Pharmaceutical Solvent Recovery Skid Purge System
Engineering Case Study
Case Study 2: Pharmaceutical Solvent Recovery Skid Purge System
Scenario A GMP-compliant solvent recovery skid was installed in a temperature-controlled cleanroom (ISO 8) at a Swiss API manufacturing facility. The skid contains distillation columns, condensers, and explosion-proof motors handling acetone and ethanol vapors (Class I, Division 1, Group D). Due to strict containment requirements and risk of static discharge, the enclosure (total internal volume = 0.78 m³) required continuous positive-pressure purge with dry air. Facility air compressors delivered air at 298.15 K; regulatory guidance (EU GMP Annex 10) required ≥45 Pa differential to prevent ingress of particulate or solvent-laden air. Air’s specific gas constant (287 J/(kg·K)) was used.
Given Data
- Volume of the Enclosure: 0.78 m³
- Pressure Differential: 45 Pa
- Temperature: 298.15 K
- Gas Constant: 287 J/(kg·K)
Calculation Using the same formula:
$$ \dot{V} = \frac{\Delta P \cdot V}{R \cdot T} = \frac{45\ \text{Pa} \times 0.78\ \text{m}^3}{287\ \frac{\text{J}}{\text{kg·K}} \times 298.15\ \text{K}} = \frac{35.1}{85,579.05}\ \text{m}^3/\text{s} \approx 0.0004101\ \text{m}^3/\text{s} $$
Rounded to 4 decimal places: 0.0004 m³/s.
Result and Decision Calculated purge flow rate = 0.0004 m³/s (1.44 m³/h). A calibrated thermal mass flow controller (with 0–2.5 m³/h range and 0.5% FS repeatability) was integrated into the dry-air supply line. To address humidity-induced condensation risk inside the skid, the air was further desiccated to <−40°C dew point upstream. Leak testing (per ISO 16000-8) confirmed ≤0.05 Pa/s decay — well within the 0.0004 m³/s design margin.
Lesson In pharmaceutical settings, purge gas quality (e.g., dew point, oil content) is as critical as flow rate — uncontrolled moisture can corrode stainless-steel internals or nucleate solvent aerosols. Always couple flow-rate calculation with gas specification compliance (e.g., ISO 8573-1 Class 2:2:2) and verify purge efficacy via tracer-gas decay testing, not just pressure hold.