4 replies to this topic

[quangbach_2003]

we have some problems with our Naphtha Stabilizer
- Steam consumption and Pressure of Steam after De-pressurize increase if we maintain bottom temperature of Naphtha Stabilizer
- We found o problem during 1st turnaround that fouling at exchanger between feed and naphtha of Naphtha Stabilizer is very serious >>2 feed pumps are running to maintain the same feed
after 3 months turnaround, that phenomenon repeats
so Plz give me some idea to solve that problem

[Technical Bard]

Have you checked whether your naphtha contains olefins and diolefins? If you have cracked material in the naphtha, the high temperatures can cause polymerization, which can (and does) foul exchangers and column internals.

In virgin material from a CDU, one shouldn't find olefins, but it is possible. Are there any recycled streams in the CDU? How high is the CDU furnace coil outlet temperature? Have you checked the CDU coil film temperature based on checkrating the furnace at operating conditions? If the CDU furnace is running too hot (>400°C film) you may be cracking larger molecules and making olefinic naphtha.

[Himanshu Sharma]

Please post a simple sketch of the system.Additionally ,please clarify that whether this naphtha is straight run naphtha,hydrotreated naphtha from Hydrocracker/DHDT or cracked naphtha from FCC/DCU.

Since you have not reported any upstream fouling issues , i am ruling out problems due to acidic corrosion and focus on olefin/diolefin fouling.

The polymerization rate and hence fouling potential is directly attributed to the following factors

Diolefin concentration. The fouling rate increases when the diolefin concentration in the column bottoms increases. MAV-maleic anhydride value analysis can be helpful in determination of the same.

Temperature and residence time. As temperature and residence time increase, the fouling rate tracks more along a logarithmic path than a linear function. Temperature also has a direct impact on the rate of free-radical formation and inhibitor consumption.

water and Oxygenates Contamination of the feedstock with oxygen may result in peroxide formation(this is a typical problem while processing feed out of storage tanks that are not properly blanketed with an inert gas), which further increases the fouling rate when heated.

Catalysis .By their very own chemistry , Transition metal such as iron and copper—also catalyze polymerization. Iron catalyzes the decomposition of peroxides into peroxy radicals and will results in increasing the rate of Diels-Alder reactions that generate initiators.

Fouling control can be done by the use of Inhibitor components.

• Antioxidant. As per their formulation they keep a tab on peroxide formation and terminate peroxy radicals during peroxide decomposition
• Antipolymerant. Special compounds that Control heat-induced alkyl (or carbon) free-radicals hydrogen donation
• Metal deactivators. Proprietary compounds typical in polymer industry that can Control iron-catalyzed free-radical polymerization and iron-catalyzed peroxide decomposition.

Edited by Himanshu Sharma, 11 February 2012 - 01:36 PM.

[obsidian_dust]

This is a very interesting topic. Further question for Mr. Himanshu Sharma: If we inject antioxidant inhibitor into naphtha stream, will it lead to further problem in the downstream unit of naphtha stabilizer such as Naphtha Hydrotreating Unit, or further, Naphtha Reforming Unit?

Any opinion or sharing of experience will be very helpful.

GBU all.

[Himanshu Sharma]

Dear Sir

Lets try and analyze the situation at hand.

NHT normally runs on hot Naphtha feed that may be coming from CDU,HCU,FCC or DCU.Most modern designs account for a cartridge filter in the feed stream to remove any contaminants.Now the problem of gum formation shall come only if feed is processed from storage tanks that are poorly blanketed,having said this it shall be once in a while scenario.

Concentration of Antioxidants that are added in Naphtha are in PPM levels but let me remind you that Phenols which are normally used as Anti-oxidant agents acts a a poison for HDS/HDN Rxns and catalysts.Phenols are assumed to inhibit the catalyst by production of water or Polymer formation.Inside an NHT, there shall be three reacting species then Sulphur being most reactive ,then water and Nitrogen being least reactive.

Bottom Line is that if antioxidant agent concentration in naphtha is below than impurity level of water and oxygenates considered for unit design ,there shall be no problem but in case it exceeds you have to consult your process licensor/catalyst supplier.