Michel Melin, Colin Baillie and Gordon McElhiney, from the Grace Davison Refining Technologies, Worms, Germany office discuss multiple options for controlling NH4Cl based salt deposition in FCCU main fractionator overhead and other areas
Ammonia chloride (NH4Cl) based salt deposition in the FCCU, particularly in the main fractionators (MF) continues to be an ongoing problem due to potentially high chloride levels in acidic feeds processed through certain refinery configurations. For example, the increasing volumes of resid processed in higher complexity facilities are typically accompanied by higher chloride content. In addition, some refiners are also bypassing the desalter with imported atmospheric residue feedstock, also impacting higher chloride feed levels.
For many of these refiners, extraction of a gasoline side-cut from the main fractionator (MF) for subsequent hydrotreatment to near-zero sulfur levels has resulted in main fractionator top temperatures as low as 100°C (212°F), compared to previous temperatures in the range of 135-145°C (275-293°F). The cooler temperatures have lead to higher salt deposition at the top of the main fractionator. Other likely causes of NH4Cl salt deposition include:
- Poor crude desalter operation
- 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 oil recovery.
In addition to these sources of salt deposition, the Grace Davison Refining Technologies technical service team has helped various refiners control FCCU salt deposition from other sources, such as chloride-rich slop injected into the main fractionator. This resulted in severe corrosion to main fractionator packing, magnified by the effects of the acidic crudes being processed.
It is important to point out that of all the possible sources of salt deposition, the technical service team from the Grace Davison offices in Worms, Germany, has shown that there is no possibility of chloride contamination generated from the use of Grace Davison’s FCC catalysts, even though chloride is an integral feature of the Grace Davison alumina-sol (Al-sol) binder system. This Al-sol binder system provides the basis for the catalyst’s formulation flexibility and the mechanism and careful process by which it has been formulated has been discussed in detail on page 36 of the March 2010 issue of Catalgram®.1 Briefly, the authors point out that high temperature calcination removes more than 80% of the chloride from the catalyst, and the chloride remaining on fresh FCC catalyst is very quickly removed in the regenerator before the catalyst makes its first transit to the FCCU’s reactor section.2
As previously noted, NH4Cl deposition primarily occurs at the top of the main fractionator, and is accompanied by an increase in pressure drop. It can also be encountered to a lesser extent in the overhead line, where the gas is passed through the air and water coolers, or the downstream FCCU gas plant. The previously noted pressure drop increase is the main symptom of NH4Cl deposition, other important symptoms include:
- Flooding of MF top section
- Plugging of top product draws
- Loss of pump duty around heat exchangers
- Loss of separation efficiency between gasoline and LCO
- Higher MF bottom temperature
- Increase in reactor/regenerator 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.
Salt deposition can cause a reduction in feed rate as well as a slight product quality deterioration. This can be a consequence of 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 also be an issue, especially for packed columns. A summary of the consequences of salt deposition include:
- Corrosion of trays/packing
- Reduced WGC capacity (lower suction pressure)
- Reduced air blower capacity (higher regenerator pressure)
- Increased unit delta coke (higher reactor pressure)
- Fouling of slurry circuit (if higher bottoms temperature)
- Poor quality heavy gasoline
- Cost associated with reduced feed rate and off-spec products during periodic water wash
- Lower depropanizer reboiler duty
- Lower debutanizer reboiler duty.
Preventing ammonia chloride deposition
Working with refiners to help solve NH4Cl deposition issues has resulting in recommendations focusing on the main fractionator column’s overhead line and the column’s trays or packing. To prevent NH4Cl 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 NH4Cl 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 NH4Cl salt deposition in various refineries over the last ten years. These additives are now considered established and effective technology. They are also said to protect against corrosion. Another recommendation is to 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 successfully practiced by Saudi Aramco. 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 full-range 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 crude desalter unit operation. Other options include installation of a dedicated FCC feed desalter, or 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 yield structure. However, this requires a large capital investment. In summary, the main methods for managing NH4Cl deposition include:
- Use of anti-fouling additives
- MF water washing
- Higher 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.
