Mechanical vapor recompression drying system and method

Abstract

The present invention discloses a mechanical steam recompression drying system and method, which includes a sludge storage tank, a sludge injection pump, a thin layer evaporation dryer, an extrusion strip forming machine, a mesh belt dryer, a dry mud bin, a steam compressor, a cyclone dust collector, a water-washing dust collector, a condensate buffer tank, and a vacuum pump connected to the condensate buffer tank and used to suck system air to maintain vacuum and discharge non-condensable gas. The present invention can save a lot of energy consumption and cooling water system consumption. There is no exhaust gas emission in the process, and the system has no cooling water system, which saves the evaporation loss of water in the cooling tower.

Description

Technical Field

[0001] The invention relates to the technical field of sludge drying, and in particular to a mechanical steam recompression drying system and method. Background Art

[0002] Mechanical Vapor Recompression (MVR) system is a new type of high-efficiency energy-saving system. MVR evaporation technology has significant energy-saving effect. Compared with multi-effect evaporation technology, MVR system fully recovers the latent heat of secondary steam, and the thermal efficiency is generally equivalent to 5-10 effect evaporator.

[0003] In industrial fields such as drying and evaporation concentration, a large amount of water vapor is generated. Most of the energy carried by water vapor is latent heat, and sensible heat is only a small part. The mechanical vapor recompression system effectively recovers the latent heat of water vapor, thereby achieving the purpose of energy saving and consumption reduction. Its working principle is that the temperature, pressure and enthalpy of the steam generated by the evaporator are increased after the mechanical steam compressor, and returned to the evaporator as a heat source for heating, replenishing the heat energy required for liquid evaporation and maintaining the evaporation temperature. The system itself can basically achieve thermal balance, and fresh steam is only used to supplement the system heat loss and the heat enthalpy of the inlet and outlet materials, thereby greatly reducing the evaporator's consumption of external fresh steam and achieving the purpose of reducing energy consumption. MVR is rarely used in the field of solid drying. Small heat transfer temperature difference, long drying time, high mechanical transmission energy consumption, system sealing, and high-performance steam compressors are all restrictive factors for its application.

[0004] Common solid steam drying technologies mostly use indirect conduction heat transfer or direct convection heat transfer. Indirect conduction heat transfer is common in hollow blade dryers, rotary disk dryers, drum dryers, and wall dryers. Direct convection heat transfer is common in superheated steam drying and hot air drying. They all require large temperature differences and mechanical stirring or forced convection to enhance heat transfer.

[0005] Research shows that the water in solids is mainly free water, interstitial water, surface water and chemically bound water. According to the evaporation and drying process of these water, the evaporation rate is divided into "constant speed stage", "reducing speed stage" and "final stage". Generally, when the sludge moisture content is lower than about 65%, the "constant speed stage" ends and enters the more difficult and more energy-consuming "reducing speed stage" drying and evaporation.

[0006] Zhou Lei of Nanjing University of Aeronautics and Astronautics' "Design and Energy-saving Analysis of Low-temperature Drying System Based on Vapor Recompression Technology" shows that in the low-temperature drying process, theoretically, the system energy consumption continues to decrease with the decrease of evaporation temperature and steam compressor pressure ratio; compared with the conventional low-temperature drying system under the same conditions, the energy consumption of the low-temperature drying system based on vapor recompression technology is only 7.7% of that of the conventional low-temperature heat recovery drying system. The application of MVR technology is based on the principle of energy saving and the limitations of existing technologies. It requires low-pressure ratio compression and small temperature difference heat exchange, which leads to long drying time, high mechanical stirring and transmission energy consumption for enhanced heat exchange, and reduces the energy saving effect of MVR. At the same time, too high evaporation temperature leads to more system heat loss, and the system heat supplement is increased for the sake of system heat balance, which further reduces the energy saving effect. At the same time, screw steam compressors that can achieve high pressure ratios are not common in the market at present, which is also one of the reasons for the difficulty in energy-saving application of MVR drying. In order to ensure the drying effect, a certain drying speed must be controlled. Combining MVR technology with vacuum low-temperature drying technology is the development direction of MVR drying, but after entering the "speed reduction stage", the evaporation rate is still unsatisfactory.

[0007] Superheated steam drying refers to a new drying method that uses superheated steam to directly contact the material to be dried to remove its internal moisture. At the same temperature, compared with hot air, superheated steam carries much more heat per unit mass than hot air due to the latent heat of superheated steam. When superheated steam is used to dry solids, there is no diffusion resistance because the water on the surface of the solid evaporates to the superheated steam. The speed of superheated steam drying is also related to the temperature of the superheated steam. The research and experiments of "Combined Superheated Steam and Vacuum Drying of Potatoes" by Lu Ye of Fujian Agriculture and Forestry University show that low-temperature superheated steam can also achieve a relatively ideal drying speed during the potato drying process.

