Dew point for RNG

[MatthewL]

Hi all,

It's been a while since I've checked in, but I have a question about water dew point in a RNG (renewable natural gas) stream. I did read the related posts, but none of them seemed to deal with the kinds of pressures and low water content I have. We are capturing landfill gas and separating the methane out for sale as RNG, but due to the location of the landfill, we have to truck the RNG to an injection point. The company we are contracting with for the trucking has a requirement of 1 lb water/MMSCF max due to condensation and freezing issues due to J-T effects during offload, plus moisture spikes that the pipeline operator doesn't much care for (and understandably so). I'm trying to wrap my head around the dew point that the 1 lb/MMSCF represents, and no matter how I have approached it, I get wildly varying results. The gas composition is approximately:

CH4 - 95%+
950 BTU/SCF
CO2, N2 - Total less than 5% (CO2 should be ~2%, N2 ~3%)
O2 - Less than 0.2%
No appreciable C2+
Total sulphur < 10 PPM
Water - Less than 1 lb/MMSCF

Some of the dew point numbers I got for a 1 lb/MMSCF water content were -116 F using a psychrometric calculator and -67 F using a calculator on the Shaw website. We will load the trailers to ~4000PSIG.

The reason I'm looking for a dew point is we have several on line dew point analyzers at process equipment prior to final compression and loading. The final quality check is performed by GC, so there is the potential to load off spec material during the time between injections and results (up to 15 minutes). If I can catch the issue earlier in the process, I can divert to flare until results meet my specs and avoid some gas processing costs.

Regards,

Matt

 

[LittleInch]

What pressures are those dew point temps?

Composition has a big impact and the is no easy way other than direct testing to get you a number, especially at 275 bar. Have you used something like HYSIS or an EOS?

I'm assuming you don't want free water in your CNG truck transport, so you might need even drier gas than 1lb/mscf.

Purely out of interest, what are you using to transport this gas? 275 v ar implies lots of small cylinders.

Some pictures would be nice....

I would be looking for something like-10C or maybe 0 at 275 barg.or -20C @ 70 bar.

If you're not heating this stuff before dropping down to pipeline pressure then you're going to hit -30 or lower off your inlet pressure is low.

A few more details would help...

 

[MatthewL]

The psychrometric was at standard pressure. The Shaw website was for 4000 PSI, but it also corrected to standard pressure. The 1 lb/MMSCF is from the transporter and has been proven empirically to not result in issues during offload.

I don't have access to HYSIS or another simulation program to model the system, hence why I thought to check with you all. Like I said, our GC will give us a water content, but my only concern is if there is an upset, we could be loading high moisture content product for 15 minutes before we find out. There are multiple dehydration steps in the process, so I don't think we will have an issue, but...

Here's a website with the trailer in question, it only has 4 tubes and is a FRP wound tank.

https://gazeo.com/automotive/technology/TITAN-53-worldapos;s-largest-composite-CNG-tanks,article,10189.html

There's always a heel left in the tank, so the useable volume for us is ~450,000 SCF at 4000 psig. We need roughly 4 a day to keep up with our faceplate capacity, but they offload it in around 2 hours, so J-T effects are pretty significant on their end.

 

[georgeverghese]

Most facilities / process engineers in the gas business should have a copy of the GPSA Handbook -see the chapter on Gas Dehydration where you will find the McKetta charts for water content of natural gas streams, covering pressures up to 10 000psia. At 4000psig, some extrapolation is required to get the dewpoint for 1 lb/mmscf, but it appears to be about 0degF.
Shaw dewpoint meters ought to be recalibrated every so often, and the sensor is prone to poisoning from contaminants in the gas.

 

[pierreick]

Another good reference:
Gas conditioning and processing. Volume 1_ The Basic Principles
by J.M Campbell

 

[LittleInch]

If they know 1lb /mmscf is a good number then fair enough.

Composition does impact the dew point so be a bit careful about charts etc based on virtually pure methane.

0F at Standard conditions is quite reasonable. What can your current dehydration do?

Thanks for the info on the trailer. That's a nice piece of kit.

 

[apetri]

for std. hydrocarbon mixtures EOS models are usually accurate within 1 .. 3 C ,
and with water one can adjust KIJ ,
there are good tools to calculate dew points, phase diagrams, regress KIJ etc. see for example

https://www.prode.com/docs/pppman.pdf

a point to discuss with yur client (to understand those specs) should be the real impact of a very small (ppm) amount of water,
as possible problems with condensed water you can, for example, consider hydrates but there are many other points to evaluate...

