The acidity of the naphtha reforming catalyst is controlled by the moisture in the feed and the chloride level on the catalyst. An increase in the feed moisture will strip chloride from the catalyst, thereby reducing its acidity. The long-term effects of excessive moisture will cause increased coking rate on catalyst and decreased catalyst activity and product yields.
Proper control of the water-chloride balance is crucial to maintain the correct acid balance on the catalyst. The recommended water level in recycle gas is ~20 ppm. For more detail about water chloride balance in reforming please view my previous blog; “Water Chloride Ballance in Naphtha Reforming Units“
On the contrary, too little moisture in the feed will severely dehydroxylate the catalyst, despite the addition of chloride in the catalyst. The water content of reformer feed is a combination of the intrinsic moisture found in the naphtha and the moisture added to the feed.
This phenomenon is most likely to occur in a fixed-bed reformer where water is injected to maintain the moisture level. While in the CCR reforming units, a significant amount of water is produced during the reduction of the metal oxides, and the water must be continuously removed to achieve optimal reduction of platinum oxides and oxides of the promoter metals.
Effects of Water Poisoning
If high water content is charged over long period, catalyst will become chloride deficient. Ultimately it will compromise the acid function, and as a result, the metal function will become dominant. Drop of chloride level on spent and regenerated catalyst is an indication of chloride strip with water.
On the other hand, excess on-stream chloride addition, moisture addition, or both can ultimately increase catalyst acidity to the point where it begins to overwhelm the metal function, and as a result, C3 and C4 formation will overtake C1 and C2 production.
Causes of Water Breakthrough
- High water dosing rates in the feed
- Poor operation of the NHT stripper, will cause water to pass to reforming section.
- Possible water contamination due to reboilers or exchangers leakage.
- High moisture from catalyst regeneration section like poor drying operation.
Control Actions
Water removal is usually achieved in the stripper of the NHT. Generally, contamination by water results from poor operation of this equipment. Water can also be introduced indirectly in the form of oxygenates which will be reacted to form water in the reformer.
- When feeding a reformer unit with low-sulfur naphtha from storage, the naphtha should be routed through the NHT stripper or splitter to remove excess water present.
- If recycle gas water content exceeds 50 ppm volume, the reactor inlet temperature must be lowered to reduce the chlorine elution (leaching) from the catalyst and sintering of the metallic sites. The following guidelines are generally accepted;
- > 50 ppm volume water, limit reactor inlet temperature to ~ 500 °C
- >100 ppm volume water, limit reactor inlet temperature to ~ 480 °C.
- Identify the the source of the water ingress and stop it. Make the appropriate process adjustments to cater the high moisture effects.
- Increase chloride dosing in the catalyst regeneration section to achieve desired Chloride concentration in the regenerated catalyst.
- If recycle gas moisture drops too far below 10 ppm volume, the catalyst surface becomes sufficiently dehydroxylated to limit its acidity. Injection of 1 ppm weight water in the feed will increase recycle gas moisture by 2~5 ppm volume depending on the operating conditions.
Top References
- Springer handbook of Petroleum Technology edited by Hsu & Robinson
- Catalytic Naphtha Reforming Process by Soni O. Oyekan
