Salt deposition in FCC gas concentration

Transcription

1 Salt Deposition in FCC Gas Concentration Units Michel Melin Director Technical Service Grace Davison Refining Technologies Europe, Middle East and Africa Colin Baillie Marketing Manager Grace Davison Refining Technologies Europe, Middle East and Africa Gordon McElhiney Director Marketing & Business Development Grace Davison Refining Technologies Europe, Middle East and Africa Various operational problems can arise when ammonium chloride deposition occurs in FCC gas concentration units, and there is a range of likely causes. 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. Introduction 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 (NH 4 Cl), but deposits can also occur of the salts ammonium hydrosulfide (NH 4 )SH and iron sulfide (FeS), although they are less common. 34 ISSUE No. 107 / 2010

2 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-sulfur 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 (212 F), compared to previous temperatures in the range of C ( F). 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 I. 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 fractionator (with severe corrosion of the fractionator packing, see Table III). The problem of salt deposition was solved by water washing (see Table IV). Chloride Contribution from the FCC Catalyst Table I Most Likely Causes of Ammonium Chloride Salt Deposition In addition to the incorporation of rare-earth chloride into FCC catalysts to stabilize 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 Alsol binder system provides the basis for formulation flexibility 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 over other catalyst suppliers. 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. Processing of imported atmospheric residue Poor crude desalter operation Recovery of a MF gasoline side cut (lower top temperature) Reprocessing of slops in MF Leaking overhead condenser (using sea water) Overflowing overhead receiver water boot Bad distribution of cold reflux stream (cold spot) Feedstocks containing organic chloride from additives used to increase the recovery of oil or for cleaning GRACE DAVISON CATALAGRAM 35

3 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 hightemperature 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 recognize 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 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. The main type of salt deposited is ammonium chloride and there is a range of likely causes. consequence, and accelerated by the steam which is also present, chloride remaining on the fresh FCC catalyst is very 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 removed in 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. Figure 1 Diagram Highlighting Where Ammonium Chloride Deposition Can Occur Overhead coolers...and in the overhead line To wet gas compressor Deposition can occur at top of MF... Overhead receiver Main Fractionator Water Wild naphtha to primary absorber Rich sponge oil To sponge oil absorber HCO recycle LCO Stripper Hydrotreater Light cycle oil (LCO) product Reactor vapours Steam Filter Decanted oil product 36 ISSUE No. 107 / 2010

4 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 II. Salt deposition can cause a reduction in feedrate 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 the salt deposition. In addition, corrosion may also be an issue especially for packed columns. A summary of the consequences of salt deposition are highlighted in Table III. Managing and Solving the Issues of Ammonium Chloride Deposition The Grace Davison Refining Technologies technical service team has worked with refiners to help solve ammonium chloride deposition issues, and the experience gained is shared in the following main recommendations. To prevent ammonium chloride deposition in the overhead line, water is usually added, with typical quantities in the range of 6-7 vol.% water on a fresh feed basis. Addition of an anti-fouling additive in the reflux stream can prevent the formation of ammonium chloride deposits on the trays and packing. The salt is carried instead with the gasoline stream, in which it is insoluble. Such additives have been used successfully to reduce ammonium chloride salt deposition in various refineries over the last ten years; for instance, at the Table II Main Symptoms of Salt Deposition Increase in MF delta P Flooding of MF top section Plugging of top products draws Loss of duty of pump around heat exchangers Loss of separation efficiency between gasoline and LCO Higher MF bottom temperature Increase in reactor/regen pressure Plugging of reflux/gasoline pump strainer Reduced reflux/tpa rate Wider opening of WGC suction valve Difficulty when using HCN for reboiling depropanizer because the HCN temperature is lower and salt may deposit in the tubes Table III Consequences of Salt Deposition Corrosion of trays/packing Reduced WGC capacity (lower suction P) Reduced air blower capacity (higher regen P) Increased unit delta coke (higher reactor P) Fouling of slurry circuit (if higher bottom T) Poor quality heavy gasoline Cost associated with reduced feed rate and off spec products during periodic water wash Lower duty of depropanizer reboiler Lower duty of debutanizer reboiler Pembroke refinery in south Wales, UK, as well as the Mongstad refinery in Norway. 2,3 These additives are now considered established and effective technology. They are also said to protect against corrosion. Another recommendation is water wash the main fractionator. Water is injected either periodically or (more rarely) continuously in the reflux stream, and the main fractionator top temperature is reduced to approximately 80 C (176 F) using the reflux rate or the tip top pumparound, to allow water to condense inside the column to dissolve the salt. The water is preferably removed on a dedicated tray, where it is separated from the heavy cracked naphtha. This procedure has been suc- GRACE DAVISON CATALAGRAM 37

