Flocculation is achieved in a batch system by stirring a destabilized suspension at

a velocity gradient of $5$ sec$-1$ for $65$ minutes. The number of particles in the

suspension is reduced by $80\%.$

a$)$ To produce the same degree of aggregation using the same velocity gradient,

what detention time is required if a single CSTR is used?

b$)$ What total detention time is required if four CSTRs$,$ each having the same

volume, are used in series?