United Statee Environmental Protection Agency Water Engineering Research Laboratory Cincinnati OH 45268 Researched Development EPA/600/S2-86/043 June 1986 &EPA Project Summary Wet Oxidation of Municipal Sludge by the Vertical Tube Reactor Jay L. McGrew, George L. Hartmann, Christina B. Cassetti, George E. Barnes, and Atal E. Eralp Evaluations were made of pilot-plant and bench-scale vertical tube reactor (VTR) systems for treating municipal wastewater sludges. The VTR system is designed to oxidize high-strength liquid organic wastes using wet combustion principles. The reactor vessel is a very long U-tube in which the waste to be oxidized flows down one leg (downcomer) and returns, through the other (upcomer). Oil well drill- ing techniques are used to install the reac- tor in the ground. The downcomer pro- vides for air injection to support combus- tion, and the upcomer is surrounded by a heat-exchange jacket that provides the thermal energy necessary to initiate and maintain temperatures appropriate for the oxidation reactions. A pilot-scale VTR system consisting of a 1,500-ft by 2-in.-diameter reactor was operated over a period of time using muni- cipal sludge to accumulate engineering data and operating experience. The infor- mation obtained was compared with that from an existing laboratory bench reactor. Pilot-plant and bench-reactor data were correlated over the range of conditions af- forded by the pilot-scale system. These correlations were sufficiently reliable to allow use of the laboratory reactor for ob- taining data over ranges of operating con- ditions approximating full-scale. The pilot-scale system achieved chem- ical oxygen demand (COD) removals of up to 50%, and the laboratory reactor (using municipal sludge from several sources and simulating deeper reactors with conse- quently higher temperatures and pres- sures) obtained COD removals of up to 80%. At similar temperature and residence time, the bench reactor results showed reasonable correlations with pilot-plant operating results and thus confirmed the prediction that a full-scale VTR would also remove 50% of municipal sludge COD. This Project Summary was developed by EPA's Water Engineering Research Laboratory, Cincinnati, OH, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report order- ing information at back). Introduction The vertical tube reactor (VTR)*'"1" is designed to treat and dispose of municipal and industrial wastes by wet-air oxidation (wet combustion). The products of this process are easily dewatered ash and a supernatant containing low-molecular- weight organics. The VTR process is especially applicable to wastes that are difficult to dewater or that have sufficiently high organic content to maintain a thermally self-sustaining (autogenous) reaction. Because oxidation occurs in the presence of liquid, it is not necessary to supply energy for the latent heat of vaporization. Though most com- bustion processes require dewatered sludge to achieve thermal self-sufficiency, considerably smaller concentrations of or- ganic matter are adequate for wet combustion. "Mention of trade names or commercial products does not constitute endorsement or recommenda- tion for use. -(U.S. Patent No. 4,272,383. Developed by the Vertical Tube Reactor Corporation, a subsidiary of Applied Science and Engineering, Inc., Englewood, Colorado. The process is currently being marketed by Ver Tech™ Treatment Systems, 12000 Pecos Street, Denver, Colorado 80234. ------- Oxygen must be added to the wet com- bustion system in stoichiometric propor- tions at a rate that will not impede com- bustion. As the waste reacts with oxygen, heat is produced. Municipal wastewater sludge has a heat of combustion of ap- proximately 3300 cal/gram of COD. So typical untreated municipal sludge with a COD of 10,000 mg/L has an overall heating value of approximately 33 cal/ gram. For reference, fuel oil has a combus- tion value of approximately 11,000 cal/ gram. Thus sludge must be processed at very high overall thermodynamic efficiency if an autogenous process is to be achieved. With sufficient conditions of tempera- ture and pressure, wet oxidation proceeds until the organic removal rate decreases to zero and the percentage of organics re- moved remains constant. The organics re- maining at this point are termed refractory organics. At low temperatures of 212° to 400 °F, this removal plateau or equilibrium is not reached for hours. Above 575 °F, it is reached in a matter of minutes. The rate of oxidation will increase up to the critical point of water (705 °F). The height of the plateau also increases with increasing temperature. Thus the extent and rate at which a material is oxidized is significantly influenced by reactor temperature, with very little oxidation occuring below 300 °F. Temperatures of 660 °F or more are re- quired for 80% COD removal. Conventional wet combustion techno- logy involves pumping waste and air into a pressure tank and heating the mixture to the desired reaction temperature. The need for high-pressure tanks, pumps, and compressors results in high capital cost and expensive, energy-consuming opera- tions. However, corrosion-resistant pres- sure vessel design, high-pressure liquid pump technology, and the state of devel- opment of heat exchangers are such that wet oxidation of municipal sludge in con- ventional pressure tanks is technically feasible. But maintenance and safety con- siderations, capital and operating costs, and the requisite high level of plant site technological skill needed for continuous operation are such that conventional pres- sure tank wet oxidation is not currently an economically attractive disposal method for municipal sludge. VTR Process The VTR process was developed to eli- minate the problems associated with con- ventional wet oxidation while providing reaction pressure, temperature, mixing, and reactant retention time for autoge- nous wet combustion. In the VTR, two 2 very long concentric tubes are suspended in a conventionally cased deep well. The waste liquid and air are injected into the center tube (downcomer) at the earth's surface at low pressure. As the waste stream and air flow down the tube, the mixture is heated by counter-flow heat exchange with the upflowing oxidized fluid. As the downflowing waste ap- proaches the bottom of the tube system, the temperature and pressure become suf- ficiently high to initiate the oxidation reac- tion. The tube lengths and diameters are appropriately sized to allow an adequate retention time in the high temperature/ pressure reaction zone to achieve a high degree of reaction completion. The oxidized water ultimately flows up the annulus between the two tubes (up- comer) and is cooled by the transfer of heat to the downflowing fluid. As the fluid ascends, the pressure also decreases as the result of decreasing hydrostatic head. Thus the pressurization and depressuriza- tion of the fluid waste and heat exchange are accomplished in a highly efficient, nearly reversible thermodynamic process. Oxygen needed for combustion is sup- plied by the injection of air into the down- comer at several depths typically ranging from 150 to 800 ft, depending on influent COD. When waste oxygen demand (COD) exceeds air supply capability, the waste is diluted with effluent or other low-COD- strength wastes. A heat-exchange jacket surrounding the outer tube from a short distance below the wellhead to the bottom of the U-tube cir- culates a heat transfer fluid (heated when necessary in a burner at the wellhead) to preheat the reactor and to add or extract heat as required to maintain appropriate temperatures during reactor operation. In- sulation to minimize heat losses to the sur- rounding rock complete the basic VTR design. Schematic views of a typical full- scale reactor are shown in Figures 1 and 2. In actual operation, as the waste stream and air flow down the tube, they undergo natural pressurization due to the hydro- static head above. Thus fluid pumps need to be designed primarily to overcome sur- face friction and pressure head at their in- fluent or injection point. They do not need to develop the high pressure actually ex- perienced at the bottom of the reactor. At some depth (typically 1,500 to 2,000 ft), temperature of the waste fluid increases to 350 °F because of heat transfer from the upcomer effluent to the downcomer influent, and wet combustion effectively begins. As the fluid flows through the reactor, oxidation proceeds until either the organic material or the dissolved oxygen are depleted, or until hydrostatic pressure and temperature decrease below those levels necessary to support combustion. Upflowing oxidized waste is gradually cooled as it transfers heat to the down- flowing fresh waste. A temperature profile for a typical reac- tor as a function of retention time is shown in Figure 3. Any excess heat which may result from the exothermic oxidation reac- tions is removed from the reaction zone by the heat exchange jacket. Excess heat can be available for use at the ground surface. Effluent temperature is expected to be within 5 °F of the influent temperature. The VTR has no moving parts below the ground surface and needs no high- pressure equipment above ground like that required for conventional wet oxidation methods. The concentric U-tube arrangements of the VTR affords very high heat recovery from the effluent to the influent waste stream. The entire reactor is, in effect, a counterflow heat exchanger, thus greatly reducing heat losses normally encount- ered in wet oxidation systems. In addition, as