Please suggest ways of reducing steam demand in our sour water stripper.Jun-2021
Marcello Ferrara, ITW Technologies, email@example.com
Sour water stripping is considered a ‘fouled service’, in particular for phenolic sour water. Among the foulants, the ones that have the greatest impact on steam consumption are hydrocarbons and polymers.
The most common fouling is hydrocarbon based, which occurs when large volumes of hydrocarbons are carried over into SWS feed. Hydrocarbons tend to agglomerate inside the equipment and form a sludge that accumulates in the heat exchangers and in the bottom of the tower, thereby directly impacting steam consumption.
It is also common for phenolic water (for instance, from the FCC unit and DCU) to contain polymer precursors which create polymeric fouling inside the unit.
In general, polymer fouling may include: amine degradation and oxidation products; red oil fouling (which frequently occurs in olefin caustic treatment); peroxides formed by the oxidation of various components (which are initiators of polymerisation reactions); olefins, in particular conjugated diolefins (which form polymers or gums in the presence of oxygen traces); thiophenes and thiophenols (which may form gum-like substances in the presence of oxygen and diolefins) and phenols (which form insoluble crosslinked polyphenols in the presence of corrosion products).
Polymeric fouling will seriously contribute not only to increased steam consumption, but to unit performance by creating high delta P.
ITW can address both of the sludge and polymeric fouling issues by applying patented ITW Online Cleaning or ITW Onstream Cleaning technologies.
Al Keller, Becht, firstname.lastname@example.org
Steam demand in a sour water stripper (SWS) can be reduced without affecting stripped sour water quality by reviewing a number of key operating parameters including:
• Sour water generation: are you generating excessive sour water or processing water in your SWS that does not require treatment? If any water going to sour water meets water treatment plant inlet specifications for ammonia, COD, and BOD, it is not necessary to send it to sour water stripper.
In addition to optimising exchanger water wash systems, if possible use low level ammonia containing water as makeup water to water washes which will increase the ammonia level. For instance, use 100 wtppm ammonia water as makeup for a water wash where the ammonia level will go up to 5000 wtppm.
• System fouling: a stripper consuming above design steam to meet spec is most likely suffering from a fouled feed effluent exchanger, fouled trays/packing, or contaminant load has increased. Simulating the tower and comparing actual pressures, temperatures and flows to design will help identify areas to optimise.
Crude or other process changes may lead to increased fouling. If fouling is detected from significant process change, routine mechanical cleaning of the reboiler and stripped water side of the feed/effluent exchangers might become necessary. Chemical cleaning may not be effective with some heavy hydrocarbon fouling materials.
Keep hard water (cooling water, fire water, city water) out of the SWS system. These contain water hardness (calcium and silicon) that can affect SWS trays and reboilers.
• Feed contaminants: the amount of ammonia allowed in the stripped water usually sets the amount of steam required in the stripper. The ability to strip ammonia from water can be affected by the level of contaminants in the sour water such as strong acids. Ammonium salts of strong acid anions cannot be decomposed by steam stripping at SWS conditions. To determine if this is happening, conduct a complete anion and cation ion chromatography scan of the sour water and compare the strong acid anion charge equivalents (formate, acetate, chloride) to cation equivalents. If anions exceed cations on a charge equivalent basis, caustic addition may be needed to convert ammonium salts to sodium salts. Measure ammonia in the bottoms with cation ion chromatography. Set the steam rate to achieve the desired ammonia level by IC. Amines used for neutralisation or from amine treating purges/upsets may interfere with field test kits based on colorimetric chemical methods. pH measurements do not give any indication of ammonia content due to presence of many different anions and cations. Do not control by pH.
• Phenolic and non-phenolic water: for systems that process both phenolic and non-phenolic water, consider using a separate SWS to process these streams. Phenolic water is usually low in ammonia and takes less steam per gallon to meet ammonia specs. Non-phenolic requires more steam per gallon, but the volumes are generally lower.
Simon Calverley, KBC, Simon.Calverley@kbc.global
Sour water strippers are typically stripped with either direct steam injection or via a steam heated reboiler. The main specifications are typically on H2S and NH3 in the stripped sour water. Modern design can produce a stripped water effluent that contains less than 10 ppmw of free NH3 and less than 1 ppmw H2S. Best practice designs will use about 0.8-1 lb of steam to the reboiler per gallon of sour water to achieve those levels. Over-stripping or over-refluxing the tower (whether overhead or via a top pumparound) will lead to higher steam rates, thus these are potential ways to reduce steam. Reducing the sour water load on the column (reducing the sour water produced) will require less steam to meet the specifications, but may not be appropriate for the operation of upstream units. A simulation of the sour water stripper can help with determining the amount of steam required to achieve the desired specifications.
Chris Claesen, Nalco Water, Chris.Claesen@ecolab.com
If some fouling is taking place in the sour water stripper this can lead to increased demand of steam to reach the desired stripped sour water quality. In such a case the use of a Nalco Water sour water stripper antifoulant programme can help control the fouling and reduce the steam demand. In severe cases and if enough sour water tankage capacity is available and recycling of that material is possible, a quick off-line cleaning can help to recover SWS efficiency.
All the water streams going to the sour water stripper should be evaluated. In some cases streams can bypass the sour water stripper and for example be sent directly to the desalter washwater. This can sometimes lead to a significant reduction in steam use but all the effects of such a bypass should be properly evaluated and the effect on ammonia and amine recycle on the CDU overhead salt formation should be checked with an ionic modelling program such as Nalco Water Pathfinder.
In some cases better removal efficiencies can be achieved with an improved stripped sour water pH adjustment to reach sour water NH3 and H2S targets