Transport phenomena

ChE 453 Transport Phenomena

Instructor: Dr.. Md. Easir Ar Dr Araf afat at Khan

Assistant Professor Department of Chemical Engineering, BUET BUET,, Dhaka-1000

July 2014

Transport Phenomena Subject: This course will deal with the three areas of study (Momentum, Heat and Mass transfer) which have certain ideas in common and the application of shell balances, equation of change for momentum, energy and mass transfer & their use. Mechanisms and analogy equation relating

momentum, energy and mass transfer.

Prerequisites:

Fluid

Mechanics,

Heat

Transfer

and

Mass

Transfer,

Engineering Thermodynamics, Material and Energy Balance.

Chemical

Transport Phenomena Instructor:

Dr. Md. Easir Arafat Khan,  Asst. Prof./ Dept. of Chemical Engineering, BUET, Dhaka-1000 Main Text:

Transport Phenomena, 1st Edition By R. Byron Bird, Warren E. Stewart and Edwin N. Lightfoot

Supplementary Text: 1. Fundamentals of Momentum, Heat and Mass Transfer, by James R. Welty

2. Momentum, Energy and Mass Transfer in Continua, by John C. Slattery 3. Transport Phenomena, by Robart S. Brodkey

Transport Phenomena Assessment: Each student’s grade will be based on performance in assignments, exams, and/or quizzes according to the following criteria:  Attendance: 10%

Homework/Assignment/Class Test/Class Assessment Test: 20%

Final Exam + Midterm exam (if applicable): 70%

Transport Phenomena What exactly are "transport phenomena"? Transport phenomena are really just a fancy way that Chemical Engineers group together three areas of study that have certain ideas in common. These three areas of study are: 

Fluid Mechanics



Heat Transfer 



Mass Transfer 



Fluid Mechanics deals with the transfer of momentum in a fluid



Heat Transfer deals with the transfer of heat energy



Mass Transfer deals with the transfer of mass

In "transport phenomena“, how are they all related? Well, They all are similar in their behavior. They all move stuff (Momentum, Heat, or  Mass) from a place where there is a lot of the stuff to a place where there is less

stuff. Here are some examples: 

In Fluid Mechanics momentum is transferred from a place where we have a lot of momentum to a place where we have less. A good analogy is the flow

of traffic on a busy freeway. The far left lane on the freeway typically move slower than the right left lane, with the lanes in the middle going faster the further right you move. This can be compared to flow over a flat plate,

where the slower flow (the left lane on the freeway) is right next to the plate, and the faster flow (the right lane on the freeway) on the surface of 

the fluid. The transfer of momentum is like the cars changing lanes, as slower cars pull into faster lanes the lanes slow down to allow the car to accelerate (and not cause a pile-up on the freeway), and the faster cars

pulling into slower lanes and speeding up the lane a little bit.



In Heat Transfer, energy moves from a place where there is a lot to a place where there is less. For example, if you heat up a brick, then drop it into cold water, the brick gets colder and the water gets warmer. Once the

brick and the water are at the same temperature, no more energy can be transferred. 

Mass Transfer. If the red dye is first dropped into the water is at a high concentration and the water is at zero concentration, as the dye spreads out, the concentration of the dye slowly increases, until, it is all at the

same low concentration everywhere. Once the concentration of the dye in the water is the same everywhere, no more mass transfer can take place. The one of the most important similarities between all of these examples is:

There is a driving force (momentum, temperature, or concentration difference or 'gradient'), which becomes smaller as time progresses in each of the examples, and eventually becomes zero when no more transfer of stuff takes place.

The math for all of these "transport phenomena" all are based on 2 ideas: 

The rate of change of stuff is proportional to some driving force, as in the examples above.  We can't destroy mass or energy (or, mass and energy must be conserved).

Driving force equation:

vx

The first idea is summed up by three similar laws for each of the three "transport phenomena" (here in one dimension and rectangular coordinates, Molecular transport of Momentum: from Newton’s law of viscosity,

  xx

 

 

dv x dx

 

 

d (  v x ) dx

Thus, Momentum flux = Momentum diffusivity × gradient of Momentum concentration

dy dx

Driving force equation: In Heat Transfer: From, Fourier’s Law of Heat Transfer,

Q  A



q

 

k 

 

 

  C  p



dT  dx d (  C  pT ) dx

,  

k  

  C  p



Thermal diffusivity

Heat energy/volume = Energy concentration

Now, Heat flux = Thermal diffusivity × gradient of concentration of heat energy

Driving force equation: In Mass Transfer: From, Fick’s Law of Mass Transfer,

 J  A

 D AB

 

dC A dx

where  D AB = diffusivity of A in B C  A = concentration of A  J  A = molar flux with respect to molar average velocity

Now, Molar flux = Diffusivity × gradient of molar concentration