Patent Publication Number: US-2023158470-A1

Title: Continuous synthesis system of urea

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This claims the benefit of priority from Chinese Patent Application No. 202210881728.4, filed on Jul. 26, 2022. The content of the aforementioned application, including any intervening amendments thereto, is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     This application relates to urea production, and more particularly to a continuous synthesis system of urea. 
     BACKGROUND 
     Urea synthesis tower is widely used in the existing urea synthesis process, but it fails to achieve the continuous-flow synthesis. In the synthesis process, a large number of raw materials are required to be introduced at one time, and the large volume of the synthesis tower leads to a small specific surface area (specific surface area=heat exchange area/volume), resulting in poor heat transfer at a center zone of the synthesis tower. Thus, it fails to ensure the product quality and consistency, and will lead to safety risks such as temperature runaway. 
     SUMMARY 
     An objective of this application is to provide a continuous synthesis system of urea, which enables the continuous urea synthesis with good heat transfer performance. 
     Technical solutions of this application are described as follows. 
     This application provides a continuous synthesis system of urea, comprising:
     a reactor;   a mixing buffer tank;   a feeding pump;   a pressure regulating valve;   a first heat exchanger; and   a back pressure valve;   wherein the mixing buffer tank is configured to accommodate a first raw material;   one end of the feeding pump is connected to the mixing buffer tank, and the other end of the feeding pump is connected to the reactor; the feeding pump is configured to pump the first raw material in the mixing buffer tank to the reactor;   the pressure regulating valve is connected to the reactor; the pressure regulating valve is configured to transfer a second raw material to the reactor and regulate a pressure of the second raw material; the second raw material is gaseous; and the reactor is configured for reaction of the first raw material and the second raw material to generate a target product;   the first heat exchanger is connected to the reactor; and the first heat exchanger is configured to regulate a temperature inside the reactor to a first preset temperature; and   the back pressure valve is connected to an end of the reactor away from the feeding pump; and the back pressure valve is configured to maintain a pressure of the continuous synthesis system at a preset pressure.   

     In an embodiment, the reactor is configured to be oscillatable to fully mix the first raw material and the second raw material in the reactor. 
     In an embodiment, the reactor comprises a first circular tube and a second circular tube; the second circular tube is provided inside the first circular tube; a side wall of the second circular tube is provided with a plurality of through holes; and the first circular tube is configured to be radially swingable to drive the second circular tube to move back and forth inside the first circular tube. 
     In an embodiment, the mixing buffer tank is provided with an agitating member; and the agitating member is configured to be axially rotatable to agitate the first raw material. 
     In an embodiment, a one-way valve is provided between the pressure regulating valve and the reactor; and the second raw material is configured to be transferred to the reactor through the pressure regulating valve and the one-way valve in sequence. 
     In an embodiment, the continuous synthesis system further comprises a preheater; a first end of the preheater is connected to the reactor, and a second end of the preheater is connected to the feeding pump; the preheater is connected to the first heat exchanger; and the first heat exchanger is configured to regulate a temperature inside the preheater to the first preset temperature. 
     In an embodiment, a gas flow controller is provided between the pressure regulating valve and the reactor; and the gas flow controller is configured to control a volumetric flow rate of the second raw material. 
     In an embodiment, the continuous synthesis system further comprises a separator; wherein one side of a top end of the separator is connected to an end of the back pressure valve away from the reactor, and the other side of the top end of the separator is connected to a tail gas treatment device. 
     In an embodiment, the continuous synthesis system further comprises a gas condensation dryer; a bottom end of the gas condensation dryer is connected to the top end of the separator, and a top end of the gas condensation dryer is connected to the tail gas treatment device. 
     In an embodiment, the continuous synthesis system further comprises at least one of a dryer, a cooler, a temperature sensor and a pressure sensor; 
     wherein the dryer is provided between the pressure regulating valve and the reactor; and the dryer is configured to dry the second raw material;   the cooler is provided between the reactor and the back pressure valve; and the cooler is configured to cool the target product output from the reactor;   the temperature sensor is configured to detect a temperature of the continuous synthesis system;   the pressure sensor is configured to detect a pressure of the continuous synthesis system.   

