Process for the production of aromatic compounds in a moving...

Details Process for the production of aromatic compounds in a moving... Process for the production of aromatic compounds in a moving...

2000-11-30

2003-12-09

Dang, Thuan D. (Department: 1764)

Chemistry of hydrocarbon compounds

Aromatic compound synthesis

From alicyclic

C585S431000, C585S432000

Reexamination Certificate

active

06660895

ABSTRACT:

The invention relates to moving bed processes for producing aromatic compounds from hydrocarbons, in which a hydrocarbon feed supplemented by a hydrogen-rich gas is transformed. More specifically, it relates to continuous reforming or still more specifically to BTX (Benzene, toluene, xylene) production with continuous catalyst regeneration.
More particularly, it relates to the step in which principally the naphthenes contain in the feed are dehydrogenated, i.e., the step carried out in the first reforming reactor, or to the production or aromatic compounds.
The catalyst generally comprises a support (for example formed from at least on refractory oxide, the support also possibly including one or more zeolites), at least one noble metal (preferably platinum), and preferably at least one promoter metal (for example tin or rhenium), at least one halogen and optionally one or more additional elements (such as alkalis, alkaline-earths, lanthanides, silicon, group IVB elements, non noble metals, group IIIA elements, etc.). As an example, such catalysts contain platinum and at least one other metal deposited on a chlorinated alumina support. In general, such catalysts are used to convert naphthenic or paraffinic hydrocarbons which can be transformed by dehydrocyclisation and/or dehydrogenation, in reforming or for the production of aromatic hydrocarbons (for example production of benzene, toluene, or ortho-, meta- or para-xylene). Such hydrocarbons originate from fractionating crude oil by distillation, or from other transformation processes such as catalytic cracking or steam cracking.
Such catalysts have been widely described in the literature.
Many chemical reactions occur during the reforming process. They are well known, reactions which are beneficial for the formation of aromatic compounds and improving the octane index which can be cited are naphthene dehydrogenation, cyclopentane ring isomerisation, paraffin isomerisation, paraffin dehydrocyclisation; the deleterious reactions include paraffin and naphthene hydrogenolysis and hydrocracking. The reaction rates of such a variety of reactions are very different and are highly endothermic for dehydrogenation reactions and exothermic for the other reactions. For this reason, the reforming process is carried out in a plurality of reactors which are subjected to varying temperature drops.
Experience has shown that naphthene dehydrogenation reactions occur in the first reactor or reactors.
Thirty years ago, reforming processes or aromatic production processes were carried out at 40 bars, while twenty years ago, it was 15 bars, and today's reforming reactors operate at pressures below 10 bars, in particular in the range 3 to 8 bars.
However, such a reduction in the hydrogen pressure is accompanied by more rapid catalyst deactivation by coking. Coke, a compound with a high molecular weight and primarily based on carbon and hydrogen, is deposited on the active sites of the catalyst. The H/C mole ratio of the coke formed is in the range about 0.3 to 1.0. The carbon and hydrogen atoms form condensed polyaromatic structures with a variable degree of crystallinity depending on the nature of the catalyst and the operating conditions employed in the reactors. While the transformation selectivity of hydrocarbons to coke is very low, the amount of coke which accumulates on the catalyst can be large. Typically, for fixed bed units, such amounts are in the range 2.0 to 20.0 or 25.5% by weight. For slurry reactor units, these amounts are in the range 3.0 to 10.0% by weight at the outlet from the last reactor. The coke is mainly deposited in the last or in the last two reactors.
Coke deposition, which is faster at low pressure, necessitates more rapid catalyst regeneration. Currently, regeneration cycles are as short as 2-3 days.
