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    Wednesday, 21 March 2018

    [How To] Design a Scrubber [ Packed column ]

    Hello Readers, Good day all.........!!!

    After many days i'm back with much more awaited post, i.e., designing a Scrubber which shall be used for safe venting of vapours / gases from reactors / vessels.

    This post was requested by Mr. Nitin kheese a long back ago, and may be i think he should have forgotten also, and Mr. Nitin sorry for being late over this query.

    Before getting into the topic lets start with some basic stuff,

    What is Scrubber & Whats its purpose ?

    A scrubber is an object used for cleaning something unwanted thing, and in Pharma terminology i..e, our words, it is simply a combination of blower system and absorption system.

    What is absorption ?

    Absorption is simply grabbing a vapour/ gas or liquid into another liquid by means of holding them in the voids spaces available in this another liquid. Also there is some term which is called adsorption, which is same but instead of another liquid, a solid comes into play.

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    What is blower ?

    This term is the main heart of the scrubber, without a blower a scrubber is simply a re-circulation column. Blower is used to blow out the vapours / gases those are produced in a reaction reactor or to remove the unwanted vapours generated because of breathing losses in  storage vessels.

    What are the major components of the scrubber ?

    There are majorly 4 major components according to me,
    1. Blower,
    2. Re-circulation pump,
    3. Column,
    4. Collection / holding tank.

    If anyone have any other major components, then kindly comment / message me, i'll add those here.

    What is the principle of scrubber ?

    May be i should have posted this question at the top, but sorry for not 😆, the principle of scrubber is absorption.

    What is Packing factor, Fp ?

    Packing factor is the ration of packing surface area per unit volume to cube of the packing's void fraction. 
    Holds the unit of m-1.

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    What are the types of packing's used in scrubbers ?

    Usually, there may be many packing's, but i know only three sort,

    1. Super intallox,
    2. Intallox saddles,
    3. Pall rings.
    4. Random packing's

    Again there will be sub categories based on the MOC, like ceramic, metal, Plastic, etc.

    By now, basic stuff is completed, 
    Lets start out show, 

    Design calculations of Scrubber,

    Out of these let us start from the damn basic thing, i.e., Collection / holding tank.

    Usually a collection tank is nothing but a vessel which contains water / caustic lye,
    Directly i've mentioned water / caustic lye because usually in a pharma field we will use that.

    If the fumes that were sucked by blower are acid then we will go for lye, and if other than that we will go with water, also the selection depends on the solubility.

    Lets have an example, the reaction that's is carried out in the reactor generates 1000 Kgs of spent in form of vapour per hour and the component to be scrubbed is HCl vapour, which is produced at a rate of 70 Kg/hr and the volume percent of HCl vapour is ~6%(v/v). Scrubbing medium is 20% caustic solution.

    Lets start,

    Input Data:

    Packing type                                   : Intallox saddles,
    Packing size                                    : 25 mm,
    Packing MOC                                 : PP,
    Gas pressure drop per m of bed : 15 mmWC /  m of bed,

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    Gas / Vapour properties :

    Gas flow rate                       : 1000 Kg/hr = 0.2778 Kg/sec,
    Gas entry pressure             : 1.00 atm,
    Gas temperature at entry : 30 degC,
    Gas mol. wt.                        : 29 g/mole

    Component to be scrubbed : 

    Component flow rate              : 70 Kg/hr,
    % component in gas                : 6%
    Molecular wt. of component : 36.5

    Scrubbing medium properties :

    Scrubbing medium : 20% caustic,
    Liquid flow rate       : 77 Kg/hr = 0.0214 Kg/sec
    Liquid density          : 1100 Kg/Cu.m
    Liquid Viscosity       : 3.5 Cp = 0.0035 N.Sec/Sq.m,

    Packing factor, Fp                          : 21 / m,
    Characteristic packing factor, Cf : 32 [ refer below table ],

    Gas Density Calculation :

    Average molecular wt. of gas = 29.45 g/mole,
    Gas in flow rate = 0.2778 / 29.45 = 0.0094 Kmol/sec,
    = (Kmole/sec) x (T in K / 273.15) x ( 1 atm / pr. in atm) x (22.414/1),
    = 0.234499 Cu.m/Sec.

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    So, now we have determined total of three flowrates for the gas,
    1. Molar flow rate = 0.0094 Kmol/Sec,
    2. Volumetric flow rate = 0.234499 Cu.m/Sec,
    3. Mass flow rate = 0.2778 Kg/Sec,

    Gas Density = Mass flow rate / Volumetric flow rate = 1.1846 Kg/Cu.m,

    Calculating L', G' & Cross sectional area of Column:

    Now, Assuming complete absorption of the desired HCl vapour into scrubbing medium,

    Component removed = molar flow rate x % component x mol. wt. 
    = 0.0094 x (6/100) x 36.5 = 0.0207 Kg/Sec,
    Liquid leaving the packing = inlet liquid flow rate + Component removed
    = 0.0207 + 0.0214 = 0.0420 Kg/Sec.

    (L' / G') x (ρg / ρl)^0.5 = (0.0420/0.2778) x (1.1846/1100)^0.5 = 0.00497,

    Use the 0.00497 as ordinate, and pressure drop of 147.1 (N/Sq.m)/m,

    find the abscissa from the below graph,

    [(G' ^ 2) x Cf x (μl^0.1) x J ] / [ρg(ρl - ρg)gc] = 0.04 [from graph],

    Now, G' = [ 0.04 x ρg(ρl-ρg)gc/(Cf x (μl^0.1) x J)]^0.5,
    = 1.6665 Kg/Sq.m.Sec,

    Column cross section area = Gas mass flow rate / G' = 0.2778 / 1.6665 = 0.1667 Sq.m,

    Column dia = 0.461 m,

    That's it....!!!

    Hope everyone understood the theory, if anyone needs the calculation for flooding & blower power requirement, pl contact me through available contact page. As the calculation will make you guys confuse, so i'm not posting it here.

    If any concern, pl do feel free to comment / contact, 

    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.
    Follow Me on Twitter AjaySpectator & Computer Innovations


    1. Hi Ajay, u had done a fabulous job. How u can take Cf as 32 and J represents what?

      1. Dear ,

        Thanks for the compliment, Cf is taken from table. 6.3, and J is conversion factor, you can take it as 1.0, which is adequate for liquid distribution and irrigation across the bed.

        Next time, kindly comment with you name .

        AJAY K

    2. Hi Ajay, How can you taken scrubbing agent flow rate as 77 Kg/Hr.In my plant we are using 10 m3/Hr centrifugal pump for transfer lye solution from lye tank to scrubber absorption column. And also tell me, how can you taken gas flow rate was 1000 Kg/Hr.


      1. Dear Ramana,

        I've considered that 77 Kg/hr w.r.t. single intallox saddle, not for whole packing bed, That 1000 Kg/hr is the case i've considered, as spent evolution from the system.

        AJAY K

    3. Hi Ajay, How you are taken liquid flow

      1. Hii,

        Liquid flow is just an assumption i made w.r.t. saddles.

    4. Hi ajay parth here we are doing our final year project on design and analysis of scrubber and fully confused regarding the project as we are not familiar with it if u could help tht could be great can u plz send ur contact no

    5. Do you have calculations on quantity of sludge generated from ETP based on TDS and COD concentrations

      1. Hii kiran,

        Please clarify me, what exactly you want to calculate ?

        AJAY K



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