17 replies to this topic

[Jiten_process]

Hello

I want to know how to decide the recirculation ratio (or outlet vapor fraction) for the verticle thermosyphon reboiler.

To brief, I have converged the reboiled absorber on unisim (similar to hysys). The feed stream is organic mixture of Water(0.318), acetic acid(0.142), p-xylene (0.048), methyl acetate(0.378), methanol(0.114), all are at mass fraction basis. Now that i have converged the column and simulation results show me the reboiler duty as 3.742e06 kj/h. Now hysys doesnt specify the type of reboiler but in my application i want to select the thermosyphon reboiler.

Now I seek method/guidlines/data as to how to decide reboiler recirculation ratio or outlet vapor fraction for vertical thermosyphon re boiler. And how to analyze the simulation results incorporating these configuration.

Kindly mention if any other data is required to be mentioned here.

plz help me...

[cea]

Hello

I want to know how to decide the recirculation ratio (or outlet vapor fraction) for the verticle thermosyphon reboiler.

To brief, I have converged the reboiled absorber on unisim (similar to hysys). The feed stream is organic mixture of Water(0.318), acetic acid(0.142), p-xylene (0.048), methyl acetate(0.378), methanol(0.114), all are at mass fraction basis. Now that i have converged the column and simulation results show me the reboiler duty as 3.742e06 kj/h. Now hysys doesnt specify the type of reboiler but in my application i want to select the thermosyphon reboiler.

Now I seek method/guidlines/data as to how to decide reboiler recirculation ratio or outlet vapor fraction for vertical thermosyphon re boiler. And how to analyze the simulation results incorporating these configuration.

Kindly mention if any other data is required to be mentioned here.

plz help me...

Hi-

General guideline about percentage vaporisation in reboiler is 30% maximum for once through flow. In order to achieve once through vaporisation, column bottom is provided with partition plate, with which one can achieve maximum vaporisation.

Now, very first thing that should be noted the reboiler is the first theoratical stage in the column where vapor liquid separation occurs. Hence,in case of once through flow in reboiler, percentage vaporisation shall be equal to percentage more volatile component that exists in liquid.

In case of multiple pass flow through reboiler, percentage vaporisation can be reduced by allowing only some percent of MVC to flash. (Heat input = mass x Lambda). Thus, less flow, less heat transfer, less vaporisation, subsequently reduced line size & reduced exchanger area. Generally, with multiple circulation, percentage vaporisation is in range of 10%.

Hope this helps you.
Regards

Edited by cea, 01 September 2009 - 07:06 AM.

[Jiten_process]

Thanks cea for your reply.

Firstly let me clear the things. i am regenerating the project for which i already have all the design data. I am attaching the simulation result and PFD for better clarity. I have rounded the stream which is in concern with red bordered circle.

Now let me take up your first statement, as u said max can be a 30%, and if i assume let say 20%, does it mean that while designing the thermosyphon reboiler i will devide the liquid rate by assumed % vaporization coz simulation assume total vaporization once thorugh. for e.g. in my case liquid going in to reboiler is 8.296e+004 kg/hr; now if i assume 20% vaporization once through, am i correct if i take liquid in flow as 8.296e+004/0.2 = 4.148e+005 kg/hr. AM I Correct in this???

@cea, i could not understand your quote " Hence,in case of once through flow in reboiler, percentage vaporisation shall be equal to percentage more volatile component that exists in liquid.". In my case if you see simulation result virtually there is no low volatile component in bottom as such. the lowest volatile component that is present is methyl acetate i.e. 0.0575. do u mean to say i have to take this much of vapor fraction???? plz clarify?

I am yet to know as to what is technical fundamental in selecting vapor fraction percentage. I urge other experts to plz comment on it. I feel i have already defined the max. data required to comment on the subject question.

Awaiting reply...

Attached Files

•  Simulation result.xls   62KB   284 downloads

Edited by Jiten_process, 01 September 2009 - 07:42 AM.

[Padmakar Katre]

Dear just find the viewlet from support web of Aspen-Tech to know how to configure the thermosyphon reboiler (In Hysys the conventional column reboiler is kettle type which you need to change). Once the configuration is over you can add one spec i.e. reboil ratio which you can adjust to get the required % vaporization at the reboiler outlet. Hope this clarifies your issue.

