Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Concentration controlled processes

When one of the reactions is limited by the rate of transport of the reactant to the metallic surface being corroded, the situation increases in complexity as illustrated in the polarization plot of the system in Fig. 5.16. The system represented here is similar to the previous one, [Pg.104]

In order to model the polarization plot, the total cathodic current corresponding to the sum of the currents of the hydrogen reaction and oxygen reduction has to be balanced by the single anodic current. The intercept where the opposing currents are balanced occurs at an E of -0.33 V vs. SHE and, since the surface area is still 1 cm an i of 8.2 pA cm or 0.1 mm y -. It should be noted that the current for the reduction of oxygen is constant across the potential range shown in Eig. 5.16 and has the value of 6.3 pA cm .  [Pg.106]

The reduction of oxygen depends, among other factors, on the level of agitation of the environment. If the limiting current of this reaction is now increased through agitation by a tenfold factor to reach the value of 63 pA cm a new situation emerges as depicted in Fig. 5.17. The marked positive shift of now -0.224 V vs. SHE, is accompanied by a marked increase in current density, now of 63 pA cm or 0.8 mm yk [Pg.106]


Concentration-controlled processes. When concentration control is added to a process, it simply adds to the polarization, as in the following equation . [Pg.49]

Concentration Control. Sequestration, solubilization, and buffering depend on the concentration control feature of chelation. Traces of metal ions are almost universally present in Hquid systems, often arising from the materials of the handling equipment if not introduced by the process materials. Despite very low concentrations, some trace metals produce undesirable effects such as coloration or instabiHty. [Pg.392]

Another purpose of inerting is to control oxygen concentrations where process materials are subject to peroxide formation or oxidation to form unstable compounds (acetylides, etc.) or where materials in the process are degraded by atmospheric oxygen. An inert gas supply of sufficient capacity must be ensured. The supply pressure must be monitored continuously. [Pg.38]

The major difference between diffusion controlled dispersion and that resulting from adsorption and desorption is that the transfer process is concentration controlled. Reiterating equation (7),... [Pg.254]

Buildings are ventilated mechanically with the ITVAC systems where it is a controlled process, as well as via air infiltration and through the openable windows and doors where it is largely an uncontrolled process. However, as discussed earlier, mechanical ventilation is one of the most energy-intensive methods of reducing indoor pollutant concentrations primarily because of the need to thermally condition air before it can be circulated inside the occupied spaces. It is estimated that the... [Pg.55]

A striking example of the interaction of solution velocity and concentration is given by Zembura who found that for copper in aerated 0-1 N H2SO4, the controlling process was the oxygen reduction reaction and that up to 50°C, the slow step is the activation process for that reaction. At 75 C the process is now controlled by diffiision, and increasing solution velocity has a large effect on the corrosion rate (Fig. 2.5), but little effect at temperatures below 50 C. This study shows how unwise it is to separate these various... [Pg.322]

For any specific BW treatment application, determining the types and concentrations of polymers that are likely to prove the most successful remains a difficult task. There are few design rules, the in-field application and control processes are still more art than dependable science, and the various reaction mechanisms are not... [Pg.440]

It is usually believed that the growth of dendritic crystals is controlled by a bulk diffusion-controlled process which is defined as a process controlled by a transportation of solute species by diffusion from the bulk of aqueous solution to the growing crystals (e.g., Strickland-Constable, 1968 Liu et al., 1976). The appearances of feather- and star-like dendritic shapes indicate that the concentrations of pertinent species (e.g., Ba +, SO ) in the solution are highest at the corners of crystals. The rectangular (orthorhombic) crystal forms are generated where the concentrations of solute species are approximately the same for all surfaces but it cannot be homogeneous when the consumption rate of solute is faster than the supply rate by diffusion (Nielsen, 1958). [Pg.73]

Given that, under the defined conditions, there is no interfacial kinetic barrier to transfer from phase 2 to phase 1, the concentrations immediately adjacent to each side of the interface may be considered to be in dynamic equilibrium throughout the course of a chronoamperometric measurement. For high values of Kg the target species in phase 2 is in considerable excess, so that the concentration in phase 1 at the target interface is maintained at a value close to the initial bulk value, with minimal depletion of Red in phase 2. Under these conditions, the response of the tip (Fig. 11, case (a)] is in agreement with that predicted for other SECM diffusion-controlled processes with no interfacial kinetic barrier, such as induced dissolution [12,14—16] and positive feedback [42,43]. A feature of this response is that the current rapidly attains a steady state, the value of which increases... [Pg.307]

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... [Pg.195]

See also Minerals recovery/processing Minerals recovery/processing, 16 595-668. See also Minerals concentration Minerals processing classification in, 16 618-622 economic aspects of, 16 606-609 environmental aspects of, 16 609—610 flow sheets in, 16 603-605 materials handling in, 16 660-663 from ocean waters, 17 695—697 ores, 16 598-603 process control in, 16 663-665 size reduction (comminution) in, 16 610-615... [Pg.589]

The effective molarity (EM) is formally the concentration of the catalytic group (RCOO- in [5]) required to make the intermolecular reaction go at the observed rate of the intramolecular process. In practice many measured EM s represent physically unattainable concentrations, and the formal definition is probably relevant only in reactions (which will generally involve very large cyclic transition states) where the formation of the ring or cyclic transition state per se is enthalpically neutral, or in diffusion-controlled processes. For the formation of small and medium-sized rings and cyclic transition states the EM as defined above contains, and may indeed be dominated by, the enthalpy of formation of the cyclic form. This topic has been discussed briefly by Illuminati et al. (1977) and will be treated at greater length in a future volume in this series. [Pg.187]

The formation of excimers and exciplexes are diffusion-controlled processes. The photophysical effects are thus detected at relatively high concentrations of the species so that a sufficient number of collisions can occur during the excited-state lifetime. Temperature and viscosity are of course important parameters. [Pg.94]


See other pages where Concentration controlled processes is mentioned: [Pg.104]    [Pg.24]    [Pg.104]    [Pg.24]    [Pg.21]    [Pg.189]    [Pg.1683]    [Pg.81]    [Pg.221]    [Pg.311]    [Pg.326]    [Pg.1245]    [Pg.879]    [Pg.993]    [Pg.81]    [Pg.990]    [Pg.249]    [Pg.269]    [Pg.559]    [Pg.52]    [Pg.301]    [Pg.273]    [Pg.394]    [Pg.317]    [Pg.318]    [Pg.351]    [Pg.326]    [Pg.353]    [Pg.149]    [Pg.14]    [Pg.76]    [Pg.551]    [Pg.136]    [Pg.661]    [Pg.180]    [Pg.249]    [Pg.220]   
See also in sourсe #XX -- [ Pg.104 , Pg.105 ]




SEARCH



Concentrate processing

Concentration process

Process control brine concentration

Processing concentrations

© 2024 chempedia.info