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Anonymous Poster #1

# Removing Moisture in Biogas Pipeline (Malaysia)

07/06/2017 5:20 AM

Hello,

can somebody help me how can I solve this problem involving thermodynamics calculations. I'm trying to use the Venturi Nozzle trap to remove the moisture in biogas inside the pipeline by condensation method (reach dew point temperature). Below is the relevant data I have had:-

T = 32-35°C

P = 120 mbar

flow rate = 150 m³/hr

amount of moisture need to remove = from 4000 ppm to 1000 ppm

I have also included the attachment that shows roughly about the problem

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#1

### Re: Removing moisture in biogas pipeline (Malaysia)

07/06/2017 5:34 AM

<...remove...> There is one stream in and one stream out: without a side stream leaving the process, there cannot be any removal, only a change of state of the materials therein.

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#10
In reply to #1

### Re: Removing moisture in biogas pipeline (Malaysia)

07/07/2017 8:38 AM

The trap of venturi nozzle trap assembly is the missing bit, perhaps like a steam trap with a float....

He might find something like Munters DF2100 Mist Eliminator works for lower cost of pressure production. These guys have lots of experience, decades of doing just this.

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#2

### Re: Removing moisture in biogas pipeline (Malaysia)

07/06/2017 8:34 AM

Where's your condensate trap?

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#3

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/06/2017 1:59 PM

Assuming your moisture figures (ppm) are mg water vapour per kg of air (or kg total air + vapour, it makes negligible difference) according to my estimate, temperature to give 4000ppm at saturation is about 2°C, and to give 1000ppm at saturation is about -15.5°C.

What's on your sketch won't achieve anything, you need a means of cooling the air.

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#6
In reply to #3

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 3:42 AM

<...saturation...> Er, um the Original Poster wants <...Removing...>. Changing the stream conditions won't achieve removal, as any removal still needs a side-stream for separated material leaving the process.

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#4

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/06/2017 2:29 PM

A little advice: start at the finish line (with the biogas and water separated in different tubings), and think your way backward.

Instead of losing a lot pressure across this Venturi, why not use an absorbent temperature cycling material such as molecular sieves. Such a system is routinely used on large volume compressors for instrument air in power generating plants, and works 100% full-time, and dries the moisture out within a small percentage of the original content.

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#5
In reply to #4

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/06/2017 3:01 PM

That's fine, but it's going away from the original post. He said "...........to remove the moisture in biogas inside the pipeline by condensation"

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#7
In reply to #5

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 4:54 AM

...and there is still no sign of a side-stream leaving the process.

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#8
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### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 5:09 AM

That's right, but if no vapour condenses there's no condensate to remove. If a cooling stage is added one would like to think it would incorporate a condensate outlet pipe.

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#9
In reply to #8

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 5:11 AM

Quite.

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#22
In reply to #7

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 9:47 AM

Obviously, installing a gas dryer "on the pipeline" infers by default:

an inlet (interrupt pipeline with some flange mounted device, for example) port for processing the gas flow to remove {water}. This is basic to all unit processing, let's not argue moot points.

an outlet (resume connection to the downstream pipeline) port for discharge of the dried gas.

a processing unit between the inlet port and the discharge (outlet) port, with a by-product port (water discharge tube), are you happy now?

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#20
In reply to #5

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 9:43 AM

Really? You are telling me that the only allowed way to think is that wrongly supposed by the OP? I am not in the bullsheite market today, neither buying nor selling.

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#11

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 8:51 AM

But is it possible to do a simple calculations about any moisture traps to find how much the moisture have been removed (relative humidity?)

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#12
In reply to #11

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 8:58 AM

Yes, indeed: for any control volume the Mass Balance calculation concept will do this:

Input = output + accumulation

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#13
In reply to #12

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 9:15 AM

I see, so which of the data do I need to use to put inside the mass balance calculation?

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#14
In reply to #13

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 9:33 AM

The flows of each component in, the flows of each component out, and the rate of accumulation of each component.

That's why it is called a Mass Balance calculation.

