June 2025

Process Optimization

Improve sour water stripper unit performance via feed sour water preconditioning

Sour water stripper (SWS) units in refinery operations are often exposed to several challenges, such as corrosion, low efficiency, decreased capacity, solids fouling and equipment plugging. This article explores some of those challenges.

IP: 10.3.106.231

Sour water stripper (SWS) units in refinery operations are often exposed to several challenges, such as corrosion, low efficiency, decreased capacity, solids fouling and equipment plugging. This article explores some of those challenges. A North American refinery had a history of problems related to the preheat exchanger and stripper tower tray fouling, which inhibited reliable unit operation. To address these issues, an onsite study was conducted to assess the feed sour water contamination profile. The study (soon to be published), concluded that feed contaminant ingress were suspended solids and emulsified hydrocarbons. These were the major root causes of the SWS fouling.  

A feed conditioning system (solids filter and liquid hydrocarbon coalescer) was installed between the sour water tanks and preheat exchangers. The coalescer removed several gallons of liquid hydrocarbons per day. Data shows that installing the filters and coalescer reduced fouling at the preheat exchangers to almost zero. The SWS unit increased the charge rate capacity significantly. In addition, the SWS achieved more consistent ammonia stripping. For these reasons, it is recommended to consider using a feed conditioning system in all refinery SWS units. 

The collected samples from different streams fed to the SWS unit. FIG. 1 displays the samples taken. The various sour water samples showed suspended solids (the presence of visible particles or in some cases haze) and liquid hydrocarbons (the presence of haze, upper dark layer or a dark ring). Sour water feed preparation or preconditioning is critical for the stable and reliable operation of SWS units. Literature indicates that inadequate removal of solids and liquid hydrocarbons in the feed sour water can significantly affect the overall process.¹ 

FIG. 1. Samples from different water streams feeding the SWS unit. 

The refinery’s SWS unit in this article had been experiencing fouling in preheat exchangers and stripping columns for a considerable time. FIG. 2 shows the effects of fouling on the SWS unit trays from the unscheduled outage. The accumulation of solids and other materials is clearly noticeable. 

FIG. 2. Fouling of the SWS trays observed during the unscheduled outage. 

An onsite contamination study was conducted on individual sour water streams and the combined sour water stream feeding the SWS unit. The results suggested that installing a filter and coalescer system would mitigate fouling. Hydrocarbons and solids were present in several of the individual streams and in the combined sour water feed. Suspended solids averaged 10 parts per million by weight (ppm/wt)─25 ppm/wt. Based on this average, approximately 7 pounds per day (lb/d)─10 lb/d of solids were entering the SWS unit. 

Fouling in SWS units can have many causes, including contaminants in the water phase (suspended solids/gels or dissolved components), as well as contaminants in the hydrocarbon phase (emulsions or dissolved components). The fouling process can result from chemical or physical reactions, such as polymerization, solubility changes due to pH fluctuations, quiescent zones leading to solid deposits on surfaces, and among other less common mechanisms. The root cause diagram in FIG. 3 shows some of the major causes involved in SWS unit fouling. 

FIG. 3. Root-cause diagram of SWS unit fouling. 

The refinery installed a sour water feed conditioning system (liquid filter and coalescer) between the sour water tanks and preheat exchangers to address unit fouling and test the system’s performance. The goal of the system was the removal of solids and liquid hydrocarbons from the sour water feed. Several studies indicate that these contaminants often cause fouling and deposits in sour water units.^2,3,4 FIG. 4 shows the location of the feed preconditioning system (filter and coalescer) in the process. 

FIG. 4. Location of sour water feed conditioning system (red box) in the process. 

A study was performed on sour water feed contamination in several streams, in addition to sampling between the sour water holding tanks and preheat exchangers. TABLE 1 details the amounts of liquid hydrocarbon captured during the sampling process. The data in the table was used to size and design the filter and coalescer. During the feed sour water sampling study, close to 1 gallon (g) of liquid hydrocarbon accumulated during the 50-hr test window. The liquid volume in the sour water correlated to about 300 ppmv of hydrocarbon entrained or emulsified in the sour water, which equates to 120 gallons per day (gpd) of hydrocarbon entering the SWS unit. Hydrocarbons not only assist in the fouling process by agglomerating solids and imparting adhesive properties to the resulting material, but also lead to problems in the sulfur recovery unit (SRU), affecting the modified Claus reaction stoichiometry and also the unit’s catalysis. 

The SWS unit operation has seen an increase in stability after the installation of the feed filter and coalescer system. FIG. 5 shows the operation of the SWS unit before the feed preconditioning system was in service, while FIG. 6 shows the operation of the SWS unit in June 2022 after the feed conditioning system was installed. 

FIG. 5. Operation of the SWS before the feed conditioning system was in service. 

FIG. 6. Operation of the SWS after the feed conditioning system was in service. 

When comparing the March and June operations of the SWS, the key differences are in ammonia removal at different flowrates. In March, at rates of 250 gallons per minute (gpm), concentrations of ammonia in the water were significantly higher than in June, when rates were increased. In the graph for June (FIG. 6), the ammonia concentration remained unchanged despite the charge rate peaking at 330 gpm at the end of the month. 