[0008] During the drying process, pollutants will enter the condensate of secondary steam during the evaporation process. "Study on the Characteristics of Urban Sludge Superheated Steam Drying Condensate" by Guan Yong of Nanchang Hangkong University and "Study on Pollutant Migration in the Distillation and Compression Process of High-Salt Organic Wastewater" by Hong Yongqiang of Shandong University show that the pollutants entering the secondary steam condensate during low-temperature evaporation are mainly alcohol organic matter and ammonia, and the content of inorganic salts is extremely low. In the high-temperature evaporation process, the pollutant composition is complex and the amount of pollution is greater. In traditional high-temperature sludge evaporation, the condensate needs to be treated in the sewage plant again before it can meet the discharge standards or be reused.

[0009] In order to solve the above problems, the following patents "CN103011546B", "CN107098562A", "CN106495427A", "CN103708697B", "CN103588375B", "CN103285637B", "CN105254147B", and "CN110040935A" have adopted corresponding technical means:

[0010] 1) "A two-stage sludge drying and energy recovery system and drying process thereof" adopts a two-stage drying process of a partition wall drying device and a belt dryer. The heat of the belt dryer is recovered in the secondary steam of the sludge in the partition wall heat exchanger, which is equivalent to the "two-effect evaporation" of water, thereby achieving energy saving.

[0011] 2) "New thermal energy cascade recycling system for two-stage sludge drying process", which includes a thin-layer evaporative dryer and a belt dryer for two-stage drying. The heat input of the system comes from the high-temperature steam of the thermal power plant. The primary steam condensate in the thin-layer evaporative dryer of stage I is flashed into low-temperature and low-pressure steam through a flash tank, which is used to reheat the hot air of stage II, thereby reducing the amount of fresh steam supplemented, achieving the purpose of saving both steam and cooling water.

[0012] 3) "MVR superheated steam sludge continuous drying system and working method", this system uses superheated steam as the drying medium, and the secondary steam generated in the drying process is divided into two paths: a small part of the steam is compressed by the steam compressor to become high-pressure superheated steam and enters the hot end of the heat exchanger, and enters the condensation pool after heat exchange and condensation; the other part of the water vapor enters the cold end of the heat exchanger and is sent back to the dryer after heating. This drying system mainly uses compression method to recover the sensible heat and latent heat of the secondary steam generated in the drying process.

[0013] 4) "Mechanical steam recompression heat pump MVR sludge drying system", the secondary steam generated after the sludge is dried by the hollow paddle dryer is compressed by the steam compressor and then returned to the hollow paddle body, recovering all the latent heat of the secondary steam generated during the sludge drying process.

[0014] 5) “A sludge MVC evaporation and drying system and a method for drying sludge therefrom”, the secondary steam from the sludge evaporation process passes through a steam compressor and enters a drying device as a steam supplementary heat source, and the high-temperature condensed water is used to wash and purify the steam.

[0015] 6) "Low-temperature vacuum dehydration and drying equipment and its process", the vacuum system evacuates the heated material chamber, and the evaporated water vapor in the chamber is condensed into liquid water in the heat exchanger and discharged, achieving the purpose of low-temperature evaporation.

[0016] 7) "Triple utilization of waste heat from superheated steam drying of sludge", recovering the sensible heat and latent heat of waste steam at the same time, recycling waste water and zero-pollution emission of waste gas. The whole device realizes the triple utilization of waste heat from superheated steam drying of sludge. The drying process recovers the sensible heat and latent heat of waste steam at the same time, preventing condensation during the drying process and shortening the drying time; during the drying process, only superheated steam needs to be introduced in the initial stage, and the steam is recycled in the later stage, which reduces the input end of the heat source and reduces energy consumption.

[0017] 8) “A multi-layer combined self-falling sludge drying device and drying method” uses indirect drying combined with superheated secondary steam to dry the sludge, and uses MVR evaporation technology to effectively shorten the drying time.

