Method for cleaning hot air closed cycle simultaneous drying / cold drying of granules

Abstract

A method for cleaning hot air closed loop synchronous drying / cold drying granules, taking three-machine shaft unit of expander, compressor and auxiliary motor as pivot, using expander to cool and dehumidify drying fluidized bed tail gas, then using cold drying fluidized bed to cold dry granules under negative pressure, using compressor to pressurize and heat cold drying fluidized bed tail gas to heat and dry granules through drying fluidized bed, thus forming a closed loop system, expander cooling and dehumidification provides cold drying medium and outputs power to reduce the power consumption of coaxial compressor, compressor provides necessary fluid dynamics conditions for two-phase flow of granule fluidization drying and provides drying heat, heat and power consumption are combined, so as to reduce drying energy consumption, converted to unit total energy consumption (electricity) is not more than 0.83 kWh / kg-water, and there is no waste gas emission and no noise interference.

Description

Technical Field

[0001] This invention relates to the intersection of energy power and clean hot air high-efficiency drying and energy saving, in particular to the use of an expander to dehydrate the water vapor enthalpy contained in the exhaust gas into kinetic energy output, and the cooled and dehumidified dry air is then used to dry granules, achieving high efficiency, energy saving and emission reduction. Background Technology

[0002] Large-scale granular material drying typically uses hot air as the drying medium. Hot air provides heat for the vaporization of moisture in the wet granules and removes the vaporized moisture, which is then emitted into the environment as exhaust gas. The problem is that the heat required to vaporize the moisture is far less than the energy emitted in the exhaust gas. In other words, most of the energy consumed in this open-air drying method is wasted, and it also generates CO2 emissions and environmental pollution such as dust and noise. Taking the drying of harvested grains during the harvest season, which is currently of great concern, as an example, freshly threshed grains contain more than 20% moisture and need to be dehumidified to below 12.5% ​​before they can be stored at room temperature. The current national standard (GB / T 16714–2007 Continuous Grain Dryer) stipulates that the unit heat consumption in the grain drying process is 6500~8800 kJ / kg-water, while the heat of evaporation required for water vaporization is only 2500 kJ / kg-water, demonstrating the significant potential for energy saving in existing technologies. Industry and science and technology departments have made continuous efforts in this regard, and recent progress (Wang Guiying. Research on the Energy-Saving Mechanism and Technology of Condensation Enhancement in Grain Hot Air Drying, 2022, Jilin University Doctoral Dissertation: Table 6.8) has achieved energy-saving effects by recycling some of the exhaust gas after condensation and dehydration, and by replacing fuel heating with electric heating, reducing the unit heat consumption to 3913 kJ / kg-water. The progress lies in reducing exhaust emissions, but its unit energy consumption, measured in electricity, is not less than the aforementioned national standard value if converted back to thermal energy. Therefore, there is still much potential for technological innovation in solving exhaust emissions and replacing carbon emissions with electricity in the drying field.

[0003] Therefore, this invention proposes a method for simultaneous drying / cooling of granules using a closed-loop clean hot air circulation system. As shown in the attached figure, the system uses a three-machine unit (1-1 to 1-3) connected in a rotating manner as the process hub. The expander cools and dehumidifies the exhaust gas from the drying fluidized bed (4) and then passes it through the cooling fluidized bed (3) to perform negative pressure cooling and drying on the granules. Then, the compressor pressurizes and heats the exhaust gas from the cooling fluidized bed (3) and passes it through the drying fluidized bed (4) to heat and dry the granules again. This constitutes a closed-loop system for simultaneous drying / cooling. The expander not only provides the cooling medium for cooling and dehumidification but also outputs power to the coaxial compressor, effectively reducing the power consumption of the auxiliary motor. The three-machine unit provides both the necessary kinetic conditions for two-phase flow and the thermodynamic conditions for drying in the fluidized bed drying of the granules, achieving two goals at once. Therefore, the thermal energy and power consumption of this invention are combined into one, and the total energy consumption per unit is no more than 3000 kJ / kg-water (0.83 kWh / kg-water). Moreover, this invention achieves zero emissions of drying exhaust gas and eliminates noise interference at the drying site. Applied to grain harvesting, it completes the entire process of new grain drying, cooling, and storage in one go, significantly reducing processing costs, shortening the storage process, and improving grain quality. Summary of the Invention

