6 replies to this topic

[marcouni08]

Hi,

 

I am a student currently completing my final design project and I am currently deciding whether the low pressure column of my double Linde process should be trayed or packed.

my oxygen purity is of 99.2%, nitrogen should contain a maximum of 10vpm of O2 ( Ar is an allowable impurity) and argon should be produced at 98% purity. 

the literature that I have explored points out that the Argon column should be packed in order to be able to reach the 98% purity without the need for catalytic conversion. however, the literature is pretty vague regarding the low pressure column.

the main advantage of packing is higher purity (lower than 1vpm of O2 in N2 rich) and lower pressure drops. however, stating the fact that i only require 10vpm, is the additional cost of packing worth? i am really struggling to justify my choice (trays or packed) as i do not have available any clear reference.

 

literature around trays is also more common which would help me when I will start designing the column in detail.

I would be very glad if you could suggest and point out advantages and disadvantages of either methods and point me towards a choice,

 

many thanks

 

Marco

[Pilesar]

In general, do not worry overmuch about the cost differences in your design project. If you find both types of internals are used successfully, then I would choose the one with the best go-by. For justification of whichever path you choose, use words like "reliable", "conservative", "proven" and other words that indicate you have confidence in your selection. Many times in the real world, technology is selected based on personal preference. The engineer who says "this will work" and backs it up with vague anecdotal experience is often the deciding factor. This method of selection is not really wrong. Successful operation is what matters in the end.

[Art Montemayor]

Marco:

 

Pilesar is citing the way that engineering projects are judged in the real world: “Successful operation is what matters in the end”.

 

If you have done any research or gathered any knowledge of how the air separation process came to be, you should know that Carl Linde developed this method of distillation in the 1890s and the double column came into industrial existence using bubble cap type of trays.  Today many other separation internals have been tested and used.  Bubble caps have successfully operated for a century.  I would say that Carl Linde proved his point.  Why try to re-invent the wheel and argue about his success?

 

Unless your assignment involves proving that packing will work just as well in an air separation column, I would stick with employing bubble caps.  Stick to the fundamentals of the assignment.  Contrary to what you state, packing DOES NOT yield higher purity.  Packing may yield a lower column height – but that is the subject of optimization and for that, you would have to also allow for the disadvantages of packing, such as bad liquid distribution, purity variances during operation, channeling, etc.

 

Are you assigned to “optimize” the column design.  I would think not.  It is a subject that can’t be attacked by one single individual.

[marcouni08]

many thanks art and pilesar you have been of great help! i will continue in my design using bubble caps cross flow and straight downcomers.

hopefully you'll be able to help me should other question arise.

 

Many thanks,

 

Marco

[marcouni08]

As I am proceeding to justify my choice of plate column against packed I have encountered the following questions and I would be glad if you could give me some suggestions:

1. liquid oxygen is a corrosive fluid and the literature advises to make use of packed column to reduce costs

2. liquid holdup is much smaller in packed columns and this is advisable for toxic/flammable liquids (i.e. O2)

3. plates are better for fouling systems. 

4. packed columns are suitable for low liquid rates

 

my questions are:

how can i justify the use of plates even though my system contains highly flammable and corrosive fluid?

 

what type of liquid flow rates should i expect? my column produces 490t/day of gaseous oxygen and 728 t/day of  gaseous nitrogen. the nitrogen is produced straight at the top of the column, whereas the oxygen is taken from the plate just above the bottom (with a total reboiler) in teh gaseous phase. my idea here is that the liquid rate i.e. oxygen flow is therefore high (am i correct?, how can i explain this better??)

 

regarding the materials to use i was thinking about stainless steel (300 series??, which one is the better?) for plates with no corrosion allowance (as customary for SS) . for packing the Mellapak (sulzer).

 

finally, Art suggested the use of bubble cap, which are the most expensive between trays but which also ensure a positive liquid seal. my question is: do i need a seal? otherwise simple sieve trays should work fine (is that correct?). i am struggling in being able to justify the use of bubble caps as these are described in the literature as the older type which has now been replaced by either valve trays or packing.

 

please do not hesitate to ask me for any clarifications,

 

Many thanks in advance for your very precious support!.

 

Marco 

[Pilesar]

Your Question: how can i justify the use of plates even though my system contains highly flammable and corrosive fluid?

Oxygen has zero flammability. It is considered an oxidizer. Oxygen toxicity is not generally a problem. Since you find both types of internals are in use, there are likely good reasons for both. You must choose. Justify your choice with persuasive words highlighting the reasons for your choice. Are you required also to explain why you did not select the alternate technology? Young engineers usually choose the newer technology. But the "tried and true" technology is often selected when real money changes hands because the consequences of failure often far outweigh the rewards of marginal improvements.

 

Your Question: what type of liquid flow rates should i expect? 

This results from your product specs and your process design. Higher yield generally comes with a more complicated process at higher capital cost.

 

Your Question: regarding the materials to use

316 SS is a safe choice. You note that you would not have a corrosion allowance. This contradicts your stated reason not to use trays in that you are dealing with a corrosive fluid. Try to reconcile this inconsistency for better understanding.

 

Your Question: finally, Art suggested the use of bubble cap

If Art is designing the process, he should use bubble cap. That is his experience and he has confidence in it. You have the final choice in your design and must weigh the evidence. In the region where sieve trays work, they will work as good as bubble caps. The big advantage to using bubble caps is that they give a higher turndown ratio. Valves also have a good operating range but not quite as good as bubble caps. Valves are replacing caps in most services because they are cheaper and usually good enough. With low liquid flows, it is of primary importance to keep the liquid on the trays. The internals of a column do not cause the separation -- the internals help provide an environment where the vapor and liquid can contact each other. Wherever you have liquid bypassing vapor, there you have inefficiency.

 

 

[Art Montemayor]

Pilesar has directly and specifically hit on the important issues involved in process design.  I would heed what he has recommended and advised.

 

I (me, myself, based on my expererience and judgment) would opt for bubble caps for this assignment.  That doesn't mean I would apply the same decision on another air separation assignment.  As Pilesar states, it is meant to be YOUR decision based on the evidence and needs at hand.  That is what sets engineers apart from most other professions: the results (and accountability) rests on us.  Welcome to the club.

 

As an additional reminder to bring into focus when considering what is expected of you and what is involved in an air separation column:  do not forget that this is a cryogenic process.  To date (and I believe in my lifetime) there are NO MANWAYS OR FLANGES allowed in cryogenic systems.  I hope this is common, accepted knowledge amonst us and the reasons are WELL UNDERSTOOD.  If not, then ask us on the Forum and we (especially I) will be more than glad to explain the whys this is so.  In other words, the column will have no conventional human entry or access to the internals (everything is seal welded).  Whatever you install as internals will have to permanently stay there without any human intervention in its future life - which is normally 15 to 20 years.  Therefore, if you propose any trays that require moving parts or maintenance, this will normally be thrown out the window by a knowledgeable or expererienced instructor.