wet air oxidation

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Wet Air Oxidation: A Review of Commercial Sub-critical Hydrothermal Treatment by Clayton B. Maugans & Claude Ellis USFilter’s Zimpro Products USA ABSTRACT Wet air oxidation (WAO) is a hydrothermal treatment process that has been commercialized for approximately 50 years. Over 200 industrial or municipal WAO systems have been constructed. The process is operated in a pressurized hydrothermal reactor in the sub-critical water temperature and pressure range with the system conditions usually under 320°C and 214 bar (608°F/3100 psig). This process is usually applied for the treatment of high strength wastewaters with components that are difficult or uneconomical to treat via conventional biological treatment or incineration. WAO has also been used for the treatment of municipal sludge to minimize the consumption of landfill capacity. In the majority of WAO wastewater applications, subsequent biological treatment of the WAO effluent is required. Another application of WAO includes the integration into industrial processes for insitu removal of impurities, such as in crystallization. Presented at: IT3’02 Conference, May 13-17, 2002, New Orleans, Louisiana

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Page 1: Wet Air Oxidation

Wet Air Oxidation: A Review of Commercial Sub-critical Hydrothermal Treatment

by

Clayton B. Maugans &

Claude Ellis

USFilter’s Zimpro Products USA

ABSTRACT Wet air oxidation (WAO) is a hydrothermal treatment process that has been commercialized for approximately 50 years. Over 200 industrial or municipal WAO systems have been constructed. The process is operated in a pressurized hydrothermal reactor in the sub-critical water temperature and pressure range with the system conditions usually under 320°C and 214 bar (608°F/3100 psig). This process is usually applied for the treatment of high strength wastewaters with components that are difficult or uneconomical to treat via conventional biological treatment or incineration. WAO has also been used for the treatment of municipal sludge to minimize the consumption of landfill capacity. In the majority of WAO wastewater applications, subsequent biological treatment of the WAO effluent is required. Another application of WAO includes the integration into industrial processes for insitu removal of impurities, such as in crystallization. Presented at: IT3’02 Conference, May 13-17, 2002, New Orleans, Louisiana

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INTRODUCTION The wet oxidation technology has been successfully commercialized for 50 years. Over 200 full scale wet oxidation systems have been constructed and operated for a variety of applications ranging from elimination of hazardous wastes to the production of vanilla flavoring. While wet oxidation chemistry is also used as a chemical production technology as well as in the manufacture of computer chips, the focus of this paper is on waste elimination. Wet oxidation, also known as wet air oxidation (WAO) or hydrothermal treatment, refers to the process of oxidizing materials suspended or dissolved in liquid water with dissolved oxygen at elevated temperature. Hydrolysis reactions also occur during the WAO process and in some cases oxygen is not necessary to achieve the treatment objectives. HISTORY OF WAO COMMERCIALIZATION The WAO technology was first patented in Sweden in 1911 for destruction of spent pulping liquor and was not initially a large commercial success.1 While WAO is considered a waste treatment technology today, it was first successfully commercialized in the 1930s and 1940s for the manufacture of artificial vanilla flavoring (vanillin) as the prime motive, with some waste elimination as a secondary bonus result. Vanillin was produced by the low temperature (160°C) wet oxidation of the lignosulfonic acids in spent pulping liquor from paper mills and by 1940, 70% of the US supply was produced by WAO.2-4 The WAO process was commercialized as the Zimmermann Process, named after its developer, and the term Zimpro® Process remains a common synonym for WAO.5

Paper mills were integral to the first commercialization of the WAO technology and as such WAO was also developed for other paper mill applications. In the 1950s, high temperature WAO was developed and commercialized for the destruction of paper mill waste liquor (black liquor) for waste elimination and energy and soda recovery.1,6-11 Following this development, the high pressure oxidation application of the WAO technology was commercialized for waste sludge destruction. A number of years later, high pressure sludge oxidation was commercially applied to paper-mill sludges for waste destruction and recovery of filler clay materials used in the paper making process.12-13

