EP1814820A2

EP1814820A2 - Method and device for producing solid particles from liquids, in particular liquid carbon dioxide

Info

Publication number: EP1814820A2
Application number: EP05815724A
Authority: EP
Inventors: Hans Quack; David Kirsten
Original assignee: Technische Universitaet Dresden
Current assignee: Technische Universitaet Dresden
Priority date: 2004-10-29
Filing date: 2005-10-27
Publication date: 2007-08-08
Also published as: WO2006045305A3; WO2006045305A2; DE102004053239A1

Abstract

The invention relates to a method for producing solid particles from liquids. Said method comprises the following steps : a) the liquid is removed from the storage container, b) is cooled to a triple point temperature, and is throttled to a desired freezing pressure, then c) drops are produced from the liquid and d) are introduced into a gas flow which, at least in parts, does not contain substances of the liquid. Then, e) the drops freeze to become solid particles due to partial evaporation of the liquid drops, and the solid particles f) are subsequently collected and evacuated. The invention further relates to a device for carrying out said method.

Description

Method and device for producing solid particles from liquids, in particular from liquid carbon dioxide

The invention relates to a method and an apparatus for producing solid particles from liquids.

In particular, the invention relates to a method and apparatus for producing carbon dioxide particles from liquid carbon dioxide. As a preferred application of carbon dioxide particles are blasting methods for abrasive surface treatment to call.

In the prior art, for example, so-called dry ice blasting for cleaning surfaces is known.

Carbon dioxide is particularly suitable for the production of solid particles of liquid carbon dioxide. The carbon dioxide differs from most other substances in that its triple point pressure is higher than 1 bar (0.1 Mpa), namely 5.2 bar. The associated triple point temperature is - 56 ⁰ C. At atmospheric pressure carbon dioxide is thus only in solid and in gaseous, but not in liquid form. The associated saturation temperature at 1 bar is about minus 80 ⁰ C. At 20 ⁰ C, the boiling pressure is about 58 bar.

These thermodynamic properties of the carbon dioxide allow a relatively simple production of solid particles of liquid carbon dioxide. The gas is stored and transported in pressure tanks or larger pressure tanks in which the liquid is in equilibrium with steam at 58 bar. Removing liquid carbon dioxide from such a storage tank and throttling it to atmospheric pressure, creates much vapor during the throttling process in the liquid and tears the liquid into very small droplets. After passing through the triple point pressure, these droplets transform into tiny ice crystals. The resulting fine crystalline product looks like snow and is also called dry ice designated. In order to be able to use dry ice for the abrasive blast treatment of surfaces, a special type of further treatment of the dry ice is required. This consists in the compression and subsequent extrusion. The end product is carbon dioxide snow pellets, hereafter referred to as pellets. The pellets have a diameter of 2 to 5 millimeters and are transported and stored in insulated containers of 200 to 700 kilograms.

A known process for the preparation of the dry ice pellets is shown in Fig. 1 by means of a conventional device. This is the liquid

Carbon dioxide from a storage tank 1 via a withdrawal line 2 and a

Precooling heat exchanger 3 is passed to the throttle valve 4, where it is relaxed and is introduced via spray nozzles 6 in the container 5 in drop form. The

Particle generation takes place in the particle-generating zone / freezer compartment 7 of the container 5. The resulting dry ice snow or the solid particles 8 accumulate in the lower part due to gravity and are over a

Sluice 9 passed into an extruder 12 and in the latter through a perforated plate

13 extruded. The resulting pellets 11 are finally isolated

Transport container 10 stored or used directly without intermediate storage for dry ice blasting using a propulsion jet.

In dry ice blasting, the pellets are usually introduced in compressed air. The mixture is expanded in a nozzle to atmospheric pressure and thereby accelerated to a high speed. Upon impact of the CO ₂ particles on the surface, the pellets transfer a locally high impact energy and, in addition, by their sublimation, cause a local strong decrease in temperature, whereby the contamination becomes brittle and can be easily removed mechanically. One advantage of dry ice blasting is that the carbon dioxide goes into the vapor state and thus disappears. There are no., Blasting agent residues that would have to be disposed of. DE 37 20 992 C2 discloses a method for cleaning radioactively contaminated workpiece surfaces and a system for carrying out the method, wherein CO ₂ ice particles are directed by means of a propulsion jet against a workpiece surface and the CO ₂ ice particles are formed by liquid CO ₂ with liquid air or liquid nitrogen is cooled below the solidification point, so that in the liquid CO ₂ crystal formation begins.

