A urea hydrolysis system hydrophobic waste heat recovery process

Abstract

The present application belongs to the technical field of industrial energy saving and wastewater resource treatment, and particularly relates to a urea hydrolysis system hydrophobic waste heat recovery process, which uses 90-100 DEG C high-temperature hydrophobic water generated by a urea hydrolysis system as a heat source, realizes waste heat cascade utilization through a single-effect heater and a preheating heat exchanger, and drives desulfurization wastewater to complete negative pressure evaporation crystallization. The present application first uses urea hydrolysis hydrophobic water as a stable heat source for zero discharge of desulfurization wastewater, so that an originally independent denitration system and a wastewater treatment system form an energy closed loop without the need of external fresh steam supply. A two-stage heat exchange mode of "high-temperature section heating circulating liquid and low-temperature section preheating original wastewater" is adopted to realize temperature matching and energy cascade utilization, and to greatly improve the utilization rate of low-grade waste heat.

Description

Technical Field

[0001] This invention belongs to the field of industrial energy conservation and wastewater resource treatment technology, specifically relating to a hydrophobic waste heat recovery process for a urea hydrolysis system. Background Technology

[0002] In coal-fired power plants, coal chemical plants, and fertilizer enterprises, selective catalytic reduction (SCR) flue gas denitrification is the mainstream technology for controlling nitrogen oxide emissions. Urea hydrolysis for ammonia production has become the preferred process for providing ammonia as a reducing agent due to its high safety and ease of operation and maintenance. During continuous operation, urea hydrolysis units consistently generate a large amount of process condensate at temperatures between 90℃ and 100℃. This condensate carries a significant amount of recoverable low- to medium-grade sensible heat. In traditional operation, this condensate is simply cooled and then directly discharged into the plant's condensate system or reused as makeup water. The heat energy it contains is not effectively utilized, resulting in significant energy waste and increasing the load on the cooling system and water consumption.

[0003] On the other hand, the desulfurization wastewater generated by wet desulfurization systems is characterized by high salt content, high hardness, high chloride ion content, and strong corrosiveness, making it difficult to treat using conventional biochemical or physicochemical processes. Evaporation crystallization technology is the core technology for achieving zero discharge of desulfurization wastewater. However, evaporation crystallization systems are highly dependent on fresh steam or electricity as a heat source, resulting in huge energy consumption and high operating costs, which has become a key bottleneck restricting the large-scale promotion of zero-emission technology.

[0004] Currently, in engineering applications, the urea hydrolysis ammonia production system and the desulfurization wastewater evaporation crystallization system are independent process units, lacking effective mechanisms for heat energy matching, cascade utilization, and system integration. Existing waste heat recovery technologies generally suffer from problems such as insufficient heat energy utilization, poor adaptability to low-grade heat sources, low coupling with the evaporation crystallization process, and insufficient system stability, making it impossible to achieve continuous, efficient, and low-cost joint operation.

[0005] Therefore, how to efficiently and stably apply the high-temperature hydrophobic waste heat from the urea hydrolysis system to the desulfurization wastewater evaporation and crystallization system in a cascade utilization manner, so as to achieve waste treatment, energy saving and consumption reduction, and resource recycling, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a hydrophobic waste heat recovery process for a urea hydrolysis system. Through two-stage heat exchange, the hydrophobic waste heat from urea hydrolysis is used to drive the entire process of preheating, evaporation, and crystallization of desulfurization wastewater. This solves problems such as waste of hydrophobic waste heat, high energy consumption in evaporation and crystallization, poor coupling between the two systems, and high operating costs, achieving the comprehensive goals of energy saving, water saving, salt resource utilization, and stable system operation.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0008] A waste heat recovery process for hydrophobic condensate from a urea hydrolysis system uses the 90℃~100℃ high-temperature hydrophobic condensate generated by the urea hydrolysis system as the sole heat source. Primary heating is achieved through a single-effect heater, and secondary preheating is achieved through a preheating heat exchanger, realizing the cascade utilization of waste heat. This drives the evaporation and crystallization of desulfurization wastewater under negative pressure conditions. The specific steps are as follows:

[0009] High-temperature hydrophobic liquid is drawn from the urea hydrolysis hydrophobic tank and enters the shell side of the single-effect heater. It serves as a heating medium to indirectly heat the circulating liquid at the bottom of the drying crystallization tower. After being heated, the circulating liquid is sent to the spray device at the top of the crystallization tower.

[0010] The drying crystallization tower operates under negative pressure to lower the boiling point of the wastewater. The heated circulating liquid is sprayed and then quickly flash-evaporated, and the water vaporizes to form secondary steam. The wastewater is continuously concentrated, and after the salt is supersaturated, it crystallizes and precipitates, forming a high-concentration crystal slurry at the bottom of the tower.

