Process Optimisation by Applying Modern Drying Technology
- Andreas Weber, R&D Manager, Ekato Systems

The whole branch of industrial production is permanently subjected to process optimisation, which serves to increase the quantity and quality of the manufactured goods while saving valuable resources. The combination of several process operations within one unit is an elemental approach to reach this sophisticated aim.

Drying and mixing of solids are fundamental process steps for a wide range of production processes. Frequently a thermal separation is carried out after a mechanical liquid solid division (eg, by centrifuge, filtration or via decanter). Since the thermal separation is usually performed at the end of the production chain, it has a massive impact on the quality of the final product. Besides compartment heaters, mostly applied for low quantities, three systems are established for gentle vacuum contact heating of moist solids horizontal, tumbling and vertical driers.

Traditionally, horizontal driers are the most commonly used units. In order to raise the heat transfer, the product is mixed via agitator that is mounted between two abutting faces. Due to the double-sided bearing the system is especially suitable for pasty and sticky material, which usually undergoes a distinctive transition phase during the drying process. This transition phase is characterised by high torque values around the shaft. The application of horizontal drying systems regularly presents problems during the processing of fine chemicals, pharmaceutical products or food. Those problems mainly occur at the bearings that are in contact with the product. This causes additional maintenance and quality control for the operator. Also the additional bearing and sealing require higher investment costs.

Due to the unfavourable geometry of the horizontal apparatus the complete discharging often causes trouble resulting from high amounts of residues. Especially for valuable products like active pharmaceutical ingredients or fine chemicals every gram of discharged product counts.

Still widely used is the tumbling drier also called double cone drier. In those systems the circulation of the product is realised by gravity. As this system has no agitator installed, the product side heat transfer coefficients are normally much lower than those compared to drying systems equipped with an agitator. This disadvantage is due to the fact that the tumbling drier needs a longer time to renew the product on the heated surfaces. Therefore, tumbling driers require a longer time to reach the desired moisture level, in comparison to competitive systems, fitted with an agitator.

Those agitated systems commonly have small wall clearances of only a few millimetres in order to avoid a further resistance for the heat transfer by generated product layers. Pasty products in particular cannot be manufactured efficiently in tumbling driers, as the product layers generated at the wall during the beginning of the process, would limit the heat transfer to the product core.

Systems with vertical design reveal clear advantages compared to the presented conventional drying units. Vertical drying systems enable far lower residues due to the conical bottom whereby the discharge yield is pretty much dependent on product properties. Beyond that, vertical systems operate with only one bearing, which is not in contact with the product. These units are especially suitable for high-value products where high discharge yields, avoidance of product contamination (eg, by contact with leaking liquids or residues from former batches) and flawless quality of the finally dry product are essential.

Despite all the mentioned advantages of vertical systems, the use of horizontal drying units is still widespread, due to their ability of providing an adequate mixing performance during the transition phase. Vertically designed systems often reveal problems during this stage of the drying process where the product tends to rotate at the same speed as the agitator (generating a vortex). Due to the lacking axial flow pattern at this stage, an effective heat transfer is not achieved.

To eliminate this problem Ekato Systems developed the Solidmix VPT technology. The ISOPAS agitator in combination with the baffle is capable of handling the moist product during the difficult transition phase, ensuring an efficient heat transfer during the whole length of the drying process. The baffle, which additionally allows the implementation of a temperature measurement directly in the product, avoids a vortex being generated.

As a result the pasty product is transported from the wall to the shaft, whereby a flow profile is created that includes the whole vessel volume (see figure 2). The capability of mixing pasty products homogeneously also facilitates the system to dry slurries efficiently.

Although the mechanical separation is usually preferred, from an energy point of view, it is sometimes necessary to pass a complete thermal separation eg, due to the solubility of some components contained in the solvent. Many times the solvent which contains the solved components is sprayed on the particle surface.

In the next process operation the solvent is separated thermally. Examples therefore concern the manufacturing of catalysts or additives for synthetic material where special product properties are generated by specific components that are adsorbed on the particle surface.

Those processes can be achieved efficiently by applying the Ekato Systems VPT technology and therefore in one single process unit. In this system the solvent is sprayed upon the particle surface while the ISOPAS agitator permanently renews the product surface in order to ensure a homogeneous distribution of the solved component. After completion of the spraying step, the solvent is evaporated via vacuum contact heating.

During the drying process the product continuously changes from one consistency to another. As a rule, the moist product is pasty at the beginning, before it starts to become friable after the transition phase. Finally, the product more and more becomes free flowing.

A lot of products tend to generate unwanted agglomerates in the transition from the friable to the free flowing consistency. Therefore, sieving may be required after completion of the drying process. To eliminate the need for sieving, it is possible to supply the Ekato Solidmix VPT unit with a chopper. Bigger agglomerates, however, tend to rise to the height of the filling level because of their higher porosity. The chopper is located exactly at this position, so as to break up those agglomerates. A reduction in the drying time is very often associated with the comminution of the particles (more surface for heat and mass transfer) and the additional power input of the chopper.

Thus the Ekato Solidmix VPT technology enables the operator to realise various process steps in one multifunctional system in order to save valuable resources and ensuring high process efficiency at the same time.

For such process optimisation Ekato Systems provides the opportunity to perform drying and mixing trials with the original product in the pilot plant scale. Not only laboratory trials with the Ekato Systems VPT technology can be performed, but trials with the solids PARAVISC are also possible. These units are part of the Ekato Solidmix VST technology and are primarily being used to dry and blend moderate till good free flowing products.

Both types of units have a GMP proven design in common and offer the facility for taking samples without process interruption. Long-term proven routines enable the scale up of the obtained results to the production size. A concept, in coordination with the customer, is developed and set to achieve the target, which provides optimum process technology, production quality, ecology and cost efficiency.