Supercritical Fluid Cleaning: Fundamentals, Technology and Applications
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GRAS is the U. Food and Drug Admin. Multi-product plants. The high capital cost of building and operating a production plant utilizing supercritical extraction promotes expanding the use of the plant to a multi-product platform.
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Selective extraction of multiple products can be accomplished by modifying the solvent power of the supercritical fluid. The solvent power is modified by varying the extraction pressure or by adding a co-solvent. Another method to extract multiple products is by sequential depressurization, in which all products are extracted simultaneously. The separation step is performed sequentially through a series of separator vessels. This process is referred to as fractional separation.
In fact, extraction of food and natural products using supercritical or liquid CO 2 can be considered a relatively mature CO 2 technology. A wide range of other applications for supercritical CO 2 has been investigated, including chemical reactions, polymer production and processing, semiconductor processing, powder production, environmental and soil remediation and dry cleaning.
Commercialization for these applications has, however, proceeded at a slower pace than for extraction. Several of these applications are highlighted here.
Chemical reactions. Supercritical CO 2 has been tested in a variety of industrially important reactions, such as alkylations, hydroformylations, and hydrogenation, as an alternative reaction medium. Relatively high rates of molecular diffusion and heat transfer are possible with a homogenous, supercritical-CO 2 reaction-medium. Limitations to the use of supercritical CO 2 as a reaction medium include a poor solubility of polar and high-molecular-weight species, b no observed improvement in reaction chemistry in some cases, and c higher capital investment cost due to higher operating pressures.
For reactions not limited by reactant-gas concentrations or other mass-transfer limitations, there is no improvement in reactivity observed when using a homogeneous, supercritical CO 2 medium. Polymer production and processing. Applications of supercritical CO 2 in polymers include polymerization, polymer composite production, polymer blending, particle production, and microcellular foaming.
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Several applications, particularly those involving low pressures, have been successfully commercialized. At moderate pressure, very few polymers, except for certain amorphous fluoropolymers and silicones, show any significant solubility in CO 2. Very high pressure is typically needed to dissolve polymers in supercritical CO 2.
Its solvent power is weaker than that of n -alkanes. However, high degrees of swelling of the polymer by CO 2 can occur at significantly lower pressure. Although many polymers have very low solubility in CO 2 , the solubility of CO 2 in polymers is typically high. This has led to the use of CO 2 as a plasticizer. One example of this application area is a process to produce fluoropolymers using supercritical CO 2 as the reaction medium that was developed by scientists at the University of North Carolina Chapel Hill.
DuPont has an exclusive license for this process until The pilot plant is capable of producing 1, metric tons per year m. Several grades of melt-processable fluoropolymers produced from this process became commercially available in However, no further progress to develop the process beyond the pilot plant phase to a large-scale industrial process has occurred. Semiconductor processing. Currently, chip manufacturing involves many wet-chemical processes that use hydroxyl amines, mineral acids, elemental gases, organic solvents and large amounts of high purity water during chip fabrication.
One potential application is the use of supercritical CO 2 in wafer processing. The low viscosity and surface tension of supercritical CO 2 allow for efficient cleaning of small feature sizes, which is of great importance with the continued miniaturization of integrated circuits. However, the main obstacle to the use of supercritical CO 2 in semiconductor cleaning is the high cost.
Powder production. One promising application for supercritical CO 2 is the production of micro- and nano-scale particles.
Supercritical Fluid Cleaning
The pharmaceutical industry currently uses supercritical CO 2 mainly to control the powder particle size of products during synthesis. In the s, a U. The use of supercritical CO 2 for micronization of pharmaceutical compounds has several potential advantages over conventional techniques such as spray drying, jet milling and grinding. These advantages include minimum product contamination, reduced waste streams, suitability for the processing of thermally, shock or chemically sensitive compounds and the possibility of producing particles with narrow size distribution in a single-step operation.
