Transpiring Wall Reactor Research Articles

Salt precipitation and scale control in supercritical water oxidation—part B: commercial/full-scale applications

Despite the potential of supercritical water oxidation (SCWO) as a viable technology for organic waste destruction, its commercial development has been hindered by the problems of corrosion and salt precipitation/solids buildup. The extremely low solubility of polar inorganic salts in the supercritical water environment causes salts present in the feed, or formed during reaction, to precipitate inside the reactor. If left unchecked, these salts can rapidly accumulate on reactor walls or process surfaces and form plugs, causing expensive and frequent downtime of the SCWO system. Other solids such as oxides exhibit low solubility in water over the range from ambient to supercritical conditions and, although they have much less tendency to adhere to process surfaces, may still hinder operations if not accommodated. Many wastes will have a combination of salt-type and oxide-type solids, and may have an intermediate tendency to stick to process surfaces. Many of the companies that have attempted to commercialize the SCWO technology over the past two decades have developed innovative approaches to dealing with the corrosion and salt precipitation/solids buildup problems. These are often the distinguishing features of each company's SCWO process. This paper objectively reviews several commercial approaches that have been developed and/or used to control salt precipitation and solids buildup in SCWO systems. The approaches reviewed consist of specific reactor designs and operating techniques, and include the following: reverse flow tank reactor with brine pool, transpiring wall reactor, adsorption/reaction on a fluidized solid phase, reverse flow tubular reactor, centrifuge reactor, high velocity flow, mechanical brushing, rotating scraper, reactor flushing, additives, low turbulence/homogeneous precipitation, crossflow filtration, density separation, and extreme pressure operation. Recent commercial SCWO applications utilizing these approaches are also discussed. A companion paper by Hodes et al. (J. Supercrit. Fluid., see this volume) reviews fundamental principles and research pertinent to scale control in SCWO processes.

Operating Conditions for a Transpiring Wall Reactor for Supercritical Water Oxidation

Operating Conditions for a Transpiring Wall Reactor for Supercritical Water Oxidation

Supercritical water oxidation (SCWO): a process for the treatment of industrial waste effluents

SuperCtical Water Oxidation (SCWO) process is investigated at the Institut of Technical Chemistry, ITC-CPV. The objectives were to determine destruction efficiencies of model compounds and industrial waste effluents and to study the feasability of the SCWO process. Two continuous SCWO bench scale plants are operated a pipe reactor and a transpiring wall reactor system (design data: T= 630°C, P = 32MPa, feed rate waste water =10 and 50kg/h, air feed rate = 20kglh, transpiring and quench water feed rate = 50 kg/h). Suspensions containing up to 5%wt solid material can be fed to the reactor using a membrane pump. With the pipe reactor, efficiencies of up to 99.99% were achieved for the oxidation of model compounds (ethanol, toluene, phenol) as well as real waste effluents (paper, chemical, pharmaceutical industry, sewage works). The use of the pipe reactor is limited to feeds without salt to avoid plugging. Salty feeds are processed using the transpiring wall reactor, which is consisting of a pressure bearing tube outside and a porous tube as reactor inside. Water is steadily running through the porous reactor preventing the formation of deposits on the wall. SCWO has a high potential at least for the destruction of halogented organic compounds using the transpiring wall reactor system and is seen to be competitive to other processes for waste destruction.

Operating Results from Supercritical Water Oxidation Plants

Two supercritical water oxidation (SCWO) plants that utilize transpiring wall reactor designs for processing organic wastes are undergoing testing and operation. One plant is designed to destroy Navy excess hazardous materials (EHM). The EHMs represent organic materials found aboard Navy ships. The plant has a nominal waste feed rate of 45 kg/h and utilizes compressed air as the oxidant. Corrosive wastes, including chlorinated solvents (diluted with kerosene) and lube oils, and a salt-producing photographic solution simulant have been processed at feed rates between 45 and 95 kg/h. Tests have been conducted at an operating pressure of 24.1 MPa and reactor temperatures between 594 and 816 °C. Destruction removal efficiencies of better than 99.99% have been obtained. Priority air pollutants NOX and CO are below 25 and 100 ppm, respectively. Liquid effluent total organic carbon (TOC) levels are consistently below 3.5 ppm. Post-test inspections have not revealed any obvious reactor liner corrosion or salt deposition. A second SCWO plant designed for the destruction of obsolete, colored smokes/dyes and pyrotechnic munitions has been fully commissioned and is currently undergoing validation testing. The plant is designed to process 145 kg/h of a 25 wt % basis slurry of smokes and dyes. The plant uses oxygen as the process oxidant. The reactor operating pressure is 26.3 MPa, and operating temperatures are between 575 and 750 °C. Off-site plant skid fabrication was completed in 1997, site preparation and construction were completed in 1998, and commissioning was completed in 1999. Validation testing will be completed in the fourth quarter of 2000. Production demilitarization processing of smokes and dyes will follow.