Recirculation rate, definition

For the optimisation work, the deposition time was increased to 24 h, which would normally give a good indication of the performance at the industrial scale. The previous testing period was so short that the effect of additives could not be properly demonstrated. The level of additives was then included in the optimisation work. Current density was definitely to be tested on a 24-h time period, considering the major impact it had on all responses. A new factor, not tested before, was the catholyte recirculation rate. A new series of optimisation tests with a five level central composite design of experiments was initiated to obtain more precise results on the effects of four factors ([HCl], [TBACl], current density and catholyte recirculation flowrate) on zinc electrowinning. The temperature was fixed at 35°C, the concentration of zinc chloride in solution was maintained at 50 g/L and the concentration of sodium chloride was fixed at 0.25 M. 

Next, we define a parallel set of NPD function in continuous flow recirculating systems. We restrict our discussion to steady flow systems. Here, as in the case of RTD, we distinguish between external and internal NPD functions. We define fk and 4 as the fraction of exiting volumetric flow rate and the fraction of material volume, respectively, that have experienced exactly k passages in the specified region of the system. The respective cumulative distribution functions, and /, the means of the distributions, the variances, and the moments of distributions, parallel the definitions given for the batch system. 

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