neqsim-adsorbent-capillary-condensation-screening

Adsorbent Capillary Condensation Screening

Use this skill to turn a sorbent pore size into a contaminant concentration limit for the gas entering a fixed bed. It answers the question "how much methanol / water / heavy hydrocarbon can this feed carry before the bed stops working", which is a different and much tighter question than "when does a bulk liquid drop out".

When to Use

• A mercury guard bed, molecular sieve, or catalyst guard bed is degrading and there is no free liquid at the inlet.
• A feed specification has to be written for a condensable polar contaminant (methanol, water, glycol, heavy ends) upstream of an adsorbent.
• An agent needs to explain why a "no free liquids" clause is not a sufficient specification.

The physics in one line

A concave meniscus in a pore condenses vapour below bulk saturation:

$$\ln a_c = -\frac{G,\gamma V_m\cos\theta}{r R T}$$

with $G = 2$ for cylindrical pores and $G = 1$ for slit pores. The bed limit is then $y_{max} = a_c \cdot y_{sat}$, where $y_{sat}$ is the bulk saturation mole fraction of the contaminant in the gas.

For methanol at 20 °C, $2\gamma V_m/RT = 0.75$ nm, so a 1.5 nm pore floods at 60 % of bulk saturation and a 6 nm pore at 88 %.

Inputs

• temperature: gas temperature in kelvin.
• surface_tension: contaminant liquid surface tension in N/m.
• molar_volume: contaminant liquid molar volume in m³/mol.
• pore_radius_nm: representative sorbent pore radius in nm.
• saturation_mole_fraction: bulk saturation mole fraction $y_{sat}$ of the contaminant in the gas at bed T and P. Obtain this from a real equation of state, not from $P^{sat}/P$ — see the coupling note below.
• contaminant_mole_fraction: optional current concentration, for a margin check.
• contact_angle_deg: default 0 (perfect wetting, conservative).
• geometry_factor: 2.0 cylindrical (default), 1.0 slit.

Outputs

• kelvin_length_nm: $G\gamma V_m\cos\theta / RT$ in nm.
• onset_relative_saturation: $a_c$.
• max_mole_fraction / max_ppmv: the contaminant limit.
• relative_saturation, margin_ratio: only when a current concentration is given.
• kelvin_valid: false below ~2 nm radius, where Kelvin is non-conservative.
• warning: ok, watch, or condensation-expected.
• assumptions: the public assumptions applied.