EP0410867B1 - Process for cleaning hot surfaces of ovens as well as installation and granules for carrying it out - Google Patents

Abstract

Process for cleaning by disintegration of the deposits on the hot surfaces of working ovens, characterised in that an oxidising agent in particulate form is sprayed in a gas stream under pressure onto the said surfaces to be cleaned.

Description

The present invention relates to a method for cleaning hot surfaces of ovens, as well as an installation and granules for its implementation.

In many fields of industry, and in particular of the petrochemical industry, ovens, boilers, air coolers are used, hereinafter called "ovens" and provided with heat exchangers having surfaces in particular metallic and in particular bundles mostly tubular. These heat exchangers often have fouling, in particular on the heating fluid side, in the form of deposits, the composition of which varies according to the fluids (liquid or gaseous) which supply the calories to the exchanger.

These deposits are responsible for fouling and possibly subsequent chemical corrosion of the surfaces, in particular of metal, of the heat exchangers, and in particular of the tubular bundles.

It therefore appeared necessary to clean the ovens to allow them to retain their heat exchange characteristics, therefore their efficiency and their yield, and also to reduce the subsequent maintenance costs resulting from degradation by chemical attack, and finally to comply with the regulations in force on the discharge of unburnt, soot and all pollutants into the atmosphere.

It is known, to clean installations of industrial furnaces and boilers, to implement a so-called "on" cleaning process. It consists in cleaning the metal surfaces using, in the dirty areas, a hyper-oxidizing solution which ensures the combustion of all unburnt materials contained in the deposits, using a supply of water in liquid form or vapor under pressure. This operation removes the combustible parts of the soot. As all these combustible elements form a binder between the different deposits, the crust which covers the tubes thus becomes more brittle. Using lances, a jet of steam is then projected onto the tubes, which removes deposits already partially broken down by the action of the hyper-oxidant.

This solution known under the name of "process in progress" however has the following drawbacks.

It is necessary to bring water to temperatures of the order of 80 °, or to the vapor state before using it, because on the one hand it would be harmful to the efficiency of the oven to introduce frigories and on the other hand the contact of a jet of cold water on the metal pipes and / or on the supports of these pipes, that is to say any thermal shock should be avoided. Finally, this treatment is done in two successive stages.

The present invention overcomes these drawbacks, in fact it relates to a method of cleaning by disintegration of deposits on the surfaces of furnaces in operation, implemented dry. According to the invention, an oxidizing agent in particulate form is sprayed onto the surfaces to be cleaned: the dry oxidizing agent itself is sprayed into a dry gas flow under pressure. By "dry" is meant substantially free of water.

Document GB-A-1 378 882 describes a method of cleaning a boiler, in particular where an oxidizing agent is brought into contact with the surface to be treated by injection into the flame or into the combustion chamber.

Furthermore, the document DD-A-34 918 describes the cleaning of sealing surfaces of coke ovens by spraying coke on the sealing surfaces.

The disadvantages of the known methods are eliminated, and moreover this treatment in dry running does not require any particular precaution, necessary on the contrary for hot water or steam, the source of compressed air used appearing generally among the utilities available on industrial sites. This aspect is particularly advantageous in refineries.

By "ovens" according to the invention is meant in particular all ovens and boilers comprising surfaces hot interiors, notably metallic, in particular refinery ovens, atmospheric distillation, vacuum distillation, reformer ovens, of the cylindrical oven or cabin oven type, or even double oven comprising two radiation zones and one or more convection zones common, as well as crackers and heat exchanger boilers.

By surface is meant more generally any hot surface of the furnace fouled by a deposit coming from a fuel, and in particular metal surfaces.

The cleaning is carried out on the fly, that is to say without the installations being stopped. Surfaces, particularly metallic ones, are then commonly brought to temperatures of 200 to 1000 ° C., the temperatures being different in the radiation zones and in the convection zones.

