U.S. patent application number 16/789050 was filed with the patent office on 2020-06-04 for urea manufacturing method and urea manufacturing apparatus.
This patent application is currently assigned to Toyo Engineering Corporation. The applicant listed for this patent is Toyo Engineering Corporation. Invention is credited to Haruyuki MORIKAWA, Keishi SATO, Kenji YOSHIMOTO.
Application Number | 20200172475 16/789050 |
Document ID | / |
Family ID | 58240692 |
Filed Date | 2020-06-04 |
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United States Patent
Application |
20200172475 |
Kind Code |
A1 |
SATO; Keishi ; et
al. |
June 4, 2020 |
Urea Manufacturing Method And Urea Manufacturing Apparatus
Abstract
Provided are urea manufacturing method and apparatus, which can
increase the conversion ratio into urea and to reduce the
consumption of steam. The temperature of the reactor is increased
by introducing the entire amount of raw material ammonia and
introducing a portion of the decomposed gas from the stripper into
the reactor. The raw material ammonia is preferably heated using
the steam condensate generated in the purification step, and/or the
steam generated by the heat of condensation of the decomposed gas
and the unreacted substances in the condensation step. The heating
temperature is preferably between 70 and 140.degree. C.
Inventors: |
SATO; Keishi; (Chiba,
JP) ; YOSHIMOTO; Kenji; (Chiba, JP) ;
MORIKAWA; Haruyuki; (Chiba, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Toyo Engineering Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
Toyo Engineering
Corporation
Tokyo
JP
|
Family ID: |
58240692 |
Appl. No.: |
16/789050 |
Filed: |
February 12, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15758295 |
Mar 7, 2018 |
10604477 |
|
|
PCT/JP2016/075505 |
Aug 31, 2016 |
|
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16789050 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 19/0053 20130101;
B01D 5/009 20130101; B01D 3/009 20130101; B01D 5/0081 20130101;
C07C 273/16 20130101; C07C 273/04 20130101 |
International
Class: |
C07C 273/04 20060101
C07C273/04; C07C 273/16 20060101 C07C273/16; B01J 19/00 20060101
B01J019/00; B01D 5/00 20060101 B01D005/00; B01D 3/00 20060101
B01D003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 8, 2015 |
JP |
2015-176433 |
Claims
1. A urea manufacturing method comprising: an ammonia introduction
step of introducing a substantially entire amount of raw material
ammonia into a reactor; a synthesis step of reacting carbon dioxide
and ammonia under a condition of excessive ammonia in the reactor,
thereby providing a synthesis mixture containing urea, ammonium
carbamate, water, unreacted ammonia, and unreacted carbon dioxide;
a decomposition step of decomposing the ammonium carbamate by
heating the synthesis mixture and stripping using at least a
portion of raw material carbon dioxide as an auxiliary agent,
thereby providing a decomposed gas containing ammonia and carbon
dioxide, and a urea synthesis solution containing ammonia, carbon
dioxide, water, and urea; a purification step of separating water
and an unreacted substances including ammonia and carbon dioxide
from the urea synthesis solution, thereby providing a purified urea
solution and recovering the separated water and unreacted
substances; a decomposed gas introduction step of introducing
between 5 and 20 wt % a portion of the decomposed gas into the
reactor, using an ejector which uses at least a portion of the raw
material ammonia as a driving fluid; a condensation step of
condensing the rest of the decomposed gas and at least a portion of
the water and unreacted substances recovered in the purification
step in a condenser, thereby providing a condensate and uncondensed
gas, separately; an off gas returning step for returning the
uncondensed gas obtained in the condenser to the purification
system; and a condensate introduction step of introducing the
condensate to the reactor using another ejector for introducing the
condensate to the reactor which uses the rest of the raw material
ammonia as a driving fluid; wherein the decomposed gas introduced
into the reactor in the decomposed gas introduction step is
condensed to increase the temperature of the reactor.
2. The method according to claim 1, wherein the raw material
ammonia is heated in the ammonia introduction step using a steam
condensate generated in the purification step and/or a steam
generated by heat of condensation in the condensation step.
3. The method according to claim 1, wherein the raw material
ammonia is heated up to from 70 to 140.degree. C. in the ammonia
introduction step.
