Electrolytic power consumption

A large cell unit like the Asahi Chemical Co. bipolar cell Figure 15 is made of metal. The partition wall is an explosion bonded titanium to steel sandwich. The anode compartment is lined with titanium and the cathode compartment is steel. Metal components allow cell operation at 90°C while maintaining the high mechanical accuracy of the cell. Anodes and cathodes with an individual area of 2.7 m are assembled into units containing up to 80 cells with a distance between the two electrodes in the range of 2-3 mm. Ohmic drops in the catholyte and anolyte are thereby minimized. Electrolytic power consumption of the Ashai Chemical Co. cell is shown in Figure 16 (67). 

The Nafion membranes utilized in the early 1970 s produced caustic soda concentrations of 10-15wt% at electrolytic power consumptions of approximately 3450 KWH/MT NaOH. Advancements in the technology of membranes by duPont, Asahi Glass Co., and Asahi Chemical Co., Tokuyama Soda Co., have achieved membranes that today can produce caustic soda concentrations of 28-40wt% with caustic current efficiency well over 90% for long term operations. 

Further development of the "Zero Gap" membrane cell technology by Asahi Glass Co., called AZEC, is reported to have achieved at laboratory scale an electrolytic power consumption of 1950 KWH/M ton NaOH at 2 KA/M current density and 35% NaOH and at a current density of 4 KA/M the power consumption is 2140 KWH/M ton NaOH (78). 

It should be remembered that the electrolytic power consumption Wcell increases as the square of the cel) current, i.e. ... 

The cathode material is stainless steel. The lead produced by this method analyzes 99.99 + %. The overall power consumption is less than 1 kWh/kg of lead, so that the electrolytic process for treating spent batteries has much less of an environmental impact than the conventional pyrometaUurgical process. 

In concentrated electrolytes the electric current appHed to a stack is limited by economic considerations, the higher the current I the greater the power consumption W in accordance with the equation W = P where is the electrical resistance of the stack. In relatively dilute electrolytes the electric current that can be appHed is limited by the abflity of ions to diffuse to the membranes. This is illustrated in Eigure 4 for the case of an AX membrane. When a direct current is passed, a fraction (t 0.85-0.95) is carried by anions passing out of the membrane—solution interface region and... 

An imperative condition for materials being used as an electrolyte is the complete (or practically complete) absence of electronic conduction. Such conduction would amount to internal short-circuiting, leading to unproductive power consumption and/ or waste of electrode materials. 

Fig. 16.12 Principle and advantages of the falling film electrolyser. Advantages include improved mass and heat transfer low gas content in electrolyte low concentration difference constant hydraulic pressure low expected voltage of 2.70 V at 3 kA m-2 leading to power consumption reduction of 70 kWh per tonne of NaOH at 4kA m-2, including circulation pumps and small element depth. 

As indicated in Fig. 17.2, the membrane process has long been characterised by substantial reductions in electric power consumption, through constant advances in membrane, electrolyser and electrode technologies. In the early years of its commercial establishment, some 25 years ago, it yielded a caustic soda concentration of 20% or lower, with less than 90% current efficiency. Today, the caustic soda concentration is 33%, the current efficiency is 97%, and the ohmic drop of the membrane has been lowered by approximately 1.0 V. During the same period, advances in electrolyser design have improved the uniformity of intracell electrolyte concentra-... 

My 7-cell electrolyser seems to be running at well over 100% efficiency. It is currently producing 48 litres per hour at about 13.96V and 6.4A. Power consumption is about 89.3W straight DC. The electrolyte is 20% pure NaOH by weight. The ambient temperature is 15C and the temperature inside cell 36C. The cell has been running for about an hour. 

The electrolyte is very conductive. In my example of 7 separate containers wired in series, there s no charge applied to the middle ones either because they are in series If you wire 7 resistors in series you have 1/7th of the total voltage across each resistor. I wanted to get about 2V dc per cell, and with 7 cells you can use a 14V dc power supply (for example a battery charger). You could scale up to any number of plates, but the voltage across the stack would be higher. If 7 cells produce 7 units of gas with a given current then 100 cells would produce 100 units of gas with the same current, but the voltage across the whole stack would need to be about 2V multiplied by the number of cells. Thus the power consumption increases approximately linearly with the number of plates. 

The electrowinning process is connected with higher power consumption but, on the other hand, the electrolytically produced zinc has higher purity. Therefore, further investigations are in progress. The main factors that must be considered in electrowinning process are (1) the electrochemical properties of the cathode materials, (2) the effect of ionic impurities in the electrolyte, and (3) the cohesion strength between the deposited metal and its substrate. 

The main advantages of bipolar membranes are no formation of gases at their surfaces or within the bipolar membranes themselves, a power consumption to dissociate water into 02 and H2 about half that used in electrolytic cells, a minimum formation of by-product or waste streams in the case of dilute (< 1 kmol/m3) acids or bases, and reduced downstream purification steps. 

The modem alkaline and PEM electrolysers are characterised by productivity from few tens litres to several hundreds cubic metres per hour at output pressure 1 to 50 atm. Some electrolysis plants can produce up to 4 tons of hydrogen under pressure of 7 atm per a day. The typical current density and efficiency are of 1.6 A/cm2 and 60-75% (power consumption 4-5 kW-h/Nm3 H2), respectively. Nowadays, high operating pressure, up to 200 atm, is used to increase the efficiency of electrolysers. Also, the electrodes covered by noble metals and advanced electrolytes are applied. In the future it is possible to increase the efficiency of the electrolysers up to 80-90% [4-6]. 

The power consumption for the production of 1 Nm3 H2 does not exceed 4.1-4.3 kW h. This value mainly depends on the required purity of the delivered hydrogen and the design features of the electrolytic cell. 

An electrolyzer with a capacity up to 500 cu ni. per hour consists of 100 chambers, is 12 m long, 2.6 m wide and 5 m high. At a current load of 7500 A the electrodes operate with a current density of 25 to 30 A/sq. dm. When a 26—27 % solution of KOH is used as electrolyte at a temperature of 75 to 80 °C, the voltage across one compartment will be approximately 2.2 V and across the entire electrolyzer 350 V the power input will then be 2600 kw the power consumption necessary to produce 1 cu. in of hydrogen will range from 5.3 to 5.4 kw-hr. The hydrogen produced is 90.0% pure and the oxygen 99.5— 90.8% pure. 

As mentioned earlier, the gas holdup in mechanically agitated contactors has been found to be, apart from geometric parameters, a function of liquid viscosity, surface tension, the electrolytic nature of the solutions, foaming character, etc. Hence, phenomenologically the reduction in power consumption should also depends on these characteristics of the system. Bruijn et al (1974) concluded that the surface tension does not affect the mechanism of cavity formation and its shape to an appreciable extent. However, liquid viscosity influences the stability of the cavity. Hughmark (1980) presented the... 

Therefore, this last approach requires a much smaller potential than that in which hydrogen reduction is the cathodic process. In addition, the overall reaction is equal to the transformation of oxygen into ozone. Although this process requires a more sophisticated cathode, it significantly reduces power consumption and thus represents a promising approach. Solid polymer electrolytes can facilitate the process even more. 

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