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  • Monday, 11 May 2020

    [How To] Design Packed distillation column

    Hiii Good Morning everyone …!! 
    Hope everyone are doing good ….!!
    From past many days, I’m receiving a query from a person and I don’t wanna disclose his name, the query is design of packed distillation column.
    Most of us were well aware of tray/plate column design and operation of packed column, but not design of packed column. But many of us know that the packed column design is similar to tray column and instead of trays we’ll use packing.
    Today I thought of posting this here. There wont be any Q & A session here.








    In general packed columns are most common equipment’s used for distillation, gas liquid absorption, sometimes liquid – liquid absorption too. These will be having some pro’s & con’s for these packed columns when compared to tray column. Below are some of them:
    i.      Packed columns are not effective for low liquid flow-rates,
    ii.     Corrosive liquids can be handled in packed column effectively than plate column,
    iii.   Liquid holdups in packed columns are low when compared with tray column, which can be profitable due to less inventory loss,


    iv.   The pressure drop per equilibrium stage can be lower for packing than that of plates,
    v.     Withdrawl of side streams can be made for tray column, but in-case of packed column it is not possible,
    vi.   There might be chances of packing damages due to variation of pressures sometimes, which requires replacement and this will impact on principle costs,
    vii.  Can be a better match to thermal sensitive liquids,
    viii.  Packing are sensitive and the chances for getting damaged during installation itself are high.


    Also Read:
    [How to] Optimize distillation setup to enhance the distillation rate 
    [How to] Check filtration feasibility in ANFD ?

    There will be two types of packing, as many of us knows these, Random and Regular packing.

    Raschig rings, Pal rings, Berl saddle, Intallox saddle, Super intallox, Metal hypac are the once which will be commonly available. Below are the properties of them:
    Packing type
    Size
    BD
    Kg/m3
    Surface area
    (m2/m3)
    Packing factor, m-1
    inch
    mm
    Ceramic Raschig
    rings
    0.5
    13
    881
    368
    2100
    1.0
    25
    673
    190
    525
    1.5
    38
    689
    128
    310
    2.0
    51
    651
    95
    210
    3.0
    76
    561
    69
    120
    Metal
    0.5
    13
    1201
    417
    980
    1.0
    25
    625
    207
    375
    1.5
    38
    785
    141
    270
    2.0
    51
    593
    102
    190
    3.0
    76
    400
    72
    105
    Pall Rings Metal
    0.6
    16
    593
    341
    230
    1.0
    25
    481
    210
    160
    1.3
    32
    385
    128
    92
    2.0
    51
    353
    102
    66
    3.5
    89
    273
    66
    52
    Plastics Saddles
    0.625
    16
    112
    341
    320
    1
    25
    88
    207
    170
    1.5
    38
    76
    128
    130
    2
    51
    68
    102
    82
    3.5
    89
    64
    85
    52
    Intallox Saddles
    Ceramic
    0.5
    13
    737
    480
    660
    1.0
    25
    673
    253
    300
    1.5
    38
    625
    194
    170
    2.0
    51
    609
    108
    130
    3.0
    76
    577
    78
    72

    Below are the packing sizes recommended ranges based on column dia:



    Column Dia, m
    Recommended Packing Size
    < 0.3 m
    < 1 in
    0.3 m to 0.9 m
    1.0 in to 1.5 in
    > 0.9 m
    2 in to 3 in

    Let’s start the design part now and for your convenience I’ll explain it with a case study :
    Design involves no. of stages, column dia, column height & Packing equivalent Dia.
    Step – 1: Estimate no. of transfer units based on McCab thiele method
    Saturated feed contains 50 % A, 50 % B. Required purity of top product A is 98 % and allowable A composition in bottom is 2 %. The relative volatility of A w.r.t. B is 3.


    [Other input: Vapour density: 2 Kg/m3, Liquid density: 650 Kg/m3; Void fraction: ~20%; Vapour viscosity: 0.0000118 Pas.Sec; Liquid viscosity: 0.000314 Pas.Sec; Vapour flow rate: 500 Kmoles/hr; Liquid flow rate: 200 Kmoles/hr; Mol. wt. of A(Vapour): 50 g/mole, Mol. wt. of B(Liquid): 78 g/mole; Vapour diffusivity: 0.0000001 m2/sec]
    Hence prepared a tabulation for Mole fractions X vs Y.
    Mole fraction, X
    Mole fraction, Y
    0.0
    0.00
    0.1
    0.25
    0.2
    0.43
    0.3
    0.56
    0.4
    0.67
    0.5
    0.75
    0.6
    0.82
    0.7
    0.88
    0.8
    0.92
    0.9
    0.96
    1.0
    1.00

    The above tabulation done by using, Y = 𝛂 X / (1+(𝛂 - 1)X)

    Based on the tabulation, plot the XY graph.
    As per the feed details, it would be a saturated liquid, hence the feed line will be straight,
    Below are some of the feed line positions for your reference:

    Ours is a saturated liquid, hence the q line will be straight.
    Also plotting the Xd, Xr points.

