Sol_HW8_v5_KS_31oct05 - Chemical Engineering 150B- Fall...

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1 Chemical Engineering 150B- Fall 2005 Problem Set #8 Solutions Problem 1. (25 Points) In the figure below, the heavier components in a slightly superheated hydrocarbon gas are to be removed by absorption at 400 psia (2,760 kPa) with a high -molecular-weight oil. [The diagram below is for N=6 stages.] Component K-value @ 97.5 o F and 400 psia C 1 6.65 C 2 1.64 C 3 0.584 nC 4 0.195 nC 5 0.0713 Oil 0.0001 a) Estimate the exit vapor flow rate, exit liquid flow rate, and exit compositions by the approximate group method of Kremser for the following number of stages of N=1, 3, 6, 10, and 30 stages. Assume that effective absorption and stripping factors for each component can be estimated from the entering values of L, V, and the component K- values, as listed below based on an average entering temperature of (90+150)/2=97.5 o F. b) Plot the percent absorption of each of the five hydrocarbons and percent absorption of the total feed gas versus the number of stages, N. c) What can you conclude about the effect of the number of stages on each component?
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2 NOTE : Since % absorption and % stripping can be interpreted in various ways, credit will be given to solutions that have the right shape of % absorption or % stripping curves and come to the proper conclusion in part c. Proper use of the Kremser method must be illustrated though. PART C 5 pt PART B 10 pt 5 pt for table; see note below 5 pt for proper use of the Kremser method
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3 Problem 2 (20 Points). 1 million lbmol / day of a gas of the composition detailed below is to be absorbed by pure n-heptane at -30 o F and 550 psia in an absorber having 10 theoretical stages so as to absorb 50 % of the ethane (C 2 ). Calculate the required flow rate of absorbent and the distribution, in lbmol / day, of all the components in the exiting gas and liquid streams. Component Mole% in Feed Gas K-value @ -30 o F and 550 psia C 1 94.9 2.85 C 2 4.2 0.36 C 3 0.7 0.066 nC 4 0.1 0.017 nC 5 0.1 0.004 The first thing we do, as always, is draw a diagram. It might be helpful to use a variant of Figure 5.8 (a) on page 242 of your text: For this problem, we are told N = 10. We are also told the composition of the incoming vapor stream. We also know that the incoming liquid stream is free of all components. We also have the K-values for all the components at the specified temperature and pressure. Finally, we are told that want to absorb 50% of the ethane. To solve the problem, we will use the Kremser Method discussed in section 5.4 of your Seader & Henley text. Specifically, we use equation (5-48): () 2 2 1 2 1 1 A N A A φ + = .
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This homework help was uploaded on 04/02/2008 for the course CHEM 150b taught by Professor Bell during the Spring '08 term at University of California, Berkeley.

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Sol_HW8_v5_KS_31oct05 - Chemical Engineering 150B- Fall...

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