Energy-saving and carbon-reducing method and device for urea device

Abstract

According to the energy-saving and carbon-reducing method and device applied to the urea device, multiple layers of radial flow type efficient tower trays are arranged in a synthesis tower, the tower plates and sealing plates play a role in preventing materials from backmixing and short circuit in the tower, and mass and heat transfer between a gas phase and a liquid phase is enhanced through the tower tray structures; one path of gas is shunted from a gas phase of the synthesis tower and enters a newly-added medium-pressure flash drum, a material which is directly decompressed to 1.2-1.7 MPa from liquid discharged from the stripping tower is fed into a distributor above a filler section of the flash drum, and the gas phase shunted from the synthesis tower enters from the lower part of a filler through the distributor and is in countercurrent contact with the liquid from the stripping tower; decomposing one part of methylamine by utilizing gas-phase carried heat and waste heat of discharged liquid of the stripping tower; the gas phase of the flash tank and the gas phase of the medium-pressure decomposition tower are mixed and then enter a heat recovery device, and the liquid phase flows into an upper filler section of the medium-pressure decomposition tower; after the materials in the medium-pressure flash tank are subjected to gas-liquid separation, the liquid phase enters the upper part of the medium-pressure decomposition tower, and the liquid phase is sprayed to the filler layer through the distributor and is subjected to mass and heat transfer with the gas phase from the circulating heating section.

Description

Technical Field

[0001] The present application relates to the technical field of urea devices, and in particular to an energy-saving and carbon-reducing method and device applied to a urea device. Background Art

[0002] With the development of science and technology, urea plants are applied in industrial scenarios and used to synthesize urea. The urea plant includes a synthesis tower, a stripping tower, a high-pressure scrubber, a medium-pressure decomposition tower, a distillation tower, and a desorption tower. The synthesis tower, the stripping tower, the high-pressure scrubber, the medium-pressure decomposition tower, the distillation tower, and the desorption tower are all separate entities, and the heat energy of each part is provided by a corresponding heating device. Each heating device needs to be additionally installed, and the waste heat of the stripping tower and the decomposition tower is not fully utilized. Summary of the invention

[0003] The object of the present invention is to provide an energy-saving and carbon-reducing method and device for a urea device, so as to solve the problems raised in the above-mentioned background technology.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] A method and device for energy saving and carbon reduction of a urea device, comprising:

[0006] S11: Liquid ammonia comes from the synthetic ammonia unit with a pressure of 2.1 MPa (absolute) and a temperature of 6°C. It enters the inlet of the high-pressure liquid ammonia pump through the liquid ammonia filter and the liquid ammonia buffer tank. The liquid ammonia is pressurized to 16.0 MPa (absolute) by the high-pressure liquid ammonia pump. A liquid ammonia preheater is provided at the pump outlet to heat the liquid ammonia to 80-110°C. The heat source is the waste heat of the system.

[0007] S12: Liquid methylammonium and a small amount of uncondensed ammonia and carbon dioxide derived from the bottom of the high-pressure condenser are respectively sent to the bottom of the synthesis tower through two pipelines; the synthesis tower is equipped with multi-layer radial flow high-efficiency trays, and the tray structure strengthens the mass transfer and heat transfer between the gas and liquid phases. Under the comprehensive effect, the carbon dioxide conversion rate can reach more than 60%;

[0008] S13: A gas stream is diverted from the synthesis tower gas phase into a newly added medium-pressure flash tank with a built-in packing section. The material directly depressurized to 1.2-1.7 MPa from the stripping tower outlet is sent to the distributor above the flash tank packing section. The gas stream diverted from the synthesis tower enters from below the packing through the distributor and contacts with the stripping tower outlet liquid in countercurrent. The heat carried by the gas phase and the residual heat of the stripping tower outlet liquid are used to decompose part of the methylammonium. The flash tank gas phase and the medium-pressure decomposition tower gas phase are mixed and then enter the heat recovery device. The liquid phase flows into the upper packing section of the medium-pressure decomposition tower.

[0009] S14: After the gas-liquid separation, the liquid phase of the material in the medium-pressure flash tank enters the upper part of the medium-pressure decomposition tower. The liquid phase is sprayed to the packing layer through the distributor, and the gas phase from the circulating heating section is transferred with mass and heat; then it is collected at the gas lifting cap baffle and descends to the bottom of the circulating heater of the medium-pressure decomposition tower, and enters the heater tube in a rising film manner. The heater is divided into two sections. The lower section is heated by the by-product steam, and the upper section is heated by the medium-pressure drum to the low-pressure steam saturated condensate. The temperature is controlled at 145-150℃ to decompose most of the unconverted methylammonium; the medium-pressure decomposition tower and its circulating heater are key equipment, with built-in packing section, stripping section and liquid level section;

[0010] S15: The gas-liquid mixture after the urine in the vacuum pre-concentration heating tube enters a newly added evaporation heat recovery section, and the urine from the urine tank is used to continue to recover the reaction heat under a higher vacuum degree;

[0011] S16: a reboiler is arranged at the bottom of the first desorption tower, and high-pressure temperature-adjusting water is used as a heat source to reduce the amount of desorption steam;

[0012] S17: The gas phase of the desorption tower enters the lower part of the packing section of the distillation tower, and the reflux condenser, the reflux condenser level tank and the reflux pump are shut down;

[0013] S18: Set up a high-pressure temperature-adjusting water supply pipeline to the low-pressure steam drum and a steam condensate pump to supply the high-pressure temperature-adjusting water pipeline, and add a regulating valve and a flow meter respectively;

[0014] S19: Add a 0.15Mpa absorption tower and ammonia purification tower to reduce the ammonia content in the tail gas of the urea inert gas discharge tube to less than 15ppm.

