Urea hydrolysis reaction device and use method

Abstract

The invention belongs to the technical field of urea hydrolyzers, and discloses a urea hydrolysis reaction device and a use method.The side face of a urea hydrolysis reactor is connected with a urea solution mother pipe through a urea solution inlet pipeline, and the bottom of the urea hydrolysis reactor is connected with a urea solution storage tank through a blow-off pipeline; a urea solution inlet pipeline test branch is arranged on the urea solution mother pipe; the blow-off pipeline is provided with a blow-off pipeline first branch of which the two ends are connected to the blow-off pipeline, and the blow-off pipeline first branch is provided with a blow-off pipeline test branch; densimeters are arranged on the blow-off pipeline test branch and the urea solution inlet pipeline test branch. According to the method, the bottom deposition condition of the urea hydrolysis reactor can be judged according to the difference value between the densities of the liquid entering the urea hydrolysis reactor and the liquid flowing out of the urea hydrolysis reactor. By comparing the liquid inlet density and the liquid outlet density of the urea hydrolysis reactor, the pollution discharge time and the pollution discharge direction are judged, and automatic pollution discharge is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of urea hydrolyzers, and in particular relates to a urea hydrolysis reaction device and a use method thereof. Background Art

[0002] The mass concentration of urea solution in the urea hydrolysis reaction is about 50%. There are some impurities in industrial urea, mainly biuret, iron ions and sulfates. These components do not undergo hydrolysis reaction and will gradually deposit at the bottom of the reactor as the operation time of the hydrolysis reactor increases, affecting the heat exchange effect and ammonia production output of the hydrolysis reactor. Therefore, regular sewage discharge is required. At present, the sewage discharge of the hydrolyzer is generally once every 1 to 2 weeks, and the sewage discharge volume is about 5% to 10% of the liquid volume in the reactor. This method cannot accurately judge the time when sewage discharge is required and whether the sewage discharge can be reused. The sewage discharge mostly goes to the wastewater pool, which easily causes waste of urea solution.

[0003] The Chinese patent publication number is CN108295657A, and the name is a patent application for a low-temperature compartment-type denitrification reactor and a flue gas denitrification method, which includes: a waste liquid recovery box; a waste liquid drainage pipe, one end of which is suitable for communicating with the waste liquid drainage port of the hydrolyzer, and one end is arranged in the waste liquid recovery box; a filter pipe, one end of which is arranged inside the waste liquid recovery box, and one end is suitable for communicating with the urea solution inlet of the hydrolyzer, and the filter pipe is provided with a first automatic valve, a Y-type filter, a Lansberg filter, a high-pressure pump, a pressure gauge, and a check valve in sequence; a desalted water pipe, one end of which is arranged in the waste liquid recovery box, and one end is suitable for connecting to the desalted water interface, and a first desalted water automatic valve is provided on the desalted water pipe near the waste liquid recovery box, and a second desalted water automatic valve is provided near the desalted water interface. This patent application can achieve sewage discharge, but it cannot judge when to discharge sewage. Summary of the invention

[0004] In order to overcome the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a urea hydrolysis reaction device and a method of use. Since the specific gravity of impurities in industrial urea is greater than that of urea particles, the density of the sedimentary liquid at the bottom of the reactor is also greater than that of the urea solution. The present invention uses a densitometer to monitor and compare the density of the hydrolyzer inlet liquid and the bottom discharge liquid to determine whether sewage discharge is required and the sewage discharge destination, thereby avoiding waste of urea solution.

[0005] To achieve the above object, the technical solution adopted by the present invention is: A urea hydrolysis reaction device comprises: a urea hydrolysis reactor, wherein the side of the urea hydrolysis reactor is connected to a urea solution main pipe through a urea solution inlet pipe, and the bottom of the urea hydrolysis reactor is connected to a urea solution storage tank through a sewage pipe; a urea solution inlet pipe test branch is arranged on the urea solution main pipe; a sewage pipe first branch with both ends connected to the sewage pipe is arranged on the sewage pipe, and a sewage pipe test branch is arranged on the first sewage pipe branch; a sewage pipe second branch and a third pneumatic switch valve are arranged in sequence on the sewage pipe between the two ends of the first sewage pipe branch along the direction from the urea hydrolysis reactor to the urea solution storage tank; the other end of the second sewage pipe branch is connected to a wastewater tank, and a fourth pneumatic switch valve is arranged on the second sewage pipe branch; a density meter is arranged on both the sewage pipe test branch and the urea solution inlet pipe test branch.

