using variable parameters, which include:

is the flow rate of the solution along the channels w,

- electric current density i;

and two independent geometric parameters:

- the linear scale of the channel specified by d _o (or interrelated quantities h, λ, δ _e );

- the parameter ψ = λ / 2h = tgα, characterizing the ratio of the length and the amplitude parameter of the generatrix of the plane wave, numerically equal to the tangent of the opening angle of the formed channel (Fig. 10).

The indicated parameters make it possible to separately influence the value of the solution concentration gradient Δρ (i, δ) (see (11), (12)) (the cause of convective perturbations of the flow) included in the left-hand side of the hydrodynamic stability condition (15) and the value of the critical Rayleigh number Ra * _o (16) on the right-hand side of condition (15), which increases hyperbolic with decreasing ψ (in proportion to 1 / ψ ⁴ = (2h / λ) ⁴ ).

5.2. Achievable effects of the method

5.2.1. Process intensity

The maximum intensity of the process q, i.e. the performance of the target product, for example, chlorine, per unit surface of the electrode of the electrolysis cell is determined by the density of the maximum permissible current stability condition i _pr :

where e is the electrochemical equivalent, e = 1.3 g Cl / Ah;

M _Cl = 35 g / mol - molecular weight of chlorine;

n = 1 is the ion charge;

F = 26.8 g / Ah - the Faraday number.

Given the linear dimensions of the channel parameters δ _e according to (24), (25) and the rate of transmission of the solution through the element w, the maximum current density i _{pr, which} is admissible under the condition of hydrodynamic stability of process (17), is

those. it is directly proportional to the base Rayleigh number Ra * _o (in a channel without a liquid flow), which in the proposed method according to (23) is regulated only by the parameter of the generating wave ψ = λ / 2h and does not depend on the overall dimensions of the electrodes.

In comparison with the known methods [2] ([3]), including passing a solution in a channel of height δ between two smooth-walled parallel surfaces of the electrodes, where the critical Rayleigh number Re * _{o (pl)} ≈π ⁴ according to formula (2), the proposed method allows by choosing the parameter ψ = λ / 2h, for example, ψ = λ / 2h → 1.5, 1.0, 0.5, to increase the hydrodynamic stability of the process relative to the prototype (at the same height of the working channels d _o and flow velocity w) in the ratio : Ra * _o / Ra ^* _{o (pl)} = [1 + 4 (2h / λ) ² ] ² , i.e. respectively at 8; 25 and 290 times and, therefore, increase the limiting current density i _pr according to (28) (see curves d, e in Fig. 11).

11 illustrates the field of flow stability and the permissible values of operating parameters in the proposed method in the form of a diagram calculated from the combination of conditions (17), (22), (23), (25), (26) and giving the dependence of the permissible values of current density on the selected δ _{e max} when the flow velocity w = 5; 10; 15 cm / s and with a fixed value of the parameters: ψ = λ / 2h = 1.5, ρ _о = 1.01 g / cm ³ , λ _el = 110 cm ² / Ohm · mol, D = 1.5 · 10 ^{- 5} cm ² / s, L = 15 cm, T = 40 ° C, M = 58 g / mol, Θ = 20 °, σ = 74 dyne / cm.

Each curve δ _{e max} = f (i) is the upper boundary of all permissible combinations δ _e -i for a given flow velocity w = const, which are represented on the diagram by the points lying below this curve w = const, i.e. all combinations δ _e -i at a flow rate w = 5 cm / s, which ensure a stable process, lie in the region below the curve w = 5 cm / s; and those lying above this curve correspond to an unstable regime with vortex disturbances of the laminar flow. With an increase in the velocity w to 10 cm / s, the region of stable regimes, respectively, expands to the boundary of the curve w = 10 cm / s, etc.

For example, with the selected channel diameter δ _e1 = 0.3 cm and the flow velocity w = 5 cm / s, the maximum permissible current density i _pr = 17 mA / cm ² (point a); when the speed w increases to 10 cm / s, the limiting current density increases to 83 mA / cm ² (point b); but when the current density increases to 100 mA / cm ² and while maintaining the previous speed w = 10 cm / s, the mode becomes unstable (point c).

The lower limit of the permissible values of δ _e in the diagram is the condition δ _e = δ _{e min} according to (26).

Curves d and c in the diagram illustrate the decrease in the stability and the magnitude of the limiting currents compared to the method in the known analogues [2], [3] - plane-parallel electrode systems with ordinary smooth electrodes; the curve d corresponding to the speed w = 10 cm / s and the curve c (w = 15 cm / s) show that the limiting current densities achieved in the method are 8 times higher than in the analogue, proportional to the above increase in the Rayleigh criterion Ra ^* _o 8 times with the parameter ψ = λ / 2h = 1.5 selected in the diagram; when choosing this parameter ψ = 1.0 or 0.8, the indicated increase will be from 25 to 290 times, respectively.

