Method and device for producing high-purity carbon dioxide particles

Abstract

The invention relates to a method for producing high-purity carbon dioxide particles, in which method liquid carbon dioxide from a storage vessel is expanded, thereby producing carbon dioxide snow and carbon dioxide gas, and the carbon dioxide snow is used as a blasting medium for cleaning surfaces by means of particle blasting. According to the invention, before expansion, the liquid carbon dioxide is brought into the supercritical state, passed through a fine filter in order to filter out impurities, and subsequently re-liquefied. Fine filtration in the supercritical state makes it possible to produce a high-purity blasting medium in the form of carbon dioxide snow, which is also in particular suitable for cleaning printed circuit boards.

Description

[0001] MG 25338 23.01.2025

[0002] 1

[0003] Method and apparatus for producing high-purity carbon dioxide particles

[0004] The invention relates to a method for producing high-purity carbon dioxide particles, in which liquid carbon dioxide is taken from a storage container, purified, and fed to an expansion nozzle where it is expanded, forming carbon dioxide snow and carbon dioxide gas. The invention further relates to a corresponding apparatus.

[0005] Highly sensitive surfaces, such as printed circuit boards, are now cleaned by irradiation with carbon dioxide snow particles, which themselves must meet the highest purity standards. In particular, it is essential to ensure that no solid or liquid particles remain on the treated surface after irradiation, as these could impair the component's functionality.

[0006] Carbon dioxide snow, typically used for surface irradiation, is produced from liquid carbon dioxide stored in a reservoir at a pressure of, for example, 20 bar. Before carbon dioxide snow can be produced, the carbon dioxide must undergo a thorough purification process. However, the relatively high viscosity of liquid carbon dioxide limits the possibilities of removing solid particles from it through filtration, as the small mesh sizes of the filters required for the necessary purity offer very little permeability for liquids.

[0007] EP 0332356 A2 proposes that carbon dioxide, stored in liquid form in a storage container, is first evaporated and then passed through a fine filter in its gaseous state before being fed to a cooling device and condensed again. The purified and re-liquefied carbon dioxide can then be used to produce carbon dioxide snow by expansion in a known manner. This snow can, for example, be fed to a blasting device and used for surface treatment. Gas filters, which have a significantly better filtration efficiency than liquid filters, can be used in this process. The equipment required for this process is described in MG 25338 on January 23, 2025.

[0008] 2

[0009] However, the regulation and processing of the carbon dioxide is considerable, and continuous operation requires the use of additional buffer tanks to store the reliquefied gas.

[0010] The invention is therefore based on the objective of reducing the equipment required for processing carbon dioxide for use as a high-purity blasting agent.

[0011] This problem is solved by a method with the features of claim 1 and by a device with the features of claim 4. Advantageous embodiments of the invention are specified in the dependent claims.

[0012] The inventive method for producing high-purity carbon dioxide particles is characterized in that the liquid carbon dioxide taken from the storage container is brought into the supercritical state by heating and, if necessary, compression, the supercritical carbon dioxide passes through a filter unit, the filtered supercritical carbon dioxide is liquefied again by cooling, and the liquefied carbon dioxide is expanded, producing carbon dioxide snow and carbon dioxide gas.

[0013] The liquid carbon dioxide is extracted from the storage container via an extraction line in which a conveying device, for example a pump, is arranged. A key aspect of the invention is that the carbon dioxide is filtration in its supercritical state, i.e., at a pressure of at least 73.8 bar (critical pressure) and a temperature of at least 31 °C (critical temperature). In the process according to the invention, the carbon dioxide therefore does not undergo a phase transition, and unlike evaporation, gas bubbles that could impede the flow of the liquid carbon dioxide do not form. Supercritical carbon dioxide has a viscosity comparable to that of gaseous carbon dioxide.This allows the use of ultra-fine gas filters in the filter unit, for example class 10 and higher HEPA filters (according to standard EN 1822), and enables the very efficient removal of impurities with a diameter down to 0.003 pm from the stream of MG 25338 23.01.2025.

[0014] 3

[0015] Supercritical carbon dioxide is removed. Its higher density compared to the gas also improves the filtration efficiency, especially when using very fine filters.

[0016] Downstream of the filter unit, the supercritical carbon dioxide is reliquefied by cooling to a temperature below the critical temperature, whereby the pressure is preferably maintained above the critical pressure, thus preventing a phase transition. The carbon dioxide, reliquefied in this way, is then fed to an expansion nozzle where it is expanded, forming a mixture of carbon dioxide snow and carbon dioxide gas. Due to its high purity, the carbon dioxide snow is particularly suitable as an abrasive for snow jet cleaning of sensitive surfaces, such as printed circuit boards.

