.. _example-phreeqc-manual-07:

07 - Gas Phase Calculations
===========================

Organic matter decomposing in a closed system, and the gas that comes off it.
The same reaction is run twice, under two different physical constraints, and
the two give different answers -- which is the point of the example.

A gas phase in contact with a water can be **fixed pressure** or **fixed
volume**, and they are not interchangeable:

* *Fixed pressure* is a bubble free to grow. It keeps its total pressure and
  changes size, which is a gas escaping upward through a sediment, or a
  headspace open to the atmosphere.
* *Fixed volume* is a pore that cannot expand. Its size is set and the
  pressure rises as gas is produced -- a sealed vessel, or gas trapped in a
  confined formation.


The reaction
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.. raw:: html
   :file: study_07 - Gas Phase Calculations.html

Organic matter is added in increments and decomposes, producing CO₂ and, once
conditions are reducing enough, methane. Nitrogen and water vapour make up
the rest of the gas.


The results
-----------

.. figure:: GasCompositionPlot.svg
   :alt: Partial pressures of methane, CO2, nitrogen and water vapour against organic matter reacted, for both cases
   :align: center

   Partial pressures of the four gases against organic matter reacted -- the
   same four species under each constraint, eight curves.

   Early on, CO₂ dominates: the organic matter is oxidising and carbon is
   leaving as carbon dioxide. Later, as the system runs out of oxidant,
   **methane** takes over, and the crossover is the transition from oxidising
   to methanogenic conditions.

   The two constraints diverge as soon as there is appreciable gas. Under
   fixed pressure, producing more gas makes a bigger bubble and the partial
   pressures are held down by the total staying constant. Under fixed volume
   the gas has nowhere to go, so the partial pressures climb.

.. figure:: PressureandVolumePlot.svg
   :alt: Total pressure and volume of the gas phase against organic matter reacted, under both constraints
   :align: center

   The constraints themselves, which makes the difference explicit: under
   fixed pressure the pressure is flat and the volume rises; under fixed
   volume the volume is flat and the pressure rises.

   This is the figure to consult when choosing between them for a real
   problem. The question to ask is whether the gas in your system can expand.


What it shows
-------------

That the physical boundary condition on a gas phase is a modelling choice
with chemical consequences, and that it has to be made deliberately. Both
calculations here are correct; they answer different questions.


Source
------

* Parkhurst, D. L. and Appelo, C. A. J. (2013). *Description of input and
  examples for PHREEQC version 3.* U.S. Geological Survey Techniques and
  Methods, book 6, chapter A43. This is Example 7 of that manual.