Chemical Engineering License Review, Second Edition
This text contains 146 problems with detailed step-by-step solutions, and is an ideal study guide and the first truly practical, no-nonsense review for the difficult PE Exam.
TABLE OF CONTENTS Chemical Engineering License Review, Second Edition
Chapter 4: Evaporation
Evaporation is the removal of a solvent, usually water, by its vaporization from relatively nonvolatile solute, usually a solid. The evaporators may be single-effect or multieffect, the latter for steam economy. The multieffect evaporators may be classified as forward feed, backward feed, parallel feed, and mixed feed. A further classification of the evaporators is natural circulation and forced circulation. Application of the various types of the evaporators is dependent upon the solution characteristics and the capacity required.
In general, the evaporation problems involve the calculation of the material and heat balances, steam economy, and required heat-transfer surfaces.
Evaporator Capacity
The heat-transfer capacity of an evaporator is given by
| where q | = total heat transferred, Btu/h |
| U | = overall heat-transfer coefficient, Btu/h ft 2 F |
| ? T | = overall temperature difference, F |
For a given heat input, the evaporation capacity is reduced if sensible heat is to be provided to bring the feed to its boiling point. The temperature difference that is available for heat transfer depends upon (1) the solution to be evaporated, (2) the liquid head in the tubes, and (3) the difference between pressure in the steam chest and that over the vapor space above the boiling liquid.
Boiling-Point Elevation
The boiling point of a solution is higher than the boiling point of the solvent because of the dissolved salts. The boiling-point elevation is small for dilute solutions but may be very high for the solutions of inorganic salts. Duhring's rule states that the boiling-point...
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