These known in the prior art process for the preparation of solid particles of carbon dioxide have serious disadvantages. The way of throttling to atmospheric pressure and the subsequent compression and extrusion of the snow into pellets according to FIG. 1 is very expensive in terms of apparatus. The resulting pellets, which consist of compressed snow, are relatively soft and not sufficiently hard for many surface treatment applications.

The production of CO ₂ ice particles on the combined cooling and pressure change is energetically very expensive, since liquid nitrogen or liquid air is required for the production of solid particles, which thermodynamically means a higher cost, which is reflected in the economic efficiency of the process negative ,

The object of the present invention is to provide a method and a device in which solid particles of liquids can be produced in a simpler manner and that the resulting particles have better properties for the blasting of surfaces. In particular, it is an object of the invention to produce harder particles for use in the blast treatment of surfaces. The invention is achieved by a method for producing solid particles from liquids, comprising the following steps: a) the liquid is removed from a storage container and b) cooled to the triple point temperature and throttled to the desired freezing pressure and then c) drops from the Liquid generated and these d) introduced into a gas stream, which consists at least in part of the substance of the liquid, whereby e) freeze the drops to solid particles by a partial evaporation of the liquid droplets, after which the solid particles f) are then collected and discharged.

The concept of the invention is that not snow-like smallest crystals are produced and then compressed, but that drops of the desired size are generated and frozen. This process occurs at a pressure that is in the order of magnitude of the triple point pressure. Essential to the invention is that needed for freezing the drops

Cold is generated by partial evaporation of a portion of the droplet or the already formed solid carbon dioxide.

In addition to the production according to the invention of the solid particles in a pressure vessel and the subsequent discharge into a transport container at ambient pressure, the solid particles produced can be supplied for immediate use in a blasting device, whereby the pressure reduction to ambient pressure is eliminated.

It is particularly advantageous that the method is energetically efficient and that a blasting agent with special, previously unavailable properties can be generated. Furthermore, it is possible by the method to obtain a very high yield of solid particles from a given initial amount of liquid, with as little gas as possible, which is ultimately difficult to recover materially and energetically.

It is particularly advantageous if precooling of the liquid takes place in method step b).

A further advantageous embodiment of the invention is that in step b) is throttled in two stages, wherein the liquid is separated after the first throttling of the resulting vapor and then throttled again.

It is expedient that the resulting vapor is used for pre-cooling the liquid.

A preferred field of application of the method according to the invention is the production of carbon dioxide grains from liquid carbon dioxide. Alternatively, this method is also advantageous for the production of methane grains, which are needed for example as a neutron brake in cold neutron sources. The gas stream used in this case is helium, hydrogen, neon or pure nitrogen. Another field of application is the production of hydrogen grains, which are required for the production of a hydrogen-solid-liquid-mixture ("slush-hydrogen"), in which case helium is used as the gas stream.

In the preferred application, the production of carbon dioxide particles, air is used as the gas stream in process step d).

The object of the invention is further achieved by a device for the production of solid particles from liquids, wherein a storage tank for the Liquid, a withdrawal line, a throttle valve and a container with spray nozzles and means for generating a gas stream are provided such that liquid droplets in free fall evaporate a portion of their mass into the gas stream and thereby freeze to solid particles and that the container a lock for pressure holding Delivery of the solid particles is provided.

The device according to the invention is preferably configured by providing a precooling heat exchanger and a separator / collector for the liquid-gas mixture produced after throttling.

As a means for generating a pre-cooled gas stream is a compressor, a dryer and a heat exchanger, wherein a distributor and a collector for the gas stream are arranged in the container.

According to the concept, the object of the invention to produce harder particles is achieved in that solid particles in the form of frozen grains can be produced by the method according to the invention.

