Floating Roof Retrofit for Crude Oil Storage in Texas Gulf Coast Refinery

Scenario

Project Type: Emission reduction retrofit for atmospheric storage tanks Location Context: Offshore-adjacent refinery near Port Arthur, TX — high ambient temperatures (avg. 28°C), humid subtropical climate, subject to TCEQ and EPA NSPS Subpart Kb compliance. Constraints: Must achieve ≥75% VOC reduction without tank shutdown; existing fixed-roof tanks lack vapor recovery; budget capped at $120k per tank; 72-hour maximum downtime allowed.

Given Data

• Tank emission factor: 0.35 kg/m²/hr (measured via tracer gas study for crude oil with low-light ends)
• True vapor pressure: 12.4 kPa (ASTM D6377 at 30°C, representative of blended crude)
• Surface area: 314 m² (diameter = 20 m)
• Henry’s law constant: 12,500 m³/kmol (for benzene in crude/water interface; conservative estimate from NIST Chemistry WebBook)
• Height above liquid: 8.2 m (tank ullage space, verified via laser level survey)

Calculation

Using the VOC Emission Estimator (aligned with AP-42 Section 7.1 methodology for fixed-roof tanks):

The estimator applies a composite model combining evaporation (vapor pressure-driven) and mass transfer (Henry’s law–informed partitioning). While proprietary weighting is embedded, the tool computes:

Emission Rate = [Tank Emission Factor × Surface Area] + [f(VP, H, h) × Surface Area]

Where the second term approximates evaporative loss correction using vapor pressure and headspace geometry:

• First component: 0.35 kg/m²/hr × 314 m² = 109.9 kg/hr
• Second component (tool internal): calibrated scaling based on VP (12.4 kPa → moderate volatility), H (12,500 m³/kmol → low aqueous solubility → higher volatilization), and h (8.2 m → larger headspace increases accumulation and diffusion gradient). Tool calculates additive contribution of 23.6 kg/hr.

Total estimated emission rate = 133.5 kg/hr

Result and Decision

The estimated 133.5 kg/hr exceeded the site’s internal action threshold of 50 kg/hr and violated TCEQ’s 2023 VOC benchmark for uncontrolled tanks (>75 kg/hr triggers mandatory controls). Engineering team selected an internal floating roof retrofit (single-deck aluminum pan-type) with liquid-mounted rim seal — validated to reduce emissions by 92% per API RP 2510. Installation completed in 68 hours (within constraint), cost $114,500. Post-installation baghouse sampling confirmed 11.2 kg/hr residual emissions.

Lesson

Field-validated tank emission factors (e.g., from tracer studies) significantly improve estimator accuracy over default AP-42 defaults — especially for complex mixtures like crude oil where composition variability skews generic factors.