6 replies to this topic

[Highlander]

Dear All,

I like to perform a process design of a Direct Fired Reboiler (Amine Reboiler). However, as I have no experience in designing such an equipment, I like to ask for help in clarifying my confusion with coming up with a decent process design.

In trying to come up with a proper design of the Reboiler, I have referred to GPSA (Chapter 8), Kern - Process Heat Transfer and relevant topics found in this forum. But I end up with more questions than answers. I am hoping that I can have my questions clarified so that I can size the reboiler correctly.

1) Based on my understanding, a Direct Fired Reboiler is a pressure vessel filled with process fluids and have Fire Tubes (connected to a Burner) running through the vessel. The Fire Tubes will heat up the process fluids as per process requirement. To size the Reboiler, the main issue is to find out the surface area required. From the calculated surface area, I will be able to find out the Fire Tube size and length. Am I right?

2) From what I have read from Forum topics, a Direct Fired Reboiler is sized based on heat flux. Do I obtain this heat flux value from the Burner vendor?

2) Based on reading, average heat flux used for Amine Reboiler is 8000 to 10000 Btu/hr.ft2. Using the given average heat flux value, can I size the Fire Tube required by dividing the Reboiler duty against heat flux?

3) From GPSA, I can calculate out the Convection Heat Flux and Radiation Heat Flux. Am I suppose to add these two heat flux together in order to obtain the actual heat flux of the reboiler? Will this calculated heat flux value be within 8000 to 10000 Btu/hr.ft2 range?

4) After the flue gas has travelled throught the Fire Tube, it will be directed to a Vent Stack. Is there any methods to determine the length of the Vent Stack required?

Any help given will be greatly appreciated. If my questions appear confusing, I apologise as I am still struggling with the concept. Please question me and I will try to clarify what my queries meant.

Thank you very much.

[Art Montemayor]

Highlander:

My responses to your questions follow:

1) Based on my understanding, a Direct Fired Reboiler is a pressure vessel filled with process fluids and have Fire Tubes (connected to a Burner) running through the vessel. The Fire Tubes will heat up the process fluids as per process requirement. To size the Reboiler, the main issue is to find out the surface area required. From the calculated surface area, I will be able to find out the Fire Tube size and length. Am I right? Correct -but the diameter has to be calculated in accordance with the allowable pressure drop.

2) From what I have read from Forum topics, a Direct Fired Reboiler is sized based on heat flux. Do I obtain this heat flux value from the Burner vendor? No. You obtain an appropriate heat flux value from your experience or from the experience of others.

3) Based on reading, average heat flux used for Amine Reboiler is 8000 to 10000 Btu/hr.ft2. Using the given average heat flux value, can I size the Fire Tube required by dividing the Reboiler duty against heat flux? The value of the correct heat flux to use is based on the experience factor – RELATED TO THE APPLICATION. The correct heat flux will vary with the application. I have used heat fluxes as low as 6,000 Btu/hr-ft² on TEG regeneration reboilers (mainly for degradation reasons).

4) From GPSA, I can calculate out the Convection Heat Flux and Radiation Heat Flux. Am I supposed to add these two heat flux together in order to obtain the actual heat flux of the reboiler? Will this calculated heat flux value be within 8000 to 10000 Btu/hr.ft2 range? No. The heat flux employed is an EMPIRICAL value.

5) After the flue gas has travelled through the Fire Tube, it will be directed to a Vent Stack. Is there any methods to determine the length of the Vent Stack required? Yes, but most people I know do it by experience.

I hope this helps out.

[Highlander]

Hello Mr. Art Montemayor,

Thank you very much for your answers.

Base on your reply, it appears that sizing an amine reboiler depends a lot more on empirical values.

However, I have additional questions that I hope you can take the time to reply.

1) You mention that in determining the size of a Fire Tube diameter, we have to take into account the allowable pressure drop. However, in this case, we will just vent the flue gas into the atmosphere. With that, I would presume that pressure drop requirement is irrelevant right?
If there is waste heat recovery in place, we will have to take into account the allowable pressure drop. Am I right?

2) If sizing a Direct Fired Reboiler is based upon empirical values, may I ask what can the Convection Heat Flux and Radiation Heat Flux calculation provided in GPSA be utilized for? Vertical-cylindrical or cabin fired heaters?

