Diffusivities in water

A = 4.05 X lO " cm/(s-kPa)(4.1 X 10 cm/(s-atm)) and = 1.3 x 10 cm/s (4)//= 1 mPa-s(=cP), NaCl diffusivity in water = 1.6 x 10 cm /s, and solution density = 1 g/cm . Figure 4 shows typical results of this type of simulation of salt water permeation through an RO membrane. Increasing the Reynolds number in Figure 4a decreases the effect of concentration polarization. The effect of feed flow rate on NaCl rejection is shown in Figure 4b. Because the intrinsic rejection, R = 1 — Cp / defined in terms of the wall concentration, theoretically R should be independent of the Reynolds... 

Early ultrafiltration membranes had thin surface retentive layers with an open structure underneath, as shown in Fig. 20-62. These membranes were prone to defects and showed poor retention and consistency. In part, retention by these membranes would rely on large retained components in the feed that polarize or form a cake layer that plugs defects. Composite membranes have a thin retentive layer cast on top of a microfiltration membrane in one piece. These composites demonstrate consistently high retention and can be integrity-tested by using air diffusion in water. 

Similarly, for diffusion in water a molecular diffusivity of 2 x 10-6 m2/h is reduced by a factor of 20 to an effective diffusivity of 10 7 m2/h, which is combined with a path length of 0.05 m to give an effective soil-to-water mass transfer coefficient of ksw 2 x 10 6 m/h. 

The mess caused by dropping sugar reflects the way nature always seeks to maximize disorder. Both examples so far, of dye diffusing in water and sugar causing a mess, demonstrate the achievement of greater disorder. But if we are specific, we should note how it is the energetic disorder that is maximized spontaneously. 

Diffusivity in water Molecular diffusion is defined as the transport of molecules (e.g., organic compounds) in either liquid or gaseous states. Typically, molecular diffusion is not a major factor under the majority of enviromnental conditions. However, in saturated aquifers with low pore water velocities (i.e., <0.002 cm/sec), diffusion can be a contributing factor in the transport of organic compounds. 

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