Exercise 2. Cooling water recycle

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

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.

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

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 = 12000 lb/h water feed Cooling water circuit flow = 250000 lb/h Water ‘hardness’ as calcium carbonate, entering system @ 10 lb/h

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?