The basic principle of fluidized bed drying is to use heated air to blow wet particles into a boiling, convective state. The hot air carries away the evaporated moisture or organic solvent, thus drying the wet particles. This involves the issue of air handling.
Currently, many domestic manufacturers configure their air handling units as follows: pre-filter—electric heating (or steam heating)—fan—medium-efficiency filter—fluidized bed dryer—it's that simple. Clearly, this is highly dependent on user requirements; lower user requirements result in lower manufacturer configurations. Here, we will only take the fluidized bed dryer in GEA's granulation line as an example to discuss the relationship between configuration and energy saving. Air handling unit configuration and parameter requirements:
(1) Inlet air temperature and humidity should be adjustable to the required process parameters: t = 80℃, RH = 20%;
(2) Chilled water cooling and dehumidification: copper tubing and aluminum finned coils; chilled water from the process chilled water system, temperature 7–12℃;
(3) Heater heat source: industrial steam; pressure and temperature consumption requirements should be specified;
(4) Filter: (G4+F8+H13) three-stage filtration; H13 requires PAO leak testing and verification; testing and replacement times should be specified;
(5) Enclosure requirements: the inner wall of the medium-high efficiency section should be... Stainless steel plate, with galvanized steel plate for medium and high efficiency sections; the wall panels have heat insulation and cooling protection functions;
(6) The inlet and outlet of cold water and steam are automatically controlled by PLC electric valves or pneumatic valves according to the set temperature and humidity;
(7) G4, F8, and H13 have differential pressure display devices, and the PLC has a differential pressure alarm function (the differential pressure is not displayed on the PLC);
(8) The filter is easy to replace and disassemble;
(9) Equipped with a drain trap for the surface cooler, the water collection tray is made of 304 stainless steel, leak-proof, with smooth drainage and no water accumulation in the collection tray;
(10) The air outlet is equipped with an electric regulating valve, the opening of which can be controlled by the PLC.
These are our requirements for the configuration of the air handling unit (AHU). We believe that many domestic manufacturers can fully meet these requirements. If domestically produced equipment is manufactured according to these requirements, it will definitely reduce the quality risk of pharmaceutical production. Furthermore, with detailed validation documentation, the technical content of the equipment will be further improved.
While meeting GMP requirements, we must also fully consider energy conservation. Energy consumption here involves the defrosting and preheating section, cold water dehumidification, heating section, and maintaining negative pressure within the fluidized bed cylinder. According to the URS, if the defrosting and preheating section is not needed, it can be eliminated; otherwise, it increases investment, airflow resistance, and energy consumption. The cold water dehumidification section and steam heater are automatically controlled by PLC solenoid valves, setting the outlet air temperature and humidity. Conventional fluidized bed drying parameters are d = 11 g/m³ and t = 80℃. The relationship between the fluidized bed's airflow and exhaust volume can be set via PLC through the negative pressure within the cylinder and the automatic adjustment of the inlet and exhaust valves. According to FDA requirements, the three-stage filter in the air conditioning unit is crucial. The main reason for the significant risks associated with domestically produced equipment lies in the filter. Filter selection is very important; the filter specifications must be clearly stated. G4, F8, and H13 must conform to international standards. Using cheap, indiscriminately made non-woven cotton filters will pose a significant quality risk. While standard filters increase airflow resistance, our primary concern is meeting quality requirements.
During fluidized bed operation, the trajectory of the particles inside is closely related to air heat exchange. Currently, air is typically blown up from the bottom, causing the particles to convect. The time the particles remain in the air is the time for moisture to evaporate. GEA's fluidized bed dryer uses fish-scale-shaped air outlets at the bottom, causing the particles to rise in a spiral shape inside the cylinder. This effectively increases the length of the streamlines and the time for heat exchange with the air, making full use of energy.
