Mercury cell performance

In a mercury cell, adjustments to the brine and caustic loops are independent but subject to the same sorts of fluctuation. Anode adjustment and the occasional stoppage to remove mercury butter can offset changes in mercury cell performance. 

It can be seen that membrane cells still have a production capability which is small compared with mercury cells, the limitation being the size of membrane sheet which can be manufactured and handled. In addition it should be recognized that the pipework for electrolyte feeds and product take-off is more numerous and much more complex than for diaphragm or mercury cells and there remains a lack of experience of and confidence in the long-term performance of the membranes. 

A. k-Factor. A popular diagnostic parameter that is measured and used in the chlor-alkali industry to monitor the performance of mercury and membrane electrolyzers and to control (manually or automatically) the anode-cathode in the mercury cells is the / -factor. The -factor is the slope of the current density-voltage curve, plotted from the cell voltages measured at dififerent loads. 

The operating voltage and current efficiency of a cell fix its unit DC power requirement. This varies with time, usually in the direction of increasing power consumption, and so cells and their auxiliaries must be designed with this fact in mind. In a diaphragm or membrane cell, performance declines over the lifetime of the anolyte/catholyte separator, usually as a result of the accumulation of impurities. The performance of a mercury cell... 

Membrane Cells. Membrane cells are not subject to the electrode poisoning suffered by mercury cells. They are in this respect similar to diaphragm cells, but the membranes themselves are exceptionally sensitive to brine impurities [77], and brine specifications for membrane cells are more onerous. Section 4.8 discussed the structure and performance of membranes and explained the reasons for this sensitivity. Certain impurities can affect cell performance and the service life of the membranes even when present at ppb levels. Their concentrations in the brine must be rigidly controlled. When this is done successfully and ultra-pure brine is consistently available, service life can be quite long, and test cells have operated well for up to 9 years [78]. 

A. Electrolytic Cells. Mercury cells are constructed with carbon steel, and the surfaces in contact with chlorine are rubber-lined. Diaphragm cells are also fabricated with carbon steel bodies and FRP or vinylester covers. The external carbon steel surfaces are usually treated with a high performance catalyzed polyamide or amine-cured painting system with 0.05-0.075-mm thick primer, 0.13-0.15-mm thick intermediate and a 0.13-0.15-mm thick finish coat, the thicknesses referring to the dry films. 

The retirement of mercury-cell plants has created a new issue, the disposal of their mercury inventory. This has been considered a waste by some. However, mercury remains a useful commodity and automatically regarding it as a hazardous waste may be counterproductive [96]. Handling of mercury is an issue that requires an accepted societal policy. The metal is available from strategic reserves, retired or converted cell rooms, and reclamation processes. Total stocks worldwide are estimated roughly to be 25-50,000 tons [97]. The gradual run-down of mercury inventory before retiring a plant is not a useful technique. The opposite approach, increasing the amount of mercury in the cells, has in fact been used in older plants to improve cell performance and reduce the loss of mercury to the environment [98]. 

The flow of liquid water in the decomposer is adjusted to obtain a 50 wt% NaOH solution and mercury is pumped back to the electrolysis cell. Typical operating conditions and performances of a mercury cell are as follows ... 

Castner had set the stage for the chlor-alkali industry in Europe with his invention of the mercury cell. The only competition then was the Billiter diaphragm cell. Unlike LeSueur s diaphragm, Billiter s was a poorer diaphragm in performance, being much less uniform than the asbestos paper used by LeSueur. So that in Europe the mercury cell reigned from the outset. 

Chemical by reaction with the components within the particulate matter. The most important components of the particulate matter for considering chemical reactions are likely to be metalhc components that may be able to react with the surface of catalysts like Pt, forming amalgams in the case of mercury, and thereby affecting fuel cell performance in a much less predictable manner. 

With the exception of the cathode in a mercury cell, there is freedom to select the anode and cathode materials according to performance. The two requirements are ... 

The performance of the mercury cell is very good since it keeps a steady voltage of approximately 1 2 volts over a wide range of current and for a fairly long time. 

Given the efficiency of VASP, electronic structure calculations with or without a static optimization of the atomic structure can now be performed on fast workstations for systems with a few hundred inequivalent atoms per cell (including transition-metais and first row elements). Molecular dynamics simulationsextending over several picoseconds are feasible (at tolerable computational effort) for systems with 1000 or more valence electrons. As an example we refer to the recent work on the metal/nonmetal transition in expanded fluid mercury[31]. 

In order to describe the geometrical and structural properties of several anode electrodes of the molten carbonate fuel cell (MCFC), a fractal analysis has been applied. Four kinds of the anode electrodes, such as Ni, Ni-Cr (lOwt.%), Ni-NiaAl (7wt.%), Ni-Cr (5wt.%)-NijAl(5wt.%) were prepared [1,2] and their fractal dimensions were evaluated by nitrogen adsorption (fractal FHH equation) and mercury porosimetry. These methods of fractal analysis and the resulting values are discussed and compared with other characteristic methods and the performances as anode of MCFC. 

Garcia-Monco Carra et al. [296] have described a hybrid mercury film electrode for the voltammetric analysis of copper (and lead) in acidified seawater. Mercury plating conditions for preparing a consistently reproducible mercury film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode , i.e., one preplated with mercury and then replated with mercury in situ with the sample, gives very reproducible results in the analysis of copper in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. 

Materials and instrumentation. Experiments were performed using a special home-made cell coupled to a dual current supply with a maximum output of 10 V/40 mA. A detailed technical description of this system is published elsewhere (10). H NMR spectra were recorded on a Varian Mercury vx300 instrument at 25 °C. GC analysis was performed on an Interscience GC-8000 gas chromatograph with a 100% dimethylpolysiloxane capillary column (DB-1, 30 m x 0.325 mm). GC conditions isotherm at 105 "C (2 min) ramp at 30 °C min to 280 °C isotherm at 280 °C (5 min). Pentadecane was used as internal standard. The ionic liquid [omim] [BF4] was prepared following a published procedure and dried prior to use (8). All other chemicals were purchased from commercial sources (> 98% pure). 

Monochromatic detection. A schematic of a monochromatic absorbance detector is given in Fig. 3.12. It is composed of a mercury or deuterium light source, a monochromator used to isolate a narrow bandwidth (10 nm) or spectral line (i.e. 254 nm for Hg), a flow cell with a volume of a few pi (optical path 0.1 to 1 cm) and a means of optical detection. This system is an example of a selective detector the intensity of absorption depends on the analyte molar absorption coefficient (see Fig. 3.13). It is thus possible to calculate the concentration of the analytes by measuring directly the peak areas without taking into account the specific absorption coefficients. For compounds that do not possess a significant absorption spectrum, it is possible to perform derivatisation of the analytes prior to detection. 

---