[0018] Although the above-mentioned technical patents or related research have solved the energy saving of solid drying or the application methods of MVR technology in the drying field, there are many problems in the implementation process, and some problems have not been effectively solved. For example, the heat recovery and utilization process of the two-stage sludge drying process of "CN103011546B" and "CN107098562A" is equivalent to the energy saving effect of "two-effect evaporation". The entire drying process uses high-temperature steam as energy, and the energy consumption is still relatively high compared with MVR technology. "CN106495427A" MVR superheated steam sludge continuous drying system and working method, the sensible heat and latent heat of secondary steam are used for preheating the feed sludge. Based on the material mass and heat balance, the latent heat of the secondary steam is much greater than the preheating heat of the sludge, which reduces the heat recovery ratio. "CN103708697B" and "CN103588375B" use MVR evaporation technology to dry sludge through hollow paddle or other indirect heat conduction drying devices. In practice, a large amount of heat needs to be added to the system. At the same time, the sludge evaporates for too long after entering the "speed reduction stage". Mechanical transmission or stirring enhances heat transfer and heat loss, resulting in a decrease in energy saving benefits. "CN103285637B" Low-temperature vacuum dehydration and drying equipment and process and "CN105254147B" A triple utilization of exhaust steam and waste heat from sludge superheated steam drying, energy consumption also comes from low-temperature (80℃-90℃) heat sources or steam, and the energy consumption is still relatively high compared to MVR technology. "CN110040935A" A multi-layer combined self-falling sludge drying device and drying method, the system adds heat to directly heat the compressed secondary steam, a part of the secondary steam directly enters the drying chamber as superheated steam and exchanges heat with the sludge by convection, and a part of the secondary steam enters the inner cavity of the indirect dryer and exchanges heat with the sludge by conduction. Theoretically, the heat transfer effect of the superheated steam in the inner cavity of the indirect dryer is not ideal compared to that of saturated steam. At the same time, after the secondary steam becomes condensed water, the sensible heat is not recovered.

[0019] In summary, it is necessary to propose a new mechanical vapor recompression drying system and method. Summary of the invention

[0020] In view of this, the present invention proposes a mechanical steam recompression drying system and method with simple operation, reliable performance, low manufacturing cost, convenient disassembly and maintenance, fast continuous drying speed, small supplementary heat and low system energy consumption.

[0021] According to the purpose of the present invention, the present invention provides a first technical solution: a mechanical steam recompression drying system, which includes a sludge storage tank for storing room temperature sludge material and having a condensate heat exchange coil, a sludge injection pump connected to the output end of the sludge storage tank and provided with a jacketed heat exchanger, a thin layer evaporative dryer connected to the output end of the sludge injection pump, an extrusion stripping molding machine connected to the output end of the thin layer evaporative dryer, a mesh belt dryer connected to the output end of the extrusion stripping molding machine, a dry mud bin connected to the output end of the mesh belt dryer, a steam compressor arranged between the thin layer evaporative dryer and the mesh belt dryer, a cyclone dust collector connected to the mesh belt dryer and used for separating secondary steam and sludge dust, a water washing dust collector connected to the output end of the cyclone dust collector and used for water washing and cleaning the secondary steam, a condensate buffer tank connected to the thin layer evaporative dryer and collecting condensate, and a vacuum pump connected to the condensate buffer tank and used to suck system air to maintain vacuum and discharge non-condensable gas.

[0022] On the basis of the first technical solution, the following subsidiary technical solutions are further included:

[0023] The thin-layer evaporative dryer includes at least one cylindrical heating bin, a scraper rotor located inside the cylindrical heating bin and having a plurality of scrapers evenly distributed on the surface, a motor located outside the cylindrical heating bin and driving the scraper rotor to rotate, a steam inlet connected to the output end of the water-washing dust collector, a steam and condensate outlet connected to the input end of the condensate buffer tank, a sludge feed port connected to the output end of the sludge injection pump, and a sludge outlet connected to the input end of the extrusion strip forming machine.

[0024] The mesh belt dryer is a closed structure, and is internally provided with a steam circulation fan and a second external heat source heater.

[0025] The water-washing dust collector comprises a condensate water spray head, a demister located above the condensate water spray head, a dosing device connected to the condensate water spray head, a first circulation pump connected to the condensate water spray head, and a sewage pump for returning sludge to a sludge storage tank.

[0026] It also includes a granulator for extruding the dry mud in the dry mud bin into granules and then discharging the materials, a second circulation pump located between the sludge injection pump and the water-washing dust collector, and a condensation water tank connected to the sludge storage tank and having an electrochemical oxidation descaling device.

[0027] The invention also comprises a first filter arranged between the thin layer evaporative dryer and the steam compressor, and a second filter arranged between the thin layer evaporative dryer and the water-washing dust collector.

[0028] The present invention provides a second technical solution: a method for using a mechanical vapor recompression drying system, comprising:

[0029] S1: The wet sludge is transferred from the sludge storage tank to the thin-layer evaporation dryer through a transfer device. During the process, the wet sludge and condensed water are heated in a jacketed heat exchanger from 5-35°C at the inlet to 65-80°C; the condensed water is cooled from 80-98°C at the inlet to 15-45°C at the outlet;

[0030] S2: The preheated sludge forms a thin layer in the thin-layer evaporative dryer under the action of the rotating scraper blades and adheres to the inner surface of the thin-layer evaporative dryer cylinder. The cylinder is a negative pressure system with a pressure setting of 25-47.5KPa, and the corresponding water evaporation temperature is 65-80℃; the heat source for heat transfer between the sludge and the cylinder wall comes from the steam in step S4, and the heat exchange temperature difference is 18-33℃, and the sludge moisture content is reduced from 80%-95% to 55%-65%;

