Note: Descriptions are shown in the official language in which they were submitted.
lOS3444
CHEMICAL PROCESS AND APPARATUS THEREFOR
FIELD OF THE INVENTION
This invention relates to chemical processes in general
wherein the reactants when admixed, form a corrosive environment
which is harmful to the reactor and wherein a method has been
devised to prevent such corrosion of the reactor. The inven-
tion also relates to an apparatus suitable to carry out such
method. Typical applications of the method of the invention
are, for example, in the process for making isopropyl alcohol
from propylene and H2SO4, or in the manufacture of hydrochloric
acid from HCL and water, or in the synthesis of urea.
THE PRIOR ART
In the synthesis of urea by reaction of ammonia and
carbon dioxide under urea synthesis pressure and temperatures in
an autoclave, for example, the process' most serious difficulty
is the occurrence of a severe local corrosion of the auto-
clave. While a number of proposals have been made to overcome
this difficulty, such as for example, by selecting special anti-
corrosive materials and passivating them with oxygen in the
autoclave, no satisfactory result has yet been achieved for
reactors having a built-in heat exchanger which is commonly
used for recovering the heat of reaction generated through the
exchanger under urea synthesis conditions.
BRIEF SUMMARY OF THE INVENTION
In accordance with one aspect of this invention there
is provided in a chemical process, wherein a plurality of fluid
A are
raw materials, at least one of which is non-corrosive, io
reacted in the header of a heat exchanger, the inner surface
of which is exposed to corrosive environment, said exchanger
forming a corrosive reaction product while concurrently exchang-
ing the heat of reaction indirectly, the improvement which com-
prises: providing a partition closely spaced from the inner
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surface of said header, flowing a non-corrosive raw material
between said inner surface of said header and one side of
said partition while flowing said plurality of raw materials
on the other side of said partition, whereby said inner sur-
face is protected from contact with said corrosive environmenti
and thereafter merging said plurality of raw materials and
said non-corrosive raw material to form said reaction product.
In accordance with another aspect of this invention
there is provided a chemical process comprising the steps of:
(a) introducing fluid raw materials into an inner member of a
reaction apparatus which includes a reactor, a heat exchanger
with tubes and a header, a feed inlet, a reaction product
outlet, a non-corrosive fluid inlet and a tube baffle dis-
posed across and within said heat exchanger to define an
inner space therein, said inner member conforming to the con-
figuration of said inner space, a plurality of openings
formed in said baffle, a first plurality of tubes attached to
said openings, and a second plurality of tubes attached to
said inner member and positioned correspondingly over said first
tubes spacedly therefrom and in fluid communication therewith;
(b) flowing said fluid raw materials from said inner member
through said second tubes into said first tubes; and (c) intro-
A ducing Daid non-corrosive material fluid between the inside
wall of said header of the heat exchanger and the outer sur-
face of said inner member and hence into said first tubes.
In accordance with another aspect of this invention
there is provided reaction apparatus comprising: a heat
exchanger; a header for said exchanger; an inlet for fluid
raw materials at one end of said header; an outlet for the
reaction products at another end of said exchanger; an inlet
for introducing a non-corrosive fluid; a tube baffle disposed
across the interior of said heat exchanger to define an inner
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space therein; an inner member inserted in said inner space
and closely spaced from the inside surface wall of said header;
said inner member substantially conforming to the configuration
of said inner space; a plurality of openings formed in said
baffle, a first plurality of tubes attached to said baffle
at said openings; a second plurality of tubes attached to said
inner member and positioned correspondingly to said first
tubes, spaced therefrom and in fluid communication therewith.
By way of added explanation, the present invention
in one aspect resides in an improvement in chemical process
and apparatus therefor whereby a plurality of reactants
including at least one non-corrosive reactant are reacted in
a reaction zone to obtain a corrosive reaction mixture, while
concurrently indirectly exchanging heat. A partition or inner
member is provided closely spaced from the surface of said
reaction zone which is exposed to corrosive environment, and
a non-corrosive reactant is flowed in the spacing between
the partition and the said surface in order to prevent the
surface from being corroded. The reaction zone, or mixing
zone, is located in the header portion of the heat exchanger.
