February 2021

Process Optimization

Specifying internals in sour water strippers—Part 1

A sour water stripper (SWS) system is a common process in petroleum refineries and other processes where hydrogen sulfide is present.

A sour water stripper (SWS) system is a common process in petroleum refineries and other processes where hydrogen sulfide (H₂S) is present. While not a revenue generator, the sour water treating system is a critical unit operation and can be a significant bottleneck to facility production rates if it is not adequately sized or if it is forced to operate at partial loads due to maintenance issues. As a result, a balance must be struck between minimizing capital costs, while still providing a reliable and flexible sour water treating system.

Part 1 provides an overview of the auxiliary separation equipment needed to remove hydrocarbons and other contaminants from the sour water prior to the stripper and reviews the design of SWS columns containing trays. In Part 2, which will be published in the March issue of Hydrocarbon Processing, the internals for packed SWS columns will be discussed, along with a summary of potential issues that may be encountered in operation of the SWS system.

SWS systems

The SWS system receives sour water from different upstream unit operations, which in a petroleum refinery may include crude units, hydrocrackers, hydrotreaters, catalytic crackers, etc. The sour water streams from each of these unit operations will vary in composition but will generally have some fraction of ammonia (NH₃) and H₂S present in solution. This article considers SWSs that have NH₃ and H₂S as the primary species to be removed; it excludes consideration of other species, such as cyanides, phenol, etc. All recommendations given are in this context.

The SWS system collects the sour water streams from different unit operations, removes hydrocarbons, solids, etc., and removes the NH₃ and H₂S from the water by heating and stripping. The liberated NH₃ and H₂S, along with a large fraction of water, flow to downstream unit operations as a vapor for further treatment. The stripped water may be disposed of as wastewater, or if it meets specifications, it may be used in other process units in the refinery, such as the crude oil desalter. A typical, simple SWS process flow diagram is shown in FIG. 1.

FIG. 1. Simplified process flow diagram for a SWS.

Different variations of the process flow shown in FIG. 1. exist. Two frequently encountered differences are:

1. The addition of live steam into the column instead of a steam reboiler. Live steam will not foul or have maintenance issues that would be associated with the steam reboiler in an SWS, but all of the steam introduced into the stripper will need to be made up in the facility’s steam system with fresh steam and additional stripped water will need to be disposed of.
2. A pumparound system in the top of the SWS instead of the conventional overhead condenser and reflux drum. In this design, a stream of water from the stripper is cooled and pumped to the top of the SWS to maintain the overheads temperature from the stripper at the same temperature it would be leaving the reflux drum in the conventional design. This design avoids the need for the stripper overhead condenser, which can be an expensive and maintenance-intensive piece of equipment. The downside to this option is that additional height is needed in the SWS for the cooling section, and the liquid pumparound equipment is made of upgraded metallurgy.

The SWS and associated equipment are not typically revenue generators in any facility; however, the unit operation is critical to the rest of the facility’s operation since most of the sour water in the facility has to be treated in the SWS before it can be reused or processed further. The sour water fed to the SWS will also change over time, with increasing or decreasing amounts of NH₃ and H₂S in the water and overall water flowrates varying daily. The designer of the SWS is challenged to design a flexible and robust system that can meet a variety of different feed conditions, while also minimizing the cost of the equipment. Above all, the SWS cannot be a bottleneck in the overall facility and must strip the sour water reliably in all operating conditions.

There is a long history of technical papers that thoroughly discuss many aspects of SWSs.^1–6 This article is not meant to be a comprehensive review of SWSs. This article reviews a few of the key design choices available for the SWS system, and then specifically focuses on some of the internals of the SWS tower. The choice of internals in the SWS can be difficult, with a range of different sources available in literature, and few very thorough technical analyses completed to guide the designer to the best solution.

Auxiliary sour water separation equipment

For the SWS tower—and the internals discussed later in this article—to function properly, they must not foul too quickly. Sour water stripping is generally considered a severe fouling service. The stripper functions much better if the chances for fouling and foaming are reduced by adequate pretreatment of the sour water. Therefore, this section examines the equipment upstream of the SWS that reduces fouling and foaming issues in the stripper tower.

