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Crude Oil Separation Process Schemes
- Thread starter MIANCH
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- #3
How to prevent crude unit corrosion-topping by 0707
faq483-1233
Posted: 13 Apr 06
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.
regards
luis
faq483-1233
Posted: 13 Apr 06
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.
regards
luis
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- #5
georgeverghese
Chemical
A 2 step dehydration process seems to be indicated for high salinity crudes. The first stage may be a batch standard dehydration tank or a concentric wash tank or an electrostatic dehydrator, followed by dilution water injection prior to final dehydration at a desalter. I am no expert on desalting equipment, there seems to be many types in the market. Feedback I got from plant operators is that desalters and dehydrators are rather finicky animals and troublesome to operate, especially when you have stable emulsions and/or high water content.
- Thread starter
- #7
George,
I know you are the guy who have practical experience in this forum and hope you can advice better. I proposed in the scheme one three phase separator, De-salter and Electrostatic dehydrator. I think need to inject more water in separator inlet and in de-salter. can you please share your advice on this scheme.
Regards
I know you are the guy who have practical experience in this forum and hope you can advice better. I proposed in the scheme one three phase separator, De-salter and Electrostatic dehydrator. I think need to inject more water in separator inlet and in de-salter. can you please share your advice on this scheme.
Regards
I was not in upstream production and separation, but to lower your feed crude salinity, of course you have to inject more water in separator inlet and also in the de-salter! The secret is in water adiction and emulsion control, otherwise the electrodes of dessalters will make electric sharps and will make dessalters to shutdown.
luis
luis
georgeverghese
Chemical
Think the order for processing should be 3phase sep, then electrostatic dehydrator, then desalter. Dilution water injection may be as you suggest. You can recycle the water from the desalter exit to the 3phase KOD to save on water usage. Pls note that some schemes are heavily reliant on heating of the crude and chemical injection to break up emulsions and to ensure disposal water meets statutory oil in water content limits. Lab tests should done to see what combinations of temp and chemical dosing is suitable in your case. Choose a model of dehydrator and desalter that is more tolerant to emulsions, even if the vendor claims slightly lower separation efficiency.
Disposal water deoiling scheme also needs to be worked out.
Disposal water deoiling scheme also needs to be worked out.
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