     Compared with the prior art, this application has the following beneficial effects. 
     In the continuous synthesis system of urea, the first raw material is pumped by the feeding pump from the mixing buffer tank to the reactor, and the second raw material is fed to the reactor through the pressure regulating valve. The first raw material is reacted with the second raw material in the reactor to generate the target product. The first heat exchanger is configured to keep the temperature inside the reactor at the first preset temperature required by the reaction. The back pressure valve is configured to maintain the pressure inside the reactor at a preset pressure required by the reaction, so as to ensure the reliable reaction. In addition, the first raw material and the second raw material can be continuously fed to the reactor, and the reactor can continuously output the target product, enabling the continuous-flow synthesis of the target product from raw materials. Moreover, since the target product can be continuously output from the reactor, the reactor can be designed in a smaller volume without influencing the synthesis efficiency. Compared with the prior art, the size of the reactor used herein is optimized to reach a larger specific surface area and enhanced heat transfer performance, thereby avoiding the occurrence of the temperature runaway. Moreover, the synthesis system provided herein enables the continuous synthesis and output of the urea, and thus can ensure the quality consistency of products of different batches. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to clearly explain the technical solutions in the embodiments of the present application or the prior art, the drawings that need to be used in the description of the embodiments or the prior art are briefly described below. Obviously, illustrated in the drawings are merely some embodiments of this application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without paying creative effort. 
         FIG.  1    is a structural diagram of a continuous synthesis system of urea according to an embodiment of this application; and 
         FIG.  2    is a structural diagram of a reactor to an embodiment of this application. 
     
    
    
     In the drawings,  10 , reactor;  11 , first circular tube;  12 , second circular tube;  1201 , through hole;  20 , mixing buffer tank;  30 , feeding pump;  40 , pressure regulating valve;  50 , first heat exchanger;  60 , back pressure valve;  70 , agitating member;  80 , one-way valve;  90 , preheater;  100 , first delivery tube;  110 , gas flow controller;  120 , dryer;  130 , safety valve;  140 , separator;  150 , ball valve;  160 , gas condensation dryer;  170 , second heat exchanger;  180 , cooler;  190 , second delivery tube;  200 , temperature sensor;  210 , pressure sensor; and 
       1 , tail gas treatment device; and  2 , product collecting tank. 
     This application will be described in detail below with reference to the embodiments and accompanying drawings to make the objectives, functions, and advantages of this application clearer. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The technical solutions of the disclosure will be described clearly and completely below with reference to the accompanying drawings and embodiments of the disclosure. Obviously, described below are merely some embodiments of the disclosure, and are not intended to limit the disclosure. Other embodiments obtained by those of ordinary skill in the art based on the embodiments provided herein without paying creative effort shall fall within the scope of the present disclosure defined by the appended claims. 
     It should be noted that as used herein, directional indications (such as up, down, left, right, front and back) are merely intended to explain the relative position relationship and movement situation among individual components in a specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication changes accordingly. In addition, relational terms such as “first” and “second” are merely used for description, and cannot be understood as indicating or implying their relative importance or the number of indicated technical features. Thus, the features defined with “first” and “second” may explicitly or implicitly include at least one of the features. Additionally, “and/or” in the disclosure includes three solutions. For example, A and/or B includes technical solution A, technical solution B, and a combination thereof. Additionally, technical solutions of various embodiments can be combined on the premise that the combined technical solution can be implemented by those skilled in the art. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist, and does not fall within the scope of the present disclosure. 
     Referring to an embodiment shown in  FIG.  1   , a continuous synthesis system of urea includes a reactor  10 , a mixing buffer tank  20 , a feeding pump  30 , a pressure regulating valve  40 , a first heat exchanger  50  and a back pressure valve  60 . The mixing buffer tank  20  is configured to accommodate a first raw material. One end of the feeding pump  30  is connected to the mixing buffer tank  20 , the other end of the feeding pump  30  is connected to the reactor  10 . The feeding pump  30  is configured to pump the first raw material to the reactor  10 . The pressure regulating valve  40  is connected to the reactor  10 . The pressure regulating valve  40  is configured to transfer the second raw material to the reactor  10  and regulate a pressure of the second raw material. The second raw material is gaseous. The second raw material is transferred to the reactor  10  through the pressure regulating valve  40  and reacts with the first raw material in the reactor  10  to generate a target product. The first heat exchanger  50  is connected to the reactor  10 . The first heat exchanger  50  is configured to regulate a temperature inside the reactor  10  to a first preset temperature. The back pressure valve  60  is connected to an end of the reactor  10  away from the feeding pump  30 . The back pressure valve  60  is configured to maintain a pressure of the continuous synthesis system at a preset pressure. 
