A plasma coupled microdroplet system for synthesizing urea and method thereof

Abstract

The application discloses a kind of plasma coupling microdroplet's synthetic urea system and method thereof, the system includes: gas supply module is used to supply carbon dioxide-containing gas;Liquid delivery module is used to deliver nitrate ion-containing aqueous solution;Spray module is used to atomize nitrate ion-containing aqueous solution into microdroplet under carbon dioxide-containing gas atmosphere;Plasma discharge module is used to generate carbon dioxide plasma, and make that plasma and nitrate ion in the microdroplet react at gas-liquid interface to synthesize urea.The system can realize efficient activation of carbon source and selective synthesis of urea product under normal temperature and pressure conditions, with the advantages of low energy consumption, compact structure, green environmental protection, etc., suitable for distributed and renewable energy driven urea production system.

Description

Technical Field

[0001] This invention belongs to the field of plasma chemistry and green chemistry synthesis, and specifically relates to a plasma-coupled microdroplet synthesis system and method for urea. Background Technology

[0002] Urea is an important nitrogen-containing chemical product, widely used in agricultural fertilizers, resins and coatings, pharmaceutical intermediates, and flue gas denitrification reducing agents, with a large market demand. Current industrial urea production typically uses synthetic ammonia and carbon dioxide as raw materials, producing urea under high temperature and pressure conditions through steps such as ammonium carbonate / ammonium carbamate formation and dehydration. This process is highly coupled with the Haber-Bosch ammonia production process, resulting in high overall energy consumption. Furthermore, the sourcing and preparation of ammonia often involve fossil fuel consumption and carbon emissions, limiting the low-carbon and distributed application of urea production.

[0003] With the increasing demand for carbon emission reduction and carbon dioxide resource utilization, the synthesis of carbon- and nitrogen-containing chemicals using CO2 as a carbon source under mild conditions has become a research hotspot. Meanwhile, nitrates, as nitrogen-containing species, are widely found in industrial wastewater and agricultural runoff, and their high-valence nitrogen has the potential to be reduced and converted into nitrogen-containing products. If the reduction and conversion of nitrates can be coupled with the activation of CO2 to achieve the direct synthesis of carbon- and nitrogen-containing compounds such as urea, it is expected that both the resource utilization of nitrogen-containing pollutants and the high-value conversion of CO2 can be realized simultaneously.

[0004] Non-thermal plasmas can generate high-energy electrons, free radicals, and excited-state molecules at room temperature and pressure, providing a new technological pathway for the activation of inert molecules. Related research indicates that plasma can promote the formation of CO and *CO2 from CO2. - It can capture active carbon-based species such as CO and drive the reduction and transformation of nitrogen-containing species in the liquid phase or near the interface. Furthermore, coupling the plasma process with the microdroplet system can leverage the large specific surface area, high interfacial reactivity, and short mass transfer path of microdroplets to enhance the enrichment and coupling reaction probability of active species at the gas-liquid interface, which is beneficial for improving reaction selectivity and continuous operation capability.

[0005] However, existing technologies for the direct synthesis of urea under mild conditions still face multiple challenges, such as: insufficient interfacial coupling efficiency between CO2 activation products and nitrogen-containing intermediates, and side reactions leading to limited product selectivity; the efficiency of plasma energy utilization and the gas-liquid contact mode in the reaction zone need to be optimized; and there is still a lack of effective solutions for continuous and stable system operation and engineering scale-up.

[0006] Therefore, there is an urgent need to provide a device and method for synthesizing urea by utilizing the interfacial reaction between carbon dioxide plasma and nitrate-containing microdroplets under normal temperature and pressure conditions, in order to meet the application requirements of distributed and renewable energy-driven applications. Summary of the Invention

[0007] To overcome the above problems, this invention proposes a system and method for synthesizing urea using carbon dioxide plasma coupled with microdroplets containing nitrate ions.

[0008] Specifically, the object of the present invention is to provide the following aspects:

[0009] On the one hand, a urea synthesis system is provided, comprising:

[0010] A gas supply module for supplying carbon dioxide-containing gas;

[0011] A liquid delivery module for delivering aqueous solutions containing nitrate ions;

[0012] A spray module is used to atomize an aqueous solution containing nitrate ions into microdroplets in a carbon dioxide-containing gas atmosphere.

[0013] A plasma discharge module is used to generate carbon dioxide plasma and cause the plasma to react with nitrate ions in the microdroplets at the gas-liquid interface to synthesize urea.

[0014] Optionally, the gas supply module includes:

[0015] CO2 cylinders store compressed gas containing carbon dioxide, which is used to provide the carbon source required for urea synthesis.

[0016] Pressure regulator, used to stabilize the output pressure of compressed air source.

[0017] Optionally, the liquid delivery module includes:

[0018] A syringe used to store and propel an aqueous solution containing nitrate ions;

[0019] The two-way adapter connects the syringe and the spray module, enabling a detachable connection of the liquid channel.

