13 replies to this topic

[A mukherjee]

In cryogenic plant we use bath type reboiler condenser.In the rb(reboiler condenser) gaseous nitrogen condenses(& eventually provide the reflux for distillation column) ,by exchanging heat with the rich liquid present in the bath.Now wat I want to know is dat the vendor has provided certain recirculation ratios for all cases...(cases means variation of heat load).The recirculation ratio is lowest in max heat load case(the case of maximum gaseous nitrogen condensation) & min at lowest one.
Can any one provide any info regarding the variation of recirculation ratio?
Thnxx in advance...

[A mukherjee]

Can any one tell me about calculation of recirculation in cryogenic bath type reboiler condenser?

[engg]

Can any one tell me about calculation of recirculation in cryogenic bath type reboiler condenser?

no answers because your question is not clear to anyone. put some sketches and some more data

[A mukherjee]

In the reboiler condenser ,Gaseous nitrogen passes through the Reboiler block.Gaseous nitrogen condenses to liquid nitrogen by exchanging heat with surrounding liquid bath.The liquid bath is oxygen rich.It is called rich liquid(60% nitrogen,39% oxygen & rest argon).Now for the liquid bath the heat exchanger vendor has given certain recirculation ratio,which is nothing but -vapor/vapor+liquid ratio.It is different for different heat loads.
I want to know how is this ratio calculated .
What factors influence it?

[srfish]

Why is the term reboiler used in this type of equipment? If all else fails you could go back to the vendor.

[Art Montemayor]

A mukherjee:

I believe you have touched on a topic dear to my heart – the Linde Double Column as applied to the Air Separation Process. However, as engg has recommended, you should supply a detailed sketch and the rest of your basic data and/or scope of work.

Since I am not going to work today due to an ice storm, I’ve decided to generate what I wanted to do for some time: a detailed drawing of the the typical Linde Double Coumn in Excel. If you are not going to spend some time in generating a detailed sketch, at least please refer to the attachment and tell us all if this is what you are referring to.

I believe this is what you are trying to describe because of the following (my responses are in RED):

You say: “In cryogenic plant we use bath type reboiler condenser. In the rb(reboiler condenser) gaseous nitrogen condenses(& eventually provide the reflux for distillation column), by exchanging heat with the rich liquid present in the bath."
The Linde Double Column uses the overhead HP nitrogen vapor condenser as the reboiler for the LP column – which is physically mounted directly on top of the HP column. All the nitrogen vapor generated in the HP column is condensed and sent to the LP column as reflux.

You say: “Now wat I want to know is dat the vendor has provided certain recirculation ratios for all cases... (cases means variation of heat load). The recirculation ratio is lowest in max heat load case (the case of maximum gaseous nitrogen condensation) & min at lowest one.”
What we don’t know is what do you mean by “heat load”. Where is this heat load applied? If it is the HP nitrogen condenser, then it is ALL the nitrogen in the HP column feed.

You say: “Gaseous nitrogen condenses to liquid nitrogen by exchanging heat with surrounding liquid bath. The liquid bath is oxygen rich. It is called rich liquid (60% nitrogen,39% oxygen & rest argon).”
The liquid in the sump of the LP column is essentially pure Oxygen – not what you claim. Please show us your sketch or give us ALL of your basic data and a full explanation.

You say: “Now for the liquid bath the heat exchanger vendor has given certain recirculation ratio, which is nothing but -vapor/vapor+liquid ratio. It is different for different heat loads.”
We don’t know what you mean by heat exchanger vendor. Are you separating air into Oxygen, Nitrogen, and Argon? If so, how can you be doing it in a separate heat exchanger? That is why we need the FULL STORY and a sketch of your process. If I can generate a detailed drawing as attached, you should be able to do better - since you are much younger, smarter, and probably better looking than I.

Await your contribution.

Attached Files

•  Air Separation.xls   929.5KB   104 downloads

[A mukherjee]

Here are the replys:

1. "The liquid in the sump of the LP column is essentially pure Oxygen "
This statement is true for ASU's where nitrogen,oxygen & argon are produced. But for plants where only Nitrogen is produced,the reboiler block is surrounded by bath, containing rich liquid, which contains 60% nitrogen,39% oxygen & rest argon. This Rich Liquid comes from the bottom of the distillation column, via a JT valve.

2. By "variation of heat load" I wanted to mean different cases. As the Nitrogen production will vary, obviously the flow through the reboiler condenser will vary & consequently the heat load.