     The first raw material is pumped by the feeding pump  30  from the mixing buffer tank  20  to the reactor  10 , and the second raw material is fed to the reactor  10  through the pressure regulating valve  40 . The first raw material and the second raw material are allowed to react in the reactor  10  to generate the target product. The first heat exchanger  50  is provided to keep the temperature inside the reactor  10  at the first preset temperature required by the reaction. The back pressure valve  60  is configured to maintain a pressure inside the reactor  10  at a preset pressure required by the reaction, so as to ensure the reliable reaction. In addition, the first raw material and the second raw material can be continuously fed to the reactor  10 , and the reactor  10  can continuously output the target product, enabling the continuous-flow synthesis of the target product from raw materials. Moreover, since the target product can be continuously output from the reactor, the reactor  10  can be designed in a smaller volume without influencing the synthesis efficiency. Compared with the prior art, the size of the reactor  10  used herein is optimized to reach a larger specific surface area and enhanced the heat transfer performance, thereby avoiding the occurrence of the temperature runaway. 
     Specifically, since specific surface area is obtain by dividing volume into heat transfer area, the smaller the volume and the larger the specific surface area, the better the heat transfer performance of reactor  10 . 
     In this embodiment, the first raw material is methanolamine and sulfur powder. Specifically, the sulfur powder is mixed in methanolamine to form a suspension. The second raw material is carbon monoxide gas, and the target products are urea and hydrogen sulfide gas. In the actual scheme, the target products also contain ammonia gas and small amounts of unreacted sulfur powder and unreacted solvent. That is, the continuous synthesis system of urea provided herein enables continuous-flow synthesis of urea. In other embodiments, the continuous synthesis system provided herein is capable of preparing other products. When the continuous synthesis system provided herein is applied to production of other products, the types of the raw materials can be varied, and the raw material can be gaseous, liquid, and/or solid. 
     In this embodiment, the second raw material is carbon monoxide gas. The carbon monoxide gas is fed by a carbon monoxide tank. In this embodiment, since the carbon monoxide gas in the carbon monoxide gas tank has a relatively high pressure, the pressure of the carbon monoxide output from the carbon monoxide tank is reduced by adjusting the pressure regulating valve  40 . 
     In this embodiment, when a pressure of the back pressure valve  60  in the continuous synthesis system is lower than the preset pressure, the back pressure valve  60  is in a blocking state, such that the pressure of the continuous synthesis system is gradually increased with the feeding process. When the pressure of the continuous synthesis system reaches the preset pressure, the back pressure valve  60  is in a conducting state, such that the target product in the system is allowed to be discharged through the back pressure valve  60 , thereby maintaining the back pressure valve  60  at the preset pressure. 
     In an embodiment, the second raw material is  13 CO, and the target product is  13 C-urea. In this embodiment, the  13 CO is mixed with an inert gas to reduce its concentration. On one hand, even though the concentration of  13 CO is reduced, the reaction can still be completed. On the other hand, the  13 CO is very expensive, and can avoid incomplete reaction of a large amount of the  13 CO after mixed with the inert gas, so as to lower the cost. 
     In this embodiment, the first preset temperature is 40-120° C., and the preset pressure is 1-2 MPa. 
     In this embodiment, the reactor  10  is configured to be oscillatable to fully mix the first raw material and the second raw material. Specifically, the reactor  10  oscillates to make the first raw material thoroughly contacted with the second raw material, so as to fully mix the first raw material and the second raw material. 
