In the production process of solid dosage forms, fluidized bed dryers are frequently chosen equipment. Fluidized bed dryers offer advantages such as excellent heat transfer, high production capacity, uniform temperature distribution, diverse operation modes, adjustable material residence time, low investment costs, and minimal maintenance.
After more than 30 years of use and improvement in China, they have demonstrated a unique position in the drying field, and their important role is increasingly evident in pharmaceuticals, chemicals, and food industries.
1. Working Principle, Process, and Characteristics of Fluidized Bed Dryers
1.1 Working Principle Fluidized bed dryers, also known as fluidized bed drying, utilize filtered clean air. Through convective heat exchange in a heat exchanger, the air temperature rises to a certain value before entering the main air distribution duct. The air is then distributed by valves into the fluidized bed dryer, while the wet material enters from the feeder. Due to air pressure, the material enters a boiling state within the dryer, ensuring sufficient contact between the hot air and the material, enhancing the heat and mass transfer process, and promoting the evaporation and separation of moisture in the material within a short time. After drying, the material is discharged from the discharge port, and the exhaust gas is discharged from the top of the fluidized bed. Solid powder is recovered by a cyclone dust collector and a bag filter before being discharged into the atmosphere.
1.2 Work Flow: The material is transported to the fluidized bed via a material cart and sealed to the bed by a sealing ring under the action of a cylinder lifting mechanism. Then, air, driven by an induced draft fan, is purified by a filter, heated by a radiator, and then distributed into the fluidized bed (drying chamber) via an airflow distribution plate (screen). The material in the hopper forms a fluidized state (i.e., fluidized bed) under the action of hot air and stirring. In the large-area gas-solid two-phase contact, the moisture (or solvent) inside the material evaporates in a short time and is carried away with the exhaust air, thus drying the material.
1.3 Technical Features (1) Excellent heat transfer effect, relatively uniform temperature within the bed, high heat capacity coefficient (or volumetric heat transfer coefficient), and large production capacity; (2) Due to the uniform temperature distribution within the fluidized bed, any local overheating of the product can be avoided, making it particularly suitable for drying certain heat-sensitive materials (such as konjac, polyacrylamide, etc.); (3) Continuous or intermittent operation can be performed within the same equipment; (4) The residence time of the material in the dryer can be adjusted as needed, resulting in stable product moisture content; (5) Independent electrical cabinet and PLC human-machine interface control, integrating all drying parameter settings, ensuring safe and convenient operation; (6) Fewer mechanical transmission components in the drying device, resulting in low equipment investment costs and minimal maintenance workload.
2. Suggestions for Improving Fluidized Bed Dryers After long-term application and development, fluidized bed dryers have seen significant improvements in structure and performance, and their quality is constantly improving. However, some problems still exist. Based on production practice, the following suggestions for improvement are proposed:
2.1 Suggestions for Improving Insufficient Heat Utilization Fluidized bed dryers are, in essence, air convection drying equipment. Compared with conduction drying equipment, their energy consumption is indeed higher. However, with certain measures, significant energy savings can be achieved. Suggestions: (1) Enhance the sealing effect of the equipment. Currently, most fluidized bed dryer hoppers are connected to the equipment body using flat flanges, resulting in poor sealing. It is recommended to use raised face flanges in the design. Many imported pump dryers use steel pipes wound with fins for heat exchange. While steel pipes can save material costs, their heat exchange effect is poor. It is recommended to use copper pipes instead. (2) Increase insulation measures. Add an insulation layer to the outer shell of the heat exchanger to reduce heat loss. 2.2 Suggestions for Improving the Dust Collection Device
The basic condition for successful fluidized bed operation is that the material has a good fluidization state, which is maintained by a high-efficiency filter dust collector. The dust removal efficiency of the filter dust collector largely determines the fluidization effect. Currently, the main dust removal methods are bag shaking dust collection and pulse jet dust collection.
Suggestion: Use clamp connections for the filter bags, select rigid materials that are not easily deformed for the suspension rods, and regularly inspect and replace the filter bags.
2.3 Suggestions for Improving the Airflow Distribution Plate (Screen)
The airflow distribution plate in the fluidized bed dryer has two functions: supporting the material layer and ensuring uniform gas distribution. The size, shape, distribution pattern, and orifice ratio of the distribution plate openings all have a crucial impact on the fluid distribution. Uneven gas distribution can cause "circulation" within the fluidized bed. In extreme cases, this can lead to "channeling" in some areas while other areas remain stagnant. In this situation, most of the gas short-circuits through certain channels in the bed, significantly worsening gas-solid contact – a situation that should be avoided. A well-designed distribution plate should suppress unevenness within the bed. That is, when pressure drop decreases and airflow velocity increases in certain areas of the bed, the resistance generated by the distribution plate should be able to suppress the increase in airflow, thereby preventing the deterioration of fluidization.
Currently, most fluidized bed dryers use a single type of airflow distribution plate, often a vertical perforated plate or a woven mesh plate. This easily leads to uneven fluidization or dead zones during material fluidization, failing to ensure the uniformity of the drug within the particles. Furthermore, the single perforation design cannot meet the production process requirements of different drugs. On the other hand, to reduce drug leakage, multi-layer mesh structures are commonly used. The airflow distribution plate and fluidized bed are often fixed with numerous bolts, making disassembly inconvenient, cleaning difficult, and prone to residue buildup leading to cross-contamination. Recommendation: Utilize computer-aided fluid dynamics models and heat and mass transfer models to perform aerodynamic and thermodynamic simulation calculations and verifications on parameters such as hole spacing, hole diameter, and open area ratio during the design of the airflow distribution plate, to meet the production process requirements of different materials. Regarding installation, the connection method should be detachable to ensure quick installation and thorough cleaning.
