I am working with body wash products and there is a problem happening with the mixing process. I use a 5000-liter tank to mix all the liquid materials including water, surfactant and preservatives. When I use a vacuum pump to add liquid materials from the bottom of tank, the content of preservative is A (g/100g). However, when I add them throug hthe top of tank, the content of preservative decreases. I am really confused because everything (amount of materials, equipment, machines) used in 2 ways is the same. And I have no idea why the content decreases. Please share your opinion. Thank you.

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    $\begingroup$ Is there any stirring of the tank or do the contents just get to settle? $\endgroup$
    – Solar Mike
    Aug 25 '19 at 13:19
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    $\begingroup$ Where are your samples being taken from? This sounds like a non-homogenous mixture and your samples will vary as a result. $\endgroup$
    – user16
    Aug 25 '19 at 15:43
  • $\begingroup$ @SolarMike: Yes, we keep stirring the solution during adding phenoxyethanol. $\endgroup$
    – Hong
    Aug 28 '19 at 14:57
  • $\begingroup$ @GlenH7: we took 3 samples from the top, the middle and the bottom of tank. And the contents of phenoxyethanol of those 3 samples is almost the same. $\endgroup$
    – Hong
    Aug 28 '19 at 15:03
  • $\begingroup$ can you provide a sketch of your setup with the tank, the pump and the inlets (top and bottom)? $\endgroup$
    – mart
    Sep 25 '19 at 11:18

It sounds like your additives are in suspension not solution. Like a fruit juice drink. You take orange powder stir it and it makes a nice drink. Leave it several hours without drinking it and you will return to a glass of water with orange stuff at the bottom. If the solution is not continually stirred the additives will sink to the bottom.

  • $\begingroup$ we took 3 samples from the top, the middle and the bottom of tank. And the contents of phenoxyethanol of those 3 samples is almost the same. I do not think that the solution is not homogenized. $\endgroup$
    – Hong
    Aug 28 '19 at 15:09
  • $\begingroup$ Now I am confused your problem was that there was more at the bottom than the top but now you say they are the same? $\endgroup$
    – Brad
    Aug 28 '19 at 15:14
  • $\begingroup$ Hi, let me clear about that. First, I add all the materials including phenoxyethanol to the tank through a valve at the bottom, and I took 3 samples. And then, I change the way, add all materials through a valve at the top of tank and I also took 3 samples. I measured all 6 samples. The results showed that with 2 different ways of adding ingredients, the content of phenoxyethanol was different. To be more specific, the content of samples in the first way is higher than that in the second way. My English is not so good. I hope you are clear now $\endgroup$
    – Hong
    Aug 28 '19 at 15:31
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    $\begingroup$ I suppose you have checked that the missing phenoxyethanol when added at the top is not forming an oily top layer. I presume your tank has a majority water mix and phenoxyethanol is only QUOTE slightly soluble in water. Possibly your are getting a better mix when adding from the bottom where it is not so easily able to seperate. Also it can degrade when it is exposed to oxygen memim.com/phenoxyethanol.html $\endgroup$
    – Brad
    Aug 28 '19 at 16:05

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