Low temperature closed cycle spray drying system and process

Abstract

This invention relates to the field of spray drying technology, providing a low-temperature closed-loop spray drying system and process. The system includes a closed-loop circuit consisting of a circulating fan, a main heater, an atomizing tower, a solid-liquid separation system, a primary condenser heat exchanger, and a secondary condenser heat exchanger connected in sequence, as well as a feeding system, a nitrogen supply system, and a control system. The process involves first purging the system with nitrogen to reduce the oxygen content below a safe threshold. Then, the circulating gas is heated to a low-temperature inlet temperature and sent to the atomizing tower. The liquid material is atomized by the atomizer and comes into contact with the low-temperature circulating gas for drying. The hot, humid airflow containing powder undergoes solid-liquid separation and enters the two-stage condenser heat exchangers for cascade condensation and dehumidification. The dehumidified, cold, dry air is reheated and returned to the atomizing tower for reuse. This invention employs closed-loop nitrogen protection and low-temperature drying, effectively avoiding thermal denaturation and oxidative degradation of heat-sensitive materials, significantly improving product activity retention rate and purity, while reducing energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of spray drying technology, and particularly relates to a low-temperature closed-loop spray drying system and process. Background Technology

[0002] Spray drying technology is a process that disperses liquid materials into fine droplets through an atomizer, which then come into contact with a hot air stream, causing the moisture to evaporate rapidly and thus obtaining a dried powder product. This technology offers advantages such as fast drying speed, good product uniformity, and continuous operation, and is widely used in the food, pharmaceutical, chemical, and biological product industries. With the rapid development of modern medicine and biotechnology, an increasing number of heat-sensitive materials (such as proteins, enzyme preparations, probiotics, active peptides, vaccines, and other bioactive substances) require spray drying technology to prepare highly active powder products.

[0003] However, these materials are extremely sensitive to temperature and oxygen, and traditional spray drying technology is difficult to meet their production requirements of high activity, high purity, and low energy consumption. Therefore, developing low-temperature, oxygen-free, closed-loop spray drying technology suitable for heat-sensitive materials has become a technical problem that the industry urgently needs to solve. Summary of the Invention

[0004] The purpose of this invention is to provide a low-temperature closed-loop spray drying system and process, which aims to solve the problems existing in the background art.

[0005] This invention is implemented as follows: a low-temperature closed-loop spray drying system, comprising:

[0006] A feeding system for storing and conveying liquid feed is connected to an atomizer at the top of the atomizing tower.

[0007] An atomizing and drying system is used to atomize and dry liquid materials. The atomizing and drying system includes an atomizing tower and an atomizer installed at the top of the atomizing tower. The air inlet of the atomizing tower is connected to a circulating fan via a main heater.

[0008] A solid-liquid separation system is used to collect dried powder. The solid-liquid separation system includes a primary cyclone separator, a secondary cyclone separator, and a bag filter connected in sequence. The primary cyclone separator is connected to the air outlet of the atomizing tower.

[0009] A condensation dehumidification system is used to perform staged condensation dehumidification on circulating air. The condensation dehumidification system includes a primary condensation heat exchanger and a secondary condensation heat exchanger connected in series. The primary condensation heat exchanger is connected to the outlet of the bag filter.

[0010] A circulating heating system is used to drive the circulating gas and heat it to the process temperature. The circulating heating system includes a circulating fan and a main heater. The circulating fan is connected to the secondary condenser heat exchanger, and the main heater is connected to the air inlet of the atomizing tower.

[0011] A nitrogen supply system is used to provide high-purity nitrogen and maintain a slight positive pressure in the system. The nitrogen supply system includes a nitrogen source and a nitrogen buffer tank, which is connected to the gas inlet of the atomizer.

[0012] Control system, used to monitor and control process parameters.

[0013] As a preferred embodiment of the present invention, the atomizer is a dual-fluid external mixing atomizer, which is provided with a liquid inlet and a high-pressure gas inlet. The liquid inlet is connected to the feeding system, and the high-pressure gas inlet is connected to the nitrogen supply system.

[0014] As a preferred embodiment of the present invention, the nitrogen supply system includes a nitrogen source and a nitrogen buffer tank, the inlet of the nitrogen buffer tank is connected to the nitrogen source, and the outlet of the nitrogen buffer tank is connected to the gas inlet of the atomizer via a pressure reducing valve.

[0015] In a preferred embodiment of the present invention, both the primary condensing heat exchanger and the secondary condensing heat exchanger are shell-and-tube heat exchangers. The shell-side inlet of the primary condensing heat exchanger is connected to the refrigeration unit, the shell-side inlet of the secondary condensing heat exchanger is connected to the shell-side outlet of the primary condensing heat exchanger, and the tube-side outlet of the secondary condensing heat exchanger is connected to the inlet of the circulating fan.

[0016] The present invention also provides a low-temperature closed-loop spray drying process using the above-mentioned low-temperature closed-loop spray drying system, comprising the following steps:

[0017] S1: The low-temperature closed-loop spray drying system is sealed with nitrogen and the internal air is emptied so that the oxygen content drops below the safe threshold.

