Liquid Mixing

Question 1

What are the objectives of mixing?

Answer 1

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

Question 2

What are the different types of mixtures and their characteristics?

Answer 2

1. positive mixtures
   - low order of mixing
   - materials can mix very well with one another via diffusion
   - e.g. mixtures of gases or mixtures of miscible liquids
2. negative mixtures
   - high order of mixing required
   - e.g. solid particle dispersed in liquid medium
3. neutral mixtures
   - intermediate order of mixing, comprise components that do not mix or segregate spontaneously.

Question 3

What are the two requirements for the mixing operation?

Answer 3

• localised mixing (shear)
  » applies shear force to the liquid
• general movement (flow)
  » takes all parts of the material to the shearing zone to produce a uniform product.

Question 4

How is liquid mixing usually performed?

Answer 4

• with a rotational device (mixing element) in a vessel

Question 5

What are the three velocity components that guide the movement of the liquid at any point in the mixing vessel?

Answer 5

longitudinal, radial and tangential

- these three velocity components need to be balanced for a satisfactory flow pattern to be established.

Question 6

What happens when the impeller shaft is vertical?

Answer 6

• satisfactory flow pattern will not be established.
• excessive radial movement –> materials taken to vessel wall
• inadequate longitudinal movement –> stratification
• dominant tangential movement –> vortexing

Question 7

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

Answer 7

• form of impeller
• position of impeller
• container shape
• presence of baffles
• liquid properties

Question 8

What are the different forms of impeller?

Answer 8

• propeller mixer: strong longitudinal component
• turbine mixer: strong radial component, weak tangential component
• paddle mixer: strong radial component, strong tangential component.

Question 9

What are the different positions of impeller?

Answer 9

• offset
• angled
• side-entering

Question 10

What are the different shapes of the container?

Answer 10

• double cone
• slant cone
• v-shape
• cylindrical

Question 11

What are the effects of the presence of baffles on mixing?

Answer 11

• used to stop the swirl in a mixing tank
• installed in cylindrical vessels to reduce vortexing
• may not allow for effective mixing throughout the liquid.

Question 12

Why is it more difficult to mix liquids of higher viscosity?

Answer 12

• liquids of higher viscosity require larger impellers (their vessel to impeller diameter is smaller) and the speed of rotation is slower.