Waste heat utilization device for stable production of nitric acid by double-pressurization method

Abstract

The utility model relates to the technical field of nitric acid production, in particular to a waste heat utilization device for stable nitric acid production by a double-pressurization method. The device comprises an evaporator and a primary air filter, the evaporator is connected with a mixer through a precious metal filter, the mixer is connected with a heat exchanger A through an ammoxidation furnace, and the heat exchanger A is connected with a nitric oxide separator through a low-pressure water cooler. The nitrogen oxide separator is connected with the tail gas expansion machine through the absorption tower, the primary air filter is connected with the cooling condenser through the air preheater, the cooling condenser is connected with the mixer through the air compressor, and the tail gas expansion machine is connected with the air preheater through a preheating pipeline. The tail gas expansion machine is connected with the air preheater through the preheating pipeline, middle-low-grade tail gas waste heat exhausted by the tail gas expansion machine is fully utilized, a heat source is provided for the air preheater, and energy consumption in the air treatment link is greatly reduced.

Description

Technical Field

[0001] This utility model relates to the field of nitric acid production technology, specifically to a waste heat utilization device for stable nitric acid production using a dual-pressure method. Background Technology

[0002] In the field of double-pressure nitric acid production, process stability, energy utilization efficiency, and adaptability to special operating conditions have always been core challenges facing the industry, and existing workshop equipment has many problems that urgently need to be solved. From the perspective of waste heat utilization, in traditional double-pressure nitric acid production systems, a large amount of waste heat, such as the 150-200℃ low-grade tail gas discharged from the tail gas expander and the circulating cooling water at the outlet of the low-pressure water cooler, is not effectively recovered. It is usually directly discharged or simply cooled, resulting in serious energy waste. At the same time, the air handling process requires additional electricity or steam to preheat the air, further increasing production energy consumption, which contradicts the current industry requirements for energy conservation and emission reduction.

[0003] In air dehumidification, when the outside temperature is low (-20-0℃) and the air moisture content is high, traditional air handling processes that directly cool and dehumidify the air are prone to frosting and clogging. The dew point of low-temperature, high-humidity air is close to or below 0℃; direct cooling causes water vapor to condense into frost that adheres to the surface of the heat exchange tubes. This not only hinders heat exchange and reduces dehumidification efficiency but also clogs the flow channels, causing a sharp drop in airflow, disrupting the ammonia-air ratio balance, and even posing safety risks. If a "direct cooling-reheating" method is used, it results in a double waste of energy: "cooling energy consumption - heating energy consumption again." Furthermore, acidic condensate can exacerbate equipment corrosion and shorten the lifespan of core equipment such as coolers and compressors. Utility Model Content

[0004] To address the shortcomings of existing technologies, the purpose of this invention is to provide a waste heat utilization device for the stable production of nitric acid using a dual-pressure method. This device connects the exhaust gas expander to an air preheater via a preheating pipeline, fully utilizing the 150-200℃ low-grade exhaust gas waste heat from the expander to provide a heat source for the air preheater. This replaces traditional electric or steam preheating methods, significantly reducing energy consumption in the air handling process. Simultaneously, a low-pressure water cooler is connected to the cooling condenser via inlet cooling condenser coils, using the waste heat from the circulating cooling water at the cooler outlet as the cooling medium for the condenser. This further improves waste heat utilization, reduces additional cooling energy consumption of the circulating cooling water, and achieves cascaded energy utilization.

[0005] This utility model is achieved using the following technical solution:

[0006] The aforementioned waste heat utilization device for stable nitric acid production using the dual-pressure method includes an evaporator, a primary air filter, an evaporator connected to a mixer via a precious metal filter, a mixer connected to a heat exchanger A via an ammonia oxidation furnace, a heat exchanger A connected to a nitrogen oxide separator via a low-pressure water cooler, a nitrogen oxide separator connected to a tail gas expander via an absorption tower, a primary air filter connected to a cooling condenser via an air preheater, a cooling condenser connected to a mixer via an air compressor, and a tail gas expander connected to the air preheater via a preheating pipe.

