Particle size discussion for Low-pressure preparative GC

Discussions about GC and other "gas phase" separation techniques.

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Hi there everyone,

So I have a fascinating yet difficult GC extraction problem to solve.

I have some thermodegradative organic acids which have a low vapor pressure, along with other compounds that need extraction.

The amount of raw materials to be processed may be on the order of tons per year, and the C.B.A. for using liquid chromatography or one of the other methods is just not good enough to justify it, the low costs associated with GC are a major selling point of the technology.

However, given the low VPs of our target compounds and the thermodegradation, a high operating temperature is an impossibility unless the process is fast enough (my research suggests it isn't).

Thus, I'm having to adapt a technique which is (as far as I can tell) only used for analytics and not production.

So my constraints are vast.

The inlet pressure needs to be close to or just slightly above or below standard atmospheric pressure (1.5 bar-0.5 bar absolute pressure). The outlet pressure should be near a vacuum (0.1-0.3 bar). The average column pressure is maintained below atmospheric so that the low temperature required can still boil the compounds I'm trying to isolate.

The only engineered silica which has the pressure drop characteristics I need are particles of 0.2mm<x<1mm. Obviously the theoretical plate number and plate height are bad compared to smaller particles.

I can increase the total plate number by lengthening the column, as the pressure drop is small enough that I can easily manage several meters of column length and still achieve the low system pressures required.

However, I'm concerned that in my quest to reach a high enough plate number, that I'm going to cause significant band-broadening due to the low efficiency of the system. How bad can this problem get with a plate height and column length as high as mine will need to be?

Obviously great care is going into the stationary phase, as we need high affinities that are quite different for each compound in order to increase our plate number/reduce plate height.

First question really, is how much lost efficiency and band-broadening becomes inevitable with particles of this size?

Thank you for the help and suggestions.
You will also need a cyrogenic cooler (prevents volatization before injection) and a headspace sampler (for volatiles analysis) as well. Do you have any assurance that these compounds are detectable by GC detectors?

HPLC may be the cheaper and your only option after all!
Indeed fascinating

From the header in your post you appear to be talking about preparative chromatography for processing tons per year of your product rather than analytical chromatography?

To the best of my knowledge preparative GC is, at most, only good for something in the order of 100mg of eluate

First question really, is how much lost efficiency and band-broadening becomes inevitable with particles of this size?


Answer - massive loss to the point of being almost ineffective even with the better approach of liquid chromatography and the huge volumes of solvent required for preparative work at your implied scale
Regards

Ralph
Can these 'volatile compounds' be adsorbed onto a matrix (like affinity chromatography) and then eluted out?
Substantial so far.

@Ralph Yes, preparative GC is done for large scale chemical production, here's a good paper to read on the topic: Pages 529-533 http://cnqzu.com/library/Anarchy%20Fold ... graphy.pdf

You probably haven't heard much about it because nobody sells these, they're pretty much all engineered from scratch, and there are only a hundred or so large scale prep-GCs in the world.

There are other papers which found that using the right packing method (tapping and rocking, strictly controlled particle diameters) column diameters of 10 cm can exhibit comparable plate height to a 1 cm column. So far nobody has asked the question of whether a larger diameter column would have different relative scaling results.

The sample preparation is indeed quite important, due to the nature of the extract this is taken from the water content luckily minimal, but bubbling through with nitrogen and vacuum purging will be done, likely ahead of time.

And while our compounds of interest are thermolabile, they're not so thermolabile that we won't get our intended goal, isolated near pure phase. It takes a temperature of around 120 C before the degradation rate really starts to pick up.

Which is to say, this even isn't a major problem, the compounds' degradation products are also desired compounds. Preparing the sample with a solution of say, ethanol (with a much lower boiling point than our target compounds), and doing a fast vaporization (mostly done with heated metal balls in-column, to get as rapid a heat transfer as possible), should result in less of the peak leading you would expect from a compound which is degrading in-column.

Granted, there's the ideal, and then there is the result when things get to the real world. But high plate heights and large diameter columns are indeed worked around when people design these. Some numbers out there are really bad. I've read papers about designs that cope with N=400/m and still do their intended job.
That is interesting.

The link to the paper doesn't appear to mention particle size and does make some "ideal" assumptions for continuous operation.

The sample preparation is indeed quite important, due to the nature of the extract this is taken from the water content luckily minimal, but bubbling through with nitrogen and vacuum purging will be done, likely ahead of time.


Without wishing to sound negative, this sounds like a very inefficient and time consuming step.

What exactly is your target analyte and matrix and proposed stationary phase to coat your particles?

The same author, along with others, has another paper on the use of preparative-supercritical fluid chromatography using CO2 with a modifier.

This may be another approach to consider. It has many advantages
Regards

Ralph
I have been looking into pSFC, and it definitely has the right thermal parameters and low-cost of mobile phase to make it attractive, we'll have to explore that more in an alternative design analysis. The run rate might be a bit long but increased capacity ultimately would deal with that issue. The only questions then would relate to capital and running costs, I'll have to poke more at it.

Particle size gets discussed in other papers, the generalized pressure drop equation divides a constant by the particle diameter squared. The effect is that a drop in particle size starts to raise the delta P to numbers in the megapascals. With a particle size of just under or at a millimeter (with very low fine particles), the generalized equation has a pressure drop of a much more comfortable 25-100 kPa.

As always with chromatography you trade one quality of your column in order to gain another. The minimum plate height increases, but that can also be managed with a good stationary phase.

Speaking of which, all our target compounds have phenyl groups, some multiple, plus different polar groups, some extra hydrocarbon rings with double bonds, some alcohols, etc. I can't discuss the details aside from that, but given the phenyl groups and differences in polarities/MW/molecular structure, existing chromatographic columns are already good at resolving these target compounds. The acids are either intentionally decarboxylated or derivatized, but a lot of the necessity to derivatize is to accomplish the analytical goals of quantification. That, and the operating schema of analytical columns requires derivatization due to the high temperatures and pressures in the tests. Where pressure and temperature are lower this is less of an issue, on an operational level that is.
GOM wrote:

Without wishing to sound negative, this sounds like a very inefficient and time consuming step.


It's not as bad if we design a unit that we can stage these samples in and purge them before use, rather than doing it with the apparatus of the column. It would be a time consuming process if we ran them one at a time, so clearly we have to run multiples at once in their own enclosed containers.
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