1 reply to this topic

[myla]

We have a naphtha hydrotreater unit designed to process straight run and cracked naphtha. The unit has 2 flash drums (hot and cold separator drums). Both separators are operating at 2800 kPag. Both flash drum liquids are sent to the debutanizer tower.

 

 

 

We checked the thermodynamics of the ammonium chloride deposition in the unit. Based on our prediction using the calculated Kp and ammonium chloride sublimation curve, the deposition of ammonium chloride is most likely seen in the flash drum sections.

 

Circuit

Kp

Reactor to Flash Drum 1

9.12 E-06

Flash Drum 1 vapor to Flash Drum 2

2.42 E-05

Debutanizer feed

3.96 E-06

Debutanizer top tray

1.13 E-06

Note: Nitrogen and chloride present in the feed are 1 ppm and 0.2 ppm, respectively.

 

However, during our offline water washing, ammonium chloride deposition in the flash drum is not significant compared to the deposition on the top trays of the debutanizer. What is the possible explanation for this deviation of the actual deposition to the thermodynamic prediction? Also, besides concentration of nitrogen and chloride in feed and hydrogen gas, what are the other parameters affecting ammonium chloride deposition?

[Killer Queen]

NH4Cl salt deposiotion depends on some numbers. If the Cl content is high than 2ppm, it give a good chance for NH4Cl to form.

 

NH4Cl salts are hygroscopic  and form a strong acid in the presence of trace amounts of water, the wet salt producing localized corrosion.

NH₄Cl → NH₃ + HCl
HCl → H⁺+Cl-
Fe²⁺ + 2 Cl־ → FeCl₂
FeCl₂ + (NH₄)₂S → FeS + 2NH₃ + 2 HCl
FeCl₂ + 2H₂O→ Fe(OH)₂↓+ 2HCl
 

The formed FeS and Fe(OH)2 are stuck under the salts, can not show itself via iron content in the water samples. That's why when you open the unit and clean the salts away, you might realize the corrosion.

 

NH4Cl is the main type of salt deposition because it can form even at high temperatures ( more than 80 C degree). Of course it likes the lower temperatures.

 

The solution is that you can use the high purity water to wash the salts. The better one is to use the salt dispersant chemical, can prevent the system from the salt deposition and under deposition corrosion and remove the existing salt deposition.

Edited by Killer Queen, 28 September 2013 - 10:52 PM.