Liquid mixing

Question 1

What is mixing?

Answer 1

Defined as an operation in which two 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

Question 2

What are the 4 broad objectives of mixing?

Answer 2

To produce a simple physical mixture
To produce a physical change
To promote or control chemical reaction
to produce a dispersion

Question 3

Chose the scenarios that require higher degrees of mixing:

1) Mixing 2 solids vs 2 miscible liquids
2) Preparation of a solution vs an emulsion

Answer 3

Solids

emulsion

Question 4

What are the three types of mixtures?

Answer 4

Positive, negative and neutral mixtures

Question 5

Which type of mixture would take place without the expenditure of work, provided that time is unlimited?

Answer 5

positive mixture

Question 6

Which type of mixture would not mix or segregate spontaneously?

Answer 6

Neutral mixture

Question 7

What are the two requirements of a mixing operation?

Answer 7

Localised mixing (shear) to the liquid

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

Question 8

What is another name for a rotational device of a mixer?

Answer 8

Mixing element or impeller

Question 9

What are the three types of forces along the circle of rotation of an impeller?

Answer 9

Longitudinal (vertical)
Radial (perpendicular to direction of movement)
Tangential (direction of movement)

Question 10

What happens when there is excessive radial movement?

Answer 10

Materials get taken to the vessel wall - solid particles would sediment to the bottom of the vessel wall, ineffective mixing

Question 11

What happens when there is inadequate longitudinal movement?

Answer 11

Stratification of the mixture (i.e separation)

Question 12

What happens when there is dominant tangential movement

Answer 12

Vortexing - liquid will swirld around the impeller shaft, air could be incorporated

Question 13

What are the factors that affect flow pattern in the vessel?

Answer 13

Form of impeller
position of impeller
container shape
presence of baffles
liquid properties

Question 14

Characteristic of propeller mixer?

Answer 14

Strong longitudinal component

Question 15

Characteristic of turbine mixer

Answer 15

Strong radial component

Weak tangential component

Question 16

Characteristic of paddle mixer?

Answer 16

Strong radial component

Strong tangential component

Question 17

Which positions of impeller can result in adequate flow patterns?

Answer 17

Offset
Angled
Side-entering

Question 18

What are the different container shapes for mixing and which is the most commonly used?

Answer 18

Double cone
Slant cone
V shaped
Cylindrical (most common)

Question 19

What are baffles used for?

Answer 19

installed to reduce vortexing

Question 20

What is a disadvantage of baffles

Answer 20

Can result in dead spots where mixing can’t be effectively carried out

Question 21

A small impeller has a higher D/d (vessel to impeller diameter). What is the relative speed of rotation and what type of liquids (viscosity) can it be used to mix?

Answer 21

High speed of rotation

Used for low apparent viscosity

Question 22

An impeller that has low D/d ratio will be more useful on liquids with high viscosity. It will usually operate at low speed of rotation. Name one example of a mixer that is like this.

Answer 22

Paddle mixer

Question 23

What types of liquid mixers are there?

Answer 23

shaker mixer
propeller mixer
turbine mixer
paddle mixer

Question 24

How are shaker mixers used?

Answer 24

Liquid contained in the vessel is agitated in an oscillary or rotatory movement.

Mixing efficiency is highly variable

Question 25

What are the characteristics of a propeller mixer?

Answer 25

High D/d ratio of 20, operates at high speeds of 8000RPM Unsuitable for liquids of high viscosity (>1000cps) and for emulsification. Accentuates longitudinal movements, but imparts little shear. Blades are inclined.

Question 26

What can be added onto a turbine mixer, and what can it do?

Answer 26

Diffuser ring, to impart more shear forces

Question 27

What are the characteristics of a turbine mixer?

Answer 27

Imparts strong radial movement and shear forces but little tangential movement Suitable for emulsification, mixing liquids of high viscosity (up to 100,000 cps) Mixing liquids that may stratify with a propeller mixer Blades are vertical, could be straight or curved

Question 28

What is a disadvantage of the diffuser ring?

Answer 28

Less suitable for use with suspending heavy solids as they may clog up the ring

Question 29

What are the characteristics of a paddle mixer?

Answer 29

Consists of flat blades attached to a vertical shaft. Blades are not inclined Low speed of around 100RPM usually employed. If high speeds are used, baffles are needed to avoid swirling and vortexing

Question 30

Different types of paddles are used for different types of liquids. For mixing of liquids with low viscosity, what kind of paddles can be used?

Answer 30

Plain paddles

Question 31

Paddles with blades that fit closely to the vessel wall are used for mixing what kind of liquids?

Answer 31

Higher viscosity

Question 32

The planetary motion mixer rotates on its own axis while travelling in a circular path round the vessel. What does this achieve?

Answer 32

Less energy required to rotate the paddle, also bring about better mixing. Overall more efficient mixing, useful for mixing highly viscous liquids.

Question 33

Vortexing and aeration are common problems encountered in liquid mixing, what can be done to overcome these?

Answer 33

Mounting impeller deep in the liquid Avoiding symmetrical positioning of impeller (offset instead of centre) Employing a push-pull propeller Employing baffles on a vessel wall

Question 34

What does a push-pull propeller do?

Answer 34

Impeller with 2 sets of blades inclined differently to bring about different flow pattern. This can reduce/ eliminate vortexing