Parallel-current flow

COUNTERCURRENT AND PARALLEL-CURRENT FLOWS. The two fluids enter at diiferent ends of the exchanger shown in Fig. 11.3 and pass in opposite directions through the unit. This type of flow is that commonly used and is called counterfiow or countercurrent flow. The temperature-length curves for this case are shown in Fig. 11.4 . The four terminal temperatures are denoted as follows ... 

F r d ic Current. The double layer is a leaky capacitor because Faradaic current flows around it. This leaky nature can be represented by a voltage-dependent resistance placed in parallel and called the charge-transfer resistance. Basically, the electrochemical reaction at the electrode surface consists of four thermodynamically defined states, two each on either side of a transition state. These are (11) (/) oxidized species beyond the diffuse double layer and n electrons in the electrode and (2) oxidized species within the outer Helmholtz plane and n electrons in the electrode, on one side of the transition state and (J) reduced species within the outer Helmholtz plane and (4) reduced species beyond the diffuse double layer, on the other. 

A possible problem of sealing the electrolyte path is found in the Foreman and Veatch cell. This can be avoided by placing the cells in a vessel. The best known example of this is the Beck and Guthke cell shown in Figure 8 (74). The cell consists of a stack of circular bipolar electrodes in which the electrolyte is fed to the center and flows radially out. Synthesis experience using this cell at BASF has been described (76). This cell exhibits problems of current by-pass at the inner and outer edge of the disk cells. Where this has become a serious problem, insulator edges have been fitted. The cell stack has parallel electrolyte flow however, it is not readily adaptable to divided cell operation. 

The field strength is uniform between parallel plates, whereas it varies in the space between concentric cylinders, being highest at the surface of the central cylinder. After corona sets in, the current flow will become appreciable. The field strength near the center electrode will be less than given by Eq. (17-18) and that in the major portion of the clearance space will be greater and more uniform [see Eqs. (17-23) and (17-24)]. 

FIG. 23-25 Typ es of industrial gas/Hqiiid reactors, (a) Tray tower, (h) Packed, counter current, (c) Packed, parallel current, (d) Falling liquid film, (e) Spray tower, if) Bubble tower, (g) Venturi mixer, h) Static in line mixer, ( ) Tubular flow, (j) Stirred tank, (A,) Centrifugal pump, (/) Two-phase flow in horizontal tubes. 

For those applications where high efficiency is important, synchronous rectification may be used on the higher current (power) outputs. Synchronous rectifier circuits are much more complicated than the passive 2-leaded rectifier circuits. These are power MOSFE B, which are utilized in the reverse conduction direction where the anti-parallel intrinsic diode conducts. The MOSFET is turned on whenever the rectifier is required to conduct, thus reducing the forward voltage drop to less than O.f V. Synchronous rectifiers can be used only when the diode current flows in the forward direction, that is in continuousmode forward converters. 

The current flowing through the parallel oscillator resistor is... 

Circuit elements may be connected in either a series or parallel configuration. In the series configuration, the same current flows through each and every element, and the circuit potential drop (or emf that is developed by the voltage source) is the algebraic sum of the potential drops of each individual element. For sources in series, the total emf developed is the algebraic sum of the emfs developed by each individual source. 

In the parallel configuration, the same potential difference occurs across each and every element with the total current being the algebraic sum of the current flowing through each individual circuit element. Table 2-35 summarizes the equivalent resistance, conductance, capacitance, and inductance of series-parallel configurations of resistors, capacitors, and inductors. 

By time/direction Protective devices responsive to the direction of current flow. These are necessary for parallel feeders or closed ring-main supplies. 

Fig. 10.38 Cathodic protection and interaction currents with parallel pipelines. Note that the pick-up area is the point on the unprotected pipe nearest to the groundbed at this point the current flows in opposite directions and attains a maximum at the points of discharge...

This value does not express the actual result since side and/or parallel reactions (e.g., H+ or 02 reduction) are not considered, but it does demonstrate the completeness of the cementation process and the effectiveness of this liquid-liquid extraction. During this extraction no external current flows through the phase boundary Hg (amalgam)/solution thereby establishing a potentiometric condition. The question of the potential difference at the phase boundary can be answered by constructing the experimentally accessible current-voltage curves for the reactions ... 

Current flow at electrode surfaces often involves several simultaneous electrochemical reactions, which differ in character. For instance, upon cathodic polarization of an electrode in a mixed solution of lead and tin salt, lead and tin ions are discharged simultaneously, and from an acidic solution of zinc salt, zinc is deposited, and at the same time hydrogen is evolved. Upon anodic polarization of a nonconsumable electrode in chloride solution, oxygen and chlorine are evolved in parallel reactions. 

Simmons JG (1964) Potential barriers and emission limited current flow between closely spaced parallel metal electrodes. J Appl Phys 35 2655-2658... 

On an inhomogeneous surface the two currents densities may vary over the surface, and need not balance locally only the total current must be zero. In this case we must replace the exchange current densities in Eqs. (11.40) - (11.42) by the corresponding exchange currents. Because of charge conservation an uneven current distribution on the electrode must be balanced by currents flowing parallel to the surface on both sides of the interface. 

The SI unit of current I is the ampere (A). An ampere was first defined as the current flowing when a charge of 1 C (coulomb) passed per second through a perfect (i.e. resistance-free) conductor. The SI definition is more rigorous the ampbre is that constant current which, if maintained in two parallel conductors (each of negligible resistance) and placed in vacuo 1 m apart, produces a force between of exactly 2 x 10-7 N per metre of length . We will not employ this latter definition. 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

---