[0020] Optionally, the spray module includes a three-way connector, a capillary needle tube, and a metal tube, which are coaxially combined.

[0021] Optionally, the capillary tube is made of stainless steel.

[0022] Optionally, a quartz sleeve is provided around the capillary needle.

[0023] Optionally, an annular air gap channel is formed between the inner wall of the quartz sleeve and the outer wall of the capillary tube.

[0024] Optionally, the plasma discharge module includes a DC high-voltage power supply, a high-voltage electrode, and a grounding electrode. The output terminal of the DC high-voltage power supply is connected to the high-voltage electrode, and the grounding terminal is connected to the grounding electrode.

[0025] In a second aspect, the method for synthesizing urea according to the system described in the first aspect includes:

[0026] S1 supplies carbon dioxide-containing gas to the urea synthesis reaction through the gas supply module;

[0027] S2, an aqueous solution containing nitrate ions is delivered via a liquid delivery module;

[0028] S3, through the spray module, atomizes the aqueous solution containing nitrate ions into microdroplets;

[0029] S4, carbon dioxide plasma is generated by the plasma discharge module. The plasma reacts with nitrate ions in the microdroplets at the gas-liquid interface to synthesize urea.

[0030] Optionally, in S1, the inlet pressure of carbon dioxide gas is 0.3 MPa-0.9 MPa.

[0031] The beneficial effects of this invention include:

[0032] (1) The plasma-coupled microdroplet urea synthesis system provided by the present invention can achieve efficient activation of carbon source and selective synthesis of urea products under normal temperature and pressure conditions. It has the advantages of low energy consumption, compact structure and green environmental protection, and is suitable for distributed and renewable energy driven urea production systems.

[0033] (2) The plasma-coupled microdroplet synthesis method for urea provided by the present invention controls the microdroplet size and plasma active species concentration by adjusting parameters such as gas pressure, liquid flow rate, and voltage, thereby adapting to the needs of laboratory pilot or pilot-scale amplification and having good engineering scalability. Attached Figure Description

[0034] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0035] In the attached diagram:

[0036] Figure 1 This diagram illustrates the structure of the plasma-coupled microdroplet urea synthesis system of the present invention.

[0037] Figure 2 Show Figure 1 A partial structural diagram;

[0038] Figure 3 A comparison graph showing the concentration of synthesized urea under different plasma voltages in Example 1 is provided.

[0039] Figure 4 A comparison chart showing the concentration of urea synthesized at different pH values ​​of silver nitrate in Example 2 is provided.

[0040] Figure 5 The diagram shows a comparison of urea concentrations synthesized under different CO2 gas inlet pressures in Example 3.

[0041] Figure 6 The graph shows a comparison of the concentrations of urea synthesized under different flow rates of silver nitrate aqueous solution in Comparative Example 1.

[0042] Figure 7 The diagram shows a comparison of the concentrations of synthesized urea under different CO2 inlet pressures in Comparative Example 2.

[0043] Figure Labels

[0044] 1-Tee connector;

[0045] 2-Two-way adapter;

[0046] 3-Capillary needle;

[0047] 4-Liquid collection unit;

[0048] 5-DC high voltage power supply;

[0049] 6- Syringe;

[0050] 7-Metal tube;

[0051] 8-Pressure regulator;

[0052] 9-CO2 gas cylinder;

[0053] 10 - Lateral gas inlet. Detailed Implementation

[0054] The following will refer to the appendix. Figures 1 to 7Specific embodiments of the invention will be described in more detail below. While specific embodiments of the invention are shown in the accompanying drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0055] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0056] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship in the working state of this invention, and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Furthermore, the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0057] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0058] On the one hand, according to the present invention, a plasma-coupled microdroplet urea synthesis system is provided, such as... Figures 1 to 2 As shown, it includes:

[0059] A gas supply module for supplying carbon dioxide-containing gas;

[0060] A liquid delivery module for delivering aqueous solutions containing nitrate ions;

[0061] A spray module is used to atomize an aqueous solution containing nitrate ions into microdroplets in a carbon dioxide-containing gas atmosphere.

[0062] A plasma discharge module is used to generate carbon dioxide plasma, which reacts with nitrate ions in the microdroplets at the gas-liquid interface to synthesize urea.

[0063] In this invention, the gas supply module includes:

[0064] CO2 cylinder 9 stores compressed gas containing carbon dioxide, which is used to provide the carbon source required for urea synthesis.

[0065] Pressure regulator 8 is used to stabilize the output pressure of the compressed air source between 0.3MPa and 0.9MPa, for example, 0.6MPa, on the one hand to maintain a stable gas flow rate, and on the other hand to avoid abnormal discharge breakdown or excessive atomization due to excessive pressure.

[0066] Furthermore, the CO2 cylinder 9 is connected to the pressure regulator 8 via a connecting pipe. Compressed gas flows out of the CO2 cylinder 9 and enters the pressure regulator 8 for pressure reduction and flow stabilization. The regulated gas continues to be transported along the connecting pipe and is connected to the tee connector 1 of the spray module, thereby providing the subsequent spray module and / or plasma discharge module with pressure controllable and flow stable carbon dioxide gas.

