Which mixing systems are suitable for pharmaceutical excipients when both dry mixing and drying are required in a single plant?
Where pharmaceutical excipients need to be both mixed and dried in a single plant, heatable vacuum mixer dryers, fluid-bed systems and, for granulation processes, one-pot granulating and drying systems come into consideration above all. The suitable design depends on whether pure residual-moisture reduction, the drying of a moist powder or filter cake, granulation, or a particularly gentle final blend is required. An apparatus that efficiently dries a moist product, however, is not necessarily also the best solution for a highly precise final blend of dry components. For particularly demanding recipes, a separate final blending step after drying can therefore be advisable.
Vertical vacuum mixer dryers and conical mixer dryers combine dry mixing, homogenising and vacuum drying in a single closed vessel. A heated jacket and a slow-running mixing tool move the product continuously over the heat-transfer surfaces. Operation under vacuum allows water or solvents to evaporate at lower temperatures. These systems are suitable for powders, granulates, crystalline excipients, moist powders, suspensions and pastes. They are particularly useful where the product needs to be moved gently and reliably during drying while an even homogenisation is required at the same time.
Horizontal vacuum shovel and paddle dryers are particularly suitable for moist filter cakes, pastes, highly viscous masses and cohesive powders. Shovels or paddles move the product within a heated vessel and thereby improve contact between the product and the heat-transfer surfaces. These systems can break up agglomerates and can also process difficult products that pass through a sticky or viscoplastic phase during drying. For fragile granulates, coated particles or particularly shear-sensitive excipients, however, tool geometry, speed and residence time must be carefully matched.
Heated ploughshare and intensive mixers with vacuum combine intensive dry mixing with simultaneous or subsequent contact drying. They are suitable where agglomerates need to be dispersed, small addition quantities mixed in reliably, or cohesive products broken open. The higher mixing intensity, however, can cause disadvantages with shear-sensitive products. The effect on particle size, abrasion and flow behaviour should therefore be checked in trials.
Rotating vacuum dryers such as double-cone or V-mixers treat the product particularly gently. The mix is redistributed by the rotation of the vessel and by gravity. They are suitable for free-flowing powders and granulates with low to medium moisture content where product protection is more important than maximum drying performance. Their limits are reached more quickly with sticky products, wet filter cakes or strongly agglomerating materials. Mixing quality should also be checked where there are marked differences in grain size, bulk density or flow behaviour.
Fluid-bed systems dry free-flowing powders and granulates using a conditioned air or inert gas stream. They enable fast and even drying and can additionally be used for granulation, agglomeration or coating. As universal dry mixers, however, they are only of limited suitability. Segregation can occur with strongly differing particle sizes, densities or shapes. Very fine, sticky or poorly fluidisable excipients require careful process-engineering assessment.
One-pot granulating and drying systems are suitable where dry mixing, liquid addition, wet granulation and drying are to be combined in a single closed plant. Depending on the system, drying can be carried out via vacuum, microwave or an integrated fluid bed. Such plants are particularly useful in the production of granulates for solid dosage forms. For a pure, particularly gentle final blend of already-dry excipients, however, they are often not the first choice.
Decisive for the selection are the product state, the desired target residual moisture, the permissible product temperature, shear sensitivity, flow behaviour, particle size distribution, and the tendency to agglomerate or segregate. Requirements for containment, inerting, explosion protection, solvent recovery, cleanability and GMP-compliant design are equally important. The final selection should be based on trials with the original product. Alongside drying time and residual moisture, mixing homogeneity, particle change, flow behaviour, segregation tendency and the cleanability of the system also need to be assessed.
How amixon® solves this task
For pharmaceutical excipients that need to be both dry-mixed and dried in a single plant, the amixon® vacuum mixer dryers of the VMT and AMT series are particularly suitable. They combine the gentle homogenisation of dry powders and granulates with vacuum contact drying of moist products in a single closed apparatus. Depending on the recipe, suspending, conditioning, de-agglomerating, coating or crystallising can also be integrated.
The VMT is a vertical vacuum mixer dryer and reactor for powders, suspensions, pastes and doughs. The AMT is a conical vacuum mixer dryer and mixing reactor. Both units are pressure- and vacuum-tight and can be temperature-controlled with water, steam or thermal oil. Under vacuum, water or solvents can be evaporated at low product temperatures. This is advantageous for temperature-sensitive, oxidation-sensitive or high-value pharmaceutical excipients.
A key advantage of the amixon® vacuum mixer dryers lies in the large, actively used heat-transfer surface. Besides the double-walled mixing chamber, the shaft, mixing arms and helical mixing tool can also be temperature-controlled. Heat is thus introduced into, or removed from, the product not only via the vessel wall but also via the product-contact surfaces of the mixing tool.
What is decisive for efficient drying is that these temperature-controlled surfaces are continuously washed over by the mix. The vertical helical mixing tool generates a three-dimensional product movement. In the outer region, the mix is conveyed upward, while it flows back down at the centre. This continuously renews the product's contact with the temperature-controlled surfaces. Heat and mass transfer remain effective throughout the entire drying process, even as the product's flow behaviour and consistency change.
SinConvex® and SinConcave® helical mixing tools are available for high mixing qualities and very good discharge. Both tool geometries were developed for demanding applications in the pharmaceutical industry. They combine effective three-dimensional product flow with particularly high mixing quality and near-complete discharge. The specific pitch of the helix supports complete product discharge and reduces product residues in the mixing chamber. For SinConcave®, residual-discharge levels of up to 99.997 percent are quoted.
SinConvex® supports an even product movement along the wall of the mixing chamber and is particularly suitable where a robust and process-reliable discharge is required alongside very good mixing quality. SinConcave®, by contrast, favours a thrust-dominated, gentle circulation. Free-flowing powders and sensitive granulates can slide downward along the steeply inclined surfaces without being unnecessarily broken down or segregated.
amixon® MultiPlane® mixing tools are available for particularly gentle mixing tasks. They are suitable for processing sensitive, breakage-sensitive or abrasion-critical excipients. They generate an even three-dimensional product movement at low tool speeds and limit mechanical stress. This allows even sensitive particles to be homogenised without particle breakage, abrasion or unwanted segregation compromising the process.
For moist, sticky or agglomerated products, cutting rotors can additionally be used. They support de-agglomeration and improve product movement during viscoplastic process phases. Their intensity can be matched to the recipe, allowing either gentle homogenisation or targeted breakdown of agglomerates.
For pharmaceutical applications, VMT and AMT can be executed hygienically and to a GMP-oriented standard. This includes suitable materials, defined surface qualities, low-dead-space constructions, cleaning-friendly connections, tight shaft seals, inerting and, on request, sterile designs to FDA, EHEDG or 3-A requirements. VMT systems can be designed as sterile mixers and reactors.
The suitability of a vacuum mixer dryer is verified in the amixon® pilot plant with the original product. Mixing quality, drying time, target residual moisture, product temperature, flow behaviour, particle change and residual discharge are examined. The trial results form the basis for selecting apparatus size, tool geometry, temperature range, vacuum level and automation.