     In an embodiment, as shown in  FIGS.  1  and  2   , the reactor includes a first circular tube  11  and a second circular tube  12 . The second circular tube  12  is provided inside the first circular tube  11 . A side wall of the second circular tube  12  is provided with a plurality of through holes  1201 . The first circular tube  11  is configured to be radially swingable to drive the second circular tube  12  to move back and forth inside the first circular tube  11 . Specifically, when the first circular tube  11  is swinged, the second circular tube  12  inside the first circular tube  11  is configured to move back and forth, such that the second raw material inside the first circular tube  11  will be fully sheared by the plurality of through holes  1201  on the side wall of the second circular tube  12 , forming a plurality of tiny bubbles. The tiny bubbles greatly increase the contact area between the first raw material and the second raw material, so as to make the first raw material fully react with the second raw material. In this embodiment, during a in the resting state, an axis of the first circular tube  11  is parallel to an axis of the second circular tube  12 . 
     In an embodiment, as shown in  FIG.  1   , the mixing buffer tank  20  is provided with an agitating member  70 . The agitating member  70  is configured to be axially rotatable to agitate the first raw material. In this embodiment, the first raw material is a suspension formed by mixing sulfur powder in methanolamine. The agitating member  70  is configured to make the sulfur powder evenly dispersed in the methanolamine to form a uniform suspension, so as to achieve uniform feeding, thereby avoiding the incomplete reaction or by-product formation due to uneven feeding. 
     Referring to the embodiment shown in  FIG.  1   , a one-way valve  80  is provided between the pressure regulating valve  40  and the reactor  10 . The second raw material is configured to be transferred to the reactor  10  through the pressure regulating valve  40  and the one-way valve  80  in sequence, and is not allowed to be returned from the reactor  10  through the one-way valve  80 . The cooperation of the one-way valve  80  and the back pressure valve  60  keeps the pressure in the continuous synthesis system to be more reliably at the preset pressure. More importantly, the pressure in the reactor  10  is reliably maintained at the preset pressure by the cooperation of the one-way valve  80  and the back pressure valve  60 . 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a preheater  90 . A first end of the preheater  90  is connected to the reactor  10 , and the second end of the preheater  90  is connected to the feeding pump  30 . The preheater  90  is connected to a first heat exchanger  50 . The first heat exchanger  50  is configured to regulate a temperature in the preheater  90  to the first preset temperature. Specifically, the preheater  90  is provided to heat the first raw material and the second raw materials to a temperature required for the reaction inside the preheater  90  before entering the reactor  10 , thereby the first raw material and the second raw material are allowed to be reacted with each other after entering the reactor  10 , improving the reaction efficiency. In this embodiment, the second end of the preheater  90  is connected to the feeding pump  30  and the one-way valve  80 , respectively. The one-way valve  80  is connected to the reactor  10  through the preheater  90 . The preheater  90  is a tubular preheater  90 . The second end of the preheater  90  away from the reactor  10  is connected to the feeding pump  30  and the one-way valve  80  respectively. 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system includes a first delivery tube  100 . Both two ends of the first delivery tube  100  are respectively connected to the first heat exchanger  50  to form a circulation loop. A middle portion of the first delivery tube  100  is in contact with an outer side wall of the preheater  9  and an outer side wall of the reactor  10 . The first heat exchanger  50  is configured to output a heat-conducting medium. The heat-conducting medium is transferred through the first delivery tube  100  and returns into the first heat exchanger  50 . When the heat-conducting medium is transferred through the first delivery tube  100 , the heat-conducting medium performs heat exchange with the preheater  90  through the side wall of the side wall of the first delivery tube  100  and the outer side wall of the preheater  90 . The heat-conducting medium performs heat exchange with the reactor  10  through the side wall of the first delivery tube  100  and the outer side wall of the reactor  10 . Specifically, the middle portion of the first delivery tube  100  is configured to wrap around the outer side wall of the preheater  9  and the outer side wall of the reactor  10  to achieve the heat exchange between the first heat exchanger  50  and the preheater  90  and the heat exchange between the first heat exchanger  50  and the reactor  10 . 
