Treated water
Hydrocarbon vapours
10-50 ppmw NH 1-25 ppmw H S
Acid gas product 50 ppmw NH
Sour water
Degasser
Ammonia vapour to ammonia puri cation and liquefaction
Feed/product exchanger
Acid gas stripper
Deoiler
Reboiler
SWSPlus feed preparation tank
Condensate Steam
Ammonia stripper
Reboiler
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
=
x
x
x
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.
98
PTQ Q1 2023
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