[0004] This invention discloses a method for simultaneously drying / cooling granular materials using a closed-loop hot air circulation system. As shown in the attached figures, the material has a moisture content of 5-25%, a particle size of 0.5-5.0 mm, and a bulk density of 500-1200 kg / m³. 3The wet granules with good fluidity at room temperature (10~25℃) are quantitatively fed into the top feed inlet of the circular cross-section drying fluidized bed (4) through a rotary feeder (9-1), falling into the uppermost fluidized drying layer of the granules. They undergo heat and mass transfer with the drying hot air that passes vertically through the layer. The temperature of the hot air drops to 35~40℃, the relative humidity rises to no more than 90%, and the pressure drops to (1.01~1.02) times the local atmospheric pressure. The hot air then leaves the drying fluidized bed (4) and enters the top of the scrubbing tower (5). The granules are then heated to 30~35℃, the moisture content decreases by 1~3%, and the overflow overturning height (measured from the distribution plate below the fluidized material layer) of 300~350mm is an arc-shaped baffle from the material drop area into the next fluidized layer, repeating the heat transfer process similar to the previous layer. During the mass transfer process, as the granules descend layer by layer, their temperature increases by 5-10°C, the hot air temperature increases by 15-30°C, the hot air pressure increases by 1.10-1.15 times the weight of the material column per unit area of ​​the fluidized bed above, and the relative humidity decreases by 10-20%. The placement of the arc-shaped baffles in each layer is 180° symmetrical with the layer above, and the height of the arc-shaped baffles decreases by 20-50mm. This forms a multi-layer fluidized drying layer with a minimum of 3 and a maximum of 6 layers. The bottom layer of granules with a temperature of 45-65°C, a moisture content of 1-16%, and an overflow height of 150-200mm falls from the material drop area into the sealed drying homogenizing tank (6). The upper gas phase space of this tank is connected to the gas phase space above the bottom fluidized bed of the drying fluidized bed (4) and maintains equal pressure. The hot air entering the bottom layer of the drying fluidized bed (4) comes from the outlet of the compressor (1-2), with a temperature not exceeding 130°C, a moisture content not exceeding 0.02 kg / kg dry air, and a pressure of 1.10 to 1.15 times the total weight of all material columns per unit area in the drying fluidized bed (4).

[0005] The granules falling from the drying fluidized bed (4) into the drying homogenizing tank (6) have an average residence time of no more than 2 hours in the tank. They are quantitatively added to the top feed port of the circular cross-section cold-drying fluidized bed (3) through the rotary feeder (9-2) at the bottom of the tank. The granules fall into the uppermost fluidized cold-drying layer and descend layer by layer. The process of heat and mass transfer and moisture vaporization of the cold-drying air that passes vertically through the material layer is similar to that of the drying fluidized bed, but the method is different. The heat for moisture vaporization of the granules in the cold-drying fluidized bed comes from the cooling and heat release of the granules themselves. The driving force comes from the low relative humidity and strong moisture absorption capacity of the cold-drying air. The negative pressure operation of the cold-drying fluidized bed at 60~80kPa (absolute pressure, the same below) further enhances the moisture absorption capacity of the cold-drying air. The cold, dry air from the outlet of expander 1-1 is cooled and depressurized to 1-2℃ and 60-80kPa. It is heated to no less than 20℃ by the dehumidifier-reheater (2) and enters the bottom layer of the cold-dry fluidized bed. Its relative humidity does not exceed 30%. It passes through the fluidized material layer layer by layer to absorb moisture. The relative humidity of the cold, dry air at the top layer does not exceed 75%. It leaves the cold-dry fluidized bed (3) and enters the compressor (1-2). It is repressurized to no more than 135kPa and heated to no more than 130℃. It is then sent back to the drying fluidized bed as drying hot air and circulated as described above. The structure of the cold-drying fluidized bed and its fluidized layer is the same as that of the drying fluidized bed. The difference is that the number of layers is at least 2 and at most 3. The temperature of the bottom layer of granules is adjustable from 18 to 20°C and the moisture content is 0.5 to 12.5%. The overflow overturning arc-shaped baffle with a height of 150 to 200 mm falls from the material drop area into the closed discharge tank (7). The upper gas phase space of the tank is connected to the gas phase space above the bottom layer of fluidized granules of the cold-drying fluidized bed (3) and maintains equal pressure (60 to 80 kPa). The average residence time of the cold-drying granules in this tank does not exceed 1 hour. The granules are quantitatively output through the rotary feeder (9-3) at the bottom of the tank.

[0006] The feeding speeds of the three rotary feeders (9-1) to (9-3) are kept the same so that the amount of granules stored in the drying fluidized bed (4), the drying homogenizer (6), the cold-drying fluidized bed (3) and the discharge tank (7) remains stable, and the average residence time of the granules also remains stable.