In the early 1960s Zimmermann found that low temperature WAO could also be applied to biological sludges, not to destroy them but to allow greatly enhanced dewaterability for landfill minimization.14-20 This specific WAO process and application is called thermal sludge conditioning (TSC) and WAO units which operate at low temperatures and low pressures to achieve TSC are known as LPO (Low Pressure Oxidation) units. LPO units were popular due to their lower capital cost compared to the higher pressure sludge destruction WAO units. The LPO units were a tremendous commercial success with a large market because the timely introduction of this technology coincided with increased sludge production created by the construction of new wastewater treatment facilities. Around 200 LPO systems were constructed in the 1960s and 1970s.1

In the late 1960s and early 1970s the Powdered Activated Carbon Treatment (PACT®) technology was developed.21 PACT is an advanced biological wastewater treatment technology which combines biological oxidation and assimilation with

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adsorption on powdered activated carbon. The impetus for the co-development of the PACT technology by Zimpro, the leading WAO producer, was to create an additional market for the sale of WAO systems. When WAO is combined with PACT, WAO is used to regenerate the spent powdered activated carbon for reuse in the PACT system.22-

24 This WAO application is referred to as wet air regeneration (WAR) and the PACT-WAR market continues to be an active market.25

In the 1970s, the application of WAO for the treatment of industrial wastewaters began to be commercialized.1 Other forms of catalytic and non-catalytic WAO for treatment of industrial wastewaters were also commercialized during this time frame.1,26-

30 During the 1980s, the market activity for the LPO technology for the treatment of sludge began to wane at the same time that WAO began to see more use as an industrial waste treatment technology. WAO was commercialized for the treatment of sulfide laden industrial spent caustics and other industrial wastes.31-35 The spent caustic and industrial market continues to be an active market for new commercial WAO applications.36-40

From the late 1990s through the present, there has been speculation of a resurgence in the municipal sludge destruction/minimization market, this time in Europe. WAO is being commercialized for this market under trade names such as Athos® and the LPO.20,26,44,45

Another recent chemical industry WAO application trend is the migration to becoming more integrated into the overall chemical production process. An example of this is in the area of improving product crystals, where the mother liquor that is purged from a crystallizer is now being wet air oxidized for the destruction of the concentrated organic buildup and allowing the recycle of the inorganic product to the crystallizer.46-50

WAO TECHNOLOGY DESCRIPTION WAO is an aqueous phase process in which soluble or suspended oxidizable constituents are oxidized by dissolved oxygen. Oxidation reactions take place in the aqueous environment where the water is an integral part of the reaction. Water provides a medium for the dissolved oxygen to react with the organics and can also react in part with the organics. It is theorized that the chemistry of wet oxidation involves free radical formation with oxygen derived radicals attacking the organic compounds and resulting in the formation of organic radicals.51,52 Catalysts, such as homogeneous Cu2+ and Fe3+, their heterogeneous counterparts, or precious metal catalysts can be used to enhance the effectiveness of the WAO reaction.28,49,53-60

A noteworthy characteristic of wet oxidation chemistry is the formation of carboxylic acids in addition to the primary end-products, CO2 and H2O. The yield of these carboxylic acids varies greatly depending on the design of the WAO system, but typically 5-10% of the total organic carbon (TOC) from the feed stream remains as carboxylic acids by-products, predominantly acetic, formic, and oxalic acid. Should these compounds be undesirable, then a more rigorous treatment at high temperature and/or catalytic operation can be used. As a practical matter however the carboxylic acids are biologically degradable and conventional biological post treatment of the WAO effluent is often the most cost effective approach. If present, the elements, nitrogen, phosphorus, sulfur, and chlorine are usually reacted to NH3, PO4

3-, SO42-, and Cl-

respectively.

