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Unformatted text preview: Exercise 8.1 Subject: Extraction vs. Distillation. Given: Dilute mixture of benzoic acid in water. Find: Why liquid-liquid extraction is the preferred separation method. Analysis: At 1 atm, the boiling point of benzoic acid is 249.2 O C. Thus, if a dilute solution of benzoic acid and water is distilled, a large amount of water, with its very high heat of vaporization (about 1,000 Btu/lb) must be vaporized, even though the relative volatility, , is large and the separation is easy. L/L extraction can be used with a solvent like methyl isobutyl ketone, MIBK, which has a normal boiling point of 119 o C and can be easily recovered from the benzoic acid by distillation and then recycled [see Trans. AIChE, 42 , 331 (1946). Exercise 8.2 Subject: Liquid-Liquid Extraction vs. Distillation. Given: Dilute mixture of formic acid in water. Find: Why liquid-liquid extraction is the preferred separation method. Analysis: At 1 atm, the boiling point of formic acid is 100.6 o C, almost identical to the normal boiling point of water. A maximum-boiling azeotrope is formed at 107.1 o C with 43.3 mol% water. Thus, it is impossible to separate formic acid from water at 1 atm by ordinary distillation . L/L extraction can be used with chloroform as the solvent to extract formic acid [see J. Chem. Eng. Data, 5 , 301 (1960) and J. Chem. Eng. Data, 4 , 42 (1959)]. Tetrahydrofuran can also be used as the solvent. Exercise 8.3 Subject: Selection of extraction device. Given: Liquid-liquid extraction of acetic acid from water by ethyl acetate. Fig. 8.8 for selecting extraction equipment. Find: Adequacy of the RDC. Other types of adequate extractors. Analysis: In the process of Fig. 8.1 for the L/L extraction of acetic acid from water by ethyl acetate, 6 equilibrium stages are needed. Pertinent properties are: &lt; 1 cP, = 0.08 g/cm 3 , I &gt; 30 dyne/cm Need stainless steel equipment. Assume no emulsion formation. Have large flow rates. Assume water-rich phase has a density of 1,000 kg/m 3 . Assume ethyl acetate-rich phase has a density of 900 kg/m 3 . Feed rate = 30,300 lb/h or 13,700 kg/h Solvent rate = 71,100 lb/h or 32,300 kg/h Total volumetric throughput = 13,700/1,000 + 32,300/900 = 49.6 m 3 /h In Fig. 8.8, this throughput is on the border line. Use a mixer-settler battery or any kind of a mechanically assisted column. Exercise 8.4 Subject: Extraction devices. Given: ARD and RDC. Find: Advantages and disadvantages of each device. Analysis: RDC: A great deal of experience exists. Hundreds of units have been installed. Can easily remove agitator assembly shaft. Easily fabricated and assembled. ARD: Get reduced back-mixing....
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This note was uploaded on 04/03/2008 for the course CHE 312 taught by Professor Ofoli during the Spring '08 term at Michigan State University.
- Spring '08