Supercritical commercial process

Supercritical Extraction. The use of a supercritical fluid such as carbon dioxide as extractant is growing in industrial importance, particularly in the food-related industries. The advantages of supercritical fluids (qv) as extractants include favorable solubiHty and transport properties, and the abiHty to complete an extraction rapidly at moderate temperature. Whereas most of the supercritical extraction processes are soHd—Hquid extractions, some Hquid—Hquid extractions are of commercial interest also. For example, the removal of ethanol from dilute aqueous solutions using Hquid carbon dioxide... 

Because of its low cost, nonhazardous chemical nature, and low critical temperature, carbon dioxide has been used in many applications. A commercial process to remove caffeine from coffee, using supercritical C02 as the solvent, is shown in Fig. 17. While actually a liquid-solid extraction process, it demonstrates principles involved in SCFE. A commercial SCFE process has been reported for recovery of hydrocarbon liquid from heavy oil. As compared with conventional propane deasphalting, this SCFE process can reduce capital and energy costs. 

A new supercritical fluid process has been developed for the continuous extraction of liquids. The most useful solvent employed in the recently patented process is supercritical or near-critical carbon dioxide(l). At the heart of the process are porous membranes. Their porosity combined with a near-critical fluid s high diffusivity create a dynamic non-dispersive contact between solvent and feed liquid. The technique is dubbed porocritical fluid extraction and will be commercialized as the Porocrit Process. 

Since the first large-scale supercritical extraction process was commercialized for the decaffeination of green coffee with carbon dioxide a decade ago, scientists and engineers in the food industry have been paying considerable attention to this technique for similar separations, i.e., removal of cholesterol from butter, removal of cocoa butter from cocoa beans, and extraction of hops, spices, and... 

Criteria for selection of equipment and components for commercial supercritical fluid processing plants for the food processing industry are listed and discussed. Unique features and designs for SCF food processing are specified. Requirements for vessels, heat exchangers, instrumentation, piping, fluid transport devices and typical ancillary equipment are reviewed. 

Passey CA. Commercial feasibility of a supercritical extraction plant for making reduced-calorie peanuts. In Rizvi SSH, ed. Supercritical Fluid Processing of Food and Biomaterials. London Blackie Academic, 1994 223-243. 

COMMERCIAL PROCESS. A practical example of a supercritical fluid extraction process is the decaffeination of coffee. Coffee beans are first soaked in water to make the extraction more selective and then are loaded into an extraction vessel through which supercritical CO2 is circulated to dissolve the caffeine. In a separate scrubbing vessel the caffeine is transferred from the CO2 to water, also at high pressure. Extraction is continued until the caffeine content of the beans, originally... 

In the examples given above, supercritical CO2 is used in what is termed clean technology with drastic reductions in the use of organic solvents, and the twenty-first century should see an increase in the use of supercritical fiuids in commercial processes. 

Fig. 8.9 Percentage contributions to the 2001 global US 960 million value of commercial production using supercritical fluid processing [data Kline Company, Inc., www.klinegroup.com].

The oxidation of light alkanes by air or O2 at supercritical temperatures and pressures was explored by Standard Oil in the mid-1920s [153]. Experiments were performed at the laboratory and then semicommercial plant level. The primary products were alcohols. For example, the oxidation of pentane was performed at supercritical conditions (240 °C, around 200 bar and a few mole per cent O2) and produced primarily C2-C3 alcohols and acids. However, the oxidation of heptane was performed at subcritical temperatures (225 °C) and produced primarily Cg-Cy alcohols. The change in selectivity was attributed to either the difference in phase or more likely the difference in temperature. Other commercial processes for the formation of alcohol denaturants or formaldehyde were reported in the same decade [154,155], but it is unclear whether those reactions were operated at supercritical pressures. Modem processes involving alkane oxidation are heterogeneously catalyzed and operated at sub-critical pressures [156]. 

In the early stages of process development in the U.S., many "promises" were made about the capabilities of, and applications for, supercritical fluid extraction. When the widely-touted applications did not materialize, as exemplified by commercialized processes, for example, the "luster" of supercritical fluid solvents rather quickly faded. It is informative to review some of the trade and association journal articles that were published during this period because of their influence on many researchers and companies in their future investigations of supercritical fluid extraction. 

Developing a commercial supercritical CO2 processing plant requires... 

Any commercialization of a process that uses supercritical fluids has to involve a cost analysis that should indicate the advantages of the new process that would offset the penalty of high-pressure operations. A variety of supercritical fluid processes have been commercialized. Details of a few such processes are given in the following text. Many other processes have been investigated on a lab or pilot plant scale and have the potential to be scaled up in the near future. 

This commercial process requires pressurization and a relatively high temperature. Oxygen is the oxidizing agent, and acetic acid (CH3COOH) is the solvent. An alternative route employs supercritical water as the solvent and hydrogen peroxide as the oxidant. This alternative process has several potential advantages, most particularly the elimination of acetic acid as solvent. 

Using its four decades of expertise in modifying the surface of fumed siUca, Cabot revolutionized the manufacture of aerogel and was the first and only company to develop a commercialized process that allows production of the material under ambient drying conditions. This process allows Cabot to control the material s porosity, pore size, and distribution and bypasses the high-cost traditional method of supercritical drying. 

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