[0011] The crystal slurry is discharged from the bottom of the tower and sent to a solid-liquid separation device to separate solid salt crystals and mother liquor; the mother liquor is returned to the drying crystallization tower for continued circulation and evaporation to improve the crystallization yield.

[0012] The condensate cooled by the single-effect heater enters the preheating heat exchanger to preheat the low-temperature desulfurization wastewater from the conditioning system, fully recovering the waste heat in the low-temperature section, increasing the temperature of the wastewater entering the tower, and reducing the system's cooling load.

[0013] The preheated desulfurization wastewater is sent to the spray device of the drying crystallization tower, where it participates in the evaporation process together with the circulating liquid in the tower bottom; the secondary steam at the top of the crystallization tower enters the condenser and is condensed into clean water, which is then reused in the desulfurization system, ash removal system, or process water system.

[0014] A vacuum pump is installed at the condenser outlet to continuously remove non-condensable gases and maintain a stable negative pressure in the drying crystallization tower; an exhaust condenser is installed at the exhaust port of the urea hydrolysis condensate tank to condense the flash vapor and return it to the condensate tank, reducing heat loss and working fluid loss.

[0015] Compared with the prior art, the present invention has the following significant advantages:

[0016] 1. Achieve deep system coupling

[0017] For the first time, urea hydrolysis and hydrophobicity are used as a stable heat source for zero discharge of desulfurization wastewater, forming an energy closed loop between the originally independent denitrification system and the wastewater treatment system, eliminating the need for external supply of fresh steam.

[0018] 2. High efficiency of waste heat utilization in stages

[0019] By adopting a two-stage heat exchange mode of "heating the circulating liquid in a high-temperature section and preheating the raw wastewater in a low-temperature section", the temperature is matched and the energy is utilized in a tiered manner, which greatly improves the utilization rate of low-grade waste heat.

[0020] 3. High evaporation efficiency under negative pressure:

[0021] By lowering the boiling point of wastewater under a negative pressure of -80 kPa, efficient evaporation can be achieved under a low-temperature heat source of around 90°C, and the system has strong adaptability.

[0022] 4. End-to-end resource utilization

[0023] All secondary steam condensate is recycled and reused, and the crystallized salt can be used as an industrial by-product, realizing the triple resource utilization of water, salt, and heat without secondary pollution.

[0024] 5. Strong engineering adaptability

[0025] The system features an independent modular design, allowing direct integration into existing power plant retrofit projects. It is stable in operation, highly automated, and easy to maintain. Attached Figure Description

[0026] Figure 1 This is a flowchart of the process system of the present invention. Detailed Implementation

[0027] This invention discloses a waste heat recovery process for a urea hydrolysis system. Using the 90℃~100℃ high-temperature hydrophobic water generated by the urea hydrolysis system as the sole heat source, primary heating is achieved through a single-effect heater, and secondary preheating is achieved through a preheating heat exchanger, realizing the cascade utilization of waste heat to drive the evaporation and crystallization of desulfurization wastewater under negative pressure conditions. The specific steps are as follows:

[0028] 1. The high-temperature hydrophobic layer is drawn from the urea hydrolysis hydrophobic tank and enters the shell side of the single-effect heater, where it is used as a heating medium to indirectly heat the circulating liquid at the bottom of the drying crystallization tower.

[0029] 2. The circulating liquid in the bottom of the tower is heated and then sent to the spray device at the top of the drying and crystallization tower. The tower body is made of 2205 duplex stainless steel, and the spray device is generally a swirl spray device. The drying and crystallization tower operates under negative pressure to lower the boiling point of the wastewater. After being sprayed, the heated circulating liquid undergoes rapid flash evaporation, and the water vaporizes to form secondary steam. The wastewater is continuously concentrated, and after the salt content becomes supersaturated, crystals precipitate out, forming a high-concentration crystal slurry at the bottom of the tower.

[0030] 3. The condensate cooled by the single-effect heater is not directly discharged, but instead enters a preheating heat exchanger to preheat the low-temperature desulfurization wastewater from the conditioning system. This fully recovers the waste heat from the low-temperature section, increases the temperature of the wastewater entering the tower, and reduces the system's cooling load. The preheating heat exchanger is a plate preheater. The two fluids undergo efficient countercurrent heat exchange within the plate heat exchanger, further cooling the condensate to a temperature that allows it to return to the power plant's conventional condensate system or be used for other applications requiring low-grade heat. The original wastewater, on the other hand, is preheated to approximately 60°C.