Sour water flash drum. As shown in FIG. 1, sour water is collected in a flash drum where hydrocarbon vapors and liquids are removed. The vapors are flashed close to ambient conditions to remove as much hydrocarbon as possible. The flashed gas is typically sent to a low-pressure destination such as a flare gas recovery system, combustion device or fuel gas as allowed by environmental regulations. At some sites, the flash gas is routed to the SWS overhead gas line; however, this can result in a significant and variable quantity of hydrocarbons being fed to the downstream unit [e.g., a sulfur recovery unit (SRU) or other technology] that can adversely impact performance of that downstream unit.⁷ Flash gas with no condensable hydrocarbons could possibly be routed to the quench tower in an amine tail gas treating unit.^7,8

The sour water fed to the flash drum often also contains liquid hydrocarbon/oil that needs to be removed to protect the rest of the SWS system from fouling and prevent foaming in the stripping column. The flash drum is usually a three-phase, horizontal vessel. A baffle system installed at one end of the flash drum is often used to skim oil from the water before it is pumped to the sour water surge tank. The oil overflows the weir into a collection compartment in the sour water flash drum for removal. Another means of collecting oil is to install a draw-off box in the sour water flash drum that could collect the oil overflowing to it. The minimum recommended residence time for the sour water inside the flash drum is 20 min, with a liquid level of 50%–60% being optimal. The sour water flash drum should include connections for level bridles on the hydrocarbon and water side of the vessel. High- and low-level alarms and pressure indication are also used. Demisting equipment or other similar plugging-prone internals are typically not used in the sour water flash drum because they may rapidly plug or corrode. The hydrocarbons collected in the sour water flash drum are often pumped to a slop system for further processing.

Sour water surge/storage tank. The sour water from the flash drum is fed to a surge/storage tank. The tank is designed with several days of storage in case the SWS goes down. With long residence times, dissolved hydrocarbon liquid and emulsions can separate from the water and collect at the interface level in the tank. The temperature of the sour water in the tank is usually less than in the flash drum, which further reduces the solubility of hydrocarbons in the sour water. Some in the industry have also observed that NH₃ or amine-laden water increases the solubility of certain types of gasoline and higher boiling range aromatic hydrocarbons (similar to benzene) in the sour water, making it difficult to separate such that significant fouling was observed in the stripper.⁶ The sour water surge tank may not remove all hydrocarbons that remain in the sour water after the surge drum, but the tank does help by removing at least some of them.

The surge tank also allows for mixing of the sour water from different time periods, so the composition is more uniform. If the sour water composition changes considerably or rapidly, the stripper may not function appropriately. By keeping the sour water flow and feed composition consistent, the stripper will be easier to control, and a more consistent treated water product can be achieved. Short-circuiting—where the sour water entering the tank inlet flows preferentially to the tank outlet without adequate mixing or residence time—is a common problem that results in higher variability in sour water composition and poor hydrocarbon separation. In the question-and-answer portion of a recent industry trade symposium⁹, measures that were said to mitigate short circuiting included (1) having the entry and exit on opposite sides of the tank and (2) having the entry and exit at different heights.

The surge tank can be a fixed or floating-roof-type storage tank. Floating roofs can be either open or internal. However, due to the potential for odors, a fixed-roof tank is often used. FIG. 2 shows an example of a surge tank with a fixed roof and internal floating roof. The floating roof may have a double-seal design to minimize emissions.

FIG. 2. Simplified schematic of a sour water surge tank (internal floating roof).

Vacuum breakers and pressure relief valves should be installed on fixed-roof tanks that are not vented to the atmosphere. By letting air in, vacuum breakers can keep the tank from collapsing during pump-out or upon cooling; however, air ingress can lead to the formation of a dangerous combination of oxygen, hydrocarbons and H₂S in the tank headspace that could lead to an explosion. Nitrogen or inert gas blanketing is often used for this reason. However, inert blanketing has its own problems. For example, using an inert blanket can lead to the formation of pyrophoric iron sulfides on exposed steel surfaces. If air is subsequently allowed into the headspace (e.g., due to a fault in the system or due to accident) and thus creates an explosive mixture, then the pyrophoric material can ignite that mixture and cause an explosion. Examples of explosions that have happened in SWS storage tanks are documented in literature.¹⁰ Much care is advised in designing inert blanketing systems for sour water tanks.

The tank may need to have a minimum of three days retention time during normal operation at 50%–60% full, including another couple of days of capacity for sour water storage. Whether the tank has a fixed or floating roof, it is common to allow a hydrocarbon layer to float above the sour water as a “blanket” to limit vapors from escaping that may be odorous or toxic. This layer may be a diesel range material and is sometimes also referred to as a rag layer. Oil skims should be used to remove oil as the floating layer grows. Floating skim nozzles with a non-metallic flexible hose are sometimes used. The design should include an automatic tank level gauge system, with provisions for measuring the thickness of the hydrocarbon rag layer on the aqueous layer, as well. A literature source¹¹ reports that nuclear signals or sound waves can be used to measure the interface, but the authors’ company is aware of successful measurement using capacitance probes, as well. In a floating-roof tank, the capacitance probe can be mounted on the floating tank roof. Level control is critical to minimize hydrocarbon carryover to the SWS; the location of the control devices is vital to accurately measure the interface.