This cleaning according to the invention makes it possible to carry out the disintegration and elimination of deposits in a single step and results from the combination of two effects. On the one hand, as the deposits contain combustible materials corresponding to the unburnt fuels generally used for reheating, these deposits can be eliminated by "chemical" or "thermal" effect due to the combustion of the deposits present on the hot surface which s 'ignite on contact with the particles of oxidizing agent, this combustion causing disintegration of the deposits and burning the combustible materials present in these deposits. On the other hand, the oxidizing agent being in the form of a particle, the projection of these particles also has a "mechanical" effect linked to the speed of the particles when they come into contact with the surfaces to be cleaned. The combination of these two effects ensures the disintegration and elimination of deposits.

For the chemical and thermal effect to be achieved, it is desirable to use a selected oxidizing agent among compounds and chemical compositions highly oxidizing and readily available.

It is possible to use the oxides, peroxides and persalts of chromium, manganese, chlorine, sulfur, nitrogen or boron such as chromates, dichromates, permanganates, chlorates, nitrates, nitrites or borates of alkali metals. , alkaline earth, or ammonium, or mixtures thereof.

So that the mechanical effect is also achieved, it has appeared preferable that the oxidizing agent according to the invention is in the form of granules. Preferably, the particle size is between 0.5 and 2.5 mm and in particular between 1 and 2 mm. More particularly, a particle size of approximately 1.5 mm for ammonium nitrate granules has been shown to be particularly effective.

Optionally, and in particular in the case of strong fouling of the surfaces, it is possible to precede the implementation of the method according to the invention by air blowing to remove particles which are not adherent to the surface to be cleaned.

Optionally, and preferably in the event of large and hard deposits, the mechanical effect due to the projection of the granules according to the invention can be amplified by simultaneously projecting a hard particulate material. Sand can be used for this purpose, for example. The grain size of the sand is then chosen between 0.5 and 2 mm.

When a mixture of oxidizing agent and particulate material is used, cleaning sand may be used, depending on the degree of fouling of the surfaces to be treated, in a proportion of up to 20% by weight.

When the oxidizing agent (s) are used in the form of granules, the density in the form of loose granules is for example between 0.5.10³kg.m⁻³ and 1.5.10³kg.m⁻³.

The dry gas flow entraining the particles in contact with the surfaces to be cleaned must have, according to the invention, a sufficient speed to entrain the particles and allow them to complete the chemical and thermal effect of local combustion by the mechanical effect of attack on the deposits. .

This mechanical effect is achieved in particular when the speed of the particles of oxidizing agent, and possibly of the hard particulate material, is between 100 and 250 m.s⁻¹. A speed of 130 to 200 m.s⁻¹ proved to be particularly effective.

The present invention also relates to an installation for cleaning by disintegration of deposits on the hot surfaces of ovens by spraying an oxidizing agent in particulate form on these surfaces in a dry gas flow under pressure. The installations according to the invention comprise a piping for transporting the dry gas flow connected at one end to a source of dry gas under pressure, and at the other end to a means for spraying the gas-particle mixture of the type nozzle.

As dry gas under pressure, compressed air or any other gas or gas mixture suitable for the safety conditions and / or the desired chemical effect can be used.

This installation also comprises a particle tank provided with a means for dosing the particles. This metering means opens into the transfer piping and for this purpose, between its two ends, the transfer piping is provided with a connection with the metering means of the reservoir.

In addition, between this connection and its end carrying for example this nozzle, the piping is at least in flexible part.

In a particular embodiment, the particle tank is pressurized, and provided with a gravity metering means, the pressure in the transfer piping and the pressure prevailing in the tank being compatible with the use of a valve. mechanical allowing the mechanical regulation of the dosing by gravity. These pressures can for example be substantially balanced.

Although other pressures can be envisaged, in particular for other pipe and nozzle diameters and other flow rates, pressures of the order of 5.10⁵ to 10.10⁵ Pa for compressed gas are preferred; the internal diameter of the piping is 25 to 50 mm; and the means for projecting the gas-particle mixture of the nozzle type, and preferably of the Venturi type, can have an internal diameter of the outlet opening of 9.5 to 11 mm, and a length of 150 to 200 mm.

It is clear that the pressure, the speed, the internal diameters of the piping and of the nozzle, as well as the length of the nozzle can vary within wide limits, in order to obtain the effective spraying speeds according to the invention. These speeds can themselves vary according to the nature of the deposits to be removed, the particle size and the nature of the oxidizing agent, and the particle size and the nature of the hard particulate material possibly associated.

