CDU Waterwash Design 1

With regard to ammonia injection to vapors to control corrosivity, it generally happens that the vapor distribution among several parallel air-cooler condensers is not totally equal. Flow maldistribution causes low-temperature dew point corrosion.

The Liebermans (Norman and Elizabeth) in A Working Guide to Process Equipment McGraw-Hill, say that those bundles nearest the inlet header tend to see perhaps 1% more flow than the tube bundles farthest from the inlet header.

Those seeing lower flows have slightly lower outlet temperatures favouring sublimation of ammonium chloride vapor to a white saltlike hygroscopic solid.

The wet chloride salts are very corrosive to carbon steel tubes forming ferric chloride on the process. This salt reacts with H₂S to produce hydrochloric acid and pyrophoric blackish-gray iron sulfide that deposits inside the tubes.

As a result the vapor flow is further restricted causing even lower outlet temperatures, which, again, promote sublimation of corrosives, in a vicious circle.

The solution for non-symmetrical flow problems -following L+L- is to make the pressure drops in both the inlet and outlet tube bundle headers very small, as compared to the bundle pressure drop itself.

I have seen this quetion somewere here is my answer...

(NH4) 2S, is obtained, in the form of micaceous crystals, by passing sulfuretted hydrogen mixed with a slight excess of ammonia through a well-cooled vessel; the hydrosulphide NH4.HS is formed at the same time. It dissolves readily in water, but is probably partially dissociated in solution. The hydrosulphide NH4.HS can be obtained as a white solid, by mixing well-cooled ammonia with a slight excess of sulfuretted hydrogen.

NH3 (g) + H2S (g) ---------- NH4SH (solid crystals)

Synonyms - Ammonium hydrogen sulfide, ammonium sulfhydrate
Ammonium bisulfide, ammonium sulfide, ammonium
Hydrosulfide, ammonium mercaptan (KI-310)

Formula .................................................. (NH4) 2S

The basic Neutralization reaction is

NH4OH + HCL --------------------- NH4CL + H2O

The use of NH3 or organic amine to control Ph produces ammonium or amine chloride as a neutralization product. The desublimation point of NH4CL may take place in the overhead condensing system ahead of water condensation point and lead to corrosion and fouling.

NH3 + HCL ------------------ NH4CL (deposit)

Resulting in under deposit pitting attack – good water wash is required

The required quality of water wash is:

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

Effects of excess Ammonia in presence of H2S

2NH4OH + H2S ---------------- (NH4) 2S + 2H2O
(NH4) 2S + Fe++ ---------------- FeS + 2NH3 +H2

The overhead H2S corrosion mechanism is as fallows:

H2S «---------------» (H+) + HS-

(Fe++) «--------------» FeSH+

The FeSH+ react further to form a complex sulphide scale: (HS-Fe-S-Fe-S-Fe-SH)

(HS-Fe-S-Fe-S-Fe-SH)

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.



On sour water systems to minimize corrosion, the sulphur content of your C.S piping and fittings shall be limited to 0.01% max. and shall satisfy the NACE requirements. I suppose that NACE MR 0175 requirements have been recently updated. To PWHT (post weld heat treat) your carbon steel piping would also increase the corrosion resistance of your piping.



A good and proper water wash would probably be the key to minimize corrosion problems. 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 ad make up water.

The standard wash water quality should be 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 of the 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.


Regards

Luis Marques