Rob
Ansys Employee

There may be a mixing tank tutorial somewhere, but I can't see one for single phase on the public side of our system. Typically, we don't have tutorials for every application but use parts of a few where needed. So, 2d to 3d is trivial; use patch rather than an inlet/outlet and review the chemistry to just look at mixing. 

There is a webinar  https://www.ansys.com/en-gb/resource-center/webinar/mixing-tank-modeling  which may be of interest.