Friday 18 March 2011

DECANTER DESIGN


Purpose
To separate water and naphtha fraction coming from water quench tower.

Liquid flow rates
The mass flow rate of water =116160 Kg/hr
Density of water =10000 Kg/m3
Volumetric flow rate of water =0.0322 m3/sec
Mass flow rate of naphtha =6900 Kg/hr
Density of naphtha =800 Kg/m3
Vol. Flow rate naphtha =0.00217 m3/sec
Total vol.flow rate   =0.034 m3/sec

Calculation for settling velocity

The settling velocity is given by,

Ud = D2d.g[ρnapwat] 18μwat

Where
Ud =settling velocity, m/sec
Dd=Diameter of dispersed phase, usually less than 150 micro-meters
Substituting values in (1),
Ud = 0.00147 m/sec
Calculation for interface area
The interface area is calculated by dividing volumetric flow rate by settling velocity,
i.e.
A1 =0.0034/0.00147 =23.3 m2
Assuming width of interface is 1/5 of length, so
A1 =L*L/5
L =10.8 m
And, W =2.16 m

Calculation for interface height

Assuming diameter of decanter is 2.5 m
The height of interface, Z, is related to the width of interface as,

Z (D-Z)    = [W/2]2……………… (2)

Substituting values,      Z = 1.88m


Calculation for residence time

Take dispersion band as 10% of interface height =0.188m
The residence time= Dispersion band / Settling velocity = 128 sec or 2 min 8 sec
Hence, the design is satisfactory

Check for droplet diameter

The settling velocity of Naphtha droplets is,

Ud = 0.00217/23.3 = 9.314*10-5

The droplet diameter is calculated by (1) using settling velocity of Naphtha droplets.
 The droplet dia, dd =27.8 micro-meter
Which is below the assumed dia150 micro-meter and hence the design calculations are satisfactory.

Piping arrangement

To minimize entertainment by the jet of fluid entering the vessel,
The entering velocity is kept below 1m/sec
Vol. Flow rate=0.034m3/sec
Pipe area =Vol. Flow area/velocity =0.034 m2
Pipe dia =0.2m
The position of interface is at a height of 1.405
Let naphtha takeoff point be at 2.1 m,
Then, the water takeoff leg is at = 2.016 m

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