In the manufacture of instant coffee, a series of around sit; fixed beds of ground coffee are connected to form a diffusion battery to extract the soluble solids. As the coffee beds become successively exhausted, they are replaced on a regular cycle. Simulating an extraction bed in a flowsheet is itself straightforward but the periodic bed changes are difficult to represent using most commercial simulators. We show how a logical stream switching module coupled with the ability to represent "delayed" time;step values overcomes the problem. A mathematical model of a diffusion battery is presented, which describes the unsteady mass transfer in the fixed beds. The discretised model equations are solved in a flowsheet using zone and hierarchical modelling techniques which are considered especially useful in food process operations. Results are presented showing the expected variations in fluid concentration with time and position during normal operation. We show how the technique can be extended to represent similar cyclic systems such as ion-exchange, including the regeneration part of the cycle.