The best possible technique for extracting flakes,
pastilles and prills
Alexander Kuppe
There are many ways in which to convert molten products into solids. It is becoming increasingly commonplace to cast ready-mades directly into the required form, rather than solidify them in various stages, as was previously the case.
Many chemical and pharmaceutical processes produce molten end products. Thanks to modern-day technology, it is possible nowadays to instantly mould even the most difficult products into the desired shape - using a variety of different techniques - thereby making processing easier, without the need for complex machinery. In addition, more stringent guidelines governing occupational health and safety, emissions and the environment have placed greater demands on the processes and equipment used. The most important issues in this respect are waste gases, waste water and occupational health and safety.
Ultimately, it is only possible to choose the best technique after weighing up the technical and economic aspects of each product.
Flakes, pastilles or prills?
Usually, the chosen product form depends on consumer demand, the various processing stages, and ultimate use. Dustproofness, accurate pourability and high bulk density are becoming increasingly important issues in this respect. There are currently three processes of solidification used commercially, covering a wide range of product forms, namely drum coolers or flakers, pastilles and prills.
Drum coolers or flakers for converting molten products have been deployed for over 80 years. Figure 1 highlights the principle. The hot molten product is poured into a heated roller. A thin layer of the liquid product adheres to the rotating and internally cooled drum. The continuous rotation of the drum ensures that the layer solidifies completely and cools down to the required temperature. An adjoining knife scrapes off the layers, and in the same instant granulates them in the form of flakes.
Fig. 1 Flow chart of a Drum Cooler
This popular method (used 80% of the time), has many variants, for instance drum cooler with an overhead applicator roll for viscous products or double drums for melts with suspended solid particles.
Thanks to the simple processes, this type of treatment is still extremely reliable and tough, as well as highly efficient, as virtually the entire cooled surface is used to transfer heat. The machine is also very compact, easy to encase and suitable for processing oxidation sensible, hygroscopic and toxic products. The following products are particularly suited to the Drum Cooler or Flaker: Caprolactam, Phtalic Anhydride and detergents. In principle, all products with a solidification point of approx. -15 °C to +400 °C and with viscosities of less than 1 mPas to approx. 100 000 mPas can be processed.
Disc Pastillator
The molten product which is to be processed is dripped onto a rotating plate using a heated dose-measuring device. The rotation of the discs ensures that the drops solidify, cool down to the required temperature, before being scraped off of the disc by a knife.
Advantages of the Disc Pastillator compared to the Flaker:
better viscosity and easier pourability,
in general, a slightly higher bulk density
dust-tight
less prone to stick together, even if stored for longer periods of time
These advantages are even more pronounced, the rounder the tablet is, i.e., the greater the diameter. The most important parameter in this respect is the surface tension of the product on the one hand and the cooling surface on the other. Products with a low surface tension turn into thin, flat layers on a stainless steel surface and do not achieve the required stability. The Disc Pastillator makes it possible to coat the disc surface with e.g. PTFE, thereby changing the surface tension. This hardly affects the heat transfer, but significantly improves the shape of the pastille, as well as making it easier to remove from the disc.
Like the Drum Cooler, the Disc Pastillator is extremely compact (Fig. 2). Owing to the modular construction, it is possible to stack up to six discs on top of each other on the same base, either immediately or at a later stage if required.
Fig. 2 Disc Pastillator
The products can show viscosities ranging from less than 1 up to 40 000 mPas as well as temperatures of fusion of approx. 25 to 200 °C.
Suitable products are Maleic Acid, fatty acids and all toxic, oxidation sensible and/or hygroscopic liquids.
Jet Priller
Prills are completely round and generally smaller than pastilles. Prill towers have been used for many years to produce prills, which cool and solidify the produced droplets by gas cooling. The disadvantage of this technique is it takes up a lot of space and requires a large amount of gas for cooling. A compact technique is currently being developed, which will make it possible to produce this product form even in smaller spaces (Fig. 3).
Fig. 3 The Closed-Loop-System produces compact prills
The process can be divided into three phases. In the first phase, droplets are generated by accurately controlled jet break-up, whereby, depending on the product, a virtually mono disperse spectrum of droplets can be achieved.
In the second phase, the produced droplets are dripped in a short downspout (nozzle) and stabilised by using liquid nitrogen, which produces a hard shell within seconds. The use of liquid nitrogen creates a large difference in temperature, as well as a high temperature evaporation. As such, this processing stage is extremely compact.
The gaseous nitrogen, together with the stabilised prills, is poured from the nozzle into a fluid bed (spiral cooler). In this third phase, the pre-solidified prills are completely solidified to the core and cooled down to the required temperature. It is possible to use liquid nitrogen as process gas. This is recycled via the inlet fan and gas cooler, creating a complete inert process.
One important factor determining the economic viability of the system is the optimum use of liquid nitrogen.
Modular construction
The system has a modular construction, i.e. various downspouts (Prill Columns) can be placed on a fluid bed (size of which can be selected). Figure 4 shows a variant with two modules.
If required, the number of modules can be increased depending on the fluid bed size.
Compared to pastilles, prills are easier to pour. The amount of dust is about the same, but the required droplet size can vary in diameter from 300 m to 1.5 mm. Depending on the product, the bulk density could be up to 10% greater compared to pastilles. The viscosity of the processed products is between 1 to 250 mPas, whereby the solidification point should not be less than 250 °C. E cav 260
