NaOH current efficiency

Eq. (60) reiterates that the disparity between the Ch and NaOH current efficiencies is a result of the alkalinity in the feed brine and the active chlorine species in the anolyte. If the feed brine is neutral and there is no active chlorine in the anolyte, it can be seen that... 

E = energy consumption, kWhrt NaOH V = operating or normalized voltage, V = NaOH current efficiency, %... 

For monitoring cell performance and comparing dififerent electrolyzer designs, the electric power required to produce one tonne of NaOH 100% is considered. This figure is determined by the voltage drop over one cell and the NaOH current efficiency. 

However, the iadustry s popular terminology is the energy consumption expressed ia terms of kilowatt hours per ton of (Pq[) oi of NaOH An estimate of this value requires a knowledge of cell voltage, current efficiency, and the efficiency of the rectifier used to convert a-c power to d-c. The energy consumption for producing a ton of is... 

H-D Tech Inc. installed a commercial electrosynthesis plant at the Muskogee mill of Fort Howard Paper in 1991. The H-D Tech reactor operates near atmospheric pressure at 0.6 kA/mz/2V/30 °C to produce solutions containing up to about 4 wt% H202 in 6wt% NaOH at about 85% current efficiency [129]. 

Fig. 17.7 Influence of Na+ concentration at interface of sulphonic acid and carboxylic layers on current efficiency 3-4 N NaCI 1.0-5.5 kA rrT2 33% NaOH.

Fig. 17.8 Current efficiency at high current densities in a high-circulation laboratory cell 32% NaOH 90°C. 

OHj OHaOH + 3NaI + 8H,0 = Na 003 + OHIs + NaOH + 5H2. The cathode should be of lead, and encased in parchment to prevent cathode hydrogen from destroying the product.2 The temperature should be 60-70° C. and anode current density 1-2 amps, per dm.2 with a voltage of 2-2 5 volts. The iodoform is produced at the rate of 500 grams per kilowatt-hour (1 3 grams per amp.-hour) and the current efficiency is about 90 per cent. 

Fig. 4. Dependence of incident photon to current efficiency (IPCE) on excitation wavelength and presence of current-doubling agent. ITO electrodes were covered with P25 (squares) or 3% [PtCl4]/P25 (circles) and biased at a constant potential of 0.7 V vs. Ag/AgCl in 0.1 M NaOH solution (a)). After addition of 0.1 M HCOONa the IPCE values increased (open circles, (b)). The inserts show the zoomed visible region.

The electroreduction of trichloroethylene (0.4 g L 1) on Cu in 0.05 M NaOH was found to be more efficient than on Ag or Cd cathodes [4], with the current efficiency increasing when the applied current density decreased. At a current density of 4 mA cm-2, the current efficiencies for the dehalogenation of monochloroacetic acid, dichloroacetic acid, chloroform, and trichloroethylene were 2%, 10%, 87%, and 29%, respectively. 5-Chlorosalicylic acid could not be dechlorinated on Cu. Nagaoka et al. [17]... 

From the loss of weight of the copper anode determine the number of faradays of electricity that were used in the cell. The ratio of the number of equivalents of NaOH to the number of faradays gives the current efficiency. 

The alkali hydroxide solution produced in this electrolyzer had a concentration of 110 to 130 grams of NaOH per litre and the average current efficiency amounted to 88 per cent. The voltage across the bath fluctuated between 3.7 and 4.2 V. 

At a voltage of 4.6 and 5.2 V a caustic solution is obtained which contains 125 to 140 grains of NaOH per a litre of solution the current efficiency is some 94 per cent. The anode current density is fairly high, 10 to 15 A/sq. dm. The temperature of the electrolyte is about 60 °C. 

The first Vorce electrolyzer was built for a load of 1000 A electrolysis was performed at 7 A/sq. dm, 75 °C, and some 3.8 V. Current efficiency was 91—95 per cent and one litre of solution produced contained about 100 grams of NaOH, 190 grams of NaCl and 0.8 grams of NaC103. The chlorine was 95—96 per cent pure and contained between 0.8 and 1.5 per cent of carbon dioxide. 

Davison et al. [73] produced hydrogen peroxide in a trickle-bed cell design (Fig. 15) with graphite chips and reticulated vitreous carbon (RVC) cathodes in 2 M NaOH electrolyte. Using graphite chips as the cathode, the current efficiency was 100% for H2O2 evolution at potentials more positive than - 0.7 V vs. SCE. Under identical conditions the RVC cathodes exhibited similar behavior up to -0.7 V. 

