A process comprising polymerizing an olefin monomer optionally together
with an olefin comonomer in the presence of a polymerization catalyst in
a diluent in a loop reactor which comprises at least 2 horizontal
sections and at least 2 vertical sections to produce a slurry comprising
solid particulate olefin polymer and the diluent wherein the Froude
number in at least 20% of the length of the vertical sections of the
reactor loop is less than 85% of the Froude number in at least 20% of the
length of the horizontal sections of the loop is disclosed.