Thrust Area C: Rate Processes
Thrust Area C, our largest, deals primarily with
solving engineering problems. Our first year has focused on equipment design
and installation. Here we have divided Thrust Area C into sections: (1)
membranes, (2) surfactant-based cleaning processes and (3) polymer systems.
Membranes
The objective of the membrane separation subgroup is
to develop the ability to separate CO2 from industrial gas streams,
organic molecules and colloidal particles to reduce re-compression costs and
aid in CO2 recycling. The membrane subgroup in has cooperated to
establish system designs for the three locations to allow testing membranes
with diverse structures and properties under relevant supercritical CO2
conditions. Sketches of the proposed
system at NCSU and the recently completed prototype at UT Austin for testing
high performance polymer membranes are shown in Appendix G Figures II.C.1 and II.C.2, respectively.
Robust Sorption-Diffusion Polymeric Membranes for
Service in High Pressure CO2 (14)
A
prototype high-pressure system for testing membranes under aggressive feed
conditions has been built. The system
is rated to feed pressures of 4000 psi with controllable transmembrane pressure
drops. The pressure of the incoming gas
stream from a standard source of CO2 is being achieved with a
Haskell booster pump capable of providing 4000 psi feeds independent of
incoming pressure. Supporting the
membrane properly for separations utilizing high feed pressures with low
permeate pressures was challenging but has been accomplished. Under extreme conditions, the support
provided by Millipore was severely deformed.
Although the cell is rated to 10,000 psi, the support required
modification to suit our testing needs.
The membranes are now supported with a sintered metal disk resting in
the modified Millipore cell.
Integrating the Haskell booster pump with the modified Millipore cell
has allowed successful testing of transport properties of membranes with
upstream pressures as high as 3000 psi for a range of downstream
conditions. A system schematic is shown
in Appendix G Figure II.C.2.
The
goal of the following studies is to utilize experimental systems for
continuous, on-line monitoring of decontamination processes to evaluate key
parameters in the cleaning domain.
An Image
Sensing System for CO2-Based Surfactant Decontamination Processes
(16) This
program has three parts: development of techniques to monitor decontamination
rates, on-line analysis of surface contaminants and phase equilibrium of
polymer systems.
I. Real-time Rate
Monitoring of Decontamination of Solid Surfaces in SCCO2: Development of reconfigurable
computing hardware has been stopped in favor of the Quartz Crystal Microbalance
(QCM) technique. QCM can be used to
monitor in real time decontamination rates of solid surfaces. This technique is very accurate and
reproducible and gives rate and thickness resolutions to Angstroms per second.
We are designing the experimental set-up, using those components of the
original design we can. Some
significant redesign will be required as QCMs are traditionally designed for
vacuum systems. Selection and sizing of components is underway.
II. Precision Surface
Cleaning with Liquid/SCCO2: This project treats the removal of cutting and machine oils,
silicone oils, and hydraulic fluids from variety of industrial substrates with
liquid / SCCO2 to <10 ug/cm2.
The equipment (Appendix G Figure II.C.3)
will provide: (1) the rate of the on-line removal of contaminants from the
substrates, (2) a spectroscopic surface analysis to determine the contaminants
left on the substrate after cleaning and (3) solubility of the contaminants in
the liquid / SCCO2. Most of this system was purchases using EPA
funding.
III. Phase Equilibrium of Polymer Systems: Determination of phase boundaries and compositions are required for the safe and efficient operation of polymer processes. We will determine the solubility of a polymer by visualizing its dew point. The advantage of this technique is that neither sampling nor extraction is required. A complete system purchased will be installed at NCSU by Fall 2000. The major system components are variable volume cell, mechanical stirrer, syringe pump, sapphire window to video camera, VCR & TV, and PC interface.
Surfactant/Supercritical
CO2 Cleaning of Contaminated Substrates (20) Research to date has examined the solubility of several
commercial surfactants in carbon dioxide. A class of fluorinated surfactants
has been found that dissolve in CO2. The cleaning performance of
this surfactant/supercritical fluid (SSCF) mixture w/ and w/o water has been
examined for the removal of model contaminants from metal surfaces. The effect of oily, polar and non-polar
contaminants has also been examined for various CO2-based systems on
metallic substrate cleaning. Additional funding has been secured from the
Department of Energy and 3M Company.
Formation of Reversible Networks (21) A “sticky reptation”
model has been developed for the dynamics of entangled solutions of associating
polymers with many stickers per chain. The process of making and breaking
reversible bonds between associating groups (stickers) controls the dynamics of
associating polymers. At a high degree of association there are very few
unassociated stickers. It is very difficult for a sticker to find a new partner
to associate with after breaking the bond with an old one. Typically a sticker
returns to its old partner following an unsuccessful search for a new one,
prolonging the effective lifetime of reversible bonds. In the “sticky
reptation” model the search for a new partner is restricted to a part of the
tube confining the entangled chain. Another important effect is the increase of
the fraction of the inter-chain associations at the expense of the intra-chain
ones with increasing polymer concentration. The “sticky reptation” model
predicts very strong concentration dependence of viscosity.
Rheology of
Polymer Melts and Solutions in CO2 (17) A high-pressure extrusion slit die rheometer was constructed to
measure the viscosity of polymer melts plasticized by liquid and supercritical
CO2. Experimental measurements of viscosity as a function of shear
rate, pressure, temperature and CO2 concentration were conducted for
three commercial polystyrene melts. CO2
was shown to be an effective plasticizer for polystyrene, lowering the
viscosity of the polymer melt by as much as 80% depending of the process
conditions and CO2 concentration.
Existing theories were used to develop a free volume model for
predicting the effects of CO2 concentration and pressure on polymer
melt rheology, dependent only on material parameters of the polymer melt. This
model provides a simple predictive equations to quantify CO2 plasticization
effects.
Coatings from
CO2 (C6) Significant progress has been
made in the area of coatings using liquid CO2. Novel negative photoresists and photoacid
generators that are soluble in CO2 have been synthesized. Carbonell and students have developed a
high-pressure spin-coating apparatus and succeeded in creating uniform
one-micron thick layers on five-inch silicon wafers using liquid CO2
as a solvent. These are being exposed
and developed in CO2 to test their resolution in photolithography. A
free meniscus coating method has been developed to create sub-micron lubricant
films on magnetic disk drives. Both of
these tools are extremely promising for many applications, especially when
combined with new CO2-soluble polymers being developed at UNC.
Plans for 8/1/00-7/31/01
Next year, the main focus of Thrust Area C will be to better coordinate individual research programs along the lines of this report. For example, research by C. Grant and K. Roberts utilizes different approaches and systems to study cleaning; increased collaboration should enhance the overall productivity. Natural collaboration with other Thrust Areas in the Center will be encouraged. For example, the need for both solubility and phase behavior information from Area B is a critical component to understand decontamination, polymer and other processes.