Liquid Mixing Flashcards

1
Q

Definition of mixing

A

An operation in which 2/more components are treated so that each particle lies as nearly as possible in contact with a particle of each of the other component

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2
Q

Objective of mixing

A

1) Produce a simple physical mixture
2) Produce a physical change
3) Promote or control chemical reaction
4) Produce a dispersion

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3
Q

How to determine degree of mixing?

A

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

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4
Q

Types of mixtures (& their behaviours)

A

1) Positive mixtures (Eg. Mixture of gases/miscible liquids)
- Formed frm materials where irreversible mixing would take place by diffusion, provided time is unlimited

2) Negative mixtures (Eg. Suspension)
- Consist of components which will separate unless work is continually expended on them

3) Neutral mixtures (Eg. Mixture of powders, ointments, suppositories)
- Consist of components that do not mix/segregate spontaneously

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5
Q

2 requirements of mixing operation?

A

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

2) General movement (flow)
- Takes all part of the material through the shearing zone to produce a uniform mixture

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6
Q

How is liquid mixing performed?

A
  • Performed with a rotational device (mixing element) in a vessel
  • > Mixing element has a suitable shape to produce shear forces & appropriate flow pattern
  • Satisfactory flow pattern depends on the balance of the 3 velocity components (longitudinal, radial, tangential)

Excessive radial movement: Materials taken to vessel wall
“ longitudinal movement: Stratification (layering effect)
“ tangential movement: Vortexing

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7
Q

Factors affecting flow pattern in the vessel

A

1) Form of impeller
Propeller mixer: Strong longitudinal component
Turbine mixer: Strong radial, weak tangential
Paddle: Strong radial & tangential

2) Position of impeller (Offset/Angled/Side-entering)

3) Container shape
(Cylindrical, Double cone, slant cone, V-Shape)

4) Presence of baffles (Vertical strips extending frm wall)
- Cons: Can result in some parts not effectively mixed

5) Liquid properties
- More difficult to mix liquids of higher viscosity

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8
Q

List types of liquid mixers

A
  • Shaker mixers
  • Propeller mixers
  • Turbine mixers
  • Paddle mixers
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9
Q

Shaker mixers (Use, advantages, disadvantages)

A

Cons:

  • High variable mixing efficiency
  • Affected by properties of the liquids & the constructional characters of the mixer
  • Limited use in practice
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10
Q

Propeller mixers (Use, advantages, disadvantages)

A
  • Accentuates the longitudinal movement but imparts little shear
  • D/d = 20 & high speed up to 8000rev/min

Use: Liquids of low viscosity & suspensions
(Resembles marine propeller of ship)
-> 2-bladed/3-bladed/Flat-bladed/Torrance ‘Trifoil’

Cons:

  • Not suitable for high viscosity liquids (>1000cps) & emulsification
  • Shearing not strong
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11
Q

Turbine mixers (Use, advantages, disadvantages)

A
  • Has circular disc attached to a no. of short & vertical blades
  • May be fitted with a diffuser ring to increase shear forces
  • Usually rotated @ lower speed than propeller impeller

Use (Strong radial & shear forces, weak tangential):

  • Emulsification (Where shearing is impt)
  • Mixing liquids of high viscosity (up to 100,000cps)
  • Mixing liquids that may stratify with propeller mixer

Cons:
- Less suitable for suspending heavy solids when fitted with a diffuser ring

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12
Q

Paddle mixers (Use, advantages, disadvantages)

A
  • Use an impeller consisting of flat blades attached to a shaft (Eg. plain/abciliary/anchor/anchor-gate paddle)
  • Low speed usually employed (100rev/min)
  • -> If high speed, need baffles to avoid swirling & vortexing

Variety of paddle mixers available:

1) Plain paddles
- Mixing liquids of LOW VISCOSITY

2) Paddles with blades fitted closely to vessel wall
- Mixing liquids of HIGH VISCOSITY

3) Planetary motion mixer (small paddle rotating on its own axis while travelling in a in a circular path round the vessel)
- Mixing HIGHLY VISCOUS liquids
- Eg. mixer for baking

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13
Q

Common problems in liquid mixing

A

Vortexing & aeration

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14
Q

How to overcome problems in liquid mixing?

A

1) Mounting impeller deep in liquid
2) Avoid symmetrical position of impeller
3) Employing a push-pull propeller (b/c diff flow patterns)
4) Employing baffles (Overcome vortexing)

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15
Q

Why is dilatant flow a prob in the production of liquid preparations?

A

Dilatant flow: Viscosity increases with increasing shear stress

  • Liquid can freeze @ very high speed bc it exhibits high resistance to flow
  • Exerts overload on motor
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