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Pre-Flash Drum Design


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#1 Leonard2511

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Posted 06 October 2021 - 03:14 AM

Hello everyone, I am facing several problems so I hope someone could help me

Now, I am doing a final project which is related to design a heat exchanger network with pre-flash drum for a crude oil distillation unit (CDU). I have some questions:

- What is the operating conditions of pre-flash drum ? I found some information about pressure that is equal or approximate to the pressure of main tower (CDU) but the temperature was not mentioned.

- How much vapor fractions that releases from the pre-flash drum ? And how the vapor fraction effect on the main tower ?

I do this project that I depend on some data about the existing CDU such as product specification, operating conditions of main tower, and the initial configuration of the CDU does not include pre-flash drum, and I wanna implement a pre-flash and do a new heat exchanger and compare the difference between the presence of pre-flash and not presence of this. 

In conclusion, I hope someone could help me to solve this problem 



#2 Pilesar

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Posted 06 October 2021 - 09:04 PM   Best Answer

When students ask questions in the student section, the responses may be in more detail and focused on the academic problem. Real world applications often require different considerations. I will consider this a student question. A drawing of the system you are describing would be helpful.

  Temperatures and vapor fractions along the crude preheat train depend upon composition and the heat exchanger network. Because of the tendency of the heavier components of crude oil to crack at high temperatures, the temperature of the feed furnace is controlled to be no higher than 700 F. Because this furnace duty is a huge operating expense, optimizing the exchanger preheat train has a large potential for energy savings. The temperature of the furnace feed is made as high as possible and the furnace firing is used to heat the feed the rest of the way. The system pressure is calculated around 'anchor points'. For a crude column the pressure at the top of the column is the 'anchor' and the upstream pressures are calculated according to their necessary pressure drops. Your question about the effect of a pre-flash drum in your system should be one of the goals of your project and cannot be answered without doing the work.



#3 Leonard2511

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Posted 07 October 2021 - 05:13 AM

When students ask questions in the student section, the responses may be in more detail and focused on the academic problem. Real world applications often require different considerations. I will consider this a student question. A drawing of the system you are describing would be helpful.

  Temperatures and vapor fractions along the crude preheat train depend upon composition and the heat exchanger network. Because of the tendency of the heavier components of crude oil to crack at high temperatures, the temperature of the feed furnace is controlled to be no higher than 700 F. Because this furnace duty is a huge operating expense, optimizing the exchanger preheat train has a large potential for energy savings. The temperature of the furnace feed is made as high as possible and the furnace firing is used to heat the feed the rest of the way. The system pressure is calculated around 'anchor points'. For a crude column the pressure at the top of the column is the 'anchor' and the upstream pressures are calculated according to their necessary pressure drops. Your question about the effect of a pre-flash drum in your system should be one of the goals of your project and cannot be answered without doing the work.

First of all, thanks for your reply. 
 

I'm a final year student and I perform this project to graduate in next year. I looked for many topics and scientific papers that related to pre-flash drum. But it might be complex and several theories that I can not understand. However, after reading all references and I tend to carry out this project following this stages:
- I will collect all required data consisting of Crude Assay, Specification Products, Operating Conditions (1)

- I will choose the vapor fraction after releasing from the Pre-Flash Drum and the conditioning temperature ( V/F = 0.03 (the composition of light ends in crude assay accounts for 2.86% so that I choose 3% and T = 200 ) (2) 

- I will do a simulation on Aspen Hysys and I will check the Specification Products (3) 

- I get all data that are relevant to pre-heat train network, for example: the residue will be used to exchange heat with crude oil (4)

- I will use pinch technology to design a HEN (5)

- Conclusion, I will make a comparison to the initial configuration (does not pre-flash) (6), which is described below:

+ comparing to Specification Products 

+ the energy consumption

+ CO2 emissions

+ operating costs

I assume that if I implement a pre-flash, the hydraulics in the main column (CDU) does not have any problems (flooding and weeping) and the ID (internal diameter) is constant.