Between the connection connecting the transfer piping by means of particle dosing and the outlet end of the nozzle, the piping comprises at least one flexible part. In fact, this installation, specific to the implementation of the method, is intended to be actuated by a man who directs the nozzle and the jet emitted by the nozzle towards the deposits in the furnace. To this end, the means of projection can be fixed on a lance, which the man directs towards the surfaces to be cleaned, starting from all the openings and / or manholes of the working oven. These can be the manholes on the periphery of the oven, the inspection door, the manholes, and even the explosion hatches.

As flexible hoses, it is preferable to use hoses with double woven reinforcement.

In a particular embodiment of the invention, these flexible pipes are chosen from a material which conducts electric current, which makes it possible to avoid the phenomena of static current, and therefore the risks of sparks and of explosion. This particular mode is very useful in refineries. The installation itself is then connected to ground.

The present invention also relates to granules comprising at least one oxidizing agent for their application to the implementation of the method according to the invention, the particle size of which is between 0.5 and 2.5 mm, preferably 1 to 2 mm.

The oxidizing agent can be chosen from the oxides, peroxides and persalts of chromium, manganese, chlorine, sulfur, nitrogen or boron such as chromates, dichromates, permanganates, chlorates, nitrates, nitrites or borates of alkali, alkaline earth metals, or ammonium or mixtures thereof.

In a particular embodiment, the oxidizing agent constituting the granules is ammonium nitrate.

Other characteristics and advantages of the invention will appear on reading the examples below:

Example 1

The process according to the invention was implemented in a cylindrical furnace of a hydrocracking unit of a oil refinery. It is an oven operating with a constant load.

We used a product quality and an oven entry temperature, always identical. As a result, gains in unit efficiency are more easily calculated.

The oxidizing agent used is ammonium nitrate with a uniform particle size of approximately 1.5 mm, the density of the compound in the form of loose granules being 1.10³kg.m⁻³.

Compresseur à air :

puissance 60 à 90 CV (1 CV = 0,735 KW)
pression : 7 bars
débit : 5 à 8 m³.mn⁻¹.

The installation consists of a compressor, a pressure device of the "sandblasting" type, flexible piping, and a spray nozzle, which have the following characteristics:

Air compressor :

power 60 to 90 CV (1 CV = 0.735 KW)
pressure: 7 bars
flow rate: 5 to 8 m³.mn⁻¹.

Pressure equipment

This "pressure sandblaster" type device is connected to the compressor. It receives the granulate and conveys it to the outlet orifice in a dry gas flow at the speed necessary for projection at the nozzle outlet. This device operates under pressure, according to the principle of gravity with regard to the passage of particles or granules from the tank to the piping used for transfer. The pressure prevailing in the particle tank is approximately equal to the pressure of the compressed air coming from the compressor through the transfer piping, which is the pressure prevailing below the granules or particles. The granule is entrained by the gaseous flow of compressed air and enters the transfer piping by means of a metering means which allows the quantity of granule to be adjusted precisely. introduced. This regulation is carried out using a metering valve with mechanical regulation.

Transfer piping

The pipes are flexible with a double woven frame; they are conductors of electric current. The internal diameter of the pipe is 32 mm.

Projection nozzle

The nozzle used is of the Venturi type and nozzles were used having an outlet opening of 9.5 mm and lengths of 150 mm. The exit speed of the granules was then between 130 and 200 ms⁻¹. The table below presents the results obtained by implementing the method described above (T / J = ton per day):

The overall efficiency of the heat exchange: heat absorbed compared to heat supplied was increased by 5.1% .

It clearly appears that other compressors could be used, in particular compressors which can develop a power between 50 and 150 CV, and which can provide a pressure between 4.10⁵ and 10.10⁵ Pa, these values being given as an indication. The choice of the projection nozzle must also take into account the type of intervention.

Example 2

The procedure of Example 1 was reproduced with the same type of installation for treating an oven where more stubborn deposits fouled the pipes. A mixture comprising the same granule and 15% by weight of dried washed sand, and calibrated between 0.5 and 1 mm was used.

Similar results have been recorded.