[0007] The arc-shaped baffles installed in each fluidized bed (4) and the cold-drying fluidized bed (3) extend vertically downwards into the next fluidized bed, 30-50 mm away from the lower distribution plate. The distance between the arc-shaped baffles and the fluidized bed wall is 20-30 mm, and the cross-sectional area of ​​the arc-shaped material drop zone formed is 5-6% of the total cross-sectional area of ​​the fluidized bed. The height of the arc-shaped baffles of each fluidized bed from the distribution plate of that layer is the dynamic height of the fluidized granular material layer. This height is designed to decrease by 6-15% from top to bottom, so that the average residence time of the granules in each layer also decreases by 6-15% from top to bottom. This adapts to the variation of the granule drying driving force with the number of material layers, making the moisture vaporization rate more uniformly distributed in space.

[0008] The drying exhaust gas with a temperature of 35~40℃ and a relative humidity of no more than 90% enters the top of the gas scrubbing tower (5) and flows down in parallel with the washing water sprayed at the top of the tower, which is 5~6 times the mass flow rate of the exhaust gas. The exhaust gas is washed and cooled to 22~25℃ saturated air. After separating impurities and mist, it enters the dehumidifier-reheater (2). The cold air at 1~2℃ from the outlet of the expander is used to further dehumidify and cool the exhaust gas to no more than 17℃. After separating the condensate through the water / gas separator (10-1), it enters the expander (1-1) to condense and expand, output shaft power, and cool and depressurize at the outlet to 1~2℃ and 60~80kPa. After separating the condensate again through the water / gas separator (10-2), it is reheated to 20℃ and enters the cold-drying fluidized bed circulation as a cold-drying medium as described above.

[0009] The clean condensate collected by the water / air separators (10-1) and (10-2) and the condensate discharged from the air scrubber (5) are thoroughly filtered and then atomized and sprayed into the fin surface of the air cooler 11 with a fan to enhance the cooling process of the wash water, so that the temperature of the wash water entering the air scrubber does not exceed 25°C.

[0010] The above steps complete the granule drying process. Its beneficial effect is that the process heat energy and power consumption are combined into one, and the total energy consumption (electricity) per unit is no more than 0.83 kWh / kg-water. Moreover, the exhaust gas is fully recycled, with no waste gas emissions or noise interference. Attached Figure Description

[0011] Appendix Figure 1 This is a schematic diagram of the method for simultaneously drying / cold-drying grains using a closed-loop hot air circulation system provided by the present invention.

[0012] Appendix Figure 1 In the middle section: 1-1 – Expander; 1-2 – Compressor; 1-3 – Auxiliary motor; 2 – Dehumidifier-reheater; 3 – Cold-drying fluidized bed; 4 – Drying fluidized bed; 5 – Gas scrubbing tower; 6 – Drying homogenizer; 7 – Discharge tank; 8 – Circulating water pump; 9-1~9-3 – Rotary feeder; 10-1~10-2 – Water / gas separator; 11 – Spray air cooler. Detailed Implementation

[0013] The following describes specific implementations of the present invention in conjunction with, but not limited to, the embodiments.

[0014] Example 1: A three-machine unit consisting of an expander, a compressor, and an auxiliary motor was used to dry 2.5 t / h of freshly harvested and threshed wet wheat. The temperature was 22℃, and the wet basis moisture content was 22%. The drying requirement was to reduce the wet basis moisture content to 12.5% ​​(dry basis). Ambient air conditions: pressure 96 kPa, dry bulb temperature 25℃, relative humidity 60%.

[0015] In this embodiment, the closed-loop dry air mass flow rate is three times the wet aggregate mass flow rate, i.e., 7.5 t / h (5820 Nm³). 3 The unit consists of an expander, a compressor, and an auxiliary motor, all connected in a single unit ( / h). The drying fluidized bed has five fluidization layers, while the cold drying fluidized bed has three fluidization layers; both beds have a diameter of 1.3m.