 

[MatthewL]

With our process, the only major constituents left are methane, carbon dioxide and nitrogen. There are three separate active media beds at the front of the plant to remove H2S, VOCs, and siloxanes, leaving us with a "raw" gas stream of ~55% CH4, 39% CO2, 5% N2 and 1% O2. It then goes to the carbon dioxide removal step, which takes CO2 down to ~1.5-2%, and finally a nitrogen removal unit which also targets O2, and gets us to the product spec I listed above. At other plants, we inject into the local pipeline, which typically has a moisture spec of ~8 lb/MMSCF and we have no issue meeting that spec. Since there are no pipelines in reasonable distance, we have to truck the RNG, and have to meet the lower moisture spec. And George, I don't really consider this a "gas business", we use either landfill gas or digester gas as a feedstock, and don't have the higher carbon constituents that typical NG facilities deal with. Our concern is removing enough atmospherics to meet tariff spec, which I also listed above, especially heat content since we don't have any higher carbons to help with that.

 

[MatthewL]

And thanks for the link to Prode calculator, apetri!

 

[MatthewL]

LittleInch,

We do three separate dehydration steps, one using fin/fan aftercoolers, then a glycol chilled HX at ~5 PSIG, 40 F, and finally a high pressure dehydration done at ~200 PSIG, again at ~40 F. Each of these is immediately after a blower/compressor, and serves both a dehydration and fluid cooling purpose. Also, the media used for polishing the gas of trace impurities remove water as a side benefit, and releases it to our flare stack (one is alumina, the others are activated carbon).

Matt

 

[LittleInch]

It's definitely that 4000 psi that makes the difference, both in the resulting free water and also the low temperatures and water drop out when you release it into the pipeline.

You have a decent system there to lower your dewpoint.

If this system is running can you not calibrate / compare the dew point measurements versus the GC allowing for the time delay?
How much does your dew point vary over time?

 

[MatthewL]

We will, but for the initial startup I don't have a good feel for what dewpoint we should be seeing. We have another plant that uses a slightly different separation tech, and it runs consistently around 0.8 lb/MMSCF. So at least I have a reference point to judge the effectiveness of the new system.

 

[georgeverghese]

So where is your concern here ? Are you uncertain of the dehydration effectiveness at the 200psig, -40degF unit, guessing from your post that you dont wish to take credit for the water vapor removed at the downstream alumina and activated carbon units ? Is this where you are currently relying on the Shaw dewpoint meter for checking effectiveness?

If so, can you describe this unit / tell us more ? Is this some temp swing or pressure swing operated mole sieves adsorber ? What contaminants in the feedgas act as deactivators / poisons for the mole sieves here ?

 

[shvet]

Natural gas dew point, hydrates, water phase...
Am I alone in concluding that the facility needs attention of an experienced process engineer to model process upsets and determine the margins&limits?

 

[MatthewL]

George,

Our process is fairly simple. The landfill gas (which is saturated, and has a dry basis composition of 54% CH4, 40% CO2, 5% N2, and PPM levels of H2S, VOCs and siloxanes) is brought into the plant through a demister vessel using a blower with a discharge pressure of around 5 PSIG. The discharge from the blower goes to a fin/fan aftercooler and then to a demister to remove any water droplets. After that, it goes to H2S removal vessels with activated carbon specifically for H2S, and then to low pressure dehydration using 35-40 F chilled glycol as the cooling medium. After another demister vessel, it goes to our feed gas compressor (discharge of 200-220 PSIG), then high pressure dehydration again using the same chilled glycol. There are an additional 2 demister vessels after the dehydration HX. From there, it goes to VOC/siloxane removal using alumina in one step and activated carbon in the second. The alumina is in PTSA (pressure/temperature swing adsorber) vessels, there are 3, one on line, one in regeneration mode using the reject (permeate) from the CO2 removal membranes, and the third in standby, and they rotate. After the PTSA vessels, there are 2 activated carbon towers in lead/lag configuration to polish any remaining VOCs or H2S (different activated carbon from the H2S vessels). The gas composition at this point is ~55% CH4, 40% CO2, 5% N2 and trace water and O2. The next step is CO2 removal using two stages of membranes. Permeate from the first stage is used to regen the PTSA media. Permeate from the second stage is sent back to the inlet of the feed compressor, as it has a relatively high concentration of CH4. The product stream is now at ~90% CH4, 9% N2, and 1% CO2, again with trace water and O2. The final purification step is nitrogen removal using VPSA beds (vacuum/pressure swing adsorbers) which get us to a final product purity of >96% CH4. From there it goes to a screw compressor to raise pressure to ~150 PSIG, then past our GC product gas analyzer (with a bypass to flare if spec is not met) and finally to a 3 stage recip compressor to load onto the tube trailers at up to 4000 PSIG. The condensate removed from the process is sent to an oil/water separator, and the water is returned to the landfill leachate system, while the oil cuff is disposed of.

 

[MatthewL]

shvet,

The only dew point of concern is the water dew point, there are no higher hydrocarbons than methane in anything beyond trace (> 0.1%) amounts.