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Predicting Fouling Factor In Reboilers


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#1 ProcessEng_12

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Posted 12 August 2021 - 11:55 AM

Hello everyone.

I am working on one of the reboilers in our plant.

My project is to develop a model to calculate and show at what rate the fouling factor is increasing.

I have developed a model of reboiler in HTRI. The reboiler is a forced flow reboiler. I have matched the geometry with the as built.

At the start of the operation, the steam chest pressure is 2.5 barG (Saturation temp: ~139 degC) and end of operation is 7 barG (Saturation temp: ~170 degC)

So, as the reboiler fouls, the steam CV opening increases and chest pressure increases and similarly the saturation temp increases. Thus, due to this LMTD across the reboiler also increases.

 

While predicting fouling factor by hit & trial method to match process outlet temperature in the model, the fouling factor predicted is on lower side. This is wrong practically. 

 

This is a bit confusing. So stating example:

 

Case 1: Steam chest pressure:3 barG. Process outlet temp: 120 degC. Calculated Fouling factor: 0.003 m2-hr-C/Btu

Case 2: Steam chest pressure: 5barG, Process outlet temp: 120.5 degC. Calculated Fouling factor: 0.0028 m2-hr-C/Btu

 

The heat transfer coefficient also increases due to better LMTD in Case 2. Case 2 is showing that my fouling has decreased which is not correct.

 

 

Please help me how can i calculate a correct fouling factor in HTRI or by calculation?

 



#2 Bobby Strain

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Posted 12 August 2021 - 02:57 PM

Your assignment is a waste of your time. Valuable time you could be seeking revenue producing projects! You don't need to know the fouling factor. You already have the information to predict when cleaning is required, if that is your goal.

 

Bobby



#3 Pilesar

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Posted 12 August 2021 - 03:16 PM

Plotting how the fouling factor changes over time can help you predict when future cleaning will be needed. Calculation depends on how precise you need an answer.
Q=UAdT so if you assume constant Duty and Area, then UdT for clean exchanger equals UdT for dirty exchanger. The temperature difference is measured, so calculate the U for both conditions. Fouling factor is a parameter of overall heat transfer coefficient U. You can use assume the other parameters of U are constant and calculate the fouling factor at both conditions.
These assumptions are not necessarily valid since all things are not necessarily equal. The surface area can change with a thick fouling layer which also can result in velocity changes, but simplification may be good enough. HTRI calc would be more sophisticated than the above description. HTRI in rating mode should give good answers when you tune the fouling factor for the process side. Manual iteration is how I would tune it. I would leave the steam side fouling factor constant.


#4 breizh

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Posted 12 August 2021 - 07:24 PM

Hi,

Consider the documents attached to support your work .

Breizh 



#5 ProcessEng_12

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Posted 17 August 2021 - 01:30 AM

Your assignment is a waste of your time. Valuable time you could be seeking revenue producing projects! You don't need to know the fouling factor. You already have the information to predict when cleaning is required, if that is your goal.

 

Bobby

Hi Bobby.

The assignment is to be done for comparing the different cleaning techniques (done in past) and show it to the management. Obliviously run length is an indication but we want to predict for our decisions for annual turnaround decisions. Your valuable input will be helpful. Thanks



#6 ProcessEng_12

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Posted 17 August 2021 - 01:41 AM

 

Plotting how the fouling factor changes over time can help you predict when future cleaning will be needed. Calculation depends on how precise you need an answer.
Q=UAdT so if you assume constant Duty and Area, then UdT for clean exchanger equals UdT for dirty exchanger. The temperature difference is measured, so calculate the U for both conditions. Fouling factor is a parameter of overall heat transfer coefficient U. You can use assume the other parameters of U are constant and calculate the fouling factor at both conditions.
These assumptions are not necessarily valid since all things are not necessarily equal. The surface area can change with a thick fouling layer which also can result in velocity changes, but simplification may be good enough. HTRI calc would be more sophisticated than the above description. HTRI in rating mode should give good answers when you tune the fouling factor for the process side. Manual iteration is how I would tune it. I would leave the steam side fouling factor constant.

 

Hi. Thank you for your prompt response.

I understand the concept you are explaining. I have done exactly the same thing in HTRI.

I am running the exchanger in rating mode. I have given input the steam flow, vapor fraction as 1, steam chest pressure, process flow, process inlet temperature. I have kept steam side fouling factor constant. 

So, to achieve the desired or the plant DCs temperature, i am changing the fouling factor of process side.

But the as the steam chest pressure is increasing, saturation temperature is increasing, due to this LMTD across the reboiler also increases. So, the calculated fouling factor is less.

The higher LMTD is compensating for the calculated fouling factor (< actual fouling factor).

Please help me with your thought process in this.



#7 ProcessEng_12

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Posted 17 August 2021 - 02:28 AM

Hi,

Consider the documents attached to support your work .

Breizh 

Thank you. I shall go through them and let you know.



#8 Pilesar

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Posted 17 August 2021 - 08:14 AM

Over time, the fouling factor will increase and the LMTD (steam chest pressure) increases to compensate. If your calculated fouling factor is not increasing over time, then the fault probably lies in an unmeasured parameter. Trending fouling factor is much more straightforward where there is no phase change such as in a refinery preheat train. Even in such a situation, the trend is upward even though the individual measurements do not form a monotonic curve. With a reboiler, perhaps the process flow rate varies along with the percent of vaporization over time. All things are seldom equal. Fouling progression does not usually have a linear slope and may require many data points to recognize a trend. It just may be that there are some important variables that you will never be able to measure adequately. I like your approach to the project, but if it is to come to a dead end then limit your efforts. Maybe plotting the steam chest pressure for the different cleaning techniques would be meaningful to your management as a representation of fouling.



#9 Bobby Strain

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Posted 17 August 2021 - 09:23 AM

What you observe is likely the impact of a changing surface area. You don't provide any information that might allow a confirmation. But if you observe an almost constant steam side pressure following cleaning, then a gradual increase, that is likely the case. It's not rocket science.

 

Bobby



#10 darshan2985

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Posted 19 October 2023 - 04:25 AM

Hi,

Consider the documents attached to support your work .

Breizh 

Hello Breizh,

 

I didnot find any document attached to this post. Could you please resend it.

 

THanks,

Darshan



#11 breizh

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Posted 19 October 2023 - 05:14 AM

Hi,

Not sure if these documents are those you are referring to.

Take a look.

Breizh






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