The invention relates to a fluid handling system and to uses of the fluid handling system.

A wide range of fluids and viscous materials are being used in food, pharmaceutical and chemical processing for applications such as purification and emulsification with use of membranes (Basic Principles of Membrane Technology, Kluwer Publishers). Membrane filtration using rotating flat disc membranes is a compact fluid handling technology that has great potential advantages in performance and costs compared to currently available (non-rotating) systems. Due to the rotation of the membrane with respect to the fluid a high shear rate is exerted on the membrane surface that prevents membrane fouling. Shear rate is defined as a gradient of the velocity of the fluid near the membrane surface. The SI unit of measurement for shear rate is sec ^"1 (or m/sec/m). It is known that an increase in shear rate applied at the membrane surface increases proportionally the flux through the membrane. It is also known that this proportionality levels off if ceramic membranes are used and/or if the pores of the membrane get clogged during operation by particles in the unprocessed liquid.

It is known that ceramic membranes have been extensively studied for atomization (creating a dispersion of the unprocessed fluid), in particular emulsification applications using a non-rotating membrane device. Typical dispersed phase fluxes of an oil in creating an oil in water emulsion using a ceramic membrane with a mean pore size of 0,8 micron are 30-100 Iitres/m ² /hour at an operating pressure of 3-10 bar. The mean emulsion droplets size is then typical a few micrometer with a broad size distribution.

The filtration of liquids typically includes the separation of unwanted particles from an unfiltered substance, e.g. a liquid, but also fluids, gases or particulate substances, the so called unfiltrate, and to provide a filtered purified substance, the so called permeate. During that filtration process particles that are filtered out from the unfiltered unfiltrate, e.g. liquid, tend to accumulate at one or more areas of the filtering unit at the retentate side, i.e. the side which is directed towards the unfiltrate, especially on the filter medium, and forming a cake layer. Methods have been offered in order to either prevent the accumulation of particles on the filter medium, or to clean the filter medium from the accumulated particles. Common practice of cleaning the filter is a process known as backwashing the filter medium, in which a cleaning liquid, e.g. the permeate itself, is pushed through the filter medium from the permeate side to the retentate side. Normally a termination of the filtering process is required, and the backwashing takes place after the filtration process. Backwashing or short backwash strokes (i.e. reversal of the flow direction through applying a negative transmembrane pressure) by pumping permeate back through the filter medium and taking place during the filtering process, i.e. without terminating the filtering process, is also known, but current solutions show difficulties of using them in an even, economical way, which would offer both a high level cleaning of the filter medium surface, while spending as little permeate and as little power as possible. Moreover, such filtering systems typically demand complicated means of incorporating the backwashing process. It is therefore desirable to find a reliable and simple means of cleaning the filter medium and inhibiting the formation of a cake layer during the filtration process.

It is known to rotate filter discs during filtration in order to diminish the thickness of the cake layer through the creation of high shear rate forces on the filter surface. Examples of such filtration units are so called single shaft disc filtration units. The filter medium, typically filter discs, are stacked on an axis which is rotated. However, also by these measures the formation of filter cakes cannot be sufficiently reduced. A consequence thereof is that rather soon after starting the filtration process filtration efficiency is reduced, depending on the substance to be filtered and the system used.

Atomization, emulsification, filtration and similar processes are processes which can be summarized as fluid handling and a system for performing said processes is consequently a fluid handling system. Known fluid handling systems have, as discussed above, several drawbacks.

It is an object of the invention to improve existing fluid handling systems, particularly with respect to efficiency. It is another object of the invention to prevent clogging of the membrane during operation by particles in the unprocessed liquid.

According to the invention a fluid handling system comprises at least one membrane provided with pores, a housing and means to rotate the membrane in the housing. The membrane is a membrane filter with a low flow resistance, in particular a microsieve. Preferably the microsieve has a pore size of between 0,1 μm and 2 μm.

Further, the use of the fluid handling system for atomization, in particular emulsification is described as is described the use of the fluid handling system for filtration.

One preferred use of the fluid handling system is its use for filtering liquid food, particularly beer or milk, both fluids which are difficult to be filtered with prior art medium.

A membrane filter with a low flow resistance is one where the pores do not pose a high resistance to the fluid passing. The pressure needed for passing fluid through the membrane is rather low. The term "low flow resistance" is well-known in the art.

