1 Fluidized bed granulation process
Fluidized spray granulation is a method of granulation technology, which is to atomize the melt or solution or suspension into mist droplets, and spray them on the basically solidified or dry fluidized moving particles (or core particles). The molten material, solution, or suspension coated on the surface will solidify or dry after a proper time, and the particles will grow into larger nuclei. Repeat this several times, and finally form a granular solid material with a certain size range and discharge as the finished product. Granulation is the processing process of particles and is almost related to all solid preparations. The process of coating growth and granulation is shown in Figure 1~ Figure 3 and illustrated by the legend.
2 Working principle of fluidized granulation
There are mainly two types of airflow nozzles and pressure nozzles. Among them, the pressure
Its working principle is to suspend the powder with air flow, even if the powder is fluidized, and then spray the binder to make the powder coagulate into particles. Since the temperature of the air flow can be adjusted, the mixing, granulating, drying and other operations can be completed in one device. The fluidized granulator is generally composed of air preheater, compressor, blower, fluidization chamber, bag filter, etc. As shown in Figure 4: The fluidization chamber is mostly inverted cone to eliminate the flow “dead zone”. The gas distributor is usually a porous inverted cone covered with a 60-100 mesh stainless steel screen. The upper part of the fluidization chamber is equipped with a bag filter and a recoil device or a vibration device to prevent the bag filter from clogging.
When working, the filtered and purified air is sent to the air preheater by the blower, and after it is preheated to the specified temperature, it enters the fluidization chamber from the lower part through the gas distributor and the secondary jet air inlet to fluidize the material. Subsequently, the adhesive is sprayed into the fluidization chamber, and the fluidization and mixing are continued for a few minutes, and then the material can be discharged. The hot and humid air is discharged after the powder is removed by the bag filter.
The granules produced by the fluidized granulation mechanism are mostly 30-80 meshes, and the granules are relatively round in shape and have better fluidity during tableting. These advantages are very beneficial to improving the quality of tablets. Because the fluidized granulator can complete a variety of operations and simplify the process and equipment, it has high production efficiency, large production capacity, and is easy to realize automation. It is suitable for the granulation of moisture or heat sensitive materials. The disadvantage is that the power consumption is relatively large. In addition, the material density cannot be too different, otherwise it will be difficult to fluidize granulation.
Sometimes the sprayed molten material, solution, or suspension droplets have solidified or dried without touching other moving particles, forming new core particles. According to the balance of the number of particles, the particle growth rate can be obtained to determine the required residence time and the height of the dense phase zone of the fluidized bed. Atomized outfit
Type nozzles are used less.
3 Classification and structure of fluidized granulation
The bed that makes the particles fluidize is generally composed of a fluidized bed and a spouted bed. When the coating is to improve the performance of the original particle material, the sprayed material and the coated particle are not the same material. Fluidized spray granulation equipment can be divided into spouted bed spray granulator and fluidized bed particle coater.
3.1 Fluidized bed spray granulator
The structure of the fluidized bed spray granulator is shown in Figure 4.
3.1.1 Working principle and advantages and disadvantages
Working principle: Fluidized bed spraying granulation is to add fine particles as seed crystals in the gas-solid fluidized bed, and then hot air to fluidize the fine particles. When the bed reaches a certain temperature, the material is liquidized with an atomizer. Spray continuously and evenly into the fluidized bed. The liquid and solid are combined (coated or agglomerated), dried by hot air and taken away by the evaporated water. The solid particles continue to grow, and when the particles grow to the specified size The product is discharged from the fluidized bed at the size.
Compared with other granulation methods, fluidized bed spray granulation has unique advantages:
(1) The mixing, granulating (coating or agglomeration), drying, cooling and other operations can be realized in the same equipment. The liquid material is made into a dust-free particle product in one step, and the average particle size is generally 0.3 ~ 3 mm
(There are also larger particles, such as fluidized bed spray granulation of urea, the particle size can reach 2 ~ 8 mm).
(2) Due to the violent agitation and mixing of gas and particles in the bed,
The bed temperature is uniform, the gas-solid contact area is large, and the heat and mass transfer rate is high.
(3) The process is simple, the equipment is compact, and the production capacity is large.