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The chemistry of wet oxidation has some attractive properties with regards to the off-gas produced. The off-gas from a WAO reaction has negligible NOx, and SOx and negligible particulates. Volatile organic compounds (VOCs), such as aldehydes, ketones, and alcohols may be in the off-gas depending on the composition of the feed stream. When VOCs are present, thermal oxidation is usually used for treatment of the off-gas. GENERAL WAO PROCESS DESCRIPTION A simple diagram of a general wet oxidation flow-scheme can be seen in Figure 1. The waste is pumped through a high-pressure pump; this can be a standard reciprocating diaphragm pump for liquids or a more exotic high pressure pump for slurries. The oxygen for oxidation is supplied by either air or pure oxygen and in this general flow scheme an air compressor is shown. The air is combined with the liquid and they pass through a feed/effluent (F/E) heat exchanger (HX) where the fluid is heated to near reaction temperatures. The two phase fluid then flows into the bubble reactor where the exothermic reaction takes place. The usual retention time in the reactor is 1 hour. The oxidized effluent and off-gas then pass through the hot side of the F/E HX to be cooled while simultaneously heating the influent. Auxiliary heaters and coolers are also employed (not shown). Depending on material of construction constraints, steam balance desires, or other factors, separate heat exchangers rather than an F/E HX have been used. After cooling, the wet oxidized effluent then passes through a pressure control valve that controls the pressure on the WAO system. A separator downstream of the pressure control valve allows the depressurized and cooled vapor to separate from the liquid. The liquid is discharged, typically to conventional biological treatment facilities for final treatment. The gas is usually vented to some form of thermal oxidation such as a boiler or a dedicated flare header.

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Fig.1

ProcessHeat

Exchanger

OxidizableWaste

FeedPump

Air Compressor

Reactor

OxidizedWastewater

PCV

PC

Figure 1. General WAO Process Flow Diagram WET OXIDATION TEMPERATURE AND APPLICATION SPECTRUM The wet oxidation reactions usually take place between 100°C and 372°C at elevated pressures to maintain water in the liquid phase. This temperature range can be further subdivided into low (100-200°C), medium (200-260°C), and high temperature (260-320°C) operation. Higher temperature (320-372°C) systems can be designed as well but are rarely used due to the high capital cost. Due to the necessity to maintain water in the liquid form, the system pressure is maintained above the steam pressure. Because steam pressure increases with temperature, the terms such as high temperature and high pressure are used interchangeably. Figure 2 is a diagram of the typical non-catalytic WAO waste treatment application spectrum.

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Figure 2. Spectrum of Typical non-catalytic WAO applications. Commercial applications of low temperature oxidation (100-200°C) include the low pressure thermal conditioning of sludge (i.e. LPO) and low strength sulfidic spent caustic treatment. Other industrial wastes treated by low temperature oxidation include cyanide and phosphorus wastes as well as non-chlorinated pesticides.

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Medium temperature (200-260°C) oxidation is used for typical ethylene spent caustics, as well as for autothermal TSC, and some industrial wastes. Ethylene spent caustics, derived from sulfidic acid gas washings in the ethylene industry are treated in the 200-220°C range. Autothermal TSC operations occur in the temperature range of 220-260°C as does WAR treatment of powdered activated carbon for use in wastewater biological treatment applications. Some other industrial wastes containing phenolic or cresylic compounds can be treated at the high end of this temperature spectrum. High temperature (260-320°C) oxidation is used for refinery spent caustic, sludge destruction, and most WAO treated industrial wastewaters. Within the 260-280°C range, refinery spent caustics containing significant concentrations of naphthenic and cresylic acids are treated. Most organic industrial wastes are oxidized in this temperature range, including pesticides, solvents, pharmaceutical wastes, and other dissolved organic chemical wastes. Sludge destruction systems are designed to operate within the higher end of this temperature range (280-320°C) and are employed for complete destruction of municipal, pulp and paper, or other organic sludges. Chemical recovery systems which are used in the treatment of pulping liquors also operate within this higher temperature range. KEY HISTORIC AND CURRENT COMMERCIAL WAO APPLICATIONS Vanillin While vanillin production doesn’t fall under the waste treatment focus of todays WAO applications of interest, it is mentioned because it is from vanillin production that WAO as a waste treatment technology has evolved. In the 1930s and 1940s, the concept of producing vanillin by low temperature wet oxidation was first commercially applied as a lower cost alternative to produce artificial vanilla flavoring.2-4 The application was to take a wood pulping waste liquor, which was rich in lignins, and apply WAO to convert a portion of the lignins into vanillin. The application was a significant commercial success, though on a small scale. In the 1940s 70% of the US supply was produced at the one reactor located in a paper mill in central Wisconsin. Eventually the process was replaced with other synthetic manufacturing techniques. However, it was from this commercial success that Zimmermann, the technology developer, was able to commercialize the WAO technology for other applications.6

