Current demand

Xylene Isomeri tion. The objective of C-8-aromatics processing is the conversion of the usual four-component feedstream (ethylbenzene and the three xylenes) into an isomerically pure xylene. Although the bulk of current demand is for xylene isomer for polyester fiber manufacture, significant markets for the other isomers exist. The primary problem is separation of the 8—40% ethylbenzene that is present in the usual feedstocks, a task that is compHcated by the closeness of the boiling points of ethylbenzene and -xylene. In addition, the equiUbrium concentrations of the xylenes present in the isomer separation train raffinate have to be reestabUshed to maximize the yield of the desired isomer. 

The conductivity of the grid plays a substantial role in a battery s abiUty to meet high current demands. The importance of grid conductivity for lead—acid batteries has been discussed (1,69). Composition and configuration are important design factors impacting grid conductivity. 

Over the last decade production capacity in the United States remained essentially unchanged, but minor fluctuations occurred in response to changes in environmental regulations (38). A similar reaction was noted worldwide (35). The current demand for activated carbon is estimated at 93% of production capacity. The near-term growth in demand is projected to be approximately 5.5%/yr (39). 

Approximately 50% of the demand for tetrachloroethylene is in the dry-cleaning industry where about 80% of all dry cleaners use it as their primary cleaning agent. Use as a feedstock for chlorofluorocarbon production accounts for 30% of current demand. Metal cleaning and miscellaneous appHcations represent 12 and 8% of demand, respectively. The miscellaneous appHcations include such varied uses as transformer insulating fluid, chemical maskant formulations, and as a process solvent for desulfurizing coal. 

According to one estimate (73), the current capacity for manufacturing dimer acids in the U.S. is around 55,000 t per year. Current demand is estimated at about 33,600 t per year, and is expected to grow at about 2—3% per year to 35,000 t in 1993. The historical growth rate for dimer acids (1980—1989) was 0.8% per year. Prices of tall oil fatty acids, the raw material for over 90% of dimers, currently fluctuates in the 0.55—0.66 per kg range. The dimer acids themselves are presently selling at about 1.10 per kg for the standard 75—80% dimer acids, and about 2.20 per kg for the distilled (90—95%) dimer acids. 

In choosing a protection method, the magnitude of the required protection current, which depends on the necessary protection current density, is of considerable importance. From Section 5.2.1.2 a rough estimate of the current demand can be made using Eq. (5-11 ). 

Determination of Current Demand, Evaluation, and Connections of the Protection Equipment... 

Based on past experience, it has been found that the protection current density for buried storage tanks coated with bitumen is over 100 /xA m. With coatings in very good condition, it can amount to a few tens of jiA but for coatings in a very poor state, it can rise to a level of mA The protection current demand can... 

The grounding resistance of different types of anodes can be calculated from the equations in Section 24.1 (see Table 24-1). The use of magnesium anodes is convenient and economical for relatively small protection currents. In the case of an increase in the protection current demand, because the voltage is fixed at about 0.6 V, the current can only be raised by lowering the grounding resistance of the anodes, i.e., by installing more anodes. Alternatively, the voltage can also be increased by an impressed current system. 

What are the best guess maximum and minimum limits of the load current and are there any intermittent characteristics in its current demand such as those presented by motors, video monitors, pulsed loads, and so forth Always add 50 percent more to what is told to you since these estimates always turn out to be low. Also what are the maximum excursions in supply voltage that the designer feels that the circuit can withstand. This dictates the design approaches of the cross-regulation of the outputs, and feedback compensation in order to provide the needs of the loads. 

Lead-acid batteries remain popular because of their capability to seiwice high and low current demand, produce high voltage, provide capacity up to 100 A-h, and recharge well. Moreover, the lead-acid battery has important material and construction advantages, such as simple fabrication of lead components, the low cost of materials (lead is abundant and much less expen-... 

It was indicated earlier that the cathodic current was a poor indicator of adequate protection. Whilst, to a first approximation the protection potential is a function of the metal, the current required for protection is a function of the environment and, more particularly, of the cathodic kinetics it entails. From Fig. 10.4 it is apparent that any circumstance that causes the cathodic kinetics to increase will cause both the corrosion rate and the current required for full (/") or partial (1/ — /, ) protection to rise. For example, an increase in the limiting current in Fig. 10.5 produced by an increase in environmental oxygen concentration or in fluid flow rate will increase the corrosion rate and the cathodic protection current. Similarly, if the environment is made more acid the hydrogen evolution reaction is more likely to be involved in the corrosion reaction and it also becomes easier and faster this too produces an increased corrosion rate and cathodic current demand. 

