EP1368158B1 - Method for the treatment with abrasives - Google Patents

Abstract

The invention relates to a method for the treatment of objects with abrasives, wherein the abrasives are blasted onto the object to be treated by means of a flow of gas. The invention is characterized in that the abrasive used is dry ice and the objects treated consist of materials or are coated with materials that have a higher hardness than dry ice but a lower thermal conductivity than the support material of the object to which the materials or coatings adhere and/or on which the materials or coatings are disposed. The abrasive treatment is preferably carried using CO2 pellets.

Description

The invention relates to a method for blasting objects with blasting media according to the preamble of claim 1, which e.g. is known from WO-A-00/67928.

The irradiation of objects (e.g. workpieces) as a treatment method with abrasives is known in numerous industrial applications. The Known beam treatment methods can be used in terms of Divide the abrasive into two groups.

The first group concerns blasting with conventional blasting media such as in particular Quartz sand (sandblasting), but also with other conventional abrasives such as steel gravel, cast iron gravel, wire grain and corundum. As an abrasive for the dry abrasive blasting treatment can also use other inorganic or organic abrasives or plant-based abrasives are used. All these conventional abrasives have the common property that the Abrasives are in a solid state under normal conditions. They work in usually abrasive. In blasting treatment using conventional, abrasive blasting media is a mixture of blasted material and blasting media which are usually disposed of with great effort and costs got to.

In addition to these conventional abrasive abrasives, there is a second group of abrasives known for blasting treatments that are in gaseous form under normal conditions or in liquid state.

These blasting media are generally softer than conventional blasting media and therefore ensure blasting treatment with less abrasiveness compared to blasting treatments with conventional blasting media in the solid state under normal conditions. Dry ice (CO ₂ ), which has proven itself in numerous applications, is to be mentioned in particular as a representative of the group of blasting agents which are present in fluid form under normal conditions.

• Know How, "Strahlreinigen nach Maß mit Cryoclean®", Dr. H.-J. Diehl, Linde AG, Werksgruppe Technische Gase, 2/96, 1996, Seiten 1 bis 5,

beschrieben. Mobile Anlagen zur CO₂-Strahlreinigung sind unter dem Namen Cryomax® und Cryomini® aus Prospekten der Fa. Linde Gas AG bekannt.Due to its physical properties, solidified carbon dioxide offers significant advantages when used as a blasting medium for blasting processing such as blasting cleaning of surfaces: carbon dioxide in a solid state (dry ice) has a temperature of around -78 ° C. When energy is supplied, carbon dioxide sublimes. Processing with carbon dioxide in a solid state is residue-free since the sublimed gaseous carbon dioxide can escape without any problems. Blasting agent preparation or blasting agent disposal is therefore not necessary for carbon dioxide. Solid carbon dioxide in the form of CO ₂ pellets is preferably used. Jet cleaning with CO ₂ pellets is, for example, in its own magazine for customers and business partners

• Know How , "Tailor-made jet cleaning with Cryoclean®", Dr. H.-J. Diehl, Linde AG, Technical Gas Group, 2/96, 1996, pages 1 to 5,

described. Mobile systems for CO ₂ jet cleaning are known under the names Cryomax® and Cryomini® from brochures from Linde Gas AG.

As a rule, solid carbon dioxide (CO ₂ ) is used in the form of compressed, generally rice-granules. These so-called CO ₂ pellets are metered into a gas stream in a blasting system, conveyed with the gas stream to a blasting nozzle and passed through the blasting nozzle onto the surface to be processed. The blasting nozzle is usually installed in a blasting gun. Mostly dry ice pellets are made in a pelletizer. For this purpose, liquid carbon dioxide is injected into the pelletizer, transferred to dry ice (snow) by expansion, compressed into a "cake" and finally pressed through a die. The result is roughly rice-sized CO ₂ pellets with a certain size distribution. The granules produced in this way typically have mean values of a length of 8 mm and a diameter of 3 mm.

The hardness of dry ice roughly corresponds to the low hardness of gypsum.

Due to the fact that compared to conventional abrasive abrasives, which are used under normal conditions are in a solid state, open the "softer" at Normal conditions in fluid form abrasive applications where removal of material is not necessary or is even undesirable.

