What is the Overall Mass Transfer Coefficient?
In chemical engineering and mass transfer operations, the overall mass transfer coefficient represents the combined resistance to mass transfer across two phases, typically a gas and a liquid. According to the Two-Film Theory, mass transfer involves crossing a stationary film on both sides of the interface. While individual coefficients (kL and kG) describe the rate in a single phase, the overall coefficient accounts for the total resistance encountered from the bulk of one phase to the bulk of the other.
Understanding the Formula
The calculation is based on the addition of resistances. The relationship between the individual film coefficients and the overall coefficients depends on the equilibrium distribution coefficient (m), often derived from Henry's Law. The formulas used in this calculator are:
1/KL = 1/kL + 1/(m · kG)
1/KG = 1/kG + m/kL
Where KL is the overall liquid-phase coefficient, KG is the overall gas-phase coefficient, and m is the slope of the equilibrium curve (y = mx).
How to Use This Calculator
To use the tool, follow these steps:
- Enter the local liquid-phase mass transfer coefficient (kL).
- Enter the local gas-phase mass transfer coefficient (kG).
- Provide the Henry's constant or distribution coefficient (m).
- Click "Calculate" to see the overall coefficients for both phases.
Frequently Asked Questions
Q: What does it mean if one resistance dominates?
A: If the gas-phase resistance is much larger than the liquid-phase resistance, the process is said to be "gas-film controlled." Conversely, if the liquid resistance is much higher, it is "liquid-film controlled." This calculator helps identify which phase provides the bottleneck in your process.
Q: What units should I use?
A: You must ensure all inputs use consistent units (e.g., m/s or kmol/m²·s·Pa). The output units will be identical to the input units provided.