12 replies to this topic

[Jiten_process]

Dear all,

I am stuck up in thermal design of kettle type reboiler. To tell you brief about the system. There is solvent recovery column where dimethylsulfoxide-water (90%-10%) seperated to their 99.9% purity. I have simulated the column on hysys and succeeded in converging the case.

I have chosen kettle type reboiler and was trying to design on HTRI. I have selected BKU type TEMA exchanger. I have used 3.5kg/cm2(g) saturated steam as heating media and put in on tube side. Shell side is solvent coming off the column at 12Kpa and at 121.7°C temp. In HTRI, i have extracted cold fluid (solvent reboiling) property from hysis case and steam side(hot fluid) property kept HTRI generated.

On HTRI process side i have specified inlet/outlet vapor fraction for both side and inlet pressure for both the fluid addition to shell side fluid mass rate, fouling and pressure drop.

Then while running for the first time even, i got HTRI design run failed with the below message.
"The calculated or specified terminal temperatures are impossible. Check the specified physical property and process conditions, and rerun the case"

When i open fail run report i found skin temp calculated was some negative value. However inlet/outlet temp calculated was correct.

Can anyone help me out finding where am i goofing exactly???

Please note that both side fluid being at saturated condition i have not specified any (inlet/outlet) terminal temperature on HTRI process side. I have just mentioned inlet/outlet vapor fraction and inlet pressure for both side fluid which i think defines the temperature by itself.

Please find attached inlet to reboiler stream property for more information. I can attach HTRI case too if anybody can help out in HTRI case directly.

Attached Files

•  Inlet_to_reboiler_stream.xls   58KB   229 downloads

[Qalander (Chem)]

Dear,

Search the forum there has been a thorough discussion somewhere back in Jan/Feb 09 or around.
This may be useful to help solve your issue.

[Jiten_process]

125 views...no more reply...is it something that is out of chemical world???

Plz mention if any data i missed out???

[katmar]

Is it correct to enter the enthalpy as a negative number? Could this be why you have negative skin temperatures?

[djack77494]

125 views...no more reply...is it something that is out of chemical world???

Jiten,
In my experience, heat exchanger designs are done by specialized mechanical engineers. While I have provided many many process specifications for heat exchangers, I was never expected to know how to do the mechanical design of an exchanger. I have seen some (not many) HTRI run reports, but have never run HTRI. I think this is true of most of our forum members. So a quick look at what it is you're asking and I realize that it is outside my area of expertise. I suspect this is common.

[Jiten_process]

@katmar

Thanks for ur reply but the property i have attached is generated by simulator only. Column is converged and i have extracted the properties of reboiler inlet stream. Honestly i dont have clarification of your doubt why it is negative may be due to reference temp sign but it is true that when i have generated properties whithin HTRI for even a water, it is also showing negative enthalpy value. I suspect it is not the reason of why my design run fails. Hope others can throw light on it...

@djack,

I am bit surprised with ur statement, it is true that mechanical design of heat exchanger is done by specialised mechanical engr but what i am doing is thermal design and HTRI is built only for thermal design not for mechanical design. I dont know at your place but in india thermal design and optimization of heat exchanger is an added skill of process engineer and HTRI is being used by many process engineer for thermal design. We in my company use PVElite for mechanical design which is operated by specialised mechanical engr.

Neways back to my problem after lot of trial and error one more thing i found is instead of specifing outlet vapor fraction if i specify outlet temp then at least my HTRI case runs on design case. I reach on conclusion based on error message by HTRI that there is some problem in cold side (solvent side) properties either by the simulator or in what HTRI generates through simulator. Just to find out out reason what i done is on cold side also i removed solvent and took just water which is in HTRI. I have kept all other cold side input as it is (inlet pressure, inlet/oultet frac, press drop, fouling, mass flow) and then run the case. Atleast i succeeded but with lot many unanswered questions.

It is to be noted here that entrainment ratio and critical flux is taken care by HTRI while designing. We dont need to specify values for it. In result we can see the boiling regime.

Neways, Finally I am looking for two answers if somebody can give

1) kettle reboiler for packed column application where pressure is in range of 12Kpa, anyone has seen outlet vapor fraction to be 1. If not then again i have to find ricirculation ratio but in kettle it can give 100% vaporization i suppose, m i right???

2) for water/water vacuum kettle vaporizer my kettle ID/Tube bundle ratio calculated is 3.
Is it feasible?? in reference i get to see this ratio in range of 1.2 - 1.8? How can i find this ratio manually???

Hope to receive answer of above...Please mention if i am not clear in my elaboration of problem.