Further details, features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings. Show it:

1: state of the art of pellet production of dry ice, FIG. 2: flow diagram for the production according to the invention of solid particles from liquids,

3 shows a flow chart for the production according to the invention of solid particles with direct use of the particles for the abrasive blast treatment of surfaces,

Fig. 4: Flowchart for the efficient design of the pre-cooling and relaxation of the liquid by use of an Expander-Komp_res_sors "and Fig. 5: Fiiefibild for two-stage throttling and recompression with condensation to improve the process in the pre-cooling and throttle stage.

The inventive method is shown schematically in Fig. 2 as a flow chart. For the realization of the method, a storage tank 1 for example, liquid carbon dioxide is shown, from which the liquid carbon dioxide is passed via a withdrawal line 2 and a Vorkühlwärmeübertrager 3 to the throttle valve 4. In the throttle valve 4 there is a relaxation of the liquid. The liquid, vapor mixture then passes into the separator / collector 14. The steam produced in the separator / collector 14 leaves the system after it has cooled the liquid CO ₂ in the pre-cooling heat exchanger 3. To regulate the system pressures, a pressure-holding valve 15 is provided.

The liquid expanded carbon dioxide now passes to the container 5 and is introduced into the container 5 via droplets 6 provided there in the form of droplets. As a result of gravity, the droplets fall down through the freezing zone 7 of the container 5. By way of a drop down, it is brought into contact with a gas stream, which is introduced via a distributor 19 into the container 5, in which a part of each carbon dioxide droplet evaporates in the freezing zone 7, whereby the cooling and solidification of the remaining droplet takes place. The carbon dioxide-enriched gas mixture is taken up by the collector 20 and passed to the heat exchanger 18. There, the carbon dioxide-enriched cold gas mixture is used for precooling the container 5 to be supplied gas stream.

The heated gas-CO 2 mixture leaves the system via a pressure-maintaining valve 15. The gas stream itself is generated by means of a compressor 16 and processed in a dryer 17 before it is pre-cooled in the heat exchanger 18 and introduced into the manifold 19 in the container 5. The diagram shows the use of air as gas into which the carbon dioxide evaporates.

The frozen grains or solid particles 8 collect in the lower part of the container 5 and are pressure-maintaining via a lock 9, for example, in an insulated transport container 10, introduced.

In Fig. 3, a further advantageous embodiment of the invention is shown, in which the solid particles 8 produced are fed directly to a jet nozzle 22. The propellant used is, for example, the carbon dioxide gas deposited in the separator / collector 14 - for example in conjunction with air which, after compression in the compressor 16 and drying in the dryer 17, are used as carrier gas for the solid particles 8.

Although no precooling of the liquid carbon dioxide is shown in FIG. 3, it is within the meaning of the invention to provide such precooling in various ways in order to make the process thermodynamically more efficient and to convert a larger proportion of the carbon dioxide used into solid particles ,

Such an increase in the efficiency of precooling and throttling is shown, for example, in FIG. In this case, extracts the extraction line 2 is shown for the liquid carbon dioxide, to which an expander 23 connects. In the expander 23, the liquid carbon dioxide is working expanded and in the separator / collector 14 a, the vapor, liquid mixture is separated. The gaseous fraction is withdrawn at the top end and compressed in a compressor 25, the compressor 25 being at least partially driven by the working-up of the liquid carbon dioxide. It is expedient, the condensed To supply carbon dioxide to an intercooler 24, after which the recooled and liquefied, high pressure carbon dioxide is fed back into the extraction line 2 before the first expansion stage. The liquid part of the carbon dioxide passes from the first separator / collector 14a in a second expansion stage, in turn, a work expander 23 is used. The expanded liquid then passes into the second separator / collector 14b, which has a pressure-holding valve 15, after which the liquid CO _{2 is} then sprayed into the container 5 with the spray nozzles 6 for generating drops.

