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Патент USA US3093654

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June 11, 1963
J. J. STEGGERDA
3,093,644
PREPARATION OF MELAMINE
Filed Aug. 17, 1962
“
INVENTOR
do/ew /V/V£$ (LSMGGEWA
ATTORNEYS
United States Patent 0 " 1C6
1
2
tained by thermal decomposition of urea is therefore sup
plied to a catalyst bed kept in the ?uidized state by means
3,093,644
Johannes J. Steggerda, Geleen, Netherlands, assignor to
PREPARATION OF MELAMINE
‘
of a current of NH3, after which the resulting vapors,
together with the NH3, are passed through a ?xed cata
lyst bed.
Stated in another way, the present invention represents
Stamicarbon N.V.
_
Patented June 11, 1963
Filed Aug. 17, 1962, Ser. No. 217,709
Claims priority, application Netherlands Apr. 2, 1959
9 Claims. (Cl. 260-2493)
an improvement over the process of 2,760,961 wherein,
instead of using one ?uidized catalyst bed as in the pat
The present invention relates to an improved method
ented process, two catalyst beds are used, ‘one being ?uid—
of preparing melamine by heating urea ‘and/or one of 10 ized and the other ?xed.
the products obtainable by thermal decomposition of
The new process offers the following advantages over
urea, e.g. biuret.
the process of 2,760,961:
It is well known that good yields of melamine can be
(1) The use of a combination of a ?uidized catalyst
obtained from urea or from any of the products obtain
bed and a ?xed catalyst bed, instead of one ?uid bed,
able by thermal decomposition of urea, ‘by heating these
means that the ?uid bed may be much smaller than if
substances at a temperature of 220~400° C., in the pres
’ only one bed is used. Thus, in the present process, the
ence of a solid catalyst of large internal surface ‘area, and
?uid bed need contain only 5—10% of the total amount
preferably in the presence of NH3 (US. Patent No.
‘of catalyst to be used. Consequently, corrosive and
2,760,961). In the abovementioned process, the urea is
erosive forces attack a much smaller and hence, less
preferably introduced into the lower part of a ?uidized
costly apparatus.
bed of catalyst, the ?uidized state being kept up by means
(2) Due to the use of a smaller ?uid bed, catalyst
of a stream of NH3.
Wear is reduced. Less catalyst dust is formed and less
The overall reaction for preparing melamine by the
method discussed above may be shown as ‘follows:
catalyst need be periodically replaced.
(3) The smaller ?uidized catalyst bed means that less
NH3 is needed to effect a suitable ?uidized state of the
catalyst at a given urea load. Thus good melamine con
This reaction is apparently built up from two consecutive
version ?gures may be obtained at an NHg/urea ratio as
reactions:
low as 1.5—5, the ‘amount of NH3 being expressed in cubic
30 meters per hour and the amount of urea in kg. per hour.
In contrast, an NH3/ urea ratio of, for example 9‘ is needed
in the known process.
(4) In view of the lower cost of the smaller ?uidized
25
catalyst bed, it becomes advantageous to connect several
thermic and proceeds considerably faster than the weakly 35 ?uidized catalyst beds to one ?xed catalyst bed. In this
way, if operational trouble should occur in the most sen
exothermic second reaction (0).
and it appears that the ?rst reaction (b) is strongly endo
sitive part of the apparatus, viz. at the place where the
supply of the starting material is effected, it is not neces
sary to stop the whole apparatus, as the function of the
propriate for a strongly endothermic reaction proceed 40 inoperative ?uidized catalyst bed can be temporarily taken
g It is known that in connection with the high treat-trans
fer coe?icient obtained with a ?uidized catalyst bed such
a ?uidized catalyst bed .is considered particularly ap
ing at a high temperature level.
I
over by the other ?uidized catalyst beds whose operation
has not been affected.
