Evaporation control

The topic of spreading rates is of importance in the technology of the use of mono-layers for evaporation control (see Section IV-6) it is also important, in the opposite sense, in the lubrication of fine bearings, as in watches, where it is necessary that the small drop of oil remain in place and not be dissipated by spreading. Zisman and coworkers have found that spreading rates can be enhanced or reduced by the presence of small amounts of impurities in particular, strongly adsorbed surfactants can form a film over which the oil will not spread [48]. 

A surfactant for evaporation control has an equilibrium film pressure of 15 dyn/cm. Assume a water surface and 25°C and calculate the distance traveled by the spreading film in 8 sec. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

The two liter carbon canister does not exhibit the HC release during the run loss portion of the test, nor does it release more than the allowable level of HC during the three day diumals. Thus, for the given vehicle configuration and the level of purge volume obtained by the vehicle, it is clear that a two liter carbon canister is required for this vehicle to pass the EPA certification requirement. This conclusion has an effect on the cost of the evaporative control system, in that the additional activated carbon volume and canister size will have an added cost, as will any additional hardware required to mount the larger canister on the vehicle. 

The reduced adsorption capacity at ORVR vapor generation rates requires increased efficiency in the canister design, in order to limit the effect on cost and performance of the evaporative control system. 

Gibaud, A. Grosso, D. Smarsly, B. Baptiste, A. Bardeau, J. F. Babonneau, F. Doshi, D. A. Chen, Z. Brinker, C. I Sanchez, C. 2003. Evaporation-controlled self-assembly of silica surfactant mesophases. J. Phys. Chem. B 107 6114-6118. 

Mg of dry mass of a non-porous solid is dried under constant drying conditions in an air stream flowing at 0.75 m/s. The area of surface drying is 55 m2. If the initial rate of drying is 0.3 g/m2s, how long will it take to dry the material from 0.15 to 0.025 kg water/kg dry solid The critical moisture content of the material may be taken as 0.125 kg water/kg dry solid. If the air velocity were increased to 4.0 m/s, what would be the anticipated saving in time if the process were surface-evaporation controlled ... 

Sewage disposal Soil conditioning Sugar refining Water clarification Water evaporation control Water repellency Wetting... 

Vanysek, P., Boone, T., Dang, T., Geiger, H., Klapperich, C.M., Fee, H., Nicewamer, D., Kurnik, R., Singh, S., Xiao, V., Zhao, M., Multiwell microfluidic plates for evaporation-controlled sub-microliter assays Design and results. Elec-trochem. Soc. Proc. 2001, 376, 383. 

Figure 4.8 A modification of the Gibbs (1970) plot shows that evaporation-controlled rivers are in arid regions, rock-controlled rivers are in intermediate rainfall areas, and atmospheric-controlled rivers are in high rainfall areas. (Modified from Gibbs, 1970.)...

Hydrocarbon slurries were analyzed by filtration on a tared micropore polymer funnel, using a pentane wash. Water based slurries were analyzed by oven evaporation. Control tests using slurries of known weight percent gave an absolute accuracy within 0.1 for both methods. 

Ozonolysis Procedure. Ten ml of olefin solution were ozonized at 0°C with an oxygen-ozone mixture. The ozone concentration was sufficiently high 4% ) to make the ozonolyses possible with relatively low 02-03 flow rates ( 0.15 mmole 03/min) in order to prevent olefin loss by evaporation. Control experiments, performed with pure 02, indicated that no appreciable amount of olefin is lost by bubbling the gas in the olefin solution in time intervals comparable with those required for the actual experiments. 

Figure7.9 Multiple-effect evaporator controls, (a) Energy flow in same direction as process flow

Overall, on the basis of ionic proportions and total concentrations, only 8.2% of the rivers (in number) in the PRISRI database can be described as evaporation controlled, 2.6% as rain dominated and vegetation controlled, and 89.2% as weathering dominated, including rivers affected by large water inputs. 

Entrainment Separation Some kind of entrainment separation is necessary to reduce losses of glycerine entrained in the vapor stream from each evaporator. Control of the vapor velocity by proper evaporator and duct sizing will minimize the amount of entrainment. An entrainment separator should be installed at the outlet of each evaporator body, and the glycerol recovered should be returned... 

The Effect of Evaporation Conditions on Film Morphology. Two extreme modes of operation were examined evaporation sufficiently rapid to avoid any accumulation of solution on the surface of the KBr disc during the run, and evaporation so mild that liquid accumulated on the disc surface and then evaporated completely. Evaporation control variables included ultrasonic nozzle power, spray temperature, gas flow rate around the disc, oven temperature, and pressure. With accumulation... 

An evaporator provides one stage of separation based on relative volatility. It is typically used with systems having a large relative volatility, such as salts and solvents. When water is being removed as an overhead vapor, multieffect operation may be used to provide improved energy efficiency. Figure 3.14 shows two alternative direct material balance evaporator control schemes. 

The droplet has a relatively short residence time (on the order of seconds) in the spray dryer, which minimizes the degradation of heat-sensitive components. In addition, the drug is exposed to a temperature much lower than that at the drying inlet owing to the cooling effect of the solvent evaporation. Control of droplet residence time and the lower temperature defines the amorphous versus crystalline nature of the material. 

Transfer of water molecules (in evaporation control), transport of solvent across monolayers at oil-water interfaces (in Ostwald-ripening of emulsions) and transfer of ions across such interfaces (as models for ion conduction in bilayers and membranes) can often be treated in terms of surface concentration fluctuations. Their magnitudes can be expressed in standard deviations (cr for the standard deviation in the surface concentration), which are measures of the probability that random holes are formed in the layer, allowing material transport. We have presented the formal treatment in sec. 1.3.7. From this section we can immediately obtain = IcTOr / 0 ), for a Gibbs monolayer, with... 

Use Additives for cosmetic creams, polymerization regulators for elastomers and plastics, detergents and viscosity-index improvers for lubricating oils, finishing and softening agents for textiles, preparation of quaternary ammonium compounds, surfactants, water evaporation control, and antifoam. 

Air velocity No significant effect High air velocities Evaporation-controlled... 

---