The issue is the value of a 316 SS quench column and 316 cladding of the quench water coolers with caustic addition on automatic pH control.
Recent new TGUs have CS quench and absorber shells. Quench coolers are CS shell and 316 SS tubes, with quench water on the shell side. Corrosion allowance in the quench tower is 0.25”.
HIC plate CS with a 0.25″ corrosion allowance for the quench towers, with 304L SS tubes in the quench water cooler.
Trend seems to be more requests for SS cladding of the quench column (and alloying up in general). One factor might be reduced cost of cladding due to improved automated fabrication techniques.
Experience with a 20-year-old unit is that CS is sufficient. Corrosion may be due to chronic SO2 breakthroughs. The key to avoiding breakthroughs is reliable H2 measurement.
Quench column design with the tail gas entering downward at a 45° angle should achieve some initial cooling by impingement on the liquid level, particularly if water recirculation is lost. It should also avoid impingement on the far vessel wall and reduce localized corrosion at the opposite side. The problem might be greater with packing, where droplets raining down are more prone to being slammed against the wall.
With a standard SCOT design (i.e., a properly sized reactor, H2 make-up available if indirect preheat, etc), CS should be adequate. Failure to control SO2 breakthrough into the quench column can result in sulfur deposits on the walls and under-deposit corrosion, including H2 blistering.
With caustic addition for pH control, low chloride caustic should be ordered – usually a more costly grade – to avoid chloride cracking of the stainless.
for the ABPG