Now I am doing at the fourth-stage and I continue to the next stage, but I am not sure that the above mentioned operating conditions of pre-flash drum are right so I do not know how to check the operating conditions. I think that I will calculate and design the size of the pre-flash drum which is able to prevent the entrainment "foaming" in pre-flash, but I do know which size is suitable for preventing this entrainment. I refer to a paper that discusses the undersized pre-flash may entrain foaming into the main column, but they do not mention which size is considered as undersized pre-flash ( a concrete number about height or diameter).

Besides that, I want to design a new HEN, I do not tend to retrofit or revamp the existing refinery because it is too complex with me.

 

That's all I want to share with you. Hope you reply me and help me to solve this. If there are any mistakes or failures in my steps, please tell me and I will fix it. Thank you so much, sir!


Edited by Leonard2511, 07 October 2021 - 05:39 AM.


#4 Pilesar

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Posted 07 October 2021 - 08:03 AM

A simple drawing of the system you are describing would be helpful. Please locate the heat exchanger network, the pre-flash drum, the furnace, and the column on the drawing. Explain where you are intending to locate the new drum and what you intend to do with the vapor and liquid products. Explain what benefit you are expecting to result from adding this new drum. Identify the units of measure when you describe process conditions.
 You write "I will choose the vapor fraction after releasing from the Pre-Flash Drum and the conditioning temperature" which seems a problem to me. How do you choose both the vapor fraction and the temperature? Concentrate your efforts on getting a high temperature and then calculate the vapor fraction that results. 
 In your work plan, I think you will need to design your heat exchanger network before you can know what the vapor fraction will be at the drum. You can make a guess (as you have done) but this will need to be an iterative process. If you are planning to design two systems (one with special drum and one without) then I would first model the system without your special drum. That will be difficult in itself so do not underestimate the effort. After that work is complete, you will have a base case to use for comparison and it will serve as a starting point for making modifications.
 You still have much work remaining. Sizing the special drum will have its own trouble and cannot be done until later unless you make conservative assumptions. If you have questions about that then, it will deserve its own thread in the forums. 
 We have not done the same research you have done on this. We are not even certain of your process flowsheet! Consider advice you get from these forums to be good intentioned, but check it with your own understanding.


#5 Leonard2511

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Posted 07 October 2021 - 10:17 PM

 

A simple drawing of the system you are describing would be helpful. Please locate the heat exchanger network, the pre-flash drum, the furnace, and the column on the drawing. Explain where you are intending to locate the new drum and what you intend to do with the vapor and liquid products. Explain what benefit you are expecting to result from adding this new drum. Identify the units of measure when you describe process conditions.
 You write "I will choose the vapor fraction after releasing from the Pre-Flash Drum and the conditioning temperature" which seems a problem to me. How do you choose both the vapor fraction and the temperature? Concentrate your efforts on getting a high temperature and then calculate the vapor fraction that results. 
 In your work plan, I think you will need to design your heat exchanger network before you can know what the vapor fraction will be at the drum. You can make a guess (as you have done) but this will need to be an iterative process. If you are planning to design two systems (one with special drum and one without) then I would first model the system without your special drum. That will be difficult in itself so do not underestimate the effort. After that work is complete, you will have a base case to use for comparison and it will serve as a starting point for making modifications.
 You still have much work remaining. Sizing the special drum will have its own trouble and cannot be done until later unless you make conservative assumptions. If you have questions about that then, it will deserve its own thread in the forums. 
 We have not done the same research you have done on this. We are not even certain of your process flowsheet! Consider advice you get from these forums to be good intentioned, but check it with your own understanding.

you said: " you will need to design your heat exchanger network before you can know what the vapor fraction will be at the drum." But I do not know the inlet temperature into pre-flash. I wish that a reference mentioned this because I have to need a temperature to design a HEN. I have the temperature in tank, the desalted temperature and the outlet temperature furnace. If I implement a pre-flash, the process temperature is described:

 

Temperature in Tank ----> Desalted Temperature -----> The inlet temperature pre-flash drum ( I am finding the value) ----> outlet temperature furnace. 
Can I calculate this inlet temperature pre-flash drum ? But I do not have any exact information. Now it is a problem. If I assume a temperature, what will I do to evaluate the conditions that I propose. Could you have any recommends ? 