[0007] The evaporator adopts a shell-and-tube structure, with liquid ammonia flowing through the shell side and the heating medium (utilizing process waste heat) flowing through the tube side. Internal baffles enhance medium turbulence and improve heat exchange efficiency. Pressure and temperature sensors are provided to monitor vaporization pressure and temperature in real time. The superheater uses a spiral plate heat exchanger, which offers a large heat exchange area and compact size. The fluid flows spirally within the heat exchanger, exhibiting high turbulence and a heat transfer coefficient of 1500-2000 W / (m²). 2 •K); Valves are installed on the inlet and outlet pipes to facilitate the adjustment of ammonia flow and temperature. The high heat exchange efficiency of the spiral plate structure ensures that the ammonia can be rapidly and stably heated to 100℃, preventing condensation during transportation; the compact size saves equipment installation space, and the design of the inlet and outlet valves facilitates precise adjustment of the ammonia temperature according to subsequent process requirements, ensuring the stability of the ammonia-air mixture.

[0008] The primary air filter employs a bag filter structure with polyester fiber filter bags and a filtration accuracy of 5μm. The housing is designed for quick opening, facilitating filter bag replacement. It is equipped with an airflow monitoring device to display the real-time air handling capacity. The secondary air filter uses a pleated filter element made of hydrophobic glass fiber with a filtration accuracy of 0.1μm. It features a differential pressure alarm that triggers when the differential pressure exceeds 0.08MPa. The low-pressure water cooler uses a shell-and-tube structure. The shell side carries NOx-containing reactant gas cooled by the economizer, while the tube side carries circulating cooling water. The heat exchange tubes are made of stainless steel for corrosion resistance. Baffles are installed inside the shell to enhance heat exchange between the reactant gas and cooling water. A temperature control device regulates the reactant gas outlet temperature by adjusting the cooling water flow. The shell-and-tube structure and baffle design ensure sufficient heat exchange between the reactant gas and cooling water, efficiently cooling the reactant gas and promoting the reaction of some NOx with water to form dilute nitric acid; the stainless steel heat exchange tubes are resistant to dilute nitric acid corrosion, extending the equipment's lifespan; the temperature control device precisely controls the reactant gas outlet temperature, preventing excessive NOx condensation in the reactant gas due to excessively low temperatures, or excessively high temperatures that affect the separation effect of the subsequent nitrogen oxide separator.

[0009] The exhaust gas expander adopts a radial-flow turbine structure with an impeller made of high-temperature alloy, capable of withstanding exhaust gas temperatures of approximately 360℃. It is equipped with a speed control system that adjusts the rotational speed based on exhaust gas flow and pressure to optimize efficiency. Inlet and outlet valves facilitate start-up, shutdown, and troubleshooting. An exhaust muffler reduces exhaust noise. The radial-flow turbine structure efficiently utilizes the pressure and heat energy of the 360℃ exhaust gas to drive an air compressor or generator, achieving energy recovery and reducing production energy consumption. The high-temperature alloy impeller resists the high temperatures and corrosion of the exhaust gas, ensuring long-term stable operation of the expander. The speed control system optimizes efficiency, maximizing the utilization of exhaust gas energy. The exhaust muffler reduces exhaust noise, meeting environmental protection requirements. Simultaneously, the inlet and outlet valves facilitate equipment maintenance and troubleshooting, ensuring continuous operation of the exhaust gas treatment system.

[0010] The evaporator is connected to a liquid ammonia inlet pipe, and a superheater is provided between the evaporator and the precious metal filter.

[0011] An economizer is provided between the heat exchanger A and the low-pressure water cooler, and a nitrogen oxide compressor is provided between the nitrogen oxide separator and the absorption tower.

[0012] A gas-liquid separator B is provided between the absorption tower and the exhaust gas expander, and a heat exchanger B is provided between the gas-liquid separator B and the exhaust gas expander.

[0013] An air inlet pipe is connected to the primary air filter, and a gas-liquid separator A is provided between the cooling condenser and the air compressor. A secondary air filter is provided between the gas-liquid separator A and the air compressor.

[0014] The air preheater is equipped with an air preheater coil inside. The primary air filter is connected to the inlet of the air preheater coil through a pipe, and the outlet of the air preheater coil is connected to the cooling condenser through a pipe.

[0015] The cooling condenser is equipped with a cooling condenser coil inside, and the low-pressure water cooler is connected to the cooling condenser coil through the inlet pipe.

