EP2048217A2 - Method for decarbonising cracking furnaces - Google Patents

Abstract

A method for decoking a cracking furnace (1) for the thermal cleavage of hydrocarbons by passing a decoking gas through coked-up plant components with formation of an off-gas (5), involves recycling the off-gas (5) through a cyclone separator (6) into the body of the furnace (1) or subjecting it to a catalytic treatment (11) so as to remove most of the solid particles from the off-gas (5).

Description

The invention relates to a process for the decoking of a cracking furnace for the thermal cracking of hydrocarbons, in which a Entkokungsgas is passed through verkokte plant parts, wherein a decoking exhaust gas is formed.

The thermal cracking of hydrocarbons, especially the thermal cracking directed to the formation of ethylene and other lower olefins, is of great industrial importance. As is known, cracking furnaces with at least one convection and radiation zone are used. The actual thermal splitting is carried out in the burner-heated radiation zone, while in the convection zone, the hydrocarbons and other fluids are heated by arranged there heat exchanger against the flue gas. Usually, hydrocarbon and process steam are preheated in the heat exchangers of the convection zone and fed to the arranged in the radiation zone coils a hydrocarbon / vapor mixture. In a downstream quench cooler, the generated cracking gases are rapidly cooled to stop the reactions. Such splitting methods and cracking furnaces are for example made DE 2830824 and DE 2854061 known.

In carrying out such processes, however, a number of side reactions occur which lead to economically uninteresting or disturbing products. Such a disturbing side reaction is the coking of system parts, in particular the split tubes and the quench cooler. Since the coke deposited in the crevices is an excellent heat insulator, coking results in a significant deterioration of the heat transfer and an increase in the temperatures of the crevices. The progressive coking and the associated increase in the tube wall temperatures of the split tubes are set material-technical limits.

It is therefore necessary to interrupt the thermal decomposition from time to time and to carry out a decoking of the plant. This is usually done by the supply of a Entkokungsgases, eg from air and / or water vapor, which through the continues to be heated externally heated cracking tubes and the still cooled fission gas cooler. At the high temperatures of the decoking, which are for example between 600 ° C and 1050 ° C, there is a burning of the deposits. Some of the deposits are mechanically discharged.

The resulting decoke exhaust gas may e.g. Peak levels up to about 20,000 ppm by volume of carbon monoxide in dry exhaust gas. An immediate release of this Entkokungsabgases to the atmosphere should therefore be avoided for environmental reasons.

After, in EP 0497155 As the prior art described method, the Entkokungsabgas for afterburning of the coke particles and the carbon monoxide is introduced into the furnace of the cracking furnace. Since the amount of Entkokungsabgases in relation to the resulting from the unterfeuerten power of the cracking furnace flue gas during decoking is large, a great deal of effort for a uniform supply of Entkokungsabgases must be operated over the entire furnace to operate the oven burner safely and trouble-free. For this purpose, a complex gas channel system is required. Additionally, at this, in EP 0497155 As described in the prior art, procedure performed a relatively large amount of exhaust gas into the furnace. This relatively large amount of exhaust gas must be heated, which increases the heating demand during decoking.

To avoid these disadvantages, is in EP 0497155 proposed not to lead the decoke exhaust gas into the furnace but to a catalytic treatment. By using a catalyst for the oxidation of incompletely combusted components in the decoking exhaust gas, the process is simplified. The carbon monoxide and other unburned components in the decoke exhaust gas are further reduced, but the mechanically discharged deposits remain in the decoke exhaust gas.

An alternative method of decoking a fission gas furnace and a subsequent quench cooler is disclosed in US Pat US 5186815 described. At the in US 5186815 described method, the hydrocarbon-containing feed is continuously fed solid particles having a diameter of less than 150 microns via a carrier gas. These small solid particles pass with the Hydrocarbon-containing resistant the gap tubes and the quench cooler and ensure a constant erosion of the coke on the interior of the fission gas tubes or the quench cooler. About a cyclone at the exit of the quench cooler, the solid particles are separated from the gaseous products of the cleavage and recycled back into the hydrocarbonaceous feedstock of the thermal cleavage.

Both prior art methods have disadvantages. At the in EP 0497155 described recycling of Entkokungsabgases into the furnace or to a catalytic treatment can be almost completely oxidize carbon monoxide to carbon dioxide, but the solid coke particles contained in Entkokungsabgas are not burned or removed. The dust or coke emission can therefore in a process after EP 0497155 partially well above the limit of 50mg / Nm ^{3 of} the European emission standard.

In a procedure such as in US 5186815 Although solid coke or dust particles are separated together with the supplied solid particles via the cyclone separator from the gas stream, but the entire product gas stream must be passed through the separator. However, since no complete separation of all solid particles can be achieved in the cyclone separator, a process is used US 5186815 solid coke and dust particles with the product gas stream into downstream equipment parts, such as the cryogenic decomposition part, where they can lead to linings.

The present invention is therefore based on the object to develop an alternative method for decoking a cracking furnace.

The present object is achieved in that the Entkokungsabgas is recycled via a cyclone separator in the furnace of the cracking furnace or is led to a catalytic treatment, wherein the majority of solid particles in the cyclone separator is removed from the Entkokungsabgas.

