Modelling And Simulation Of The Fluid Distribution In The Hollow Fiber Membrane Modules

Non-ideal performance of the hollow fiber membrane module is identified that arises from the non-uniform fluid distribution, which is a result of the special module structure. Theoretical models have been developed to characterize the fluid flow and mass transfer both in the fiber- and module-scales. Relationship between the fluid distribution and module performance has been investigated through computational fluid dynamics (CFD) and supplemental experiment. Several distribution technologies have been developed to enhance the membrame process. On basis of the simulation data, the correlation equation of water production has been obtained through curve-fitting. Simulation of the entire process in the large-scale hollow fiber membrane module has been achieved. Relationship has been established between the microscale flow field and the module performance, which enables better design and optimization of the module. Three aspects of this research are presented as follows:(1) Uniformity of the flux distribution and energy utilization of single fiberThe analytical expression of fiber flux distribution has been established. Based on the analytical model, three non-dimensional numbers have been proposed to determine the location of maximum local flux, and characterize the uniformity of flux distribution and energy utilization, respectively. CFD model has been developed to simulate the dead-end outside-in hollow fiber membrane filtration process in clean water system. How the uniformity of flux distribution and energy utilization change with the geometrical parameters has been revealed by CFD simulation. A decrease in the uniformity of flux distribution caused by a change in one of the geometrical parameters will always accompany an increase in the energy utilization. To avoid deterioration of the flux distribution and energy utilization, the following parameters has been recommended for the fiber:a length shorter than 2 m, an internal diameter wider than 0.4 mm and a shell void fraction larger than 0.4. According to theoretical deduction, fibers that have the parabolic-shape membrame resistance will theoretically achieve a completely uniform initial flux distribution.(2) Structure optimization of the external cartridgeCFD simulation and experiment have been performed to investigate the fluid distribution inside the empty external cartridge and the proportion of the enegy consumption of every individual part of the cartridge. The uniformity of fluid distribution is independent of the volumetric flow rate. However, the energy consumption ratio of every individual part to the whole cartridge changes with the volumetric flow rate. The inertial loss consumes approximately half of the total energy input of the cartridge. Three-dimentional CFD model has been developed to simulate the dead-end outside-in filftration in the hollow fiber membrane module fed with clean water. The effect of the inlet manifold has been studied numerically on the energy consumption, fluid distribution in the shell and lumen sides, and flux distribution of the module. Conclusions can be drawn that pressure loss caused by the inlet manifold results in a higher water-production performance of single fiber than the one of the module. A higher fractional hole area of the inlet manifold causes less inlet pressure loss. The non-uniform flow from the inlet manifold causes non-uniform flow in the shell side, whereas the fluid distribution in the lumen side is hardly affected by the manifold. Non-uniformity of the flux distribution exists both in the axial and radial directions. With given fibers, the key method of improving the uniformity of the flux distribution is to keep uniform fluid distribution of the shell side. At last, a new pattern of inlet manifold has been designed.(3) Numerical simulation of the waste water filtration in the large-scale hollow fiber membrane moduleBased on the mechanism of cake formation during membrane filtration, a CFD model has been built to simulate the dead-end outside-in filtration in the hollow fiber membrane module. Various fiber length, diameter, permeability, packing density, fouling index, and trans-membrane pressure have been chosen during numerical simulation of the dynamic evolution of flux distribution and permeate volumetric flow rate. The simulation reveals that the uniformity of flux distribution improves as the filtration processes. The self-adjustment of the flux distribution is more pronounced with longer, narrower, more permeable fibers, higher packing density, fouling index and trans-membrane pressure. The inverse of the water volumetric flow rate increases linearly with the accumulated volume of the permeate. Due to the non-uniformity of the flux distribution and its dynamic evolution, the linear relationship differs from the one presented in the classic cake filtration model. A correlation equation to characterize the dead-end outside-in cake filtration in the hollow fiber membrane module is obtained through curve fitting of the simulation data. Next, CFD simulation has been performed to investigate the waste water filtration in the large-scale hollow fiber membrane module which is equipped with the newly-designed inlet manifold. Conclusions can be drawn that the uniformity of the shell side flow is nearly constant as filtration processes. Therefore, proper design of the inlet manifold determines the uniformity of the shell side fluid distribution in the entire filtration phase. In the constant-pressure operating mode, the inlet pressure loss decreases as filtration processes. Inter-adjustment with time between the flux and cake spatial distribution occurs in the module. The improvement in the uniformity of the cake distribution with time is less pronounced than the one of the flux distribution because the adjustment effect of the cake on the flux distribution is a result of the cake accumulation, whereas the adjustment effect of the flux on the cake distribution depends only on the flux of that moment. The inverse of the water volumetric flow rate still increases linearly with the accumulated volume of the permeate in the large-scale module. Although the inlet pressure loss decreases with time, the aforementioned correlation equation can describe the linear-relationship in the module. With simple experiment, the correlation equation enables the direct monitor of the transient inlet pressure loss.