Gas void fraction

Most of the voltage savings in the air cathode electrolyzer results from the change in the cathode reaction and a reduction in the solution ohmic drop as a result of the absence of the hydrogen bubble gas void fraction in the catholyte. The air cathode electrolyzer operates at 2.1 V at 3 kA/m or approximately 1450 d-c kW-h per ton of NaOH. The air cathode technology has been demonstrated in commercial sized equipment at Occidental Chemical s Muscle Shoals, Alabama plant. However, it is not presentiy being practiced because the technology is too expensive to commercialize at power costs of 20 to 30 mils (1 mil = 0.1 /kW). 

Zuber showed that Eq. (70) corresponded to the result obtained experimentally by Armand (A7), with K = 0.S33, and that it could correlate gas void-fractions as a function of quality for values of Ra to 0.8 for vertical flow. 

In many engineering applications, gas-liquid flows of larger gas void fractions are encountered. In the next section, a method is described that makes use of the peak ultrasound echo intensity to detect the location of the gas-liquid interface in such flows. 

The results presented show the gas-liquid interface can be detected by two different methods. First, we can detect changes in the velocity gradient near the interface. In this method, only velocity information of the liquid is needed however, this method is restricted to low gas void fractions and bubbles larger than 102. The main motivation for applying this technique is in the development of a UVP-based sensor to reduce frictional drag. This sensor can be incorporated into ships or pipelines. Our results are part of an intermediate step crucial in the development of such a device. 

The bubbles in the inter-electrode gap (bubble diffusion region and bulk region) increase the inter-electrode resistance, as they affect the electrical conductivity of the electrolyte. The parameter describing this increase is the gas void fraction e, defined as the fraction between the volume of gas and the total volume of liquid and gas. Several relations are used in the electrochemical literature to quantify this effect. The most widely used are the relations from Bruggeman [16] ... 

Figure 3.9 Relative conductivity k /k of an electrolytic solution as a function of the gas void fraction according to Bruggeman [16] and Maxwell [87].

Let us first start with the description of the bubble diffusion region that is directly accessible to visual observations. The important parameters that characterise the bubble diffusion region are the gas void fraction e and the geometrical shape of the bubble layer, both of which are dependent on the cell geometry. We will restrict the discussion to the case that is most representative of micromachining with electrochemical discharges the case of a thin electrode dipped inside an electrolyte. The electrochemical cell is much larger than the electrode. 

To describe the contribution to the inter-electrode resistance a simplified model will be used. Let us assume that the bubble diffusion layer has a mean thickness d, and that the gas void fraction is constant and equal to e over the whole electrode height h. The efficient conductivity of the electrolyte is... 

Considering the gas void fraction s Fco = = 300 kmol/h But, under normal conditions, the volumetric flow is ... 

The calculation of the distance between the anode and the separator is slightly complex as the asbestos diaphragm is deposited on the cathode screen, and the anode blade is located 4-5 mm from the diaphragm with the anolyte in between, and contains dispersed chlorine gas bubbles. The gas-solution mixture is circulated effectively by the convection of the two-phase flow resulting in a relatively low gas void fraction in the electrolysis zone. The superficial resistivity, Pmix. based on Eq. (163), would be about 1.2 times the resistivity of the solution free of gas bubbles, which is 1.2/0.59 = 2.03 cm. The ohmic drop between the anode and the diaphragm can then be calculated from Eq. (165) and the current as ... 

The gasket thickness sets the electrode spacing, and either finite- or narrow-gap configuration is possible. The anode frame is titanium and the cathode frame is stainless steel or nickel. The anolyte volume is larger than it is in competing elements, therefore, the electrolyte gas void fraction is smaller. This feature also reduces the change in liquid... 

Certain organics will degrade the performance of chlor-alkali cells by producing foam, which increases the gas void fraction in the cells and can cause the membranes to dry out, and by blocking the active sites in the membranes, resulting in lower current efficiency. Organics also affect the catalytic activity of coated anodes and shorten their... 

The electrical conductivity of the gas-solution mixture is a function of the gas void fraction, e. The larger the void fraction, the larger is the resistivity, since... 

At the top of the cell, the gas separates from the solution and the solution free of bubbles falls to the bottom of the cell. This creates an internal circulation, thereby reducing the gas void fraction, and hence, the voltage drops in the solution. In other words, the design of the solution downcomer in the cells is of great importance for vertical cells containing gas-evolving electrodes. 

The problem of two-phase flow has been investigated extensively in the field of chemical engineering since the 1950s. Still, the factors involved in two-phase flow are not completely clarified, as it is related to many variables such as the physical properties of the liquid and gas, the volume fraction of gas or the gas void fraction, the flow velocity, and the geometry of the space where the two-phase flow occurs. 

We have considered reactors operating at low current densities. At higher values when bubble motion is hindered and coalescence occurs, the rise velocity is likely to be affected by the local rate of gas generation at the electrode. The ability of the electrolyte to circulate between the electrodes will influence the distribution of gas voidage. Hine has addressed this problem and has obtained expressions for the average gas void fraction. 

Szekely and co-workers used this equation to determine gas void fraction thus,... 

Castillejos et al. [52,53] solved the quasi-single-model equations considering that the ladle is occupied by a single-phase fluid with spatially variable density. Gas void fraction is prescribed with an empirical model developed from the experimental data of Castillejos and Brimacombe [18,52,53] thus ... 

The first fiow characteristics of single phase stirred tank reactors were performed using the HFA technique. For example, Gtinkel and Weber [77] used a hot-wire technique to measure instantaneous fluid velocities in the bulk flow and between the impeller blades in baffled stirred vessels. The HFA technique was later used to measure the first instantaneous fluid velocities in dispersed two-phase (gas-liquid) dispersions. Attempts were also performed to measure the local gas void fraction. Many examples of such investigations of two-phase bubble column flows can be found in the literature [67, 72, 76, 135, 199]. 

Hills [88] was among the first to measure the local gas void fraction in a two-phase bubble column using a One-Point Electro Conductivity Probe (OPECP). The column was made of perspex with ID 138 mm and height 1370 mm. Three different sieve plates were used as gas distributors. Hills [88] did perform measurements at... 

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