Boiling dryers are often the equipment of choice in the production of solid formulations. Boiling drying has the advantages of good heat transfer effect, large production capacity, uniform temperature distribution, various operation modes, adjustable material residence time, low investment cost and small maintenance workload. After more than 30 years of use and improvement in China, it has shown its unique position in the field of drying, and it is now increasingly playing an important role in pharmaceutical, chemical, food and other aspects.

1. The working principle of boiling dryer is boiling drying, also known as fluidized bed drying. It uses the filtered clean air and the convective heat transfer through the heat exchanger to raise the air temperature to a certain value and enter the main engine air duct. Distribute into the boiler and the wet material enters the dryer from the feeder. Due to the wind pressure, the material forms a boiling state in the dryer, so that the hot air is in full contact with the material, thereby enhancing the heat and mass transfer process, and promoting the evaporation and separation of the moisture in the material in a short period of time. After the material is dried, it is discharged from the discharge port, and the exhaust gas is discharged from the top of the boiling bed. After the solid powder is recovered by the cyclone dust collector and the bag filter, it is emptied.

2. The working process of the boiling dryer

The material is transported to the ebullating bed by the skip, and is sealed with the ebullating bed by the sealing ring under the action of the cylinder jacking. Then, under the action of the induced draft fan, the air is purified by the filtering device, heated by the radiator, and then distributed into the boiling bed (drying chamber) through the air distribution plate (screen). The material in the hopper forms a boiling state (ie, fluidization) under the action of hot air and agitation. In the large-area gas-solid two-phase contact, the moisture (or solvent) inside the material evaporates in a short time and discharges with the air. Take away and the material is dried.

3. Technical characteristics of boiling dryer

(1) The heat transfer effect is good, the temperature inside the bed is relatively uniform, and the heat capacity coefficient (or volume heat transfer coefficient) is high, and the production capacity is large;

(2) Due to the uniform temperature distribution in the fluidized bed, any local overheating of the product can be avoided, so it is especially suitable for the drying of certain heat sensitive materials (such as grinding, polyacrylamide, etc.);

(3) Continuous operation can be performed in the same equipment, and intermittent operation can also be performed;

(4) The residence time of the material in the dryer can be adjusted as needed, so the moisture content of the product is stable;

(5) Independent electrical cabinet and PLC human-computer interaction operation control, integrated all drying parameter settings, safe and convenient operation;

(6) The drying device has few mechanical transmission components, the investment cost of the equipment is low, and the maintenance workload is small.

4. Suggestions for improvement of boiling dryer

After long-term application and development, the boiling dryer has obvious improvement in structure and performance, and the quality is also constantly improving, but there are still some problems. The following suggestions are proposed in combination with production practice:

4.1 Suggestions for improvement of insufficient thermal energy utilization

The boiling dryer is formally an air convection drying device. Compared with the conduction drying device, the energy consumption is indeed larger, but if some measures are taken, a good energy saving effect can be achieved. Suggestions: (1) Strengthen the sealing effect of the equipment. At present, most of the boiling dryer hoppers and the equipment body are connected by flat flanges, and the sealing effect is poor. It is recommended to use the concave-convex flange connection when designing; the imported pump (2) dryers use steel tube winding fins for heat exchange. Although the steel pipe can save material cost, but the heat transfer effect is not good, it is recommended to use copper pipe; (3) increase insulation measures. Add insulation to the outer casing of the heat exchanger to reduce heat loss.

4.2 Suggestions for improvement of air distribution plate (screen)

The air distribution plate in the boiling dryer has two functions, one is to support the material layer, and the other is to make the gas distribution uniform. The size, shape, distribution potential, and opening ratio of the opening of the distribution plate all have a crucial influence on the distribution of the fluid. Uneven gas distribution will cause a "circulation" in the bed, which tends to cause "channeling" in some parts of the bed, while the rest is a deadbed. At this time, most of the gas follows the bed. Some of the channels are short-circuited through the bed in the form of "channeling", which makes the gas-solid contact greatly deteriorated, which should be avoided. A good distribution plate design should be able to suppress the occurrence of unevenness in the bed. That is, when the pressure drop is lowered and the airflow speed is increased in some parts of the bed, the resistance generated by the distribution plate should suppress the increase of the airflow, thereby suppressing The deterioration of fluidization. At present, most of the boiling air dryers use a single airflow distribution plate, and most of them use vertical perforated plates or mat-shaped mesh plates. The materials are prone to fluidization unevenness or dead angles during fluidization, and cannot be ensured in the particles. The uniformity of the drug, while the single opening form can not meet the production process requirements of different drugs. On the other hand, in order to reduce the leakage loss of the drug, a multi-layer mesh structure is generally used at present, and the airflow distribution plate and the boiling bed body are mostly fixed by a large number of bolts, which is inconvenient to disassemble, difficult to clean and cause residual pollution and cross-contamination. Suggestion: Using computer fluid dynamics model, heat transfer and mass transfer model, aerodynamic and thermodynamic simulation calculation and verification of parameters such as hole spacing, aperture and opening ratio in the design of air distribution plate to meet the production of different materials. Process requirements. In the installation mode, the connection method is made detachable to ensure quick installation and thorough and complete cleaning.

4.3 Suggestions for improvement of control system At present, the operating parameters of fluidized bed equipment are generally set by the experience of the operator, but it can be fully intelligent, and the process parameters can also be traced. This is for the fluidized bed. The electrical control system of the class of equipment puts forward higher requirements. In the electrical control system, it is necessary to have a series of devices for detecting temperature, humidity, pressure, differential pressure, wind speed, operating time, dust concentration, etc., and obtain basic data, which is then transported and stored in the touch screen through the transmitter, and the data is touched by the touch screen. Intelligent control can be achieved by performing a storage analysis and then developing a suitable routing.

4.4 temperature control

The common hot air heating control mode adopts a simple "on" and "off" mode. When the temperature reaches the set value, the steam is stopped, but the heat exchanger still has residual heat to keep the air temperature rising, and vice versa, which will cause temperature fluctuation. Too large, affecting the dry quality of the equipment. Suggestion: By controlling the steam flow to maintain the temperature of the inlet air, the steam flow is increased when the temperature starts to rise, so that the inlet air temperature is close to the set value as soon as possible, and then the steam is automatically adjusted to slowly approach the set value. Maintaining a steady amount of steam keeps the inlet air temperature stable.
This paper starts with the working principle of boiling dryer, combines the process operation parameters to summarize some problems in the production process, and briefly puts forward suggestions for the improvement of such equipment. It is hoped that equipment manufacturers can develop more products to meet pharmaceutical production in the future. Pharmaceutical drying equipment with advanced process requirements, advanced performance parameters, operability, environmental protection, energy saving and advanced economic and technical indicators.

Pharmaceutical Intermediates

Pharmaceutical Intermediates

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