[0031] S3: The sludge with a moisture content of 55%-65% is squeezed into strips with a width of 2-5mm and then enters the net belt dryer;

[0032] S4: The sludge entering the mesh belt dryer exchanges heat with the superheated steam during the mesh belt transmission process, and enters the dry mud bin after further evaporation and drying, and is discharged after extrusion and granulation by the granulator. The moisture content of the sludge is further evaporated and dried from 55%-65% to 20%-45% moisture content sludge, completing the drying process in the mesh belt dryer; the saturated steam at the outlet of the steam compressor enters the mesh belt dryer, is heated by the second external heat source heater to increase the superheat degree by 20-50°C, and is forced to circulate by the steam circulation fan and exchange heat with the sludge by convection; the surplus superheated steam becomes clean saturated steam after cyclone separation, water washing and dust removal, and filtration, and enters step S2 again;

[0033] S5: The secondary steam generated in the evaporation process in step S2 is filtered by the first filter, and then enters the steam compressor for compression and then enters step S4. The condensed water from the condensed water buffer tank is sprayed into the steam compressor suction inlet. The compression ratio is 2.0-4.5, and the steam temperature rise is 18-33°C.

[0034] S6: A portion of the condensed water enters step S5 to reduce the exhaust superheat of the steam compressor; a portion enters the water-washing dust collector in step S4 and is discharged into the sludge storage tank; the remaining portion of the condensed water enters the condensed water tank after heat exchange in the jacketed heat exchanger in step S1, and is discharged or reused after removing trace organic matter through an electrochemical oxidation descaling device;

[0035] S7: The non-condensable gas in the condensate buffer tank and the condensate pool is removed by the vacuum pump until the end.

[0036] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0037] The use of MVR technology in the field of solid drying can save a lot of energy consumption and cooling water system consumption compared to traditional evaporation technology using heat sources. There is no exhaust gas emission in the process, and the system has no cooling water system, which saves the evaporation loss of water in the cooling tower. The secondary steam condensate produced by low-temperature evaporation has low pollutants, and its main component is volatile organic matter. After low-cost treatment by methods such as "electrochemical oxidation", the condensate can be directly discharged in compliance with the standards. However, the secondary steam condensate produced by high-temperature evaporation often needs to be treated in the sewage treatment plant again.

[0038] During the "low-temperature vacuum evaporation" process, the material inlet temperature and evaporation temperature are low, which reduces the preheating heat of the material; during the "low-temperature superheated steam evaporation" process, the material outlet temperature is relatively low and the moisture content is low, which reduces the heat loss of the outlet material; the temperature difference between the overall operating temperature of the system and the ambient temperature is low, which also reduces the heat loss of the system's heat dissipation.

[0039] During the entire evaporation process, only the sludge with high viscosity and low evaporation rate (water content below 55%-65%) is subjected to convective heat exchange by "superheated steam evaporation" with relatively large temperature and heat exchange temperature difference. While shortening the overall drying time, it is beneficial to heat recovery and the amount of supplementary heat is also small. The cost of supplementary heat in this system has nothing to do with the grade of energy, but is related to the amount. The supplementary heat is mainly used to heat the secondary steam to increase the sensible heat of the secondary steam. When the superheated steam and sludge are convectively exchanged, the heat exchange temperature difference is high and the heat exchange effect is good.

[0040] Without affecting the continuous evaporation rate, by properly adjusting the parameters of the "superheated steam" evaporation stage, sludge particles with different dryness can be produced, with a drying range of 70%-95%.

[0041] All the latent heat of evaporation of water with the largest heat consumption can be recovered and utilized, and all or most of the heat of high-temperature condensed water can be recovered and utilized.

[0042] The temperature of the secondary steam in the system can reach 120℃-150℃, but the pressure is lower than 0.1MPa. The system pipelines and containers are not "pressure vessels", which can effectively reduce the production and maintenance costs. The gas that the sludge directly contacts is secondary steam, and there is no air pollution emission. In a low-oxygen environment, there is no risk of combustion and dust explosion, and the safety is better. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0044] Figure 1 It is a structural schematic diagram of the present invention. DETAILED DESCRIPTION

[0045] like Figure 1 The present invention provides a first embodiment of a mechanical steam recompression drying system, which includes a sludge storage tank 110 for storing room temperature sludge material and having a condensed water heat exchange coil 111, a sludge injection pump 120 connected to the output end of the sludge storage tank 110 and provided with a jacketed heat exchanger 121 and a first external heat source heater 122, a thin layer evaporative dryer 130 connected to the output end of the sludge injection pump 120, an extrusion strip forming machine 140 connected to the output end of the thin layer evaporative dryer 130, a mesh belt dryer 150 connected to the output end of the extrusion strip forming machine 140, and a mesh belt dryer 150 connected to the output end of the mesh belt dryer. A dry mud bin 160 connected to the output end of the dryer 150, a steam compressor 220 arranged between the thin layer evaporative dryer 130 and the mesh belt dryer 150, a cyclone dust collector 230 connected to the mesh belt dryer 150 and used for separating secondary steam and sludge dust, a water washing dust collector 240 connected to the output end of the cyclone dust collector 230 and used for washing and cleaning the secondary steam, a condensate buffer tank 310 connected to the thin layer evaporative dryer 130 and used for collecting condensate, and a vacuum pump 400 connected to the condensate buffer tank 310 and used for sucking system air to maintain vacuum and discharge non-condensable gas.