[0015] Optionally, in S12, high-efficiency tower trays are used to replace conventional tower trays, which can eliminate short circuits and material backmixing, enhance material mass transfer and heat transfer, and thus improve the CO2 conversion rate.

[0016] Optionally, in S13: the 183°C gas phase of the synthesis tower is split into the lower part of the packing section of the medium-pressure flash tank, and is in countercurrent contact with the decompressed material from the high-pressure stripping tower to perform mass and heat transfer, thereby promoting the decomposition of the methylammonium liquid and playing a role in corrosion protection of the medium-pressure system pipelines and equipment.

[0017] Optionally, in S14, the medium-pressure decomposition tower and its circulating heater are a comprehensive core equipment with built-in packing section, stripping section and liquid level section, which fully utilize the by-product steam for heating and reduce the medium-pressure steam consumption. CO2 is introduced into the stripping section to balance the ammonia-carbon ratio of the low-pressure system.

[0018] Optionally, in S15, the mixed reaction of the medium-pressure gas phase and the methylammonium liquid from the low-pressure system releases heat to heat the urine in the vacuum pre-concentration tube, and the incompletely reacted gas-liquid mixture continues to condense and release heat in an evaporation heat recovery section. Under the condition of a higher vacuum section, the temperature difference between the inside and outside of the tube increases, thereby increasing the condensation amount of the unreacted gas phase and releasing more heat, thereby saving steam consumption.

[0019] Optionally, in S11, the vapor-liquid mixture of the medium-pressure flash tank and the medium-pressure decomposition tower gas phase after heat recovery enters the medium-pressure methylammonium condenser, and the remaining gas phase components continue to condense and react, and the heat released is removed by a closed independent medium-pressure temperature regulating water. The setting of the medium-pressure condenser increases the operational flexibility of the medium-pressure system, which is beneficial to increase the gas phase flow rate of the synthesis tower and reduce the load of the high-pressure scrubber.

[0020] Optionally, in S16, the high-pressure temperature-adjusted water from the high-pressure scrubber is used in a newly added reboiler to heat the ammonia water at the bottom of the first desorption tower, thereby reducing the amount of by-product steam in the desorption system.

[0021] Optionally, in S17, the gas phase of the first desorption tower is directly sent to the bottom of the filling section of the distillation tower after pressure adjustment, and is countercurrently contacted with the urine from the top of the distillation tower to complete mass transfer and heat transfer, thereby discarding the reflux condenser and the liquid level tank, stopping the reflux pump, recovering heat, simplifying the process, saving electricity and maintenance costs.

[0022] Optionally, in S18, by utilizing the high temperature characteristic of the high-pressure temperature-adjusting water, a portion of the water is led into the low-pressure steam drum as makeup water, thereby reducing the cooling water consumption of the high-pressure temperature-adjusting water and increasing the by-product steam production, which is beneficial to the thermal balance of the system;

[0023] In S19, the gas phase of the low-pressure absorption tower and the low-pressure methylammonium condenser liquid level tank is introduced into the newly added 0.15Mpa absorption tower, and the lower liquid of the low-pressure absorption tower is used as the absorbent. After circulating and concentrating, a part of it is recovered and enters the shell side of the low-pressure methylammonium condenser; the upper part of the atmospheric absorption tower is changed to the overflow liquid of the clean ammonia tower as the absorbent, which is sent to the ammonia water tank after washing by the circulating pump to reduce the concentration of the ammonia water tank; a clean ammonia tower is set in the inert gas discharge cylinder, and the vented tail gas is sucked into the clean ammonia tower by the fan, and a packing section is set in the lower section. The washing volume is increased by the circulating pump, and the lower liquid level overflows to the upper part of the atmospheric absorption tower after being full of liquid; a tower plate is set in the upper section of the clean ammonia tower, and desalted water mixed with CO2 is used as the absorbent to reduce the ammonia in the vented air to below 15ppm.

[0024] An energy-saving and carbon-reducing device for a urea device uses the energy-saving and carbon-reducing method for the urea device.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The invention provides an energy-saving and carbon-reducing method and device applied to a urea device. Liquid methylammonium and a small amount of uncondensed ammonia and carbon dioxide derived from the bottom of a high-pressure condenser are respectively sent to the bottom of a synthesis tower through two pipelines. A multi-layer radial flow high-efficiency tower plate is arranged in a synthesis tower. The tower plate and a sealing plate are used to prevent materials from being mixed back and short-circuited in the tower. The tower plate structure strengthens the mass transfer and heat transfer between gas and liquid phases. Under the comprehensive effect, the carbon dioxide conversion rate can reach more than 60%. A gas is diverted from the gas phase of the synthesis tower to enter a newly added medium-pressure flash tank. The material directly reduced to 1.2-1.7MPa from the liquid outlet of the stripping tower is sent to the built-in packing section of the flash tank. The gas phase diverted from the synthesis tower enters from the lower part of the packing, countercurrently contacts with the liquid from the stripping tower, and part of the methylammonium is decomposed by using the heat carried by the gas phase and the residual heat of the liquid outlet of the stripping tower. The gas phase of the flash tank and the gas phase of the medium-pressure decomposition tower are mixed and then enter a heat recovery device. After the gas-liquid separation, the liquid phase of the material in the medium-pressure flash tank enters the upper part of the medium-pressure decomposition tower. The liquid phase is sprayed onto the packing layer through a distributor, and conducts mass and heat transfer with the gas phase from the circulating heating section. The high-pressure loop load is appropriately shifted backward, which not only reduces the medium-pressure steam consumption, but also makes full use of the by-product steam. The gas phase of the desorption tower is directly introduced into the low-pressure system, which not only simplifies the process but also recovers part of the heat. The waste heat is used to heat the liquid ammonia, which not only produces more by-product steam but also reduces the amount of circulating water. The cascade utilization of high-pressure temperature-adjusting water is strengthened to save steam consumption. The above routes and facilities are adjusted in combination to achieve the energy-saving and carbon-reduction effects of the urea plant. Reasonable setting of tail gas treatment facilities, without additionally increasing the amount of ammonia water, reduces the ammonia content of the tail gas discharged from the device to below 15ppm, which not only recovers ammonia but also solves environmental protection problems.

