The de-oiled water from De-Oiling Unit enters the HLS vessel for softening. The primary function of HLS is to remove the Ca and Mg hardness and bring the total hardness to less than 30ppm and total silica to less than 50ppm.
The After-Filter package reduces turbidity from 50 to less than 2 NTUs and total TSS to less than 5 ppm. WAC system reduces the total hardness to less than 0.1 ppm.
Feed streams are fed to the HLS into the vapour space above the reaction zone through separate spray boxes to partially liberate DO. Steam is injected into the HLS in two locations to maintain the temperature at 105°C. Operation at this temperature increases the reaction and sediment rates and improves overall efficiency of the process.
The non-condensable gases including DO that are liberated from the water are vented at the top of HLS.
The lime reacts with the bicarbonate alkalinity in the feed waters, forming calcium carbonate (CaCO3) alkalinity and a slight Magnesium Hydroxide (Mg(OH)2) alkalinity. Removal of silica is accomplished through adsorption on magnesium hydroxide precipitate. Magnesium oxide slurry is added to provide adsorbent material. Flocculant is fed to promote particle agglomeration and the formation of large particles of floc. These floc particles settle quickly to the bottom of the HLS for eventual removal.
The softened and clarified water flows up through an internal path and overflows into the De-Aerated Water Compartment. Dissolved oxygen is removed from water by steam stripping.
The water from the HLS is pumped by the After-Filter Feed Pumps through the After-Filter Package. The suspended fines are removed by filtration. Filtered water is sent to the WAC package for further softening before being used as BFW.
The removal of the hardness is done in two stages; the primary removal is done in the Primary WAC vessels, where the hardness levels are reduced to approximately 2 ppm; the secondary hardness removal is done in the Secondary WAC vessels, which remove remaining hardness to below 0.1 ppm. The softened water becomes BFW.