Crossflow ultrafiltration (UF) is designed to continuously purify the CNC product. Removal of small molecules can be conducted in a diafiltration mode , where water is continuously added to wash away small molecules, namely sugars and acid. The diafiltration mode commonly keeps a constant retentate volume . A polysulfone (PS) membrane was selected since PS is resistant to acidic pH. Sartorius AG suggests that molecular weight cut off (MWCO) of the selected membrane should typically be 1/3 ~ 1/2 of the retained molecules to balance the flux rate and recovery rate . As molecular weight of CNC is ~ 30 kDa, we select a PS membrane with a 10 kDa MWCO. With this MWCO, we assume that CNC is fully retained (retention coefficient of CNC is 1), while the small molecules can freely move through the PS membrane (their retention coefficients are 0). Existing pilot plants suggest that CNC concentration should be kept at 1 wt.%, since a higher CNC concentration can dramatically decrease the flux .