[0006] Optionally, the urea solution inlet pipe is connected to a high-temperature water washing pipe through a connecting pipe, one end of the high-temperature water washing pipe is connected to a high-temperature flushing pipe, and the other end is connected to the sewage pipe between the connection between the second branch of the sewage pipe and the sewage pipe and the connection between the first branch of the sewage pipe and the sewage pipe.

[0007] Optionally, a first check valve is provided on the connecting pipeline.

[0008] Optionally, a third check valve is provided on the high-temperature water washing pipeline.

[0009] Optionally, both the first branch of the sewage pipe and the second branch of the sewage pipe are provided with a check valve.

[0010] Optionally, a twelfth manual ball valve and an eleventh manual ball valve are respectively provided at both ends of the third pneumatic switch valve on the first branch of the sewage pipe; a thirteenth manual ball valve and a fourteenth manual ball valve are respectively provided at both ends of the fourth pneumatic switch valve on the second branch of the sewage pipe.

[0011] Optionally, a first thermometer is provided on the urea solution inlet pipe, and a second thermometer is provided on the first branch of the sewage pipe.

[0012] Optionally, a back pressure valve is provided on the first branch of the sewage pipe, between the connection between the first branch of the sewage pipe and the sewage pipe and the sewage pipe test branch, and the set pressure of the back pressure valve is lower than the hydrolysis reaction pressure.

[0013] Optionally, the urea solution inlet pipe is connected to a discharge pipe, and a third manual ball valve is provided on the discharge pipe.

[0014] The method for using the urea hydrolysis reaction device comprises the following steps: Calculate the density deviation between the sewage pipe test branch and the urea solution inlet pipe test branch; when the density deviation is less than 2.6%, no sewage discharge is required; When the density deviation is between 2.6% and 5.2%, the third pneumatic switch valve is opened to discharge the urea solution into the urea solution storage tank until the density deviation is less than 2.6%; When the density deviation exceeds 5.2%, the fourth pneumatic switch valve is opened to discharge the urea solution into the wastewater pool until the deviation is less than 5.2%. The fourth pneumatic switch valve is closed, and then the third pneumatic switch valve is opened to continue discharging the urea solution into the urea solution storage tank until the density deviation is less than 2.6% and the discharge is stopped.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention can judge the bottom sedimentation of the urea hydrolysis reactor by the difference between the density of the liquid entering the urea hydrolysis reactor and the liquid flowing out of the urea hydrolysis reactor. There will be salt solution with high density deposited in the urea hydrolysis reactor. In order to discharge sewage in time, by connecting the sewage pipe to the bottom of the urea hydrolysis reactor and setting a branch with a density meter, the bottom sedimentation of the salt solution of the urea hydrolysis reactor can be judged. When the density increases, it indicates that the bottom salt solution is deposited. By controlling the third pneumatic switch valve to open, the flow to the urea solution storage tank can be increased; by controlling the fourth pneumatic switch valve to open, the discharge flow can be further expanded, and the solution can be discharged to the wastewater pool, so as to timely discharge sewage from the urea hydrolysis reactor. The present invention judges the sewage discharge time and destination by comparing the inlet and discharge densities of the urea hydrolysis reactor, and realizes automatic sewage discharge. It is beneficial to save urea consumption, avoid hydrolyzer fouling, and affect the heat exchange effect.

[0016] Furthermore, the first branch of the sewage pipe and the second branch of the sewage pipe of the present invention are both provided with check valves, which can prevent the urea solution from flowing back to the hydrolyzer.

[0017] Furthermore, the urea solution inlet pipe and the sewage discharge pipe of the present invention are both connected to the high-temperature water washing pipe, which can flush the blocked parts of the valves and pipes.

