Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUNG 0~ THE INV~NTION
rrhe present invention is related to an apparatus for
mixing very rapidly reacting gases at the inlet of an oxygen
reforming reactor, where synthesis gas is produced.
In the oxygen reforming technology, a process gas is
subjected to a partial oxidation reaction by reacting with
a free oxygen containing gas, in a refractory lined reactor,
packed with a bed of catalyst, ana operating under essential-
ly adiabatic conditions. Said process gas is composed of ga-
seous hydrocarbons, and possibly of steam, hydrogen, carbon
oxides and nitrogen. Said oxygen containing gas has a high
percentage of molecular oxygen, and possibly nitrogen, steam
and carbon dioxide. In industrial practice, it is found advan-
tageous to p~eheat reacting gases as much as possible before
injecting into the reactor, in order to save on oxygen con-
sumpt~on, and thereby impro~ing the overall yield of the re-
action.
It is then found that beyond a certain preheat tempe-
rature, which may vary according to other process parameters,
the reaction starts instantaneously as the two reacting gases
come into contact in the mixing zone of the reactor, long
before they reach the catalyst. It is known in the prior art
that the reaction occuring in the gas phase in said mixing
zone may lead to undesirable effects such as excessive tempe-
ratures and the formation of carbon particules which deposit
on the catalyst and reduce its performance. In a great number
of cases, these undesirable effects are due to the gas mixing
- apparatus used in the mixing zone, because it mixes the reac-
ting gases at a slow rate compared to that of the reaction
30, in the gas phase.
In the oxygen reforming reactors presently used in
the industry, which can be classified into two categories,
said undesirable effects are avoided for reasons specific to
each category:
(a) In the preparation of ammonia synthesis gas by a
primary steam reforming followed by a secondary reforming
with air, the inlet temperature of the reacting gases is suf-
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ficiently high to initiate the reaction instantaneously in the
gas mixing zone; however, the oxygen partial pressure in the
mixture of reacting gases is very low, in the order of 1.0 to
1.5 atm only, and the process gas flowing from the primary
reformer does not contain any hydrocarbon other than methane,
which is much more stable thermally than other hydrocarbons;
furthermore, the methane partial pressure in the mixture of
reacting gases i5 very low, in the order of 1.3 to 1.8 atm.
Under these conditions, no particular precautions are necessary
when mixing the two reacting gases, except that the upper layer
of catalyst, as well as the refractory lining in the gas mixing
zone, are designed to withstand temperatures much higher than
that of the catalyst bed.
(b) In the preparation of synthesis gases by direct
oxygen reforming of hydrocarbons, the preheat temperature of
the two reacting gases is generally limited to about 450C, and
an appreciable amount of steam is introduced with the process
gas, and usually also with the oxygen stream. Under these con-
ditions, and due to the absence of hydrogen in the process gas,
the reaction barely starts in the gas phase before reaching the
catalyst, and the mixing devices presently used are able to
achieve an essentially homogeneous mixture over the catalyst
bed. However, such mixing devices would not be satisfactory
if the reacti~n were to start appreciably in the gas phase,
by virtue of higher preheat temperatures, that is higher than
500C, and lower steam ratios.
In the French patent application serial number 77-08459,
corresponding to Canadian patent 1,076,361, the operating con-
ditions of the secondary oxygen reforming are precisely more
- 30 severe than those presently practicefl in the industry, essentially
due to the much higher oxygen partial pressure in the reacting
mixture, that is in the range of 6 to 8 atm, and to the higher
methane partial pressure in said mixture, in the order of 10
to 15 atm, and also due to the fact that the process gas may
contain hydrocarbons heavier than methane, while containing
an appreciable percentage of hydrogen and feeding in the reac-
~` tor at a temperature higher than 600C. Therefore, for the
~- conditions of the aforesaid patent applications, a new concept
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of gas ~ixing apparatus is required~
In a similar way, if one i9 aiming to produce
a ~Aethanol synthesis gas or a synthesis gas having a low
H2/CO ratio by direct oxygen reforrning of a hydrocarbon
feedstock, usin~ high preheat temperatures and low steam
rates, a new concept of gas mixing apparatus is also required
for the same reasons as exrlained above.
