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What Is Henry's Law? | Process Engineering Glossary
What Is Henry’s Law?
In piping engineering and process engineering, Henry’s Law states that the amount of gas dissolved in a liquid at constant temperature is directly proportional to the partial pressure of that gas above the liquid surface. Formulated by William Henry in the early nineteenth century, it is the foundational equilibrium relationship for all gas-liquid mass transfer operations. Engineers apply it to design absorbers, stripping columns, gas scrubbers, and deaeration systems, and to predict where dissolved gas will come out of solution as pressure drops along a pipeline.
Applications of Henry’s Law
Natural Gas Sweetening
Amine absorption units remove hydrogen sulphide and carbon dioxide from natural gas using reactive solvents that combine Henry’s Law physical dissolution with rapid chemical reaction. The Henry’s Law constant for hydrogen sulphide in amine solution is effectively very small because the chemical reaction converts the dissolved gas immediately to a non-volatile amine salt. This allows amine absorbers to achieve very high acid gas removal at a liquid-to-gas ratio that would be impractically high for a purely physical solvent.
Brewery and Carbonated Beverage Production
Carbonation of beverages uses Henry’s Law to dissolve carbon dioxide into liquid at elevated pressure. The carbonation level, measured in volumes of carbon dioxide per volume of liquid, directly reflects the Henry’s Law equilibrium at the carbonation temperature and carbon dioxide partial pressure. Once the pressure is released at the point of serving, the dissolved carbon dioxide becomes supersaturated and evolves as bubbles, as Henry’s Law no longer supports the elevated dissolved concentration at atmospheric pressure.
Environmental Groundwater Remediation
Air stripping towers and diffused air aeration systems apply Henry’s Law to remove volatile organic contaminants from contaminated groundwater. The Henry’s Law constant of the target contaminant governs the air-to-water ratio required and the column height needed to achieve the required treatment standard. Contaminants with high Henry’s constants strip easily with modest air flows. Those with low Henry’s constants, meaning they are relatively water-soluble, require higher air-to-water ratios or chemical oxidation to achieve the treatment standard.
Boiler Feed Water Deaeration
Thermal and mechanical deaeration systems remove dissolved oxygen and carbon dioxide from boiler feed water before it enters the boiler, preventing pitting corrosion. The deaerator operates at a steam partial pressure that drives the Henry’s Law equilibrium to near-zero dissolved gas concentration. Residual dissolved oxygen is then scavenged chemically by oxygen scavenger injection as a final polishing step.
Benefits of Understanding Henry’s Law
Correct Column Sizing
Henry’s Law provides the equilibrium relationship needed to calculate the theoretical minimum liquid-to-gas ratio for an absorber or the minimum stripping gas-to-liquid ratio for a stripper. These minimum ratios determine the operating cost benchmark and allow the engineer to select an appropriate actual ratio with a meaningful safety margin above the minimum.
Process Pressure Optimisation
Because Henry’s Law predicts that gas solubility increases with pressure, operating an absorber at higher pressure directly reduces the liquid-to-gas ratio needed to achieve a given outlet gas quality. Comparing the capital cost of higher-pressure equipment against the operating cost saving from reduced liquid circulation allows the engineer to find the optimum operating pressure for each absorption application.
Safe Dissolved Gas Management
Henry’s Law calculations predict where dissolved gas comes out of solution along a pressure profile, enabling engineers to design pipeline systems, pump suctions, and separation vessels to handle dissolved gas release safely. Without this understanding, gas pockets form in unexpected locations, causing pump cavitation, metering errors, and two-phase flow in lines sized only for single-phase service.
Limitations to Consider
Ideal Behaviour Assumption
Henry’s Law assumes ideal behaviour for both the gas phase, where partial pressure equals mole fraction multiplied by total pressure, and for the liquid phase, where the dissolved gas concentration is dilute enough that interactions between dissolved molecules are negligible. These assumptions fail at high pressures, high dissolved gas concentrations, and for gases that associate or react with the solvent molecules. Engineers must use more sophisticated equation-of-state and activity coefficient models for systems operating outside the Henry’s Law ideal regime.
Temperature Dependence of H
Henry’s Law constants are temperature-dependent and must be evaluated at the actual process temperature rather than at a standard reference temperature. Using a room-temperature Henry’s constant for a high-temperature absorber or a cold-temperature pipeline gives incorrect solubility predictions that can lead to significant design errors in column sizing and dissolved gas release calculations.
Reactive Systems
Henry’s Law describes only physical dissolution without chemical reaction. When the dissolved gas reacts with the solvent, as in amine scrubbing or caustic scrubbing, the effective capacity of the liquid to hold the gas far exceeds the Henry’s Law prediction for the unreacted gas alone. Designing reactive absorption systems requires coupling the Henry’s Law physical equilibrium with the chemical equilibrium of the absorption reaction to calculate the true effective solubility of the gas in the reactive solvent.
Henry’s Law FAQ
What is Henry’s Law in process engineering? Henry’s Law states that the dissolved concentration of a gas in a liquid is directly proportional to its partial pressure above the liquid at constant temperature. Process engineering applies it to design absorption columns that remove gases from process streams, stripping columns that remove dissolved gas from liquids, and gas scrubbers that control emissions. It also predicts where dissolved gas releases from solution as pressure drops in pipelines and separators.
How does temperature affect Henry’s Law and absorption system design? For most gases, Henry’s Law constant H increases with temperature, meaning gas solubility decreases as temperature rises. In an absorption column handling an exothermic absorption, the liquid heats up as it absorbs gas, reducing the equilibrium capacity and degrading the absorption efficiency. Engineers cool the absorber, either through intercooling between column sections or through a cooled solvent circuit, to maintain a low operating temperature that keeps H small and preserves the absorption driving force. In stripping operations, heating the liquid has the opposite effect: higher temperature reduces solubility and improves stripping efficiency.
How is Henry’s Law used in water and wastewater treatment systems? Water treatment plants use Henry’s Law to design aeration and stripping systems that remove dissolved gases and volatile organic contaminants. Thermal deaeration for boiler feed water drives the Henry’s Law equilibrium to near zero by raising the water temperature to its boiling point at reduced partial pressure of dissolved gases. Air stripping towers remove volatile organics from groundwater by providing a large surface area for gas-liquid contact at conditions where Henry’s Law predicts that the contaminant strongly partitions into the air stream. Chemical dosing systems inject oxygen scavengers after deaeration to ensure any residual dissolved oxygen below the Henry’s Law detection limit is chemically neutralised before the water enters the boiler. Instrumentation such as dissolved oxygen analysers and online gas chromatographs confirms that the treatment system is achieving the required dissolved gas specification at the outlet.
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