Development and design of a high pressure carbon dioxide system for the separation of hazardous contaminants from non-hazardous debris Page: 3 of 8
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The objective of this joint AlliedSignal KCP/Sandia National Laboratories project is to
demonstrate the feasibility of using high pressure carbon dioxide to segregate hazardous oils,
greases, and organic solvents from non-hazardous solid waste such as rags, wipes, swabs,
coveralls, gloves, etc. Carbon dioxide possesses many of the characteristics desired in an
"environmentally acceptable" solvent system. It is nontoxic, inexpensive, and recyclable. A
schematic of an ideal cleaning apparatus is shown in Figure 1. Contaminants dissolved in the
CO2 solvent are separated out by expansion of the fluid to a lower pressure where CO2 is a
gas and the dissolved materials precipitate out (usually as a liquid or solid). The gaseous CO2
can then be recompressed and recycled. The cycle describing a typical supercritical
extraction is schematically represented on the carbon dioxide phase diagram in Figure 2.
2. EXPERIMENTAL EQUIPMENT AND METHODS
The experimental activity currently in progress is directed at determining the extractability
and solubility of the major oils, greases, and solvents. Both the "extractability" and the
"solubility" of the contaminant affect the cleaning/recycle process. Solubility is a
thermodynamic equilibrium property of the solvent-contaminant system. Extractability, on
the other hand, is the amount of contaminant that can be removed from the substrate. It is
affected by the chemical nature of the contaminant and substrate, and the mass transfer in the
cleaning system. The extractability of a contaminant cannot exceed its solubility. The
solubility as a function of pressure and temperature determines the conditions that the
separator must be operated at in order to achieve a given level of carbon dioxide purity in the
recycle. Since the pressure drop must be recovered in the recompression step, the economics
of the process are greatly affected by the separator conditions.
At Sandia, a high pressure view cell is used to measure the thermodynamic equilibrium
solubility of contaminants in high pressure carbon dioxide. Pressures up to 68.9 MPa and
temperatures of 373 K can be achieved. Visual determination of the phase condition of a
contaminant/CO2 mixture as a function of temperature and pressure is made. The dew point
(contaminant solubility in CO2) or bubble point (CO2 solubility in contaminant) of the
mixture can be measured. From this data, equilibrium phase plots can be constructed as a
function of concentration and solubility information obtained.
At AlliedSignal KCP, a recycling high pressure carbon dioxide cleaning system was used to
measure the extractability of oils, greases, and solvents from laboratory paper wipes as a
function of the pressure, temperature, flowrate, and total mass flow of supercritical carbon
dioxide. The recirculating/recycle feature allows continual exposure of the contaminated
solids to clean carbon dioxide for as long as necessary without consuming large amounts of
CO2. For example, at 358 K and 32.4 MPa, the cleaning system contains only 7 kg of CO2,
but in a one hour run, 27 kg of fluid are circulated through extraction vessel. At the end of a
cleaning cycle, the clean solids are removed from the extractor to be reused or discarded as
non-hazardous waste. The oils/greases/solvents, now in concentrated form, are drained from
the separator and disposed of as hazardous waste.
Oils, greases, and typical solvents common to DOE/USAF facilities were identified. The list
included 131 oils and greases in use at the Kansas City Plant and USAF bases. These were
sorted by type and by monthly usage in order to allow a priority ranking for the experimental
work. Hydraulic and cutting oils were most commonly used. A similar organic solvent list
contained 29 solvents in use within DOE and USAF facilities consisting mostly of cleaning
solvents, paint thinners, and aircraft fuels. Isopropanol, methyl ethyl ketone, and
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Adkins, C. L. J.; Russick, E. M.; Smith, H. M. & Olson, R. B. Development and design of a high pressure carbon dioxide system for the separation of hazardous contaminants from non-hazardous debris, article, July 1, 1995; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc619232/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.