Q & A > Question Details
We have been facing issues in one of our sour water stripping units wherein we observe continuous plugging of the feed control valve to the stripper column with greenish black hard deposits. The temperature of sour water passing through this control valve is in the range 146-152 deg C. Could anyone suggest the testing we should carry out to understand this heavy deposit? Does anyone have experience with hard greenish black deposits in their sour water stripping units?
21/07/2020 A: Utkarsh Jain, BPCL Mumbai Refinery, jainutkarsh.20@gmail.com
A:@Pronab Mistry : thanks for your inputs. In our SWS unit we do not have any filtration or liquid coalescer upstream or downstream of the surge drum. Since this is a 2-stage sour water stripping unit, in the 1st stage column where specifically H2S is separated out from the column overhead, MP steam is used as the reboiling medium. Since the column bottom temperature is required to be maintained in excess of 165oC, feed water exchanges heat with the feed/bottom exchanger to achieve the desired feed temperature in excess of 147 oC so as to minimize the load on reboiling.
@Lindsay McRae : thanks for your inputs. We do not have a liquid coalescer or a feed pre-filter installed in our setup. However, at what HC or TSS content should we consider installation of a coalescer?
20/07/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
First step is to determine exactly what this deposit contains. A gc-mass spec would help identify organic constituents. Atomic absorption analysis would identify the metals present. A thorough analysis of the feed water is also appropriate. Carry over hydrocarbons must be removed as should filterable solids
20/07/2020 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
The deposits could be analyzed for loss on ignition, for it is almost certain that it is coked up hydrocarbon. Checking the oil skimmers’ performance would help. If the feed water is coming from a tank, then check the floating skimmer’s best operating level range for the tank. If the skimmer is settled at the bottom (due to float malfunctioning, inadequate level for keeping the skimmer afloat) the bulk of the oil will get into the stripper which gets coked up. I have an experience where the stripper used to get coked up every four months, limiting the refinery run length.
06/07/2020 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
Fouling in SWS unit (especially feed exchanger and sometime SWS column) is a fairly common issue in in refineries and can often be attributed back to poor separation in the Sour Water drum just before the SWS. This is usually a quite difficult separation to make as this is a quite stable emulsion with a low IFT and the residence time and separation efficiency of a simple large separation drum is just not good enough to for a difficult separation like this.

High efficiency PhaseSep Liquid Coalescers equipped with suitably sized pre-filters have proven to be able to break this emulsion and protect downstream SWS without being disarmed.

It might be worthwhile for you to check for carryover of HC's from SW Drum. Suspended solids ( corrosion products) may also be present due to the very sour service, high temp and various CS materials.

Our commercial experience shows this is often a quite variable and a fouling stream too so it pays to conduct a characterisation study on the SWS feed including PSD, TSS and HC liquid loading to ensure the pre-filter & coalesce is sized correctly.
03/07/2020 A: Pronab Mistry, ADNOC Gas Processing, pronab88@yahoo.com
I presume that your system has feed water filtration before entering the stripping tower. Even after filtration sour water will contain some residual hydrocarbon and at the freed water temperature you mentioned, hydrocarbon could be decomposing and precipitating inside the control valve. Moreover, dissolved H2S also can decompose. You should analyse the greenish black materials. I wonder why the feed water temperature is so high? Normally we maintain a 120-125 Deg C feed water temperature. Which media you are using in the stripper reboiler? If it is steam, it should be low pressure ( 5~ 7 barg) saturated steam. You can try to reduce the feed water temperature gradually to 125 Deg C as a trail and see the effects.