Bottom spray film

Bottom-spray film coating is accomplished by means of the Wurster system, originally developed by Dr. D.E. Wurster (Wurster 1959). A scheme of the bottom-spray fluid bed is given in Figure 16.4. 

Fig. 9. A bottom spray (Wurster) film coating, surface and...

In a spray-coating process, an inert soluble core composed of sugars, salts, polymers, or a combination of these are flnidized within a fluid bed and an enzyme solution is sprayed on the core to make a continuous film on the surface. The process can be carried out either in a top spray mode, that is, where the spray nozzles are placed in the top of the fluid bed chamber or as a bottom spray... 

Another variant (the cone spray) allows the sample solution to flow down the sides of an inverted cone and through a hole at the bottom of which flows a fast stream of argon gas. As the liquid film meets the gas, it is ripped apart into a finely dispersed aerosol (Figure 19.15). 

In the spray tower, the gas enters at the bottom and the liquid is introduced through a series of sprays at the top. The performance of these units is generally rather poor, because the droplets tend to coalesce after they have fallen through a few metres, and the interfacial surface is thereby seriously reduced. Although there is considerable turbulence in the gas phase, there is little circulation of the liquid within the drops, and the resistance of the equivalent liquid film tends to be high. Spray towers are therefore useful only where the... 

Its formation is accompanied by the generation of a spray, resulting from the vibrations at the liquid surface and cavitation at the liquid-gas interface. The quantity of spray is a function of the intensity. Ultrasonic atomization is accomplished using an appropriate transducer made of PZT located at the bottom of the liquid container. A 500-1000 kHz transducer is generally adequate. The atomized spray which goes up in a column fixed to the liquid container is deposited onto a suitable solid substrate and then heat treated to obtain the film of the material concerned. The flow rate of the spray is controlled by the flow rate of air or any other gas. The liquid is heated to some extent, but its vaporization should be avoided. 

The monoglycerides of the raffinate (the bottom product) is shown in Figure 9 as a function of the superficial velocity of the gas phase at a phase ratio of 19. At an superficial velocity of 10 mm/s the raffinate obtained with different packings is nearly the same except for the Sulzer Packing SMV. At lower superficial velocities the wire mesh packings (Sulzer CY) provide the best yields. The experiments are made at conditions where a falling film disintegrates into drops. Therefore, it seems understandable that the efficiency of the spray column and that of the collector-distributor installations do not much differ. 

Spln-on and Cure Procedure. The spln-on applications of SOG were carried out with a manual photoresist spinner (Headway, Carrolton, Texas) under a laminar flow hood. In order to minimize the splash back problem, spray of the substrate by the mist or droplets formed by the SOG solution thrown off against the spinner bowl wall, it was necessary to apply a downward exhaust through the bottom of the bowl. The films were spun on 3 in. or 4 in. diameter single-crystal silicon wafers using an acceleration of 20,000 rpm/sec and a spin-time of 20 sec. at the desired speed. For most of the characterizations, the SOG film was cured at 100 C for 15 min. followed by 400°C for 60 min. in ambient air. 

In gas-liquid spray towers the liquid is atomized and enters as a fine spray at the top and the gas is introduced at the bottom. The gas flow rate has to be kept sufficiently low to permit the liquid to fall. It is generally chosen in such way that the liquid drops of mean diameter fall at 20 percent of their free-fall velocity, as calculated from Stokes law. An efficient dispersion of the liquid requires the openings of the distributor to be small and the pressure high. Thereby a fraction of the drops hits the wall and flows down the wall as a film. Furthermore, a certain degree of coalescence of the drops is inevitable, so that the drop size, velocity, and therefore residence time vary strongly with position. A rigorous hydrodynamic analysis of such a situation is extremely complicated so that only the overall behavior has been studied. 

Fig. 4.7 Hematite films created by USP show a platelet morphology by cross-sectional SEM analysis (top, fi om [101]), and an increased IPCE at longer wavelengths compared to films created by conventional spray pyrolysis (bottom, from [94], used with permission)...

In packed columns, liquid reflux flows as a falling film, or as a streamlet, from top to bottom counterflow to the upflowing vapor. Both liquid and vapor phases are in continual contact (Fig. 2-5 8 b and c). Mass and heat transfer occur at the inside and outside surfaces of the randomly packed filling material or the arranged packing elements in reflux film. The exchange area is the surface area. In the case of spraypack fabrics, the reflux liquid is sprayed. The contact area is the total surface area of the liquid droplets. 

A butyl rubber sheet, or high temperature plastic film, is held taut aeross two vertieally stacked metal frames or rings, which are themselves sealed between the two platens of a press. The requisite number of prepreg plies is formed into a preform, which is sandwiched between sheets of PTFE coated glass cloth, or a non-perforated PTFE film with a perforated PTFE film next to the preform, and the paek placed on a steel plate sprayed with a fluorocarbon release agent and positioned on the bottom heated platen of the press. The preform... 

Some major applications are thickener in paint industry paper coating, hair sprays shampoos adhesives biodegradable products for the feminine hygiene diapers bottoms water soluble packaging films (detergents, disinfectants, scouring powder, pesticides, fertilizer, laundry, etc.) fibers for concrete reinforcement lubricant for eye drops and contact lenses and material for chemical resistant gloves. 

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