A 3-unit process (2 evaporators and 1 mixer) is used to concentrate salt water.

Question

A 3-unit process (2 evaporators and 1 mixer) is used to concentrate salt water. 2000 kg/min of a NaCl/water solution of unknown mass fractions is flowed into an evaporator, where 500 kg/min of pure water is removed as vapor. The concentrated solution stream is sent to a mixer, where 1000 kg/min of a 20 wt% NaCl solution in water is added. The mixture is then sent to an evaporator, where 1000 kg/min pure water is removed as vapor. The final stream contains 50 wt% NaCl in water at an unknown total flow rate. Which of the following unit balances has zero degrees of freedom (DOF)?

Question 1 options: Evaporator 1, Mixer, Evaporator 2, Overall, None of the above

Explanation / Answer

In this 3-unit process at all the stages addition of substraction of water or concentration of solution is constantly changing.

Zero degree of freedom means it is a fixed state/point.

Thus non of the above unit balances zero degree of freedom (FOD)

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