[srfish]

I don't understand your math. What is the 4.148E+05 if the inlet liquid flow is 8.296E+04. This would be creating mass.

The exact recirculation ratio is a complicated calculation. It involves a piping and heat exchanger circuit where the slightest change will change the recirculation ratio. The ratio is especially sensitive to the size and configeration of the outlet piping. In the planning stages I would make sure the outlet mixed phase density is above 1.0 lb/cu-ft.

[Padmakar Katre]

Dear,
Please let me know have you configured the thermosyphon reboiler if so just find the vapor flow in the column environment and the total circulation or the inlet flow and find the % vaporization. I didn't get anything from your previous reply what you want to say. Please be more clear. The circulation rate will be calculated based on the reboiler duty calculated for the given specification to your column. Waiting for your comment.

[Padmakar Katre]

Dear Jiten_Process find below the steps to configure the thermosyphon reboiler in hysys.
I assume that you have a converged column.
1. Go to the column environment. In the tray section add one tray which will act as the bottom sump.
2. Then Click on the kettle reboiler and unattach the Liq outlet product stream from reboiler an attach another stream say Liq Out.
3. Click on the main tray section and inlet streams connect this Liq Out from reboiler to the one last tray (Bottom Sump) and click on side options and check the Liq product stream as side stream as liquid product.
4. This will complete the configuration.
5. Come to main window of column runner page. You will find an additional degree of freedom which will insist you to give another specification. You give the Column Reboil Ratio spec value such that you find the required % vaporization in reboiler out. You can check this in column environment i.e. Mass Flow of Stream Boil-Up (Kettle reboiler vapor outlet stream) divided by the reboiler inlet stream mass flow.
Over and above if you are going to ooperate the column for different feed cases you can calculate this spec in the spreadsheet and export to the active spec so it will iterate until you get the desired vaporization at reboiler out (Here you can use If Then Else function in spreadsheet). I hope there won't be any more confusion.

[abhi_agrawa]

Jiten,

In my opinion:

1) The outlet vapor fraction from a thermosyphon should depend on the composition of the feed to reboiler. I have seen as low as 10% vaporization (for a very wide boiling feed to reboiler) to 50% (for very narrow boiling eg. C2 splitter in an Ethylene Plant).

2) In your simulation summary, the difference in inlet and outlet temperature of the reboiler is 1.1 deg C. With this small difference I'd say that you should be able to get 30% vaporization without any difficulty.

3) In your feed to reboiler you have water (31.8% by wt) and p-xylene (4.8% by wt). Do check that there is no phase separation between these components, else you may have a problem.

4) As far as any simulation software is concerned, if you configure a thermosyphon with whatever % vaporization you want, it will give your a result. It is up to you to check if the result makes any sense.

Hope this helps,
Abhishek

[Jiten_process]

@padmakar

thank u, i didnt know how to configure thermosyphon on hysys. But i ma bit confused with adding an additional tray section to act as a sump. In simulator, after configuring the thermosyphon reboiler, i got two seperate stream as vapor to column and liquid to column. But the both stream enter to same tray no. i.e. 1. It means liquid part from thermosyphon will be fed on above the first tray(which i have added to configure the thermosyphon) and pass through the first tray to bottom. and vapor part will be fed below that tray and pass through the first tray. Eventually it means the liquid and vapor part coming off the so called thermosyphon reboiler again will get contacted in that first tray which is not the case in real life. Practically in column liquid portion from thermosyphon will go directly to sump for recirculation and vapor directly pass through the first tray. Can you throw some light on it???

I am attaching the simulation result for clarity.

Now back to my question, now i understand how to set fraction ratio in column simulation. But as abhi rightly pointed out, how to judge how much %of vapor i should consider.

@abhi, could you please explain some more on your point no. 2 in your below post. you said with 1.1°C temp driving force i could get vapor fraction 30%, on what basis it is decided on temp driving force. is there any such calculation reference? It is needed to point out here that while analysing performance on simulation result, i could see that when i increase the outlet vapor fraction the temperatue difference between inlet to reboier and outlet to reboiler gets increased and lower the exit vapor fraction increases the temp difference. for example, @30% exit vapor fraction it is 0.6°C, @17% exit vapor fraction it is 0.3°C only. In attached simulation you can this temp difference at 16.6% vapor fraction. so what is the temp. driving force that there should be?

