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  • Wednesday 20 December 2017

    [How to] Reduce Time-Cycle of ANFD drying - Convective Drying

    Gud evening guyz.....!!!!

    Back after many days on blogger to share some technical stuff here, today what i gonna explain takes some guts to grab & implement, because wrong understanding will lead to blunders,

    Drying is the most difficult and typical task which will decide the time cycle fate, Many a time i've came across through process engineers who are being a part of Technical services trying to reduce the time cycle of product drying, especially in ANFD, because in other dryers the time-cycles depends upon the operating conditions, but in ANFD there will be uniform mixing and we can apply heat through three areas i.e., Jacket, Heating coil over the agitator shaft & bottom dish.

    Being an Engineer / Operator have you ever tried reducing the time-cycle of drying ?
    If yes, then whats the result ?
    Whether the try resulted in positive result or negative result ?

    If positive, then skip this post,
    If negative, then go through the post.

    Before going deep inside the post, lets have some terminology,

    What is Heat Transfer ?

    I'm much interested in Thermodynamics, so i'll explain you in terms of predefined  thermodynamic laws, As per second law of thermodynamics, Heat is a form of energy, which flows from body at higher temperature to a body at lower temperature.

    I love thermodynamics, because thermodynamics is a mix of Heat transfer, Mass transfer and Momentum Transfer. And the second reason is Thermodynamics is a nightmare for many of the chemical engineers 😃.

    What are the modes of Heat transfer ?

    There are three modes of heat transfer, 
    1. Conduction - through material contact / medium,
    2. Convection - through fluid contact / medium,
    3. Radiation - without any medium through electromagnetic waves.

    What is Convective drying ?

    Drying with hot gas which gets in contact to the material which we need to dry is simply Convective drying.

    Examples of Convective drying: Fluidization, Dryer: Fluid bed dryer.

    Whats the drying principle of ANFD ?

    Continuous agitated conductive drying. Lemme explain you clearly, i've mentioned continuous agitated because it is having a Agitator and Conductive because The shell surface acts as a material medium by transferring heat from jacket to inside. Hope it is clear for you now.  

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    Lets get into the topic of reducing drying time cycle of drying in ANFD.

    Usually as per the principle of ANFD, the mode of heat transfer is Conduction. 
    And there will be a filter screen with perforations at the bottom of the dryer.

    ANFD will have comparison with reactor, 

    Vapor column of a reactor is replaced by dust collector,
    Bottom valve is replaced with discharge manhole,
    Condensers remains same,
    Equalization line is located between dust collector and bottom dish, 
    but for reactor in between secondary condenser and receiver.

    In this subject concept, the Equalization line located between the dust collector and bottom dish plays a major role.

    For what the Equalization line is there for ANFD ?

    As  ANFD has an agitator, and during drying the due to continuous agitation some of the vapours which are generated due to heating of material will be suppressed beyond the filter screen and due tot he high volume of material the vacuum cant enter the other side of filter screen. To avoid these kind of issues the Equalization line is added to ANFD setup.

    If any vapours were collected on the other side of filter screen, then automatically the applied vacuum from the bottom will suck the vapours and enhances the drying efficiency.

    The dotted line indicates the Equalization line.

    Now, lets introduce Convective drying in the ANFD which is somewhat innovative, and can enhance the drying rate by 20-30%.

    The same ANFD, shall undergo some minor physical changes, check it below:
    If you can clearly observe, i've added a nitrogen tapping to the equalization line through a flow meter for purging(i made it clear, its purging not flushing).

    You may also like:
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    And the nitrogen used for the purging process should be pure, i.e., free from moisture.

    And a steam heater setup should be arranged in the nitrogen line, it can be a plate type ehat exchanges with a control valve setup, as shown below:
    The Nitrogen inlet temperature should be ~10% less than that of the drier outlet temperature.

    As the hot nitrogen is entering from bottom then automatically it resembles the fluidization process and from top vacuum will suck the entering nitrogen, then by this action the material which wont get in contact with hot nitrogen will get dried. 

    And the entering nitrogen flow shall be regulated through the flow meter instead of flushing. If flushing done then automatically the nitrogen will lift the material from the bottom screen and vacuum will suck the material automatically. 