[0016] The air preheater is equipped with an air preheater coil made of 316L stainless steel, which is corrosion-resistant and has good thermal conductivity. The chamber between the shell and the coil is vented with waste heat exhaust gas from the exhaust gas expander, resulting in counter-current heat exchange between the exhaust gas and air. A temperature control module is included to regulate the air preheating temperature by adjusting the exhaust gas flow rate. The 316L stainless steel coil is resistant to corrosion from trace amounts of NOx in the exhaust gas, extending the equipment's lifespan. The counter-current heat exchange design improves heat exchange efficiency, allowing the air to fully absorb the waste heat from the exhaust gas and stably preheat to 65-75℃. The temperature control module ensures accurate air preheating temperature, preventing frost formation on the subsequent cooling condenser due to excessively low temperatures or increased cooling load due to excessively high temperatures. The cooling condenser adopts a shell-and-tube structure. Preheated air is vented through the shell side, while the tube side contains the cooling condenser coil, which is circulated with cooling water from a low-pressure water cooler. Baffles are installed inside the shell to guide airflow and enhance heat exchange with the coil. A condensate outlet is located at the bottom, connecting to the gas-liquid separator A. The shell-and-tube structure combined with the baffle design allows for full heat exchange between air and cooling water, efficiently cooling the air to 15-20℃ and promoting water vapor condensation. The connection between the condensate outlet and the gas-liquid separator A ensures timely discharge of condensate, preventing it from accumulating inside the shell and affecting heat exchange efficiency. Utilizing the circulating cooling water from the low-pressure water cooler as the cooling medium enables the secondary utilization of waste heat, reducing cooling energy consumption.

[0017] The working principle of this utility model is as follows:

[0018] Liquid ammonia is transported to the evaporator via a pipeline. The pressure inside the evaporator is controlled at 0.52 MPa. An external heat source is used to vaporize the liquid ammonia, converting it into gaseous ammonia at 0.52 MPa. The vaporized gaseous ammonia then enters the superheater, where it is heated to 100°C to ensure temperature stability and prevent condensation in subsequent pipelines. The superheated gaseous ammonia then enters a precious metal filter to remove oil and other solid impurities. The filtration precision reaches below 10 μm, preventing contamination of the platinum mesh catalyst in the subsequent mixer and ammonia oxidation furnace.

[0019] Air enters the primary air filter through the air inlet pipe, removing large particles such as dust with a filtration efficiency of ≥99%, ensuring that subsequent equipment is not damaged by impurities. The primary filtered air then enters the air preheater, which is connected to the exhaust gas expander via a preheating pipe. The preheater utilizes the waste heat from the 150-200℃ exhaust gas to preheat the air from ambient temperature (down to -20℃ under low-temperature conditions) to 65-75℃, reducing the relative humidity to below 1%. The preheated air then enters the cooling condenser. The cooling condenser coils inside the cooling condenser are connected to a low-pressure water cooler via inlet pipes. The circulating cooling water from the low-pressure water cooler outlet serves as the cooling medium, cooling the air to 15-20℃, causing water vapor in the air to condense and precipitate. After cooling and dehumidification, the air enters the gas-liquid separator A, where condensate is separated (separation efficiency ≥99%). The air then enters a secondary air filter to remove residual tiny water droplets and impurities, achieving a filtration accuracy of 0.1μm, ensuring that the air entering the air compressor is dry and clean. The dry and clean air enters the air compressor, is pressurized to 0.45MPa, and during compression, the air temperature rises to 236℃. The outlet air parameters are stably controlled at 0.45MPa and 236℃.

[0020] Filtered ammonia gas at 100℃ and 0.52MPa and compressed air at 0.45MPa and 236℃ (primary air) enter the mixer. The ammonia concentration in the mixture is precisely controlled at 9.5% by the ammonia-air ratio adjustment system to ensure that the mixture is outside the safe explosion limit. The ammonia-air mixture is evenly distributed on the surface of the platinum mesh catalyst by a distributor at the top of the ammonia oxidation furnace. The reaction temperature inside the furnace is controlled at 860℃ and the pressure is maintained at 0.4-0.45MPa, where the ammonia oxidation reaction (4NH3 + 5O2 → 4NO + 6H2O) occurs, generating a reaction gas containing NOx, with a NOx conversion rate of ≥96%.

