ChEE 4520 Design Projects
A Solvent Recovery System for a Fiber Spinning Plant.
Asymmetric hollow-fiber polymeric membranes are used in a wide variety of applications
including the production of nitrogen from air and the removal of cell metabolism byproducts in
dialysis. Such membranes range in size from approximately 100 to 400 microns outer diameter
and 75 to 300 microns inner diameter. Asymmetric membranes possess a porous wall in which
pore size depends on position in the wall. Typically, the smallest pores are adjacent to one wall
and the largest adjacent to the other wall.
The manufacturing process for these materials involves mixing the polymer with one or more
solvents to form a ‘spin dope.’ The spin dope is spun to produce a hollow filament in a fiber
spinning process similar to that used to produce textile and structural fibers. Such fibers are
found in clothing or carpet.
During the spinning process, the fibers contact multiple water baths in which the solvent(s) are
exchanged for water. To produce uniform fibers, the solvent concentration(s) in these baths must
be maintained within certain limits.
Your project is to design a system to keep the baths at acceptable solvent concentrations and
temperatures and, potentially, recover the solvents for reuse in the spinning process. The
following process specifications may be useful in developing your design:
1. The spin dope consists of 32.5 weight percent N-methyl pyrrolidinone (C5H9NO), 15.5
weight percent ethylene glycol, and the balance polycarbonate.
2. The spin dope is extruded at a rate of 5 lbm/min from three spinnerets which each produce 60
fibers.
3. The 180 liquid filaments enter into a water bath maintained at 5 C and spend approximately 5
seconds in the bath.
4. Upon exiting from the first bath, approximately 65 percent of the ethylene glycol is removed
and 45 percent of the N-methyl pyrrolidone. An equal volume of water replaces the organic
solvents.
5. The total solvent composition of the first bath must be kept below one weight percent.
6. The fiber enters a second bath maintained at a temperature of 85 C and spends approximately
10 minutes in the bath.
7. Upon exiting from the second bath, virtually all of the organic solvents have been removed
and replaced by an equal volume of water.
8. The total solvent composition of the second bath must be kept below 10 weight percent.
9. Makeup water for the baths comes from city water lines but must be distilled to purify it.
10. Makeup water should be returned at the temperature of the bath. However, you do not have
to provide temperature control for either bath.
11. To reuse the solvents, the water content must be less than one weight percent.
In the course of your design, you should answer the following questions:
1. What is the required makeup water flow rate for each bath? At what rate is water removed
from each bath for treatment?
2. What are the design specifications for each unit operation?
3. Provide a complete PFD for the process including a table of stream and unit operation
properties.
4. Estimate process costs and compare to the cost of simply returning the waste water to the city
sanity sewer at a concentration of less than 0.1 weight percent organics.
For more information on such a process, I recommend you read US Patent 4,772,392 (you can
access it through IBM’s patent server, http://www.patents.ibm.com). However, note that the
process specifications given here are not identical to those in the patent.
CO2 Recovery from Winery Off-Gas
You have been contacted by a well-known environmental organization to evaluate the cost of
recovering carbon dioxide produced by the mega-wineries, such as Gallo, in California’s
Sacramento Valley. The environmentalists are concerned about the impact of CO2 production on
smog production.
The wineries have rejected the environmentalists’ request to evaluate the cost of CO2 capture
technologies. They claim the link between their CO2 emissions and smog generation is too
tenuous to justify further consideration. Consequently, the environmentalists have asked you to
provide a preliminary cost evaluation.
Since the wineries refuse to share any process data with you, you will have to estimate CO2
production rates from published wine production rates and fermentation chemistry. The
environmentalists have suggested that you consider two processes: 1) cryogenic condensation
and distillation and 2) an absorption process utilizing diethanolamine (or triethanolamine) as the
absorption liquid.
Your contract with the environmentalists requires that you provide a written and oral report
detailing:
1. An estimate of CO2 production rates at a typical mega-winery.
2. Process flow sheets for the two processes they suggested and process flow sheets for
alternative or hybrid processes of your own design.
3. Provide a complete PFD for each process including a table of stream and unit operation
properties.
4. A preliminary cost estimate for CO2 capture per liter of wine produced.
5. An estimate of the value of the CO2 captured, in either gas or liquid form, if sold on the open
market.
You might find the following references useful in developing your process flow sheets:
1. Spillman, R. W., “Economics of Gas Separation by Membranes,” Chem. Eng. Prog., 85, 41
(1989).
2. Schendel, R. L., Mariz, C. L., and Mak, J. Y., “Is Permeation Competitive,” Hydrocarbon
Process., 62, 58 (1983).
While these references discuss membrane separations relative to CO2 stripping and cryogenic
condensation for CO2/CH4 separations, I believe you will find the discussion useful in
developing your process. Additionally, I do not expect you to consider membrane processes
unless you are motivated to do so.
Vapor Recovery System for Gas Storage Tanks
An important source of hydrocarbon emissions is due to vaporization of gasoline in gasoline
storage tanks at gas stations. US Patent 5,367,882 (available through the IBM Patent server,
http://www.patents.ibm.com) describes the problem in some detail and provides useful diagrams.
These emissions are becoming significant, relative to other air pollution sources, due to the
increased use of environmental controls.
When a gas tanker is filled with gasoline at a central distribution point such as the Sun or BP
refinery (or retail gas service station tank is filled from a tanker), the gas vapor present in the
tanker (or underground storage tank) must be vented. This gas stream is commonly vented
without treatment to the environment. A process that recovers the gasoline vapors eliminates the
pollution problem and produces a valuable by-product.
Lievens and Tiberi disclose in US Patent 5,367,882 a membrane process for recovery of the
gasoline vapor. Your company has asked you to evaluate the economics of alternative processes
such as cryogenic condensation or compression and condensation. The results of your study will
be used to decide whether to license the patent or market an alternative process.
Your boss expects to receive the following information in an oral presentation and written report:
1. An estimate of the rate at which vapor is produced at a typical retail and distribution site.
You may ask gas stations for estimates of their volumetric sales or search the open literature
for rates.
2. Process flow sheets for each of the processes you considered.
3. A complete PFD for each process including a table of stream and unit operation properties.
4. An estimate of the sensitivity of your processes to seasonal and brand variations in gasoline
composition.
5. A preliminary cost estimate for vapor recovery at retail and distribution sites.
6. An estimate of the value of the recovered gasoline.
You do not have to account for the presence of water for the purpose of this project. However, I
would recommend you consider how water might affect your design. For an informative
discussion of what constitutes gasoline, I recommend you visit the Usenet FAQ devoted to
gasoline, http://www.cs.ruu.nl/wais/html/na-dir/autos/gasoline-faq/.html.