the concentrations of the acid and base species. In FCC units, ammonia is typically abundant due to cracking reactions, making the acid component the dominant factor affecting the K p value. Salt deposition commonly occurs in areas where fluid temperatures drop. Corrosion risk by salt is then magnified when moisture is present. Typical locations include the top trays or packing in the main fractionator’s heavy naphtha section, heavy naphtha extracting pumps, pump strainers, pumparound piping, heat exchangers, and distributors. Other locations include overhead piping, headers and air-fin cooler tubes, trim coolers, and occasionally the aftercoolers of gas compressors and the internals of absorbers (see Figure 1 ).
Wash water
Gas
38 37 36 35 34 33 32 31 30 29 28 27 26 25 24
SW
SWS
Naphtha
Delta T
Delta P
LGO
Figure 3 RFCC unit case study problem location
formation from unavoidable acidic components. However, this measure is difficult to adopt when FCC units operate in diesel maximisation mode. Remedial measures are typically applied when salt dep- osition has already occurred. The most well-known reme- dial measure is water washing. Continuous water injections are commonly practised in overhead systems to dilute or remove corrosive components before they condense or deposit. Periodic wash water injections into the tower inter- nals are also practiced, 4 usually as an emergency operation for several hours to a day or so. In salt deposition conditions, bulk temperatures are often significantly higher than the water dew point, and insufficient water injections may fail to provide enough liquid for effective washing. During such operations, the FCC unit’s throughput must be reduced by 20-30%, and tower temperatures must be significantly low - ered, requiring the reprocessing of affected fractions. Another common remedial measure is the use of chem- ical additives, which are broadly categorised into oil-solu- ble and water-soluble types. Oil-soluble additives detach and disperse deposited salts into the process fluid. While both chemical types are effective in salt removal, oil-sol- uble types can spread salts and corrosive substances into downstream equipment, necessitating isolation and repro- cessing of intermediate products. Water-soluble additives, on the other hand, are not dis- persants. They reduce the corrosivity of salt deposits and improve fluidity. Therefore, although they do not enhance the solubility of acids into oil, they can be used continu- ously to mobilise the salt deposits safely out to the unit. Kurita’s proprietary Ammonium Chloride Free technology, a programme that involves the application of a water-sol- uble salt resolving additive, belongs to this category and
In addition to corrosion, operational issues include increased differential pressure in towers, piping and heat exchangers, frequent plugging of the pump strainer, and reduced heat transfer efficiency in exchangers. These salt-related fouling problems can lead to operational restrictions, emergency shutdowns, or major incidents. When FCC units operate in diesel maximisation modes by lowering the naphtha end point, the top section temper- ature of the main fractionator is reduced, promoting salt deposition and moisture absorption. 5 If this is accompanied by an increase in chloride and bromide content in the feed- stock, the overall salt formation would be greater than that caused by either factor alone (see Figure 2 ). Countermeasures Countermeasures against fouling and corrosion caused by salt deposition can be broadly classified into preventive and remedial measures. Preventive measures aim to minimise the introduction of sulphur, chloride, and bromide into the system. These include improving salt removal and desul- phurisation prior to FCC units, as well as selecting feedstock with lower acidic components. However, selecting high- er-quality feedstocks inevitably impacts profitability. Reducing caustic soda injection into crude oil prior to the atmospheric crude distillation unit (CDU) can lower chlo- ride migration to the heavy oil stream. However, this will increase the migration of acidic components to the CDU tower mid-section and overhead system, accelerating corrosion and fouling. Temperature control is also a key preventive measure. By calculating salt deposition temper- atures based on process conditions and analytical data, and maintaining sufficient margins, it is possible to suppress salt
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PTQ Q1 2026
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