[0027] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings herein are incorporated into the specification and constitute a part of the specification, showing embodiments consistent with the present application, and together with the specification, are used to explain the principles of the present application. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. In the drawings:

[0029] Figure 1 and Figure 2 A schematic flow chart of a method for energy saving and carbon reduction in a urea plant is provided for an embodiment of the present application. DETAILED DESCRIPTION

[0030] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in a variety of forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that this application will be more comprehensive and complete and fully convey the concept of the example embodiments to those skilled in the art.

[0031] In addition, described feature, structure or characteristic can be combined in one or more embodiments in any suitable manner. In the following description, many specific details are provided to provide a full understanding of the embodiments of the present application. However, those skilled in the art will appreciate that the technical scheme of the present application can be put into practice without one or more of the specific details, or other methods, components, devices, steps, etc. can be adopted. In other cases, known methods, devices, realizations or operations are not shown or described in detail to avoid blurring the various aspects of the application.

[0032] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities may be implemented in software form, or in one or more hardware modules or integrated circuits, or in different networks and / or processor devices and / or microcontroller devices.

[0033] The flowcharts shown in the accompanying drawings are only exemplary and do not necessarily include all the contents and operations / steps, nor must they be executed in the order described. For example, some operations / steps can be decomposed, and some operations / steps can be combined or partially combined, so the actual execution order may change according to actual conditions.

[0034] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only examples of a part of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

[0035] Example

[0036] See also Figure 1 , a method for energy saving and carbon reduction of a urea device, the method comprising:

[0037] S11: Liquid ammonia comes from the synthetic ammonia unit with a pressure of 2.1 MPa (absolute) and a temperature of 6°C. It enters the inlet of the high-pressure liquid ammonia pump through the liquid ammonia filter and the liquid ammonia buffer tank. The liquid ammonia is pressurized to 16.0 MPa (absolute) by the high-pressure liquid ammonia pump. A liquid ammonia preheater is provided at the pump outlet to heat the liquid ammonia to 80-110°C. The heat source is the waste heat of the system.

[0038] At this time, for the compression and purification of liquid ammonia and carbon dioxide, the liquid ammonia enters the inlet of the high-pressure liquid ammonia pump through the liquid ammonia filter and the liquid ammonia buffer tank, and enters the next process in the preliminary filtration state. The liquid ammonia is pressurized to 16.0MPa (absolute) by the high-pressure liquid ammonia pump, and the liquid ammonia preheater provided at the pump outlet heats the liquid ammonia to 80-110°C; the high-pressure liquid ammonia is sent to the high-pressure ejector as an ejection power source, and the methylammonium liquid from the high-pressure washing is sucked into the high-pressure condenser to absorb carbon dioxide and ammonia gas, and the waste heat of the system, such as medium-pressure temperature regulating water and high-pressure temperature regulating water, is used for more by-product steam to reduce the consumption of circulating water.

[0039] In S11, the vapor-liquid mixture of the medium-pressure flash tank and the medium-pressure decomposition tower after heat recovery enters the medium-pressure methylammonium condenser, and the remaining gas phase components continue to condense and react. The heat released is removed by the closed independent medium-pressure temperature-adjusting water. This heat is used to heat liquid ammonia, which plays a role in increasing by-product steam and reducing circulating water consumption. The setting of the medium-pressure condenser increases the operational flexibility of the medium-pressure system, which is beneficial to increase the gas phase diversion flow of the synthesis tower and reduce the load of the high-pressure scrubber.

[0040] S12: Liquid methylammonium and a small amount of uncondensed ammonia and carbon dioxide derived from the bottom of the high-pressure condenser are respectively sent to the bottom of the synthesis tower through two pipelines; the synthesis tower is equipped with multi-layer radial flow high-efficiency trays. The trays and sealing plates are used to prevent the materials from mixing and short-circuiting in the tower. The tray structure strengthens the mass and heat transfer between the gas and liquid phases. Under the comprehensive effect, the carbon dioxide conversion rate can reach more than 60%;

[0041] At this time, the synthesis tower is equipped with multi-layer radial high-efficiency trays, and the trays and sealing plates are used to prevent the materials from mixing and short-circuiting in the tower, and the tray structure strengthens the mass transfer and heat transfer between the gas and liquid phases. Under the combined effect, the carbon dioxide conversion rate can reach more than 60%. In S12, high-efficiency trays are used to replace conventional trays, and the short circuit and material mixing are eliminated, and the material mass transfer and heat transfer are strengthened, thereby achieving the purpose of improving the CO2 conversion rate.

[0042] The gas at 185-187℃ is discharged from the top of the stripping tower and enters the top of the high-pressure methylammonium condenser together with fresh ammonia and methylammonium liquid from the high-pressure scrubber at 14.1MPa (absolute). The high-pressure condenser is a shell-and-tube heat exchanger. The material flows through the tubes, and water flows between the tubes to produce by-product steam. The condensation degree of ammonia and carbon dioxide can be adjusted according to the by-product pressure. However, a part of the gas should be retained to condense in the synthesis tower in order to compensate for the heat required for the conversion of methylammonium into urea in the synthesis tower and achieve self-heating balance. Controlling the by-product steam pressure is one of the conditions for controlling the temperature and pressure of the synthesis tower.

