# ITP: What is Transport Phenomena?

Transport Phenomena is the study of the transfer of momentum, heat and mass. These three "modes" of transport are usually taught/grouped together because:

• they have similar molecular origins
• they yield similar governing equations/principles
• they often occur simultaneously
• they require similar mathematical/conceptual tools

A few examples of Transport in action may serve to better put this in perspective ...

##### EXAMPLE:

TWO-PHASE REACTOR In a two-phase reactor the catalyst is carried in one phase (say, the oil phase) along with one monomer, while the other monomer is carried in the other phase. The reaction will clearly go fastest if we can maximize the surface area between the two phases so that the "contact" of the reactants is better.

What sort of device would be best suited for this task? How would you run the device?

Perhaps you would run this in a tank with a bladed impeller. This would allow you to prevent the fluids from separating via gravity and also to create small droplets. The small droplets would be useful from a mass transfer perspective because there would be more contact area between the phases.

What properties of the fluid and device will be most important? What operating conditions?

The two fluid densities would determine how rapidly they would settle. The viscosities would determine the power required to move the impeller (and the impeller speed would dictate the mixing rate and droplet size). The conductivity of the fluids would determine what the cooling requirements might be if this is an exothermic reaction.

##### EXAMPLE:

UREASE A person undergoing kidney failure needs a device to aid them until a transplant become available. You decide to encapsulate the urea-destroying enzyme urease in a solid pellet and build a "reactor" (artificial organ) to do the task.

What size of pellet should be used?

A smaller pellet would allow better contact between the bulk fluid (blood) and the pellet reducing mass transfer limitations; however, small particles will also make it more difficult to "push" the fluid past the particles (ever try to pack flour into a straw and then suck water through it?!).

What material properties of the pellet will impact its effectiveness?

As in the previous example, we need to worry about heat transfer either to or from the pellet (center) depending on whether the reaction requires or releases heat. Other than that, it would be useful if the pellet was porous so that we could put more catalyst on the "surface", but we need to worry about how fast mass transfer will occur through those pores.

What should the flow-field around the pellet look like?

Generally, the faster the fluid flow past the pellet, the better the heat and mass transfer to/from the pellet will be; however, as with small pellets, fast flow might require too much (mechanical) energy, that is, too "big" of a pump.

##### EXAMPLE:

HEATER You wish to design a better home heater to offset the rising prices of oil. You plan on designing a solar collector to go on the roof of your house and harness the energy of the sun.

How will Pittsburgh's weather impact your design?

The most obvious thing is that cloudy days will absorb much of the radiant energy from reaching the collectors. Also, the efficiency of the collectors may depend on temperature so that you may need to devise a way of keeping them warm.

Once you "capture" the heat how will you move it into the house?

The traditional way is to heat up a fluid like air or water and simply flow that through the house. Air is nice because it requires little energy to move, but it also has a very low heat capacity (so it cannot carry much heat). Water on the other hand is much more viscous, but it can also carry much more heat and has a better conductivity so you can move heat in/out of water more rapidly.