[0018] S2: Start the circulating fan and main heater to heat the circulating gas to the low-temperature inlet air temperature and then send it into the atomization tower;

[0019] S3: The liquid feed is delivered to the atomizer via the feeding system, and high-pressure nitrogen is introduced into the atomizer at the same time. The liquid feed is dried by contacting the low-temperature circulating gas in the atomization tower.

[0020] S4: The hot, humid airflow containing powder is introduced into the solid-liquid separation system for gas-solid separation, and the dried powder is collected;

[0021] S5: The separated hot and humid airflow enters the first-stage condenser heat exchanger and the second-stage condenser heat exchanger in sequence for staged condensation and dehumidification, and the moisture is separated by condensation.

[0022] S6: The dehumidified cold and dry air is sent to the main heater via a circulating fan to be reheated to the low-temperature inlet air temperature and then returned to the atomizing tower for recycling;

[0023] S7: During the circulation process, nitrogen is continuously replenished to maintain the slightly positive pressure of the low-temperature closed-loop spray drying system.

[0024] As a preferred embodiment of the present invention, in the stepped condensation dehumidification, the first-stage condensation uses 0°C chilled water to cool the hot and humid airflow to 8-9°C, and the second-stage condensation uses 0°C chilled water to further cool the airflow to 1-2°C.

[0025] As a preferred embodiment of the present invention, it further includes an online high-temperature sterilization step: before low-temperature drying, the inlet air temperature is raised to the sterilization temperature and maintained for a certain period of time, and after sterilization, the temperature is lowered to the low-temperature inlet air temperature to enter the drying stage.

[0026] The low-temperature closed-loop spray drying system and process provided by this invention have the following beneficial effects:

[0027] The low-temperature closed-loop spray drying system and process provided by this invention achieves low-temperature oxygen-free drying of heat-sensitive materials by adopting low-temperature drying conditions of 50°C air inlet and 35°C air outlet, combined with a closed-loop nitrogen protection system, a two-stage cascade condensation dehumidification design, and a precisely matched 2.2m diameter atomizing tower.

[0028] This invention effectively avoids thermal denaturation and oxidative degradation of materials under high temperature and oxygen conditions, significantly improves the activity retention rate and product purity of heat-sensitive materials such as proteins and enzymes, and ensures that the product's microbiological indicators consistently meet pharmaceutical-grade standards. At the same time, the closed-loop circulation design allows for the recycling of thermal energy, significantly reducing energy consumption and eliminating exhaust gas and dust emissions, thus achieving green and environmentally friendly production.

[0029] The 35℃ outlet air temperature keeps the water vapor in the airflow in an unsaturated state, completely solving the problem of moisture accumulation and clogging in bag filters, significantly extending the filter bag life and reducing maintenance costs; the 2.2m diameter atomizing tower ensures uniform airflow distribution and sufficient droplet residence time, resulting in stable product moisture content and uniform particle size distribution.

[0030] This invention has significant application prospects and market value in the field of drying heat-sensitive materials. Attached Figure Description

[0031] Figure 1 This is a process diagram of the low-temperature closed-loop spray drying system of the present invention;

[0032] Figure 2 This is a diagram showing the results of the process calculations.

[0033] In the attached diagram: 1. Atomizing tower; 2. Main heater; 3. Circulating fan; 4. Primary cyclone separator; 5. Secondary cyclone separator; 6. Bag filter; 7. Primary condenser heat exchanger; 8. Secondary condenser heat exchanger; 9. Nitrogen buffer tank; 10. Feed pump; 11. Primary condensate collection tank; 12. Secondary condensate collection tank. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0035] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0036] like Figure 1 As shown, the low-temperature closed-loop spray drying system provided by the present invention includes a feeding system, an atomization drying system, a solid-liquid separation system, a condensation dehumidification system, a circulating heating system, a nitrogen supply system, and a control system.

[0037] (a) Feeding system

[0038] The feeding system is used to store and transport the liquid feed, including a liquid feed tank, a feed pump 10, and a flow meter. The liquid feed tank has a volume of 50-100L and is equipped with a stirring device to prevent solid sedimentation. The feed pump 10 is a high-pressure plunger pump with a maximum pressure of 5000mbar and an adjustable flow rate of 0-20kg / h, transporting the liquid feed from atmospheric pressure to the atomizer. The flow meter is an electromagnetic flow meter with an accuracy of ±1%, monitoring the liquid feed flow rate in real time. The piping is made of 316L stainless steel with an inner diameter of 10mm, connecting the storage tank, pump, flow meter, and atomizer. The feeding system is connected to the atomizer at the top of the atomization tower 1.

[0039] (ii) Atomizing Drying System

[0040] The atomizing and drying system is used to atomize and dry the liquid material, including an atomizing tower 1 and an atomizer installed at the top of the atomizing tower 1.

[0041] Atomizing tower 1 is a cylindrical tower with a diameter of 2.2m, a height of 2.5m, and a cone base angle of 60°. It is made of 316L stainless steel with an inner wall polished to Ra≤0.8μm. The insulation layer is 100mm thick rock wool, covered with an aluminum cladding to reduce heat loss to 0.91kW. The air inlet is located at the center of the tower top, with a diameter of 300mm, and is equipped with a guide plate to ensure uniform airflow distribution. The air outlet is located in the lower middle part of the tower side wall, with a diameter of 250mm, and is tangentially led out. The discharge port is located at the bottom of the cone, with a diameter of 150mm, and is equipped with a pneumatic butterfly valve.