[0016] As shown in the attached diagram, fresh wet wheat is fed into the top inlet of the drying fluidized bed (4) at a feeding rate of 41.7 kg / min via a rotary feeder (9-1), falling into the uppermost fluidized drying layer. Through heat and mass transfer with the drying hot air, the hot air temperature drops to 40°C, the relative humidity rises to 76.1%, and the pressure reaches 97.5 kPa. The wheat then leaves the drying fluidized bed and enters the top of the air scrubbing tower (5). The wet wheat is heated to 25°C, its moisture content decreases by 1.3%, and it overflows over an arc-shaped baffle with a height of 350 mm, falling from the discharge area into the next fluidized layer, repeating the heat and mass transfer process similar to the previous layer. In the process of purifying, the temperature of each layer of granules increases by 5°C, the moisture content decreases by 1.33~1.34%, the hot air temperature increases by 17°C, the hot air pressure increases by 2.1~2.5 kPa, the relative humidity of the hot air decreases by 17~18%, and the height of the arc-shaped baffle decreases by 50 mm. The temperature of the bottom layer of granules is 45~48°C, the moisture content is 15.5~16%, and the overflow overturning height is 150 mm. The granules fall from the material drop area into the closed drying homogenizing tank (6). The upper gas phase space of the tank is connected to the gas phase space above the bottom layer of granules in the drying fluidized bed (4) and maintains an equal pressure of 107~108 kPa. The hot air entering the bottom layer of the drying fluidized bed (4) comes from the outlet of the compressor (1-2). Its temperature is 105~108°C, the relative humidity is very low, the moisture content does not exceed 0.016 kg / kg dry air, and the total gas pressure is 115~120 kPa.

[0017] The granules falling from the drying fluidized bed (4) into the drying homogenizing tank (6) have an average residence time of 1 hour in the tank. They are then fed into the top feed inlet of the cold-drying fluidized bed (3) at the same rate as the rotary feeder (9-1) via the rotary feeder (9-2) at the bottom of the tank. The uppermost fluidized cold-drying layer, with a material layer height of 300 mm, is fed down layer by layer for a total of 3 layers, with the material layer height decreasing by 50 mm each time. The cold-drying air passing vertically through the material layer transfers heat and mass to vaporize moisture. The temperature of the wheat grains decreases by 8~10℃ and the moisture content decreases by 0.95~℃ layer by layer. 1.05%, until the temperature of the lowest layer of wheat grains drops to 18~20℃, the moisture content drops to no more than 12.5%, and the overflow overturning arc baffle with a height of 200mm falls from the material drop area into the sealed discharge tank (7). The upper gas phase space of the tank is connected to the gas phase space above the lowest layer of fluidized wheat grains in the cold-dry fluidized bed (3) and maintains an equal pressure of 65~66kPa. The average residence time of the cold-dry grains in the tank is 0.5 hours. The grains are discharged through the rotary feeder (9-3) at the same rate as the rotary feeder (9-1). The cold and dry air entering the cold and dry fluidized bed (3) comes from the outlet of expander 1-1, with a temperature of 0.5~1.0℃ and a pressure of 68~68.5kPa. After passing through the dehumidifier-reheater (2), the temperature is raised to no less than 20℃ and the relative humidity is no more than 30%. It passes through the fluidized material layer layer by layer to absorb moisture. When it reaches the top layer, the temperature is no more than 25℃ and the relative humidity is no more than 70%. After leaving the cold and dry fluidized bed (3), it enters the compressor (1-2), is pressurized to 116~120kPa, and heated to 105~108℃. It is then sent back to the drying fluidized bed as drying hot air for circulation.

[0018] The drying exhaust gas with a temperature of 40℃, relative humidity of 76.1%, and pressure of 97.5kPa enters the top of the gas scrubbing tower (5) and flows down in parallel with the 40t / h washing water sprayed from the top of the tower, so that the exhaust gas is purified, cooled, and dehumidified to 22~23℃ saturated air. After separating impurities and mist, it passes through the dehumidification-reheater (2) and uses the cold dry gas at the outlet of the expander of 0.5~1.0℃ to dehumidify and cool the exhaust gas to 16~17℃. After separating the condensate through the water / gas separator (10-1), it enters the expander (1-1) to condense and expand to output shaft power. At the outlet of the expander, it is cooled and depressurized to 0.5~1.0℃ and 68~68.5kPa. After separating the condensate again through the water / gas separator (10-2), it is reheated to not less than 20℃ and enters the cold dry fluidized bed circulation as a cold dry medium.

[0019] The clean condensate collected by the water / air separators (10-1) and (10-2) and the condensate discharged from the air scrubber (5) are thoroughly filtered and then atomized and sprayed into the surface of the fins of the air cooler 11 with a fan to enhance the cooling process of the wash water, so that the temperature of the wash water entering the air scrubber does not exceed 25°C.

[0020] This example completes the drying of 2.5 t / h fresh threshed wet wheat, reducing the moisture content from 22% to 12.5%, with a dehydration vaporization rate of 237.5 kg / h. The isentropic efficiency of the expander in the three-machine-coil unit is 0.88, the isentropic efficiency of the compressor is 0.83, and the actual power consumption of the auxiliary motor is 184.1 kW. This translates to a total energy consumption (electricity) of 0.775 kWh / kg-water.

