How much CO₂ is normally present in combined acid gases produced in refinery amine systems, and how much can the CO₂ concentration be decreased by switching to a more selective amine? To what extent can Claus plant pressure drop be decreased, by reducing the concentration of CO₂ in the acid gas?
 

TOM BACON (Dow Chemical Company)

The ratio of H₂S to CO₂ in typical refinery gases is usually high enough to provide a Claus feed of more than 70 % H₂S. Using the typical selectivity of MDEA, this ratio can be increased to 90 %.
 

TOFIK KHANMAMEDOV (The Tofik K. Khanmamedov Company)

There is a new patented family of HIGHSULF™ processes that allows to decrease CO₂ concentration in acid gases by using generic MDEA in amine unit. For example the CO₂ concentration in acid gas can be reduced from 57.3% to 47.1% for gas plants, and from 79.11% to 68.4% for gasifier. The CO₂ concentration in acid gas of SCOT type tail gas treating units can be reduced by about 10% in generic MDEA amine system. Besides of these, HIGHSULF™ process significantly reduces concentrations of hydrocarbons, COS, CO, H₂ in acid gases of gas plant, gasifier and refinery.
HIGHSULF™ controls hydraulic loading of sulfur recovery unit. The reduction of CO₂ concentration in acid gases of main amine units reduces pressure drop of sulfur unit and leads to capacity increase of sulfur unit minimum by 3%.Reduction of CO₂ concentartion in acid gas of SCOT type tail gas treating unit also leads to reduction of pressure drop of sulfur unit and the capacity of sulfur unit can also be increased at least by 3%. For sulfur recovery units with oxygen injection configuration the reduction of CO₂ concentration in acid gas (main amine and tail gas treating units) will translate to reduced oxygen consumption or increased throughput at the same oxygen consumption level.
HIGHSULF™ tail gas treating process increases CO₂ rejection in the absorber and may reduce the lean amine cooling requirements. This translates to reduced utility cost for an existing tail gas treating unit or reduced equipment cost for a new unit. The application of the HIGHSULF™ processes is based on utilizing existing equipment and solvents (amine). A significant advantage of HIGHSULF™ is attaining the same level of CO₂ rejection with generic amines that are attained with specialty amines. This translates into significant amine saving, particularly where losses are high. Justification of this process based
on computer simulations by using different commercially available and well proved simulators.
 

DAVID BURNS (Union Carbide Corporation):

Typically a refinery acid gas feed to a Claus process is in the 50-100 mol% H₂S range and can be accommodated in a straight-through configuration. If the concentration is in the 20-50% range then a split-flow Claus configuration would be
required but this is less common in the refineries as the number of unit operations producing significant levels of CO₂ is limited.
By way of example, consider a refinery using MEA or DEA and achieving essentially complete CO₂ removal and producing a Claus feed of 60 mol% H₂S and 40 mol% CO₂ (an oversimplification). Switching to a more selective amine such as MDEA might be expected to reduce the CO₂ pickup by 60%. The resulting Claus feed would be about 79 mol% H₂S and 21mol% CO₂ - and the overall volume would be reduced by about 24%. If a more selective formulated solvent such as UCARSOL® Solvent HS-115 is used the overall CO₂ pickup could be reduced even further to around 66%, with the result that the Claus feed would be about 82 mol% H₂S and 18 mol% CO₂, reducing the overall volume by about 26%. Actual CO₂
removal rates are a function of the partial pressures involved and the number of absorber stages, amongst other things, but this simple analysis gives an idea of the impact of using a selective amine on the Claus feed composition and volume.