No abstract

Water management at alumina refineries and associated power plants is a major concern, especially if water resources are scarce and liquid discharges are not permitted. This paper describes the case of one such plant in Jamaica recovering 350 m 3 /h of digester regenerative condensate. The steam and power for the alumina refinery is supplied by a cogeneration facility, which has to use a mix of well water and return condensate from the steam users in the alumina production process. The refinery produces alumina from Jamaican bauxite ore, using the Bayer process. This process includes the digestion of bauxite with sodium hydroxide at 135 deg.C, hence the requirement for both steam and power. In the course of this extraction, large quantities (350 m 3 /h) of digester and evaporator condensate are produced. In view of the limited liquid discharges permitted from the site, this condensate was stored on site in large ponds, and reused in the process. Pressured from three sides, with a limitation on raw water abstraction from the wells by the Water Resource Authority, a requirement for minimized liquid discharge of wastewater, and high cost of energy there was a need to utilize the regenerative condensate from the digesters and the evaporators as boiler feedwater. Studies carried out by international water treatment companies had concluded that Total Organic Carbon (TOC) would not allow condensate re-use as boiler feedwater, with an attached US$10 million cost for the removal of TOC. Extensive laboratory work and thermodynamic analysis were carried out and the condensate are used as boiler feedwater, without treatment. The avoided cost was US$10 million and energy savings of US$3 million realized.

The conventional energy analysis evaluates the performance of a thermodynamic system generally on its quantity only. It gives no information about the effect of irreversibilities on performance that occurs inherently during any thermodynamic process. On the other hand, exergy analysis, based on the second law of thermodynamics recognizes magnitudes and locations of the losses due to these irreversibilities. This paper deals with the improved energy performance of two-alumina refineries in Jamaica. One refinery produces alumina from Jamaican bauxite ore, using the Bayer process. The first process includes the digestion of bauxite with sodium hydroxide at 135 deg.C, (low temperature digestion). The second process includes the digestion of bauxite with sodium hydroxide at 245 deg.C. Hence, the requirement for both steam and power. Steam is generated using heavy fuel oil. There is auxiliary power from Internal Combustion Engines, Gas Turbine Engine and the Power Grid. The engine (the machine) produces mechanical work and then electrical power. According to the Second Law of Thermodynamics, the machine gives back heat (a part of the useful energy delivered by CHP) to a cold sink (low temperature sink), before rejecting the remaining heat to the environment at the reference temperature. Losses to the low temperature sink is minimized. This resulted in daily fuel savings of 400 barrels of Heavy Fuel Oil (HFO) and an annual savings of 146,000 barrels of Heavy Fuel Oil.