Method and apparatus for vacuum drying of products, in particular membrane filters, such as dialyzers, especially after washing or functional testing, using microwave energy

Abstract

Method for vacuum drying of products, in particular membrane filters (12), such as dialyzers, especially after washing or functional testing, using microwave energy and an atmospheric multi-mode microwave chamber (14) and a sealed, microwave-transparent vacuum housing (26), for example made of polytetrafluoroethylene, PTFE, in the multi-mode microwave chamber (14), wherein the vacuum housing (26) has at least two separate compartments (28) and a vacuum sensor (30) in each of the compartments (28) for individually monitoring a vacuum in the respective compartment (28), the method comprising: - Arranging at least one product in each of the at least two compartments (28) and sealing the at least two compartments (28), - Creating a vacuum in the sealed compartments (28), - Closing the multi-mode microwave chamber (14), - Generating and coupling microwaves into the multi-mode microwave chamber (14), - Measurement of individual vacuum pressures (P1, P2, P3, P4, P5) in the at least two compartments (28) using the vacuum sensors (30), and - Control of microwave vacuum drying depending on the individually measured vacuum pressures (P1, P2, P3, P4, P5) in the at least two compartments (28).

Description

[0001] The present invention relates to a method and a device for vacuum drying of products, in particular membrane filters, such as dialyzers, especially after washing or functional testing, using microwave energy.

[0002] The problems underlying the present invention are described below only by way of example in the context of membrane filter technology.

[0003] The importance of membrane filtration technology has increased in many areas of food, biotechnology (BioTec), and especially pharmaceutical applications over the last 10 to 15 years. Increasingly finer membrane structures allow for ever-improving filtration and increasingly selective separations. Rapid drying of the membrane modules is required after, for example, wet testing. Compared to air drying alone, microwave drying can reduce drying times from hours to minutes. Microwave drying is therefore well-suited for automated processes.

[0004] German patent DE 10 2006 051 656 B4 describes a microwave batch system in which membrane modules, such as dialyzers, are rapidly dried atmospherically in plastic housings using microwaves and purge air. The individual membrane modules (filter modules) must be properly connected so that purge air is forced or transported through the filter housing to remove the free water vapor.

[0005] German patent DE 103 33 639 B3 describes a microwave vacuum system for drying membrane modules and filter modules, in which individual membrane or filter modules are arranged in a circular pattern on several levels and can be dried with microwaves in a vacuum at temperatures below 60°C. This system also allows for the quick and gentle drying of pre-packaged membrane modules in water vapor-permeable bags.

[0006] German patent DE 10 2012 003 466 B3 describes an extended design in which dialyzers are pressurized with vacuum via connections instead of purge air. In this design, the dialyzer chamber is placed under vacuum and must withstand the pressure difference compared to the ambient atmosphere. There is a risk of deformation of the dialyzer housings, particularly at temperatures above 50°C.

[0007] German patent DE 10 2020 105 340 B3 describes a frequency-controlled microwave generator that supplies an atmospheric drying chamber for drying dialyzers. The drying of the dialyzers is achieved through frequency and microwave power control as well as humidity measurement of the exhaust gas stream.

[0008] German patent application DE 10 2021 121 051 A1 describes a frequency-controlled microwave generator that supplies a drying chamber containing a single dialyzer. The system described therein includes a device for determining the reflected microwave power and the frequency at which the reflected power is lowest, as well as a device for determining and adjusting the frequency, with feedback, to a frequency at which the reflected power is minimal.

[0009] Furthermore, if it is a single-mode microwave chamber, then usually only one or two fundamental microwave modes are possible, onto which the applied microwave power is concentrated. This limits the microwave power to prevent membrane damage caused by high field concentrations (hot spots).

[0010] The connection of components such as filter modules is always a critical part of a microwave drying system. If the microwave drying system operates in a vacuum, this connection is unnecessary. However, the microwave power densities that can be transmitted are then limited by the breakdown field strength, which is dependent on the vacuum pressure. The highest microwave field strengths occur particularly at the microwave coupling points into a vacuum chamber. This limits the microwave power input, which in turn results in shorter drying times.

[0011] US 11 035 612 B1, CN 1 11 829 295 A, CN 1 06 839 661 A, WO 00 / 23 861 A1 and US 4 606 650 A provide the technological background for the invention claimed here.