[0046] The sludge storage tank 110 stores the incoming sludge at room temperature. There is a condensate heat exchange coil 111 on the inner wall of the sludge storage tank. The condensate heat exchange coil 111 contains condensate, which is discharged into the condensate tank 320 after heat exchange with the sludge. The sludge injection pump 120 provides power to transport the sludge in the sludge storage tank 110 to the sludge inlet of the thin-layer evaporative dryer 130, and the pressure of the internal solid extrusion and pushing can ensure that the sludge storage tank 110 and the inner cavity of the thin-layer evaporative dryer 130 are sealed without gas cross-talk. A jacketed heat exchanger 121 is externally provided on the sludge conveying pipeline, and the jacketed heat exchanger 121 contains condensate, which is discharged into the sludge storage tank 110 after heat exchange with the sludge. The first external heat source heater 122 provides heat to the system to supplement heat to heat the sludge and increase the temperature.

[0047] The thin layer evaporative dryer 130 includes at least one cylindrical cylindrical heating bin and a scraper rotor therein. A motor is provided at one end of the cylindrical heating bin, and the scraper rotor is driven by the motor. Scraper blades are evenly distributed on the surface of the scraper rotor. The scraper blades push the sludge from one end of the thin layer evaporative dryer 130 to the other end, and make the sludge evenly spread over the cylindrical inner wall of the entire cylindrical heating bin. A steam inlet 131 is provided at one end of the cylindrical heating bin, and a steam and condensate outlet 132 is provided at the other end of the cylindrical heating bin. A sludge feed port 133 is provided at the other end of the cylindrical heating bin, and a sludge outlet 134 is provided at one end of the cylindrical heating bin. Furthermore, the cylindrical heating bin is a partition wall structure, and steam can flow inside. An insulation layer is added to the outer surface of the cylindrical heating bin, and the inner surface of the cylindrical heating bin is coaxial with the scraper rotor.

[0048] The extrusion strip forming machine 140 is a sludge extrusion device with a mesh. The sludge from the sludge outlet 134 in the thin layer evaporative dryer 130 is squeezed and shredded into "strips" by the extrusion strip forming machine 140 and then scattered on the conveyor belt of the mesh belt dryer 150. The solid pushing pressure in the extrusion strip forming machine 140 can ensure that the cavities of the thin layer evaporative dryer 130 and the mesh belt dryer 150 are sealed without air leakage. The mesh belt dryer 150 is a closed structure with a steam circulation fan 151 with steam inside and a second external heat source heater 152. Furthermore, the steam circulation fan 151 of the mesh belt dryer 150 is located in the jacketed heat exchanger 121. The first and second external heat source heaters 122 and 152 preferably have heat sources of electric heating, high temperature heat pumps or high temperature steam directly introduced from the outside. Furthermore, the mesh belt dryer 150 delivers the superheated steam heated by circulation to the steam compressor 220 for compression, forming water vapor with increased temperature and pressure, which is the secondary steam of the sludge evaporated by the thin layer evaporation dryer 130. The superheated steam is forced to circulate under the action of the steam circulation fan 151, and convectively exchanges heat with the sludge on the mesh belt to dry the sludge moisture. The dry mud bin 160 is used to store the dry sludge produced by the mesh belt dryer 150 and the cyclone dust collector 230. The granulator 161 extrude and granulate the dry mud in the dry mud bin 160 and then discharges the material. The solid pushing pressure in the granulator 161 can ensure that the granulator 161 and the external atmospheric environment are sealed and not cross-gased. The first and second filters 210 and 250 are air high efficiency filters for filtering dust in the gas. The steam compressor 220 is a water vapor compressor, preferably a twin-screw steam compressor, with a compression ratio of 2.0-4.5. The steam compressor 220 inlet injects high-temperature condensate from the condensate buffer tank 310 to reduce the exhaust superheat. The cyclone dust collector 230 is used to separate the secondary steam and sludge dust from the chamber of the mesh belt dryer 150. The secondary steam enters the water-washing dust collector 240, and the sludge dust enters the dry mud bin 160. The water-washing dust collector 240 is used to wash and clean the secondary steam and reduce its superheat. The system water replenishment of the water-washing dust collector 240 comes from the high-temperature condensate of the condensate buffer tank 310. Under the action of the first circulating pump 244, the condensate spray head 241 of the water-washing dust collector 240 sprays the water-washed secondary steam. The dosing device 243 adds organic acid to the circulating water to clean and remove the free ammonia in the secondary steam. There is a sewage outlet at the bottom of the water-washing dust collector 240, which returns to the sludge storage tank 110 through the sewage pump 245. The demister 242 is used to separate the steam and water in the secondary steam after water washing. After dust removal and ammonia removal, the secondary steam enters the second filter 250. The clean and saturated secondary steam returns to the steam jacket of the thin layer evaporative dryer 130. The condensate buffer tank 310 is used to store the condensate of the thin layer evaporative dryer 130. The upper part of the condensate buffer tank 310 is gas, and the lower part is liquid water. The condensate pool 320 is a water storage tank with a closed structure, with gas in the upper part and liquid water in the lower part.Furthermore, the condensate pool 320 is provided with an electrochemical oxidation descaling device 321, which is used to reduce volatile organic matter in the condensate, reduce the COD of the condensate, and discharge or reuse it in compliance with the standards. The vacuum pump 400 is used to suck the system air to maintain vacuum and remove non-condensable gases during operation. The regenerable activated carbon filter 410 is used to deodorize and filter a small amount of exhaust gas from the vacuum pump 400.

