2 replies to this topic

[aragorn]

Hi

I am a process engineer and work in crude distillation unit . can someone explain to me about the calculation of injection caustic flow rate in the feed ?

 

[P.K.Rao]

Injection rate depends upon the salts present in the crude and efficiency of desalter removing salts and organic acidity of crude. Mostly magnesium chloride hydrolyzes giving volatile hydrochloric acid. Besides this, there would be volatile organic acids such as formic and acetic acids. You have to plan to maintain a pH of 6.5 to 7 of the overhead condensate. For this purpose, you have to doze caustic soda in crude to neutralize the acids till the pH of overhead condensate attains the .required value. A pH of  more than 7 is detrimental to condenser.  Ofcourse, corrosion inhibitor is also needed.  I have seen some refineries doze 4-6 ppm NaOH on crude..

Edited by P.K.Rao, 29 December 2014 - 09:00 AM.

[Himanshu Sharma]

Hey there !

 

Well this one is my favorite and if you are looking for a calculation i have none ,its just experience the hard way.

 

Caustic is often typically Injected directly in crude either upstream or downstream of Desalter and refiners have claimed to benefit from both of them,each has some specific advantage/disadvantages, at least in theory.

 

Crude Column Overhead Chloride levels shall be maintained between 10 and 30 ppm with lower limit set to avoid over-treatment with caustic. Over treatment with caustic can result in contamination of the heavy products from the crude distillation tower with sodium, which can affect downstream cracking units such as cokers, visbreakers, and Fluid Catalytic Cracking (FCC) Units.

 

Caustic Injection Upstream of Desalter

 

Refiners inject Caustic directly in Crude upstream of Desalter to take care of Chloride corrosion in Crude Overhead circuit.These chlorides are present as salt of Na,Mg and Calcium wherein NaCl of-course can be removed with water  as Brine from Desalter. Thus refiner aim at Hydrolyzing salt of Mg and Ca to Nacl,this normally requires a good deal of caustic Injection  since rate of Hydrolysis at Desalter operating temperature of 125-135 deg C is low and huge reactant is required to put reaction to completion.

 

This is an effective solution as almost all the Chloride salts move out of crude at lower operating temperatures.The life is not easy in reality,Caustic upstream of the desalter can react with naphthenic acids to form sodium naphthenates or what we know as Soaps. These naphthenates will promote emulsion stability and make desalter operation more difficult.This may worsen the corrosion rates for high TAN Crudes altogether rather than improving the situation

 

Another problem is that unreacted caustic goes off as brine and is not available (in crude) for suppression of HCl formed in crude overhead with leftover Mg and Calcium salts.

 

Even with all that discussion of negatives, this is my preferred option to choose from in case i can assess the amount of Naphthenic acids(not just TAN no but both Type-I/Type-2 Napthenic acids) present in feed.

 

Caustic Injection Downstream of Desalter

 

This is less common put in action but the proponents of the theory says that Desalters are designed with high salt removal efficiency and the left-over chlorides shall be nailed only at Desalter Outlet wherein the concentration is already much lower than the original design number.The lower caustic injected downstream can effectively hydrolyze the magnesium and calcium salts with lower amount of caustic at elevated pre-heat temperatures and Heater.

 

There are chances that sieves or valves of the tray may be choked with Nacl if enough water is not injected at the overhead.Another,problem is that caustic is present in much longer section of the unit and possibility of caustic embrittlement which may happen above 20 ppm of Hdroxyl ion concentration.

 

I know,i have talked a lot without giving any solutions blame it on my profession as a Consultant.In your shoes ,I would have straight ways contacted the chemical vendors like GE ,Nalco and similar league and asked about the possibility of Injecting corrosion inhibitor/Neutralizing amine/Filming amines rather than going for Caustic Injection which is simple,easy and dirt cheap but can play havoc if turned otherwise.In the business of refining,downtime hurts more than the sulking galfen or wife for that matter  .

 

Stop yawning,its time for lecture closure.

 

Thanks and Regards

Himanshu