4. The method according to claim 1, wherein the condenser is a
shell and tube condenser, and wherein the decomposed gas from the
stripper and the unreacted substances from the purification system
are introduced to the shell side of the condenser.
5. The method according to claim 1, wherein the condenser is a
bubble column vertical condensation reactor.
6. The method according to claim 1, further comprising: a carbon
dioxide introduction step of introducing a part of raw material
carbon dioxide directly into the reactor and the rest of the raw
material carbon dioxide into the stripper, using a carbon dioxide
introduction line that is directly connected to the reactor.
7. The method according to claim 1, wherein the pressure of the
urea synthesis solution obtained in the decomposition step is
reduced using a control valve to between 15 and 20 bar to obtain a
gas-liquid mixture, and the obtained gas-liquid mixture is
introduced into the purification step.
8. The method according to claim 1, wherein the molar ratio of the
NH.sub.3 component to the CO.sub.2 component (N/C) in the synthesis
step is between 3.0 and 4.0.
9. The method according to claim 1, wherein the molar ratio of the
NH.sub.3 component to the CO.sub.2 component (N/C) of the
condensate obtained in the condensation step is between 2.5 and
3.5.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. National Phase
patent application Ser. No. 15/758,295, filed Mar. 7, 2018,
entitled Urea Manufacturing Method And Urea Manufacturing
Apparatus, which claims benefit of and priority to International
Patent Application No. PCT/JP2016/075505, International Filing Date
Aug. 31, 2016, entitled Urea Production Method And Urea Production
Device; which claims benefit of Japanese Patent Application No.
2015-176433 filed Sep. 8, 2015; all of which are incorporated
herein by reference in their entireties.
TECHNICAL FIELD
[0002] The present invention relates to urea manufacturing method
and manufacturing apparatus, more specifically to urea
manufacturing method and manufacturing apparatus, which can
increase the ratio of conversion into urea and consume less
steam.
BACKGROUND ART
[0003] Urea is manufactured by the following method: first, ammonia
(NH.sub.3) and carbon dioxide (CO.sub.2) are subjected to reaction
to produce ammonium carbamate (NH.sub.2COONH.sub.4) as represented
by Formula (1), and then, ammonium carbamate is subjected to
dehydration reaction to produce urea (NH.sub.2CONH.sub.2) and water
(H.sub.2O) as represented by Formula (2).
2NH.sub.3+CO.sub.2.fwdarw.NH.sub.2COONH.sub.4 (1)
NH.sub.2COONH.sub.4.fwdarw.NH.sub.2CONH.sub.2+H.sub.2O (2)
Both reactions are the equilibrium reaction but the reaction of
Formula (1) is the exothermic reaction while the reaction of
Formula (2) is the endothermic reaction. For this reason, various
schemes have been studied to increase the conversion ratio from the
raw materials of ammonia and carbon dioxide to urea.
[0004] Patent Literature 1 has described the improved urea
synthesis method with the characteristics below. In this method,
ammonia and carbon dioxide react with each other under the urea
synthesis temperature and pressure in the urea synthesis zone. The
resulting urea synthesis solution containing urea, unreacted
ammonia, unreacted carbon dioxide, and water is brought into
contact with at least a portion of the raw material carbon dioxide
under heating and under the pressure substantially equal to the
urea synthesis pressure in the stripping zone. This causes the
unreacted ammonia and the unreacted carbon dioxide to be separated
as the mixed gas of ammonia, carbon dioxide, and water. The urea
synthesis solution containing the unreacted ammonia and the
unreacted carbon dioxide which are not separated is processed
further; thus, the urea is obtained. Meanwhile, the mixed gas
separated in the stripping zone is introduced to the bottom of the
vertical condensation zone and is brought into contact with the
absorbing medium while being cooled. This causes the mixed gas to
be condensed. The resulting condensate circulates in the urea
synthesis zone.
[0005] In the third example of Patent Literature 1, ammonia as the
raw material is heated up to 175.degree. C. in the heat exchanger
and then introduced into the ejector. The ejector plays the role of
sending the solution from the condenser to the reactor under the
boosted pressure. Carbon dioxide (CO.sub.2) is introduced into the
reactor and the stripper. The temperature of the solution in the
condenser is adjusted to 185.degree. C. The solution goes through
the ejector to be introduced into the reactor. In the reactor, urea
is synthesized through the dehydration reaction of the ammonium
carbamate. This reaction is endothermic reaction. By increasing the
temperature of the raw material ammonia up to 175.degree. C., the
temperature of the reactor is maintained so as not to decrease
below 185.degree. C.
