A method for continuous crystallization of large particle d-allulose crystals

Abstract

The application provides a method for continuous crystallization of large-particle D-psicose crystal, based on the growth kinetics characteristics of D-psicose crystal, through the mode of "single stimulation nucleation + constant temperature crystal growth + circulating seed reservation + mother liquor recovery", the synergism of continuous production and accurate regulation of crystal habit is realized, and high-purity, large-particle and regular rod-shaped D-psicose crystal is prepared, which is a continuous preparation process for realizing large-particle, high-purity and easy-to-filter D-psicose crystal through "circulating seed reservation and crystal growth + mother liquor recovery cycle". Meanwhile, the process is simple, the degree of automation is high, the product quality is stable, there is no batch difference, the raw material utilization rate is high, there is no organic solvent addition in the whole process, the energy consumption is low, the cost is controllable, and the process is suitable for industrialized large-scale continuous production.

Description

Technical Field

[0001] This invention relates to the field of industrial crystallization and functional sugar production technology, and in particular to a method for continuous crystallization of large-particle D-allulose crystals. Background Technology

[0002] D-Allulose (D-psicose) is a hexose ketose, an epimer of fructose, with a sweetness 70% that of sucrose but only 0.3% of its calories. It does not participate in human glucose metabolism and is known as a "third-generation functional sweetener." Besides its low-calorie characteristics, it also exhibits various physiological functions such as anti-oxidation, anti-hypertensive, anti-hyperlipidemia, anti-inflammatory, improvement of insulin resistance, and anti-atherosclerosis. Therefore, allulose is an ideal alternative to sucrose and is widely used in low-calorie foods, functional beverages, and pharmaceutical excipients.

[0003] Currently, the industrial production of D-allulose mainly uses fructose as a raw material through an isomerization reaction. The subsequent crystallization process mostly adopts the traditional batch cooling crystallization, which has many technical defects: Firstly, commercially available D-aloxose crystals are mostly in powder form, with poor crystal habit and small particle size, which makes filtration and separation difficult, results in high molasses residue, and limits product purity and yield. Furthermore, subsequent drying consumes a lot of energy and packaging and storage costs are high.

[0004] Secondly, the traditional crystallization method for D-allulose is usually intermittent, which has low production efficiency and may lead to inconsistent quality between product batches, as well as increased energy consumption and production costs.

[0005] Third, some crystallization processes add organic solvents to increase yield, which poses environmental and food safety risks. In addition, the processes are complex and have low production efficiency, making it difficult to meet the needs of large-scale industrial production.

[0006] Fourth, existing crystallization processes often fail to achieve efficient recovery and utilization of mother liquor, resulting in raw material waste and low overall yield. At the same time, the synergistic problem of recycling seed retention and crystal habit control in continuous production has not been solved, which limits the industrial application of the process.

[0007] CN112574263A discloses a method for preparing allulose crystals, mainly employing an intermittent combination process of "evaporation crystallization + crystallization + cooling crystallization," which achieves a single crystallization yield of over 55% for allulose. However, this process requires adding seed crystals when the solid content is controlled at 70-75% during the evaporation stage, and the crystallization process requires simultaneous water replenishment and evaporation, making the process cumbersome and energy-intensive. CN116178460A discloses a method for D-allulose crystallization, employing an intermittent pre-crystallization + cooling crystallization mode. During the process, it is necessary to intermittently introduce completely dry gas to prevent crystal agglomeration, and the pre-crystallization time is as long as 50-60 hours, limiting production efficiency. CN115974939A discloses a method for D-allulose crystallization, which requires the addition of ethanol as an auxiliary crystallization medium, posing a risk of organic solvent residue. It also relies on intermittent ultrasonic-assisted mass transfer, and the ethanol recovery issue needs to be considered after crystallization, resulting in high costs and complex process control.

[0008] While the aforementioned patent documents have optimized the D-allulose crystallization process to some extent, they all employ intermittent D-allulose crystallization methods, resulting in low production efficiency and unstable particle size and crystal habit of the obtained D-allulose crystals. The issue of synergistic control between continuous production and crystal habit regulation remains unresolved. Therefore, developing a process that requires no organic solvents, allows for continuous production, and can produce large-particle D-allulose crystals is crucial for addressing the current industry challenges. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a method for continuous crystallization of large-particle D-allulose crystals.

