As the pharmaceutical industry has matured, concerns about the safe handling of drug compounds during the manufacturing process have increased. To meet exposure limits and protect batches from any kind of contamination, the industry’s need for improved containment has increased. Vendors have responded by providing glove boxes, better seals, more robust dust-collection systems and improved vessel design.
One such piece of equipment that incorporates many of these improvements is the agitated filter dryer, which Rosenmund Guedu, Liestal, Switzerland, invented in 1971. “The agitated filter dryer is a unique piece of processing equipment to separate solids from liquid in a single vessel,” says Todd Peterson, manager of filters and dryers for De Dietrich Process Systems Inc., Charlotte, N.C., which acquired Rosenmund in 1999.
The filter dryer typically is designed to provide a closed environment for separating, washing and drying in a single vessel, and is continually evolving to meet manufacturers’ needs. Vendors have taken various approaches to address contained powder discharge, including glove boxes, continuous liner technology and specially designed valves and scrapers, promising product recovery upward of 99%.
Although generally used for pharmaceutical applications, chemical producers such as Eastman Chemical Co., Kingsport, Tenn., also are making use of the technology. “It’s easier to justify the cost for pharmaceuticals,” says Bryan Suggs, business analyst for Eastman. “But we also use filter dryers for general chemicals production.”
Machines that multi-task
Depending on the application, the agitated filter dryer might use either cloth (generally polyethylene or polypropylene) or metal mesh media; a manufacturer might even alternate between the two. The filter media can be held in place by bolts or clamps, and the metal mesh has the added option of being welded. Most advanced designs now use a boltless mechanism to secure the filter media by simply clamping it between the two halves of the filter dryer. This also provides a sterile design, says Chad Ranpuria, process manager for Powder Systems Ltd. (PSL), Liverpool, England. Cloth filter media is more likely to be used when processing specialty chemicals. Although many pharmaceutical manufacturers prefer metal mesh since product can more easily be recovered, cloth is ideal when used for single runs. The types and pore sizes of filter media that are available are continually expanding, while the cost is relatively stable, Ranpuria says.
Agitated filter dryer vessels separate into two pieces to allow access to the vessel internals, mainly for changing or repairing filter media. The two parts are generally sealed with an O-ring and can be held together with bolts, C-clamps or a bayonet closure (Figure 1); these last two are easier to assemble.
Once the vessel is charged with slurry, pressure is either applied from the top of the filter dryer using a gas, such as nitrogen or compressed air, or a vacuum is pulled from beneath the filter media, thereby forcing or pulling liquid through the cloth or mesh. Low pressures are generally used (1 barg to 2 barg) to keep the cake from becoming so compressed that the crystals fuse together. The liquid exits at the bottom of the vessel where it might be recycled or reused in another part of the process.
While the crystals are collecting on the filter media, the smooth edge of the agitator acts to smooth the surface of the cake so there are no crevices. The other edge of the agitator, which can rotate in both directions, might have teeth for digging into the cake to help break it up and remove it from the filter media (Figure 2). The cake might be broken up and washed several times to remove all trace solvents or impurities.
The powder is then dried by applying heat to the vessel. “Heating has become more sophisticated and efficient,” Ranpuria says. Not only are PSL’s filter dryer vessels fitted with jackets on the walls and top dome, but the agitator shaft and blades are also heated, providing direct contact with the powder. Several vendors’ designs also supply heat to the bottom filter plate.
Lewis Fabricius, P.E., manager of reactor systems for Pfaudler Inc., Rochester, N.Y., says the company’s design speeds drying by using an internal heating grid that directly conducts heat to the product. The company also offers an optional heated nitrogen distribution system, which can fluidize the cake, thereby reducing drying times.
Vendors now can provide programmable logic controllers (PLCs) that integrate with a site’s distributed control system (DCS), as well as an interlock system. “If the unit is under pressure or vacuum, you don’t want someone to open the side-discharge valve,” Ranpuria says. “So you add a pressure sensor and an interlock system so the valve can’t be opened until the vessel is at atmospheric pressure.”
Customized control systems can be used to set the nitrogen purge, temperature and pressure cycles, Ranpuria says. “It also makes it possible to dry to a specific end point which is a great benefit for processes that need to be repeatable.”
For smaller, lab-scale units, however, it probably doesn’t pay to automate units, says Lawrenzo Heit, New Brunswick, N.J.-based group leader for Bristol-Myers Squibb Co.
The final step is to remove the finished product from the filter dryer vessel.
PSL’s approach is to attach a glove box directly to the side of the vessel so no powder can escape. Operators then discharge the dried product using the agitator controls and transfer the loose powder from the vessel to containers, such as drums or bottles. Since the agitator is designed so that it can’t damage the filter media, there is always some product left on it, which is called the heel. This can be removed through the glove box with a specially designed rake, Ranpuria says.