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Aug-2014

Sour water stripping Part 1: non-phenolic water

Most non-phenolic sour water strippers for removing ammonia and hydrogen sulphide from are built with 30 to 60 trays and are usually designed using equilibrium stages and stage efficiencies.

Nathan A Hatcher, Clayton E Jones and Ralph H Weiland
Optimized Gas Treating, Inc.
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Article Summary
However, overall tray efficiency values are quoted over the rather wide range from 15% to 45%, corresponding to a three-fold range in the potential tray count. Recently a mass transfer rate-based simulator has become available for designing and troubleshooting SWSs. This article is the first of a three part series that uses a rate model to gain better understanding of sour water stripping. Part 2 will deal with sour waters containing heat stable salts (HSSs) and caustic soda injection. Part 3 will address the effect of HCN and phenol on stripping. In the present article, the focus is on the proper description of phase equilibrium in uncontaminated sour water and the use of a mass transfer rate-based model to develop better information on efficiency factors.

Introduction
Dealing with sour water is a reality that most refiners must face. Most refinery sour water sources contain relatively little carbon dioxide; it is the hydrogen sulphide content that makes water “sour”. When present together, ammonia and hydrogen sulphide have almost unlimited solubility in water. Gaseous ammonia will continue to be absorbed as long as it becomes protonated as a result of H2S co-absorption. Thus, it is conceivable that a particular sour water stream may be a lot more concentrated in both ammonia and hydrogen sulphide than the solubility of either component alone would suggest is even remotely possible.

The sour water generated in refineries is generally classified as either phenolic or non-phenolic. Non-phenolic water contains almost exclusively NH3, H2S, and possibly a trace of CO2. Stripped, non-phenolic water can typically be recycled to certain refinery processes such as HDS units (wash water), or reused as makeup water in a crude desalter. Phenolic (also called non-HDS) water contains contaminants (e.g., salts, phenols, and caustic) that prevent reuse. Other sources of water to SWS units are process drums, crude desalting units, scrubbing of hydrocarbons following caustic treatment for mercaptans, COS and final H2S removal, TGU quench columns, and various effluent drains for removing the water used to prevent salt deposition in equipment1.

Basic Sour Water Stripping Process
At first glance, sour water stripping appears to be an extremely simple process in which either external steam, steam generated by a reboiler, or even a hot hydrocarbon stripping vapour is used to drive ammonia and hydrogen sulphide into the vapour phase, first by heating the sour water, and then by providing a diluent for the stripped gases. Stripping vapour is the “gaseous solvent” used to remove and carry the ammonia and hydrogen sulphide out of the system. Stripping occurs by:
• Heating the sour water feed to boiling point
• Reversing any chemical reactions
• Diluting the partial pressure of the gases stripped by furnishing a diluent

Figure 1 shows a typical SWS column with live steam injection. When live steam is used as a stripping agent, the additional water generated as condensate is simply added to the refinery’s water inventory. Typical steam consumption in the stripping process is in the range 0.1–0.2 kg steam (saturated at 4–5 bara) per kg of sour water. Other aspects of SWS operations have been discussed elsewhere2.

There are good reasons for the fact that trays have historically been used in SWSs. Firstly, because sour water is often highly contaminated with salts, SWSs are usually considered a fouling service. However, in units processing relatively clean water, random packing is beginning to find use. The second reason for preferring trays, however, is that sour water stripping is a rate process controlled by mass transfer resistance in the water phase. The froth on trays is highly agitated which greatly reduces this resistance. On the other hand, flow over packing is relatively quiescent and, therefore, packing is not conducive to rapid stripping. The penalty for the sometimes higher throughput of packing may be a taller column.

Stripped water specifications for ammonia and hydrogen sulphide depend on the locale where the unit is installed and the final discharge requirements. Ammonia is harder to strip to really low levels than hydrogen sulphide and typical targets for NH3 are 10-80 ppmw in the stripped water, versus undetectable to less than 0.1 ppmw for H2S. Typical recent installations3, 4 involve 35-45 actual trays with quoted overall tray efficiencies anywhere from 15 to 45%.

Chemistry of Ammonia with Acid Gases

The solution chemistry and thermodynamic behaviour of sour water seems to be a generally misunderstood or at least poorly understood subject. The only molecular components are ammonia, hydrogen sulphide, and water. All other species in solution are ions! Ammonia is a relatively weak base capable of being mono-protonated. For example, in aqueous solution it forms ammonium ion (protonated ammonia) but because of its weakly basic nature, it protonates only to a limited extent:

NH3 + H+  √  NH4+                      (1)

This equilibrium reaction perfectly parallels amine protonation, so ammonia can be thought of as nothing more than just another reactive amine. There is a great deal of uninformed writing in various books and other publications concerning the reactions of H2S and CO2 with ammonia. All reactions involve ionic species and all reaction products are ions. Hydrogen ion is common to all the reactions. In solution, ions do not pair to form ionic compounds; they do this only in the solid state. As long as the species are in solution, they exist solely as individual ions, albeit with interactions that result in solution nonideality. Thus, there is no such thing as ammonium carbonate, ammonium bicarbonate, ammonium bisulphide, or diammonium sulphide in solution. It is quite hard to make sense of reaction equilibria unless one discards the notion of the existence of such compounds, especially when stronger acids or bases are present.

The equilibrium reactions that occur when H2S and CO2 dissolve in solution are the same as in any other aqueous, primary, or secondary amine system:

H2O √    H+ + OH–                                               (2)

H2S √    H+ + HS-                                                 (3)

HS- √ H+ + S=                                                      (4)
           
CO2 + H2O √ H+ + HCO3-                                   (5)
           
HCO3-  √ H+ + CO3=                                            (6)
       
NH3 + CO2 √ 〖NH2COO- + H+                           (7)
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