Liquid mixing Flashcards
What is mixing?
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
What are the 4 broad objectives of mixing?
To produce a simple physical mixture
To produce a physical change
To promote or control chemical reaction
to produce a dispersion
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
Solids
emulsion
What are the three types of mixtures?
Positive, negative and neutral mixtures
Which type of mixture would take place without the expenditure of work, provided that time is unlimited?
positive mixture
Which type of mixture would not mix or segregate spontaneously?
Neutral mixture
What are the two requirements of a mixing operation?
Localised mixing (shear) to the liquid
General movement (flow) - takes all parts of the material through the shearing zone to produce a uniform product
What is another name for a rotational device of a mixer?
Mixing element or impeller
What are the three types of forces along the circle of rotation of an impeller?
Longitudinal (vertical)
Radial (perpendicular to direction of movement)
Tangential (direction of movement)
What happens when there is excessive radial movement?
Materials get taken to the vessel wall - solid particles would sediment to the bottom of the vessel wall, ineffective mixing
What happens when there is inadequate longitudinal movement?
Stratification of the mixture (i.e separation)
What happens when there is dominant tangential movement
Vortexing - liquid will swirld around the impeller shaft, air could be incorporated
What are the factors that affect flow pattern in the vessel?
Form of impeller position of impeller container shape presence of baffles liquid properties
Characteristic of propeller mixer?
Strong longitudinal component
Characteristic of turbine mixer
Strong radial component
Weak tangential component
Characteristic of paddle mixer?
Strong radial component
Strong tangential component
Which positions of impeller can result in adequate flow patterns?
Offset
Angled
Side-entering
What are the different container shapes for mixing and which is the most commonly used?
Double cone
Slant cone
V shaped
Cylindrical (most common)
What are baffles used for?
installed to reduce vortexing
What is a disadvantage of baffles
Can result in dead spots where mixing can’t be effectively carried out
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?
High speed of rotation
Used for low apparent viscosity
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.
Paddle mixer
What types of liquid mixers are there?
shaker mixer
propeller mixer
turbine mixer
paddle mixer
How are shaker mixers used?
Liquid contained in the vessel is agitated in an oscillary or rotatory movement.
Mixing efficiency is highly variable
What are the characteristics of a propeller mixer?
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.
What can be added onto a turbine mixer, and what can it do?
Diffuser ring, to impart more shear forces
What are the characteristics of a turbine mixer?
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
What is a disadvantage of the diffuser ring?
Less suitable for use with suspending heavy solids as they may clog up the ring
What are the characteristics of a paddle mixer?
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
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?
Plain paddles
Paddles with blades that fit closely to the vessel wall are used for mixing what kind of liquids?
Higher viscosity
The planetary motion mixer rotates on its own axis while travelling in a circular path round the vessel. What does this achieve?
Less energy required to rotate the paddle, also bring about better mixing.
Overall more efficient mixing, useful for mixing highly viscous liquids.
Vortexing and aeration are common problems encountered in liquid mixing, what can be done to overcome these?
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
What does a push-pull propeller do?
Impeller with 2 sets of blades inclined differently to bring about different flow pattern.
This can reduce/ eliminate vortexing