According to Liebermann, Franz, ,,Dynamic cross flow filtration with Novoflow's single shaft disk filters ", Desalination Journal, Volume 250, Issue 3, 30 January 2010, Pages 1087-1090, the basic property of low flow resistance membranes is the flux rate difference through channels vs. orifices. In theory, under the assumption of ideally round tubes and pores, the comparison of the flow rates can be described as follows (C. J. M. van Rijn, Nano- and micro-engineered membrane technology, Elsevier Amsterdam, ISBN 0444 514899, (2004), p. 89 ff):

Φ = ([ττ ^* r ⁴ ] / [8 ^* η ^* L]) ^* ΔP for round tubes, and φ = ([ΔP ^* r ³ ] / [3 ^* η]) / (1 + [8 ^* L] / [3 ^* ττ ^* r]) for orifices r ~ L, where:

Φ = volume flow rate r = radius η = dynamic viscosity ΔP = pressure drop

L = tube length

Based on these formulas, channels only 10 times longer than the nominal pore size, reduce flow rates already by multiples. In reality the channels of porous membranes, e.g. ceramics, tend to be longer and tortuous at the same time. A low flow resistance membrane having a relatively thin membrane layer (< 1 ,5 micron) with a high pore density (open areas of 30 %) and a narrow pore size distribution (+/- 10 %) on a wide open support show a better separation behaviour and much higher flow rates at lower pressures.

Therefore, compared to other microfiltration membranes, low flow resistance membranes have an extremely small flow resistance. Like ceramic membranes they are chemically inert, temperature resistant and can be back pulsed.

Membranes having low flow resistance typically have channels which are short in relation to pore diameters. Examples are wafer membranes or microsieves. In contrast to other membranes, e.g. ceramic or polymer membrane filters, wafer membranes are support-free. This means that no internal support structures are present. The membrane is single-layered; it may be coated, however.

As announced above, the term support-free means that no internal support is present. A typical microsieve for use in the invention has a thickness of max. 1 ,5 μm, or even thinner, which is, consequently, also the pore length. Pore diameters typically range between 0,1 - 10 μm. However it has to be noted that microsieves have uniform pore diameters. Since the material (SiN) is very hard, though elastic, such a membrane does not need any support structure up to membrane field width of ca. 1 mm and a length of several millimeters.

As announced above it is an object of the invention to provide for an optimized performance of the fluid handling system having rotating membranes. The fluid handling system is therefore characterized in that the applied membrane is a membrane filter with a low flow resistance, in particular a microsieve. With the means according to the invention it is possible to extend the said proportionality between shear rate and flux through the membrane and thus obtaining high operational process fluxes. It is an insight of the invention that known ceramic membranes are characterized by a relatively high flow resistance. In order to have still a minimum flux through the membrane these membranes are being operated at transmembrane pressures exceeding easily 3-5 bar both for filtration and considerably higher for emulsification applications. It is a further insight of the invention that pores of the membrane get easily clogged when relatively high transmembrane pressures are being applied. Microsieves are known e. g. from U.S. 5,753,014 and are manufactured by etching pores in a very thin layer, rendering a membrane with a very low flow resistance. In this patent application microsieves could be defined as a membrane with a clear water flux at 20 ⁰ C of at least 100.000 Iitres/m ² /hour/bar at a pore size of 1 ,0 micrometer and at least 10.000 I itres/m ² /h our/bar at a pore size of 0,1 micrometer. Clear water fluxes at intermediate pore sizes are obtained by direct interpolation, e.g. at a pore size of 0,5 micrometer the clear water flux at 20 ⁰ C is at least 50.000 Iitres/m ² /hour/bar.

According to the invention using a fluid handling system having rotating microsieve membranes it has very surprisingly been found that using a microsieve with slit shaped pores with a pore size of 0,45 micron and 0,8 micron typical dispersed phase fluxes of an oil in creating an oil in water emulsion are between 2.000-20.000

Iitres/m ² /hour at an operating pressure of less than 0.5 bar. Even more surprisingly it has been found that the emulsion is relatively monodisperse with a mean emulsion droplets size of typically twice the pore diameter.

EP0879635 describes a non-rotating filtering device in which a microsieve is operated in a crossflow filtration process by applying a shear of the unfiltered liquid over the membrane. The described device is capable of achieving a beer flux of about 15.000 litres per hour per square metre of filter surface at one bar of pressure difference over the filter. Typically this membrane is operated at a pressure of 0,05 bar and the beer process flux is in the order of 750-3.000 litres/hour/m ² . According to the invention using a fluid handling system having rotating microsieve membranes it has surprisingly been found that beer filtration is possible with a beer process flux is in the order of 5.000-20.000 litres/hour/m ² at a pressure of 0,05-0,2 bar.

According to one preferred aspect of the invention there are provided means to rotate the membrane in the filter housing. The membrane is subjected during fluid handling, e. g. the filtration process, with small periods of a negative transmembrane pressure through an increase and decrease in rotational speed. The change in rotational speed is preferably a periodic one.