[0017] Preferably, the liquid carbon dioxide is stored in the storage tank at a pressure above the critical pressure of 73.8 bar; for example, the storage tank is a medium-pressure or high-pressure tank with an operating pressure above 80 bar. In this case, the liquid carbon dioxide only needs to be heated to a temperature above the critical temperature to reach the supercritical state, and the installation of a suitable heating device, for example an electric one, is sufficient for this purpose.

[0018] Otherwise, a compression device is required between the storage tank and the filter unit, which compresses the liquid carbon dioxide to a pressure value above the critical pressure without evaporating it, in order to subsequently bring it to the supercritical state by heating it in a heating device.

[0019] The carbon dioxide snow obtained from purified and reliquefied carbon dioxide is preferably used as an abrasive for irradiating sensitive surfaces, for example for cleaning electronic circuit boards.

[0020] The object of the invention is also achieved by a device with the features of claim 4. MG 25338 23.01.2025

[0021] 4

[0022] A device for producing high-purity carbon dioxide particles, comprising a storage container for liquid carbon dioxide and a discharge line in which a cleaning device for purifying the carbon dioxide is arranged and which is fluidically connected to an expansion nozzle for producing carbon dioxide snow from the liquid carbon dioxide, is characterized according to the invention in that a pump for conveying and, if necessary, compressing the liquid carbon dioxide is provided upstream of the cleaning device and, viewed in the direction of flow of the carbon dioxide, a heating device for bringing the liquid carbon dioxide into the supercritical state, a filter unit and a cooling device for reliquefying the supercritical carbon dioxide are arranged successively in the cleaning device.

[0023] The pump serves to convey the liquid carbon dioxide through the extraction line and—if the liquid carbon dioxide is not already present in the storage tank at a pressure above its critical pressure—to compress it to such a pressure. In the heating unit located downstream of the pump, the liquid carbon dioxide is heated to a temperature above the critical temperature, bringing it into a supercritical state. In this supercritical state, it is passed through the filter unit and filtered. The supercritical carbon dioxide is then reliquefied by cooling in the cooling unit. The purified and reliquefied carbon dioxide is then fed to the expansion nozzle to produce carbon dioxide snow.

[0024] The pressure of the carbon dioxide is therefore maintained at a value above the critical pressure, at least while passing through the filter unit. To ensure this, an additional pressure-maintaining valve, for example a bypass valve, can be used, which is installed downstream of the filter unit and preferably downstream of the cooling device in the extraction line. Alternatively, the pressure relief valve can also be designed such that the pressure of the liquid carbon dioxide is maintained at a value above the critical pressure upstream of the pressure relief valve.

[0025] The expansion nozzle is preferably used in a blasting device for irradiating a surface with the carbon dioxide snow generated at the expansion nozzle MG 25338 23.01.2025

[0026] 5

[0027] The integrated carbon dioxide snow, and optionally the simultaneously generated carbon dioxide gas, is propelled, possibly with the use of a propellant gas, from a jet nozzle towards the surface to be treated. Due to its high purity, the carbon dioxide snow ensures a thorough and virtually residue-free cleaning of the surface.

[0028] The filter unit is a filter or an arrangement consisting of several filters that enables the efficient purification of supercritical carbon dioxide. Preferably, the filter unit comprises at least one particulate filter (EPA, HEPA, or ULPA filter) belonging to filter class 10 (according to EN 1822) or higher, and particularly preferably to filter class 12 or higher. Filtration with such fine filters allows for the efficient removal of impurities with a diameter of less than 1 pm, preferably less than 0.1 pm and down to 0.003 pm, from the carbon dioxide stream. The filter unit can also consist of several filters, for example, a combination of a pre-filter and a fine filter.

[0029] In an advantageous embodiment of the invention, the liquid carbon dioxide extracted from the storage tank is cooled upstream of the pump. This prevents overheating of the liquid carbon dioxide, which could impair the pump's functionality. In this embodiment, a cooling device, for example a heat exchanger, is provided between the storage tank and the pump to cool the liquid carbon dioxide from the storage tank.

[0030] To avoid pressure spikes, the pump can be integrated into a ring main that returns to the storage tank. Such a ring main branches off downstream from the supply line to the pump, but upstream to the heating element and the filter unit, and is equipped with a pressure-reducing valve or a bypass valve.

[0031] Preferably, the device is equipped with an electronic control unit by means of which pressure and flow settings in the device can be regulated by control. MG 25338 23.01.2025

[0032] 6

[0033] The heating device arranged upstream of the filter unit and / or the cooling device arranged downstream of the filter unit can be electrically operated devices or designed as heat exchangers, in which the carbon dioxide absorbs heat energy from or releases it to a heat transfer medium. To utilize the energy used in the operation of the device as efficiently as possible, a preferred embodiment of the invention provides that the heating device and the cooling device are thermally coupled, for example via a suitable heat exchanger arrangement in a heat transfer circuit or via a heat pump.