The fluidized bed spray granulation also has some shortcomings:
(1) The gas flow distribution in the bed is not uniform enough, and many gases pass through the bed in the form of bubbles, and the entrainment phenomenon is serious if the operation is not good.
(2) Due to the violent collision between the particles, the particles will be broken,
Produce fine powder, so the efficiency of its fine powder recovery equipment is required to be high.
(3) Once the amount of spraying is too much, it is easy to cause large-scale agglomeration of particles and particles, and the fluidization state will deteriorate sharply, resulting in gas short circuit and dead bed.
The fluidized bed spray granulation process is an organic combination of fluidization technology, spray technology and drying technology. It has been widely used in the batch operation of the pharmaceutical industry and the continuous operation of the chemical industry.
3.1.2 Category
It can be divided into:
(1) Conical fluidized bed spray granulation dryer The fluidized bed spray granulation dryer is generally a conical bed, with a gas distribution plate at the bottom of the cone. The distribution plate is generally a perforated sieve plate, at a certain height from the gas distribution plate. There are one or more nozzles for material liquid. Multiple nozzles are usually evenly distributed on the same bed section to make the material liquid evenly atomized in the bed.
(2) Stirred fluidized bed spray granulation dryer In order to overcome the shortcomings of ordinary fluidized bed spray granulation, a stirred fluidized bed spray granulation dryer appeared. This is a kind of granulation device commonly used in China, and the nozzle can be sprayed from side to side or from top to bottom. This is an intermittently operated granulation device, which is mostly used in the pharmaceutical industry. Compared with ordinary fluidized bed spray granulation, the advantages of stirred fluidized bed spray granulation are as follows.
1) The agitator can break the air bubbles and make the fluidization process more stable.
Reduce the bursting degree of bubbles on the surface of the bed, thereby reducing the pinch
band. As the bubbles are broken, the contact area of the gas-solid two-phase is increased, and the heat and mass transfer efficiency is improved.
2) Stirring makes the material liquid distribute more evenly on the surface of the particles, and the heat and mass transfer area is increased.
3) Even if the amount of spray is too much, the bed will not be dead immediately, because the agitator can break the agglomerated material, remove the agglomeration dead zone and force flu
(3) Fluidized bed spray granulation dryer with annular air distributor The hot air of this dryer is sprayed toward the wall of the vessel.
(4) Fluidized bed spray granulation dryer with grading air The grading air is introduced into the central discharge pipe, and the fine particles can be blown into the fluidized bed, and the coarse particles are discharged as the product, which is set in the screw discharger. The fluidized-bed spray granulation dryer of the classification device is in the screw discharger. The classification device can return the fine particles to the bed, and the coarse particles are discharged out of the dryer as a product.
3.1.3 Mechanism
Mechanism: The melt or solution is sprayed into the dense phase surface by the airflow nozzle installed on the side wall of the conical bed and buried in the dense phase zone. The sprayed mist is coated on the surface of the moving particles, and the particles grow up after being solidified or dried. The conical bed makes the fluidized particles flow with the center upwards and the four walls downwards, so that the cyclic spraying grows. Granules that have grown to a certain size and solidified or dried are discharged by the grading airflow of the center tube on the plane of the distribution plate. Granules with qualified granularity are used as finished products, and the oversized granules (that is, the oversize particles) are melted or dissolved. use. The gas entrained with fine powder is discharged from the top of the vessel, and the entrained fine powder particles are returned to the bed as nuclei through a filter tube or cyclone installed inside or outside the vessel. The air used for cooling and solidification or evaporation and drying is introduced from the distribution chamber under the distribution plate after being cooled by a cooler or heated by a heater. The subdivided exhaust gas is recovered and discharged into the atmosphere by the exhaust fan. The powder spray adhesive coating granulation device is shown in Figure 5.
What is sprayed is the binding fluid, which binds the powder on the surface of the particles to form a coating.
Fluidized bed spray granulation has a longer residence time than spray granulation. Each particle can remove more cooling and solidification heat or evaporate more water, and the finished particles obtained are also larger (the particle size is 0.5 ~ 5
mm), so that the height of the equipment can be reduced, generally 3 ~ 5 times the diameter,
Less than 10 m. Used for granulation of urea, ammonium nitrate and compound fertilizers, raw materials
The production capacity can reach 50 t/h.