[0012] The present invention is therefore based on the objective of enabling faster microwave drying of products, in particular membrane filters, such as dialyzers.

[0013] According to the invention, this problem is solved by a method according to claim 1.

[0014] This can involve, for example, creating a vacuum before closing the multi-mode microwave chamber and vice versa.

[0015] Furthermore, this problem is solved by a system according to claim 13.

[0016] The pressure curve corresponds to the drying process. The more water vapor is produced, the higher the pressure rises. When the water vapor decreases at constant microwave power, the vacuum pressure also drops, and the product is dry. This relationship and curve are shown in Figure 3, which will be discussed further below.

[0017] If there are deviations in the drying processes or in the corresponding vacuum pressures between the compartments, mode stirrers can be used to reduce these deviations. For example, the mode stirrers can be "taught" and specific angular positions with specific dwell times can be achieved in test series. Once these have been determined for a particular product, these angular positions are then always used for that product by the control system during microwave drying.

[0018] The process can be designed to spatially modify the microwave field strength distribution in the multi-mode microwave chamber during microwave vacuum drying, preferably by means of at least one mode stirrer. Ideally, this ensures that each compartment, and thus, for example, each filter, receives the same microwave energy. The mode stirrer(s) shifts the modes back and forth on a time-averaged basis, thereby generating a uniform field distribution (→avoiding static hotspots).

[0019] Advantageously, the control of microwave vacuum drying includes power control and, preferably automatically, optimization of an impedance matching transformation using a tuner (especially an auto-tuner) to minimize microwave reflections. Control parameters for uniform drying can include the microwave power, the tuner, and / or the mode stirrer(s). These two or three control parameters continuously modify the microwave field so that each filter receives the same average microwave energy over time. The cyclical movements of the mode stirrer(s) ensure that the modes / field strength maxima are shifted within a range of, for example, 5–20 cm, thus preventing local hotspots.The drying process is controlled by the vacuum pressure in each compartment, which initially rises due to the released water vapor during drying and then reaches equilibrium with the vacuum pump's output and the hose cross-sections used to extract the water vapor. During the final drying stage, the amount of water vapor decreases and the vacuum pressure drops. Once a certain final vacuum pressure is reached, the microwave drying process is terminated and the filters are sufficiently dry.

[0020] According to another particular embodiment, the control of the microwave vacuum drying process includes determining the end of the drying of the product by means of individually measured vacuum pressures and terminating the generation and coupling of microwaves upon detection of the end of the drying process.

[0021] Advantageously, the drying time is less than 5 minutes.

[0022] According to another embodiment, during microwave vacuum drying, a microwave power in the range of 1.0 kW / product to 3.0 kW / product, preferably in the range of 1.0 kW / product to 2.0 kW / product, is transmitted.

[0023] Favorably, the standardized performance of microwave vacuum drying is η > 0.75 kWh / kg evaporated water.

[0024] Advantageously, the efficiency of microwave vacuum drying from generated microwave power to electrical power is 70%.

[0025] Furthermore, it may be provided that the sealed closure of the at least two compartments comprises a single sealed closure of the at least two compartments.

[0026] Alternatively, it may be provided that the sealed closure of the at least two compartments includes a joint sealed closure of the at least two compartments.

[0027] Advantageously, the arrangement of the products in the compartments includes arranging the products in one plane, preferably in columns and / or rows.

[0028] The system may be designed to include a lockable door for the multi-mode microwave chamber.

[0029] Furthermore, the microwave generating device may include a magnetron or a semiconductor assembly.

[0030] Advantageously, the system has at least one mode stirrer in the multi-mode microwave chamber and the control unit is designed to control the at least one mode stirrer.

[0031] According to another special embodiment, the system includes a separate vacuum pump for each of the compartments.

[0032] Furthermore, it may be provided that each of the at least two compartments can be individually sealed and locked.

[0033] Advantageously, at least two compartments can be sealed and closed together.

[0034] In particular, it may be provided that the system has a vacuum housing closure door for the joint sealed closure of at least two compartments.

[0035] The microwave chamber can be loaded from the side or from above, for example, whereby the vacuum housings are then not vertical but arranged horizontally.

[0036] Finally, a system is provided comprising a system according to one of claims 13 to 20, with several said multi-mode microwave chambers arranged, for example, in a circular arrangement.

