Unformatted text preview: he VOC
as a product. Freezing
Absorbent may stripped into the
May have to incinerate the spent
May require high pressure and/or
May require a low temperature.
May be difficult to obtain high
selectivity. May require a very
high pressure. With absorption, may be able to distil the VOC from the absorbent.
With adsorption, may be able to incinerate the VOC or recover it.
With condensation, can recover the VOC as a product.
With freezing, can recover the VOC as a product.
With a membrane, can recover the VOC as a product.
The process shown on the following page shows a process for recovering acetone from
air. In the first step, the acetone is absorbed with water. Although water is far from
being the most ideal solvent because of the high volatility of acetone in water, the air will
not be contaminated with an organic solvent. The acetone-water mixture is then easily
separated by distillation, with recycle of the water. Exercise 1.24 (continued)
One possible process-flow diagram: Clean gas Makeup
Water Acetone Absorber
Gas Feed Recycle Water Exercise 1.25
Subject: Separation of air into nitrogen and oxygen.
Given: Air to be separated.
Find: Three methods for achieving the separation.
Analysis: Three methods are used commercially for separating air into oxygen and
1. Gas permeation mainly for low capacities
2. Pressure-swing gas adsorption for moderate capacities.
3. Low-temperature distillation for high capacities. Exercise 1.26
Subject: Separation of an azeotropic mixture of water and an organic compound.
Given: An azeotropic mixture of water and an organic compound such as ethanol.
Find: Suitable separation methods.
Several suitable methods are:
Heterogeneous azeotropic distillation
Liquid-Liquid extraction. Exercise 1.27
Subject: Production of magnesium sulfate from an aqueous stream.
Given: An aqueous stream containing 5 wt% magnesium sulfate.
Find: Suitable process for producing nearly pure magnesium sulfate.
Analysis: Use evaporation to a near-saturated solution, followed by crystallization to
produce a slurry of magnesium sulfate heptahydrate crystals. Use a centrifuge or filter to
remove most of the mother liquor from the crystals, followed by drying. A detailed flow
sheet of the process is shown and discussed near the beginning of Chapter 17. Exercise 1.28
Subject: Separation of a mixture of acetic acid and water.
Given: A 10 wt% stream of acetic acid in water
Find: A process that may be more economical than distillation.
Analysis: The solution contains mostly water. Because water is more volatile than
acetic acid, distillation will involve the evaporation of large amounts of water with its
very high enthalpy of vaporization. Therefore, it is important to consider an alternative
method such as liquid-liquid extraction. Such a process is shown and discussed near the
beginning of Chapter 8. Exercise 2.1
Subject: Minimum work for separating a hydrocarbon stream.
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