[0067] The CO2 cylinder 9 is preferably a high-purity CO2 steel cylinder.

[0068] In this invention, the liquid delivery module includes:

[0069] Syringe 6, which is used to store and propel an aqueous solution containing nitrate ions as a supply source for liquid-phase reactants;

[0070] The two-way adapter 2 is used to connect the syringe 6 to the spray module, enabling a detachable connection of the liquid channel.

[0071] Furthermore, the syringe 6 can be a manual syringe or an electric syringe pump, preferably any commercially available electric syringe pump, which has a constant flow rate output capability to ensure continuous and controllable liquid supply. The volume of the syringe 6 is selected according to experimental needs, such as 1 mL, 5 mL, or 10 mL, suitable for reactions of different scales. In one embodiment, the syringe 6 is an electric syringe pump, specifically a TYD01-01 type laboratory syringe pump.

[0072] In this invention, the spray module includes a three-way connector 1, a capillary needle tube 3, and a metal tube 7, which are coaxially assembled. The three-way connector 1 is used to introduce a compressed gas source containing carbon dioxide into the metal tube 7; the capillary needle tube 3 is used to spray an aqueous solution containing nitrate ions in the form of a jet; and the metal tube 7 is located outside the capillary needle tube 3.

[0073] In this invention, the syringe 6 is connected to the stainless steel capillary tube 3, and the two are coaxial. The capillary tube 3 extends to the central axis of the metal tube 7, and its outlet has a conical or needle-shaped structure to enhance the focusing effect of the liquid jet.

[0074] Preferably, the capillary needle 3 is a hollow circular tube structure with an inner diameter of 80-120 μm and an outer diameter of 25-300 μm, for example, an inner diameter of 100 μm and an outer diameter of 300 μm. The constraints for setting these parameters are: if the inner diameter is too small, it is prone to blockage due to particles or crystals in the nitrate solution, affecting the stability of continuous operation; if the inner diameter is too large, the generated microdroplets will be too large, which is not conducive to the efficient reaction of carbon dioxide plasma and nitrate ions at the gas-liquid interface.

[0075] The capillary needle tube 3 is surrounded by a quartz sleeve, and the two are coaxially nested in a non-contact manner. A gas annular gap channel is formed between the capillary needle tube 3 and the quartz sleeve, which not only ensures sufficient cross-sectional area for gas flow, but also maintains the mechanical rigidity of the capillary needle tube 3 and prevents bending during installation or operation.

[0076] The quartz sleeve has an outer diameter of 600-700 μm and an inner diameter of 500-550 μm, for example, an outer diameter of 690 μm and an inner diameter of 530 μm. The constraint for setting these parameters is that its inner diameter matches the outer diameter of the capillary needle tube 3, forming a gas annular channel between them. This annular gap width ensures that the carbon dioxide-containing gas flows at a sufficiently high velocity to apply effective shear force to the liquid jet for efficient atomization, while also preventing blockage or excessive pressure drop due to an overly narrow channel. The wall thickness formed between the outer and inner diameters of the metal tube 7 meets the mechanical strength requirements of the quartz material to prevent breakage during installation or plasma discharge, and also ensures a reliable sealing connection with the tee connector 1.

[0077] Preferably, the capillary needle 3 is used to guide the liquid flow to the nozzle area, while avoiding the impact on delivery efficiency due to bending or blockage. Its length is 8-15cm, for example, 12cm. The constraints for setting this parameter are: this length range takes into account the spatial layout requirements of the experimental system, the flow resistance control of liquid delivery, and the stability of nozzle coaxial alignment; if the length is too short, it is difficult to achieve a reliable connection, and if it is too long, it will lead to excessive flow resistance, requiring increased injection pressure and increasing the risk of seal failure.

[0078] In this invention, the capillary needle 3 is made of stainless steel, such as 316L stainless steel, which has excellent resistance to nitrate solution corrosion and is suitable for long-term continuous operation. The quartz sleeve has excellent electrical insulation, high-temperature resistance, and chemical stability, enabling it to work stably for a long time in a plasma discharge environment and preventing electrode short circuits or corrosion.

[0079] like Figure 1As shown, the syringe 6 is detachably connected to the capillary tube 3 via a two-way connector 2. Specifically, the front end of the syringe 6 is connected to the capillary tube 3, and the two-way connector 2, located near the syringe 6, clamps the capillary tube 3, facilitating the assembly and maintenance of the liquid channel. The two-way connector 2 is a cylindrical metal joint made of metal materials such as stainless steel or brass, possessing good corrosion resistance and mechanical strength.

[0080] Preferably, a polytetrafluoroethylene (PTFE) gasket with a thickness of 0.3-0.5 mm is arranged between the mating surfaces of the quartz sleeve surrounding the capillary needle tube 3 and the two-way connector 2 to achieve a reliable seal, prevent liquid leakage during pressurized delivery, and ensure the stable operation of the system.

