Salt deposition in FCC gas concentration units
Various operational problems can arise when ammonium chloride deposition occurs in FCC gas concentration units, and there is a range of likely causes
Michel Melin, Colin Baillie and Gordon McElhiney, Grace Davison Refining Technologies Europe
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Salt deposition in FCC gas concentration units can lead to various operational problems if it is not dealt with in an appropriate manner. It is therefore important for refiners to be aware of the main causes of salt deposition so that the correct procedures can be applied to manage this phenomenon.
Troubleshooting of FCCUs in terms of cyclone problems, catalyst circulation issues or coking has been discussed in much detail.1 However, less information has been reported about ways of dealing with salt deposition issues. The salt that is deposited most in FCC gas concentration units is ammonium chloride (NH4Cl), but deposits can also occur of the salts ammonium hydrosulphide (NH4)SH and iron sulphide (FeS), although they are less common.
This article is intended to provide refiners with useful information regarding the most likely causes of salt deposition, the associated symptoms and resulting consequences, as well as approaches that can be taken to handle such situations. The Grace Davison Refining Technologies technical service team has helped various refiners manage the issue of salt deposition and this valuable experience will be discussed.
Ammonium chloride deposition: likely causes
There are two reasons for an increasing occurrence of ammonium chloride deposits. First, refiners are processing a higher amount of residue feedstocks, which typically have a higher chloride content. Some refiners are also bypassing the desalter with imported atmospheric residue feedstock, which contributes to higher feed chloride levels. Second, due to the need to produce low-sulphur gasoline, a gasoline side cut is extracted from the main fractionator (MF) and subsequently hydrotreated. This leads to main fractionator top temperatures as low as 100°C, compared to previous temperatures of 135–145°C.
While these are the most likely origins of ammonium chloride deposits, there are other circumstances that can cause this problem, and a summary is listed in Table 1.
During troubleshooting for a salt deposition issue, all of these possibilities should be considered, individually and in combination. For example, one refinery that experienced issues with ammonium chloride deposition performed such a troubleshooting exercise, and the problem was finally attributed to the injection of slop to the main fractionator. This slop was rich in chloride and, together with the effects of acidic crudes that were being processed, resulted in ammonium chloride deposition on the main fractionator (with severe corrosion of the main fractionator packing, see Table 3). The problem of salt deposition was solved by water washing (see Table 4).
Chloride contribution from the FCC catalyst
In addition to the incorporation of rare earth chloride into FCC catalysts to stabilise the zeolite and steer product selectivities, chloride is an integral feature of the Grace Davison Al-sol binder system, which was first commercialised in the early 1980s, with the Worms plant in Germany being the pioneer site. This Al-sol binder system provides the basis for formulation flexibility, including the EnhanceR Technology Platform, which generates the high performance associated with Grace Davison FCC catalysts. Indeed the uniqueness of this binder system is one of the main reasons why Grace Davison FCC catalysts have maintained a performance advantage (approx-imately 70% of the FCCUs in the EMEA region are using Al-sol FCC catalysts). The question as to whether chloride from this binder can contribute to salt deposition is occasionally raised, and in this context the following facts are relevant.
During the FCC catalyst manufacturing process, the Al-sol binder is “set” using a high-temperature calcination to provide attrition resistance over a wide range of formulations. This high-temperature calcination step also removes most (>80%) of the chloride from the catalyst. If necessary, additional processing steps can be used to further reduce the fresh catalyst chloride content. In use, the fresh catalyst is added to the FCCU via the regenerator, and it is important to recognise that typical temperatures in the FCCU regenerator are significantly higher than those used in calcination in the standard catalyst manufacturing process, which in turn are higher than typical reactor temperatures in the FCCU. In consequence, and accelerated by the steam that is also present, chloride remaining on the fresh FCC catalyst is quickly removed in the regenerator before the catalyst makes its first transit to the reactor section. Typically, 80–95% of the fresh catalyst chloride is therefore removed in the FCCU flue gas, depending on the regenerator design. It is therefore recommended to avoid adding the fresh catalyst to a zone where it can bypass the regenerator bed and travel directly to the riser/stripper.
Ammonium chloride deposition: symptoms and consequences
Ammonium chloride deposition takes place primarily at the top of the main fractionator, although it can be encountered to a lesser extent in the overhead line, where the gas is passed through the air and water coolers, or the downstream FCC gas plant. Figure 1 shows a schematic diagram of where ammonium chloride deposition is most likely to occur.The main symptom of ammonium chloride deposition is an increase in pressure drop at the top of the main fractionator. Further symptoms are listed in Table 2.
Salt deposition can cause a reduction in feed rate as well as a slight deterioration of product quality. This can be a consequence of the salt deposition itself, but will also temporarily be observed during any resulting period of water wash applied to reduce salt deposition. In addition, corrosion may be an issue, especially for packed columns. A summary of the consequences of salt deposition are shown in Table 3.
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