Water - Mass transfer Flashcards
What is mass transfer?
The moment of something that has mass (e.g. matter)
What does mass have a tendency to do?
Move from an area of high concentration to an area of low concentration
What kind of engineering processes does mass transfer apply to?
Membran processes, Aeration processes, spray drying, extractions, jet engines, cooling fans (remember 2)
What law does mass transfer follow? Describe it in general terms
Ohm’s law, Flow = driving force/resistance
What are the types of mass transfer driving forces?
Concentration differences ∆C, pressure differences ∆P, osmotic pressure differences ∆π, density differences ∆ρ
What are the two mechanism of mass transfer?
Convection and diffusion
When does convective mass transfer occur? What isa fluid system?
- In mixed fluid systems
- Fluid systems are a continuous amorphous substance whose molecules move freely past one another and have the tendency to assume the shape of its container (e.g. a gas or liquid)
When does convection work? How does it work? What causes it to work?
- When the bulk motion of the mass is caused by an externally forced fluid motion
- A driving forces causes mechanical motion of the substance
- Driving force can be created by temperature gradient, density differences, physical stirring etc.
What causes mass transfer to occur at a higher rate, (forced) convection or diffusion?
Forced convection
Which one of these situations is not convection? Why are they classified as convection/diffusion: Dispersion of ink in still water or dispersion of pollutants in flowing river
Dispersion of ink = diffusion, still water ∴ no mechanical driving force
Pollutant in flowing river = convection, moving water from driving force of gravity
What is molecular movement?
When single molecules move
What is bulk movement?
When many molecules are moving
Is diffusion a process of molecular or bulk movement? Why?
Molecular as there is no bulk movement of the fluid due to NO DRIVING FORCES (that is convection)
What is the driving forces of molecular diffusion?
Concentration differences
When there is no barrier separating a solution, what creates the resistance to the flow of molecules? What is the formula for this resistance?
-Resistance is created by the collision of fluid molecules with each other and with the walls of the container
- R = L / DA
R = resistance
L = Length of container
D = constant of diffusion
A = Area of container
What is the formula for mass transfer? How is it calculated?
m = DA (c1-c2) / L
m = mass flow rate by diffusion (units depend on volumes used)
D = diffusion constant
L = length of container
c1, c2 = concentration difference, MUST be positive
-Formula created by substituting the resistance formula (R = L/DA) and the driving force equation of diffusion (c1-c2) into the Flow rate equation (Driving force / Resistance)
What does the diffusivity constant describe? What do large value of D indicated
How fast an object diffuses, larger D = faster diffusion
What and how can the diffusivity constant be affected?
The temperature, increasing temperatures cause increased rates of diffusion
What are barriers? What do they do and why?
Solid objects (such as membranes) which molecules may diffuse through -Causes diffusion to occur at a slower rate as it creates a significant resistance to diffusion
What is the formula for the mixed mass transfer equation? When and why is this used?
m = kA (c1-c2) m = mass transfer k = overall mass transfer co-efficient A = area of container c1,c2 = concentration differences -This is used to calculate the mass transfer when diffusion and convection are both occurring and when the length is not well defined (these circumstances are more likely to happen)
