Dysonian effective medium


In diffusion charging, particles are too small and mobile for rapid charging by ion bombardment. They become charged by coUision caused by motion of the gas molecules. Diffusion charging becomes efficient on particles smaller than 0.2 p.m and has been demonstrated to be effective on particles down to 0.05 p.m. These fine particles are charged rapidly and have higher charge density at saturation. Unlike field charging, the rate of charging by diffusion is independent of electric field strength. For particles between 0.2 p.m and 1.5—2.0 p.m neither charging mechanism is highly efficient, but both operate. Particles in this size range are the most difficult to collect efficiendy. Figure 12 compares experimental and calculated particle charge levels for 0.3-p.m particles. The field has been reviewed (177) and a particle-charging equation which has been used in computer-modelled precipitator design has been proposed (178,179).  [c.400]

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor.  [c.280]

Jigs, generally very effective for relatively coarse (typically 0.5—200 mm) material, have relatively high unit capacities. The sharpness of separation is a function of size distribution. Fine specific gravity separation is possible for a closely sized material. The principal usage of jigs is in coal beneficiation. Other apphcations include concentration of cassiterite, tungsten, gold, barites, and iron ores. The basic jig (Fig. 10) has a large tank or hutch divided in the upper portion into two main sections. One section contains the stationary screen with the mineral bed on it, the bed depth being many times the thickness of the largest particle, and the other section contains the pulsating device, most commonly using air pressure rather than mechanical means. Mineral separation is achieved by applying a vertical oscillatory (pulsating) motion to the soHds—fluid bed. This pulsating motion produces dilation of the bed and subsequent stratification. The denser and larger particles form a lower layer whereas the finer lighter particles ate on top. The processes occurring in a fliU cycle of operation may be considered differential initial acceleration, hindered settling, and consoHdation trickling. Several other theories have been developed, however, notably the center-of-gravity theory (6,10). The pulsing action is supplemented by using additional water in the hutch during the settling period. This extends the open state of the bed for a longer time. The dense minerals are collected either on the screen or under the screen depending on the screen aperture size. In the latter case, a layer of dense (ragging) particles larger than the aperture size are placed on the screen to regulate the collection of dense fraction. Examples ate feldspar in coal cleaning, and hematite in cassiterite and scheelite separation. Several stages of jigging are used to achieve efficient separation. The commercial jigs have a variety of designs for the pulsating device and the removal of products. Some examples are shown in Figure 11. The Batac jig, which uses multiple air chambers under the screen, is the industry standard in coal cleaning. A more recent development is the Kelsey centrifugal jig (33,34). The circular or radial jig is a variation of the conventional rectangular design of hutches in series. The pulp is fed at the center and flows radially over the jig bed and exits at the circumference. A raking mechanism ensures an even bed depth throughout. It is mechanically simple and has very high capacity, up to 300 m /h for a maximum particle size of 25 mm. It achieves a fast compression/slow suction stroke with virtually no hutch water. The slow suction stroke allows more time for the fines to settle to the bed.  [c.404]

With the advent of DCSs, PLCs, smart transmitters, smart valves, and digital communications, it is becoming more and more imperative to take an overall system approach ia analysing the benefit—cost ratios of alternative systems to meet control objectives. It is no longer appropriate to view benefits or costs on a loop-by-loop basis. For example, a minimal DCS configuration may consist of one operator station, a data highway, and one control unit. Thus, for the controls up to the capacity of the control unit, the per-loop cost iacludes the cost of the operator station and the data highway. However, it is usual to find that the capacity of both the operator station and the data highway is much greater than that of a single control unit. Therefore, adding another control unit, ie, doubling the control capacity, can be done without investing ia another operator station or data highway. Much modem control equipment consists of prepackaged modularized components configured iato a system. Thus similar types of overall considerations are required ia arriving at a cost-effective control design for a particular faciUty.  [c.79]