[0043] At this time, for synthesis and stripping, liquid methylammonium and a small amount of uncondensed ammonia and carbon dioxide discharged from the bottom of the high-pressure condenser are respectively sent to the bottom of the synthesis through two pipelines; the synthesis tower is equipped with seventeen layers of radial flow high-efficiency tower plates, forming several reactors similar to those in series. The material moves from the bottom of the tower to the top of the tower with a designed residence time of about one hour, and the carbon dioxide conversion rate can reach more than 60%, thus ensuring the carbon dioxide conversion rate.

[0044] The synthesis tower, stripping tower, high-pressure methylammonium condenser and high-pressure scrubber form the high-pressure loop, which is the core of the carbon dioxide stripping process. The operating conditions of these four devices are considered uniformly in order to achieve the maximum yield of urea and the maximum heat recovery to produce steam as a by-product.

[0045] Liquid methylammonium and a small amount of uncondensed ammonia and carbon dioxide are respectively sent to the bottom of the synthesis through two pipelines. The liquid phase plus gas phase material NH3 / CO2 (molar ratio) is 2.9-3.1, and the temperature is 168-170°C. There are multiple layers of trays in the synthesis tower, forming a series of reactors. The function of the trays is to prevent the materials from mixing back in the tower. The material has a designed residence time of about one hour from the bottom to the top of the tower, and the carbon dioxide conversion rate can reach more than 60%.

[0046] S13: A gas stream is diverted from the synthesis tower into a newly added medium-pressure flash tank with a built-in packing section. The material directly depressurized to 1.2-1.7 MPa from the effluent of the stripping tower is sent to the distributor above the packing section of the flash tank. The gas stream diverted from the synthesis tower enters from below the packing through the distributor and contacts with the effluent from the stripping tower in countercurrent. The heat carried by the gas phase and the residual heat of the effluent from the stripping tower are used to decompose part of the methylammonium. The gas phase of the flash tank and the gas phase of the medium-pressure decomposition tower are mixed and then enter the heat recovery device, and the liquid phase flows into the upper packing section of the medium-pressure decomposition tower.

[0047] In S13: the urea synthesis reaction liquid rises from the tower to the normal liquid level, the temperature rises to 183-185℃, is discharged from the lower outlet of the tower through the overflow pipe, enters the upper part of the stripping tower through the liquid level control valve, and is evenly distributed to each stripping tube through the liquid distributor in the tower. It descends along the tube wall in the form of a liquid film; the carbon dioxide gas introduced from the lower part of the tower meets the synthesis reaction liquid in the tube in countercurrent; the tubes are heated with steam, and the excess ammonia and unconverted methylammonium liquid in the synthesis reaction liquid will be stripped and decomposed and discharged from the top of the tower; urine and a small amount of undecomposed methylammonium are discharged from the bottom of the tower; the outlet temperature of the stripping tower is controlled at 168-170℃; the bottom liquid level of the tower is controlled at 220mm to prevent the carbon dioxide gas from flowing into the decomposition section of the back section with the liquid, causing overpressure of the medium and low pressure equipment.

[0048] Gas at 183-185°C is discharged from the top of the stripping tower and enters the top of the high-pressure methylammonium condenser together with fresh ammonia and methylammonium liquid from the high-pressure scrubber at 14.1MPa (absolute). The high-pressure condenser is a shell and tube heat exchanger. The material flows through the tubes, and water flows between the tubes to produce by-product steam. The condensation degree of ammonia and carbon dioxide can be adjusted according to the by-product pressure. However, a part of the gas must be retained to condense in the synthesis tower in order to compensate for the heat required for the conversion of methylammonium into urea in the synthesis tower and achieve self-heating balance. Controlling the by-product steam pressure is one of the conditions for controlling the temperature and pressure of the synthesis tower.

[0049] The gas discharged from the top of the synthesis tower has a temperature of about 183°C. A new gas phase pipeline is added to the medium-pressure system, and the other pipeline still enters the high-pressure scrubber; here, the ammonia and carbon dioxide in the gas are condensed and absorbed by the pressurized methylammonium in the medium-pressure absorption section, and then returned to the synthesis tower through the high-pressure condenser; the inert gas and a certain amount of ammonia are discharged from the high-pressure scrubber to the high-pressure system, washed by the low-pressure absorption tower, and then vented to the atmosphere, and the liquid phase is sent to the low-pressure system;

[0050] In the high-pressure scrubber, part of the tail gas from the synthesis tower is further condensed and recovered, and is washed with methylammonium liquid sent from the circulation process by the high-pressure methylammonium pump to recover NH3 and carbon dioxide; for safety reasons, the gas phase components of the high-pressure scrubber need to be moved out of the explosion range, and the gas absorption amount is controlled by high-pressure water temperature adjustment. The tail gas is decompressed and sent to the low-pressure absorption tower to continue to recover NH3 and carbon dioxide, and the inert gas and trace NH3 are discharged here.

[0051] Among them, the 183℃ gas phase of the synthesis tower is diverted into the lower part of the packing section of the medium-pressure flash tank, and is in countercurrent contact with the material from the high-pressure stripping tower under reduced pressure for mass and heat transfer, which promotes the decomposition of methylammonium liquid and reduces the load of the high-pressure scrubber. It also provides anti-corrosion air for the medium-pressure system, reduces the requirements for the selection of equipment, pipelines and fittings, and plays an anti-corrosion role for the pipelines and equipment of the medium-pressure system.

[0052] S14: After the gas-liquid separation, the liquid phase of the material in the medium-pressure flash tank enters the upper part of the medium-pressure decomposition tower. The liquid phase is sprayed onto the packing layer through a distributor, and conducts mass and heat transfer with the gas phase from the circulating heating section. Then it is collected at the gas lifting cap baffle and descends to the bottom of the circulating heater of the medium-pressure decomposition tower, and enters the heater tube in a rising film manner. The heater is divided into two sections. The lower section is heated by the byproduct steam, and the upper section is heated by the medium-pressure drum to the low-pressure steam saturated condensate. The temperature is controlled at 145-150℃ to decompose most of the unconverted methylammonium. The medium-pressure decomposition tower and its circulating heater are key equipment, with built-in packing section and stripping section, which play the role of mass and heat transfer, heating decomposition, stripping decomposition, and gas-liquid separation.

