Analysis of desalination alternates for phosphoric acid plant in Tunisia

Abstract

A sulfuric acid/phosphoric acid production facility being constructed in Tunisia will require an average of 485 m 3/hr of process and potable water. There is insufficient fresh water available and desalination of brackish ground water or sea water is required. Three basic cases were studied: thermal sea water desalination using heat recovery; reverse osmosis desalination of brackish water and; part reverse osmosis and part thermal sea water desalination. Total heat rejected through acid coolers from the two 1750 tonne/day sulfuric acid lines is 58×10 6 KCal/hr. This is at a high enough temperature level to operate either an MSF type plant or MED type plant. A 10 stage MSF plant and a 3 effect MED plant with a flash stage were designed. Heat recovery is accomplished by plate type heat exchangers. The brackish water reverse osmosis system will produce water of three different qualities using two product stages. Raw water supply has 3434 ppm chlorides and some of this water is blended with the first stage product water for the required process water. In the combined systems, process water and potable water are produced in th RO system while high purity boiler make-up water is produced in thermal evaporators using the brine reject from the RO plant as feed. Three types of evaporators are examined: heat recovery, thermal compression and steam. Capital and operating cost estimates are given for all six cases. The RO system is lowest in capital costs but the MED system with heat recovery is lowest in operating costs. The system is also lowest in total present worth costs. The high costs of electricity and water wells at this site combined to make sea water desalination using heat recovery the most economical solution. A subsequent analysis using lower cost, internally generated electricity from waste heat resulted in lower present worth costs for all systems and the all brackish water RO system was decided upon.