Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including heat exchanger for reaction chamber or reactants...
Reexamination Certificate
1999-09-03
2001-11-27
Yildirim, Bekir L. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including heat exchanger for reaction chamber or reactants...
C422S200000, C422S214000, C202S114000, C208S132000
Reexamination Certificate
active
06322760
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a process for the pyrolysis of a hydrocarbon feed containing at least two carbon atoms which is carried out simultaneously with decoking of the coke deposited on the reactor walls.
It is of particular application to the continuous production of acetylene or acetylenic compounds such as methyl acetylene.
In processes for the high temperature thermal transformation of hydrocarbons containing at least one carbon atom, for example pyrolysis between 900° C. and 1500° C. or steam cracking at about 850° C. near the end of the heating zone, coke forms and deposits on the surface of the reactor walls. The reactor is then decoked, normally using an air/steam mixture at temperatures which are usually below 900° C., attempting in the case of metal furnaces to avoid any overheating or hot spots which could damage the metal tubes of the furnace. Exothermic decoking thus requires the whole unit to be shut down and in particular, it requires the furnace to be disconnected from the downstream heat exchangers, reducing the total productivity of the unit. Further, safety regulations require the hydrocarbon introduction lines to be disconnected and replaced by air introduction lines, thus requiring a very long downtime for the unit.
The same disadvantages are there when rebuilding the unit for the pyrolysis phase, with the additional necessity of purging the reaction zone and lines with an inert gas.
Pyrolysis of hydrocarbons containing at least two carbon atoms to produce olefinic or acetylenic hydrocarbons has been described, in particular in our French patent application FR-A-2,715,583 (U.S. Pat. No. 5,554,347), European patent EP-A-0 733 609 and FR 95/15527, which are hereby incorporated by reference.
The prior art is illustrated by patents EP-A-0,542,597 and FR-A-1,501,836.
Pyrolysis reactors of ceramic material have been used in which non impermeable walls which are advantageously of ceramic material determine channels in which the feed and the reaction effluents circulate. These walls advantageously have a shape which is adapted to create turbulence and which, for example, comprise cells or cavities about the heating means. These latter are generally sleeves containing an electric heating element or a gas burner.
These high technology reactors, however, require high investment costs and their energy requirement, in particular of electrical energy, results in high operating costs. European patent application EP-A-0 733 609 describes the possibility of using a steam cracking effluent, as a feed for the pyrolysis reactor as it already contains unsaturated hydrocarbons. The energy required to convert the feed to acetylenic would then be greatly reduced.
However, industrial steam cracking must be stopped every two to three months for decoking. A pyrolysis furnace operating at a higher temperature must be decoked more frequently, for example every four to five days. During the decoking step, the furnace must be isolated. Unfortunately, sealing valves which operate between 800° C. and 900° C. do not exist. An alternative thus consists of sending the effluent from the steam cracker to the pyrolysis furnace, following cooling by passage through a transfer line exchanger, but the benefit of using hot gas is lost and the gain is thus small. Further, the dead volume of the transfer line exchanger encourages secondary reactions to the detriment of the yield of ethylene.
A further disadvantage is linked to the frequency of tube decoking, every two to three months. At the end of a cycle, the inside of the tubes is covered with a thick layer of coke. Coke tends to detach itself at times and is entrained by the gas, stream at speeds which are of the order of 200 m/s, risking damage to the ceramic material sleeves in the pyrolysis furnace downstream of the steam cracking furnace.
SUMMARY OF THE INVENTION
One aim of the invention is to provide a process which can pyrolyse a hydrocarbon feed without stopping the unit, but which can decoke the unit.
A further aim is to reduce the investment and operating costs of the unit.
A still further aim of the invention is to maintain the temperature of the facility substantially constant during operation to avoid thermal stresses which would otherwise occur, in particular during use of a gas containing oxygen for the decoking step, which is an exothermic reaction, while the pyrolysis step uses an endothermic reaction.
Because of the presence of non impermeable (and therefore cheap) walls in the pyrolysis zone, it has been observed that it is possible to carry out a continuous pyrolysis process for a hydrocarbon feed and a reaction zone decoking process with no deleterious effects.
It has been observed that by combining a steam cracking furnace operating with a high degree of dilution of the feed with steam and at least one very high temperature pyrolysis furnace, with no transfer line exchanger between the steam cracking furnace and the pyrolysis furnace, excellent selectivity towards the desired products is obtained for a reduced overall enthalpy requirement.
More precisely, the invention concerns a continuous pyrolysis and decoking process carried out in a reaction zone comprising a pyrolysis zone (
40
) which is of refractory material, which zone is elongate in one direction (one axis), and which comprises a heating zone and a cooling zone following the heating zone, the heating zone comprising at least two rows (
1
,
2
) which are substantially parallel to the axis separated by a wall (
70
), which is advantageously non impermeable, of refractory material and located between two successive rows, at least one of said rows (
1
) receiving hydrocarbons and steam, at least one other (
2
) of said rows receiving essentially steam, said rows comprising heating means (
8
) surrounded by sleeves (
7
) which are substantially parallel to each other and substantially perpendicular to the reactor axis, wherein coke is deposited in the reaction zone, the process being characterized in that a hydrocarbon feed comprising at least one hydrocarbon containing at least two carbon atoms is circulated at a temperature which is sufficient for steam cracking in a steam cracking zone containing at least two steam cracking tubes, at least one of said tubes communicating with a supply for feed and steam and being heated so as to crack the feed, at least one other of said tubes communicating with a supply for a fluid consisting essentially of steam and being heated so as to decoke said tube on which coke has been deposited, the flow rate of steam in the steam cracking tube being such that the steam/feed weight ratio is in the range 0.5 to 20, a steam cracking gas stream comprising hydrocarbons and steam and a decoking gas stream comprising essentially steam are obtained and the steam cracking stream is circulated in at least one row of the heating zone in the pyrolysis zone (
40
) which is adjacent the steam cracking zone so as to pyrolyse the steam cracking stream and to produce a temperature of at least 850° C. at the outlet from said heating zone, and the decoking stream is circulated in at least the other row of the heating zone so as to decoke said row at least in part, and to produce a temperature of at least 850° C. at the outlet from said heating zone, and in which hydrocarbons comprising at least one acetylenic compound, for example acetylene, are recovered, along with a decoking effluent.
In one feature of the invention, the temperature at the outlet from the steam cracking zone is generally lower than the temperature at the outlet from the heating zone of the pyrolysis zone.
The temperature in the steam cracking tube or tubes in which steam cracking of the feed is carried out is advantageously kept substantially equal to the temperature in the tubes in which decoking is carried out. Similarly, the temperature in the row or rows in which pyrolysis of the gas stream leaving the steam cracking zone is carried out is advantageously kept substantially equal to the temperature in the row or rows in which decoking is carried out.
Accor
Burzynski Jean-Pierre
Busson Christian
Dubois Christian
Marache Pierrr
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Yildirim Bekir L.
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