In the proposed method, the main disadvantage of the prototype [8], [9], [13] is eliminated, including passing the solution through circular cylindrical channels, where the base Rayleigh number Ra ^* _{o (q)} according to (3) is determined by the square of the ratio of the channel height δ to the inner radius electrode [7, p. 92]:

where k _R = (5 ... 7) · 10 ³ .

The maximum permissible current density in this case according to (28) is:

those. intensively decreases with increasing electrode diameter 2r _{B in} proportion to 1 / r _B ² .

To achieve acceptable industrial current densities i _pr ≈0.1 A / cm ² in this method, the ratio δ / r _in ≈0.2 is adopted.

Since the channel height is always strictly limited by the condition of minimizing ohmic energy losses in the element (in the method δ = δ _min = 0.12 cm), this in turn forces us to limit the limiting radius of the electrodes r _{to max} ≤ 0.12 / 0.2 = 0, 6 cm (in the prototype method r _{in max} ≈0.4 ... 0.6 cm), i.e. fundamentally limits the possibility of developing their working surface S (in the prototype S = πr _in L ² ≤50 · cm ² ).

A numerical comparison of i _pr with the prototype is given in the examples of the method.

5.2.2. Overall process performance

The total performance of the electrolysis cell for the target product:

- in the proposed method, taking into account (27), the maximum productivity of the element with the channel length L _o , the total width of the electrodes H - is:

Q _c = i _pr S S _Э = i _pr S S _M k _ψ = k k _ψ i _pr HL _o ,

those. with an increase in the width of the electrode, H increases unlimitedly;

(here: S _М = Н L _o - element overall area, equal to the area of the separation diaphragm; S _Э - working surface of the electrode; S _Э = S _M · k _ψ = S _M (1 + 1 / ψ ² ) ^1/2 );

- in a cylindrical element [8] of the same length L _o , taking into account (30), ceteris paribus, δ _{e the} productivity is:

Q _c = i _pr ES = k _i k _R (δ / r _B ) ² (2πrL _o ) Э = 2πЭk _i k _R δ ² L _o / r _{in (max)} ,

those. with increasing electrode diameter _E d = 2r _B - Q _p falls performance because the limiting current i _ave decreases with the square of the radius r _to the electrode. The ratio of the quantities Q _c / Q _c grows proportionally to H / r _{in (max)} and does not have fundamental restrictions on the upper limit;

k _i = w ^7/3 (K _o / δ _o L ^0.25 ) ^4/3 - constant according to the conditions of comparison.

The features of the proposed method is that:

- the set of developed conditions for ensuring the stability of the process (17), (22), (23), being fulfilled for one separate channel, is automatically performed for the entire system of channels, without imposing any restrictions on the size of the electrodes and the performance of electrolysis cells up to developed industrial scale;

- the set of essential features of the method also does not impose any restrictions on the general geometry and design of the electrodes and electrolysis cells, which can be performed both planar and cylindrical (see Fig. 12, 13).

This feature is almost the most important, because solves the problem of creating filter-press multi-cell batteries with bipolar electrodes for the industrial scale of processing weak water-salt solutions.

5.2.3. Process energy efficiency

A feature of the method is a significant increase in the energy efficiency of the process of processing weakly concentrated water-salt solutions.

The main energy loss ΔN during the processing of such solutions is the ohmic loss on the resistance of the electrolyte ΔR _el :

where I is the current through the element;

P is the resistance of the electrolyte;

ρ is the electrical resistivity of the electrolyte;

b is the thickness of the layer of electrolyte between the electrodes, b = b _o = d _o + δ _M for wave electrodes of the method; b = b _C = 2d _o + δ _M - for the cylindrical electrodes of the prototype;

δ _M is the thickness of the separation diaphragm;

S is the surface of the electrodes.

Comparing, with equal current strength I, the energy loss ΔN _o in the element with a wave-like surface of the electrodes and the loss ΔN _L in the element with cylindrical electrodes having the same channel height δ = d _o in the cathode and anode cavities and the same working surface along the separation diaphragm S _M , we obtain :

For values of the parameter ψ from 1 to 2 and for the usual order, the relations δ _M / d _o ≈0.5 / 0.12 = 0.4 the energy loss in an element with a wavy surface of the electrodes is from 40% to 50% relative to the cylindrical element. This is a consequence of the development of the surface of the electrodes and the reduction of the working interelectrode distance in the cell with a wavy surface of the electrodes.