The tank is typically made from carbon steel, and a suitable durable coating may be used on all interior surfaces to minimize corrosion of the tank surfaces.

Solids and heavy oils will sink to the bottom of the tank. For this reason, the tank bottom should be designed to slope (e.g., approximately 3 in. for every 100 in.) to a low point drain. The tank discharge to the pump is also generally elevated somewhat above the tank bottom to allow room for heavy materials to accumulate without exiting the tank with the sour water. The sour water is pumped using flow control to the stripper.

Angled ports are sometimes installed on the sour water tank so that it can be vigorously circulated (e.g., with a large portable pump) to stir up solids and then filter the solids out during turnarounds. This reduces the frequency with which persons will have to go inside the tank to clean it out. Images of the usually uninsulated exterior walls of the tank from a thermal camera can sometimes be used to evaluate solids levels. In addition, the surge tank should have a bypass line around it so that it can be bypassed (e.g., for inspection), if needed.

Sour water solids filtration/coalescing filters. Additional solids filtration and coalescing technology may be installed downstream of the sour water charge pump and upstream of the feed/effluent exchanger. Solid particle filters should be used upstream of a liquid/liquid coalescer. Suspended solids removal improves the efficiency of the coalescer by weakening the hydrocarbon emulsion and minimizes fouling from solids in the sour water heat exchangers, stripper reboiler (if used) and stripper trays or packing. Some refineries reportedly have used a strainer instead of a more expensive filter.

The liquid/liquid coalescer helps to control hydrocarbon fouling in the same sour water equipment. Disposable, microfiber-based coalescers are reported to give adequate separation of hydrocarbon emulsions.¹¹ During a recent industry symposium⁹, use of liquid/liquid coalescers for partial treatment of the feed sour water was reported to increase the time between cleaning from once every 9 mos to twice that length in a refinery with three SWSs.

Hydrocarbons in the stripper overhead gas can also cause operational issues in the downstream SRU or other processing technology. Hydrocarbons in the stripper bottoms that are routed to a water treatment plant can pose environmental/regulatory concerns, as well. Therefore, using filters and liquid coalescers can benefit not only the SWS but also overall refinery operations.

SWS diameter and feed water feed locations

Many SWSs experience severe foaming, which needs to be accounted for when sizing the column. As such, the capacity should be de-rated to account for foaming, and a system factor of 0.6–0.7 is typically recommended. This can make the SWS much wider in diameter than would be anticipated for a column that, at least on first appearance, is basically boiling water.

The location of the sour water feed in the stripper can vary based on several factors, including whether trays or packing are used, number of trays used, the desire for lower steam usage, inlet H₂S and NH₃ concentration and treatment specification, as well as operating temperature and pressure. If a pumparound cooling system is used in lieu of an overhead condenser and reflux drum, the feed location will be below these trays, as well. Optimal feed location can be determined in a process simulation, and the feed location is usually located within the top several trays in trayed columns. In addition, if the column is constructed from carbon steel, it may be lined with a corrosion-resistant durable coating or made of corrosion-resistant alloy above the liquid feed nozzle, where corrosion is more significant.

Tray tower design for SWS

Most SWS systems are designed with trayed towers. Trays can be designed to be fouling resistant. However, even in trayed systems, the selection of an inappropriate tray can lead to poor performance of the SWS. General recommendations for SWS tray selection include:

• Trays should be a fixed-valve type and should be designed for vapor to flow horizontally out of the valves to minimize bridging of deposits on the fixed valves. Tray designs like this are readily available from major distillation internals vendors. Sieve trayscan also be fouling resistant in some services. For example, the authors know of acceptable sieve tray use in aqueous systems with solid particles circulating (i.e., in slurry service). However, sieve trays have shown severe fouling in SWS service, with vapor flow area decreasing by as much as 90%. This may be due to the vertical direction of the vapor leaving the tray deck, which allows precipitation on the tray deck that can foul the tray.^12,13 FIG. 3 shows an example of fouling that can occur on sieve trays in SWS service. This level of fouling occurred over a typical SWS run between maintenance intervals of 5 mos.¹³
  
  

  FIG. 3. Fouling of sieve tray in SWS service.¹³
• All trays should be constructed of 300-series stainless steel or better. Depending on the sour water processing demand, the tower may be too small for personnel to physically install the trays. In this instance, cartridge trays could be used.
• If a pumparound system is installed, the trays used for the pumparound loop should not be counted as active mass transfer trays.
• In a fouling service like with an SWS, the downcomers are potential traps for fouling material and can adversely affect the capacity of a tray. Special designs that are available from the internals suppliers to address fouling material in the downcomers should be used.