[0010] The technical solution adopted in this invention is: A method for continuous crystallization of large-particle D-allulose crystals employs a production model of "single-stimulation crystallization + constant-temperature crystal growth + cyclic seed retention": First, D-allulose syrup with a purity of over 95% is used for full-tank stimulation crystallization. During the crystallization stage, the temperature is controlled at 45-50℃ and the supersaturation is 1.03-1.1 to complete full-tank stimulation crystallization. The crystals are then grown at a constant temperature of 45-50℃ for 16-24 hours to form a stable seed crystal system. After the crystal growth is completed, 20-30% of the liquid is reserved in the crystallization tank as mother crystals. Fresh high-purity D-allulose syrup is added for cyclic crystal growth, and no further addition of seed crystals is required. The crystal slurry is continuously fed into a four-stage series crystallizer. After stepped gradient cooling crystallization and post-treatment, large-particle D-allulose crystals are obtained. The crystallization mother liquor generated by centrifugation can be purified and concentrated and then re-entered into crystallization.

[0011] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the stirring power is set differently, with 2.2-3.2 kW / m³ during the crystallization stage. 3 To ensure seed dispersion, the crystal growth and crystallization stages require a power output of 1.5-2.5 kW / m³.3 To prevent crystal breakage, supersaturation is strictly controlled throughout the process, and the insulated feed pipe design ensures stable liquid temperature.

[0012] The above-mentioned method for continuous crystallization of large-particle D-allulose crystals has a single crystallization yield of over 50%, an overall yield of ≥80% after mother liquor recycling, a purity of ≥99%, and over 90wt% of crystals with a particle size greater than 250μm, exhibiting regular rod-shaped crystals with smooth surfaces and uniform particle size distribution; no organic solvents are used in the crystallization process.

[0013] Preferably, the method for continuous crystallization of the above-mentioned large-particle D-allulose crystals comprises the following specific steps: (1) Stimulating crystal formation: Add D-allulose syrup to a crystal formation tank (seed buffer tank), control the system temperature at 45-50℃ and the supersaturation at 1.03-1.10, add D-allulose seed crystals with an average particle size of 150-200 mesh, the amount of seed crystals added is 1-3wt% of the syrup mass, and the concentration is 2.2-3.2kW / m 3 Stirring with a frame paddle for 2-3 hours at the stirring power to stimulate crystal formation in the full tank and form seed syrup. During the stirring process, the pressure inside the crystallization tank is at atmospheric pressure, without any additional vacuum or pressurization operations. (2) Constant temperature crystal growth and circulating seed retention: Maintain the temperature inside the crystal growth tank at 45-50℃, and adjust the stirring power to 1.5-2.5kW / m 3 The same frame-type paddle of the same specifications as the crystallization stage is used for constant temperature crystal growth for 16-24 hours. During the cyclic crystal growth, the supersaturation of the system is controlled at 1.05-1.10, and precise control is achieved by periodically detecting the concentration and temperature of the feed liquid. The crystal growth flow field is consistent with the crystallization flow field to avoid crystal breakage or secondary nucleation. After the crystal growth is completed, 70-80% of the crystal slurry is sent to a four-stage series crystallizer. 20-30% of the liquid is reserved in the crystallization tank as seed crystals. Fresh D-allulose syrup (purity 95-99%, concentration 78-88wt%) is added to fill the tank. The constant temperature crystal growth step is repeated to achieve cyclic seed crystal retention. Subsequent crystal crystal growth does not require repeated addition of seed crystals. (3) Gradient cooling crystallization: After crystal growth is completed, the crystal slurry in the crystallizing tank is sent to a four-stage series crystallizer for cooling crystallization, maintaining a constant 1.5-2.5 kW / m throughout the process. 3 The stirring power adopts the same frame-type impeller as the crystallization stage. The stirring flow field of the frame-type impeller is consistent with the crystallization flow field. Each crystallizer is equipped with an independent programmable temperature control system. By periodically detecting the concentration and temperature of the liquid, and combining the D-allulose solubility curve, the cooling rate is adjusted in real time to control the supersaturation of each crystallizer at 1.05-1.15. The four-stage gradient cooling is as follows: cooling to 40-45℃ at 0.1-0.3℃ / h, cooling to 35-40℃ at 0.3-0.5℃ / h, cooling to 25-30℃ at 0.5-0.8℃ / h, and cooling to 15-20℃ at 0.8-1.0℃ / h. (4) Post-processing: The crystal slurry from the fourth-stage crystallizer is successively centrifuged, washed at low temperature and dried in a fluidized bed to obtain large-particle D-allulose crystals; the crystallization mother liquor generated by centrifugation is collected and returned to the separation and purification step of D-allulose syrup. After purification to a purity of ≥95%, it is vacuum evaporated and concentrated to a concentration of 78-88wt% to participate in crystallization again.

