Posted 26 February 2012 - 03:50 PM
Shirish D:
After looking at your schematic drawing, I agree with Breizh: although both versions of your configuration may work, I would opt for the simplest and most direct method to expel the possible non-condensables leaking into your process during its operation.
Allow me to note the basic scope of work of the intended vacuum system you would place on this type of distillation column:
The location of the nozzle that allows expelling the possible non-condensables in the system through the use of a downstream vacuum system should be done where there is more likely accumulation of the non-condensables due to possible physical traps or “dead spots” within the total condenser and reflux drum assembly. The non-condensables that are being extracted by the vacuum system are those that are either part of the process stream because they were originally dissolved (or entrained) in the feed liquid or if they were generated by a previous reaction within the system or prior to it. Additionally, there are non-condensables that are “filtering” into the distillation system through all the joints in the process and its piping. Every gasketed joint or flange is subject to this invading leakage from the outside, atmospheric environment that is at a higher pressure.
Therefore, in order to maintain the proposed vacuum distillation operation running at design conditions, it is imperative that a continuous purging (or extraction) of the non-condensables take place as fast as they appear within the distillation system. The most opportune location to extract the non-condensables is at the end of the vapor system – the overheads total condenser / reflux drum area. This is so because one can rationalize that all in-coming non-condensables (regardless of their origin) will confront a “dead-end” at the junction where the overhead vapors are condensed into a liquid and the non-condensables (as such) have no place to go except either accumulate or be purged. In other words, this is the opportune place because there is a phase separation – condensed liquid solvent product and saturated non-condensables. Note that I describe SATURATED non-condensables as being available for extraction at that point.
Applying engineering common sense to the above Scope of Work, one can rationalize that the ideal spot to extract the non-condensables is where they are most apt to accumulate. And that would be at the spot in the combined condenser / reflux drum assembly where the condensed stream is at its coldest and highest location – since this ensures that most of the condensable solvent vapor has been converted to liquid and can be gravity-drained, stored, refluxed, and produced. If your non-condensables are relatively light (which is the normal case– such as hydrogen, methane, etc. ) then the physically most desirable manner of dealing with the task at hand is to have a total condenser that ensures that the total amount of vapor (solvent vapor and non-condensables) that enters the condenser is subjected to complete and thorough cooling within the condenser and that this will allow subsequent phase separation between the resulting condensate and the non-condensables. If this can be assured as taking place in a physical location (such as at the end of the condenser) where the non-condensables are prone to accumulate if the condenser is higher than the reflux drum, then this is naturally – and logically – the optimum place to allow the vacuum system to extract the non-condensables.
Please take careful note of the following:
You can also force removal of the non-condensables from the top of the reflux drum. However, this forces you to ensure that all the condensate and non-condensables are evacuated from the condenser into the reflux drum and physically separated into two phases there. This requires a special drain nozzle between the condenser and drum that is designed for 2-phase flow and self-venting characteristics. Additionally, it should be considered that the longer trajectory between the top of the condenser and the reflux drum will take more time for the non-condensables to reach their exit point and, therefore, more solvent vapor may have to be evacuated due to the pressure instrument’s response. If you opt to use the reflux drum as the site for non-condensable evacuation, then you cannot rely on a dip pipe to transfer the 2-phase flow; you must totally rely on the proposed “equalization” line as the transport means for the non-condensables. This is not a logical description (nor use) for the equalization line. The equalization line is used to merely equalize pressures between both vessels, not to transport fluid or vapor. What is additionally not logical is the fact that if you are relying on the equalization line to transport the non-condensables from the condenser into the reflux drum, why do you go to all the trouble and expense of doing it that way, when you could just as well do it the simple, direct way and expel the non-condensables at the end of the total condenser? The answer is obvious: you would not logically do that because it makes no sense. You would be more logical and practical if you simply extracted the non-condensables at the extreme end of the condenser – the way that Breizh recommended in the first place.
I believe – and highly recommend – that you pay careful attention to the physical design of the total condenser and the type and layout of internal baffles employed in it. Remember: you are trying to create a phase separation at the end of the condenser, so it is important to provide sufficient and efficient surface area and a place at the end of the condenser where you can effectively separate the non-condensables. This also means that the type and shape of baffles should conform to the scope of work: do not employ horizontal cut baffles if you employ the scheme in your sketches. Use vertical cut baffles instead to ensure that you can transfer the differentially formed condensate from baffle compartment to baffle compartment until all the condensate reaches the end. You can also opt to use individual drains in each compartment to drain away the condensate formed there. In that case, you can employ horizontal cut baffles. Each individual drain, however, must be liquid-sealed by a looped drain in order to maintain each baffle section at its respective pressure condition and not allow the drains to serve as vapor by-passes. In short, what all this means is that you should seriously study and learn to dominate the physical characteristics and tradeoffs of the various TEMA type of exchanger designs in order to ensure that you have a reliable and well-engineered physical design. This type of practical engineering solution does not employ a single academic “formula” or equation that quickly resolves the problem at hand. It requires practical, common sense and hard application and understanding of the available equipment to do the job.
Also, another bit of advice: In the future, try to furnish a schematic sketch - that can be commented on - in an Excel spread sheet. That way, we can avoid a lot of the above verbiage and go directly to the drawing, showing what and where we make our comments. This method is much simpler and more direct - a pure engineering way of communicating that is precise and direct. Words are good for story-telling and poetry; an engineering sketch tells what a thousand words can't convey. We can't make mark-ups and indicated comments on a word processing document as we can in a spread sheet.
Good Luck.
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Condenserconfiguration.doc 159.5KB
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