Fluid mixing

Question 1

what is mixing

Answer 1

Mixing is 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

The objectives of mixing may be broadly classified as follows:

Answer 2

1)To produce a simple physical mixture
(2 or more miscible fluid / solid; diluting potent drug; need high degree of mixing)

2)To produce a physical change (eg salt mixing no need high degree of mixing)

3)To promote or control chemical reaction
(eg acid and base; if sensitive to ph change then need good mixing if not nvm)

4)To produce a dispersion (suspension / emulsion)

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

Question 3

Mixtures may be divided into three types that differ fundamentally in their behaviour

Answer 3

Positive mixtures (low mixing)
these are formed from materials where irreversible mixing would take place by diffusion, without the expenditure of work provided that time is unlimited.
(tend to be miscible)

Negative mixtures (high mixing)
These consist of components which will separate unless work is continually expended on them.
(suspension and emulsion)

Neutral mixtures (intermediate mixing) 
These consist of components that do not mix or segregate spontaneously.
eg powder and pastes

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

Question 4

The degree of mixing needed depends on the

Answer 4

objective of the mixing, as well as the type of mixture to be produced

Question 5

The fluid mixing operation requirements:

Answer 5

(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 6

Liquid mixing is usually performed with

Answer 6

a rotational device (mixing element) in a vessel

The mixing element has a suitable shape to produce shear forces and appropriate flow pattern

Question 7

The movement of the liquid at any point in the mixing vessel has _____ components

Answer 7

1) longitudinal (parallel to shaft)
2) radial (perpendicular to shaft)
3) tangential (tangent to circle of rotation)

Question 8

Assuming shaft is vertical

excessive radial movement cause

Answer 8

material to be taken to vessel wall.

solid particles will sediment under the influence of gravity and rotate at the bottom and wont achieve good mixing.

Question 9

Assuming shaft is vertical

inadequate longitudinal movement cause

Answer 9

stratification

eg 2 immiscible liquid will remain as 2 phase and rotate within its components layer.

Question 10

Assuming shaft is vertical

dominant tangential movement cause

Answer 10

vortexing

not effective mixing
cone of air the separate the impeller from the liquid.

Question 11

vortexing is form because

Answer 11

a vortex form when the centrifugal force imparted to the liquid by the propeller blades causes it to back up round the sides of the vessels and form a depression around the shaft.

as the speed of rotation is increased, air may be sucked into the fluid due to the formation of a vortex; this can cause frothing and possible oxidation

Question 12

factor affecting flow pattern

Answer 12

Form of impeller 
Position of impeller 
Container shape 
Presence of baffles
Liquid properties

Question 13

components of propeller mixer

Answer 13

strong longitudinal component

Question 14

components of turbine mixer

Answer 14

strong radial component,

weak tangential component

Question 15

components of paddle mixer

Answer 15

strong radial component

strong tangential component

Question 16

position of impeller

Answer 16

offset
angled
side-entering

Question 17

container shape

Answer 17

cylinder or symmetrical container tends to form vortex

which can be overcome by baffles

Question 18

presence of baffles

Answer 18

create dead spot
(liquid near here dont mix well with other)

guide movement of fluid and prevent vortex so mix well

create turbulence by diverting the rotating fluid from its circular path to the centre of the vessel where the vortex would otherwise form.

Question 19

liquid properties

Answer 19

Mixing parameters depend on the viscosity of the liquid

Apparent Viscosity = LOW
Speed of rotation = HIGH
D/d = 20

Apparent Viscosity = HIGH
Speed of rotation = LOW
D/d = ~ 1

D= vessel diameter; d=impeller diameter

Question 20

types of fluid mixer

Answer 20

Shaker mixers
Propeller mixers
Turbine mixers
Paddle mixers

Question 21

shaker mixer

Answer 21

The vessel containing the liquid is agitated in an oscillary or rotatory movement
Mixing efficiency is highly variable
It is affected by the properties of the liquids and the constructional characters of the mixer
Such liquid mixers have limited use in practice

Question 22

propeller mixer

Answer 22

The impeller usually resembles the ordinary marine propeller
It accentuates the 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

stratification occur if mixing high viscosity fluid

Question 23

Turbine mixer

Answer 23

The impeller has a circular disc to which are attached a number of short and vertical blades which may be straight or curved.
It may be fitted with a diffuser ring to increase the shear forces

The turbine mixer produces strong radial movement and shear forces but little tangential movement
The turbine impeller is usually rotated at a lower speed than a propeller impeller

It is suitable for:
Emulsification
Mixing liquids of high viscosity (up to 100000 cps)
Mixing liquids that may stratify with a propeller mixer Fitted with a diffuser ring, it is less suitable for suspending heavy solids

Question 24

principle of turbine mixer

Answer 24

The impeller has four flat blades surrounded by perforated inner and outer diffuser rings. The rotating impeller draws the liquid into the mixer ‘head’ and forces the liquid through the perforations with considerable radial velocity, sufficient to overcome the viscous drag of the bulk of the fluid. One drawback is the absence of an axial component, but a different head with the perforations pointing upwards can be fitted if this is desired. As the liquid is forced through the small orifices of the diffuser rings at high velocity, large shear forces are produced. When mixing immiscible liquids, if the orifices are sufficiently small and velocity sufficiently high, the shear forces produced enable the generation of droplets of the dispersed phase which are small enough to produce stable dispersions (water-in-oil or oil-in-water). Turbine mixers of this type (homogenizers) are therefore often fitted to vessels used for the large-scale production of emulsions and creams. Turbine-type mixes will not cope with liquids of very high viscosity since the material will not be drawn into the mixer head. These liquids are best treated as semi-solids and handled in the same equipment as used for such materials

Question 25

paddle mixer

Answer 25

They use an impeller which consists of flat blades attached to a vertical shaft.
Low speed of the order of 100 rev/min is usually employed.
If high speed is used, baffles are needed to avoid swirling and vortexing.

paddle mixer accentuates tangential movement so mixing is often accompanied by vortexing which is made worse when rotating at high speed.

Question 26

speed of impeller

Answer 26

fastest
propeller
turbine
paddle
slowest

Question 27

types of paddle mixer

Answer 27

A variety of paddle mixers is available for mixing liquids of different viscosities

Plain paddles
For mixing liquids of low viscosity

Paddles with blades that fit closely to the vessel wall For mixing liquids of higher viscosity

Smaller paddle that rotates on its own axis while travelling in a circular path round the vessel
Known as planetary motion mixer
For mixing highly viscous liquids

Question 28

Vortexing and aeration are common problems encountered in liquid mixing
how to overcome

Answer 28

a) Mounting impeller deep in the liquid
b) Avoiding symmetrical positioning of impeller c)Employing a push-pull propeller
d) Employing baffles on vessel wall

Question 29

planetary motion mixer principle

Answer 29

small clearance between the vessel and the paddle in order to ensure sufficient shear.
scrapping down of the bowl is usually necessary several times during a run to mix the contents well, since some materials are forced to the top of the bowl.

The problems that arise during the mixing of semi-solids (ointment and pastes) stem from the face that, unlike liquids, semi-solid will not flow easily.
Material that finds its way to a dead spot will remain there.
therefore mixer must have rotating elements with narrow clearances between themselves and the mixing vessel wall and they must produce a high degree of shear mixing since diffusion mixing cannot occur.