CHEMICAL ENGINEERING TECHNOLOGY 3B (CMTB321)

Evaporation Tutorial Questions

Question 1: Heat transfer in a single effect

A continuous single-effect evaporator concentrates 9072 kg/h of a 1.0 wt % salt solution entering at 38oC to a final concentration of 1.5 wt %. The vapor space of the evaporator is at 100 kPa and the steam supplied is saturated at 140 kPa. The overall coefficient U = 1704 W/m2.K. Assuming the solution is dilute and has the same boiling point as water, calculate the:

i. amounts of vapour and liquid productii. quantity of steam required

iii. heat transfer area required

Question 2:

A sugar company ITM in KZN produces a juice concentrate at rate of 4800kg.hr-1 in a single stage evaporator. The surface area for heat transfer is 150m2 and the overall heat transfer coefficient is 1800W.m-2K-1. Heat is supplied as saturated steam at a rate 7075kg.hr -1 and pressure of 150kPa. The vapour space pressure is maintained at pressure of 65kPa. If the feed is supplied at 25oC, and has a ; Calculate:

i. The boiling point rise for this unit

ii. The ratio of the mole fractions product and feed streams

Question 3: Single-effect evaporation (Use of Dühring and enthalpy-concentration charts)

An evaporator is used to concentrate 4536 kg/h of a 20% solution of NaOH in water entering at 60 oC to a product of 50% solids. The pressure of the saturated steam used is 170 kPa and the pressure in the vapour space of the evaporator is 12.33 kPa. The overall heat-transfer coefficient is 1560 W/m2.K. Calcuate:

i. amounts of vapour and liquid productii. quantity of steam used

iii. the steam economy in kg vaporized/kg of steam usediv. heat surface area required