[0034] An embodiment of the invention will be explained in more detail with reference to the drawing. The single drawing (Fig. 1) schematically shows a device according to the invention.

[0035] The device shown in Fig. 1 has a storage tank 2 for storing liquid carbon dioxide. The storage tank 2 is, for example, a medium-pressure tank in which liquid (subcritical) carbon dioxide is stored at a pressure of 80 bar and a temperature below 31 °C (for example, 20 °C). The storage tank 2 is connected to a discharge line 3 in which a pump 4 is integrated. At a branch point 5, a ring line 6, in which a pressure maintenance device, for example, a bypass valve 7, is arranged, leads from the discharge line 3 back to the storage tank 2. A cooling device 8 is optionally provided in the discharge line 3 upstream of the pump 4.

[0036] From branch point 5, the extraction line 3 leads to an application in which the liquid carbon dioxide is used. In the embodiment shown here, the application is a jet device 10 for generating CO2 snow jets. The jet device 10 is equipped, in a manner known per se, with an expansion nozzle 11, at which liquid carbon dioxide is converted into a mixture of carbon dioxide gas and carbon dioxide snow by expansion to a pressure of less than 5.18 bar. The carbon dioxide snow thus produced can be used MG 25338 23.01.2025

[0037] 7

[0038] subsequently, in a manner also known per se, it is used as an abrasive for irradiating a surface not shown here.

[0039] Downstream of branch point 5, a cleaning device 12 for filtering impurities from the liquid carbon dioxide is arranged in the extraction line 3. In the embodiment shown here, the cleaning device 12 comprises – viewed in the direction of carbon dioxide flow – a heating device 13, a filter unit 14, a valve 15, and a cooling device 16. The heating device 13 and the cooling device 16 can be electrically operated devices or heat exchangers.

[0040] Downstream of the cleaning device 12, a branch point 17 can optionally be arranged, at which the purified carbon dioxide can be distributed via lines 18a, 18b, 18c to a plurality of applications not shown here, for example, a plurality of jet devices. For this purpose, the branch point 17 can be equipped with a valve arrangement by means of which the distribution of the carbon dioxide to the lines 18a, 18b, 18c can be controlled.

[0041] During operation of the device 1, liquid carbon dioxide is extracted from the storage tank 2 by means of the pump 4. The optional cooling device 8 prevents the liquid carbon dioxide from overheating upstream of the pump 4. If the liquid carbon dioxide in the storage tank 2 is not yet at a pressure above the critical pressure (73.8 bar), it is compressed by the pump 4 to a pressure above the critical pressure; however, in the example shown here, the liquid carbon dioxide is already at a pressure of 80 bar in the storage tank 2. To avoid pressure spikes, if a limit pressure specified at the overflow valve 7 is exceeded, at least a partial flow of the carbon dioxide is returned to the storage tank 2 via the ring main 6.

[0042] The liquid carbon dioxide, conveyed downstream via the extraction line 3 to the branch point 5, is heated in the heating unit 13 to a temperature above its critical temperature of 31 °C. This causes the carbon dioxide to reach the supercritical state without undergoing a phase transition. In the supercritical state, the carbon dioxide passes through the filter unit 14. DaMG 25338 23.01.2025

[0043] 8

[0044] Since supercritical carbon dioxide has a viscosity similar to that of gaseous carbon dioxide, the filter can be designed to be very fine; for example, the filter unit 14 has a particulate filter, in particular an EPA filter (Efficient Particulate Air Filter), a HEPA filter (High Efficiency Particulate Air Filter) or a UEPA filter (Ultra Efficient Particulate Air Filter), which is, for example, able to remove particles with diameters of more than 0.3 pm, preferably more than 0.03 pm, from the gas stream with a separation efficiency of 99.995% or above.

[0045] After passing through filter unit 14, the supercritical carbon dioxide is cooled in cooling unit 16 at approximately constant pressure and thus returned to its liquid state. The valve 15, which may be a bypass valve or a control valve, prevents an unwanted pressure drop in the area of ​​filter unit 14 and ensures that the carbon dioxide is safely kept in the supercritical state there.

[0046] A pressure-reducing valve 19, which can be, for example, a bypass valve, is installed upstream of the jet apparatus 10. This valve serves to adjust the pressure of the reliquefied carbon dioxide to the pressure required by the application, in this case, the jet apparatus 10. For this purpose, the valve 19 can also be installed upstream of the cooling unit 16, but downstream of the filter. Valves 15 and 19 can also be implemented as a single valve upstream or downstream of the cooling unit 16.

[0047] The liquid carbon dioxide reaches the jet apparatus 10 and is expanded at the expansion nozzle 11, producing a mixture of carbon dioxide gas and carbon dioxide snow. The mixture, or the carbon dioxide snow, is then ejected, optionally with the assistance of a carrier gas stream supplied via a carrier gas supply line 20, from a nozzle 21 located on the jet apparatus 10 towards a surface (not shown here).

