Typical PFD for Sour Water Stripper Column

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Typical PFD for Sour Water Stripper Column In upstream crude oil production and processing f acilities, the produced water f rom oil wells is separated in the slug catcher. Generally this produced water contains salts, hydrogen sulphide, carbon dioxide etc. Salts are removed using a desalting unit. Due to presence of acidic gases present in the produced water, sometimes it is called as sour water. For removal of these gases such as H2S, CO2 etc, the sour water is sent to sour water stripper columns. Even in crude oil ref ining f acilities, steam and water are of ten used in many units such as atmospheric distillation unit, naphtha hydrotreater unit, kerosene hydrotreater etc. T hese units produce sour water as a result of steam/ water coming in contact with sulphur products in the hydrocarbons. T his sour water is also then sent to sour water stripper columns f or H2S removal. T he H2S removed is sent to sulphur recovery unit to produces liquid sulphur.

Figure 1 – Typical PFD f or a sour water stripper column

Typical process f low in a sour wat er st ripping unit Typically a sour water f low in crude oil processing f acility or in a ref inery is not steady. Sour water is gathered f rom a number of dif f erent sources and treated in a single column. For upstream oil production and storage f acilities, the produced water comes along with crude oil in a pipeline. During pigging of these pipelines vast quantities of sour water are produced in the slug catcher, which may f ar exceed the sour water stripper f low handling capacity. Hence a buf f er vessel is generally provided upstream to the stripper column to accommodate extra sour water quantities in case of a peak e.g. pigging.

Sour water f rom buf f er vessel is sent to the sour water stripper column under level control. In case the f low to the column sees major f luctuations which may f requently disturb the column operation, f low control valve on the column inlet water line can be used with a cascading signal f rom buf f er vessel level control. Gases f lashed in the buf f er vessel can be f lared or sent to the sulphur recovery unit, if they are rich in H2S. T he stripper column may use either steam or f uel gas to strip of f the H2S dissolved in water. Water is introduced f rom top of the packing or trays section and steam/f uel gas f lows upwards f rom the bottom of the trays/packing section as shown in the typical PFD in f igure-1. Contact between water and vapor phase allows transf er of H2S f rom water to the gas phase (steam or f uel gas). Gases rich in H2S are removed f rom the top of the column and treated water is taken out f rom the bottom of the column, under level control f or the liquid level of the bottom section of stripper column. If steam is used in the stripper column f or removing H2S, then overhead condenser and ref lux drum are required at the top outlet of the column as shown in typical PFD in f igure-1. T he vapor outlet line of the stripper column is also equipped with a pressure control valve to maintain the sour water stripper column overhead pressure. Steam used can be either taken directly f rom the utility steam and sent to column or the treated water f rom bottom of the column can be sent to a reboiler to produce steam which is circulated back to the column f or H2S removal. If f uel gas is used instead of steam to remove H2S, reboilers, condensers, ref lux drums etc. are not required. But the gases f rom column overhead outlet have a high hydrocarbon content. An analyzer is installed to detect the H2S content in the sour water stripper outlet. T his analyzer generally transmits signal to the control room and steam/f uel gas f low can be adjusted f rom the control room in order keep the H2S content in treated water well below the allowable H2S content limit. Alternatively, a controller can be used to directly throttle the steam/f uel gas f low based on a setpoint of H2S content in the treated water.