[0081] Preferably, the length of the two-way connector 2 is not strictly required, as long as it can fix the capillary tube 3. Its length is usually 2-5cm, for example, 3cm.

[0082] Furthermore, the axial distance between the two-way connector 2 and the syringe 6 is determined by the length of the capillary tube 3, which is usually 5-8 cm. This length ensures the stability of liquid delivery and facilitates the assembly and maintenance of the experimental system.

[0083] In this invention, one end of the metal tube 7 extends to the right side of the two-way connector 2 (i.e., the side near the reaction area) and is coaxially fitted with the quartz sleeve to form a liquid transport channel; the other end extends into the interior of the reaction area (i.e., extends to the right end of the three-way connector 1) to form a transition section between the gas conduit and the discharge gap.

[0084] Furthermore, the length of the metal tube 7 is 3-5 cm longer than that of the tee connector 1. This length is sufficient to cover the plasma discharge gap, ensuring that the entire reaction area is under insulation protection. At the same time, this length helps the liquid to be stably output through the tube opening at the front end of the capillary tube 3, improving the uniformity and repeatability of microdroplet generation.

[0085] In this invention, the metal tube 7 is sleeved outside the quartz sleeve surrounding the capillary needle tube 3, and the two are coaxially nested in a non-contact manner. Specifically, the capillary needle tube 3 and the quartz sleeve pass through the central hole of the metal tube 7 from the rear end opening and extend to its front end outlet; the rear ends of the capillary needle tube 3 and the quartz sleeve are fixed by two through-holes 2, and the front end extends slightly beyond the outlet end of the metal tube 7 by 2-3 mm, with the extended part having a 1-2 mm tapered or needle-like structure at the end of the capillary needle tube 3.

[0086] Furthermore, the metal tube 7 is axially limited at its front end by a tee connector 1, and optionally radially supported on its outer wall by an external bracket.

[0087] In this invention, the tee connector 1 is a hollow circular tube structure located on the outer periphery of the metal tube 7. The tee connector 1 and the metal tube 7 are connected by threads and are coaxially aligned. Preferably, a polytetrafluoroethylene sealing gasket is arranged between the connecting surfaces to achieve a reliable airtight connection and ensure that the carbon dioxide-containing gas can uniformly surround the liquid jet.

[0088] In this invention, the inner diameter of the tee connector 1 is 0.8-1.5 mm and the outer diameter is 2-5 mm, for example, the inner diameter is 1 mm and the outer diameter is 2 mm. Its size design matches the metal tube 7 to ensure smooth gas flow without turbulence interference.

[0089] In this invention, the length of the tee connector 1 is 5-10 cm, for example, 8 cm. The constraint for setting this parameter is that this length allows the gas containing carbon dioxide to complete flow field reshaping before entering the annular gap channel, reducing turbulence and uneven velocity distribution, thereby improving the stability of microdroplet generation.

[0090] In this invention, the length of the exposed axial metal tube 7 between the two-way adapter 2 and the three-way connector 1 is 3-5cm, usually 4cm.

[0091] Preferably, the three-way connector 1 is provided with a lateral gas inlet 10, which is connected to the gas supply module. After the compressed gas containing carbon dioxide enters through the three-way connector 1, it flows into the annular air gap channel between the quartz sleeve and the capillary tube 3 along its internal channel, and flows out from its front end outlet, forming an annular air gap around the capillary tube 3.

[0092] In this invention, the spray module achieves efficient atomization through a coaxial gas-liquid structure: an aqueous solution containing nitrate ions enters the capillary needle tube 3 through the syringe 6 via the two-way connector 2 and is ejected from its outlet in the form of a jet; simultaneously, compressed gas containing carbon dioxide enters from the side gas inlet 10 of the three-way connector 1, flows into the annular channel between the quartz sleeve and the capillary needle tube 3 after passing through its internal cavity, forming a high-speed surrounding airflow; under the action of pneumatic shearing, the liquid jet breaks, generating microdroplets; the quartz sleeve is a complete hollow structure, serving as both a gas conduit and an insulating barrier for the plasma discharge region; the microdroplets then enter the discharge gap and undergo an interfacial reaction with the carbon dioxide plasma, achieving efficient synthesis of urea at room temperature and pressure.

[0093] In this invention, the spray module is connected to the plasma discharge module, so that the microdroplets immediately enter the discharge gap after generation and react with the carbon dioxide plasma to promote the formation of carbon-nitrogen bonds, thereby achieving efficient synthesis of urea at room temperature and pressure.

[0094] The plasma discharge module includes a DC high-voltage power supply 5, a high-voltage electrode, and a grounding electrode. The output terminal of the DC high-voltage power supply 5 is connected to the high-voltage electrode, and the grounding terminal is connected to the grounding electrode. The metal tube 7 serves as the high-voltage electrode, forming a discharge gap of 8-12 mm between it and the grounded liquid collection unit 4 (which serves as the grounding electrode, its surface covered with conductive copper foil for good grounding). The DC high-voltage power supply 5 can be any commercially available type, such as the TRC2025P20-1000 model.

