# Design & Sizing of Acid Gas Scrubbers for Batch API Reactors: A Step-by-Step Engineering Guide

# Executive Summary & Industrial Context

In commercial Active Pharmaceutical Ingredient (API) and specialty chemical manufacturing, key synthetic transformations involve highly reactive chlorinating, sulfonating, and deprotecting reagents such as Phosphorus Oxychloride ($\text{POCl}_3$), Thionyl Chloride ($\text{SOCl}_2$), Oxalyl Chloride, and Thiols. During batch processing, these reactions liberate large quantities of hazardous, corrosive acid gases—primarily Hydrogen Chloride ($\text{HCl}$), Sulfur Dioxide ($\text{SO}_2$), Chlorine ($\text{Cl}_2$), and Hydrogen Sulfide ($\text{H}_2\text{S}$).

Because batch reactions operate with time-varying addition rates and exothermic heat spikes, acid gas evolution is non-steady-state. If the scrubbing system is under-sized, sudden gas surges can over-pressurize reactor headspaces, blow out liquid seals, release toxic fumes into manufacturing bays, or exceed environmental emission limits ($< 5\text{ ppm}$).

This engineering guide provides a rigorous blueprint for designing and sizing Wet Packed Acid Gas Scrubbers, covering gas generation stoichiometry, carrier gas dilution factors ($D_f$), GPDC column diameter sizing, HTU/NTU mass transfer integration, exhaust blower static pressure calculations ($250\text{ mmWC}$ motor head baseline), recirculation pump hydraulics, and Heat of Neutralization Plate Heat Exchanger (PHE) & Cooling Tower Water Sizing.

# 1. System Anatomy of an Industrial Acid Gas Scrubber

An industrial batch scrubber system consists of five primary interconnected unit operations:

  +-----------------------------------------------------------------------------------------+
  | BATCH REACTOR ACID GAS SCRUBBING SYSTEM SCHEMATIC                                       |
  +-----------------------------------------------------------------------------------------+
  |                                                                                         |
  |  [ Batch Reactor ] ──► Acid Gas Ducting ──► [ Packed Scrubbing Column ] ──► [ Blower ] ─► Stack
  |   (POCl3/SOCl2)       (with N2 Purge)         │ (Pall Rings / Saddles)    │ (Exhaust)   |
  |                                               ▼                           │             |
  |                                       [ Caustic Sump ] ◄──────────────────┘             |
  |                                       (10% NaOH Liquid) ──► [ Pump ] ──► [ PHE Cooler ] 
  |                                                                               (CT Water)
  +-----------------------------------------------------------------------------------------+

1. Reactor Vapor Ducting & Nitrogen Purge: Collects acid gas fumes and carrier gas ($N_2$ or air in-leakage).
2. Packed Absorption Column: Random plastic packing (Pall Rings or Intalox Saddles) providing gas-liquid interfacial area.
3. Liquid Recirculation Sump & Neutralization: Stores circulating scrubbing solution ($10\% \text{ NaOH}$ caustic or water).
4. Recirculation Pump & Titanium PHE Cooler: Delivers cooled scrubbing liquid to top spray distributors at a continuous irrigation rate ($15 - 25 \text{ m}^3/\text{m}^2 \cdot\text{h}$) while removing neutralization heat via Cooling Tower Water.
5. Exhaust Blower & Demister Pad: Maintains negative duct pressure and removes entrained liquid droplets prior to stack discharge.