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#15
In reply to #11

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 9:44 AM

If by moisture trap you mean a physical separator, that on its own will not remove moisture.

If as implied in original post, you want to do it by cooling the gas, you can calculate the mass of water condensed. You first need to know or calculate the density ρ of the biogas. Decide whether your 150m3/h is at actual or standard conditions, and use ρ at same conditions. That gives you gas mass flow, and water condensed, kg/h = gas mass flow*(4000 - 1000)*10-6.

But that doesn't give you the temperature you need to cool the gas to. To get that you need to find the temperature (hence saturated vapour pressure) which results in 1000ppm in the gas after cooling.

The figures I gave in #3 were based on water vapour in air. In biogas I make it temperatures about -0.5 and -17.5°C, assuming biogas 70% methane, 30% CO2 (unless I've made any mistakes).

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#17
In reply to #15

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/08/2017 1:28 PM

Just want to ask those calculations you stated are for refrigeration method to remove the moisture (chiller) or for the moisture trap (nozzle)? Thank you.

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#19
In reply to #17

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 3:15 AM

The only way to achieve those heat transfer surface temperatures, other than by moving to the polar regions, is using refrigeration. However, at those temperatures, the moisture will solidify.

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#23
In reply to #15

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 9:54 AM

IF there is a large amount of carbon dioxide in biogas, and not arguing that point, then surely, any water condensed will also be saturated in CO2 that the percentage mentioned for biogas composition. Apparently, there will also appear a means for recycling the carbon dioxide back to the source in the water condensed, if means are controlled to the point that an alkaline absorber is added, and thermally cycled.

Better yet, use a (1) gas dryer (molecular seive thermal cycle), and a (2)CO2 absorber, to collect these separately, and recycle them both.

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#24
In reply to #23

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 10:32 AM

<...recycling the carbon dioxide back to the source...> is probably not needed. The biogas is needed as a fuel and <...carbon dioxide...> is one of the end products of combustion of fuels. Sending it back to the source is abstruse.

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#25
In reply to #24

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 10:34 AM

Clean up the fuel to the highest possible BTU content is never a bad idea, sorry, but you are just wrong.

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#26
In reply to #25

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 10:36 AM

Sending CO2 back to the source would be abstruse, as it is waste from combustion and the biogas is probably going to be used as a fuel anyway, producing even more of it.

Adding recycled CO2 to the biogas at source would reduce its calorific value, and that too would be abstruse.

There has been no suggestion of removing all the CO2 at the moisture separation point from the Original Poster, only the moisture. At this juncture the Original Poster has not revealed to the forum the need to remove the moisture.

The plot thickens...

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#28
In reply to #26

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 11:03 AM

Do you actually understand photobiochemical pathways?

CO2+ sunlight + algae (or other) = biomass

biomass + bacteria = rot gas (biogas)

and it really is a tremendously inefficient process.

You will find this to be true --- someday.

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#42
In reply to #25

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 12:12 PM

I'm with PWSlack on this.

I don't understand your #23 Apparently, there will also appear a means for recycling the carbon dioxide back to the source in the water condensed, if means are controlled to the point that an alkaline absorber is added, and thermally cycled.

Some CO2 is dissolved in the condensed water, and the more the better as it increases the biogas CV, but not all of it and who cares anyway? It will lower the water pH, but this is likely to be recycled to the works inlet (assuming it's a sewage or effluent plant) and the volume will be << plant flow. There's no reason I can think of that the CO2 would be recycled.

If the gas is used locally, CO2 isn't a problem if the burner or engine is designed to suit. If it's to be injected into a natural gas pipeline and the CO2 has to be removed, I'd guess the easiest way is to compress the gas till the CO2 liquefies (critical temperature 31°C) run it off and sell it to somebody.

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#44
In reply to #42

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 1:40 PM

Yes, the problem is much easier to solve by compressing first, then cooling followed by adiabatic expansion. The liquids will separate out (mostly likely water in liquid CO2), then the operator can side stream out the liquids, and separate them by flash boiling, etc.