[0021] The NOx-containing reaction gas at 860℃ from the ammonia oxidizer outlet first enters heat exchanger A for initial cooling through heat exchange with the subsequent low-temperature medium. It then enters the economizer, where the waste heat from the reaction gas preheats the boiler feedwater. Next, it enters the low-pressure water cooler, where circulating cooling water cools the reaction gas. During this process, some NOx reacts with water to form dilute nitric acid. The cooled acid-gas mixture enters the nitrogen oxide separator, where dilute nitric acid (concentration approximately 10-15%) is separated. The dilute nitric acid is pumped to the corresponding tray in the absorption tower, while the NOx gas proceeds to subsequent treatment stages. The NOx gas mixes with secondary air from the bleaching tower and enters the nitrogen oxide compressor, where it is pressurized to 1.1 MPa. After compression, the gas temperature rises to 194℃, and then it enters the tail gas preheater and high-pressure water cooler for further cooling. The cooled NOx gas at 1.1 MPa enters the bottom of the absorption tower, where it comes into countercurrent contact with water sprayed from the top and circulating dilute nitric acid. NOx is absorbed by the water to form nitric acid, and 60% of the final product, nitric acid, is produced at the bottom of the absorption tower. The operating pressure inside the tower is maintained at 1.0-1.1 MPa, and the temperature is controlled at 40-50℃. The 60% nitric acid is then sent from the bottom of the absorption tower to a bleaching tower, where secondary air is introduced to remove dissolved NOx gas. It then enters an acid cooler to be cooled to 25-30℃ before finally being sent to the finished acid storage tank.

[0022] The exhaust gas from the top of the absorption tower first enters the gas-liquid separator B, where entrained nitric acid droplets are separated. It then enters heat exchanger B, where it exchanges heat with a low-temperature medium to increase its temperature. The heated exhaust gas then enters the air preheater, where it further absorbs heat and reaches approximately 360°C. It then enters the exhaust gas expander to perform work. After performing work, the exhaust gas temperature drops to 150-200°C and is discharged through the exhaust stack. The NOx content in the exhaust gas is <200×10⁻⁶. -6 .

[0023] Compared with the prior art, the beneficial effects of this utility model are:

[0024] (1) This utility model connects the exhaust gas expander to the air preheater through a preheating pipe, making full use of the waste heat of the low-grade exhaust gas (150-200℃) discharged from the exhaust gas expander to provide a heat source for the air preheater, replacing the traditional electric or steam preheating method and significantly reducing energy consumption in the air handling process. At the same time, the low-pressure water cooler is connected to the cooling condenser through the inlet cooling condenser coil pipe, using the waste heat of the circulating cooling water at the outlet of the low-pressure water cooler as the cooling medium for cooling the condenser, further improving the waste heat utilization rate, reducing the additional cooling energy consumption of the circulating cooling water, realizing the cascade utilization of energy, and conforming to the industry development trend of energy conservation and emission reduction.

[0025] (2) This utility model adopts an air handling process of "air preheater - cooling condenser". First, the waste heat of the exhaust gas is used to preheat the air after primary filtration to 65-75℃ (low temperature condition) to reduce the relative humidity of the air to below 1%. Then, the air enters the cooling condenser to cool to 15-20℃ for dehumidification. This process fundamentally avoids the frosting and clogging problem of traditional direct cooling methods, ensures smooth airflow, and maintains the ammonia-air ratio balance. At the same time, the preheating process keeps the acidic impurities in the air in a gaseous state. During cooling and dehumidification, the acidic substances are discharged with the condensate by the gas-liquid separator A, avoiding the corrosion of the air compressor by acidic vapors, extending the service life of the equipment, and improving the stability and reliability of the air handling system.

[0026] (3) This utility model achieves deep synergy between the liquid ammonia treatment, air treatment, NOx reaction, and tail gas treatment units. In the liquid ammonia treatment unit, the superheater ensures that the liquid ammonia is stably heated to 100°C after vaporization, meeting the requirements of the subsequent mixing process. The air treatment unit, through precise preheating and cooling control, keeps the outlet air of the air compressor stably maintained at the design parameters of 0.45MPa and 236°C, ensuring the precise control of the 9.5% ammonia concentration in the ammonia-air mixer, and providing a foundation for the stable reaction at 860°C in the ammonia oxidation furnace. In addition, the linkage between the tail gas treatment unit and the air preheater, as well as the heat exchange between the low-pressure water cooler and the cooling condenser, form a complete energy and medium circulation system, reducing parameter fluctuations between units, improving the stability of the entire nitric acid production process, and thus improving the yield and quality of nitric acid.

