Many processes in a refinery use steam as a stripping medium in distillation and as a diluent to reduce the hydrocarbon partial pressure in catalytic or thermal cracking . The steam is eventually condensed as a liquid effluent commonly referred to as sour or foul water. The two most prevalent pollutants found in sour water are H2S and NH3 resulting from the destruction of organic sulfur and nitrogen compounds during desulfurization, denitrification, and hydrotreating. Phenols and cyanides also may be present in sour water.
The purpose of sour water pretreatment is to remove sulfides (H2S, ammonium sulfide, and polysulfides) before the waste enters the sewer. The sour water can be treated by stripping with steam or flue gas, air oxidation to convert sulfides to thiosulfates, or vaporization and incineration.
Sour water strippers are designed primarily for the removal of sulfides and can be expected to achieve 85-99% removal. If acid is not required to enhance sulfide stripping, ammonia will also be stripped, the percentage varying widely with stripping pH and temperature. Depending on pH, temperature, and contaminant partial pressure, phenols and cyanides can also be stripped with removal as high as 30%.
There are many different types of strippers, but most of them involve the downward flow of sour water through a trayed or packed tower while an ascending flow of stripping steam or gas removes the pollutants. The stripping medium can be steam, flue gas, fuel gas, or any inert gas. Owing to its higher efficiency, the majority of installed refinery sour water strippers employ steam as both a heating medium and a stripping gas . Some of the steam strippers are provided with overhead condensers to remove the stripping steam from the overhead H2S and NH3. The condensed steam is recycled or refluxed back to the stripper. The results of a 1972 survey by the American Petroleum Institute suggested that, overall, refluxed strippers remove a greater percentage of H2S and NH3 than nonrefluxed strippers .
The operating conditions of sour water strippers vary from 0.1 to 3.5 atm (1-50 psig) and from 38 to 132°C (100-270°F). The sour water may or may not be acidified with mineral acid prior to stripping. H2S is much easier to remove than NH3. In pure water at 100°F, for example, the Henry's Law coefficient for NH3 is 38,000 ppm/psia, whereas that for H2S is 184 ppm/psia . To remove 90% of the NH3, a temperature of 110°C (230°F) or higher is usually employed, but 90% or more of the H2S can be removed at 100°F.
Two-stage strippers are installed in some refineries to enhance the separate recovery of sulfide and ammonia. Acidification with a mineral acid is used to fix the NH3 in the first stage and allow more efficient H2S removal. In the second stage the pH is readjusted by adding caustics for efficient NH3 removal. One example is the Chevron WWT process, which is essentially two-stage stripping with ammonia purification, so that the H2S and NH3 are separated. The H2S goes to a conventional Claus sulfur plant and the NH3 can be used as a fertilizer . Figure 13 shows a schematic flow diagram of the Chevron WWT process.
Another way to treat sour water is air oxidation under elevated temperature and pressure. Compressed air is injected into the stream followed by sufficient steam to raise the reaction temperature to at least 88°C (190°F). Reaction pressure of 3.7 to 7 atm (50100 psig) is required. Oxidation proceeds rapidly and converts practically all the sulfides to thiosulfates, and about 10% of the thiosulfates to sulfate . Air oxidation, however, is much less effective than stripping in reducing the oxygen demand of sour waters, as the remaining thiosulfates can later be oxidized to sulfates by aquatic microorganisms. Air oxidation is sometimes carried out after sour water stripping as a sulfide polishing step.
Stripping of sour water is normally carried out to remove sulfides, hence the effluent may contain 50 to 100 ppm of NH3, or even considerably more, depending on the influent ammonia
concentration. Values of NH3 have been reported to be as low as 1 ppm, but generally the effluent NH3 concentration is held to approximately 50 ppm to provide nutrient nitrogen for the refinery biological waste treatment system. Because of more stringent effluent requirements for NH3, many refineries seek to improve the sour water stripping systems for NH3 removal. This can be done by (1) increasing the number of trays, (2) increasing the steam rate, (3) increasing tower height, and (4) adding a second column in series. All these methods are now available to the refining industry .
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