[0053] At this time, the liquid phase of the material in the medium-pressure flash tank enters the upper part of the medium-pressure decomposition tower after gas-liquid separation. The liquid phase is sprayed onto the packing layer through a distributor, and conducts mass and heat transfer with the gas phase from the circulating heating section. Then, it is collected at the gas lifting cap baffle and descends to the bottom of the circulating heater of the medium-pressure decomposition tower, and enters the heater tube in a rising film manner for heat energy utilization. The heater is divided into two sections, the lower section is heated by byproduct steam, and the upper section is heated by medium-pressure drum to low-pressure saturated condensate. The temperature is controlled at 145-150°C to decompose most of the unconverted methylammonium. The medium-pressure decomposition tower and its circulating heater are key equipment, with built-in packing section and stripping section, which play the role of mass and heat transfer, heating decomposition, stripping decomposition, and gas-liquid separation.

[0054] In S14, the medium-pressure decomposition tower and its circulating heater are a comprehensive core equipment with built-in packing section, stripping section and liquid level section. They make full use of by-product steam for heating and reduce medium-pressure steam consumption. CO2 is introduced into the stripping section to balance the ammonia-carbon ratio of the low-pressure system.

[0055] S15: The gas-liquid mixture after the urine in the vacuum pre-concentration heating tube enters a newly added evaporation heat recovery section, and the urine from the urine tank is used to continue to recover the reaction heat under a higher vacuum degree.

[0056] At this time, the urine from the urine tank is used to continue to recover the reaction heat under a relatively high vacuum degree, thereby reducing the consumption of by-product steam and the load of the medium-pressure methylammonium condenser. In S15, the medium-pressure gas phase and the methylammonium liquid from the low-pressure system are mixed to release heat to heat the urine in the vacuum pre-concentration tube, and the incompletely reacted gas-liquid mixture continues to condense and release heat in an evaporation heat recovery section. Under the condition of a relatively high vacuum section, the temperature difference between the inside and outside of the tube is increased, the heat exchange effect is improved, the consumption of by-product steam is reduced, and the medium-pressure methylammonium condensation heat exchange area is reduced at the same time, thereby achieving the purpose of increasing the condensation amount of the unreacted gas phase and releasing more heat, thereby saving steam consumption.

[0057] S16: a reboiler is set at the bottom of the first desorption tower, and high-pressure temperature-adjusted water is used as a heat source to reduce the amount of desorption steam. In S16, the high-pressure temperature-adjusted water from the high-pressure scrubber is used in the newly added reboiler to heat the ammonia water at the bottom of the first desorption tower, thereby reducing the amount of by-product steam in the desorption system.

[0058] S17: The gas phase of the desorption tower enters the lower part of the filling section of the distillation tower, and the reflux condenser, liquid level tank and reflux pump are stopped.

[0059] At this time, the original process of the gas phase of the first desorption tower is to send it to the desorption reflux condenser, but now it is changed to enter the lower part of the filling section of the distillation tower, and the reflux condenser, liquid level tank and reflux pump are stopped, which simplifies the process and recovers part of the heat. In S17, the gas phase of the first desorption tower is directly sent to the lower part of the filling section of the distillation tower after pressure adjustment, and countercurrently contacts with the urine from the top of the distillation tower to complete mass transfer and heat transfer, and the reflux condenser and liquid level tank are thrown away, and the reflux pump is stopped. While recovering heat, the process is simplified, saving both electricity and maintenance costs.

[0060] S18: Set up a high-pressure temperature-controlled water supply pipeline to the low-pressure steam drum and a steam condensate pump to supply the high-pressure temperature-controlled water pipeline, and add a regulating valve and a flow meter respectively to keep the high-pressure temperature-controlled water system stable;

[0061] In S18, by utilizing the high temperature characteristic of the high-pressure temperature-adjusting water, a portion of it is led into the low-pressure steam drum as makeup water, thereby reducing the heat exchange load of the high-pressure temperature-adjusting water and increasing the output of by-product steam, which is beneficial to the thermal balance of the system.

[0062] S19: A new 0.15Mpa absorption tower and ammonia purification tower are added to reduce the ammonia content in the tail gas of the urea inert gas discharge tube to less than 15ppm. By reducing ammonia losses, it achieves the purpose of carbon reduction and environmental protection. At the same time, it does not increase the amount of ammonia water after ammonia absorption, which is beneficial to the water balance of the system.

[0063] In S19, the gas phase of the low-pressure absorption tower and the low-pressure methylammonium condenser liquid level tank is introduced into the newly added 0.15Mpa absorption tower, and the lower liquid of the low-pressure absorption tower is used as the absorbent. After circulating and concentrating, a part of it is recovered and enters the shell side of the low-pressure methylammonium condenser; the upper part of the atmospheric absorption tower is changed to the overflow liquid of the clean ammonia tower as the absorbent, which is sent to the ammonia water tank after washing by the circulating pump to reduce the concentration of the ammonia water tank; a clean ammonia tower is set in the inert gas discharge cylinder, and the vented tail gas is sucked into the clean ammonia tower by the fan, and a packing section is set in the lower section. The washing volume is increased by the circulating pump, and the lower liquid level overflows to the upper part of the atmospheric absorption tower after being full of liquid; a tower plate is set in the upper section of the clean ammonia tower, and desalted water mixed with CO2 is used as the absorbent to reduce the ammonia in the vented air to below 15ppm.

