Liquid mixing

Question 1

What is the definition of mixing?

Answer 1

operation in which 2 or more components are treated so that each particle lies as nearly as possible in contact with a particle of each of the other component. (seen in ideal mixing which is hard to achieve)

Question 2

What are the objectives of mixing? (there are 4)

Answer 2

1) to produce a simple physical mixture (mix 2 solids or 2 liquids)
2) to produce a physical change (E.g. Soluble salt (solid) changed to liquid form if can mix the salt in a solvent)
3) to promote or control chemical reaction (Product obtained by chemical rxn by reactant/compound A and B)
4) to produce a dispersion (e.g. suspensions, emulsions)

degree of mixing needed depends on the objective of the mixing, as well as the type of mixture to be produced

Question 3

• Mix 2 solids VS 2 miscible liquids
• prepare a solution VS an emulsion

Which requires a higher degree of mixing?

Answer 3

Solids and emulsion require higher degree of mixing

Question 4

What are positive mixtures? What are some examples?

Answer 4

Irreversible mixing take place by diffusion, without the expenditure of work provided that time is unlimited (no time limit)
doesn’t require high order of mixing

E.g. Mixtures of gas/miscible liquids

Question 5

What are negative mixtures and examples?

Answer 5

Consist of components which will separate unless work is continually expended on them
Require high degree of mixing

e.g. suspensions

Question 6

What are neutral mixtures and examples?

Answer 6

Components that don’t mix or segregate spontaneously.
Intermediate mixing

E.g. Mixtures of powder; ointments; suppositories containing drugs

Question 7

What are the 2 requirements in mixing operation?

Answer 7

1) Localised mixing (shear) - applies shear to the liquid

2) General movement (flow) - takes all parts of the material through the shearing zone to produce a uniform product

Question 8

What are the 3 velocity components in the movement of the liquid at any point in the mixing vessel?

Answer 8

Longitudinal
Radial
Tangential

Question 9

What if there is excessive radial movement?

Answer 9

Materials taken to vessel wall

If e.g. Liquid with solid particles; solid will move to wall the solid will sediment to the bottom of the vessel

If there is excessive radial movement in suspension; solid particles at bottom and mixing not effectively conducted

Question 10

What if there is inadequate longitudinal movement? What is the term called?

Answer 10

Stratification

One layer of liquid above the other layer of immiscible liquid (layering effect)

If mixing suspension: solid particles in layer separate from liquid medium

Question 11

What if there is dominant tangential movement?

Answer 11

Vortexing
Liquid will swirl around the impeller shaft; liquid separated by shaft by cone of air; if cone of air reaches the tip of blade –> introduce air - bubbles ; if any component prone to oxidation; bubbles air - degradation of product

Question 12

What are the factors affecting flow patten in vessel?

Answer 12

1. Form of impeller
2. Position of impeller
3. Container shape
4. Presence of baffles
5. Liquid properties

Question 13

What are the 3 types of impeller and its properties (movement)?

Answer 13

Propeller mixer: strong longitudinal component

Turbine mixer: Strong radial component, weak tangential component

Paddle mixer: strong radial component, strong tangential component

Question 14

What are the 3 positions of impeller?

Answer 14

Offset (not in centre)
Angled
Side-entering

Question 15

What are 4 types of container shape?

Answer 15

• Double cone
• Slant cone
• V-shape
• Cylindrical

Question 16

What does presence of baffles do?

Answer 16

Vessel can have more than 1-2 baffles; baffles reduce vortexing

In cylindrical vessels + baffles

BUT Installation of baffles might have dead spots; mixing cannot be effectively carried out

The solid and liquid will not mix properly if it’s at dead spots

Question 17

Is more viscous or less viscous more difficult to mix liquids?

Answer 17

More difficult to mix liquids of higher viscosity

Question 18

What if the apparent viscosity of liquids is low?

Answer 18

Speed of rotation is high
D/d 20
(smaller impeller is used)

D= vessel diameter
d = impeller diameter

Question 19

What if the apparent viscosity of liquids is high?

Answer 19

Speed of rotation is low
D/d ~1

D= vessel diameter
d = impeller diameter

The size of impeller is as wide as the vessel diameter
Typically rotated at low speed
To mix high viscosity liquids large impeller with low speed

Question 20

What are the 4 types of liquid mixers?

Answer 20

Shaker mixers
Propeller mixers
Turbine mixers
Paddle mixers

Question 21

What are the properties of shaker mixers?

Answer 21

• Vessel containing the liquid is agitated in an oscillary or rotatory movement
• Mixing efficiency is highly variable
• Affected by properties of liquids and the constructional characters of the mixer
• limited use in practice

Question 22

What are the properties of propeller mixers?

Answer 22

• Accentuates longitudinal movement but imparts little shear
• suitable for mixing liquids of LOW VISCOSITY and SUSPENSIONS
• D/d 20 and high speed up to 8000 rev/min
• Unsuitable for liquids of high viscosity (>1000 cps) and for emulsification

There are inclined blades as well

Question 23

What are the properties of turbine mixers?

Answer 23

• Impeller has circular disc to which are attached a number of short and vertical blades which may be straight or curved
• May be fitted with a diffuser ring to increase the shear forces

Smaller the holes; greater shear force exerted on liquid as liquid move out from impeller to diffuser ring to other parts in vessel

• Produces STRONG RADIAL MOVEMENT and shear forces but little tangential movement
• turbine impeller rotated at lower speed than propeller mixer
• Suitable for:
  ~ Emulsification
  ~ Mixing liquids of HIGH VISCOSITY (up to 100000cps)
  ~ Mixing liquids that may stratify with a propeller mixer
  (Despite strong longitudinal component; may not be efficient to mix the liquids - stratification; possible to separate into layers; thus turbine may be used)
• Fitted with a diffuser ring, it is less suitable for suspending heavy solids

Question 24

What are the properties of paddle mixers?

Answer 24

• flat blades attached to a vertical shaft; The blades will lie flat on surface; no inclined (propeller is inclined and turbine blades are vertical but curved or straight)
• low speed of the order of 100 rev/min usually employed
• If high speed used, baffles needed to avoid swirling and vortexing; Strong tangential movement will promote vortexing; to minimise that in paddle mixers; baffles are usually employed

Question 25

What are the 3 variations of paddle mixers for mixing liquids of different viscosities?

Answer 25

1. Plain paddles (mixing liquids at low viscosity)
2. Paddles with blades that fit closely to the vessel wall (mixing liquids of higher viscosity)
3. Smaller paddle that rotates on its own axis while travelling in a circular path round the vessel - aka planetary motion mixer (mixing highly viscous liquids); Require less energy; circular path - better mixing of liquid - more efficient mixing using less energy

Question 26

How to overcome vortexing and aeration in liquid mixing? (There are 4 approaches)

Answer 26

1) Mounting impeller deep in liquid; Dont have the blade too close to the surface of the liquid
2) Avoiding symmetrical positioning of impeller (offset/ off-centre)
3) Employing a push-pull propeller; They are inclined differently - Note the different flow patterns
4) Employing baffles on vessel wall