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Key words: drying; horizontal multi-chamber fluidized bed; stirred fluidized bed; vibrating fluidized bed; centrifugal fluidized bed; pulsed fluidized bed fluidization technology originated in 1921, the earliest application in drying industrialization production was 1948 The Dole-Olie Solid Fluidizer was established in the United States in the United States and it was not until 1958 that our country began to develop this technology. During the fluidized bed drying process, the bulk material is placed on the orifice plate, and the gas is transported under the lower part so that the material particles are in a suspended state, as if the liquid boils, so that the material particles are in full contact with the gas, and rapid heat transfer and moisture transfer are performed. Fluidized drying has been widely developed and applied because of its advantages of good heat transfer, uniform temperature distribution, various operating modes, adjustable material residence time, low investment cost, and low maintenance workload.
1 Classification of Fluidized Bed Drying Equipment Fluidized bed drying equipment has achieved considerable development and widespread application in less than 100 years after continuous improvement and innovation by researchers. There are many different kinds of fluidized bed, depending on the nature of the material to be dried, the fluidized bed used is also different. According to its structure, it can be roughly divided into: single-layer and multi-layer cylinder type fluidization type, horizontal multi-chamber fluidization type, stirring Fluidization, vibration fluidization, centrifugal fluidization, pulse fluidization and other types.
1.1 The earliest fluidized bed for single-layer and multi-layer cylindrical fluidized bed is a single-layer cylinder. The material is plain carbon steel coated with epoxy phenolic coating. The gas distribution plate is porous sieve. Hole radius 1.5 mm, regular hexagonal arrangement.
The entire drying process is as follows: the wet material is conveyed by the belt conveyor to the material feeder, and then uniformly thrown into the fluidized bed, and is fully contacted with hot air and dried, and the dried material continuously overflows from the overflow. After the air enters the blower and the heater, it enters the bottom of the sieve plate and passes upwards through the sieve plate so that the wet material in the bed fluidizes to form a fluidized layer. The exhaust gas enters the cyclone separator group, removes the entrained fines, and is discharged by the exhauster to the atmosphere. This dryer is easy to operate, has low labor intensity, good working conditions, and long running cycles.
However, due to the small diameter of the single-layer cylinder fluidized bed and the long residence time of the material, the humidity of the resulting product after drying is not uniform. Therefore, a multi-layered fluidized bed has been developed. The fluidized bed not only can improve the efficiency, but more importantly can obtain a more uniform residence distribution time. In order to control the in-diffusion of materials, the multi-layer fluidized bed has successively experienced three stages: overflow pipe type, down pipe type, and flow plate type. The material in the multi-layer fluidized bed has a uniform degree of drying, and the drying quality is easy to control. High thermal efficiency, suitable for drying materials with longer drying stages and materials with high moisture content (moisture content >14%).
1.2 Horizontal Multi-chamber Fluidized Bed Due to the difficulties of multi-layered fluidized bed, large bed resistance and complicated structure, to overcome these shortcomings, a horizontal multi-chamber flow was developed in the late 1960s and early 1970s. Beds. The device is simple in structure and convenient in operation, and is suitable for drying various difficult-to-dry powdery granular materials and heat-sensitive materials. It can be said that the horizontal multi-chamber fluidized bed dryer corresponds to a plurality of square interface fluidized bed series systems.
Its main features:
(1) Baffles are installed between adjacent compartments, so that a uniform and dry product can be obtained, and the distribution of the residence time of the materials is improved;
(2) The cooling and drying of materials can be combined in the same equipment, simplifying the process and equipment;
(3) Due to the division into multiple chambers, the amount of air in each chamber can be adjusted, and the added baffle can prevent the material from being short-circuited.
The equipment is widely used in the pharmaceutical industry. At present, there are dozens of factories in China that use this equipment to dry a variety of tablet-granule medicines, powdery granular materials, and sheet-like materials. If the air volume and the air temperature in each room are adjusted in operation, or the hot air in the last few rooms is used again, or an internal heater or the like is added to the bed, the thermal efficiency can also be improved.
1.3 Stirring Fluidized Bed In order to make certain wet granular materials or materials that have aggregated into clusters also adopt fluidized drying technology, researchers are equipped with stirring blades in the bed near the feeding port, so that the materials in a lump or block form can be played in time. Crushed, in order to facilitate the formation of fluidization, such a fluidized bed equipped with a stirrer is called a stirred fluidized bed.
Its advantages are:
(1) It is suitable for the drying of materials with large moisture content, which are not easily dispersed in hot gas flow, or materials that may be agglomerated;
(2) Channeling, sag and dead bed phenomena can be avoided, a uniform fluidization state can be obtained, and the heat and mass transfer intensity can be improved. In recent years, agitation fluidized bed has been widely used in the pharmaceutical industry, and it is often used as a drying device for the subsequent process of the pharmaceutical process to simplify equipment and processes and reduce costs.
