Keeping powder dry
Current trends and systems in the field of drying engineering

A review of what suppliers of drying systems have to offer shows that many companies are clearly focusing new development work on the pharmaceutical industry. Many producers also appear to be specializing to an increasing extent on customers in the food industry (who are perhaps more willing to invest). It is also interesting to observe Atex 100a implementation. Many suppliers offer total containment.

Technical experts from Glatt define drying as “the art of effectively evaporating water”. To be a master at this art, you have to pay special attention to four requirements of the drying process. Drying must be as homogenous as possible, ideally the process should be dust free, there should be a reproducible amount of residual moisture, and above all the process should of course be as gentle on the product as possible. Glatt believes that fluidized bed drying is effective because the ideal mixture can be achieve very quickly. The entire surface of each individual particle is available for drying during the airborne phase, heat transfer is very good and drying times are short. The drying process is gentle on the product. Continuous, complete blending results in homogenous product temperature and uniform drying of the product. The company produces systems with a range of capacities for drying granulated material, agglomerate and pellets. The continuous fluidized bed systems can handle throughput rates between 20 kg and 5 tons per hour. The batch fluidized bed systems have capacities ranging between 2 g and 1500 kg per batch.

Mixer-dryers: gentle handling of bulk goods
Drying is often performed in several stages. Thermal drying does not begin until removal of water by mechanical means has reached its limits. The reason is obvious. Drying is relatively energy intensive, and many producers make every effort to reduce the amount of energy they use. Filtration (mechanical dewa-tering) and drying during pre-processing and processing are based on different methods, so the two are often looked at separately, but Krauss-Maffei Process Technology believes that only a combination of the two will produce the best results. This view is based on the following consideration. The classic distinction between mechanical and thermal process leads inevitably to the creation of interfaces that can increase cost and cause problems during installation and ongoing production. To exclude problems right from the outset, Krauss-Maffei engineers look at the entire task involved in a separation process, from loading the suspension to filling the dried products. The goal is to avoid interfaces. To cite one practical example, mechanical solid-liquid separation in centrifuges or suction filters is often followed by drying to remove residual moisture. The selection of a suitable dryer is based on process-specific criteria. Mixer-dryers are primarily used for batch drying of chemical and pharmaceutical products, which possibly may only withstand limited mechanical and thermal stress. The Krauss-Maffei product range includes mixer-dryer models with top or bottom drive. The company says that the designs offer a number of variations to ensure that the machines can be modified to suit nearly any application.
The engineers at Heinen Trocknungstechnologie use a similar line of argument. The company can integrate various process steps into a single compact system. The company offers mixer-granulating machines, batch fluidized bed systems, screen-mills, lift columns, docking stations and other components which are used in the production of pharmaceuticals in solid form. Total containment for individual machines and entire granulation lines safeguards human health and the environment. Mobatch is a GMP enabled laboratory fluidized bed system for batch processing featuring 12 bar, shock-proof design (Atex 100a compliant). This exceptional batch fluidized bed system is suitable for mobile, flexible dry-
ing, agglomerating, granulating, instantizing and coating in the laboratory environment. Suitable replaceable containers and a variety of distributor plate systems can be used to handle between 150 g and 6 kg per batch.
Drying systems for sensitive products
Sandvik Process Systems specializes in drying and pre-drying technology for sensitive products. The drying duct has been specially designed to achieve a uniform moisture level at the outlet. The dryer is made up of several modules which can be assembled to provide the capacity that the customer needs. Every module has its own air circulation system, and temperature control is also separate. The dryer is made largely of stainless steel elements and has special washing units to meet stringent hygiene standards (pharmaceuticals, food).
The S.B.S.-M system supplied by the Italian firm Steel Belt Systems, a subsidiary of the Austrian company Berndorf Band, is suitable for drying very sensitive products (the system is typically used in the food industry). The system features a modular design and is divided into two zones. The product is fed in and pre-dried in
zone 1. Finishing and homogenizing takes place in zone 2. The belt in zone 1 is perforated to assist air circulation (in a controlled temperature and humidity environment). All parts that come into contact with the product are made of stainless steel.
Bucher-Guyer realizes that vacuum belt dryers are ideal for continuous drying of temperature-sensitive products (pharmaceutical and chemical industry: enzymes, proteins, vitamins and antibiotics; chemical industry: dyes, metal oxides and pigments and pellets). There are two types of products to have to be dried:
-concentrates that can be pumped, and pasty mixtures of solids which also can be pumped;
-lumpy products that can be poured or granulate/powder that can be loaded into the vacuum with suitable equipment.
The following conditions are important for vacuum belt drying:
-continuous operation, five to ten days of uninterrupted production;
-high solid content in the wet product (min. 50 percent solid matter, ideally more than 80 percent solid matter).
The system is a vacuum belt dryer containing a series of conveyers arranged in series or in parallel, which are contained in an enclosure and run at constant speed over heating plates. The product lies on the conveyers from the inlet to the outlet. This drying technique is ideal for products that go through a sticky and/or high viscosity phase during the drying process and for products that can be damaged by mechanical treatment.
Mixer-dryers and reactors made of Hastelloy
amixon offers the VMT series of vertical mixers which are used in the production of sterile goods and active pharmaceutical substances. The working volume varies between 0.2 and 2 m3. Ideal mixing quality can be achieved at loading levels between 10 and 100 percent. A rotating spiral tool in the temperature-controlled mixer transports the product upwards on the periphery of the mixing chamber. The product then flows down through the center. Because some of the components are flammable and can cause dust explosions, the circumferential speed is somewhat less than 1 m/s. The parts which come into contact with the product are made of alloy 59 and are fully temperature-controlled. Because the bearing and seal are on the top of the mixer, operation is maintenance-free for the most part.
The Aachen firm Misch- und Knetmaschinenfabrik has introduced an enhanced version of its AMK VIS inclined mixer which is designed for fast, gentle drying of moist
and powdery materials using heat and vacuum. The company offers a special version for the pharmaceutical industry (GMP/FDA).
Drying bulk goods with screw heat exchangers
Engelsmann heating and cooling screw conveyors are contact dyers and heaters designed to handle pourable powder, granulate and similar solids. A combination of fixed and moving heat exchange surfaces produces indirect heat exchange. Only a small flow of air or inert gas is needed to remove the vapor. The manufacturer emphasizes the fact that the systems run at low rotational speeds. This reduces mechanical stress and provides gentle cooling or heating of sensitive solids. The heating and cooling screws have significant advantages including good thermal efficiency and large heat exchange surfaces. Multi-shaft versions and cascade configurations can usually be retrofitted into process lines where space is at a premium.