4 replies to this topic

[Ammart42]

Hello, 

 

I am a 3 year process operator for a stabilization facility and am trying to learn as much as i can about process and optimization. and this is my first post so bare with me. and thank you for any time.

 

at my facility we have condensate which is composed of C1- C10+. we stabilize the product for sale. here recently I have noticed senior board operators encouraging and practicing the use of our reboiler temperature bypass. I disagree with this, here is a process overview, what I have seen and my theory. please help my argument of it not being okay to do, or inform me better on why it is okay. 

 

With our tower pressure we need a finished liquid temperature of say 280F degrees to meet RVP spec, the process enters the tower midway, travels through the bubble cap trays, to the hat tray where it then flows to the reboiler at 200F, then achieving 280F at the discharge which then enters the towers bottom area where it has a chimney to allow the vapors to go through the tower and help the stabilization process and from the bottoms the stabilized condensate goes through cooling then to storage. 

 

We maintain bottoms/stabilized condensate temperature by adjusting the reboilers bath temperature. we enter a setpoint and the system automates the burner system to reach the reboiler bath set point.

 

Tower flow is dependent on our surge vessels level which is dependent on flow from the field which swings always. so if the surge vessels are too high or to low we manually adjust flow to each tower to maintain surge level. 

 

the stabilizer towers have a bypass valve upstream of the reboiler that can split flow of up to I'm going to assume 50% (same pipe size, valve opens up to 100%). this bypass ties directly downstream of the reboiler on the piping of the reboiler discharge to tower bottoms. 

 

and what I am seeing is we are not changing bath temps anymore when flow is decreased to the tower because the reboiler bypass is being set for 5-10 degrees above the spec temp and we are relying on the bypass valve to open and allow cooler condensate ( 200F) to go directly into the bottoms and mix with the flow from the reboiler achieving desired temp. I have seen the valve from 30% open to 100% open (full split flow) 

 

my biggest issue and argument is, if at full flow 7500bpd the bath set point is required to be 315F to achieve 280F bottoms, and if flow then gets decreased to 4,000bpd. the automated system that controls heat is the reboiler bath setpoint. thus it will never change unless an operator does it. so what happens, less flow to reboiler so hotter bottoms product. for the example I will say 300F out of the reboiler.  the bypass then opens to achieve the setpoint and lets say now were at a 50% split. (bypass valve is open 100%) 2,000 bpd to the reboiler and 2,000 bpd bypassed the reboiler to mix and cool the bottoms.

 

1. Shouldn't we worry that if at 4,000bpd to the reboiler its hotter than we desire. what temperature is the 2,000 bpd through the reboiler getting up to? is this going to cause fouling issues?

2. Even though bypassing the reboiler is allowing the desired product temp, is it still technically stabilizing the product? 

3. what other issues can this cause?

 

Thank you for any help. I look forward to hearing any input.

 

 

 

 

[Pilesar]

I think you are a good operator who wants to understand the system and wants to operate the plant correctly. Is the product meeting specifications? From your description, that does not seem to be the concern. A picture would help, but you did pretty good with the word description. RVP can be viewed as an indirect measurement of the butane in the stream. As the butane content is removed, the RVP is reduced. So the column is designed to send C4 and lighter out the overhead. Where is this separation taking place? It is not primarily at the reboiler! The separation takes place a little bit on every tray above the reboiler. The vapor from the reboiler has very lttile C4. This vapor travels up the column and removes a portion of the C4 from the liquid coming down the tower. The vapor from each tray going up the tower has a little bit more C4 in it when it leaves the tray from the top than when it enters the tray from the bottom. On the liquid side, the liquid leaving each tray from the bottom has a little bit less C4 than the liquid entering the tray from the top. The reboiler's job is NOT to make a separation between C4 and heavier. There is a little separation there, but not very much because the liquid entering the reboiler does not have much C4 in it! The job of the reboiler is to make vapor that will travel up the column to mix with the liquid traveling down the column so the separation can happen on each tray. 

  The temperature of the product leaving the bottom of the column is key. This temperature is the boiling point temperature of the liquid. The boiling point is greatly influenced by the lightest component in the liquid bottoms product. So if the bottoms temperature is high enough, you can judge 'there cannot be much C4 in this liquid or the boiling point would be lower.' Because there are many components in the liquid, there may be a different boiling temperature target required to meet specifications. But if your tower feed composition is fairly constant, operations will be able to determine how hot is hot enough. 

