6 replies to this topic

[S. Biswas]

It's very common to inject wash water for vapor services containing NH3 and H2S to dissolve NH4HS (form at higher temperature) in water and thus to avoid corrosion at the downstream air cooler inlet manifold. What is the maximum wt% NH4HS allowed in water at a moderate temperature below 250°F? This is to determine water injection rate.

[Zauberberg]

From what I know, water flow is determined based on dew-point calculations, i.e. to inject that amount of washwater which will result in presence of liquid H2O at overhead condenser inlet. It has nothing to do with NH3/H2S concentration (and corresponding pH value) since you cannot affect pH in significant proportion by changing the amount of washwater. Keep in mind that any excess water will put additional load on overhead condenser.

pH-value of overhead condensate should be maintained by proper neutralizer injection, and not by adjusting the flow of washwater; NH3 and H2S are present in the feed (crude) and there isn't much you can do to affect those values.

[S. Biswas]

Hi Zauberberg,

This is for Diesel Hyrotreating unit - HHPS vapor water wash upstream of air cooler, not for the crude or vacuum unit. Feed Nitrogen and Sulfur converts in the upstream Reactor to NH3 and H2S respectively, which form NH4HS at appropriate pressure and temperature. Deposition of NH4HS and thus corrosion at air cooler inlet header is a common problem in refineries. Wash water is injected to dissolve NH4HS. There is a limit of NH4HS concentration in water whoever design this kind system. Wash water injection is not required if there is no NH3/H2S or if the operating condition oes not permit formation of NH4HS.

From what I know, water flow is determined based on dew-point calculations, i.e. to inject that amount of washwater which will result in presence of liquid H2O at overhead condenser inlet. It has nothing to do with NH3/H2S concentration (and corresponding pH value) since you cannot affect pH in significant proportion by changing the amount of washwater. Keep in mind that any excess water will put additional load on overhead condenser.

pH-value of overhead condensate should be maintained by proper neutralizer injection, and not by adjusting the flow of washwater; NH3 and H2S are present in the feed (crude) and there isn't much you can do to affect those values.

[smalawi]

Hi Zauberberg,

This is for Diesel Hyrotreating unit - HHPS vapor water wash upstream of air cooler, not for the crude or vacuum unit. Feed Nitrogen and Sulfur converts in the upstream Reactor to NH3 and H2S respectively, which form NH4HS at appropriate pressure and temperature. Deposition of NH4HS and thus corrosion at air cooler inlet header is a common problem in refineries. Wash water is injected to dissolve NH4HS. There is a limit of NH4HS concentration in water whoever design this kind system. Wash water injection is not required if there is no NH3/H2S or if the operating condition oes not permit formation of NH4HS.

From what I know, water flow is determined based on dew-point calculations, i.e. to inject that amount of washwater which will result in presence of liquid H2O at overhead condenser inlet. It has nothing to do with NH3/H2S concentration (and corresponding pH value) since you cannot affect pH in significant proportion by changing the amount of washwater. Keep in mind that any excess water will put additional load on overhead condenser.

pH-value of overhead condensate should be maintained by proper neutralizer injection, and not by adjusting the flow of washwater; NH3 and H2S are present in the feed (crude) and there isn't much you can do to affect those values.

Hi,

The titile for this question is water wash for distilation tower overhead, not for HHPs, hence Zauberberg asnwer

For HHPS finfan NH4HS conc. limit is realted to the material (CS, SS, duplex, monel ... etc). Check your lisensor limit first and your design velocity and material

cheers,

sm

[Zauberberg]

Also take into account the capacity of overhead cooler/condenser, and downstream equipment as well. It may require modifications in order to handle liquid water (second liquid phase), if it has not been design for such conditions at the first place.

The main purpose of water injection is to dissolve salt deposits which would otherwise induce under-deposit corrosion, after sublimation from process stream. No need to inject excessive amounts of liquid H2O, there's no purpose of doing so.

[suhas]

S. Biswas

You should add min water so that after injection point 20-30% water should remain in unvaporized form.

[TPR]

It's very common to inject wash water for vapor services containing NH3 and H2S to dissolve NH4HS (form at higher temperature) in water and thus to avoid corrosion at the downstream air cooler inlet manifold. What is the maximum wt% NH4HS allowed in water at a moderate temperature below 250°F? This is to determine water injection rate.

Hi Nwebie,
I have read a little about this topic to do with HDS systems and their reactor effluent cooling woes.
The %NH3HS2 (AmmoniaBiSulphide - ABS) is governed by pipe material and the velocity in said pipe.
Generally the higher the %ABS the lower the allowed velocity or the higher the spec of pipe material.
There is also a low velocity point where settling of the condensed ABS occurrs = plugging etc.
It appears that this % concentration of the solution is anything from 2% to 10% for carbond steel depending on where you read. May tend to be cautious as I have run into some trouble before.
RL Piehl has a paper out with some rules of thumb for the design of such systems.
You can search this on the net and get substantial info.
Also, a good summary is to be found at www.hghouston.com under their special corrosions topics.
In that summary they talk of this in overhead systems.
As the others said, it would be a fine line between dilution it enough and overloading your condensers.
NOte - it appears that very low dissolved oxygen in important to avoid corrosion <50ppb.
Cheers