For details on how to order the full report, contact the author using the contact information on p. In the chemicals industry, there is no room for compromises. View More. This publication contains text, graphics, images, and other content collectively "Content" , which are for informational purposes only. Certain articles contain the author's personal recommendations only. Latest Issue. Connected Plant Directory. Tokyo, Japan; www. Figure 1. The phase diagram for carbon dioxide shows its supercritical region. Click here for full pdf version of this article — includes all graphs, charts, tables, and author information.
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Partner Content BrandConnect is content both created and aggregated by Marketers from leading companies in the industry - featuring the most up-to-date information on the latest industry news, trends and product information to help make informed purchase decisions. In order to deliver a constant flow of CO 2 it is desired that the CO 2 supply in the storage tank 56 be kept at constant pressure.
The preferred pressure is psi and is maintained by control of temperature. Any other temperature and pressure setting below the critical point can be chosen. In order to maintain the integrity of the phases of the process chemical, inlet and outlet lines of the process chamber 10 , and valves 52 , 54 are heated to the desired process temperature as indicated by the dotted lines in FIG. The CO 2 delivery system 50 allows charging the process chamber 10 with either the process chemical in the gas, the liquid or the supercritical state. Pressurizing the process chamber with supercritical process chemical is desired for some processes such as dry resist develop.
The Equilibrium Thermal Physics of Supercritical Fluids
Using supercritical process chemical instead of liquid one prevents pattern collapse of high aspect ratio structures, as noted in Environmental News , Vol. Process steps 1 and 6 are necessary pre and post process steps to the core steps of the process; fill, soak, agitate, and rinse.
These principle process steps 2 through 5, or alternatively, steps 3 and 4 or step 4, within the sequence of steps 2 to 5, can be repeated as many times as needed in a process cycle to satisfy the user's requirement. It will be readily apparent to those skilled in the art that the soak and agitation steps are fundamental to the cleaning process of the invention, and that repetition of these two steps or of other combinations of the basis process steps will provide additional cleaning effect to the wafer under process.
Process parameters can be modified between or during a process cycle, and the process chemicals can be varied if required, as well. The basic process steps are described in detail, in the context of the enabling apparatus illustrated in the figures, as follows:. Once the wafer is positioned on studs 34 as described above, lid 30 moves in a linear motion towards the process chamber While the lid is moving, gaseous CO 2 flows through a dedicated line 92 into the process chamber with the outlet valve 54 closed. When lid 30 FIG. Gaseous CO 2 is now leaving the process chamber through this gap generating a forced flow of CO 2 gas from the inside towards the outside of the chamber through the gap.
The flow of CO 2 gas into the process chamber is adjustable but kept constant by means of a mass flow controller Therefore, with the decreasing gap between seal and seal seat, the CO 2 flow through the gap from the inside of the process chamber to the outside is increasing in velocity. The outflow velocity reaches its maximum in the moment just before the lid is locked to close the chamber, i.
This gaseous CO 2 flow carries any particulates that may be generated when the seal touches its seat away from the wafer and the chamber. The wafer is thus prevented from being contaminated by particulates of this source. Once the chamber is closed and locked, the CO 2 continues to flow, which results in an increase in pressure in the process chamber. The gaseous CO 2 pressure is a variable but typically limited to 28 psi by a pressure regulator The outlet valve 54 of the chamber as well as valve and backpressure regulator will be opened and the chamber will be purged with a continuous flow of gaseous CO 2 to replace the air that is trapped in it.
The process control parameter is the purge time. Pressure and flow rates of gaseous CO 2 are preset. Prior to pressurization of the chamber, the gas supply system is conditioned such that all supply lines 62 , 72 , 82 are filled with CO 2 and are set to a pressure of psi up to the inlet valve of the process chamber. The process chamber is then charged with gaseous CO 2 through another dedicated line 94 out of the CO 2 storage tank 56 and pressurized to a pressure of psi, which is the equilibrium pressure of the CO 2 vapor in the storage tank.