[0018] Furthermore, the present invention provides a back pressure valve on the first branch of the sewage pipe. By setting the pressure of the back pressure valve to be lower than the pressure of the urea hydrolysis reactor, the bottom solution of the urea hydrolysis reactor can be continuously transferred to the urea solution storage tank through the first branch. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings described herein are for explanation purposes only and are not intended to limit the scope of the present invention in any way. In addition, the shapes and proportional dimensions of the components in the drawings are only for illustration purposes to help understand the present invention, and are not intended to specifically limit the shapes and proportional dimensions of the components of the present invention. In the drawings: Figure 1 It is a schematic diagram of the structure of the device of the present invention; Among them, 1. urea hydrolysis reactor; 2. high-temperature flushing pipe; 3. urea solution main pipe; 4. urea solution inlet pipe; 41. urea solution inlet pipe first branch; 42. urea solution inlet pipe test branch; 5. high-temperature water washing pipe; 51. connecting pipe; 6. sewage pipe; 61. sewage pipe first branch; 611. sewage pipe test branch; 62. sewage pipe second branch; 7. urea solution storage tank; 8. wastewater tank; 101. first manual ball valve; 102. second manual ball valve; 103. third manual ball valve; 104. fourth manual ball valve; 105. fifth manual ball valve; 106. sixth manual ball valve; 107. seventh manual ball valve 10. the first pneumatic switch valve; 102. the second pneumatic switch valve; 103. the third pneumatic switch valve; 104. the fourth pneumatic switch valve; 301. the first check valve; 302. the second check valve; 303. the third check valve; 304. the fourth check valve; 305. the fifth check valve; 401. the first thermometer; 402. the second thermometer; 501. the back pressure valve. DETAILED DESCRIPTION

[0020] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0021] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only and are not intended to be the only embodiment.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0023] The present invention is described in detail below with reference to the accompanying drawings.

[0024] like Figure 1 As shown, a urea hydrolysis reaction device of the present invention comprises: a urea hydrolysis reactor 1, wherein the side of the urea hydrolysis reactor 1 is connected to a urea solution main pipe 3 through a urea solution inlet pipe 4, and the bottom of the urea hydrolysis reactor 1 is connected to a urea solution storage tank 7 through a sewage pipe 6; a urea solution inlet pipe test branch 42 is arranged on the urea solution main pipe 3; a sewage pipe first branch 61 with both ends connected to the sewage pipe 6 is arranged on the sewage pipe 6, and the sewage pipe first branch 61 A sewage pipe test branch 611 is arranged on the sewage pipe 6; a sewage pipe second branch 62 and a third pneumatic switch valve 203 are arranged in sequence on the sewage pipe 6 between the two ends of the first sewage pipe branch 61 along the urea hydrolysis reactor 1 to the urea solution storage tank 7; the other end of the second sewage pipe branch 62 is connected to the wastewater tank 8, and the second branch 62 is provided with a fourth pneumatic switch valve 204; both the sewage pipe test branch 611 and the urea solution inlet pipe test branch 42 are provided with a density meter.

[0025] The present invention detects the density of the liquid entering and flowing out of the urea hydrolysis reactor 1 through the urea solution inlet pipe test branch 42 and the sewage pipe test branch 611, respectively, and can reflect the bottom sedimentation condition of the urea hydrolysis reactor according to the difference between the inflow and outflow liquid densities.

[0026] Since there will be high-density salt solution deposited in the urea hydrolysis reactor, in order to discharge sewage in time, the sewage discharge pipe is connected to the bottom of the urea hydrolysis reactor to determine the salt solution deposition at the bottom of the urea hydrolysis reactor.

[0027] When the density increases, it indicates that the salt solution is deposited at the bottom. By controlling the third pneumatic switch valve to open, the flow to the urea solution storage tank can be increased; by controlling the fourth pneumatic switch valve to open, the discharge flow can be further expanded and the solution can be discharged to the wastewater pool, thereby discharging sewage from the urea hydrolysis reactor in a timely manner.

[0028] Example 1 like Figure 1As shown, in this embodiment, the side of the urea hydrolysis reactor 1 is connected to the urea solution main pipe 3 through the urea solution inlet pipe 4, and the bottom of the urea hydrolysis reactor 1 is connected to the urea solution storage tank 7 through the sewage pipe 6.

