Page 60 - Separation process principles 2

Exercises 25

permeator cell, can selectively and continuously remove sulfur 1.16 A mixture of ethanol and benzene is separated in a network
dioxide and nitrogen oxides from the flue gas of power-generating of distillation and membrane separation steps. In one intermediate
plants. Prepare a detailed drawing of a possible device to carry out step, a near-azeotropic liquid mixture of 8,000 kgih of 23 wt%
such a separation. Show all locations of inlet and outlet streams, the ethanol in benzene is fed to a pervaporation membrane consisting
arrangement of the hollow fibers, and a method for handling the of a thin ionolneric film of perfl~~orosulfonic acid polymer cast on a
membrane liquid. Should the membrane liquid be left in the cell or porous Teflon support. The membrane is selective for ethanol such
circulated? Would a sweep fluid be needed to remove the oxides? that the vapor permeate contains 60 wt% ethanol, while the non-
permeate liquid contains 90 wt% benzene.
Section 1.5 (a) Draw a flow diagram of the pervaporation step using the ap-
propriate syn~bol froill Table 1.2 and include on the diagram all of
1.10 Explain the differences, if any, between adsorption and gas-
solid chromatography. the given information.
(b) Compute the component flow rates in kglh in the feed stream
1.11 In gas-liquid chromatography, is it essential that the gas
flow through the packed tube in plug flow? Discuss in detail. and in the two product streams and enter these results on the dia-
gram.
Section 1.6 (c) What separation operation could be ~~sed to further purify the
vapor permeate?
1.12 In electrophoresis, explain why most s~nall suspended parti-
cles are negatively charged.
Section 1.8
1.13 In field-flow fractionation, could a turbulent-flow field be
used? Why or why not? 1.17 The Prism gas permeation process developed by the
Monsanto Company is highly selective for hydrogen when using
Section 1.7 hollow-fiber membranes of materials such as silicone-coated
polysulphone. In a typical application, a gas at 16.7 MPa and 40°C,
1.14 The feed to Column C3 of the distillation sequence in Fig-
containing the following components in kmollh: 42.4 HZ, 7.0 CH4,
ure 1.9 is given in Table 1.5. However, the separation is to be al-
and 0.5 N2, is separated into a nonperlneate gas at 16.2 MPa and a
tered so as to produce a distillate that is 95 mol% pure isobutane
permeate gas at 4.56 MPa.
with a recovery of isobutane in the distillate (SF) of 96%. Because
(a) If the membrane is nonpermeable to nitrogen, the Prism mem-
of the relatively sharp separation in Column C3 between iC4 and
brane separation index, on a mole basis (SP) for hydrogen relative
nC4, assume all propane goes to the distillate and all C5s goes to the
to methane is 34.13, and the split fraction (SF) for hydrogen to the
bottoms.
permeate gas is 0.6038, calculate the kmollh of each component
(a) Compute the flow rates in lbmol/h of each component in each
and the total flow in kmolth of both the nonpermeate gas and the
of the two products leaving Column C3.
permeate gas.
(b) What is the percent purity of the normal butane in the bottoms
(b) Compute the percent purity of the hydrogen in the permeate gas.
product?
(c) Using an average heat capacity ratio, y, of 1.4, estimate the
(c) If the purity of the isobutane in the distillate is fixed at 95%,
outlet temperatures of the two exiting gas streams by assuming the
what percent recovery of isobutane in the distillate will maximize
ideal gas law and reversible expansions for each gas and no heat
the percent purity of normal butane in the bottoms product?
transfer between the two exiting gas streams.
1.15 Five hundred knlolth of a liquid mixture of light alcohols
(d) Draw a process-flow diagram of the membrane process and in-
containing, by moles, 40% methanol (M), 35% ethanol (E), 15%
dicate on the diagram for each stream the pressure, temperature,
isopropanol (IP), and 10% normal propanol (NP) is distilled in a
and component flow rates.
sequence of two distillation columns. The distillate from the first
column is 98% pure M with a 96% recovery of M. The distillate 1.18 Nitrogen gas can be injected into oil wells to increase the re-
covery of crude oil (enhanced oil recovery). Usually, natural gas is
from the second column is 92% pure E with a 95% recovery of E
from the process feed. Assume no propanols in the distillate from produced with the oil and it is desirable to recover the nitrogen
from the gas for reinjection into the well. Furthermore, the natural
Column Cl, no M in the bottoms from Column C2, and no normal
gas must not contain more than 3 mol% nitrogen if the natural gas
propanol in the distillate from Column C2.
is to be put into a pipeline. A total of 170,000 SCFH (based on 60°F
(a) By material balances, assuming negligible propanols in the dis-
and 14.7 psia) of natural gas containing 18% N2, 75% CH4, and 7%
tillate from the first column, compute the flow rates in kmoVh of
CzH6 at 100°F and 800 psia is to be processed for N2 removal.
each component in each feed, distillate, and bottoms. Draw a la-
A two-step separation process has been proposed consisting of
beled block-flow diagram like Figure 1.9. Include the results of the
(1) membrane separation with a nonporous glassy polyiinide mem-
material balances in a table like Table 1.5 and place the table below
brane, followed by (2) pressure-swing adsorption using molecular
your block-flow diagram.
sieves to which the permeate gas is fed. The membrane separator is
(b) Compute the mole-percent purity of the propanol mixture leav-
highly selective for N2 (SPN2,CH, = 16) and completely imperme-
ing as bottoms from the second column in the sequence. able to ethane. The pressure-swing adsorption step selectively
(c) If the recovery of ethanol is fixed at 95%, what is the maximum adsorbs methane, giving 97% pure methane product in the adsor-
purity that can be achieved for the ethanol in the distillate from the bate, with an 85% recovery of CH4 fed to the adsorber. The non-
second column? permeate (retentate) gas from the membrane step and adsorbate
(d) If instead, the purity of the ethanol is fixed at 92%, what is the from the pressure-swing adsorption step are combined to give a
maximum recovery of ethanol (based on the process feed) that can methane stream that contains 3.0% N2. The pressure drop across
be achieved? the membrane is 760 psia. The permeate at 20°F is compressed in