3.2 Spouted bed spray granulator
The structure of the spouted bed spray granulator is shown in Figure 6:
The jet gas velocity near the jet nozzle is very large, even large particles can be jetted upwards, and large particles will not form a dead bed. The circulating flow of particles is also upward in the center and downward in the peripheral wall. The finished particle discharge outlet is in the middle of the side wall of the container, near the dense phase surface, which is convenient for the larger particles to roll off Unload. The discharging pipe can still introduce the grading airflow to avoid the mixing and discharging of fine nuclei. The process is basically the same as fluidized bed spray granulation.
The spouted bed spray granulator can be used for the granulation of chemical fertilizers and fertilizers, especially when larger particle size products are required, it is better to use spouted bed spraying.
3.3 Fluidized bed particle applicator
The coating of larger particles can be carried out in a rotating drum or a rotating disk rolling granulator. When smaller particles need to be coated, it is often carried out in a specially designed fluidized bed. The core of the fluidized bed particle applicator is the mist of the membrane liquid
Figure 6 Spout bed coating granulator
1 －Heater 2 －Nozzle 3 －Large sieve pulverizer
In general, there are three kinds of installation methods: top, side tangential, and bottom.
The installation position is shown in Figure 7.
The nozzle is usually a pressure nozzle. With the different installation positions of the nozzles, the structure of the fluidized bed is also quite different.
3.3.1 Top nozzle
Most of them are used in a conical fluidized bed. The particles flow upward in the center of the container, and after receiving the sprayed liquid from the top, they fall down to the surroundings, and are cooled and solidified by the fluidized gas or evaporated to dryness.
3.3.2 Side tangential nozzle
The rotating disc is placed flat at the bottom, with a cone protruding in the middle, fluidizing gas is introduced into the annular gap between the chassis and the wall of the vessel, and the particles receive atomized droplets from the tangential nozzle, and rotate upward along the wall to the direction near the dense phase surface. When it descends and hits the cone of the bottom plate, it is forced to flow outward again. When it rotates upward along the wall, it is cooled and solidified by the fluidized gas introduced in the annular gap or evaporated and dried to form a film layer.
The side-jet fluidized bed adopts rotating fluidized bed technology, which is used for pill manufacturing and powder coating in the pharmaceutical industry.
3.3.2.1 Side jet fluidized bed treatment process
The main difference between a centrifugal fluidized bed and a conventional fluidized bed is a rotating disk with adjustable speed, which has a narrow slit with the bed.
The material is put into the bed, and is subjected to three forces: centrifugal force, air lifting force and gravity, moving in a spiral rope strand shape. As far as a single particle is concerned, its trajectory is basically constant. The atomizer is set on the wall of the bed and sprays in the same direction along the direction of rotation.
3.3.2.2 Pill making
Figure 7 Fluidized bed film applicator structure
1-Nozzle 2-Fluidized bed dense phase 3-Fluidized bed dilute phase 4-Air flow 5-Annular gap 6-Rotating disc (adjustable height)
7-Air distribution plate 8-Atomized coating material 9-Coating area 10-Particle flow direction
3.3.3.2 Granule (pill) coating
The material rotation, revolution and mutual friction generated by the material in the centrifuge bed can produce pills with high sphericity and smooth surface.
The factors that affect the sphericity of the pill are: spray liquid flow, air volume, rotor speed, fog size and powder supply speed. For a certain pill, these parameters must be matched to obtain a good repeatability process. In addition to equipment parameters, proper formulations are necessary for high-quality pills.
3.3.3 Bottom nozzle
Most of them are used in the guide tube type fluidized bed. The particles are at the bottom of the guide tube and receive the sprayed liquid foam at the bottom. The fluidized gas moves upward in the guide tube, and when it reaches the top of the tube, it goes outwards, and from the guide tube to the top of the tube. Falling in the annular space between the walls, the fluidizing gas is cooled and solidified or evaporated to dry in the co-current or counter-current flow of the fluidized gas during the upward and downward processes of the inside of the cylinder. The opening rate in the projection area of the guide tube is larger, and the opening rate outside the region is smaller, so that the air flow velocity in the guide tube is high, and the particles in the tube are guaranteed to flow upward and stable particle circulation flow.