    Now intersecting the extended q-line by drawing a line from Xd extrapolated to q-line point on the curve, which will result in enriching section operating line & similarly for Xw extrapolation to q-line point on curve, which will result in stripping section operating line.

    Now drawing the steps from Xd to Xw over the enriching & stripping section lines by limiting inside the curve. Now the curve will look like this:


    Now if we count the steps / stages, they are 18. So NTU is 18.

    Also Read:
    [How to] Scale-up process equipments in pharma 
    [How to] Vent Sizing for pressure vessels

    Step – 2: HTU (Height of Transfer Unit) Calculation:



    Let’s begin by selecting ceramic Saddles for the packing of Dia 1.5”.
    For this we need to calculate the equivalent diameters (as these are non-spherical the diameter cannot be calculated, hence the equivalent diameter need to be estimated):
    Equivalent Diameter, d = 4 x (Ɛ/ω);
    Ɛ – Void fraction (~20%); ω – Surface area per unit column area (m2/m3)
    d = 4 x (0.2/194) = 0.004 m.


    Archimedes number, Ar = d3 x (ρ1 – ρ2) x ρ2 x g / µ22 
    = (0.004)3 x (650 – 2) x 2 x 9.81 / (0.0000118)= 5843728.
    Reynolds number, Re = 0.15 x Ar0.57 x (V/L)0.43 = 0.15 x (5843728)0.57 x (500/200)0.43 = 1600.36.
    Linear vapour velocity, V = Re x µ2 / d x ρ2 = 1600.36 x 0.0000118 / 0.004 x 2 = 2.36 m/sec.
    Mass transfer coefficient, K = 0.35 x Re0.8 x D x (µ2 / (ρ2 x D))0.35 / d2
    = 0.35 x 1600.360.8 x (0.0000001) x (0.0000118/(2 x 0.0000001))0.35 / 0.0042
    = 3.34 Sec-1
    Height of transfer unit, HTU = V / K = 2.36 / 3.34 = 0.71 m,


    Step – 3: Column Height & Diameter estimation:

    Column height, H = NTU x HTU =18 x 0.71 = 12.74 m;
    Column Diameter, Dc = ((3 x ν)/(π x V))0.5
    For estimating the column diameter, we need to calculate the volumetric flowrate of vapour,

    Volumetric flowrate, ν = molar flowrate x Vapour Mol. wt. / Vapour density
    = 500 x 50 / 2 x 3600 = 3.472 m3/sec



    Dc = ((3 x 3.472)/(3.141 x 2.36))0.5 = 1.19 m.

    So, finally we have derived our requirements,

    Height of column is 12.72 m ~13 m,

    Dia meter of column is 1.19 m,

    Packing is ceramic super intallox and height of each packing is 0.71 m ~0.7 m with voidage of 20 %,

    Dia of packing is 1.5" with interfacial surface area of 194 m2/m3 of column and

    Equivalent diameter of each packing is 0.004 m 
    [Don't miss-understand there is a difference between Diameter and Equivalent diameter, for spherical structures both are same, but for non-spherical they will vary.]


    /* Also we need to confirm our vapour velocity is below flooding velocity, that i'll update later */

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

    If any queries, pl feel free to comment ....!!!

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

    /* We are done, most of the input values are considered and some of them were grabbed from my Mass transfer operations lab observation notes */






    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.
    Follow Me on Twitter AjaySpectator & Computer Innovations


    3 comments:

    1. Vishal here
      What should the U value be while finding out vapor load from a batch reactor. Ssr and glr ?

      ReplyDelete
      Replies
      1. Hii Vishal,

        It depends on the MOC of the batch reactor,
        There will be general considerations like, if its SS316 then the U would be ~250 Kcal/hr.m2.C,
        if its MSGL then the U would be ~150 KCal/hr.m2.C,
        for hastelloy it would be nearly equivalent to SS316.

        Delete
    2. Sir vishal here. What will be the basis of rupture disc sizing on RVDs rotary vacuum driers? or even PDVs on ANFs but most importantly the first question

      ReplyDelete

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