# 2. Stoichiometric Gas Generation Rates & Peak Surge Factors ($K_{peak}$)

# 2.1 Reaction Stoichiometry Equations

The mass of acid gas liberated per batch depends on reactant stoichiometry:

• $\text{POCl}_3$ Chlorination:

• $\text{SOCl}_2$ Thionyl Chloride Reaction:

• Thiol Deprotection:

# 2.2 Peak Acid Gas Mass Flow Rate ($W_{g,peak}$)

Because reagent addition is non-uniform, average gas evolution rate must be multiplied by a peak surge factor ($K_{peak} = 1.5 - 2.5$):

# 3. Carrier Gas Dilution Factor ($D_f$) & Gas Concentration Analysis

# 3.1 Dilution Factor ($D_f$)

In industrial fume extraction headers, carrier dilution air or $N_2$ purge gas ($Q_{purge}$) is introduced to sweep acid fumes from multiple reactor pick-up points. The Dilution Factor ($D_f$) is defined as:

Where:

• $Q_{acid\_gas}$: Volumetric flow rate of pure acid gas liberated ($m^3/h$)
• $Q_{purge}$: Volumetric flow rate of carrier dilution air/nitrogen ($m^3/h$)
• $Q_{total}$: Total combined gas flow rate entering scrubber ($m^3/h$)

# 3.2 Volumetric & Mass Concentration %

The acid gas concentration in the entering gas stream is calculated as:

Safety Engineering Importance: Maintaining a minimum dilution factor ($D_f \ge 5 - 10$) prevents localized high concentrations of flammable vapors (for solvent-containing fumes) and lowers acid dew point temperatures inside FRP ductwork.

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

Extraction ducting connecting reactor manholes to the scrubber header must maintain an economical gas velocity:

• Design Extraction Velocity ($v_{duct}$): $8.0 - 12.0 \text{ m/s}$ (Recommended: $10.0 \text{ m/s}$).
  • Velocities below $8.0 \text{ m/s}$ cause liquid droplet fallout.
  • Velocities above $12.0 \text{ m/s}$ create excessive noise and frictional static head drop.

# Duct Diameter Equation:

Standard nominal FRP/PP duct sizes are selected ($150\text{ mm}, 200\text{ mm}, 250\text{ mm}, 300\text{ mm}, 400\text{ mm}, 500\text{ mm}, 600\text{ mm}$).

# 5. Packed Column Diameter Sizing ($D_c$) & Flooding Optimisation

# 5.1 Sherwood-Leva-Eckert (GPDC) Correlation

Column diameter is governed by the gas flooding velocity ($u_{flood}$), calculated using the generalized pressure drop correlation:

Where:

• $F_p$: Packing factor ($m^{-1}$) (e.g. $F_p = 85 \text{ m}^{-1}$ for 50mm PP Pall Rings)
• $\rho_g, \rho_l$: Gas and liquid densities ($kg/m^3$)
• $u_{flood}$: Superficial flooding gas velocity ($m/s$)

# 5.2 Target Flooding % vs Commercial Vessel Selection & Operating Flooding %

Design gas velocity is set at a user-selected Target Flooding % (typically $50\% - 60\%$, Max: $70-80\%$):

# Step-by-Step Commercial Sizing Example:

• For $Q_{total} = 1551 \text{ m}^3/\text{h}$ and $u_{flood} = 2.17 \text{ m}/\text{s}$:
  • At 50% Target Flood: $D_{calc} = 711 \text{ mm} \implies$ Standard Shell Selected = $700 \text{ mm}$ (Operating Flooding = 51.6%).
  • At 60% Target Flood: $D_{calc} = 649 \text{ mm} \implies$ Smart Economical Shell Selected = $600 \text{ mm}$ (Operating Flooding = 70.2%).
  • At 70% Target Flood: $D_{calc} = 601 \text{ mm} \implies$ Next Standard Shell = $600 \text{ mm}$ (Operating Flooding = 70.2%).

Key Takeaway: Both 60% and 70% target flooding map to the exact same $600\text{ mm}$ commercial shell size, resulting in an identical safe operating flooding velocity of 70.2%!

# 6. Height of Packing ($H_{packed}$) & Equivalent Multi-Column Criteria

# 6.1 HTU x NTU Mass Transfer Integration

Packed bed height ($H_{packed}$) is calculated using the Transfer Unit method:

For $99.5\%$ absorption efficiency ($\eta_{removal} = 0.995$):

Where $\text{HTU} \approx 0.55 \text{ m}$ for 50mm PP Pall Rings, yielding $H_{packed} = 3.35 \text{ m}$.