Sidebar: the adiabat must be calculated to still have sufficient residual pressure for liquid separation and energy recovery from the compression. One never gets it all back, but can get at least some of it back.

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#16
In reply to #11

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/07/2017 10:01 AM

OK. Here goes:

• Removal of 3000ppm of H2O from a mix of 70% CH4 and 30% CO2 flowing at at 150m3/h results in a flowrate of ...<tap...tap...tap...> ....er.... <...tap...tap... tap...> around 0.5kg/h.

"1 pint per hour" territory.

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#45
In reply to #16

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 1:42 PM

Yep, glycol unit here we come.

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#18

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/08/2017 1:40 PM

Actually my main problem here is that, I have to use the moisture/steam trap method (Venturi nozzle or simply a nozzle) to remove the water vapor inside the biogas. And I have to proof that method by showing manual calculations on how to reduce the water content from 4000 ppm to 1000 ppm. So I decided just to show simple calculations of nozzle gas flow where at the exit water will condensed due to velocity changes -> pressure drops -> temperature drops -> condensation occurs. And I also need to find the dew point temperature. So the only data i've been given as stated as above where I have temperature, pressure and flow rate at input.

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#21
In reply to #18

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 9:46 AM

Fine, but that is not a separation process with which one might effect <...removal...>.

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#29
In reply to #18

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 12:21 PM

I think the dew point for water doesn't change in the gas mixture, providing it hasn't chemically combined with one of your other gases. So find the thermodynamic difference between your 4000 and 1000 ppm states, and then what pressure drop produces that shift?

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#31
In reply to #29

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 12:59 PM

You know that your mixture is above saturation point at 4000ppm incoming conditions...

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#32
In reply to #31

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 1:45 PM

Based upon rough order of magnitude equivalence of the molecular weights of gases involved methane (16), water (18), and carbon dioxide (44), where the gas is supposedly 30% carbon dioxide, and 4000 ppm water in 120 mbar (12 kPa) gas,

the approximate partial pressure of water vapor is around 0.05 kPa, and the vapor pressure of water at 35 °C is clearly 5.627 kPa from data tables, thus I see this as about 1% saturated.

Arden_Buck_equation

This equation has a closed form solution for vapor over water or over ice, that is exceedingly accurate. It is clear to me, the temperature would have to be lowered well below the triple point, and thus it is impossible to remove liquid water from this stream by temperature shift alone, one must resort to absorbent technology, or be prepared to remove ice.

The energy required is more than the value of the product as fuel.

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#33
In reply to #32

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 2:42 PM

I suspect the (unknown component) gas mixture is considered at equilibrium water saturation at 4000ppm, especially if you think you can drop moisture out with the negative heat of air expansion (consider as adiabatic process). Compressed air vortex coolers can produce well below freezing air temperatures, using this same process.

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#34
In reply to #33

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 3:39 PM

please explain that term: "equilibrium water saturation at 4000ppm"

It is either 4000 ppm, or it is at saturated equilibrium, the two are not equal, sir. Nor are those two terms congruent with each other in his temperature and pressure range.

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#35
In reply to #34

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 3:46 PM

The water condensing in the pipe is at equilibrium with the water in the vapor phase, and I don't really know how you can know that they are not at saturation?

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#38
In reply to #35

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 8:48 AM

Simple physical chemistry tells me what the vapor pressure at saturation is. They are no where near such a condition at 120 mbar total pressure, 4000 ppm vapor, and 30-35 °C. Not going to condense as liquid. It might be possible to reduce T below the ice point and get solid water to drop out, but we all know what a sticky wicket that is.

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#41
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### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 11:36 AM

I don't think that's right. According to my calcs, at 32°C saturated vapour pressure is 47.5mbar, giving vapour content 33.7gm/m3. Density of the mixture comes to 1.06kg/m3, and w/w gas content 33.7/1.06*103gm/kg = 31800ppm w/w. So at 4000ppm it's nowhere near saturation.