~he object of the present invention is precisely
to fulfil~ such a need, by providing a new concept of gas
mixing apparatus which enables to obtain quasi instantane-
ously an essentially homogeneous gas mixture before the par-
tial oxidation reaction proceeds significantly.
In the field of partial oxidation of hydrocarbons
for the production of synthesis gases, the reaction of oxygen
with the feedstock is carried without the aid of a catalyst,
and therefore the small amount of carbon particules that may
be formed by side reactions is not detrimental to the success~
ful operation of the process. ~urthermore, the reactors in
these processes operate at appreciably higher temperatures
: 20 ( 13pO-1500 C ) than the oxygen reforming reactors ( 900-
1100 C ), and therefore the main concern is to protect the
mixing apparatus used therein from the excessive heat of the
reactor, by an internal cooling water circulation for example.
In said partial oxidation field, several mixing
devices have been conceived, such as described in US patents
2,582,938 - 2,7q2,l49 - 2,621,1177- 2,~38,105 -
and in British patents 7i26,206 - 780,120 - 832,385. In these
mixing devices, the oxygen stream is generally injected
~` through a single channel, which must have a large cross sec-tion for handling the total flow: consequently, even if the
oxygen is injected at high velocity through said cross section,
the rate of dispersion of the oxygen molecules in the reacting
gas mixture is slow compared to the reaction rate. In addition,
the oxygen jet, as it leaves the orifice, is surrounded usu-
ally by low velocity gas in the reaction chamber, which is
; not favourable for rapid dispersion of the oxygen molecules.
In the particular case of US patent 2,772,149 the
mixing of the reacting gases is achieved on the surface of a
porous diaphragm or barrier; this has the advantage of a rapid
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mixing of the reacting gases. ~lowever, due to the necessa-
rily low velocity of the gas stream in the pores o~ the dia-
phragm, the reaction is procee~ing to a great extent on the
outlet surface of said diaphragm, which accordingly should
be designed to withstand the corresponding high temperatures.
In large capacity plants, this ~ystem would require a very
large surface ~or the porous diaphragm, which would make it
unpractical and expensive.
In the case of British patent 726,206 the mixing
apparatus is particularly designed for a liquid feedstock,
which is injected in the reaction chamber as a thin film sub-
jected to impingement from both sides by jets of oxygen at a
certain angle. Such impingement is particularly suitable to
- disperse the liquid feed into very fine droplets; however,
this does not achieve very rapidly a homogeneous gas mixture,
because the oxygen jets are not surrounded by high velocity
reacting gases. If the feedstock were a gaseous hydrocarbon,
and especially if containing/appreciable amount of hydrogen,
the corresponding volume flow rate would be much larger than
; 20 the equivalent liquid feed and than the oxygen flow rate; as
the oxygen jet has only one of its sides contacted with the
process gas jet, the rate of dispersion of the oxygen molecu-
les would be too slow compared to the reaction rate.
For all the reasons developped above, the mixing de-
vices conceived for partial oxidation processeæ are not sui-
table for oxygen reforming of a gasified hydrocarbon feedsto-
ck, under severe conditions as outlined above, because the
~- risk of carbon formation should be entirely eliminated.
SUMMARY OF THE INVENTION
. ~ .
An objeot of the present invention is to provide an
apparatus for mixing very rapidly the reacting ga~es at the
inlet of an oxygen reforming reactor, in which a highly exo-
thermic reaction is taking place between a rich free oxygen
stream and a process gas stream containing hydrocarbons, and
possibly steam, hydrogen and carbon oxides.
` To obtain this object, the apparatus i~ essentially
` composed of two parts:
a) A refractory envelope in which the process gas is injected
tangentially, thereby subjecting it to a helical movement in-
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side said envelope.
b) A distributor and a multitude of parallel channels, all
located inside said envelope; the oxygen rich gas is injec-
ted in said distributor, which feeds one end of said paral-
lel channels, and it issues at the other end of each channel
through an orifice of which at least one dimension is less
than 20 mm, and preferably less than 8 mm.
The above restriction regarding the size of the out-
let orifice of the rich oxygen stream, at the point where it
comes into contact with the process gas, imparts to the oxy-
:
gen stream a shape of a thin layer or a thin thread, therebydispersing the oxygen molecules within a very short distance
in said process gas.
