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Desalter Operation upset

Desalter Operation upset

(OP)
Hello every one,

We have field small refinery to produce diesel ~10000 BFD API =36, BS&W=0.05 and salt= max 4 PTB in our plant already designed one stage deslater my question in such case need to operate this unit I mean mix crude with service water or no need. See below the water and crude desalter in/out result: (see attached file)

We faced problem with over head water accumulator with high chloride and iron content and ph fluctuating value out of range (5.5-6.5) as you can see from attached file results.

Note that we injecting neutralized amine which is diluted with service water 1:3 with total rate is 7 gpd ( naphtha reflux to crude tower injection point) and corrosion inhibitor with rate 8 gpd (upstream of overhead condenser exchanger).

Moreover, over head exchanger is experienced high corrosion, tubing crack at inlet ends no fouling problem has been reported for desalter downstream (Heat exchanger tubing) we used Antifoulant chemical (crude discharge pumps).

Using water wash is the root cause of corrosion and high chloride level at over head accumulator. What the source of high chloride level (our crude is very low PTB & BS &W).
It is worth to mention that we closed deslater operation for long time but we face same problem high chloride, iron and ph value fluctuating. What the source of chloride our crude feed low salt and BS&W values.

Your recommendation is highly appreciated

Zayed

RE: Desalter Operation upset

Your file isn't working for me, so I'm going to throw some general questions out that need to be asked...

What is the wash water source (both for the desalter and for the overhead)

PTB exiting the desalter?

Have you done a salt speciation? Some salts hydrolyze MUCH easier than others, so while 1 ptb may be good if it's mostly sodium chloride, if it's magnesium chloride, you'll be sending huge amounts of chlorides overhead.

Have you done a chloride balance to determine % hydrolysis?

Have any non-extractable chloride tests been done?

Do you have caustic injection?



Lots, lots of stuff can go into something like this.

Nathan Brink

RE: Desalter Operation upset


zayed78

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.

I hope it helps

lm


RE: Desalter Operation upset

(OP)
Nathan Brink & 0707 Thanks for prompt response,

First I will go through Nathan Questions:

What is the wash water source (both for the desalter and for the overhead?)

We are using service water for desalter no application for the overhead

We are using service water (potable water) with below specifications:

Analysis Inlet
pH 6.9
TDS, mg/L 263
Conductivity, uS/cm @ 25 0C 399
Total Iron ( Fe), mg/L 0.23
Chloride (Cl-), mg/L 66


PTB exiting the desalter?

Salt content of our crude is max 4 PTB – API =36, SG=0.845
I can say 4 PTB = 13.5 mg/L (ppm) is that correct (~16mg/kg)
1 ppm=( ptb*2.85)/SG

Analysis Inlet Outlet
BS&W 0.05 0.05
Salt PTB 2.5 2


Have you done a salt speciation? Some salts hydrolyze MUCH easier than others, so while 1 ptb may be good if it's mostly sodium chloride, if it's magnesium chloride, you'll be sending huge amounts of chlorides overhead.

Never had we done a salt speciation what do you mean Salt Speciation? Percentage of that slat
Can we do laboratory test to find out the content of salt such organic nonromantic.
General speaking {The chloride percentage in crude oil salt as type of inorganic Chlorides (75% NaCl , 15% CaCl and 10% MgCl )}

Salinity of produced brine
The salinity of produced brine varies widely, but for most produced water, it ranges from 5,000 to 250,000 ppm of equivalent NaCl. Crude oil that contains only 1.0% water with a 15,000-ppm salt content has 55 lbm of salt per 1,000 bbl of water-free crude. The chemical composition of these salts varies, but nearly always is mostly NaCl, with lesser amounts of calcium and magnesium chloride.
http://petrowiki.org/Desalting


Have you done a chloride balance to determine % hydrolysis?


No; how to do such test

Have any non-extractable chloride tests been done?
No; also how can we do it

Do you have caustic injection?

No; as I said above only we are using neutralized amine & corrosion inhibitor filming amine

Zayed

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