Contact drying in a flash

Frank Bessner

The orbiting screw of a conical screw dryer with its internal gears in the product chamber is complicated and susceptible to problems, and the risk of product contamination is always present. For some operators, the annual maintenance costs run to almost six-figure amounts owing to the complicated drive design, which makes the entire drying system not very profitable. For this reason, the main development aims of the double-helix cone dryer were to shorten drying times while retaining the gentle product mixing process and also to increase the availability of the dryer and hence the drying system as a whole. A helical agitator is therefore used for the double-helix cone dryer, which is installed on the central shaft in the dryer vessel. The agitator rotates with a very small clearance to the vessel wall. Depending on the product properties, a single or double-helix agitator can be used. Instead of the internal gear in the product chamber, the drive unit was simplified in line with standard commercial gear motors. The bearing and shaft seals are located outside the product area.

Compared to the orbiting screw, which only moves a small portion of the product during the drying process, the helical agitator mixes the complete batch. This significantly improves heat transfer on the heated vessel wall and, as a result, permits much shorter drying times. This effect is further enhanced by the significantly higher turbulence and product agitation in the double-helix agitator cone dryer. AVA double-helix cone dryers are operated with a filling capacity of up to 80 % of the gross vessel volume, while dryers with orbiting screws can only be operated with a filling capacity of 50 to 60 %, since the internal gears are installed in the product area and may come into contact with the product material. A double-helix agitator cone dryer with a smaller gross volume can thus be chosen for the same batch size due to the higher filling capacity.

The agitator mixing gear is free-floating, i. e. not fixed at the bottom. The entire discharge cross-section is therefore utilised as a discharge opening. This ensures optimum emptying of the dryer. In the case of sticky or poorly flowing products, the discharge process can be additionally improved by operating the double-helix agitator in reverse. While dryers with orbiting screws can only be used for mechanically pre-dehumidified products, a double-agitator cone dryer is also suited for evaporating fluids (suspensions). The evaporation can be effected up to dry material. This procedure is preferred for difficult products or products that cannot be processed in a centrifuge. The dryers are designed and manufactured to comply with GMP regulations according to each user’s process requirements and the space available. As a result, every unit is different.

A few years ago, large machines for mono production were in demand. Today – due to increasingly active and highly concentrated agents and additives – customers opt almost exclusively for smaller machines up to 1000 l in multi-purpose production plants. The challenge for the mechanical engineer is often how to integrate the dryer into an existing plant or drying room with limitations on the dryer height and width. Dryers are often installed downstream of a mechanical solid-liquid separation stage. This can be a suction filter, a filter press or a centrifuge, whereby the combination of a centrifuge and a vertical dryer tends to be particularly successful. To meet the requirement for a low ceiling height, the dryer is installed on a rotating lifting column. During the filling procedure, it is docked onto the centrifuge and handles several centrifuge cycles. For drying, the cone dryer is uncoupled and swung away from the centrifuge. The centrifuge can be cleaned during the drying process for the next production cycle or a product change. The dryer is raised prior to evacuation, so that barrels, containers or other mobile transport means can be pushed underneath it and docked. To allow the dryer to be swung, its heater connections, the connections for the vapour filter and the electrical cables must be flexible and rotatable. Systems with two dryers on one lifting column, which are filled alternately by the centrifuge and swung to the side for drying, are already being planned to enable the centrifuge capacity to be optimally utilised.

To increase the capacity of production systems for bulk materials, old machines are either replaced with new ones or additional production lines envisaged. However, in addition to the actual investment, extra costs are incurred due to the replacement or expansion of the instrumentation technology through supplementary effort for planning, installation and assembly. This effort, and the associated expense, can be significantly reduced by careful selection of the process plant or by upgrading the existing equipment to innovative technologies. Ideally, by adapting the old plant, the system can be brought into line with the state of the art, so that both the container and the internal mixing gear are designed and matched to current production requirements. If the funds for new investments are lacking but the operator wishes to increase production capacity nevertheless, it is possible to convert the orbital screw dryer to a vertical agitator system. In this case, the container of the mixer or dryer can still be used and only the old, inefficient mixing gear is replaced with a new agitator. The production supply lines, the connections for the peripheral equipment and all instruments can remain in the system. Only the orbital screw arm with the internal gear need be exchanged – simply and economically – for a single or double agitator. This modification results in an optimized process as well as substantially less mainte-nance. The advantages of the vertical helix agitator system can therefore be leveraged for an existing plant without additional planning or conversion work. This is also true if the existing dryer has a fixed-welded (non-removable) cover. At the modern AVA technology test centre, users have a chance to test their original products in the double-agitator cone dryer/evaporator and witness the dryer’s performance for themselves. Product temperatures up to 380 °C and vacuum operation up to 1 mbar abs. are supported. If the products cannot be tested outside the production plant, several machine types are available for renting.

Hall 9, Booth 310

cpp 402

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