PTQ Q1 2023 Issue

Treated water

Hydrocarbon vapours

10-50 ppmw NH 1-25 ppmw HS

Acid gas product 50 ppmw NH

Sour water


Ammonia vapour to ammonia purication and liquefaction

Feed/product exchanger

Acid gas stripper



SWSPlus feed preparation tank

Condensate Steam

Ammonia stripper


Condensate Steam

Degassing and feed preparation

Acid gas stripping

Ammonia stripping

Figure 1 Segregation of NH 3 from H 2 S using a two-column distillation approach

This simplified mass-flow approach shows that NH3 is a very inefficient use of the available space in an SRU. Thus, one way to expand SRU capacity is to remove the NH 3 , which is how the SWSPlus unit can become a valuable asset in the refinery. SWSPlus unit description The SWSPlus process segregates the NH 3 from the H 2 S using a two-column distillation approach. The SWSPlus process consists of four main processing steps: 1) degas- sing and feed preparation, 2) acid gas stripping, 3) NH 3 stripping, and 4) NH 3 purification and liquefaction (see Figures 1 and 2 ).  Degassing and feed preparation Sour water feeds from a single or multiple sources are cooled and passed through a degasser where dissolved hydrogen, methane, and other light hydrocarbons are removed. The degassed sour water is pumped to a SWSPlus feed preparation tank, which serves to attenuate flow rate and composition changes while also providing the opportunity to settle and remove entrained oil and solids.  Acid gas stripping From the SWSPlus feed preparation tank, the degassed sour water feed is pumped to the feed/ product exchanger and fed to the reboiled acid gas strip- per. H 2 S and carbon dioxide (CO 2 ) are stripped to the over- heads, and a water wash reduces NH 3 contamination in the overhead H 2 S stream. The resulting acid gas is of high purity and is an excellent feed for an SRU or a sulphuric acid plant. It contains negligible NH 3 (less than 50 ppmw) and very few hydrocarbons since the plant feed has been degassed.  NH 3 stripping The acid gas stripper bottoms, which con- tain NH 3 and some residual acid gases, are fed directly to the NH 3 stripper. The NH 3 stripper is a reboiled and refluxed distillation column. In this column, essentially all the NH 3 and acid gas are removed from the water, which leaves as

NH 3 + 0.75 O 2 → 0.5 N 2 + 1.5 H 2 O + heat [Mass = 41 kg per kg-mole of NH 3 ]

As before, this is not the whole picture. We should con- sider atmospheric nitrogen and humidity present in the combustion air. In addition, the sour water stripper acid gas (SWSAG) contains additional water vapour. If we assume a 1:1:1 ratio of NH 3 :H 2 S:H 2 O in the sour water stripper acid gas, we have a more complete summary of NH 3 combustion: NH₃ + 0.75 O2 + 2.8 N 2 + 1.2 H 2 O → 3.3 N 2 + 2.7 H 2 O + heat [Mass = 142 kg per kg-mole of NH 3 ] Combining the above, we get an overall equivalent mass of H 2 S to NH 3 :

142 kg

kg-mole of H 2 S

kg-mole of NH 3

34 kg H 2 S

2.7 kg H 2 S





17 kg NH 3

kg-mole of H 2 S kg NH 3

kg-mole of NH 3

105 kg

This indicates that an additional 2.7 kg H 2 S can be pro- cessed for each kg of NH 3 removed from the Claus SRU feed. Considering the hydrocarbons typically present in the sour water stripper acid gas, the replacement value of the NH 3 removed increases to about 3 kg H 2 S per kg NH 3 . Advantages of SWSPlus Bolt-On Absorber • Existing sour water stripper equipment is already installed and operational (sunk cost) • Takes the SWSAG as the starting point rather than the sour water • Captures all NH 3 and lets ~50% of the H 2 S slip through to the SRU • The other 50% of the H 2 S enters the SWSPlus before being routed to the SRU • 15-35% Opex and 20-40% Capex savings for the Bolt-On Absorber feeding a smaller SWSPlus vs a con- ventional SWSPlus.


PTQ Q1 2023

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