Heat recovery from sulphuric acid plants for seawater desalination

It is a well known fact that sulphuric acid production is a highly exothermic process. In that sense, combined with the increased energy cost, surprisingly little heat is recovered from the process. Most often the heat is just cooled off by means of seawater or in air coolers. When recovering heat from the absorption and drying tower coolers, only minor changes to the process are needed. With plate heat exchangers (PHEs), the heat in the sulphuric acid can be recovered with negligible temperature losses and minimum space requirements. The PHE can take crossing temperature programs, temperature pinches of less than 5°C and still having k-values of about 2500 kW/(°C·m2). The heat recovered can be used for district heating of cities, boiler feed water preheating or as heating media in connected processes or adjacent plants. A very interesting use of this heat is for the production of fresh water by desalination of sea or brackish water, when the sulphuric acid plants are situated close to the coast such as several installations in Middle East and Northern African countries. The thermal duties in the acid coolers are very well adapted for heat recovery in combination with production of fresh water by means of multi effect distillation (MED). Evaporation temperature in a MED system is typically lower than 80°C, and the heating energy required is in the range of 60–360 kWh per produced m3 of fresh water, depending on the number of effects. There are many advantages, both installation-wise and operation-wise, using PHE for highly efficient sulphuric acid heat recovery and falling film plate evaporators for the thermal exchange surface in the desalination system. The high k-value of the plate system gives smaller heat transfer areas and compact installations.

This paper indicates a new trend in heat recovery in sulphuric acid production: heat recovery of low enthalpy from the circulating acid for desalination of sea and brackish water. The implementing of this heat recovery can be done with only minor changes to the existing systems. Hence, it should be attractive to both retro-fitting projects of existing plants as well as to completely new acid plants. The analysis shows that, both economically and environmentally, the investment for recovering heat from circulating sulphuric acid enhances profits and reduces dependence on external distributors of high quality water.