Seawater is boiled in a 5 liter kettle to produce drinking water. In class we solved the steady-state version of this problem. Here you are asked to consider the transient period while the process is started up. To start up the process, two liters of seawater (3.4% salt) are initially added and the kettle is heated.
Water vapor is removed and condensed continuously to produce (150. ml/min of liquid (pure water).
Feed seawater contains 3.4 wt % salt (NaCl). (note: the actual composition is more complex).
Concentrated solution collects in the kettle during the boiling process — no liquid leaves — thus both water and salt accumulate. The process operates in this manner until the solution concentration reaches
12.0 % and the total volume in the kettle is 5 liters.
• Calculate the mass of water and of salt remaining in the kettle at the end
• Determine the required feed rate of seawater
• Determine the time to reach 12% salt in the kettle
• Plot the concentration in the kettle versus time
Hint: Since the rate of feed seawater and the rate of pure water vapor leaving are constant, an integral form of the balances can be used to answer the first few questions. A rate form is needed to obtain the data for the plot. Set up your diagram, write the equations and then think about a strategy to complete the problem.
Pure water Seawater Concentrated brine
(feed, initially in boiler) (in boiler at end and in exit stream)
Mass Fraction Salt 0.00 0.034 0.12
Density g/cm³ 1.000 1.026 1.086
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