LPO Typical existing municipal sewage treatment plants of the early 1960s used simple primary settling of solids from raw sewage and applied anaerobic digestion to the sludge. To meet new federal regulations most plants added secondary aerobic treatment to the primary effluent. This additional step greatly increased the amount of sludge produced and made the anaerobic digestion impractical. The secondary sludge was difficult to dewater with the equipment that was available at that time thus opening a market for more efficient methods to minimize sludge disposal. The LPO treatment of sludge solved this dewatering problem by conditioning the sludge, allowing the water to drain more effectively during the dewatering step. This thermal treatment of the sludge is called thermal sludge conditioning (TSC) and was accomplished at the lower temperature and pressure end of the WAO application range.

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The LPO process flow scheme is as shown in Figure 1 and is operated at 175-200°C (350-395°F) and 200-500 psig.19,20 Due to the low operating temperature, relatively little oxidation occurs and autothermal (self-sustaining) operation is not achieved in the LPO. Steam is injected into the reactor to maintain the operating temperature. The separator is often a large settling tank that allows the solids to readily settle to the bottom while the off-gas is vented to an odor control system. The supernatant from the LPO generally has a high carbon and nitrogen content, which adds an additional 15% loading when recycled to the headworks of the wastewater plant. While the LPO off-gas is not considered hazardous, aldehydes and ketones in the off-gas produce a characteristic LPO odor that some municipalities choose to eliminate by bubbling the off-gas through the secondary treatment aeration basins. After settling, the solids are pumped to conventional dewatering equipment, such as belt presses, where the sludge is dewatered to around 30-60% solids. Most facilities reported an order-of-magnitude or greater decrease in the amount of landfill material produced, which is now of an acceptable quality and is often used as the daily “top layer” at most landfills rather than dirt. The LPO was a tremendous commercial success in the US in the 1960s and 1970s as it optimized WAO capital and operating costs and solved the burgeoning sludge issue of that period. Although roughly 1/3 of those constructed are still in use today, the popularity of the LPO waned in the 1980s with an EPA policy change encouraging the agricultural reuse of the wet sludge. This policy is not expected to change in the US for some time. However, recent regulatory changes in Europe are closing most of the low-cost sludge disposal options. The option of ocean disposal as well as land application of the sludge are being eliminated and may cause a resurgence in the popularity of the LPO as well as other hydrothermal treatment options for sludge disposal.44

Spent Caustic The spent caustic treatment system represents todays most successful WAO application. There are two distinct spent caustic WAO applications: ethylene spent caustic and refinery spent caustic. The ethylene spent caustic WAO application is traditionally a low temperature treatment and is operated in the range of 120-220°C. 36-

37,43 The main purpose of this treatment is to destroy the odorous sulfides present in the spent caustic. The refinery spent caustic WAO application operates at 240-260°C.40-42

For ethylene production, ethane, propane, or naphtha gas is cracked in a furnace. Carbon dioxide (CO2) and hydrogen sulfide (H2S) by-products are also formed. A caustic scrubbing tower is used to remove these by-products which are absorbed out of the gas phase and into the caustic solution. Over time the caustic becomes loaded, or spent, and is disposed of. This disposal is difficult because the spent caustic is considered a hazardous waste due to the high pH and the presence of the sulfides. The sulfides represent a significant odor issue. WAO is used to destroy the odorous sulfides and allow acid neutralization of the spent caustic. Conveniently the WAO reaction converts the sulfides to sulfuric acid, an acid which partially consumes the alkalinity of the spent caustic. This WAO application was commercially introduced in the 1980s and today is considered the standard technique for ethylene spent caustic treatment. WAO is considered the state of the art for spent caustic36 and virtually all new ethylene facilities employ WAO for the treatment of their ethylene spent caustic.

