# Type of Reactors, Key Considerations in Selection & Material of Construction (MOC)

# Executive Summary & Engineering Scope

Choosing the right reactor configuration and its Material of Construction (MOC) is one of the most critical decisions in chemical and pharmaceutical process design. A reactor is not merely a vessel where reactions occur; it is a highly engineered system that must balance reaction kinetics, thermodynamic heat transfer, fluid dynamics, pressure/vacuum limitations, and material corrosivity.

This comprehensive guide details:

11 Major Industrial Reactor Types complete with fundamental design performance equations ( $V, t, W, u_{mf}$ ) and practical numerical sizing examples for each.
Step-by-step Reactor Selection Matrices based on reaction phase, kinetics, heat load, and operating regime.
Detailed Material of Construction (MOC) Selection Guidelines covering Carbon Steel, SS304L, SS316L, Hastelloy C-22, Titanium, Glass-Lined Steel (GLS), PTFE/PFA lining, and Silicon Carbide (SiC).
Physical degradation mechanisms including Pitting Resistance Equivalent Number (PREN), corrosion rates (mpy), and glass-lining thermal shock limits ( $\Delta T_{shock}$ ).

# 1. Fundamental Reactor Design Performance Equations

The design of any chemical reactor begins with the mole balance for chemical species $A$ :

\text{Input} - \text{Output} + \text{Generation} = \text{Accumulation}

F_{A,0} - F_A + \int r_A \, dV = \frac{dN_A}{dt}

    BATCH REACTOR               CONTINUOUS STIRRED TANK (CSTR)               PLUG FLOW (PFR)
┌──────────────────┐               ┌──────────────────┐               ┌──────────────────┐
│  Accumulation    │               │  Perfect Mixing  │               │  No Radial Gradient│
│  dN_A / dt != 0  │               │  dC_A / dt = 0   │               │  dC_A / dV != 0  │
└──────────────────┘               └──────────────────┘               └──────────────────┘

# 2. Detailed Breakdown of 11 Industrial Reactor Types

# 1. Batch & Semi-Batch Agitated Tank Reactor

# A. Process Characteristics:

Widely used in Active Pharmaceutical Ingredient (API) synthesis, specialty chemicals, and fine chemical manufacturing. Operates under unsteady-state conditions where reagents are charged, reacted over time $t$ , and discharged.

# B. Governing Design Equation:

For a liquid-phase reaction with rate $-r_A$ ( $f(C_A)$ ):

t_{reaction} = N_{A,0} \int_0^{X_A} \frac{dX_A}{-r_A \cdot V}

For a constant-volume liquid reaction ( $V = V_0$ ) with first-order kinetics ( $-r_A = k \cdot C_A$ ):

t_{reaction} = \frac{1}{k} \ln \left( \frac{1}{1 - X_A} \right)

# C. Practical Engineering Example:

Problem Statement: A $5.0\text{ m}^3$ jacketed batch reactor is charged with $C_{A,0} = 2.0\text{ kmol/m}^3$ of Reactant A. The reaction is first-order with $k = 0.50\text{ h}^{-1}$ . Calculate the reaction time $t$ required to achieve $95\%$ conversion ( $X_A = 0.95$ ), and total batch cycle time assuming $1.5\text{ hours}$ for charging and discharging.
Calculation:

t_{reaction} = \frac{1}{0.50} \ln \left( \frac{1}{1 - 0.95} \right) = 2.0 \times \ln(20) = 2.0 \times 2.9957 = \mathbf{5.99\text{ hours}}

\text{Total Cycle Time} = 5.99 + 1.50 = \mathbf{7.49\text{ hours per batch}}

# 2. Continuous Stirred-Tank Reactor (CSTR)

# A. Process Characteristics:

Operates at steady state with thorough mechanical agitation. The composition and temperature inside the reactor are identical to the exit stream.

