Re-typed article from Oil & Gas Journal
Static mixer improves desalting efficiency
An in-line static mixer has increased salt removal from crude oil at a large West Coast refinery, says the mixer supplier, Komax Systems Inc.
The mixer was installed at a 150,000 bid crude distillation unit’s desalter.
Crude at this refinery is a mixture of local production and imports from Indonesia and Alaska.
In the past, the refiner used a typical globe-type mix valve to mix fresh water with crude at the desalters. The crude is heated to 300 degrees F., mixed with 5% fresh water, and then fed to desalters.
Chemical and electrostatic treatment is used in the desalters to remove salt and water from the crude. Pressure drop in the mix valve was typically set at 10 psi. However, when the processing of heavy (14 degree API) naphthenic crudes was attempted, the oil/water mixture formed a stable emulsion in the desalter. This prevented economic processing of this low cost, domestic crude. Several problem areas were identified which could contribute to stabilizing emulsions. One was the high shear-rate mixing found in the mix valve.
The crude oil and water are then simultaneously mixed though two-by-two division, cross-current mixing, and back-mixing, which improves turbulence and increases mixing efficiency without requiring high fluid shear velocities.
The modified desalter system has operated well on 14″ and 22″ API naphthenic crudes, with less that 5% oil in the effluent water.
As an unexpected bonus, salt removal also increased as a result of using the static mixer (Table 1). Depending on the type of crude oil, the refiner has been able to remove between 5% and 10% more salt than by the mix valve method. With less salt carried over out of desalter, less corrosive HCI will be generated in the crude unit furnaces. This will require less ammonia to neutralize the atmospheric column overhead stream.
At the same time, the mixer has helped reduce emulsions formed by too much pressure drop created by the mix valve. With less oil carry under, less fuel is consumed from having to reheat recycled oil up to 300 degree F before it re-enters the crude unit.
In addition, the new mixer has a better turndown ratio than the mix valve. The Komax unit has performed satisfactorily at rates as low as 35% of its design capacity.
Previous types of static mixers were unable to operate effectively at these levels.
Also, pressure drop due to the mixing device was decreased for 10 psi to 1.5 psi. Depending on the particular situation this could have energy conservation or through put incentives.
The refiner calculates that the mixer will pay for itself in its first year of operation with combined savings of $4,000/year in lower power consumption and chemical costs and $1,000/year in lower fuel costs.
|Salt in,||Salt out,||% Removal|
|90,000 b/d 22º Crude||42||4.4||89%|
|14º API Crude
|90,000 b/d 22º API Crude||41||1.6||96%|
|45,000 b/d 14º Crude||43||º||97%|
|* Desalters, mix valve, and static mixer all designed for full design crude unit feed rate of 150 MBPD.|
|* † Pounds NaCl per 1000 bbl crude.|
|* § Crude Unit did not run 14º crude before desalter modifications, which included the static mixer, because stable emulsions were formed.|