CPE Mixing (6)

How can we scale up a liquid mixer effectively?

geometric similarity

dimensionless numbers should be constant (Re for similar flow regime, power number Np for similar energy dissipation, Froude number Fr for similar surface behaviour)

constant P/V

How can mixing quality be described?

degree of homogeneity

mixing time

intensity of segregation or concentration fluctuations

What approaches assess mixing quality?

statistical methods (variance or standard deviation of concentration)

tracer methods

residence time distribution

sampling analysis

visual analysis

What causes segregation?

differences in particle size and shape

differences in density

What measures prevent segregation?

reduce size and density difference

increase mixing intensity or time

choose appropriate mixer type

add baffles

minimize free fall and uncontrolled flow path

Which two-phase mixing processes do you know?

liquid-liquid (emultions)

gas-liquid (sparging, aeration)

solid-liquid (suspension, dissolution)

gas-solid (fluidized bed)

solid-solid (powders, granulates)

What does the Froude number describe?

compares inertial forces to gravitational forces

describes how strongly the material is lifted relative to gravity

What are typical ranges of Fr for free-fall and throwing mixers?

less than 1 for free-fall mixers

larger than 1 for throwing mixers

What is mixing?

material combination of different material components

What is the aim of mixing processes?

homogenization of the mixture (small partial volumes contain components in largely the same composition)

What purpose can mixing serve for heat and mass transfer?

eliminating temperature and concentration gradients that limit transfer rates

Define stirring

production of material movement by mechanical means for achieving the most homogeneous conditions in a system

Examples of stirring tasks

homogenization

suspension

emulsification

dispersion

gas sparging

heat transfer

What is the objective of suspension?

achieving a quasi-homogeneous distribution of solid particles in a liquid

What is the objective of emulsification?

production of a stable dispersion of a two insoluble liquid phases

What is the objective of dispersion?

to intensify material exchange between two insoluble liquid phases or liquid/solid phase

What is the objective of gas sparging?

substance exchange between liquid and gas phases

What is the objective of heat transfer?

to intensity the heat exchange in a homo- or heterogeneous mixture or between a mixture and a reactor wall

What factors need to be considered for a stirring task?

physical properties of the system

performance goals

thermal management

shear limitations

What reactors type exist?

batch reactor

plug flow reactor

mixed flow reactor

Batch reactor

perfectly mixed

no dead zones

no concentration gradients

Plug flow reactor

unmixed

no mixing in longitudinal direction

same residence time for each component

Mixed flow reactor

perfectly mixed with continuous flow

no dead zones

no concentration gradient

exit concentration = vessel concntration

What are the objectives of agitators?

mixing liquids and suspending solids unformly

promoting chemical reactions through enhanced contact

maintaining homogeneous bulk properties during storage

increasing heat and mass transfer coefficients

How can scale up be performed?

numbering up (increasing the number of similar equipment)

geometrically

What problems are associated with geometric scale up?

cooling problem

mixing problem

What is mixing time determination based on?

introduction of a small amount of detectable liquid into the tank and monitoring the concentration using a probe

How is the extent of mixing quantified?

by how large are the oscillations of the measured concentration of a detectable liquid in a tank

What does mixing time t₉₉ mean?

time required for the concentration oscillations to be within 1% of the true value

What is the mixing rule of thumb?

mixing is not a challenge if reaction half time is at least 8 times the mixing time

When is 99% mixing reached?

if the tank volume makes five passes through the impeller pump during turbulent conditions

What is the specific power input (P/V)?

energy dissipation rate pr unit of volume

What does specific power input represent?

how much mechanical energy is introduced per unit volume of fluid per unit time

What does higher P/V mean?

more intense turbulence

faster mixing

faster mass transfer

smaller droplets/bubbles in dispersions

higher shear rates

Name common types of stirrers

turbine stirrer

impeller stirrer

cross beam, grid or blade stirrer

anchor stirrer

helical ribbon stirrer

What are anchor stirrers used for?

highly viscous sistems

Why are anchor stirrers edged?

to intensity heat transfer

What are the characteristics of a helical ribbon stirrer?

slowly conveys high viscosity fluids to the edge and bottom of the tank

What are the two possible ways of particle movement in solid mixtures?

convective transport

dispersive mixing

Define convective transport

bulk movement of groups of particles from one region of the mixer to another

How does convective transport occur?

the mixer applies force to push large areas of material, creating a directional movement of particle groups

Why is loosening important for convective transport?

it reduces friction to allow bulk to be transported

Define dispersive mixing

breaking up particle clusters and agglomerates and redistribution of individual particles

How does dispersive mixing occur?

due to directional movement and shear forces particles slide, collide and separate from each other

Requirements for solid mixers

short mixing time (for productivity)

gentle operation (to ensure material doesn’t degrade or primary particles don’t break)

enhanced randomness (to suppress systematic movement causing segregation)

Name the types of solid mixers based on their operation principle

drum mixer (mixer with moving container, or free fall mixer)

screw mixer (mixer with slow moving mixing tools, or thrust mixer)

shovel mixer (mixed with fast moving mixing tools. or throwing mixer)

pan mixer (moving container + tools)

mixing silos (pneumatic mixer)

Working principle of a jet mixer

spiral upward flow through the air with exiting at high speed

Characteristics of a jet mixer

strong turbulence and large velocity differences

What are the advantages of a jet mixer?

extensive loosening

short mixing times

uniformity even of segregated products

What are the disadvantages of a jet mixture?

large air requirement

high pressure compressor is required