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  #1  
Old 02-04-2012
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Understanding fuel and e10 gasoline tank vent driers

Yes, I know they dry air by surface absorption. I'm chemical engineer, I know the theory, and I've designed and operated industrial systems. What I'm wondering about is applicability to gasoline and fuel tanks.

No, I do not intend to bash or boost H2OUT. I'm glad to see someone put their foot forward. I do want to better understand the applicability of the product.

First, an analogy. Carbon is often used to control VOC and odor emissions. If you put a small measured flow through the carbon in the lab, you can calculate a projected capacity. If you then calculate reactions, evaporation and breathing, you can calculate a projected lifespan. However, when observing real systems in intermittent service (breathing in and out), one generally sees many years of service, not a few months.

In the case of odor control, such as holding tank carbon filters, the carbon catalyzes break down reactions, cleaning itself over time. In the case of more resistant chemicals, it still buffers the highs and lows, preventing the odor from reaching a noticeable threshhold. If the filter is large enough, it lasts until it fouls with dust, clogs with non-volitile reaction products, or the pore structure breaks down; perhaps 3-5 years. In the case of petroleum vapors on tank systems we similar effects; if the load is not too high (pipe line deliveries are too much but normal breathing is not) carbon can last a long time, if the canister is big enough. In cars we draw combustion air through the canister to regenerate the carbon, and so it lasts the life of the car, give or take.

Can a silica gel canister, if large enough, function in the same manner? Do we need to dry the air to a low dew point, or do we simply need to average out the high points, so that water absorbs into the gel during the night and then breaths out during the day? In art cases it's common practice to place enough gel in the case to buffer seasonal variations, without intending to replace the gel. They're are more interested in stability than very low RH.
http://talasonline.com/photos/instru...a_gel_info.pdf

If the user is really going through fuel, sailing every week, the flow through the canister is greater BUT he has no need for breather control anyway; it's mathematically impossible for the tanks to breath an important amount of water until the fuel half life exceeds 6 months for e10 gasoline and a few months for diesel. If there is water, there is another source, either leaks or delivery.

If all we really need to prevent is precipitation of condensate in the gasoline tank (or phase separation), is it enough to simply take out the highs and lows? Does a small amount of dissolved water make a difference? In the case of e10 it is the alcohol itself that is corrosive to aluminum, reacting with the oxide film; water cannot do that and doesn't contribute to the process.

In the case of ordinary diesel, water is needed for bugs to grow, but with biodiesel dissolved water may be enough; the research I've read (I have also done some lab testing related to biocides) is not conclusive on this point. It is known that biodiesel is more susceptible to infection--those that reject that are in denial. It is also understood that biocide can hydrolyze to fatty acids in the presence of trace water. With conventional diesel, simply eliminating free water should do; with biodiesel, if you can't possibly avoid it, which I would, drier is always better.

I'm wondering if a larger gel container, perhaps several times the size of the H2OUT package, would be functionally permanent for non-biodiesel applications, requiring no maintenance, or at least going 5 years? If I was stuck with biodiesel, I would go even bigger, 3x, and be finished with it. With carbons systems I have a good feel for the amount of over-sizing required; here, I am guessing. A typical 25 gallon tanks might be able to absorb ~ 2-5 ounces of water per year by breathing, and any unit that holds 6-10 times that should serve as a permanent buffer; the smaller H2OUT unit (AVD23) holds 19 ounces and should serve the average sailor with a tank of less than 30 gallons (it's rated for 60 gallons) very well. Beyond that, I'm not guessing.

Also, by it's placement in the line, any container reduces fresh breathing, since about half of that volume is recycled and is not fresh.

(note: I think the web site has the dimensions and volumes reversed for the AVD2 and AVD3. The dimensions and bulk density seem about right. Someone should tell them.)

What do we think about an obstruction in the line? I'll not discuss that, other than to say I would be tempted to provide a spring check valve in a bypass as an emergency relief. Maybe not.

Like so many things, just oversize.

Your thoughts?
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A possible blockage in the vent line could cause a shut down at a very bad time. I briefly read the H2OUT site and didn't see anything about a bypass valve, similar to oil filters? Other than that, if it does what it says it does, better to keep the water from forming in the first place rather than trying to capture it or disolve it after it is in the fuel.

Dabnis
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I used to actually do this during my term in dehydration of air/gas engineering using various pressure swing and vacuum swing adsorption beds.

