Author Topic: Stirring P10 by rolling the cylinder?  (Read 4220 times)

Doug_Meier

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Stirring P10 by rolling the cylinder?
« on: June 10, 2016, 09:59:21 am »
I've had some version of the following conversation rather frequently with colleagues in EPMA, so I'd like to visit it on this forum. Here goes:

Sometimes during the course of a flow counter's life, it will fail to function correctly, even though there is still P10 flowing and plenty of pressure in the cylinder. The reason that this happens, I am told, is that the gas mixture in the cylinder has settled out. That is, heavier argon has sunk to the bottom, lighter methane has floated to the top, and thus the cylinder must be remixed in order to work properly. So then, the conventional recommendation goes, one must disconnect the cylinder, lay it down on the ground, roll it around for a while, stand it back up, reconnect, and purge for a while. The counts should then recover.

Can someone here point me to any literature or data demonstrating that this settling actually happens and that roll-mixing actually works? I cannot find anything that convincingly supports this line of reasoning; what's more, it runs counter to everything I know about compressed gas cylinder use.

First and foremost is the safety issue. Every cylinder safety guide I've every read warns strongly against laying a cylinder down for any reason. Doing so greatly increases the risk of damaging the valve, potentially creating a ballistic hazard. Secondly, rolling cylinders on the ground can compromise the strength of the container, increasing the risk of rupture upon refill.

Even assuming we're all very careful and can perform this rolling operation safely, is there any reason to believe it would be effective? I don't believe that there is.

First, these are compressed gas cylinders, not condensed gas cylinders. Condensed mixed gases could indeed separate into separate fluid phases, provided the fluids were immiscible. If this happened, one would hear or feel them sloshing around inside the cylinder as it was being rolled and mixed. However, in a P10 cylinder, neither gas can condense. At full pressure of a new cylinder, the argon is supercritical (P=1800 psig > Pcrit = 705 psi while T > Tcrit = 151 K) and the methane is in the gas phase (P = 200 psig > Pcrit = 677 psi while T > Tcrit = 190 K). Thus, there can be no formation of phase boundaries.

One of the basics of gas physics is that gases expand to fill their containers, with the only density gradients corresponding to changes in gravitation from one end of the container to the other. Since the cylinders are under 2 meters tall in a field in which the center of mass is around 6,000,000 meters away, that gradient ought to be negligible.

Finally, thermodynamics generally doesn't favor spontaneous separation of any mixture, as that requires a reduction in system entropy.

So, barring a GC or MS trace counter to this, or if anyone knows a physical law that explains spontaneous gas separation (I sure couldn't find one, and if anyone can, I'm sure Praxair would be delighted to learn of it), I can't come up with any reason rolling a cylinder helps with anything. Yet I know people who swear by this dangerous practice of questionable value. After all, it is working for them. Why? I propose that when one disconnects a cylinder to roll it around, one inadvertently fixes the actual problem upon system reassembly -- poorly regulated gas flow.

In many circumstances, I have seen regulators attached to these systems that are poorly suited for the task. Our flow counters require just a slow gas flow (about a bubble per second), which is achievable at a pressure just above atmospheric. To do this most effectively, one requires a low-pressure (range from 0.1 - 2 psig) dual stage regulator. I haven't seen every system in operation so this is admittedly anecdotal, but the ones I have seen often have the regulator usually found in the general stockroom, which is a 125 psig single stage. The trouble with using this regulator is that it isn't built to control under 2 psig, and flow counters probably don't use 0.2 psig. So when these are placed in use, people open them just enough to leak, which isn't how they're designed and result in poor pressure and flow control, and perhaps even purity (this kind of regulator isn't sealed all that well). Worse, as the cylinder is depleted, single stage regulators need to be continually adjusted to maintain a pressure even within their operating range, let alone outside their range. So the longer they are allowed to operate in this unregulated condition, the worse things get. However, upon reassembly and purge, operators are usually very careful about resetting their unmaintainable conditions.

So naturally, everything works again after rolling the cylinder. I claim that, if the operator would do all of the steps they normally do when rolling the cylinder, *except for actually rolling the cylinder*, they would achieve the same result.

 In conclusion, I recommend swapping regulators on P10 cylinders out for dual stage low pressure regulators. After doing this in my former lab, I never had another cylinder that "settled out," my cylinders lasted longer and could be used down to the last 100 psig, and my PHA control needed considerably less adjustment on my flow counters from session to session.

Again, I'd be glad to hear other opinions on this. I felt I should initiate this conversation and generate some thought, discussion, and new practices before someone gets hurt by doing something that is ultimately fruitless.