[0014] The aforementioned method for continuous crystallization of large-particle D-allulose crystals employs a continuous crystallization system. This system utilizes a seed crystallizer (or seed buffer tank) to stimulate crystal growth, followed by gradient cooling crystallization through a series of primary to secondary cooling crystallizers, and post-processing with a centrifuge and fluidized bed. The entire process is continuous, automated, and allows for material recycling. The seed crystallizer (or seed buffer tank) is equipped with a level sensor, feed pump, discharge pump, and insulation device. Each cooling crystallizer is equipped with an independent programmable temperature control system, a frame-type paddle agitator, and a temperature / concentration detection port. The seed crystallizer (or seed buffer tank) is connected to the primary cooling crystallizer via a feed pipe, and the cooling crystallizers at each stage are connected in series via feed pipes. The discharge port of the tertiary cooling crystallizer is connected to the post-processing unit's centrifuge and fluidized bed. The centrifuge is connected to the mother liquor recovery and purification unit, and the purified and concentrated mother liquor is returned to the seed crystallizer (or seed buffer tank) via a feed pipe. This continuous crystallization system can operate stably for more than 3 months, during which the product crystal purity, particle size, and crystal habit remain stable.

[0015] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the purity of the D-allulose syrup in steps (1) and (2) is 95-99%, preferably >98%.

[0016] Preferably, in the method for continuous crystallization of large-particle D-allulose crystals described above, the total content of miscellaneous sugars and polysaccharides in the D-allulose syrup is less than 5%, preferably less than 2%.

[0017] Preferably, in the method for continuous crystallization of large-particle D-allulose crystals, the D-allulose syrup is obtained by separating, purifying, and concentrating the enzyme conversion solution under vacuum evaporation, with a concentration of 78-88 wt%, preferably 80-86 wt%.

[0018] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the mother liquor is deemed qualified and can be re-crystallized when it has a purity ≥95% and a concentration of 78-88wt% after separation and purification.

[0019] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the full-tank stimulation crystallization is carried out in a crystallization tank, and the supersaturation of the crystallization tank is precisely controlled by periodically detecting the concentration and temperature of the feed liquid.

[0020] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the flow field formed during the crystal growth process is the same as the flow field formed during the stimulation of crystal formation, thereby avoiding crystal breakage or secondary nucleation.

[0021] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the crystallization and continuous production process of D-allulose syrup is carried out under normal pressure without additional vacuum or pressurization operations.

[0022] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the crystallizing tank is equipped with a liquid level sensor linked with the feed pump and the discharge pump to achieve precise control of the feed volume and the reserved bottom liquid volume (20-30%), ensuring that the residence time of the liquid during circulating crystal growth is stably controlled at 16-24h.

[0023] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the stirring flow field during the cooling crystallization process is consistent with that during the crystal growth process.

[0024] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, each crystallizer is equipped with an independent programmable temperature control system. By periodically detecting the concentration and temperature of the feed solution and combining the D-allulose solubility curve, the cooling rate is adjusted in real time to achieve precise control of supersaturation.

[0025] Preferably, in the above-mentioned continuous crystallization method for large-particle D-allulose crystals, the crystal slurry obtained by cooling crystallization is subjected to centrifugation, washing, and drying in sequence. The centrifugation rate is 3000-4000 r / min, and the centrifugation time is 30-60 min. The washing water is sterile deionized water at a temperature of 5-10℃, and the water volume is 0.2-0.5 times the mass of D-allulose. The drying is carried out by fluidized bed drying, with an inlet air temperature of 70-90℃ and an outlet air temperature controlled at 40-50℃. The drying endpoint is set at a crystal moisture content of ≤0.5%, and the drying time is 1-2 h.