2.4 Recommendations for Improving Intake Air Treatment
Hot air intakes are generally located in the auxiliary equipment room, installed together with heating devices and silencers. The auxiliary equipment room and the clean area do not have direct doors or windows. The air cleanliness level in the auxiliary equipment room is often relatively low, which will affect the quality of pharmaceutical hot air. This requires the equipment itself to have a good purification device; otherwise, unpurified air will contaminate the medicines, making it difficult to meet GMP requirements.
Currently, many domestic equipment systems configure their air handling units as follows: pre-filter—medium-efficiency filter—steam heating (or electric heating)—(sub)high-efficiency filter. Although the air handling system is equipped with pre-filters, medium-filters, and high-efficiency filters, the high-efficiency filters may become clogged or damaged over time. Currently, the need for replacement can only be determined visually, lacking theoretical basis. Premature replacement increases costs, while delayed replacement risks deteriorating air quality, thus affecting product quality. Recommendation: Add differential pressure displays before and after the high-efficiency filters, triggering an alarm to prompt replacement when the differential pressure reaches a certain value.
Furthermore, most equipment lacks dehumidification devices, resulting in persistent air dehumidification issues, especially in late spring and summer when humidity is high. Failure to dehumidify can significantly impact material drying. Recommendation: Add dehumidification devices.
Many devices lack a coordinated system between the induced draft fan and the damper, potentially causing air backflow between fan shutdown and damper closure. Recommendation: Link the fan's start/stop to the damper's opening and closing. The damper should open simultaneously when the fan starts and close synchronously when the fan stops to prevent air backflow. 2.5 Suggestions for Improving the Integration of Equipment and Production Process
An unreasonable drying process flow and equipment design can lead to significant energy losses. To thoroughly solve these problems, a systematic study of the product's drying characteristics is necessary to determine the optimal drying process parameters, such as studying the properties of the material being dried. The properties of the material itself are the most important factor affecting drying; the material's shape, size, packing thickness, moisture binding method, and chemical properties all affect the drying rate. Except for a few domestic companies, most equipment manufacturers lack understanding of formulation process technology and the necessary conditions for conducting process experiments. Their understanding of the usage conditions of various materials is also insufficient, resulting in insufficient research and development and difficulty in developing new products.
2.6 Suggestions for Improving the Control System
Currently, the operating parameters of fluidized bed equipment are generally set based on operator experience. However, it is entirely possible to achieve intelligent control and traceability of process parameters. This places higher demands on the electrical control system of fluidized bed equipment. In electrical control systems, a series of devices are needed to detect temperature, humidity, pressure, differential pressure, wind speed, operating time, dust concentration, etc., and obtain basic data. This data is then transmitted and stored on a touchscreen via transmitters. The touchscreen stores and analyzes the data, and then formulates a suitable process route to achieve intelligent control.
2.6.1 Temperature Control
Common hot air heating control methods use a simple "on" and "off" mode. When the temperature reaches the set value, steam supply stops, but the heat exchanger still has residual heat, causing the air temperature to continue to rise, and vice versa. This results in excessive temperature fluctuations, affecting the drying quality of the equipment. Recommendation: Maintain the inlet air temperature by controlling the steam flow rate. Initially, the steam flow rate should be higher to quickly bring the inlet air temperature close to the set value. Then, the steam flow rate should be automatically adjusted to gradually approach the set value, and finally, a stable steam flow rate should be maintained to keep the inlet air temperature stable. 2.6.2 Airflow Control
Most airflow control equipment uses frequency conversion speed regulation, but lacks airflow measurement elements. During production, airflow can only be manually adjusted based on the fluidization state of the material, thus failing to guarantee stable and relatively constant airflow. Changes in material composition and filter bag resistance can affect airflow stability, which in turn affects drying speed. Recommendation: Install airflow measurement elements in the air inlet duct for automatic control, automatically adjusting the frequency based on airflow volume to maintain a relatively constant airflow during production.
2.6.3 Online Humidity Detection
Add an online humidity detection device. This allows users to adjust parameters according to actual conditions, improving drying efficiency.
2.6.4 Repeatability and Traceability of Fluidized Bed Drying Process
In actual production, operators must reset and modify equipment process parameters for each production run. This makes it impossible to guarantee that the same product is produced using the same equipment process parameters, thus compromising traceability. According to GMP, equipment is required to store a certain amount of production process parameters to ensure production repeatability and traceability. Each user sets this according to the number of product varieties. Fluidized bed dryers generally require the ability to store 50 production processes, but most domestically produced equipment currently cannot achieve this. It is recommended to improve and expand the PLC control system and mechanical actuators to make the functions more complete. For example, sufficient memory should be provided to store multiple production processes, offering on-site parameter printing, data saving, and data connection to a PC.
3. Conclusion
This article starts with the working principle of fluidized bed dryers, summarizes some problems in the production process based on process operation parameters, and briefly proposes suggestions for improving this type of equipment. It is hoped that equipment manufacturers can develop more pharmaceutical drying equipment that meets the process requirements of pharmaceutical production, has advanced performance parameters, is highly operable, environmentally friendly, energy-saving, and has advanced economic and technical indicators.