[0037] The present invention is based on the surprising finding that when products, particularly membrane filters such as dialyzers, are encapsulated in microwave-transparent vacuum housings within atmospheric multi-mode microwave chambers (drying chambers), they can be dried much faster under vacuum, even with high microwave power densities, than with conventional vacuum systems. This is because microwave coupling at atmospheric pressure of approximately 1000 mbar allows for breakthrough field strengths that are about 30 times higher. As a result, drying times of less than 5-6 minutes are possible, instead of the previous 20 to 30 minutes.

[0038] Furthermore, the modes can be spatially modified, for example using mode stirrers, to ensure a uniform microwave energy distribution while avoiding hot spots. This is difficult to achieve in a vacuum, as metallic structures tend to form plasmas and arcs in a vacuum.

[0039] Likewise, the volume of the vacuum section (the space or chamber) is significantly smaller, which considerably reduces vacuum pumping times and allows for a more favorable design of the vacuum system.

[0040] Since membrane modules, for example, are temperature-sensitive, microwave vacuum drying offers significant advantages. Among other things, the evaporation temperature is limited to <60°C. This reliably prevents membrane damage even at high microwave power densities.

[0041] If the vacuum housings are made of, for example, PTFE, this leads to a significant shortening of the microwave wavelength, which allows for a more compact design of the microwave chamber.

[0042] According to a particular embodiment, at least one circulator can be provided as idling protection.

[0043] At least in one particular embodiment, the feedback-based determination of the measured dew point allows a statement to be made regarding the determination of the endpoint of the drying process.

[0044] Further features and advantages of the invention will become apparent from the attached claims and the following description, in which several exemplary embodiments are explained in detail with reference to the schematic drawings. These show: Fig. 1 a graph of the breakthrough field strength as a function of pressure (from “Industrial Microwave Heating”, AC Metaxas & RJ Meredith ISBN 0 906048 89 3); Fig. 2 a perspective schematic representation of a system for vacuum drying of products, in particular membrane filters, such as dialyzers, using microwave energy, in particular after washing or functional testing, according to a particular embodiment of the present invention; Fig. 3. Time courses of radiated microwave power, pressure in compartments, dew point temperature and performance when carrying out a method according to a particular embodiment of the present invention; and Fig. 4 a schematic representation of an atmospheric multi-mode microwave chamber with a vacuum housing contained therein of a system according to a particular embodiment of the present invention.

[0045] As in the Fig. As shown in 1, the breakthrough field strength Ê DAt 1000 mbar, the breakdown field strength is approximately 30,000 V / cm. In the typical vacuum drying range at 20 mbar, however, the breakdown field strength is 1000 V / cm and is therefore about 30 times lower than at 1000 mbar.

[0046] Fig. Figure 2 shows a particular embodiment of a system 10 for vacuum drying products, in this example membrane filters 12, using microwave energy, particularly after washing or functional testing. The system 10 comprises an atmospheric multi-mode microwave chamber 14, a microwave generating unit 16, a microwave phase shifting unit 18, a microwave frequency changing unit 20, a microwave coupling unit 22 for coupling microwaves generated by the microwave generating unit 16 into the multi-mode microwave chamber 14, a vacuum pump (not shown), a mode stirrer 24 driven by a motor M, and a sealed, microwave-transparent vacuum housing 26, in this example made of polytetrafluoroethylene (PTFE), within the multi-mode microwave chamber, wherein the vacuum housing 26 has five separate compartments 28 and a vacuum sensor 30 connected to each of the compartments.for individual monitoring of a vacuum in the respective compartment, wherein the compartments 28 are connected to the vacuum pump (not shown) for generating a vacuum in the respective compartment, and a control unit (not shown) connected to the microwave generating unit 16, the microwave phase-shifting unit 18, the microwave frequency-shifting unit 20, the microwave coupling unit 22, the vacuum pump and the vacuum sensors 30 for controlling the vacuum pump, the microwave generating unit, the microwave phase-shifting unit, the microwave frequency-shifting unit and the microwave coupling unit depending on vacuum pressures (or pressures) P1 to P5 determined with the vacuum sensors 30. While atmospheric pressure prevails in the multi-mode microwave chamber, a vacuum is generated in the compartments of the vacuum housing (each).