(5) Due to the combination of ?uid and ?xed cata
In principle, it is therefore correct that the ?rst reac
tion (b) is, made to proceed in a ?uidized catalyst bed.
beds, a portion of the catalyst, viz. the portion con
Moreover, this ?uidized bed is favorable in connection 45 lyst
tained
in the ?xed ‘bed, may have a considerably larger
with the required rapid mixing with the catalyst bed of
particle diameter than is normally suitable in a ?uidized
the urea to be added as solid substance or as melt, since
catalyst bed. Thus, it has been found that the use of ‘a
in the case of the much slower mixing, which takes place
catalyst in the ?xed bed with a particle diameter several
if a ?xed catalyst bed is used, it is possible that urea does
not decompose according to reaction (b) but according 50 times, or even ten times as large as is usual in a ?uid
bed, can be effectively used without having an unfavor
to reaction:
able e?ect on the yield.
where lumps of cyanic acid form on the catalyst that
block the passage through the bed.
.
,It has appeared now that in a ?uidized catalyst bed at
a relatively high load the reaction (b) proceeds prac
This possibility of using particles of larger diameter
means that the resistance experienced by gas ?owing
through the bed is smaller. This in turn means that the
55 process is more economical vfrom the point of view of
energy.
In ‘order that the reaction (0) may
In addition to its process aspects, the present inven
tion also contemplates a new and improved apparatus ‘for
be caused to proceed to completion and consequently a
larger yield of melamine may be obtained, either the load
matically showing a longitudinal section of one form of
tically to completion, but the reaction (0) proceeds only
to a ‘small extent.
on the ?uidized bed would have to be decreased or a
carrying out the process, the annexed FIGURE diagram
‘apparatus contemplated herein.
larger ?uidized bed reactor would have to be use. For
the reaction (c), however, the known advantages of a
As shown, the apparatus comprises a fluid bed reactor
In view of these considerations the reaction (0) should
cyclone 6, a feed conduit 7 for feeding the starting ma
terial into the ?uidized catalyst bed, and a feed conduit
8 for the supply of the ?uidization gas which keeps the
1, connected to a tube reactor 3 by a conduit 2. The
?uidized bed are of no importance, since the reaction (0) 65 reactors are made of appropriate corrosion-resistant ma-v
terial. Both aluminum and austenitic chrome nickel
has little calori?c elfect; a properly constant tempera
steel, type V 4a, have proven to be very suitable for this
ture is of little importance here, and no more is the satis
purpose. The ?uid bed reactor 1 comprises a grate 4
factory mixing of the reactants with the catalyst, since
with a catalyst bed 5 resting thereon, a dust collector
in the case of the reaction (c) the reactants are gaseous.
preferably take place in a ?xed catalyst bed.
'
According to the invention, the urea ‘or a product eb
speaeaa
4
bed in the ?uidized state. The tube reactor 3 contains
a bundle of tubes 9 ?lled with catalyst. If desired, a
coolant or a heating agent may be sent through the space
between the tubes. At their top and bottom, the tubes
9 of the bundle are ?xed in tube plates 10 and 11. Over
the tube plate 11 is the discharge conduit 12 for the gases
tween about 220° C. and 450° C. The same or different
temperatures may be used in the two beds. However, ac
cording to a modi?cation of the invention herein, the
amount of catalyst may be even further reduced with
considerable saving in catalyst and cost of apparatus in
vestment by varying the temperature of the ?xed catalyst
bed by, for example, controlling the degree of cooling
?owing through the reactor.
In the ?uid bed 5, there are tube- or rod-shaped heating
elements 13 through which a heating agent may be passed.
The elements may be hollow to permit the passage of,
in the bed. More particularly, this modi?cation contem
plates feeding the vapors from the ?uidized bed at a tem
perature of about 350° C. into the ?rst part of the ?xed
e.g. steam, or they may be solid and serve as heating 10 catalyst bed, allowing the temperature of this part of the
rods which are brought to a high temperature by passing
?xed bed to be raised from about 350° C. to 375-425° C.
an electric current through them. To counteract wear
as a result of corrosion and erosion, the rods are prefer
ably made of a hard conducting material, e.g. SiC or
by the exothermic reaction and lowering the temperature
through the cyclone 6, catalyst dust being caught therein
?rst part of the ?xedbed which is traversed within the
of the remaining part of the bed which is subsequently
15 traversed by cooling so that vapors issuing therefrom
MoSi2.
again have a temperature of about 350° C. or lower,
When the above described apparatus is in operation,
e.g. 330° C.
the catalyst is kept in the ?uidized state by the supply of
Stated another way, the modi?cation described in the
NH3 through conduit 8. Solid or liquid urea is supplied
preceding paragraph comprises feeding the vapors from
to the bed through conduit 7. The decomposition prod
the ?uidized bed into the ?xed bed at a temperature of
ucts formed from the urea together with NH3 pass
about 320-375° C., maintaining the temperature of the
and returned to the ?uid bed.