# B. Governing Design Equation:

V_{CSTR} = \frac{F_{A,0} \cdot X_A}{-r_{A,exit}} = \frac{v_0 \cdot C_{A,0} \cdot X_A}{k \cdot C_{A,0}(1 - X_A)} = \frac{v_0 \cdot X_A}{k(1 - X_A)}

# C. Practical Engineering Example:

Problem Statement: Calculate the required CSTR volume ( $V$ ) for a continuous liquid feed rate $v_0 = 2.0\text{ m}^3/\text{h}$ with $k = 0.50\text{ h}^{-1}$ to achieve $95\%$ conversion.
Calculation:

V_{CSTR} = \frac{2.0 \times 0.95}{0.50 \times (1 - 0.95)} = \frac{1.90}{0.025} = \mathbf{76.0\text{ m}^3}

(Notice: Achieving 95% conversion in a single CSTR requires $76.0\text{ m}^3$ , whereas a batch reactor needed only $5.0\text{ m}^3$ working volume! This highlights why CSTRs are often arranged in series).

# 3. Plug Flow Tubular Reactor (PFR)

# A. Process Characteristics:

Fluid flows through a pipe or tube at high velocity with zero axial mixing. Concentration changes continuously along the length of the reactor tube.

# B. Governing Design Equation:

V_{PFR} = F_{A,0} \int_0^{X_A} \frac{dX_A}{-r_A}

For constant volumetric flow rate $v_0$ and first-order kinetics:

V_{PFR} = \frac{v_0}{k} \ln \left( \frac{1}{1 - X_A} \right)

# C. Practical Engineering Example:

Problem Statement: For the same continuous feed rate $v_0 = 2.0\text{ m}^3/\text{h}$ , $k = 0.50\text{ h}^{-1}$ , and $X_A = 0.95$ , calculate the required PFR volume and tube length ( $D_i = 0.10\text{ m}$ ).
Calculation:

V_{PFR} = \frac{2.0}{0.50} \ln \left( \frac{1}{1 - 0.95} \right) = 4.0 \times 2.9957 = \mathbf{11.98\text{ m}^3}

\text{Cross-Sectional Area } A_{cross} = \frac{\pi}{4} (0.10)^2 = 0.007854\text{ m}^2

\text{Required Tube Length } L = \frac{11.98\text{ m}^3}{0.007854\text{ m}^2} = \mathbf{1,525.4\text{ meters}} \quad (\text{Configured as a coiled tube or multi-pass bank})

# 4. Fixed-Bed Catalytic Reactor (PBR - Packed Bed)

# A. Process Characteristics:

Gas or liquid reactants pass through a packed bed of solid spherical or cylindrical catalyst pellets (3–8 mm). Operates in adiabatic mode (no heat transfer) or near-isothermal mode (using diluents).

# B. Governing Design Equation:

W_{catalyst} = F_{A,0} \int_0^{X_A} \frac{dX_A}{-r_A'}

Where $-r_A'$ is the reaction rate per unit mass of catalyst ( $\text{kmol/kg}_{cat}\cdot\text{h}$ ).

# C. Practical Engineering Example:

Problem Statement: Determine the required catalyst bed mass ( $W$ ) for a gas feed $F_{A,0} = 50.0\text{ kmol/h}$ achieving $X_A = 0.80$ , where $-r_A' = k' \cdot C_A$ and average rate $-r_{A,avg}' = 0.040\text{ kmol/kg}_{cat}\cdot\text{h}$ .
Calculation:

W_{catalyst} = \frac{F_{A,0} \cdot X_A}{-r_{A,avg}'} = \frac{50.0 \times 0.80}{0.040} = \mathbf{1,000.0\text{ kg of Catalyst}}

# 5. Multi-Tubular Catalytic Reactor

# A. Process Characteristics:

Resembles a shell-and-tube heat exchanger. Solid catalyst is loaded inside hundreds or thousands of narrow tubes ( $25 - 50\text{ mm}$ diameter), while heat transfer fluid (molten salt, thermic oil, or boiling water) circulates in the shell to handle extreme reaction enthalpy.

