in order to improve mixing efficiency of highly viscous fluid , while structural simplicity being maintained , two types of stirring impellers are employed in a single stirring apparatus . one of such stirring impellers is a large flat impeller and the other is a slanted or screw - shaped impeller to cause an up - and - down flow . this principle is further utilized in a stirring tower type polymerization reaction apparatus which has an array of mixing areas , each of which corresponds to the stirring apparatus above , and partitions between the mixing areas . the partitions are disposed so that the temperature of reaction can be controlled easily . undesirable effects such as &# 34 ; dead space &# 34 ;, space in which flow is insufficient , and the attachment of gelled material to the rotational shaft can be avoided while rather obvious , but no less important , advantages of efficient and uniform mixing , etc ., are secured .