1. Exercise 2. Cooling water recycle

2. Important ChemCad notes
I talked with a ChemCad rep early Friday afternoon;
She is sending better instructions on how to do ChemCad for Macs, and info on the possible loss of the license key after you have used the system for a while
Hopefully, we will be able to implement these fixes

3. Process recycle streams
Recycle stream arise from:
Low reactor yield
Poor separation efficiencies
Poor separation thermodynamics
Success in modeling processes with recycle streams is somewhat dependent on the choice of ‘cut’ stream, and order of computation.

4. Notes:
do not specify stream 4; its value is computed by ChemCad
Specify streams 1 and 6; water flow of stream 5 is set by specifying the divider outflow to 5
Specify the heat load by setting the temperature of stream 9 = 110 F
Remember to select water/hydrocarbons as immiscible in the thermodynamics screen
Remember to specify CaCO3 as a solid – we will use this on Monday

5. Problem conditions
Recycle loop with cooling tower, divider to remove blowdown, mixer to add make-up, pump, and heat exchanger to simulate the cooling load of the plant
Air to water ratio: ~ 1 lb air/1 lb cooling water; cooling tower
Blowdown stream acts as a purge
Cooling tower; no condenser, no reboiler, 4 stages, atmospheric pressure
Make-up stream = lb/h water feed
Cooling water circuit flow = lb/h
Water ‘hardness’ as calcium carbonate, entering 10 lb/h

6. Given the PFD, what is your strategy for…
Setting process conditions?
Be ready to explain your process
Verifying material balances for water only?
Verifying calcium carbonate material balances?
Which stream is the ‘cut’ stream? Why might this be a good choice?