[0042] The atomizer is a dual-fluid external mixing atomizer, equipped with a liquid inlet and a high-pressure gas inlet. The liquid nozzle has a diameter of 0.5 mm and is made of 316L stainless steel. The gas nozzle is an annular slit with a width of 0.2 mm. The atomizing gas pressure is 3 barg, and the flow rate is 12 kg / h. The droplet size D50 is 15-25 μm, and the span is 1.5-2.0. The liquid inlet is connected to the feeding system, and the high-pressure gas inlet is connected to the nitrogen supply system.

[0043] The air inlet of the atomizing tower 1 is connected to the circulating fan 3 via the main heater 2.

[0044] (III) Solid-Liquid Separation System

[0045] The solid-liquid separation system is used to collect dried powder and includes a primary cyclone separator 4, a secondary cyclone separator 5 and a bag filter 6 connected in sequence. The primary cyclone separator 4 is connected to the air outlet of the atomizing tower 1.

[0046] First-stage cyclone separator 4: Diameter 0.50m, height 2.0m (cylinder 1.2m + cone 0.8m). Overflow pipe diameter 0.25m, insertion depth 0.4m. Inlet is rectangular, dimensions 0.25m × 0.125m, tangential entry. Discharge port diameter 100mm, equipped with a rotary valve. Separation efficiency >95% (for particles >10μm).

[0047] Two-stage cyclone separator 5: Diameter 0.44m, height 1.76m (cylinder 1.06m + cone 0.70m). Overflow pipe diameter 0.22m, insertion depth 0.35m. Inlet is rectangular, dimensions 0.22m × 0.11m, tangential entry. Discharge port diameter 80mm, equipped with a rotary valve. Separation efficiency >90% (for particles >5μm).

[0048] Baghouse Dust Collector 6: Filtration area 30m². 20 filter bags, single bag size Φ130mm×2000mm. Filter media is polyester needle-punched felt, temperature resistance ≤150℃. Cleaning method is pulse backflushing, compressed air pressure 6bar. Dust hopper volume 0.5m³, equipped with a discharge valve. Filtration efficiency >99.9%.

[0049] (iv) Condensation Dehumidification System

[0050] The condensation dehumidification system is used to perform cascade condensation dehumidification on the circulating gas, including a primary condensation heat exchanger 7 and a secondary condensation heat exchanger 8 connected in series. The primary condensation heat exchanger 7 is connected to the outlet of the bag filter 6.

[0051] Both the primary condensing heat exchanger 7 and the secondary condensing heat exchanger 8 are BEM350×2500 type shell-and-tube heat exchangers. The shell side has an inner diameter of 350mm and a length of 2500mm. The tube side consists of 162 stainless steel tubes with an outer diameter of 19.05mm, an inner diameter of 14.83mm, and a length of 2.5m, arranged in a single tube pass. The tubes are arranged in an equilateral triangle with a spacing of 23.81mm and an angle of 30°. There are 5 baffles, arc-shaped, with a 44.11% cut and a spacing of 350mm. The heat exchange area is 24.2m² (effective 23.5m²). The shell-side inlet and outlet are both Φ88.9mm. The tube-side inlet is Φ168.28mm and the outlet is Φ114.3mm.

[0052] The shell-side inlet of the primary condensing heat exchanger 7 is connected to the refrigeration unit, the shell-side inlet of the secondary condensing heat exchanger 8 is connected to the shell-side outlet of the primary condensing heat exchanger 7, and the tube-side outlet of the secondary condensing heat exchanger 8 is connected to the inlet of the circulating fan 3.

[0053] The condensate collection system includes a primary condensate collection tank 11 and a secondary condensate collection tank 12, each with a volume of 20L, for periodically discharging condensate.

[0054] (v) Circulating heating system

[0055] The circulating heating system is used to drive the circulating gas and heat it to the process temperature. It includes a circulating fan 3 and a main heater 2. The circulating fan 3 is connected to the secondary condenser heat exchanger 8, and the main heater 2 is connected to the air inlet of the atomizing tower 1.

[0056] The circulating fan 3 is a centrifugal fan with an air volume of 1800 kg / h (approximately 1400 m³ / h, calculated based on density at 1℃), a pressure head of 60 mbar (600 Pa), a power of 8.27 kW, and is made of 304 stainless steel, which is corrosion resistant.

[0057] The main heater 2 is an electric heater or a steam heat exchanger with a heating power of 25.5kW, a temperature control accuracy of ±1℃, and a heating medium of electric heating wire or saturated steam (0.3MPa).

[0058] (vi) Nitrogen supply system

[0059] The nitrogen supply system is used to provide high-purity nitrogen and maintain a slight positive pressure in the system. It includes a nitrogen source and a nitrogen buffer tank 9, which is connected to the gas inlet of the atomizer.

