Sizing Information

The most important parameters affecting final selection of the vacuum system are the suction pressure and the throughput. 

---

T abular. A typical distributioa as measured by modem iastmmeatatioa can iaclude size information on tens of thousands and even millions of iadividual particles. These data can be Hsted ia a computer and then sorted iato a series of successive size iatervals, keeping track of the measured quantity, such as number, surface area, or mass, within each group. For narrow size distributions it may be sufficient to group the data ia linear iatervals, such as 0—1, 1—2, 2—3 p.m, etc, and then Hst the iatervals as a perceat value of the whole. 

A wide variety of particle size measurement methods have evolved to meet the almost endless variabiUty of iadustrial needs. For iastance, distinct technologies are requited if in situ analysis is requited, as opposed to sampling and performing the measurement at a later time and/or in a different location. In certain cases, it is necessary to perform the measurement in real time, such as in an on-line appHcation when size information is used for process control (qv), and in other cases, analysis following the completion of the finished product is satisfactory. Some methods rapidly count and measure particles individually other methods measure numerous particles simultaneously. Some methods have been developed or adapted to measure the size distribution of dry or airborne particles, or particles dispersed inhquids. 

On-Hne Procedures The growing trend toward automation in industry has resiilted in many studies of rapid procedures for generating size information so that feedback loops can be instituted as an integral part of a process. Many of these techniques are modifications of more traditional methods. The problems associated with on-line methods include allocation and preparation of a representative sample analysis of the sample evaluation of the results. The interface between the measuring apparatus and the process has the potential of high complexity, and consequently, high costs [Leschonsld, Paiticle Cha racterization, 1, 1 (July 1984)]. 

For dilute systems, Stoke s law is applicable to particle settling. References cited at the end of this chapter provide design and sizing information. 

At the end of this chapter you will find three annexes. The first of these is a list of nomenclature used in the chapter. There are quite a few equations that are sununarized in the foregoing sections and hence, you will need to refer to this annex from time to time. The second annex is a list of recommended references that I have relied on over the years, plus some interesting Web sites for you to visit for vendor-specific information as well as supplemental design and equipment sizing information. The final annex is the Questions for Thinking and Discussing. Remember to refer to the Glossary at the end of the book if you run across any terms that are unfamiliar to you. 

Theory. We will outline theory developed earlier (11,12) for converting the detector response F(v) from a turbidity detector into particle size information. F(v) is related to the dispersion-corrected chromatogram W(y) by the integral equation... 

The key to obtaining pore size information from the NMR response is to have the response dominated by the surface relaxation rate [19-26]. Two steps are involved in surface relaxation. The first is the relaxation of the spin while in the proximity of the pore wall and the other is the diffusional exchange of molecules between the pore wall and the interior of the pore. These two processes are in series and when the latter dominates, the kinetics of the relaxation process is analogous to that of a stirred-tank reactor with first-order surface and bulk reactions. This condition is called the fast-diffusion limit [19] and the kinetics of relaxation are described by Eq. (3.6.3) ... 

A number of alternative sizing methods are available, and these are described in Table 8. The American Association of Pharmaceutical Scientists, Inhalation Focus Group conducted a comprehensive review of available methods, which was published in a series of articles identified in the last column of the table. All of the methods described either have been or are currently employed in the development of aerosol products. However, at this time only the inertial samplers, cascade impactors and impingers appear in compendial standards and in regulatory guidelines [44-46], Other methods such as thermal imaging are also under development and may give complementary size information to the current methods. 

More recent work has focused on understanding the mechanism or mechanisms of selectivity. Some of these studies have been performed on well-characterized catalysts about which particle size information is available. Still others have been performed on single crystals. So conclusions may be reached about the effects on chemoselectivity of planes, edges, and corners that are related to particle size (structure sensitivity). A number of these studies, mostly on Pt, are summarized in Table 2.6. Since these studies have usually been performed in the vapor phase, information about solvent effects and their possible influence on chemoselectivity is unavailable. 

Each of the detailed sub-models feed into the thermodynamic system model, and provides sizing information directly to the conceptual design and configuration. The thermodynamic system... 

The relationship of particle size to molecular composition is highly significant. (2). Particles below 1-3 fun (submicron size class) in diameter are considered respirable by animals, in that this material can reach the lower alveoli of the lung. Particle size information may be used to determine the sources of various atmospheric pollutants. Particulate matter formed from gaseous pollutants tends to be found in the less than the 1-3 /nm particle diameter size range. Supermicron sized particles (particles greater than 1- 3 typically originate from primary emission sources, (i.e., stack emissions, vehicles, soil). 