Probeman

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Re: Stirring P10 by rolling the cylinder?
« Reply #1 on: June 14, 2016, 10:39:09 am »
Hi Doug,
It's an interesting question. Here is my take based on my own experience.

I don't think that rolling a cylinder on the floor is dangerous except maybe for the possibility of rolling the cylinder over ones' toes.  However, if mixing the P-10 gases is the goal, I would simply suggest a small greenhouse seed starter heating pad (~15 watts) which when placed under the base of the cylinder will create a slight temperature differential enough for convection mixing.

As for seeing detector degradation over time I would suggest that outgassing of contaminants from the cylinder walls is more likely as the cylinder pressure falls below 500 psi.  We have a high pressure gauge which "phones home" to our engineer when the cylinder pressure drops below 500 psi. See here for the Sensaphone device we use for all sorts of alarms:

http://www.sensaphone.com/

Here's a picture of the gauge:

http://probesoftware.com/smf/index.php?topic=332.msg2077#msg2077
« Last Edit: June 14, 2016, 10:50:20 am by Probeman »
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Doug_Meier

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Re: Stirring P10 by rolling the cylinder?
« Reply #2 on: June 23, 2016, 09:35:19 am »
Thanks for including those observations, Probeman. I would agree that rolling the cylinder in a controlled manner is not all that dangerous; however, the increased probability of dropping it while either laying it down or picking it back up adds a bit more hazard. Perhaps oversensitivity to liability is the cause for the stern warnings included in the compressed gas safety guides. Perhaps the floor-rolling procedure really does reduce the reusability of the cylinders? I wish I knew an honest-to-goodness compressed gas industrialist who would provide an unfiltered answer to that question. If I get one, I'll relay it here.

I like the suggestion of the convection pad or some other means of heating the gas differentially instead of rolling it. However, I've already convinced myself that a P-10 cylinder, being filled with either a supercritical fluid or a compressed gas for the entirety of its service life, should never have to be mixed in any way for any reason. Granted, I've arrived at this conclusion by invoking theoretical constructs such as critical temperatures and system entropy arguments. Naturally, a real-world GC, IR, or other experiment showing a concentration gradient of the gas that leaves the bubbler as well as the gas that leaves the cylinder would change my opinion, not to mention serve as the foundation for an excellent technical note in an appropriate journal.

Pressure-dependent cylinder outgassing as a contamination mechanism is interesting to think about. I normally would consider this question from the high-vacuum side of the problem, where one may invoke either the Langmuir adsorption model (non-interacting gas molecules occupying non-interacting adsorption sites) or possibly Henry's law (treating the adsorbed gas as if it were dissolved in the wall of the chamber). Either way one considers this problem, the proportional coverage of each of the adsorbed gases is proportional its own partial pressure. So as one pumps a vacuum chamber down to ultrahigh vacuum, one certainly sees the gases with low proportionality constants (nitrogen, oxygen, and argon) removed rapidly while the "sticky" gases (like water and CO) with much higher proportionality constants hang around forever, and must be baked out to be removed from the system. This process plays out over several orders of magnitude, however (I'd usually graph coverage as a function of the log of pressure, and some systems would require pressurizing by about 4 orders of magnitude to saturate one molecular layer).

For a P-10 cylinder, which we usually rotate out of service after the pressure drops a single order of magnitude, would we expect the relative proportions of contaminants (probably water from the argon and CO and CO2 from the methane) to change noticeably? Many of the Langmuir assumptions are invalid at high pressure in real cylinders (water, CO, CO2, and even methane are quite interactive with each other and with steel, adsorption sites are most certainly non-equivalent, multi-layer adsorption or some contaminants is possible). Referring to Owen and Anette's discussion in another channel, I had only used storeroom P-10 previously, which was standard purity. It is true that we did need the wires cleaned from time to time, but I don't know if we were more or less frequent than other instruments' cleaning schedules. Once again, an experiment verifying the magnitude of the change would make for an interesting technical note.

Thanks, all, for the discussion!

 

Anette von der Handt

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Re: Stirring P10 by rolling the cylinder?
« Reply #3 on: June 24, 2016, 02:10:17 pm »
Very interesting discussion.

As mentioned in the other thread, I found the following discussion on P10 in an XRF forum from 2001, with some anecdotes regarding "demixing" and attempts to make sense of all this.

