Crude overhead water wash system 1

[hanon]

We are currently designing a water wash for the overhead crude condenser (the unit have two condensing stages: hot and cold drum) in order to to be able to manage lower temperatures and to avoid corrosion in the first condenser due to a lack of neutralizing injection in this stage (not recommended for being a "dry" overhead system)and a flow maldistribution in the three parallel condenssers (which produce a low-temperature dew point corrosion). I would be grateful in receiving any further information about practical tips needed for a correct work of this system (water quality, flowrate of recirculated water, design of drum, water boot & riser height, type of spray nozzle, filter, water purge, restriction orifice...). I have read an answer in thread483-111706 posted by 25362 about a link to a PTQ paper (PTQ04314.pdf) which is not longer available. Could anyone pass me a copy or a new link?

I am especially interested in the subject about the necessity of a make-up water stream to avoid concentrating the salts in the recirculated water. I am interested if this stream is needed and if so, the standard flowrate for this stream. Any recomendations are welcome.

If someone is also interested I have found another interesting thread483-101342 about this subject and some design practices in the Lieberman´s book "Process Design for Reliable Operations" and NACE paper No. 615 - 585 - 591 - 1538

Regards

 

[EmmanuelTop]

Hello Hanon,

You are right: injecting neutralizing amine upstream of 1st stage condensers would be a disaster. But, if you have decided to apply waterwash system, keep on mind the following:

- Continuous washing implies 1st condensers duty reduction, due to decrease in LMTD (by having forced H2O condensation at 1st stage condensers inlet). This also means higher hot drum temperature (if condensing capacity is now at its maximum) and bigger heat load of 2nd stage condensers. As a result, more intensive flaring could occur. Also, smaller amount of reflux naphtha will be available.

- How do you control the amount of extra-liquid (naphtha) from hot reflux drum, which is not used as column reflux? In one of the units I had seen, this extra-quantity is routed to heavy naphtha product tank, together with downstream naphtha splitter bottoms; this arrangement (pressure is controlled via PCV on the cold drum) gives much flexibility to the unit, compared to designs where system pressure is controlled through bypass valve around 1st stage condensers (in other words, it is not possible to control system pressure and naphtha end point at the same time, because there is no additional line provided for extra-amounts of hot drum naphtha to storage).

- What is your average salt and BS&W content in desalted crude and what test method do you use? Generally, as you probably well know, the best way to battle crude unit corrosion is to optimize desalter operation up to its maximum.

 

[EmmanuelTop]

Concerning the flowrate of wash water: you must calculate the amount of liquid H2O which will be big enough to promote forced condensation of steam at 1st stage condensers inlet, and to provide sufficient flow of liquid H2O to (continuously?) wash away salt deposits. This can be done by knowing overhead composition and system parameters (T, P). In my opinion, it is best to use process simulators for such purpose, or to consult chemical vendors. One interesting and highly experienced guy is Henk P. Helle (NACE member); I had a few contacts with him. You can find him at:

http://www.corrosioncontrol.nu/

FIC control of washwater should be more than enough. I think there is no such thing as "standard flowrate": it strongly depends on your system characteristics, water dew-point and overhead naphtha cut-point.