High: [Drying] Energy terms multiply molar drying rates by mass-basis heat quantities

[bernalde]

Maintainer triage (June 29, 2026)

• Status: Verified bug against the current master source in this fork.
• Severity: High.
• Area labels: area:drying.
• Tackle batch: P2 - shared thermo, phase, and basis correctness.

The source review confirmed that the cited code path and failure mode are still present. The original audit details are preserved below for reproduction notes and suggested fixes.

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Original audit details

Independent Codex verification: NEW FINDING

severity_assessment: high
evidence: PharmaPy/Drying_Model.py:72 and PharmaPy/Drying_Model.py:209 make dry_rate a molar volumetric rate, but PharmaPy/Drying_Model.py:373 multiplies dry_rate.sum(axis=1) by mass-basis cpg_mix and PharmaPy/Drying_Model.py:402 multiplies dry_rate by mass-basis latent_heat. These energy terms need molar-to-mass conversion and remain dimensionally wrong even after removing the separate factor-of-2 issue.
fix_note: Use component mass rates, dry_rate * MW / 1000, before multiplying by cpg_mix or latent_heat.

[andres9403]

Coordination note (re: PR #103 / #20). The same root cause was fixed locally for the saturation balance in PR #103 (dry_rate_mass block in material_balance, mw in g/mol → /1000). The energy terms this issue tracks still multiply the molar dry_rate by mass-basis quantities:

sensible_heat = cpg_mix * (temp_gas - temp_wb) * dry_rate.sum(axis=1)            # :373, cpg in J/kg·K
drying_terms  = (dry_rate[:, self.idx_volatiles] * latent_heat * 2).sum(axis=1)  # :402, latent_heat in J/kg

Cleanest fix: convert self.dry_rate → mass rate once in unit_model (~lines 266–269) so every consumer sees kg·m⁻³·s⁻¹, then remove PR #103's local conversion (and #21's, if landed by then). Note :402 also carries a spurious * 2 factor — that's #24, so coordinate the two fixes since they touch the same expression.

After #20/#21/#48/#24 all close, a small doc-only PR adding docstrings that document the units basis of each material_balance/energy_balance term would be preferable to keep this from recurring.

[Mazhar331]

Coordination note from PR #108 / issue #21:

PR #108 converts self.dry_rate from molar to mass basis once in Drying.unit_model, immediately after get_drying_rate, and then passes that mass-basis rate into both material_balance and energy_balance.

That means the MW/1000 conversion part of this issue should be covered by #108 once it lands. I am not closing #48 from that PR because #48 still needs energy-specific validation, and the latent-heat expression also has the separate * 2 factor tracked in #24.