the surrounding earth approaches equi- librium with a continuously operating VTR, operations will be less affected by waste quality changes or above-ground (climate) influences because the surrounding earth will act as a thermal buffer. The VTR requires little real estate. At the same time, its vertical configuration and its compactness make downhole acces- sibility difficult should temperature- and pressure-measuring devices or other well components need to be unplugged, in- spected, or replaced. Thus mechanical reliability and maintenance of the VTR system are of critical importance. Pilot-Plant Reactor Tests The main objective of the pilot-plant re- actor (PPR) study was to operate the system on a continuous basis over an ex- tended period to monitor, measure, ana- lyze, and otherwise evaluate all significant operating parameters and reaction infor- mation during operation. The reactor of this project consisted of a 1.75-in.-diameter downcomer in a 2-in.- diameter upcomer. The reactor, air line, heat exchanger lines, instrumentation, and insulation were all suspended in a 5-in.- diameter well casing. The PPR extended to a depth of 1,000 ft below the ground surface in a 9,000-ft-deep abandoned oil well located 25 miles east of Denver, Colorado. The PPR was large enough to provide essential concept verification and engine- ering data. Designed to process up to 8 ------- L/min of municipal sludge with a COD of 2,000 mg/L, the PPR would operate at a maximum downhole temperature of 500 °C at 1000 ft, with a retention time of 20 to 30 min to accomplish up to 50% COD removal. To achieve the downhole pressure of 680 psi required by the increased tem- perature of 500 °F, air and sludge inlet pressures to the reactor were increased to 400 psi, necessitating the installation of a larger compressor. These modifications in effect simulated the reaction conditions of a 500 °F reactor 3,000 ft long — con- ditions approximately those expected with a full-size reactor. The COD reductions in this test series fell within the expected range of 40% to 55%. The results of PPR . experiments are summarized in Table 1. The PPR was operational for four test periods between July'1979 and March 1981. Although a considerable amount of Air Heat Exchanger Line (On) f— Heat Exchanger line (Out) time was spent in identifying and remedy- ing physical problems, enough data were collected to evaluate the process and to demonstrate the correlation between bench-scale reactor and PPR experiments. Detailed accounts of the problems en- countered and their solutions in reactor design and process control are given in the final report. Laboratory-Bench-Reactor Tests The laboratory-bench-reactor (LBR) ves- sel was fabricated from stainless steel pipe 3.3 ft long with an inside diameter of 2.0 in. Through one end of an otherwise seal- ed vessel was a 1/4-in, stainless steel tube for monitoring pressure and providing en- try and exit for sample and air. Electrical resistance rods and cooling tubes were ex- ternally attached to the vessel. To control heating and cooling rates, the electrical resistance rods were powered by a motor- Reactor Influent (Downcomer) 33°F - 100°F Reactor Effluent (Upcomer) 5°F to 20° F Above Influent Start of Reaction Zone 350°F Heat Exchanger Oil Bottom of Reactor Temperature Varies Reactor Casing (Pressure Vessel) Well Casing Cement Grout Surrounding Rock ^ Not to Scale Figure 1. Typical VTR profile. driven, variable-voltage transformer. Room temperature tap water flowed through the cooling tubes and was shut off or metered by a needle valve to control cool-down rates. Thermocouples to monitor temper- ature were welded on the vessel's outer surface at several locations. The reactor vessel and attached thermocouples, elec- trical resistance rods, and cooling tubes were insulated. This arrangement was mounted to permit 180 ° rotation in a ver- tical plane at 7.5 sec/oscillation to provide internal mixing of sample and air. The objectives for LBR testing were to establish the oxidation characteristics of sludges from different sources and to sim- ulate as closely as possible the pilot-scale VTR conditions with the LBR to obtain treatability data that predicts the COD re- moval rates expected in a full-scale VTR. Five types of sludges identified as digested sludge, blended sludge, primary sludge, pri- mary sludge with food processing waste, and domestic waste were used in LBR ex- periments. The sources and character- istics of these sludges are detailed in the main report. Table 2 summarizes the COD reduction data for LBR runs by sludge type. In gen- eral, as the reaction temperature increases, the COD reduction increases. Increasing reaction time from 30 to 60 min also re- sults in greater COD removals. Typically, larger differences in COD removals