# 6.2 Industry Multi-Column Split Criteria ($H_{total} > 4.0\text{ m}$)

Adding sump ($1.5\text{ m}$) and distributor/demister space ($1.2\text{ m}$) gives a single column height $H_{total} = 6.05\text{ m}$.

• Single Column Limit: $H_{total} \le 4.0 \text{ m}$ (Standard indoor API plant ceiling clearance).
• Scale Chem Industry Recommendation ($H_{total} > 4.0\text{ m}$):
  • Install 2 Equivalent Columns in Series:
    • Column 1 (Primary Bulk Scrubber): Takes $50\%$ of packed bed ($1.68\text{ m}$ bed, $4.38\text{ m}$ total height). Absorbs $85-90\%$ acid.
    • Column 2 (Polishing Scrubber): Takes remaining $50\%$ of packed bed ($1.68\text{ m}$ bed, $4.38\text{ m}$ total height). Polishes emissions to $< 5\text{ ppm}$.

# 7. Exhaust Blower Motor Power Sizing (250 mmWC Static Baseline)

The exhaust blower must draw gas through ducting, packed beds, and demisters under negative pressure:

1. Packed Bed Pressure Drop:

2. System Pressure Drop:

3. Motor Power Sizing Baseline ($250\text{ mmWC}$):
   • While calculated system drop may be $120 - 180 \text{ mmWC}$, chemical engineering best practice sizes the blower motor against a $250 \text{ mmWC}$ design static head to handle duct fouling and surge resistance.

# 8. Recirculation Pump Sizing & Liquid Distributor Velocity

# 8.1 Irrigation Rate & Pump Flow ($Q_L$)

To ensure thorough packing wetting, liquid irrigation rate is maintained at $L_{rate} = 20 \text{ m}^3/\text{m}^2 \cdot\text{h}$:

# 8.2 Spray Distributor Nozzle Velocity ($1.8 \text{ m/s}$)

Each spray nozzle orifice ($10\text{ mm}$) is sized for an economical liquid discharge velocity of $1.8 \text{ m/s}$:

# 8.3 Pump Motor Power:

# 9. Caustic Neutralization Consumption Math

Neutralizer solution ($10\% \text{ NaOH}$, density $1.10 \text{ kg/L}$) consumption per batch is calculated stoichiometrically:

# 10. Heat of Neutralization ($\Delta H_{neut}$) & Sump Recirculation PHE Cooler Sizing

# 10.1 Exothermic Heat Release Math ($Q_{neut}$)

Gas-liquid absorption accompanied by chemical neutralization is strongly exothermic:

The heat generation rate ($Q_{neut}$) at peak gas evolution ($W_{g,peak}$) is calculated as:

Adding a $15\%$ safety margin for sensible gas cooling yields total cooler duty:

# 10.2 Cooling Tower Water (CT Water) Flow Requirement ($Q_{CW}$)

For a standard plant Cooling Tower Water loop ($30^\circ\text{C}$ supply $\to 35^\circ\text{C}$ return, $\Delta T_{CW} = 5^\circ\text{C}$):

# 10.3 Recirculation Plate Heat Exchanger (PHE) Area ($A_{HEX}$)

To prevent caustic sump overheating and loss of absorption efficiency, an inline Titanium Plate Heat Exchanger (PHE) is installed on the pump recirculation discharge line:

Where:

• $U = 1350 \text{ W/m}^2\cdot\text{K}$ (High-efficiency Titanium PHE plates for corrosive caustic vs CT water)
• $\text{LMTD} \approx 8.5 \text{ K}$ Logarithmic Mean Temperature Difference

# Summary Sizing Table (Sample Batch API Case Study)

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