SOUR WATER

Sour water is any water from a refinery that contains hydrogen sulfide. In addition, sour water may contain ammonia, phenol and cyanide. Historically, sour water from refinery process units is treated to remove hydrogen sulfide prior to disposal.  Today, the removal of ammonia from sour water is more important due to recent regulation reducing nitrogen into estuaries.  Selenium is also important as regulators seek to reduce selenium because of the mutagenic effects found in wildlife with high selenium concentrations.  While ammonia, selenium, phenol, salts and other constituents are removed while during hydrogen sulfide treatment, removal efficiencies of these chemicals are lower due to the physics and chemistry of hydrogen sulfide treatment systems.

Traditionally, sour water is sent to a stripping unit where steam is used to heat the sour water to above 1800 F. Hydrogen sulfide and ammonia are released at the top of the stripping tower and sent to the Claus unit where they are converted into sulfur and nitrogen.  The stripped sour water is treated in a biological wastewater treatment plant where the any remaining ammonia is nitrified and then denitrified.

Interestingly, the physical conditions for the efficient removal of both hydrogen sulfide and ammonia from the sour water are not possible in a traditional hydrogen sulfide treatment system.  The ideal pH for stripping hydrogen sulfide is below 5.5 and at a pH above 5.5 sulfide is primarily found in the form of ions and cannot be as effectively removed. Conversely, efficient ammonia stripping requires a pH above 10 to prevent the formation of ammonium ion that cannot be stripped. Most refineries target a pH of about 8 in the steam stripper which allows removal of both ammonia and hydrogen sulfide but at lower efficiencies.  Selenium is not removed during the stripping process and it finds its way into the stripped water.

After stripping the stripped sour water usually contains 50-100 million ppm of ammonia plus the selenium.  Some of the ammonia found in the stripped water is resistant to removal.  It has been hypothesized that acidic materials such as thiosulfuric acid, thiocyanic acid and weak organic acids may cause ammonia fixation.  This issue may go away if the pH of the stripped water increases to above10.

There have been proponents of using dual strippers to increase the removal of ammonia from sour water.  The first stripper removes the hydrogen sulfides under acidic conditions and the second stripper would remove the ammonia under caustic conditions.  While hydrogen sulfide and the removal of ammonia from sour water would increase, and ammonia fixation avoided, the selenium would remain in the stripped sour water.

CASTion’s Sour Water Treatment Solution

CASTion’s  CAST and RCAST systems when combined with reverse osmosis and other traditional filtration systems can be a very efficient and cost effective sour water treatment for hydrogen sulfide, ammonia, selenium and salts.  Because of our flash vacuum distillation platform our sour water system would be less costly to operate than a stripper.   Our sour water treatment system can be either single or dual staged with the first stage removing hydrogen sulfide and selenium and the second stage targeting the removal of ammonia from sour water.  In addition, we could retrofit our RCAST system after the hydrogen sulfide stripper to remove just the ammonia

A Project Example

A CASTion CAST system was combined with a vibrating membrane to treat the stripped sour water for selenium at a West Coast refinery.  CASTion was able to remove 93% of the selenium in the stripped sour water.