[0060] The nitrogen source is a liquid nitrogen vaporizer or a PSA nitrogen generator, with a purity ≥99.99%. The nitrogen buffer tank 9 has a volume of 1 m³ and a pressure of 5 bar. A pressure reducing valve reduces the nitrogen pressure from 5 bar to 3 barg for atomization. The flow meter is a mass flow meter with a range of 0-20 kg / h and an accuracy of ±2%. The inlet of the nitrogen buffer tank 9 is connected to the nitrogen source, and the outlet of the nitrogen buffer tank 9 is connected to the gas inlet of the atomizer via a pressure reducing valve.

[0061] (vii) Control system

[0062] The control system is used to monitor and control process parameters.

[0063] Temperature control: Measuring points include inlet air temperature, outlet air temperature, temperature of each layer of atomizing tower 1, and inlet and outlet temperatures of the heat exchanger. Actuators include power regulation of main heater 2 and refrigerant flow regulation. The control strategy is PID control, with inlet air temperature set at 50℃ and outlet air temperature set at 35℃.

[0064] Pressure control: Measuring points include the pressure of atomizing tower 1, the inlet and outlet pressures of the cyclone separator, and the inlet and outlet pressures of the filter bag. The actuator is a variable frequency drive for the circulating fan 3. The control strategy is to maintain the pressure of atomizing tower 1 at 1019 mbar and the pressure drop of the filter bag at <10 mbar.

[0065] Flow control: Measuring points include liquid flow rate, nitrogen flow rate, circulating air volume, and refrigerant flow rate. Actuators include the feed pump speed (10 kg / h), nitrogen flow valve, and refrigerant pump speed. The control strategy is to set the liquid flow rate to 10 kg / h and the nitrogen flow rate to 12 kg / h.

[0066] Safety interlocks include low material level shutdown protection, high temperature alarm (>60℃) and emergency shutdown, abnormal pressure alarm (<1000mbar or >1100mbar), and low nitrogen purity alarm (<99%).

[0067] like Figure 1 As shown, the connection relationships of each component are as follows:

[0068] Main circulation loop: Circulating fan 3 → Main heater 2 → Atomizing tower 1 → Primary cyclone separator 4 → Secondary cyclone separator 5 → Bag filter 6 → Primary condenser heat exchanger 7 → Secondary condenser heat exchanger 8 → Circulating fan 3, forming a closed loop.

[0069] Feed circuit: Feed tank → Feed pump 10 → Atomizer (located at the top of atomizing tower 1).

[0070] Nitrogen supply circuit: Nitrogen source → Nitrogen buffer tank 9 → Pressure reducing valve → Atomizer gas inlet.

[0071] Refrigerant circulation loop: Refrigeration unit → First-stage condensing heat exchanger 7 shell side → Second-stage condensing heat exchanger 8 shell side → Refrigeration unit.

[0072] Product collection loop: Primary cyclone separator 4 discharge port → product collection tank; Secondary cyclone separator 5 discharge port → product collection tank; Bag filter dust collector 6 discharge port → product collection tank.

[0073] Condensate discharge loop: Bottom of tube 7 of primary condensing heat exchanger → condensate collection tank → discharge; Bottom of tube 8 of secondary condensing heat exchanger → condensate collection tank → discharge.

[0074] Implementation of the low-temperature closed-loop spray drying process described in this invention:

[0075] Step S1: Initial nitrogen purging and system sealing

[0076] Before formal drying, the entire closed-loop circulation system is purged with nitrogen. High-purity nitrogen (≥99.99% purity) is introduced into the system through the nitrogen inlet (flow rate 12 kg / h, pressure 3 barg). The existing air in the system is then purged, reducing the oxygen content to below 0.5%. All inlets and outlets are sealed to ensure system airtightness (leakage rate <0.1% / h). A slightly positive pressure environment is established (system pressure 1019-1043 mbar) to prevent outside air infiltration.

[0077] The nitrogen flow rate of 12 kg / h is the optimal value determined based on the atomization requirements (gas-liquid ratio 1.2:1, feed flow rate 10 kg / h) and system leakage compensation (approximately 2 kg / h). The pressure of 3 barg ensures that the atomizer obtains sufficient kinetic energy to form droplets with a reasonable particle size distribution (D50 = 15-25 μm).

[0078] As a preferred implementation method, for pharmaceutical-grade products, online high-temperature sterilization can be performed before low-temperature drying: the inlet air temperature is increased to 150℃ and maintained for 30-60 minutes, with the circulating air volume maintained at 1800 kg / h, ensuring that all parts of the system reach the sterilization temperature, with the inner wall temperature of the atomizing tower 1 ≥ 130℃, and the temperatures of the cyclone separator and bag filter 6 ≥ 120℃. After sterilization, the temperature is gradually reduced to 50℃ before formal drying. This method utilizes the airtightness of the closed-loop circulation system to achieve online sterilization without disassembly, and immediately switches to low-temperature drying after sterilization, with full oxygen-free protection, avoiding secondary contamination caused by reassembly after traditional sterilization.

[0079] Step S2: Low-temperature air intake heating

[0080] The circulating nitrogen gas is heated to a precisely controlled low temperature. The inlet air temperature is 50℃ (±1℃), supplied by the main heater 2 (25.5kW). The inlet air flow rate is 1800kg / h, driven by the circulating fan 3 (8.3kW). The inlet air pressure is 1019mbar, ensuring positive pressure operation within the atomization tower 1. The dew point temperature of the inlet air humidity is below 0℃ (absolute moisture content <0.004kg water / kg dry air).