The saturation within desiccant beds is an isotere front that travels through the 'bed' physically in a 'funnel' shape ... one side of the front unsaturated the other side full saturated. The obvious problem in an atmospheric pressure system ('natural aspiration') is relative inefficiency of adsorption ... desiccants need system pressure to affect such adsorption to any degree of efficiency - simply look at the isotere curves as would be supplied by RhonePoulenc, etc. (or No.Am.Norite etc. for 'carbons')

With the potential of hydrocarbon vapor fouling of a desiccant bed presents a special problem in that regeneration MUST be done via a vacuum swing (approx.-6"Hg. abs.) or you will 'coke' the vast surface area of the desiccant (normal regen. at ~350°F/8hrs).

Carbon has an equilibrium preference for water vapor vs. hydrocarbon vapor and you really need to 'trim' the flow to carbon with a low dewpoint gas or the carbon will 'swell' and eventually block the mass flow. Since vacuum swing regeneration is a PITA, especially on a boat, that leaves you with the need to constantly 'replace' any such desiccant with 'fresh'.

I actually had a mixed-bed desiccant chamber on my boats (100+gallons cap.) vent line. Tank-->carbon--->activated alumina--->vent and after about 2 yrs. of minimal performance, and then simply removed it - a royal PITA.

If you 'really' want to keep 'fresh/dehydrated' fuel from saturating just follow what the 'oil patch' people do .... constant small volume 'tank padding/blanketing' with dry nitrogen or -40° dp air... just a few cc's / hr. will do this. Another possibility, again from the oil patch, is to use an inert 'blanketing foam'. If you had a 'vac. rated' fuel tank the 'real solution' would be a simple gas tight ****-valve on the vent line !!!!!!!

For me, 'condensation' is only a symptom that the oil has already become fully saturated by vapor/mass transfer equilibrium. The obvious then, would be to keep an absolute minimum of fuel + 'reserve'; ... fuel only (if possible) purchased from a high turn-over source (fresh and relatively water free) such as from 'truck stop' or an 'active' fuel depot that serves 'watermen'/commercial marine.
I use a bio-blocking (0,2µ abs. liquid rated) hydrophobic membrane filter to prevent the aspiration of fungal, etc. spores (typically @ 1-2µM) ... a 0.5 sq. ft. bio-pharma 'vent capsule filter' works well (also on my potable water tank).
An active, independent 'recirc.' filter (5-10µM abs. @ 3gpm/100 gal.) to keep the 'titre' of microorganisms at 'below trace'
What I now use. Fresh (dry) fuel (if possible) and 'just enough', bio-blocking filter on the vent, recirc. system. I only take on the fuel that I NEED. I drain/empty the fuel tank when the boat is long-term (month or two+) stored.

If emulsified water or free water becomes a problem then you can switch out the 'particle' recirculation/polishing filter for a 'water shedding' or water aBsorbing filter (containing HydroxyMethylCellulose, etc.) or if the fluid velocity is low just drain the recirc filter bowl occasionally ... use a tygon, etc. pigtail with a **** on the filter bowl so you can 'visualize' - a simple gravimetric 'knock out pot' BEFORE the recirc pump.

BTW ... the common fungals (Cladosporium Resinae, etc. - principally 'resin formers') that contaminate fuel oil systems really dont need 'free'/visible water to propagate, probably use the emulsified or molecular water/oil to stimulate their metabolism, the oil is their nutrient source. An active recirc-polish system constantly removes such spores and the detritus of cellular decomposition.

As stated, I used to actually do 'desiccant work', had such a chamber on my fuel tank but ultimately abandoned it ... and simply adopted 'best practices' as used in the oil patch and ultra-high purity chem process industry, etc. You simply have to get inside that tank every now and then to mechanically clean it ... no use in 'filtering', desiccant vents, recirculation polishing, adding biocides, etc. ..... until you 'start' with a clean/hygienic tank.

Bio-diesel .... if from 'reclaim' usually contains a whopping amount of oleic acid ... need to change out ALL of the fuel system 'elastomers'. Oleic goes through BUNA, Neoprene, etc. 'like it was soft butter'. Too much shysterism with (reclaimed) Bio-diesel.