The invention has for its object a method for the removal of materials containing sulfur or soot or coke or coatings in apparatus or plant parts for the thermal splitting of Show hydrocarbons or in parts of Claus plants.

This object is achieved by the method according to claim 1.

With the invention, an effective blast treatment of surfaces - in particular an effective cleaning of surfaces - with the help of dry ice, i.e. mitttels abrasives present in normal form in fluid form are guaranteed even if accumulations, deposits, buildup, deposits or Coatings made of materials with a greater hardness than dry ice become.

Object surfaces on or on which material adheres or which are coated with material which has a greater hardness than that has blasting media present in fluid form under normal conditions, one Blasting treatment with carbon dioxide as at least one blasting agent effectively subjected become .

It is probably due to the fact that harder materials can be effectively irradiated with dry ice on a primarily not chemical, but complex chemical / physical Mode of action.

Dry ice has a very low temperature of around -78 ° C. The low one Temperature means a high temperature difference ΔT to the one to be removed Material. The high flow rate (preferably 100 to 300 m / s) of the Carrier gas leads to high turbulence and thus high heat transfer coefficients α. High heat transfer coefficients α and a high temperature difference ΔT mean very quick, superficial cooling. With material that is bad It has thermal conductivity, as is the case with sulfur, for example high temperature gradients within the contamination layer. This in turn lead to high thermal voltages that are not reduced by heat conduction. As a result, the material becomes brittle due to the low temperature structural changes, the weakening of the material up to cracks after pull yourself.

The blasting treatment is advantageously carried out using CO ₂ pellets.

In this case, the crack formation is reinforced by a second effect: CO ₂ pellets, which hit a surface at high speed, generate high pressure at certain points at the point of impact. Solid CO ₂ becomes liquid, similar to water ice under ice skates. Liquid CO ₂ is a good solvent for many substances, especially organic materials. In the case of sulfur in particular, the locally occurring solution leads to a change in the crystal structure, which can regress again after some time. In the meantime, however, the different volumes of the crystal structures create stresses, which in turn promote crack formation.

For this reason, it is advantageous if the dry ice or the CO ₂ pellets hit the materials, coverings and / or coatings to be removed at high speed. The abrasion, which would require particularly hard blasting material for hard coverings, is only of minor importance as an effect in the blasting treatment.

Particular advantages can be achieved with the invention if the thermal conductivity the materials or coatings have a value less than 20 W / m ° K, preferably less than 15 W / m ° K, particularly preferably less than 9 W / m ° K. Relating to the carrier material of the object on which the materials or coatings stick and / or on which the materials or coatings are located it is advantageous if the thermal conductivity of the carrier material of the object is one Value greater than 30 W / m ° K, preferably greater than 35 W / m ° K, particularly preferred greater than 45 W / m ° K. The carrier material is often steel, the one Has thermal conductivity of approx. 46 W / m ° K, or stainless steel with a Thermal conductivity of approx. 35 W / m ° K. An example of a successfully to be replaced Material represents carbon black, which has a good thermal conductivity of approx. 8.9 W / m ° K has. Materials with poorer thermal conductivity are therefore easier to remove. As a result, this applies to sulfur with a thermal conductivity of approximately 0.45 W / m ° K too.

The flow velocities of the carrier gas flow in the work area on the In a further development of the invention, the nozzle is preferably from 50 m / s to 400 m / s from 100 to 350 m / s, particularly preferably from 200 to 300 m / s.

a) Die Flüssigkeit entspannt sich und verdampft schlagartig unter Volumenvermehrung um ca. einen Faktor 500 bis 600. Dadurch entsteht eine lokale Sprengwirkung, die Beläge von der darunterliegenden Oberfläche ablöst.

b) Die Druckentlastung führt zusätzlich zur Bildung von CO₂-Eis-Partikeln. Diese kleinen Partikel sublimieren jedoch sehr schnell - fast ohne Zeitverzögerung - unter starker Volumenvermehrung, was wiederum zu der genannten Sprengwirkung auf das abzulösende Material führt.

