Application Note: Ph Control in Sour Water Stripping

pH Control in Sour Water Stripping In petroleum refining, so called sour water (process water that contains dissolved hydrogen sulfide – H2S) is produced in various processes. Due to environmental legislation and the fact that H2S is extremely toxic, refineries nowadays adhere to Application Note strict sulfur management which involves removing the H2S from the sour water through steam stripping. Background Sulfur compounds such as mercaptans and hydrogen sulfide occur naturally in crude oil or are formed in certain process Sour gas to SRU steps. Water and steam are extensively used in petroleum refining, and sour water is formed in the presence of hydrogen Sour water sulfide. The increased use of sour crudes obviously leads to an increase in sour water formation. Apart from H2S, sour water contains ammonia, phenols, HCN, CO2, acids, salts and many other water soluble waste compounds. After stripping, the wa- Stripper ter may be used as process water, for example in the desalter, or be treated as waste water. Caustic Process After the removal of solids and hydrocarbons, the sour water Processss waterwater is fed to the top of the stripper column. A reboiler provides heat reuse or or steam to the bottom of the stripper, or steam is injected to treatmentatment directly. In a counter current flow the steam liberates the dis- Reboiler solved gases from the sour water. Subsequently, the overhead gas flow is directed to the Sulfur Recovery Unit where elemen- tal sulfur is produced through catalytic oxidation of H2S. Unfortunately things are slightly more complicated than this and the use of steam alone is not enough to remove all the dissolved (sour) gases. Gas concentrations vary and both hydrogen sulfide and ammonia occur in different forms. Fur- ther, depending on the pH of the sour water the gases can be The ideal solution would be to use two strippers: one working at found either in ionic or gas form. It is even possible that hydrogen low pH for the removal of hydrogen sulfide, and the other working sulfide and ammonia are found bound as ammonium bisulfide, at high pH for the removal of ammonia. Most refineries however a solid salt. The following chemical equilibria take place: do not have the benefit of multiple sour water strippers, so alter- native remedies have to be found. H2S <—> H+ + HS– HS– <—> H+ + S2– Increasing the carbon dioxide content by adding flue gas helps NH3 + H2O <—> NH4+ + OH– lower the pH of sour water and the release of hydrogen sulfide in NH4+ + HS– <—> H2S + NH3 the upper part of the tower. For the same purpose, some refineries still use sulfuric or acetic acid for preacidification of the sour Application Note Application As long as these components are present in an ionic state they will water. Although acidification improves the stripping of hydrogen not be completely stripped from the water. So in order to increase sulfide, some of these acids form ammonium salts which make the efficiency of the sour water stripper it is important to force the the stripping of ammonia more difficult. Injection of caustic at components into their gas form. the bottom of the tower to keep the pH above 8 will improve ammonia stripping. This is where the pH of the sour water plays an important role. A common misconception is that sour water by definition is acidic. Instrumentation This is not the case, as in this context “sour” refers to the pres- The foul conditions, the presence of ammonia and sulfides and ence of hydrogen sulfide. Sour water definitely can be alkaline in the elevated temperatures, are extremely tough on pH electrodes. nature. (In fact alkaline sour water is one of the most underesti- Diaphragms easily clog up and the reference electrode can quick- mated sources of corrosion in oil refining and natural gas pro- ly become poisoned. This leads rapidly to unstable measurements duction.) Being a weak acid in solution, hydrogen sulfide re- and sensors may not last longer than two weeks. mains dissociated and dissolved under alkaline conditions and is difficult to strip from the solution. At pH < 5.5 though, it returns The InPro 4260 i pH sensor with its open reference junction and to its gas form and, thanks to its increased partial pressure, strip- Xerolyt solid electrolyte has a proven track record in sour water ping becomes easy with less steam required. Ammonia would stripper service. It resists fouling from hydrocarbon contami- behave similarly if it were not for the fact that ammonia is a weak nants and the solid electrolyte provides an excellent barrier base when dissolved in water. Contrary to hydrogen sulfide it re- against poisoning from sulfides, ammonia or cyanides, guaran- quires a pH > 10 to return it to its gas form. The process condi- teeing high accuracy and fast response throughout many months tions for efficient and economic stripping of both gases are thus of operation. rather contradictory. Featuring Intelligent Senor Management (ISM) technology, the sensor provides full diagnostics that advise when maintenance is due or when the sensor needs replaced. The corresponding transmitter for the sensor is the M420, a two- wire pH analyzer that is fully certified for hazardous area use, and offers HART® communication and ISM diagnostics func- tionality. For more information: pH electrode InPro 4260 i Transmitter M420 www.mt.com/pro-petrochem Mettler-Toledo AG www.mt.com/pro Process Analytics Visit for more information Im Hackacker 15 CH-8902 Urdorf Switzerland.

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