[0026] Preferably, in the above-mentioned method for continuous crystallization of large-particle D-allulose crystals, the crystallization mother liquor generated by centrifugation is collected and returned to the upstream separation and purification step for processing until it reaches the qualified purity and concentration indicators before being re-crystallized.

[0027] A method for preparing large-particle D-allulose crystals using the above-described continuous crystallization method.

[0028] Preferably, the above-mentioned large-particle D-allulose crystals have a crystal purity of ≥99%, a single crystallization yield of >50%, an overall yield of ≥80% after recycling the crystallization mother liquor, and the crystals are regular rod-shaped with smooth surfaces and uniform particle size distribution; more than 90 wt% of the large-particle D-allulose crystals have a particle size greater than 250 μm; and no organic solvents are added during the entire crystallization process, making it green and environmentally friendly.

[0029] The beneficial effects of this invention are: The aforementioned method for continuous crystallization of large-particle D-allulose crystals, based on the growth kinetics of D-allulose crystals, achieves synergistic continuous production and precise crystal habit control through a "single-stimulation crystallization + isothermal crystallization + cyclic seed preservation + mother liquor recovery" model. This method produces high-purity, large-particle-size, regular rod-shaped D-allulose crystals. It is a continuous preparation process for large-particle-size, high-purity, and easily filterable D-allulose crystals through "cyclic seed preservation and crystallization + mother liquor recovery and recycling." Simultaneously, the process is simple, highly automated, produces stable product quality with no batch-to-batch variation, has high raw material utilization, requires no organic solvents, has low energy consumption, and controllable costs, making it suitable for large-scale industrial continuous production. The large-particle D-allulose crystals obtained by this method are characterized by high purity, large particle size, and easy filtration, making them suitable as food additives, functional beverage ingredients, low-calorie baked goods ingredients, and pharmaceutical excipients.

[0030] The continuous crystallization system used in the method for continuous crystallization of large-particle D-allulose crystals can operate stably for more than 3 months. The prepared D-allulose crystals have a purity of ≥99%, a yield of >50%, and an overall yield of ≥80% after mother liquor recycling. The crystals are regular rod-shaped with smooth surfaces, and more than 90 wt% of the crystals have a particle size greater than 250 μm. Compared with traditional batch crystallization, the production efficiency is increased by more than 50%. Specifically: 1. Adopting the "single-stimulation crystal formation + constant temperature crystal growth + cyclic seed retention" mode, 20-30% of the liquid is reserved as the mother crystal seed after the first crystal formation. Subsequent replenishment of the liquid for crystal growth does not require repeated addition of the crystal seed. Through 16-24 hours of long-term constant temperature crystal growth and precise control of supersaturation, a stable crystal seed system is formed. This simplifies the operation process, reduces energy consumption, and ensures the consistency of crystal seed concentration and crystal habit, significantly improving production repeatability and product uniformity.

[0031] 2. Through "gradient cooling crystallization in a four-stage series crystallizer", each stage is equipped with an independent programmable temperature control system. Combined with precise control of supersaturation and volume matching design, it adapts to the growth kinetics of D-allulose crystals, avoids secondary nucleation and crystal aggregation, and finally produces regular rod-shaped crystals with a particle size greater than 250μm and a content of more than 90wt%, with stable product quality.

[0032] 3. Achieve efficient recycling of crystallization mother liquor. After purification and concentration to qualified standards, the mother liquor is re-entered into crystallization, increasing the overall yield from over 50% per run to over 80%, significantly improving raw material utilization, reducing material costs, and reducing wastewater discharge, thus meeting green production requirements.

[0033] 4. The entire process uses a standardized frame-type paddle agitator with a consistent flow field to avoid the risk of crystal breakage; no organic solvents are added during the crystallization process, making it green and environmentally friendly, and meeting food safety and environmental protection requirements.