Polymerizer design variables such as length /diameter ratio of the polymerizer, type and speed of agitation, and materials of constmction, have important effects on operation of the process but Httie effect on the product itself. Polymerization reactors are usuaHy jacketed or contain cooling coHs (or both) for temperature control. Stainless steel is common in modem reactors and agitation is usuaHy designed for low shear and high pumping rates, the objective being to aHow good heat transfer while avoiding the production of smaH particle size latex.  [c.520]

Typical flow sheets for alternative energy-recovery systems are shown in Fig. 25-65. Perhaps the most common flow sheet for the production of electric energy involves the use of a steam turbine-generator combination (see Fig. 25-65). As shown, when solid wastes are used as the basic fuel source, four operating modes are possible. A flow sheet using a gas-turbine-generator combination is shown in Fig. 25-65. The low-energy gas is compressed under high pressure so that it can be used more effectively in the gas turbine. Use of low- or medium-Btu gas for gas turbines has been attempted, and success requires good design and operation of gas cleaning equipment prior to introduction into the combustor of the gas turbine.  [c.2244]

Rossiter and Douglas (1986) attempted to optimize a given flowsheet by determining best values of design variables. The scope of such studies can span from preliminary design where process alternatives may have to be compared and screened economically, to determining the best conditions for equipment trails or for detailed final design. In both contributions, the focus was on the optimization of median size. Simplified cost correlations derived for various units were used to evaluate the impact of average size on system economics. Flowsheet decomposition analysis was carried out to identify the independent subsystems. From individual models for crystallizers, primary product separation and drying units, a cost model for the whole process was developed. The effects of perturbations in a given design parameter while keeping others constant were used to determine an optimal set of values for all the design parameters.  [c.272]

The use of an unnecessarily hot heating medium led to the runaway reaction at Seveso, Italy, in 1976, which caused a fallout of dioxin over the sun ounding countryside, making it unfit for habitation. Although no one was killed, it became one of the best-known chemical accidents, exceeded only by Bhopal, and had far-reaching effects on the laws of many countries.  [c.376]

Recent developments in computer technology in combination with increasingly accurate quantum chemical methods allow for the detailed investigation of molecular reactions, properties, and bulk effects. However, studies of systems with several (adjacent) heteroatoms are hampered owing to strong electron correlation effects, spin contamination in open-shell molecules, and strong charge polarization which requires very flexible basis sets. These problems limit the reliability of semiem-pirical methods and simple Hartree-Fock ab initio calculations for this class of molecules. Nevertheless, many questions could be solved based on such calculations. But the accuracy of modem gradient-corrected density functionals and a new generation of fast ab initio post-Hartree-Fock methods allow the investigation of molecules of up to 50 atoms on standard personal computers at much higher levels. Since these methods now permit a quantitative interpretation rather than a qualitative one, computational chemistry has become increasingly important in traditional heterocyclic chemistry, life sciences, and dmg design. It is thus the purpose of this contribution to provide an overview of the recent advances of modem quantum chemical methods in heterocyclic chemistry. Many of these methods are no longer blackbox in nature, they require a sound knowledge of quantum mechanics. Thus particular care must attend the choice of method and the interpretation of results. The last aspect is even more important when reference data are lacking.  [c.2]

The pressure drop of these trays is usually quite low. They can be operated at an effective bubbling condition wnth acceptable efficiencies and low pressure drops. For more efficient operation the clear liquid height on the tray appears to be. similar to the sieve tray, i.e., 1.5-2-in. minimum. This is peculiar to each system, and some operate at 1 in. with as good an efficiency as when a 2-in. is used. When data is not available, 2 in. is recommended as a median design point.  [c.203]

