Separation Process Principles- 2n - Seader & Henley - Solutions Manual

Given ard and rdc find advantages and disadvantages

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Unformatted text preview: is seen to be 6. The value is independent of the feed condition because at total reflux, there is no feed. The temperature of the bottoms is 93.5oC. The temperature of the distillate is 64.7oC 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 249.2OC. 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, α, 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 119oC 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 100.6oC, almost identical to the normal boiling point of water. A maximum-boiling azeotrope is formed at 107.1oC 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: µ < 1 cP, ∆ρ = 0.08 g/cm3, σI > 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/m3. Assume ethyl acetate-rich phase has a density of 900 kg/m3. 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 m3/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. Can not easily remove agitator assembly shaft. More complex unit to fabricate and assemble. Exercise 8.5 Subject: Selection of extraction devices. Given: Cascade of mixer-settler units. Find: Conditions for selection of this device. Analysis: Mixer-settler units are preferred for: 1. Less than 5 equilibrium s...
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