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I am working in a solvent extraction-based unit for benzene production, we opened the unit’s extractive distillation column after a year of operation to troubleshoot the reasons for high pressure drop (2 barg), where we found a thick layer of heavy, coked material on each tray. Feed for the unit is reformate from a CCR unit (4% benzene). What are the possible reasons for this?”
 
Answers
21/09/2007 A: KengYong Chan, Air Products & Chemicals, Inc, chanky@apci.com
Based on what you described, the material found on the trays is heavy coke material which most likely comes from the solvent polymer or degraded solvent by-products (and not metal corrosion).
Reference responses from others on possible solvent degradation causes (the most common cause is oxidation, followed by high temperature), does your extraction unit have a solvent filtration system (that recycles part of the main solvent flow through filters) and/or a solvent regeneration system to remove the heavy polymers? Filtering and solvent regeneration will help to remove the heavy solvent polymer materials.
It is unusual for an extractive distillation column to be fouled within such a short time frame; possible due to "harsh" conditions that affect the solvent quality (oxygen, temperature or possibly contaminants that degrade the solvent). Ensure that the operating temperature of all the columns, separators are much lower than the solvent decomposition temperature. Check the compatibility of the anti-foam (if you are injecting) with solvent, just to ensure it does not degrade solvent. Lastly, for solvent oxidation, if there is a vacuum solvent regeneration unit, there is a simple way to check for air ingress. Let me know if you want to do this vacuum leak check.......
11/09/2007 A: Sjaak van Veelen, MPR Services, MPReurope@TKInet.com
Solvent degradation can take place in sulfolane systems but also in glycol systems (TEG and TTEG). Degradation involves acids and acidic polymers. The acids can lead to increased corrosion. The deposits usually contain corrosion products and polymers. At high temperatures the deposits get “coked”. The best way to check whether the solvent is in a good shape is the total acid number (TAN). This should be low to prevent problems, preferably < 0.04 mg KOH /g. Relatively small amounts already cause problems because the water fraction in the solvent in BTEX extraction systems is low. Since the degradation products are acidic, they can be perfectly removed by using a special adsorption process. This can be done online on a mobile basis using trailer mounted equipment offered by MPR services or it can be done by their permanent technology.......
11/09/2007 A: Amarjit Bakshi, Refining Hydrocarbon Technologies LLC, abakshi@rhtgulfcoast.com
This is mainly due to solvent degradation resulting from temperature/and or oxidation, which might have increased with pressure drop as that would have further increased the temperature.
It is a good idea to keep an eye on solvent quality from time to time to avoid this problem.
Hope this will help or would be happy to provide consulting services in this area.......
10/09/2007 A: Vladimir Gagic, Oil Refinery Pancevo, vladimir.gagic@rnp.co.yu
Solvent degradation can cause heavy deposits on column trays.
Sulfolane solvent degradation can be caused by high temperature or the presence of oxygen. Even small quantities of oxygen will degrade solvent rapidly. As result solvent will change color to dark or black and pH will go down. Did you notice any changes in solvent appearance?
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