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We are operating an aromatic recovery unit producing benzene and toluene. The extraction section uses Sulfolane as solvent. The extract is stored in a charge day tank and is used for charging the benzene column. To remove olefins from the feed, there is a clay tower prior to the benzene column that operates at at inlet temp of 170 deg C and a pressure of 13 kg/cm2. There is an exchanger for heating the clay tower feed (tube side). We are observing a frequent issue of plugging in this exchanger. This leads us to shut down the fractionation section for almost a day every five months for cleaning/replacing the tube bundle. The olefin content in the light reformate feed varies between 5% and 7%. Is there any way this issue can be resolved? Is the olefinic content in the feed too high? The plugging material seems black in colour. Is there any method that can be used for identifying the fouling type? Is is it due to polymerisation of olefins? Any solution to avoid such frequent plugging in this exchanger?
 
Answers
02/10/2020 A: Julia Moore, Sulzer GTC, julia.moore@sulzer.com
From the problem description, the fouling on the heat exchanger tube side likely is due to the polymerization of the olefins.

The 5-7% olefins in light reformate is on the high side, but is not unusual when reformer is operating at high severity for max aromatics or octane number. The root of the problem is the Sulfolane extraction unit, especially if it’s liquid-liquid extraction that has poor separation efficiency between olefins and aromatics. It results in olefins leakage to the extract. Those olefins in the extract are stable when storied for a short time in the day tank. But when they are heated up at high temperature through the heat exchanger, olefins tend to polymerize and foul on the surface of the tube. Another problem is that those olefins will significantly shorten the clay’s lifetime, and if not careful, jeopardize the BI/Doctor Test spec in the B & T product.

The fundamental solution is to conduct a simple revamp to the Sulfolane extraction unit, and use a higher extraction efficiency solvent, to reject nearly entire olefins to the raffinate. Sulzer GTC has done many of such revamps to easily convert Sulfolane extraction unit to GT-BTX technology that uses proprietary Techtiv solvent. The Techtiv solvent in GT-BTX technology has significantly higher extraction efficiency and selectivity than Sulfolane to reject olefins. As the result, olefins content in extract will be trace, fouling problem in the heat exchanger will be greatly reduced or eliminated, clay lifetime will be extended significantly, and any risk of product BI/Doctor Test to be off-spec will be eliminated. If desired, such a revamp also provides significant capacity increase, aromatics recovery improvement, and energy saving on the extraction unit.

Should you find it interesting, we would be happy to look further into the details with more info of the current unit and its performance, to provide preliminary project cost and expected result.
02/10/2020 A: Manoj, Lummus Global Pte. Singapore, parasharmk@yahoo.com
The fouling is most likely gums formed from diolefins being exposed to oxygen in the day tank. The tank should be nitrogen blanketed. An even better solution is to bypass the tank altogether and feed directly to the column.
01/10/2020 A: Sridhar Balakrishnan, Bharat Oman Refineries Limited , laksrid@yahoo.com
One of the probable reasons may be due to conjugated olefins which are highly prone to polymerisation and help to allow auto-oxidation reactions leading to gum formation. You need to check for diene value by the maleic anhydride addition reaction UOP -327 method in light reformate. This can give an idea of the problem.
30/09/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Yes, this sounds like a polymerisation problem. I’d suggest you consider four approaches:
1. Is it possible to bypass the tank? The residence time and possible exposure to oxygen in the tank could be making this problem worse. Is the tank really necessary?
2. If the tank is necessary, can the flow be rerouted so that the extract goes through the clay tower then the tank then the benzene column? Heat integration might make this difficult.
3. If not, try to minimise inventory (and hence residence time) in the tank. Also minimise contact with oxygen. If it is a fixed roof tank, add a nitrogen blanket to the vapour space. Floating roof tanks reduce contact between the product and air, but there may still be air pockets below the roof. If it is an internal floating roof tank, you could consider a nitrogen purge between the floating roof and the dome.
4. Speak to your chemical vendor about the use of an antifoulant. Obviously this would need to be compatible with the downstream processes. Antifoulants are not always successful, but it would be a straightforward project so may be worth trying.


30/09/2020 A: Nagarathinam S Murthy, Ashphil Consultancy, Chennai, nssvdvr@gmail.com
Typically, a Sulfolane system should never have olefinic feed and even it exists the olefins content in the feed should be in ppm or at best less than 0.5wt%. Further, Sulfolane has a tendency to become degraded in the presence of oxygen; this could get into the unit either via feed (from a storage tank) and/or through the vacuum distillation system for solvent recovery. I see the following options:
1. Change the solvent to NMP which has no such issues with oxygen, or
2. Reduce feed olefins to less than say 0.1%wt to get longer runs.
30/09/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The fouling is most likely gums formed from diolefins being exposed to oxygen in the day tank. The tank should be nitrogen blanketed. An even better solution is to bypass the tank altogether and feed directly to the column.
30/09/2020 A: Marcello Ferrara, ITW SrL, mferrara@itwtechnologies.com
Please contact me privately