1. A water stream contaminated by benzene is to be steam stripped. The goal of this process is to
greatly decrease the volume of the benzene-containing phase so that it can be efficiently
treated downstream. In other words, a contaminated water stream at a relatively high flow
rate is to be treated with steam at a much lower flow rate, transferring the volatile benzene
contaminant into the steam.
Temperature and pressure are such that benzene has an effective uniform K-value of 40.5
throughout the column. The water feed flow rate is 100 kmol/h. The water feed contains a
benzene mole fraction of 0.002 (i.e. 0.2 mol%). Use Kremser's group method to respond to the
questions below. Assume constant molar overflow; that is, assume that the total liquid and
vapor flow rates do not change from stage to stage over the column.
a. For the water to be decontaminated in compliance with regulatory requirements, it must
contain a benzene concentration less than 0.0001 mol%. In a ten-stage column, is a steam flow
rate of 5 kmol/h capable of achieving this? What is the benzene concentration in the resulting
vapor product with a 5 kmol/h steam flow rate?
b. Changes to downstream processing require that the benzene-containing steam product stream
must now have a flow rate less than 3 kmol/h. How many stages must be added to the
column in order to achieve this? (Find the smallest whole number of stages that will give the
required separation.) What is the benzene concentration in the resulting vapor product with
this number of stages?
2. One million pound-moles per day of a gas of the following composition is to be absorbed by
pure n-heptane at -30 ºF and 550 psia in an absorber with 10 theoretical stages. The goal of this
process is to absorb 50% of the ethane from the feed. Calculate the required flow rate of
absorbent and the distribution, in lbmol/h, of all the components between the exiting gas and
liquid streams.
Component Mole % in Feed K-value at -30 ºF and
550 psia
methane 94.2 2.65
ethane 4.9 0.34
propane 0.7 0.062
n-butane 0.1 0.015
n-pentane 0.1 0.003