Progress in High-Power-Particle Beams and Pulsed Power for Industrial Applications at Forschungszentrum Karlsruhe, страница 3

Effective use and regeneration of natural resources

Strong pulsed electric fields can induce irreversible pores in the membranes of biological cells and their organelles. This effect, which is called electroporation can be used to extract raw materials for pharmaceutics, cosmetics, sustainable chemistry and food industry.

A second application of electroporation is the destruction of cells (bacteria, yeast etc.) and microorganisms to increase the shelf life of food products or to eliminate nuisance in aquatic systems.

The treatment of biological cells with pulsed electric fields is a physical method that can avoid undesirable side effects. In addition it is an energy efficient method and therefore has both economic and ecologic advantages.

At the last Beams meeting we have reported on the application of pulsed electric fields to improve the production of sugar from sugar beets [1]. These results had been obtained with the mobile electroporation device (KEA), which had a throughput limited to 300 kg/h. The KEA results demonstrated significant process advantages, especially reduction of energy consumption and providing of cleaner raw juice. To demonstrate that these advantages can also be realized under semi industrial conditions we have built the facility KEA-ZAR on the site of a sugar factory.

KEA-ZAR uses a hoisting drum to transport entire beets into the reaction zone where high voltage electrodes are installed in the jacket of the outer cylinder of the coaxial set-up. The ground electrodes are stainless steel ribbons mounted on the jacket of the inner cylinder. The high voltage pulses were supplied from two 7-stage Marx generators each delivering pulses with an amplitude of 350 kV at a frequency of 20 Hz. The complete facility is shown in Figure 3.

FIGURE 3. View of the semi-industrial facility KEA-ZAR for the treatment of sugar beets with pulsed electric fields. KEA-ZAR allows a throughput of up to 36 t/h

KEA-ZAR demonstrated that the design throughput of 36t/h could be achieved exposing each sugar beet to 30 pulses with electric fields of up to 10 kV/cm. However KEA-ZAR also revealed a few problems and deficiencies. The main problem was the inhomogeneous field distribution in the reaction zone. Another problem was the strong dependence of the electroporation result on the beet temperature. Since beets are harvested in autumn and stored outside, their temperatures can become close to 0° C. Successful electroporation at this temperature requires higher electric fields or a preheating of the beets. It has still to be determined which of these options is the better strategy.

Based on the experience with KEA-ZAR we are now designing a new reactor with a more homogeneous field distribution using segmented electrodes supplied by independent, yet synchronized Marx generators. This concept seems to be scalable to the huge throughputs of large factories (up to 15.000 t/d).

Synchronizing the Marx generators requires triggerable spark gaps. Conventional trigger electrode set-ups of spark gaps are however subjected to large burn-up and thus to unacceptably short lifetime. Therefore a new trigger concept has been developed at Forschungszentrum Karlsruhe, which avoids a wearing trigger electrode. In this concept the Marx generator charging coils become the secondary coils of a pulse transformer. Thereby the trigger pulses can be superimposed on the charging voltage of the Marx capacitors. In general only the first spark gap needs to be triggered in this way. The design can be made potential free using the Marx charging current also to supply the trigger generators. Synchronization of several Marx generators is achieved with light pulses guided by optical fibers to photosensitive solid state switches in the trigger circuit.

Naturally there is still a certain wear of the switch electrodes and their lifetime will be limited. However a lifetime of 2´10⁸ pulses corresponding to an annual sugar beet campaign is considered to be sufficient. Replacement of the electrode end-pieces after each campaign is considered to be economically acceptable. We have now set-up a Marx test stand to demonstrate that the required number of shots can be achieved with one set of switch electrodes.