[0034] 5. The post-processing adopts low-temperature washing + fluidized bed drying, which effectively prevents crystals from absorbing moisture and clumping, ensuring that the product moisture content is ≤0.5%; the prepared large-particle crystals are easy to filter and have high bulk density, which greatly reduces the costs of separation, drying, packaging and storage, and enhances the product's market competitiveness; the process has a high degree of automation and can run continuously and stably for more than 3 months without batch differences. The production efficiency is more than 50% higher than that of traditional intermittent crystallization, making it suitable for large-scale industrial production and having significant economic and social benefits. Attached Figure Description

[0035] Figure 1 This is a process flow diagram of the method for continuous crystallization of large-particle D-allulose crystals in the embodiments.

[0036] Figure 2 This is an optical microscope image of D-allulose crystals after crystallization in Example 1.

[0037] Figure 3 This is an optical microscope image of D-allulose crystals after crystallization in Example 2.

[0038] Figure 4 This is an optical microscope image of D-allulose crystals after crystallization in Example 3. Detailed Implementation

[0039] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] Example 1 A method for continuous crystallization of large-particle D-allulose crystals, wherein large-particle D-allulose crystals are continuously crystallized using a continuous crystallization system, such as... Figure 1As shown, the continuous crystallization system utilizes a seed crystallizer (seed buffer tank) to stimulate crystal growth, followed by gradient cooling crystallization through a series of primary to secondary cooling crystallizers, and post-processing with a centrifuge and fluidized bed. The entire process achieves continuous, automated control and material recycling. The seed crystallizer (seed buffer tank) is equipped with a level sensor, feed pump, discharge pump, and insulation device. Each cooling crystallizer is equipped with an independent programmable temperature control system, a frame-type paddle agitator, and a temperature / concentration detection port. The seed crystallizer (seed buffer tank) is connected to the primary cooling crystallizer via a feed pipe, and the cooling crystallizers at each stage are connected in series via feed pipes. The discharge port of the tertiary cooling crystallizer is connected to the post-processing unit's centrifuge and fluidized bed. The centrifuge is connected to the mother liquor recovery and purification unit, and the purified and concentrated mother liquor is returned to the seed crystallizer (seed buffer tank) via a feed pipe. The specific steps include the following: (1) Stimulate crystal formation: Inject 80wt% D-allulose syrup with a purity of 98.5% after upstream separation and purification into a 1m... 3 In a crystallizing tank, 3 wt% D-allulose seed crystals with a particle size of 150-200 mesh were added, and the mixture was stirred at 2.8 kW / m³. 3 At the specified stirring power, a frame-type paddle was used for stirring for 2 hours to complete the full-tank stimulation crystal formation process. (2) Constant temperature crystal growth and circulating seed retention: The seed syrup obtained in step (1) is kept at a temperature of 45℃ and the stirring power is adjusted to 1.5kW / m 3 During the cyclic crystal growth process, the supersaturation was controlled at 1.03-1.1, achieved through precise control by periodically monitoring the concentration and temperature of the feed solution. Crystal growth was carried out at a constant temperature for 16 hours. After crystal growth, 0.8m... 3 Crystallization slurry (80%) is fed into a 1m... 3 The primary crystallizer and crystallizing tank have a 0.2m allowance. 3 Using the base solution (20%) as the seed crystal, add 80wt% fresh syrup (98.5% purity) to a final volume of 1m. 3 Once the container is full, repeat the constant-temperature crystal growth process for 16 hours to form a new batch of seed syrup. This process is then repeated to retain seed syrup. (3) Gradient cooling crystallization: The crystal slurry is fed into a four-stage crystallizer for cooling crystallization, with a stirring power of 1.5 kW / m throughout the process. 3 The supersaturation is maintained at 1.05-1.15; the first-stage crystallizer cools from 45℃ to 40℃ at a rate of 0.1℃ / h, the second-stage at 0.3℃ / h, the third-stage at 0.5℃ / h, and the fourth-stage at 0.8℃ / h; each crystallizer is equipped with an independent programmable temperature control system to adjust the cooling rate in real time. (4) Post-processing: After the material is discharged from the four-stage crystallizer, the crystal slurry is centrifuged (3000 r / min, 60 min), washed with sterile deionized water at 5℃ (using water at 0.2 times the wet crystal weight), and dried in a fluidized bed (70℃ air intake, 40℃ air outlet, 1 h) to obtain D-alokulose crystals; the crystallization mother liquor generated by centrifugation is collected and sent to the mother liquor recovery and purification unit for purification to a purity of 98.2%, vacuum evaporated and concentrated to 80 wt%, and returned to the crystallizing tank to participate in crystallization again.

