This keyword data block is
used to define the composition of a fixed-total-pressure or a fixed-volume
multicomponent gas phase. The thermodynamic properties of the gas components
are defined with PHASES input. If
the critical pressure and temperature are defined for a gas component with PHASES, the Peng-Robinson equation of
state (EOS) will be used for calculating the relation between pressure and
molar volume, and fugacity coefficients will be calculated for the gases. If
the critical temperature and pressure are not defined, the ideal gas law will
be used. Ideal gases and Peng-Robinson gases cannot be mixed in a GAS_PHASE. A GAS_PHASE data block is not needed if
fixed partial pressures of gas components are desired; use EQUILIBRIUM_PHASES instead. The gas
phase defined with this keyword data block subsequently may be equilibrated
with an aqueous phase in combination with pure-phase, surface, exchange, and
solid-solution assemblages in batch-reaction calculations. Either Henry’s law
(ideal gases) or the Peng-Robinson EOS (nonideal gases) is used for calculating
the solubility of the gases. As a consequence of batch reactions, a
fixed-pressure gas phase may exist or not, depending on the sum of the partial
pressures of the dissolved gases in solution. A fixed-volume gas phase always
contains some amount of each gas component that is present in solution. The
initial composition of a fixed-pressure gas phase is defined by the partial
pressures of each gas component. The initial composition of a fixed-volume gas
may be defined by the partial pressures of each gas component or may be defined
to be that which is in equilibrium with a fixed-composition aqueous phase. When
the Peng-Robinson EOS is used and the GAS_PHASE has a pressure
higher than about 10 atmospheres, the initial gas-phase composition calculated
for a fixed-composition aqueous phase is only an approximation of the true gas
composition.
Line 0: GAS_PHASE 1-5 Air
Line 1: -fixed_pressure
Line 2: -pressure 1.001
Line 3: -volume 1.0
Line 4: -temperature 25.0
Line 5a: CH4(g) 0.0
Line 5b: CO2(g) 0.000316
Line 5c: O2(g) 0.2
Line 5d: N2(g) 0.78
Line 0: GAS_PHASE [ number ] [ description ]
GAS_PHASE is the
keyword for the data block.
number --A positive number
designates the gas phase and its composition. A range of numbers may also be
given in the form m - n ,
where m and n are positive integers, m is less than n
, and the two numbers are separated by a hyphen without intervening spaces.
Default is 1.
description --Optional comment that
describes the gas phase.
-fixed_pressure --Identifier defining the gas phase to have a fixed total
pressure; that is, a gas bubble. A fixed-pressure gas phase is the default if
neither the -fixed_pressure nor the -fixed_volume identifier is used. Optionally fixed_pressure or -fixed_p [ ressure ].
-pressure
--Identifier defining the fixed pressure of the gas phase that applies during
all batch-reaction and transport calculations. Optionally pressure
or -p [ ressure ].
pressure --The pressure of the gas
phase, in atm (atmosphere). Default is 1.0 atm.
-volume --Identifier defining the
initial volume of the fixed-pressure gas phase. Optionally, volume
or -v [ olume ].
volume --The initial volume of the
fixed-pressure gas phase, in liters. The ideal gas law or the Peng-Robinson EOS
is used to calculate the initial moles, n, of each gas component in the
fixed-pressure gas phase. Default is 1.0 L (liter).
-temperature
--Identifier defining the initial temperature of the gas phase. Optionally, temperature
or -t [ emperature ].
temp --The initial temperature of
the gas phase, in °C (degree Celsius). The temp along with volume
and partial pressure are used to calculate the initial moles of each
gas component in the fixed-pressure gas phase. Default is 25.0 °C.
Line 5: phase name, partial pressure
phase name --Name
of a gas component. A phase with this name must be defined by PHASES input in the database or input
file.
partial pressure --Initial
partial pressure of this component in the gas phase (atm). The partial
pressure along with volume and temp are used to
calculate the initial moles of this gas component in the fixed-pressure gas
phase.
