CS233544B1 - Method for determining slip characteristics - Google Patents

Abstract

A method for determining slip characteristics of a flowable fluid comprising electrochemically active substances using a measurement of unsteady diffusion flows. A step change in the polarization potential of the working electrode embedded in the viscometer wall causes the transient current to be recorded and a velocity profile determined from this record. The working electrode has a circular cross-section. The described method can be used in physicochemical and reodynamic analysis of microdisperse liquids for scientific research or industrial laboratories.

Description

The invention relates to a method for determining the slip characteristics of a wall flow, e.g., slip velocity, wall velocity gradient, separated solvent layer thickness, and the like. and can be used in physicochemical and rheodynamic analysis of microdisperse fluids such as polymer solutions, colloidal suspensions, etc., in scientific research or in industrial laboratories, eg in the chemical and food industries, in drilling, petrochemistry, medical diagnostics, etc.

In the flow of microdisperse liquids, such as polymer solutions, colloidal suspensions, and the like, a liquid layer is formed in the immediate vicinity of the wall due to liquid-wall interactions whose properties, eg dispersed phase concentration, apparent local viscosity, differ significantly from those liquid. Macroscopic consequences of these downstream phenomena, referred to as slip respectively. wall effects, strongly influence the results of visometric measurements, but also eg mass transfer coefficients, which is important for the sizing of dialysers and other separation devices. On the other hand, a more detailed study of slip effects can provide new information on the internal structure of microdisperse fluids, eg degree of coagulation, concentration profiles of the dispersed phase, etc.

The extent and actual nature of slip effects has so far been judged only indirectly by comparing the results of rheological measurements obtained on a set of capillary resp. Rotary viscometers with different capillary diameters slot width between rollers. These indirect methods for determining slip characteristics are already described (Mooney M., The Rheology of Raw Elastomer in the Encyclopedia of Rheology, ed. F.R. Eirlch, Academic Prese (1958), Vol. II. Ooldemith H. L., Mason S. G., The Microrheology of Dispereions, ibid., (1967), Vol. IV; Kozichi W., Paeari S. N., Rao A. R. K., Tiu C., Anomalous Effects in Flow of Polymer Solutions, Chem. Eng. Sci 25. 41 (1970).

The disadvantage of these methods is the high laboriousness and considerable scattering of the results, both of which are due to the necessity to perform measurements in several viscometers under identical conditions and identical samples.

The use of optical methods for direct measurement of the velocity profile in the pulp, eg laser Doppler anemometry, is avoided, since slip effects occur in a very thin surface layer of liquid with a thickness below .00 / m.

Since the mass-sharing intensity is strongly dependent on the extent of the slip effects, measurements of this intensity give the possibility to infer the extent of the slip effects. Measuring the overall intensity of steady-state mass-sharing in a single geometric arrangement / e.g. Mashelkar, R. A., Dutta, A .: Convective Diffusion in Structured Fluid: Need for New Analysis and Design Strategies, Chem. Eng. Sci. 37. 969 (1982) / however, do not give the possibility to quantify slip effects quantitatively.

The advantage of the proposed method of determining the slip characteristics is the possibility of taking the necessary measurements in a single viscometer or other suitable vessel, within a very short time, 1 to 2 s, and the possibility of obtaining detailed data about the actual velocity profile in the liquid wall layer. wall surface.

SUMMARY OF THE INVENTION The present invention provides a method for determining the slip characteristics of a supernatant flow of a liquid containing electrochemically active substances utilizing the measurement of unsteady diffusion flux, recording the transient current I = I (t) u = u (z) according to relations

Z = c ₀ DF / I (t), u (z) = L / t '(1.36 I (oo) (I (t)) ^p , where c _Q is the concentration and D diffusivity of the electrochemically active substance, F is Faraday's constant, L is the effective electrode length, K,, = ⁸ is the current density from steady-state conditions, t = = £ (1 + p) b] ^ ^{2 + p} is the settling time, a, b, p are parameters in the following three-parameter theoretical description of the transient current versus time, for t <to,:

t ^{/ 2} I (t) = ad + bt ^{1 + p / 2} ).

In said method of determining slip characteristics, a working electrode having a circular cross-section of radius R is used, the effective electrode length L = 1.64 R.

In order to better explain the nature of the invention, a drawing is attached, wherein Fig. 1 shows an example of an embodiment of the proposed method of solution and Fig. 2 shows in a specific example a method of processing transient current data.

The viscometric flow of liquid in the space between the cylinder 1 and the cylindrical sleeve 2 is due to the rotation of the sleeve 1. The electric current passing between the working electrode 3 and the auxiliary electrode 4 is registered by a rapid action recording galvanometer 6, e.g. The working electrode 3 is electrically insulated from the cylindrical housing 2 by a layer 6.

Example

The commercial rotary viscometer RHEOTEST 2 has been modified to determine the slip characteristics according to the described method. In this case, the platinum working electrodes 3 have a circular cross section of diameter 2.03 mm _t o and the auxiliary electrode is made of a nickel foil with an area 3 cm. Transient currents were recorded on a loop oscillograph.

A 2% carboxymethylcellulose solution was added with the addition of 0.025 kmol / ml potassium ferrocyanide and ferrocyanide. The working electrode was connected as a cathode at a polarizing voltage of 0.7 V.

The course of the transient currents, measured at a peripheral velocity of cylinder 1 of 18.8 mm / s and a slot width of cylinder 1 of cylindrical sleeve 2 of 0.4 mm, is shown in Fig. 2.

P = 0, a = 35.4 Am ^-2 s -1 ^{/ 2} , b = 0.994 s ^-1 .

The corresponding values t (0 = 1.006 s, Κ ») = 70.8 Am ^-2 are fully consistent with the direct data on the settling time (i s) and the steady-state current density (71 Am). Also the diffusion coefficient value D = 0.676. 10 ® m ² s je is consistent with a value (0.68 ± 0.1) of 10 m ² s stanov determined independently, from the transient currents in the stationary liquid.

On the basis of the stated values of parameters p, t,, Ι (») e for L = 1.64 x 1.015 = 1.66 mm, the speed profile in the aft area was determined:

u = 1.7 mm s ¹ for 0 z z ^ 14 µm in accordance with a rough estimate of the slip velocity (1 to 3 mm s) ¹ ) obtained by the known visometric method.

Claims (2)

1. A method for determining the slip characteristics of a wall flow of a liquid containing electrochemically active substances utilizing measurement of unsteady diffusion flux, characterized by recording the transient current I = I (t) induced by a step change in the polarization potential of the working electrode embedded in the viscometer wall; profile u = u (z) according to z = c ₀ DF / I (t), uCz) = L / t '(1.36 I (') / I (t)) ^p , where c _Q is the concentration a D diffusivity of electrochemically active substance, F is Faraday constant, L is effective electrode length, I („) = ⁸ is steady-state current density, too = [(1 + p) b] ^-2 ^ (2 + p) j _U8 the ele- ments, a, b, p are parameters in the following three-parameter theoretical description of the transient current versus time, for t <too: t ^{/ 2} I (t) = ad + bt ^{1 + p / 2} Method according to claim 1, characterized in that a working electrode having a circular cross-section of radius R is used, the effective electrode length L = 1.64 R.