It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporisation is 2100 kJ/kg and the specific heat of the solution is 4 kJ/kg.K. The rate at which the heat has to be supplied in (kJ/min) to the evaporator is A)3.06 × 10 ⁵ B)6.12 × 10 ⁵ C)7.24 × 10 ⁵ D)9.08 × 10 ⁵

Question

Quizplus · Accepted Answer

The correct answer is A because the rate of heat supply is equal to the sum of the heat required to raise the temperature of the feed to the boiling point, the heat required to vaporize the water, and the heat required to raise the temperature of the vapor to the boiling point. The heat required to raise the temperature of the feed to the boiling point is:Q1 = m * Cp * (Tb - Tf) = 300 kg/min * 4 kJ/kg.K * (110°C - 30°C) = 24000 kJ/minThe heat required to vaporize the water is:Q2 = m * Lv = 300 kg/min * 2100 kJ/kg = 630000 kJ/minThe heat required to raise the temperature of the vapor to the boiling point is:Q3 = m * Cp * (Tb - Tv) = 300 kg/min * 4 kJ/kg.K * (110°C - 110°C) = 0 kJ/minTherefore, the total heat required is:Q = Q1 + Q2 + Q3 = 24000 kJ/min + 630000 kJ/min + 0 kJ/min = 654000 kJ/min = 3.06 × 10^5 kJ/min

It Is Desired to Concentrate a 20% Salt Solution (20