Thermal Desalination Output Affected by Chlorine Dosing

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We have Thermal Desalination units working on a multi-effect principle, 8.33MIGD capacity. Whenever a chlorine is dosed to control biofouling, we see a drop in distillate output.

Can anyone explain why this happens? We suspect change in surface tension of sea water that affects flashing.

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3 Answers

  1. Dear Solomon,

    I am a licensor for MED desalination technology and I never experimented that missing production in any of my licensed units.

    I cannot believe in either changes in the surface tension nor in the formation of complex moleculas as envisaged by McGowen, such to affect the production capacity.

    Most probably the free chlorine stripped by vacuum cannot be vented by your vacuum system because not sufficiently overdesigned. The excess of non-vented NCG causes a significant reduction of the working delta T and of the production accordingly.

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  2. Dear Solomon,  17 Oct 2017

    From your mention of "flashing" it seems you have a MSF system rather than ME?  Can you describe what are the effects that you observe on flashing?   What other observations have you recorded or noticed - e.g. greater steam consumption and reduced brine heater output temperatures -  that accompany the observed lower distillate output?  If the chlorine application that you practice is done to control growth of foulants in the intake system (please clarify) then I think it is possible that you are not only degrading but also dislodging some of the accumulated and attached biofouling material.  This probably fouls the heat transfer surfaces in heat exchangers and condensing tubes, consequently affecting  distillate outputs.  This might occur intermittently.   Fine particulate material is known to cause “foaming” effects in many processes and surfactants could perhaps be released.  Can you observe effects like foaming etc. in the lab scale?  If biological attachment and growths in the intake system and in the entire system are kept under tighter control, intermittent bio particulate releases might be reduced?  Perhaps your chlorination approach can be studied and adjusted?  I hope this suggestion is of some interest and value to you.  It would be good to learn of any success or further problems! 

    Bruce Hendry Pr Eng,

    Private Consultant,

    Cape Town, South Africa.  

    +27(0)822006898

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  3. Hi Soloman,

    Dependent on how much "chlorine" you use, this may be due to production of a non-condensible gas that is evolved during the thermal degradation of the chlorine.  Non-condensible gases mean that less heat can be recovered via the multiple effects, thus less distillate being produced.  

    The type of non-condensible gas (probably ClO2 but potentially Cl2 or even O2) and reaction pathway is dependent on the pH/composition of the solution, the presence of certain metals, temperature and the type of chlorine being used (hypochlorite, ClO2 etc).  

    Cheers,

    Matt.

     

    Dr Matthew Brannock  CEng, CSci, MIChemE, RPEQ

    matthew@​​​saltwatersolutions​​​​​.​com.au ​ ​​

    www.​​saltwatersolutions​​​.​com.au  ​​

    www.aqmb.net 

     

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