The dedusted gases are
range of 375425“ C. by the resulting exothermic re
then fed from the top of the cyclone 6 into the tube bun
action, cooling the balance of the ?xed bed so that it is
dle 9 where further conversion takes place. A mixture 25 maintained at a lower temperature not above about 350°
of NH3 and melamine vapor is discharged through con
C. and withdrawing the gases from said bed at said
duit 12, the melamine being separated from the NH3 in
lower temperature.
As a typical illustration of the advantages of this last—
The invention is illustrated, but not limited, by the
mentioned modi?cation, it has ‘been found that the ?xed
following examples:
30 bed need contain only 505 kg. of catalyst if the vapors
from the ?uidized bed enter the bottom of a ?xed bed
Example 1
‘at a temperature of 350° C., and are allowed to raise the
Using the apparatus described above and a silica gel
temperature in the lower part (e.g. the bottom half) of
catalyst, the particle diameter of which was 0.25-0.50
the bed to 400° C. or maintain this part of the bed at that
mm. in the ?uid bed, and 4 mm. in the tube bundle, in
temperature while the upper part of the ‘bed is cooled to
conventional, known manner.
conjunction with a temperature of 350° C. in the ?uid
bed and a temperature of 330° C. in the ?xed bed, urea
was introduced through conduit 7 at the rate of 55 kg.
per hour and NH3 was fed through conducit 8 at the rate
of 250 cu. m. per hour. The NHS/urea ratio was, there 40
fore, 4.5.
Analysis of the gas mixture discharged through con
In contrast, under otherwise identical conditions, 825 kg.
of catalyst ‘are necessary for the ?xed bed if this bed is
kept ‘at 350° C. throughout its mass.
This means a
further saving in catalyst of 38%.
It will be appreciated that the amount of the ?xed bed
catalyst which is operated at the raised temperature, and
consequently the amount of this bed which is cooled,
duit 12 showed that the urea fed in had been converted
into melamine with a yield which amounted to 90% of
that theoretically attainable.
Example 2
reduce the temperature of the bed so that the vapors
leaving the same again have a temperature of 350° C.
45 can be varied.
As a typical example, it can be stated
that from about 50% up to about 80% of the length
or amount of the bed which is ?rst traversed should be
Under conditions similar to those described in Exam
operated at the elevated range of 375—425° C. while the
ple l, but with molten urea sprayed onto the ?uid bed at
balance
of the bed is cooled to give the desired exit tem
the rate of 100 kg. per hour, and NH3 fed in at the rate 50
perature of 350° C. or lower for the gas issuing from‘ the
of 300 ‘cubic meters per hour, an 85% yield was obtained
bed. Contact times are such that the gas is essentially
in the conversion to melamine.
at
the same temperature as the bed at any stage of the
The procedures described in the foregoing examples
treatment and it will be appreciated that the raios of NH3
were operated at atmospheric pressure. However, the
to urea or other starting material, catalyst particle size
invention may also be practical at elevated pressures, e.g.
and further operating details will conform to' those out
up to about 20 atmospheres pressure.
lined hereinabove. The desired degree of cooling for the
Other operating conditions of the process described
?xed bed may be accomplished by indirect cooling with
herein may also be widely varied. Thus, for example, in
water through tubes or coils positioned within the bed.
lieu of the silica gel catalyst mentioned above, any of the
As will be evident, it is possible to use several series
catalysts conventionally used for converting urea or its 60
conneoted ?xed catalyst beds, e.g. two, instead of a single
decomposition products like biuret, to melamine may be
?xed bed, in carrynig out the present invention. Thus,
used. Typically, this includes such catalysts of large in
for example, ‘following the modi?cation described above,
ternal surface area (e.g. 180 m.2/ g. or more) as activated
the temperature in the ?xed catalyst bed ?rst traversed
bauxite, silica alumina catalyst.