[0064] In an embodiment of the present invention, by the method in the embodiment of the present invention, the liquid methylammonium and a small amount of uncondensed ammonia and carbon dioxide derived from the bottom of the high-pressure condenser are respectively sent to the bottom of the synthesis tower through two pipelines; a multi-layer radial flow high-efficiency tray is provided in the synthesis tower, and the function of the tray and the sealing plate is to prevent the material from mixing back and short-circuiting in the tower. The tray structure strengthens the mass transfer and heat transfer between the gas and liquid phases, and the carbon dioxide conversion rate can reach more than 60% under the comprehensive effect; a gas is diverted from the synthesis tower gas phase to enter the newly added medium-pressure flash tank with a built-in packing section, and the material directly reduced to 1.2-1.7MPa from the stripping tower outlet is sent to the distributor above the flash tank packing section, and the gas phase diverted from the synthesis tower enters from the bottom of the packing through the distributor, and contacts with the liquid from the stripping tower in countercurrent, and the heat carried by the gas phase and the residual heat of the stripping tower outlet are used to decompose a part of the methylammonium; the flash tank gas phase and the medium-pressure decomposition tower gas phase are mixed and enter the heat recovery device, and the liquid phase flows into the upper packing section of the medium-pressure decomposition tower. After gas-liquid separation, the liquid phase of the material in the medium-pressure flash tank enters the upper part of the medium-pressure decomposition tower. The liquid phase is sprayed to the packing layer through the distributor, and mass and heat are transferred with the gas phase from the circulating heating section. Therefore, heat exchange is carried out between the stripping tower and the medium-pressure decomposition tower, and the waste heat of the stripping tower is fully utilized, avoiding the external heating device, thereby achieving the energy-saving and carbon reduction effect of the urea plant.

[0065] Example

[0066] An energy-saving and carbon-reducing device for a urea device uses the energy-saving and carbon-reducing method for a urea device, wherein the energy-saving and carbon-reducing device for a urea device is as follows: Figure 2 It is stated that the energy-saving and carbon-reduction device of the urea plant includes a synthesis tower, a stripping tower and a medium-pressure decomposition tower, a vacuum pre-concentration heater, a heat recovery section, a liquid ammonia preheater, and tail gas recovery and treatment equipment (0.15Mpa absorption tower, ammonia purification tower).

[0067] High-voltage coil transformation content:

[0068] High-efficiency radial flow trays replace the original trays to improve the CO2 conversion rate.

[0069] Heat recovery from high pressure synthesis tail gas

[0070] The tail gas temperature of the urea synthesis tower is 183℃, and the gas phase volume is about 16000kg / h. The normal process is to enter the high-pressure scrubber, and the methylammonium liquid sent by the circulation system is washed, condensed and recovered and then returned to the synthesis tower. The heat released by the condensation of methylammonium is taken away by high-pressure temperature-adjusting water, and this part of heat is actually eventually removed by the circulating cooling water. This transformation considers optimizing the configuration of the medium-pressure system and increasing the pipeline from the synthesis tower gas phase to the medium-pressure system, which can not only reduce the load of the high-pressure scrubber, but also directly use the heat of the synthesis tower gas phase to heat the urine in the subsequent process, saving steam consumption, and at the same time playing a role in the corrosion prevention of the subsequent process equipment.

[0071] Adjustment of working conditions of stripping tower and methylammonium condenser

[0072] Under normal circumstances, the stripping efficiency needs to be maintained at around 80%. The 2.5Mpa steam consumption in the decomposition stage of the high-pressure stripping tower is about 900kg / t.ur per ton of urea. The 0.35Mpag steam produced as a by-product of the high-pressure methylammonium condenser is about 200kg / t.ur in excess for self-use. This excess by-product steam is sent to the low-pressure steam network. Due to its low quality, it affects the maximization of its utilization value. After the transformation, the stripping efficiency is appropriately reduced in operation, the 2.5Mpa steam consumption is reduced, and the by-product steam pressure is increased (about 0.38-0.40Mpag).

[0073] New medium pressure flash tank

[0074] A medium-pressure flash tank is added before the urine at the outlet of the high-pressure stripping tower enters the medium-pressure decomposition tower. Due to the sudden drop in pressure, the solution uses its own heat to decompose part of the methylammonium, and then enters the medium-pressure decomposition tower and the circulating heater for further decomposition. Adding a medium-pressure flash tank can reduce the medium-pressure decomposition load by more than 5% and reduce the steam consumption of the medium-pressure decomposition system. The flash tank has a built-in packing section to facilitate heat recovery.

[0075] Since the amount of methylammonium liquid returning to the high-pressure circle increases after the medium-pressure system is put into operation, the problem of insufficient capacity of the high-pressure injector is solved by modifying the injector needle, so that the increased methylammonium liquid after the modification can be completely digested by the high-pressure scrubber, without the need to add a new methylammonium pump to the high-pressure methylammonium condenser pipeline.

[0076] New medium pressure decomposition and condensation system

[0077] The urine at the outlet of the high-pressure stripping tower enters the medium-pressure flash tank, and uses its own heat and the waste heat of the synthesis tower gas phase to decompose part of the methylammonium, and then enters the packing section of the medium-pressure decomposition tower to continue mass transfer and heat transfer. The circulating heater of the decomposition tower adopts two-stage heating, one section uses by-product steam, and the other section uses medium-pressure drum steam condensate to control the urine in the decomposition tower at about 145°C. The new medium-pressure decomposition and condensation recovery system added in this transformation requires equipment including a medium-pressure flash tank, a medium-pressure decomposition tower and circulating heater, a medium-pressure methylammonium condenser and methylammonium gas-liquid separation level tank, a medium-pressure tail gas scrubber, an evaporation pre-concentration heater and separator, a heat recovery section, a medium-pressure condenser temperature control water system, two medium-pressure methylammonium centrifugal pumps, two medium-pressure scrubber circulating pumps, and a medium-pressure steam condensate pump.

