Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By dehydrogenation
Reexamination Certificate
1998-02-23
2001-02-20
Knode, Marian C. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By dehydrogenation
C585S911000, C585S921000, C585S924000, C422S109000, C422S198000
Reexamination Certificate
active
06191332
ABSTRACT:
The invention concerns a method and apparatus for converting hydrocarbons, particularly to produce olefinic hydrocarbons with at least one double bond, from a charge of aliphatic hydrocarbons with 2 to 20 carbon atoms in the presence of a catalytic composition. The conversion entails an endothermic reaction to produce hydrocarbons, followed by an exothermic reaction to regenerate the catalyst with coke deposited on it. The invention also concerns the use of the apparatus. It more particularly concerns the synthesis of isobutene, which is used especially in the preparation of MTBE (methyl tertiarybutyl ether) with a view to improving the octane number of petrols.
The upgrading of cuts, particularly aliphatic ones with a low boiling point such as the C4 cut from steam cracking or catalytic cracking and the LPGs, explains the importance which may be attached to using hydrocarbon conversion methods which are efficient, selective and economic and which also contribute to the formation of hydrogen.
The technological background is illustrated by patent JP-A-63197534.
The reaction producing olefinic hydrocarbons has been described especially in U.S. Pat. No. 4,704,497. It uses a catalyst containing alumina.
The basic processes used in converting aliphatic hydrocarbons to olefinic hydrocarbons are chiefly dehydrogenation of paraffins. Taken as a whole, the reaction is endothermic, the reaction speed is sensitive to temperature variations and the successive reactions are accompanied by deposition of coke on the catalyst and reduction of the metal oxides contained in the catalyst, with the result that the catalyst is deactivated very rapidly and the cycle is shortened.
One of the problems to be solved is thus how to ensure uniform heating of the reaction zone in the region of 500 to 600° C. so as to obtain the flattest possible temperature profile therein, with the knowledge that the catalyst is sensitive to a temperature increase and may be destroyed when the critical temperature is exceeded.
It has in fact been found that the heat requirement does not remain constant as the endothermic reaction producing olefinic hydrocarbons progresses.
Another problem to be solved relates to regeneration of the catalyst: it must be rapid and of variable frequency according to the reaction temperature, which is directly dependent on the charge to be treated, and thus according to the quantity of coke deposited. Regeneration is generally carried out, e.g., every ten hours. It must be gentle enough to maintain the performance of the catalyst and to minimize its replacement rate.
During the used catalyst regenerating phase it is moreover preferable to reach the coke combustion temperature as rapidly as possible, in order to maintain a substantially constant temperature level right along the tube and hence a homogeneous level of activity, in order to keep the catalyst active for as long as possible.
It has been recommended that the regenerating gases should be introduced at a very high temperature level (600-700° C.), so as not to overcool the catalyst at the reactor inlet and so as to compensate for the shortage of coke required to initiate the reaction.
It has also been recommended that a gaseous or liquid fuel should be added, in a large enough quantity to raise the temperature of the regenerating gas before it enters the reactor.
This solution has the disadvantage of initiating cracking of the hydrocarbon molecules and fostering the appearance of undesirable byproducts.
The object of the invention is to deal with the problems raised above so as to improve the rates of conversion to olefinic hydrocarbons and the durability of the catalyst.
More particularly, the invention concerns a method of producing olefinic hydrocarbons from a charge including aliphatic hydrocarbons with 2 to 20 carbon atoms which can be dehydrogenated in at least one reaction chamber, the chamber having a plurality of substantially parallel tubes arranged in rows and containing a fixed bed of catalyst, characterize in that:
a) a reaction phase producing olefinic hydrocarbons is carried out, during which the possibly preheated charge is circulated in the tubes containing the catalytic composition under appropriate conditions, and an effluent rich in olefinic hydrocarbons is collected:
b) a phase of purging the tubes with at least one appropriate gas is carried out, after the reaction phase and after a catalyst-regenerating phase defined below, and a purge effluent is collected:
c) and a phase of regenerating the catalyst is carried out in the tubes of the chamber under appropriate regenerating conditions, the catalyst being in a fixed bed and having had coke deposited on it during the reaction phase, and a regeneration effluent is recovered,
the method further being characterize in that the tubes are heated during the reaction phase by a plurality of appropriate radiant heating means arranged in layers which are substantially parallel, independent of one another and substantially perpendicular to the tubes, the heating means being adapted to heat a first part (the feed side) of the tubes with a heat flow greater than the mean heat flow of the reaction chamber necessary for the production of hydrocarbons, and adapted to heat a second part of the tubes, downstream of the first part, with a heat flow no more than equal to said mean heat flow, so that the isothermicity of the catalyst is substantially maintained, and any combustion fumes emanating from the heating means eventually discharged from the chamber.
The word “charge” refers to a mixture of aliphatic hydrocarbons and hydrogen or steam.
The term “olefinic hydrocarbons” refers to a hydrocarbon with at least one double bond.
One feature of the process is that 1 to 50% of the length of the reaction tubes, at the feed side, may be heated with a heat flow from 101 to 500% of the mean heat flow of the reaction chamber, and the remaining part of the tubes with a heat flow from 10 to 100% of the mean heat flow.
The mean heat flow of the chamber is defined by the ratio of the power absorbed by the tubes of the chamber for a given reaction to the total external area of the tubes.
From 1 to 40% and preferably 1 to 35% of the length of the reaction tubes at the feed side may advantageously be heated with a heat flow from 120 to 300% and preferably 150 to 200% of the mean heat flow, and the remaining part of the tubes with a heat flow from 20 to 85% and preferably 40 to 75% of the mean heat flow.
Under these conditions the temperature profile right along the tube is substantially flat, allowing for a higher heat demand in the first part of the tube than in the remaining part. In this case use of the catalyst may be optimized relative to the quasi maintenance of its activity for the longest possible time.
The heating means used may advantageously be those described in U.S. Pat. No. 4,664,620, comprising burners of substantially cylindrical, elongated shape with a matrix of ceramic fiber, which burn a mixture of gaseous fuel and air, without a flame, in the interstitial spaces between the fibers and transfer the heat by radiation. The use of these ceramic fiber burners is thus a subject of the invention.
They further have the advantage of liberating little NO
x
and little CO and hydrocarbon during combustion. Their use is also very flexible: the quantity of heat liberated may be controlled and may preheat the charge and the purging and regenerating gases.
In addition they have very little inertia. This is important in the event of inopportune termination of the feeding of the charge, or an excessive coke deposit on the catalyst which may interrupt the feeding of the charge.
Another feature of the process is that a purging phase is normally carried out in between the phase producing olefinic hydrocarbons and the phase regenerating the used catalyst. For this purpose the feeding of the charge to the tubes is reduced or stopped and the tubes are purged at least once with an inert gas such as nitrogen, under flow and temperature conditions such that the temperature of the catalyst remains subst
Burzynski Jean-Piere
Duee Didier
Leger Gerard
Mank Larry
Minkkinen Ari
Dang Thuan D.
Institut Francais du Pe'trole
Knode Marian C.
Millen White Zelano & Branigan P.C.
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