In other words, at least one membrane is rotated in a first rotational speed. During the process the first rotational speed is changed to a second rotational speed whereby a negative transmembrane pressure is effected on the membrane. This change in rotational speed is performed during the process, e. g. filtration. There is no need to stop the filtration process, as is necessary for classical backwashing step. Further, in contrast to known on-line backwashing process the energy charge for the new method freeing the membrane surface from particles is neglectable; to the contrary, as the membrane is much better penetrable as in prior art filtering method the overall energy consumption is even reduced.

What matters to effect the negative transmembrane pressure is the change between first and second rotation speed. This change is preferably of a short term, e.g. occurs only within one second. Thus, the change can also be said to be a rather fast one. Necessary is that the difference between first and second rotational speed should not be too small, as otherwise the effect will be too small to be measured. As will be clear to the expert, exact data depend on the filtration circumstances, i.e. the kind of unfiltrate, the filtration apparatus, the filter medium chosen, the first rotation speed etc. Consequently, the expert will be able to understand the invention by relative terms and will take the necessary measures in order to adapt the invention to his needs and conditions. By way of example, in experiments performed by the applicant changes of 1 ,5-20 times of the first rotational speed, more preferably 1 ,5-5 times, proved useful.

Surprisingly it has been found that by changing the rotational speed also a change in the transmembrane pressure over the membrane is induced. The transmembrane pressure can even become negative and this means that a back wash stroke (i.e. a reversal of the flow direction through the membrane) occurs that releases the membrane surface of accumulated particles.

Good results have been obtained when the membrane is subjected to an average rotation speed between 1 and 100 cycles per second and the duration of the, preferably periodic, increase and decrease in rotational speed is less than 50 % of the total filtration time. In other words, during the filtration process at least half of the time the membranes are rotated in the average rotation speed. During the rest of the time the rotational speed is changed in order to induce a backwashing effect. In first experiments performed applicant found that to effect a measurable improvement of flux rate 80-95% of the total filtration time the first rotation speed can be maintained. During the rest, 5-20 %, of the overall filtration time the second rotation speed is assumed in a periodically manner.

By the term average rotation speed is meant the rotation speed used in prior art to effect a first improvement of membrane cake formation, as briefly explained in the introductory part. As the rotation of membrane in an average rotation speed is well known in the prior art the average skilled artisan will know that the exact velocity depends on the type of membrane used and its pore size, on the unfiltrate, on the filter apparatus size etc. Consequently the artisan is able to determine the average rotational speed without any inventive activity. This average rotational speed is termed first rotational speed.

According to the invention the rotational speed is during filter process changed from a first rotational speed to a second rotational speed, the first and second rotational speed being different from each other. This change can be a decrease in rotational speed as well as an increase. Important to effect a change in transmembrane pressure over the membrane is the change in rotational speed. This change in rotational speed preferably is not done constantly but the membrane is rotated in the first rotational speed and transiently the velocity is changed to the second rotational speed. Thereafter the first rotational speed is resumed. This change is effected preferably periodically.

According to experiments good negative transmembrane pressures can be induced wherein the membrane is subjected to an average rotation speed and the difference in periodic increase and decrease in rotational speed is between 10 and 50 cycles per second. Because this way of operation will reduce cake layer formation on the membrane, in some cases even free the membrane from cake layer, it is advantageous to use membranes with a low flow resistance, such as microsieves, in order to obtain high fluxes at relatively low transmembrane pressure. Microsieves are filtration membranes having pores with a pore diameter smaller than ten times the pore length. However, the use of a microsieve as membrane is only a preferred embodiment. Other membranes such as microporous ceramics, metals sieves, plastics etc. can also be used, as long as they have a low flow resistance.

According to another preferred embodiment of the invention the fluid handling system can also be used to make emulsions, oil in water emulsions as well as water in oil emulsions. For this purpose, rotational speeds of between 10 and 100 cycles per second are used, preferably. According to one preferred embodiment the pore size is smaller than 2 μm for making a monodisperse emulsion, particularly a monodisperse oil in water emulsion. Also, microsieves having slit shaped pores having a width smaller than 2 μm may be used for atomization purposes. If a microsieve is placed in a rotatable disc circa 5 cm from the rotation axis then the estimated shear rate at the surface of the microsieve filter is 10000 sec ^"1 at a rotation speed of 40 Hz. At a height of 1 micrometer above the microsieve surface the estimated cross flow velocity of e.g. water is then 10 mm/sec. If at the same time e.g. oil is flowing through the pores with a mean velocity of about 10 mm/sec then the oil will be uptaken by the water having a comparable speed. In case the microsieve is sufficient hydrophilic and no coalescence occurs, the oil will be dragged into the water phase by the formation of long jets that break up further downstream by the Rayleigh breakup process. The emulsion droplets will be monodisperse and have a typical size of twice the pore size.

