PAT REPORT PnalrTlcbL
AVI(I.AR.E
TECnhO.OGV
air oxidation comes L
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Inc. has 118 WAO installations on five continents Ile sludges from sewage plants, recover chemical S trom paper mills and helps with incineration emissions Today, wet air oxidation provides some of the best practical available technology to destroy waste substances, recover valuable inorganic materials from the waste stream, and condition sludge solids for easy disposal. And since the oxidation is exothermic, substantial energy recovery is also feasible. Developed and patented by Zimpro Inc. (Rothschild. Wis.), wet air oxidation (WAO) is based on the discovery that any organic material in aqueous solution or suspension can be oxidized to any desired extent by air, under pressure, at temperatures from 350-700°F. The degree of oxidation (from 0-100%) depends on the temperature and the amount of air supplied. The WAO of some compounds may be effectively catalyzed. At the Burnie mill of Associated Pulp and Paper Mills Ltd. (APPM-Tasmania, Australia), a WAO system has been processing soda pulping black liquor since 1966. A hemisphere away, on Michigan's Upper Peninsula, a WAO system is being constructed at the Hoerner Waldorf Corp.3 Ontonagon paper mill to recover energy and pulping chemicals and destroy mill sludge. These two plants are the first and-the latest industrial members of a family of 118 WAO installations.
Down under experience At APPM's Australian mill, WAO provides practical and economical recovery of chemicals and energy from the soda pulping of eucalyptus wood, according to APPM's senior research officer, John Morgan. "The WAO unit processes liquor from the production of about 30 long tOnS of oven-dried pulp per day and returns better than 99% of the pulping chemicals to the pulping operation, with sampling and pump gland leakage being the largest losses," he reports. "The inorganic chemicals-sodium carbonate and some sodium bicarbonate-remain in the residual solution and can be passed to the recausticizing plant. Energy recovery is in the form of saturated process steam and electric power. The process is sufficiently flexible to allow the pro300
Environmental Science?. Technology
portion of steam and power generated to be varied over wide limits to suit local mill requirements." Morgan points out that, used in this way, WAO can replace the conventional evaporation and dry incineration approach to heat and chemical recovery in the pulp and paper industry. Evaporation may not be required in the WAO process; in fact, dilution may be required prior to the oxidization reaction. "The process has operated SUCcessfully right from the plant commissioning," Morgan says. "The unit has demonstrated 93-plus% availability. I feel that our working relationship with Zimpro Inc. is good and Ithat our joint development program with them has been conducted successfully during the past eight years the unit has been in operation, even though we are located almost on the other side of the globe."
produce a recovered organic-free filler that is acceptably bright, without changing the abrasive qualities of the
reea IS pumpea to system pressure. mixed with compressed air, and heated by he: 3t exchange with outgoing products. In an enclosed reaction .. ,. '.. . vessel, the t r w w r e 1s n e ~ u ~ u r a specified length of time and the oxygen in the air reacts with the organic matter in the feed stream. This reaction i s accompanied by a rise in temperature. The gas and liquid phases
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In this country The Ontonagon mill presently produces 240 tons per day (tpd) of semichemical pulp using a mixture of sodium carbonate-sodium hydroxide in the cooking liquor. In late 1975, capacity will be expanded to 440 tpd, and the present recovery boiler will be replaced by a Zimpro WAO unit. The unit will oxidize the black liquor, returning a sodium carbonate-sodium bicarbonate solution directly to the pulping process. It will also produce steam for process use at the rate of 3.2 Ib of steam per pound of .oiacn ~ * - ,!.~... -,~ 1-L--iquur JUIIUS. fib ail e i i i a " m e fit, the unit will also be able to oxidize about five tpd of activated sludge solids, fed to the unit at a 99% water content from the mill's new secondary wastewater treatment plant. Another unique application of WAO in the pulp and paper industry is the recovery of filler materials, such as clay and titanium dioxide, from paper manufacturing or de-inking wastes. In a conventional incineration operation, the fillers are recrystallized and can cause abrasion to papermaking machinery when the recovered material is recycled. The low temperatures encountered during WAO, however,
APPM's Morgan "Unit operated well from start-up" are separated after the reaction. The liquid and solids are passed on for recovery or final disposal; the gas is scrubbed or treated in an afterburner before release to the atmosphere. The degree of oxidation can be selected, and can range from 0-100% At high temperatures (above 600°F) and pressures (above 2000 psig), organic materials can be oxidized almost completely to carbon dioxide, water, and elemental nitrogen, leaving behind inorganic materials and ash suspended or dissolved in the carrying water. At temperatures between 350-600°F and at correspondingly lower pressures (300-2000 psig) partial oxidation takes place.