The bottom jet fluidized bed, because the ascending zone is formed in the center of the fluidized bed, and the annulus forms the descending zone, resembling a fountain, changing the random fluidization into a regular flow, therefore, it is used for coating operations in industry.
3.3.3.1 Powder coating
The bottom jet fluidized bed combines fluidization and air transport to form a central rising zone and a circumferential fluidized zone. Because the gas velocity in the rising zone is large, the powder is in a highly dispersed state and will not stick. Therefore, use The bottom jet fluidized bed can be used for powder coating.
The bottom jet fluidized bed achieves the “fountain” effect is the prerequisite for the coating. For powder, the fluidity of the material, the size of the powder, and the thermophysical properties should be considered, and the air volume ratio of the rising zone to the fluidization zone can be reasonably matched. Good completion of the coating operation.
The coating of granules or pills is mainly done by bottom jet fluidized bed or side jet fluidized bed. The particles used for coating should meet the basic conditions: certain strength, small surface area, few micropores and compact surface. Therefore, the granules and pellets that need to be coated should be prepared by using a rotating fluidized bed for pellets and pellets.
The fluidized bed particle applicator has a wide range of applications. The particle size of the pharmaceutical coated particles is not too large. The film-forming substance can be melted at a not too high temperature or can be formulated into a solution. The fluidized bed particle applicator can be used. .
4 The main factors affecting the quality of finished products by fluidized granulation equipment
In summary of the above three different fluidized granulation equipment, according to the working principle and structural characteristics, it is concluded that the main factors affecting the quality of the fluidized granulation equipment are the physical state of the airflow, which is mainly affected by the sieve plate and the spray gun. The four points of the fluidized bed itself, the bed properties, temperature, the physical properties of the particles and the membrane liquid, are simply: airflow, bed properties, temperature, and material properties.
4.1 Airflow
Without a bed, there is no fluidized bed. Therefore, the main role of the gas flow forming the bed is self-evident. At the same time, it also plays a major role in the growth of particles, that is, the heat and mass transfer process. During the fluidization process, the air flow must be uniform and the flow rate must be appropriate. Therefore, the main factors affecting the physical properties of the airflow are the sieve plate and the spray gun.
4.1.1 Sieve plate
Also called a gas distribution plate, it is used to support the material on the one hand, and on the other hand to evenly distribute the airflow, resulting in good initial fluidization conditions and inhibiting the instability of the polymer fluidized bed. If the air distribution is uneven and the local flow velocity of the bed is too high, it will cause the porosity of the bed to increase, reduce the bed resistance at this place, and further increase the wind speed at this place, and finally cause serious channeling and damage Bed-level operation stability.
The sieve plate is equivalent to many parallel pipelines. To make the air flow evenly distributed, the pressure loss at both ends of each channel must be the same. But in fact, there are many factors that make them unequal, mainly: the dynamic pressure head of the inlet air flow under the sieve plate is not equal everywhere, and the flow velocity at the gas inlet is higher, and the dynamic pressure head is larger, so the central part of the sieve plate The air velocity of the small holes is higher; the violent fluctuation of the bed makes the height of the particle column at each point of the sieve plate different, and the void ratio is different, so that the static pressure head of each point on the sieve plate is also different. Therefore, in order to make the air flow distribution tend to be uniform, the pressure drop of the fluid through the pores must greatly exceed the deviation caused by the above factors, so that the latter can be ignored, so that the flow velocity of the pores is basically the same. According to experiments, the pressure drop of the distribution plate is generally taken as 10%-40% of the bed pressure drop. When there is a stirring device in the bed, a small value can be used.
The resistance of the gas through the pores depends on the ratio of the total area of the sieve pores to the area of the sieve plate, that is, the opening rate of the sieve plate. Tests have shown that for general fluidized bed dryers, the larger the opening rate, the worse the fluidization quality; reducing the opening rate can improve the fluidization quality. However, if the opening rate is too small, the resistance will be too large and the power consumption will increase. Generally, the opening rate of the distribution plate of the fluidized bed dryer is 3%~8%, and the lower limit is often used for low fluidization speed, that is, to dry materials with fine particles and low density. The aperture is usually 1.5~2.5 mm, and sometimes the aperture can reach 5 mm.