TODAY'S REFINERY
REFINING DETAILS NOTEBOOK
SOUR WATER STRIPPING
Today's Refinery / June 1996
$10.00 per copy
SOUR WATER STRIPPING Tim Armstrong, Bruce Scott, Kin Taylor and Art Gardner Sour water strippers do not enjoy the same constant feed rate and composition that most refinery hydrocarbon processes thrive on. Sour water systems have been described as garbage disposals or toilets of refineries. Any water soluble waste, whether produced continuously, intermittently or in a slug will be dumped into this system. Becau se sour water is produced from most refinery processes, a refinery processing sour crudes will supply sour water from such units as: atmospheric crude columns, vacuum crude towers, hydrodesulfurization (HDS) units, steam crackers and fluid catalytic cracking (FCC) units. Sour water is considered to be any refinery process water containing hydrogen sulfide. Sour water may also contain ammonia, phenol and cyanide. Concentrations of both hydrogen sulfide and ammonia contaminants are highest in waters from HDS a nd FCC units. SOUR WATER SOURCES: Atmospheric crude columns and vacuum crude towers produce sour water from condensed stripping steam removed by overhead condensing systems. Also, vacuum towers produce sour water from equipment, such as ejectors and barometric condensers, designed to maintain proper operating vacuum. Thermal and catalytic cracking units produce sour water as condensates from steam used in injection, stripping and aeration. Heavy viscous feeds, rich in sulfur, produce high hydrogen sulfide concentrations when hydrogenated. Ammonia is produced from hydrogenation of organic nitrogen compounds. Another major source of sour water is hydrotreater wash water. As more sulfur is removed to meet more stringent requirements, more nitrogen is converted to ammonia, which accumulates in the wash water. Phenols are produced from reactions between steam and cyclic hydrocarbons. Demineralized water may be added to the FCC gas concentration unit so that there is sufficient water to dissolve and carry off hydrogen sulfide, ammonia and cyanides and so that hydrogen sulfide and ammonia partial pressures are decreased reducing the potential for hydrogen blistering and corrosion. Because of the nature of cracked products, condensates can form stable chemical emulsions. Some additional sour water is defined as hot condensates that have only accidentally contacted hydrocarbon products and, consequently, do not contain high concentrations of contaminates. SELENIUM: Crudes containing selenium, a toxic material, will contribute significant concentrations of selenium to sour water. Selenium tends to concentrate in sour water tank sludges and stripper reboiler deposits and is removed with special treatment in the refinery effiuent waste water treatment system. AIR STRIPPING: Air stripping calls for (1) acidifying condensates to 5.5 pH, (2) so that hydrogen sulfide is freed (3) into a sufficient volume of counter current air flow. However, vapors, because they contain oxygen, cannot be sent to a flare and must be incinerated locally. Of course, hydrocarbons must be removed from condensates so that combustible off gases are not produced during air stripping, which is why some refineries prohibit air stripping. Designed to process sour water of very low contaminant concentrations, air stripping has advantages of (1) being able to operate cold and (2) possibly being the least expensive to construct and operate. This process used to be popular in Europe but is being phased out because of (1) ammonia retention, (2) high explosion risks, (3) costly residual air incineration and (4) new environmental emission regulations. AIR OXIDATION: If sour water contains low sulfide concentration, no mercaptans, low oxide concentration and few suspended solids, catalytic oxidation, using ferrous or ferric iron, can remove sulfides at atmospheric pressure. Process requirements are (1) four to eight times excess oxygen, (2) 6.5 pH to 7.0 pH, (3) large open concrete basins, because of necessary high flow rates and residence times and (4) low water temperature, such as 68 F. While environmental emission regulations may prevent this process from being used in the United States, air oxidation operated at 200 F, 100 psig and 45 to 60 minutes residence time with an air rate to supply twice the oxygen needed for stoichiometric oxidation of sulfides is being used in the United States. PREACIDIFICATION: A strong acid at 5.5 pH to 6.0 pH or flue gas, because of the carbon dioxide content, can disassociate volatile ammonium salts releasing hydrogen sulfide and producing stable water soluble ammonium sulfate or ammonium carbonate salts. Environmental regulations requiring (1) ammonium reductions in water discharges and (2) water reuse have resulted in acidification with strong acids to be phased out. Using hot flue gas reduces steam consumption but increases pipe, valve and fan sizes. Particulates must be removed from flue gas before use. PREPARING SOUR WATER FOR STEAM STRIPPING: Sour water feed is first pumped into the flash drum which has two purposes: (1) to remove hydrocarbon vapors and (2) to remove hydrocarbon liquids. After sour water enters the flash drum, vapors are flashed off through a pressure controlled vent connected to a low pressure system, such as a sour water system flare knock out drum. A single weir permits liquid hydrocarbons, that rise to the top of the water during approximately 20 minutes residence time, to spillover into the drum's hydrocarbon collection compartment. Sour water
