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Koch-Glitsch research on impact of improved fractionation on FCC Main Fractionator product sulfur distribution was presented at the Spring 2002 NPRA Annual Meeting.

Equipment | References | Case Studies | Technical Papers | FAQ


Koch-Glitsch Equipment


Conventional Valve Trays - Still the workhorse in many FCC applications.

Fixed Valve Trays - Provide improved fouling and corrosion resistance in the top section of the FCC Main Fractionator.

High Capacity Trays - Often the most cost effective way to increase the capacity of an existing Main Fractionator. Our wide selection of high capacity trays enables us to tailor our offering to your needs.

Structured Packing - Offers the ultimate in capacity and reduced pressure drop. We have successfully applied structured packing in FCC Main Fractionators up to C-Factors of 0.5. Our experience enables us to design structured packing systems to provide the performance and reliability required of an FCC Main Fractionator.

FLEXIGRID® Metal Grids - Specially designed for highly fouling systems such as the slurry pumparound and top of the Main Fractionator, FLEXIGRID® metal grids provide unparalleled fouling resistance and cleanability. Our distribution technology provides superior fouling resistance while maintaining distribution quality.

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References

We have successfully revamped more than 75 FCC main fractionators since the mid 80’s with a combination of high capacity trays, structured packing and grids with the largest main fractionator at 26’ ID. If you would like specific information regarding our experience please contact us.

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Case Studies (Currently Under Construction)

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Frequently Asked Questions

What fractionation options exist to help control FCC Gasoline Sulfur?

What is the best design and operating practice to prevent coking in the slurry pumparound section of the FCC Main Fractionator?

What can be done to prevent salt fouling in the top of the FCC Main Fractionator?

Q: What fractionation options exist to help control FCC Gasoline Sulfur?

A: There are nearly as many ways of dealing with FCC gasoline sulfur as there are FCCs. Fractionation options take advantage of the fact that FCC gasoline sulfur is concentrated in the heavy end of the gasoline while octane is more evenly distributed and olefins are concentrated in the front end. Heavy, high sulfur fractions can therefore be hydrotreated with minimal loss of FCC Naphtha octane due to saturation of olefins. Fractionation options that are well known are:
1. Undercutting FCC naphtha sending the heavy fraction out with the LCO for hydrotreating in the diesel hydrotreater. In most cases at least some of this stream is recovered as naphtha from the hydrotreater product stripper. The downside to this approach is some loss in naphtha yield and lower overhead temperatures in the Main Fractionator, thus increasing the risk of fouling and corrosion.
2. Withdrawing heavy FCC naphtha as a side cut from the FCC Main Fractionator for treating either in existing naphtha hydrotreaters or dedicated units as in “FCCU Main Fractionator Revamp for CARB Gasoline Production”, Hydrocarbon Processing, Feb. 1998. This method is well proven and maintains high naphtha yields. However, some fractionators may not have sufficient height to employ this approach. Also, it requires the same considerations regarding lower overhead temperature as option 1.
3. Splitting FCC Naphtha in a downstream distillation column. This option provides the cleanest separation between the light and heavy naphtha streams and offers the greatest operational flexibility. However it requires much greater capital investment than either option above.
One paper1 suggests the potential for achieving a less distinct cut using a hot and cold overhead receiver as is commonly practiced in crude units. This offers the additional advantage of reducing the potential for localized cold spots or water refluxed to the Main Fractionator and therefore reduces the potential for corrosion. It has the disadvantage of less separation between the olefins and sulfur containing molecules than any of the options above. To our knowledge this has not been employed commercially.

The question specifically addresses FCC gasoline sulfur. In our opinion, however, there is an equal incentive to address LCO sulfur content. While LCO is typically a much smaller portion of the diesel pool than FCC Naphtha of the gasoline pool it is typically the lowest quality and the most difficult to desulfurize. In fact, studies have shown that most of the sulfur remaining in samples of diesel treated to 500 ppm sulfur is contained in sterically hindered dimethyl dibenzo thiophenes2. The heavy tail of LCO contains a large concentration of these molecules. Reducing this tail can therefore significantly impact the requirements for meeting the proposed new diesel specifications.

One thing that has been largely overlooked by industry is that there is the potential to impact FCC product sulfur distrbution by improving fractionation in the Main Fractionator. While this will probably not make the difference in sulfur reduction technology selection it may make meeting the new specifications easier for some people. Koch-Glitsch conducted research in this area and presented a paper documenting the results at the 2002 NPRA Annual Meeting.

1 Stoley. 2001
2 Tippett, et. al. 1999

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Q: What is the best design and operating practice to prevent coking in the slurry pumparound section of the FCC Main Fractionator?