occur between 60- and 30-min LBR runs at 400°F than at 500° or 600 °F. At 650°F, the COD removal between 60- and 30-min reaction times is virtually the same. This effect is probably due to the production of low- to medium-molecular-weight com- pounds that require elevated temperatures rather than increased reaction times to undergo further oxidation. Correlation Between LBR and PPR COD Reduction Data The COD reductions obtained from LBR and PPR runs are compared in Table 1 cor- responding to reaction for similar times and temperatures. Data from PPR runs in October 1979 are low compared with those for 15- and 20-min LBR runs per- formed at 445 °F. The remaining COD reductions for PPR runs fall within the standard deviation of COD reductions established by multiple LBR runs at the temperatures and times listed in Table 1. These results support the use of LBR- generated treatability studies to model expected COD reduction rates. Though COD reductions approach 50% in the present PPR experiments, as much as 80% COD reduction is expected for a full-scale VTR where higher temperatures ------- •Stainless Steel Upcomer Stainless Steel Downcomer Rock .. High Temperature Ceramic Insulation Figure 2. Typical 1/77? cross section. Cement Grout , Steel Well Casing Steel Reactor Casing "'' (Pressure Vessel) ''X""'" ~._S?a//7/ess Steel Air Line — Heat Exchanger Oil Steel Heat Exchanger Lines 700. 600 . 500 . 400 . | 300 J 1 § 200 . 100 . • Downcomer • Upcomer Reactor Bottom Reaction Time Reactor Entrance Reactor Exit i i i i r i i [ i i 70 15 20 25 30 35 40 45 50 55 Time, Minutes Figure 3. Effect of retention time on temperature. 4 and pressures can be attained and where the reaction time would be longer than 30 min because of the longer VTR. Conclusions The pilot-scale system achieved COD re- movals of up to 50%, and the laboratory reactor tests using municipal sludge from several sources and simulating deeper reactors with consequently higher temper- atures and pressures obtained COD re- movals of up to 80%. At similar temperature and residence time, the bench reactor results showed reasonable correlations with pilot-plant operating results, confirming the predic- tion that a full-scale VTR would also remove 80% COD of municipal sludge COD. The pilot-scale test program provided in- formation on structural, mechanical, and other operational problems important to the design of full-scale systems. In summary, the VTR was shown to be a potentially successful method for stabi- lizing organic wastes when significant sludge volume reduction is required, where stringent requirements for sludge disposal exist, when destruction of toxics or path- ogenic organisms is necessary, and/or where potential energy recovery from high-strength wastes is good. The full report was submitted in fulfill- ment of Contract No. 68-03-2812 by Ap- plied Science and Engineering, Inc., under the sponsorship of the U.S. Environmen- tal Protection Agency. ------- Table 1. Correlation of Data from Pilot- and Bench-Scale Reactors Pilot-Plant Reactor Data Laboratory-Bench Reactor Data Sampling Period October 1979 July 23, 1980 July 24-25, 1980 November 1980 December 1980 March 1981 Reaction Temperature (°F) 435 440 440 420 433 510 Reaction Time (min) 17.5 27 30 10.6 14 28 COD Reduction (%) 11.0±6.8% 38.1% 29.7±5.5% 24.9±8.9% 22.0 + 9.4% 42.6±9.4% Reaction Temperature (°F> 445 445 445 445 445 445 500 Reaction Time (min) 15 20 30 30 15 15 • 30 COD Reduction (%l 21.4±2.0% 24.6 + 6.3% 35.9 ±14.0% ^35.9 ±14.0% 21.4 + 2.0% 21.4±2.0% 47. 2± 10.1% Table 2. Total % COD Reduction at Various Reaction Times and Temperatures 30-min Reaction Time 60-min Reaction Time Item 400° F 500° F 600° F 650° F 400° F 500° F 600° F 650° F Type of Sludge: Digested Blended Primary Primary W/Food Processing Waste Domestic Waste Average & Standard Deviation 13% 27% 9% 54% 52% 50% 21% 20% 30% 44% 20 + 9% 44±14% 75% 75% 63% 75% 50% 68±11% 82% 8O% 74% 86% 79 ±5% 35% 63% 79% 83% 37% 65% 78% 81% - 53% 69% 78% 15% - 61% 80% 16% 55% 7O% 77% 26+12% 59±6% 71 ±7% 80±2% U. S. GOVERNMENT PRINTING OFFICE: 1986/646-116/20855 ------- ------- ------- JayL. McGrew, George L Hartmann, Christina B. Cassetti, and George E. Barnes are with Applied Science and Engineering, Inc., Englewood, CO 80110; the EPA author AtalE. Eralp (also the EPA Project Officer, see below) is with the Water Engineering Research Laboratory, Cincinnati, OH 45268. The complete report, entitled "Wet Oxidation of Municipal Sludge by the Vertical Tube Reactor," (Order No. PB86-183 936/A S; Cost: $16.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, MA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Water Engineering Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 Official Business Penalty for Private Use $300 EPA/600/S2-86/043 ------- |