3 replies to this topic

[shan]

Hi Everybody,

I know, usually, an oil electrostatic treater of oil production facilities is operated at 150F and 50 psig to satisfy oil pipeline requirement 0.5 BS&W. However, the treater temperature has to be 284 F to break the water/oil emulation for my specific case. The problem is a lot of HC light components and water are vaporized at that high temperature, which result loss of oil volume and salt drop-off. Is it a good idea to pump treater inlet stream pressure higher than saturate pressure 160 psig to avoid the vaporization? Is there any other problem, such as de-composition of long chain hydrocarbons and equipment fouling, because of such high temperature?

Regards,

Shan

[kkala]

I suppose that mentioned electrostatic treater (http://www.frames-gr...at=5&product=21) is doing something less than a desalter. One desalter met in a local refinery operated at a pressure range 12-16 kgf/cm2 g and temperatures 120-140 oC (248-284 oF). According to Operators, main problem in piping & equipment was corrosion; scales and hydrocarbon decomposition were not mentioned (so these issues were not serious). Of course kind of crude plays a role on this. It is noted that, downstream desalter, crude is heated further, passing from another exchanger train and a furnace, then enters crude distillation tower.
So increase of operating pressure to avoid vaporization looks possible, after a check of all around design pressures.
Likelihood of decomposition, polymerization, and especially scales at new higher temperatures had better be based on operating experience of the specific crude oils treated; probably there will not be a problem, or can be solved through e.g. anti scaling agent. I do not have experience on these matters though.

Edited by kkala, 22 July 2011 - 02:38 PM.

[Technical Bard]

Depending on the gravity / viscosity of the oil, the temperature needs to be set to give you good density difference between oil and water and to reduce the oil viscosity (all of which helps break the emulsion, along with chemicals and the electrostatic field.
I have seen treaters/desalters running up to 150°C (300°F), although many in heavy oil service and refinery desalters are specified for ~130°C to keep pressures down. This limit is pretty hard, based on the maximum operating temperature of the bushings around the electrical parts entering the pressure vessel. The operating pressure of an electrostatic treater/desalter should be maintained above the vapour pressure of the oil. Vapourisation will tend to impede separation because it increases turbulence and velocity. And you are correct that if any water vaporises it could cause salt to precipitate (although unlikely if only a little water evaporates). I also try to keep the operating pressure (i.e. backpressure from downstream valve) about 200 kPa above the oil vapour pressure at the maximum treating temperature.

I wouldn't worry about polymerization or decomposition of large molecules - these temperatures are well below the temperatures that occur in a the Crude Distillation Unit. If your crude contains cracked material (rare, but not unheard of), or material produced from a fire-flood (in-situ combustion), there is more risk due to the partially oxidized components.

The biggest stability question you should answer is "will asphaltenes precipitate" at my treater condition? This is best done through lab testing.

[shan]

Technical Bard/Kkala,

Thank you for your valuable opinions. I am considering pumping-up the treater inlet and specifying stainless as the treater building materials.

Best regards,

Shan