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What Is Partial Pressure? | Process Engineering Glossary
What Is Partial Pressure?
In piping engineering and process engineering, partial pressure is the pressure a single gas in a mixture would exert if it alone occupied the whole volume at the same temperature. In an ideal gas mixture, the partial pressures of all components add up to the total pressure, a relationship known as Dalton’s law. Partial pressure, rather than total pressure, governs how each component condenses, dissolves, reacts, and corrodes.
Calculating Partial Pressure
For an ideal gas, the partial pressure of a component equals its mole fraction in the vapor multiplied by the total pressure. In a mixture of 20% CO2 and 80% methane at 50 bar, for example, the CO2 partial pressure is 10 bar. At liquid-vapor equilibrium, Raoult’s law links a component’s partial pressure to its liquid mole fraction and its pure-component vapor pressure.
Because each component responds to its own partial pressure, two streams at the same total pressure can behave very differently. A wet gas starts to condense water when the water partial pressure reaches the vapor pressure of water at that temperature, which defines the water dew point.
Applications
Condensation and Separation
Designers of a partial condenser use component partial pressures to predict which compounds condense and which stay in the vapor, and the same principle underlies every flash calculation.
Gas Absorption
In absorbers, the driving force for transfer is the difference between a component’s partial pressure in the gas and its equilibrium value over the liquid, often described by Henry’s law.
Corrosion and Materials Selection
CO2 and H2S partial pressures set the severity of sweet and sour corrosion, and H2S partial pressure is a key criterion for whether piping must meet sour service material requirements.
Benefits
Predicts Phase Behavior
Partial pressures show when a component will condense, evaporate, or dissolve under a given set of conditions.
Sizes Mass Transfer Equipment
They provide the driving force used to size absorbers, strippers, and dryers.
Guides Material Selection
Knowing acid gas partial pressures helps engineers choose materials and corrosion allowances that suit the service.
Limitations
Ideal Gas Assumption
Dalton’s and Raoult’s laws lose accuracy at high pressure or with strongly non-ideal mixtures, where fugacity and activity coefficients are needed.
Requires Accurate Composition
Small errors in measured composition, especially for trace components like H2S, translate directly into errors in partial pressure.
Temperature Dependence
Equilibrium relationships change with temperature, so calculations must be repeated across the full operating range.
FAQ
What is Dalton’s law of partial pressures?
It states that the total pressure of an ideal gas mixture equals the sum of the partial pressures of its individual components.
How is partial pressure different from vapor pressure?
Partial pressure describes a component’s share of the pressure in a gas mixture. Vapor pressure is a property of a pure liquid at a given temperature. When a component’s partial pressure reaches its vapor pressure, it begins to condense.
Why does partial pressure matter in vacuum operations?
Lowering the total pressure lowers the temperature at which a component’s vapor pressure reaches the system pressure, which lets vacuum distillation and drying run at lower temperatures and protect heat-sensitive materials.
How do I find partial pressure from a gas analysis?
Convert the analysis to mole fractions, which for ideal gases equal volume fractions, and multiply each by the total pressure. Always use absolute pressure rather than gauge pressure in the calculation.
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