Reactors

This lesson covers reactor hardware. Learn pressure-vessel design (wall thickness, corrosion allowance, codes), agitator selection and power, heat-transfer jackets and coils, materials of construction (steel, stainless, glass-lined), nozzles, relief and scale-up.

Kinetics tells you the reactor volume; equipment design turns it into a vessel that holds pressure, mixes properly, removes heat and resists corrosion — safely for decades.

Most chemical reactors are stirred pressure vessels or packed tubes built to pressure-equipment codes.

An analogy

A reactor vessel is like a pressure cooker with a built-in mixer and cooling jacket — engineered so it never leaks, bursts or overheats.

Vessel design basics

Design pressure and temperature include margins above operating. Shell thickness comes from hoop stress (codes: ASME VIII, EN 13445, PD 5500), plus corrosion allowance (typically 1.5–3 mm). Heads: ellipsoidal (2:1) or torispherical.

Agitation

Rushton turbines (gas dispersion, N_p ≈ 5), pitched-blade and hydrofoil impellers (blending, suspension, N_p ≈ 0.3–1.3), anchors for viscous fluids. Baffles prevent swirling. Power per volume: 0.1–0.5 kW/m³ blending, 1–3 kW/m³ gas dispersion.

Heat transfer hardware

Conventional jackets, half-pipe coils (higher pressure/velocity), internal coils (more area, harder to clean), external loops with exchangers for large exothermic reactors. Area-to-volume ratio falls with scale.

Materials, relief and scale-up

Carbon steel, stainless (304/316L), duplex, nickel alloys, glass-lined steel for acids, PTFE linings. Relief devices sized for runaway (DIERS two-phase methods). Scale-up keeps geometric similarity and chooses a criterion (P/V, tip speed, mixing time) depending on the process.

Formulas

• Cylindrical shell (ASME VIII): t = P R / (S E − 0.6 P) + corrosion allowance
• Agitator power (turbulent): P = N_p ρ N³ D⁵
• Impeller Reynolds number: Re = ρ N D² / μ
• Jacket duty: Q = U A ΔT_lm

Worked example: Shell thickness

Given: P = 1 MPa, inside radius 1000 mm; S = 138 MPa, E = 0.85, CA 3 mm

Find: Thickness.

• t = 1 × 1000/(138 × 0.85 − 0.6) = 8.6 mm
• + 3 mm CA = 11.6 → 12 mm plate

Answer: 12 mm

Worked example: Agitator power

Given: Rushton N_p = 5, D = 0.6 m, N = 2 rps; Water, 5 m³ batch

Find: Power, Re, P/V.

• P = 5 × 1000 × 8 × 0.0778 = 3.1 kW
• Re = 1000 × 2 × 0.36/0.001 = 720 000 (turbulent)
• P/V = 0.62 kW/m³

Answer: 3.1 kW, 0.62 kW/m³

Common mistakes

• Wrong: Scaling up at constant speed. — Right: Choose a scale-up criterion (P/V, tip speed) — constant rpm explodes power.
• Wrong: Forgetting corrosion allowance and relief. — Right: Include CA and size relief for runaway scenarios.

Applications

• Pharma & fine chemicals: Glass-lined and stainless jacketed reactors.
• Polymers: Large agitated reactors with external cooling loops.
• Bioreactors: Aerated fermenters with Rushton/hydrofoil impellers.

Key takeaways

• t = PR/(SE − 0.6P) + CA.
• P = N_p ρ N³ D⁵; choose impeller for duty.
• Jackets/coils/external loops remove heat.
• Scale up with a chosen criterion; size relief.

Going further

• DIERS relief sizing: Two-phase venting methods for runaway reactions.
• CFD mixing: Predicts blend time, gas holdup and shear distribution.
• Fatigue & creep: Cyclic batch operation and high temperature affect vessel life.