CL 351                                   Homework # 7

 

Due date: 24 October 2005

 

1.       One hundred kilogram-moles per hour of a saturated liquid mixture of 12% ethyl alcohol in water is distilled continuously by direct steam at 1 atm introduced directly to the bottom plate.  The distillate required is 80 mol% representing 90% recovery of the alcohol in the feed.  The reflux is saturated liquid with reflux ratio is 3.  Feed is on the optimal stage.  Find steam requirement, number of theoretical stages, the optimum feed location and minimum reflux ratio.  Vapour liquid equilibrium data is tabulated below.

 

x

0.02

0.07

0.1

0.12

0.17

0.23

0.26

0.33

0.40

0.51

0.57

0.68

0.75

0.89

y

0.17

0.39

0.44

0.47

0.51

0.54

0.56

0.58

0.61

0.66

0.68

0.74

0.78

0.89

 

 

2.       A distillation column is separating mixture of water (MVC) and acetic acid input to the column at two locations.  Both feeds are 100 kmol/hr but one contains 75% water and the other 50%.  The top and bottom products are saturated liquids containing 98 mol% and 5 mol% water.  Determine the optimal feed stage for each feed and the number of stages required, if the column is operated at a reflux ratio 20% larger than the minimum reflux ratio..  The vapour liquid equilibrium data is given below.

 

x

0.005

0.05

0.13

0.21

0.30

0.51

0.65

0.80

0.96

Y

0.011

0.13

0.24

0.34

0.44

0.63

0.75

0.87

0.97

 

 

1.       This problem concerns estimation of duties of condenser and reboiler of a distillation column separating an equimolar binary mixture.  The vapour-liquid equilibrium can be modeled by a constant relative volatility = 3.0.  Product streams are saturated liquids with 99.5 mol% purity.  Estimate minimum reflux ratio for q values changing from -1.0 to 2.0 in steps of 0.4.  For each minimum reflux ratio, calculate the duties of condenser and reboiler expressed as ratio of heat duty to product of latent heat of vapourization and feed flow.  Make a plot individual duties and their sum versus q.  Assume CMO is valid.

 

2.       A distillation column with a partial reboiler and a total condenser is separating benzene, toluene and cumene.  Feed flow is 10,000 kgmoles/hr and mole fractions in the feed are: benzene = 0.15; toluene = 0.25; cumene = 0.6.  We wish to recover 99% of toluene in the distillate and 98% of cumene in the bottoms.  Reflux is a saturated liquid and reflux ratio = 1.0.  Vapour-liquid equilibrium can be modeled with constant relative volatilities αBT = 2.25, αTT= 1.0, αCT = 0.21.  CMO is valid.  Find the composition of the liquid leaving the second stage above the partial reboiler and vapoure leaving second stage from the top of the column.

 

3.       Suppose that we have a ternary mixture of pentane (A), hexane (B) and heptane (C).  We take the relative volatilities as constant αAB = 2.59, αBC= 2.45, αAC = 6.35.  The feed contains equal molar amounts of all components.  You want to determine which of the two sequences, shown below, is a better way of separating the components.  One good way is to compare the minimum vapour flows needed in each sequence.  Using Underwood equations and assuming that the non-key components do not distribute, for 99% fractional recoveries of key components,  find which sequence is better.