Tubular modules flow velocity

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

The following data are obtained for an ultiafiltration experiment with an oil emulsion in a tubular module with an internal diameter of 17 mm and a length of 0.5 m at 25°C. The applied pressure is 6 bars and the cross flow velocity is 4 m/s. The viscosity of the emulsion q = 1.5.10- Pa.s and the density is equal to the density of water (1 gfml). The emulsion droplets have a diameter of 0.2 (tm. 

In the side-stream configuration, the MLSS is pumped through the membrane module. Side-stream systems typically use tubular membranes. Fouling is controlled by a well-defined flow velocity in the range of l-4m/s, generating a turbulent crossflow. Figure 9.7 shows a schematic view of a side-stream system. 

The concept of cross-flow microfiltration is shown in Figure 16.11, which represents a cross-section through a rectangular or tubular membrane module. The particle-containing fluid to be filtered is pumped at a velocity in the range 1-8 m/s parallel to the face of the membrane and with a pressure difference of 0.1-0.5 MN/m2 (MPa) across the membrane. The liquid penneates through the membrane and the feed emerges in a more concentrated form at the exit of the module.1617 All of the membrane processes are listed in Table 16.2. Membrane processes are operated with such a cross-flow of the process feed. 

As a consequence of the permeate flux, the pressure gradient as well as the mean velocity and concentration will vary along the membrane surface-in principle at the feed side as well as at the product side. Depending on module design, this has to be taken into account for one or both sides. If, for example, the membrane is of tubular design with feed-flow inside, the following has to be expected along the tube-axis ... 

---