Line 5 must be repeated as necessary to define all of the
components initially present in the fixed-pressure gas phase as well as any
components which may subsequently enter the gas phase. The initial moles of a
gas component that is defined to have a positive partial pressure in GAS_PHASE
input will be computed using either the ideal gas law, n = PV/RT ,
where n is the moles of the gas, P is the defined partial
pressure (Line 5), V is the initial volume, given by -volume
, R is the gas constant (0.08207 L K -1 mol -1 , liter
per degree kelvin per mole), and T is given by -temperature (converted
to kelvin), or the Peng-Robinson EOS (see keyword PHASES for the equations). Thus, in
Example data block 1 and with the wateq4f.dat database, which does not define
critical temperatures and pressures, the moles of all gases are calculated by n
= (0.000316 + 0.2 + 0.78) × 1.0 / (298 × 0.02807) = 0.04 mol.
If this gas phase reacts with a solution with a very small amount of water
so that n does not change (that is, the dissolution of gas is negligible), the
volume becomes V = 0.04 × (298 × 0.02807) / 1.001 =
0.979 L. It is likely that the sum of the partial pressures of the defined
gases will not be equal to the pressure given by -pressure . However, when the GAS_PHASE
reacts with a solution during a batch-reaction simulation, the moles of gases
and volume of the gas phase will be adjusted so that each component is in
equilibrium with the solution while the total pressure (sum of the partial
pressures) is that specified by -pressure . It is possible that the gas phase
disappears if the sum of the partial pressures of dissolved gases is less than
the pressure given by -pressure .
A gas component may be defined to have initial partial pressure of
zero. In this case, no moles of that component will be present initially, but
the component may enter the gas phase when in contact with a solution that
contains that component. If no gas phase exists initially, the initial partial
pressures of all components should be set to 0.0; a gas phase may subsequently
form if batch reactions cause the sum of the partial pressures of the gas
components to exceed pressure .
Line 0: GAS_PHASE 1-5 Find composition from solution 1
Line 1: -fixed_volume
Line 2: -volume 1.0
Line 3: -temperature 25.0
Line 4a: CH4(g) 0.0
Line 4b: CO2(g) 0.000316
Line 4c: O2(g) 0.2
Line 4d: N2(g) 0.78
Line 0: GAS_PHASE [ number ] [ description ]
GAS_PHASE is the
keyword for the data block.
number --a positive number
designates the gas phase and its composition. A range of numbers may also be
given in the form m - n ,
where m and n are positive integers, m is less than n
, and the two numbers are separated by a hyphen without intervening spaces.
Default is 1.
description --Optional comment that
describes the gas phase.
-fixed_volume --Identifier defining the gas phase to be one that has a fixed
volume (not a gas bubble). A fixed-pressure gas phase is the default if neither
the -fixed_pressure nor the -fixed_volume identifier is used. Optionally fixed_volume or -fixed_v [ olume ].
-volume --Identifier defining the
volume of the fixed-volume gas phase, which applies for all batch-reaction or
transport calculations. Optionally, volume or -v
[ olume ].
volume --The volume of the
fixed-volume gas phase, in liters. Default is 1.0 L.
-temperature
--Identifier defining the initial temperature of the gas phase. Optionally, temperature
or -t [ emperature ].
temp --The initial temperature of
the gas phase, in °C. Default is 25.0 °C.
Line 4: phase name, partial pressure
phase name --Name
of a gas component. A phase with this name must be defined by PHASES input in the database or input
file.
partial pressure --Initial
partial pressure of this component in the gas phase, in atm. The partial
pressure along with volume and temp are used to
calculate the initial moles of this gas component in the fixed-volume gas
phase.
Line 4 may be repeated as necessary to define all the components
initially present in the fixed-volume gas phase, as well as any components
which may subsequently enter the gas phase. The initial moles of a gas
component with a positive partial pressure will be computed using either the
ideal gas law, n = PV/(RT) ,
where n is the moles of the gas, P is the defined partial
pressure (Line 4), V is given by -volume , R is the
gas constant, and T is given by -temperature (converted
to kelvin), or the Peng-Robinson EOS. When the gas phase reacts with a solution
during a batch-reaction simulation, the total pressure, the partial pressures
of the gas components in the gas phase, and the partial pressures of the gas
components in the aqueous phase will be adjusted so that equilibrium is
established for each component. A constant-volume gas phase always exists
unless all of the gas components are absent from the system. The identifier -pressure
is not used for a fixed-volume gas phase.