Thats my 'story' and Im sticking to it. ;-)

--------
Dabnis, a simple 12vdc differential vac.-pressure switch wired to an 'alarm' with the vacuum 'set-point' adjusted to 'below' the vac. levels in the fuel delivery system that cause an engine to 'stumble' at WOT is standard practice on 'important'/critical engine or 'prime mover' systems ... lets you know long before the engine stumbles that you'd better be 'looking' and long before you become 'surprised' with a shutdown.
;-)

Last edited by RichH; 02-04-2012 at 03:08 PM.
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Old 02-04-2012
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Quote:
Originally Posted by RichH View Post
I used to actually do this during my term in dehydration of air/gas engineering using various pressure swing and vacuum swing adsorption beds...

... and simply adopted 'best practices' as used in the oil patch and ultra-high purity chem process industry, etc. You simply have to get inside that tank every now and then to mechanically clean it ... no use in 'filtering', desiccant vents, recirculation polishing, adding biocides, etc. ..... until you 'start' with a clean/hygienic tank.

Bio-diesel .... if from 'reclaim' usually contains a whopping amount of oleic acid ... need to change out ALL of the fuel system 'elastomers'. Oleic goes through BUNA, Neoprene, etc. 'like it was soft butter'. Too much shysterism with (reclaimed) Bio-diesel.

Thats my 'story' and Im sticking to it. ;-)
Thanks for taking the time to respond. The effort and expereince show. I know only enough about carbon to support my work, plus some boat specific testing I'm doing with Practical Sailor on holding tanks vents. As for desiccants, I've placed them on a few chemical tanks but not studied the real performance in any depth. Your response was detailed and made abundant sense to me.

The bit about coking during regeneration is both obvious and missing from the vent filter product literature. Really, can you imagine baking-out gasoline saturated absorbent in the kitchen oven? I'm in love my wife and would like to stay married. Kidding aside, that's plain stupid, without engineering details.

My personal interest is in e10, and I'm quite sure that it cannot absorb enough though normal breathing to be a problem; calculations show a gain of 0.1 - 0.2% water for a boat that is used a fair amount (the fuel can absorb ~ 0.4% without risk of separation) and I have not seen the problem on my boat. I have big Raycor filters and would have observed it. My last boat had terrible problems with separation, but I am certain some minor infiltration was the root cause (the fill cap was located in a run-off channel). I've also learned to keep my tank full, which clearly helps; minimal breathing.

I know the holding tank vent filters work--there will be an article in Practical Sailor in the next issue (there are also other good solutions to holding tank odor, but that would require a LOT of thread drift). In spite of the fact--FACT--that descant filters are almost certainly not needed on fuel tanks, I do wonder if there might be some small benefit and an intelligent way to do it. And it's winter and I've got to ponder something. I'm pretty sure you are right though; fuel and e10 problems aren't generally solved by fixing the air vent. The problem is elsewhere.

Again, thanks.
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Last edited by pdqaltair; 02-04-2012 at 05:50 PM.
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PD & Rich H,

Very interesting and complex. As you mentioned , try to get good quality fuel to start with. Maybe the easiest approach is to have a good water separator/filter system and service it often?

Dabnis
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For holding tank vents, consider 'extruded' carbon (almost like an open celled rigid 'foam')... wont swell as bad as 'loose' carbon when wetted.
There literally 1000s of 'carbon forms' used for ad/absorption ..... .
Suggest you check with the NorthAmericanNorite website for carbon-blends best applicable for 'stink' adsorption ... you really have to 'match' the carbon (source of carbon -- coconut shell, coal, soft/hard wood, and 'blends' etc.) for your exact application. Most of the hardware store stuff is mismatched inefficient 'hooo-dooo' and is principally for 'decolorization' of water. The carbon form (source) you use really should be molecule 'specific'. Also with ground/granulated/powdered carbon you really need to be careful to keep it 'contained' ... or mega-mess.
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Quote:
Originally Posted by dabnis View Post
PD & Rich H,

Very interesting and complex. As you mentioned , try to get good quality fuel to start with. Maybe the easiest approach is to have a good water separator/filter system and service it often?

Dabnis
free water removal from fuels is VERY simple .... on a boat simply install an empty small filter housing with the 'bowl' pointing down, keep the flow 'non-turbulent' ... and the free water will 'drop out' or settle by gravity into the 'bowl'. The knock-out pot should be at the 'lowest' part of the entire fuel distribution run. Called a 'knock out pot'.
Use a small run of clear/translucent tubing (tygon or PTFE tube is best) with a cockvalve at the terminal end, all attached to the 'bowl' ... and you can 'see' when to empty it.
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Yes, I did consider these things. They are famiar.

