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# GAS_PHASE

This keyword data block is used to define the composition of a fixed-total-pressure or a fixed-volume multicomponent 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. 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. The thermodynamic properties of the gas components are defined with PHASES input.

### Example data block 1. Fixed-pressure gas phase

```Line 0:  GAS_PHASE 1-5  Air
Line 1:       -fixed_pressure
Line 2:       -pressure       1.0
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```

### Explanation 1

Line 0: GAS_PHASE [ number ] [ description ]

GAS_PHASE is the keyword for the data block.

number --Positive number to designate the following 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.

Line 1: -fixed_pressure

-fixed_pressure--Identifier defining the gas phase to be one that has 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].

Line 2: -pressure pressure

-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 atmospheres. Default is 1.0 atm.

Line 3: -volume volume

-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 volume along with temp and partial pressure are used to calculate the initial moles of each gas component in the fixed-pressure gas phase. Default is 1.0 liter.

Line 4: -temperature temp

-temperature--Identifier defining the initial temperature of the gas phase. Optionally, temperature, or -t[ emperature].

temp --The initial temperature of the gas phase, in 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.

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, in atmospheres. 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.

### Notes 1

Line 5 may 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 any gas component that is defined to have a positive partial pressure in GAS_PHASE input will be computed using 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 given by -volume, and T is given by -temperature (converted to Kelvin). 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 initial moles of gas components are brought in contact with a solution during a batch-reaction simulation, the moles of gases and volume of the gas phase will adjust so that each component is in equilibrium with the solution and the total pressure (sum of the partial pressures) is that specified by -pressure. It is possible that the gas phase will not exist if the sum of the partial pressures of dissolved gases does not exceed the pressure given by -pressure.

Some gas components may be defined to have initial partial pressures 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 .

### Example data block 2. Fixed-volume gas phase: Define initial moles of components with partial pressures

```Line 0:  GAS_PHASE 1-5  Air
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```

### Explanation 2

Line 0: GAS_PHASE [ number ] [ description ]

GAS_PHASE is the keyword for the data block.

number --positive number to designate the following 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.

Line 1: -fixed_volume

-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].

Line 2: -volume volume

-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 liter.

Line 3: -temperature temp

-temperature--Identifier defining the initial temperature of the gas phase. Optionally, temperature, or -t[ emperature].

temp --The initial temperature of the gas phase, in Celsius. Default is 25.0.

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 atmospheres. 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.

### Notes 2

Line 4 may be repeated as necessary to define all of 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 any gas component that is defined to have a positive partial pressure will be computed using 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, and T is given by -temperature (converted to Kelvin). When the initial moles of gas components are brought in contact 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 adjust 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.

Some gas components may be defined to have initial partial pressures 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.

### Example data block 3. Fixed-volume gas phase: Define initial moles of components by equilibrium with a solution

```Line 0:  GAS_PHASE 1-5  Air
Line 1:       -fixed_volume
Line 2:       -equilibrium with solution 10
Line 3:       -volume         1.0
Line 4a:      CH4(g)
Line 4b:      CO2(g)
Line 4c:      O2(g)
Line 4d:      N2(g) ```

### Explanation 3

Line 0: GAS_PHASE [ number ] [ description ]

GAS_PHASE is the keyword for the data block.

number --Positive number to designate the following 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.

Line 1: -fixed_volume

-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].

Line 2: -equilibrium number

-equilibrium--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.

Line 3: -volume volume

-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 liter.

Line 4: phase name

phase name --Name of a gas component. A phase with this name must be defined by PHASES input in the database or input file.

### Notes 3

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. 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.

### Example problems

The keyword GAS_PHASE is used in example problem 7.

### Related keywords

EQUILIBRIUM_PHASES, PHASES, SAVE gas_phase, and USE gas_phase.

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