Which factors determine the design of a ring-layer granulator for free-flowing, low-dust granules in the chemical industry?
The design of a ring-layer granulator is an overall process engineering task. Product properties, the desired granule quality, process management and technical framework conditions have to be matched to one another. At its heart is the formation of a stable, thin ring layer in which agglomeration, compaction and drying proceed under control.
One essential influencing factor is the process mechanics. The ring layer arises through high rotor speeds and centrifugal forces that press the product against the vessel wall. The Froude number, the circumferential speed, the residence time and the specific energy input are important here. These variables determine how intensively the material is moved, compacted and built up into low-dust granules.
The Froude number relates to rotating free-fall, drum and throw mixers as well as to systems with a horizontally mounted mixing tool. In vertical mixers with forced restratification it is not a design or scale-up criterion. What is decisive there is that the mixing principle remains the same across all sizes and that the restratification covers the entire mixing chamber independently of the fill level; transferability is secured in the pilot plant with the original product.
The properties of the starting material also play a major role. Particle size distribution, fines content, bulk density, moisture, hygroscopicity, stickiness and thermal sensitivity markedly influence the process behaviour. Added to these are abrasiveness and chemical aggressiveness, which have a direct effect on the choice of materials, the seals and the bearings.
The desired end product must likewise be clearly defined. Particle size distribution, bulk density, grain strength, residual moisture and the lowest possible dust content are decisive. For free-flowing granules, a narrow grain size band is important above all, so that the product can be conveyed, stored and dosed well.
A further design area concerns the supply of binder and liquid. Quantity, concentration, viscosity, addition rate and nozzle arrangement determine how uniformly the particles are wetted and how stably the agglomerates are built up. Precise liquid distribution helps to avoid lump formation and to achieve the desired granule structure.
Not least, temperature control, design details and the connection to the overall process are important. Double-jacket temperature control, tool geometry, drive power, feeding, drying, classification, extraction and explosion protection have to be matched to one another. In practice, the design is usually established through laboratory and pilot plant trials before the parameters are transferred to the production scale.
How amixon® sets granule size and structure reliably in the RMG ring-layer mixer-granulator
The operating principle: controlled granulation
The substances to be granulated are fed in continuously, accelerated into rotation and guided along the wall as a ring layer. The pin tools intensively blend, de-agglomerate, compact and convey this layer. Solid and liquid bridges form in the process, from which granulation nuclei arise and grow further. As soon as granules exceed a critical size, they become more fragile and break down again. The fines are re-attached and rounded off. With a suitable mode of operation, a stable granule within narrow grain limits arises in this way.
The control variables
Control is exercised via the circumferential speed in the range of approximately 8 to 35 m/s, the residence time and the application of liquid or binder. Temperature control also plays an important role, since the mixing chamber, inlet and discharge nozzles can be temperature-controlled with a double jacket using steam or water. An integrated sampler enables ongoing monitoring of the particle size distribution and granule stability.
Constructional prerequisites for a narrow grain band
The mixing chamber is machined by cutting and is particularly round. The pin tools maintain a uniform clearance from the wall, which supports a narrow grain band. The tools are hardened and, on request, made of carbide or ceramically armoured. Larger RMGs are mounted with vibration damping and dynamically balanced. The mixing chamber is designed for ATEX Zone 20. The sizes range from Type 10 with approximately 0.4 to 1.7 m³/h up to Type 3000 with approximately 10.1 to 49.5 m³/h.
Design through trials
The actual volume flows and the optimum mode of operation are product-specific. amixon® therefore conducts design trials with the original product in its works pilot plants and transfers the results to the target size. For chemical applications, corrosion-resistant materials up to Alloy 59, Hastelloy-C22 and nickel as well as an ATEX Zone 20 design, inerting and pressure-tight and vacuum-tight reactor designs are available.
Verification in the pilot plant
Before the investment, the process is examined in the amixon® pilot plant with the original product, at real fill levels and under the intended temperature and pressure conditions. Mixing quality, product protection, energy input, cleanability and reproducibility are assessed. The results are evaluated in documented form and constitute a robust basis for decision-making. amixon® can offer, manufacture and service ring-layer mixers and ring-layer mixing granulators of the RMG design; this class is not, however, developed further, because it works with a horizontally supported mixing shaft. The development focus lies on precision mixers with vertical or inclined mixing shafts.