Author:

Keywords:

Gravity Pressure Vessel, Process Intensification, Mineral Carbonation, Asbestos Remediation, Oil Sand Tailings Treatment

Abstract:

A multitude of chemical reactions rely on high temperatures and pressures to attain suitable kinetics and reach desirable conversions, which are commonly delivered in autoclave reactors. Traditional reactor designs (e.g. CSTRs) have large energy demands (due to pressurization and mixing demands, and heat losses) and costly construction specifications (to meet pressurized vessel codes and safety provisions), which make certain processing routes prohibitively expensive. An alternative reactor technology, which cleverly applies the principles of process integration and process intensification [1], is the Gravity Pressure Vessel (GPV). This is a special kind of autoclave with a built-in heat exchanger, plug flow configuration, and gravity driven pressurization/depressurization. Residence time is controlled by the reactor length that can reach up to 2400 m, resulting in hydrostatically built pressures that can exceed 120 bar. By continuously recycling exothermic reaction heat, up to 70% of the energy can be conserved, and high end temperatures can be achieved (up to 500 °C). In the case of slurry flow, the generated turbulence promotes particle-particle interaction, removing passivating layers and autogenously milling the reacting material, permitting post-processing separation of the mineral phases into valuable product streams. The first patent of the GPV technique was granted in 1981 (US4272383) for wet-air oxidation of sewage sludge. This process was in operation for 12 years in Apeldoorn (the Netherlands), reducing chemical oxygen demand (COD) by >70% and generating up to 10 MW in heat output. Recently, Innovation Concepts B.V. patented the ‘CO2 Energy Reactor™’ (WO2011/155830A1), an application of GPV to mineral carbonation. This is an economically viable, socially acceptable and environmentally sustainable process that permanently sequesters CO2; an alternative to underground storage. It utilizes alkaline minerals (virgin or waste-derived), rich in calcium (e.g. wollastonite, steel slags and incineration ashes) or magnesium (e.g. olivine, serpentine, asbestos and mine tailings) as carbon sinks. Besides carbon capture, valuable product streams also emerge: precipitated carbonates, amorphous silica, and enriched metal residues. This solution results in the stabilization or detoxification of hazardous industrial wastes, and in the valorisation of abundant low-value minerals. The GPV technology is also being developed for the oil sands industry. Bituminous sands are a type of unconventional petroleum deposit consisting of a mixture of sand, clay, water, and a dense and extremely viscous form of petroleum (the bitumen or “tar”). GPV technology is a sustainable solution to one main problem area: oil sand tailings treatment. Wet-air oxidation permits the conversion of MFT (Mature Fine Tailings) to TTT (Thermally Treaded Tailings), wherein residual bitumen in MFT is used as energy source for the process. The outcome is reduced settling time, metals oxidation, reduced contaminants leaching, and freed water that can be re-used in the separation process. This work reports on the technical aspects of the GPV technology, and on the latest developments, challenges and outlook for its adaptation to the considered applications and adoption by the mainstream industry. [1] Santos, R.M., Van Gerven, T. (2011) Process intensification routes for mineral carbonation. Greenhouse Gases: Science and Technology 1(4), 287–293.