Not sure how "proper" it is to paste in some of their discussion but the links give the entirety of the discussion including the original authors. There are more posts but I copied what I felt might add most to the discussion here:

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... Another tip I have heard from my instrument manufacturer (Bruker) is to allow the P10 to rest (don't move) prior to placing in line.  Organics (other than methane) should settle in the bottom of the tank.  Also recommended... I have my backup standing next to my primary tank waiting for several weeks and I don't disturb to allow the impurities to settle.  For similar reasons avoid running your P10 completely empty so not to suck up the potential residue lurking down there.

Our GC chemist also "bleeds" the main valve prior to placing the regulator on his GC gas tanks.  For similar reasons avoid running your P10 completely empty so not to suck up the potential residue lurking down there.....

https://listserv.syr.edu/scripts/wa.exe?A2=ind0104&L=xrf-l&D=0&P=43141
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and

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One thing to be careful of is tanks of P-10 that are in storage for a long period of time. We had a tank delivered by our internal stores (who managed the tanks at the time) and I could not see any peaks with the flow counter. Turns out that the tank was in storage for a few years and diffusion set in changing the composition of the gas. Got a new tank and the problem was solved.

https://listserv.syr.edu/scripts/wa.exe?A2=ind0104&L=xrf-l&D=0&P=44495
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but then in another thread on P10 temperature effects the following month:

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...The question I am trying to answer is can the Argon and Methane really separate? I realise the whole world but me believes this to happen. Till now I assumed this was only possible if one of the gasses started to condense, but I never had the numbers to know at what temperature methane condensed at 2000psi or so cylinder pressure. Robert said it was not possible for either gas to condense as you needed a temperature below -82C. So its only really now that I make the argument that the whole temperature stability issue is because of changing gas density and not composition. Why would the difference in the densities of the 2 different gasses cause them to separate? Isn't the mobility of the molecules of the 2 gasses sufficient to give a completely homogeneous mixture? Presumably not if your theory is correct. But then wouldn't the gases separate anyway even at room temperature. If that were the case then a full cylinder would give less methane than an empty one and pulse shift would occur as the cylinder was used up. I never heard anyone complain about pulse shift until the cylinder was almost empty. So this also supports my gut feeling that the whole world but me has it wrong.

https://listserv.syr.edu/scripts/wa.exe?A2=ind0105&L=xrf-l&D=0&P=43998
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And one that goes more into much more detail with thoughts on the Soret effect:


https://listserv.syr.edu/scripts/wa.exe?A2=ind0105&L=xrf-l&D=0&P=45872

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I have enjoyed reading the commentary on this topic and have decided, after years of wondering what was really going on in my P-10 bottle, to try and work out the actual phases and their concentrations present in the bottle at various pressures and temperatures.  I started with a basic assumption that many people believe in:  gas suppliers use acetone to clean tanks (?) and their may be some residual in my tank.  What I have discovered is this:  If there is acetone in my bottle of P-10 there must be at least 2% by weight (inclusive of both the gas and liquid phases) in order to have any liquid at 2000 psi and 60 deg. F.  The transition point where all of the acetone is in the gas phase occurs between 200 and 250 psi at 60 deg. F.  Interestingly enough, raising the temperature of the bottle to 80 deg. F will also bring all of the acetone into the gas phase at 2000 psi.  I have included some calculated values in the form of a chart ... hopefully the mail server can handle it!

It is interesting to note that the concentrations of Argon and Methane in the gas phase change very little in this system.  The maximum concentration of both at 60 deg. F is achieved at a pressure between 500 and 1500 psi -- it seems that with acetone as a contaminent, a full bottle is not the best thing to have.  Argon and Methane dissolved in the liquid Acetone act to buffer the system.

My thanks to the people at Quest Consultants, Inc. who have an excellent web site (http://www.questconsult.com/~jrm/thermot.html) with a tool for doing phase calculations.  It works in mole fraction, so you must do your own conversions to weight percent.

It would be my opinion at the moment that, if contaminants in the bottle are the source, PHD shifts are probably due to a relatively low vapor pressure contaminant that is not significantly present in the gas phase until the bottle is nearly empty.

https://listserv.syr.edu/scripts/wa.exe?A2=ind0105&L=xrf-l&D=0&P=48119
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Probeman

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Re: Stirring P10 by rolling the cylinder?
« Reply #5 on: June 24, 2016, 03:32:50 pm »
and there are various more interesting posts there. For (my) time sake, I will just put the links of relevant posts here so that it is easier to follow their discussion.

Hi Anette,
So in your reading of this vast amount of material, did you come across any discussion of contaminants outgassing from the inside of the cylinder walls as the P-10 pressure drops below a few hundred psi?
john
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