Absorption and Stripping of Dilute Mixtures

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Absorption and Stripping of
Dilute Mixtures
Chapter6
Exercises
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Exercise6.1

• Prior to 1950, only two types of commercial
  random packings were in common use Raschig rings
  and Berl saddles. Starting in the 1950s, a wide
  variety of commercial random packings began to
  appear. What advantages do these newer packings
  have? By what advances in packing design and
  fabrication techniques were these advantages
  achieved? Why were structured packings introduced?

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Exercise6.2

• In Example 6.3, a lean oil of 250 MW is used as
  the absorbent. Consideration is being given to
  the selection of a new absorbent. Available
  streams are
• Rate, Density,
• gpm 1b/gal MW
• C5s 115 5.24 72
• Light oil 36 6.0 130
• Medium oil 215 6.2 180
• Which stream would you choose? Why? Which
  streams, if any, are unacceptable?

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Exercise6.3

• A straw oil used to absorb benzene from coke-oven
  gas is to be steam-stripped in a sieve-plate
  column at atmospheric pressure to recover the
  dissolved benzene. Equilibrium conditions at the
  operating temperature are approximated by Henry's
  law such that, when the oil phase contains 10
  mol CfiHft, the CeHfi partial pressure above the
  oil is 5.07 kPa. The oil may be considered
  nonvolatile. The oil enters containing 8mol
  benzene, 75 of which is to be recovered. The
  steam leaving contains 3 mol C6H6. (a) How many
  theoretical stages are required? (b) How many
  moles of steam are required per 100 mol of
  oil-benzene mixture? (c) If 85 of the benzene is
  to be recovered with the same oil and steam
  rates, how many theoretical stages are required?

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Exercise6.4

• One thousand kilomoles per hour of rich gas at
  70F with 25 C1, 15 C2, 25 C3, 20 nC4, and
  15 nC5 by moles is to be absorbed by 500kmol/h
  of nC10 at 90F in an absorber operating at 4atm.
  Calculate by the Kremser method the percent
  absorption of each component for four, ten, and
  thirty theoretical stages. What do you conclude
  from the results? (Note The K-value of nC10 at
  80F and 4atm is 0.0014.)

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Exercise6.5

• The absorption operation of Examples 6.1 and 6.4
  is being scaled up by a factor of 15, such that a
  column with an 11.5-ft diameter will be needed.
  In addition, because of the low efficiency of 30
  for the original operation, a new tray design has
  been developed and tested in an Oldershawtype
  column .The resulting Murphree vapor-point
  efficiency, EOV, for the new tray design for the
  system of interest is estimated to be 55.
  Estimate EMV and E0. (To estimate the length of
  the liquid flow path, ZL, use Figure 6.16. Also.
  assume that u/de 6

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Exercise6.6

• Conditions at the bottom tray of a reboiled
  stripper are as shown in Figure 6.48. If valve
  trays are used with a 24-in. tray spacing,
  estimate the required column diameter for
  operation at 80 of flooding.

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Exercise6.7

• A wastewater stream of 600gpm, containing 10ppm
  (by weight) of benzene, is to be stripped with
  air in a packed column operating at 25C and 2atm
  to produce water containing 0.005ppm of benzene.
  The packing is 2-in. Flexirings made of
  polypropylene. The vapor pressure of benzene at
  25C is 95.2torr. The solubility of benzene in
  water at 25C is 0.180 g/100 g. An expert in VOC
  stripping with air has suggested use of 1,000scfm
  of air (60F, 1atm), at which condition one
  should achieve for the mass transfer of benzene
• kLa0.067s-1 and kga0.80s-1
• Determine
• (a) The minimum air stripping rate in scfm. Is it
  less than the rate suggested by the expert? If
  not, use 1.4 times your minimum value.

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• (b) The stripping factor based on the air rate
  suggested by the expert.
• (c) The number of transfer units, NOG required.
• (d) The overall mass transfer coefficient, KGa,
  in units of mol/m3-s-kPa and s-1. Which phase
  controls mass transfer?
• (e) The volume of packing in cubic meters.