3. We are producing nitrogen by conventional method only. But the problem is that we don't manufacture plate & fin heat exchangers. For that reason we have to rely on vendor only. Thats why I wanted to know in detail about the recirculation ratio.

I will upload a sketch definitely to make the reboiler process more clear.

[Art Montemayor]

A mukherjee:

OK. So now we know that you are not producing Oxygen, Nitrogen, and Argon pure products in your cryogenic Air Separation Unit (ASU). You have now given us another piece of your basic data: you are really only producing pure nitrogen.

But you still persist in not addressing MY questions. Are you working with a Linde Double Column? We don’t know what you call “the conventional method”.

Again, why do you need to “rely” on a plate and fin heat exchanger? Are you planning on modifying your unit? If so, why? Why not contact the designer and fabricator of your ASU instead? I am not asking to extract industrial secrets; I ask so that I can answer your queries more intelligently and without giving you bad comments or recommendations that could lead to a mistake or - worse - an unsafe act or procedure.

Await your detailed drawing and the rest of the story.

[A mukherjee]

Now I 've uploaded a drawing.Hopefully it will help to understand the process.

Actually I meant to say by "Conventional Method" that it is actually same as Linde double column method..only difference is that no. of column is one.There is no low pressure column,only high pressure column.
Here also,like the Linde double column , the rich liquid(composition has been mentioned in earlier post) exits from high pressure column bottom.Then it enters the JOULE THOMPSON valve .At the downstream of valve liquid becomes 2 phase.The liquid enters the bath.Vapor leaves the bath along with vapor generated from the bath itself.At steady state the amount of liquid entering the bath & vapor generated from it is equal.Rich liquid of the bath condenses the gaseous nitrogen entering the reboiler block to liquid nitrogen,as I've mentioned in the earlier post.
Now we do not design the plate & fin heat exchangers.We specify only the heat load & allowable pressure drop.
The vendor has provided the offer for bath type reboiler condenser with certain recirculation ratio in the bath.I have already mentioned my idea about this ratio in earlier posts.I want to know in detal about it.

Hopefully I'm now clear.

Attached Files

•  REBOILER CONDENSER DESIGN.xls   22KB   85 downloads

[A mukherjee]

Still any confusion????

[A mukherjee]

?????????

[Art Montemayor]

A mukherjee:

Unfortunately I don’t see things as “clear” as you do and I was hoping other members would see the clearness and respond. Part of your postings that make it hard to read and interpret is the fact that you do not follow the rules of grammar and writing for the English language. Please separate your sentences by at least ONE SPACE – PREFERABLY TWO SPACES – in order to see the separate and independent sentences and thoughts. Use paragraphs to separate different subjects, thoughts, and topics. This will help tremendously in interpreting your English prose.

Some confusion is created by you asserting that “it is actually same as Linde double column method..only difference is that no. of column is one. There is no low pressure column, only high pressure column”. This can’t be true. If it were, then you could not transfer liquid from the sump of the lower portion to the top where the presumed condenser/reboiler is located. What I suspect you have is TWO COLUMNS (in the Linde tradition) with one mounted on top of the other. The top section may not be a “distillation Column” in the conventional sense, but it is another pressure vessel – and at a lower pressure than the distillation below it. This top vessel serves as the sump where you use the expansion product of crude oxygen from the sump of the lower (High Pressure) column to function as the cooling medium for the condensation of gaseous nitrogen that emanates from the top section of the lower (High Pressure) column. This nitrogen is liquefied and is the product LIN (liquefied nitrogen) as well as reflux for the lower (High Pressure) column. In other words, the process is EXACTLY A DUPLICATE OF WHAT I DREW IN MY DETAILED DRAWING. Am I not correct?

All your sketch shows is a lack of distillation trays (and separation) in the upper (Low Pressure) vessel. Isn’t this true? Rather than wait for your confirmation, I will assume that what I state is true and proceed to comment on your last post.

In an air separation unit – which yours clearly is – the separation of air into its components is done in a steady, continuous, flow. The process calculations done around - and related to – the process are all considered as steady state. There is no fluctuation in the main air feed stream fed into such a process. Consequently, the heat and mass balance calculations done for such a process reveal the flow rates and compositions expected in each of the main streams. There is NO “variation of recirculation ratio” as you state. What you have is a conventional distillation process that requires a constant REFLUX ratio. The flow rate of LIN returned to the HP column as reflux is constant – as is also the flow rate of LIN taken off as condensed, saturated product.