5.3. The technical result of the method

The set of distinguishing features of the method described in paragraph 5, allows to obtain a qualitatively new technical effects:

5.3.1. To increase the hydrodynamic stability of the processes of electrochemical processing of laminar flows of weakly concentrated aqueous solutions to perturbations from gravity-concentration convection in large-sized plane-gap electrode channels.

5.3.2. To increase the intensity and energy efficiency of the process; to qualitatively increase the overall productivity of flowing electrolysis cells relative to analogues, eliminating the fundamental performance limitations of known methods.

5.3.3. Get the basis for creating large-scale structures of elements with bipolar electrodes and multi-cell electrolysis batteries of industrial capacity for the processing of weakly concentrated aqueous solutions.

6. Examples of constructive implementation of the method.

Examples of the method in various designs are shown in Fig.12, 13, 14.

12, 13 illustrate the construction of a planar electrolysis cell, including:

2 - cathode of flat rectangular geometry with a plane wave surface,

3 - an anode with a similar surface located symmetrically to the anode relative to the axis of the diaphragm 4 with a shift of the surface relative to the anode by the phase angle π of the generating wave, at a distance h + δ _M normal to the surface of a flat membrane and, accordingly, at a distance b = (h + δ _M ) · Sinα along the normal to the surfaces of the electrodes at each point (α is the angle of the channel profile, determined by the parameter of the generating wave ψ = tgα = λ / 2h, where λ is the wavelength, h is the wave amplitude parameter, δ _M is the diaphragm thickness,

4 - separation diaphragm thickness δ _M.

The cathode 2 and anode 3, made with surfaces (see Fig. 13) that meet the conditions (17) ... (25) and are located in this way, form, together with the diaphragm 4 in the cathode and anode chambers of the element, a system of parallel channels, through which the processed solutions 1 are passed, withstanding the strength of the electrolysis current and the flow rate of solutions in accordance with condition (17) of the hydrodynamic stability of the flows against concentration-gravitational perturbations.

At the outlet of the chambers, anolyte 6 with an acid reaction and disinfecting properties and catholyte 5 with an alkaline reaction are respectively obtained (see Fig. 12).

A feature of this design is the possibility of a substantial development of the working area of electrodes 2, 3 and, consequently, the performance of electrolysis cells, since the stability condition (17) does not impose any restrictions on the size and overall configuration of the electrodes and elements as a whole, in contrast to the limitations (type (3)) inherent in the known methods [8] and structures [9].

The second significant design feature is the ability to make electrodes bipolar, with two working surfaces: one in cathode mode, the second in anode mode, when cells are connected in series to the electrolysis battery (see dotted line in Fig. 12).

This feature allows you to implement industrial-scale electrolysis batteries of a modern filter-press type for the treatment of weakly concentrated solutions.

The features of the proposed method allow, if necessary, to realize on its basis also cylindrical designs of electrolysis cells, illustrated in Fig. 14, where 2 is a cylindrical cathode and 3 is a cylindrical anode with profiled surfaces according to (17) ... (25), 4 - aperture.

However, in contrast to the known constructions of the type [9], there are no restrictions on the element diameter d _B and, therefore, on its working surface is not imposed.

On Fig shows the appearance of the electrodes with a plane wave surface made according to the conditions of the method.

With the dimensions of the working surface of the electrodes with options 75 × 150 mm and 100 × 200 mm, the value of their working surface in the assembly of the element is about 140 cm ² and 240 cm ² , i.e. from 3 to 4.5 times the surface of a single cylindrical element in known structures [9], [13].

Literature

1. L.A. Kulsky et al. "Technologies for the purification of natural waters", Kiev, Higher School, 1986

2. US patent No. 5427658 from 06/27/1995,

3. Medical electrochemical activator. Information leaflet No. 03049 dated 02.27.1987

4. USSR author's certificate No. 904394 of 05/27/1980

5. “Applied Electrochemistry”, ed. Tomilova A.P., M., Chemistry, 1984

6. Getling A.V. “Rayleigh-Benard Convection”, M., URSS Editorial, 1999

7. G.Z. Gershuni, E.M. Zhukhovitsky. "Convective stability of an incompressible fluid." M., Science, 1972

8. RF patent №2155719 from 12.25.1998

9. RF patent No. 2038322 of 04/03/1992

10. Japan Patent No. 1104387 of 09.21.1987

11. B.B.Damaskin. "Introduction to electrochemical kinetics", M., Higher school, 1975

12. S. S. Kutateladze. "Fundamentals of the theory of heat transfer", Novosibirsk, Science, 1970

13. V.M. Bakhir. "Electrochemical activation", M., VNIIMT, 1992

List of figures:

Figure 1. Scheme of the process of electrochemical treatment of water-salt solution.