After several years of operation, a membrane life of 20,000 h was reached for the BPMs, while the CEMs have been replaced after 18,000 h. The electrodes have not been changed until now. The customer has increased the capacity in several steps by adding several EUR20 and EUR40 stacks and an additional capacity expansion is being considered. By changing the operation from batch mode to feed and bleed, the base concentration could be increased from 6 to 8 wt% with the same current efficiency (for a new plant, the NaOH could be produced at 10 wt%). The main operating costs are power at 0.88 kW h/kg of produced acid and membrane (electrodes) replacement at 0.077 /kg. 

The current efficiency amounts to ca. 95% again. The reason for losses lies in the fact that the membranes are not completely impermeable for OH- -ions, so that reactions (6) to (8) can take place to a certain amount. Caustic alkali is generated with ca. 33 wt%, in theory it would be free of chloride-ions, but in reality Cl--ions can penetrate the membranes, too, by a certain amount. So, the NaOH generated will contain up to 50 ppm Cl-. 

According to Burkhardt (.22), the current efficiency of the Nafion membrane exhibits unusual characteristics with respect to NaOH concentration in the electrochemical experiment. Three distinct regions exist in the plots of current efficiency versus caustic con-... 

Some of these features are illustrated in Figures 14-18. A rather typical literature plot of current efficiency vs, sodium hydroxide concentration for perfluorosulfonate membranes is shown in Fig. 14. Nation 427 is a 1200-EW sulfonate membrane with fabric reinforcement. Poor hydroxide rejection occurs at catholyte concentrations above 10 wt % but a minimum is seen at higher concentrations, wtih increasing current efficiency from 28 to 40% caustic (9-14 M). The current efficiency of a 1200-EW homogeneous perfluorosulfonate film is shown in more detail over this concentration region in Fig. 15. Sodium ion transport number niol F ), which is equivalent to caustic current efficiency, is plotted vs. both brine anolyte and caustic catholyte concentration. These values were determined using radiotracer techniques, which have proven to be rapid and accurate methods for the determination of membrane performance. " " " A rather sharp maximum is seen at 14 M NaOH, and the influence of brine con-... 

Mauritz and co-workersand Hopfinger have reported spectroscopic and sorption studies of perfluorosulfonate membranes. Several conclusions drawn in these studies are useful in the interpretation of current efficiencies as a function of caustic solution concentration. Sorption measurements of an 1100-EW film in equilibrium with NaOH solutions from 7.5 to 18 M show that the... 

The variation of current efficiency with solution concentration in the chlor-alkali environment is an added complicating feature of these membranes behavior. Kruissink (9) has performed elaborate calculations to yield the effect of electro-osmotic water transport on permselectivity, in classical terms. Results suggest that the minimum seen in t a+ (at lower NaOH concentrations than used here)... 

For the application of these membranes to the electrolytic production of chlorine-caustic, other performance characteristics in addition to membrane conductivity are of interest. The sodium ion transport number, in moles Na+ per Faraday of passed current, establishes the cathode current efficiency of the membrane cell. Also the water transport number, expressed as moles of water transported to the NaOH catholyte per Faraday, affects the concentration of caustic produced in the cell. Sodium ion and water transport numbers have been simultaneously determined for several Nafion membranes in concentrated NaCl and NaOH solution environments and elevated temperatures (30-32). Experiments were conducted at high membrane current densities (2-4 kA m 2) to duplicate industrial conditions. Results of some of these experiments are shown in Figure 8, in which sodium ion transport number is plotted vs NaOH catholyte concentration for 1100 EW, 1150 EW, and Nafion 295 membranes (30,31). For the first two membranes, tjja+ decreases with increasing NaOH concentration, as would be expected due to increasing electrolyte sorption into the polymer, it has been found that uptake of NaOH into these membranes does occur, but the relative amount of sorption remains relatively constant as solution concentration increases (23,33) Membrane water sorption decreases significantly over the same concentration range however, and so the ratio of sodium ion to water steadily increases. Mauritz and co-workers propose that a tunneling process of the form... 

Figure 9. Current efficiency vs. NaOH catholyte concentration for Nafion 227 membrane in a chlor-alkali cell (34). Conditions current density, 31 A/dm2 temperature, 85° C anolyte concentration, 4.4 N NaCl cell voltage, 4.6 V.

NEOSEPTA-F C-1000 membrane optimum catholyte concentration is about 20 wt% NaOH. NEOSEPTA-F C-2000 give a maximum value of current efficiency at nearly 30 wt% catholyte current ratio (68). 

The Hooker MX, Allied PCF, and Ionics Chloromate electrolyzers are all bipolar designs that utilize plastic cell frames The electrolytic area of these cells is typically 1.2-1 5 per cell unit The electrolyzers are typically composed of 10-50 cell units Power consumptions are reported to be 2750 KWH/metric ton of NaOH operating at 3KA/M current density at 95% current efficiency producing 35% caustic soda (65,69,70) ... 

---