Seawater is to be desalinized in a reverse osmosis plant. 1000 lb_m/h of seawate

Question

Seawater is to be desalinized in a reverse osmosis plant. 1000 lb_m/h of seawater with a salt concentration of 3.1% is mixed with a recycle stream. This combined stream has a salt concentration of 4.0% and is pumped into the process where desalinized water is produced and a waste brine solution is discarded. What is the mass flow rate of recycled brine? What is the production rate of desalinized water (also called potable water)? How many pounds of salt are removed per ft^3 of potable water produced? If one cell can handle a fresh feed of 1000 lb_m /h of seawater, how many units would be required for a city requiring one million gallons per day of potable water?

Explanation / Answer

Basis: One hour

Overall balance, 1000 = B+D

Salt balance = (1000*3.1) =5.25 B + 0.050    {500ppm = 0.05%}

on solving we get,

B = 586.5 lb ; D = 423.5 lb

R = 5.35%

Overall balance ( at entry to cell), 1000 + R =A

Salt balance, 1000(0.031) + R(0.0525) = A(0.04)

on solving we get,

A = 1720 lb and R = 720 lb

If once cell handles 1000 ld/h of seawater then for 1 million gallon ( 8345404 lb) is,

8345404/1000 = 8345.404 = 8345 cells required

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