Method and apparatus for treating a fluid

The invention relates to a method and apparatus for treating a fluid and also to a fluid filter for use in such a method and apparatus. Within the scope of this patent application, a “filter” comprises any object through which a fluid can flow, apart from the question whether this object is used to stop particles from the fluid, or passes all particles.

From an article in Food Research International 29(2), pages 117-121, entitled “Inactivation of Escherichia Coli by combining pH, ionic strength and pulsed electric field hurdles” by H. Vega-Mercado, U.R. Pothakamurt, F.J. Chang, G.V. Barbarossa-Canovas and B.G. Swanson, a method is known for pasteurizing milk with the aid of an electric field. Such a technique is also known from IEEE Transactions on Industry Applications 34, pages 43-50, 1998.

By exposing milk to an electric field having a strength of 2-4 MV/m, the cell membranes of microorganisms present in the milk break open through the agency of the charge carriers present thereon. This leads to deactivation of the microorganisms.

The above article also describes an apparatus in which the milk is exposed to the field. This apparatus is provided with a pipe through which the milk is passed. The pipe wall consists over the major part of the length of a conductive material, which is locally interrupted by a ring of insulating material. The ring separates the conductive material into two parts. Between the parts an electric tension is applied to generate the electric field in the milk.

The employed electric tension is within the range of 20-40 kV. The energy consumption as a result of this voltage is rather high. In order to eliminate the microorganisms, however, short voltage pulses, of the order of a microsecond, will suffice. These must be repeated whenever the treated fluid has flown from the space within the insulating ring. Thus the energy consumption can be limited.

The use of such a high voltage and short pulses, however, makes the apparatus expensive and impractical for use on a large scale.

It is inter alia an object of the invention to provide a method and an apparatus in which fluids can be exposed to high electric fields without high voltages being necessary.

The invention provides a method for treating a fluid, which comprises passing the fluid through pores in a membrane having surfaces connected by the pores, on which conductive layers are arranged, which are interrupted at the pores, the membrane containing a material that electrically insulates the conductive layers practically from each other, in which method a voltage is applied between the two layers. According to the invention, the fluid, for instance milk, is exposed to an electric field in the pores of an insulating membrane.

The electric field is generated by means of voltage on conductive layers on opposite surfaces of the membrane. The membrane preferably consists of insulating material, but any material that admits a voltage drop between the conductive layers is useful. The pores connect the two surfaces, and the layers are interrupted at the pores, so that the fluid can flow through the pores. It is possible to generate a high electric field strength with a relatively low voltage between the layers, because a thin membrane having a large amount of small pores is possible, which together pass sufficient fluid flow.

Although the method can be used specifically for pasteurizing milk, the method is applicable to treatment of all kinds of bulk fluids, for instance for disinfecting liquid foods, water purification or for the extraction of the cell content of biomass, for instance for the extraction of juice from plant cells et cetera, in which the cell wall of biological cells must be perforated.

As voltage an alternating voltage is preferably used. Thus disintegrating effects are inhibited in the fluid, and the membrane is prevented from clogging as a result of charge effects. In principle, however, direct current voltage is also useful.

The invention also relates to an apparatus with two chambers and such a membrane between them. In one embodiment of the apparatus according to the invention, there is used a packet of such membranes through which the fluid is successively passed. After passage through each membrane the fraction of cells that are not broken open will fall by a factor. Thus a very high effectiveness can be obtained. Preferably, the same voltage is applied across each membrane. Thus one voltage source will suffice. The polarity with which the voltages are applied between the different layers makes no difference to the effectiveness. Preferably, the polarity is selected such that the layers on the outside of the packet have the same potential, corresponding to that of the rest of the apparatus (earth).

In a further embodiment, such a series of membranes is obtained by means of a sandwich construction of alternately non-insulating membranes and conductive layers. Thus each conductive layer between a pair of membranes serves as a pole for applying voltage across two membranes, on both sides of the layer. In this embodiment, the polarity of the voltage in successive layers interchanges. Thus one voltage source will suffice for the sandwich.

The invention also relates to a fluid filter with a membrane for use in the method or apparatus according to the invention.

These and other objectives and advantageous aspects of the method and apparatus according to the invention will be described in more detail with reference to the following figures.

FIG. 1 shows an apparatus for treating a fluid.

FIG. 2 shows a membrane element.

FIG. 3 shows a top view of a membrane element.

FIG. 4 shows a cross-section of a detail of a membrane element.

FIG. 5 shows a packet of membranes.

