Particles concentration

The following sections discuss many of the major particle types and provide bioconjugation options for the coupling of ligands to the surface of functionalized particles. Some additional nanoparticle constructs, including gold particles, dendrimers, carbon nanotubes, Buckyballs and fullerenes, and quantum dots are discussed more fully elsewhere (see Chapter 7 Chapter 9, Section 10 Chapter 15 and Chapter 24). 

Aerosol concentrations are defined in different ways depending on the application. For example, particle riumber concentrations (particles per unit volume of gas) are used to characterize dean rooms and atmospheric cloud condensation nuclei federal air pollution standards both for the atmosphere and for industrial emissions are usually stated in terms of aerosol mass per unit volume of gas. Effects of particles on viscosity depend on the ratio of the volume of particulate matter per unit volume of gas. For aerosols composed of particles all the same size, it is easy to relate the different methods of characterizing the concentration. For aerosols of mixed sizes, concentration measures are easily related only in certain cases as discussed below. 

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Atmospheric ions are important in controlling atmospheric electrical properties and conmumications and, in certain circumstances, aerosol fomiation [128. 130. 131. 138. 139, 140. 141. 142, 143, 144 and 145]. In addition, ion composition measurements can be used to derive trace neutral concentrations of the species involved in the chemistry. Figure A3.5.11 shows the total-charged-particle concentration as a frmction of... 

Altliough tire behaviour of colloidal suspensions does in general depend on temperature, a more important control parameter in practice tends to be tire particle concentration, often expressed as tire volume fraction ((). In fact, for hard- sphere suspensions tire phase behaviour is detennined by ( ) only. For spherical particles... 

Doppler Flow Meters. Doppler flow meters sense the shift in apparent frequency of an ultrasonic beam as it is reflected from air bubbles or other acoustically reflective particles that ate moving in a Hquid flow. It is essential for operation that at least some particles ate present, but the concentration can be low and the particles as small as ca 40 p.m. CaUbration tends to be influenced by particle concentration because higher concentrations result in mote reflections taking place neat the wall, in the low velocity portion of the flow profile. One method used to minimize this effect is to have separate transmitting and receiving transducers focused to receive reflections from an intercept zone neat the center of the pipe. 

Air Entrainment. Fine particles generally have a lower permeabiUty than coarse particles, and therefore tend to retain air longer in void spaces. Heavier particles settie more quickly in a duidized mixture than lighter particles. Thus, when a mixture of particles is charged into a bin, it is not uncommon to find a vertical segregation pattern, where the coarser, heavier particles concentrate at the bottom of the bed and the finer, lighter particles concentrate near the top. 

The relative viscosity of a dilute dispersion of rigid spherical particles is given by = 1 + ft0, where a is equal to [Tj], the limiting viscosity number (intrinsic viscosity) in terms of volume concentration, and ( ) is the volume fraction. Einstein has shown that, provided that the particle concentration is low enough and certain other conditions are met, [77] = 2.5, and the viscosity equation is then = 1 + 2.50. This expression is usually called the Einstein equation. 

Fig. 2. (a) Particle concentration profile of liquid flowing in a pipe, where YjD = the ratio of the distance along the diameter to the diameter ( ) (b)... 

The flocculation rate is deterrnined from the Smoluchowski rate law which states that the rate is proportional to the square of the particle concentration by number inversely proportional to the fluid viscosity, and independent of particle size. 

Orthokinetic flocculation is induced by the motion of the Hquid obtained, for example, by paddle stirring or any other means that produces shear within the suspension. Orthokinetic flocculation leads to exponential growth which is a function of shear rate and particle concentration. Large-scale one-pass clarifiers used in water installations employ orthokinetic flocculators before introducing the suspension into the settling tank (see Water,... 

In x-ray sedimentation, a collimated beam of x-rays permits particle concentration detection as a function of mass. The relationship between the fraction of x-rays transmitted and the mass concentration of particles of atomic weight >12 is expressed as in equation 13 ... 

Instead of monitoring x-ray transmission, some instmments use visible light to monitor concentration (53). This optical technique is generally referred to as turbidimetry or photoextinction. Light is usually used for low particle concentrations and detection of low molecular weight particles. 

The behavior of colloidal suspensions is controlled by iaterparticle forces, the range of which rarely extends more than a particle diameter (see Colloids). Consequentiy suspensions tend to behave like viscous Hquids except at very high particle concentrations when the particles are forced iato close proximity. Because many coating solutions consist of complex mixtures of polymer and coUoidal material, a thorough characterization of the bulk rheology requires a number of different measurements. 

Hindered Settling When particle concentration increases, particle settling velocities decrease oecause of hydrodynamic interaction between particles and the upward motion of displaced liquid. The suspension viscosity increases. Hindered setthng is normally encountered in sedimentation and transport of concentrated slurries. Below 0.1 percent volumetric particle concentration, there is less than a 1 percent reduction in settling velocity. Several expressions have been given to estimate the effect of particle volume fraction on settling velocity. Maude and Whitmore Br. J. Appl. Fhys., 9, 477—482 [1958]) give, for uniformly sized spheres,... 

