Preventing salt fouling in FCC main fractionators
Results of using salt dispersant additives on a badly fouled FCC unit has shown that the method allows refiners to avoid more costly methods of naphtha fractionation and, potentially, water washing of the main fractionator.
David O Martin, Nalco/Exxon Energy Chemicals
Richard O Allen, Texaco Ltd
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Low sulphur gasoline specifications for both European and US markets will require new technologies to allow refiners to meet these requirements. Process licensors are introducing new processes for sulphur removal from gasoline, focusing in particular on the FCC naphtha. To take advantage of these processes and minimise octane loss, some form of FCC naphtha fractionation will usually be required, followed by treatment of the heavy naphtha fraction.
Depending on the fractionation option chosen, modifications and operating changes to the FCC main fractionator may be required. These changes may include direct undercutting of the FCC naphtha, separating the heavy, high-sulphur naphtha via a heavy naphtha draw.
At the same time, FCC feeds are becoming increasingly heavy. Processing of atmospheric residue is becoming more common. Residues typically contain at least traces of chloride salts.
These trends can lead to conditions in which ammonium chloride salts can deposit in FCC main fractionators and overheads, due to the combination of low temperature and relatively high concentrations of ammonia and hydrogen chloride. The build-up of salt deposits normally occurs in the mid-to-upper sections of the main fractionator column and top pumparound circuits. If not removed, these salts will accumulate to the point where they can cause a number of problems in maintaining optimal tower operation, and ultimately affect the operation of the entire FCC unit. Salt deposition is becoming an increasing concern.
Traditional methods of preventing salt deposition or removing salt deposits have included maintaining the fractionator temperature high enough to ensure the sublimation of all the salts or, where this is not possible, water washing of the tower. FCC feed desalting is also an option to consider if long-term processing of heavy, contaminated feeds is anticipated. However, the use of salt dispersant additives can allow refiners to avoid more costly means of FCC naphtha fractionation, and potentially avoid the practice of water washing of the main fractionator.
Salt deposit formation
Ammonium chloride salt deposition is a problem in FCC fractionator towers when sufficient amounts of both ammonia and hydrogen chloride are present. The formation of solid ammonium chloride occurs when the salts “precipitate” from the vapour phase. The deposition occurs when the product of the partial pressures of the ammonia and hydrogen chloride are greater than the stability constant (Kd) of ammonium chloride at the system temperature.
NH3 (g) + HCl (g) Æ NH4Cl (s)
Ammonium salts are highly water soluble, with negligible solubility in hydrocarbons. Depending on the level of reactants and the conditions inside the tower, salts can deposit and accumulate at temperatures above the water dewpoint. The accumulated salts can cause severe restrictions in the operation of the FCC. In addition, the ammonium chloride salts will readily absorb water. The damp salts can set up extremely corrosive conditions on fractionator trays, piping, and exchanger surfaces.
European Union specifications for gasoline call for a maximum of 30ppm sulphur content by 2005, with a maximum of 150ppm until that time. Germany is already pushing to achieve 10ppm sulphur gasoline by 2003, and has introduced a proposal to the European Union for “sulphur-free” fuels by 2007 [Nocca J L et al, The domino interaction of refinery processes for gasoline quality attainment; NPRA Annual Meeting, March 2000].
In the USA and Canada, gasoline sulphur content will be 30ppm maximum by 2005. To meet this quality, pool gasoline sulphur will have to be reduced by more than 90 per cent compared to current levels [Billon A et al, A novel approach to attain new fuel specifications; European Refining Technology Conference, Paris, 22-24 November 1999].
FCC naphtha makes up roughly 39 per cent of the US gasoline pool composition, but contributes over 90 per cent of the current pool sulphur. Therefore, reduction of FCC naphtha sulphur content is essential to ensure regulatory compliance.
Examination of a typical full range (C5-220°C) FCC naphtha composition shows that most of the sulphur is concentrated in the heavy naphtha fraction. The majority of the olefins are found in the lighter naphtha fractions (Figure 1, on previous page).
Since olefins have a higher octane value than the corresponding paraffins, it is desirable to minimise the saturation of olefins. Therefore, fractionation of the FCC naphtha followed by hydrotreatment of the heaviest fraction allows the refiner to remove most of the sulphur and minimises the overall loss in octane. Further sulphur removal from the light naphtha fraction can be accomplished by mercaptan sweetening.
One route to fractionating FCC naphtha is to install a heavy naphtha draw off the main fractionator tower – 15-20 per cent of the naphtha can be drawn off and hydrotreated. This is typically a minimal cost option for fractionating the naphtha, and lessens the loading on the overhead condensing system. With limited capital available, refiners may initially opt for this lower cost option.
However, direct undercutting of the FCC naphtha results in a lower temperature in the upper part of the main fractionator tower. If sufficient levels of hydrogen chloride are present, the lower tower temperature will result in deposition of ammonium chloride in the top of the column (Figure 2).
Another refining practice that contributes to the ammonium chloride problem is the economic incentive to process atmospheric residue through the FCC. Residue commonly contains inorganic salts, primarily NaCl, which dissociates to Na and Cl in the riser. The sodium primarily deposits on the catalyst but the chloride exits the reactor in the form of hydrogen chloride.
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