[0095] In operation, a 1-15kV DC voltage is applied to excite carbon dioxide-containing gas around the metal tube 7, forming a non-equilibrium plasma. During the discharge process, microdroplets generated by the spray module pass through the plasma region, and their surfaces react with the active carbon and nitrogen at the interface, achieving in-situ synthesis of urea. The plasma region can be understood as the ionized gas space formed in the discharge gap between the metal tube 7 and the liquid collection unit 4 after being broken down by a 1-15kV DC voltage. In this space, carbon dioxide gas is ionized into electrons, ions, and excited-state molecules, exhibiting typical plasma characteristics such as conductivity, luminescence, and the generation of active particles.

[0096] In this invention, urea is synthesized via a gas-liquid-plasma multiphase coupling reaction pathway:

[0097] First, in the gas-liquid atomization stage, nitrate ions (NO3) are present. - The aqueous solution is injected into the capillary tube 3 through the two-way connector 2 of the syringe 6 and ejected from its outlet in the form of a stable jet. At the same time, the compressed gas containing carbon dioxide (pressure 0.3-0.9 MPa) after being stabilized by the pressure regulator 8 enters from the side gas inlet 10 of the three-way connector 1 and flows at high speed along the annular gap channel between the quartz sleeve and the capillary tube 3, applying a strong aerodynamic shear force to the central liquid jet, causing it to become interfacially unstable and break, forming microdroplets.

[0098] Secondly, during the plasma activation stage, the DC high-voltage power supply 5 applies a DC voltage of 1-15 kV between the high-voltage electrode and the downstream liquid collection unit 4, forming a discharge gap of 8-12 mm. Within this gap, carbon dioxide gas is excited to generate non-equilibrium plasma, producing various highly reactive species, including carbon monoxide radicals (·CO) and carbon dioxide anions (CO2). - ), oxygen negative ions (O - ), high-energy electrons (e - ) and excited-state CO2 molecules (CO2*).

[0099] Subsequently, during the gas-liquid interface reaction stage, the aforementioned microdroplets traverse the plasma region, and their surfaces undergo interfacial chemical reactions with active species: nitrate ions (NO3)- Under the action of plasma electrons and reducing free radicals, urea is gradually reduced to amino radicals (·NH2) or ammonia (NH3); at the same time, carbon-containing active species (such as ·CO) diffuse to the surface of the droplet and undergo CN coupling reaction with nitrogen-containing intermediates. After condensation, rearrangement and other steps, urea ((NH2)2CO) is finally synthesized in situ.

[0100] Finally, during the product collection stage, the microdroplets that have completed the reaction fall into the liquid collection unit 4 located downstream of the discharge zone, and urea is enriched in the collection liquid, which facilitates subsequent quantitative analysis or separation and purification.

[0101] The entire process is carried out at normal temperature and pressure, without the need for additional catalysts or high temperature and high pressure conditions. It not only realizes the resource utilization of carbon dioxide, but also simultaneously completes the high-value conversion of nitrate-containing wastewater, which has significant environmental and energy synergistic benefits.

[0102] On the other hand, according to the method for synthesizing urea using the system described in the first aspect provided by the present invention, the method includes:

[0103] S1 supplies carbon dioxide-containing gas to the urea synthesis reaction through the gas supply module;

[0104] S2, an aqueous solution containing nitrate ions is delivered via a liquid delivery module;

[0105] S3, through the spray module, atomizes the aqueous solution containing nitrate ions into microdroplets;

[0106] S4, carbon dioxide plasma is generated by the plasma discharge module. The plasma reacts with nitrate ions in the microdroplets at the gas-liquid interface to synthesize urea.

[0107] In S1, the inlet pressure of carbon dioxide gas is preferably controlled between 0.3 MPa and 0.9 MPa, for example 0.6 MPa, to ensure that a stable annular gas flow is formed between the quartz sleeve and the capillary needle tube 3, which satisfies the requirements for efficient atomization and maintains the stability of plasma discharge.

[0108] In S2, the pH value of the aqueous solution containing nitrate ions is preferably controlled between 5 and 9 to avoid corrosion of stainless steel parts by strong acids / bases; the aqueous solution containing nitrate ions is pushed into the capillary needle 3 at a flow rate of 0.01-0.05 mL / s to ensure the repeatability of droplet generation and the continuity of reactant supply.

[0109] In S3, under the shearing action of the annular gas, the liquid jet breaks down to form microdroplets with a particle size of 10-200 micrometers. The entire spray unit operates at room temperature, such as 25°C, without the need for additional heating or cooling, which helps to reduce energy consumption and simplify the system structure.