[0041] Optical microscope images of D-allulose crystals after cooling crystallization are shown below. Figure 2 The D-allulose crystals are large and uniform in size, with a relatively smooth surface, and are generally rod-shaped. The purity of the large-particle D-allulose crystals is 99.8%, the single crystallization yield is 53%, and the overall yield reaches 82% after three cycles of mother liquor circulation. More than 92 wt% of the crystals have a particle size greater than 250 μm and a water content of 0.3%. The entire system was shut down after 4.5 months of stable operation. During the operation, there was no secondary nucleation or crystal agglomeration, and the product quality indicators remained stable.

[0042] Example 2 A method for continuous crystallization of large-particle D-allulose crystals includes the following steps: (1) Stimulating crystal formation: D-allulose syrup with a purity of 96.3% and 83wt% after upstream separation and purification is injected into 1m 3 In a crystallizing tank, 2 wt% D-allulose seed crystals with a particle size of 150-200 mesh were added, and the mixture was heated at 3.2 kW / m³. 3 At the specified stirring power, a frame-type paddle was used for stirring for 2.5 hours to complete the process of stimulating crystal formation in the full tank. (2) Constant temperature crystal growth and circulating seed retention: The seed syrup obtained in step (1) is kept at a temperature of 48℃ and the stirring power is adjusted to 2.0kW / m 3 During the cyclic crystal growth process, the supersaturation was controlled at 1.03-1.1, and the crystal growth was carried out at a constant temperature for 20 hours. After the crystal growth was completed, 0.75m 3 Crystallization slurry (75%) is fed into a 1m... 3 The primary crystallizer and crystallizing tank have a reserved space of 0.25m. 3 Using the base solution (25%) as the seed crystal, add 83wt% (96.3%) fresh syrup to a final volume of 1m. 3 The container is filled and the constant temperature crystal growth process is repeated for 20 hours to achieve cyclic seed retention. (3) Gradient cooling crystallization: The crystal slurry is fed into a four-stage crystallizer for cooling crystallization, with a stirring power of 2.0 kW / m throughout the process. 3The supersaturation was maintained at 1.05-1.15; the temperature of the first-stage crystallizer was reduced from 48℃ to 43℃ at a rate of 0.2℃ / h, the second-stage at 0.4℃ / h, the third-stage at 0.6℃ / h, and the fourth-stage at 0.9℃ / h; the temperature control accuracy of each stage was ±0.1℃. (4) Post-processing: After the material is discharged from the fourth-stage crystallizer, the crystal slurry is centrifuged (3500 r / min, 45 min), washed with sterile deionized water at 8℃ (water volume 0.35 times the wet crystal weight), and dried in a fluidized bed (air inlet at 80℃, air outlet at 45℃, 1.5 h) to obtain D-alokulose crystals; the centrifuged mother liquor is purified and concentrated to a purity of 97.5% and 83 wt%, and then returned to the crystallizing tank to participate in crystallization again.

[0043] Optical microscope images of D-allulose crystals after cooling crystallization are shown below. Figure 3 The D-allulose crystals are large and uniform in size, with a relatively smooth surface, and are generally rod-shaped. The purity of the large-particle D-allulose crystals is 99.5%, the single crystallization yield is 51%, and the overall yield reaches 80% after three cycles of mother liquor circulation. More than 93wt% of the crystals have a particle size greater than 250μm and a water content of 0.15%. The system was shut down after seven months of stable operation, with no production interruption during operation and stable product quality.

