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.