Question

In most mass transfer operations........... flow is desired to increase the rate of transfer per unit area.

• A. Laminar
• B. Turbulent
• C. Transitional
• D. Streamline

Hint:

Turbulence = Chaos = Faster mixing. While turbulent flow requires more pumping energy (due to pressure drop), the benefit of faster production usually outweighs the cost.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question relates to transport phenomena (mass and heat transfer) and seeks the most efficient flow regime for industrial processing applications.

Step 2: Detailed Explanation:

• Mass transfer is the movement of chemical species from one phase to another (e.g., aroma leaving food during drying or oxygen entering a fermenter).

• In Laminar (or streamline) flow, fluid moves in parallel layers. Transfer between these layers occurs only via molecular diffusion, which is a slow process.

• In Turbulent flow, the fluid is characterized by chaotic fluctuations and eddies.

• These eddies provide "eddy diffusion," which is orders of magnitude faster than molecular diffusion. It ensures vigorous mixing and rapid transport of mass/energy through the bulk of the fluid.

• Consequently, the mass transfer coefficient (\(k\)) is much higher in turbulent flow.

• To maximize productivity and reduce the size of equipment (increase transfer per unit area), industrial processes (like evaporation, extraction, or aeration) are designed to operate at high Reynold numbers in the turbulent regime.

Step 3: Final Answer: Turbulent flow is desired in mass transfer operations for its high transfer rates.