We would like to assert that we did not introduce any approximations within the local interaction zone concept because it is based on the olution of the exact Dyson equation. Of course, strictly speaking one has to consider the LIZ of infinite size. But actually the application of a judiciously chosen effective medium leads to a minimal size of the local interaction zone. Really, one immediately observes from eq. (3) that although the off-diagonal blocks of the desired Green s function matrix gnn may be long-ranged this becomes irrelevant for the on-site matrix element if an effective medium can be found such that gnni has short range in which case the sum over lattice sites may be restricted to a finite cluster, i.e. a local interaction zone containing M atoms.  [c.117]

Filter Design. Modem leukocyte-reduction filters have become highly efficient as a result of catefijl filter design and advanced biomaterials. The binding of leukocytes to the filter media is weak, and hence the flow of blood components through the filter must be well controlled. Effects, such as channeling and bypass, ate detrimental to the quaUty of the product. General design considerations include high flow rate through the filter, low retention volume, and hydrophilic filter media that does not requite priming prior to filtration. The filter should also give a high yield of the source product, and loss of platelets or red cells in the filter as a result of either adhesion or retention volume should be minimal.  [c.523]

An ideal shock-absorbiag medium, eg, for football ia the United States, would combine a reasonable softness ia normal shoe contact with a high capacity for dissipation or distribution of kinetic energy iavolved ia the impact of a player s fall. Various foamed elastomers are suitable for this purpose (see Elastomers, synthetic). The design criterion is the abiUty to dissipate energy of motion by reduciag impact deceleration through hysteresis losses ia the material. A useful device for characterising the required properties is a dynamic mechanical impact tester (ASTM F355). It employs an iastmmented missile that is allowed to fall freely from a specific height onto the surface. Sensing components record electronically the force- and displacement-time profiles of the missile throughout the iaterval of first penetration and rebound from the playiag surface. The plots shown ia Figure 1 illustrate the acceleration and displacement between initial and final contact of the missile with the playiag surface, both vs time. The deceleratioa forces iacrease as the missile peaetrates the surface, reach a maximum, and decrease as the missile rebounds from the surface. The effectiveness of the shock-absorbiag medium is iadicated by the height of the maximum or, mote accurately, the iategrated profile throughout the duratioa of impact, calculated according to the severity iadex, dt  [c.533]

Bipolar circuits tend to dissipate much more power because large currents are always flowing. NMOS circuits dissipate more power than CMOS circuits because the PFET is replaced by a depletion mode NFET which always conducts. Thus, traditionally, CMOS has been regarded as a low power circuit technology. However, this is only tme for circuits operating at low frequency, with low activity, or at low supply voltages. Modem microprocessor chips contain millions of transistors, operate at high frequencies, and hence dissipate substantial power. For example. Digital Equipment Corporation s (DEC) 21064 Alpha chip, introduced in 1992, contained 1.68 million transistors with 0.75 p.m gate lengths and dissipated 30 W when operating at 200 MHz from a 3.3-V supply (41). The Alpha chip is mentioned because its design emphasized achieving maximum clock frequency for a given level of technology, as defined by gate length. Clearly, because power dissipation depends quadraticaHy on supply voltage, lowering the supply voltage is the most effective way to lower power dissipation. If the DEC Alpha chip were operating at 5 V it would dissipate almost 70 W.  [c.353]

Tires are designed with a very high factor of safety, about six for radial medium tmck tires and ten or higher for radial passenger tires. The reason for such a higher factor of safety is that the cord tensile strength is measured at 23°C and 55% relative humidity (ASTM D885-64). However, tires can reach short-term operating temperatures up to 150°C in some appHcations. At these temperatures some organic tire cord materials can degrade resulting in loss of strength <50% of room temperature value. Adding 10—20% degradation in cord strength during the life cycle of the tire, drops the effective tire strength to 150—200% of designed strength, which may be sufficient for burst but may fail to meet several important performance criterias. Therefore, selecting cords only on burst strength requirements may not be the most desirable approach for overall tire design.  [c.89]

