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Reactor Conversion
Measures the fraction of a limiting reactant consumed in a chemical reactor relative to what entered.
Interpretation
Conversion: X = (F_A0 − F_A) / F_A0, measures extent of reaction. Example: Inlet 100 mol/h, outlet 30 mol/h → X=0.7 (70%).
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| X | Fractional conversion | |
| F_A0 | Molar feed rate of reactant A | mol/s |
| F_A | Molar outlet rate of reactant A | mol/s |
What it means
Conversion is a key performance indicator for chemical reactors. It is defined as the fraction of reactant that has been transformed into products. For a reactant A, conversion X is calculated as (F_A0 − F_A) / F_A0, where F_A0 is the molar flow of A entering the reactor and F_A is the molar flow of A leaving. Conversion ranges from 0 (no reaction) to 1 (complete reaction). It is used in reactor design to determine the required volume for a given conversion, and to compare the performance of different reactor types. Conversion is also used to calculate selectivity and yield. In industrial practice, high conversion is often desired to reduce separation costs, but it may require larger reactors or higher temperatures/pressures. Conversion is a fundamental concept in chemical reaction engineering and is essential for process optimisation and scale‑up.
Worked example
Reactor Conversion – Two Examples
Real‑World| Parameter | Value |
|---|---|
| F_A0 | 10 mol/s |
| F_A | 2 mol/s |
| Parameter | Value |
|---|---|
| F_A0 | 5 |
| F_A | 1 |
Common mistakes
- Inlet and outlet flows: F_A0 and F_A are molar flow rates of reactant A – must be at the same reference conditions.
- Conversion X: Between 0 and 1 (or 0‑100%); cannot exceed 1.
- Stoichiometry: If the reaction has multiple reactants, conversion is defined per reactant – specify which one.
- Flow vs. batch: Conversion is defined the same way for batch (using moles) and flow (using molar flow).
- Inert species: Inerts do not affect conversion; they dilute the reactant.
Applications
Reactor conversion, X = (F_A0 − F_A) / F_A0, quantifies the fraction of reactant converted to products. It is a key performance indicator for chemical reactors, measuring how effectively the feed is transformed. Engineers use conversion to size reactors, to compare different catalyst or process conditions, and to evaluate plant performance. Conversion is linked to reactor type (batch, CSTR, PFR), operating temperature, and residence time. In design, the required conversion determines the reactor volume and operating costs. For existing plants, monitoring conversion helps detect catalyst deactivation or fouling. By optimising conversion, engineers can maximise product yield and selectivity while minimising by‑products and energy consumption.
- Design of batch, CSTR, and PFR reactors for given conversion targets
- Performance monitoring of industrial reactors
- Comparison of catalyst activity and selectivity
- Optimisation of reaction conditions (temperature, pressure, residence time)
- Troubleshooting reactor under‑performance
Frequently Asked Questions
Conversion (X) is the fraction of the limiting reactant that has been converted to products: X = (F_A0 – F_A) / F_A0, where F_A0 is the molar flow rate of reactant A entering the reactor, and F_A is the molar flow rate of A leaving. It is a measure of how far the reaction has progressed.
- Using a reactant that is not the limiting reactant – conversion is always based on the limiting reactant for a given stoichiometry.
- Ignoring the effect of volume change on concentration – for gas‑phase reactions, the total molar flow may change, affecting concentration.
- Using conversion for multiple reactions – for complex networks, use selectivity or yield instead.
Higher conversion requires more reactor volume (or longer residence time). The design equations for CSTR and PFR incorporate conversion to calculate volume.
Conversion is a dimensionless fraction. The extent of reaction (ξ) has units of moles and is related to conversion by X = –ξ·ν_i / F_A0, where ν_i is the stoichiometric coefficient.
Measure the inlet and outlet concentrations of the reactant. For a constant‑density system, X = (C_A0 – C_A) / C_A0. For variable‑density, account for volumetric change.
Conversion is the primary design variable. The reactor volume is determined by the required conversion and the reaction kinetics. It also affects product separation and recycle.
Conversion is still valid, but the design equations become non‑linear and may require numerical integration. The relationship between conversion and rate must be integrated.
The theoretical maximum is 1 (complete conversion), but in practice it is limited by equilibrium (reversible reactions) or economics (cost of separation).