Figure 2. The phenomenon of gravitational convection with a gradient of electrolyte concentrations in the channel of the electrolysis cell.

Figure 3. Structures of vortex convection.

Figure 4. A method of processing a solution in an element with an annular channel.

Figure 5. The circuit of the annular channels.

6. The concentration gradient in a stationary electrolyte.

7. The concentration gradient during the flow of an electrolyte with a boundary diffusion layer.

Fig. 8. Dependence of the concentration gradient in the element channel on the density of the processing current and the solution flow rate.

Fig.9. The principle of separation of the solution flow into a system of parallel channels in the electrolysis cell.

Figure 10. Profiles of plane-wave equidistant electrode surfaces.

11. Diagram of stability zones of laminar flow of a solution as a function of current densities, flow velocity, and sizes of electrode channels.

Fig. 12. The structural scheme of planar electrolysis cells with plane wave electrodes.

Fig.13. Profiles of electrodes with an equidistant plane-wave surface in a planar electrolysis cell.

Fig.14. Profiles of electrodes with an equidistant wave surface in a cylindrical element.

Fig.15. Appearance of electrodes with a plane-wave surface (material - titanium).

Claims (1)

The method of electrochemical treatment of water-salt solutions to obtain disinfectants and detergents and control the acid-base state of solutions, which includes passing electrolyte solutions in separate streams through the chambers between the surfaces of the cathode and anode of the electrolysis cell, respectively, and the surface of the diaphragm separating them, permeable to electrolyte, characterized the fact that the total flow of the initial solution at the entrance to the cathode and anode chambers is evenly divided into a system of parallel flows ; flows are passed over the surfaces of the electrodes in a laminar flow regime at a velocity w through a system of parallel, hydrodynamically separated channels that form in the chambers the surface of the separation diaphragm and the equidistant surfaces of the cathode and anode having a plane wave configuration with parameters of length λ, amplitude h and surface curvature radii r ; processing is carried out at current density i, maintaining the ratio of parameters within the limits determined by the criterion of hydrodynamic stability of the laminar flow of the solution to disturbances from concentration-gravity convection and by the condition of the bubble regime of evacuation of electrolysis gases: K ₁ + K ₂ · 0,0208Θ (σ / ρ ₀₎ ^1/2 ≡δ _emin <δ _e, <δ _emax ≡K _0, Ra ₀ ^{* w 1,75 / (i 0,75 L} 0,25) and conditions for the equidistance of electrode surfaces: r _max -r _min ≡b = (h + δ _m ) · [1+ (2h / λ ² ] ^{- (1/2)} ; 0 <r _min <(1/2) · [λ (1 + λ ² / 4h ² ] ^{- (1/2)} -b], where δ _e = 2h [1+ (1 + 4h ² / λ ² ) ^{l / 2} ] ^-1 is the equivalent hydrodynamic diameter of the channel; r _max , r _min - the maximum and minimum radii of curvature of the surface profile of the electrodes; b is the normal distance between the equidistant surfaces of the anode and cathode in the channels; L is the channel length downstream; δ _m - the thickness of the diaphragm; Ra ₀ ^* = π ⁴ [1 + 4 (2h / λ) ² ] ² - the minimum critical value of the diffusion analogue of the Rayleigh criterion in a channel with a stationary fluid; To ₀ = [(4,5 · 10 ^-3 · ρ ₀ · R · Т · (1 + t _a ) · λ _el · D ^2/3 ) · (g · F · ν · M) ^-1 ] ^{0, 75} - the constant of the physicochemical parameters of the solution; ₁ K = r _max [1- (1 + 4h ^{2/2) 1/2]} · [1+ (1 + 4h ² / λ ^{2) -1/2] -1} K ₂ = [1+ (1 + 4h ² / λ ² ) ^-1/2 ] ^-1 , ρ ₀ is the density of the solution, g / cm ³ ; R is the universal constant; T is the temperature; t _a is the anion transfer number in the electrolyte; λ _e - mole equivalent conductivity solution ^cm2 / (ohm · mol); D is the diffusion coefficient of the salt solution in the electrolyte, cm ² / s; g is the acceleration of gravity, cm / s ² ; F is the Faraday number, A · h / mol; ν is the kinematic viscosity coefficient, cm ² / s; M is the molecular weight of the salt of the solution, g / mol; σ is the surface tension of the solution, dyne / cm; Θ is the contact angle of the electrode surface with the solution.

Images