This sounds like an unusual application, especially with the pipe size.  The normal operating pressure could be an advantage or disadvantage. The simple solution would be either an atmospheric standpipe or expansion leg.

With a closed circulation loop, at some point near the center of the loop the discharge pressure and the suction pressure should be only slightly positive. At that point, it should be possible to provide a means for some expansion volume.

Getting anything to mix in a 10mm diameter pipe will be difficult since you will be in laminar flow with very little axial mixing.

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