Agitator and Reactor Mechanical Design

This topic sizes stirred vessels and their drives. You compute the impeller Reynolds number to fix the flow regime, read the power number from the impeller-type chart, calculate power draw, and select impeller type, baffling and shaft/seal arrangement for the mixing duty — blending, suspension, or gas dispersion.

(impeller Reynolds number)

(power draw, power number N_p)

(blend time in turbulent mixing)

Relation 1 —

(impeller Reynolds number)

Write this relation with symbols exactly as in Bhattacharya Chemical Equipment Design — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

(power draw, power number N_p)

Write this relation with symbols exactly as in Bhattacharya Chemical Equipment Design — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

(blend time in turbulent mixing)

Write this relation with symbols exactly as in Bhattacharya Chemical Equipment Design — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

The power law P = N_p ρ N³ D⁵ shows why mixing scale-up is tricky: power rises with the cube of speed and the fifth power of diameter, so a modest size increase demands a large motor. The power number N_p is roughly constant in fully turbulent flow but rises steeply in the laminar regime, so the impeller Reynolds number must be checked first. Baffles convert wasteful swirling into effective top-to-bottom turnover and prevent the central vortex that would otherwise pull in air. For gas–liquid contacting the mass-transfer coefficient k_L a is driven by power per unit volume, which is why scale-up often holds P/V constant.

State assumptions explicitly before using any relation for agitator and reactor mechanical design — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Process Equipment Design viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Process Equipment Design papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to agitator and reactor mechanical design.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Agitator and Reactor Mechanical Design appears in EPCM and fabrication. In Indian chemical curricula this topic is tested because it connects theory to mechanical design of vessels and columns.

GATE and semester exams often combine agitator and reactor mechanical design with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use agitator and reactor mechanical design?" — answer with a lab, mini-project, or plant visit example if possible.

Students treat the power number as constant even in laminar flow, forget the fifth-power diameter dependence during scale-up, and omit baffles (grossly under-predicting power). Using vessel diameter instead of impeller diameter in the power equation is a frequent slip.

Before attempting agitator and reactor mechanical design problems, confirm you can:

1. Baffles prevent vortexing — four baffles are standard for turbulent mixing
2. Gas–liquid k_L a rises with power per volume and gas superficial velocity
3. Mechanical seal versus packing for the shaft penetration

Revise the solved examples in Bhattacharya Chemical Equipment Design — Standard reference and one previous-year GATE or university paper for this unit.