Chlor-alkali energy efficiency

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

Only after viewing the membrane as a thin film semiconductive phase can one begin to seriously evaluate its potentialities. It is a multidimensional problem, and in the chlor-alkali cells the water transport is controlled by brine concentration while caustic strength controls the cathode efficiency. The membrane provides a low energy pathway for the phase change and separation process. 

The terminology employed in the chlor-alkali industry for comparing the performance characteristics of various cells is the energy consumption expressed in kilowatt hours per metric ton (kW h/M.T) of Cl2 or NaOH. This is related to the cell voltage ( ) and the current efficiency (17) as... 

Diaphragm cells used in chlor-alkali production are also effectively a parallel-plate flow reactor but they are constructed in a very different way they will be discussed in the next chapter. While the potential distribution in a parallel-plate cell is good and the mixing conditions can be made to meet most requirements, the space time yield leaves much to be desired and it is often difficult to reduce the inter-electrode gap sufficiently to give the required space time yield and energy efficiency. These problems have led to the development of many novel cell designs at the present time they remain laboratory or pilot-plant concepts but it is to be expected that some will eventually have an impact on the industrial scene. 

The major use of perfluorinated membranes, at present, is as separators in chlor-alkali ceiis. The combination of low resistances, high current efficiencies at high solution concentrations, and high temperatures that can be achieved results in 20-30% lower energy requirements than those achieved with diaphragm or mercuiy cells. The long membrane lifetime (typically 2 years) results in low cost for membrane replacement. (Asbestos diaphragms usually last for only a year or less.)... 

The units of the energy consumption figures calculated using Eq. (5) are in DC kWhr/unit product. However, some chlor-alkali plants require data on the energy consumption expressed in AC kW hr/unit product, in which case the rectifier efficiency, rectifien has to be taken into account ... 

In a chlor-alkali plant, rectifier specifications are not set in isolation but are part of the larger question of circuit design. Rectifiers are matched with groups of cells or electrolyzers, and the goals of high rectification efficiency and low cost must be compromised in the interest of safety, fluctuating energy demand, and an efficient cell layout. 

FIGURE 10.2.7. Energy distribution in a membrane chlor-alkali cell (MGC-26) operating at 5kAm . (Energy consumption 2,607 kW hr ton" of CI2 Cell voltage 3.35 V Current efficiency 97%.)... 

The major objective of a test run will be to demonstrate that the plant can produce the required amount of product at the expected energy efficiency. The relationship between these two parameters is especially important in a chlor-alkali plant. It is usually possible... 

Today, the membrane process is state of the art. Its energy demand is about 25-30 % lower its operation is easier, safer, more efficient and flexible and needs less maintenance in comparison with the traditional processes, without environmental problems of asbestos or mercury (see entry Chlorine and Caustic Technology, Overview and Traditicaial Processes ). Investment costs and space requirements are significantly reduced. AU new chlor-alkali electrolysis plants use the membrane process since about 20 years. 

The mercury process needs the most electrical energy, but no steam is required to concentrate the caustic solution. The overall primary energy needed (if a power plant efficiency of 40% is assumed) for the mercury and diaphragm process is about the same, whereas the membrane process is more efficient (Table 6.19.6). Today, 54% of all chlor-alkali plants are membrane plants (Table 6.19.7) in the last 20 years, all new plants utilize this technology (Behr, Agar, and Joerissen, 2010). 

Three processes of chlor-alkali electrolysis are currently used, namely, the mercury process, diaphragm process, and membrane process. With regard to energy consumption and environmental concerns, the membrane process is the most efficient. 

The ideal chlor-alkali process is one that is energy-efficient and does not use mercury. A type of cell offering these advantages is the membrane cell, in which the porous diaphragm of Figme 19-24 is replaced by a cation-exchange membrane, normally made of a fluorocarbon polymer. The membrane permits hydrated cations (Na" " and H3O ) to pass between the anode and cathode compartments but severely restricts the backflow of Cl and OH ions. As a result, the sodium hydroxide solution produced contains less than 50 ppm chloride ion contaminant. 

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