    If there is any operational range that need to be maintained then trial and error method should be implemented for the nitrogen flow against the vacuum with material inside the drier, so that we can establish a healthy relation between vacuum and nitrogen without much material loss.

    There is no much calculation included in this theory but just some feel that can make a huge difference during drying ANFD.

    If it is clear thumbs up, 

    Hope everyone understand, if any doubts feel free to 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. Dear Ajay.. why u choose nitrogen tapping on equilization line? I thing the entering hot nitrogen should be sucked by vacuum then its creates vacuum down in the ANFD. I have the why u preferred 10% below heated nitrogen? Please clarify me

      1. Dear,

        during continuous flow of nitrogen there may be suction in the bottom dish but not vacuum, and as like mentioned above there is every need to maintain the continuous flow of nitrogen by trial and error method by adjusting the rotameter flow.

        Coming to 10%, usually in some manufacturing companies the outlet of ANFD shall be monitored, but if thats the case we need to maintain the temperature ~10% less than that of outlet so that the material wont get degrade,

        If the material temperature is monitored than there is nothing like ~10%, simply we can maintain the material temperature.

        Ajay K

    2. Dear Ajay,
      In convective drying, why can't we use vacuum from bottom & Nitrogen/Hot air supply from Top,
      impact: reduce product loss through cyclone seperator

      1. Dear,

        Usually using a vacuum balancing line we will be regularly applying vacuum from bottom, but purging from bottom through the voids spaces makes the convective drying different.

        And also the drying time cycles will be high and the clearance in the bottom dish shall be low, so applying vacuum from bottom and nitrogen/air from top makes no sense.

        Also i've mentioned nitrogen purging from bottom to the ANFD instead of flushing, that to purging shall be based on the trial and error method to avoid material escaping through the dust collector/Cyclone separator some times due to high vacuum.

        AJAY K

    3. Dear Ajay,
      1) In ANFD for convective drying is it possible for High viscous filtered materials and can we do scaleup..?.
      2)Is the principle in FBD can apply here in ANFD..?
      3) in this case the material will expose to atmosphere (which is opposite to ANFD principle).
      4) In this convective process type, the solvent traces from the bottom may trapped to the material which is in between shaft clearance and may effect purity also.... can u explain..?

    4. What sort of materials nature's can be filtered through anfd

      1. Dear Karthik,

        Materials having high density and free from gummy nature can be filtered through ANFD.

        AJAY K

    5. Can explain if some times in commercial batches residual solvents are not complies means then wt are possible methods remove it

      1. Improper drying, improper suck dry/press dry after filtration.

    6. Can u explain any bottle neck problems that u handled

      1. I've faced some bottlenecks, and i don't want to discuss those.

    7. Normally y achnor type agitation is in cryallation ares y

      1. As per my knowledge, there wont be anything like that, you can go with paddle, PBT, whatever, but with low RPM, i mean the agitation scale should be mild, it shouldn't disturb the crystal growth.

    8. can you u explain any trouble shootings issues have sortouted till date now

      1. Dear,

        Yeah, but i cant list out the products here.

        Next time pl comment with your good name.

        Best Regards,
        AJAY K

      2. Hi ajay,
        1. what can be the maximum batch size for 2kl ANFD with material having 0.690 specific gravity.

      3. Dear Allu,

        First of all, filtration trial need to be taken in LAB and considering the cake height in the lab we need to match it with plant, so, let the cake height be 20 cms in 2KL ANFD.
        20 cms in 2 KL ANFD would be 400 L (2 x 0.2 = 0.4 Cu.m = 400 L).
        as you have mentioned BD is 0.69, the the wet cake can be 400 x 0.69 = 276 Kgs.
        The operational wet cake would be 276 Kgs, and based on this the batch size can be fixed.

        Best Regards,
        AJAY K

    9. Dear Ajay,
      Please let me know, calculation for drying time in ANFD with 30% occupancy of wet material.
      Initial MC: 25%
      Final MC: 5%


      1. Dear Khan,

        There is no direct calculation for drying, as the time is majorly attributed by falling rate period, but its not impossible to do, i'll demonstrate you in my next post.

        AJAY K


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