In a preferred embodiment it has very surprisingly been found that using a microsieve with slit shaped pores (e.g. with a pore size of 0,8 micron x 3,0 micron) typical dispersed phase fluxes of an oil in creating an oil in water emulsion are 2.000-20.000 Iitres/m ² /hour at an operating pressure of only 0,3 bar. The mean emulsion droplets size is then typical 0,8 micrometer with a size distribution between 0,1 and 1 ,0 micrometer.

In the following the invention will be further explained by way of examples.

Example 1 , filtration of skimmed milk:

A microsieve filter with a pore size of 0,8 μm is placed in a rotatable disc in a distance of circa 5 cm of the rotation axis. The filtration system is filled with skimmed milk at 45 ⁰ C and a constant transmembrane pressure is set at 0,2 bar while rotating the disc with the filter medium at a constant speed of 12 cycles per second. The initial filtration flux was about 8.000 l/m ² /hour but within a few seconds was reduced to less than 500 l/m ² /hour. Such a reduction in the flux rate is typically for prior art filtration apparatus using this type of filter medium. When the rotating speed is increased from the average filtration speed of 12 cycles per second to a second rotation speed of 30 cycles per second during 1 second a negative transmembrane pressure is induced and some permeate is being pushed back through the microsieve, thus backwashing occurring. When the rotating speed is next decreased to the average rotation speed of 12 cycles per second the initial filtration flux is being restored. For constant operation mode the rotating parameters were set to 10 seconds at 12 cycles per second and after each 10 seconds the rotation is set to 30 cycles per second during one second. In this way an average milk filtration flux is measured of about 2.500 l/m ² /hour (at 0,2 bar and 45° C) and was maintained for more than two hours. Example 2, filtration of beer:

A microsieve filter with a pore size of 0,8 μm is placed in a rotatable disc circa 5 cm from the rotation axis. The filtration system is filled with unfiltered beer at 15 ⁰ C and a constant transmembrane pressure is set at 0,22 bar while rotating the disc with the filter medium at a constant speed of 12 cycles per second. The initial filtration flux is about 10.000 l/m ² /hour but within ten seconds reduces to less than 1.000 l/m ² /hour. When the rotating speed is increased to 30 cycles per second during 1 second a negative transmembrane pressure is induced and some permeate is being pushed back through the microsieve. When the rotating speed is next decreased to 12 cycles per second the initial filtration flux is being restored. The rotating parameters were set to 10 seconds at 12 cycles per second and after each 10 seconds the rotation is set to 30 cycles per second during 1 second. In this way an average beer filtration flux is measured of about 3.000 l/m ² /hour (at 0,22 bar and 15 ⁰ C) and was maintained for more than four hours.

Example 3, oil in water emulsion for parenteral solution:

A microsieve filter with a pore size of 0,8 μm is placed in a rotatable disc circa 5 cm from the rotation axis. The emulsification system is filled with water with a 1 % tween surfactant at 35 ⁰ C while rotating the disc with the membrane at a constant speed of 40 cycles per second. The estimated shear rate at the surface of the microsieve filter is 10.000 sec ^"1 . At a height of 1 micrometer above the microsieve surface the estimated cross flow velocity of the water is 10 mm/sec. At the permeate side of the microsieve filter the system has been filled with sunflower oil. After opening of the permeate valve a large negative pressure is exerted on the sunflower oil due to the rotation of the disc. The sunflower oil is next dragged into the water phase by the formation of long jets that break up further downstream by the Rayleigh breakup process. The flux of the sunflower oil is typically 3.500 Iitres/m ² /hour. The sunflower oil emulsion droplets size have a typical size of 1 ,6 micrometer with a variation in size distribution of 0,2 micrometer.

Example 4, oil in water emulsion for lotion:

A microsieve filter with a slit shaped pore size of 0,8 μm x 3,0 μm is placed in a rotatable disc circa 5 cm from the rotation axis. The emulsification system is filled with hot water with a 1 % polysorbate surfactant at 75 ⁰ C while rotating the disc with the filter medium at a constant speed of 50 cycles per second. At the permeate side of the microsieve filter the system has been filled with a wax mixture containing beeswax. After opening of the permeate valve a large negative pressure is exerted on the hot wax due to the rotation of the disc. A typical dispersed phase flux is obtained of 10.000 Iitres/m ² /hour. The mean emulsion droplets size is typically 0,8 micrometer with a size distribution of between 0,1 and 1 ,0 micrometer.