Calculating NH4Cl deposition in the main fractionator
Consider an FCC unit processing atmospheric residue feedstock under the following conditions:
- Feed rate = 440 tonne/h (440 000 kg/h) (968,000 lb/hr)
- Feed nitrogen content = 1645 ppmw
- Feed chloride content = 1.93 ppmw
- MF top pressure = 1.89 bara (27.8 psig)
- Steam to the MF = 50.5 tonne/h = 2806 kmol/h
- Dry gas = 53584 Nm3/h = 2392 kmol/h
- LPG = 100.3 tonne/h = 1937 kmol/h
- LCN + reflux = 344.2 tonne/h = 3843 kmol/h
- Total flow to the MF top = 10978 kmol/h
The following example assumes that 15% of the feed nitrogen goes to NH3:
- the production of nitrogen (from the feed) = 440 000 × 0.1645 wt.%
= 723.8 kg/h
= 51.7 kmol/h
- the resulting production of ammonia = 51.7 × 15%
= 7.79 kmol/h
- the partial pressure of NH3 = 1.89 × (7.79/10978)
= 1.34 × 10-3 bara
- the production of chloride (from the feed) = 440 000 × 0.000193 wt.%
= 0.85 kg/h
= 2.4 × 10-2 kmol/h
- The partial pressure of HCl = 1.89 × (2.4 × 10-2/10978)
= 4.13 × 10-6 bara
- ppNH3 × ppHCl = 5.53 × 10-9 bara
Using the following formula:
ln (Kp) = – 21183.4/T + 34.17
where Kp = ppNH3 × 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).
Other types of salt deposition
The other main types of salt deposition are from ammonium hydrosulfide (NH4SH) and iron sulfide (FeS). The deposition of (NH4)SH is controlled by the equilibrium reaction:
NH3 (g) + H2S (g) ↔ (NH4)SH (s)
The reaction takes place at a lower temperature than for ammonium chloride. Deposition of this salt is most likely to occur in the overhead line coolers and sometimes in the wet gas compressor itself, particularly when processing feeds with high nitrogen and sulfur content. The deposition of (NH4)SH is best controlled by adjusting the temperature where the deposition occurs, as well as by the use of a water wash. Iron sulfide is a corrosion product, which, to a large extent, is found on main fractionator trays and packing. Since the salt is pyrophoric, its accumulation is a potential hazard during the opening of the column. It has been reported that the additives used to prevent NH4Cl deposition also help prevent the accumulation of FeS in the main fractionator. Proper procedures for shutdown of the FCCU, and the application of a chemical wash to oxidize the FeS prior to vessel entry, should be considered.
Various operational problems can arise when salt deposition occurs in FCC gas concentration units. The main type of salt deposited is NH4Cl and there is a range of likely causes, with the increased processing of imported atmospheric residue being a major contributor. The vast majority of FCCUs using alumina-sol (Al-Sol) based FCC catalysts never experience problems due to the chloride content associated with this binder. It is important to clarify, moreover, that there are a number of ways to manage and solve any issues of NH4Cl deposition.
Indeed, various refineries have enlisted the support of the Grace Davison Refining Technologies technical service team with respect to chloride management, so that they can avoid having to change to a chloride-free FCC catalyst, with the resulting sacrifice in performance and economics.
- Melin, M., Baillie, C. and McElhiney, G., “Salt Deposition in FCC Gas Concentration Units,” Catalgram®, Issue No. 107, Spring 2010, p. 36.
- Mott, R. W., “Troubleshooting FCC Standpipe Flow Problems.” Catalagram® publication No. 83, pps. 25-35, 1992.
Note: This article is based on a paper published in the Grace Davison Catalgram®, Issue No. 107, Spring 2010, pp.s 34-40.