Problem in sour water stripper

[Mohammad Ka]

Hi everyone
Mixed sour water and hydrocarbon stream from a quench tower is fed to a drum to separate heavy hydrocarbons from water. Then the separated sour water is sent to a coalescer for removing pygas from top and heavy components from bottom. The sour water from effluent of the coalescer has max. 10 ppm oil content and 10 ppm TSS (total suspended solid). This water is fed to top of a stripping tower having 12 sieve trays. At the bottom of the tower dilution steam (DS) is directly injected in. The mass flow ratio of DS to inlet sour water is about 1 to 10 to reach the bottom temperature of the tower to 117.8C. Although the ratio was increased from 1/10 to 1/6, the temperature has been remained 102C constantly. Dispersant chemical is injected to sour water at the inlet of tower and a corrosion inhibitor is injected to the process water at the bottom outlet of the tower. The process water then is sent to a dilution steam drum and heat exchangers. Heat exchangers are suffering from fouling in tubes where the process water flows in.
What's the reason of fouling in heat exchangers and the low temperature of the stripping tower?

 

[georgeverghese]

What is the minimum normal stripper tower top pressure? It has to be at least 90-100kpag to enable tower bottoms temp to be 117.8degC.

Corrosion on sour water stripper preheat HX tubeside with incoming cold sour process water may be a mix of microbial and chemical fouling I suspect. What materials of construction do you have for these tubes? Chloride content here?

 

[Mohammad Ka]

Dear georgeverghese
The stripper tower top pressure is 75 kpag (based on PFD).
Material of the tubes is carbon steel.
Based on the Lab. data, there is no chloride in dilution samples. Also pH=8.5-9.5 and conductivity=10-12 micS/cm.
It should be noted that blowdown from steam drum to dilution steam drum has phosphate content. The HX is connected to the dilution steam drum as a thermosiphon reboiler. MP steam is also in shell side of the HX.

 

[georgeverghese]

If you take a look at steam tables, you can see it is impossible to obtain liquid water at the tower bottoms at 118degC at 75+5(?)kpag. Am assuming tower dp is approx 5kpa and so, much less than 20kpa. If this assumption is valid, then can you increase tower top pressure to be > than 100kpag?

Can we see a sketch of this arrangement at tower stripped water exit / steam drum / reboiler HX and MP steam ?

 

[Mohammad Ka]

Dear George
I checked steam table and steam properties calculators and it is found that the saturated temp. at 75 kpag is 116.3C and at 85 kpag is 117.9 C. Here is the link:

https://www.spiraxsarco.com/resources-and-design-tools/steam-tables/dry-saturated-steam-line

By the way, the fouling in the DSG HX was analyzed by the lab. It contains hydrocarbons, Fe and Sulfur.

 

[georgeverghese]

Okay, from the sketch, it does look like you may not be getting sufficient steam at 600kpag, 190degC for live injection at SWS bottoms. And the main suspect would be thermosyphon reboiler at the dilution steam mixing drum, since this drum also receives dirty blowdown from all over the plant. So suggestion would be to divert this fouling blowdown elsewhere, since thermosyphon reboilers cannot tolerate fouling feedstreams that will affect pressure drop and hence duty. This is to me a design weakness. The plant design contractor / licensor should make the necessary corrections if possible. Installing a pump on this thermosyphon circuit may help a little, but fouling will still occur at the HX.

I redid interpolation with steam tables in DQ Kern, and yes 118degC at 85kpag at tower bottoms is correct.

 

[Mohammad Ka]

I do not understand why there is no sufficient steam @ 600kpag, 190C for injection at SWS bottom?!
However, due to low thermal efficiency of DSG HX, MP is directly injected to DS via PV-B where the operating conditions of injected DS is 6.2barg and 210 C.
It worth noting that the noticeable fouling (composition:Fe, S, HC) and corrosion are observed in SWS.

 

[georgeverghese]

Yes, I just realised MP steam bypass PV-B can be used to makeup for fouling at thermosyphon reboiler DSG HX, provided it has sufficient capacity.

 

[0707]

verify what is happening with the dispersant chemical injected in the sour water at the inlet of tower and the corrosion inhibitor injected to the process water maybe you are injecting too much of these chemicals.

luis

 

[Dhurjati Sen]

Check for presence of ammonia (ammonium salts) in the sour water. There could be a chance of leakage from the upstream units (like hydro-cracker).

The ammonium bi-carbonate salts could foul / choke the exchangers.



DHURJATI SEN
Kolkata, India

 

[0707]

Dhurjari Sen

Is also wright

luis