[0029] The urea solution inlet pipe 4 is fed with liquid from the urea solution main pipe 3 to input the urea solution into the urea hydrolysis reactor 1. A first manual ball valve 101, a first pneumatic switch valve 201, a second pneumatic switch valve 202, a second check valve 302, a fourth manual ball valve 104 and a fifth manual ball valve 105 are sequentially arranged on the urea solution inlet pipe 4 along the urea solution main pipe 3 toward the urea hydrolysis reactor 1.

[0030] The inlet end of the first branch 41 of the urea solution inlet pipeline is connected to the urea solution inlet pipeline 4 between the urea solution main pipe 3 and the first manual ball valve 101, and the outlet end of the first branch 41 of the urea solution inlet pipeline is connected between the fourth manual ball valve 104 and the fifth manual ball valve 105 of the urea solution inlet pipeline 4. A second manual ball valve 102 is provided on the first branch 41 of the urea solution inlet pipeline. A urea solution inlet pipeline test branch 42 is provided between the outlet end of the first branch 41 of the urea solution inlet pipeline and the fifth manual ball valve 105, and a density meter is provided in the urea solution inlet pipeline test branch 42. A first thermometer 401 is provided on the urea solution inlet pipeline 4 between the fifth manual ball valve 105 and the urea hydrolysis reactor 1.

[0031] The high-temperature water washing pipeline 5 is connected between the first pneumatic switch valve 201 and the second pneumatic switch valve 202 of the urea solution inlet pipeline 4 via a connecting pipeline 51. The connecting pipeline 51 is provided with a first check valve 301 and a fifteenth manual ball valve 115 in sequence along the high-temperature water washing pipeline 5 toward the urea solution inlet pipeline 4.

[0032] A third manual ball valve 103 is connected between the second pneumatic switch valve 202 and the second check valve 302 of the urea solution inlet pipe 4 through a discharge pipe. The third manual ball valve 103 is normally closed and is used to control sewage discharge.

[0033] One end of the high-temperature water washing pipe 5 is connected to the high-temperature flushing pipe 2, and the other end is connected to the sewage pipe 6. The high-temperature water washing pipe 5 is sequentially provided with a third check valve 303 and a sixth manual ball valve 106 along the direction from the high-temperature flushing pipe 2 to the sewage pipe 6.

[0034] The sewage pipe 6 is provided with a seventh manual ball valve 107 , a ninth manual ball valve 109 , a fifth check valve 305 , an eleventh manual ball valve 111 , a third pneumatic switch valve 203 and a twelfth manual ball valve 112 in sequence along the direction from the urea hydrolysis reactor 1 to the urea solution storage tank 7 .

[0035] Among them, the inlet end of the first branch 61 of the sewage pipe is connected between the seventh manual ball valve 107 and the ninth manual ball valve 109 , and the outlet end of the first branch 61 of the sewage pipe is connected between the twelfth manual ball valve 112 of the sewage pipe 6 and the urea solution storage tank 7 .

[0036] The first branch 61 of the sewage pipe is provided with an eighth manual ball valve 108 , a fourth check valve 304 , a back pressure valve 501 , a second temperature gauge 402 and a tenth manual ball valve 110 in sequence from the inlet end to the outlet end.

[0037] By closing the tenth manual ball valve 110 , the urea hydrolysis reactor 1 can be isolated from the urea solution storage tank.

[0038] By providing the second temperature meter 402, the temperature of the discharged solution can be monitored to avoid condensation and blockage of the pipeline.

[0039] Among them, a sewage pipe test branch 611 is arranged on the first branch 61 of the sewage pipe between the back pressure valve 501 and the second thermometer 402, and the inlet end and the outlet end of the sewage pipe test branch 611 are both arranged between the back pressure valve 501 and the second thermometer 402, and a density meter is arranged in the sewage pipe test branch 611.

