Rotation-induced Taylor vortex enhances filtrate flux in plasma separation

Techniques of plasma filtration with rotating flat membranes are capable of producing a Taylor vortex and concomitantly reducing concentration polarization by formed elements. The present study describes filtration characteristics of a small-sized rotating filter that is superior to a capillary filter in terms of filtrate flux. The rotating filter with a surface area of 60 cm2 was composed of two concentric cylinders, with the inner cylinder mobile, with a flat polycarbonate membrane (pore diameter: 0.8 micron), and the outer cylinder fixed. Filtration experiments were performed using bovine blood to determine filtrate flux at rotational speeds from 600 to 4,500 rpm. Photographic observations using aluminum powder revealed that many Taylor vortices were present in the radial gap (0.6 mm). Filtrate flux increased with transmembrane pressure until a maximum filtrate flux of 0.02 cm/min at 600 rpm, 0.42 cm/min at 1,800 rpm, 0.60 cm/min at 3,600 rpm, and 0.69 cm/min at 4,500 rpm were reached at a blood flow rate of 100 ml/min. Based on the filtration resistance model, values for fractional filtration resistance of the red blood cell polarization layer ranged from 0.25 to 0.80 at a rotational speed of 3,600 rpm, whereas those of a capillary filter range from 0.80 to 0.90. This indicates that Taylor vortices promote back diffusion of red blood cells accumulated on membrane surfaces. Thus, the filtrate flux of a rotating filter is two orders of magnitude higher than that of a capillary filter because of the back diffusion of red blood cells facilitated by Taylor vortices.