Medium effects resistances

A model for the bulk effective resistivity of a dilute suspension (disperse phase) of noninteracting conducting spheres (not necessarily mono-dispersed) of material resistivity 9id and void fraction ad suspended in a continuous medium of material resistivity 9ic was derived by Maxwell (1954). His result is... 

There is no reason why there should be no general medium effect upon the resistivities. The approximate nature of Eqs. (44) and (48a) gives rise to speculations regarding their improvement, which however do not belong to the field of macro-dynamics. The self diffusion of a medium containing solvated macro-molecules was studied by Wang. ... 

Hardness of nanocomposites is related to the tribological properties an enhancement in this property implies that the nanocomposites would be more effectively resistant to the penetration of the abrasive medium and as a result, the amount of material removal would be decreased [67]. 

Butadiene elastomers have a medium thermal resistance. The CL measurements on this type of polymers, whose macromolecules are consisting of l,4-c/5,, A-trans and 1,2-vinyl units, provide a realistic description on the stability behavior in connection with the place on which oxygen attack occurs [94J3]. The values of induction time (Fig. 37a), oxidation rate (Fig. 37b) and maximum oxidation time Fig. 37c) underline the electronic effects of substituents ... 

According to Ohm s law, the potential difference arising from driving a current I through a medium with resistance is IR. Hence the overpotential associated with overpotential increases with driven current, and so with scan rate. At megavolt-per-second scan rates, specialised equipment is required to disentangle useful information given by Faradaic currents from current distortions due to resistive and capacitive effects. 

Added to the medium effect on activity is the hindering of the pH glass functioning by the solvent dehydrating the glass, by high sample resistance, and by large liquid junction potential developed at the reference electrode. These factors make nonaqueous pH difficult to measure and interpret. 

Medium Cone. Temp. Effective Resistance factor ... 

The scale-up of conventional cake filtration uses the basic filtration equation (eq. 4). Solutions of this equation exist for any kind of operation, eg, constant pressure, constant rate, variable pressure—variable rate operations (2). The problems encountered with scale-up in cake filtration are in estabHshing the effective values of the medium resistance and the specific cake resistance. 

Benefits of Prethickening. The feed soHds concentration has a profound effect on the performance of any cake filtration equipment. It affects the capacity and the cake resistance, as weU as the penetration of the soHds into the cloth which influences filtrate clarity and medium resistance. Thicker feeds lead to improved performance of most filters through higher capacity and lower cake resistance. 

The effect on soHds yield can be easily demonstrated using the foUowing equation (neglecting medium resistance) ... 

A reverse-wipe appHcation is seen ia the clothes dryer fabric softener sheet whereia the spunbonded fabric is coated with a complex combination of compounds that are released iato the environment of a hot clothes dryer to soften and perfume the clothes, as weU as provide an antistatic quaHty. The spunbonded sheet, which must be made of polyester or nylon for temperature resistance, provides a simple and cost-effective medium to store the chemical compounds prior to release ia the dryer. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

Cake Filters Filters that accumulate appreciable visible quantities of solids on the surface of a filter medium are called cake filters. The slurry feed may have a solids concentration from about 1 percent to greater than 40 percent. The filter medium on which the cake forms is relatively open to minimize flow resistance, since once the cake forms, it becomes the effective filter medium. The initial filtrate therefore may contain unacceptable sohds concentration until the cake is formed. This situation may be made tolerable by recycling the filtrate until acceptable clarity is obtained or by using a downstream polishing filter (clarifying type). 

When the resistance opposing fluid flow is small, gravity force effects fluid transport through a porous filter medium. Such a device is simply called a gravity filter. 

The fluids available cover a range of chemical constituents, physical characteristics and costs, so the user is able to choose the medium that offers the best compromise for operational satisfaction, fire-resistance and cost effectiveness. Four basic types of fluid are available and are shown in Table 52.7. 

---