9 replies to this topic

[Pleks14]

Hello everyone,

I was wondering if anyone has any experience with the use of antifoulants in crude preheat trains. Our chemical vendor is proposing substantial savings in both higher throughputs (due to higher inlet temperature to our atmospheric furnace) and less mechanical/chemical cleanings of our heat exchangers. This is based on the fact that the antifoulant decreases the rate of fouling on the preheat exchangers, particularly the hotter exchangers at the end of the train.

Of course, the program itself has a fair cost, and installation/commissioning of the facilities will add on to that. We would be doing a trial first, so I was just curious what others may have seen.

Thanks,
Pleks14

[Zauberberg]

My experience with using antifoulants in hot preheat train suggests that you should not expect miracles - because they will not happen. Chemicals can reduce the fouling up to 10-15% as compared with running the plant without having any anti-fouling philosophy.

The main contributor to the fouling in hot preheat exchangers is lower solubility of inorganic salts in water, which is carried off from the desalter anyway. So as the temperature of crude oil raises through the exchangers, mass transfer of water into the crude is accelerated while the solution of salts in water becomes supersaturated and they simply deposit on the outer surface of exchanger tubes (crude is usually on the shell-side). In addition, if crude oil is rich in asphaltenes, temperature rise can cause deposition of these complex compounds and what you can usually see when you dismantle heat exchangers is something like a black pancake around the tubes.

In some occasions, applying online water wash can help: simply connect the water source to the crude-side of exchangers and wash them for several hours or longer (while the CDU is online). If inorganic salt deposition is the main culprit of extended fouling, additional water will wash away these deposits.

Good luck,

[Pleks14]

My experience with using antifoulants in hot preheat train suggests that you should not expect miracles - because they will not happen. Chemicals can reduce the fouling up to 10-15% as compared with running the plant without having any anti-fouling philosophy.

The main contributor to the fouling in hot preheat exchangers is lower solubility of inorganic salts in water, which is carried off from the desalter anyway. So as the temperature of crude oil raises through the exchangers, mass transfer of water into the crude is accelerated while the solution of salts in water becomes supersaturated and they simply deposit on the outer surface of exchanger tubes (crude is usually on the shell-side). In addition, if crude oil is rich in asphaltenes, temperature rise can cause deposition of these complex compounds and what you can usually see when you dismantle heat exchangers is something like a black pancake around the tubes.

In some occasions, applying online water wash can help: simply connect the water source to the crude-side of exchangers and wash them for several hours or longer (while the CDU is online). If inorganic salt deposition is the main culprit of extended fouling, additional water will wash away these deposits.

Good luck,

Thanks for the response. I was hearing numbers in the 60-70 % reduction in fouling rates through the use of antifoulants, and I was quite skeptical, especially since I don't have much background data/info to look at.

Regarding the foulant, it is predominantly asphaltenes, which goes along with your statement about temperature being proportional to the deposition of these compounds. With desalter operation working pretty well (>95 % removal of salts), and the heat exchangers that exhibit the most fouling being after the desalter, I'm not too worried about salt deposition. But thanks for the recommendations.

[Zauberberg]

Analyze the deposit thoroughly - it will definitely point out the main culprit(s) of fouling. For single-stage desalting, 95% efficiency is generally OK but you should also look at absolute figures (ppm of salt which are carried away with crude). These salts will precipitate along the hot preheat train as water solubility in crude oil increases with temperature, making the residual free water supersaturated with inorganic salts. Do you inject caustic downstream of desalter, and what are the chloride levels in overhead receiver?

Also, it will be quite useful to visit website of Henk Helle: http://www.corrosioncontrol.nu/
This guy is well known for corrosion management in Crude and Vacuum units, and attending one of his seminars or getting his excelent technical book will give you more insightful views into CDU corrosion mechanisms and management.

Best regards,

[Pleks14]

For desalted crude salt levels, usually were 0.5 ptb or lower (single stage desalter). Our corrosion control is quite extensive I would say, although it could be better. We do have ER probes in most of our pinch points, and TAN inhibitor injection points in many streams.

Off the top of my head I'm not sure of the chloride levels in the overheads, however I know they're quite low. When we have the exchangers out next year, I'll take a close look at the foulant/get appropriate testing done. My predecessor had taken some pictures from the last cleaning, and it was mostly asphaltene fouling.

Btw, thanks for the link.

[Subra]

The topic was really wonderful and informative.
I have read in the one reply about use of wash water for reducing fouling when unit is online.

We are planning to start water washing downstream of desalter mainly to decrease fouling in preheat exchangers . How much water can we use for this purpose. There must be a limit as excess of water may result in upset of pretopping column. I have attached the PFD of our crude distillation column in excel format.
Kindly suggest.
Thanks in advance.
I am a new process engineer looking after atmospheric distillation unit in Indian Oil, India.

regards
S.Subramanian

Attached Files

•  CDU_PFD_.xls   160KB   82 downloads

[Zauberberg]

At first look, it appears strange that you have decided to apply water-washing of heat exchangers. Why? Based on which facts the cleaning/washing of heat exchangers has been judged as required?
The design of your preheat section is what I call "fouling resistant" since you have crude booster pump downstream of preflash tower. Such network configuration is provided mainly for the purpose of handling lighter crude oils, and for maintaining high velocity/high pressure drop in hot preheat exchangers, which should be beneficial for effective and long-lasting heat transfer.

