Improve Fluid Bed Dryer Production Through Desiccant Technology, Part 1
By Scott Haynes and Mario Ranieri, Munters Corporation-Cargocaire Division
Editor's note: This two-part article covers the use of desiccant technology in a pharmaceutical manufacturing plant. Part 1 examines critical issues in drying pharmaceuticals, while part 2 discusses specific desiccant techniques, including the Cargocaire HoneyCombe desiccant drier.
An audit at a New Jersey pharmaceutical company manufacturing more than a dozen prescription drug products showed an opportunity to increase the throughput during the summer months by decreasing process humidity. How can we achieve winter time humidity levels during even the hot humid, dog days of summer? Through desiccant dehumidification. For any process or production line that seems to run better, faster or with higher product quality in the winter vs. summer, there is a very good chance that it is a problem related to excess humidity.
Three variables operate in drying pharmaceutical products: temperature, air velocity, and humidity. The three variables are sometimes limited by the product itself. For example large, fragile particles can be damaged by high air velocities and thus require higher temperatures or drier air (less humidity) to compensate for the reduction in velocity.
Many pharmaceutical products are temperature sensitive. Therefore, to have adequate throughput in production, one must compensate for a lack of heat by increasing the air velocity and decreasing the humidity. Higher air velocity is achieved by using a fluidized bed, while an efficient way to achieve drier air is by passing the air through a desiccant before the air comes into contact with the product.
The combination of the desiccant dehumidification system and the fluidized bed make up the fluidized bed processing and drying system. Fluidized bed dryers are well known for their ability to decrease drying time dramatically over tray dryers and vacuum dryers. The drying process is isothermal and the high drying rate results in evaporative cooling which keeps product temperatures low while supplying high drying temperatures from the air handling system to the fluid bed dryer.
Fluidized bed systems are normally configured such that the air for fluidization and drying is drawn from outside the building through pre-filters, and in our example, through a freeze protection coil, desiccant dehumidifier, a humidifier, final heater, temperature control device and HEPA final filters. Exiting the product bed, the moisture laden air passes through some type of product/air filter, a fan, possibly a final filter and is exhausted to the atmosphere.
Fluidized bed drying is rapid because the entire product surface area is exposed to the high volume air stream. Heat is transferred to the product surface by convection. The vigorous mixing action of the fluidized bed causes the process to be isothermal, a desirable condition for operating with temperature-sensitive materials. By employing a desiccant dryer pharmaceutical manufacturers can reduce the time required to dry a batch even further than with conventional refrigeration technology due to desiccant technology's extremely low dewpoint.
How Desiccant Dehumidification Works
At the plant, before the outdoor air reaches the fluid bed dryer, it advances through preheating and cooling coils. The process air then passes through a Cargocaire HCD-4500 desiccant dehumidifier that dries the air to a very low dew point of 12°C or 10 gr/lb, matching the best winter operation dew points. Finally, the air moves through cooling and heating coils again and is delivered to the Glatt dryer at the required moisture and temperature level of between 54.5°C and 77°C at 10 gr/lb.
Since installing the desiccant dehumidifier in August 1996, the drying times for one batch of the cardiovascular product granulation's have decreased by 15 percent during the summer. Due to significant improvements such as this, the majority of pharmaceutical manufacturing facilities use cooling-based and/or desiccant dehumidification systems to remove moisture from operations.
Cooling Based Dehumidification
Cooling-based dehumidification is the most common method of removing moisture from air. Most air conditioning units dehumidify as they cool. The basic principle is that cold air cannot hold moisture as well as warm air - air is cooled to force out moisture by condensation. To illustrate this principle, think of a cold glass of soda or beer during summer and winter. In the summer, high moisture condenses a large amount of moisture - In the winter, air has less moisture-so less condenses on the cold glass surface. Direct expansion (DX) cooling is the most widely applied method-most A/C units are DX-type. The basic principle is that heat is moved from one air stream to another by heating a gas and moving it with a compressor. A specific unit is the "Cool-Reheat" dehumidifier, a "Sears" style, which chills the air to dehumidify it, then uses the rejected heat to reheat the air after it has been dehumidified. Since all BTUs are used effectively, this method is considered the most energy-efficient of all dehumidifiers-all BTUs are used effectively.
Advantages of Cooling Based
- Very energy-efficient at high temperatures, and at dew points above about 5°C
- High-volume production allows very low equipment cost
- Technology is familiar to most engineers and equipment service companies
- Not practical to use cooling dehumidification below about 5°C (Risk of freezing & high energy cost)
- If low relative humidity is required, must spend money for reheat after cooling
- Comparatively complex mechanical equipment
Dry desiccant acts on each water vapor molecule directly-attracting them by differences in vapor pressure. The fundamental process of desiccant dehumidification is to absorb and release moisture based on differences between: vapor pressure at their surface and vapor pressure in the air around their surfaces. Desiccant dehumidification changes the vapor pressure of the desiccant by changing its temperature and moisture content. Low temperature and low moisture content-low vapor pressure (attracts moisture from air). High temperature and moisture content-high vapor pressure (gives off moisture to air). Desiccants follow four major steps in removing moisture from the air:
- Sorption, where the moisture is removed by the desiccant, the surface vapor pressure rises as the desiccant collects moisture.
- Equilibrium, at which point the surface vapor pressure is equal to the vapor pressure of the surrounding air. Without a vapor pressure differential, there is no further moisture movement to desiccant.
- Reactivation of the desiccant, when the surface vapor pressure rises as desiccant is heated. Since the vapor pressure is higher than the surrounding air -- moisture leaves the desiccant.
- Dessicant cooling, where the desiccant surface vapor pressure reduces rapidly with cooler temperature. When the desiccant is cool, its surface vapor pressure is once again lower than the surrounding air, it can again collect moisture.
- Energy to drive the cycle is proportional to the mass that is heated and cooled (More desiccant and water, more energy)
- All desiccants move through the same cycle-no escaping thermodynamics. All must be heated, and all must be cooled
- Cost of operation depends primarily on the cost of: reactivation energy plus cooling energy
Graph 1: The Desiccant Cycle
End of Part 1
For more information, contact Mario Ranieri, Business Development Engineer, Munters Corporation-Cargocaire Division, 79 Monroe St, Amesbury, MA 01913. Tel: 508-388-0600.
Go to "Improve Fluid Bed Dryer Production Through Desiccant Technology, Part 2"