Quote:
Originally Posted by RichH View Post
For holding tank vents, consider 'extruded' carbon (almost like an open celled rigid 'foam')... wont swell as bad as 'loose' carbon when wetted.
There literally 1000s of 'carbon forms' used for ad/absorption ..... .
Suggest you check with the NorthAmericanNorite website for carbon-blends best applicable for 'stink' adsorption ... you really have to 'match' the carbon (source of carbon -- coconut shell, coal, soft/hard wood, and 'blends' etc.) for your exact application. Most of the hardware store stuff is mismatched inefficient 'hooo-dooo' and is principally for 'decolorization' of water. The carbon form (source) you use really should be molecule 'specific'. Also with ground/granulated/powdered carbon you really need to be careful to keep it 'contained' ... or mega-mess.
We did test extruded carbon; not impressive. Oddly, moisture and damp are not a problem, unless you splash waste or seawater into the filter, which we didn't explore. Clearly, if you're splashing water or wastewater in the filter, it's going to be dead in days anyway. Condesation is not a factor in this application (boat or industrial scale) in my expereince.

From what I can tell (marine vendors are tight-lipped) all others were about 4/20 bituminous and all worked well enough, at least at one-year. The reality is that pet-store carbon is much more easily available, and that is what will be used for refills. It works well enough to be nearly indistinguishable in the real world.

If I were in the business of making the filters, I would wade through the options--I've done that before in chemical polishing applications--but this application doesn't seem so demanding, not at these flow rates.
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RichH:

a. How was the desiccant trap a PIA? Any difficulties other than changing the absorbent? Minimal benefits I understand.

b. As part of an experiment for Practical Sailor I left a number of 1 L jars loosely sealed and exposed to the weather (sheltered from rain and mist) for six months. This particular set of experiments was never reported as it was only intended to get a feel for the materials involved. Several were plain e10, and the balance were treated with a variety of "stabilizing" additives. During the initial six months, none of the samples separated and the experiment was terminated. However, I ignored them and checked on them today, at about 10 months. It seems, that as the lighter portions of the gasoline evaporated, including the ethanol, the water was able to drop out. Interestingly, all samples had exactly the same amount of precipitate, and the water was exactly the amount that e10 could have held in saturation at the end of the summer. in other words, every sample was saturated-- probably saturated early in the past-- but none separated until the alcohol evaporated and the temperatures dropped. this was not an example of phase separation; the amount of water is far too small and too uniform for that. It seems that ethanol's tendency to absorb water is self-limiting; it saturates but does not spontaneously separate unless there is another source of water, perhaps leaks.

Nothing new under the sun, just an interesting confirmation of my prior understanding.
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Old 02-05-2012
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Quote:
Originally Posted by pdqaltair View Post
RichH:

a. How was the desiccant trap a PIA? Any difficulties other than changing the absorbent? Minimal benefits I understand.

b. As part of an experiment for Practical Sailor I left a number of 1 L jars loosely sealed and exposed to the weather (sheltered from rain and mist) for six months. This particular set of experiments was never reported as it was only intended to get a feel for the materials involved. Several were plain e10, and the balance were treated with a variety of "stabilizing" additives. During the initial six months, none of the samples separated and the experiment was terminated. However, I ignored them and checked on them today, at about 10 months. It seems, that as the lighter portions of the gasoline evaporated, including the ethanol, the water was able to drop out. Interestingly, all samples had exactly the same amount of precipitate, and the water was exactly the amount that e10 could have held in saturation at the end of the summer. in other words, every sample was saturated-- probably saturated early in the past-- but none separated until the alcohol evaporated and the temperatures dropped. this was not an example of phase separation; the amount of water is far too small and too uniform for that. It seems that ethanol's tendency to absorb water is self-limiting; it saturates but does not spontaneously separate unless there is another source of water, perhaps leaks.

Nothing new under the sun, just an interesting confirmation of my prior understanding.
I did a similar but simpler test. Half a coffee can of gas, not sure how much ethanol was in it, about 2 ounces of water, and some Berryman's B-12 fuel additive. After a few minutes the water was no longer visible. After about 6 months the only thing left was a very small amount of thick dark goo, about the consistency of 90 wt gear oil. To this day it has not dried, still very thick
oil like substance. That was about a year ago. Have no idea what it means but I suppose if it was in the bottom of an empty float bowl new gas coming in may melt it and cause no problems? As mentioned in an earlier post I did have a float bowl that had solid deposits in it, kind of like course salt, after forgetting to drain it.

Dabnis
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