Plug Flow & Stirred Tank Reactors: Characteristics and Operation

Plug Flow (Tubular Piston) Reactor

General Characteristics

Features: A plug flow reactor (also called a tubular piston reactor) operates under continuous flow. The reaction mixture moves through a tube in constant motion. All reagents travel in a selected direction, and the fluid moves like a piston or plug. All elements take the same residence time through the reactor without back-mixing. Properties are essentially constant at each cross section but vary longitudinally. These conditions occur when axial dispersion is negligible and there is complete mixing in the radial direction.

Phases

1. Homogeneous

Used for gas-phase reactions and some fast liquid-phase reactions. In this type of reactor only the reacting fluid is present. It often operates under isothermal conditions.

2. Heterogeneous (Fixed Bed)

For catalytic reactions the reactor is packed with solid catalyst particles or granules; this configuration is known as a fixed-bed reactor. Typical applications include synthesis of ammonia and methanol.

Forms and Construction

Plug flow reactors may be a large-diameter cylinder or a set of parallel tubes inserted between two heads. Tubes may be a few inches in diameter up to much larger widths and may be several meters long.

Flow Behavior

Reagents typically flow axially down a tube, but some systems have more complex flow patterns. For example, in platforming (reforming) reactions, the reacting gas flows in from a perforated cylinder and through catalyst packed between that cylinder and a central tube that contains the product. Although the local flow is radial, the system can satisfy plug flow conditions because adequate mixing between substances prevents back-mixing.

Composition Variation

The composition changes along the direction of flow. Composition can also vary in directions perpendicular to the flow due to temperature gradients, velocity gradients, or both.

Heat and Temperature

Plug flow reactors sometimes operate under adiabatic conditions and other times with heat transfer through the wall. In the adiabatic case, temperature rises along the flow direction if the reaction is exothermic and falls if the reaction is endothermic. In many cases it is necessary to preheat the reactants before they enter the reaction zone; once the reaction is initiated, it is often necessary to remove heat through the wall to avoid formation of undesirable secondary reactions.

Stirred Tank Reactors (CSTR)

Continuum Hypothesis and Characteristics

A stirred tank reactor (continuous stirred-tank reactor, CSTR) is based on the continuum hypothesis and is characterized by a tank through which a continuous, constant flow of reacting material passes. The outlet composition equals the composition inside the reactor (well-mixed assumption). CSTRs are typically operated at steady state.

Operation and Mixing

CSTRs are most suitable for liquid-phase reactions. For best performance, multiple CSTRs are often arranged in series. The goal is for each vessel to be well mixed so the entire volume participates in the reaction without any dead volume.

Design Considerations and Problems

• Temperature control: CSTRs allow temperature control, but complications arise when reactions are highly exothermic or endothermic.
• Viscosity and mixing: If viscosity is high, or impeller design is poor, dead zones or short-circuiting can appear, reducing reactor performance.
• Series operation: Multiple tanks in series approximate plug flow behavior and often give better conversion for certain kinetics.

Assumptions

The key working hypothesis for a CSTR is perfect mixing so that concentration and temperature are uniform throughout the tank.