In a gas stream cariying dust or fume, some degree of particle flocculation will exist, so that both discrete particles and clusters of adhering particles will be present. The discrete particles composing the clusters may be only loosely attached to each other, as by van der Waals forces [Lapple, Chem. Eng., 75(11), 149 (1968)]. Flocculation tends to increase with increases in particle concentration and may strongly influence collector performance. 

Several investigators have attempted to modify the basic Deutsch equation so that it would more nearly describe precipitator performance. Cooperman ( A NewTheoiy of Precipitator Efficiency, Pap. 69-4, APCA meeting, New York, 1969) introduced correction fac tors for diffusional forces arising from variations in particle concentration along the precipitator length and also perpendiciilar to the collecting surface. Robinson [Atmos. Environ. 1(31 193 (1967)] derived an equation for collec tion efficiency in which two erosion or reentrainment terms are introduced. 

Techniques used in bioseparations depend on the nature of the product (i.e., the unique properties and characteristics which provide a handle for the separation), and on its state (i.e., whether soluble or insoluble, intra- or extracellular, etc.). All early isolation and recovery steps remove whole cells, cellular debris, suspended solids, and colloidal particles, concentrate the product, and, in many cases, achieve some degree of purification, all the while maintaining high yield. For intracellular compounds, the initial harvesting of the cells is important... 

It is known that the vertical distribution of diffusing particles from an elevated point source is a function of the standard deviation of the vertical wind direction at the release point. The standard deviations of the vertical and horizontal wind directions are related to the standard deviations of particle concentrations in the vertical and horizontal directions within the plume itself. This is equivalent to saying that fluctuations in stack top conditions control the distribution of pollutant in the plume. Furthermore, it is known that the plume pollutant distributions follow a familiar Gaussian diffusion equation. 

For a given combination of filter design and dust, the effluent particle concentration from a fabric filter is nearly constant, whereas the overall efficiency is more likely to vary with particulate loading. For this reason, fabric filters can be considered to be constant outlet devices rather than constant efficiency devices. Constant effluent concentration is achieved because at any given time, part of the fabric filter is being... 

Hence, the application of these formulas only applies to very dilute systems. At high particle concentrations, mutual interference in the motion of particles exists, and the rate of settling is considerably less than that computed by the given expressions. In the latter case, the particle is settling through a suspension of particles in a fluid, rather than through a simple fluid medium. 

Hygiene requirements for particle concentrations in the air have been based on concentrations of particles smaller than 10 pm (particulate mass, P.M,)). Studies have shown a direct connection between the death rate and finer particles, and official requirements are under review, in both Europe and the United States, to be based on the concentration of particles smaller than 2.5 pm (PM2.5). 

In most cyclone applications in industrial ventilation, the particle concentration in the cyclone is very low and it may be assumed that the particles have very little chance of colliding with each other in the main body of the cyclone. Hence, the fate of the particles, whether they are collected or they penetrate the cyclone, may be determined by tracking the motions of individual, isolated particles suspended in the fluid flow. 

A relatively simple modification of the Deutsch equation is obtained by assuming that the particle concentration near the collection surface is linearly... 

Most of the results presented in the previous chapters are based on idealized conditions. In practice, the performance of an electrostatic precipitator can be significantly influenced by the dust layers on discharge and collection electrodes i.e., dust layers may alter the electrical properties of the system. It is also possible that dust layers are not stable i.e., collected particles become loose, increasing the particle concentration in the outlet of the precipitator. These problems play a much smaller role if the surface collection electrode is continuously flushed with water. These wet elearostatic precipitators, however, cannot be used in all applications. 

Particle Collection Particles in the extracted partial volume flow are retained in the collector filter. Tlie particle mass emitted is determined by the weight difference of the filter before and after the collection. Factors crucial to the measuring precision and the smallest measuring range of particle concentration are ... 

Figure 5.12 The relationship between volumetric abrasion rate and particle concentration after Nienow and Conti, 1978)...

Increased morbidity and mortality is associated with increases in outdoor particle concentrations (U.S. Environmental Protection Agency 1995). Of particular concern are the particles smaller than 2.5 micrometers in diameter, which are more likely to deposit deep inside the lungs (U.S. Environmental Protection Agency 1995). Some particles, biological in origin, may cause allergic or inflammatory reactions or be a source of infectious disease. 

Particle sizes are measured in microns, p, A micron is 1/1000 millimeter or 1/25,400 inch. A millimicron, m,U, is 1/1000 of a micron, or 1/1,000,000 millimeter. Usually particle size is designated as the average diameter in microns, although some literature reports particle radius. Particle concentration is often expressed as grains/cubic feet of gas volume. One grain is 1/7000 of a pound. 

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