     Referring to the embodiment shown in  FIG.  1   , a gas flow controller  110  is provided between the pressure regulating valve  40  and the reactor  10 . The gas flow controller  110  is configured to control a volumetric flow rate of the second raw material. Specifically, the volumetric flow rate refers to the mass of fluid flowing through an effective cross section of a closed pipe or an open tank per unit time. The gas flow controller is provided between the pressure regulating valve  40  and the one-way valve  80 . 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a dryer  120 , which is provided between the pressure regulating valve  40  and the reactor  10 , and configured to dry the second raw material. Specifically, since the gas flow controller  110  will be easily damaged under the exposure to the liquid, the dryer  120  dries the second raw material to avoid the damage to the gas flow controller  110  caused by the moisture contained in the second raw material. In this embodiment, the dryer  120  is provided between the pressure regulating valve  40  and the gas flow controller  110 . 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a safety valve  130 . The safety valve  130  is provided between the pressure regulating valve  40  and the reactor  10 . When the pressure of the continuous synthesis system is too high, the safety valve  130  is configured to relieve pressure. In this embodiment, the safety valve  130  is provided between the gas flow controller  110  and the one-way valve  80 . 
     Specifically, the safety valve  130  includes a first end, a second end and a third end, all of which are provided spaced apart. The first end is connected to the gas flow controller  110 , the second end is connected to the one-way valve  80 , and the third end is connected to the external environment. When the pressure of the continuous synthesis system is normal, the first end is configured to be in communication with the second end of the safety valve  130 , so as to enable normal transportation of the second raw material. When the pressure of the continuous synthesis system is excessively high due to system fault or blockage or any other reasons, the first end is configured for communication with the second end of the safety valve  130  to discharge the second raw material, so as to avoid a continuous rise of the pressure in the continuous synthesis system. 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a separator  140 . One side of a top end of the separator  140  is connected to an end of the back pressure valve  60  away from the reactor  10 , and the other side of the top end of the separator  140  is connected to a tail gas treatment device  1 . Specifically, the products generated in the reactor  10  e.g., urea and hydrogen sulfide, enter the separator  140  through the back pressure valve  60 . A non-gas product, that is, urea solution remains at a bottom of the separator  140 , while the gas products (hydrogen sulfide gas, ammonia gas), which are lighter and not dissolved in the methanolamine, enter the tail gas treatment device  1  from the top of the separator  140  for tail gas treatment. In this embodiment, the separator  140  is a gas-liquid-solid multi-phase separator. The tail gas treatment device  1  is an external device. In other embodiments, the tail gas treatment device  1  may also be a part of the continuous synthesis system. 
     Referring to the embodiment shown in  FIG.  1   , a bottom end of the separator  140  is connected to a product collecting tank  2 . The non-gas product (urea) at the bottom of the separator  140  is transferred to the product collecting tank  2  for collection. In this embodiment, the product collecting tank  2  is an external device. In other embodiments, the product collecting tank  2  may also be a part of the continuous synthesis system. 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a ball valve  150 . One end of the ball valve  150  is connected to the bottom end of the separator  140 , and the other end of the ball valve  150  is connected to the product collecting tank  2 . The ball valve  150  is opened and closed to respectively control the conduction and blocking between the separator  140  and the product collecting tank  2 . Specifically, when the reaction starts, the ball valve  150  is closed. When the product in the separator  140  reaches a certain level, the ball valve  150  is opened to allow the product to be continuously transferred from the separator  140  to the product collecting tank  2 . 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a gas condensation dryer  160 . A bottom end of the gas condensation dryer  160  is connected to a top end of the separator  140 , and a top end of the gas condensation dryer  160  is connected to the tail gas treatment device  1  for tail gas treatment. Specifically, the gas (hydrogen sulfide gas) in the separator  140  is transferred to the tail gas treatment device  1  via the gas condensation dryer  160 . The gas condensation dryer  160  is provided, such that when the gas (hydrogen sulfide gas) passes through the gas condensation dryer  160 , the moisture contained in the gas is subjected to condensation and separation from the gas, and then flows back into the separator  140 , thereby reducing the loss of non-gas (urea). 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a second heat exchanger  170 . The second heat exchanger  170  is connected to the gas condensation dryer  160 . The second heat exchanger  170  is connected to maintain a temperature in the gas condensation dryer  160  at a second preset temperature. The second preset temperature is lower than the first preset temperature. At the second preset temperature, the moisture contained in the gas (hydrogen sulfide gas) will be condensed and separated from the gas. 