Page 61 - Separation process principles 2

26 Chapter 1 Separation Processes

two stages to 275 psia and cooled to 100°F before entering the ad- (2) isobutane-rich, (3) n-butane-rich, and (4) combined pentanes-
sorption step. The adsorbate gas, which exits the adsorber during rich. However, the distillate from the first column is to be the
regeneration at 100°F and 15 psia, is compressed in three stages to propane-rich product; the distillate from Column 2 is to be the
800 psia and cooled to 100°F before being combined with nonper- isobutane-rich product; and the distillate from Colnrnn 3 is to
meate gas to give the final pipeline natural gas. be the n-butane-rich product, with the combined pentanes being the
(a) Draw a process-flow diagram of the separation process using bottoms from Column 3. The recovery of each main component in
appropriate symbols from Tables 1.2 and 1.3. Include the gas com- each product is to be 98%. For example, 98% of the propane in the
pressors and heat exchangers. Label the diagram with all of the data feed stream is to appear in the propane-rich product, and 98% of the
given above, and number all process streams. combined pentanes in the feed stream is to appear in the bottoms
product from Column 3.
(b) Compute by material balances, using the data above, the com-
ponent flow rates of N2, CH4, and C2H6 in lbmol/h for all process (a) Draw a process-ilow diagram, similar to Figure 1.9. i
streams entering and exiting the two separation operations. Place (b) Complete a material balance for each column and summarize
the results in a material-balance table similar to Table 1.5. the results in a table similar to Table 1.5. To complete the material 1
balance, you will have to make some assumptions about the flow i
rates of: (1) isobutane in the distillates for Columns 1 and 3 and
Section 1.9 I
(2) n-butane in the distillates for Columns 1 and 2, consistent with \
1.19 A mixture of ethylbenzene (EB) and the three isomers
the specified recoveries. Assume that propane will not be found in
(ortho, meta, and para) of xylene is widely available in petroleum the distillate from Column 3 and pentanes will not be found in the '
refineries. distillate from Column 2.
(a) Based on differences in normal boiling points, verify that the sep- I
(c) Calculate the mol% purities of each of the products and sum- j
aration between meta-xylene (MX) andpara-xylene (PX) by distilla- marize your results in a table similar to Table 1.7, but without the
tion is far more difficult than the separations between EB and PX, and
specifications, which are not given here.
MX and ortho-xylene (OX).
1.23 The need to remove organic pollutants from wastewater is
(b) Prepare a list of properties for MX and PX similar to Table 1.11.
common to many industrial processes. Separation methods that
From that list, which property differences might be the best ones to
may be considered are: (1) adsorption, (2) distillation, (3) liquid-
exploit to separate a mixture of these two xylenes?
liquid extraction, (4) membrane separation, (5) stripping with air,
(c) Explain why melt crystallization and adsorption are used com- and (6) stripping with steam. Discuss the advantages and disadvan-
mercially to separate MX and PX. tages of each method for this application. Be sure to consider the
1.20 When a mixture of ethanol and water is distilled at ambient fate of the organic material.
pressure, the products are a distillate of ethanol and water of near- 1.24 Many waste gas streams in processing plants contain volatile
azeotrope composition (89.4 mol% ethanol) and a bottoms product organic compounds (VOCs), which must be removed. Recovery of
of nearly pure water. Based on differences in certain properties of the VOCs may be accomplished by several separation methods, in-
ethanol and water, explain how the following separation operations cluding: (1) absorption, (2) adsorption, (3) condensation, (4) freez-
might be able to recover almost pure ethanol from the distillate: ing, and (5) membrane separation. Discuss the advantages and
(a) Extractive distillation disadvantages of each method, paying particular attention to the fate
(b) Azeotropic distillation of the VOC. For the case of a stream containing 3 mol% acetone in
(c) Liquid-liquid extraction air, draw a flow diagram for a process based on absorption. Choose a
reasonable absorbent and include in your process a means to recover
(d) Crystallization
the acetone and recycle the absorbent.
(e) Pervaporation membrane
1.25 Describe three methods suitable for the separation of air into
(f) Adsorption
nitrogen and oxygen.
1.21 A stream containing 7,000 kmoh of water and 3,000 parts
1.26 What separation methods can be used to separate azeotropic
per million (ppm) by weight of ammonia at 350 K and 1 bar is to be
mixtures of water and an organic chemical such as ethanol?
processed to remove 90% of the ammonia. What type of separation
operation would you use? If it involves a mass-separating agent, 1.27 An aqueous stream contains 5% by weight of magnesium
propose one. sulfate. Devise a process, complete with a process-flow diagram,
for the production of nearly pure magnesium sulfate from this
1.22 A light-hydrocarbon feed stream contains 45.4 kmob of
stream.
propane, 136.1 kmolih of isobutane, 226.8 kmol/h of n-butane,
181.4 kmoh of isopentane, and 3 17.4 kmol/h of n-pentane. This 1.28 Explain why the separation of a stream containing 10 wt%
stream is to be separated in a sequence of three distillation columns, acetic acid in water might be more economical by liquid-liquid
similar to that in Figure 1.9, into four products: (1) propane-rich, extraction with ethyl acetate than by distillation.