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Unformatted text preview: Exercise 6.1 Subject: Simultaneous absorption and stripping. Given: 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 Subject: Column packings since 1950. Given: History of the development of column packings. Find: Advantages of new packings. Advances in packing design and fabrication. Need for structured packings. 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 Subject: Selection of alternative absorbent for Example 6.3 Given: 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 C 1 to nC 6 hydrocarbons, with mostly C 3 . Object of absorber is to absorb most of the nC 4 . The absorbent is an oil of 250 MW and 21 o API, at a flow rate, L , of 368 lbmol/h. An alternative absorbent must also be of higher molecular weight than nC 4 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, S=KV/L. The entering gas rate, V , is 946 lbmol/h. Column operating conditions are about 94 psia and a maximum temperature of 126 o F. Pertinent properties and factors for the three potential absorbents are: Absorbent gpm , lb/gal lb/h MW lbmol/h K-value S=KV/L C 5 s 115 5.24 36,200 72 500 0.9 2.3 light oil 36 6.0 13,000 130 100 0.005 0.013 medium oil 215 6.2 80,000 180 440 0.0005 0.0013 The light oil can not be used because its flow rate of 100 lbmol/h is much lower than the necessary 368 lbmol/h. The C 5 s can not be used because their stripping factor is very high. The only possible alternative is the medium oil. only possible alternative is the medium oil....
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- Spring '11