level is controlled to about three inches from the top of the weir. From its collection compartment, hydrocarbons are pumped to a system for reprocessing, such as a slop system. FEED STABLIZATION TANK: Sour water is pumped to a sour water storage tank, which is operated about 60% full, increasing residence time to permit (1) additional hydrocarbon removal and (2) longer sour water mixing to minimize feed composition fluctuations due to the many sources of refinery sour water. Each refinery water draw boot contains a different sour water composition. Also, flow rate from each boot can vary with crude type, processing severity and manual level control operation or how well level control instrumentation functions. Sometimes significant increases in hydrocarbons in the sour water system are first to alert operators that a level control is stuck open. Significant composition changes cause poor stripper operation, resulting in either not consistently meeting product specification or wasting steam by overstripping to meet the worst composition. Whether the feed stabilization tank has a fixed or floating roof, about two feet of hydrocarbons should be allowed to float on top of the sour water for odor reduction. Cone roof tanks may require nitrogen blanketing for complete odor control and to prevent explosive mixtures from forming. Environmental regulations may require vent gases to be treated for hydrocarbon and or hydrogen sulfide removal. Of course, hydrocarbons should be removed before emptying the tank to prevent the sour water stripper from becoming a hydrocarbon stripper. From the storage tank, sour water is pumped on flow control to the sour water stripper. SOUR WATER STRIPPER: Sour water from the storage tank is heated in a heat exchanger by hot stripped water from the stripper bottoms and fed to the tower. As the sour water falls down the tower, hydrogen sulfide and ammonia are stripped by steam entering the bottom of the tower. Rather than adding more water to the tower by introducing live steam, a reboiler is used to boil sour water at a minimum tower operating pressure with low pressure steam allowing recovery and return of steam condensate to the boiler house. Hydrogen sulfide, ammonia and steam rise to the tower cooling section, which is controlled at 180 F by cooled, pumped around sour water from the middle of the tower. Overhead gases flow by pressure control to a low pressure system, such as sour water gas flare knock out drum or sulfur recovery unit. Stripped water is collected from the tower bottom and flows through a heat exchanger in which it is cooled by sour water feed to the tower. After being cooled, stripped water is pumped on level control off site for further water processing, such as to the crude unit desalter which operates as a liquid/liquid extractor by transferring up to 95% of the phenols in water into atmospheric crude feed. PUMP-AROUND COOLING SYSTEM: Most modern sour water stripper cooling systems are pump-arounds rather than overhead condensers, because there is less corrosion in pump- around systems and, consequently, they require less capital for construction and maintenance. In the absence of chlorides, pump-around coolers can be stainless steel. FCC sour water with cyanides requires titanium overhead condensers to prevent corrosion under deposits. Ammonium 'bisulfate and ammonium carbonate salts sublimate to solid salts directly from the gas stream off the sour water stripper overhead condenser. Pump-arounds, which are liquid filled systems, will not permit solid salt deposits to form. However, even with a pump-around, gases, which must be removed from the top of the tower, can form salts if gas temperatures fall below 180 F. Ammonium salts will plug everything, including the overhead piping from the tower. Proper steam tracing and insulation are required. Electrical tracing has not been proven in this service. For systems with overhead condensers, water wash is effective for removing salt deposits from overhead condensers, except from dead line segments. SOUR WATER STRIPPER OPERATION: Operating variables affecting hydrogen sulfide and ammonia in stripped water are (1) steam rate, (2) tower pressure and (3) sour water pH. Raising steam rate increases stripping of hydrogen sulfide and ammonia. However, because of diminishing returns, the recommended maximum useful steam rate is about three pounds of steam per gallon of combined sour water feed and reflux. Of course, this recommendation would be higher for cold feed that is not heated by hot stripped water product. Lowering tower pressure will increase stripping effectiveness at constant steam rate or will permit lowering steam rate, while meeting product specifications. Generally, towers should operate at the lowest pressure possible, which is about 17.5 psig, that will allow overhead gases to get through the control system and into the disposal system. High pH water tends to hold hydrogen sulfide in solution and aids in releasing ammonia. Conversely, low pH water tends to hold ammonia in solution and improves the stripping of hydrogen sulfide. By injecting a small amount of caustic near the tower bottom, ammonia stripping in the bottom is improved. Hydrogen sulfide is still stripped in the upper part of the tower. If caustic is added, injection control is critical and stripped water should have about a 8.0 pH. In some sour water strippers dispersants have been successful in keeping heavy water-soluble oils and calcium dissolved. When deposits do form in towers, sometimes they can be removed by shutting off steam flow and washing tower insides with sour water feed. Tim Armstrong, who manages sulfur technology, can be contacted at TPA, Inc., 9101 LBJ Freeway, LB 2, Dallas, Texas 75243 USA, (214) 669- 2908 (voice), (214) 234-1954 (fax),
[email protected] (e-mail).
Kin Taylor, who manages process engineering, can be contacted at Parsons Process Group, Inc., 9920 Gulf Freeway, Houston, Texas 77034 USA (713) 943-5225 (voice), (713) 943-5499 (fax). Bruce Scott, who, having retired from Chevron USA, provides consultation on sour water treater operation, can be contacted at Bruce Scott, Inc., 182 Irwin Street, San Rael, California 94901 USA, (415) 485-5626 (voice & fax).
Reprinted with permission from “Today’s Refinery”
