Sour water/corrosion

Various patents (22—24) have been issued claiming the use of tetrakis(hydroxymethyl)phosphonium sulfate in, for example, water treating, pharmaceuticals (qv), and in the oil industry where this compound shows exceptional activity toward the sulfate-reducing bacteria that are a primary cause of hydrogen sulfide formation and consequent problems associated with souring and corrosion (25). 

When inorganic chlorides (e.g., ammonium chloride) and organic chlorides collect (usually in the tops of columns and equipment connected to the tops of columns), mild to severe corrosion occurs. When organic nitrogen compounds in the feed exceed 0.05 wt% (500 ppm), cyanides and ammonia form. These compounds collect in the aqueous phases and cause corrosion of certain materials. The corrosion problems in the aqueous phases are discussed in the following section on overhead systems, in the Chapter Two section on characteristics of sour water, and in the Chapter Three section on coolers. 

The main corrosion problems in a nonacidified condensing sour water stripper occur in the overhead system. Exchanger tubes in the overhead condenser are often made of commercially pure titanium. The reflux pump is often alloy 20 (CN7M). In very corrosive waters, such as those containing phenols or large quantities of salts, Hastelloy1141 C276 is used. Two API surveys (API Standards 944 and 950) concluded that the location and severity of corrosion vary with the type of unit as follows ... 

API Standard 944. "Survey of Materials Experience and Corrosion Problems in Sour Water Equipment. Washington, DC American Petroleum Institute, latest revision. 

Corrosion problems have also occurred in compressors, sour water valve trims, expansion bellows, and hydrocracker fractionators. Type 4140 (UNS G41400) compressor impellers have sulfide cracked in sour gas when the hardness exceeded 235 Brine)i(e>. Hardened 12Cr valve trim has sulfide cracked to prevent this problem, 18Cr-8Ni SS trim in sour water service is commonly used. Sulfide cracking has not occurred significantly in pumps. 

For areas where sour water collects, see Table A-S. Where mild corrosion is expected, use carbon steel with 1A in. (3 mm) CA on piping. 

MR0175, For carbon steeft the hardness limit shoiid be 200 Srlneil Rockwell B 95 see NACE Standard RP0472) p4] Avoid use of cement linings or Monel 400 for sour water. See Number 14 for corrosion allowance on carbon steel exposed to sour water. 

Maximum corrosion allowance. Primarily used for wet carbon dioxide equipment when the carbon dioxide partial pressure is less than 4 psf (30 kPa) and for sour water equipment when the hydrogen sulfide partial pressure exceeds 10 psi (70 kPa)... 

A slightly different but equivalent definition of acids and bases is suggested by these equations. The first reaction results in an increase in hydronium ions (H O ). The second reaction results in an increase in hydroxide ions (OH ). Therefore, an acid can be defined as a chemical substance that, when added to water, results in an increase in the concentration of hydronium ions. A base is a chemical substance that, when added to water, results in an increase in the concentration of hydroxide ions. Hydronium ions give acid solutions the properties we associate with acids—sour taste, corrosiveness, and the ability to turn blue litmus paper red. Hydroxide ions give basic (or alkaline) solutions the properties of feeling soapy, causticity, and the ability to turn red litmus paper blue. 

Corrosion in overhead system of a sour water stripper... 

The conditions in the sour water system are most severe when the redox potential is low and the pH is high. The main corrosion problem is general... 

Corrosion of materials in sour water stripping operations involves complex mixtures of sulfides, cyanides, carbonates, and chlorides. Corrosion control is by alloy selection. 

Thermal processes such as coking, cat cracking, hydrodesulfurization, and hydrocracking ail produce both NF 3 and H2S. The resulting aqueous-phase ammonium hydrosulfide is antagonistic to carbon steel heat-exchanger tubes. Most of the ammonium hydrosulfide winds up in sour-water streams. Before disposal, the sour water must be steam stripped. The overhead condenser used for this stripper has an extremely corrosive environment. Titanium is preferred for this service. 

Acid A classification of substances that liberate hydrogen ions in water, and are normally sour and corrosive, with a pH lower than 7. A compound or atom that donates protons. 

Type 430 stainless is resistant to chloride stress corrosion cracking and elevated sulfide attack. Applications are found in nitric acid services, water and food processing, automobile trim, heat exchangers in petroleum and chemical processing industries, reboilers for desulfurized naphtha, heat exchangers in sour-water strippers and hydrogen plant efluent coolers. The compatibility of type 430 stainless steel with selected corrodents is provided in Table 7.4, which is taken from Reference [1]. 

For many waste water strippers used in refinery services, rapid rates of reboiler fouling are experienced, not due to corrosion, but as a consequence of heavy hydrocarbon sludge, that settles out in the bottom of the sour water stripper feed tank. In particular, slurry oil that is produced in a refinery catalytic cracking unit can accumulate in the stripper s feed tank. This slurry oil, being denser than water, settles out in the tank and eventually is drawn into the suction of the sour water stripper feed pump. The heavy hydrocarbon phase then accumulates in the stripper s reboiler, and with time and temperature fouls the reboiler s tubes. 

The sour water stripper system represents a very simple separation process, which generally operates in a predictable and straightforward manner. The three main problems encountered in many installations are (1) the presence of fixed anunonia, (2) the presence of phenols, and (3) corrosion. 

Sour water strippers are generally constructed of carbon steel and only minor ctorosion is reported in the tower, trays, and feed-to-bottoms heat exchangers (Gantz, 1975). Materials experience and corrosion problems in sour water strippers are discussed in two API reports (1974, 1976). Significant corrosion has been encountered in the overhead systems of refluxed towers when the overhead systems were constructed of carbon steel. The API reports indicate that titanium is very resistant to cmrosion in such situations. 

American Petroleum Institute, 1976, A Study of Variables That Effect the Corrosion of Sour Water Strippers, API 948, May. 

The term sour water denotes various types of process water containing H2S,NH3HCN, and small amounts of phenols, mercaptanes, chlorides, and fluorides. High concentrations of ammonia can saturate process water with ammonium bisulfide (NH HS) and canses serious corrosion of carbon steel components. Ammonium bisulfide will also rapidly attack admiralty metal (C44300) tubes. Only titanium Grade 2 (R50400) tubes have sufficient resistance to be used in this case. 

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