The refinery periodically steamed out the preheat exchanger to combat high ammonia content and its deposits. Operators conduct the steam-outs to regain inlet temperature to the SWS. The primary role of the filter and coalescer system is to prevent fouling at the preheat exchanger. Low fouling rates allow for more consistent ammonia removal in the SWS unit by maintaining inlet temperatures between 197°F and 200°F (92°C and 93°C). FIG. 7 shows data from before the filter and coalescer system was in place. As shown in the figure, the fouling rate caused a 3.4°F loss per day. 

FIG. 7. Pre-heat exchanger inlet temperature before the feed conditioning system was in place. 

The graph in FIG. 8 shows the preheat temperature to the SWS unit after the installation of the filter and coalescer preconditioning system. Using regression analysis, the fouling rate with the filter and coalescer system decreased to a 0.02°F loss per day, or a 0.5°F loss per month. The number of steam-outs of the heat exchanger was reduced from weekly to almost zero after the refinery started operating the sour water feed conditioning system. 

FIG. 8. Pre-heat exchanger inlet temperature after the feed conditioning system was in place. 

The commissioning of the filtration and coalescer system led to a successful removal of liquid hydrocarbon in the sour water feed. The refinery drains hydrocarbon from the coalescer vessel 2 times/d─4 times/d with levels ranging from 40%─100%. The estimated liquid hydrocarbon removal, based on average operational notes and coalescer system volume, ranged between 5 gpd and 20 gpd. This is lower compared to the amount of hydrocarbon in the SWS feed water during the onsite study. This can be attributed to the variability in hydrocarbon content in SWS feed water. 

The onsite study was conducted within a time window of 4 d─5 d, and it is possible that during that interval, hydrocarbon content was much higher. In addition, as the pressure drop across the coalescer vessel increases, the separated liquid hydrocarbon volume decreases. At a pressure drop of > 15 psig, the frequency of hydrocarbon drainage in the vessel decreased to once every 2 d. Liquid coalescer systems are not recommended to operate above a 8 psig─10 psig pressure drop, as liquid coalescing is hindered and hydrocarbon carryover increases. 

The filter and coalescing media lifespan was also evaluated. Upon startup, the filters had a 20-micron pore size filtration media (99.95% efficiency). With the 20-micron media, the filter's greatest pressure drop was only 10 psig, lasting approximately 90 d in operation. Conversely, the coalescer media lasted nearly 45 d with a pressure drop reaching about 15 psig. The filter media was then adjusted and changed to a 10-micron pore size (99.95% efficiency): since then, the overall operational lifespan of the entire system increased to 90 d─120 d. 

FIG. 9 shows the simulated distillation analysis of the hydrocarbons present in the feed sour water collected during onsite testing. The hydrocarbons were consistent with a mixture of kerosene and naphtha by weight. This information is usually instrumental in determining the potential origins of the hydrocarbons from the many upstream units sending water to the SWS. Addressing hydrocarbon carryover at the source is often the best route to lowering the total hydrocarbons in the feed sour water. However, more samples should be analyzed to confirm the actual carbon distribution in the feed sour water and further narrow down the hydrocarbon properties and unit(s) of origin. 

FIG. 9. Simulated distillation of the sour water feed hydrocarbon contamination. 

Takeaway. Sour water stripping operations have become far more reliable since the installation of the sour water feed preconditioning system (liquid filter and coalescer). The data has proven that the filter can stop particulates efficiently, and the coalescer can remove liquid hydrocarbons with a high degree of recovery. Removing both contaminants has significantly reduced the fouling rate in the preheat exchangers to the SWS unit, to the point that operations have stopped performing steam-outs. With the low fouling rates, the SWS unit was also able to increase the sour water charge rate by 30 gpm─50 gpm. The filter system has also increased the flexibility of the unit by improving control of the levels in the sour water stabilization tanks. The feed conditioning system allowed the unit to send water with little to no consequence to other SWS units. 

The installation of a sour water feed conditioning system upstream of the stripper preheat exchanger is highly recommended. Having an effective filter and coalescer system permanently installed to remove particulates and liquid hydrocarbons from the sour water feed significantly improves overall unit performance and reliability, while lowering operational costs, operator maintenance and exposure. In addition, these systems improve stripping performance and asset integrity. Furthermore, the feed conditioning system enhances the operation of the SRU by minimizing hydrocarbons in the sour water acid gas. 

LITERATURE CITED 

1 Engel, D., P. Le Grange, M. Sheilan and B. Spooner, “The seven deadly sins of sour water stripping,” Sulphur Magazine, 2016. 

2 Ridge, C., D. Engel and S. Williams, “SWS fouling evaluation,” Nexo Solutions Internal Report, 2021. 

3 Engel, D., “Reducing hydrocarbons and solids contamination in sour water strippers,” Hydrocarbon Processing, 2013. 

4 Engel, D., “Reducing hydrocarbons in SWS acid gas,” Sulphur Journal, 2012. 

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