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a cooler  180 . The cooler  180  is provided between the reactor  10  and the back pressure valve  60 . The cooler  180  is configured to cool the target product output from the reactor  10 . Specifically, the cooler  180  is a tubular cooler  180 . In this embodiment, the second heat exchanger  170  is connected to the cooler  180 . The second heat exchanger  170  is configured to keep the temperature inside the cooler  180  at the second preset temperature. 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a second delivery tube  190 . Both two ends of the second delivery tube  190  are respectively connected to the second heat exchanger  170  to form a circulation loop. A middle portion of the second delivery tube  190  is in contact with an outer side wall of the cooler  180  and an outer side wall of the gas condensation dryer  160 , respectively. The second heat exchanger  170  is configured to output a heat-conducting medium. The heat-conducting medium is transferred through the second delivery tube  190  and returns to the second heat exchanger  170 . When the heat-conducting medium is transferred through the second delivery tube  190 , the heat-conducting medium performs heat exchange with the cooler through a side wall of the second delivery tube  190  and an outer side wall of the cooler  180 , and the heat-conducting medium performs heat exchange with the gas condensing dryer  160  through the side wall of the second delivery tube  190  and the outer side wall of the gas condensing dryer  160 . Specifically, the middle portion of the second delivery tube  190  is configured to wrap around the outer side wall of the cooler  180  and the outer side wall of the gas condensation dryer  160  to achieve the heat exchange between the second heat exchanger  170  and the cooler  180  and the heat exchange between the second heat exchanger  170  and the gas condensation dryer  160 . 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a temperature sensor  200 . The temperature sensor  200  is configured to detect the temperature of the continuous synthesis system, Specifically, according to the difference between the temperature detected by the temperature sensor  200  and the actual required temperature in this embodiment, the temperature of the first heat exchanger  50  is adjusted to keep the temperature of the system at the required temperature, e.g., to keep the temperature in the reactor  10  and the temperature in the preheater  90  at the first preset temperature. 
     Referring to the embodiment shown in  FIG.  1   , a plurality of temperature sensors  200  are provided. The plurality of temperature sensors  200  are configured to reliably detect the temperature of the continuous synthesis system. In this embodiment, the number of temperature sensors  200  is eleven. A first temperature sensor  200  is located between the feeding pump  30  and the preheater  90 . A second temperature sensor  200  is located between the preheater  90  and the reactor  10 . The first temperature sensor and the second temperature sensor  200  are configured to detect a temperature of the material (here is referred to the first raw material and the second raw material) before entering the preheater  90  and a temperature of the material after leaving the preheater  90 . A third temperature sensor  200  is located at a front end of the reactor  10 , and a fourth temperature sensor  200  is located at a rear end of the reactor  10 . The third temperature sensor and the fourth temperature sensor  200  are configured to detect the temperature of the material just entering the reactor  10  and the temperature at which the material (here is referred to the pre-determined product) leaves the reactor  10 . A fifth temperature sensor  200  is located on the first delivery tube  100  between the first heat exchanger  50  and the reactor  10 , a sixth temperature sensor  200  is located on the first delivery tube  100  between the reactor  10  and the preheater  90 , and a seventh temperature sensor  200  is located on the first delivery tube  100  between the preheater  90  and the first heat exchanger  50 . The fifth temperature sensor, the sixth temperature sensor and the seventh temperature sensor  200  are respectively configured to detect the temperature of the heat-conducting medium at different positions of the first delivery tube  100  to obtain the heat transfer effect of the first heat exchanger  50  on the reactor  10  and the preheater  90 . An eighth temperature sensor  200  is located between the cooler  180  and the back pressure valve  60 . The eighth temperature sensor  200  is configured to detect the temperature of the material after leaving the cooler  180 . A ninth temperature sensor  200  is located on the second delivery tube  190  between the second heat exchanger  170  and the cooler  180 , a tenth temperature sensor  200  is located on the second delivery tube  190  between the cooler  180  and the gas condensing dryer  160 , and an eleventh temperature sensor  200  is located on the second delivery tube  190  between the gas condensing dryer  160  and the second heat exchanger  170 . The ninth temperature sensor, the tenth temperature sensor and the eleventh temperature sensor  200  are respectively configured to detect the temperature of the heat-conducting medium at different positions of the second delivery tube  190  to obtain the heat transfer effect of the second heat exchanger  170  on the cooler  180  and the gas condensing dryer  160 . 