The heat load (or duty) of the total overheads condenser (which the heat exchanger you describe truly is) is determined by the heat and mass balance done around the process. This duty will depend on the flow rate of the gaseous nitrogen released at the top of the HP column and its latent heat of condensation.

Another part of your post that adds to the confusion is that you fail to tell us EXACTLY what you are doing and what your scope of work is. You tell us you are producing pure nitrogen. Again, I have to assume that it is with this existing column. If so, why are you concerned as to sizing, building, or replacing the nitrogen condenser? Is the present one too small? Is is leaking? Additionally, why don’t you resort to the original designer, and builder of the unit as well as to the detailed process calculations that you should have? For example, WHO designed and built the unit? Are the process calculations available? Are the Equipment DataSheets not available? If you don’t have this basic information available to you now, you (or another process engineer) will have to develop it in order to come up with a normal PFD supported by a complete, detailed heat and mass balance on the unit. I have assumed that you already have this information in hand. Is this not correct?

As shown, there is NO recirculation ratio in the “bath” of the nitrogen condenser. There may be natural convection currents built-up, but these are anybody’s guess. What you have is a conventional, submerged coil (or plates) condensing the nitrogen gas within it while evaporating some of the crude liquid oxygen it finds itself submerged in. The total heat transfered is LATENT HEAT.

Please confirm ALL my assumptions and answer my questions in order to have a complete and better understanding of what you are facing.

[A mukherjee]

I will reply point wise:

1. On the top of high pressure distillation column,there is a pressure vessel,where Nitrogen is condensed by rich liquid(around 66%nitrogen,33% oxygen,1% argon) not by oxygen.It is called reboiler-condenser,because nitrogen is condensed here & Rich liquid boils.I have already given details of the bath arrangement in earlier post.
Generally when oxygen is produced along with nitrogen,a low pressure distillation column is present on the top high pressure distillation column.Between the two columns ,a reboiler condenser is present ,where N2 is condensed & O2 is boiled,as described by ART.
As we are producing only N2,LP column is missing,only reboiler -condenser is present.

2.I am concerned about RECIRCULATION RATIO, not REFLUX RATIO.I have already mentioned about recirculation ratio in bath in the earlier post.It is about the natural convection current as correctly stated by ART.PLATE & FIN HE vendor has given certain recirculation ratio for the bath side.I just wanted to know about its calculation procedure & significance.

3.We are taking these vendor offers for a NEW PLANT,which is yet to be built.So there is NO question of replacement.We have specified the heat loads for reboiler condenser,allowable pressure drop & accordingly vendor has given the offer.In the offer they have mentioned certain recirculation ratio for the bath side.Thats why I am interested in knowing the details of it.We have asked the VENDOR to provide detail of it,but he has refused.So I am seeking help from this community.

Hopefully this post will eradicate some confusion.

[Art Montemayor]

A mukherjee:

You haven't confirmed that my assumptions are correct but by repeating what I have already stated (although not drawing a sketch), you admit that I have been correct. You have a total condenser that is acting as the reboiler for the vessel mounted on top of the High Pressure Air Separation Column. We could have saved a lot of my time (& yours) if you had only stated and drawn that sketch from the very outset.

I have already addressed and responded to your concern regarding the fact that there are natural convection currents created in a totally submerged type of reboiler coil. And I repeat my response:

"As shown, there is NO recirculation ratio in the “bath” of the nitrogen condenser. There may be natural convection currents built-up, but these are anybody’s guess. What you have is a conventional, submerged coil (or plates) condensing the nitrogen gas within it while evaporating some of the crude liquid oxygen it finds itself submerged in. The total heat transfered is LATENT HEAT."

You or your supplier can claim whatever "recirculation ratio" you like; the real-life fact still remains: no one can prove that the recirculation ratio exists or verify its existance. Natural recirculation is pure guess work - at best - and it is based on a lot of simplifying assumptions that are subject to change in the real world. All that matters in your application is that the condenser/reboiler perform as specified. And to do that, it has to have sufficient heat transfer area. Who cares if the "recirculation ratio" is the "correct" one - but the unit does not perform? I don't believe anyone would care. All your company wants is a condenser/reboiler that works according to the specifications on the DataSheet. And that should drive the selection and purchase of the unit.