While complying with the above set restriction, said
outlet orifice of the rich oxygen stream may have any shape
such as : circle j; ellipse, slot, cross, star, crescent,
and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
, ~
The apparatus of the present invention i8 illustrated
schematically in the dra~ings by means of cross sections,
wherein:
Fig. 1 represents a vertical cross section of the
mixing apparatus ; for the sake of simplification, only three
parallel channels have been represented, and the inlet nozzles
of the process gas have not been represented.
~ ~ Fig. 2 represents a cross section along A-A' of Fig.
',J' ~ 1~ and includes all the parallel channels, supposed to be-
cular, and two process gas inlet nozzles.
Fig. 3 represents a cro~ section along A-A' of Fig.l,
; on ~hich the parallel channels have a cross section in the
form of a crescent, instead of a circular form as on Fig.l.
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DESCRIPTION OF THE PRE?~R~ BODI~:E~TS
The basic concept und~rlayin~ the present invention
is that, at the point where the t~o reacting gases come into
contact, the cross section of the oxygen channel should be
such as to impart to the oxygen stream a shape of a thin la-
yer or a thin thread, surrounded on all sides by high veloci-
ty process gas, thereby dispersing ~Jithin a ~ery short dis-
tance the oxygen molecules in said process gas. As the oxygen
flow rate used in industrial plants is very large, this implies
that said oxygen stream be injected in the process gas throu-
~h a multitude of parallel channels, each one ending with an
outlet orifice having at least one very reduced dimension.
Said outlet orifice may have the shape of a slot, continuous
or discontinuous, having a width of less than 20 mm, and pre-
ferably of less than 8 ~m; said orifice may also have a cir--
; cular or elliptical shape, of which the smallest diameter
should be less than 20 mm, and preferably less than 8 mm.
Furthermore, in order to increase the dispersion rate of oxy-
gen into the process gas, the latter is flowing at high ~elo-
city in a helical movement around the zforesaid parallel chan-
nels, such movement being obtained by a tangential injection
of the process gas along the internal surface of the apparatus.
Due to the elevated temperature and pressure prevai-
ling in the oxygen reforming reactor, and the need to position
the mixing apparatusclose to the catalyst, the latter and the
former are both located in the same metallic vessel, interna-
lly lined ~ith a layer of brickR or refractory cement. Further-
` more, the catalyst bed requires a large diameter, whereas the mixing apparatus needs only a reduced space, the volume of
which is about 3 to 9 per cent of the catalyst volume.
The apparatus of the present invention is thus compo-
sed of two parts: firstly an envelope internally lined with
refractory, in which the process gas is injected, ans second-
ly a distributor and a multitude of parallel channels inwhich
the oxygen stream is flowing. The shape of the envelope can
be either cylindrical, with a circular or elliptical cross-
section, or like a truncate~ cone. The tie-in ~ith the cata-
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lyst zone is usually achieved through a truncated conical
section.
In order to describe the invention in more detail, ref-
erence will now be made to the drawings, which illustrate dia-
grammatically useful forms of mixing apparatuses in accordancewith the present invention. In the drawings, like parts are
designated by like numerals throughout.
Fiy. 1 of the drawings is a vertical section through
a mixing apparatus embodying a cylindrical round shell. Fig.
2 and Fig. 3 represent horizontal sections along A-A' of Fig.
1, and embodying different shapes of parallel channels for
the oxygen stream.
Fig. 1 represents the shell in its preferred embodi-
ment: the cylindrical round metal wall 1 is connected at the
lower side with a truncated conical section 3 to tie-in with
; the catalyst section of the reforming reactor, and it termin-
; ates at the upper side with a spherical end 2 through which
passes line 5 where the oxygen stream is flowing. The inlet
connections for the process gas have not been repreF~ented on
Fig. 1 to facilitate its reading; however, their axes are
located at level A-A', and they are positioned in such a way
as to impart a tangential movement to the process gas along the
P~ inner refractory wall 4, as indicated on Fig. 2 and Fig. 3.
~`~~ As the process gas is then flowing downward, it follows a
; 25 helical movement inside the shell, around and between the
multiple channels containing the oxygen rich gas.