As indicated heretofore, one of the advantages of the 65 by the vapors may be kept at 375—425° C. and that in the
next ?xed catalyst bed at 350° C. or lower.
present process is that lower NH3/urea ratios (as low as
The modi?cation involving two different sets of tem
1.5-5) may be used than heretofore possible. However,
perature conditions for the ?xed bed or beds is illustrated
higher ratios may be used if desired. Desirably, this bed
by the following examples using the apparatus heretofore
will contain from 0.1 to 1 kgram of catalyst per 1 kilo
described:
gram per hour of urea. The ?xed bed will usually con
tain from 10 to 20 times more catalyst than the ?uid bed.
Particle size of the catalyst in the ?uid bed usually ranges
from 0.1 to 0.5 mm. As indicated hertofore, the particle
size in the ?xed bed may be as much as ten times greater.
The process may be carried out at temperatures be
70
Example 3
Molten urea was sprayed onto a ?uidized bed of 700 kg.
silica gel catalyst (particle diameter averaging ‘between
0.25-0.50 mm.) at the rate of 100 kg. per hour. NHs
gas was fed into the reactor to maintain the bed in the
3,093,644
5
5
?uidized state at the rate of 300 cubic meters per hour.
The temperature of the bed was maintained at 350° C.
presence of NH3 gas and a catalyst of large internal sur
face area, the improvement which comprises providing a
?uidized catalyst bed in which the catalyst is ?uidized by
said NH3 gas, providing a ?xed catalyst bed, the amount
of catalyst in the ?uidized bed comprising 5 to 10% of
the total amount of catalyst utilized and the particle di
ameter of the catalyst in the ?xed bed being several times
greater than the particle diameter in the ?uid bed, main
Decomposition vapors and NH3 ?uidizing gas were
withdrawn from the ?uidized bed reactor at 350° C. and
fed into the ‘bottom of a ?xed bed of 505 kg. silica gel
catalyst similar to that used in the ?uidized bed. The
resulting exothermic reaction was allowed to heat the
catalyst to a temperature varying from 375° C. up to
about 425° ‘C. through the ?rst half of the bed. The
taining both of said beds at a temperature between 220°
balance of the catalyst bed was cooled so that the vapors
issuing at the top of the bed were cooled to about 350° C.
and 450° C., feeding said compound into the ?uidized
Analysis of these vapors showed that the urea starting
material had been converted into melamine with a yield
of about 94% of that theoretically attainable.
with NHS gas from said ?uidized catalyst bed, the ratio
of NH3 in cubic meters to said compound in kg. being
between 1.5 and 5, then passing this mixture of gases
15 through the ?xed catalyst bed and recovering melamine
from the product.
Example 4
catalyst bed, withdrawing the resulting vapors together
Example 3 was repeated except that the ?xed bed of
6. In a process for preparing melamine from a starting
catalyst was maintained at 350° C. throughout its mass
material selected from the group consisting of urea and
thermal decomposition products thereof wherein said
by cooling. It was found necessary to use 825 kg. of the
catalyst under these conditions to obtain a yield equivalent 20 starting material is ?rst passed through a heated ?uidized
catalyst bed using NH3 gas as the ?uidizing gas, the re
to that resulting from the process outlined in Example 3
sulting vapors including the NH3 ?uidizing gas are then
where the temperature ‘of the ?xed bed was allowed to
passed through a heated ?xed catalyst bed and melamine
rise in the manner described.
is recovered from the gaseous product issuing from said
Example 5
25
?xed bed, the improvement comprising feeding the vapors
from said ?uidized bed into said ?xed bed at a tempera
Example 3 was repeated except that, in lieu of the
ture of 320-375° C., maintaining the ?rst part of the
single ?xed catalyst bed used in Example 3 above, two
?xed catalyst bed which is traversed by said vapors at a
separate ?xed catalyst beds were used in series, the vapors
temperature between about 375° C. and 425° C., and
‘leaving the ?rst such bed being fed through the second.