# B. Governing Heat & Mass Equations:

Q_{total} = F_{A,0} \cdot X_A \cdot (-\Delta H_{rxn}) = U \cdot (N_{tubes} \cdot \pi \cdot D_i \cdot L) \cdot \Delta T_{lm}

# C. Practical Engineering Example:

Problem Statement: A gas-phase oxidation reaction generates $Q = 500,000\text{ kcal/h}$ heat duty. Using tubes of $D_i = 0.038\text{ m}$ (1.5 inch), $L = 6.0\text{ m}$ , $U = 150\text{ kcal/h}\cdot\text{m}^2\cdot^\circ\text{C}$ , and $\Delta T_{lm} = 40^\circ\text{C}$ , calculate the number of reactor tubes ( $N_{tubes}$ ).
Calculation:

\text{Surface Area per Tube } A_1 = \pi \cdot (0.038) \cdot (6.0) = 0.7163\text{ m}^2

\text{Total Required Area } A_{total} = \frac{500,000}{150 \times 40} = 83.33\text{ m}^2

N_{tubes} = \frac{83.33}{0.7163} = 116.3 \approx \mathbf{117\text{ Tubes}}

# 6. Fluidized Bed Reactor

# A. Process Characteristics:

Fine solid catalyst powder ( $20 - 100\ \mu\text{m}$ ) is suspended by an upward gas stream, causing the bed to behave like a boiling liquid. Provides outstanding gas-solid contact and uniform isothermal temperature profiles.

# B. Governing Hydrodynamic Equation (Minimum Fluidization Velocity $u_{mf}$ ):

u_{mf} = \frac{d_p^2 \cdot (\rho_p - \rho_g) \cdot g}{150 \cdot \mu_g} \quad (\text{for } Re_p < 20)

# C. Practical Engineering Example:

Problem Statement: Calculate $u_{mf}$ for catalyst particles ( $d_p = 80\ \mu\text{m} = 8.0 \times 10^{-5}\text{ m}$ , $\rho_p = 1,800\text{ kg/m}^3$ ) suspended in gas ( $\rho_g = 1.2\text{ kg/m}^3$ , $\mu_g = 1.8 \times 10^{-5}\text{ Pa}\cdot\text{s}$ ).
Calculation:

u_{mf} = \frac{(8.0 \times 10^{-5})^2 \times (1,800 - 1.2) \times 9.81}{150 \times 1.8 \times 10^{-5}} = \frac{6.4 \times 10^{-9} \times 1,798.8 \times 9.81}{0.0027} = \mathbf{0.0418\text{ m/s}} \quad (4.18\text{ cm/s})

# 7. Reactive Distillation Reactor

# A. Process Characteristics:

Combines chemical reaction and fractional distillation inside a single column. As volatile reaction products form, they are continuously distilled overhead, shifting chemical equilibrium forward per Le Chatelier's principle.

# B. Governing Equilibrium Shift:

K_{eq} = \frac{[C] \cdot [D]}{[A] \cdot [B]}

By continuously removing product $D$ ( $[D] \to 0$ ), the reaction achieves $100\%$ conversion even for reversible reactions with low equilibrium constants ( $K_{eq} \ll 1$ ).

# C. Practical Engineering Example:

Application: Esterification of Acetic Acid and Ethanol to Ethyl Acetate ( $K_{eq} = 4.0$ ). In a batch reactor, maximum conversion is limited to $\sim 66\%$ . In a Reactive Distillation column, continuous overhead removal of Ethyl Acetate/Water azeotrope achieves $> 99.5\%$ conversion.

# 8. Photo-Chemical Reactor

# A. Process Characteristics:

Reactions initiated by absorption of ultraviolet or visible light photons (e.g., chlorinations, brominations, vitamin D synthesis). Light lamps (UV-A, UV-C, LED arrays) are housed inside high-transmittance Quartz glass sleeves.

# B. Governing Photometric Equation (Beer-Lambert Radiation Attenuation):

I(r) = I_0 \cdot e^{-\alpha \cdot C_A \cdot r}

# C. Practical Engineering Example:

Application: Free-radical side-chain chlorination of Toluene to Benzyl Chloride. Using 365 nm UV LED lamps inside quartz sleeves eliminates chemical initiators, increases selectivity to $> 94\%$ , and prevents over-chlorination to Benzal Chloride.

# 9. Packed Scrubber Reactor

# A. Process Characteristics:

Packed columns designed for rapid gas-liquid absorption reactions (e.g., gas scrubbing, acid gas neutralization, phosgenation). Gas flows upward while liquid reagent recirculates downward over high-surface-area packing rings (Pall rings, Raschig rings).