Many patents concern processes for reforming or producing aromatic compounds with continuous or sequential catalyst regeneration.
The processes employ at least two reactors in which a moving bed of catalyst circulates from top to bottom traversed by a feed composed of hydrocarbons and hydrogen, with the feed being reheated between each reactor.
Experience has shown that the first reactor is the home of rapid reactions producing large amounts of hydrogen.
The Applicant's French patent FR-A-2 657 087 describes such a reforming process.
FIG. 1
reproduced in this document (corresponding to
FIG. 2
of FR-A-2 657 087) employs 4 reactors. An initial feed composed of hydrocarbons and hydrogen is circulated through at least two reaction zones disposed in series, side by side, each of these reaction zones being of the moving bed type, the feed circulating successively in each reaction zone, and the catalyst also circulating in each reaction zone and flowing continuously in the form of a moving bed from top to bottom in each zone, the catalyst being withdrawn from the bottom of each reaction zone and being transported in a stream of hydrogen to the top of the next reaction zone, the catalyst that is continuously withdrawn from the bottom of the last reaction zone traversed by the feed then being sent to a regeneration zone.
Referring to
FIG. 1
, the feed composed of hydrocarbons and hydrogen in a set H
2
/HC ratio traverses reactor
1
(
29
) and is re-heated, traverses reactor
2
(
42
), is re-heated traverses reactor
3
(
55
a
), is re-heated, traverses reactor
4
(
55
), and is sent to a separation section.
The catalyst drops into reactor
1
(
29
), is traversed by the feed and is withdrawn from (
29
) via lines (
31
) and (
32
). It is recovered in a hopper (
34
a
), lifted to the upper surge drum (
39
) of reactor
2
via a lifting means (
34
) and (
36
), it flows from the surge drum (
39
) via lines (
40
) and (
41
) towards reactor
2
(
42
); it is withdrawn from (
42
) via lines (
44
) and (
45
), is recovered in a hopper (
47
a
), lifted to upper surge drum (
52
a
) of reactor
3
via a lifting means (
47
) and
49
a
); it flows from the surge drum (
52
a
) via lines (
53
a
) and (
54
a
) towards reactor
3
(
55
a
); it is withdrawn from (
55
a
) via lines (
62
a
), is recovered in a hopper (
47
b
), lifted to upper surge drum (
52
) of reactor
4
via a lifting means (
47
c
) and (
49
); it flows from the surge drum (
52
) via lines (
53
) and (
54
) towards reactor
4
(
55
); it is withdrawn from (
55
) via lines (
62
), is recovered in a hopper, lifted to upper surge drum (
7
a
) of regenerator (
10
) via a lifting means (
60
a
), (
6
a
) and (
6
b
); it flows from this surge drum (
7
a
) via line (
9
) towards regenerator (
10
); it is withdrawn from (
10
) via lines (
16
) and is recovered in a hopper (
17
a
), lifted to upper surge drum (
63
) of reactor
1
via a lifting means (
17
) and (
19
); it flows from this surge drum (
63
) via a line (
66
) to a reduction drum (
20
) where the catalyst at least partially regains it metallic form; finally, it flow via lines (
27
) and (
28
) towards reactor
1
(
29
).
The feed in the reactor(s) for reforming or producing aromatic compounds is generally treated at pressures of 0.1 to 4 MPa, preferably 0.3-0.8 MPa, 400-700° C., preferably 480-600° C., at space velocities of 0.1 to 10 h
−1
, preferably 1-4 h
−1
, and with recycled hydrogen/hydrocarbon (mole) ratios of 0.1 to 10, preferably 3-10, more particularly 3-4 for regenerative reforming and 4-6 for the aromatic compound production process.
Traditionally, after the last reactor, a first separation is carried out between the hydrocarbons and a recycled hydrogen which is re-injected into fresh feed.
The non-recycled effluent undergoes a separation process to produce hydrogen known as exported hydrogen, which may contain up to 10% by volume or preferably 4% by volume of light hydrocarbons such as ethane and propane. By comparison, recycle hydrogen can contain more than 10%, generally more than 12% or 15% by volume of C
2
+
, C
2
H
4
to C
10
aromatic compounds.
The coked catalysts are regenerated.
The catalyst is generally regenerated in three principal steps:
(a) a combustion step wher

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