[0044] Example 3 A method for continuous crystallization of large-particle D-allulose crystals includes the following steps: (1) Stimulating crystal formation: D-allulose syrup with a purity of 97.1% and 88 wt% after upstream separation and purification was injected into a 1.5m jar. 3 In a crystallizing tank, 1 wt% D-allulose seed crystals with a particle size of 150-200 mesh were added, and the mixture was stirred at 2.8 kW / m³. 3 At the stirring power, a frame paddle was used to stir for 3 hours to complete the full tank stimulation crystal formation; (2) Constant temperature crystal growth and circulating seed retention: The seed syrup obtained in step (1) is kept at a temperature of 50℃ and the stirring power is adjusted to 2.5kW / m 3 During the cyclic crystal growth process, the supersaturation was controlled at 1.03-1.1, and the crystal growth was carried out at a constant temperature for 24 hours. After the crystal growth was completed, 1.05m 3 Crystallization slurry (70%) is fed into a 1.5m... 3 The primary crystallizer and crystallizing tank have a reserved space of 0.45m. 3 The base solution (30%) was used as the seed crystal, and 88wt% (97.1%) fresh sugar syrup was added to a final volume of 1.5m. 3 The container is filled and the constant temperature crystal growth process is repeated for 24 hours to achieve cyclic seed retention; (3) Gradient cooling crystallization: The crystal slurry is fed into a four-stage crystallizer for cooling crystallization, with a stirring power of 2.5 kW / m throughout the process. 3The supersaturation was maintained at 1.05-1.15; the temperature of the first-stage crystallizer was reduced from 50℃ to 45℃ at a rate of 0.3℃ / h, the second-stage crystallizer at 0.5℃ / h, the third-stage crystallizer at 0.8℃ / h, and the fourth-stage crystallizer at 1.0℃ / h, and the fourth-stage crystallizer at 1.0℃ / h, respectively. (4) Post-processing: After the material is discharged from the four-stage crystallizer, the crystal slurry is centrifuged (4000 r / min, 30 min), washed with sterile deionized water at 10℃ (using water at 0.5 times the weight of wet crystals), and dried in a fluidized bed (air inlet at 90℃, air outlet at 50℃, 2 h) to obtain D-alokulose crystals; the centrifuged mother liquor is purified and concentrated to a purity of 98.0% and 86 wt%, and then returned to the crystallizing tank to participate in crystallization again.

[0045] Optical microscope images of D-allulose crystals after cooling crystallization are shown below. Figure 4 The D-allulose crystals are large and uniform in size, with a relatively smooth surface, and are generally rod-shaped. The purity of the large-particle D-allulose crystals was 99.6%, the single crystallization yield was 55%, and the overall yield reached 85% after three mother liquor cycles. More than 95 wt% of the crystals had a particle size greater than 250 μm and a water content of 0.1%. The system was shut down after four months of stable operation. During operation, there was no secondary nucleation or crystal agglomeration, and the overall continuous production was uninterrupted.

[0046] As can be seen from the above embodiments, the continuous crystallization method for large-particle D-allulose crystals described in this invention can achieve continuous production, improve production efficiency, and reduce downtime. By precisely controlling the crystallization conditions, product consistency and high quality can be ensured, and energy consumption and production costs can be significantly reduced. Furthermore, this method is easily automated, reducing manual intervention and thus improving production safety and reliability.

[0047] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for continuous crystallization of large-particle D-allulose crystals, characterized in that: First, D-allulose syrup with a purity of over 95% is used to stimulate crystal formation in a full tank. During the crystal formation stage, the temperature is controlled at 45-50℃ and the supersaturation is 1.03-1.1 to complete the full-tank stimulation crystal formation. The crystals are then kept at a constant temperature of 45-50℃ for 16-24 hours to form a stable seed crystal system. After the crystal formation is completed, 20-30% of the liquid is reserved in the crystal formation tank as the mother crystal. Fresh high-purity D-allulose syrup is added to circulate the crystal formation, and no further addition of seed crystals is required. The crystal slurry is continuously fed into a four-stage series crystallizer, and after stepped gradient cooling crystallization and post-treatment, large-particle D-allulose crystal products are obtained.

2. The method for continuous crystallization of large-particle D-allulose crystals according to claim 1, characterized in that: Differentiated stirring power settings: 2.2-3.2 kW / m³ during the crystallization stage. 3 Crystallization and growth stage: 1.5-2.5 kW / m 3 .

