Understood. The key point in combating against CDU overhead corrosion is to ensure the following:
- Removal of inorganic salts below 5ppm in the equipment upstream (desalter). Organic Chlorides must be removed anyway to avoid poisoning of catalyst in downstream units (NHT, CCR).
- Maintaining top pumparound return temperature above the condensation temperature of water vapors flowing up the tower, which once condensed, act like a spider web for absorption of HCl vapors, creating extremely corrosive environment. The areas most affected by corrosion are top pumparound trays (which is why they are usually made of Monel 400), overhead condensers, and top pumparound exchangers.
The key mechanism in overhead corrosion lies in the fact that the first condensed water droplet (inlet section of the exchanger/fin-fan) will contain almost concentrated HCl, with HCl being extremely hydroscopic substance. The solution that worked in many places (and elaborated by several esteemed authors such as Norm Lieberman) is to employ waterwash of the overhead condenser, by recycling some of the water from the overhead receiver back to the exchanger inlet, in quantities sufficient to ensure presence of liquid water at the exchanger inlet - for the purpose to dillute HCl and to dissolve sublimed NH4Cl salt deposits which are the main cause of so called under-deposit corrosion. I would employ this concept if I am ever to design a CDU tower, together with the two bullet points from above. Rather prevent than cure.
To answer your questions:
1. I have never seen a chemical injection scheme that worked properly. Vendor will tell their stories of course, but if the above two conditions are not met the injection will help so little, if any. We used to work with Nalco, Chimec, Petrolite, but without major success. You may try any of them, but I would make sure that laboratory tests are conducted with the crude oil (or crude blends) which are going to be processed in my unit.
2. I have seen SS316L trays used in the top pumparound loop. The material selection will largely depend on how much Chloride salts escapes into the tower. There is a limit for usage of austenitic SS depending on Chloride concentration. You want to be in a safe region.
3. If you still want to go for injection, a common sparger designed for proper atomization of liquid chemical should be more than sufficient. Unfortunately I do not have any typical drawings of such assembly. Usually the chemical vendor specifies the level of atomization and "ideal" injection configuration.