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#40
In reply to #18

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 10:54 AM

You won't get temperature low enough to achieve 1000ppm by expansion etc, specially from 120mbar(g). To do it by cooling (as opposed to desiccant drying) there must be some external means of cooling the gas.

To estimate the required temperature, one way is to assume a required temperature and calculate the mixture density at that temperature, from gas MW, molar volume (22.7m3 at 0°C and 1bar) and the gas laws. Then you use that to convert 1000ppm = 1000mg/kg to gm/m3. From that, and using molar volume and water MW, you work out saturated vapour pressure to give that density.

You then find temperature to give that saturated vapour pressure, either from a table or from the Antoine equation solved for T in terms of P. Then put that in as the initial temperature, and iterate to get a closer answer. It helps a good deal if you have Mathcad!

As others have said, the water will be ice at that temperature, but I assume this a course-work question and you only want a theoretical answer.

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#27

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 10:37 AM

Why is this removal process a requirement, please?

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#30
In reply to #27

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 12:50 PM

It's usually done to raise the LHV (Lower Heating Value) of the fuel gas to be closer to the HHV (Higher Heating Value) since excess moisture in the fuel lowers the overall combustion efficiency, which in turn leads to lower draft and/or potential corrosion/erosion as the flue gas passes over cooler exhaust surfaces.

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#46
In reply to #30

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 2:32 PM

Thanks. :-)

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#47
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### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 4:39 PM

Not only that, but the extra CO2 is not going to help anything, except add more mass flow through whatever combustion device is present.

It would OK to not do anything to the gas if this gas were being fed to a pressurized, closed combustion system with pure oxygen as the oxidizer, where the prime mover was a supercritical CO2 Brayton cycle engine. The water (including the extra water), and the carbon dioxide (including the extra carbon dioxide) are tapped off, and sent their merry way down other pipeline as (shocker here) pure water and food grade carbon dioxide.

Cycle efficiency on this bad boy can be north of 70%, IMHO. That makes for a quite a comfortably low heat rate for the electric power (or other) generated.

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#36

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 5:27 PM

Here is some information that might be helpful...

http://naturalgas.org/naturalgas/processing-ng/#water

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#37
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### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/10/2017 6:55 PM

Nice link, led to this, might give the desired answers, in my comfortable English units, being old & backward....

93F, ~16 psia, 190 lbm/MMSCF water shows equilibrium at 42,000 PPM, maybe a wild goose chase. But the target temperature for 1,000 PPM is shown, at 10F, so maybe this is more of a theoretical exercise, or a case of misplaced decimal points. Or someone trying to desperately avoid building a glycol dehydration plant.

1000 ppm is an order of magnitude higher than commercially saleable fuel gas, which makes this application more like wellhead transmission pre-treatment, normally saturated.

http://petrowiki.org/File:Vol3_Page_199_Image_0001.png

Doesn't cover 'bio-gas', which could be its own set of compounds.

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#43
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### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 1:35 PM

Until water reaches its own saturation vapor pressure, there ain't no stinking liquid water. It is just a mixture of various molecular gases (hopefully no inert gases such as Helium, or Argon), each with its own partial pressure. Surely, you did not confuse partial pressure with vapor pressure. Vapor pressure is a property of the liquid of the substance in question, modified only slightly be the solutes present (in liquid), and by the overall mechanical pressure on the system. At such low pressures overall, less than 1 bar by almost an order of magnitude, I expect no correction needed.

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#39
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### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/11/2017 8:56 AM

Nicely done, Mr. Rixter. That is how the world goes around, alright.

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#48

### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/28/2017 11:32 AM

Just want to ask, if/whether a thermodynamic properties table and psychometric chart specifically for biogas exist?

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#49
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### Re: Removing Moisture in Biogas Pipeline (Malaysia)

07/28/2017 1:17 PM

Biogas might not be a definitive term, I might assume cellulose based product, for instance, someone else might be decomposing soy beans, probably some differences there. Difference in extraction method for the same feedstock could influence compound mixture, which would affect the dew point.

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