A gas component may be defined to have an initial partial pressure
of zero. In this case, no moles of that component will be present initially,
but the component will enter the gas phase when in contact with a solution
containing the component.
Line 0: GAS_PHASE 1-5 Air
Line 1: -fixed_volume
Line 2: -equilibrate with solution 10
Line 3: -volume 1.0
Line 4a: CH4(g)
Line 4b: CO2(g)
Line 4c: O2(g)
Line 4d: N2(g)
Line 0: GAS_PHASE [ number ] [ description ]
GAS_PHASE is the
keyword for the data block.
number --A positive number
designates the gas phase and its composition. A range of numbers may also be
given in the form m - n ,
where m and n are positive integers, m is less than n
, and the two numbers are separated by a hyphen without intervening spaces.
Default is 1.
description --Optional comment that
describes the gas phase.
-fixed_volume --Identifier defining the gas phase to be one that has a fixed
volume (not a gas bubble). A fixed-pressure gas phase is the default if neither
the -fixed_pressure nor the -fixed_volume identifier is used. Optionally fixed_volume or -fixed_v [ olume ].
-equilibrate
--Identifier indicates that the fixed-volume gas phase is defined to be in
equilibrium with a solution of a fixed composition. This identifier may only be
used with the -fixed_volume
identifier. Optionally, equil ,
equilibrium , -e [ quilibrium
], equilibrate , -e [ quilibrate ].
number --Solution number with which
the fixed-volume gas phase is to be in equilibrium. Any alphabetic characters
following the identifier and preceding an integer (“with solution” in Line 2)
are ignored.
-volume --Identifier defining the
volume of the fixed-volume gas phase, which applies for all batch-reaction or
transport calculations. Optionally, volume or -v
[ olume ].
volume --The volume of the
fixed-volume gas phase, L. Default is 1.0 L.
phase name --Name
of a gas component. A phase with this name must be defined by PHASES input in the database or input
file.
Line 4 may be repeated as necessary to define all of the
components that may be present in the fixed-volume gas phase. The -equilibrate
identifier specifies that the initial moles of the gas components are to be
calculated by equilibrium with solution 10. This calculation is termed an
“initial gas-phase-composition calculation”. During this calculation, the
composition of solution 10 does not change, only the moles of each component in
the gas phase are calculated. This calculation is approximate for a
Peng-Robinson GAS_PHASE due to the
fugacity coefficient, which is used for calculating the activity of the gas in
the solubility equation. Alternatively, for Peng-Robinson gases, keyword GAS_PHASE_MODIFY may be used, but
this is still approximate for a gas-mixture at high pressure. A constant-volume
gas phase always exists unless all of the gas components are absent from the
system. When the -equilibrate identifier is used, the
identifiers -pressure and -temperature are
not needed and initial partial pressures for each gas component need not be
specified; the partial pressures for the gas components are calculated from the
partial pressures in solution and the temperature is equal to the solution
temperature. The -equilibrate identifier cannot be used with a
fixed-pressure gas phase.
A gas component may have an initial partial pressure of zero
because the solution with which the gas phase is in equilibrium does not
contain that gas component. In this case, no moles of that component will be
present initially, but the component may enter the gas phase when the gas is in
contact with another solution that does contain that component.
After a batch reaction has been simulated, it is possible to save
the resulting gas-phase composition with the SAVE keyword. If the new composition
is not saved, the gas-phase composition will remain the same as it was before
the batch reaction. After it has been defined or saved, the gas phase can be
used in subsequent simulations through the USE keyword. TRANSPORT and ADVECTION calculations
automatically update the gas-phase composition and SAVE has no effect during these
calculations.
ADVECTION, COPY, DELETE, DUMP, EQUILIBRIUM_PHASES, GAS_PHASE_MODIFY, PHASES ,
SAVE gas_phase
, TRANSPORT, and USE gas_phase
.