Barrer: Gas Permeability Unit for Polymer Membranes
The barrer is a composite unit expressing gas permeability through non-porous membranes. One barrer equals 10⁻¹⁰ (cm³(STP)·cm)/(cm²·s·cmHg), capturing how flux depends on diffusivity, solubility, membrane thickness, and pressure differential.
Membrane engineers often tabulate permeability in barrers while quoting permeance in gas permeation units (GPU). Cross-reference this article with the darcy explainer to clarify differences between porous and dense-media transport.
Definition and Conversions
Permeability P relates molar flux J to driving pressure difference Δp and membrane thickness l through J = (P/l) Δp. Converting barrers to SI yields 1 barrer = 3.35 × 10⁻¹⁶ mol·m/(m²·s·Pa). Permeance equals permeability divided by thickness and is reported in GPU, where 1 GPU = 10⁻⁶ barrer/cm.
When comparing literature values, confirm whether authors normalise pressure to standard temperature and pressure (STP) volumes or to molar units. Consistent units prevent errors when sizing membrane areas or estimating compressor duties.
Historical Perspective
The barrer honours Richard M. Barrer, a pioneer in polymer membrane science who studied gas diffusion through rubber in the mid-20th century. His work established the solution-diffusion model, separating permeability into diffusivity and solubility contributions, which remains foundational to modern membrane research.
Early applications focused on separating oxygen and nitrogen for breathable atmospheres. Today, barrer-based performance metrics underpin desalination pretreatment, biogas upgrading, hydrogen purification, and carbon capture systems.
Key Concepts
Solution-Diffusion Model
Permeability equals the product of diffusivity D and solubility S: P = D × S. Diffusivity describes molecular motion through the polymer matrix, while solubility captures how readily the gas dissolves into the membrane. Additives, crosslinking, and temperature directly influence both parameters.
Selectivity
Membrane performance is assessed by permeability-selectivity trade-offs. Robeson upper bound plots chart the highest selectivity achievable at a given permeability. Expressing permeability in barrers ensures comparability across studies and materials.
Permeance vs. Permeability
Thin-film composite membranes exhibit high permeance because of nanometre-scale selective layers supported by porous substrates. Engineers must report both permeability (intrinsic property) and permeance (application-specific) to avoid misinterpretation of pilot data.
Applications
Hydrogen purification. Proton exchange membrane fuel cells and hydrogen pipelines require membranes that balance high hydrogen permeability with low crossover of contaminants such as CO₂ or H₂S. Planning tools like the hydrogen compression cost calculator help translate permeability targets into equipment sizing.
Carbon capture. Post-combustion capture systems use polymeric or hybrid membranes to separate CO₂ from flue gas. Designers evaluate barrer-level permeability alongside sorbent or solvent alternatives when optimising capture trains and downstream compression, pairing techno-economic models with the 45Q credit revenue calculator.
Biogas upgrading. Membranes enrich methane while removing CO₂ and H₂S. Process engineers reference barrer data when selecting membrane modules that integrate with dehydration, chilling, or pressure swing adsorption stages.
Reporting and Best Practices
Always state test temperature, feed composition, and pressure differential when publishing permeability data. Include membrane thickness, preconditioning steps, and aging history, because physical aging and plasticisation can shift permeability by orders of magnitude.
When benchmarking new materials, link to complementary measurements such as BET surface area and sorption isotherms. Providing complete datasets accelerates material screening and helps project teams integrate membrane options into techno-economic models supported by calculators like the hydrogen tax credit tool.