# Design & Sizing of Acid Gas Scrubbers for Batch API Reactors

Comprehensive Engineering Guide: For an in-depth step-by-step chemical engineering analysis, reaction kinetics, and case studies, visit the full blog post: Design & Sizing of Acid Gas Scrubbers for Batch API Reactors.

# 1. Process Overview & Gas Evolution Stoichiometry

In commercial Active Pharmaceutical Ingredient (API) and fine chemical manufacturing, batch reactions using Phosphorus Oxychloride (POCl₃), Thionyl Chloride (SOCl₂), Oxalyl Chloride, and Thiols generate sudden, exothermic acid gas evolution.

# Primary Reactions & Stoichiometry:

POCl₃ Chlorination:

R\text{-OH} + \text{POCl}_3 \to R\text{-Cl} + \text{POCl}_2\text{OH} + \text{HCl} \uparrow \quad (1 \text{ mol POCl}_3 \to 3 \text{ mol HCl})

SOCl₂ Thionyl Chloride Reaction:

R\text{-COOH} + \text{SOCl}_2 \to R\text{-COCl} + \text{SO}_2 \uparrow + \text{HCl} \uparrow \quad (1 \text{ mol SOCl}_2 \to 1 \text{ mol SO}_2 + 1 \text{ mol HCl})

Thiol Deprotection:

R\text{-S-Trt} + \text{TFA} \to R\text{-SH} + \text{Trt-OH} + \text{H}_2\text{S} \uparrow \quad (1 \text{ mol Thiol} \to 1 \text{ mol H}_2\text{S})

# 2. Main Header Duct Sizing & Multi-Point Pick-up Velocity

Contaminated fumes and acid gas from multi-reactor pick-up points are extracted into the main FRP/PP duct header.

Design Duct Gas Velocity ( $v_{duct}$ ): 8.0 to 12.0 m/s (Economical Default: 10.0 m/s) to prevent droplet condensation while minimizing duct friction loss.
Required Duct Diameter ( $D_{duct}$ ):

D_{duct} = \sqrt{\frac{4 \cdot Q_{total}}{\pi \cdot v_{duct}}}

# 3. Mass Balance & Peak Gas Surge Factor ( $K_{peak}$ )

Because batch dosing is non-steady-state, peak mass evolution rate ( $W_{g,peak}$ ) accounts for exothermic addition surges:

W_{g,peak} = \left( \frac{M_{react} \cdot \frac{P}{100}}{MW_{react}} \right) \cdot \text{Stoich\_Factor} \cdot MW_{gas} \cdot \frac{1}{t_{rxn}} \cdot K_{peak} \quad [\text{kg/h}]

Where:

$M_{react}$ : Mass of reagent charged (kg)
$t_{rxn}$ : Dosing / addition time (hours)
$K_{peak}$ : Peak-to-average surge multiplier (1.5 to 2.5)

# 4. Packed Column Diameter Sizing ( $D_c$ )

Column hydraulic diameter ( $D_c$ ) is determined using the Sherwood-Leva-Eckert (GPDC) Flooding Correlation:

u_{flood} = \sqrt{ \frac{C_{s,flood}^2 \cdot g \cdot (\rho_l - \rho_g)}{\rho_g \cdot F_p \cdot \mu_l^{0.2}} }

Design Velocity ( $u_{design}$ ): Operated at 50% to 70% of flooding velocity ( $u_{design} = 0.60 \times u_{flood}$ ).
Required Area ( $A_c$ ): $A_c = \frac{q_G}{u_{design}} \quad [\text{m}^2]$ .
Column Diameter ( $D_c$ ): $D_c = \sqrt{\frac{4 A_c}{\pi}} \quad [\text{m}]$ .

# 5. Height of Packing ( $H_{packed} = \text{HTU} \times \text{NTU}$ ) & Multi-Column Threshold

Number of Transfer Units (NTU) for 99.5% removal efficiency:

\text{NTU} = \ln\left( \frac{1}{1 - 0.995} \right) = \ln(200) = 5.30

Height of Transfer Unit (HTU): Typical values range between 0.40 and 0.60 m for random plastic packings (50mm Pall Rings / Saddles).
Height Constraint Alert: If Total Height ( $H_{total}$ ) exceeds 4.0 m, the system automatically recommends 2 Equivalent Columns in Series operating at half packed height per column.

# 6. Blower & Recirculation Pump Sizing

# 6.1 Exhaust Blower Fan

System Pressure Drop ( $\Delta P_{total}$ ): Sum of packed bed drop, demister pad drop, duct friction, and sump submergence.
Blower Airflow ( $Q_{fan}$ ): Total gas volumetric flow rate plus 15% design margin ( $Q_{fan} = Q_{total} \times 1.15$ ).

# 6.2 Recirculation Pump

Irrigation Rate ( $L_{rate}$ ): Minimum 15 to 25 m³/m²·h across the column area to ensure total bed wetting.
Economical Liquid Spray Nozzle Velocity ( $v_{liq}$ ): 1.5 to 2.5 m/s.
Total Dynamic Head (TDH): Static lift + spray nozzle drop + piping friction + column packed height.

# 7. Full Engineering Blog Post

For detailed numerical case studies, dynamic kinetics, and equipment procurement specifications, refer to the published blog article:
👉 Read the Full Acid Gas Scrubber Design Blog Post