[0006] According to Patent Literature 2, the temperature of the
reaction zone where the urea synthesis is carried out is increased
by introducing at least a portion of a gas mixture discharged from
the stripping zone into the reaction zone and condensing the
introduced gas mixture.
CITATION LIST
Patent Literature
PATENT LITERATURE 1: JP-A-H-10-182587
PATENT LITERATURE 2: EP 0329215 A, Specification
PATENT LITERATURE 3: JP-A-61-109760
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0007] As the reactor has the higher temperature, the conversion
ratio of ammonium carbamate into urea is increased and the
unreacted substances are therefore decreased. Ammonium carbamate is
decomposed by the heating with steam. Therefore, as the ammonium
carbamate is decreased, less steam is required to separate the
unreacted substances. In addition, as the temperature of the
reactor is higher, the heat to enter the stripper is increased.
Increasing the temperature of the reactor is therefore effective to
reduce the steam to be consumed in the stripper.
[0008] The third example of Patent Literature 1 has examined the
method of increasing the temperature of the raw material ammonia in
order to increase the temperature of the reactor. The temperature
of the raw material ammonia, however, has risen to 175.degree. C.
To increase the temperature further, heating with the high-pressure
steam is considered but in this case, more steam is consumed.
[0009] In another method, the temperature of the reactor is
increased by introducing a portion of carbon dioxide into the
reactor but in this case, the amount of carbon dioxide to enter the
stripper is reduced. The reduction of carbon dioxide makes it
difficult to decompose and separate the unreacted substances in the
stripper. Therefore, more unreacted substances remain in the urea
synthesis solution exit from the bottom of the stripper. The
unreacted substances are separated in the downstream urea
purification step. The separated unreacted substances are recovered
by decomposing and absorbing in the purification step. In order to
recover the substances, however, water is required as the absorbent
solvent. Using the water here will result in more recovered
solution, and more water returns to the reactor as the recovered
solution. The presence of water reduces the conversion ratio into
urea, which is determined by the synthesis equilibrium. For these
reasons, the amount of water returned to the reactor is preferably
as small as possible. In this sense, the urea synthesis solution
from the bottom of the stripper preferably contains as little
ammonium carbamate as possible. Into the stripper, as large amount
of carbon dioxide as possible is preferably introduced. If the
amount of carbon dioxide introduced into the stripper is reduced,
the conversion ratio into urea is decreased and the unreacted
substances will increase. Thus, more steam is consumed in the
manufacture of urea. Moreover, less gas is generated by the
decomposition and separation of the unreacted substances in the
stripper. This reduces the heat of condensation in the condenser.
Accordingly, less steam is generated from the heat of condensation.
That is to say, it has been impossible to increase the temperature
of the reactor without suppressing the decrease in carbon dioxide
introduced into the stripper as much as possible or without
continuing to heat until the temperature of ammonia becomes
high.
[0010] It is an object of the present invention to provide urea
manufacturing method and apparatus, which can increase the
conversion ratio into urea and consume less steam.
Solution to the Problems
[0011] A urea manufacturing method of the present invention
includes: an ammonia introduction step of introducing an entire
amount of raw material ammonia into a reactor; a synthesis step of
reacting carbon dioxide and ammonia reaction under a condition of
excessive ammonia in the reactor, thereby providing a synthesis
mixture containing urea, ammonium carbamate, water, unreacted
ammonia, and unreacted carbon dioxide; a decomposition step of
decomposing the ammonium carbamate by heating the synthesis mixture
and stripping using at least a portion of raw material carbon
dioxide as an auxiliary agent, thereby providing a decomposed gas
containing ammonia and carbon dioxide, and a urea synthesis
solution containing ammonia, carbon dioxide, water, and urea; a
purification step of separating an unreacted substances including
ammonia, carbon dioxide, and water from the urea synthesis
solution, thereby providing a purified urea solution and recovering
the separated unreacted substances; a decomposed gas introduction
step of introducing a portion of the decomposed gas into the
reactor; a condensation step of condensing the rest of the
decomposed gas and at least a portion of the unreacted substances
recovered in the purification step in the condenser, thereby
providing a condensate; and a condensate introduction step of
introducing the obtained condensate to the reactor using an ejector
which uses at least a portion of the raw material ammonia as a
driving fluid.
