we apply the model introduced in fig.2.1 to a desalter Typically a desalter consists of a contacting between a process stream (oil) and a water stream The contaminant salt is transferred from the contaminant-rich phase (oil) to the water stream We can apply the contaminant-rich process-stream representation directly to a desalter In the figure we represent the limits of contaminnant concentration of the water inlet and outlet as limiting concentration at the outlet and inlet respectively of the process stream We only require that the water stream enter the water-using unit at a contaminant concentration less than limiting concentration at the process atream outlet and water leave the water-using unit at a contaminant concentraticeon less than the limiting concentration at the process stream inlet