Agitated Vessels

CHE 482 – EQUIPMENT DESIGN     

AGITATED AND STIRRED VESSELS Engr. Ronnie V. Flores   

2nd Semester AY 2009-2010

TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES ‐ MANILA DEPARTMENT OF CHEMICAL ENGINEERING

 

2 | P a g e

AGITATED AND STIRRED VESSELS 

REQUIREMENTS FOR THE DESIGN OF AGITATED VESSELS: 1. Impeller Design a. Type of Impeller b. Impeller Diameter c.

Position of impellers

2. Power requirement to drive the agitation 3. Stirrer Speed 4. Baffle design (if needed) 5. Mixing Time SELECTION OF TYPE OF IMPELLER Use figure 10.57 Coulson and Richardson’s Chemical Engineering Volume 3, 4th edition – a function of tank diameter and liquid’s viscosity IMPELLER SIZE For standard turbine design (source: Unit Operations for ChE by McCabe, 7th edition)

1 3 1 1 12

1 5 1 4

ChE 482 – Equipment Design   

 

3 | P a g e

AGITATED AND STIRRED VESSELS 

From Chemical Process Equipment Selection and Design by Stanley Walas,

THE VESSEL ; A dished bottom requires less power than a flat one. ; When a single impeller is to be used, a liquid level equal to the diameter is optimum, with the impeller located at the center for an all-liquid system. Economic and manufacturing considerations, however, often dictate higher ratio of depth to diameter. BAFFLES ; Except at very high Reynolds numbers, baffles are needed to prevent vortexing and rotation of the liquid mass as a whole. ; A baffle width one-twelfth the tank diameter, w = 4/12; a length extending from one half the impeller diameter, d/2, from the tangent line at the bottom to the liquid level, but sometimes terminated just above the level of the eye of the uppermost impeller. ; When solids are present or when a heat transfer jacket is used, the baffles are offset from the wall a distance equal to one sixth the baffle width. ; Four radial baffles at equal spacing are standard; six are only slightly more effective, and three appreciably less so. ; When the mixer shaft is located off center (one-fourth to one-half the tank radius), the resulting flow pattern has less swirl, and baffles may not be needed, particularly at low viscosities.

ChE 482 – Equipment Design   

 

4 | P a g e

AGITATED AND STIRRED VESSELS 

DRAFT TUBES ; A draft tube is a cylindrical housing around and slightly larger in diameter than the impeller. ; Its height may be little more than the diameter of the impeller or it may extend the full depth of the liquid, depending on the flow pattern that is required. ; Usually draft tubes are used with axial impellers to direct suction and discharge streams. ; An impeller-draft tube system behaves as an axial flow pump of somewhat low efficiency. Its top to bottom circulation behavior is of particular value in deep tanks for suspension of solids and for dispersion of gases. IMPELLER LOCATION Source: Chemical Process Equipment Selection and Design by Stanley Walas Impeller clearance

Maximum level,

Number of

H/Dt

impellers

Lower