A Closer Look at Existing Dryer
How Desiccant Dehumidification Works
Introduction (Back to Top)
As the US pharmaceutical unit of BASF Corp. and part of the global pharmaceutical business BASF Pharma, Knoll Pharmaceutical Co. (Mount Olive, NJ) markets prescription medications for cardiovascular disease, pain management, oncology and immunology, endocrinology, and central nervous system disorders. At its Whippany, NJ manufacturing facility, Knoll produces more than a dozen prescription drug products in tablet, liquid and ointment forms.
Several years ago, Knoll conducted an audit of its production history and saw an opportunity to increase productionand profitsduring the humid summer months. The study closely compared production rates during the summer and winter seasons and proved that productivity for its cardiovascular product Isoptin SR was higher in winter because humidity in the airand in processing operationswas lower.
According to Brendan Mooney, senior project engineer for Knoll, high humidity increased the drying time of the granulation process in the fluidized bed drier, especially in the summer months when the outdoor air used for product drying typically carries more moisture. "It took significantly longer to dry one batch of product granulations during the summer, delaying production of tablets," stated Glenn Herring, tablet department supervisor at Knoll. "Staff had to work longer hours each time a batch was processed and this increased overtime expenses."
Additionally, a new product Knoll was manufacturingan analgesic compoundneeded to be dried at a lower dry bulb temperature than the other products. "We needed to examine our drying system and update the equipment with a dryer that could decrease drying times for existing products and provide adequate drying at a lower temperature for the new product," Mooney said.
A Closer Look at Existing Dryer (Back to Top)
At the Knoll plant, a Glatt fluidized bed drier utilizing 100% outside air is used to dry product granulations. During the winter, when outside air is typically drier, the outside air would pass through a preheating steam coil which delivered the air at a specified temperature with a low grain (humidity) level of between 2 and 10 grains per pound (gr/lb). In the summer months, however, the air had to also pass through a cooling coil utilizing chilled water to condense as much moisture as possible. The air then moved through reheat coils that conditioned the air to the required drying temperature.
"While we were able to condense moisture out of the air by cooling the air temperature to a lower dew point in the summer using a cooling coil with chilled water, we could not condense as much moisture as necessary to improve production," Mooney said. "In fact, the air at the 55 F dew point contained a higher level of saturation64 gr/lband we knew it could be drier using a desiccant system."
After careful consideration, Knoll decided to install a Cargocaire desiccant dehumidifier as part of the preconditioning system for the Glatt fluid bed dryer. The desiccant unit dried the air beyond the capabilities of the existing mechanical refrigerator that was used.
"In addition to its fair pricing, Cargocaire promised to have the equipment up and running within one monthin time to help us through the most humid period of the year, August," Mooney said.
How Desiccant Dehumidification Works (Back to Top)
At Knoll, 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 F (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 130 F and 170 F at 10 gr/lb.
The HoneyCombe rotor is the heart and soul of Cargocaire desiccant technology. Air enteromg the dehumidifier passes through the HoneyCombe desiccant wheel, which attracts water molecules directly from the air.
The majority of pharmaceutical manufacturing facilities use cooling-based and/or desiccant dehumidification systems to remove moisture from operations. In cooling-based dehumidification, the air is chilled below its dewpoint temperature, causing moisture to condense on the nearest surface. In this process, the air is dehumidified through cooling and condensation. This is the operating principle behind most commercial and residential air conditioning systems.
The amount of moisture removed depends on how cold the air can be chilled. The lower the temperature, the drier the air. However, if temperatures need to be consistently maintained at or around 40 F, the cooling coils freeze and cannot operate efficiently. This creates many additional problems.
Condensing-type dehumidifiers cannot effectively dry below a 40 F dew point. And they would not be effective at Knoll where products are dried at much lower dew points. When drier air is needed, desiccant dehumidification in necessary. Desiccant dehumidifiers are quite different from cooling/condensing units. Instead of cooling the air to condense its moisture, desiccants attract moisture from the air by creating an area of low vapor pressure at the surface of the desiccant. The pressure exerted by the water in the air is higher, so the water molecules move from the air to the desiccant to make the air drier.
In fact, most solid materials can attract moisture, but they tend to have very low moisture capacities. Desiccants, on the other hand, can attract and hold from 10 to more than 10,000% of their dry weight in water vapor. They are very effective in removing moisture from the air at low humidity levels and do not freeze when operated at low temperatures, which makes them ideal for pharmaceutical processing and manufacturing applications.
The essential characteristic of desiccants is low surface vapor pressure. A cool, dry desiccant can attract moisture from the air because its surface vapor pressure is low. When the desiccant becomes wet and hot, creating high surface vapor pressure, it will give off vapor to the surrounding air. Vapor moves from the air to the desiccant and back again depending on the vapor pressure differences. Desiccant dehumidifiers use the changing vapor pressures to dry air continuously in a repeating cycle.
In Cargocaire dehumidifiers from Munters Corp. (Amesbury, MA) the desiccant support structure is a HoneyCombe wheel. The desiccanteither silica gel, lithium chloride or molecular sieveis infused into a corrugated composite material that resembles the honeycombs of a bee hive, and this is formed into a wheel. The wheel rotates slowly between the process and reactivation air streams. Moisture extracted from the process air is removed from the desiccant by the heated exhaust air stream.
After comparing the mechanical refrigeration method Knoll was using to desiccant dehumidification, it was clear that desiccant technology was essential to obtain the desired air temperature and dew point levels.
Since installing the desiccant dehumidifier in August 1996, the drying times for one batch of Isoptin SR product granulations have been decreased by 15% during the summer. "Installing a desiccant dehumidifier has resulted in increased productivity and more consistent drying times between the summer and winter seasons," Mooney said. "Knoll also has decreased overtime expenses."
For more information: David Simkins, Business Development Manager, Munters Corp., Cargocaire Division, 79 Monroe St., Amesbury, MA 01913. Tel: 978-241-1100. Fax: 978-241-1215.