     Referring to the embodiment shown in  FIG.  1   , the continuous synthesis system further includes a pressure sensor  210 . The pressure sensor  210  is configured to detect the pressure of the continuous synthesis system. Specifically, according to the difference between the pressure detected by the pressure sensor  210  and the required pressure provided herein, the conducting state of the safety valve  130  is adjusted, or the volumetric flow rate of the second raw material is adjusted via the gas flow controller  110 , so as to maintain the pressure of the system at the required pressure. 
     Referring to the embodiment shown in  FIG.  1   , a plurality of pressure sensors  210  are provided. The plurality of pressure sensors  210  are configured to reliably detect the pressure of the continuous synthesis system. In this embodiment, the number of pressure sensors  210  is two, one pressure sensor  210  is provided between the one-way valve  80  and the tubular preheater  90  to detect the pressure of the second raw material before entering the tubular preheater  90 , i.e., the pressure before the reaction. The other pressure sensor  210  is provided between the cooler  180  and the back pressure valve  60  to detect the pressure of the product output from the cooler  180 , i.e., the pressure after the reaction, such that the two pressure sensors  210  cooperate to achieve the monitor of the pressure of the system. 
     As shown in  FIG.  1   , a workflow of the preparation of  13 C-urea performed by the continuous synthesis system of urea is briefly described as follows. 
     Methanolamine and sulfur powder are added to the mixing buffer tank  20 , and the agitating member  70  agitates the methanolamine and sulfur powder to form a homogeneous suspension. The feeding pump  30  transfers the suspension from the mixing buffer tank  20  to the reactor  10  via the preheater  90 . 
     An external carbon monoxide tank provides  13 CO. The  13 CO is transferred to the reactor  10  through the pressure regulating valve  40 , the dryer  120 , the gas flow controller  110 , the safety valve  130 , the one-way valve  80  and the preheater  90  in sequence. 
     The suspension reacts with the  13 CO in the reactor  10  to generate  13 C-urea and hydrogen sulfide. The  13 C-urea and hydrogen sulfide in the reactor  10  are sequentially transferred to the separator  140  through the cooler  180  and the back pressure valve  60 . Since the hydrogen sulfide gas is lighter, the hydrogen sulfide gas moves upward in the separator  140  and enters the tail gas treatment device  1  for tail gas treatment through the gas condensing dryer  160 . When the hydrogen sulfide gas passes through the gas condensation dryer  160 , the moisture contained in the hydrogen sulfide gas (such as  13 C-urea liquid) is subjected to condensation and separation from the hydrogen sulfide gas, and flows back into the separator  140 . The  13 C-urea liquid entering the separator  140  directly flows into the bottom of the separator  140 . Then the  13 C-urea flows into the product collecting tank through the ball valve  150 , so as to obtain the  13 C-urea. In addition, the plurality of temperature sensors  200  and the plurality of pressure sensors  210  monitor the temperature and the pressure of the continuous synthesis system of urea in real time. According to the data fed back by the plurality of temperature sensors  200  and the plurality of pressure sensors  210 , the temperature and the pressure of the continuous synthesis system of urea are adjusted in real time by the first heat exchanger  50  and the gas flow controller  110 , so as to keep the temperature and the pressure of the continuous synthesis system at the temperature and pressure required by the reaction. 
     The continuous synthesis system of urea provided herein has high integration, and enables precise control of the reaction parameters such as temperature, pressure and flow rate, such that it is easy to identify the fault in the event of system fault, facilitating fast maintenance. 
     Described above are merely preferred embodiments of the disclosure, which are not intended to limit the scope of the application. It should be understood that any replacements, modifications and changes made by those skilled in the art without departing from the spirit of the application shall fall within the scope of the present application defined by the appended claims.