The apparatus of the present invention is actually an
improvement in a reactor having an inlet for the reactants at one
end and an outlet for the reaction products at another end, an
inlet for introducing a non-corrosive fluid and a multi-tube heat
exchanger. The improvement provides an inner member which is
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positioned within the header of the exchanger or of thereactor, with walls closely spaced therefrom, and conformed
to t:he configuration of the inner space defined by the inner
surface of the mixing zone of the reactor and the tube sheet
or baffle of said multitubular heat exchanger disposed across
the interior of the reactor. Further provided is a plurality
of smaller tubes connected to said inner member and in fluid
communication with the tubes of the heat exchanger.
The reactor with the improvement afforded by the
present invention can be of any conventional type, namely a
vertical, horizontal or inclined reactor.
The present inventlon will be described hereinafter
in greater detail using as illustrative example the synthesis
process of urea from ammonia and carbon dioxide, although the
present invention is not limited to merely the urea synthesis,
but is applicable to many other e~uivalent chemical processes.
For example, the invention is applicable to the synthesis of
iso-propyl alcohol from propylene and sulfuric acid. The
same apparatus used for urea synthesis and described herein-
after, can be used, in which case, however, a propylene flow
is maintained in the spacing between the header of the exchanger
and the inner member contained therein. Another application
resides in the preparation of hydrochloric acid from hydrogen
chloride and water; again, the same apparatus can be used, in
which case a water flow is maintained in the said spacing.
THE DRA~INGS
For a better understandlng of the invention, a few
embodiments thereof will be described more fully hereinbelow
with reference to the accompanying drawings, in which:
Fig. 1 is a schematic cross-sectional, partial
elevational view of a heat exchanger of a reactor, containing
lOS3~4
the improvement in accordance with one embodiment of the in-
ntiOn;
Fig. 2 is a fragmentary enlarged sectional view of
the exchanger shown in Fig. l;
Fig. 3 is a fragmentary enlarged sectional view of
another embodiment of heat exchanger of the invention; and
Fig. 4 is a fragmentary enlarged sectional view of
still another embodiment of the exchanger of the invention.
Taking the synthesis of urea as an illustrative
example, ammonia and carbon dioxide (and ammonium carbamate
solution, if necessary) are fed into a mixing zone located in
the upper part of the inner member through an inlet positioned
at one extremity of the header. From the mixing zone, the
reactants flow down along a plurality of tubes connected with
the inner member. If desired, the reactants may be mixed
prior to being introduced into the mixing zone. Liquid
ammonia is supplied at one end of the header and is caused to
flow down through the spacing defined by the inside surface
of the header and the outside surface of the inner member,
and then passes between the outslde of the inner member and
the header to be ultimately fed into and run through the
clearances between the tubes of the heat exchanger and the
tubes integrally connected with the inner member. Since the
free ends of these latter tubes open into the exchanger tubes
- or are communicating therewith through small clearances, the
liquid ammonia is subsequently mixed with the reactants dis-
charged from the tubes of the inner member. The contact
effected by the inside surface of the header and by the tube
baffle with the flow of liquid ammonia is sufficient to in-
hibit corrosion.
Reffering now speclfically to Fig. 1, there is
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shown in cross-section the upper part of a heat exchanger con-
taining an embodiment of 1:he invention, which may be used in
the urea synthesis. The exchanger is constructed so that,
prior to feeding into the synthesis zone, liquid ammonia,
carbon dioxide and an aqueous ammonium carbamate solution
(under urea synthesis pressure) are passed inside the tubes
of the multitubular heat exchanger from which the heat
generated during the formation of ammonium carbamate from
carbon dioxide and ammonia is recovered as steam on the shell
side of the heat exchanger.