In short, the current demand for cathodic protection varies according to the aggressiveness of the corrosive environment. It is for this reason that cathodic protection finds its greatest application where the pH is close to neutral. The more acid environments entail a current output that rapidly becomes uneconomic. The more alkaline environments prove less aggressive to the structure and therefore often do not justify cathodic protection. Table 10.5 provides some estimated current densities for cathodic protection that illustrate the point. 

This section is not intended to deal with those environmental factors which influence cathodic current demand (e.g. oxygen availability or the presence of calcareous deposits) but those which directly affect the performance of the anodes. 

The capacity of an anode is dependent on the anode current density. To some extent it will be governed by the exposure environment but, in part, is within the control of the design. Certainly wholly unsuitable current densities can usually be avoided. At lower operating current densities some anodes exhibit reduced capacity this is shown in Fig. 10.17. Long periods of low operating current density can lead to passivation. This may result in failure to activate when the current demand increases (as can occur with anodes on coated structures when the coating deteriorates). 

Fig. 10.18 The effect of soil resistivity and current demand on the choice between impressed-current and sacrificial anode protection (after Ashworth et al )...

Cathodic Current Densities for Protecting Steel Examples of current density requirements for the protection of steel (to achieve a steel potential of —0-8 V vs. Ag/AgCl/seawater) are given in Tables 10.13 and 10.14. It should be realised that the current demand of a structure will be influenced by, inter alia, temperature, degree of aeration, flow rate, protective scales, burial status, presence of bacteria and salinity. 

Obviously, the total weight of the anode material must equal or be greater than the total weight, IF, calculated above. Similarly each anode must be of sufficient size to supply current for the design life of the cathodic protection system. The anodes must also deliver sufficient current to meet the requirements of the structure at the beginning and end of the system life. That is, if current demand increases (as a result of coating breakdown, for example) the output from the anodes should meet the current demands of the structure. 

Initial effective electrical resistance of tapes, as evidenced by the cathodic protection current demand, has been outstanding. There have been reports of increasing current demand with time which indicate a need for investigation. The current demand increase has been found, on occasion, to be due to poor construction practice, but not all tapes are affected in this way. 

The technical aspects influencing these changes are reviewed in this paper. Discussion of these trends is limited to the steel-based materials. The current demand for easy-open ends for food containers has led to the development of many scored easy-open ends. This is a subject in itself and is not included in this discussion. 

In general, most converters are tested on the bench with the electronic load set to constant current (CC mode). True, that s not benign, nor as malignant as it gets. But the implied expectation is that converters should at least work in CC mode. They should, in particular, have no startup issues with this type of load profile. But even that may not be the end of the story Some loads can also vary with time. For example, an incandescent bulb has a resistive profile, but its cold resistance is much lower than its hot resistance. That s why most bulbs fail towards the end of their natural lifetime just when you throw the wall switch to its ON position. And if the converter is powering a system board characterized by sudden variations in its instantaneous supply current demand, that can cause severe problems to the converter, too. The best known example of this is an AC-DC power supply inside a computer. The 12V rail goes to the hard disk, which can suddenly demand very high currents as it spins up, and then lapse back equally suddenly into a lower current mode. 

The couplings between the production decisions over the periods due to the storage ofbatches and due to late sale are shown in Figure 9.1. The storage enables to produce the products for future demands earlier, e.g., when the demand in the next period exceeds the capacity in the next period. The late sale with a tardiness of one period allows for the compensation of a previously incurred deficit, e.g., when current demands cannot be satisfied from current storage and production. 

The counter electrode is the current carrying electrode and it must be inert and larger in dimension. Platinum wire or foil is the most common counter electrode. For work with micro- or ultramicroelectrode where the maximum current demand is of the order of few microamperes, the counter electrode is not necessary. At very low current, a two-electrode system with the reference electrode can function as the current-carrying electrode with very little change in the composition of the reference electrode. Many commercial glucose sensors and on-chip microcells have such electrode configuration. 

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