A: The primary design aspect is good liquid and vapor distribution. With shed or baffle trays liquid distribution is accomplished by feeding the slurry pumparound onto each panel of the top tray using feed pipes with appropriately sized holes. FLEXIGRID® metal grids in combination with trough distributors provide high quality liquid distribution with a fouling resistant design. The Koch-Glitsch slurry distributor utilizes a combination slot with a V-notch to resist fouling while accomplishing good distribution. Whether sheds or packing is utilized, it is important to consider distribution of the liquid from the wash section as this liquid can adversely impact the reliability of the slurry pumparound zone. Vapor distribution is also critical regardless of the type of internals in the slurry pumparound. High vapor velocity results in vapor maldistribution which can dry out sections of the pumparound zone resulting in coking. Distribution is improved either through the use of a vapor distributor such as Koch-Glitsch's proprietary design or by reducing the feed velocity by installing a larger nozzle and upstream piping.

Maximum slurry temperature is often used as a guideline to prevent coking in the slurry section of the main fractionator. When following this guideline it is important to note the difference between the temperature of the slurry leaving the pumparound shed trays or packing and the temperature of the main fractionator bottoms product. Since most refiners control the bottoms temperature using a slipstream of the slurry pumparound as bottoms quench, the bottoms product temperature can be significantly lower. The key temperature for preventing pumparound zone coking is the temperature leaving the sheds or packing. This temperature is a function of the amount of LCO range material left in the slurry product. There is no universally safe temperature to run. For each unit the maximum safe temperature is a function of equipment design, unit feed type, conversion level, catalyst carryover. A well designed slurry pumparound can maximize the temperature for an individual unit and maximize LCO recovery.

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Q: What can be done to prevent salt fouling in the top of the FCC Main Fractionator?

A: Chlorides must be present in the feed for deposition to occur so this problem is characteristic of units feeding unhydrotreated resid or gas oil that has been contaminated with salt water (typically from a barge). Organic chlorides are not generally present in sufficient quantities to cause a problem. Therefore the best solution is to determine if the source of the chlorides can be eliminated or reduced. Salt deposition can often be reduced by improving the operation of the crude unit desalters. In some cases (sodium content greater than 3 ppm) refiners have found it economically attractive to desalt the FCC feed, although the economics of this are based more on catalyst consumption than Fractionator fouling.

Ammonium chloride salt deposition in the top of the main Fractionator occurs when the temperature at any point in the Fractionator drops below the salt dew point. Therefore the simplest approach is to maintain the temperature well above the dewpoint. It is important to note that localized cold spots can exist which will cause salt deposition even though the bulk overhead temperature is above the dew point. For this reason trough distributors which introduce cold reflux or naphtha pumparound to the top bed of a packed main Fractionator can be subject to maldistribution that will be difficult to diagnose. The cold liquid causes salt to build up on the lower portion of the distributor plugging the distribution holes and causing poor distribution. In a trayed Fractionator localized corrosion is often apparent where the reflux is introduced.

Various chemical dispersants are available which are advertised to prevent salt buildup or to clean already fouled equipment. These have met with mixed success in industrial applications. In general they are most successful when applied to trays rather than packing. For cleaning salt deposits the most universally successful approach has been to water wash the Fractionator while operating at reduced rates and overhead temperatures and routing the LCO to slop. It is important that proper procedures be followed during the water wash to prevent column upsets and damage to the internals.

New low sulfur gasoline regulations are causing more refiners to undercut FCC gasoline or pull a heavy naphtha product from the Main Fractionator. This will increase the incidence of salt fouling in the industry. Units which have never had a problem may experience fouling for the first time. It is therefore important to consider the potential when modifying the equipment or operation in any way. Fractionator internals can be designed to minimize the impact of salt deposition and to provide for convenient water washing when fouling occurs. High capacity fixed valve trays are much more forgiving than packing in highly fouling systems. In addition, proper design of a water wash system will minimize the impact of water washing the fractionator when salt deposits occur.

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Technical Papers

› "The Impact of Improved Fractionation on FCC Gasoline and Light Cylcle Oil Sulfur Content", NPRA 2002 Annual Meeting (Paper available after Conference)

› "Maximizing FCC Main Fractionator Revamp Potential - Conception Through Start Up" , KG Symposium @ the 1998 NPRA Q & A

› "FCCU Main Fractionator Revamp for CARB Gasoline Production", Hydrocarbon Processing, Feb. 1998

› "Ethylene & FCCU Gas Plant Revamps for High Capacity and Efficiency"

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