Each of these series-connected ?xed catalyst beds in~ 30 then cooling the balance of said ?xed catalyst bed so that
the vapors issuing therefrom have a temperature not in
eluded about 252.5 kg. of catalyst. The ?rst lbed was
excess of about 350° C.
kept at a temperature between 375° and 425 ° C., averag
7. The process of claim 6 wherein said ?xed catalyst
ing about 400° C., while the second ?xed bed was kept at
bed
comprises a plurality of beds connected in series, the
a little below 350° C. The yield of melamine was about
35 catalyst in the ?rst bed of said series being kept at a tem
94% of the theoretical amount.
perature between 375° and 425° C. and the catalyst in
This application is a continuationdn-part of my co
the other beds of said series being cooled so that the
pending applications Serial No. 17,481, ?led March 28,
temperature of the gas issuing from the last such bed is
1960, now abandoned, [and Serial No. 122,625, ?led July
not above 350° C.
10, 1961, now abandoned.
40
8. In a process for preparing melamine from a starting
It will be ‘appreciated that various modi?cations may
material selected from the group consisting of urea and
be made in the invention described above without deviat
thermal decomposition products thereof wherein said
ing from the scope thereof as set forth in the following
starting material is ?rst passed through a heated ?uidized
claims.
catalyst bed using NH3 gas as the ?uidizing gas, the re
I claim:
sulting vapors including the NH3 ?nishing gas are then
1. In a process for preparing melamine by heating ‘a
passed through a heated ?xed catalyst bed and melamine
compound selected from the group consisting of urea
is recovered from the gaseous product issuing from said
and products obtainable 'by thermal decomposition of
?xed bed, the improvement comprising feeding the vapors
urea, at a temperature between ‘about 220° and 450° C.,
from said ?uidized bed into said ?xed bed at a tempera
in the presence of NH3 gas and a catalyst ‘of large internal 50 ture of 320-375 ‘’ ‘C., maintaining the ?rst part of the ?xed
sur?ace area, the improvement which comprises providing
catalyst bed which is traversed by said vapors at a tem
a ?uidized. catalyst bed in which the catalyst is ?uidized
perature between about 375° C. and 425° C., and then
by said NH3 gas, providing a ?xed catalyst bed, main
cooling the balance of said ?xed catalyst bed so that the
vapors issuing therefrom have a temperature not in excess
220° C. and 450° C., feeding said compound into the 55 of about 350° C., the ?rst part of said ?xed catalyst bed
?uidized catalyst bed, withdrawing the resulting vapors to
comprising from about 50% to about 80% of the total
taining both of said beds at ‘a temperature between about
gether with NH3 gas from said ?uidized catalyst bed,
then passing this mixture of gases through the ?xed cata
lyst bed and recovering melamine from the product.
2. The process of claim 1 wherein the ratio of NH3 60
in cubic meters per unit to urea in kg. per unit time is
from 1.5 to 5.
3. The process of claim 1 wherein the amount of
catalyst in the ?xed bed is 10 to 20 times greater than the
65
amount in said ?uidized bed.
4. The process of claim 1 wherein the particle diameter
of the catalyst in the ?xed bed is several times greater
than the particle diameter in the ?uid bed.
5. In a process for preparing melamine by heating a
compound selected from the group consisting of urea and
products obtainable by thermal decomposition of urea, at
a temperature between about 220° and 450° C., in the
?xed catalyst bed.
9. The process of claim 6 wherein the vapors from
the ?uidized bed have a temperature of about 350° C.
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,760,961
Mackay _________ _‘______ Aug. 28, 1956
2,768,882
Mattson ______________ __ Oct. 30, 1956
2,779,777
2,783,131
Mungen ______________ __ Ian. '29, 1957,
Mackay et a1 __________ __ Feb. 26, 1957
2,943,088
Westfall ______________ __ June 28, 1960
2,943,997
MacLaren 'et al. ________ __ July 5, 1960
FOREIGN PATENTS
767,344
32—1531
Great Britain __________ __ Jan. 30, 1957
Japan _______________ __ Mar. 27, 1957
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