[0049] This embodiment also provides an MVR low-temperature vacuum combined with superheated steam drying method, which uses the above-mentioned related structures and devices to perform drying work, specifically including the following steps:

[0050] S1: The wet sludge enters the sludge injection pump 120 through the transmission device, and is transmitted from the sludge storage tank 110 to the thin layer evaporation dryer 130. During the process, the wet sludge and condensed water exchange heat in the jacketed heat exchanger 121, and are heated from 5-35°C at the inlet to 65-80°C. The insufficient heat is supplemented by the first external heat source heater 122. The condensed water is cooled from 80-98°C at the inlet to 25-55°C at the outlet, and then enters the condensed water heat exchange coil 111, cooled to 15-45°C, and then discharged into the condensed water tank 320;

[0051] S2: The preheated sludge forms a thin layer in the thin-layer evaporative dryer 130 and adheres to the inner surface of the evaporator cylinder under the action of the rotating scraper blades. The cylinder is a negative pressure system with a pressure set to 25-47.5KPa, and the corresponding water evaporation temperature is 65-80°C. The sludge and the cylinder wall conduct heat exchange, and the cylinder jacket contains saturated steam from step S4. The preferred heat exchange temperature difference is set to 18-33°C. The sludge begins to evaporate in a vacuum negative pressure environment, and moves along the cylinder wall under the push of the rotating scraper blades and is enriched at the sludge outlet 134; the sludge in the thin-layer evaporative dryer 130 is a "vacuum low-temperature drying" process, and the drying process in the thin-layer evaporative dryer 130 is completed when the sludge moisture content is reduced from 80%-95% to 55%-65%; the saturated steam in the jacket of the thin-layer evaporative dryer 130 becomes liquid condensed water after heat exchange and enters the condensed water buffer tank 310;

[0052] S3: The extrusion strip forming machine 140 squeezes the sludge with a moisture content of 55%-65% at the sludge outlet 134 into viscous strips with a diameter of 2-5 mm, and then enters the mesh belt dryer 150;

[0053] S4: The sludge entering the mesh belt dryer 150 undergoes convection heat exchange with the superheated steam during the mesh belt transmission process, and after further evaporation and drying, it enters the dry mud bin 160, and is extruded and granulated by the granulator 161 before being discharged. The sludge in the mesh belt dryer 150 undergoes a "superheated steam drying" process, and the sludge moisture content is further evaporated and dried from 55%-65% to 20%-45% moisture content sludge, completing the drying process in the mesh belt dryer 150. After the saturated steam at the outlet of the steam compressor 220 enters the mesh belt dryer 150, it is heated by the second external heat source heater 152 to increase the superheat degree by 20-50°C, and then forced to circulate by the steam circulation fan 151 for convection heat exchange with the sludge. The abundant superheated steam enters the cyclone dust collector 230, the water-washing dust collector 240, and the filter 250 to become clean saturated steam and enter step S2 again;

[0054] S5: The secondary steam generated during the evaporation process in step S2 is filtered by the first filter 210, and then enters the steam compressor 220 for compression and enters step S4. The condensed water from the condensed water buffer tank 310 is sprayed into the air intake of the steam compressor 220 to reduce the steam compressor rotor temperature and the outlet water vapor superheat. The preferred compression ratio is 2.0-4.5, and the steam temperature rise is 18-33°C;

[0055] S6: The temperature of the condensed water in the condensed water buffer tank 310 is 80-98°C. A portion of the condensed water enters step S5 to reduce the exhaust superheat of the steam compressor; a portion of the condensed water enters the water-washing dust collector 240 in step S4 and is discharged into the sludge storage tank 110; most of the condensed water enters the condensed water tank 320 after heat exchange through the jacketed heat exchanger 121 and the condensed water heat exchange coil 111 in step S1, and is discharged or reused after removing trace organic matter through the electrochemical oxidation descaling device 321;

[0056] S7: The non-condensable gas in the condensate buffer tank 310 and the condensate pool 320 is removed by the vacuum pump 400 to ensure the system vacuum degree and evaporation pressure; a small amount of non-condensable gas is deodorized by the regenerable activated carbon filter 410 and then discharged.