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Refineries also generate spent caustic solutions, though of a different composition and usually in smaller quantities than ethylene plants do. There are three types of refinery spent caustics which are known as sulfidic, naphthenic, and cresylic spent caustic.61 Often the refinery will mix the three spent caustics prior to disposal. The sulfidic spent caustic comes from caustic washing of LPG for sulfur removal and can be treated much like ethylene spent caustic. The naphthenic spent caustic comes from the Merox™ type treatment of diesel and jet fuels for sulfur and naphthenic acid removal. Naphthenic spent caustic contains a much higher Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) content than ethylene spent caustic due to the organic naphthenic acids. Naphthenic acids are also known to foam under low temperature WAO treatment conditions, so higher temperatures are employed to destroy the naphthenic compounds and to control foaming. Cresylic spent caustic comes from the Merox type treatment of gasoline for sulfur and cresylic acid removal. Cresylic spent caustic is very similar to naphthenic spent caustic in content of COD and TOC, behavior, and treatment requirements. The refinery spent caustic WAO application was commercially introduced in the 1990s. At the present time, WAO is emerging as a successful refinery spent caustic waste treatment technology. EXAMPLE WAO FACILITIES This section describes four examples of currently operating commercial WAO facilities. A photograph of each facility is provided for reference. Refinery Spent Caustic - RPDM - Rio de Janeiro, Brazil40,41

In 1995, a WAO system was put into operation for the treatment of refinery spent caustic from gasoline sweetening, gasoline and LPG prewashing, and from gasoline and LPG mercaptans extraction at the Refinaria de Petroleos de Manguinhos, S.A. (RPDM) in Rio de Janeiro. RPDM is a producer of liquid fuels and is located near residential property within Rio de Janeiro. Prior to installing a WAO system, the refinery was dependent on off-site disposal of their spent caustic. In considering on-site treatment, it was desired to avoid odors since the refinery was in a populated area. The WAO system was installed in 1995 and operates at 2 gpm and 260°C with a 1 hour residence time. Figure 3 is a photograph of the WAO system. After treatment by WAO, the oxidized spent caustic is sent to the refinery’s biological treatment system for final treatment. System performance includes odor-free operation with complete removal of sulfides, mercaptans, and thiosulfates as well as significant reductions in phenols and overall chemical oxygen demand (COD).

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Figure 3. RPDM Refinery Spent Caustic WAO Unit Ammonium Sulfate Crystallizer Mother Liquor - ATOFINA - Rho, Italy49,50

ATOFINA Italia has incorporated WAO into its approach for dealing with the large quantities of unavoidable by-product formed from the methyl methacrylate production process. Through utilizing crystallizers, by-product ammonium sulfate (AMS) salt is recovered in high quality form. In order to ensure that the quality of the AMS is, and remains, acceptable for agricultural use, a purge stream is generated from the operation of the crystallizers. This purge stream, which contains the organic by-products, passes through a WAO system where the organic components are destroyed and the treated AMS returns to the crystallization system. The exothermic WAO process produces surplus energy (steam) which is returned to the crystallizers. AMS in the WAO effluent is returned to the crystallizer system for recovery of AMS product. Large volumes of crystallizer blowdown liquor are no longer trucked off-site for 3rd party disposal. The WAO reactor is operated at 280°C using a homogenous copper catalyst, which is recycled within the system. A photograph of the WAO system is shown in Figure 4.

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Figure 4. ATOFINA Italia AMS Crystallizer Mother Liquor Catalytic WAO System Ethylene Spent Caustic – Chinese Petroleum Corporation – Kaohsiung, Taiwan

In the late 1980s, Chinese Petroleum Corporation (CPC) contracted with the M.W. Kellogg company in Houston, Texas to build a 500,000 Mt/year grass roots ethylene plant. The refinery where this facility was to be located was in a heavily populated area and the whole project came under considerable public scrutiny because of concerns over odor issues surrounding the spent caustic that the plant would produce. Public pressure was so great that the entire ethylene expansion project was being threatened. CPC evaluated a number of alternatives including using WAO as a means of treating their spent caustic. Ultimately CPC chose to construct three WAO systems, each with a capacity of 7.3 m3/hr to handle the spent caustic from the new ethylene facility, plus what was produced from the existing ethylene units at the refinery. In the late fall of 1989, the 200°C WAO units were started up for testing to evaluate their performance. The effluent produced by the WAO systems easily met the guarantee requirements. The most important result was that the effluent quality and the operation of the WAO systems completely satisfied the local neighbors such that a positive solution to the spent caustic problem had been implemented and protest of the continued construction of the new ethylene plant subsided. A photograph of one of the units is shown in Figure 5.