[0110] In S4, carbon dioxide plasma is generated by a plasma discharge module, causing microdroplets to react with active species at the gas-liquid interface to synthesize urea. Specifically, a DC high-voltage power supply 5 applies a voltage of 5-15kV, corresponding to a discharge current of 8-12mA, forming a non-equilibrium plasma between the outer wall of the quartz sleeve and the downstream liquid collection unit 4. After generation, the microdroplets pass through the plasma region, and NO3- is generated on their surface. - Urea is synthesized in situ at room temperature and pressure through a reduction and CN coupling reaction.

[0111] In this invention, the system and method provide guidance for a scheme to produce urea on a larger scale and at a lower cost.

[0112] Example

[0113] The present invention is further described below through specific examples; however, these examples are merely exemplary and do not constitute any limitation on the scope of protection of the present invention. All the following examples were conducted at 20-30°C and 101.325 kPa; the syringe 6 used was a TYD01-01 type laboratory syringe pump; the DC high-voltage power supply 5 was a TRC2025P20-1000; and the liquid collection unit 4 was a large-diameter bottle.

[0114] Example 1: Synthesis of urea under different plasma voltages

[0115] like Figures 1 to 2 The system shown consists of a spray module coaxially assembled from a three-way connector 1, a stainless steel capillary needle tube 3, and a metal tube 7; a CO2 cylinder 9 (containing 99.99% pure carbon dioxide) is depressurized to 0.6 MPa by a pressure regulator 8 and then connected to the three-way connector 1; a syringe 6 delivers silver nitrate aqueous solution at a flow rate of 0.01 mL / s; the discharge gap between the high-voltage electrode and the ground electrode is 8 mm; and the overall operating temperature of the system is 25℃.

[0116] CO2 gas with an inlet pressure of 0.6 MPa was introduced into the three-way connector 1, and the CO2 gas entered the annular gas channel between the quartz sleeve and the capillary needle tube 3; a silver nitrate aqueous solution with pH=5 and a concentration of 5 mmol / L was injected into the capillary needle tube 3 at a flow rate of 0.01 mL / s; under the action of the annular gas flow, the liquid jet was sheared into microdroplets with a particle size of 10-200 μm and ejected from the nozzle; a DC voltage was applied between the two electrodes, and the urea yield was tested under conditions of 0, 3, 6, 9, 12, and 15 kV.

[0117] After the reaction was completed, the urea concentration in the collected solution was determined by the M-DAMO method, and the results were as follows: Figure 3As shown, the urea concentration initially increases and then decreases with increasing applied DC voltage: Within the 0-6 kV range, the urea concentration increases significantly (approximately 1.0 mg / L at 0 kV, approximately 1.7 mg / L at 3 kV, and reaches a maximum of approximately 3.3-3.4 mg / L at 6 kV, more than three times that at no discharge); with further voltage increases, the urea concentration decreases (approximately 2.1 mg / L at 9 kV, approximately 1.2 mg / L at 12 kV, and only approximately 0.5 mg / L at 15 kV). This indicates that the CO2 spray discharge system has an optimal discharge intensity; a moderate discharge (approximately 6 kV, 8-12 mA adaptive current) is most conducive to the generation and utilization of effective active species to promote urea production. However, excessively high voltage may lead to side reactions / further decomposition or oxidation of products, or inhibition of effective reaction pathways, resulting in a decrease in urea yield.

[0118] Example 2 Synthesis of urea at different pH values ​​of silver nitrate

[0119] Urea was synthesized in a manner similar to that in Example 1, except that the pH of the silver nitrate aqueous solution was different. Specifically:

[0120] like Figures 1 to 2 The system shown consists of a spray module coaxially assembled from a three-way connector 1, a stainless steel capillary needle tube 3, and a metal tube 7; a CO2 cylinder 9 (containing 99.99% pure carbon dioxide) is depressurized to 0.6 MPa by a pressure regulator 8 and then connected to the three-way connector 1; a syringe 6 delivers silver nitrate aqueous solution at a flow rate of 0.01 mL / s; the discharge gap between the high-voltage electrode and the ground electrode is 8 mm; and the overall operating temperature of the system is 25℃.

[0121] CO2 gas with an inlet pressure of 0.4 MPa is introduced into the three-way connector 1, and the CO2 gas enters the annular gas channel between the quartz sleeve and the capillary needle tube 3; silver nitrate solutions (concentration 5 mmol / L) with pH=5, 7 and 9 are prepared respectively, and the silver nitrate aqueous solutions are injected into the capillary needle tube 3 at a flow rate of 0.01 mL / s; under the action of the annular gas flow, the liquid jet is sheared into microdroplets with a particle size of 10-200 μm and ejected from the nozzle; a DC voltage of 6 kV is applied between the two electrodes, and a current of 8 mA is maintained to allow the microdroplets to pass through the plasma region and react.