[Art Montemayor]

Jiten:

HTRI has offices in Vadodara, Gujarat, India and offers Technical Support there as well as from representatives in Italy and South Africa:

Worldwide
Support@HTRI.net (e-mail is preferred)
+1.979.690.3250 fax (“Fine” resolution ensures clarity)
+1.979.690.5055 office

Asia-Pacific
Support@HTRI.net
+81.43.297.0354 fax
+81.43.297.0353 office

Have you – or your member company – contacted HTRI? HTRI is an organization established by industrial companies with a membership of contributing companies. In the past, only those member companies have the access to HTRI. If your company is a member – which would normally be the only way for you to have access to HTRI – it has the right and privilege to seek and obtain help and assistance in the software from HTRI. You should have full, hands-on training from HTRI available to you with technical backup.

I don’t understand what is meant by:
“1) kettle reboiler for packed column application where pressure is in range of 12Kpa, anyone has seen outlet vapor fraction to be 1. If not then again i have to find ricirculation ratio but in kettle it can give 100% vaporization i suppose, m i right???”
ALL kettle reboilers produce a 100%, saturated vapor - ONLY. Therefore, I guess the vapor fraction is 1.00, but I don’t see any reason to mention it. There is NO RE-CIRCULATION around a kettle reboiler. All the liquid is introduced once, and part of it is vaporized while the rest is removed as bottoms product. In fact, that is sometimes a bad feature with a kettle – it will produce a concentration of solids around the tube bundle if there are contaminants in the liquid. It is really a true vaporizer.

I also am confused by:
“2) for water/water vacuum kettle vaporizer my kettle ID/Tube bundle ratio calculated is 3.
Is it feasible?? in reference i get to see this ratio in range of 1.2 - 1.8? How can i find this ratio manually???”
I have never given any importance or value to the “kettle ID/Tube bundle ratio”. As long as the tube bundle is well submerged in the liquid – as fixed by the overflow weir – you have a good, working design. The amount of submergence is usually 6” to 12” – depending on the fluid’s characteristics. I have never found anyone that gives this ratio importance and I don’t see how its calculation is of any value. The bottom line is to follow the basic concepts of the design: keep the tube bundle submerged so that pool boiling is ensured.

What does the word “Neways” mean?

[djack77494]

I am bit surprised with ur statement, it is true that mechanical design of heat exchanger is done by specialised mechanical engr but what i am doing is thermal design and HTRI is built only for thermal design not for mechanical design. I dont know at your place but in india thermal design and optimization of heat exchanger is an added skill of process engineer and HTRI is being used by many process engineer for thermal design.

Every locale, I suppose, operates a bit differently. Where I have worked and doing what I have done (usually greenfield units), the process engineers generally completed only the process specific portions of a datasheet for a heat exchanger. They, for instance, supply the temperatures, pressures, viscosities, densities, and surface tensions. They supply phases and flowrates for both sides. If there's a phase change, a heat release curve would be supplied. Then they stop and a heat transfer engineer does the rest.

When doing a brownfield project we would have to, of course, rate existing exchangers. Then we would use other software such as STX to allow inputing of the specific hardware configuration so the exchanger could be rated. (I've also used just ProII's rigorous heat exchanger module for this.) These results are considered preliminary and are checked/confirmed by a heat transfer engineer running HTRI. Though I'm not certain about this, I do not think that the heat transfer engineer does the detailed mechanical design of the exchanger. I think that is pushed forward to the exchanger manufacturer. In days gone by, we tended to do more of the work and the manufacturers tended to be more just fabricators. But modern tendencies seem to be to push more of the engineering work "downstream".

[fallah]

2) for water/water vacuum kettle vaporizer my kettle ID/Tube bundle ratio calculated is 3.
Is it feasible?? in reference i get to see this ratio in range of 1.2 - 1.8? How can i find this ratio manually???

Do you mean Kettle ID/Port ID?

[narendrasony]

Dear Jiten,
My experience is mainly in operations, so I can not comment directly.
But I think that due to very low pressure and resulting high velocities , more settling space is provided with Kettle Dia /tube bundle dia ratio of 3 to avoid entrainment. Another deciding factor could be low surface tension of the boiling liquid which will require more space.

I don't know how much should be the value of this ratio. However a minimum value of 1.2 for this ratio (Kettle/Tube bundle dia) seems to be very less, should it not be ~2.

Regards
Narendra

[Jiten_process]

Thanks all for their replies...

