heat and mass Flashcards
What is evaporation?
The phenomenon of change of a liquid into its vapors at any temperature below its boiling point.
What is the main operation in evaporation processes?
The removal of large amounts of water from solution.
What is a typical industrial application of evaporation?
Concentration of aqueous solutions of fruit juices, sodium hydroxide, sodium chloride, sugar, glycerol, glue, and milk.
What are the main benefits of evaporation?
- Reduction in mass and volume
- Preservation by reduced water activity
- Required operation for other treatments like crystallization
What is the difference between evaporation and drying?
- Evaporation removes large amounts of water from liquids
- Drying removes small amounts of water from solids
What factors affect evaporation?
- Solubility
- Temperature sensitivity of material
- Foaming or frothing
- Pressure and temperature
- Scale deposition
How does solubility affect evaporation?
The maximum concentration of solution obtained depends on the solubility of the solute in the solution.
What is boiling-point-rise (BPR)?
The phenomenon where the boiling point of the solution may rise as evaporation proceeds.
What is the role of pressure in evaporation?
The higher the operating pressure, the higher the boiling temperature of the solution.
What are the general types of evaporators?
- Open Kettle or Pan Evaporator
- Horizontal Tube Evaporator
- Short Tube Vertical Evaporator
- Falling Film Evaporator
- Forced Circulation Evaporators
- Agitated Thin Film Evaporator
- Open-pan Solar Evaporator
What characterizes an Open Kettle or Pan Evaporator?
It consists of an open pan where liquid is boiled, often operated at atmospheric pressure or under vacuum.
How does a Horizontal Tube Evaporator work?
The feed solution is inside the tubes, and steam condenses outside, causing liquid to rise naturally.
What is a Short Tube Vertical Evaporator known for?
It uses the formation of vapor bubbles to give rise to high liquid velocities and is suitable for foamy liquids.
What is the purpose of a Falling Film Evaporator?
The liquid flows down the inner wall of vertical tubes as a film, allowing for rapid vaporization and concentration.
What is a Forced Circulation Evaporator used for?
To concentrate highly viscous solutions by pumping liquid to enhance heat transfer.
What is the significance of mechanical agitation in an Agitated Thin Film Evaporator?
It improves the heat transfer coefficient by spreading the feed as a thin film on the heating surface.
What is a Single-effect evaporator?
An evaporator used when the required capacity is small or steam is cheap, with all operations occurring in one effect.
What is Multiple effect evaporation?
A process where vapors produced in one effect are reused to heat the feed in the next effect, improving steam economy.
What is the steam economy formula?
Steam economy = kg of vapor produced / kg of steam used.
What defines Forward-feed Multiple-effect Evaporators?
Both feed and steam are introduced in the first effect, with the boiling temperatures decreasing from effect to effect.
What is a key characteristic of Backward-feed Multiple-effect Evaporators?
The feed enters the last effect first, allowing for a more concentrated solution to be produced as it moves backward.
What is the last effect under when using steam at a pressure of 1 atm?
Under vacuum pressure
Vacuum pressure is used to prevent damage to the final concentrated product at high temperatures.
In backward feed multiple-effect evaporators, where does the feed enter?
The last effect (coldest effect)
This arrangement allows effective concentration due to high heat availability in the first effect.
What happens to viscosity in backward feed multiple-effect evaporators?
Viscosity reduces from effect to effect
High temperature in the first effect significantly reduces viscosity.
In mixed feed multiple-effect evaporators, where does the dilute feed liquid enter?
An intermediate effect
This allows for further concentration of the liquor before being pumped back.
What is the main advantage of the mixed feed arrangement in multiple-effect evaporators?
It eliminates some pumps needed in backward configuration
Flow occurs due to pressure difference whenever applicable.
In parallel feed multiple-effect evaporators, what is introduced in each effect?
Fresh feed
Products are withdrawn from the same effect, and there is no liquid transfer between effects.
What is the typical condition of vapors from the last effect in multiple-effect evaporators?
Usually under vacuum
These vapors must be condensed and discharged as liquid at atmospheric pressure.
What is a surface condenser?
A type of condenser where vapor and cooling water are separated by a metal wall
This prevents mixing and is often a shell and tube design.
What types of noncondensable gases may be present in the vapor stream?
- Air
- CO2
- N2
- Other gases
These gases can enter as dissolved gases or from decomposition.
What is the energy balance equation for direct contact condensers?
Heat IN = Heat OUT
This relationship involves vapor and cooling water flow rates.
How does feed temperature affect evaporator design?
Lower feed temperature requires more heat to reach boiling
This may necessitate a larger heat transfer area.
What is boiling point elevation (BPR)?
The difference between the boiling temperature of a solution and that of pure water at the same pressure
BPR increases as the solution becomes more concentrated.
What does Duhring’s chart represent?
The relationship between the boiling point of a pure liquid and that of a mixture
It is used for estimating boiling point rise (BPR).
Fill in the blank: The pressure in an evaporator for a solution of 30% NaOH is given as ______.
25.6 kPa (0.256 bar)
This pressure is crucial for determining the boiling point and BPR of the solution.
