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    Saturday, 29 December 2018

    Design of a decanter for Layer Separations [Decanter design for Pharma operations]

    This is a property of Pharma Engineering

    Hiiii Alll....!!!

    Being busy at work, unable to concentrate much here....!!
    Recently received a comment asking about the settling time for separating two layers during workups / Extractions. Requested by  Mr. HariKrishna.

    As per me it is nothing but a decanter design.

    Decanters are nothing but settlers. Mostly in our pharma operations we use reactors for separations and somewhere Vertical Agitation Tanks are also used for the purpose.

    As this is a simple topic and can be illustrated with simple basics, here i wont waste time, but before that its quite necessary to retrieve the laws which we have used a long back ago.




    What is settling velocity ?

    The name itself indicates, but again, the velocity with which the solvent particles settle is called settling velocity, lets consider our workup's(extractions/washing's) we'll have double layers. In that case the bottom layer particles will settle with some velocity and that velocity is called settling velocity.

    What is terminal velocity ?

    Terminal velocity is the consistent velocity attained by a particle which is freely falling and at that case the particles shouldn't gain any acceleration or that should be at its peak acceleration.


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    What is Stokes's law ?  

    Stokes law relates the case of a freely falling particle in a viscous medium with maximum velocity with the resisting force. In the decanter design we can use it to calculate the settling velocity of particles.




    Stokes's law expression: Ud = ( Dd ^2) x g x ( 𝝆d𝝆c ) / ( 18 x 𝝁c ).

    where,

    Dd - Droplet diameter in m,
    Ud - Terminal velocity of the particles in m/sec,
    𝝆c - density of the bottom solvent in Kg/Cu.m,
    𝝆d - density of the top solvent in Kg/Cu.m,
    𝝁c - Viscosity of the bottom solvent in Pas.Sec,
    g   - Acceleration due to gravity in m/Sq.S.

    What is Interfacial area ?


    Interfacial area can be defined as the area of contact between two solvents during settling till separation.

    Interfacial width expression: W = 2 ( D x Z - Z^2 )^(0.5) 

    Interfacial Area expression: A = Settling flowrate / Settling velocity.

    Z - height of interface from vessel base,
    D - diameter of cylinder.

    Now lets start designing our decanter, and i'll explain you with a case study.


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    We need to perform a workup in our process and the process solvents be Ethyl acetate and water.




    Below are the stages of settling: 


    This is a property of Pharma Engineering


    Ethyl acetate : 1000 L, Density: 0.89 Kg/L, Viscosity: 1.5 mN s/m2
    Water             : 2000 L, Density: 1.00 Kg/L, Viscosity: 1 mN s/m2.

    Here, i'll consider one assumption, i.e., the droplet size be 100 𝝁m.

    Settling velocity of Ethyl acetate  = [ ( 100 x 10^-6 ) ^2] x 9.81 x (890-1000)
                                                                                              18 x 1 x (10^-3)                                                   

    = - 0.0006 m/s = -0.6 mm/s.




    Now, we need to calculate the bottom phase(water) settling rate

    = ( 2000 / 1000 ) * ( 1 / 3600 ) = 0.000556 m3 / s.


    Interfacial area required = 0.000556 / 0.0006 = 0.92 m2

    Dia of the vessel = 2 x [ ( 0.92 / 𝚷 ) ^0.5 ] = 1.08 m.

    Consider the height based on requirement, 

    Now lets check the time required for settling of the layers.


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    Let the height be 2 m and i'll take the dispersion band(i.e., the area where the settling will take maximum time) as 30% of the total height.

    ** I have taken the worst case where there is much emulsion formation.




    Let it be 2 x 0.30 = 0.6 m.

    Time required for settling  = 0.6 / 0.0006 = 1000 s = 16.67 mins ~17 mins. 

    This will be the theoretical time, and we can consider 20% excess while performing plant operations. If any abnormality, then the agitation/mixing(i.e., RPM) shall be controlled.


    That's it.........!!!

    Any queries feel free to ask/comment.....!!!

    Comments are most appreciated .........!!!





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    About The Author


    Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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