3. The method for continuous crystallization of large-particle D-allulose crystals according to claim 1, characterized in that: The specific steps are as follows: (1) Stimulating crystal formation: Add D-allulose syrup to a crystal formation tank, control the system temperature at 45-50℃ and the supersaturation at 1.03-1.10, add D-allulose seed crystals with an average particle size of 150-200 mesh, the amount of seed crystals added is 1-3wt% of the syrup mass, and the concentration is 2.2-3.2kW / m 3 Stirring with a frame paddle at the stirring power for 2-3 hours to stimulate crystal formation in the full tank and form seed syrup; (2) Constant temperature crystal growth and circulating seed retention: Maintain the temperature inside the crystal growth tank at 45-50℃, and adjust the stirring power to 1.5-2.5kW / m 3 The same frame-type paddle of the same specifications as the crystallization stage is used for isothermal crystal growth for 16-24 hours. During the cyclic crystal growth, the supersaturation of the system is controlled at 1.05-1.10, and the crystal growth flow field is consistent with the crystallization flow field. After the crystal growth is completed, 70-80% of the crystal slurry is sent to a four-stage series crystallizer. 20-30% of the liquid is reserved in the crystallization tank as seed crystals. Fresh D-allulose syrup is added to fill the tank. The isothermal crystal growth steps are repeated to achieve cyclic seed crystal retention. Subsequent crystal crystal growth does not require the addition of seed crystals. (3) Gradient cooling crystallization: After crystal growth is completed, the crystal slurry in the crystallizing tank is sent to a four-stage series crystallizer for cooling crystallization, maintaining a constant 1.5-2.5 kW / m throughout the process. 3 The stirring power adopts the same frame-type impeller as the crystallization stage. The stirring flow field of the frame-type impeller is consistent with the crystallization flow field. Each crystallizer is equipped with an independent programmable temperature control system to control the supersaturation of each crystallizer at 1.05-1.

15. The four-stage gradient cooling is as follows: cooling to 40-45℃ at 0.1-0.3℃ / h, cooling to 35-40℃ at 0.3-0.5℃ / h, cooling to 25-30℃ at 0.5-0.8℃ / h, and cooling to 15-20℃ at 0.8-1.0℃ / h. (4) Post-processing: The crystal slurry from the fourth-stage crystallizer is successively centrifuged, washed at low temperature and dried in a fluidized bed to obtain large-particle D-allulose crystals.

4. The method for continuous crystallization of large-particle D-allulose crystals according to claim 3, characterized in that: The D-allulose syrup is obtained by separating, purifying, and concentrating the enzyme conversion solution under vacuum evaporation, with a concentration of 78-88 wt%.

5. The method for continuous crystallization of large-particle D-allulose crystals according to claim 3, characterized in that: The flow field formed during the crystal growth process is the same as the flow field formed during the stimulation of crystal formation, and the stirring flow field during the cooling crystallization process is consistent with that during the crystal growth process.

6. The method for continuous crystallization of large-particle D-allulose crystals according to claim 3, characterized in that: The crystallization tank is equipped with a liquid level sensor that is linked with the feed pump and discharge pump to achieve precise control of the feed volume and the reserved bottom liquid volume, ensuring that the liquid residence time is stable at 16-24 hours during the circulating crystal growth process.

7. The method for continuous crystallization of large-particle D-allulose crystals according to claim 3, characterized in that: Each crystallizer is equipped with an independent programmable temperature control system, which monitors the concentration and temperature of the liquid at regular intervals and adjusts the cooling rate in real time based on the D-allulose solubility curve to achieve precise control of supersaturation.

8. The method for continuous crystallization of large-particle D-allulose crystals according to claim 3, characterized in that: The crystal slurry obtained from cooling and crystallization is sequentially centrifuged, washed, and dried. The centrifugation rate is 3000-4000 r / min, and the centrifugation time is 30-60 min. The washing water is sterile deionized water at a temperature of 5-10℃, and the water volume is 0.2-0.5 times the mass of D-allulose. The drying is carried out in a fluidized bed with an inlet air temperature of 70-90℃ and an outlet air temperature controlled at 40-50℃. The drying endpoint is defined as a crystal moisture content of ≤0.5%, and the drying time is 1-2 h.

9. A method for preparing large-particle D-allulose crystals by continuous crystallization of large-particle D-allulose crystals as described in any one of claims 1-8.

Images