[0081] 50℃ is the optimal inlet air temperature, verified through extensive experiments. Below 45℃, the driving force for moisture evaporation is insufficient, resulting in decreased drying efficiency; above 55℃, some heat-sensitive proteins begin to denature. The air volume of 1800 kg / h is calculated based on the atomizing tower 1 diameter of 2.2 m, the average air velocity inside the tower of 0.15 m / s, and the residence time of 15-20 seconds.

[0082] Step S3: Atomization and drying of the liquid feed

[0083] The feed liquid is atomized and dried within atomizing tower 1. The feed liquid flow rate is 10 kg / h (80% water content, 2 kg / h solid content). The feed liquid temperature is 25℃, and it is delivered by feed pump 10 (5000 mbar pressure). The atomization method is high-pressure nitrogen atomization, with an atomizing gas flow rate of 12 kg / h and a pressure of 3 barg.

[0084] Atomizing tower 1 has a diameter of 2.2m and a height of 2.5m. The droplet size distribution is as follows:

[0085] Small particles: average particle size 4.16μm, initial velocity 185.7m / s, rapidly decelerated to 0.125m / s at the top of the tower (0-0.1m), moisture content decreased from 80% to 0.1%, and temperature increased from 25℃ to 36.4℃.

[0086] Medium particles: average particle size 7.09 μm, initial velocity 185.7 m / s, deceleration to 0.126 m / s, drying characteristics similar to small particles.

[0087] Large particles: average particle size 10.4 μm, initial velocity 185.7 m / s, deceleration to 0.128 m / s, requiring a longer residence time (descending to the 0.8-2.5 m region inside the tower) to be fully dried.

[0088] The temperature distribution inside the tower is as follows: at the top of the tower (0m), the gas phase temperature is 49.5℃ and the droplet temperature is 25℃; in the middle of the tower (1.25m), the gas phase temperature is 36.3℃ and the droplet temperature is 36.4℃ (reaching wet-bulb temperature equilibrium); at the bottom of the tower (2.5m), the gas phase temperature is 35.1℃ and the powder temperature is 35.1℃.

[0089] Air outlet parameters: temperature 35.1℃ (±0.5℃), flow rate 1825kg / h (including 10kg / h of evaporated water vapor), humidity is 0.0147kg water / kg dry air (relative humidity about 45%).

[0090] The 2.2m diameter of the atomizing tower is the optimal size, verified through precise fluid dynamics calculations and long-term practical application. A diameter that is too small (e.g., 1.8m) will result in excessively high airflow velocity within the tower (>0.2m / s), insufficient droplet residence time, and inadequate drying of large particles; a diameter that is too large (e.g., 2.5m) will lead to uneven airflow distribution within the tower, creating dead zones and short-circuit flow, increasing energy consumption. The 2.2m diameter achieves optimal drying uniformity and energy efficiency under conditions of 10kg / h evaporation rate, 50℃ inlet air, and 35℃ outlet air.

[0091] Step S4: Gas-solid separation

[0092] The dried gas-solid mixture is then sequentially fed into a solid-liquid separation system for separation.

[0093] The particles first enter the primary cyclone separator 4. The cyclone diameter is 0.50m, the overflow pipe diameter is 0.25m, the inlet velocity is 9.31m / s, the separation efficiency is >95% (for particles >10μm), the pressure drop is 15mbar, and the outlet temperature is 35.1℃ (consistent with the inlet temperature, with no additional heat loss). The separated coarse powder (approximately 60-70% of the total output) is discharged from the bottom discharge port of the cyclone and enters the product collection system.

[0094] Fine powder not collected by the primary cyclone enters the secondary cyclone separator 5 with the airflow. The cyclone diameter is 0.44m (smaller than the primary separator, improving the collection efficiency of fine powder), the overflow pipe diameter is 0.22m, the inlet velocity is 12.11m / s (higher than the primary separator, enhancing centrifugal force), the separation efficiency is >90% (for particles >5μm), the pressure drop is 25mbar, and the outlet temperature is 35.1℃. Fine powder collected by the secondary cyclone (approximately 20-30% of the total output) is also discharged into the product collection system.

[0095] After two-stage cyclone separation, the remaining ultrafine powder (particle size <5μm, accounting for about 5-10%) enters the bag filter 6 with the airflow. The inlet temperature is 35.1℃, the filtration velocity is 0.8-1.0m / min (low velocity, reducing pressure drop), the pressure drop is 8.75mbar (far lower than the 20-30mbar of traditional systems), the filtration efficiency is >99.9%, and the outlet temperature is 35.1℃.

[0096] 35℃ is a "balance temperature" that has been verified through extensive practical experience. At this temperature, the water vapor in the airflow is in an unsaturated state (relative humidity 45%), preventing condensation on the filter bag surface and avoiding secondary drying and agglomeration of powder due to overheating. Long-term continuous operation (>168 hours) shows that the filter bag pressure drop remains stable, eliminating the need for frequent dust cleaning and completely solving the persistent problem of moisture accumulation and clogging in traditional systems.