I am yet to decide as to what is criteria/guidline to decide this exit vapor fraction. i know the range but i want to reach to the firm answer. Any book/reference would also serve my purpose so please suggest me good book for the subject question.

awaiting more replies...

Attached Files

•  Simulation result rev. 1.xls   99.5KB   148 downloads

[SSWBoy]

@abhi, could you please explain some more on your point no. 2 in your below post. you said with 1.1°C temp driving force i could get vapor fraction 30%, on what basis it is decided on temp driving force. is there any such calculation reference? It is needed to point out here that while analysing performance on simulation result, i could see that when i increase the outlet vapor fraction the temperatue difference between inlet to reboier and outlet to reboiler gets increased and lower the exit vapor fraction increases the temp difference. for example, @30% exit vapor fraction it is 0.6°C, @17% exit vapor fraction it is 0.3°C only. In attached simulation you can this temp difference at 16.6% vapor fraction. so what is the temp. driving force that there should be?

I am yet to decide as to what is criteria/guidline to decide this exit vapor fraction. i know the range but i want to reach to the firm answer. Any book/reference would also serve my purpose so please suggest me good book for the subject question.

awaiting more replies...

1. I think you misunderstand abhi, 1.1°C refers to the difference in inlet and outlet temperature of your reboiler, what this shows is that you have a relatively narrow boiling range. As this is the case you can use a higher outlet vapour fraction (I believe that with wide boiling liquids and high %vapour fraction you run the risk of tube dryout)

2. You are focusing too much on a precise answer, do you want somebody to tell you use 24.3% outlet? Let me explain to you the real sequence of events that will occur in designing such a circuit.

i - Simulation will be made with a guideline vapour fraction, say 25%
ii - Hydraulics will be run by a process engineer liasing with a heat exchanger engineer to determine what vertical head would give EXACTLY 25%, hydraulics would be based on an assumed routing by the process engineer.
iii - Actual pipe layout will be completed by a piping engineer, exchangers may be raised or lowered slightly as required for piping etc.
iv - At a later stage in the project the thermosyphon loop will be checked against actual pipe routings, is the %vapour fraction within say 10% of the original intent? Yep, thats good then lets move onto something else...

It is very nice to put on the PID that there should be a elevation difference of 7413mm between the bottom tubesheet and the NLL of the bottom sump but as engineers our we should realise our designs are not an academic excercise, they just need to work.

I'd say just use 30% and be done with it.

[abhi_agrawa]

Jiten,

1) By saying that the difference in reboiler inlet and outlet is only 1.1 ^oC, I imply that the feed to reboiler is not wide boiling. So you can get high vaporization. My take(just like sswboy) would be to take 30% and move on. Also remember that the flow in a thermosyphon is driven by density difference and not temperature difference.

2)If you are doing the basic design, then the approach by sswboy is what should be followed. If you need exact answers, size the line, get the reboiler elevations, go to the heat exchanger design people, ask them to size the reboiler and then check if everything is ok.

3) Please refer to "Distillation Troubleshooting" by Henry Kister, Chapter 23 (or Hower and Kister, "Solve Column Process Problems - Part 2", Hydrocarbon Processing June 1991) for an excellent case study wherein and forced circulation reboiler was changed to thermosyphon reboiler in a revamp and the hydrocarbon and water phase began to separate out in the new reboiler. The would lead to the thermosyphon reboiler not to work every few weeks. The column would have to be shut-down. The reboiler will have to be drained then. The drained liquid was water.

So, do check for any possibility of phase separation in the reboiler. If there is some, then the design will have to address this issue.

Hope this helps,
Abhishek

[narendrasony]

Dear Abhishek,
How do you infer that recirculation ratio (high or low) depend on whether the reboiler feed is wide boiling or not ? I think it is the density difference (liquid - vapor density diff.) which governs this ratio irrespective of whether the reboiler feed is wide boiling or not. Greater the density diff., lower recirculation ratio may suffice and vice a versa.