5 cessfully practised by Saudi Aramco. 4 Alternatively the main fractionator top temperature can be increased (for instance, to above 135 C (275 F)) for a given period of time to enable dissociation of the salt. Obviously, this results in a fullrange gasoline leaving overhead during the time period. Other recommendations include improving water settling in imported feed tanks by allowing more time and the use of additives. Hardware modifications could include the design of the main fractionator s reflux distributor to avoid cold spots at the top of the column. Alternatively the main fractionator s tray design could be revised. For example, the installation of a water boot in one of the trays will allow water (and the dissolved salt) to be removed without contaminating the heavy cracked naphtha. The installation of a two-stage desalter could also be considered to optimize the operation of the crude desalter unit. Other options include the installation of a dedicated FCC feed desalter, 5 or the installation of a gasoline splitter and then collecting the thermally cracked naphtha overhead of the main fractionator. Finally, a very effective solution is to hydrotreat the FCC feed, as this removes most of the feed chloride and significantly improves the yield structure. However, this requires a large capital investment. Table IV Methods for Managing Ammonium Chloride Salt Deposition Use of anti-fouling additives Water washing of the MF Increased MF top temperature Improved water settling in imported feed tanks Modification of MF reflux distributor to avoid cold spots at the top of the column Installation of a water boot in one of the MF trays Installation of a two-stage crude desalter Installation of a FCC feed desalter Installation of a gasoline splitter Hydrotreating the FCC feed Avoid adding FCC catalyst in a zone where it can bypass the regenerator bed The main methods for managing ammonium chloride deposition are highlighted in Table IV. 38 ISSUE No. 107 / 2010

6 Ammonium Chloride Deposition in the Main Fractionator Consider an FCC unit processing atmospheric residue feedstock under the following conditions: Feed rate Feed nitrogen content Feed chloride content MF top pressure Steam to the MF = 50.5 tonne/h Dry gas = Nm 3 /h LPG = tonne/h LCN+reflux = tonne/h Total flow to the MF top = 440 tonne/h ( kg/h) (968,000 lb/hr) = 1645 ppmw = 1.93 ppmw = 1.89 bara (27.8 psig) = 2806 kmol/h = 2392 kmol/h = 1937 kmol/h = 3843 kmol/h = kmol/h The following example assumes that 15% of the feed nitrogen goes to NH 3 the production of nitrogen (from the feed) = wt.% = kg/h = 51.7 kmol/h the resulting production of ammonia = % = 7.79 kmol/h the partial pressure of NH 3 = 1.89 (7.79/10978) = bara the production of chloride (from the feed) = wt.% = 0.85 kg/h = kmol/h The partial pressure of HCl = 1.89 ( /10978) = bara ppnh 3 pphcl = bara Using the following formula: ln (Kp) = /T where Kp = ppnh 3 pphcl, and T is the minimum main fractionator top temperature required to avoid salt deposition (measured in K), the minimum top temperature required to avoid salt deposition under these conditons is 125 C (257 F). GRACE DAVISON CATALAGRAM 39