As it appears in the previous examples, the method and installation according to the invention are particularly useful for finding or maintaining in oven installations an efficiency during use and even after a long use equivalent to the start-up efficiency. , that is to say without waiting for the fouling of the annual shutdowns of industrial sites.

Claims (16)

1. Process for cleaning by disintegration of the deposits on the hot surfaces of running furnaces, characterized in that an oxidizing agent in particulate form and sand as hard particulate material, in a dry gas stream under pressure, are sprayed simultaneously onto the said surfaces to be cleaned, the spraying velocity, sufficient to attack the deposits using a mechanical effect, being between 100 m s⁻¹ and 250 m s⁻¹.
2. Process according to Claim 1, characterized in that the said agent is in the form of granules with a particle size of between 0.5 and 2.5 mm, in particular between 1 and 2 mm, and more particularly approximately 1.5 mm.
3. Process according to Claim 1 or 2, characterized in that the said oxidizing agent is chosen from chromium, manganese, chlorine, sulphur, nitrogen or boron oxides, peroxides and persalts, such as alkali or alkaline-earth metal chromates, dichromates, permanganates, chlorates, nitrates, nitrites or borates or mixtures thereof.
4. Process according to Claim 3, characterized in that the said oxidizing agent is chosen from chromium, manganese, chlorine, sulphur, nitrogen or boron oxides, peroxides and persalts, such as alkali or alkaline-earth chromates, dichromates, permanganates, chlorates, nitrates, nitrites or borates, or mixtures thereof, preferably ammonium nitrate.
5. Process according to one of Claims 1 to 4, characterized in that the temperature of the surfaces is between 200° and 1000°C.
6. Process according to one of Claims 1 to 5, characterized in that up to 20 % by weight of sand is sprayed simultaneously.
7. Process according to one of Claims 1 to 6, characterized in that the velocity is between 130 and 200 m s⁻¹.
8. Process according to one of Claims 1 to 7, characterized in that the spraying of the oxidizing agent is optionally preceded or followed by blowing through with air, in particular with air under pressure.
9. Plant specifically designed for the implementation of the process according to Claims 1 to 8 for cleaning by disintegration of deposits on hot surfaces of running furnaces, by spraying onto these surfaces a particulate oxidizing agent in a dry gas stream under pressure, characterized in that it comprises at least one storage vessel for particles of oxidizing agent, provided with a metering means, a pipework for transfer of the dry gas stream which is connected at one of its ends to a source of dry gas under pressure and at the other end to a means for spraying the mixture of gas and agent of the nozzle type, and provided between its two ends with a connection to the metering means of the storage vessel, the pipework being at least partially flexible between the connection and the said spraying means; and the pressure, the pipework and the means for spraying being constructed so as to ensure that the spraying velocity is between 100 m s⁻¹ and 250 m s⁻¹.
10. Plant according to Claim 9, characterized in that the storage vessel is under pressure and is provided with a metering means using gravity.
11. Plant according to Claim 9 or 10, characterized in that the pressure is between 5×10⁵ and 10×10⁵ Pa and is preferably equal to approximately 7×10⁵ Pa.
12. Plant according to one of Claims 9 to 11, characterized in that the means for spraying the mixture of gas and agent is a nozzle of the Venturi type, which has a diameter of between approximately 9.5 and 11 mm at its outlet end, and a length of between approximately 150 and 200 mm.
13. Plant according to one of Claims 9 to 12, characterized in that the pipework has an internal diameter of between 25 and 50 mm, preferably approximately equal to 32 mm.
14. Plant according to one of Claims 9 to 13, characterized in that the pipework is made of a material which conducts electrical current, the plant being earthed.
15. Use of a granulate comprising at least one oxidizing agent chosen from chromium, manganese, chlorine, sulphur, nitrogen or boron oxides, peroxides and persalts, such as alkali or alkaline-earth metal chromates, dichromates, permanganates, chlorates, nitrates, nitrites or borates or mixtures thereof, having a particle size of between 0.5 to 2.5 mm simultaneously with sand, as hard particulate material, for the implementation of the process according to one of Claims 1 to 8.
16. Use according to Claim 15, characterized in that the granulate consists of ammonium nitrate.