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Figure 5. CPC Ethylene Spent Caustic WAO System; Insert: Feed and Treated Samples Polyol Ether / Styrene Monomer Wastewater - Repsol Quimica - Tarragona, Spain In 2000, a wastewater treatment system was constructed to accommodate a new Repsol facility in Tarragona, Spain. A WAO system operating at 295°C using pure oxygen and a 1 hour retention time was built for the treatment of propylene oxide / styrene monomer (PO/SM) wastewaters. A photograph of the WAO system is shown in Figure 6. The impetus for WAO usage was to destroy the biologically refractory components in the PO/SM wastewater and to decrease the COD loading on the overall wastewater plant.

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After WAO treatment, the effluent is mixed with all other facility wastewaters and is routed through a PACT-WAR system. The PACT sludge is regenerated in a 240°C WAR/WAO reactor. The WAR reactor and part of the PACT system is shown in Figure 7.

Figure 6. Repsol Quimica PO/SM Wastewater Pure Oxygen WAO system

Figure 7. Repsol Quimica WAR reactor (right) and PACT system (left)

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CONCLUSIONS Wet oxidation has been used for many years and has been applied to a wide variety of waste treatment projects. These applications range from the destruction of sludge to the destruction of waste liquors with extremely toxic components such as cyanides and pesticides. Wet oxidation has also been used to enhance the biodegradability of certain wastewaters and for the general disruption of the COD of other wastewaters. At present, WAO is widely used for the treatment of spent caustic liquors in the ethylene and refining industries, and custom applications are made in the chemical process industry. With the high thermal efficiency of WAO, it offers an economical alternative to liquid waste incineration. Since the WAO system utilizes a direct chemical oxidation process, it is not subject to toxic upsets as would be the case with biological treatment. Besides the existing applications for the technology, additional applications will surely be realized as more stringent discharge criteria are applied by regulatory agencies. ABBREVIATIONS AMS...........................Ammonium Sulfate COD ...........................Chemical Oxygen Demand CPC............................Chinese Petroleum Corporation EPA............................US Environmental Protection Agency F/E HX.......................Feed / Effluent Heat Exchanger LPG............................Liquid Propane Gas LPO............................Low Pressure Oxidation (specific type of TSC) PACT .........................Powdered Activated Carbon Treatment PO/SM........................Propylene Oxide/Styrene Monomer RPDM ........................Refinaria de Petroleos de Manguinhos, S.A. TSC ............................Thermal Sludge Conditioning VOC ...........................Volatile Organic Compound WAO..........................Wet Air Oxidation WAR ..........................Wet Air Regeneration of spent powdered activated carbon using

WAO

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REFERENCES 1 Mishra, V. S., Mahajani, V. V., and Joshi, J. B. “Wet Air Oxidation” Ind. Eng. Chem. Res., 34, pp2-48 (1995) 2 Zimmermann, F. J. “Process of Making Vanillin”, US Patent 2399607, (1943) 3 Zimmermann, F. J. “Process for Making Vanillin”, US Patent 2434626, (1945) 4 Gitchel, W. Former Zimpro Vice President of Research, Interview (2002) 5 “Preserving the Legacy: Thermal Treatment Technologies”, PTL 0401, Audiovisual VHS, INTELECOM,

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2696424, (1954) 8 Schoeffel, E. W. “Recovery of Cooking Liquor from Spent Semi-Chemical Pulping Liquors”, US Patent

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2774666, (1956) 11 Zimmermann, F. J. “New Waste Disposal Process”, Chemical Engineering, August 25, 117-120 (1958) 12 Flynn, B. L. “Swiss Mill is Trying Wet Air Oxidation to Get Rid of Sludge and Recover Filler”, Paper Trade

Journal, May 1 (1976) 13 Ellis, E. C. “Disposing of Paper Mill Sludge and Recovering Filler Clay”, Industrial Wastes, March/April

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Effluents” 46th Purdue Industrial Waste Conference Proceedings, (1992) 27 Flynn, B. L. and Flemington, W. Va. “West Air Oxidation of Waste Streams” Chemical Engineering

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36 Grover, R. and Gomaa, H. M. “Proven Technologies Manage Olefin Plant’s Spent Caustic” Hydrocarbons Processing, pp61-69, September (1993)

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