Table 13.6 contains some of the analytical techniques that allow particle size determination. Before a technique is chosen, the relative particle size and type of size information... 

Because PCS relies on the determination of the particle diffusion coefQcient, it is not a direct method for the determination of particle sizes. Information on the particle size can be obtained via the Stokes-Einstein equation... 

The advantage of utilizing the standardized form of the variable is that quantities of different types can be included in the analysis including elemental concentrations, wind speed and direction, or particle size information. With the standardized variables, the analysis is examining the linear additivity of the variance rather than the additivity of the variable itself. The disadvantage is that the resolution is of the deviation from the mean value rather than the resolution of the variables themselves. There is, however, a method to be described later for performing the analysis so that equation 16 applies. Then, only variables that are linearly additive properties of the system can be included and other variables such as those noted above must be excluded. Equation 17 is the model for principal components analysis. The major difference between factor analysis and components analysis is the requirement that common factors have the significant values of a for more than one variable and an extra factor unique to the particular variable is added. The factor model can be rewritten as... 

This result may be deceptive, however, as we had to eliminate the many studies from Cols. 2 and 3 that contained no size Information, so the two sets of averages are not for the same populations of plants. A further problem with these data may also be the Inclusion of particles up to 2.5-pm dlam, whereas, recent studies (44, 46) indicate that the large changes of composition occur below about 0.5-pm dlam. Thus, our "fine" particles In Table IV Include particulate mass between 0.5 and 2.5 ym that Is chemically similar to larger particles, which reduces the Rvalues. We made the cut at 2.5 ym to develop components for separate CEBs of fine and coarse particles In ambient air, which are commonly divided at about that point. 

While electrostatic precipitators have relatively high collection efficiencies (99-100%) over a wide range of particle sizes ( 0.05-5 /im), there are a number of disadvantages. These include the lack of size information, particle reentrainment due to sparking, and practical problems such as high cost and shock hazards. As a result, they have not been widely used in ambient air studies. 

Electronic properties semiconductor and metal conductivity, magneto-resistance, emission of electrons, electronic devices of the molecular size, information recording, diodes, field transistors, cold cathodes, materials for displays, quantum wires and dots, cathodes for X-ray radiation, electric probes, etc. 

There is only a severely limited amount of mass available for analysis, and efforts to increase size information through more stages simply makes the available mass even less. Attempts to collect more mass by longer runs are limited by particle-bounce effects. 

The linewidth (corrected for instrumental effects) may also provide important chemical information of several types. For example, if the chemical environment of a resonant atom is not the same for all of the atoms in the sample, then a broadening of the observed resonance is expected. That is, the observed resonance is a sum of the contributions from each atom, the latter not all having the same Mossbauer parameters. Thus for a small catalyst particle, interesting particle size information might be contained in the linewidth due to the contribution from the surface atoms to the Mossbauer spectrum. The distribution (clustered or uniform) of resonant atoms throughout a multicomponent catalyst particle may also be reflected in the linewidth. 

It should be standard for each newly prepared batch of nanoparticles to characterize all chemical as well as physical properties, and report all data necessary to prove unequivocally purity and size/size distribution including, but not limited to, 1H NMR (absence of free, non-bound ligands, ammonium salts, or other impurities/ reagents) and elemental analysis and/or inductively coupled plasma spectroscopy, ICP-OES/MS (providing information about purity as well as monolayer coverage in conjunction with size information provided by TEM, X-ray diffraction/scattering or DLS). 

The whole-pattern-fitting structure-refinement method, which was first introduced by Rietveld and used for neutron diffraction powder patterns, does yield from x-ray diffraction patterns correct, refined structural information for linear polymers. Remarkably precise lattice parameters are obtained incidentally in the use of the method. The method lends itself to improved estimations of the fraction of amorphous and crystalline materials, or of two polymorphic forms, present. As improved profile functions come in to use, the method promises to provide crystallite size information, almost as a spin-off benefit. 

The particle size information for the latexes selected for structural characterization is shown in Table XIV. The homopolymer polystyrene seed latex was used to prepare the two-stage... 

If the dispersed phase concentration is not too high, and the species are very small, light-scattering can yield size information. The theory underlying the determination of size distribution for a colloidal dispersion is quite involved [13,75,76], When a beam of light enters a suspension some light is absorbed, some is scattered, and some is transmitted. Many dilute, fine emulsions and suspensions show a noticeable turbidity given by,... 

---