Liquid ammonia line 1, carbon dioxide line 2 and
aqueous ammonium carbamate solution line 3 are connected to-
gether in an inlet zone 4 for admission of the fluid mixture
into a header 5. After mixing, the carbon dioxide is not
entirely converted into liquid carbamate under the normal
pressure used for urea synthesis, but there remains a sub-
stantial proportion of carbon dioxide gas together with an
excess amount of ammonia. The resulting gas-liquid mixture
causes severe corrosion of the tube baffle 7, in particular
where tubes 9 of the exchanger are welded to the tube baffle
7. Branch lines 6 from line 1 introduce liquid ammonia into
the header 5 through its top for controlling the corrosion
thereof. Horizontal tube baffle 7 extends across the entire
header 5 to define a space therewith. An inner member 8 made
of a corrosion-resistant material such as an 18 Cr-8 Ni stain-
less steel is inserted and positioned in the space thus
defined; fastening means such as bolts or the like secure it
to the header 5. Inner member 8 is open at its top for
admission of ammonia, carbon dioxide and ammonium carbamate
solution thereinto. Liquid ammonia flows down in the clearance
or spacing kept between the inner member 8 and the inside
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surface wall of the header 5 in an amount sufficient to main-
tain an uninterrupted flow, The liquid ammonia accumulates
on the tube baffle 7, and then flows down the plurality of
tubes 9 through a plurality of openings 10 formed in the tube
baffle 7, or more specifically through the gaps formed between
these tubes 9 and tubes 11 which latter are integrally
connected with the bottom surface member 8, The gas-liquid
mixture comprising ammonia, carbon dioxide and ammonium
carbamate solution is discharged from the openings existing
in the lower end of the tubes 11, whereby it is mixed with
the flow of liquid ammonia which is delivered down the gaps
between tubes 9 and tubes 11. The distance through which the
tubes 11 extend into tubes 9 is predeterminedly chosen to
provide an adequate protection against corrosion of the
header walls while avoiding a loss in the heat transfer
effect, Alternatively the extremities of tubes 11 may open
spacedly over the top ends of the exchanger tubes 9. As
mentioned previously, the heat generated by the reaction and
removed by the exchanger is utilized to produce steam,
Fig, 2 is a fragmentary enlarged view further
illustrating the relationship between tube baffle 7, and
tubes 9 and tubes 11 in the embodiment shown in Fig. 1, Tubes 9
are joined such as at welds 12 to the wall of the openings 10
in the baffle 7 so as to extend downwardly therefrom, and
tubes 11 are individually inserted into the respective tubes
9, Tubes 11 may be suitably thinner in wall thickness and
are open at their lower end, The solid arrows shown in
figure 2 indicate the main mixed flow, while the broken
arrows indicate the flow of the liquid ammonia which passes
through tubes 9 and the smaller tubes 11. Both flows merge
together below the lower extremity of the tubes 11.
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Fig. 3 shows a modification similar to ~he previous
embodiment except that tubes 11 have fitting means or caps 13
which bear against the openings formed in the bottom of the
inner member 8 and into which the tops of the tubes 9 are fit
so as to have them protrude a little into the member 8. In
addition, tubes 9 are formed with side apertures 14 for the
passage of liquid ammonia as indicated by the arrows and
broken lines. This arrangement is advantageous in that the
inner member 8 and the tubes 11 can be manufactured separately
from each other.
In a further embodiment of the invention, tubes 11
are connected with the bottom of the inner member 8 as clearly
shown in figure 4 and are positioned over the openings of
tubes 9 ~joined with baffle 7) leaving a clearance between the
two tubes 11 and 9. Through the clearance liquid ammonia
flows into tubes 9.
~ hile in the above description, both flows of the
gas,liquid mixture and of the liquid ammonia are directed
downwardly, these flows may both be directed upwardly with
equally satisfactory effect. Furthermore, although the above
desc~ibed embodiments of the apparatus are of the so-called
vertical type, the reactor may also be disposed in the
horizontal or in an inclined position. The gas-liquid mixed
flow in the inner member 8 or the liquid ammonia flowing
between the header of the heat exchanger and the inside
member 8 can be introduced into the tubes of the heat exchanger
provided in a reactor of horizontal or inclined type in the
same way as in one of the vertical type, because of the
pressure exerted by the continuously introduced non-corrosive
single flow,
The invention has been described hereabove as
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utilizing the heat exchanger specifically for removal of the
heat of reaction generated; however, it will be appreciated
that it can equally be utilized for other purposes: in the
case of urea synthesis for example, when unreacted ammonium
carbamate contained in the melt is heat-stripped with carbon
dioxide by introducing it into the shell side of the heat
exchanger to decompose into ammonia and carbon dioxide, the
ammonia can be replaced by a small quantity of carbon dioxide.
From the foregoing description, it will be under-
stood that the invention provides an improved structure forreactors having a built-in heat exchanger, which prevents
corrosion of the inside of the wall of the header of the
exchanger and which also facilitates replacement thereof in
the event this is needed.
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