[0122] After the reaction was completed, the urea concentration in the collected solution was determined by the M-DAMO method, and the results were as follows: Figure 4As shown, the pH of the silver nitrate solution has a significant effect on urea formation: the urea concentration decreases significantly with increasing pH, with the highest concentration (approximately 2.3 mg / L) at pH=5, decreasing to approximately 0.8 mg / L at pH=7, and further decreasing to approximately 0.4 mg / L at pH=9. This indicates that a weakly acidic environment is more conducive to urea formation in this spray plasma system, while neutral and alkaline conditions inhibit urea yield.

[0123] Example 3: Synthesis of urea under different CO2 gas inlet pressures

[0124] Urea was synthesized in a manner similar to that of Example 1, except that the CO2 gas inlet pressure was different. Specifically:

[0125] like Figures 1 to 2 The system shown consists of a spray module coaxially assembled from a three-way connector 1, a stainless steel capillary needle tube 3, and a metal tube 7; a CO2 cylinder 9 (containing 99.99% pure carbon dioxide) is depressurized by pressure regulator 8 to 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8 MPa respectively before being connected to the three-way connector 1; a syringe 6 delivers silver nitrate aqueous solution at a flow rate of 0.01 mL / s; the discharge gap between the high-voltage electrode and the ground electrode is 8 mm; and the overall operating temperature of the system is 25℃.

[0126] CO2 gas is introduced into the three-way connector 1, and the CO2 gas enters the annular gas channel between the quartz sleeve and the capillary needle tube 3; a silver nitrate aqueous solution with pH=5 and a concentration of 5mmol / L is injected into the capillary needle tube 3 at a flow rate of 0.01 mL / s; under the action of the annular gas flow, the liquid jet is sheared into microdroplets with a particle size of 10-200 μm and ejected from the nozzle; a high voltage is applied between the two electrodes to induce an arc discharge, and the detection current is kept stable at about 10mA, so that the microdroplets can pass through the plasma region and react.

[0127] After the reaction was completed, the urea concentration in the collected solution was determined by the M-DAMO method, and the results were as follows: Figure 5 As shown, the urea concentration generally increases with increasing pressure, reaching its peak around 0.7 MPa: when the pressure increases from 0.3 MPa to 0.7 MPa, the urea concentration gradually increases from approximately 0.4 mg / L to approximately 4.2 mg / L; after further increasing to 0.8 MPa, the urea concentration decreases slightly / remains relatively stable (approximately 4.0 mg / L). This indicates that appropriately increasing the CO2 inlet pressure can enhance the annular gas shear and atomization effect, improve the contact and mass transfer efficiency between microdroplets and plasma, thereby increasing the urea yield. However, when the pressure is too high, the gain tends to saturate or even slightly decreases, indicating the existence of an optimal pressure range (approximately 0.7 MPa).

[0128] Comparative Example 1

[0129] Urea was synthesized in a manner similar to that in Example 1, except that a plasma discharge module was not used, and the flow rate of the silver nitrate aqueous solution was 0.01-0.05 mL / s. Specifically:

[0130] The spray module is composed of a three-way connector 1, a stainless steel capillary needle tube 3 and a metal tube 7 coaxially combined; the CO2 cylinder 9 (carbon dioxide with a purity of 99.99%) is depressurized to 0.6 MPa by the pressure regulator 8 and then connected to the three-way connector 1; the syringe 6 delivers silver nitrate aqueous solution at a flow rate of 0.01 mL / s, and the overall operating temperature of the system is 25℃.

[0131] CO2 gas with an inlet pressure of 0.6 MPa is introduced into the tee connector 1, and the CO2 gas enters the annular gas channel between the quartz sleeve and the capillary needle tube 3; silver nitrate aqueous solution with pH=5 and a concentration of 5 mmol / L is injected into the capillary needle tube 3 at flow rates of 0.01, 0.02, 0.03, 0.04, and 0.05 mL / s, respectively; under the action of the annular gas flow, the liquid jet is sheared into microdroplets with a particle size of 50-200 μm and ejected from the nozzle.

[0132] After the reaction was completed, the urea concentration in the collected solution was measured using M-DAMO, and the results were as follows: Figure 6 As shown, the effect of the inlet flow rate on urea production exhibits a "first increase, then decrease" pattern: when the flow rate increases from 0.01 mL / s to 0.02 mL / s, the urea concentration rises from approximately 0.7 mg / L to a maximum of approximately 1.3 mg / L; after further increasing to 0.03, 0.04, and 0.05 mL / s, the urea concentration decreases to approximately 0.82, 0.63, and 0.60 mg / L, respectively. This indicates that under a CO2 inlet pressure of 0.6 MPa, a moderate inlet flow rate (approximately 0.02 mL / s) is more conducive to atomization and effective residence / mass transfer reactions of microdroplets in the plasma region, while excessively low or high flow rates will reduce reaction efficiency and urea yield.

[0133] Comparative Example 2

[0134] Urea was synthesized in a manner similar to that of Example 1, except that a plasma discharge module was not included. Specifically:

[0135] The spray module is composed of a three-way connector 1, a stainless steel capillary needle tube 3 and a metal tube 7 coaxially combined; the CO2 cylinder 9 (CO2) is depressurized by pressure regulator 8 to 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8 MPa respectively before being connected to the three-way connector 1; the syringe 6 delivers silver nitrate aqueous solution at a flow rate of 0.01 mL / s, and the overall operating temperature of the system is 25℃.