[0021] The present invention is not limited to the above embodiments, and its technical solutions have been described in the invention content section.

Claims

1. A method for simultaneously drying / cooling granular materials using a closed-loop hot air circulation system, characterized by using... The unit, consisting of an expander, a compressor, and an auxiliary motor, is a hub. The expander cools and dehumidifies the exhaust gas from the drying fluidized bed, and then passes it through a cold-drying fluidized bed to perform negative pressure cold drying on the granules. The compressor then pressurizes and heats the exhaust gas from the cold-drying fluidized bed, and passes it through the drying fluidized bed to heat and dry the granules again. This forms a closed-loop system for drying / cold drying of the drying medium. The expander's cooling and dehumidification provides the cold-drying medium and outputs power to the coaxial compressor, effectively reducing the power consumption of the auxiliary motor. The compressor pressurizes the hot air to no more than 135 kPa and raises the temperature to no more than 130°C. The hot air passes vertically through all the material layers and granules of the drying fluidized bed for heat and mass transfer. The temperature of the hot air drops to 35-40°C, the relative humidity rises to no more than 90%, and the pressure drops to 1.01-1.02 times the local atmospheric pressure. It leaves the drying fluidized bed and enters the top of the scrubbing tower. It flows down in parallel with the wash water sprayed at the top of the tower, which is 5-6 times the mass flow rate of the air. The air is washed and cooled to saturation at 22-25°C. After separating impurities and mist, it enters the dehumidifier-reheater. It is further dehumidified and cooled to no more than 17°C by the cold air at 1-2°C from the expander outlet. After the condensate is separated by the water / air separator, it enters the expander for condensation, expansion, cooling and depressurization to 1-2°C and 60-80 kPa. After the condensate is separated again, it is reheated to no less than 20°C and used as a cold drying medium to circulate in the cold drying fluidized bed. The relative humidity of the cold dry air entering the cold dry fluidized bed does not exceed 30%. It absorbs moisture by passing through the fluidized material layer layer by layer until the relative humidity of the cold dry air at the top layer does not exceed 75%. After leaving the cold dry fluidized bed, it enters the compressor to be repressurized to not more than 135 kPa and heated to not more than 130°C. It is then sent back to the drying fluidized bed as drying hot air for circulation. All clean condensate collected from the water / air separator and condensate discharged from the air scrubber are thoroughly filtered and then atomized and sprayed onto the outer surface of the air cooler fins with a fan to enhance the cooling of the wash water, ensuring that the temperature of the wash water entering the air scrubber does not exceed 25°C.

2. The method for simultaneously drying / cooling granules using a closed-loop hot air circulation system according to claim 1, characterized in that: Both the drying fluidized bed and the cold-drying fluidized bed are multi-layer fluidized granular material layers with circular cross-sections. The drying fluidized bed has 3 to 6 layers, and the cold-drying fluidized bed has 2 to 3 layers. Wet granules added from the top of the drying fluidized bed fall into the uppermost fluidized granular material layer, overflowing layer by layer until they fall from the bottom layer into a sealed drying homogenizing tank. The average residence time does not exceed 2 hours. The granules are then fed into the top inlet of the cold-drying fluidized bed through a rotary feeder at the bottom of the drying homogenizing tank, falling into the uppermost fluidized cold-drying granular material layer. The granules overflow layer by layer until they fall into a sealed discharge tank. The average residence time does not exceed 1 hour. Finally, the granules are quantitatively discharged through a rotary feeder at the bottom of the tank. After being added to the top of the drying fluidized bed, the granules move downwards until the entire drying / cold-drying process is completed.

3. The method for simultaneous drying / cooling of granules using a closed-loop hot air circulation system according to claim 1, characterized in that: The bottom layer of granules in the drying fluidized bed overflows and overturns an arc-shaped baffle with a height of 150-200 mm from the material drop area into a sealed drying homogenizing tank. The upper gas phase space of the drying homogenizing tank is connected to the gas phase space above the bottom fluidized layer of the drying fluidized bed and maintains isobaric pressure. The bottom layer of granules in the cold-drying fluidized bed overflows and overturns an arc-shaped baffle with a height of 150-200 mm from the material drop area into a sealed discharge tank. The upper gas phase space of the discharge tank is connected to the gas phase space above the bottom fluidized granule layer of the cold-drying fluidized bed and maintains isobaric pressure.

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