Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method of keeping
the water content in a urea reaction vessel low and to an
apparatus suitable therefor.
Prior art processes for the manufacture of urea by
reaction of carbon dioxide and ammonia under increased pressure
and high temperatures (e.g. 170 bars and 180C) are character-
ized in that in addition to the urea formed, comparable
quantities of principally carbamate are produced. If the
pressure does not exceed 200 bars, urea yields up to 70% can
be obtained.
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To separate the carbamate from the urea, the
carbamate is decomposed with the aid of heat. Various
methods are known for reintroducing into the reactor the
resultant gaseous substances. One of the earliest of
these known methods dates back to the 1950's and is based
on the fact that under suitable conditions, ammonia can
be selecti~ely separated from carbon dioxide with the:aid
of an ammonium nitrate solution (cf. e.g., Swiss Patent
-No. 290,289; French Patent No. 1,085,316). Although this
b~ known process proved successful in industrial operations,
it was soon replaced by less costly and more efficient
processës (cf. e.g. U.S. Patent No. 3,317,601).
. Common to all of these prior art known methods
is the separation of the carbamate at pressures lower than
the reactor pressure in more or less numerous stages, and
- the absorptive return into the pressure autoclave of the
unconverted reactants.
Another method which has met with some success is
- the stripping process of Netherlands Staatsmijnen (see,
i ~o e.g., U.S. Patent No. 3,356,723). This method utilizes the
principle of counter-flow (countercurrent). The carbamate
i8 decomposed and the resuLting products are returnet
isobaricall~ to the reactor.
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All of these prior art processes are disadvantageous
in that the melt in the reaction chamber contains a large
amount of water and the conversion into urea relative to carbon
dioxide at the outlet of the reactor is relatively slight.
Moreover, in all cases the urea must be released from the
reaction water at low pressure and at low temperature. During
"prilling" the melt must again be brought to a high temperature.
All of these steps increase the energy requirement for the
process and large unused quantities of energy are released
directly into the environment at low temperatures. U.S.
Patent No. 2,527,315 teaches that through the use of a large
excess of ammonia it is possible to obtain a urea-melt having
a lower water content. However, ~he use of a large excess of
ammonia requires extensive equipment and the input of con-
siderable quantities of energy.
The present invention overcomes the aforementioned
disadvantages of the known processes. It should be noted that
in developing this invention it was found that an NH3 to CO2
mole ratio of 2 or sligntly more could be employed. This is
remarkable because it is now possible to dispense with the use
of usual excess ammonia while maintaining very high urea yields.
When the present invention is carried out continuously, the
melt in the reactor is continually drained and the concentration
of water is kept low. -~
Surprisingly, it was discovered that the free, sub-
critical water does not stay in the liquid phase, as would be
expected, but instead is distributed almost equally between
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the two phases, liquid and gas. The ammonia, which is above
its critical temperature, is surprisingly found to a large
extent in the liquid phase; while carbon dioxide at higher
temperatures prefers the gaseous phase almost exclusively.
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Accordingly, the present invention concerns a process
for the continuous production of urea from the reaction of
ammonia and carbon dioxide in a reactor. The process is
characterized in that water is continually removed from the
urea melt in the reactor. Preferably, the water is removed at
a rate such that the mole ratio of water to urea produced in
the reactor is always less than 1. This is achieved by con-
tinually feeding the urea melt with a mixture of recirculated
gases consisting of CO2, NH3 and H20 which has been at least
partially dried before entering the urea reactor. The process
is further characterized in that a reaction temperature of at
least 160C is employed, a temperature of 170-200C being
preferred. The preferred mole ratio of ammonia to carbon
dioxide is 2 : 1.
Thus the process exhibits the following advantages:
1. The urea conversion in the melt relative to CO2
is large (e.g., 93~ and more).
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2. The content of water in the melt
is ver~ low.
3. There is a modest equilibrium of reaction
- pressure even at high temperatures.
4. Although the operation is effected with or
without a very slight excess of NH3, the liquid phase is
strongly alkaline.
5. Due-to the high alkalinity, little biuret
i8 created despite the low H2O content.
