Q & A > Question Details
I am facing an unusual problem of a localized fouling in vacuum column top section and i am trying to develop a solution for the ongoing problem , i am looking for any advice or insights or even prior experience with similar problems , any contributions are highly welcomed .
My problem is periodic formation of semi-solid fouling in the top section of the tower despite of operating at relatively low temperatures (Tray temperature 185 C ) and low Pressures (-0.955 kgf/cm2) , i assumed that cracking or coking at this conditions is highly unlikely at this conditions (correct me if i am wrong) and i assume that the problem might be caused by phase separation of asphaltenes entrapped in light hydrocarbons .... is there any way to exactly determine the problem , what kind of lab tests can be done? any one faced similar problems in vacuum columns?

Thanks all for your valuable answers , I want to add some missing information to the original posts , first of all the fouling color is blackish and the top tower temperature is nearly 85 C ....the fouling seems to be of a hydrocarbons origin.......... it was noted that the fouling increase with the increase of overhead temperature
what steps and lab tests can i do to exactly characterize the fouling?

We Analyzed the solid fouling using x-ray analysis , it was 98.9 % Hydrocarbon, 0.7 % Sulfur , the rest are trace metals with various low percentages (0.01 ~0.02 % ) ... the lab analysis didn't indicate any chlorides , i am not sure if the x-ray analysis can or can't detect chlorides but will discuss it with the lab chemist , most of the replies suggested ammonium chlorides , but apparently it isn't the case....
26/10/2017 A: Mayur Talati, Reliance Industries Limited, mayur.talati@ril.com
This is an interesting phenomena i.e. organic (Hydrocarbon) fouling in vac top . Is this resolved / root cause could be determined ?
21/04/2017 A: Albert VELD, Wintech Global, aveld@wintech-global.net
We recently noticed that many communications are posted on blogs and forums dealing with fouling problems in top sections of crude distillation columns (CDU/VDU). These fouling problems often are associated with increased pressure drops, thus leading to production loss and degradation of the economic benefits of the crude distillation units. Although not frequently mentioned in the forum and blog discussions, the occurrence of fouling and increase of pressure drop in the top section of the distillation columns should alert us of another and unfortunately more serious issue, being Equipment Integrity. Severe corrosion of column wall, internals and draw-off piping is generally associated with the observed fouling and increased pressure drops in the top sections of CDU and VDU.
It is widely agreed that extremely corrosive chloride salts such as ammonium chloride or amine chloride play a major role in the degradation and attack of column internals and draw off piping. Corrosive attack of column internals (trays, packings, distributors, …) will lead to operational problems and economic losses, but the corrosive degradation of piping represents a major integrity risk for the refinery. In addition to the above, one shall note that the corrosion of the circulation reflux piping by e.g. ammonium chloride will accelerate fouling of the top section of the column by the formation of Iron sulfide, a sticky and black product often called “coke” by site inspectors. In the end, it is necessary to absolutely avoid the risks of damage that could lead to accidents or even fires in the distillation unit. The unfortunate experience of some refiners shows that the danger is real.
01/04/2017 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
The localised and intermittent nature of the apparent fouling problem implies ammonium chloride deposition is a likely culprit. If you are operating close to the water dew point at the top of the column, it is possible water droplet formation is dissolving the salt deposits but when conditions change and the liquid temperature increases the water phase is driven off leaving the salt deposits behind. In addition to causing sub-optimal operation in the short term, this effect can cause corrosion damage to the column internals in the water dew point location which may eventually lead to premature shutdown if yields decline dramatically (due to tray collapse) or column integrity is threatened (internal HCl acid attack). I would recommend optimising CDU desalter performance to remove more chlorides from VDU feed and operating with a higher margin above water dew point temperature at the top of the VDU column.
01/04/2017 A: Ganesh Maturu, Self, maturu.ganesh@gmail.com
It is mainly because of chlorides at vacuum column top. we cant avoid chlorides but maintain vac column top temp above nh4cl sublimation temp and also ensure water dewpoint is above column top temperature
31/03/2017 A: Dave Collings, Jacobs, dave.collings@jacobs.com
It's not unusual to have fouling in a vacuum column where temperatures are below 200degF (93degC). Years ago we had problems in our vacuum overhead precondenser. To remedy the pressure drop associated with this fouling, we would periodically (perhaps once every 3 months) increase the temperature substantially for about 6 hours. As I recall, we raised the temperature by approximately 100degC (55 degF). You lose yield during this short time, but it helped the pressure drop. The fouling material was present in the precondenser when we opened it and was identified as salts. Since then, I've run across other customers with similar problems, but most do not have overhead precondensers.
31/03/2017 A: NS Murthy, Suez, murthy.ns.ext@suez.com
Check whether the deposits are water soluble or not. If it is water soluble with white type deposits, we safely assume the foulant is from NH4Cl formation. However, if the foulant is not water soluble, then start look at the feedstock and the operating parameters in vac. heater. Check for presence of conjugated di-olefins formation or presence in feed to get such experience. Take to your nearest GE Water and Process Technology representative to analyze the deposit and offer the lasting solution.
31/03/2017 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I the top of the vacuum the most likely cause of fouling is from ammonium chloride salt formation that comes from running the tower top too cold. The salt point can be estimated from the concentration of ammonia and chlorides in the overhead water. Foul at samples, if you could get them, would show very high levels of N and Cl-.