[0048] By means of an electronic control unit 22, components 4, 8, 10, 13, 15, 16, and 17 can be controlled manually or according to a predefined program, thus enabling largely automated operation. (MG 25338 23.01.2025)

[0049] 9

[0050] To ensure particularly economical operation of the device 1, the heating device 13 and the cooling device 16 can be integrated in a heat transfer circuit 23 and, for example, thermally coupled to each other via a heat pump 24.

[0051] Furthermore, the cooling device 16, which reliquefies the supercritical carbon dioxide, need not be located directly downstream of the filter unit 14; rather, it is equally conceivable to route the supercritical carbon dioxide to the branch point 17 and distribute it to the lines 18a, 18b, and 18c. In this case, it is conceivable to carry out reliquefaction by means of a cooling device in only one line 18a, 18b, or 18c, or in a portion thereof. It is also conceivable to use the carbon dioxide snow generated at the expansion nozzle 10 in a manner other than for surface irradiation.

[0052] Due to the supercritical purification of the carbon dioxide in filter unit 14, the carbon dioxide emitted from nozzle 21 is a highly purified blasting medium suitable for treating sensitive surfaces such as printed circuit boards. MG 25338 23.01.2025

[0053] 10

[0054] Reference symbol list

[0055] 1 Device

[0056] 2 storage containers

[0057] 3 extraction line

[0058] 4 pump

[0059] 5 Junction

[0060] 6 Ring main

[0061] 7 Overflow valve

[0062] 8 Cooling unit

[0063] 9

[0064] 10. Beam apparatus

[0065] 11 Relaxation nozzle

[0066] 12 Cleaning device

[0067] 13 Heating system

[0068] 14 filter units

[0069] 15 valve

[0070] 16 Cooling unit

[0071] 17 Junction

[0072] 18a, 18b, 18c Management

[0073] 19 Pressure reduction valve 20 Carrier gas supply line

[0074] 21 nozzle

[0075] 22 Control unit

[0076] 23 Heat transfer circuit 24 Heat pump

Claims

MG 25338 23.01.2025 11 Patent claims 1. Method for producing high-purity carbon dioxide particles, in which liquid carbon dioxide is taken from a storage container (2), purified and fed to an expansion nozzle (11) where it is expanded to form carbon dioxide snow and carbon dioxide gas, characterized by that the liquid carbon dioxide taken from the storage container (2) is brought to the supercritical state by heating and, if necessary, compression, the supercritical carbon dioxide passes through a filter unit (14), the filtered supercritical carbon dioxide is liquefied again by cooling and the liquefied carbon dioxide is expanded at the expansion nozzle (11) producing carbon dioxide snow and carbon dioxide gas.

2. Method according to claim 1, characterized in that during operation of the device (1) the pressure of the liquid carbon dioxide in the storage container (2) is at a value which is above the critical pressure of carbon dioxide (79.8 bar).

3. Method according to claim 1 or 2, characterized in that the produced carbon dioxide snow is used as an abrasive for a blasting device (10) for cleaning surfaces.

4. Device for producing high-purity carbon dioxide particles, comprising a storage container (2) for liquid carbon dioxide, with a withdrawal line (3) in which a cleaning device (12) for cleaning the carbon dioxide is arranged and which is connected by flow to an expansion nozzle (11) for producing carbon dioxide snow from the liquid carbon dioxide, characterized in that that a pump (4) is provided upstream of the cleaning device (12) for conveying and, if necessary, compressing the liquid carbon dioxide, and in the cleaning device (12) - viewed in the direction of flow of the carbon dioxide - a heating device (13) is successively provided for transferring the liquid MG 25338 23.01.2025 12 Carbon dioxide is brought to the supercritical state, a filter unit (14) and a cooling device (16) for reliquefying the supercritical carbon dioxide are arranged.

5. Device according to claim 4, characterized in that the expansion nozzle (11) is integrated in a blast device (10) for irradiating a surface with the carbon dioxide snow generated at the expansion nozzle (11).

6. Device according to claim 4 or 5, characterized in that the filter unit (14) has a particulate filter.

7. Device according to one of claims 4 to 6, characterized in that a cooling device (8) for cooling the liquid carbon dioxide from the storage container (2) is arranged between the storage container (2) and the pump (4) in terms of flow technology.

8. Device according to one of claims 4 to 7, characterized in that the pump (4) is integrated in a ring line (6) leading back to the storage container (2).

9. Method according to one of claims 4 to 8, characterized in that pressure and flow settings in the device (1) can be controlled by means of a control unit (22).

10. Device according to one of claims 4 to 9, characterized in that the heating device (13) and the cooling device (16) are thermally coupled to each other.

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