The latter conditions are popular among municipal sewage treatment systems that use low oxidation to convert difficult-to-dewater sewage sludges to sterile, easily dewatered residues. Such major cities as Toronto, Ont.. Canada; Cincinnati and Columbus, Ohio; Davenport and Dubuque, Iowa; Louisville, Ky.: Seoul, Korea; and Port Elizabeth, South Africa, have selected the iowpressure oxidation method of sewage sludge treatment. Unlike open air burning, the feed in WAO does not need to be dewatered. In fact, water must be present at ail times; the system's pressure accomplishes this by preventing water from being converted to steam during heating. Likewise, WAO eliminates many of the air pollution problems associated with conventional incineration. No smoke or particulate matter is carried to the atmosphere. Since oxidation temperatures are comparatively low, the exhaust gases are free of nitrogen oxides and sulfur oxides, which are retained in the liquid and solid phases. WAO exhausts may, however, contain small amounts of low-molecular-weight organic compounds that can be easily removed by incineration. The WAO reaction is exothermic, jUSt as in conventional oxidation and, in most cases, sufficient heat is generated to make the process thermally self-sustaining. Under certain conditions, surplus energy is liberated during the reaction, and energy recovery becomes practical. For start-up, and in those cases where energy is not sufficient for self-sustaining operation, additional energy can be added by direct steam injection or by heat exchange between the reactor feed and a heat transfer medium. Most WAO operations, whether high or low oxidation, are similar. They have an air compressor to supply the air, a pump to deliver the feed material in a continuous flow, a heat exchanger, a reaction vessel, and a boiler or other heat source for start-up. Accessory equipment, such as a heat exchanger washing system. instrumentation, thickening tanks, chemical feed systems, or other equipment, is added as required. Chemical feed is used in industrial or high-oxidation plants where control of pH or hardness is desirable. Other WAO uses I n addition to pulp and paper and municipal waste treatment, Zimpro inc.'s WAO is being increasingly applied to other pollution and recovery problems. WAO is being used to detoxify acrylonitrile wastes by destroying cya-
nide. Recent process developments have made it possible to produce a water-white effluent containing less than 2% of the influent COD. Ammonium sulfate of marketable quality can be recovered from the effluent. Sufficient heat is liberated during the oxidation not only to make the process thermally self-sustaining, but also to concentrate the ammonium sulfate by evaporation. To date, four acrylonitrile waste plants have been built. WAO will also be used by the U.S. Navy to safely destroy, without air pollution, off-specification and outdated propellants, explosives, and munitions. Such materials have been disposed of in the past by open-pit burning. In WAO, however, a slurry of ground explosives is oxidized with air; the residue is a small volume of inert ash and salts. The Naval Ordnance Station at Indian Head, Md., will be the site of the first such installation. The WAO process is also applica-
ble to the regeneration of powdered activated carbon. Zimpro Inc. is marketing such systems in combination with waste treatment plants, or as individual wastewater reclamation units. i n this application, spent powdered activated carbon is processed through a WAO unit where adsorbed and associated impurities are destroyed, and about 95% of the carbon is regenerated for reuse. i n the past, high-oxidation plants often have been characterized by reiativeiy high capital costs and low operating costs. Where inexpensive fuel and electric power have been available, the economics of the situation might not have justified the capital investment needed for the high-temperature, high-pressure, high-oxidation equipment. Recently, however, the energy squeeze and increasing fuel and power costs have changed the economics of the situation so that high oxidation plants once again appear to be attractive for the municipal, as well as the industrial, market.
Status of wet air oxidation installations
Far East and Australia
Europe Africa
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Various indtistrial wastes Municipal sewage sludge and night soil Acrylonitrile waste liquors Pulp and paper wastes Food processing wastes Petrochemicalwastes Municipal sewage sludge Municipal sewage sludge and night soil
Source: Zimpro Inc..
Volume 9, Number 4, April 1975
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