Solid Mixing Flashcards
Occurs when the granular bed undergoes solid body rotation and then slides, usually intermittently, against the rotating tumbler walls.
The Slipping Regime
In this regime, flow consists of discrete avalanches that occur as a grouping of grains travel down the free surface and come to rest before a new grouping is released from above.
Avalanching flow, also referred to as slumping
At higher tumbling speeds, discrete avalanches give way to continuous flow at the surface of a blend (Shinbrot et al., 1999a). Grains beneath this surface flowing layer rotate nearly as a solid body with the blender until they reach the surface.
Rolling
As the rotation
speed of the tumbler is increased, the surface becomes increasingly sigmoidal until grains become airborne, and at higher speeds yet, the grains centrifuge against the tumbler wall.
Cascading, Cataracting, and Centrifuging
Exhibit stick-slip motion so that flow becomes intermittent rather than continuous. This is a situation of practical importance since most industrial applications use particles across a broad range of sizes and materials.
Weakly Cohesive Material
Processing blends of dissimilar grains almost invariably promotes ________, characterized by the spontaneous emergence of regions of nonuniform
composition.
de-mixing, also referred to
as segregation
Large grains rapidly segregate radially, producing a central core of fine grains surrounded by larger grains.
Radial De-mixing
A second stage of segregation occurs in drum tumblers as grains in the core migrate along the tumbling axis. Numerical and experimental investigations have attributed this migration to conflicting causes.
Axial De-mixing
These patterns are believed to arise from a competition between surface segregation of coarse grains flowing over a radially segregated core of fine grains and interactions with the boundaries of the tumbler.
Competitive Patterned De-mixing
Three of the most common geometries
used in pharmaceutical operations are the:
Double cone
V-blender
Bin blender
In these simulations, particles are treated as individual entities with physical properties
appropriate to the problem of interest, and Newton’s laws of motion are integrated for each particle.
Particle-dynamic simulations
Two of the most common classes of particle-dynamic simulations are termed:
Hard-particle and Soft-particle methods
calculate particle
trajectories in response to instantaneous, binary collisions between particles, and allow particles to follow ballistic trajectories between collisions.
Hard-particle methods
allow each particle
to deform elastoplastically and compute responses using standard models from elasticity and tribology theory. This approach permits enduring particle contacts and is therefore the method of choice for tumbler applications.
Soft-particle methods,
The faster and more efficient mixing mechanism, yet at the same time it suffers from the same mixing limitations known for
fluids
Convection in grains (as in fluids)
Transport associated with flow driven by gravity (in
tumbling blenders) or impellers (in intensified, ribbon, or
other blenders).
Convection in the context of granular blenders
The random
relocation of individual grains due to collisions
between adjacent particles and can take hundreds to thousands of revolutions to act.
Dispersion, or diffusion
The second class of blenders commonly used in industrial applications_______. It
primarily mix by transporting materials throughout a mixing
vessel by the motion of a stirring device.
Convective Blenders
It is one of the most common general-purpose mixers, as it can effectively perform a wide range of mixing processes including
liquid, solid, and liquid-solid blending.
Ribbon Blender
Scope of Solid–Solid Mixing (5):
- Mixing of product for homogenization of quality or reduction of variance
- Mixing of active ingredient onto a carrier material
- Mixing of multicomponent mixtures as a formulation
- Coating of a cohesive component onto a carrier particle
- Mixing of fine powders to create a homogeneous mixture at the particulate level
Any sample randomly taken from the mixture will contain the same proportion of each particle as the proportions present in the mixture taken as a whole.
Perfect Mixture or Perfect mixture of two types of particles
When two non-interacting components (e.g., free-flowing pellets) with similar properties (size,
shape, elasticity,etc.) are mixed in an ideal mixer, the quality of mixing reaches an asymptotic
limit of random mixing.
Random or Stochastic Mixture
When two interacting components are mixed together, a structure or order can build up into the mixture. The units ordered could be a result of agglomeration or cohesion of one component to the other or a mixture of the two.
Ordered Mixture
Once the cohesive fraction or minor component saturates available sites on the carrier particles, the remaining fraction will agglomerate.
Partially Ordered Random Mixture
When the ordered units contain different numbers of adherent particles and the carrier particles are randomly mixed.
Pseudorandom Mixture
Mixing ingredients in any amount in individual batches in an individual mixer or a vessel. All ingredients are loaded into a mixer and agitated for a certain period until
they are homogeneously distributed or mixed.
Batch Mixing
Used to mix ingredients continuously in a mixer in a single pass. The
ingredient quantity to be mixed may vary in any range.
Continuous Mixing
Occurs due to small scale random motion of particles when they roll over a free surface.
Diffusive mixing