FIG. 1 diagrammatically shows an embodiment of an apparatus for treating a fluid. The apparatus is provided with an inlet 16 and an outlet 18 for fluid, first chambers 12a-e, second chambers 14a-e and membrane elements 10a-e. Filter units, which each contain one of the first chambers 12a-e and one of the second chambers 14a-e separated by one of the membrane elements 10a-e, are inserted parallel to each other between the inlet 16 and the outlet 18. The number of chambers 12a-e depends on the desired processing capacity. If necessary, one chamber will suffice. Optionally, the membrane elements 10a-e are arranged on carriers (not shown) to increase their firmness. In operation, a fluid to be treated flows from the inlet 16 to the outlet 18 via successively one of the first chambers 12a-e, a membrane element 10a-e and one of the second chambers 14a-e.

FIG. 1 only shows an illustrative embodiment of the invention. Each arrangement of fluid channels with membrane units therein having any shape (not necessarily flat and not necessarily having a fixed time-independent shape) can be used.

FIG. 2 shows a side view of a membrane element (not to scale). The element contains a membrane 20 with electrically conductive layers 22a-b thereon and, on the layers, connections 24a-b to a voltage source 26. The voltage source is used to apply a voltage within the range of 10-20 Volts between the layers 22a,b. The membrane is for instance about 20 micrometers in thickness and has a diameter of a number of centimeters. The circumference of the membrane may have any desired shape, for instance round, square, et cetera. The membrane contains small pores (not shown in FIG. 2) having a diameter of the order of 10-20 micrometers. Although in FIG. 2 the metal layers 22a are thinner than the membrane 20, thicker metal layers may be used in practice.

The invention is of course not limited to flat membrane elements, as shown in FIG. 1. In general, the surface of the membrane can be given any three-dimensional shape desired for insertion in a fluid flow. Even time-dependent shapes are possible. The membrane elements may for instance also be cylindrical, so that a membrane forms a separation between a cylindrical inner space and an outer space, with the layers 22a,b on respectively the inner and the outer side of the cylinder. By pressing the fluid into the cylinder, it is forced to flow out of the cylinder through the pores.

FIG. 3 shows a top view of a detail of the membrane 20. In the membrane 20 a pore 32 is visible. The diameter D of the pore 32 is indicated. The diameter of the membrane 30 is much larger, many times larger than the diameter of the pore 32, typically at least of the order of centimeters. The membrane comprises a large number of pores, such as pore 32.

FIG. 4 shows a cross-section of the membrane 20 and the conductive layers 22a,b in side view along the line I-I of FIG. 3. The cross-section runs through a pore 32. At the pore the membrane 20 and the layers 22a are interrupted so that a fluid flow is possible through the membrane (from the top to the bottom in FIG. 3). The voltage that in use is present between the layers 22a,b provides an electric field in the membrane 20 and the pore 32.

Some field lines 30a-d of this electric field are indicated in FIG. 4. In the membrane 20 the field lines run practically straight from the first layer 22a to the second layer 22b. In the pore, field lines 20a-d run in curves from the edge of the first layer to the edge of the second layer. In general, the field strength will decrease according as the distance to the edge between the pore and the conductive layers 22a,b increases, but as long as the radius of the pore is of the same order or smaller than the thickness of the membrane 20 this decrease is not strong, and therefore about the same field strength will prevail in the pore as in the membrane 20, that is to say a field strength of about the voltage between the conductive layers 22a-b divided by the thickness of the membrane. At a voltage of 20 Volts and a membrane of 10 micrometers in thickness this is therefore a voltage of about 2 MV/m.

Thus it is possible with a relatively low voltage to generate sufficient field strength in the fluid that flows through the pores 32 in the membrane to perforate cell walls of cells in the fluid. This simplifies the apparatus for treating a fluid considerably in comparison with a high-voltage installation.

Through the small thickness of the membrane it is possible to renew the fluid in the membrane almost immediately. Thus even at a voltage continuously present there hardly occurs energy dissipation, while the pore is filled with “treated” fluid, which is not combined with perforation of cell walls. This renders an efficient treatment possible without complicating pulse techniques.

The diameter of the pores is selected on the basis of the largest particles that occur in the fluids to be treated. Thus these particles can pass these pores. The cross-section of the pores need not necessarily be circular. Any shape is useful. The thickness of the membrane 20 is preferably not smaller than the radius of the pores (or, more in particular for non-circular pores, than the distance from one of the conductors 22a,b to any point in the pore in the plane of the respective conductor) or at least a small factor of for instance at most 5 times that radius or distance. Thus sufficient field strength is left across the whole pore.

