Water - Mass transfer

Question 1

What is mass transfer?

Answer 1

The moment of something that has mass (e.g. matter)

Question 2

What does mass have a tendency to do?

Answer 2

Move from an area of high concentration to an area of low concentration

Question 3

What kind of engineering processes does mass transfer apply to?

Answer 3

Membran processes, Aeration processes, spray drying, extractions, jet engines, cooling fans (remember 2)

Question 4

What law does mass transfer follow? Describe it in general terms

Answer 4

Ohm’s law, Flow = driving force/resistance

Question 5

What are the types of mass transfer driving forces?

Answer 5

Concentration differences ∆C, pressure differences ∆P, osmotic pressure differences ∆π, density differences ∆ρ

Question 6

What are the two mechanism of mass transfer?

Answer 6

Convection and diffusion

Question 7

When does convective mass transfer occur? What isa fluid system?

Answer 7

• 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)

Question 8

When does convection work? How does it work? What causes it to work?

Answer 8

• 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.

Question 9

What causes mass transfer to occur at a higher rate, (forced) convection or diffusion?

Answer 9

Forced convection

Question 10

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

Answer 10

Dispersion of ink = diffusion, still water ∴ no mechanical driving force
Pollutant in flowing river = convection, moving water from driving force of gravity

Question 11

What is molecular movement?

Answer 11

When single molecules move

Question 12

What is bulk movement?

Answer 12

When many molecules are moving

Question 13

Is diffusion a process of molecular or bulk movement? Why?

Answer 13

Molecular as there is no bulk movement of the fluid due to NO DRIVING FORCES (that is convection)

Question 14

What is the driving forces of molecular diffusion?

Answer 14

Concentration differences

Question 15

When there is no barrier separating a solution, what creates the resistance to the flow of molecules? What is the formula for this resistance?

Answer 15

-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

Question 16

What is the formula for mass transfer? How is it calculated?

Answer 16

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)

Question 17

What does the diffusivity constant describe? What do large value of D indicated

Answer 17

How fast an object diffuses, larger D = faster diffusion

Question 18

What and how can the diffusivity constant be affected?

Answer 18

The temperature, increasing temperatures cause increased rates of diffusion

Question 19

What are barriers? What do they do and why?

Answer 19

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

Question 20

What is the formula for the mixed mass transfer equation? When and why is this used?

Answer 20

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)