Vacuum pyrolysis process for remediation of hydrocarbon‐contaminated soils

Abstract

Around the world, hazardous wastes and contaminated soils have become a major environmental concern. Over the last five years, the traditional treatment of contaminated soils by incineration has been challenged by thermal separation techniques, which frequently involved a rotary kiln or a conveyor kiln. In these thermal pyrolytic processes, the contaminants may be removed by vaporization at high temperature a n d or by a continuous stream of hot inert gas. An extensive review of North American demonstration and commercial cleanup methods for hazardous waste sites can be found in the literamre (U.S. EPA, Centre SaintLaurent, 1993). A few examples of thermal pyrolytic processes for the treatment of contaminated soils are given below.Rotary kilns were initially used for incineration, but have since been used in less destructive thermal techniques. Some of the North American processes operating with a rotary kiln since 1992 are described here. The anaerobic thermal processor (U.S. EPA, 1992; Air and Waste Management Association, 1993) treats soils at 600" C in a 25 tonshrunit. In the X ' W process (US. EPA, 1992), the contaminants are volatilized by a continuous N, stream at 500" C in a 7 tonshr unit. The low temperature thermal aeration process ( U S . EPA, 1992 Reintjes, 1985) and the Siemens process (Bege and Meininger, 1989) both use two kilns. In the Ecotechniek BV, the combustion of the gases produced in the first reactor is used to heat the second reactor. In addition, a part of the hot soil exiting the second reactor is reintroduced into the frrst reactor to heat the soils entering the system. In the second kiln used in the Siemens process, organic compounds are oxidized by a stream of hot gas consisting of N,, CO, and 0,. In the NBM process (De Leur, 1988), the amount of oxygen in the kiln's atmosphere is controlled. The reactor walls are heated to a temperature that will cause spontaneous combustion of the pyrolytic products to consume the oxygen that is present. One of the main advantages of the rotary kiln is its ability to treat a large variety of wastes, both solid and liquid. Conveyor kilns using screws and paddles are used to treat sludges and relatively solid material. A limitation of some of these reactors is the need to attain rather high temperatures for efficient treatment. The use of a carrier gas is also inconvenient, as it necessitates a subsequent treatment or separation of the particles and contaminants contained in the carrier gas.Thus, it would be advantageous to design a reactor that combines the strong points of both systems-i.e, treats a large variety of wastes, permits a rather precise control of temperature and pressure, does not require a high operating temperature or a carrier gas, and is more energy-efficient.In this article, we would like to describe a reactor with a new configuration, specially developed for the treatment of soils contaminated with organic material. However, the kiln's inside configuration cannot be revealed at this point, as a patent describing the reactor ha...