[0047] The geometries of the compartments 28 are usually determined by the geometry of, for example, the membrane filters and are usually identical for all membrane filters in a given embodiment.

[0048] Typically, the membrane filters 12 are loaded with approximately 20 g to 100 g of water.

[0049] A typical drying process with approximately 5 x 60 g loaded membrane modules in approximately 4 minutes is shown in the Fig. Figure 3 shows that in this example, 5.5 kW are transferred to the five membrane modules 12 during the main drying stage. This results in a microwave power density of 1.1 kW per membrane module in this example. Typically, however, 1.0 to 2.0 kW are transferred per membrane module. This corresponds to a microwave power density of approximately 18–36 W / g of water at the start of microwave drying and 100 to 200 W / g with 10 g of residual water per membrane filter before final drying. Maximum microwave power densities of over 2.0 to 3.0 kW are also possible without product damage.

[0050] In the Fig. Graph a shows the time course of the radiated microwave power, graph b shows the vacuum pressures (pressures) measured in compartments 28 as a function of time, graph c shows the dew point temperature as a function of time, and graph d shows the performance as a function of time.

[0051] A vacuum pressure of, for example, approximately 30 mbar is generated in the loaded vacuum housings via the vacuum pump. The vacuum pressure is measured in each vacuum housing. The average of all five measured vacuum pressures is also calculated (see graph b in [reference]). Fig. 3) The microwave power is then switched on in, for example, two stages from, for example, 3.0 to, for example, 5.5 kW. The reflected microwave power is in the range of, for example, 0.5 to, for example, 1.5 kW (see graph a in [reference]). Fig. 3) The dew point is also measured both as an absolute value and as a gradient in [K / min] (see graph c in Fig. 3) The difference between the generated microwave power and the reflected microwave power is summed and calculated as absorbed microwave energy in kWh. The efficiency of evaporating water relative to the absorbed microwave energy is used as a factor for the efficiency of the thermal conversion and allows for the calculation of the amount of water evaporated.

[0052] During the main drying stage, using, for example, 5.5 kW microwave power, the vacuum pressure rises to approximately 80 mbar based on the generated water vapor. After approximately 79-80% of the water has evaporated, the microwave power is initially reduced to, for example, 2.5 kW; then, at approximately 90% evaporation, it is reduced to approximately 1 kW. The drying process is complete when the dew point reaches approximately 25°C. This corresponds to a residual water content of less than approximately 5 g per filter module.

[0053] The performance in the example shown is 0.28 kg of water evaporated with an absorbed microwave energy of 0.215 kWh (see curves c and d). This results in a normalized performance of η > 0.75 kWh / kg evaporated water. The efficiency of generated microwave power to electrical power in this example is approximately 77%.

[0054] The wavelengths of the microwaves are shortened by approximately 30% in a plastic version (e.g. PTFE) of the vacuum housing 26, which also generates more microwave modes, thus benefiting the distribution of the multi-mode system with the mode stirrer 24.

[0055] Multi-mode microwave chambers typically have microwave power outputs of 3-60 kW per microwave system.

[0056] Fig.Figure 4 shows details of a particular embodiment of a multi-mode microwave chamber 14 in which a sealed, lockable, microwave-transparent vacuum housing 26, e.g., made of polytetrafluoroethylene (PTFE), is arranged. The vacuum housing 26 has a total of ten separate compartments 28, arranged in two columns 32, each containing five compartments. In this example, the ten compartments 28 are individually sealed and lockable from one another by means of an O-ring. A single vacuum housing door is preferably used for all vacuum housings. However, particular embodiments can also be segmented, so that, for example, in the case of sixteen vacuum housings, four vacuum housings are sealed by a single vacuum housing door.

[0057] The features of the invention disclosed in the foregoing description, in the drawings and in the claims can be essential for the realization of the invention in its various embodiments, both individually and in any combination. Reference symbol list 10 Annex 12 membrane filters 14 Multi-mode microwave chamber 16 Microwave generating unit 18 Microwave phase shift device 20 Microwave frequency changing device 22 Microwave coupling device 24 Fashion Stirrers 26 vacuum housings 28 subjects 30 vacuum sensor 32 columns M Motor P1, P2, P3, P4, P5 Vacuum pressures (pressures)

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