A two-stage throttling makes sense, so that a part of the resulting vapor is present at a higher pressure and thus has more driving force when using the steam in the jet nozzle 22 according to FIG. 3. It can be used in the way three propellant streams, on the one hand carbon dioxide from triple point pressure, the second carbon dioxide from the medium pressure and third compressed air. The inclusion of a Gegenstromwärmeübertragers to increase the yield and to heat the steam, which gets a higher driving energy, is advantageous.

Another alternative advantageous embodiment of the pre-cooling and throttling is shown in Fig. 5 as a flow chart.

In this case, in turn, throttled in two stages, in each case a recompression by means of the compressor 25 of the gaseous carbon dioxide, which is fed after Zwischenkühiung in the intercooler 24 back into the extraction line 2 before the first throttling in the throttle valve 4. It close in a known manner, the two separator / collector 14a, b, before the liquid carbon dioxide is introduced into the container 5 in drop form. Although the gas stream in which the carbon dioxide evaporates does not have to be colder than the freezing carbon dioxide droplet for the evaporative freezing process, precooling of the gas stream in a heat exchanger 18 with, for example, CO ₂ vapor from the separator / collector 14 according to FIG. 3 is thermodynamically meaningful , In contrast, the propellant gas supplied to the nozzle 22 need not be pre-cooled because no cooler gas is needed in the blasting unit.

The drying of the air as a gas stream is useful because otherwise water in the heat exchanger 18 freezes and clogged. The cold for precooling of the gas stream, which via the valve 21 and the manifold 19 in the

Freezing zone 7 of the pressure vessel 5 is introduced, comes alternatively or cumulatively from the gas / carbon dioxide mixture, which evaporates in the

Particle production / freezing zone 7 is formed and leaves the system via the collector 20 and a heat exchanger 18 and a pressure-maintaining valve 15, provided that the CO _{2 is} not supplied for reuse.

The gas-carbon dioxide mixture produced during the evaporation process can alternatively also be used to assist the cooling of the starting liquid and, in the direct use of the solid particles in a blasting unit, the heated gas mixture can subsequently be used as propellant gas in the blasting nozzle 22.

The evaporation freezing process is characterized in that the precooled and throttled carbon dioxide is sprayed directly or after intermediate storage in a separator / collector 14 in drop form into the gas stream of, for example, air. Contact with the gas stream causes part of the liquid to evaporate on the surface. This cools the

Remainder of the liquid to the triple point temperature and even lower, until a necessary for crystal formation "supercooling temperature is reached. The

Evaporation is not associated with the formation of ice on the surface but it continues to evaporate from already frozen solid carbon dioxide until all the liquid is frozen. This method of evaporative freezing is characterized in that the heat transfer takes place by evaporation, wherein the temperature difference between the droplet and gas flow is irrelevant.

The drop generation can be realized in various ways. Known devices for producing drops are rotary plates, perforated baskets, spray nozzles, spray tubes and perforated plates. So that the formation of the droplets is not already superimposed and influenced by the onset of freezing, a carbon dioxide atmosphere as pure as possible should be present in this area directly around the droplet formation point.

As soon as the drops are formed and enter the gas stream, evaporation starts at the surface. This leads to a supercooling of the liquid on the surface until freezing begins. If the mass transfer is uniform over the entire surface, then the entire surface freezes and an ice layer surrounds the liquid core. With further evaporation, the ice surface cools significantly below the triple point temperature and heat flows from the liquid surface into the ice layer. As a result, more liquid freezes and the ice layer continues to grow inwards. Since freezing is associated with a reduction in volume, a reduced pressure is formed in the liquid and thereby a vapor bubble. The prerequisite for the emergence of such a hollow sphere is that the mass transfer at the periphery of the droplet is uniform and on the entire surface at the same time an ice layer is formed. If the mass transfer is asymmetrical, as for example when a drop falls through a stagnant gas, a frozen-through, but no longer globular particle will be formed. The volume of the resulting grains amounts to about 50% of the volume of the original trumpet._36 _^ % of the mass-evaporate-and the rest of the mass contracts due to volume reduction during freezing.