1.4 Vibrating fluidized bed With the development of multi-stage drying, a vibrated fluidized bed (VFB) is used. Its basic structure is similar to that of a normal fluidized bed, which is a kind of mechanical vibration added to the fluidized bed. Improve the product. The material is fluidized by the double action of mechanical vibration and perforated airflow, and moves forward in the form of a piston under the action of vibration. The use of convection, conduction, and radiation to supply heat to the material layer can achieve the purpose of drying.
The vibrating fluidized bed is completed by the vibration of the material. The supplied hot air is only used for heat transfer and mass transfer, so the energy consumption can be significantly reduced. In addition, due to the strong vibration of the bed, the resistance to heat transfer and mass transfer is reduced, the drying rate of the vibrating fluidized bed is increased, and at the same time, a lot of entrained lump or high-dispersion materials are liable to be generated when the fluidization or fluidization is difficult. It can also dry smoothly, which overcomes the problems of back-mixing, channeling, and sticking in the common fluidized bed.
1.5 Centrifugal fluidized bed Centrifugal fluidized bed is a new type of drying equipment for fluidized drying in a centrifugal force field. The principle is to distribute the granular material in the screen covered cylinder under the action of centrifugal force caused by mechanical rotation. On the porous wall, the hot gas flow passes through the porous wall to fluidize and dry it. Due to centrifugal force field, the centrifugal acceleration can be several times to several tens of times of the gravity acceleration. Therefore, compared with the ordinary gravity fluidized bed, the migration process of moisture within the material is strengthened, the drying time is short, and the heat and mass transfer rate is high. It can effectively inhibit the formation of bubbles and the entrainment of materials, and it can effectively dry the low-density, heat-sensitive, and easily-bonded solid materials that are difficult to dry in a gravity fluidized bed.
1.6 Pulsed Fluidized Beds Pulsed fluidized beds have been developed for difficult-to-flow materials and crystalline drugs where the drying temperature does not exceed 50-80 °C. The pulsed fluidized bed changes the constant air supply of the traditional fluidized bed into a periodic air supply. By adjusting the pulse frequency of the air flow or the pulse air flow conductivity, the gas flow or fluidized area passing through the orifice plate periodically changes. Dry the material.
The main structural feature is that several hot air inlet pipes are installed around the dry and wet bottom, and pulse valves are installed on each hot air pipe. They are opened at a certain frequency and order. The opening time is related to the bed thickness and material properties. When the gas is suddenly introduced, a pulse is formed in a short time to make the particles violently fluidized, which promotes a strong heat transfer and mass transfer between the materials. When the valve is closed, the fluidized state of the bed gradually disappears. At rest, a portion of the gas is still passed through the bed so that the next pulse can be effectively transferred in the bed.
Its advantages are: high heat transfer coefficient, short drying time, reduced air consumption, and low power consumption. Pulsed fluidized bed can effectively overcome the common drawbacks of the traditional fluidized bed such as channel flow, dead zone and local overheating, so it can be used to deal with viscous, agglomerating and heat-sensitive materials, such as tetracycline antibiotics.
1.7 Inert Particle Fluidized Bed The inert particle fluidized bed dryer has the characteristics of evaporating, crystallizing, drying and pulverizing the material in the same equipment. In this dryer, glass beads with a diameter of 1 to 2 mm are preliminarily installed, which are fluidized under the action of hot air, and the materials enter the fluidized bed and are quickly crushed and dried under the action of mutual grinding of the glass beads. . At present, such fluidized bed dryers are used less in the pharmaceutical industry.
2 Problems and Solutions of Fluidized Bed Drying Equipment Through the joint efforts of engineers and technicians, the fluidized bed drying equipment in China has made considerable progress in recent years. At present, fluidized bed dryers commonly used in China include built-in heat pipe fluidized bed dryers, ZLG vibrating fluidized bed dryers, and boiling fluidized bed dryers. The structure and performance of the fluidized bed dryer have been significantly improved, and the quality has been continuously improved, but there are still some problems.
2.1 Insufficient thermal energy utilization Fluidized bed drying equipment has the problem of insufficient thermal energy utilization, resulting in unsatisfactory drying efficiency and increased energy consumption. If the contact between the material and the hot air is not tight during the fluidized drying process, there is a problem that the heat exchange is incomplete, not only reducing the drying efficiency but also increasing the energy consumption.
