Unformatted text preview: location for extractive distillation
Temperature leaving cooler
Makeup phenol rate
1 Exercise 5.41
Subject: Specifications for a distillation column with sidestream Given: Distillation column in Fig. 5.33 with specifications.
Find: If given specifications are sufficient. If not, give additional specifications. Analysis: For a distillation column with a total condenser and sidestream, Example 5.5
applies, giving ND = 2N + C +11.
The following specifications are given:
Pressures of stages, reboiler, condenser, divider
Heat transfer for stages and divider
Feed completely specified
Feed stage and sidestream stage locations
Sidestream rate and purity
Number of stages
1 2N + C + 10 Therefore, one more specification is needed. It might be saturated liquid leaving
condenser or temperature of reflux if subcooled. Exercise 6.1
Given: Simultaneous absorption and stripping.
Fig. 6.1 in which simultaneous absorption and stripping occur. Find: Whether absorption and stripping is more important.
Analysis: Stripping of water = 22 kmol/h
Absorption of acetone = 10.35 kmol/h
Therefore, more stripping than absorption occurs.
However, the operation is primarily absorption because a very large percentage, 99.5%, of the
acetone is absorbed. Only a very small fraction of the water is stripped. Exercise 6.2
Given: Column packings since 1950.
History of the development of column packings. Find: Advantages of new packings. Advances in packing design and fabrication. Need for
Analysis: The newer packings provide more surface area for mass transfer, a higher flow
capacity, and a lower pressure drop. They provide more effective through flow. Many are
plastics made from molds or thin metal strips that can be inexpensively fabricated into intricate
shapes. Structured packings largely eliminate the problems of channeling and at the same time
give improvement in efficiency, capacity, and pressure drop. Exercise 6.3
Subject: Advantages of bubble-cap trays Given: Characteristics and performance of bubble-cap trays
Find: Characteristics that give bubble-cap trays a very high turndown ratio.
Analysis: Unlike sieve and valve trays, bubble-cap trays do not allow liquid to weep.
Therefore, bubble-cap trays can be operated at very low liquid flow rates. Also, bubble caps
force the vapor to flow out sideways, rather than vertically up, thus allowing a relatively high
vapor rate. Exercise 6.4
Given: Selection of alternative absorbent for Example 6.3
Flow rate, density, and MW of three potential absorbents listed below. Find: Select the best absorbent with reasons why. Are any of the absorbents unacceptable?
Analysis: In Example 6.3, the rich gas contains C1 to nC6 hydrocarbons, with mostly C3. Object
of absorber is to absorb most of the nC4. The absorbent is an oil of 250 MW and 21oAPI, at a
flow rate, L, of 368 lbmol/h. An alternative absorbent must also be of higher molecular weight
than nC4 and must have a flow rate of at least 368 lbmol/h. If its molecular weight is too low, a
significant amount of it will be stripped. This can be judged by its K-value and stripping factor,...
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