# B. Governing Height Equation:

H_{packed} = \text{HTU} \times \text{NTU} = \left( \frac{G}{K_G \cdot a \cdot P} \right) \times \int_{y_2}^{y_1} \frac{dy}{y - y^*}

# C. Practical Engineering Example:

Problem Statement: Size packed bed height for an $\text{HCl}$ gas scrubber where Height of a Transfer Unit ( $\text{HTU}$ ) = $0.60\text{ m}$ and Number of Transfer Units ( $\text{NTU}$ ) = $8.5$ for $99.9\%$ gas absorption.
Calculation:

H_{packed} = 0.60\text{ m} \times 8.5 = \mathbf{5.10\text{ meters}}

# 10. PTFE / PFA Lined Circulating Loop Reactor

# A. Process Characteristics:

Used for extremely corrosive chemical services where all metals (including Hastelloy and Titanium) and glass linings fail (e.g., hot aqueous Hydrofluoric Acid $\text{HF}$ , mixture of $\text{HCl} + \text{HNO}_3$ ).

# B. System Configuration:

Because PTFE-lined vessels cannot support internal jackets or heavy agitators due to liner tearing risks, the reactor operates as a circulating loop:

PTFE-Lined Reactor Vessel ──► PFA-Lined Magnetic Pump ──► External Silicon Carbide (SiC) Heat Exchanger ──► Return

# 11. Rotary Kiln Reactor

# A. Process Characteristics:

Long, inclined cylindrical steel shell lined with refractory brick, rotating slowly ( $1 - 5\text{ RPM}$ ) at temperatures up to $1,500^\circ\text{C}$ . Used for high-temperature solid-gas reactions (calcination, cement clinker, fluorite roasting).

# B. Governing Residence Time Equation:

\tau = \frac{11.8 \cdot L}{D \cdot N \cdot S}

Where $L$ is length (m), $D$ is diameter (m), $N$ is rotational speed (RPM), and $S$ is slope (%).

# C. Practical Engineering Example:

Problem Statement: Calculate solid residence time $\tau$ in a kiln of $L = 30\text{ m}$ , $D = 2.0\text{ m}$ , $N = 2.0\text{ RPM}$ , and slope $S = 3.0\%$ .
Calculation:

\tau = \frac{11.8 \times 30.0}{2.0 \times 2.0 \times 3.0} = \frac{354.0}{12.0} = \mathbf{29.5\text{ minutes}}

# 3. Comprehensive Reactor Selection Matrix

Reaction Phase & System Characteristics	Recommended Reactor Type	Primary Design Justification
Liquid Phase / Multi-Step Batch API	Jacketed Agitated Batch Reactor	Flexible cleaning validation; multi-solvent compatibility; batch traceability.
Liquid Phase / High Volume Continuous	CSTR Train in Series (3-5 CSTRs)	Approximates PFR conversion while maintaining simple mechanical temperature control.
Gas Phase / Fast Homogeneous Reaction	Plug Flow Tubular Reactor (PFR)	High volumetric rate; zero back-mixing; uniform residence time.
Gas-Solid / Catalytic / Exothermic	Multi-Tubular Fixed Bed Reactor	Massive heat transfer surface area; prevents catalyst thermal sintering.
Gas-Solid / Fluidized / High Heat Flux	Fluidized Bed Reactor	Isothermal bed; continuous catalyst regeneration; no localized hot spots.
Reversible Liquid Reaction / Low $K_{eq}$	Reactive Distillation Column	Le Chatelier product removal drives $100\%$ conversion.
Gas-Liquid / Rapid Absorption	Packed Scrubber Reactor	High mass transfer coefficient ( $K_L a$ ); low gas-side pressure drop.
Corrosive Acids ( $\text{HF}$ , Hot $\text{HCl}$ )	PTFE-Lined Loop + SiC Exchanger	Complete chemical immunity; external loop heat transfer.
Solid Calcination / High Temp ( $> 1,000^\circ\text{C}$ )	Rotary Kiln Reactor	Refractory lining protection; continuous solid transport.

# 4. Materials of Construction (MOC) & Corrosion Engineering

Selecting the correct reactor MOC requires evaluating chemical resistance, mechanical allowable stress, thermal conductivity ( $k_w$ ), and cost.

