On top of overhead CDU as pH increases toward the neutral region the concentration of bisulphide ion in solution increases. It has been demonstrated by several investigators that there is a sharp increase to the rate of corrosion in the region of pH 6.8 to 7.3. The sharp increase in corrosion rate is apparently the result of faster reduction of bisulphide ions both from the scale lattice and solution. As the scale lattice is altered FeS is released, exposing unreacted iron, and the FeS enters the water phase. In a crude unit this phenomena is recognized as black water.
The dissociation of H2S in HS- and S++ is minimum at 5 pH.
A good and proper water wash would probably be the key to minimize corrosion problems on overhead distillation units. The quality of water wash is very important.
In our crude unit we use continuous water washing of the overhead air coolers. The water from the dessalters is refluxed through a feed water drum to the inlet nozzles of air coolers header boxes. When water is saturated and chloride contents on overhead drum are above 50 PPM we empty the feed water drum and we add make up water. The optimum water wash should be with demineralised but this is not economic.
The standard wash water quality normally is as fallows:
Ph ? 5.5-7.5 Total hardness- < or = 50PPM Total HCO3- and CO3- content < or = 50PPM Ammonium hydrosulphide (NH4HS) < or = 100PPM Chloride < or = 2000PPM Sulphate < or = 200PPM Oxygen < or = 1PPM
Some years ago because of ammonium chloride under deposit corrosion and difficulties in stabilizing pH we give up injecting liquid NH3, we start controlling Ph with a neutralize inhibitor up stream overhead air coolers. On overhead line of the column we have a corrosion inhibitor injection. Down stream of dessalters we have a caustic injection to neutralize the chlorides. This caustic injection should be as much as possible stequiometric, we should inject caustic OHNA in the same proportion of the salts we have at dessalters outlet. The caustic injection is to form sodium chloride salts at dessalters outlet and not to control PH. The caustic limit injection of 12 PPM of OHNA at dessalters outlet is to avoid caustic embritllement downstream dessalters equipment minimizing as much as possible coking formation at crude heaters. The caustic injection should be performed through a proper quill. From times to times is important to check quill condition. Good dessalters performance is also a good way to control crude overhead corrosion.