-10 6. All the compounds in the liquid phase can
be decomposed into ~ and CO2. Thus it is possible, at a
relatively low reaceor pressure and low biuret content, to
employ a high temperature. In this manner, a high reaction
~peed is achieved, a small reactor can be utilized and a
~all C2 compressor output is requiret.
The figure illustrates, in schematic representation,
typical flow diagram of a preferred embodiment of the
present invention. As is apparent from the figure the apparatus
consists of horizontally disposed reaction vessel 1, vertical
! 20 partial condenser 2, gas circulating blower 3, urea work-up
unlt 12, separsting unit 10 and piping (represented by
connecting lines and arrows) belonging to the system.
The mode of operation of this process is as follows:
The water-saturated gas leaves reaction vessel 1 and reaches
partial condenser 2 at its lower end via a short pipe 4, which
~8 hydrodjnamically well designed. Together with the constituents
flowing in from pipe 5 and hereinafter descri~ed, the mixture
of gases is conveyed upwardly through the condenser tubes, being
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partially condensed as the temperature falls in the direction
of flow. The ascending saturated vapor is consequently
conveyed in counter-current relationship to the film of
condensate trickling down the walls, the condensate accumulating
at the lower end of the condenser 2 surprisingly showing a
very high concentration of water. This condensate also
contains carbamate. The water is separated in separating unit
10 and drawn of f via the pipe 11. The gaseous carbamate
mixture is conveyed into the condenser 2 via the pipe 6. The
uncondensed vapor now low in water is drawn in by the circulating
blower 3 and introduced tangentially into the urea melt at a
plurality of points at the periphery of the reaction vessel 1
via the pipe 7. By momentum exchange between the gas and the
liquid, the latter is kept in rotary movement, in such manner,
in fact, that in the interior of the reaction vessel 1 there
is formed a ring of liquid from the hollow space of which the
saturated vapor mixture is drawn off towards the partial con-
denser 2. The gas circuit described is thereby closed. From
the reaction vessel 1, the urea is conveyed together with CO2,
20 NH3 and H20 via the pipe 9 into the work-up unit 12 and is
separated therein in known manner, the urea being drawn off
through the pipe 13 and the other constituents flowing into
the gas circuit through pipe 5 into pipe 4.
The reaction vessel 1 is preferably placed in a
horizontal position. As is apparent from the foregoing, it
is desirable to bring the gas entering the reaction vessel
and low in water into intensive contact with the urea melt
in order to saturate the gas with as much water as possible.
This operation could be suitably carried out in a vertical
bubble column. In order to keep investment costs low, pressure
apparatus of small diameter is preferred. For a given
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residence-time, however, the bubble-dolumn reaction vessel
and, consequently, the static liquid pressure acting on the
bottom of the reaction vessel, becomes high. This static
and very important liquid pressure must be supplied, in
addition to the other pressure losses occurring in the cir-
culating system, by the circulating blower 3, whereby its
energy requirements increase markedly. Due to the horizontal
reaction vessel with the melt revolving at above-critical
speed, a liquid ring is obtained. If this ring is developed,
then there is obtained, together with the length of reaction
vessel in accordance with the above residence-time, a layer
of liquid with a considerable base area and of small height
or depth. The area increases with increasing length of the
reaction vessel. In accordance with the small layer height
(ring thickness), the feed or charging of this layer with gas
requires a comparatively small energy consumption. The large
reaction vessel surface (base area) may have a large number ~-`
of tangentially drilled holes through which the melt is
intensively charged with gas in consequence of the large area
of the bubbles formed. This arrangement consequently avoids
- high energy requirements by the circulating blower 3 with
low investment costs for the reaction vessel 1. At the same
time, an equal residence-time and an equally intensive
exchange of substances is ensured in comparison with a bubble
column. The advantage achieved in this way is decisive for
the economy of the method, since the circulating mass of gas
for an average 500 tons per day plant, is, for example, 650
tons per hour. Another advantage of the method is that the
heat liberated by the partial liquefaction of the gas can be ~
30 used for generating 6-bar steam. ~ ~ -
Examples 1-4 hereafter are batchwise experiments in
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which an NH3/CO2 molar ratio of 2:1 was employed. The ratio
of water and urea introduced to the autoclave to the volume
of the reactor was 530 kg/m3. The mole ratio of H20 : CO2
was varied between 0.5
Example 1
H20/co2 mole ratio = 1 Temperature = 160C.
