FR2473313A1 - Constant pressure bacterial filter partic. to sterilise milk or wine - has ceramic membrane supported by rigid ceramic porous tubular element - Google Patents

Abstract

A continuous steriliser for liqs. comprises a filter capable of trapping bacteria from the liq. The filter has a cylindrical, tubular element made of porous ceramic with pore dias. of 1 to 10 microns. the interior surface of the tube is lined with a ceramic membrane with pore sizes of 0.1 to 0.22 micron. Liq. to be sterilised is pumped continuously into one end of the passage through the tubular element, where pressure is maintained at a predetermined value by an automatic pressure relief system. Sterilised liq. passes radially outwards through the wall of the tubular ceramic element to be collected in a surrounding reception tank. The pressure relief system pref. comprises a pressurestat controlling a relief valve which releases liq. for recycling back to the pump, or for passing onwards to a secondary bacterial filter system, or for drawing off laboratory samples. The element tube pref. has a wall thickness of between 0.5 and 2 mm. and the thickness of the membrane lining is between 10 and 50 microns. Sterilising liq. food prods. esp. milk or wine. There is no loss of nutritive value or deterioration of flavour of the prod. in this sterilisation process. The filter is robust, corrosion resistant, easy to clean and withstands high pressures.

Description

Liquid sterilization device
The present invention relates to a device for sterilizing a liquid.

It is known that it is possible to sterilize a liquid by heat treatment at a temperature of the order of 140 ° C., followed by rapid cooling. This method has the disadvantage of requiring a large installation and a high energy expenditure.

In addition, when the liquid is a food product such as milk or wine, its nutritional and taste qualities are generally altered.

 A device for sterilizing a liquid is also known, comprising organic screen filters through which the liquid is passed in order to retain the bacteria. The use of a device of this type makes it possible on the one hand to reduce the importance of the installation and the expenditure of energy, and on the other hand to preserve the taste and the nutritive qualities of the product if it it is a food product.

However, this device also has drawbacks.

Indeed, organic filters are fragile, resist corrosion badly and clog up quickly. In practice, it is necessary to operate at very low pressure and to perform a preliminary filtering through larger pore filters; however, the filters should be replaced frequently.

 The aim of the present invention is to overcome these drawbacks and to provide a filter sterilization device comprising filters capable of operating at higher pressures, under conditions which allow them to resist clogging for much longer, these filters being, in addition, easy to regenerate when clogged.

The present invention relates to a device for sterilizing a liquid comprising a filter capable of retaining the bacteria contained in this liquid, characterized in that - the filter is a porous cylindrical tube, formed of a tubular ceramic support whose pores have a size between 1 and 10 microns and a ceramic membrane disposed on the inner cylindrical surface of the support, the pores of this membrane having a dimension between 0.1 and 0.22 microns, - and that behaves
a first pumping circuit connected to the cylindrical volume
inside the tube to pass the liquid in this volume,
from a first end to the second end of the tube,
means for maintaining the pressure of the liquid in
the tube at a first fixed value high enough to
that part of the liquid passes through the wall of the porous tube
and a container surrounding the outer cylindrical surface
of the tube to collect said part of liquid as well
sterilized.

An embodiment of the object of the present invention is described below, by way of example, with reference to the appended drawing in which the single figure schematically represents an embodiment of the device according to the invention.

In this figure, the end of a pipe 1 is disposed above a liquid 2 contained in a tank 3. The pipe 1 comprises a valve 4 whose opening and closing is controlled by a servo system 5. The system 5 is connected to a sensor 6 of the equilibrium level 7 of the liquid 2.

 The tank 3 has a lower opening 8 connected by a pumping circuit to one end 18 of the interior volume of a tube 12.

This circuit comprises in series a pipe 9, a pump 10 and a pipe 11.

 The tube 12 is formed of a tubular support 13 and of a membrane 14 deposited on the inner cylindrical surface of the support 13. The support 13 consists of a poreUse ceramic which can be, for example, alumina. The pores of the ceramic constituting the support 13 have a size of between 1 and 10 microns. The thickness of this support can be between 0.5 and 2 mm, and its diameter is of the order of one cm. The membrane 14 is also formed of a porous ceramic which can comprise, for example, alumina, zirconia or silica. The pores of the membrane 14 have a size between 0.1 and 0.22 microns and its thickness can be between 10 and 50 microns.

The outer cylindrical surface of the tube 12 is surrounded by a coaxial cylindrical container 15. A pipe 16 provided with a valve 17 is connected to a lower opening of the container 15.

A recycling circuit is constituted by a pipe 19 starting from the other end 20 of the internal volume of the tube 12 and ending above the equilibrium surface 7 of the liquid contained in the tank 3.

A valve 21 is arranged on the pipe 19 at the outlet of the tube 12. A pressure gauge 22 is arranged in bypass on the pipe 19 between the end 20 and the valve 21. The information of the pressure read by the pressure gauge 22 is transmitted by an electrical connection 23 to a servo system 24 controlling the opening and closing of the valve 21.

