Spray mass distribution

The fundamental issues to be addressed in the process modeling include spray enthalpy, gas consumption, spray mass distribution, microstructure of solidified droplets, and droplet-substrate interactions. The effects of atomization gas chemistry, alloy composition and operation conditions on the resultant droplet properties are also to be investigated in the process modeling. 

Another type of distributor, not shown in Fig. 14-64, is the spray nozzle. It is usually not recommended for hquid distribution for two reasons. First, except for small columns, it is difficult to obtain a uniform spray pattern for the packing. The fuU-cone nozzle type is usually used, with the need for a bank of nozzles in larger columns. When there is more than one nozzle, the problem of overlap or underlap arises. A second reason for not using spray nozzles is their tendency toward entrainment by the gas, especially the smaller droplets in the spray size distribution. However, some mass transfer in the spray can be expected. 

Consumer products require expensive magazine advertisements and television commercials. This can amount to 30% or 40% of the selling price. If a new laundry product is introduced, a large amount of money must be spent to convince supermarkets and/or mass distribution outlets to grant it shelf space. They must be convinced it will sell before they will put it on their shelves. They are only interested in items that will move. In 1968 Colgate introduced its new pre-soak, Axion . In the first 9 months it spent over 3,000,000 for advertisements alone- 640,000 for newspaper ads, 865,000 for network television, 886,000 for spot local radio commercials, 800,000 for spot local television commercials, and 199,000 for magazine ads.8 Airwick spent over 7,000,000 to introduce a new disinfectant-deodorant spray in 1973.7... 

Generally, 3-D models are essential for calculating the radial distributions of spray mass, spray enthalpy, and microstructural characteristics. In some applications, axisymmetry conditions may be assumed, so that 2-D models are adequate. Similarly to normal liquid sprays, the momentum, heat and mass transfer processes between atomization gas and metal droplets may be treated using either an Eulerian or a Lagrangian approach. 

For a stationary spray without scanning, a Gaussian shaped mass distribution typically develops with an annular-jet or discrete-jet atomizer. The radial mass distribution in the spray can be formulated in terms of mass flux)632]... 

The mass flux in the spray scales with liquid metal flow rate. Gas pressure tends to narrow the spray whereas melt superheat tends to flatten the spray)3] By changing the process parameters and/or manipulating the configuration and/or motion of the spray, the mass distribution profile can be tailored to the desired shape. For example, a linear atomizer produces a relatively uniform mass distribution in the spray. The mass flux distribution in the spray generated with a linear atomizer has been proposed to follow the elliptical form of the Gaussian distribution)178]... 

Selectivity of the template effect. The selectivity of the method of synthesis is illustrated by the narrow distribution of the polyanions in solution. Dilute mixtures of M02O2S2 and carboxylic acid at pH = 4.5 have been examined by Electro Spray Mass Spectroscopy (ESMS). Each ESMS spectrum reveals that the parent peak of the expected anion is unambiguously observed in solution as predominant species. The experimental m/z values correspond exactly to that deduced from single-crystal X-ray diffraction. Table 1. ... 

Figure 7-13 gives also size distributions for the concentration of sea-salt particles about 6 m above the ocean surface. They were derived by Blanchard and Woodcock (1980) from mass distributions measured by Chaen (1973) on board ships. Durbin and White (1961) and Woodcock (1972) had earlier obtained similar results by aircraft sampling at elevations between 600 and 800 m. Figure 7-13 shows that the size spectrum of sea-salt particles corresponds to that of sea-spray production, indicating that in the size range... 

The validation of the CAB model has been performed by means of experimental data for non-evaporating, evaporating and reacting sprays under cmitroUed conditions in either a constant-volume or a constant-pressure combustion vessel. Particular attention has been given to the spray structure in the near-nozzle region by comparing the mass distribution with data from X-ray measurements reported in Ref. [19]. 

Spray nozzles can be categorized based on mass flow rate, liquid mass distribution, spray pattern, spray angle, spray impact, and droplet size. 

Liquid distribution refers to the amount of liquid being distributed at different areas of the spray impact. Some applications use multiple nozzles at once where an even liquid distribution is critical, while others use only one nozzle, where an even liquid distribution is not required. Designing nozzles to evenly distribute liquid is a challenge, since most nozzles do not have even liquid distribution. Figure 24.2 shows a pattemator (composed of a series of vials) which is used to measure the mass distribution across the spray. It is evident from it that the vials near the edges do not collect as much liquid as those at the center. 

Tong, X. Henion, J. Ganem, B. Studies on the distribution of oUgodeoxynucleotide charge states in the gas phase by ion spray mass spectrometry. J. Mass Spectrom. 1995, 30, 867-871. 

Pattemators may comprise an array of tubes or concentric circular vessels to coUect Hquid droplets at specified axial and radial distances. Depending on the pattemator, various uniformity indexes can be defined using the accumulated relative values between the normalized flow rate over a certain sector or circular region and a reference value that represents a perfectly uniform distribution. For example, using an eight-sector pie-shaped coUector, the reference value for a perfectly uniform spray would be 12.5%. The uniformity index (28) could then be expressed as foUows, where is the normalized volume or mass flow rate percentage in each 45-degree sector. 

The flow rate of liquid in the HPLC-electrospray system is paramount in determining performance both from chromatographic and mass spectrometric perspectives. The flow rate affects both the size and size distribution of the droplets formed during the electrospray process (not all droplets are the same size) and, consequently, the number of charges on each droplet. This, as we will see later, has an effect on the appearance of the mass spectrum which is generated. It should also be noted that the smaller the diameter of the spraying capillary, then... 

Coating Mass Uniformity and Distribution. Basically, there have been two approaches to model the accumulation of mass (coating material) on the surface of bed particles (i) the use of population balances and (ii) the probabilistic modelling of the spray-particle interaction. We will look at each of these approaches and see how it may be possible to combine these methods to give a fuller picture of coating performance. 

In general, different sized particles may have different cycle time distributions and different mass deposition distributions in the spray zone. One approach would be to use small discrete size distribution increments and then to apply Eq. (12) to each size fraction. Inherent in this approach is the assumption that each particle size fraction acts independently. This assumption may not be valid, especially if different particles take different circulation paths within the bed. From the population balance approach, Randolf and Larson (1988) have suggested the use of an effective growth diffusivity coefficient to account for random fluctuations in growth rate. Thus Eq. (6) would be modified to give ... 

Figure 13.4 presents the mass spectrum and distribution of the molecular ion of salbutamol (mass 240 u), the asthma drug, which is commercially produced as a coating on micron-sized sugar beads. For analysis a Bi3++ cluster gun was used. Before measurements the sample was sprayed onto a silicon substrate in order to disperse the beads [7],... 

It should be indicated that a probability density function has been derived on the basis of maximum entropy formalism for the prediction of droplet size distribution in a spray resulting from the breakup of a liquid sheet)432 The physics of the breakup process is described by simple conservation constraints for mass, momentum, surface energy, and kinetic energy. The predicted, most probable distribution, i.e., maximum entropy distribution, agrees very well with corresponding empirical distributions, particularly the Rosin-Rammler distribution. Although the maximum entropy distribution is considered as an ideal case, the approach used to derive it provides a framework for studying more complex distributions. 

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