[0031] The preheated desulfurization wastewater is fed into the drying and crystallization tower, where it participates in the evaporation process along with the circulating liquid in the tower bottom. It is then mixed with the heated circulating liquid (approximately 85°C) from the single-effect heater in the main pipe before entering the spray system of the drying and crystallization tower (or sprayed directly into the tower separately). This step significantly reduces the "cold shock" of the cold material to the evaporation process inside the tower, reduces the additional steam consumption required to heat this portion of the feed water from 30°C to its boiling point, and fully utilizes the waste heat from the hydrophobic stage.

[0032] 4. The secondary steam at approximately 70°C at the top of the crystallization tower enters the condenser and is condensed into clean water at 50-55°C. This high-quality recycled water is stably pumped to the desulfurization process water system or the ash removal system. A vacuum pump is installed at the condenser outlet to continuously remove non-condensable gases and maintain a stable negative pressure in the drying crystallization tower. An exhaust condenser is installed at the exhaust port of the urea hydrolysis condensate tank to condense the flash steam and return it to the condensate tank, reducing heat loss and working fluid loss.

[0033] 5. The crystal slurry is discharged from the bottom of the tower and sent to the solid-liquid separation equipment to separate solid salt crystals and mother liquor. The mother liquor is temporarily stored in the mother liquor tank. Part of the mother liquor is returned to the drying crystallization tower for continued circulation and evaporation to adjust the crystallization environment in the tower and improve the crystallization yield. Part of the mother liquor is returned to the upstream conditioning system or subjected to specific treatment depending on the water quality.

[0034] The present invention will be further described in detail below with reference to several embodiments. Those skilled in the art will understand that any modifications, substitutions, and combinations made without departing from the concept of the present invention are within the protection scope of the present invention.

[0035] Example 1 (Standard operating conditions of a coal-fired power plant)

[0036] A hydrophobic waste heat recovery process for a urea hydrolysis system is applied to a zero-discharge system for desulfurization wastewater in a 300MW coal-fired power unit.

[0037] The urea hydrolysis condensate drain tank outputs high-temperature condensate: temperature 95℃, pressure 0.5MPa (gauge pressure), and flow rate 10t / h.

[0038] High-temperature hydrophobic fluid enters the shell side of the shell-and-tube single-effect heater to heat and dry the circulating liquid at the bottom of the crystallization tower: the circulating liquid flow rate is 60t / h, the temperature is 75℃, and it is heated to 85℃.

[0039] The drying crystallization tower maintains a negative pressure of -80 kPa, and the circulating liquid is sprayed and flashed to vaporize the water and crystallize the salt, forming a crystal slurry with a solid content of 15% at the bottom of the tower.

[0040] The crystal slurry was discharged at a flow rate of 4t / h and separated by a horizontal screw centrifuge to obtain NaCl salt crystals with a water content of <5%. The mother liquor was refluxed to the drying crystallization tower.

[0041] The outlet condensate temperature of the single-effect heater drops to 80℃ and enters the plate heat exchanger, where the 30℃ desulfurization wastewater (TDS 80000mg / L, flow rate 4t / h) in the clear liquid tank is preheated to 60℃ before being sent to the crystallization tower.

[0042] The secondary steam at the top of the crystallization tower is condensed into clean water at 50-55℃ by a condenser, with a water production rate of 3.2t / h, which is reused in the desulfurization system.

[0043] The exhaust gas from the condensate tank is condensed and returned by a small condenser to reduce heat loss; the vacuum pump maintains a stable negative pressure in the system.

[0044] Operational results: No external steam supply is required, saving 4.5t / h of steam, reducing operating costs by 62%, and ensuring continuous and stable system operation.

[0045] Example 2 (High salinity condition)

[0046] A hydrophobic waste heat recovery process for a urea hydrolysis system is applied to the treatment of high-salinity desulfurization wastewater:

[0047] Urea hydrolysis hydrophobicity: temperature 93℃, pressure 0.45MPa (gauge pressure), flow rate 9t / h.

[0048] Desulfurization wastewater: temperature 28℃, TDS 100000mg / L, flow rate 3.5t / h.

[0049] The single-effect heater heats the circulating liquid in the crystallization tower from 72℃ to 83℃; the negative pressure in the crystallization tower is -75kPa.

[0050] The hydrophobic material is cooled to 78°C through a primary heat exchanger, and then the original wastewater is preheated to 58°C.

[0051] The solids content of the crystal slurry is 18%. The salt crystals are separated by centrifugation, and part of the mother liquor is refluxed and part is returned to the conditioning system.

[0052] The secondary steam condensate is 3.0 t / h, which is reused in the ash and slag removal system.

[0053] Operational performance: Adaptable to high-salinity wastewater, no scaling or clogging, and waste heat utilization rate of over 92%.

[0054] Example 3 (Low-flow chemical operation)

[0055] A hydrophobic waste heat recovery process for a urea hydrolysis system is applied to a urea hydrolysis + desulfurization wastewater system in a chemical industrial park.

