What steps do we need to take to keep our sour water stripper at peak performance following a shutdown?
Responses to a question in the Q1 2021 issues Q&A Feature
Various from Axens & Sulzer
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Shutdown operations of the sour water stripper unit should maximise the removal of H2S and NH3 from water, especially for long duration shutdowns, to minimise the safety concerns and corrosion issues during maintenance. If the sulphur recovery unit is not available, the sour gas should be routed to flare, then start the shutdown by decreasing the sour water feed to the unit manually and slowly.
If applicable, by-pass the storage tank, stop the stripped water pumps, and open the start-up recirculation line from stripped water outlet to surge drum. Demineralised water can be introduced into the surge drum to maintain the flow. As H2S and NH3 content in the sour water in the closed loop will start to decrease, sour gas production will be reduced accordingly and should be diverted to the sour flare, if not done previously.
Maintain the temperature of the bottom section of the sour water stripper by keeping steam supply. Continue stripping the water. As H2S and NH3 content in the water decreases in the circulating water, the stripper pressure will start to decrease and the flare pressure control valve will continuously close to maintain the stripper overhead pressure. If required, the stripper can be manually pressurised slightly using nitrogen. Continue water circulation until the water no longer contains any H2S and NH3, approximately two hours. Shut the unit down and, depending on scheduled activities, keep the unit in stand-by with a slight pressure of nitrogen (failing that, by opening the flare PV to float on flare header pressure if nitrogen is not available), or drain all equipment using nitrogen for pushing, depressurise and inert the unit, or prepare the unit for safe entry. During the shutdown procedure, care should be taken to not admit air into the system until all hydrocarbons/acid gases are removed from the system and it is thoroughly washed.
Is this a scheduled or unscheduled shutdown? The peak performance of the unit is a function of the condition of the column internals (no mechanical damage or fouling).
For a start-up following an unscheduled shutdown:
1. Do not try to re-establish 100% of the feed flow rate when the column is hot by injecting cold feed (there may not be any bottom product for the feed/bottom exchanger). Large quantities of cold feed can cause sudden condensation of the vapour, creating a local vacuum. Sudden vacuum can cause trays around the feed injection to collapse.
2. If the bottom liquid level exceeds the level indicator, shut off the heat source to the reboiler.
a. It is important that the bottom liquid level never reaches the reboiler return. If it reaches the reboiler return, it will generate severe entrainment and uplift forces to the bottom column internals (trays or packing) that can damage them.
b. While trying to re-establish the bottom liquid level, never exceed the maximum liquid flow rate of the trays in the column.
c. If the liquid level reaches the trays and the liquid is drawn at a faster rate than the trays can drain, the liquid column above the trays can produce a mechanical failure of the trays.
3. If the column liquid inventory requires make-up, you can use feed as long as the flow rate never exceeds the maximum and the column is not hot.
For scheduled shutdowns:
1. Make sure the feed in the sour water tank has been skimmed to remove the maximum amount of hydrocarbons. Hydrocarbons in the feed create foaming in the column and contaminate the sour gas with hydrocarbons, increasing air consumption in the sulphur plant.
2. Follow the unit start-up procedure.
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