EVOLUTION OF BROMINE IN SEA WATER MSF DESALINATION PLANTS1

Chlorine is the most economical biocide widely used in SWCC sea water MSF desalination plants to prevent biofouling in the intake and flow through systems. However, chlorine has a negative contribution towards the corrosion aspect of the plant. Excess of chlorine when present in sea water oxidizes its higher homologues in the periodic table such as bromide present in sea water to bromine which finds its way in the venting system causing corrosion damage to the materials in the venting system of the MSF plants. Evolution of bromine is influenced by several parameters such as residual chlorine, pH and temperature of the sea water streams. It is deemed necessary to study the magnitude of their influence on the evolution of bromine in order to reduce its production to the minimum levels thereby keeping the corrosion probability to the lowest possible extent. This paper summarises the results of detailed studies carried out in the Research and Development Center on the evolution of bromine in sea water containing bromide as a function of several parameters such as residual chlorine, pH, temperature, contact time etc. These shtdies were carried out using a laboratory scale set up. Several water samples from an operating MSF plant in Al-Jubail Phase-II area were also analysed for bromide contents in order to assess the extent of bromine conversion in various process streams. A new analytical method was developed during the course of these studies to determine bromide in sea water samples containing complex matrix. Finally, vent gases from various stages of a MSF plant also were collected for evaluating the bromine contents. The results indicate that significant amounts of bromine evolution occur even at high pH (8.3-9.0) and low levels of residual chlorine normally present in an additive based MSF plant. Further, sea water used in barometric condensers was found to be inefficient in stripping bromine completely from the vent gases thereby creating a potential corrosion problem to the materials down stream of the condensers. Some suggestions have been put forward to avoid corrosion problems in vent systems in future MSF plants. I