[0056] Urea hydrolysis hydrophobicity: temperature 90℃, pressure 0.4MPa (gauge pressure), flow rate 6t / h.

[0057] Desulfurization wastewater: temperature 32℃, TDS 70000mg / L, flow rate 2t / h.

[0058] The single-effect heater heats the circulating liquid to 82℃; the crystallization tower is under negative pressure of -80kPa.

[0059] After passing through two stages of heat exchange, the temperature of the condensate drops to 45℃ and is reused in the plant's hot water system.

[0060] Condensate flow rate is 1.6 t / h, which is recycled to the desulfurization system.

[0061] Operational results: Stable operation at low flow rates, complete recovery of waste heat, and strong system adaptability.

[0062] Example 4 (with salt separation process)

[0063] A hydrophobic waste heat recovery process for a urea hydrolysis system is applied to a zero-discharge system for desulfurization wastewater salinization.

[0064] Urea hydrolysis hydrophobicity: temperature 98℃, pressure 0.55MPa (gauge pressure), flow rate 11t / h.

[0065] The clarified liquid of desulfurization wastewater after conditioning and salt separation has the following characteristics: temperature 30℃, TDS 85000mg / L, and flow rate 4.5t / h.

[0066] The single-effect heater heats the circulating liquid to 88°C; the crystallization tower is under negative pressure of -80 kPa.

[0067] After preheating, the wastewater temperature reached 62℃, and high-purity NaCl salt was obtained through crystallization.

[0068] The condensate flow rate is 3.6 t / h, which is recycled for desulfurization process water.

[0069] Operational results: It can be used in conjunction with salt separation processes to produce high-purity salt, with high waste heat utilization and strong system compatibility.

[0070] In summary, this embodiment successfully converts the waste heat of the urea hydrolysis system into an effective energy source for zero-discharge treatment of desulfurization wastewater, constructing an efficient, energy-saving, and resource-efficient integrated wastewater treatment and energy recovery system, thus verifying the feasibility and superiority of the present invention.

Claims

1. A process for recovering hydrophobic waste heat from a urea hydrolysis system, characterized in that, Includes the following steps: 1) The 90℃~100℃ high-temperature hydrophobic water generated by the urea hydrolysis system is introduced into the single-effect heater as a heating medium to heat the circulating liquid at the bottom of the drying crystallization tower; 2) The heated circulating liquid in the tower bottom is sent to the spray device of the drying crystallization tower, where flash evaporation and spray evaporation are carried out under negative pressure to concentrate the wastewater and precipitate salt crystals to form crystal slurry; 3) The cooling condensate discharged from the single-effect heater is sent to the preheating heat exchanger to preheat the low-temperature desulfurization wastewater. The preheated desulfurization wastewater is then sent to the drying crystallization tower. 4) The secondary steam generated at the top of the drying crystallization tower is condensed by a condenser into recycled water, which is then reused in the production system; 5) The crystal slurry at the bottom of the drying crystallization tower is separated into salt crystals and mother liquor through solid-liquid separation, and part of the mother liquor is returned to the drying crystallization tower.

2. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, The high-temperature hydrophobic material comes from the hydrophobic tank of the urea hydrolysis ammonia production unit, with a temperature of 95°C, a pressure of 0.5 MPa, and a flow rate of 8–12 t / h.

3. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, The drying crystallization tower maintains a negative pressure environment with a gauge pressure of -75kPa to -85kPa.

4. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, The single-effect heater is a shell-and-tube heat exchanger, with high-temperature condensate flowing through the shell side and circulating liquid flowing through the tube side.

5. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, The preheating heat exchanger is a plate heat exchanger that preheats the desulfurization wastewater at a temperature of 25℃~35℃ to 55℃~65℃.

6. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, The low-temperature desulfurization wastewater is a clear liquid that has undergone pretreatment and conditioning and salt separation. It is stored in a clear liquid buffer tank with a total dissolved solids of 70,000 mg / L to 100,000 mg / L and a flow rate of 2 to 5 t / h.

7. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, A vacuum pump is installed at the condenser outlet to maintain negative pressure operating conditions for the drying crystallization tower.

8. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, An exhaust condenser is installed at the exhaust port of the urea hydrolysis system's condensate tank to condense the steam in the exhaust and return it to the condensate tank.

9. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, Solid-liquid separation was performed using a horizontal screw centrifuge, and the resulting salt crystals had a water content of less than 5%.

10. The hydrophobic waste heat recovery process for the urea hydrolysis system according to claim 1, characterized in that, In step 3), the preheated desulfurization wastewater is mixed with the heated tower bottom circulating liquid in the main pipe or separately connected to the spray device before entering the drying crystallization tower.

Images