3) With regards to determining the length of the Vent Stack, unfortunately I am just a newbie process engineer with no prior experience in this issue. Can you recommend me any literature that explains the method used in determining length of Vent Stack?

4) Is a Direct Fired Reboiler an uncommon design to utilize? Reason I ask is that I have been asking vendors that specialise in heat transfer equipment, however none of them seems to design such a Reboiler which utilize a Burner to provide hot flue gas through Fire Tube and heat up the process fluid.

Thank you very much. Any reply would be greatly appreciated.

[Art Montemayor]

Highlander:

Here are my additional replies:

1)You mention that in determining the size of a Fire Tube diameter, we have to take into account the allowable pressure drop. However, in this case, we will just vent the flue gas into the atmosphere. With that, I would presume that pressure drop requirement is irrelevant right?
If there is waste heat recovery in place, we will have to take into account the allowable pressure drop. Am I right?
No. Check with your burner manufacturer. With him/her, the pressure drop is very relevant. You will always have a pressure drop through the fire tube – no matter how small; and that pressure drop affects the diameter of the fire tube. Bear in mind that you haven’t even told us whether you are discussing a forced, induced, or natural draft firing chamber.

2)If sizing a Direct Fired Reboiler is based upon empirical values, may I ask what can the Convection Heat Flux and Radiation Heat Flux calculation provided in GPSA be utilized for? Vertical-cylindrical or cabin fired heaters?
Yes. Convection and radiation play a significant role in more complex, more efficient firing chambers and designs.
3)With regards to determining the length of the Vent Stack, unfortunately I am just a newbie process engineer with no prior experience in this issue. Can you recommend me any literature that explains the method used in determining length of Vent Stack?
I don’t have any available. Presumably your concern for the stack design is due to your using natural draft. This is an issue that points out the fact that this is not that easy a design. A process engineer can’t simply make some pencil and paper calculation and go out and build a successful unit. It probably is possible – but I have never heard about it and doubt anyone is fully capable of doing this. All the people I have known who are in this business in Oklahoma and Texas (where the book was written on this type of reboiler) have consistently had to resort to pilot units or have learned from prior sad experiences based on theory. This is why “empirical” is the color of this design and it is rich in Lessons Learned.

4)Is a Direct Fired Reboiler an uncommon design to utilize? Reason I ask is that I have been asking vendors that specialise in heat transfer equipment, however none of them seems to design such a Reboiler which utilize a Burner to provide hot flue gas through Fire Tube and heat up the process fluid.
The direct, gas-fired reboiler was innovated and developed out here in Texas, Oklahoma and surrounding states during the first half of the last century. The driving force was the need for natural gas transport and marketing. These units are seen everywhere around here, in remote gas well sites doing both amine reboiling as well as TEG. They are also employed as water bath heaters in well-head crude oil production prior to heater-treaters. They are “a dime a dozen”.

You stated you would "like" to design this unit. That's fine; you have a lot of learning ahead of you. But if you have to obtain such a unit due to an industrial assignment or application, then I would suggest that it is far more expedient to import one from the USA and learn from that design in the future.

[Propacket]

1)You mention that in determining the size of a Fire Tube diameter, we have to take into account the allowable pressure drop. However, in this case, we will just vent the flue gas into the atmosphere. With that, I would presume that pressure drop requirement is irrelevant right?
If there is waste heat recovery in place, we will have to take into account the allowable pressure drop. Am I right?
No. Check with your burner manufacturer. With him/her, the pressure drop is very relevant. You will always have a pressure drop through the fire tube – no matter how small; and that pressure drop affects the diameter of the fire tube. Bear in mind that you haven’t even told us whether you are discussing a forced, induced, or natural draft firing chamber.

[/color]

Highlander,

When sizing the fire tube, an important factor is heat density through the fire tube. Heat density is heat transferred per unit cross-sectional area of fire tube. A typical value is to keep heat density below 21,000 Btu/hr.in2. Higher values cause instability of flame. But the above value should not be taken as standard because heat density can be best provided by burner manufacturer.

[Propacket]

Just imagine that you have an undersized fire tube. High pressure drop through the fire tube will increase the velocity of flue gases causing high heat transfer coefficients. As a result, heat density through the fire tube will increase which must be limited for adequate operation of burner. Therefore, you can’t allow pressure drop to increase to any value.

[Highlander]

Mr. Art Montemayor & P. Engr,

Thank you very much for your explanations. I have much to learn. )