Thermo-economic analysis of heat-driven ejector system for cooling smelting process exhaust gas

Abstract This study presents a new solution for cooling the exhaust gas of an aluminum smelting process, thus increasing the efficiency of the Gas Treatment Center (GTC). High-temperature waste gas enters the GTC, where contaminants are removed before release to the atmosphere. After the smelting process, the gas temperature varies between 100 °C (in winter) and 150° (in summer). It should not exceed 135 °C at the GTC inlet to ensure good scrubbing process performance. A heat-driven ejector system is proposed for this purpose. Its main advantage is that it can produce a double effect cooling by utilizing waste heat from the exhaust gas. The peculiar feature of the proposed design is that both the generator and the evaporator of the ejector system are used to produce a cooling effect for the smelter exhaust gas. A simulation model of the proposed system is developed, and a two-step approach is applied to complete the thermo-economic analysis. At the first step, formulated as a thermodynamic evaluation, the total heat transfer area was minimized as a function of the generator temperature difference. In the second step, the optimum value of this parameter is used to design the heat exchangers and the ejector. The results show the existence of an optimum value for the generator temperature difference that minimizes the total heat transfer area. An economic analysis based on evaluating the Equivalent Uniform Annual Cost (EUAC) criterion concludes the investigation. It is shown that the new optimization strategy for an ejector system improves its viability as a potential solution for cooling smelting process exhaust gas. The proposed ejector system can reduce the total heat transfer area and the capital cost up to 34% and 57% compared to a commercially available cooling system.