[0040] The back pressure valve 501 is operated by the elastic force of the built-in spring: when the system pressure is smaller than the set pressure, the diaphragm blocks the pipeline under the action of the spring force; when the system pressure is greater than the set pressure, the diaphragm compresses the spring, the pipeline is connected, and the liquid passes through the back pressure valve 501. The operating pressure of the urea hydrolysis reactor 1 is 0.55~0.6MPa, and the set pressure of the back pressure valve is lower than 0.55MPa, so that the back pressure valve is always in the open state, and the liquid flows through the density meter 611.

[0041] The inlet end of the second branch 62 of the sewage pipe is connected between the fifth check valve 305 and the eleventh manual ball valve 111 of the sewage pipe 6, and the outlet end of the second branch 62 of the sewage pipe is connected to the wastewater tank 8. The second branch 62 of the sewage pipe is provided with a thirteenth manual ball valve 113, a fourth pneumatic switch valve 204 and a fourteenth manual ball valve 114 in sequence from the inlet end to the outlet end.

[0042] By providing the thirteenth manual ball valve 113 and the fourteenth manual ball valve 114 , when the fourth pneumatic switch valve 204 is damaged, the fourth pneumatic switch valve 204 can be replaced by closing the thirteenth manual ball valve 113 and the fourteenth manual ball valve 114 .

[0043] By providing the eleventh manual ball valve 111 and the twelfth manual ball valve 112 , when the third pneumatic switch valve 203 is damaged, the third pneumatic switch valve 203 can be replaced by closing the eleventh manual ball valve 111 and the twelfth manual ball valve 112 .

[0044] Example 2 During the daily operation of the urea hydrolyzer, it is impossible to determine the exact time of sewage discharge.

[0045] A urea hydrolysis reaction device of the present invention, when used, comprises the following steps: The density displayed by the density meter on the urea solution inlet pipeline test branch 41 represents the hydrolyzer inlet density, and the density displayed by the density meter on the sewage pipeline test branch 611 represents the drainable density. The hydrolyzer inlet density and the drainable density values ​​are uploaded to the DCS screen at the same time, and the deviation percentage between the two is calculated.

[0046] When the deviation is less than 2.6%, no blowdown is required.

[0047] When the density deviation is between 2.6% and 5.2%, the third pneumatic switch valve 203 on the sewage pipe 6 to the urea solution storage tank 7 is automatically opened to discharge the urea solution to the urea solution storage tank 7 until the density deviation drops to within 2.6%.

[0048] When the density of the discharged liquid suddenly increases by more than 5.2%, the fourth pneumatic switch valve 204 on the sewage pipe to the wastewater tank 8 is automatically opened, and the urea solution is discharged to the wastewater tank 8 until the deviation is less than 5.2%. The fourth pneumatic switch valve 204 is closed, and then the third pneumatic switch valve 203 on the sewage pipe 6 to the urea solution storage tank 7 is opened to continue discharging the urea solution to the urea solution storage tank 7 until the density deviation is less than 2.6% and the sewage is stopped.

[0049] When the urea solution main pipe 3 needs to be flushed at high temperature, the fifteenth manual ball valve 115 is opened; when the sewage pipe 6 needs to be flushed at high temperature, the sixth manual ball valve 106 is opened.

[0050] The system high-temperature flushing is divided into two routes: the urea solution inlet pipeline and the urea hydrolyzer sewage pipeline. When the hydrolyzer is shut down, open 115, close 105, 201, and 101, flush the urea solution inlet pipeline, and the sewage after flushing is discharged to the ditch through 103; when flushing the sewage pipeline, close 107 and open the valves after 108 and 109.

[0051] As shown in the following table, the density of urea solution at different temperatures and different mass concentrations is input into the DCS system. The inlet concentration of the urea hydrolysis reactor 1 is 50%. As the impurities at the bottom continue to settle, the density of the bottom solution continues to rise. By setting a critical density for starting sewage discharge (60% urea solution density at the current temperature), sewage is automatically discharged to the urea solution storage tank 7. If the density exceeds 70% urea solution density at the current temperature, sewage is automatically discharged to the wastewater tank 8.

