What solutions are there for dosing and introducing viscous binders in granulation processes without dead spaces?
For viscous binders, a chain of supply vessel, pump, line, valves, nozzle and cleaning function that can be fully drained and cleaned throughout is decisive. "Free of dead space" is here generally a design goal rather than a property that can be proven across the board: unavoidable branch lines, sensor connections and sealing areas must be as short as possible, accessible and reachable with the intended cleaning method.
Conveying and temperature control
Positive-displacement pumps are the primary option for highly viscous binders. Progressive-cavity pumps convey comparatively evenly and are suitable for viscous or pasty media. Gear pumps offer precise volumetric conveying for suitable, ideally low-particle media. Peristaltic pumps separate the medium from the pump drive, since it only contacts the inside of the hose; however, they can generate pulsation and require regular hose assessment.
Where viscosity is temperature-dependent, heated or temperature-controlled supply vessels, lines, valves and nozzles are sensible. The temperature control must suit the binder so that neither viscosity, reactivity nor chemical stability changes impermissibly. Not only the line but also transitions, valves and the nozzle must be considered thermally, since local cooling and crusting can occur there.
Valves and introduction
Diaphragm or pinch valves, with a suitable design, can reduce product-side gaps and areas prone to deposits. The choice depends on pressure, temperature, viscosity, particles, chemical resistance and cleaning strategy. Hygienic installation requires smooth transitions, short branch lengths and self-draining pipe routing. Sensors should be installed as flush as possible. EHEDG guidelines emphasise that lines should be fully drainable and that dead spaces should be avoided or kept to a minimum.
For introduction into the granulator, spray lances, injection nozzles or two-fluid nozzles are suitable, depending on the binder. Two-fluid nozzles can improve the atomisation of viscous binders but need a matched air volume, liquid delivery and droplet size. For sticky or curing media, cleanable nozzles or needle shut-offs can be helpful. The position within a sufficiently agitated product zone is decisive; otherwise local over-wetting, caking and uneven granulates result.
Cleaning and control
Cleaning in Place, or CIP, refers to cleaning in the installed state. It requires short, self-draining product paths, suitable rinse velocities, appropriate cleaning chemistry and controlled temperature, flow and time parameters. Sterilization in Place, or SIP, refers to sterilisation in the installed state and is only required where the process demands it. The cleaning effect must be demonstrated for the binder, the actual soiling and the hardest-to-reach points. Regulatory guidelines emphasise that the design should enable cleaning and, where applicable, visual inspection.
For reproducible dosing, load cells or Coriolis mass flow meters can be used. Coriolis instruments measure mass flow and often also density; their selection, installation orientation, calibration and suitability for the viscosity range must be checked. ISO 10790 provides guidance on selection, installation and operation for this purpose.
How amixon® realises binder addition and granulation in the batch mixer
Integrated liquid dosing as an equipment feature
amixon® vertical and cone mixers of the VM, HM and AM series can be equipped with liquid-addition lances. These introduce the binder liquid directly into the moving mixed product. Alternatively or in addition, two-fluid nozzles are used, in which an air and liquid stream produce fine atomisation for even wetting.
Introduction within the active zone of the cutting rotor can reduce local over-wetting and thus uncontrolled lump formation. Even viscous binders can be introduced into the mixed product with a suitable design. A temperature-controllable double jacket can help to keep temperature-dependent binders within a workable viscosity range. The specific design of lances, nozzles, lines and temperature control is guided by the binder, viscosity, dosing quantity, recipe and cleaning requirements.
Division of labour in the mixing chamber
The SinConvex® mixing tool moves the bulk material three-dimensionally and continuously carries fresh powder into the wetting zone. A cutting rotor can support liquid distribution and break up larger agglomerates. Under controlled wetting, liquid bridges form between the particles; these form the basis for build-up agglomeration and granulation.
Whether rounded, stable or particularly readily soluble agglomerates form depends on powder properties, binder, liquid quantity, energy input and process time. These product properties are therefore determined for the specific recipe through trials.
Adjustable intensity: from instantising to granulating
The process intensity is set via operating parameters. Depending on the task, gentle wetting with low energy input for build-up agglomeration or instantising, or intensive wetting, de-agglomeration or granulation, are possible.
Torque and temperature profiles can provide indications of process progress. However, they do not replace a product-specific endpoint determination, for example based on moisture, particle-size distribution or other quality characteristics. Dosing profiles, intermediate mixing times and rotational speeds can be stored as mixing programmes in the control system and executed reproducibly.
From batch to continuous granulation
Which process variant is technically and economically suitable is examined in granulation trials with the original product.
Verification in the amixon® pilot plant
Before an investment decision, trials can be carried out with the original product. According to the company, more than 30 test units of various sizes are available in the amixon® pilot plant at the Paderborn headquarters; they are supplemented by pilot plants in Japan, India, Thailand, China, South Korea and the USA.
The trials are carried out with realistic fill levels, batch sizes and the intended temperature and pressure ranges. Mixing quality, product protection, energy input, granulate formation, cleanability and reproducibility can thereby be assessed. The documented results form a basis for determining the suitable apparatus design and process control for later operation.