Tray efficiency is reported in several different ranges for SWS service, but generally will vary from 15%–50% depending on different factors. The number of trays present in the SWS will then also vary widely; a common range on the number of actual trays installed may be 20–60. On a 24-in. spacing, this translates to 40 ft–120 ft of height for trays, which may mean an SWS as tall as 150 ft in some applications.

From the authors’ discussions with a few refinery subject matter experts (SMEs), a rough rule of thumb for design tray efficiency in an SWS is 3 actual trays per 1 theoretical stage or 33% efficiency. This is probably a conservatively low efficiency for most systems. For example, one SME acknowledged this rule of thumb, but noted that actual tray efficiencies experienced in sour water service (presumably well designed) were closer to 50%. In designing a trayed system, one could probably rely on the rule of thumb to result in a system with significant over-design built in. For a less conservative and more economical design, careful engineering analysis and comparison with the actual performance of other similar SWS systems is needed.

Some factors that influence the efficiency of the trays include the following:

• Perhaps most importantly, tray efficiency is a chemical engineering factor that is applied to equilibrium-based designs to account for the fact that operating trays do not reach equilibrium conditions. Hatcher and Weiland¹⁴ show that component efficiencies for H₂S and NH₃ will vary widely across the stripper column, and could depend heavily upon the stripped water specification for the water leaving the bottom of the stripper, the steam rate to the stripper or reboiler, etc. Therefore, the efficiency of the tray is not a static value throughout the stripper, varies from one component to another and may be different in the top of the tower than it is in the bottom. To reduce uncertainty, the designer may need to do a more rigorous simulation of the column.
• The most important consideration for SWSs is that they work reliably. As a result, designs for SWSs tend to be conservative. One way of introducing conservatism into the SWS design is to specify a low tray efficiency that, when installed, will allow the stripper to operate and meet specifications in a more heavily fouled state and to meet specifications if the impurities present in the sour water exceed the initial design values. If there is access to an existing stripper in the same service, then operating data can be obtained to verify the design parameters.
• In many instances, the actual composition of the sour water feeding the SWS system may be uncertain. Crude oil slates in a refinery can change frequently, with the nitrogen and sulfur contents of the different hydrocarbon changing over time as the refinery processes different crudes, or different unit operations are added to the refinery. Ideally, the SWS can handle most or all these changes without major modifications to the stripper. A conservative estimate of tray efficiency will provide more flexibility in the design to account for the uncertainty of the feed composition.

As previously mentioned, numerous SMEs use an initial rule-of-thumb tray efficiency of 33%, or three actual trays in the SWS for every equilibrium stage in the process simulation. To further refine the cost estimate or proceed with detailed design, it may be prudent to build a mass-transfer rate model of the SWS. This can be more easily done once the column internals have been selected, since accurate information about the tray—such as weir height, active tray area, etc.—are critical to building an accurate mass-transfer rate model. Reliable estimates of the sour water composition will also be necessary to help ensure the appropriateness of the SWS design.

Another important factor in the design of the column is tray hydraulics. The actual hydraulics on the tray is dependent on the tray device such as fixed-valve trays. The number of valves and size of the opening is important to maintain liquid on the tray and get proper contacting of the vapor and liquid; thus, the proper operating range for the design becomes important. If the trays are overdesigned, then the tray may weep or dump liquid, resulting in poor operation. The design must account for the low end, as well as the high end of operations. One reason 24-in. tray spacing is often used is to give more capacity, especially when fouling or foaming is expected.

