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

Introduction

Choosing the right reactor configuration and its Material of Construction (MOC) is one of the most critical decisions in chemical 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, pressure limitations, and material corrosivity.

Here is a comprehensive breakdown of the various reactor types, criteria for their selection, and material compatibility guidelines for industrial applications.

11 Types of Industrial Reactors

1. Agitated Vessel Type Reactor

• Glass Lined Reactor: Typically features a Carbon Steel (CS) or Stainless Steel (SS) shell with a 1.0 mm glass lining. These are selected when CS or SS is ruled out due to process corrosivity, and glass provides suitable compatibility. They are widely used in pharmaceutical synthesis due to their ease of cleaning and sanitization. Operating limits are normally restricted to 6 barg internal design pressure and 200 deg C operating temperature.
• Metallic Reactors (Unclad): Solid vessels fabricated from plates of Carbon Steel, SS-304, SS-316, Alloy 904L, Inconel, Hastelloy, or Titanium. Carbon Steel or Stainless Steels are preferred due to lower fabrication costs when compatible.
• Metallic Reactors with Cladding: Cladded reactors utilize a thin (3-4 mm) liner of an exotic, expensive material (such as Inconel, Hastelloy, Alloy 904L, or Titanium) on the process side, backed by a thicker Carbon Steel or Stainless Steel shell that provides mechanical strength. This combination substantially reduces equipment cost. Furthermore, thick-walled solid SS reactors suffer from poor heat transfer due to low thermal conductivity. Cladding a 3-4 mm SS layer onto a CS backup sheet improves heat transfer, as Carbon Steel's thermal conductivity is 3 to 4 times higher than SS.

2. Reactive Distillation Reactors

In these units, a distillation column is mounted directly on top of the reactor. They are utilized when the reaction products have a lower boiling point than the starting materials. Removing the volatile products as they form drives the reaction forward (shifting equilibrium), accelerates the reaction rate, and reduces impurity formation caused by side reactions between products and reactants.

3. Tubular Reactors

• Double Pipe Reactors: The simplest form of continuous flow reactors. They are used when the heat of reaction is low and a jacketed pipe provides sufficient heat transfer area.
• Reaction Pipe in Fuel-Fired Furnace: A multi-limb reactor pipe placed inside a fuel-fired furnace. These are used for high-temperature reactions (like gas cracking) where heat transfer requirements are high (e.g., Naphtha crackers for Ethylene/Propylene or Ethylene Dichloride cracking for Vinyl Chloride).
• Reaction Pipe in Electrical Furnace: A reactor pipe or coil placed in an electrical furnace. These are used for high-temperature endothermic gas-phase cracking reactions (up to 1000 deg C) where the overall heat load is low and the plant capacity is small-to-medium (e.g., Tetrafluoroethylene production from R-22).

4. Fixed Bed Catalytic Reactor

Used for gas-gas reactions catalyzed by a solid catalyst (typically 3-8 mm spheres or pellets) packed inside a cylindrical shell. Since solid catalysts have very low thermal conductivity, cooling or heating cannot be efficiently done using a standard vessel jacket. They operate in two modes:

• Adiabatic: No heat is added or removed. The temperature of the outlet gas increases (exothermic) or decreases (endothermic) depending on the reaction enthalpy. This mode is used when temperature changes do not significantly impact reaction rate or selectivity.
• Near Isothermal: If temperature control is crucial, an inert raw material or one with high specific heat capacity is fed in large excess to act as a heat carrier, dampening temperature variations. The excess material is later separated and recycled.

5. Multi-tubular Catalytic Reactor

For gas-phase catalytic reactions with high heat loads, a multi-tubular configuration (similar to a Shell & Tube heat exchanger) is preferred. The catalyst is packed inside the tubes (reaction side) while a heating or cooling utility circulates in the shell. While much more expensive than fixed beds, they are essential for highly exothermic or endothermic reactions.

6 Fluidized Bed Reactor

Consists of a reaction vessel containing fine solid catalyst powder fluidized by an upward gas or liquid stream, behaving like a boiling liquid. It provides excellent gas-solid contact and uniform temperature profiles. Heat is removed or added using excess reactants as a heat carrier. Because heat transfer is highly effective, it eliminates local hot spots on the catalyst, resulting in extended catalyst life and minimal side-reaction impurities.

7. Photo-Reactors

Utilized when a reaction is catalyzed by light (e.g., chlorinations, brominations, and oxidations). Light lamps of specific wavelengths are housed inside Quartz tubes, which do not absorb light, allowing maximum energy transmission. The reaction mass flows through the shell outside the tubes. Exothermic heat is removed by circulating the mass through an external heat exchanger.

8. Scrubber Reactors

Packed columns used for fast or instantaneous gas-liquid reactions. Gaseous reactants enter the bottom while liquid reactants and catalysts enter from the top. The packed bed ensures excellent interfacial contact. Heat of reaction is managed via external circulation loops.

9. PTFE-Lined Reactors

Used for highly aggressive services where no metallic or glass lining is compatible (e.g., mixtures of Hydrofluoric Acid, Hydrochloric Acid, and water). Since PTFE-lined vessels cannot support internal agitators or cooling jackets easily, agitation and heat transfer are achieved by circulating the reaction mass via a PFA-lined pump through external Silicon Carbide (SiC) tube heat exchangers.

10. Rotary Kiln Reactors

Long, slightly inclined cylindrical reactors that rotate slowly (5-10 RPM). They operate in the 250 to 1500 deg C range. High-temperature kilns are refractory-lined for solid-solid reactions (e.g., cement clinker, alumina) and fired by fossil fuels. Low-temperature kilns are used for solid-liquid reactions (e.g., Calcium Fluoride and Sulfuric Acid to produce HF) and are heated by flue gas jackets.

11. Open Hearth Furnace Reactors

Refractory-lined furnace chambers fired by oil or gas. Used for high-temperature solid-solid reactions (1200 - 1500 deg C) resulting in molten products (e.g., glass production and Sodium/Potassium Silicate manufacturing).

Reactor Selection Guidelines

The following matrix provides a guide to choosing a reactor type based on the phase and reaction characteristics:

Considerations for Selection of Reactor MOC

When selecting the Material of Construction for a chemical reactor, five parameters must be evaluated:

1. Chemical Compatibility: Resistance of the material to corrosion from raw materials, intermediates, and final products at all process stages.
2. Catalytic Inertness: The MOC must not act as a catalyst for side reactions that generate impurities or degrade the product.
3. Mechanical Strength: Ability of the vessel walls to withstand design pressures and vacuum at the maximum operating temperature.
4. Thermal Conductivity: Crucial if heat transfer occurs primarily through the reactor wall.
5. Cost: Fabrication and materials cost.

Indicative Comparison of Main MOCs (Specialty Chemicals Plants)

[!NOTE] After going through this article, look back in depth at the work you have done in the past to see whether you would have done anything differently. Whether your answer is yes or no is not important. What is important is that you went into depth. That is what Process Engineering is!

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