The freezing of the grains takes a certain amount of time. This time is the longer, the larger the original drop and the lower the amount of gas flow, that is, the greater the partial pressure of the carbon dioxide in the gas mixture. The required height of the pressure vessel 5 results from the required freezing time and the falling speed of the drops. In order to reduce the height of the container 5, internals are preferably arranged in the container 5, which prevent the drops or the already formed grains from falling free. However, it is necessary to prevent the remaining liquid in or on the grains from wetting the internals, since otherwise the internals are surrounded by an ice layer, which over time adds the container 5. Wetting is prevented by the surface of the internals is formed poorly wettable.

Another advantageous possibility is to design a part of the surface of a gas-permeable material, for example a sintered material, via which at least part of the gas flow enters the pressure vessel 5.

When freezing the carbon dioxide droplets by evaporative cooling in air evaporates about 30% of the mass of the drops. This CO ₂ vapor is now in the mixture with air. The inventive method is advantageously designed particularly efficient when the carbon dioxide is recovered from this mixture by the mixture is compressed and then cooled. Part of the carbon dioxide condenses, depending on pressure and temperature, after compression and cooling. The more carbon dioxide you can recover the less air that evaporates. In this regard, advantageously designed is a process in which the evaporation takes place with a mixture of, for example, 10% air and 90% carbon dioxide. According to an alternative embodiment of the invention, methane grains are produced as solid particles by the process according to the invention. As a gas stream, into which parts of the liquid droplets evaporate in, gases such as helium, hydrogen, neon or pure nitrogen are used.

According to a further alternative embodiment of the invention, hydrogen atoms are produced as solid particles by the process according to the invention. The gas helium is used as the gas stream into which parts of the liquid droplets evaporate.

LIST OF REFERENCE SIGNS

1 storage tank for liquid

2 sampling line

3 pre-cooling heat exchanger

4 throttle valve

5 (pressure) Bθhälter

6 spray nozzles

7 snow production, freezing zone

8 solid particles

9 lock

10 insulated transport container

11 pellets

12 extruders

13 hole bends

14a, b separator / collector for liquid-vapor mixture

15 pressure-retaining valve

16 compressor

17 dryers

18 heat exchangers

19 distributors

20 collectors

21 valve

22 jet nozzle

23 expander

24 intercoolers

25 compressors

Claims

1. A method for producing solid particles from liquids, comprising the following steps: a) removal of liquid from a storage container, b) cooling the liquid to the triple point temperature and throttling the liquid to the desired freezing pressure, c) producing drops from the liquid, d) introducing the droplets into a gas stream which at least in part does not consist of the substance of the liquid, e) freezing the droplets to solid particles by partial evaporation of the liquid droplets, f) collecting and discharging the solid particles.

2. The method according to claim 1, characterized in that in

Process step b) a pre-treatment of the liquid takes place.

3. The method according to claim 1 or 2, characterized in that the liquid is throttled in two steps in step b), wherein the liquid is separated after the first throttling of the resulting vapor and then throttled again.

4. The method according to claim 3, characterized in that the

Steam is used for pre-cooling the liquid.

5. The method according to any one of claims 1 to 4, characterized in that liquid carbon dioxide is used as the liquid.

6. The method according to any one of claims 1 to 5, characterized in that air is used as the gas stream in step d).

7. Device for the production of solid particles from liquids according to a method according to one of claims 1 to 6, characterized in that a storage tank (1), a withdrawal line (2), a throttle valve (4) and a container (5) with spray nozzles (6) and means for generating a gas stream are provided such that liquid droplets in free fall evaporate a portion of their mass in the gas stream and thereby freeze to solid particles and that on the container (5) a lock (9) for pressure-holding delivery of solid Particles is provided.

8. Apparatus according to claim 7, characterized in that a

Precooling heat exchanger (3) and a separator / collector (14) are provided for the liquid.

9. Apparatus according to claim 7 or 8, characterized in that as a means for generating a gas stream, a compressor (16), a dryer (17) and a heat exchanger (18) are provided and that in the container (5) a distributor (19) and a collector (20) are arranged for the gas flow.

10. Device according to one of claims 7 to 9, characterized in that at least a part of the surfaces of sintered material for introducing the gas stream into the container 5 are provided in this.