Even when the designer is confident in the design of the column, some additional precautions are recommended. These include:

• NH₃ will be the more difficult component to remove in most sour water streams. NH₃ has a high affinity for water and will almost always strip out of the sour water after the H₂S is almost completely removed. It is possible to reach a stripped water condition where the remaining NH₃ is fixed in the stripped water, meaning that it is bound to a non-volatile or strong acid in the stripped water and will not come out of solution regardless of the energy input into the bottom of the column. In this case, it is prudent to install a nozzle in the lower section of the column to allow for caustic addition, if necessary, under some or all conditions. The strong base will displace the NH₃ and allow it to be more easily stripped from the column. By placing the nozzle in the lower section of the tower, the caustic will not interfere with H₂S stripping.
• Although trays can be designed for fouling service, some reduction in efficiency will likely be noticed over time. Even with adequate solids removal and hydrocarbon phase removal, some fraction of these materials will enter the column periodically. Some slightly-water-soluble hydrocarbons may enter the tower and precipitate in the lower section of the SWS as the water heats to near boiling. Other salts may be present in the water that precipitate in the higher temperature areas of the SWS. Adequate access to the column for quick maintenance and some additional design margin may be prudent to address fouling concerns. FIG. 4. shows an example of fouling that can occur in SWS service over a 5-mos period of operation, which corresponded to the 10%–15% reduction in vapor flow area noted by the authors.^12,13
  
  

  FIG. 4. Example of tray fouling in SWS service.¹³

Takeaways

Stripping sour water is a demanding process in a refinery or gas treating facility. The sour water will contain a multitude of contaminants in addition to the NH₃and H₂S stripped out of the water in the process. These contaminants make reliable operation of the SWS a challenge, but one that can be realized with appropriate design of the SWS and the equipment that surrounds it. Proper sizing and level control of the three-phase separators in the sour water system are critical to removing contaminants—such as hydrocarbons—that can severely impact SWS performance by causing fouling and foaming in the column. Solids filtration and liquid/liquid coalescing equipment should also be considered as additional means to further clean the sour water prior to the stripper. The SWS needs to be designed to handle variations in inlet feed composition and flowrates, as well as allow a margin for fouling and foaming. The selection of tray internals should consider the severity of the service and the presence (or absence) of good sour water cleanup steps prior to the stripper. The tray design should take into consideration fouling, efficiency and hydraulics, among other factors.

Part 2

Part 2 discusses the design of packed tower SWS and details operating problems that can occur in an SWS system.

NOTE

This article was originally prepared for the Brimstone Sulfur Recovery Symposium. It has been edited and separated into two parts for publication in Hydrocarbon Processing.

LITERATURE CITED

1. B. Scott, “Processes for Treating Refinery Sour Waters,” Brimstone Sulfur Recovery Symposium, Vail, CO, 1995
2. Asquith and A. Moore, “Sour Water Processing—Balancing Needs,” Brimstone Sulfur Recovery Symposium, Vail, CO, 2000
3. D. Stevens, A. Mosher and D. Ogg, “Fundamentals of Sour Water Stripping,” Brimstone Sulfur Recovery Symposium, Vail, CO, 2007
4. J. Stavros and A. Keller, “Reducing Nitrates in Refinery Wastewater,” Brimstone Sulfur Recovery Symposium, Vail, CO, 2013
5. N. Hatcher, R. Alvis and R. Weiland, “Sour Water Stripper Performance in the Prescence of Heat Stable Salts,” Brimstone Sulfur Recovery Symposium, Vail, CO, 2012
6. A. Keller, “Fundamentals of Sour Water Strippers,” Brimstone Sulfur Recovery Symposium, Vail, CO, 2015
7. P. Le Grange, “Operational Challenges in Sour Water Stripping,” Digital Refining, online: www.digitalrefining.com/article/1002335, July 2009
8. D. Stevens and A. Mosher, “Fundamentals of Sour Water Stripping,” Brimstone Sulfur Recovery Symposium, Vail, CO, 2008
9. McIntush, K., J. Farone, B. Piggot and C. Beitler, “Specifying internals in sour water strippers,” Brimstone Sulfur Recovery Symposium (Virtual), 2020
10. American Fuel & Petrochemical Manufacturers, “AFPM Process Safety Bulletin—Hazards of Purged Tanks—Formation of Pyrophoric Iron Sulfide in Low Oxygen Environments,” No. 14-01, 2014
11. B. Spooner, “Reduce Hydrocarbons and Solids Contamination in Sour Water Strippers,” VWR International, 2013
12. R. Hauser and R. T. Kirkey, “Refinery Tests Demonstrate Fixed Valve Trays Improve Performance in Sour Water Stripper,” New Orleans, LA, AIChE, 2003
13. F. Bela, Comprimo, “SWS Fixed Valve Trays,” 2003
14. N. Hatcher and R. Weiland, “Reliable Design of Sour Water Strippers,” PTQ, September 2012

The Authors

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