Residence Time Distribution in Aseptic Processing Systems

RESIDENCE TIME DISTRIBUTION IN ASEPTIC PROCESSING SYSTEMS 

In flow systems not all fluid and solid food particles remain for the same time periods, that is, the particles have a distribution of residence times. Danckwerts (1953) proposed the concept of residence time distribution (RTD), and the theoretical and experimental principles of RTD have been well reviewed by Levenspiel (1972) and 

Himmelblau and Bischoff (1968). The RTD functions and the role of RTD in continuous pasteurization systems was reviewed (Rao and Loncin, 1974a, 1974b). Here only the necessary principles of RTD are discussed and for additional information the above references must be consulted. 

we note that with few exceptions RTD studies represent lumped parameter analysis. Only in a few instances, such as that of fully developed laminar flow of fluids in tubes, one can attempt distributed parameter analysis in that the expressions for the RTD functions C and F can be derived from known velocity profiles. 

Experimental RTD data on any equipment can be obtained by imposing at the inlet a pulse, step or sinusoidal impulse and monitoring the response at the outlet. The tracer used must have physical properties such as density and viscosity similar to the test fluid food, and it should not in any marmer alter the properties of the test fluid food. The sinusoidal impulse and response require more care and sophisticated equipment than the pulse and the step inputs. The response to a pulse impulse is called a C-curve, while the response to a step change is called a F-curve. For a closed vessel, that is, in which material passes in and out by bulk flow (i.e., no difFusion at inlet and outlet) the Cand F functions are related as  

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