[0027] (4) In response to the low temperature environment, the air preheater in the device effectively preheats the air, and the evaporator and superheater in the liquid ammonia treatment unit work together to avoid the condensation and freezing blockage of liquid ammonia in the pipeline, ensuring that the device can still operate stably in the low temperature environment of -20-0℃, thus expanding the applicability of the device. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the waste heat utilization device for the dual-pressure method of nitric acid stabilization production according to this utility model;

[0029] In the diagram: 1. Evaporator; 2. Superheater; 3. Precious metal filter; 4. Mixer; 5. Ammonia oxidation furnace; 6. Primary air filter; 7. Air preheater; 8. Cooling condenser; 9. Gas-liquid separator A; 10. Secondary air filter; 11. Air compressor; 12. Heat exchanger A; 13. Economizer; 14. Low-pressure water cooler; 15. Nitrogen oxide separator; 16. Nitrogen oxide compressor; 17. Absorption tower; 18. Gas-liquid separator B; 19. Heat exchanger B; 20. Tail gas expander; 21. Liquid ammonia inlet pipe; 22. Air inlet pipe; 23. Air preheater coil; 24. Cooling condenser coil; 25. Preheating pipe; 26. Inlet pipe to cooling condenser coil. Detailed Implementation

[0030] To make the objectives and technical solutions of this utility model clearer, the present utility model will be further described in detail below with reference to the accompanying drawings.

[0031] Example 1

[0032] like Figure 1As shown, the waste heat recovery device for the dual-pressure nitric acid stabilization production includes an evaporator 1, a primary air filter 6, an evaporator 1 connected to a mixer 4 via a precious metal filter 3, a mixer 4 connected to a heat exchanger A12 via an ammonia oxidation furnace 5, a heat exchanger A12 connected to a nitrogen oxide separator 15 via a low-pressure water cooler 14, a nitrogen oxide separator 15 connected to a tail gas expander 20 via an absorption tower 17, a primary air filter 6 connected to a cooling condenser 8 via an air preheater 7, a cooling condenser 8 connected to a mixer 4 via an air compressor 11, and a tail gas expander 20 connected to the air preheater 7 via a preheating pipe 25. The absorption tower 17 adopts a packed tower structure, filled with corrugated packing to increase the gas-liquid contact area; a spray device is installed at the top of the tower to uniformly spray dilute nitric acid onto the packing layer; the tower body is made of stainless steel with a corrosion-resistant lining; multiple temperature and pressure monitoring points, as well as a liquid distribution uniformity detection device, are installed. A liquid ammonia inlet pipe 21 is connected to evaporator 1, and a superheater 2 is installed between evaporator 1 and precious metal filter 3. An economizer 13 is installed between heat exchanger A12 and low-pressure water cooler 14, and a nitrogen oxide compressor 16 is installed between nitrogen oxide separator 15 and absorption tower 17. A gas-liquid separator B18 is installed between absorption tower 17 and tail gas expander 20, and a heat exchanger B19 is installed between gas-liquid separator B18 and tail gas expander 20. An air inlet pipe 22 is connected to primary air filter 6, and a gas-liquid separator A9 is installed between cooling condenser 8 and air compressor 11. A secondary air filter 10 is installed between gas-liquid separator A9 and air compressor 11. Gas-liquid separator A9 adopts a composite structure combining cyclone separation and wire mesh mist capture. The cyclone separation section uses centrifugal force to separate large-diameter water droplets, and the wire mesh mist capture section captures small water droplets. An air outlet is set at the top, and a condensate drain is set at the bottom. A float-type condensate drain is equipped to automatically discharge condensate. The air preheater 7 has an internal air preheater coil 23. The primary air filter 6 is connected to the inlet of the air preheater coil 23 through a pipe, and the outlet of the air preheater coil 23 is connected to the cooling condenser 8 through a pipe. The cooling condenser 8 has an internal cooling condenser coil 24, and the low-pressure water cooler 14 is connected to the cooling condenser coil 24 through a cooling condenser coil inlet pipe 26.