[0012] A urea manufacturing apparatus of the present invention
includes: a reactor in which carbon dioxide and ammonia are reacted
under a condition of excessive ammonia, thereby providing a
synthesis mixture containing urea, ammonium carbamate, water,
unreacted ammonia, and unreacted carbon dioxide; an ammonia
introduction line that is used to introduce an entire amount of raw
material ammonia into the reactor; a stripper that decomposes the
ammonium carbamate by heating the synthesis mixture and stripping
using at least a portion of raw material carbon dioxide as an
auxiliary agent, thereby providing a decomposed gas containing
ammonia and carbon dioxide, and a urea synthesis solution
containing ammonia, carbon dioxide, water, and urea; a purification
system that purifies urea by separating the unreacted substances
including ammonia, carbon dioxide, and water from the urea
synthesis solution, and recovers the separated unreacted
substances; a decomposed gas introduction line that is used to
introduce a portion of the decomposed gas into the reactor; a
condenser that condenses the rest of the decomposed gas and at
least a portion of the unreacted substances recovered in the
purification system in the condenser, thereby providing a
condensate; and a condensate introduction line that is used to
introduce the obtained condensate to the reactor using an ejector
which uses at least a portion of the raw material ammonia as a
driving fluid.
Effects of the Invention
[0013] According to the present invention, the urea manufacturing
method and apparatus, which can increase the conversion ratio into
urea and consume less steam, can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a diagram illustrating a configuration example of
a urea manufacturing apparatus according to the present
invention.
[0015] FIG. 2 is a diagram illustrating another configuration
example of a urea manufacturing apparatus according to the present
invention.
DESCRIPTION OF THE EMBODIMENTS
[0016] FIG. 1 illustrates a configuration example of a urea
manufacturing apparatus according to the present invention. The
apparatus illustrated in FIG. 1 includes a reactor A, a stripper B,
a purification system C, and a condenser D.
[0017] In the reactor A, ammonia (NH.sub.3) and carbon dioxide
(CO.sub.2) are subjected to reaction to produce ammonium carbamate,
and further ammonium carbamate is subjected to dehydrogenation
reaction to produce urea and water (urea synthesis step). In the
urea synthesis step, ammonia is excessive in consideration of the
equilibrium pressure of the synthesis mixture to be obtained. In
the urea synthesis step, the molar ratio of the NH.sub.3 component
to the CO.sub.2 component (N/C) is preferably between 3.0 and 4.0,
more preferably between 3.5 and 4.0 (3.7 for example).
[0018] The NH.sub.3 component contains, in addition to the actually
present ammonia, ammonia converted into ammonium carbamate and
ammonia converted into urea. Therefore, the molar amount of the
NH.sub.3 component corresponds to the total value of twice as much
as the molar amount of urea, twice as much as the molar amount of
ammonium carbamate, and the molar amount of ammonia. The CO.sub.2
component contains, in addition to the actually present carbon
dioxide, carbon dioxide converted into ammonium carbamate and
carbon dioxide converted into urea. Therefore, the molar amount of
the CO.sub.2 component corresponds to the total value of the molar
amount of urea, the molar amount of ammonium carbamate, and the
molar amount of carbon dioxide.
[0019] The two stage reactions of the urea synthesis step are both
the equilibrium reaction. Therefore, in the urea synthesis step,
the synthesis mixture containing urea (including a small amount of
biuret), ammonium carbamate, water, unreacted ammonia, and
unreacted carbon dioxide is obtained. The ammonium carbamate
contained in the synthesis mixture is decomposed in the next
decomposition step, and the unreacted raw materials need to be
separated. Therefore, it is more preferable that the conversion
rate to urea in the reactor A be higher. The reactor A is
accordingly operated at the high temperature (from 175 to
200.degree. C.) and high pressure (from 130 to 200 bar).