In the method according to the invention, a decoking gas is passed through the coked plant parts during a cleaning phase, whereby a decoking off-gas is produced. The resulting decoking exhaust gas contains beside Oxygen, nitrogen and water vapor predominantly coke particles (dust particles), carbon monoxide and carbon dioxide. For the purposes of the invention, the decoke exhaust gas is first passed through a cyclone separator. In such a cyclone separator, solid components may be separated from gaseous parts. In Zyklonabscheider therefore the majority of solid particles (coke particles, dust particles) is removed from the Entkokungsabgas. The gaseous part of the Entkokungsabgases is performed by the cyclone in a suitable manner in the furnace of the cracking furnace, where at the prevailing temperatures carbon monoxide is almost completely converted into carbon dioxide and a large combustion of the non-deposited coke particles. Alternatively, the gaseous portion of the decoking exhaust gas may be passed from the cyclone separator to a catalytic treatment. In the catalytic treatment (oxidation) carbon monoxide is converted to carbon dioxide. The decoke exhaust gas, as it ultimately leaves the cracking furnace or the catalytic treatment, is therefore almost free of solid particles such as coke particles and almost free of carbon monoxide. The process according to the invention thus represents an environmentally friendly alternative to the decoking of cracking furnaces.

According to a preferred embodiment of the invention, water is injected into the decoking exhaust gas upstream of the cyclone separator. By injecting water into the decoking exhaust gas, the separation efficiency of the cyclone separator is significantly improved. The emission of dust or coke particles can thus be further reduced. The solid particles of the decoking exhaust gas can be humidified by the regular injection of water, whereby the separation of the solid particles is improved by the cyclone.

In another embodiment of the invention in a plant with an existing multi-gap ovens Ofenstraße incurred during the cleaning decoking exhaust gases are conveniently performed via a single cyclone separator for afterburning in the furnace of a selected slit furnace or in a catalytic treatment.

With the present invention, it is possible in particular to provide an alternative and environmentally friendly method to the prior art. With the help of the present invention, both coke and dust emissions and the Carbon monoxide content of decoke exhaust gas is minimized prior to venting to the atmosphere.

In the following, the invention will be explained in more detail with reference to an embodiment shown in the figure.

Show it:

FIG. 1

an embodiment of the invention with the return to the furnace of the cracking furnace and

FIG. 2

an embodiment of the invention with a catalytic treatment.

FIG. 1 shows a radiation zone (1 a) and convection zone (1 b) existing cracking furnace (1). During decoking, a mixture of air and water vapor is sent into the cracking tubes (2) and the subsequent quenching gas coolers (3). During decoking, the radiation zone (1a) of the cracking furnace (1) is kept at a temperature of about 1100 ° C. via the burners (4). The Entkokungsabgas (5) resulting in the crevices is directed to a cyclone separator (6). Before the cyclone separator (6) water (7) is injected into the Entkokungsabgas (5) at regular intervals. In the cyclone separator (6), solid particles, such as coke particles or dust particles, are separated from the gaseous decoke exhaust gas (8). The freed from solid constituents Entkokungsabgas (9) is fed into the combustion chamber of the radiation zone (1 a) of the cracking furnace (1). At the temperatures prevailing there, the carbon monoxide still present in the decoking exhaust gas (9) is almost completely converted to carbon dioxide. The, the cracking furnace escaping, exhaust gas (10) is thus now exempt from coke or dust particles and carbon monoxide.

The measured in this embodiment of the invention dust and carbon monoxide concentrations in the exhaust gas at the various positions (5, 9 and 10) are shown in the following table.

Working Example: position 5 9 10 Gas quantity [Nm ³ / h] 29,000 29,000 71,000 (including flue gas) Pressure [bar over atmosphere] 12:25 12:15 0 Temperature [° C] 450 450 180 Carbon monoxide * [ppmv] 1..800 1800 <20 Dust (coke) * [mg / Nm ³ ] 1050 50 <20 * Mean over the decoking period

FIG. 2 shows an embodiment of the invention with a catalytic treatment. The Entkokungsabgas (5) produced in the crevices is passed into a cyclone separator (6) after regular humidification (7), where from the Entkokungsabgas (5) solid components such as coke and dust particles (8) are separated out. The decoking exhaust gas (9) freed from solid components is passed into a catalytic treatment (11), where carbon monoxide is oxidized to carbon dioxide and the remaining coke particles at temperatures dependent on the respective catalyst. The exhaust gas flow guided into the smoke outlet (10) is thus now freed from coke or dust particles and also carbon monoxide.

Claims (3)

Process for the decoking of a cracking furnace (1) for the thermal cracking of hydrocarbons, in which a decoking gas is passed through coked plant parts, whereby a decoking exhaust gas (5) is formed, characterized in that the decoking offgas (5) via a cyclone separator (6) into the combustion chamber the cracking furnace (1) is returned or to a catalytic treatment (11) is performed, wherein the majority of solid particles in the cyclone separator (6) is removed from the Entkokungsabgas. A method according to claim 1, characterized in that water (7) is injected into the Entkokungsabgas (5) in front of the cyclone separator (6). Method according to claim 1 or 2, characterized in that the decoking flue gases (5) of several simultaneously decoked cracking furnaces (1) are brought together and passed through a cyclone separator (6) into the furnace of a cracking furnace (1) or into a catalytic treatment (11).

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