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NEWSFRONT

upercritical fluids (SCFs), which combine key performance proper- S ties of both liquids and gases (box,

p. 33), are closer to reaching the tremendous growth predicted for them in the 1980s. First commercialized in the 1970s as extractants in “natural” decaffeination processes, SCFs-par- ticularly carbon dioxide and water- are finding new applications, as better, less-expensive equipment lowers pro- cessing costs, and regulations drive the chemical process industries (CPI) away h m organic solvents.

SCFs’ extraction capabilities are now being exploited in a range of new phar- maceutical and environmental applica- tions, while supercritical extraction, ox- idation and precipitation are being applied to waste cleanup challenges. Even in established food processing markets, regulatory pressures and health trends are driving increased use of SCFs. In the US., SCFs are filling a void left by trichloroethylene and meth- ylene chloride, formerly used in coffee and tea decaffeination, and ethylene dichloride, used in spice extraction. In Japan, six food plants have already shifted from solvent to SCF extraction.

The trend to low-fat foods, mean-

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chloride and acetone. In Japan, Takeda Chemical Industries (Osaka) is using supercritical CO, commercially to re- move acetone from antibiotics.

Another of C0,’s assets is that its solvent properties can be varied by ad- justing pressure and varying the num- ber of polar groups interacting with it. This flexibility makes it useful for pu- rification, explains Carsten Tiegs, a re- search chemist in Hoffmann-La Roche AG‘s (Basel) pressure and hydrogena- tion laboratory. Naphthalene, for ex- ample, is nearly insoluble in CO, at low pressure, but its solubility rises to 10 glL at 100 bars and 50 g/L at 200 bars.

Hoffmann-La Roche is pilot testing supercritical CO, extraction to purify vitamin E. The process may eventually replace vacuum distillation, which re- quires 200°C temperatures, resulting in some thermal decomposition and lower yield. Roche is now running a 16- m column to purify 50 m.t/yr.

In the U.S. and Europe, the push has been to develop supercritical fluid tech- nology to treat waste streams, as an al- ternative to incineration. Several tech- niques use either supercritical extraction or oxidation, or combinations of the two. In one recent Westinghouse

I ES FOR E R C RlTl CAL

while, is pushing fat extraction routes based on SCFs. Nearing scaleup to the million-lbdyr-level is one such process, developed by Liquid Carbonic Super- critical (Allentown, Pa.), which re- moves up to 80% of the cholesterol from egg yolk. Similarly, NutraSweet Co. (Chicago) plans to commercialize a su- percritical GO, process to extract 90% of the fat and 95% of the cholesterol from eggs. Liquid Carbonic has other processes for extracting vitamin E from soybean oil and decaffeinating coffee.

SCF extraction extends far beyond food processing. In pharmaceuticals processing, supercritical CO, is being investigated as an alternative to such conventional solvents as methylene

32 CHEMICAL ENGlNEERlNGlMARCH 1994 \ In this thermoplastic extruder, supercritical CO, is used to create a dispersion of micro- scopic bubbles, or voids, to form a product combining lightness and strength. The unit runs continuously, and is said to be easier to scale up than batch extruders

HOW TO BECOME SUPERCRITICAL Compressing and heating a liquid or gas above a critical level - in C0,'s case, 31 .o" C and 72.9 atm - gives it both liquid and gas properties. To address the dearth of data on SCFs, whose properties vary with tiny temperature and pressure changes, data are being gathered by the National Institute of Standards and Technology (Gaithersburg, Md.) and University of Massachusetts' (Lowell) professor Samuel

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I Sawan. Jaaan's Education Ministrv has funded a $4.4-million project to collect phase equilibra and other basic data.

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Hanford Co. (Richland, Wash.) project, a supercritical CO, extraction process deve loped by P h a s e x Corp . (Lawrence, Mass.), removed over 95% of the diesel oil, polychlorinated biphenyls (PCBs) and phthalates at its nuclear site (CE, May 1993, pp. 21 - 23).

Many projects are testing or using a technique called supercritical water ox- idation (SCWO), in which organic sub- stances are treated with supercritical water at 700 to 1,100"F and 3,400 psi. Organic liquids and gases mix with the water, and are transformed into CO, and water, while inorganics dissolve only slightly, allowing them to be recov- ered and concentrated. The technique destroys over 99.9% of organics, and is being applied to !waste at pulp and paper, chemical, and weapons plants.

SCWO poses various problems, as acids from halogenated waste streams

In the 1,000-Ud unit above, a Roche re- searcher evaluates supercritical oxidation of polluted wastewater. A commercial process FLUIDS is outlined below

I SUPERCRITICAL WATER TREATS TOXIC WASTE I

CRITICAL FLUID PARAMETERS

can corrode reactor vessels, while solids can precipitate and build up within the reactors. Nevertheless, the technique shows great promise, and is already being applied in pilot plants and a few commercial-scale projects. The US. Departments of Energy and Defense (DOD and DOE; Washington, D.C.), and the National Institute of Stan- dards and Technology (NIST; Gaithers- burg, Md.) are spending $15-20 mil- liodyr to develop SCWO.

