>If you pump natural gas through liquid nitrogen you get ... slightly colder natural gas.
I am not proposing to bubble raw natural gas through liquid nitrogen.
>It's not like people haven't thought of these things before. Thermodynamics is well understood.
I don't claim to be the first to come up with the idea of using liquid nitrogen to liquefy raw natural gas. I am entirely open to the idea that using liquid nitrogen to liquefy RNG is somehow uneconomical or infeasible. I was hoping to get a more constructive reply: not references of how expensive or hard a smaller scale liquefaction plant is, but rather papers detailing why specifically using a heat exchanger with liquid nitrogen is infeasible, if as you say "It's not like people haven't thought of these things before."
What is raw natural gas? Predominantly methane, and some slightly longer alkanes, with impurities (H2S, CO2, N2, He,...) see [1].
What are the atmospheric boiling points of the alkanes? see [2]:
alkane, boiling point:
methane -183 °C
ethane -183 °C
propane -190 °C
butane -138 °C
pentane -130 °C
hexane -95 °C
heptane -91 °C
octane -57 °C
nonane -51 °C
decane -30 °C
undecane -25 °C
dodecane -10 °C
eicosane 37 °C
triacontane 66 °C
Observe that the lowest boiling point of the alkanes (the fuel in natural gas) is for propane at -190 °C.
The atmospheric boiling point of liquid nitrogen is −195.795 °C [3], wich is colder than the boiling point of any alkane!
What does this mean by definition? that a balloon filled with gaseous alkanes, when submerged in a heat bath of liquid nitrogen, and left to equilibrate temperature, will fully liquefy.
>Thermodynamics is well understood.
^ indeed
(This is my first order interpretation, assuming the mixing entropy does not decrease the boiling point of a mixture of alkanes, if however this is the case, I would like to see an explicit reference to a paper describing this for alkanes.)
So essentially heat exchangers and a supply of liquid nitrogen should be able to liquefy raw natural gas.
Now what about the economics of having to generate liquid nitrogen?
Let's take a step back: domestic natural gas pipelines connected to the gas grid doesn't contain liquid but gaseous natural gas. So at some point the liquid natural gas is expanded to a gas, and this absorbs heat at a low temperature. (I don't know at what point in the distribution chain this happens, but it necessarily happens somewhere.) This is the ideal point to colocate the liquid nitrogen generation plant: use the evaporation of LNG to gaseous natural gas to help the heat pump liquefy air/nitrogen, so the energy required to liquefy nitrogen is largely recycled.
The storage of liquid nitrogen at the gas well should be an entirely solved problem: at the exact sciences campus (de Sterre) of the Ghent university, at the solid state research block, there is a large (fenced off) tank of liquid nitrogen, and once in a blue moon a tank wagon comes to refill it...
[1] https://www.e-education.psu.edu/fsc432/content/natural-gas-c...
[2] http://chemistry.elmhurst.edu/vchembook/501hcboilingpts.html
[3] https://en.wikipedia.org/wiki/Nitrogen