Hi,
We issued Functional Specification to supplier of the Gas Processing Plant which includes, ‘Produced Water Treatment (PWT) Unit'. In the functional spec, we specified that design of the PWT unit is the responsibility of the supplier and unit design shall meet max 10 mg/lit Oil in water and negligible amount of H2S specification in the treated water going to evaporation pond.
Feed to the Slug Catcher is light (C1-C2 =83 mol%; C3-C5= 5 mol%; C6-C8= 2 mol%, C9+ =3 mol%, Co2 =3 mol%, H2S =2mol%, Water =2 mol%). Properties of C9+ are - density=859kg/m3, mol wt=249). Slug Catcher is sized as 3 Phase separator with Weir and boot. Produced water from the boot of the Slug Catcher is sent to PWT unit.
Treated water downstream of the PWT is simply stored and send to Evaporation pond.
Proposed vendor design of the PWT unit is shown in the attached sketch. The unit is provided with 2 phase Produced water surge drum (or degasser) to de-gas the flashed gas generated during pressure let down from 60 barg – 10 barg followed by packed bed Stripper Column to strip the dissolved H2S present in the water using Fuel Gas. During HAZOP, we discussed the possibility of Oil (HC Condensate) carryover from boot of the Slug Catcher to PWT unit and put the recommendation to Supplier to review his design.
Supplier has confirmed that no oil should occur in the Produced Water Surge Drum. Controls on the water boots of the Feed Gas Slug Catchers have a long history of successful performance provided the plant is operated within design parameters. The also confirmed that the treated water will not contain more than 10 mg/l of “oil”. However, the treated water may contain “dissolved” methane and ethane in quantities greater than 10 mg/l. Supplier’s understanding of “oil” in water is free oil that would float on the water, causing a sheen which might lead to environmental harm. Supplier states that in the event oil is carried over to the Produced Water Surge Drum, it could be removed by draining the vessel through the 2 inch gate valve in the bottom of the vessel.
We feel that present supplier design may not be adequate and secondary treatment such as CPI (Corrugated Plate pack Interceptor) / IGF (Induced Gas Floatation) / Nutshell Filters may is necessary. However, Supplier is confident about his design and not going to add anything / change anything at this stage.
Request to share your experience of Gas Field with HC Condensate (and not crude Oil).
Attached Files
Sketch of PWT.pdf 260.33KB
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