CO ₂ penetrates into the cracks described above in the state of the transient liquid phase at high pressure. After that, however, there is a sudden pressure relief because the impact energy of the CO ₂ pellet does not reach down to the depth of the crack. The pressure relief can have two consequences:

a) The liquid relaxes and evaporates suddenly with volume increase by a factor of about 500 to 600. This creates a local explosive effect that detaches deposits from the underlying surface.

b) The pressure relief also leads to the formation of CO ₂ ice particles. However, these small particles sublimate very quickly - with almost no time delay - with a large increase in volume, which in turn leads to the explosive effect mentioned on the material to be removed.

The gas stream for conveying the dry ice or the CO ₂ pellets can be composed of any suitable gas or gas mixture. As a rule, compressed air is used to supply the blasting system, for example at a pressure of 5 to 20 bar, with a dew point of 5 ° C or drier. The dry ice particles or the CO ₂ pellets are metered into the air stream, conveyed through the jet nozzle with the aid of the air stream, accelerated to a speed of up to 400 m / s and directed onto the surface to be cleaned.

In an embodiment of the invention, the materials or coatings to be treated have at least partially different from that of the object surface Coefficient of thermal expansion.

In this case, if the material to be removed and the surface on which it adheres, have very different expansion coefficients when cooling occur the cooling of the material by the blasting agent dry ice also thermal stresses between material and surface. This in turn leads to thermal stresses and thus shear forces that separate the material from the underlying surface.

Advantageously, coatings with a medium Thickness of over 2 µm can be irradiated, including centimeter thick layers can be successfully replaced. It has been shown that with very thin layers of a solid, the method according to the invention loses its effectiveness. In such thin layers is also poor heat conduction through the material to be removed on the underlying surface due to the small thickness, so good that, despite poor thermal conductivity, there is only a small temperature difference between material and surface. This also creates only low shear forces. In addition, the thin layer is only mechanically resilient, so that it adapts to the movements of the surface without detaching.

Within the scope of the invention, in addition, under normal conditions in a solid state existing blasting media can be used. Under certain circumstances There can be advantages that have the disadvantage of blasting agent preparation or Abrasive disposal can weigh up or exceed.

In a further development of the invention, organic materials or coatings can be irradiated.

Form in parts of the plant such as sulfur condensers from Claus plants Sulfur, salts and catalyst wear from upstream reactors be called "Sulfurcrete". Sulfurcrete is used, for example, in the Publication: H.G. Paskall, J.A. Sames: "Sulfur Recovery", Calgary (Canada), 1989, mentioned in the chapter "Sulfur condenser function and problem areas".

Sulfurcrete is a solid whose hardness is similar to that of granite. Such toppings were previously with the help of drills or similar mechanical devices away. Damage to the apparatus and object surfaces due to this mechanical Cleaning cannot be completely avoided and lead to a low one Service life of the apparatus, for example the sulfur condensers mentioned. It has been shown that Sulfurcrete coverings using the invention Procedure can be effectively removed. With appropriate attempts Sulfurcrete coverings could be confirmed that blasting with dry ice particles despite their low hardness, the pads are made of harder Allow material to be separated from the surface of the object. Responsible for this unexpected fact is probably the special properties mentioned above of carbon dioxide and the lower thermal conductivity of Sulfurcrete the carrier material, usually steel. Specifically, the thermal conductivities are Steel approx. 46 W / m ° K, from Sulfurcrete approx. 0.5 to 1 W / m ° K.

The invention has been particularly in the removal of soot or Proven coke-containing materials or coatings on objects. Here is especially the cleaning of can, quench coolers and the like Plant parts used in the thermal cracking of hydrocarbons be mentioned. In the following, this should be based on the example of cleaning Quench coolers in cracking furnaces of ethylene plants can be illustrated.

In the thermal cracking of hydrocarbons, especially heavy ones Hydrocarbons are known to use cracking furnaces in which the thermal fission is carried out. The fission gases are then subordinate Quench coolers rapidly cooled to interrupt the reactions.

Coke settles during the cracking process or the cooling of the cracked gases on a wide variety of plant parts and apparatus, for example in the Canned tubes, but also in quench coolers. The coke deposits narrow the free cross section of the gas flow. The coke deposits also act as Thermal insulation in the externally heated reaction tubes. This leads to the system parts must be decoked at regular intervals.