Eventually, DOD could use up to 25 SCWO plants, each with around 6,000 g d d throughput, says Richard Kirts, a team leader for hazardous waste mini- mization at the U.S. Navy's Facilities Engineering Service Center (Port Hueneme, Calif.) The center is testing a 50-gaVd unit, running at 650°C and 3,400 psig, with a 10-s residence time. to destroy a mixture containing paint, oil, chlorinated solvents, PCBs and other hazardous compounds. E r t s says that SCWO could save the Navy be- tween $3 and $28/gal of such waste.

The Army, meanwhile, is evaluating SCWO to treat the 8,000 tons of smoke and dye target markers and tear gas stored at its McAlester plant in Okla- homa. Sandia National Laboratories (Livennore, Calif.) is working on a process under contract with the Army's Armament Research, Development and Engineering Center (Picatinny Arsenal, N.J.), and, with Foster Wheeler Develop ment Corp. (Livingston, N.J.), will design a 100-gaVh pilot plant due next October.

DOE, meanwhile, is collaborating with several companies to develop a 50- galh pilot plant in Idaho Springs that could eventually treat mixed radioac- tive waste. In Oak Ridge, Tenn., Mar- tin Marietta Energy Systems, Inc. is teaming up with Model1 Development Corp. (Modec; Framingham, Mass.) on another SCWO pilot.

CHEMICAL ENGINEERING/MARCH 1994 33

NEWSFRONT

Private industries are also evaluat- ing Modec‘s process.Last year, the com- pany joined forces with Noram Engi- neering and Constructors Ltd. (Vancouver, B.C.) to commercialize its SCWO system for pulp and paper treat- ment. A 5-todd plant is due onstream this year, to treat 4,040 tons/d (10% solids) of waste. The process costs about $225ldry ton, one-third as much as offsite incineration.

Also working in wastewater treat- ment is GNI Group Inc. (Deer Park, Tex.), which recently began building a $6-million, 5,000-gaud SCWO unit in Deer Park to treat contaminated wastewater, using technology devel- oped by Modar, Inc. (Natick, Mass.) and Boston’s Stone & Webster Engi- neering Corp. (CE, Dec.,1993, p. 17).

Last year, Texaco Chemical Co.’s (Austin, Tex.) research laboratories se- lected an SCWO process developed by ECO Waste Technologies (EWT; Austin, Tex.) to treat 5 gaumin of ethyl- ene- and propylene-based organic wastes (CE, July 1993, p. 17).

In Europe, the Kernforschungszen- trum Karlsruhe GmbH (W, Karl- sruhe, Germany) is developing a process that combines supercritical CO, extraction with either SCWO or indirect electro-oxidation (IEOX). IEOX uses metals with high oxidation potentials, such as Co(III), to oxidize organics to CO,, then regenerates them electrolytically. Pilot trials just began in a new 30-L/h plant to treat metal- laden sludge from a glass grinding fa- cility. This April, a lO-kg/h SCWO pilot

plant will be built to treat a halo- genated organic wastestream.

The Fraunhofer Institut fur Chemis- che Technologie (ICT; F’finztal, Ger- many) is also evaluating Modec’s and other SCWO processes to destroy such toxic organic pollutants as lindane, mu- nitions, and pharmaceutical wastes. Bench tests in a 250-cm3 autoclave show that alkaline supercritical water can destroy NDT-3, a mix of nitrocellu- lose, nitroglycerine, TNT and DNT, in 30 min. with less than 3% residue.

Supercritical water is being used in a precipitation route to recover organics. Developed in Ohio, at the University of Akron’s Chemical Engineering depart- ment, the technique removed over 99% of the polycyclic aromatic hydrocarbons (PAHs) in soil, and is being aimed at some 2,000 sites in the U.S. and Canada where town gas had been pro- duced from coal.

Process applications In addition to their other new roles, SCFs show promise as alternative sol- vents for chemical synthesis. Mass transfer improvements ranging from 50 to 1,000-fold have been observed in laboratory catalytic hydrogenation re- actions in the fluids, says Hoffman-La Roche’s Regs. Roche is investigating their possibilities in autoclaves ranging from 25 mL to 10 L, and plans to build a pilot plant within the next two years. SFCs could also play a role in such bio- catalytic processes as lipid hydrolysis, says Michel Perrut, CEO of Separex, Inc. (Champigneuelles, France).

In Japan, most SCF research is fo- cusing on chemical processes. The Min- istry of International Trade & Industry (MITI; Tokyo) started up a pilot plant in Izumi City last year, to fine-tune a process that uses supercritical CO, to purify ethanol. Last year, a 1-kIJd pilot plant started up in Izumi City. Its goal is a process that costs 30-40% less to in- stall than conventional distillation, and 66% less to run.

Clearly, SCFs are already doing a lot more than decaffeinating coffee. New applications (table, above) could soon include ceramics processing, paper deacidification, metal cleaning, and plastics and textiles processing. Stephen Moore and Sam Samdani,with

Gerald Ondrey and Gerald Parkinson

CHEMICAL ENGINEERING/MARCH 1994 35