[0020] Ammonia as the raw material is introduced into the reactor A
through an ammonia introduction line 1 (ammonia introduction step).
Carbon dioxide as the raw material is introduced into the reactor A
through carbon dioxide introduction lines 2 and 2a. Carbon dioxide
to be introduced here is usually approximately 10 wt % of the
necessary amount as the raw material. Some carbon dioxide and
ammonia are also supplied from the condenser D, which will be
described below, through a condensate introduction line 6a and a
raw material introduction line 1a. In addition, other portions of
carbon dioxide and ammonia are also supplied through decomposed gas
lines 4b and 4d and the raw material introduction line 1a as a
portion of a decomposed gas separated in the stripper B to be
described below. The condensate introduction line 6a and the
decomposed gas line 4d are connected to an ejector 12a. The ejector
12a uses as a driving fluid, at least a portion of the raw material
ammonia introduced through the ammonia introduction line 1.
[0021] In the present invention, the entire amount of raw material
ammonia is introduced into the reactor A through the ammonia
introduction line 1. This can achieve higher N/C, which results in
the larger conversion ratio of CO.sub.2 into urea. Accordingly,
less steam is consumed to separate the decomposed gas in the
stripper B.
[0022] In the present invention, using the ejector 12a lower the
ammonia introduction line 1 and the condenser D elevation, and also
makes the operation more stable. If the entire amount of raw
material ammonia is introduced into the reactor A without using the
ejector 12a, it is necessary to send the condensate into the
reactor A under gravity. In this case, the condenser D needs to be
located above the top of the reactor A by at least 5 m, preferably
10 m. In this case, the operation fluctuation affects the pressure
balance between the reactor A and the condenser D. This may affect
the introduction of the condensate into the reactor A and result in
the instable operation but the present invention will solve such a
problem.
[0023] In the present invention, preferably, the raw material
ammonia before being introduced into the reactor A is heated in an
ammonia pre-heater 11. This can increase the temperature in the
reactor A and therefore increase the conversion ratio of CO.sub.2
into urea. Accordingly, less steam is consumed to separate the
decomposed gas in the stripper B. To heat the raw material ammonia,
preferably, the steam condensate generated in the purification step
(steam condensate generated by condensing the steam used in the
heating in the purification step and/or the subsequent
concentration step of heating and concentrating the purified
aqueous urea solution) and/or the steam (LP steam) generated by the
heat of condensation in the condensation step.
[0024] The raw material ammonia is preferably heated up to from 70
to 140.degree. C. More specifically, the raw material ammonia is
heated up to approximately from 70 to 90.degree. C. by the steam
condensate, and heated further by the LP steam up to approximately
from 120 to 140.degree. C. as necessary.
[0025] The synthesis mixture obtained in the reactor A is
introduced into the stripper B through a synthesis mixture line 3a.
In the stripper B, the synthesis mixture is heated so that ammonium
carbamate is decomposed into ammonia and carbon dioxide, and
further stripped using at least a portion of raw material carbon
dioxide as an auxiliary agent. Thus, the decomposed gas containing
ammonia and carbon dioxide is separated (decomposition step).
However, the ammonia and carbon dioxide cannot be fully separated
from urea and water in the synthesis mixture in the stripper B;
therefore, the urea synthesis solution containing ammonia, carbon
dioxide, water and urea is obtained. Carbon dioxide is contained in
the urea synthesis solution as the ammonium carbamate generated
from the reaction with ammonia, and the urea synthesis solution
from the stripper B usually contains ammonia, including the ammonia
as ammonium carbamate, by approximately from 10 to 15 wt %.
[0026] Carbon dioxide as the auxiliary agent in the stripping is
introduced into the stripper B through carbon dioxide introduction
lines 2 and 2b. The stripper B is heated by a heating medium
introduced through a stripper heating medium introduction line 21.
The heating medium is discharged through a stripper heating medium
discharge line 22. The heating medium is usually steam (water
vapor). The pressure of the steam is set to, for example, 20
bar.
