Dynamic operability analysis is applied to an industrial whey ultrafiltration process with 12 stages to determine the best feedback control structure. Feedback control is used to maintain the properties of the product stream within their desired ranges. The process is inherently difficult to control because of the extensive recycling of retentate and permeate streams. Using all-pass factorization of the process dynamics models and the Internal Model Control framework, the best achievable closed-loop performances of mitigating the effects from variations in the flowrate and compositions of whey from different control system designs are determined. The best control performance can be achieved by manipulating the recycle flowrate at the 12th stage. The control performance is further improved after a buffer tank is installed upstream of the process. However, control performance will deteriorate with operating time as a result of long-term membrane fouling. When necessary, membrane washing in the middle of process operation should be used to restore control performance. (C) 2011 Elsevier Ltd. All rights reserved.