Fouling in the feed/effluent exchangers to our naphtha hydrotreater is at unacceptable levels. The feed is straight-run naphtha. Solutions please. (PTQ Q&A)
Response to a question in the Q3 2020 issue of PTQ
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Shone Abraham, Senior Naphtha Specialist, Honeywell UOP - Shone.Abraham@Honeywell.com
Preheat exchanger fouling can be a persistent problem in some naphtha hydrotreaters (NHT). Listed below are the most common foulants that are observed in NHT feed/effluent (F/E) exchangers which include NHT units that only process straight-run naphtha:
- Gums (feed-side fouling)
- Corrosion products (feed-side fouling)
- Salts (effluent-side fouling)
The primary cause is polymers forming due to olefins in the naphtha that are exposed to oxygen during transport or storage. This leads to the formation of combined oxygen compounds, such as peroxides. When heated in process units these combined oxygen compounds form polymers which cause fouling of heat exchangers. The most effective means of preventing the problem is to prevent the exposure of the olefins to oxygen. This is done by feeding the hydrotreater directly from an upstream processing unit rather than route the naphtha through intermediate storage. If that is not feasible, the next best preventative measure would be to ensure that any intermediate storage tank is effectively blanketed with nitrogen. These methods work by preventing the exposure of olefins to oxygen. Where these approaches are not adequate or practical, it is possible to use additives such as antioxidants and dispersants to mitigate the rate of fouling. However, the use of dispersants while the unit is online has a risk of dislodging material from the exchangers, creating pressure drop issues in the downstream reactor.
The most severe fouling problems typically occur in units where the naphtha has been imported to the hydrotreating unit. There is an increased chance that the naphtha will be exposed to oxygen during transit and the time that elapses between exposure and processing increases the total amount of peroxides formed. In these cases, a reboiled oxygen stripper is the preferred means of preventing excessive exchanger fouling. The purpose of the oxygen stripper is to break down the polymer forming combined oxygen compounds and to provide sufficient vapour and liquid contact for stripping free oxygen from the naphtha. Typically, only hydrotreaters designed to directly process imported naphtha have such oxygen strippers. Those that attempt to process such naphtha without first oxygen stripping inevitably have exchanger fouling problems.
Corrosion products like iron oxides (Fe2O3) or iron sulphides (FeS or FeS2) can also contribute to fouling of F/E exchangers. Scale flakes off the metal surfaces and is entrained in feed flow. This could happen either from the seals of floating roof storage tanks scraping rust from the tank walls or corrosion products from the crude unit itself. Filtering the feed can be helpful in such cases. Nonetheless, the effectiveness of having a feed filter will depend on the relative size of particles to feed filter mesh size (microns). FeS particles can be as small as 5 microns, and not all may be captured by a typical feed filter.
Effluent side fouling in F/E exchangers is typically caused by ammonium sulphides. The more nitrogen in the feed, the greater the risk. Ammonium chloride could also be the cause of fouling if feed contains organic chlorides. This type of fouling is typically observed in the last two bundles where the temperature falls below the desublimation point. The most effective way to reduce the effluent side fouling is by intermittent wash water injection to dissolve the salts. There should be a water wash injection point towards the last set of exchanger bundles. Wash water can temporarily be injected at this location to dissolve salts and a higher than normal rate of injection (typically 7-10 lv% of feed rate) would be required to maintain at least 25% of the injected water in the liquid state (due to the higher process temperature).
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