Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2002-04-16
2004-05-04
Bennett, Henry (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S492000, C034S337000, C034S339000, C516S104000
Reexamination Certificate
active
06729042
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for enhancing replacement of one fluid in a porous medium by a second fluid. Particularly, the present invention relates to the use of pressure fluctuation to enhance the mass and heat transport in a porous medium and, in particular, the replacement of one fluid in a porous medium by a second fluid. More particularly, the present invention relates to such methods wherein at least one of the fluids is compressible. Various applications include drying, solvent exchange, removal of soluble impurities, and the like.
BACKGROUND OF THE INVENTION
Many processes involve the replacement of one fluid in a porous media by a second fluid. For example, the process of drying involves the replacement of a liquid, frequently water, with a gas, usually air, through a process of evaporation. In another example, the extraction of caffeine from coffee beans can be considered as imbibing a solvent for the caffeine into the bean and replacing the solvent containing the caffeine with pure solvent, thereby extracting the caffeine from the bean. Also, aerogel products, after wet gel formation, are conventionally prepared by a process of solvent exchange between liquid CO
2
and the solvent that was utilized to form the wet gels, followed by a supercritical CO
2
extraction.
It often is desirable to perform these processes in a shorter period of time. Frequently, heat is used to hasten or sustain or support such processes. However, the heat transfer inside the porous medium can be very slow, and there are times when the application of high temperature heat can degrade the product. In drying, often vacuum is applied to hasten the process without degrading by heat. However, vacuum application requires extra equipment and expense, and still may require considerable time periods for completion of the process. Further, repressurization or depressurization may require care to avoid harm to the product.
Supercritical fluids can be used as solvents in extraction instruments, chromatographs and other related instruments. In supercritical fluid extraction, an extraction vessel is held at a temperature above the critical point and is supplied with fluid at a pressure above the critical pressure. Under these conditions, the fluid within the extraction vessel is a supercritical fluid. In supercritical fluid chromatography, a similar process is followed except that the supercritical fluid moves the sample through a column, separates some of the components of the sample one from the other, and removes the components from the column.
The critical temperature is the temperature above which the distinction between gases and liquids disappears—that is, where there is one fluid phase for all pressures, and where, no matter how much pressure is applied, a liquid phase cannot be condensed. The supercritical region is defined by all temperatures and pressures above the critical temperature and pressure. Supercritical fluids are a useful hybrid of gases and liquids as we commonly perceive them, possessing gas-like viscosities, liquid-like densities, and diffusivities greater than typical liquid solvents. The liquid-like density of a supercritical fluid imparts a variable liquid-like solvent power by an essentially linear function of density. This allows the solvent power, usually considered a chemical interaction, to be set (“dialed in”) simply by adjusting a physical parameter, namely density or pressure.
The supercritical fluid transport properties of relatively low viscosity and relatively low diffusivity allow enhanced mass transport within complex matrices, such as coal, plant or animal tissue, or packed beds. In other words, supercritical fluids penetrate better and dissolve almost as well as typical liquids. Therefore, supercritical fluids are more efficient to use for extractions of complex matrices.
Carbon dioxide is the principal extracting fluid used in supercritical fluid extraction systems because it is cheap, innocuous, readily available at high purities, and has a relatively low critical temperature of about 31° C. Thus, it is useful for thermally labile compounds and to avoid the hazards of high temperature flammable solvents. Furthermore, it is mutually soluble with many common liquid solvents.
It has been found that carbon dioxide has a solvent power similar to that of hexane. Hence, many applications exist that require great solvent power, the advantageous properties of supercritical fluids, and mild operating temperatures for thermally labile compounds. Mixtures of carbon dioxide plus modifiers can meet these requirements. As is well known to those of ordinary skill, supercritical fluids can be used as solvents in extractions and chromatography; in such applications carbon dioxide is the preferred solvent. Other fluids, e.g., ethane, nitrous oxide, ethylene, or sulfur hexafluoride, that have critical points near ambient temperature (25° C.) can also function as the base solvent. The capability to utilize these alternative solvents is preferably not exploited because of the potential danger in using these solvents.
U.S. Pat. No. 5,133,859 describes a sample preparation device, which extracts sample components from complex matrices using supercritical carbon dioxide as the principal extracting solvent, and which presents the resulting extract in a user-chosen sample collection vessel. Traditional preparative procedures such as solvent extraction, Soxhlet extraction, liquid/liquid extraction, concentration, and evaporation are replaced with the solvent power stepwise settable by the parameters of density, modifier concentration, and temperature.
The supercritical fluid extractor can mimic column chromatography sample fractionation in some applications. Accordingly, the fluid flow system apparatus comprises control apparatus having a variable and controllable flow restriction and a sample container section. The sample is inserted into the sample container section, the temperature, pressure, flow rate and extraction time setpoints are inputted into the control apparatus, and pressurized fluid is provided.
By directing the fluid to a pump—which injects the fluid into the flow system apparatus at the input flow rate setpoint—the extraction process is initiated. The system pressure is sensed as fluid is pumped into the system, and the variable flow restriction is regulated to achieve and to maintain the setpoint pressure. Extraction is accomplished by directing fluid through the sample at the setpoint flow rate, and by directing a fluid mixture leaving the sample container section to an expansion nozzle section.
Preferably, the methods include maintaining the controlled setpoints of flow, pressure, and temperature until the input extraction time is achieved. The methods also contemplate opening and closing the orifice in order to control the variable flow restriction, or closing the orifice until setpoint pressure is achieved and controlling the restriction of the orifice to maintain the setpoint pressure.
In one class of supercritical fluid extraction of soluble components from a sample using a supercritical fluid, the components dissolved in the extraction fluid are separated from the fluid by allowing the extraction fluid to vaporize. For extraction, supercritical fluid flows through material to be extracted.
As described in U.S. Pat. No. 6,149,814, the fluid flows through a heat exchanger so that the heat exchanger is at the same temperature as a pressure vessel and an extraction tube. Before using the extraction system, the pump is set to the desired pressure and the heater block is set to the desired temperature. The internal cavity is then filled or partly filled with sample to be extracted. Pressurized fluid flows through a valve into the heat exchanger so that it is at the desired supercritical temperature, and flows into the cavity. This supercritical fluid flowing through the interior sample cavity of the extraction cartridge extracts the soluble components from the sample contained within the cavity.
In making aerogel products via a conve
Aspen Systems Inc.
Bennett Henry
Edwards & Angell LLP
Neuner George W.
Nguyen Camtu
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