[0097] Step S5: Cascade condensation dehumidification

[0098] The separated hot and humid airflow sequentially enters the primary condenser heat exchanger 7 and the secondary condenser heat exchanger 8 for cascade condensation and dehumidification.

[0099] The hot, humid airflow (35℃, moisture content 0.0147 kg / kg) after bag filter dust collection enters the primary condenser heat exchanger 7. The heat exchanger type is a BEM-type horizontal shell-and-tube heat exchanger with a heat exchange area of ​​24.2 m² (effective area 23.5 m²). The tube side carries the hot, humid airflow with an inlet temperature of 50.67℃ (pressurized and heated by the circulating fan 3) and an outlet temperature of 8.73℃. The shell side carries refrigerant (0℃ chilled water) at a flow rate of 20,000 kg / h, with an inlet temperature of 7℃ and an outlet temperature of 8.15℃. The condensate flow rate is 4.97 kg / h (approximately 50% of the total evaporation). The heat transfer coefficient is 113.1 W / (m²·K). The pressure drop is 1.94 kPa for the tube side and 2.53 kPa for the shell side.

[0100] A refrigerant temperature of 0℃ is the optimal value, verified through precise calculations and practical application. Temperatures that are too low (e.g., -5℃) can cause ice buildup and blockage in the heat exchanger pipes, especially in areas with high water vapor concentrations in the airflow. Temperatures that are too high (e.g., 5℃) result in insufficient condensation efficiency, failing to adequately capture moisture, leading to excessive humidity in the circulating air and reduced drying capacity. Condensation at 0℃ can lower the airflow dew point to 8.5℃ and the absolute water content to 0.0052 kg / kg, creating favorable conditions for subsequent secondary condensation.

[0101] The gas flow after primary condensation enters the secondary condenser heat exchanger 8 (with the same structural parameters as the primary stage). The tube-side gas flow inlet temperature is 8.15℃, and the outlet temperature is 1.16℃. The shell-side refrigerant inlet temperature is 0℃, and the outlet temperature is 0.58℃. The condensate flow rate is 4.96 kg / h (approximately 50% of the total evaporation). The cumulative condensation efficiency is 99.3% (the two stages of condensation remove a total of 9.93 kg / h of moisture).

[0102] After two-stage condensation, the dew point of the airflow drops below 0.58℃, and the absolute water content drops to 0.0004kg / kg, forming "dry air" and significantly improving the water-carrying capacity of the circulating gas.

[0103] Two-stage condensation forms a "tiered dehumidification" system. The first stage quickly removes most of the moisture, while the second stage deeply dehumidifies to an extremely low dew point. This design avoids the risk of icing caused by single-stage deep cooling, while achieving highly efficient dehumidification. The condensed dry air re-enters the circulation, restoring its water-carrying capacity to its initial state, forming a virtuous cycle of "evaporation-condensation-re-evaporation".

[0104] Step S6: Reheating of circulating air

[0105] The deeply dehumidified, cooled, dry air (1.16℃) is pressurized by the circulating fan 3 and then reheated to 50℃ by the main heater 2. The circulating fan 3 has a power of 8.27kW and provides the system pressure head (overcoming the resistance of cyclones, bag filters, heat exchangers, etc., with a total pressure drop of approximately 60mbar). The main heater 2 has a power of 25.5kW and heats the 1800kg / h airflow from approximately 10℃ to 50℃. The heating efficiency is >95% (using electric heating or steam heating).

[0106] This completes a full closed-loop cycle, and the airflow re-enters atomizing tower 1 to begin the next round of drying.

[0107] Step S7: System Balancing and Fine-tuning

[0108] During continuous operation, the following measures are taken to maintain system stability:

[0109] Nitrogen replenishment: Continuously replenish nitrogen (approximately 2 kg / h) to maintain a slight positive pressure, depending on the system leakage situation.

[0110] Condensate drainage: Drain condensate regularly (9.93 kg / h) to prevent liquid accumulation.

[0111] Temperature fine-tuning: Based on the feedback of the air outlet temperature, fine-tune the power of the main heater 2 (±2kW) to ensure that the air inlet temperature is stable at 50±1℃.

[0112] Pressure balancing: The system pressure is maintained at 1019-1043 mbar by adjusting the speed of the circulating fan 3.

[0113] The continuous operation test for 168 hours (7 days) showed that the fluctuation rate of various system parameters was <3%, the product moisture content was stable at 3-5%, the active ingredient retention rate was >95%, and there were no equipment failures, which fully proved the reliability of the process design.

[0114] IV. Working Principle

[0115] like Figure 1 and Figure 2 As shown, the working principle of this invention is based on the innovative concept of "low-temperature evaporation - closed-loop circulation - cascade condensation - dry air regeneration":

[0116] (1) Low-temperature evaporation: Although the inlet air temperature of 50℃ is lower than that of traditional processes, the precise design of the atomizing tower 1 size (diameter 2.2m) and the reasonable gas-liquid ratio (1.8:1) ensure that the droplets have sufficient residence time (15-20 seconds) and heat and mass transfer area to achieve thorough drying. The droplets undergo three stages in the tower: "rapid heating-constant drying-deceleration drying", and finally reach the target moisture content (3-5%) at the outlet air temperature of 35℃.

