Q & A > Question Details
What is the reaction chemistry behind the formation of Ammonium Bisulfide salt and how does the deposition take place with respect to temperature?
18/03/2016 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
For any given hydroprocessing reactor effluent system, ammonium bisulphide (NH4HS) deposition increases as the reactor effluent stream is cooled below the NH4HS sublimation temperature (typically in the reactor effluent air cooler). A continuous supply of wash water is typically injected upstream of the reactor effluent air cooler to reduce corrosion and fouling in the cooler.
If the feed or makeup gas to the hydroprocessing unit is contaminated with chlorides, ammonium chloride (NH4Cl) salt deposition can also occur. However NH4Cl has a higher sublimation temperature than NH4HS (this typically occurs in the reactor effluent exchangers upstream of the air cooler). NH4Cl salts can be highly corrosive at the moisture dew point location and can cause severe fouling of the reactor effluent exchanger train. An intermittent wash water injection system is typically provided upstream of the exchanger where precipitation is predicted and corrosion-resistant materials of construction (eg. Alloy 625) can be specified to mitigate damage to the exchanger by HCl acid dew point corrosion.
08/06/2009 A: Rupesh Patel, Saudi Aramco Total Refining and Petrochemical Company, rupeshgpatel@gmail.com
The reaction chemistry is simple an NH3+H2S=(NH4)HS. The deposition temperature means vapor to solid depends on product of partial pressure of NH3 and H2S in effluent. Higher the partial pressure higher the deposition temperature and it may shift upstream of air cooler or inside air cooler.
07/06/2009 A: AMIYA LAHIRI, AK Associate, lahiri04k@rediffmail.com
H2S and NH3 being acidic and alkaline, they react to form NH3-bi sulphide. When formed at high temperatures downstream of reactors it is in vapour phase. The compound however sublimates. When cooled in condensor / air cooler the vapours get directly deposited as solid at about 70C
05/06/2009 A: keith bowers, B and B Consulting, kebowers47@gmail.com
H2S +Nh3 = trouble. Temperature is not the important variable - water is. One MUST keep enough water injected into the reactor effluent when temperature goes below about 400F to dissolve ALL the would-be deposits. Excess water is required, of course. Naturally, deposits accumulate on colder surfaces, and poor flow distribution, dead zones, etc in the exchangers are common. Bulk fluid temperature is not a reliable indicator of deposition probability in those 'cold' areas.
All too often the first indication is tube failure (and lots of hydrocarbons escaping to the cooling tower or air cooler draft. It is absolutely awesome and frightening to see the resulting fire if the failure is in the air cooler. 2500# loop pressure pushing the reactor effluent through 2 -1" openings of a tube fracture results in tremendous amount of fuel.