Coagulation and Flocculation-I

SOLUTION

Design a conventional vertical-shaft rapid mix tank unit for uniformly dispersing coagulant in 10 MLD of settled raw water as per design parameters given below:

Detention time (t):                                                                     20 – 60 s

Ratio of tank height (H) to diameter (D):                                    (1:1 to 1:3)

Ratio of impeller diameter (DI) to tank diameter (D):      (0.2:1 to 0.4:1)

Gt:                                                                                            10000 – 20000

Tank diameter (D):                                                                   < 3 m

Paddle tip speed (vp):                                                                1.75 – 2.0 m/s

### Velocity of paddle relative to water (v):0.75 x paddle tip speed

Paddle area (Ap)/Tank section area (AT):                                  10:100 – 20:100

Coefficient of drag on impeller blade (CD):                                 1.8

### Maximum length of each impeller blade (L):0.25 x impeller diameter

Maximum width of impeller blade (B):                           0.20 x impeller diameter

Impeller height from bottom (HB):                                              1.0 x impeller diameter

Kinematic viscosity :                                                            1.003 x 10-6 m2/s

Dynamic viscosity of water :                                                1.002 x 10-3 N.s/m2

Determine tank dimensions (provide a freeboard of 0.5 m), impeller diameter, paddle dimensions, number of paddles, clearance of the impeller from tank bottom, paddle rotation speed and power input requirement.

Solution:

Let the detention time (t) be 40 s.

Therefore, volume of tank (V) =

Let the tank diameter (D) be 2 m

Tank cross-sectional area (Acs) = 3.14 m2

Tank height (H) = 1.47 m, provide freeboard of 0.5 m

Total height (HT)= 1.97 m, say 2 m.

Tank height (H) to tank diameter (D) ratio =

(within the range of 0.33–1.0, hence okay)

Let velocity gradient, G = 400 s-1

Therefore, G.t = (400).(40) = 16000 (within the limit of 10000-20000, hence okay)

Let the paddle tip speed (vp) be 1.8 m/s

Hence, velocity of paddle relative to water (v) = 0.75.(1.8) = 1.35 m/s

,    or,

or, Ap = 0.335 m2

Tank sectional area = (D).(H) = (1.47).(2) = 2.94 m2

, which is around 0.15, hence okay.

Let the impeller diameter be 0.8 m, i.e., , which is okay

Choose length of each impeller blade (L) as 0.20m, i.e.,  (okay)

Area of each blade = (L.B) = (0.2).(0.15) = 0.03 m2

Therefore, number of blades to be provided = , say 12

Clearance of the paddles from the tank bottom = 0.8 m