                CORROSION RESISTANCE vs THERMAL CONDUCTIVITY OVERVIEW
  High Thermal Cond (54 W/mK)  ◄──────────────────────────────► Low Thermal Cond (1.0 W/mK)
  ┌───────────────────────────┬───────────────────────────┬───────────────────────────┐
  │     CARBON STEEL (CS)     │    STAINLESS STEEL 316L   │    GLASS-LINED STEEL (GLS) │
  │ Low Cost | Hydrocarbons   │ Mid Cost | General Solvents│ High Cost | Hot Acids     │
  └───────────────────────────┴───────────────────────────┴───────────────────────────┘

# 4.1 Pitting Resistance Equivalent Number (PREN)

For stainless steels and nickel alloys exposed to chloride environments:

\text{PREN} = \% \text{Cr} + 3.3 \left( \% \text{Mo} + 0.5 \% \text{W} \right) + 16 \left( \% \text{N} \right)

SS 304L: $\text{PREN} \approx 19$ (Susceptible to pitting in $> 50\text{ ppm}$ $\text{Cl}^-$ ).
SS 316L: $\text{PREN} \approx 25$ (Resistant up to $1,000\text{ ppm}$ $\text{Cl}^-$ at ambient temp).
Duplex 2205: $\text{PREN} \approx 35$ (Excellent seawater / high-chloride resistance).
Hastelloy C-22: $\text{PREN} \approx 65$ (Immune to wet chlorine and hot mineral acids).

# 4.2 Comprehensive Reactor MOC Comparison Table

Material of Construction (MOC)	Thermal Conductivity $k_w$ ( $\text{W/m}\cdot\text{K}$ )	Allowable Temp Range (°C)	Chemical Compatibility & Strengths	Severe Incompatibilities & Limitations	Relative Cost Index
Carbon Steel (SA 516 Gr 70)	54.0	$-29 \text{ to } 425^\circ\text{C}$	Alkaline solutions, dry amines, concentrated $\text{H}_2\text{SO}_4 (>93\%)$ , hydrocarbons.	Aqueous acids, rust contamination, moist air.	1.0 (Base)
SS 304L (UNS S30403)	16.2	$-196 \text{ to } 600^\circ\text{C}$	Nitric acid, alcohols, dry solvents, food grade.	Chlorides ( $>50\text{ ppm}$ ), $\text{HCl}$ , pitting corrosion.	2.2
SS 316L (UNS S31603)	16.3	$-196 \text{ to } 650^\circ\text{C}$	Organic acids, general API solvents, dilute acetic acid.	Hydrochloric acid, warm sulfuric acid, bleach.	2.8
Hastelloy C-22 (N06022)	10.1	$-196 \text{ to } 400^\circ\text{C}$	Mixed hot acids ( $\text{HCl} + \text{H}_2\text{SO}_4$ ), wet chlorine gas, ferric chloride.	Fluorine gas, hot hydrofluoric acid ( $\text{HF}$ ).	10.5
Titanium Gr 2 (R50400)	21.9	$-40 \text{ to } 300^\circ\text{C}$	Wet chlorine gas, chlorine dioxide, seawater, nitric acid.	Dry chlorine gas (ignites!), hydrofluoric acid.	8.5
Glass-Lined Steel (GLS)	1.2 (Glass)	$-20 \text{ to } 200^\circ\text{C}$	All mineral acids ( $\text{HCl}, \text{HNO}_3, \text{H}_2\text{SO}_4$ ) at all concentrations.	Hydrofluoric acid ( $\text{HF}$ ), hot alkalis ( $\text{NaOH} > 50^\circ\text{C}$ ), thermal shock.	3.2
PTFE / PFA Lining	0.25	$-20 \text{ to } 180^\circ\text{C}$	Total chemical immunity to all acids, bases, and solvents.	Vacuum collapse if unbonded; mechanical abrasion.	4.5
Silicon Carbide (SiC)	120.0	$-30 \text{ to } 300^\circ\text{C}$	Extreme thermal conductivity; immune to $\text{HF}, \text{HCl}, \text{HNO}_3$ .	Brittle ceramic; susceptible to mechanical impact.	14.0

# 4.3 Glass-Lined Steel (GLS) Thermal Shock Operating Limits

Glass-Lined reactors are standard in API plants, but the glass lining ( $1.0 - 2.0\text{ mm}$ enamel) will crack if exposed to sudden temperature differentials ( $\Delta T_{shock}$ ):

\Delta T_{shock} = |T_{vessel\_contents} - T_{incoming\_jacket\_utility}|

Safe Operating Limit: $\Delta T_{shock} \le 120^\circ\text{C}$ when vessel is at $20^\circ\text{C}$ .
High-Temperature Limit: $\Delta T_{shock} \le 70^\circ\text{C}$ when vessel is at $180^\circ\text{C}$ .