The oxygen rich gas is admitted through line 5, which
ends at distributor 6 having here the shape of a cone, but
could also have other shapes of revolution around the axis
of said shell such as a torus, a sphere or a saucer. The
central distributor 6 feeds all the multiple channels which
- are parallel to the axis of the distributor and welded to it,
and regularly distributed over its area. The diameter of
distributor 6 is appreciably smaller than that of the inner
refractory lining 4, in order to leave enough space for the
process gas flow. The distributor and the channels are all
made of an adequate stainless and refractory alloy, and they
are kept in position by any appropriate mechanical means.
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According to a first representation of the present in-
vention, shown on Fig. 1, each oxygen channel is made of a pipe
7 of a small diameter, that is less than 80 mm. diameter, end-
ing by a double-edged fish-tail 8; the oxygen rich gas flows
between the two edges, through the slot thus formed, said
slot being continuous or discontinuous, and having a width of
less than 20 mm, and preferably less than 8 mm. Each fish-
tail is oriented either in a parallel direction to the nearest
inner refractory wall, or at a small angle with respect to the
latter, in the direction which facilitates the helical flow of
the process gas. Only three channels have been represented on
Fig. 1, for the sake of clarification, but Fig. 2 represents
all the oxygen channels intended on Fig. 1.
According to a variation of the above representation,
the outlet end of the multiple channels has a form of a cross
or a star, each branch of said cross or star being double-
edged, and the slot thus formed having a width of less than 20
mm, and preferably less than 8 mm.
The above described representation is particularly
suitable for small or medium capacities, requiring a limited
number of parallel channels, each having a small diameter. When
the rich oxygen gas rate is very large, it is necessary to in-
crease either the number or the diameter of said channels, or
; both simultaneously; the effect of this is to counteract the
~- 25 helical movement of the process gas, thus converting it very
; rapidly to a movement parallel to the channels. One way to
correct this situation, at least partially, is by imparting to
some channels, mainly those located closer to the inner refract-
ory wall, a helical form oriented in the same direction as that
~; 30 of the process gas.
A second representation of the present invention is
shown on Fig. 3, which is a cross section along A-A' of Fig.
1, but with different shape for the oxygen channels. Said
second representation is conceived to avoid the above mentioned
drawbacks, and therefore is suitable for very large capacities.
Each channel 10 for the oxygen rich gas has a cross section in
the form of a double-edged crescent, oriented in such a way as
to facilitate the helical flow of process gas between the
channels. Furthermore, the slot through which the oxygen
rich gas is injected in the process gas is less than 20 mm
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wide, and preferably less than 8 mm wide, and is obtained
by narrowing the space between the two parallel edges of
the crescent, and therefore has the same shape as the channel
itself. In this manner, each layer or film of oxygen rich
gas, at the instant it leaves the slot, is trapped between
two la~ers of process gas, there~y is being dispersed verv
rapidly in the latter.
As for the first representation, it is possible also
for the second representation to impart to at least some
channels a helical form, in the same direction as that of
the process gas, in order to faciliate the dispersion of the
oxygen rich gas in the latter, while reducing the impact of
the hot reacting gases on the catalyst in the reaction zone.
It will be appreciated, from the above description,
that the smallest dimension of the orifice at the outlet end
of the oxygen channels, or the width of the slot thereat, will
depend essentially on the severity of the oxygen reforming
reactionl that is the reaction rate of oxygen with the process
gas in the gas phase, and the risk of carbon formation and
excessive temperatures that may result from a high reaction
rate in a very heterogeneous gas mixture. Accordingly, the
higher the reaction rate, or the risk of carbon formation,
and the smaller should be said smallest dimension of the outlet
oxygen orifice, or said slot width. In the most severe cases,
said smallest dimension or said slot width should be less than
3 mm.
Similarly, the velocity of the rich oxygen stream
through the outlet orifice of the parallel channels, and the
velocity of the process gas around said channels, shall take
into account the severity of the oxygen reforming reaction.
It is contemplated that in all cases, said velocity of the
oxygen stream should be at least 50 meters/second, and said
velocity of the process gas, expressed per cross section of
free area, shoul~ be at least 30 meters/second.
While particular embodiments of the present invention
have been described, it will be understood that this invention
is not limited thereto, and it is therefore contemplated to
cover by the appended claims any and all modifications that
may be made, as may fall within the true spirit and scope
of the present invention.
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