In ethylene plants, this happens in practice every four to ten weeks in hot Oven and quench cooler. So far, cleaning and decoking has been carried out with a hot oven by interrupting the feed stream normally supplied to the furnace is and a mixture of steam and air is introduced, which the deposited Coke burns off The amount of air is metered in such a way that local overheating occurs the cooling pipes in the cracking furnace and in the quench cooler is excluded However, coke is not complete with this method.

The ovens and coolers must therefore be cooled every three to four months Condition can be cleaned mechanically. According to the state of the art Technology uses high pressure water of up to 2500 bar with which the deposits be removed from the pipe walls. This treatment leads to one sufficiently good cleaning, but leaves on the treated surfaces Damage, often shell-shaped washouts. In particular, by Corrosion of areas already attacked on the pipes or system parts is preferred washed out so that incipient damaged areas are further damaged. The treated surface is also treated with high pressure water not 100% metallic bright. Some impurities remain, so that Surface appears roughened and new deposits preferentially settle there.

For such mechanical cleaning, including cooling and Reheating the oven typically takes three to four days. The The availability of the furnace and thus the amount of ethylene produced drops significantly from.

According to the invention, however, the greatly different thermal conductivity and Thermal expansion of the deposits and the pipe material or the system parts used. In this way, apparatus, pipes or Remove any deposits from the system components. Experiments have shown that Dry ice as a blasting agent already contains the soot or coke-containing impurities allow a pressure of about 12 bar to be removed. Air is preferably used as the blowing agent or nitrogen is used. This creates a 100% shiny metallic surface achieved, while experience has shown that cleaning with high pressure water only about 80% leaves bare surfaces.

A major advantage of the method according to the invention is that during the surfaces can be polished, if necessary with the addition of Abrasionshilfsmitteln. According to the invention, the roughness of the surfaces can be Reduce below 5 microns. As a result, new deposits on the smooth surface find no starting point and therefore not or only very much attach later. This significantly extends the life of the ovens. In practice can be a quarter to half of the mechanical cleaning required are eliminated, which means the availability of the furnaces and thus the amount of ethylene produced is increased by about 2 percent per year.

Claims (10)

1. Process for the blasting treatment of objects using blasting agents, the blasting agents being blasted on to the object to be processed with the aid of a gas stream, the blasting agent used being at least dry ice, and the object blasted being an object which comprises support material provided with materials or coatings, the materials or coatings having a greater hardness than dry ice and a lower thermal conductivity than the support material, characterized in that materials or coatings which contain sulphur or carbon black or coke in apparatuses or installation parts, in particular tubes, cracking tubes or quench coolers, for the thermal cracking of hydrocarbons, in particular ethylene, or in installation parts of Claus installations, are blasted.
2. Process according to Claim 1, characterized in that the blasting treatment is carried out using CO₂ pellets.
3. Process according to Claim 1 or 2, characterized in that the thermal conductivity of the materials or coatings is less than 20 W/m°K, preferably less than 15 W/m°K, particularly preferably less than 10 W/m°K, and more particularly preferably less than 9 W/m°K.
4. Process according to one of Claims 1 to 3, characterized in that the thermal conductivity of the support material of the object is greater than 30 W/m°K, preferably greater than 35 W/m°K, particularly preferably greater than 45 W/m°K.
5. Process according to one of Claims 1 to 4, characterized in that the flow velocities of the carrier gas stream in the working region at the nozzle are from 50 m/s to 400 m/s, preferably from 100 to 350 m/s, particularly preferably from 200 to 300 m/s.
6. Process according to one of Claims 1 to 5, characterized in that the coatings or materials to be treated have a coefficient of thermal expansion which is different from that of the object's surface.
7. Process according to one of Claims 1 to 6, characterized in that coatings with a mean thickness of over 2 µm are blasted.
8. Process according to one of Claims 1 to 7, characterized in that in addition blasting agents which are in the solid state under standard conditions are used.
9. Process according to one of Claims 1 to 8, characterized in that organic materials or coatings are blasted.
10. Process according to one of Claims 1 to 9, characterized in that the pressure of the gas stream is from 10 to 100 bar, preferably 10 to 20 bar.