[0078] Low pressure decomposition circulation system

[0079] After adding the medium-pressure system, the load of the low-pressure system is reduced by about 50%, and the ammonia component of the solution increases significantly. It is necessary to introduce a line of CO2 after desulfurization from the compressor interstage into the low-pressure system, adjust the ammonia-carbon ratio of the low-pressure system to 2.2-2.4, and use low-pressure temperature regulating water to lower the temperature operation, thereby increasing the operational control flexibility of the circulation system.

[0080] Evaporation system

[0081] The original evaporation flash tank was changed to an evaporation pre-concentration heater and separator, maintaining the flash pressure at 0.0045Mpa (A). The shell side of the pre-concentration heater was heated by the heat released by the gas phase condensation from the medium-pressure decomposition, so that the urine concentration to the urine tank can be increased to 80-82% (W). The first stage of evaporation heating reduces the amount of by-product steam by about 120kg / t.ur, which is beneficial to the system steam balance. In order to reduce biuret, the liquid level is set on the shell side of the first stage of the evaporation heater to reduce the heat exchange area of ​​one steam. In addition, the first desorption tower ammonia water reboiler is set to use high-pressure temperature-adjusting water as a heat source to reduce the amount of by-product steam.

[0082] In order to reduce the quality fluctuation of the finished product caused by flushing of the first and second stage evaporation separators and the excessive entrainment of urea in the separator gas phase, the separator internals are modified to achieve non-flushing operation of the separator, and the urea content in the ammonia tank is reduced first.

[0083] Water desorption system

[0084] The desorption tower gas phase is sent to the low-pressure system, and the reflux condenser, reflux condenser liquid level tank, and reflux pump are stopped. This simplifies the process and recovers part of the heat. It also saves electricity and equipment maintenance costs.

[0085] Steam system and condensate modification

[0086] The newly added decomposition system needs to consume more low-pressure steam produced as a byproduct of the high-pressure methylammonium condenser. In order to balance the steam system, the amount of byproduct steam needs to be increased. Therefore, the system waste heat is considered to be used to heat the liquid ammonia at the outlet of the ammonia pump. The heat sources include high-pressure temperature-adjusting water, medium-pressure temperature-adjusting water, flash steam of steam condensate, desorption waste liquid, etc., which not only recovers heat but also reduces the amount of circulating cooling water. At this time, the system waste heat (desorption waste liquid, medium-pressure temperature-adjusting water, flash steam condensation heat) is used to heat liquid ammonia to increase the amount of byproduct steam; a high-pressure temperature-adjusting water to low-pressure steam drum replenishment pipeline and a steam condensate pump to high-pressure temperature-adjusting water system pipeline are set up, and flow meters and regulating valves are added respectively to keep the high-pressure temperature-adjusting water system stable. A steam condensate flushing pump is added for abnormal medium-pressure system process treatment.

[0087] Add ammonia pump and methylammonium pump outlet main pipe filter

[0088] Since the filler fragments of the plunger pump can easily block the small holes of the liquid distributor of the high-pressure stripping tower after entering the high-pressure system, it will not only affect the stripping efficiency, but also cause main pipe corrosion and bring safety hazards. Therefore, we use proprietary technology to design a high-pressure pipeline filter, and add pipeline filters at appropriate positions on the outlet main pipes of the ammonia pump and methylammonium pump to solve the problem of filler fragments entering the high-pressure system.

[0089] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0090] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more features.

[0091] Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and core idea of ​​the present application. At the same time, for technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A method for energy saving and carbon reduction in a urea plant, It is characterized in that include: S11: Liquid ammonia comes from the synthetic ammonia unit with a pressure of 2.1 MPa (absolute) and a temperature of 6°C. It enters the inlet of the high-pressure liquid ammonia pump through the liquid ammonia filter and the liquid ammonia buffer tank. The liquid ammonia is pressurized to 16.0 MPa (absolute) by the high-pressure liquid ammonia pump. A liquid ammonia preheater is provided at the pump outlet to heat the liquid ammonia to 80-110°C. The heat source is the waste heat of the system. S12: Liquid methylammonium and a small amount of uncondensed ammonia and carbon dioxide derived from the bottom of the high-pressure condenser are respectively sent to the bottom of the synthesis tower through two pipelines; the synthesis tower is equipped with multi-layer radial flow high-efficiency trays, and the tray structure strengthens the mass transfer and heat transfer between the gas and liquid phases. Under the comprehensive effect, the carbon dioxide conversion rate can reach more than 60%; S13: A gas stream is diverted from the synthesis tower gas phase into a newly added medium-pressure flash tank with a built-in packing section. The material directly depressurized to 1.2-1.7 MPa from the stripping tower outlet is sent to the distributor above the flash tank packing section. The gas stream diverted from the synthesis tower enters from below the packing through the distributor and contacts with the stripping tower outlet liquid in countercurrent. The heat carried by the gas phase and the residual heat of the stripping tower outlet liquid are used to decompose part of the methylammonium. The flash tank gas phase and the medium-pressure decomposition tower gas phase are mixed and then enter the heat recovery device. The liquid phase flows into the upper packing section of the medium-pressure decomposition tower. S14: After the gas-liquid separation, the liquid phase of the material in the medium-pressure flash tank enters the upper part of the medium-pressure decomposition tower. The liquid phase is sprayed to the packing layer through the distributor, and the gas phase from the circulating heating section is transferred with mass and heat; then it is collected at the gas lifting cap baffle and descends to the bottom of the circulating heater of the medium-pressure decomposition tower, and enters the heater tube in a rising film manner. The heater is divided into two sections. The lower section is heated by the by-product steam, and the upper section is heated by the medium-pressure drum to the low-pressure steam saturated condensate. The temperature is controlled at 145-150℃ to decompose most of the unconverted methylammonium; the medium-pressure decomposition tower and its circulating heater are key equipment, with built-in packing section, stripping section and liquid level section; S15: The gas-liquid mixture after the urine in the vacuum pre-concentration heating tube enters a newly added evaporation heat recovery section, and the urine from the urine tank is used to continue to recover the reaction heat under a higher vacuum degree; S16: a reboiler is arranged at the bottom of the first desorption tower, and high-pressure temperature-adjusting water is used as a heat source to reduce the amount of desorption steam; S17: The gas phase of the desorption tower enters the lower part of the packing section of the distillation tower, and the reflux condenser, the reflux condenser level tank and the reflux pump are shut down; S18: Set up a high-pressure temperature-adjusting water supply pipeline to the low-pressure steam drum and a steam condensate pump to supply the high-pressure temperature-adjusting water pipeline, and add a regulating valve and a flow meter respectively; S19: Add a 0.15Mpa absorption tower and ammonia purification tower to reduce the ammonia content in the tail gas of the urea inert gas discharge tube to less than 15ppm.

