Experimental Investigations On Pressure Field And Microfiltation Mechanism Of An Outside Helical Flow Module

Outside helical flow microfiltration assembly is a new kind of equipment by use of centrifugal force field to enhance permeate flux. Suspensions to be separated flow into the assembly in the tangential direction, forming high-speed rotating cross flow in the annular chamber between the membrane tube and the shell of this module, which could effectively weaken the negative influence of concentration polarization and membrane fouling. In this thesis, the fluid pressure field in the module was tested for the first time. Furthermore, systemic investigation of the separation mechanism of the membrane module was made with the measurement of permeate flux.First, a new lab rig was set up, with which pressure tests and flux tests could be carried out simultaneously. By means of changing suspension concentration and operation pressure, the pressure fields and fluxes under different operation conditions were probed. In addition, in the same operation circumstances, the comparison tests of cross-flow microfiltration flux in two different flow patterns (outside helical flow and outside axial flow) were completed.Secondly, the experimental data were analyzed and the pressure profiles in longitudinal half-section of the annular chamber were discovered. The pressure profile presents "saddle-like" distributions in radial direction, that is, pressures close to the membrane tube and inner side of the shell are relatively greater. On the other hand, pressures at a given radius increase from top to end in axial direction, and they descend lightly if the weight influence has been excluded, thus a conception of"maximum effective length for outside helical module" was put forward. There are fluctuations of pressure in both radial and axial directions due to the large eddies like Taylor vortexes in the longitudinal section. It is also found that the increase of operation pressure and concentration of inlet flow would result in the rise of pressure field, but the rising slope decreases gradually.Thirdly, two new mathematical models of permeate flux were presented. One is an empirical equation for steady state flux, in which influence factors have been summed up into some important dimensionless numbers like Reg and Eu to obtain a feasible formula that provides basis for scale-up of the module. Another is a power function equation for the dynamic attenuation of flux, which shows the real varying process of flux with time and fits the experimental data better than the existed linear one. The deviations between the data predicted in these two equations and the experimental ones are less than 13.9% and 30%, respectively.Finally, for given flux and pressure drop, the critical particle sizes were determined by analyzing motion of particles and forces exerted on them in rotational flow field. Particles larger than the critical size could not enter the boundary on the membrane surface or could not deposit in the boundary. Furthermore, the formula of fouling resistance Rf was derived which depends on the pressure along membrane surface and permeate flux. It is found, based on Rf calculated under different operation conditions, pressure distribution measured on membrane surface, and flux tested, that increasing suspension concentration or operation pressure would lead to raise both pressure field within the assembly and Rf, however, due to their different changing slope, the permeate flux would drop sharply with increasing inlet flow concentration and rise slowly with increasing operation pressure.In sum, the outside helical flow microfiltration could effectively make permeation flux larger than traditional cross-flow filtration and be especially feasible to separate suspensions thinner than 0.05%(wt.) under relatively low operation pressure.