Density skimmers

Design of an underflow structure requires the knowledge that the water table will not drop below the bottom of the baffle system. If the seasonal water table fluctuation is not known, use another approach. 

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Density skimmers are a special type of bailer that are designed to remove LNAPL from the water surface in wells (Figure 7.4). Most of these circular skimmers are constructed with a hydrophobic (water-repelling) membrane around the intake area. When installed in a well (or other structure), such that the membrane is at or slightly below the water surface, the LNAPL seeps into the storage compartment. Periodically, the bailer is manually retrieved and LNAPL is recovered. Density skimmers are very useful at locations that have slow recovery and are in remote locations. A properly designed skimmer unit can remove LNAPL down to a thickness of /x in. (0.01 ft or 3 mm). 

Black Liquor Soap Recovery. Black Hquor soap consists of the sodium salts of the resin and fatty acids with small amounts of unsaponifiables. The soap is most easily separated from the black Hquor by skimming at an intermediate stage, when the black Hquor is evaporated to 25% soHds (7). At this soHds level, the soap rises in the skimmer at a rate of 0.76 m/h. At higher soHds concentrations, the tall oil soap is less soluble, but higher viscosity lowers the soap rise rate and increases the necessary residence times in the soap skimmer beyond 3—4 hours. The time required for soap recovery can be reduced by installing baffles, by the use of chemical flocculants (8,9), and by air injection into the suction side of the soap skimmer feed pump. Soap density is controUed by the rate of air injection. Optimum results (70% skimmer efficiency) are obtained at a soap density of 0.84 kg/L (7 lb/gal). This soap has a minimum residual black Hquor content of 15% (10—12). 

In a DAF thickening process, air is added at pressures in excess of atmospheric pressure (2.1 to 4.9 kg/cm1 2 3 4 30 to 70psig) either to the incoming sludge stream or to a separate liquid stream. When the pressure is reduced and turbulence is created, air in excess of that required for saturation at atmospheric pressure leaves the solution as very small bubbles of 50 to 100 pm in diameter. The bubbles adhere to the suspended particles or become enmeshed in the solids matrix. As the average density of the solids-air aggregate is less than that of water, the agglomerate floats to the surface. The floated solids build to a depth of several inches at the water surface. Skimmers continuously remove the float.58... 

A minimum residence time of 10 to 30 minutes should be provided to assure that surges do not upset the system and to provide for some coalescence. As discussed previously, potential benefits of providing more residence time probably will not be cost efficient beyond this point. Skimmers with large residence times require baffles to attempt to distribute flow and eliminate short-circuiting. Tracer studies have shown dial skniimei tanks, even those with carefully designed spreaders and baffles, exhibit poor flow behavior and short-circuiting This is probably due to density and temper-atuie differences, deposition of solids, corrosion of spreaders. etc... 

Od/water separation is enhanced because both the particle size and density difference have been increased The net result is a higher velocity rise rate for the froth which is continuously removed from the surface of the water by a skimmer arm. 

Vortex or centrifugal skimmers use the difference in density between oil and water to separate them and then selectively remove most of the oil. A water whirlpool or vortex is created that draws the oil into a collection area where it is rotated in a centrifuge, using either the force of the water itself or an external power source. The oil is forced to the centre by the heavier water and pumped from the chamber. These skimmers are not used often, however, because they are susceptible to debris. They... 

Figure 11.3 schematically shows the experimental setup. The approach consists of a pulsed nozzle, skimmer, and reflectron TOF mass analyzer. The 125 nm VUV beam was introduced along the jet axis and focus in the ionization volume. A doubled NdiYAG laser (532 nm) was used for desorption, with pulse energy densities of 10 to 100 mj /cm and a spot size of about 1 mm diameter. Vaporization was caused by substrate heating only. Optimal injection of the vapor into the jet expansion occurred when the desorption spot was about 1 mm in front of and 0.5 mm below the nozzle, and the desorption laser was fired near the peak of the jet gas pulse. The pulsed valve was operated with Ar or Xe at a backing pressure of 8 atmospheres. 

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