In Crude Distillation Unit CDU, we inject caustic soda (NaOH) for treating salts in crude oil (<= 20ppm ), but if the concentration of the salt in crude became too high (e.g 40ppm), desalter (desalting system) is required for that. The query is, if the desalter is out of service, what procedures are to be taken for treating salts? And what are the disadvantages of increasing the injection of caustic soda (NaOH)?May-2022
Sarah Bird, Quadra Chemicals, Sarah_Bird@QuadraChemicals.com
The most common chlorides found in crude oil are MgCl2 CaCl2 and NaCl. The chlorides that are not removed via desalting make their way onto your distillation unit and undergo hydrolysis forming hydrochloric acid which can lead to aggressive corrosion in your overhead and pump around. In some instance, your inlet salt content is low enough that you can get away with a caustic injection only, however it is important to understand that by injecting caustic you are potentially only moving the problem to a different unit in the refinery.
The purpose of the caustic injection to convert the MgCl2 and CaCl2 to NaCl. You may be wondering why is NaCl a lesser evil than MgCl2 or CaCl2, well it is a function of temperature. NaCl will not hydrolyse until you reach temps > 1,000F (537 C) while CaCl2 will hydrolyse at 400-450 F (204 - 232 C) and MgCl2 250 F (121 C) forming HCl.
So by injecting caustic you are mitigating the risk for corrosion in your crude/vac unit, but you are moving all the salts out with the bottoms that could potentially cause issues in the coker etc. so its important to be mindful of the corrosion limits for these units when discussing an increased amount of chlorides that you plan to handle with a caustic injection.
Another approach to mitigating the increased chlorides in the feed without adjusting caustic is the addition of a filmer and neutraliser in your overhead. A fiming amine will help to prevent corrosion by preventing completion of the corrosion cells, this is a continuous injection and the neutraliser is used to adjust pH. You will also need to monitor your chloride concentration in the your overhead and salt formation temperatures to ensure you run high enough temps to not salt up.
Salem Ghoul, Mellitah O&G Co, email@example.com
If Desalter is out of service, try to inject washing (hot water) at the inlet of separator or increase the water flow rate, plus inect the neutralizing filming amine at the top of CDU upstream of condenser.
Rigoberto Santaella Rigores, LIpesa, firstname.lastname@example.org
Whenever caustic soda is used to reduce corrosion at the top of the crude distillation column, there is a risk of fouling the cold train, the hot train and the furnace due to the excess of unreacted caustic soda that could stay.
If you have the desalter out of service, you could manage the contingency by increasing the consumption of neutralizing amine and filmic amine on top and thus you can balance the increase in the injection of caustic soda with the top amines.
Suru Patel, Patex Controls Ltd, email@example.com
At a Steam Cracker plant, the field plant Operator would measure the spent caustic strength using an At Line analyzer to control the amont of fresh Caustic added to the process. The chemistry is quite complex!