[0057] Advantages of the present invention:

[0058] Based on the characteristics of the drying process and combined with the optimal energy saving of MVR technology in the field of water evaporation, the "constant speed section" drying process of the sludge is treated by "low temperature vacuum evaporation", and the "speed reduction section" drying process of the water-containing solid material is treated by "superheated steam". "Superheated steam" comes from the reheated secondary steam in the "low temperature vacuum evaporation" process; the steam after "superheated steam" evaporates enters the "low temperature vacuum evaporation" dryer again to release all the latent heat of water vapor and become high temperature condensed water, and the sensible heat of the high temperature condensed water is recovered by preheating the water-containing solid material.

[0059] During the entire evaporation process, the part with the largest heat consumption, that is, all the latent heat of the secondary steam in the water evaporation process is recycled; during the "speed reduction stage" drying process, the steam temperature rise is only a process of increasing or releasing sensible heat, which significantly improves the convective heat transfer temperature difference, shortens the drying time and minimizes the heat demand. The source of all steam in the convective heat transfer process is the secondary steam generated by indirect conduction heating of the sludge, that is, the entire evaporation process does not waste the latent heat of steam when recovering it, and at the same time, the high quality of energy is not wasted in the heating process of supplementing the heat source.

[0060] The secondary steam generated by "superheated steam drying" is indirectly heat-exchanged with the "vacuum low-temperature drying" sludge to generate condensed water after cyclone dust removal, high-efficiency filtration and water washing. An automatic dosing device is set up in the water washing process to clean and eliminate most of the free ammonia to generate ammonium salt dissolved in the sewage. The washing water source comes from the system condensed water, and the sewage washed by water washing is discharged into the sludge storage tank to recover all its heat and pollutants. A small amount of volatile organic matter such as alcohols and esters enters the condensed water after condensation with water vapor. After the trace volatile organic matter in the condensed water is removed by "electrochemical oxidation", it is discharged or reused in compliance with the standards. There is no pollutant discharge in the condensed water during the entire drying process. The "superheated steam" after water washing reduces the pollutants in the steam and increases the saturation of the steam. Saturated steam is conducive to improving the indirect heat transfer efficiency and saving the drying time of the "low-temperature vacuum evaporation" stage of indirect heat exchange.

[0061] The amount of water evaporated in the "superheated steam drying" process accounts for 10%-25% of the total water volume in sludge drying. All the system's supplementary heat is used to increase the sensible heat of the "secondary steam". The same supplementary heat can bring higher superheat and convective heat transfer temperature difference.

[0062] The moisture content of the outlet sludge is controlled by adjusting and matching the speed of the scraper or mesh belt, and the combination of "vacuum low-temperature drying" and "superheated steam drying" improves the overall drying speed of the sludge.

[0063] All the secondary steam generated during the drying process recovers all its latent heat and most of its sensible heat during the "vacuum low-temperature drying" and sludge preheating stages. Compared with the traditional steam heating drying evaporation method, there is no need to set up a cooling water system.

[0064] Low-temperature evaporation can prevent pollutants from migrating to the condensate system. The condensate system has low pollutant content, and the main components are alcohols and esters of volatile organic compounds. Through simple and low-cost treatment, it can meet the discharge standards or be reused. Compared with the traditional high-temperature drying and evaporation method, the condensate does not need to be returned to the sewage treatment plant or undergo complicated treatment on site.

[0065] The system condensation pressure is less than 100KPa, and the system is not a "pressure vessel". Compared with the traditional high-temperature drying and evaporation method, it improves system safety and reduces equipment manufacturing costs and system maintenance costs. The convection heat exchange method of superheated steam is adopted, and the oxygen content in the gas is extremely low, avoiding the explosion and fire hazards of the system.