Marine coatings ate the most important, cost-effective means to preserve steel (qv) and other metals in the marine environment. These coatings impart physical and chemical properties, eg, antifouling, color, and sHp resistance, to surfaces that cannot be obtained in any other way. Modem coatings ate sophisticated mixtures of polymers and other chemicals, and they requite careful control of surface preparation and appHcation conditions. These materials ate not as adaptable as eadiet coatings to the variety of metals, design features, and surface conditions encountered in marine constmction, but they provide long-lasting cost-effective protection when used as part of a corrosion control program (see Corrosion and corrosion control). In 1989 a total of 38 X 10 L of marine coatings were sold in the United States. Shipbuilding consumed 4 x 10 L, constmction of pleasure boats requited 3 x 10 L and 31 x 10 L were employed for maintenance and repair. The selection and appHcation of marine coatings has become a highly specialized discipline in which governmental regulations ate a dominant influence. Many paints (qv) and painting procedures used in the past ate no longer permitted or ate extremely cosdy to use.  [c.363]

Fabric Filters Fabric filters, commonly termed bag filters or baghouses, are collectors in which dust is removed from the gas stream by passing the dust-laden gas through a fabric of some type (e.g., woven cloth, felt, or porous membrane). These devices are surface filters in that dust collects in a layer on the surface of the filter medium, and the dust layer itself becomes the effective filter medium. The pores in the medium (particularly in woven cloth) are usually many times the size of the dust particles, so that collection efficiency is low until sufficient particles have been collected to build up a precoat in the fabric pores (Billings and Wilder, Handbook of Fabric Filter Technology, vol. I, EPA No. APTD-0690, NTIS No. PB-200648, 1979). During this initial period, particle deposition takes place mainly by inertial and flow-line interception, diffusion, and gravity. Once the dust layer has been fully established, sieving is probably the dominant deposition mechanism, penetration is usually extremely low except during the fabric-cleaning cycle, and only limited additional means remain for influencing collection efficiency by filter design. Filter design is related mainly to choices of gas filtration velocities and pressure drops and of fabric-cleaning cycles.  [c.1600]

When water is used as the quench medium and the effluent stream is a hydrocarbon or organic, separate hquid phases are often formed. In this case, heat transfer is the predominating mechanism during the quench. To achieve effective heat transfer, there must be a sufficient difference between the quench liquid temperature and the bubble point of the incoming effluent stream. The minimum temperature difference occurs at the end of the discharge, when the quench pool temperature is highest. A rule of thumb, from industry prac tice, is to allow a 10 to 20°C (IS to 36°F) AT. For atmospheric tank operation, the final quench hquid temperature is then set 10 to 20°C (IS to 36°F) below the normal boihng point of the final quench pool mixture. For nonvented or controllea venting operation, the final boihng point is elevated, permitting a greater design temperature rise and the use of less quench liquid. Therefore, the quench pool final temperature must tDe set 10 to 20°C (IS to 36°F) lower than the saturated temperature of the discharge effluent at the design maximum quench tank pressure.  [c.2299]

The arc pla.sina i,s caused during an interruption of the live contacts, and also just before closing the contacts, when the contact gap falls. short of the required dielectric strength to withstand the impressed voltage. When an tire is caused the gases present in the arcing chamber, under the influence of high temperature of the arc plasma and the high pressure or high vacuum maintained within the arcing chamber, become ionized. They liberate protons (positive ions, positively charged, heavier particles) and neutrons (uncharged particles) surrounded by electrons (negatively charged lighter particles (Figure 19.3)). The theory of arc extinction relates to the physics and behaviour of these elecfrically charged particles that are responsible for a restrike of the TRV even after a current zero. The effectiveness of the medium and the design of the arc chamber to diffuse these electrically charged particles to neutrons as quickly as possible determines the ability of one type of breaker over others. In fact, the theory of arc extinction is the theory of deionization (neutralization) of the electrically charged protons and electrons. The theory may be briefly explained as follows.  [c.631]