[0027] The urea synthesis solution obtained in the stripper B is
discharged through a urea synthesis solution line 4a connected to
the bottom of the stripper B. The pressure is reduced using a
control valve 13 and the discharged urea synthesis solution becomes
a gas-liquid mixture (pressure reduction step). With the control
value 13, usually the pressure is reduced to between 15 and 20 bar
(for example, 17 bar), and thus the gas-liquid mixture with a
temperature of between 130 and 140.degree. C. is obtained. The
concentration of each of ammonia and carbon dioxide contained in
the gas-liquid mixture is preferably between 10 and 15 wt %. An
apparatus for heating the obtained gas-liquid mixture may be
provided.
[0028] The gas-liquid mixture is introduced into the purification
system C. In the purification system C, the unreacted substances
including ammonia, carbon dioxide, and water is separated from the
gas-liquid mixture. This provides the purified urea solution and
moreover the separated unreacted substances are recovered
(purification step).
[0029] In the purification system C, the gas-liquid mixture is
placed under the pressure suitable to separate the unreacted
substances including ammonia, carbon dioxide, and water. By heating
with the steam, moreover, the substantial aqueous urea solution is
obtained. In general, when the total amount of ammonia and carbon
dioxide remaining in the gas-liquid mixture is approximately 15 wt
% or more, for example, the two-stage system as disclosed in Patent
Literature 3 is used. This system includes the medium-pressure
decomposition column of between 15 and 20 bar (for example, 17
bar), and the low-pressure decomposition column of between 2 and 5
bar (for example, 2.5 bar). If the total amount of residual ammonia
and carbon dioxide is less than 15 wt %, the system including only
the low-pressure decomposition column is used.
[0030] In the purification system C, ammonia and carbon dioxide
remaining in the gas-liquid mixture are removed. The heat required
for that removal can be obtained from the LP steam generated in the
condenser D as described below. The pressure of the LP steam is
decided by the operation temperature of the condenser D. As the
operation pressure in the synthesis zone is higher, the temperature
of the condenser D is higher and the pressure of the LP steam to be
generated is also higher. The pressure of LP steam is generally
between 4 and 6 bar (between 151 and 164.degree. C.). In the
purification system C, such LP steam is used for the heating, but
the temperature that can be attained by the medium-pressure
decomposition column and the low-pressure decomposition column
(especially, the medium-pressure decomposition column) is limited.
If the saturated temperature of the steam and the process
temperature are different by 10.degree. C., the temperature of the
medium-pressure decomposition column heater can be increased up to
141.degree. C. in the case of the LP steam of 5 bar and up to
154.degree. C. in the case of the LP steam of 6 bar. The
temperature can be increased further but in this case, the heat
transfer area of the heater is increased and from the economical
point of view, the further temperature increase is not adopted. If
the temperature of the medium-pressure decomposition column is
increased, ammonium carbamate and ammonia as the unreacted residue
contained in the aqueous urea solution from the medium-pressure
decomposition column are decreased and the duty on the low-pressure
decomposition column on the downstream side is reduced.
[0031] The aqueous urea solution obtained in the purification
system C contains a small amount of ammonia and carbon dioxide. The
aqueous urea solution may be sent to a urea concentration step
through an aqueous urea solution line 5a. In the urea concentration
step, the aqueous urea solution may be concentrated by heating in
vacuum condition. The urea resulting from the concentration is sent
to a production step, where the solid urea is manufactured as a
final product.
[0032] Ammonia and carbon dioxide separated in the medium-pressure
decomposition column and the low-pressure decomposition column are
recovered by water as the absorbent solvent in absorbers for each
pressure level. The recovered solution obtained in the low-pressure
absorber has the absorbing capability under the higher pressure
condition, so that this recovered solution is sent to the
medium-pressure absorber for condensing gas from the
medium-pressure decomposition column and used as the absorbent
solvent. The recovered solution obtained in the medium-pressure
absorber, which absorbed separated ammonia and carbon dioxide
therein is pressurized up to the necessary pressure and then sent
to the condenser D. The less water in the recovered solution
obtained in medium-pressure absorber contributes to higher
conversion ratio into urea in the synthesis step. Thus, the smaller
amount of water sent to the low-pressure absorber is therefore
preferable. The water to be sent to the low-pressure absorber can
be reduced by reducing the unreacted substances separated in the
low-pressure decomposition column. To reduce the unreacted
substances in the low-pressure decomposition column, preferably, a
larger amount of unreacted substances to be separated in the
medium-pressure decomposition column, and this can be achieved by
increasing the temperature in the medium-pressure decomposition
column. For synthesizing urea, it is preferable to remove as many
unreacted substances as possible by increasing the temperature of
the medium-pressure decomposition column. The method of heating the
medium-pressure decomposition column without using the steam
generated in the urea synthesis step may be adopted.