 A line 25 is connected in bypass to the line 19 at the outlet of the valve 21. The line 25 is provided with a valve 26 and terminates above the equilibrium level of a liquid contained in a tank 28. An apparatus 29 is connected in diversion to the pipe 25 to perform periodic withdrawals of the liquid in circulation. A system 31 controls the opening and closing of the valve 26.

 As shown in the figure, the sterilization device may also include another pumping circuit connected to a lower opening 32 of the tank 28.This circuit comprises a pipe 33 starting from the opening 32 and ending at a pump 34. A at the outlet of the pump 34 the pumping circuit is divided into two branches 35 and 36. The branch 35 ends in a chamber 37 of which a part of the wall is formed by a filtering plate 38.

 The plate 38 comprises a porous ceramic support plate which may for example be alumina. The pores of the support plate have a size between 1 and 10 microns. The thickness of the support plate can be between 0.5 and 2 mm. The plate 38 also includes a membrane deposited on one face of the support plate.

This membrane also consists of a porous ceramic and can comprise, for example, alumina, zirconia or silica.

The thickness of this membrane can be between 10 and 50 microns and its pores have a dimension between 0.1 and 0.22 microns.

On the pipe 35, a valve 39 is arranged in series and, bypass, a pressure gauge 40, the measurements of which are transmitted by an electrical connection 41 to a servo system 112 controlling the opening and closing of the valve 39.

The branch 36 also leads to a chamber 43, part of the wall of which is formed by a filtering plate 44 identical to the plate 38. On this branch are also arranged, as on the branch 35, a pressure gauge 45 and a valve 46 whose opening and closing is controlled by a servo system 47 electrically connected to the pressure gauge 45.

The sterilization device described above and shown in the figure operates as follows.

 The liquid to be sterilized, which can for example be milk, arrives via the pipe 1. In this case the pipe 1 can be connected directly to a reservoir (not shown) receiving the milk leaving a milking machine.

 When the valve 4 is open, the milk leaving the pipe 1 flows into the tank 3. The pump 10 circulates the milk 2 filling the tank 3 in the interior volume of the tube 12, from the end 18 to the end 20 of this tube. The pressure of the milk circulating in the tube 12 is read by the pressure gauge 22 and the control system 24 which receives the information of the pressure measured by the pressure gauge 22 regulates the opening of the valve 21 so as to maintain this pressure at a fixed value, high enough for part of the milk to radially pass through the wall of the tube 12 in the direction indicated by the arrow 48. This fixed pressure value is however relatively moderate, for example of the order of 5 bars, for avoid too rapid clogging of the pores of the tube 12. it should be noted that the anisotropy of the porous tube allows significant flow rates without appreciable fouling of the membrane.

The membrane 14, the pore size of which has been defined above, retains the bacteria but lets the organic matter of the milk pass, the porous support 13 playing here a purely mechanical role, so as to allow the tube 12 to resist the internal pressure. milk.

 The part of the milk which has passed through the pores of the tube 12 is thus sterilized. This part is collected in the container 15. The sterilized milk contained in the container 15 can be withdrawn by opening the valve 17 for packaging.

The major part of the milk which entered by the end 18 leaves the tube 12 by the end 20 without having crossed the pores of the tube. This part is returned to the tank 3 by the pipe 19.

The control system 5 which receives from the sensor 6 the information of the level 7 of the milk in the tank 3 regulates the opening of the valve 4 so as to maintain this level substantially constant.

 The volume of milk contained in the closed circuit constituted by the tank 3, the pipe 9, the pump 10, the pipe 11, the tube 12 and the pipe 19 therefore remains constant, the supply of milk through the pipe 1 compensating for the quantity milk filtered through the pores of tube 12. As a result, this volume is gradually enriched with bacteria.

 The apparatus 29 takes milk samples at regular time intervals, the level of bacteria contained in the sampled liquid being determined for example by an optical method of counting in a petri dish. When this rate reaches a predetermined threshold, the system 31 makes it possible to control the opening of the valve 26, thus causing the emptying of the closed circuit in the tank 28. It is also possible to control the opening of the valve 26 at time intervals predetermined during previous tests, these intervals depending on the type of milk to be sterilized.

 The predetermined threshold is preferably chosen at a level low enough to avoid clogging of the pores of the tube 12.

In small installations, additional filtering of the liquid 27 charged with bacteria is not profitable. The installation is then limited to the part of the diagram located in the figure above the tank 28.

 If it is a large installation, the device preferably comprises the entire diagram shown in the figure. Thanks to the pump 34, the liquid 27 filling the tank 28 is led through the branch 35 into the chamber 37, the valve 46 being kept closed. The liquid 27 which is thus compressed in this chamber passes through the pores of the filter plate 38. The liquid pressure in the line 35 is measured by the pressure gauge 40, and the servo system 42 closes the valve 39 when this pressure exceeds a predetermined value. Closing the valve 39 makes it possible to avoid rupture of the porous plate 38 when it is completely clogged.

 In this case, the valve 39 is then kept closed and, the valve 46 being released, the rest of the liquid 27 is sent by the pump 34 to the chamber 43 through the pipe 36. This liquid is compressed in the chamber 43 so to be filtered by the porous plate 44, and so on.