[0117] (2) Closed-loop circulation: The system adopts a completely sealed closed-loop design. The circulating gas (nitrogen) circulates between "atomizing tower 1 - cyclone separators 4 and 5 - bag filter 6 - condenser heat exchanger 7 and 8 - main heater 2" without contacting the outside air. This design achieves three major advantages: an oxygen-free environment to prevent material oxidation; no external pollution to ensure high product purity; and heat energy recycling to significantly reduce energy consumption.

[0118] (3) Step-by-step condensation: Two-stage condensers work in series. The first stage cools the airflow from 50°C to 8.7°C, condensing out 50% of the moisture. The second stage further cools the airflow to 1.2°C, condensing out the remaining 50% of the moisture. Step-by-step condensation avoids the icing risk of single-stage deep cooling and achieves 99.3% efficient dehumidification, restoring the circulating gas to a "dry air" state.

[0119] (4) Dry air regeneration: The dew point of the circulating gas after deep dehumidification drops to 0.58℃ and the absolute water content is only 0.0004kg / kg, restoring the water carrying capacity to the initial state. After this "dry air" is reheated to 50℃, it can once again undertake the task of evaporating 10kg / h of water, forming a sustainable virtuous cycle.

[0120] (5) Balanced temperature design: The 35℃ outlet air temperature is one of the core innovations of this invention. At this temperature, the water vapor in the airflow is in an unsaturated state (relative humidity 45%), which will not condense on the surface of the filter bag (avoiding moisture accumulation and blockage), nor will it cause secondary drying and agglomeration of the powder due to overheating. The discovery of this "balanced temperature" completely solves the long-standing problem of bag dust collectors in traditional spray drying systems.

[0121] like Figure 2 As shown, the process calculation parameters of the system of this invention include: inlet air mass 1800 kg / h, atomizing gas volume 12 kg / h, atomizing tower diameter 2.2 m, atomizing tower height 2.5 m, heat transfer coefficient 3.8 W / (m²·K), ambient temperature 25℃, inlet air temperature 50℃, feed water volume 10 kg / h, pump pressure 5000 mbar, atomization pressure 3 barg, refrigerant temperature 0℃, refrigerant flow rate 20000 kg / h, total evaporation rate 9.98751 kg / h, and heat loss to the environment -0.911. The system features the following parameters: 226 kW power output, outlet air temperature 35.1036℃, primary condensate flow rate 4.97175 kg / h, secondary condensate flow rate 4.95531 kg / h, primary cyclone pressure drop 14.9904 mbar, primary cyclone diameter 0.5002 m, secondary cyclone pressure drop 24.9773 mbar, secondary cyclone diameter 0.4418 m, bag filter pressure drop 8.75 mbar, chiller unit energy consumption -7.29458 kW, circulating fan energy consumption 8.27431 kW, and main heating power 25.4853 kW. These parameters verify the rationality and feasibility of the system design.

[0122] Example 1: Spray drying of enzyme preparations

[0123] The material is an aqueous solution of α-amylase with a solid content of 20% and an initial enzyme activity of 10,000 U / g.

[0124] Process parameters: liquid feed flow rate 10 kg / h, temperature 25℃; inlet air temperature 50℃, flow rate 1800 kg / h; outlet air temperature 35℃; atomizing gas flow rate 12 kg / h, pressure 3 barg; refrigerant temperature 0℃, flow rate 20000 kg / h.

[0125] Results: Product moisture content 4.2%; enzyme activity retention 96.5% (9650 U / g); product yield 98.5%; the system was stable and fault-free after 72 hours of continuous operation.

[0126] In comparison, the traditional 80℃ air inlet process only achieves an enzyme activity retention rate of 68% (6800U / g).

[0127] Example 2: Probiotic spray drying

[0128] The material is lactic acid bacteria fermentation broth with a solid content of 15% and an initial viable count of 1×10¹¹ CFU / g.

[0129] Process parameters: liquid flow rate 10 kg / h, temperature 20℃; inlet air temperature 48℃ (slightly lower than in Example 1, to further protect live bacteria); outlet air temperature 33℃; other parameters are the same as in Example 1.

[0130] Results: Product moisture content 3.8%; viable bacteria count 9.2 × 10¹ 0 CFU / g, survival rate 92%; product yield 97.8%; total number of microbial contamination colonies <10 CFU / g, no miscellaneous bacteria.

[0131] In comparison, the survival rate of live bacteria in traditional processes is only 55%, and there is a risk of contamination by other microorganisms.

[0132] Example 3: Spray drying after online sterilization

[0133] The material is pharmaceutical-grade protein powder with a solid content of 25%.

[0134] Process steps: (1) The system is filled with nitrogen and the oxygen content is reduced to 0.3%; (2) The air inlet temperature is increased to 150°C and maintained for 60 minutes to complete online sterilization; (3) The temperature is reduced to 50°C and the spraying begins; (4) Other parameters are the same as in Example 1.

[0135] Results: The total number of colonies on the inner wall of the sterilization equipment decreased from 500 CFU / 100cm² to <1 CFU / 100cm²; the product moisture content was 4.5%; the protein denaturation rate was <3%; and the total number of colonies was <5 CFU / g, meeting the pharmacopoeia standards.