[0033] The unreacted substances (recovered solution) recovered in
the purification system C are introduced into the condenser D
through a recovered unreacted substance line 5b. Some of the
decomposed gas separated in the stripper B (preferably from 80 to
95 wt %) is introduced into the condenser D through decomposed gas
lines 4b and 4c. In the condenser D, the unreacted substances and
the decomposed gas are cooled by the cooling medium and condensed.
Thus, the condensate is obtained (condensation step). The N/C of
the condensate obtained in the condenser D is preferably between
2.5 and 3.5, more preferably between 2.8 and 3.2.
[0034] Ammonia and carbon dioxide introduced into the condenser D
react with each other to produce ammonium carbamate, and a portion
of ammonium carbamate is turned into urea through the dehydration
reaction. Thus, the resulting condensate is preferably retained in
the condenser D for a certain length of time. Since the condensate
can be retained in the condenser D for a sufficient period of time
(25 minutes, for example), the bubble column type vertical
condensation reactor (also called condenser) is preferably used.
The vertical type condensation reactor is preferably the one
disclosed in Patent Literature 1, for example.
[0035] The cooling medium of the condenser D may be, for example,
water. By supplying water to a condenser cooling medium
introduction line 31, the LP steam (from 4 to 6 bar) is discharged
through a condenser cooling medium discharge line 32. As described
above, the LP steam is usually used to heat the medium-pressure
decomposition column and the low-pressure decomposition column. But
in the present invention, the LP steam is preferably used to heat
the raw material ammonia in the ammonia pre-heater 11.
[0036] The condensate obtained in the condenser D still contains
much unreacted raw material. Thus, the condensate is introduced
into the reactor A through a condensate introduction line 6a and
the raw material introduction line 1a (condensate introduction
step). The condensate is introduced using the ejector 12a, and the
ejector 12a uses at least a portion of the raw material ammonia as
a driving fluid. The off gas generated from the condenser D
(uncondensed gas, mainly including ammonia, carbon dioxide, and
inert gas) is returned to the purification system C through an off
gas line 6b.
[0037] Meanwhile, a portion of the decomposed gas separated in the
stripper B is introduced into the reactor A through the decomposed
gas lines 4b and 4d and the raw material introduction line 1a
(decomposed gas introduction step). By introducing a portion of the
decomposed gas directly into the reactor A, the reactor A can be
heated.
[0038] Here, it is preferable that between 5 and 20 wt % of the
decomposed gas be introduced into the reactor A. If 20 wt % or less
of the decomposed gas is introduced into the reactor A, the effect
of increasing the temperature of the reactor A by the condensation
of the decomposed gas is increased. If 5 wt % or more of the
decomposed gas is introduced into the reactor A, the temperature of
the reactor A is increased effectively. Moreover, the conversion
ratio into urea is increased, and the consumption of steam can be
reduced efficiently.
[0039] In the apparatus illustrated in FIG. 1, the condensate
introduction line 6a and the decomposed gas line 4d are connected
to the same ejector 12a, i.e., the condensate and the decomposed
gas are introduced into the reactor A through the same raw material
introduction line 1a. However, how the condensate and the
decomposed gas are introduced is not limited to the aforementioned
procedure. The condensate introduction line 6a and the decomposed
gas line 4d may alternatively be connected to the different
ejectors 12a and 12b as illustrated in FIG. 2. The condensate and
the decomposed gas may be introduced into the reactor A through the
different raw material introduction lines 1a and 1b. Here,
preferably, the ejector 12b uses at least a portion of the raw
material ammonia as a driving fluid. The introduction of the
decomposed gas from the decomposed gas line 4d to the reactor A is
not necessarily by the ejector 12b.