@Art,

yes, my company is a member of HTRI. infact my company till last year was a representative of HTRI for india but now we are a member. Reason of putting up query on this forum is to have informative and detailed fundamentally discussion. I just do not want to run my case on HTRI besides i want to understand the stuff fundamentally and share with others too so that everybody on this forum can learn and share their views. I am not only concerned about running HTRI but more concerned about knowing the fundamental of it with respect to what is practice.

now to your questions,
1) i knew that kettle reboiler is meant for 100%vaporization. but due to my run case fail on HTRI i got confused. But now i am clear.

2) By finding the tube bundle/kettle id ratio eventually i wanted to find how much space on above tube bundle is requried for disengagement of liquid particles from vapor. And i suppose that depends on entrainment ratio.

and lastly i regret to use short character language; by 'Neways' i actually meant 'Anyways'.

@narendrasony

I have referred the ludwig and there this values i could found. But the cases for which the values mentioned were not of my use as my partiular case is different altogether.

I wanted to calculate it for my case but dont know how to do it.

Thanks once again to all

[Art Montemayor]

Jiten:

No one can deny that when it comes to dictating or judging how an HTRI program should be inputted or run on their program, HTRI IS THE EXPERT. Therefore, it stands to reason that it is HTRI that should make the call or decide on how to operate their program. No one’s opinion or preference on this Forum can supersede that fact. That is why I highly recommend you obtain HTRI’s answer to your program problem.

As to the fundamentals of what makes a kettle reboiler operate properly and what are its important characteristics, I can point to the basic facts:

1) As stated previously, the basic objective is to submerge the entire tube bundle in order to obtain the basic pool boiling effect;

2) When you submerge the tube bundle as I stated, you will find that your physical dimensions will, by natural selection, give you a natural, low velocity of emitted vapors from the surface of the boiling liquid. I know of no “entrainment ratio” – and much less of how to calculate it. I can tell you that everyone I know who has designed and operated kettle reboilers has never used it or knows how to calculate it. What’s more important is the knowledge that it is unimportant if your vapor disengagement velocity is very low – as it is in almost all kettle reboilers built with experienced know-how.

3) Trying to inject a “factor” of tube bundle diameter to kettle ID is another means of confusing the entire issue. It makes no importance if you follow the basic concepts and principles applied to good engineering judgment. If the tube bundle is sufficiently submerged and located well within the kettle, you will find that the vapor disengagement space and subsequent velocity will be very appropriate to good level control and vapor generation – without having any liquid entrainment occur.

I believe what is happening is that a lot of Chemical Engineers who are thermally designing (or trying to design) kettle reboilers, have never actually mechanically fabricated one. If you take the time and actually sketch or draw the details of a kettle reboiler on CAD or on a sketch pad, you will realize what I am saying is the truth. I have designed and fabricated several TEMA BKU/CKU type reboilers for Amine strippers as well as for refrigeration applications. I have never had the problems or concerns you list. And ALL of my reboilers operated exactly as predicted. The most important item that is almost hardly ever mentioned is one that I have mentioned before in our Forums and which I believe only Fallah has picked up on: the actual mechanical fabrication of the U-tube bundle and its support inside the kettle is very, very important and can contribute to a lot of problems like it can also make for a 100% trouble-free operation.

The most important item is making sure that ALL condensate formed in all U-tubes is drained freely and efficiently (by gravity) – without causing any liquid pools or pockets within the bundle. This is easy to produce when your bundle starts to be rather long – such as longer than 8 feet. The U-tubes will sag – even slightly – and cause a low spot in the length of the bundle. This will accumulate liquid pools and cause a pressure drop and slug-type flow with erratic operation. I always resolved / prevented this problem by having my U-tube bundle actually designed in the shape of a V-shaped bundle. This feature always produced constant and efficient condensate drainage 100% of the time. By doing this, I automatically decreased the factor you call “tube bundle diameter to kettle ID” also and produced a conservative vapor velocity above the boiling liquid – with no entrainment problems.

I offer this experience in response to your “wanting to understand the stuff fundamentally and share with others too so that everybody on this forum can learn and share their views….. not only concerned about running HTRI but more concerned about knowing the fundamental of it with respect to what is practice.”

I hope the experience helps.

Sorry I am jumping in between, but according to me what Mr. Jiten is facing problem is not related to fabrication or type of reboiller. I hope or should I say before using a tool like a HTRI or any other one should go through the basic calculations or the algorithms the software is following. So my advice is to anyone who is using a tool even a simple tool like excel one should go through the basic literature and procedure.

Based on my experience I will just say the present case is Junk in and Junk out.

Regards