[0136] Advantages: No need to disassemble the equipment, production can begin immediately after sterilization, avoiding secondary contamination and significantly shortening the production cycle.

[0137] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-temperature closed-loop spray drying system, characterized in that, The low-temperature closed-loop spray drying system includes: A feeding system for storing and conveying liquid feed includes a feed pump (10) connected to an atomizer at the top of an atomizing tower (1); The atomizing and drying system is used to complete the atomization and drying of the liquid material. The atomizing and drying system includes an atomizing tower (1) and an atomizer set at the top of the atomizing tower. The air inlet of the atomizing tower (1) is connected to a circulating fan (3) via a main heater (2). A solid-liquid separation system is used to collect dried powder. The solid-liquid separation system includes a primary cyclone separator (4), a secondary cyclone separator (5), and a bag filter (6) connected in sequence. The primary cyclone separator (4) is connected to the air outlet of the atomizing tower (1). A condensation dehumidification system is used to perform cascade condensation dehumidification on circulating gas. The condensation dehumidification system includes a primary condensation heat exchanger (7) and a secondary condensation heat exchanger (8) connected in series. The primary condensation heat exchanger (7) is connected to the outlet of the bag filter (6). The condensate collection system includes a primary condensate collection tank (11) and a secondary condensate collection tank (12), which are respectively connected to the bottom of the tube side of the primary condensate heat exchanger (7) and the secondary condensate heat exchanger (8) for collecting condensate; A circulating heating system is used to drive the circulating gas and heat it to the process temperature. The circulating heating system includes a circulating fan (3) and a main heater (2). The circulating fan (3) is connected to the secondary condenser heat exchanger (8), and the main heater (2) is connected to the air inlet of the atomizing tower (1). A nitrogen supply system is used to provide high-purity nitrogen and maintain a slight positive pressure in the system. The nitrogen supply system includes a nitrogen source and a nitrogen buffer tank (9). The nitrogen buffer tank (9) is connected to the gas inlet of the atomizer. Control system, used to monitor and control process parameters.

2. The low-temperature closed-loop spray drying system according to claim 1, characterized in that, The atomizer is a dual-fluid external mixing atomizer, which is provided with a liquid inlet and a high-pressure gas inlet. The liquid inlet is connected to the feeding system, and the high-pressure gas inlet is connected to the nitrogen supply system.

3. The low-temperature closed-loop spray drying system according to claim 1, characterized in that, The nitrogen supply system includes a nitrogen source and a nitrogen buffer tank (9). The inlet of the nitrogen buffer tank (9) is connected to the nitrogen source, and the outlet of the nitrogen buffer tank (9) is connected to the gas inlet of the atomizer via a pressure reducing valve.

4. The low-temperature closed-loop spray drying system according to claim 1, characterized in that, Both the primary condensing heat exchanger (7) and the secondary condensing heat exchanger (8) are shell-and-tube heat exchangers. The shell-side inlet of the primary condensing heat exchanger (7) is connected to the refrigeration unit, the shell-side inlet of the secondary condensing heat exchanger (8) is connected to the shell-side outlet of the primary condensing heat exchanger (7), and the tube-side outlet of the secondary condensing heat exchanger (8) is connected to the inlet of the circulating fan (3).

5. A low-temperature closed-loop spray drying process using the low-temperature closed-loop spray drying system described in any one of claims 1-4, characterized in that, The process includes the following steps: S1: The low-temperature closed-loop spray drying system is sealed with nitrogen and the internal air is emptied so that the oxygen content drops below the safe threshold. S2: Start the circulating fan (3) and the main heater (2) to heat the circulating gas to the low temperature inlet air temperature and then send it into the atomizing tower (1). S3: The liquid feed is delivered to the atomizer via the feeding system, and high-pressure nitrogen is introduced into the atomizer at the same time. The liquid feed is dried in contact with low-temperature circulating gas in the atomizing tower (1). S4: The hot, humid airflow containing powder is introduced into the solid-liquid separation system for gas-solid separation, and the dried powder is collected; S5: The separated hot and humid airflow is sequentially fed into the first-stage condenser heat exchanger (7) and the second-stage condenser heat exchanger (8) for cascade condensation and dehumidification, and the moisture is separated by condensation. S6: The dehumidified cold dry air is sent to the main heater (2) via the circulating fan (3) to be reheated to the low temperature inlet air temperature and returned to the atomizing tower (1) for recycling; S7: During the circulation process, nitrogen is continuously replenished to maintain the slightly positive pressure of the low-temperature closed-loop spray drying system.

6. The low-temperature closed-loop spray drying system according to claim 5, characterized in that, In the cascade condensation dehumidification process, the first-stage condensation uses 0°C chilled water to cool the hot and humid airflow to 8-9°C, and the second-stage condensation uses 0°C chilled water to further cool the airflow to 1-2°C.

7. The low-temperature closed-loop spray drying system according to claim 5, characterized in that, It also includes an online high-temperature sterilization step: before low-temperature drying, the inlet air temperature is raised to the sterilization temperature and maintained for a certain period of time, and after sterilization, it is cooled down to the low-temperature inlet air temperature to enter the drying stage.

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