  You did not tell the heat source for the reboiler. I think from your description that the bypass may be what is used to control how much heat the reboiler adds to the tower. It is the total heat input that matters more than the reboiler outlet temperature. The total heat to the reboiler directly affects the amount of vapor traveling up the column. You want enough vapor so that the bottom specs are met. Too much vapor is a waste of energy (and money.) The heat transfer in the reboiler exchanger decreases as the flow rate is reduced. The outlet stream gets hotter, yes, but the total amount of vapor generated is smaller as long as the hot side stays the same.

  These temperatures are very low to be concerned about excessive fouling unless you have some evidence of that. Petroleum streams are usually very stable at those temperatures. 

  If I have misinterpreted your questions or you want more discussion, please continue. I have learned much from operators and you are welcome to be in these forums.

[Pilesar]

Reading your question again... You describe a burner and a bath for the reboiler. The heat control seems to be 1) burner firing, 2) reboiler bypass. If I were operator, I would want both of these to be in control range so that I could go up or down with either and see the effect. It may be that one or the other control mechanisms is faster when adding heat and the other is faster when reducing heat. As long as the duty is controllable, I don't think it matters much. If I were an engineer at your plant, I would defer to the operator experience as long as product specs are met. I don't see there would be a maintenance problem with using the bypass as you described. I cannot say the bypass is required, but I don't see the harm.

[breizh]

Hi,

To add to Pilesar's answer, have a chat about your concern with the process engineer in charge of the facility. 

Don't put yourself in trouble with your colleagues/ management.

Review the operation manual about this part.

I've added a document to help you to better understand the process. Not that easy.

 

Separation Processes, Second Edition

 

A Google search AI:

" A bypass valve upstream of a stabilizer tower's reboiler is typically used for maintenance, startup, and to maintain proper temperature and vapor-liquid balance under specific conditions. 

 

Common purposes for the bypass valve

 

Maintenance and isolation

A bypass line, paired with isolation valves on the main reboiler line, allows the reboiler to be taken offline for maintenance or repair without shutting down the entire stabilizer tower. An operator can perform the following steps: 

1. Open the bypass valve.
2. Close the main isolation valves on the reboiler line.
3. Continue operating the column, albeit at reduced capacity, while technicians service the reboiler. 

 

Startup

During the startup sequence of a stabilization unit, the reboiler bypass can provide more precise control over the initial heat input to the column. This is particularly useful for achieving and maintaining the proper temperature profile without overshooting the target, which could cause process upsets. 

 

Fouling mitigation

Fouling is a common issue in reboilers and can reduce their heat transfer efficiency. By partially opening the bypass valve, operators can temporarily reduce the flow through a fouled reboiler, which may help to: 

• Maintain a steadier heat input to the tower, preventing fluctuations in the product composition.
• Help manage the process until the unit can be taken offline for cleaning. 

 

Flow and temperature control

In certain operating schemes, a bypass is used to manage the flow of the hot reboiled liquid back into the column sump. A flow controller on the bypass, sometimes called a reboiler overflow valve, can regulate this flow to achieve a specific temperature or to pre-heat the feed to the column. For example, in some designs: 

• Liquid from the bottom of the column is sent through the reboiler.
• The reboiled and vaporized fluid is sent back to the bottom of the column.
• A bypass allows a portion of the hot bottoms liquid to avoid the reboiler and be routed elsewhere, often to a heat exchanger to pre-heat the incoming feed. 

 

Managing reboiler circulation

The bypass can also be used to manage the thermosiphon effect in certain types of reboilers. During low-load operation, a portion of the flow can be diverted, or the bypass can be used to initiate circulation by preventing an excessive pressure drop that can cause instability. This helps ensure the reboiler's tube bundle remains fully submerged, preventing dryout and damage. "

 

 

Good luck ,

Breizh

[Pilesar]

Thanks, Breizh. Interesting info. I think the reboiler bath temperature is what will help control fouling the best as it sets the max tube wall temperature. The 'fouling mitigation' paragraph of your quote indicates to me that bypass would not reduce fouling, but might be needed if the tubes had a fouling layer which greatly increased pressure drop. Reducing flow by itself might help if it kept the percent vaporization within the optimal range.The rest I think I understand somewhat. I've modeled stabilizer columns but never operated them or designed them in detail.

  For this plant operation, there should exist an operating manual explaining the design intentions. While some plants operate better by not following the operating manual, it may be a good resource to understand why the system was originally built this way.