It is therefore seen that at a given instant the liquid circulates only in one of the two branches 35 or 36. In fact as soon as a filtering plate for example the plate 38 is clogged, it is dismantled from the chamber 37 to regenerate it by a pyrolysis treatment or by chemical leaching. The pyrolysis treatment consists of placing the clogged plate in an oven at a temperature between 600 and 7000C so as to calcine the bacteria which clog the pores, the carbon produced then burning spontaneously. The plate thus regenerated is then reassembled on the chamber 37, so that it can be used when the plate 44 is in turn clogged.

 The sterilized liquid having passed through the filter plates 38 and 44 is collected in a container, not shown.

The complete device shown in the figure therefore makes it possible to obtain a larger quantity of sterilized liquid for the same quantity of liquid to be treated. This result is obtained by adding, to the device operating in continuous mode by filtering through the tube 12, an auxiliary device operating by filtering through the plates 38 and 44 in discontinuous mode each time the draining operation is carried out. of the closed circuit by opening the valve 26.

In practice, the liquid pressure in the tube 12 and the maximum rate of bacteria tolerated in the liquid circulating in the closed circuit can be chosen so that the tube 12 is only clogged very rarely. Of course, this tube is then regenerated under the same conditions as the plates 38 and 44.

 The support 13 of the tube 12 can be produced, by a known method, by spinning a paste, the tube thus obtained is then scoured and sintered. The membrane 14 is then deposited on the internal cylindrical surface of the support 13 by coating with a slip.

 This is then sintered to obtain pores whose dimensions are located in the narrow range which was specified above.

 The device according to the invention can be applied in particular to the sterilization of liquid food products such as milk, wine or fruit juices. In the case of milk in particular, the device according to the invention allows sterilization to be carried out at the place of production, thus eliminating costly transport.

Of course the present invention is not limited to the embodiment described and shown which has been given only by way of example. In particular, it is possible, without departing from the scope of the invention, to replace certain technical means by equivalent means.

Claims (9)

1 / Device for sterilizing a liquid comprising a filter capable of retaining the bacteria contained in this liquid, characterized in that - the filter is a porous cylindrical tube formed of a ceramic tubular support whose pores have a dimension between 1 and 10 microns and a ceramic membrane disposed on the inner cylindrical surface of the support, the pores of this membrane having a dimension of between 0.1 and 0.22 microns, - and that it comprises a first pumping circuit connected to the internal cylindrical volume of the tube to pass the liquid in this volume, from a first end to the second end of the tube,. means for maintaining the pressure of the liquid in the tube at a first fixed value sufficiently high for a part of the liquid to pass through the wall of the porous tube. and a container surrounding the outer cylindrical surface of the tube for collecting said portion of liquid thus sterilized. 2 / Device according to claim 1, characterized in that the wall of the tubular support has a thickness between 0.5 and 2 mm, and that the membrane has a thickness between 10 and 50 microns. 3 / Device according to claim 1, characterized in that the means for maintaining the pressure of the liquid in the tube at a first fixed value comprise a first manometer and- a first valve arranged in series in the first pumping circuit at the outlet of the tube, and a first control system regulating the opening of the first valve, so as to maintain this pressure at said first fixed value. 4 / Device according to claim 3, characterized in that it comprises a recycling circuit in series with the first pumping circuit, this recycling circuit being capable of returning to the first end of the tube the liquid leaving the second end. 5 / Device according to claim 4, characterized in that it further comprises a first tank located upstream of the first pumping circuit and a pipe bringing into the tank the liquid to be sterilized. 6 / Apparatus according to claim 5, characterized in that the pipeline comprises a second valve and that it further comprises a sensor of the liquid level in the first tank and a second servo system acting on the opening of the second valve to keep the liquid level in the first tank substantially constant. 7 / Device according to claim 6, characterized in that it further comprises, downstream of the first valve, a pipe connected in bypass on the recycling circuit, a third valve in series in this pipe, a device mounted in bypass on this pipe for carrying out periodic withdrawals of the liquid in circulation, means for measuring the level of bacteria in the withdrawn liquid and means for controlling the opening of the third valve when the level of bacteria in the liquid reaches a predetermined threshold. 8 / Device according to claim 7, characterized in that it further comprises - a second tank receiving the liquid leaving the third valve - a porous ceramic filter plate capable of retaining the bacteria contained in the liquid - and a second pumping circuit capable of bringing the liquid from the second tank into a chamber, part of the wall of which is formed by the filtering plate, this circuit comprising in series, between the second tank and the plate, a fourth valve and a second pressure gauge and a third servo circuit capable of closing the fourth valve when the liquid pressure measured by the second pressure gauge exceeds a second fixed value. 9 / Device according to claim 8, characterized in that the filtering plate comprises - a ceramic support plate whose thickness is between 0.5 and 2 mm, and whose pores have a dimension between 1 and 10 microns, - and a ceramic membrane deposited on one face of the support plate, this membrane having a thickness between 10 and 50 microns and pores whose size is between 0.1 and 0.22 microns.