[0052] Table 1

[0053] Unless otherwise specified, the device elements involved in the above embodiments are all conventional device elements, and the structural settings, working modes or control modes involved are all conventional settings, working modes or control modes in the art unless otherwise specified.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in the field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A urea hydrolysis reaction device, characterized in that: include: A urea hydrolysis reactor (1), wherein the side of the urea hydrolysis reactor (1) is connected to a urea solution main pipe (3) via a urea solution inlet pipe (4), and the bottom of the urea hydrolysis reactor (1) is connected to a urea solution storage tank (7) via a sewage pipe (6); a urea solution inlet pipe test branch (42) is provided on the urea solution main pipe (3); a sewage pipe first branch (61) whose two ends are connected to the sewage pipe (6) is provided on the sewage pipe (6); and a sewage pipe test branch (42) is provided on the sewage pipe (6). A second branch of the sewage pipe (62) and a third pneumatic switch valve (203) are sequentially arranged on the sewage pipe (6) between the two ends of the first branch of the sewage pipe (61) in a direction from the urea hydrolysis reactor (1) to the urea solution storage tank (7); the other end of the second branch of the sewage pipe (62) is connected to a wastewater tank (8), and a fourth pneumatic switch valve (204) is arranged on the second branch (62); and a density meter is arranged on both the sewage pipe test branch (611) and the urea solution inlet pipe test branch (42).

2. A urea hydrolysis reaction device according to claim 1, characterized in that: The urea solution inlet pipe (4) is connected to a high-temperature water washing pipe (5) via a connecting pipe (51); one end of the high-temperature water washing pipe (5) is connected to a high-temperature flushing pipe (2), and the other end is connected to the sewage pipe (6) between the connection between the second branch (62) of the sewage pipe and the sewage pipe (6) and the connection between the first branch (61) of the sewage pipe and the sewage pipe (6).

3. A urea hydrolysis reaction device according to claim 2, characterized in that: The connecting pipe (51) is provided with a first check valve (301).

4. A urea hydrolysis reaction device according to claim 2, characterized in that: The high-temperature water washing pipeline (5) is provided with a third check valve (303).

5. A urea hydrolysis reaction device according to claim 1, characterized in that: Both the first branch (61) of the sewage pipe and the second branch (62) of the sewage pipe are provided with check valves.

6. A urea hydrolysis reaction device according to claim 1, characterized in that: A twelfth manual ball valve (112) and an eleventh manual ball valve (111) are respectively arranged at both ends of the third pneumatic switch valve (203) on the first branch (61) of the sewage discharge pipeline; a thirteenth manual ball valve (113) and a fourteenth manual ball valve (114) are respectively arranged at both ends of the fourth pneumatic switch valve (204) on the second branch (62) of the sewage discharge pipeline.

7. A urea hydrolysis reaction device according to claim 1, characterized in that: The urea solution inlet pipe (4) is provided with a first thermometer (401), and the first branch (61) of the sewage discharge pipe is provided with a second thermometer (402).

8. A urea hydrolysis reaction device according to claim 1, characterized in that: A back pressure valve (501) is provided on the first branch (61) of the sewage pipe, between the connection between the first branch (61) of the sewage pipe and the sewage pipe (6) and the sewage pipe test branch (611), and the set pressure of the back pressure valve (501) is lower than the hydrolysis reaction pressure.

9. A urea hydrolysis reaction device according to claim 1, characterized in that: The urea solution inlet pipe (4) is connected to a discharge pipe, and a third manual ball valve (103) is provided on the discharge pipe.

10. The method for using the urea hydrolysis reaction device according to any one of claims 1 to 9, characterized in that: The following steps are involved: Calculating the density deviation between the sewage pipe test branch (611) and the urea solution inlet pipe test branch (42); when the density deviation is less than 2.6%, no sewage discharge is required; When the density deviation is between 2.6% and 5.2%, the third pneumatic switch valve (203) is opened to discharge the urea solution into the urea solution storage tank (7) until the density deviation is less than 2.6%; When the density deviation exceeds 5.2%, the fourth pneumatic switch valve (204) is opened to discharge the urea solution into the wastewater tank (8) until the deviation is less than 5.2%. The fourth pneumatic switch valve (204) is closed, and then the third pneumatic switch valve (203) is opened to continue discharging the urea solution into the urea solution storage tank (7) until the density deviation is less than 2.6%, and then the discharge is stopped.

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