310204f Drying, Humidification And Dehumidification Flashcards
What are the 2 most common methods of dehumidification?
- Compression followed by refrigeration or cooling
- Desiccants to absorb moisture
When drying solid materials, the liquid molecules are activated and separated from the material. What are 4 ways of increasing the motivation or activity of the liquid molecules
- Raise the temperature - causes the liquid molecules to move faster and eventually escape from the material to form a gas
- Reduce the atmospheric pressure - creates more space for the liquid molecules in the ambient atmosphere and makes it easier for them to migrate out of the drying material
- Move the ambient air layer over the solids - sweeps away the molecules of the liquid that have left the solid material and encourages the remaining liquid molecules to break away
- Agitate the drying solid material - exposes more of the surface to the air, which allows more uniform heating
What are the 2 main categories of heat that are required for drying?
- Direct heat
- Indirect heat
What is one concern or thing to keep in mind when using direct heat drying
The hot gas that contacts the material can cause contamination
What are five advantages of indirect heat drying over direct heat drying
- Requires less heat then direct heat drying
- The dried salts are not contaminated by the heated carrier gas
- The liquid solvent can be recovered more easily
- There are no fine dust losses
- The pressure can be manipulated
What type of materials are tray dryers, also called shelf dryers, suitable for?
They are suitable for granular materials such as sugar or powdered milk
Define sublimation
Sublimation is when water changes directly from a solid to a vapour
What is the difference between a vacuum tray dryer and a tray dryer?
Tray dryers use direct heating
Vacuum tray dryers operate under vacuum and are indirectly heated
What is vacuum tray drying useful for?
Vacuum drying is useful for drying heat sensitive materials such as powdered milk.
Vacuum freeze drying reduces the flavor lost caused by drying foods
The product is cooled, then frozen to a boat, -30°C and dried under high vacuum pressure
The combination of the low temperature and high vacuum causes the water to evaporate below its freezing point
Dehumidification removes moisture from air or gas
What are 3 methods of dehumidification?
- Compression and refrigeration
- Desiccant beds
- Deliquescent desicant beds
Define dew point
The dew point is the temperature at which the air or gas can no longer hold all the water vapor and some of the vapor must condense into liquid form
This dew point range is the lowest achievable dew point with a refrigerated dehumidifying system. Because the condensate begins to freeze at 0°C
What is a desiccant bed?
Desiccant beds hold a drying agent called desiccant
When air or gas pass through the desiccant, the desicant absorbs the moisture from the air
When a desiccant absorbs moisture, it holds the moisture in a thin layer on its surface.
Define sorption
Sorption is a mass transfer process or a component or multiple components in a fluid are transferred to another fluid or solid
The transferred component transfers in 1 of 2 ways:
1. On the interface (adsorption processes)
2. By dissolving in the other phase (absorption processes)
Define adsorption
Adsorption is a surface process where gas or liquid accumulates on a liquid or solid surface
Adsorption is based on the strength of the interaction between the adsorbent and the adsorbed molecules
To add adsorption processes are physisorption and chemisorption
Define absortion.
Absorption uses the properties of solid liquid or gas
The absorbing material draws in liquid or gas molecules from the wet material
The absorbent is the substance that absorbs the other substance
The absorbate is the substance being absorbed
Absorption can be either physical or chemical
Why would a deliquescent desiccant dehydration system be suitable for remote gas drying applications?
Because it has no moving parts other than the liquid draining equipment and does not require power
A deliquescent desiccant natural gas dehydration the vessel has no pump or reboiler produces no emissions and consumes no fuel gas.
What is the minimum number of vessels used in a regenerative desiccant system?
A regenerative desiccant system uses a minimum of two vessels
This allows drying to occur in one vessel and regenerating in the other