[0033] The above-mentioned waste heat recovery device for the dual-pressure nitric acid stabilization production includes the following steps during operation:

[0034] (1) Liquid ammonia enters the evaporator 1 via the liquid ammonia inlet pipe 21 and is vaporized. It is then heated by the heater 2 and filtered through the precious metal filter 3 to remove impurities, resulting in clean ammonia gas. Air enters the primary air filter 6 via the air inlet pipe 22 and is then preheated by the exhaust gas expander 7. It is then cooled and dehumidified by the exhaust gas expander 20, separated by the gas-liquid separator A9, filtered by the secondary air filter 10, and finally pressurized by the air compressor 11. (2) Clean ammonia gas and pressurized air are mixed in the mixer 4. The mixed gas enters the ammonia oxidation furnace 5 and undergoes an oxidation reaction to generate NOx-containing reaction gas. The NOx-containing reaction gas is cooled by the heat exchanger A12, the economizer 13, and the low-pressure water cooler 14 in sequence, and then enters the nitrogen oxide separator 15 to separate dilute nitric acid. The NOx gas is pressurized and cooled by the nitrogen oxide compressor 16 and then enters the absorption tower 17, where it is absorbed to generate nitric acid. The nitric acid at the bottom of the tower is treated and then sent to the storage tank. (3) The exhaust gas at the top of the absorption tower 17 is treated by the gas-liquid separator B18 and the heat exchanger B19, then enters the air preheater 7 to raise the temperature, and finally enters the exhaust gas expander 20 to do work, and the exhaust gas is discharged in compliance with the standard.

Claims

1. A waste heat utilization device for stabilizing nitric acid production using a dual-pressure method, characterized in that, The system includes an evaporator (1), a primary air filter (6), an evaporator (1) connected to a mixer (4) via a precious metal filter (3), a mixer (4) connected to a heat exchanger A (12) via an ammonia oxidizer (5), a heat exchanger A (12) connected to a nitrogen oxide separator (15) via a low-pressure water cooler (14), a nitrogen oxide separator (15) connected to a tail gas expander (20) via an absorption tower (17), a primary air filter (6) connected to a cooling condenser (8) via an air preheater (7), a cooling condenser (8) connected to a mixer (4) via an air compressor (11), and a tail gas expander (20) connected to an air preheater (7) via a preheating pipe (25).

2. The waste heat utilization device for stabilizing nitric acid production using the dual-pressure method according to claim 1, characterized in that, The evaporator (1) is connected to a liquid ammonia inlet pipe (21), and a superheater (2) is provided between the evaporator (1) and the precious metal filter (3).

3. The waste heat utilization device for stabilizing nitric acid production using the dual-pressure method according to claim 1, characterized in that, An economizer (13) is provided between the heat exchanger A (12) and the low-pressure water cooler (14), and a nitrogen oxide compressor (16) is provided between the nitrogen oxide separator (15) and the absorption tower (17).

4. The waste heat utilization device for stabilizing nitric acid production using the dual-pressure method according to claim 1, characterized in that, A gas-liquid separator B (18) is provided between the absorption tower (17) and the tail gas expander (20), and a heat exchanger B (19) is provided between the gas-liquid separator B (18) and the tail gas expander (20).

5. The waste heat utilization device for stabilizing nitric acid production using the dual-pressure method according to claim 1, characterized in that, An air inlet pipe (22) is connected to the primary air filter (6), a gas-liquid separator A (9) is provided between the cooling condenser (8) and the air compressor (11), and a secondary air filter (10) is provided between the gas-liquid separator A (9) and the air compressor (11).

6. The waste heat utilization device for stabilizing nitric acid production using the dual-pressure method according to claim 1, characterized in that, The air preheater (7) is equipped with an air preheater coil (23) inside. The primary air filter (6) is connected to the inlet of the air preheater coil (23) through a pipe, and the outlet of the air preheater coil (23) is connected to the cooling condenser (8) through a pipe.

7. The waste heat utilization device for stabilizing nitric acid production using the dual-pressure method according to claim 1, characterized in that, The cooling condenser (8) is equipped with a cooling condenser coil (24) inside, and the low-pressure water cooler (14) is connected to the cooling condenser coil (24) through the cooling condenser coil inlet pipe (26).

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