Mineral oils: processes and products – Chemical conversion of hydrocarbons – With prevention or removal of deleterious carbon...
Reexamination Certificate
1999-12-14
2001-03-20
Yildirim, Bekir L. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
With prevention or removal of deleterious carbon...
C196S046100, C196S099000, C196S102000, C202S180000, C202S185100
Reexamination Certificate
active
06203693
ABSTRACT:
The present invention relates to a process and an apparatus for strongly reducing the coke dust in the effluent flue gases and vapours during the decoking of cracking ovens for the production of olefins, particularly ethylene, with a dry content of coke in the gas effluents lower than 50 mg per Nm
3
gas, without other liquid and/or solid effluents to be treated.
It is known that the inside of the tubes of the radiating section of hydrocarbon cracking ovens for the production of olefins is subjected to get dirty because of the very hard carbon deposits that occur during the operation so that they should be removed periodically by a so-called “decoking”.
Such carbon deposits grow up because of the temperature of the process, typically in the range of 850-870° C., with skin temperatures of the coil of 950-980° C. under cleaned conditions of the oven. In operation, such temperature increases gradually because of such deposits and reaches maximum values of 1110-1150° C. At this point the oven must be stopped for “decoking”.
The decoking operation consists of removing the coke by combustion controlled by air and vapour, being careful the combustion temperature is keeping in the range of 700-800° C.
Such operation is very delicate and is carried out gradually, being careful to avoid a too fast combustion of the coke which would damage the material of the tubes. To this end, the decoking is started initially by vapour that is gradually reduced and replaced with air supporting the combustion preferably in three following steps.
At the beginning of the operation, an almost mechanical removal of the coke from the inside surface of the coil is caused by the thermal contraction of the material of the coil which causes coke to crush and/or crumble so that coke scales of a certain size are present in the effluents. Said contraction of the coil occurs because the decoking temperature is essentially lower than the temperature of a normal operation.
When the combustion starts, the size of the particles in the combustion gases decreases progressively and the oxygen content in the effluents is lowered while CO
2
increases.
As the combustion continues and when the removal of coke is almost completed, the amount of air is increased and CO
2
in the effluent begins to decrease. When CO
2
measured in the effluent is lower than 0.5% referred to the volume of the total gas, the decoking is to be regarded as finished.
The mixture of air, coke and flue gases at about 805-810° C. is passed through a heat exchanger that generates vapour at a very high pressure of the order of 120 bar, with the purpose of quenching the reaction gases to prevent undesired reactions. During decoking the jacket of such heat exchanger, so-called “quench boiler”, is filled with water. When passing through such exchanger, the above mixture is quenched to about 340-350° C., whereupon it passes through a cyclone so that the gas escapes from the coke particles before being vented to the atmosphere.
Typical compositions and flow rates of gaseous and solid effluents for ovens of little and medium capacity (25,000 t/y and 50,000 t/y) during the three different steps of decoking are shown in the following Tables 1, 2 and 3.
Up to now the Italian standards prescribed that the solid content in the gaseous effluent should be lower than 30 mg per Nm
3
total volume (vapour plus gas). The regulations of some European countries that will presumably be followed as European Regulations prescribe that the maximum coke content which can be admitted in the effluents is lower that 50 mg per Nm
3
dry gas.
The following Tables 4 and 5 show the maximum content of coke dust equivalent to the above-mentioned prescribed value which can be admitted in the total effluent with reference to the ovens of capacity 25,000 t/y and 50,000 t/y, respectively. The present technology allows ovens of greater capacity (up to 120,000 t/y) to be manufactured, for which similar Tables may be shown.
More severe regulations have drastically reduced the maximum content of coke dust which can be admitted by a factor 10 to 20. From now on 1.5 to 3 mg/Nm
3
instead of 30 mg/Nm
3
according to the preceding regulations are admissible in the vapours.
Actually the need of a suitable apparatus for removing coke particles from decoking gases has been known to those skilled in the art for long time, however, it has not drawn designer's attention as such even delicate operation is essentially regarded as a discontinuous operation of maintenance for cleaning ovens. In fact the decoking is only necessary every 60-70 days for an oil cracking oven, and every 80-120 days for a gas cracking oven.
The time between two successive decoking operations is further prolonged if lower cracking temperatures and vapour dilution ratios (ratio between weight of the vapour and weight of hydrocarbons) greater than the conventional ratios are reached: typically 0.25-0.30 for ethane, 0.45-0.55 for virgin naphtha, 0.60-0.80 or even 1.0 for batches of heavier hydrocarbons.
The nature of the coke to be removed strongly depends on the nature of the hydrocarbon which is more metallic with gaseous batches, more bituminous, tarry, softer with very heavy batches.
By the way, it is necessary to point out that the market does not offer at present any technology tested for such application that guarantees limited investment costs, reduced overall dimensions, easy handling, and reduced maintenance.
In fact, the new regulations make illegal those cracking plants having a conventional cyclone installed in the decoking effluent, whose maximum retention of 99.9% allows 20 mg/Nm
3
to be reached in the best case instead of 1.5-3 mg/Nm
3
prescribed by said regulations.
Therefore, the invention seeks to provide an apparatus which is sturdy, flexible and above all not inclined to get dirty, be corroded, clogged and so on but capable of strongly reducing the coal dust contained in the effluents during decoking, irrespective of any severe problem due to the properties of the solid content to be separated from the gaseous effluent having, as previously mentioned, different nature and morphology during the decoking steps.
A better understanding of the invention will result from the following description with reference to the accompanying drawing that shows the preferred embodiment over other technologies only by way of a not limiting example.
The apparatus and the technology described allows a strong reduction of the coal dust carried by the gas up to the values requested by the specifications of the regulations mentioned above.
Particularly, the invention has been conceived and compared with a lot of alternative solutions and the results have been shown in Tables T1 and T2 mentioned above. It is evident from those tables that the present invention gives optimum results under any point of view.
The most modern cracking technologies actually provide a lining of the coil either consisting of a ceramic coating or materials such as to prevent the deposit of coke. Other technologies intended to reduce the deposit of coke by the addition of a chemical sulphur composition are being developed, especially as far as the gaseous hydrocarbons such as ethane/propane, e.g. dimethyl disulphide, etc., is concerned.
There is still the tendency, that has found many opponents up to now, to convey decoking vapours and flue gases inside the combustion chamber where the coke should be definitively removed. There are, however, troubles such as erosion and dirt problems both in the radiating and convection area, the presence of black particles in the flue gases, etc.
In order to prevent such troubles, there is the tendency, especially in Europe, to use water scrubbers that have, however, a poor dust removal efficiency and above all transfer the pollution from air to water, thus causing problems for the treatment of the latter. At last, in order to make the washing more efficient, there is the tendency to condense all of the vapours with high cost of investment as well as cost of providing pumping means and operation fluids such as cooling wat
Beltrame Dario
Di Cintio Roberto
Picciotti Marcello
Stevens Davis Miller & Mosher LLP
Technip Italy S.p.A.
Yildirim Bekir L.
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