2. The method for energy saving and carbon reduction of a urea device according to claim 1, It is characterized in that In S12, high-efficiency tower trays are used to replace conventional tower trays. By eliminating short circuits and material backmixing, material mass transfer and heat transfer are enhanced, thereby improving the CO2 conversion rate.

3. The energy-saving and carbon-reducing method for a urea device according to claim 2, It is characterized in that In S13: the 183℃ gas phase of the synthesis tower is split into the lower part of the packing section of the medium-pressure flash tank, and is in countercurrent contact with the material from the high-pressure stripping tower under reduced pressure to carry out mass and heat transfer, thereby promoting the decomposition of the methylammonium liquid and playing a role in corrosion protection of the medium-pressure system pipelines and equipment.

4. The method for energy saving and carbon reduction of a urea device according to claim 3, It is characterized in that In S14, the medium-pressure decomposition tower and its circulating heater are a comprehensive core equipment with built-in packing section, stripping section and liquid level section. They make full use of by-product steam for heating and reduce medium-pressure steam consumption. CO2 is introduced into the stripping section to balance the ammonia-carbon ratio of the low-pressure system.

5. The method for energy saving and carbon reduction of a urea device according to claim 4, It is characterized in that In S15, the medium-pressure gas phase and the methylammonium liquid from the low-pressure system react to release heat to heat the urine in the vacuum pre-concentration tube, and the incompletely reacted gas-liquid mixture continues to condense and release heat in an evaporation heat recovery section. Under the condition of a higher vacuum section, the temperature difference between the inside and outside of the tube increases, thereby increasing the condensation amount of the unreacted gas phase and releasing more heat, thereby saving steam consumption.

6. The method for energy saving and carbon reduction of a urea device according to claim 5, It is characterized in that In S11, the vapor-liquid mixture of the medium-pressure flash tank and the medium-pressure decomposition tower after heat recovery enters the medium-pressure methylammonium condenser, and the remaining gas phase components continue to condense and react. The heat released is removed by the closed independent medium-pressure temperature regulating water. The setting of the medium-pressure condenser increases the operational flexibility of the medium-pressure system, which is beneficial to increase the gas phase flow rate of the synthesis tower and reduce the load of the high-pressure scrubber.

7. The method for energy saving and carbon reduction of a urea device according to claim 6, It is characterized in that In S16, the high-pressure temperature-adjusted water from the high-pressure scrubber is used in a newly added reboiler to heat the ammonia water at the bottom of the first desorption tower, thereby reducing the amount of by-product steam in the desorption system.

8. The method for energy saving and carbon reduction of a urea device according to claim 7, It is characterized in that In S17, the gas phase of the first desorption tower is directly sent to the bottom of the filling section of the distillation tower after pressure adjustment, and is countercurrently contacted with the urine from the top of the distillation tower to complete mass transfer and heat transfer, thereby discarding the reflux condenser and the liquid level tank, and stopping the reflux pump. While recovering heat, the process is simplified, saving both electricity and maintenance costs.

9. The method for energy saving and carbon reduction of a urea device according to claim 8, It is characterized in that In S18, the high temperature of the high-pressure temperature-adjusting water is used to lead a part of it into the low-pressure steam drum as makeup water, which reduces the cooling water consumption of the high-pressure temperature-adjusting water and increases the by-product steam production, which is beneficial to the thermal balance of the system; In S19, the gas phase of the low-pressure absorption tower and the low-pressure methylammonium condenser liquid level tank is introduced into the newly added 0.15Mpa absorption tower, and the lower liquid of the low-pressure absorption tower is used as the absorbent. After circulating and concentrating, a part of it is recovered and enters the shell side of the low-pressure methylammonium condenser; the upper part of the atmospheric absorption tower is changed to the overflow liquid of the clean ammonia tower as the absorbent, which is sent to the ammonia water tank after washing by the circulating pump to reduce the concentration of the ammonia water tank; a clean ammonia tower is set in the inert gas discharge cylinder, and the vented tail gas is sucked into the clean ammonia tower by the fan, and a packing section is set in the lower section. The washing volume is increased by the circulating pump, and the lower liquid level overflows to the upper part of the atmospheric absorption tower after being full of liquid; a tower plate is set in the upper section of the clean ammonia tower, and desalted water mixed with CO2 is used as the absorbent to reduce the ammonia in the vented air to below 15ppm.

10. An energy-saving and carbon-reducing device for a urea plant, It is characterized in that A method for energy saving and carbon reduction in a urea plant as claimed in any one of claims 1 to 9.

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