🛑 Caution

Thermal Shock Warning: Injecting cold utility water (

20^\circ\text{C}

) into the jacket of a Glass-Lined reactor containing hot batch mass at

160^\circ\text{C}

(

\Delta T = 140^\circ\text{C}

) causes immediate glass spalling, exposing the carbon steel shell to corrosive acid attack!

# Technical Conclusion

Designing an optimal reactor requires integrating chemical kinetics, thermodynamic heat transfer, and corrosion engineering. By applying exact design equations ( $V, t, W$ ), selecting appropriate vessel geometry, and specifying compatible MOCs, process engineers build safe, high-yield, and reliable chemical plants.

# Applicable Engineering Standards & Codes Used

The engineering methodologies, design correlations, and safety criteria detailed in this article adhere to the following international standards and industry codes:

ASME Boiler and Pressure Vessel Code (BPVC) Section VIII Division 1 & 2: ASME Boiler and Pressure Vessel Code (BPVC) Section VIII Division 1 & 2
API 620 & API 650: Welded Tanks for Oil, Chemical and Liquid Storage
TEMA Class R, C & B: Tubular Exchanger Manufacturers Association Standards
DIN EN 13445: Unfired Pressure Vessels European Standard
IS 2825: Code for Unfired Pressure Vessels (Bureau of Indian Standards)

# Type of Reactors, Key Considerations in Selection & Material of Construction (MOC)

# Executive Summary & Engineering Scope

# 1. Fundamental Reactor Design Performance Equations

# 2. Detailed Breakdown of 11 Industrial Reactor Types

# 1. Batch & Semi-Batch Agitated Tank Reactor

# A. Process Characteristics:

# B. Governing Design Equation:

# C. Practical Engineering Example:

# 2. Continuous Stirred-Tank Reactor (CSTR)

# A. Process Characteristics:

# B. Governing Design Equation:

# C. Practical Engineering Example:

# 3. Plug Flow Tubular Reactor (PFR)

# A. Process Characteristics:

# B. Governing Design Equation:

# C. Practical Engineering Example:

# 4. Fixed-Bed Catalytic Reactor (PBR - Packed Bed)

# A. Process Characteristics:

# B. Governing Design Equation:

# C. Practical Engineering Example:

# 5. Multi-Tubular Catalytic Reactor

# A. Process Characteristics:

# B. Governing Heat & Mass Equations:

# C. Practical Engineering Example:

# 6. Fluidized Bed Reactor

# A. Process Characteristics:

# B. Governing Hydrodynamic Equation (Minimum Fluidization Velocity umfu_{mf}umf​):

# C. Practical Engineering Example:

# 7. Reactive Distillation Reactor

# A. Process Characteristics:

# B. Governing Equilibrium Shift:

# C. Practical Engineering Example:

# 8. Photo-Chemical Reactor

# A. Process Characteristics:

# B. Governing Photometric Equation (Beer-Lambert Radiation Attenuation):

# C. Practical Engineering Example:

# 9. Packed Scrubber Reactor

# A. Process Characteristics:

# B. Governing Height Equation:

# C. Practical Engineering Example:

# 10. PTFE / PFA Lined Circulating Loop Reactor

# A. Process Characteristics:

# B. System Configuration:

# 11. Rotary Kiln Reactor

# A. Process Characteristics:

# B. Governing Residence Time Equation:

# C. Practical Engineering Example:

# 3. Comprehensive Reactor Selection Matrix

# 4. Materials of Construction (MOC) & Corrosion Engineering

# 4.1 Pitting Resistance Equivalent Number (PREN)

# 4.2 Comprehensive Reactor MOC Comparison Table

# 4.3 Glass-Lined Steel (GLS) Thermal Shock Operating Limits

# Technical Conclusion

# Applicable Engineering Standards & Codes Used

Discussion

# B. Governing Hydrodynamic Equation (Minimum Fluidization Velocity $u_{mf}$ ):