The subsidiary company [1] of German GEA Group has specially designed a sieve plate with more uniform and smooth intake air. The sieve plate can make the heat exchange in the fluidized drying process more fully and rationally and reduce the energy consumption caused by heated air. , can greatly increase economic efficiency. The built-in heat pipe fluidized bed dryers manufactured by Sulzer of Switzerland and Japan's Nara Machinery [2] changed the method in which the heat of the fluidized bed was always brought in by the hot air. It immersed the tube heat exchanger in the flow. In the desiccating material, the heat required for dehydration of the material is provided by the borehole heat exchanger and the hot air for fluidization, respectively, and the small amount of air is required to meet the requirements of fluidized drying of the material, because part of the heat comes from the conduction heat transfer of the heat medium in the tube. Therefore, the thermal efficiency is greatly improved, and the heat medium is generally steam, heat transfer oil, and the like.
In addition, we can use new heating technology or new energy sources to increase the thermal efficiency of the equipment. For example, heat exchange tubes are embedded in the bottom of the bed, latent heat of the liquid is used for spray heat exchange, and microwave heating is used to further increase the thermal efficiency of the fluidized bed drying equipment, thereby reducing energy consumption and achieving economical and environmental protection.
2.2 On-line monitoring of the moisture content of dried materials There is also the problem of the inability to monitor the moisture content of the dried material in the fluidized bed drying process. If the moisture content of the material can be detected online during the drying process, the parameters can be adjusted according to the actual conditions to improve the drying efficiency. .
K. Seppälä [3] et al. designed a device for on-line detection of the moisture content of a fluidized bed during drying. The device uses a double cuvette system to measure the moisture content of the material by measuring the conductivity of the material, leaving the entire drying process at Can control the status.
In addition, there are reports of on-line moisture detection of dry materials using techniques such as weight change and infrared scanning [4]. These methods allow the entire drying process of the fluidized bed to be timely controlled.
2.3 Differences in Drying Results There are common problems in the fluidized bed drying that have large differences in drying results:
(1) There is a difference in the drying results of the same material and different feeding amounts;
(2) There are major differences in the drying results of different types of materials and the same amount of feed.
Therefore, for these two problems, the structure of the fluidized bed is generally improved to make the design more reasonable, thereby increasing the types of dry materials, and can improve the drying efficiency and reduce the difference in the drying results.
The German GEA Group [5] has designed two-layer cylindrical fluidized bed dryers suitable for smaller amounts of dry materials and four layers suitable for medium-scale dry volumes for problems with the same material and different drying rates. Tandem fluidized bed dryers and rectangular three-layer fluidized bed dryers suitable for large-scale drying. For different types of materials and different drying results with the same feed amount, a fluidized dryer specially designed for drying and easy to agglomerate solid materials and a fluidized dryer suitable for drying liquid materials are designed.
The German company Glatt [6] designed intermittent fluidized bed and continuous fluidized bed for different materials with different drying rates. The intermittent production capacity was 2 kg/h to 1.5 t/h. The continuous production capacity is 20 kg/h to 5 t/h. This design can effectively solve the problem of different drying results of the same kind of material and different feed quantities.
2.4 Dry stability of special materials Heat sensitive materials, oxygen sensitive materials, and solvent-containing flammable materials require high drying conditions. Irrational drying conditions can lead to degradation and deterioration of the dried materials. The fixed fluidized bed dryer designed by Bepex (USA) [7] is a relatively new fluidized drying equipment suitable for the drying of heat-sensitive materials, oxygen-sensitive materials, and solvent-containing flammable materials. The fluidized bed dryer has multiple zones that can be heated and cooled in the same device. Each zone can independently control the temperature, dew point, and fluidization rate by adjusting the cofferdam height in each zone. The residence time of the material in the drier can differ by up to 4 times. Due to the lower vapor pressure, the heat-sensitive material can be dried at a temperature lower than the boiling point, and an indirect heat transfer surface is added to improve the drying ability.
In addition, the plant uses a closed circulating fluidized bed system capable of recovering inert gas, which can handle oxygen-sensitive materials and solvent-containing flammable materials.
2.5 Part of the cooling system Fluidized bed drying equipment lacks a cooling system, so the cooling process is generally longer. At present, researchers and manufacturers both at home and abroad are considering designing and adopting more reasonable and efficient cooling devices when designing and manufacturing fluidized beds, so that the dried materials can be quickly cooled and cooled to simplify the production process, or directly The purpose of the package.
3 Looking forward, the fluidized-bed drying equipment produced by foreign companies adopts advanced technology to ensure better production quality. Because of the late start in China, the fluidized-bed drying equipment has a certain degree of comparison with the international advanced level. The gap, but with the development of China's pharmaceutical industry, based on the study of advanced foreign technology and practical experience, select specific breakthrough points, increase the intensity of independent innovation, we can certainly narrow the gap with the international advanced level.
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Research on the Progress of Fluidized Bed Drying Equipment
Abstract : Fluidized bed dryers are widely used in chemical, food, ceramic, pharmaceutical and other industries because of their high thermal mass transfer rate, compact structure, and ease of operation. The types and prevalence of fluidized bed drying equipment are widespread. A brief review of some of the issues and solutions.