[0066] Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A mechanical vapor recompression drying system, characterized in that The invention comprises a sludge storage tank (110) for storing room temperature sludge material and having a condensed water heat exchange coil (111) therein, a sludge injection pump (120) connected to the output end of the sludge storage tank (110) and provided with a jacketed heat exchanger (121), a thin layer evaporative dryer (130) connected to the output end of the sludge injection pump (120), an extrusion strip forming machine (140) connected to the output end of the thin layer evaporative dryer (130), a mesh belt dryer (150) connected to the output end of the extrusion strip forming machine (140), a dry sludge bin (160) connected to the output end of the mesh belt dryer (150), and a heat exchanger (150) disposed between the thin layer evaporative dryer (130) and the mesh belt dryer (150). A steam compressor (220), a cyclone dust collector (230) connected to a mesh belt dryer (150) and used for separating secondary steam and sludge dust, a water-washing dust collector (240) connected to an output end of the cyclone dust collector (230) and used for washing and cleaning the secondary steam, a condensate buffer tank (310) connected to a thin-layer evaporative dryer (130) and used for collecting condensate, a granulator (161) for extruding dry mud in a dry mud bin (160) into granules and then discharging the granules, a first filter (210) arranged between the thin-layer evaporative dryer (130) and the steam compressor (220), and a second filter arranged between the thin-layer evaporative dryer (130) and the water-washing dust collector (240). (250), a second circulation pump (311) located between the sludge injection pump (120) and the water-washing dust collector (240), a condensate pool (320) connected to the sludge storage tank (110) and having an electrochemical oxidation descaling device (321), and a vacuum pump (400) connected to the condensate buffer tank (310) and used to suck system air to maintain vacuum and discharge non-condensable gas; wherein the thin-layer evaporative dryer (130) comprises at least one cylindrical heating bin, a scraper rotor located inside the cylindrical heating bin and having a plurality of scrapers evenly distributed on the surface, a motor located outside the cylindrical heating bin and driving the scraper rotor to rotate, and a steam inlet ( 131), a steam and condensate outlet (132) connected to the input end of the condensate buffer tank (310), a sludge feed inlet (133) connected to the output end of the sludge injection pump (120), and a sludge outlet (134) connected to the input end of the extrusion strip forming machine (140); wherein the water-washing dust collector (240) comprises a condensate spray head (241), a demister (242) located above the condensate spray head (241), a dosing device (243) connected to the condensate spray head (241), a first circulation pump (244) connected to the condensate spray head (241), and a sewage pump (245) for returning the sludge to the sludge storage tank (110).

2. The mechanical vapor recompression drying system according to claim 1, characterized in that: The mesh belt dryer (150) is a closed structure, and is internally provided with a steam circulation fan (151) and a second external heat source heater (152).

3. A method for using a mechanical vapor recompression drying system as claimed in claim 1, comprising: S1: The wet sludge is transferred from the sludge storage tank to the thin-layer evaporation dryer through a transfer device. During the process, the wet sludge and condensed water are heated in a jacketed heat exchanger from 5-35°C at the inlet to 65-80°C; the condensed water is cooled from 80-98°C at the inlet to 15-45°C at the outlet; S2: The preheated sludge forms a thin layer in the thin-layer evaporative dryer under the action of the rotating scraper blades and adheres to the inner surface of the thin-layer evaporative dryer cylinder. The cylinder is a negative pressure system with a pressure setting of 25-47.5KPa, and the corresponding water evaporation temperature is 65-80℃; the heat source for heat exchange between the sludge and the cylinder wall comes from the steam in step S4, and the heat exchange temperature difference is 18-33℃, and the sludge moisture content is reduced from 80%-95% to 55%-65%; S3: The sludge with a moisture content of 55%-65% is squeezed into strips with a width of 2-5mm and then enters the net belt dryer; S4: The sludge entering the mesh belt dryer exchanges heat with the superheated steam during the mesh belt transmission process, and enters the dry mud bin after further evaporation and drying, and is discharged after extrusion and granulation by the granulator. The moisture content of the sludge is further evaporated and dried from 55%-65% to 20%-45% moisture content sludge, completing the drying process in the mesh belt dryer; the saturated steam at the outlet of the steam compressor enters the mesh belt dryer, is heated by the second external heat source heater to increase the superheat degree by 20-50°C, and is forced to circulate by the steam circulation fan and exchange heat with the sludge by convection; the surplus superheated steam becomes clean saturated steam after cyclone separation, water washing and dust removal, and filtration, and enters step S2 again; S5: The secondary steam generated in the evaporation process in step S2 is filtered by the first filter, and then enters the steam compressor for compression and then enters step S4. The condensed water from the condensed water buffer tank is sprayed into the steam compressor suction inlet. The compression ratio is 2.0-4.5, and the steam temperature rise is 18-33°C. S6: A portion of the condensed water enters step S5 to reduce the exhaust superheat of the steam compressor; a portion enters the water-washing dust collector in step S4 and is discharged into the sludge storage tank; the remaining portion of the condensed water enters the condensed water tank after heat exchange in the jacketed heat exchanger in step S1, and is discharged or reused after removing trace organic matter through an electrochemical oxidation descaling device; S7: The non-condensable gas in the condensate buffer tank and the condensate pool is removed by the vacuum pump until the end.

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