Regards & thanks
Narendra

[SSWBoy]

Dear Abhishek,
How do you infer that recirculation ratio (high or low) depend on whether the reboiler feed is wide boiling or not ? I think it is the density difference (liquid - vapor density diff.) which governs this ratio irrespective of whether the reboiler feed is wide boiling or not. Greater the density diff., lower recirculation ratio may suffice and vice a versa.

Regards & thanks
Narendra

In this instance I am afraid you are wrong.

With wide boiling liquids you are more likely to get tube dryout in the exchanger ==> fouling. Therefore to maintain a liquid film on the exchanger tube you have more liquid circulating, i.e. lower vapourisation %.

I think you are coming at this from the wrong angle, your reasoning assumes a constant static head driving force. IF this were the case then you would be right, a greater delta (Density) would mean that hydraulics would dictate that to balance the pressure drop in the system with lower 2-phase static head loss (w.r.t to liquid static gain) the liquid circulation rate would be higher. Design all your thermosiphons for 30% by all means, but expect excessive fouling in cases where you have a wide boiling liquid.

[abhi_agrawa]

Narendra,

Just to extend what SSWBoy has said.

When you have a wide boiling mixture, as you progressively move along the length of the tube, the lighter material from the reboiler feed will get stripped off. So the liquid film inside the tube will become progressively heavier and its boiling point increases, vaporization from the film decreases. You will soon reach a limit and the vaporization from the film will cease. At this point the film takes up heat corresponding to sensible heat transfer only. This restricts you % vaporization in the thermosyphon reboiler.

hope this clarifies,
Abhishek

Edited by abhi_agrawa, 08 September 2009 - 08:05 AM.

[Zauberberg]

You may find this article from A. Sloley to be useful: http://www.distillat...question016.htm

Edited by Zauberberg, 08 September 2009 - 01:08 PM.

[jcazenave]

Hi

if you have access to the latest HYSYS and Shell&Tube, you can model the thermosiphon rigorously (including the recirculation flow rate based on the inlet/outlet circuit) and the column model together.

[narendrasony]

Dear Abhishek and SSWBoy,
Thankyou very much for youe nice explanation and correction.

I think you misunderstand abhi, 1.1°C refers to the difference in inlet and outlet temperature of your reboiler, what this shows is that you have a relatively narrow boiling range. As this is the case you can use a higher outlet vapour fraction (I believe that with wide boiling liquids and high %vapour fraction you run the risk of tube dryout)

SSWBoy : Excessive fouling will take place in case of wide boiling liquids with high vaporization. But, there is little confusion on the term "tube dry out" with wide boiling liquids. Dry out should mean the loss of liquid layer in some parts of reboiler which should happen with narrow boiling liquids. Article from A. Soley also tells that.

Regards
Narendra

[Jiten_process]

Firstly apology for late reply and thanks to all for their active participation.

As said, i took 30% vapor fraction as primary value and run my HTRI. i could converge the design with minor vibration which i could vanish by optimisation. In result, for 30% vapor fraction i got required static head 1.13 meter. By digging into HTRI manuals i got to read some points which i have listed down in italic font for everyone's kind information.

"As heat flux increases, the high pressure drop of the two-phase flow in the restrictive tube eventually causes film boiling, mist flow, or instability. The heat flux at which these behaviors occur are usually above the 31.5-47.3 kw/m2 range formerly accepted as a design limit."

My HTRI case result shows this value in tube side monitor as 23kw/m2 - 27kw/m2

"Customarily, vertical tubeside thermosiphons should not be designed to have an exit vapor fraction greater than 0.35 for hydrocarbons and 0.1 for water. However, values up to 0.5 are acceptable for hydrocarbons boiling under vacuum if mist- and choke-flow limits are carefully checked."

My case i checked boiling regime in tube side monitor and it is not in mist flow regime. However to check i kept on increasing vapor fraction and i found at higher vapor fraction above 0.6 the flow regime is in mist flow where my heat transfer rate reduced substantially.

"If the outlet vapor fraction is greater than the mist flow vapor fraction, ymist, mist flow can occur in part of the tube, lowering heat transfer rates and possibly causing accelerated fouling and corrosion."

In my case, i could see it when i increase the vapor fraction beyond acceptable value.

Once again thanks to all for their valuable inputs. It was indeed fruitful discussion.

Edited by Jiten_process, 11 September 2009 - 02:19 AM.