[0040] According to the present invention as described above, the
temperature of the reactor A can be increased while the decrease in
amount of carbon dioxide to be introduced into the stripper B is
suppressed as much as possible and without heating the raw material
ammonia too high. As a result, according to the present invention,
it is possible to increase the conversion ratio into urea and to
reduce the consumption of the steam.
EXAMPLES
Example 1
[0041] Urea is synthesized using the apparatus illustrated in FIG.
1. The pressure of the synthesis zone (reactor A and stripper B) is
set to 160 bar. The condition is set so that the molar ratio (N/C)
of the NH.sub.3 component to the CO.sub.2 component in the reactor
A is 3.7. In addition, the condition is set so that the molar ratio
of the H.sub.2O component to the CO.sub.2 component in the reactor
A is 0.58. The H.sub.2O component is calculated by excluding the
amount of water generated by the urea synthesis from the amount of
water existing actually. That is to say, the molar amount of the
H.sub.2O component is obtained by subtracting the molar amount of
urea from the molar amount of water.
[0042] To the reactor A, 10 wt % of carbon dioxide necessary as the
raw material is introduced. In addition, the entire amount of raw
material ammonia is heated up to 140.degree. C. and introduced into
the reactor A. Thus, the operation temperature of the reactor A is
maintained at 182.degree. C. The reaction is caused at this
temperature. The synthesis mixture obtained in this reactor A is
sent to the stripper B. In the stripper, stripping is performed
using rest of raw material carbon dioxide as an auxiliary agent
while heating is carried out with the steam of 20 bar, so that the
urea synthesis solution and the decomposed gas are separated. The
steam is consumed by 0.66 tons per ton of urea in the stripper
B.
[0043] To the shell side of the vertical submerge type condenser D,
90 wt % of the decomposed gas from the stripper B is sent. The sent
decomposed gas is condensed in the presence of the recovered
solution from the purification system C, and thus the condensate is
obtained. The heat of condensation is removed by generating the
steam from the condensate supplied to the tube. The generated steam
is used as the steam for heating, which was necessary in the
purification step and the subsequent urea concentration step. The
condensate generated in the condenser D is returned to the reactor
A using the ejector 12a, which uses the raw material ammonia heated
up to 140.degree. C. as the driving fluid.
[0044] The rest of the decomposed gas from the stripper B (10 wt %)
is sent to the reactor A together with the condensate by the
ejector 12a. Here, the temperature of the reactor A is increased by
4.degree. C. This increases the conversion ratio of carbon dioxide
into urea by 1%. The consumption of steam (20 bar) in the stripper
B is reduced by 0.045 tons (approximately 7 wt %) per ton of
urea.
Example 2
[0045] The urea is synthesized under the same condition as that of
Example 1 except that 80 wt % of the decomposed gas from the
stripper B is sent to the condenser D and the rest 20 wt % is sent
to the reactor A together with the condensate by the ejector 12a.
Here, the temperature of the reactor A is increased by 4.degree. C.
In addition, when the operation pressure is increased up to 165
bar, the temperature of the reactor A is increased by 8.degree. C.,
in which case the conversion ratio of carbon dioxide into urea
increases by 2% and the consumption of steam (20 bar) in the
stripper B is reduced by 0.1 tons (approximately 15 wt %) per ton
of urea.
DESCRIPTION OF NUMERALS
[0046] A Reactor [0047] B Stripper [0048] C Purification system
[0049] D Condenser [0050] 1 Ammonia introduction line [0051] 1a Raw
material introduction line [0052] 1b Raw material introduction line
[0053] 2 Carbon dioxide introduction line [0054] 2a Carbon dioxide
introduction line [0055] 2b Carbon dioxide introduction line [0056]
3a Synthesis mixture line [0057] 4a Urea synthesis solution line
[0058] 4b Decomposed gas line [0059] 4c Decomposed gas line [0060]
4d Decomposed gas line [0061] 5a Aqueous urea solution line [0062]
5b Recovered unreacted substance line [0063] 6a Condensate
introduction line [0064] 6b Off gas line [0065] 11 Ammonia
pre-heater [0066] 12a Ejector [0067] 12b Ejector [0068] 13 Control
valve [0069] 21 Stripper heating medium introduction line [0070] 22
Stripper heating medium discharge line [0071] 31 Condenser cooling
medium introduction line [0072] 32 Condenser cooling medium
discharge line
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