ITP: Relation to Thermodynamics

The subject of this course is Transport Phenomena. In particular, we will examine the transfer of momentum, heat and mass. While Thermodynamics tells us that heat and/or mass will be transfered as a system moves from one equilibrium state to another, it does not tell us how or how rapidly this transfer will take place. (because this transfer is inherently a non-equilibrium process!)

Nevertheless, there is still a strong connection between the energy balances done in Thermodynamics and Transport...

Energy Balance

There are multiple forms of energy that a material may possess:

These forms may interconvert (i.e., from chemical to thermal), but energy must be conserved (if we exclude nuclear reactions). This idea is expressed in Thermodynamics as the First Law: Energy can neither be created nor destroyed.

Conservation expressions will play a central role in this course! One clear difference from Thermodynamics lies in the fact that in Transport we will focus on specific forms of energy (thermal, mechanical). A more subtle difference is our focus on rates in the conservation equations that are to be used.

Interphase Mass Transfer

We know from Thermodynamics that when two phases are in contact with each other, they will (eventually) reach an equilibrium state where the composition of the two phases will be determined (by the temperature, perhaps pressure, and the total composition of the mixture). In Transport Phenomena, we will discuss how rapidly the transfer of material will occur from the one phase to the other, and what the composition will be at different points in space and time within the phases.

NOTE:

One somewhat tricky thing about the mass transfer part of the class is that we will often assume that the phases are in equilibrium at the interface between them. WHY?!

OUTCOME:

Discuss the relationship between Thermodynamics and Transport Processes

TEST YOURSELF

Determine whether Thermo or Transport can answer the following questions?

  • What temperature will a glass of ice-water be when left in the classroom for 1 hour? How long will it stay that temperature?

In the first case we will assume that 1 hour is long enough for the liquid-ice system to reach equilibrium (probably true unless it is a huge glass), so thermo is your ticket and about 0C is your answer. How long it will stay that temperature, however, requires transport, as you need to know how rapidly heat is being sucked out of the surrounding room and into the glass (by convection and/or radiation).

  • How many trays/stages should there be in a liquid-liquid extraction column? How big should they be?

In the second case, you can determine the "ideal" number of stages using only thermo and material balances, as the mass equilibrium at each stages is assumed (note that real trays are not "ideal" because of transport not being fast enough for them to always reach equilibrium!). In order to figure out how big the trays need to be, however, you need transport to help you determine what flow pattern you want and what the rate of interphase mass transfer will be.