The ionization process is very sensitive to the medium. The effects of the medium depend on the charge type of the reactants. These relatioiiships follow the general pattern for solvent effects discussed in Section 4.10. Ionization of a neutral substrate results in charge separation at the transition state, and solvent polarity will have a greater effect at the transition state than for the reactants. Solvents of higher dielectric constant will lower the energy of the transition state more than will solvents of lower polarity. Ionization of cationic substrates such as alkyl diazonium ions or trialkylsulfonium ions leads to dispersal of charge in the transition state and is moderately enhanced by less polar solvents, because the reactants are more strongly solvated than the transition state.  [c.266]

Often, complex apparatus and systems, process piping arrangements and even support structures utilize different metals, alloys or other materials. These are often employed in corrosive or conductive environments and, in practice, the contact of dissimilar materials cannot be avoided totally. It is important that the designer minimize the damaging effects of corrosion by optimizing the compatibility of materials either by selection or arrangement in the overall design. Compatible materials are those that will not cause an uneconomic breakdown within the system, even though they are utilized together in a particular medium in appropriate relative sizes and compositions. In addition to material influences on each other by virtue of inherent or induced differences of electric potentiality, adverse chemical reactions can occur as a result of changes in materials caused by environmental variations. All these possibilities must be examined thoroughly by the designer.  [c.36]

Coagulation during the sticky period can be prevented by reducing the surface tension of droplets and minimizing the force with which they collide. The latter is controlled by proper reactor design and stirring force. The force with which the droplets collide can be reduced by decreasing the stirring speed, but the stirring speed must be kept high enough to prevent the droplets from aggregating and separating in the sticky period. Reducing surface tension is achieved by using a small amount of a suitable droplet stabilizer as a coagulation inhibitor. In o/w suspension polymerization, a highly effective droplet stabilizer is a small amount of water-insoluble inorganic salt or organic polymer that is insoluble in the monomer droplets and has relatively low solubility in the suspension medium to the suspension system. Organic polymers are usually preferred over insoluble inorganic salts because they are removed more easily from the surface of the beads. Examples of inorganic droplet stabilizers used for suspension polymerization include calcium sulfate, calcium phosphate, and benzonite. Among the most commonly used organic stabilizers for suspension systems are polyvinyl alcohols (75-98% hydrolyzed) and polyvinylpyrrolidone. A wide range of other water-soluble polymers such as methylcellulose, gelatin, and other natural gums are also used. In general, a relatively low concentration of the stabilizer (0.15 to 1%) is sufficient to maintain a stable suspension system under constant stirring conditions. The droplet stabilization is a surface phenomenon. Therefore, the minimum stabilization concentration required for a full monolayer coverage of droplets increases with decreasing particle size.  [c.5]

The North American P-51 Mustang, designed at the outset of World War II, was the first production aircraft to employ a laminar flow airfoil. Flowever, laminar flow is a sensitive phenomenon it readily gets unstable and tries to change to turbulent flow. For example, the slightest roughness of the airfoil surface caused by such real-life effects as protruding rivets, imperfections in machining, and bug spots can cause a premature transition to turbulent flow in advance of the design condition. Therefore, most laminar flow airfoils used on production aircraft do not yield the extensive regions of laminar flow that are obtained in controlled laboratory tests using airfoil models with highly polished, smooth surfaces. From this point of view, the early laminar flow airfoils were not successful. However, they were successful from an entirely different point of view namely, they were found to have excellent high-speed properties, postponing to a higher flight Mach number the large drag rise due to shock waves and flow separation encountered near Mach 1. As a result, the early laminar flow airfoils were extensively used on jet-propelled airplanes during the 1950s and 1960s and are still employed today on some modem high-speed aircraft.  [c.10]


See pages that mention the term Dysonian effective medium : [c.117]    [c.2213]    [c.407]    [c.533]    [c.425]    [c.2346]    [c.284]    [c.255]   
The science and technology of carbon nanotubes (1999) -- [ c.95 , c.100 ]