The H20/CO2 mole ratio refers to the liquid and
the gaseous phase. The experiments were carried through
isochorically.
~ith these parameters the following results are
obtained:
Pressure = 85 bar
Composition of liquids (w' = percentage by weight):
w' = 37.8 w' = 25.4 w' = 24.8 w' = 11.3 w' = ~.7
Urea NH3 C02 H2o Biuret
Composition of gases (w" = percentage by weight):
w" = 1~.8 w" = 78.7 w" = 2.5
NH3 C2 H20
Urea conversion in the melt relative to CO2 = 60.4%.
Example 2
H20/Co2 mole ratio = 1 Temperature = 170C.
Pressure = 120 bar
Composition of liquids (w' = percentage by weight):
w' = 40.7 w' = 23.3 w' = 23.1 w' = 11.5 w' = 1.4
Urea NH3 CO2 H2o Biuret
Composition of gases (w" = percentage by weight):
w" = 18.1 w" = 77.1 w" = 4.80
NH3 C2 H20
Urea conversion in the melt relative to CO2 = 63.8%.
Example 3
H20/CO2 mole ratio = 0.63 Temperature = 170C
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With these parameters the following results are
obtained:
Pressure = 108 bar
Composition of liquids (w' = percentage by weight~:
w' = 55.3 w' = 23.7 w' = 16.8 w' = 2.5 w' = 1.7
Urea NH3 C02 H2o Biuret
Composition o~ gases (w" = percentage by weight):
w" = 23.1 w" = 72.0 w" = 4.9
NH3 C2 2
Urea conversion in the melt relative to C02 = 76.7%.
Example 4
H20/Co2 mole ratio = 0.63 Temperature = 180C
With these parameters the following results are
obtained:
Pressure = 127.5 bar
Composition of liquids (w' = percentage by weight):
w' = 64.6 w' = 24.8 w' = 5.0 w' = 3.3 w' = 2.3
Urea NH3 C02 H2o Biuret
Composition of gases (w" = percentage by weight):
w" = 21.2 w" = 76.2 w" = 2.6
NH3 C2 2
Urea conversion in the melt relative to CO2 = 92.8%. ~ -
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Example 5 ~ 5
This Example illustrates the continuous operation of
the present invention. Reference to the apparatus corresponds
to the figure.
For an output of 500 tons of urea per day, reaction vessel 1
i8 fed with 4.24 kg/s CO2 and 3.28 kg/s ~H3.
The pressure in the reaction vessel is 125 bars and
the temperature is adjusted to 180C. The gas circulates
in the circu~t 1-4-2-3-7. The gas leaving the reaction vessel
through pipe 4 consists of 136.4 kg/s C02, 37.95 kg/s NH3
and 4.65 kg/s ~ O.
When this gas stream is combined with the constituents
coming from the urea work-up unit by way of pipe 5, 136.83 kg/s
C2 J 40.09 kg/s NH3 and 4.93 kg/s H20 enter the lower part
of the part~al condenser 2 at a temperature of 180C. This
apparatus is operated at the same pressure as the reaction
vessel, the heat being carried off in the ~scket by 6-bar steam.
The liquid leaving the tubes ~through pipe 8) consists of
2.7 kg/s C02, 2.08 kg/s NH3 and 1.86 kg/s H20.
The mixture of gases drawn in by the blower (composed
of gas from pipe 6 and gas not condensed in condenser 2) has
a temperature of 165C and contains 136.83 kg/s C02, 40.09 kg/s
and 3.2 kg/s H2O, this being returned to reaction vessel 1
through pipe 7. The melt leaving the reaction vessel through
pipe 9 consists of the following mixtures:
5.78 kg/s urea, 0.43 kg-~s- C02, 2.14 kg/s NH3 and 0.28 kg/s ~ O.
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This mixture is split up by a work-up unit 12, known
in the art, the urea being drawn off through pipe 13, while
the other constituents are introduced into the gas circuit
(through pipe 5~ into pipe 4~
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