Electrofiltration
The electrostatic precipitation is a method in which the membrane filtration with the electrophoresis is combined in a dead-end process.
Electrofiltration has proven to be a suitable process for the concentration and fractionation of biopolymers . The build-up of the top layer on the filter membrane, which is a hindrance to the filtration, can be reduced or even prevented by an electric field , thus increasing the filtration performance, but also its selectivity (in the case of fractionations). This significantly reduces the costs in the downstream process of bioprocesses.
Procedure
The electrofiltration is a process for the separation or concentration of colloidal substances such. B. Biopolymers. The principle of electrofiltration is based on the fact that an electrical field is superimposed on a common dead-end filtration . As a result, if the polarity is skilful, the usually charged biopolymers are subjected to an electrophoretic force which counteracts the resistance of the filtrate flow. This drastically reduces the build-up of the top layer on the micro- or ultrafiltration membrane and reduces the filtration time from several hours in the case of filtration to a few minutes in the case of electrofiltration. Compared to tangential flow filtration , electrofiltration not only shows a greater permeate flow , it is also a particularly gentle separation process due to the low shear stress on the often sensitive biopolymers.
The use in biotechnological product processing is very promising, since biopolymers are difficult to filter on the one hand, but are charged on the other hand due to the amino or carboxy groups that are often present . In electrofiltration, the aim is to counteract cake build-up in order to improve the filtration kinetics of products that are difficult to filter.
If an electric field is superimposed on the filtration process, electrophoresis of the particles and electroosmosis occur . In electrofiltration, an electric field (DC) is superimposed on conventional filtration, which acts parallel to the direction of flow of the filtrate. If the electrophoretic force F E opposing the flow exceeds the hydrodynamic drag force F W , charged particles migrate away from the filter medium, so that the thickness of the filter cake on the membrane is significantly reduced.
If the solid particles to be separated are negatively charged, they migrate to the anode (positive pole) and are deposited on the filter cloth there. As a result, only a very thin cover layer is formed on the membrane on the cathode side (negative pole), so that almost all of the filtrate flows off through this membrane.
Figure 1 shows a schematic representation of an electrofiltration chamber with flushed electrodes . A buffer solution is used for the rinsing circuit . A patent was granted for this process in 2002.
Basics
The hydrodynamic drag force can be estimated using Stokes' law .
The electrophoretic force can be estimated using Coulomb's law .
In these equations, r H is the hydrodynamic radius of the colloid , ν the electrophoretic migration speed, η dynamic viscosity of the solvent, ε 0 the permittivity of the vacuum (electrical field constant), ε r the relative permittivity of water at 298 K, ζ zeta potential and E electrical Field. The hydrodynamic radius is the sum of the particle radius and the stationary solvent boundary layer.
During the stationary electrophoretic migration of a charged colloid, this electrical force and the hydrodynamic drag force are in equilibrium, and the following applies:
Due to these effects, the electrofiltration acts on the biopolymers, which can be charged, in addition to the hydrodynamic resistance force, as well as the electric field force. If you look at the cathode side, the electric field force acts there on negatively charged particles against the hydrodynamic resistance force. This hinders the build-up of the filter cake on this side , in the best case no filter cake is formed at all. The field strength from which this is the case is referred to as the critical field strength E crit . An electrical force also acts on the liquid because it is charged due to the neutrality condition. In addition to the applied hydraulic pressure difference, the electroosmotic pressure P e also acts .
The extension of the basic equation of cake-forming filtration of Darcy's law with the electrokinetic effects by integration under the assumption of constant values of the electroosmotic pressure P e , the critical field strength E crit and the effective field strength E is obtained:
In this equation, α c is mass-specific cake resistance, c concentration, A filtration area, V L filtrate volume, Δ P H hydraulic pressure.
Previous work in the field of bioprocess engineering at the Institute for Biotechnology and Food Technology at the University of Karlsruhe has shown that electrofiltration works for the concentration of charged biopolymers. Very good results have already been achieved with the purification of the charged polysaccharide xanthan . A filter cake of xanthan gum is shown in Figure 2.
literature
- Eugène Vorobiev, Nikolai Lebovka (ed.): Electrotechnologies for Extraction from Food Plants and Biomaterials (= Food Engineering Series ). Springer, New York NY 2008, ISBN 978-0-387-79373-3 .
Individual evidence
- ↑ patent WO02051874 : Electro filtration of biopolymer. Registered on December 20, 2001 , published on July 4, 2002 , applicants: Clemens Posten, Michael Herrenbauer, Karsten Weber, Ralph Hofmann, inventors: Clemens Posten, Michael Herrenbauer, Karsten Weber, Ralph Hofmann.
- ↑ Ralph Hofmann, Clemens Posten: Improvement of dead-end filtration of biopolymers with pressure electrofiltration . In: Chemical Engineering Science . tape 58 , no. 1 , 2003, ISSN 0009-2509 , p. 3847-3858 , doi : 10.1016 / S0009-2509 (03) 00271-9 .