Although in FIG. 2 the pore runs straight on, it will be clear that pores that rather coil from one side of the membrane to the other are also useful. In fact, it will suffice that the field lines run through the pores.

Preferably, there is used an alternating voltage between the conductors 22a,b, at a frequency that is preferably at least so high that, at the employed flow rate of the fluid through the pores, fluid particles cannot flow through the pore from one side of the membrane to the other within a small part of the period of the alternating voltage (for instance less than a quarter of a period). Preferably, the frequency is so high that the fluid particles will take at least one whole period to flow through the pore. Thus disintegration of the fluid is inhibited, and clogging as a result of charge effects is prevented.

The manner in which the element with the conductive layers 22a,b and the membrane 20 with the pores therein is made is not essential to the invention. In one embodiment, there is started from a membrane of plastic foil, but a ceramic material et cetera could also be used. Subsequently, the conductors are arranged on the membrane (for instance by sticking metal foil thereon, or by evaporating, arranging a metallic paint by sputtering et cetera). The technique by which the metal layer is applied is not essential either. Preferably, there is used a rather thin metal layer, but the desired field may also be generated with thicker metal layers.

With laser technology holes are burned through the packet of the conductors layers 22a,b and the membrane 20. By locally heating the metal layers and the membrane with a focused laser beam, there can thus be made pores running through the metal layers and the membrane and having a diameter adjustable up to the micron. Of course, without departing from the invention, another membrane, another manner of arranging the conductive layers and/or another manner of arranging pores can be used, Thus, for instance, there can be used etching through apertures in a photolithographic layer to make the pores.

Essential to the selection of the material of the membrane 20 is only that this admits the existence of an electric field between the conductors conductors 22a,b. Preferably, an insulating material is used, but also a hardly conductive material will be satisfactory, on condition that at the employed voltages a significant part of the voltage drop between the two conductive layers remains present. Less insulation means in this case a higher energy consumption, but not that the cell wall perforating effect is lost. If necessary, a semi-conductive material may be used. Furthermore, a combination of material layers may also be used in the membrane 20.

FIG. 5 shows a packet of membrane elements 50, 51, 52, 53 of the type of FIG. 2. Each membrane element 50, 51, 52, 53 comprises an insulating membrane 500, 510, 520, 530 and a pair of conductive layers 502a-b, 512a-b, 522a-b, 532a-b. The membrane elements 50, 51, 52, 53 are separated by insulating layers 56a-c. Connections 58a,b to the conductive layers 502a-b, 512a-b, 522a-b, 532a-b render it possible to apply voltage between pairs of layer 502a-b, 512a-b, 522a-b, 532a-b around each of the membranes 500, 510, 520, 530. A first and second electrode 55a,b of the voltage source 54 are connected to the layers 502a-b, 512a-b, 522a-b, 532a-b.

In one embodiment, the insulating layers 56a-c may be left out, on condition that the same potential is applied to directly successive conductive layers 502b-512a, 512b-522a, 522b-532a. In that case there may even be used an integrated layer packet in which between each pair of membranes there is only one conductive layer, which is connected with both membranes.

The packet is inserted as a membrane element 10a-e between a first and a second chamber 12a-e, 14a-e of the apparatus of FIG. 2. Thus the fluid, when flowing through the packet, will be subjected a number of times to a high electric field in pores of successive membranes. Thus the fraction of the cells that remains unperforated can be limited in the fluid. In order to promote the flow, the pores may be arranged, if desired, after the membranes and the layers have been arranged on each other, so that the pores in different layers are automatically aligned with each other. This, however, is not necessary, certainly not if flow space is left between successive layers.

As shown, the voltages across the successive membranes 500, 510, 520, 530 are, in each case, applied with opposite polarity. Thus there will not arise any problems with fields between successive pairs of membranes 500, 510, 520, 530. Preferably, an even number of membranes is used. There is thus no potential difference between outer conductive layers 500a, 530b of the packet, with which the packet can enter into communication with its surroundings.

Although in FIG. 5 the membranes and layers are separated, there may of course also be used a single flexible membrane, with associated layers, which is folded over itself a number of times or is wound around a tube with a number of windings, after which continuous pores are arranged.

Although the connections 58a,b are shown as pins that cut through the layers 502a-b, 512a-b, 522a-b, 532a-b, with which they make contact, in practice preferably one or more electrodes are contacted with the surface of the relevant layers, for instance by making a part of the different layers, seen in FIG. 5 from the top to the bottom, accessible to the electrodes, or by folding the layers to the electrode.