In the novel thermal softening process, water is heated with directly sparged steam in a pressure vessel to temperatures in the order of 200 °C. The pressure is kept high enough to prevent water from boiling. Caustic is added to the feed water to raise its pH as necessary. At high temperatures and pH, bicarbonate converts to carbonate ions I resulting in the precipitation of calcium carbonate and magnesium hydroxide which are subsequently removed by filtration.
Experiments have been conducted in the Esso heavy oil pilot project in Cold Lake, Alberta, to demonstrate the technical feasibility of the method. A 8L/min apparatus was built at Cold Lake to soften produced water from wellheads after the produced water was de-oiled. Experiments have indicated that the thermal softening process is capable of reducing hardness to a level comparable to the "conventional" water treatment process. A larger pilot has been built and connected to a steam generator, to produce 14 MPA pressure and 80% quality steam at a rate of 110 m3/d.
The thermal softening process simplified the water treatment concept and reduces operating and capital costs.
Water Reuse plays an important factor in in-situ bitumen production due to large volumes of steam injection and water production at Cold Lake.
The chief objective of the project has been to maximize water recycle thus minimizing fresh water reuse and water disposal volumes. This in turn greatly reduces environmental impact and enables the process to use most of the heat in the produced fluids for better economics.
Following research over a number of years, a water reuse pilot was started up in 1978 to tryout various deoiling and softening unit operations. The produced water from the pilot was fed continuously to a well instrumented steam generator pilot to generate 140 m3/d, 80% quality steam at 14 MPa pressure. Over a two year period, various experiments were conducted in the pilot to find the optimum combination of units using already known and proven technology wherever possible. This then culminated into a larger pilot at Leming site for further refinements and developments. In 1985 first commercial production project was built, the first commercial project of its kind, producing 3000 m3/d bitumen and recycling 6200 m3/d produced water. This was immediately followed by second and third production projects all utilizing water recycle system developed earlier.
Simultaneously, research and development has continued to simplify the water recycle concept. One result of this has resulted in yet another pilot currently under operation. This so-called patented Thermal Softening Process is the subject of this paper.
Thermal Softening, as the name implies, is softening of hard water by the application of heat. A good example of Thermal Softening can be cited with respect to a well known home appliance, an electric kettle used for boiling water.