[0136] CO2 gas with an inlet pressure of 0.6 MPa is introduced into the three-way connector 1, and the CO2 gas enters the annular gas channel between the metal tube 7 and the capillary needle tube 3; a silver nitrate aqueous solution with pH=5 and a concentration of 5 mmol / L is injected into the capillary needle tube 3 at a flow rate of 0.01 mL / s; under the action of the annular gas flow, the liquid jet is sheared into micro-droplets with a particle size of 50-200 μm and ejected from the nozzle.

[0137] After the reaction was completed, the urea concentration in the collected solution was determined by the M-DAMO method, and the results were as follows: Figure 7 As shown, even without the plasma discharge module, urea can still be generated through CO2 gas atomization alone, and the urea concentration gradually increases with increasing CO2 inlet pressure: as the pressure increases from 0.3 MPa to 0.8 MPa, the urea concentration rises from approximately 1.0 mg / L to approximately 2.7 mg / L, indicating that increasing the gas pressure enhances the annular gas shear and atomization effect, increases gas-liquid contact and mass transfer, thereby promoting urea generation. However, compared to Example 1, under the same CO2 inlet pressure, the urea concentration in the pure microdroplet system is significantly lower than that in the plasma microdroplet system. For example, at the optimal pressure of 0.7 MPa, the urea concentration in the pure microdroplet system is only about 2.5 mg / L, while that in the plasma microdroplet system reaches about 4.0 mg / L, an increase of about 60%. The above results indicate that although gas-liquid mass transfer enhanced by airflow atomization can promote urea production to some extent, plasma discharge plays an irreplaceable and crucial role in urea synthesis. Plasma discharge can effectively activate CO2 molecules and nitrogen-containing species in the solution, generating reactive intermediates such as high-energy electrons and active free radicals, significantly reducing the energy barrier for C–N bond formation, thereby greatly increasing urea yield. The synergistic effect of plasma discharge and microdroplet atomization is far superior to that of a single microdroplet atomization system.

[0138] The present invention has been described in detail above with reference to preferred embodiments and exemplary examples. However, it should be noted that these specific embodiments are merely illustrative explanations of the invention and do not constitute any limitation on the scope of protection of the invention. Various improvements, equivalent substitutions, or modifications can be made to the technical content and embodiments of the present invention without departing from the spirit and scope of protection of the invention, and all such modifications fall within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A urea synthesis system, characterized in that, include: A gas supply module for supplying carbon dioxide-containing gas; A liquid delivery module for delivering aqueous solutions containing nitrate ions; A spray module is used to atomize an aqueous solution containing nitrate ions into microdroplets in a carbon dioxide-containing gas atmosphere. A plasma discharge module is used to generate carbon dioxide plasma and cause the plasma to react with nitrate ions in the microdroplets at the gas-liquid interface to synthesize urea.

2. The system according to claim 1, characterized in that, Preferably, the gas supply module includes: CO2 cylinder (9) stores compressed gas containing carbon dioxide to provide the carbon source required for urea synthesis; Pressure regulator (8) is used to stabilize the output pressure of the compressed air source.

3. The system according to claim 1, characterized in that, The liquid delivery module includes: Syringe (6), which is used to store and propel an aqueous solution containing nitrate ions; Two-way adapter (2) connects the syringe (6) to the spray module to enable detachable connection of the liquid channel.

4. The system according to claim 1, characterized in that, The spray module includes a three-way connector (1), a capillary needle tube (3), and a metal tube (7), which are coaxially combined.

5. The system according to claim 4, characterized in that, The capillary needle (3) is made of stainless steel.

6. The system according to claim 4, characterized in that, The capillary needle (3) is surrounded by a quartz sleeve.

7. The system according to claim 6, characterized in that, An annular air gap channel is formed between the inner wall of the quartz sleeve and the outer wall of the capillary tube (3).

8. The system according to claim 1, characterized in that, The plasma discharge module includes a DC high voltage power supply (5), a high voltage electrode and a ground electrode. The output terminal of the DC high voltage power supply (5) is connected to the high voltage electrode, and the ground terminal is connected to the ground electrode.

9. The method for synthesizing urea according to any one of claims 1-8, characterized in that, The method includes: S1 supplies carbon dioxide-containing gas to the urea synthesis reaction through the gas supply module; S2, an aqueous solution containing nitrate ions is delivered via a liquid delivery module; S3, through the spray module, atomizes the aqueous solution containing nitrate ions into microdroplets; S4, carbon dioxide plasma is generated by the plasma discharge module. The plasma reacts with nitrate ions in the microdroplets at the gas-liquid interface to synthesize urea.

10. The method according to claim 1, characterized in that, In S1, the inlet pressure of carbon dioxide gas is 0.3MPa-0.9MPa.

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