There is no universal recipe for water volume required for online washing - it depends on the nature of deposits, and time between two cleaning cycles. If you are suffering from asphaltene deposition, water washing will simply not help - these are insoluble in water. On the other hand, if inorganic salt deposition is the cause of reduced heat transfer rates, water will act as disolving agent. You can start with water amount which corresponds to maximum water carryover from desalter, and increasing the flow slightly untill you observe, for example, excessive foaming in the preflash tower (watch split-level gauge glasses), or sudden temperature rise in the V-1 overhead receiver, or coloured (blackish) naphtha product from V-1 vessel. Preflash towers are in general very susceptible to foaming, and additional water will act quite detrimental.

Reduced CDU throughput while online washing is applied is definitely something which I strongly recommend. But first, make sure that you really need water washing.

Good luck,

[Qalander (Chem)]

At first look, it appears strange that you have decided to apply water-washing of heat exchangers. Why? Based on which facts the cleaning/washing of heat exchangers has been judged as required?
The design of your preheat section is what I call "fouling resistant" since you have crude booster pump downstream of preflash tower. Such network configuration is provided mainly for the purpose of handling lighter crude oils, and for maintaining high velocity/high pressure drop in hot preheat exchangers, which should be beneficial for effective and long-lasting heat transfer.

There is no universal recipe for water volume required for online washing - it depends on the nature of deposits, and time between two cleaning cycles. If you are suffering from asphaltene deposition, water washing will simply not help - these are insoluble in water. On the other hand, if inorganic salt deposition is the cause of reduced heat transfer rates, water will act as disolving agent. You can start with water amount which corresponds to maximum water carryover from desalter, and increasing the flow slightly untill you observe, for example, excessive foaming in the preflash tower (watch split-level gauge glasses), or sudden temperature rise in the V-1 overhead receiver, or coloured (blackish) naphtha product from V-1 vessel. Preflash towers are in general very susceptible to foaming, and additional water will act quite detrimental.

Reduced CDU throughput while online washing is applied is definitely something which I strongly recommend. But first, make sure that you really need water washing.

Good luck,

Dear Subra hello, Just to add little more caution.
Probably with water you implied either BFW(boiler feed water) or treated water suiting local (point process) conditions of pressure and temperature.
However It may result into possibility of
1) Water hammer,
2) Thermal shocks&
3) Dislodging of some deposits actually passivating certain surfaces ; thereby increased risks to smooth operation.
Hope you do realize all this!
Regards

[Qalander (Chem)]

At first look, it appears strange that you have decided to apply water-washing of heat exchangers. Why? Based on which facts the cleaning/washing of heat exchangers has been judged as required?
The design of your preheat section is what I call "fouling resistant" since you have crude booster pump downstream of preflash tower. Such network configuration is provided mainly for the purpose of handling lighter crude oils, and for maintaining high velocity/high pressure drop in hot preheat exchangers, which should be beneficial for effective and long-lasting heat transfer.

There is no universal recipe for water volume required for online washing - it depends on the nature of deposits, and time between two cleaning cycles. If you are suffering from asphaltene deposition, water washing will simply not help - these are insoluble in water. On the other hand, if inorganic salt deposition is the cause of reduced heat transfer rates, water will act as disolving agent. You can start with water amount which corresponds to maximum water carryover from desalter, and increasing the flow slightly untill you observe, for example, excessive foaming in the preflash tower (watch split-level gauge glasses), or sudden temperature rise in the V-1 overhead receiver, or coloured (blackish) naphtha product from V-1 vessel. Preflash towers are in general very susceptible to foaming, and additional water will act quite detrimental.

Reduced CDU throughput while online washing is applied is definitely something which I strongly recommend. But first, make sure that you really need water washing.

Good luck,

Dear Subra hello, Just to add little more caution.
Probably with water you implied either BFW(boiler feed water) or treated water suiting local (point process) conditions of pressure and temperature.
However It may result into possibility of
1) Water hammer,
2) Thermal shocks&
3) Dislodging of some deposits actually passivating certain surfaces ; thereby increased risks to smooth operation.
Hope you do realize all this!
Regards

In addition even the distillation tower internals may be somewhat at slight risk.
Regards
qalander

[Pleks14]

The risk of blowing out the trays of the pre-flash tower obviously depends on how much water you plan to use, increasing with additional water. So as you're cleaning gets potentially better, your risk goes up (I would say the risk goes up faster than the cleaning efficiency).

Then you have the risk of excessive water going to the atmospheric pipestill. You have to ensure the overhead drum on the pre-flash tower can handle the water you plan to add; residence time calculations for separation, as well as hydraulic limitations on the water line.

I guess Zauerburg hit it on the head - with starting at the maximum water carry-over from the desalter. And the fact that if it is asphaltene fouling, which I would think it would predominantly be, the water wash will be ineffective at improving heat transfer.