South Africa has amongst the cheapest energy supplies in the world and it is rich in mineral resources. Economic considerations have resulted in a relatively inefficient use of resources including thermal energy when bench marked against world best practice. While this may provide a temporary economic competitive advantage, it results in poor environmental performance and has led to South Africa being classified as environmentally dirty. Furthermore, South Africa has limited water resources and measures are being taken to conserve the water that is available. Most of the water is used for agriculture and irrigation (59%), forestry (4%), rural (4%), urban (25%), power generation (2%) and mining and bulk industry (6%). (Mukheibir and Sparks, 2003)
Industries with thermal processes (as opposed to electrical processes and excluding electrical power generation), are a significant sub-class of the industrial water-using sector. Energy which is consumed in excess has to be dissipated. This is frequently through the use of wet cooling towers, where the excess energy is used to evaporate water. This results in a two-fold problem for the aqueous environment. Firstly the evaporated water is lost from the national water cycle. Secondly, the salts contained in the cooling water remain behind for discharge or disposal as cooling tower blow-down. The blow-down consists of highly soluble sodium or chloride ions and of less soluble calcium and magnesium cations, and sulphate and carbonate anions. The soluble salts contribute to the salination of the aquatic system, while the sparingly soluble salts contribute to scaling problems either where the blow-down is being treated, or in the receiving water.
Two separate but similar process integration techniques (thermal pinch analysis and water pinch analysis) have been developed to optimise the thermal efficiency or the water efficiency in industrial complexes. These techniques have evolved to meet the needs for increased thermal efficiency (firstly because of the first "energy crisis" in the 1970's and then the subsequent global warming / carbon dioxide issue of the 1990's) and for reduced water consumption (primarly intake water volume in well endowed, water rich regions). Thermal pinch is a mature technique, while water pinch is evolving rapidly. Combining these two techniques could result in significant energy and water savings in South African industries.
Objectives of the study
The overall aim of this project was to promote both water and energy savings in the South African process industry through more efficient use of both process water and cooling and heating utilities. This was to be achieved by creating awareness of potential water and energy savings in the process industry and through the development and promotion of tools incorporating water and thermal pinch and mathematical modelling for the optimisation of water and heat exchanger networks. Specific objectives were:
- Conserve the use of water by the process industry through thermal energy conservation.
- Reduction of salination through the reduced cooling need by the process industry.
- Develop a rigorous and rational technique to optimize cooling circuits for simultaneous water and energy conservation.