Thanks a lot! Sir 



#6 Pilesar

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Posted 08 October 2021 - 09:45 AM

A simple drawing of the system you are describing would be helpful.



#7 SilverShaded

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Posted 10 October 2021 - 04:09 AM

Preflash drum pressure must be higher than the CDU tower, otherwise the vapours won't get to the CDU, it's kind of self controlling.  Pressure needs to be high enough to push the vapours to the CDU flash zone, the temperature is up to you but your mostly trying to flash off naphtha.  The more vapour you flash off, the lower the CDU yields will be (this is not obvious and most people get it wrong for non-obvious reasons, even so called experts).  Flash off enough vapour that the feed to the furnace is liquid at whatever pressure and temperature you have designed the furnace feed for.

 

 



#8 Pilesar

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Posted 10 October 2021 - 03:46 PM

SilverShaded... Would you explain further what you mean by 'lower CDU yields'? I think this means the atmospheric resid will be higher proportion due to the lower amount of stripping vapor? So the only advantage to a preflash drum is a more efficient feed furnace? (I've only modeled refineries, not designed or worked in them. In modeling, I integrated the feed furnace with the column for easier solution.) Thanks.



#9 SilverShaded

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Posted 11 October 2021 - 03:44 AM

SilverShaded... Would you explain further what you mean by 'lower CDU yields'? I think this means the atmospheric resid will be higher proportion due to the lower amount of stripping vapor? So the only advantage to a preflash drum is a more efficient feed furnace? (I've only modeled refineries, not designed or worked in them. In modeling, I integrated the feed furnace with the column for easier solution.) Thanks.

Sure, yes if you take the naphtha out of the crude you will vaporise less in the furnace at any given temperature or pressure, therefore you get more atmospheric residue, more VDU feed and a lighter VGO material in the VDU, which may impact ejector performance to some degree.  Increasing VDU feed will also increase VDU furnace duty and vaporising something under vacuum takes more energy than vaporising it at atmospheric pressure.  The upside is you might vaporise more in the VDU furnace and end up with less vacuum residue (or not).

 

So preflash drums may be usefull for debottlenecking a CDU furnace, or designing for light crudes/lower pressure prehehat train, but don't save energy overall.  Preflash towers maybe slightly better.

Theres more CDU/VDU designs out there than stars in the sky (well allmost).


Edited by SilverShaded, 11 October 2021 - 03:44 AM.


#10 SilverShaded

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Posted 18 October 2021 - 05:26 AM

 

SilverShaded... Would you explain further what you mean by 'lower CDU yields'? I think this means the atmospheric resid will be higher proportion due to the lower amount of stripping vapor? So the only advantage to a preflash drum is a more efficient feed furnace? (I've only modeled refineries, not designed or worked in them. In modeling, I integrated the feed furnace with the column for easier solution.) Thanks.

Sure, yes if you take the naphtha out of the crude you will vaporise less in the furnace at any given temperature or pressure, therefore you get more atmospheric residue, more VDU feed and a lighter VGO material in the VDU, which may impact ejector performance to some degree.  Increasing VDU feed will also increase VDU furnace duty and vaporising something under vacuum takes more energy than vaporising it at atmospheric pressure.  The upside is you might vaporise more in the VDU furnace and end up with less vacuum residue (or not).

 

So preflash drums may be usefull for debottlenecking a CDU furnace, or designing for light crudes/lower pressure prehehat train, but don't save energy overall.  Preflash towers maybe slightly better.

Theres more CDU/VDU designs out there than stars in the sky (well allmost).

 

Actually forgot to say theres not only the stripping effect of the Naphtha but with a preflash drum the cool vapour enters at the flash zone and cools down the main feed vapour, so this is another, and possibly the main reason, you get more residue.  The preflash tower is slightly better as the preflash vapour does not need to enter at the flash zone of the main column, but you still lose the stripping effect.


Edited by SilverShaded, 18 October 2021 - 05:27 AM.





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