The overall mass transfer co-efficient for the absorption of SO₂ in air with dilute NaOH solution can be increased substantially by

ANS:A - increasing the gas film co-efficient.

Increasing the gas film coefficient involves enhancing the efficiency of mass transfer between the gas phase (containing SO2) and the liquid phase (containing dilute NaOH solution) by reducing the resistance to mass transfer in the gas film. Here's how increasing the gas film coefficient can be achieved and why it improves mass transfer:

1. Enhancing Turbulence or Agitation: One way to increase the gas film coefficient is by enhancing turbulence or agitation in the gas phase. This can be achieved by introducing mechanical agitation devices such as stirrers, baffles, or spargers into the absorption vessel. These devices promote mixing and turbulence in the gas phase, increasing the contact between the gas and liquid phases. As a result, the concentration gradient between the gas phase (containing SO2) and the liquid phase (containing NaOH solution) is maintained, facilitating faster mass transfer.
2. Increasing Gas Velocity: Increasing the velocity of the gas phase can also enhance the gas film coefficient. Higher gas velocities create greater turbulence and increase the surface area available for mass transfer. This allows more SO2 molecules to come into contact with the liquid phase, resulting in higher mass transfer rates.
3. Reducing Thickness of Gas Film: The gas film coefficient is inversely proportional to the thickness of the gas film. Therefore, reducing the thickness of the gas film can increase the gas film coefficient and improve mass transfer efficiency. This can be achieved by optimizing the design of the absorption column or by using packing materials that promote better contact between the gas and liquid phases.
4. Optimizing Temperature and Pressure: In some cases, optimizing the temperature and pressure conditions of the system can also affect the gas film coefficient. For example, increasing the temperature can reduce the viscosity of the gas phase, leading to increased turbulence and improved mass transfer. However, this effect should be considered along with its impact on other factors such as solubility and chemical reactions.

By increasing the gas film coefficient through methods such as enhancing turbulence or agitation, increasing gas velocity, and reducing the thickness of the gas film, the resistance to mass transfer in the gas phase is reduced. This allows more SO2 molecules to transfer from the gas phase into the liquid phase, leading to higher overall mass transfer rates and improved efficiency in the absorption process.