Kenkichi Izumi

Formation of oxides and hydroxides of iron in high-temperature water.

The heat transfer coefficient of heat exchangers decreases with operation time due to the fouling of heat transfer surfaces. The fouling substance consists of the corrosion products produced by the plant construction materials, and their constituents are mainly oxides and hydroxides of iron. The mechanism of formation of these iron oxides in water at high temperature was experimentally investigated. The experiments showed that the main products are Fe3O4, α-Fe2O3, and amorphous iron oxides. The products were morphologically controlled by the water temperature and the ratio of dissolved oxygen to Fe (II) ion concentration in water. In addition, the rate constants of the oxidation of Fe (II) ion in water were obtained.

The PH control in the large-scale MSF plants by using drop structure type decarbonator

The pH control by means of decarbonation is well known as the effective method for scale prevention. However, decarbonation and pH control characteristics for the large-scale plants are not so clear. So, we studied scale-up techniques for the module of MSF plants with a capacity of 100,000 m3/d under the sponsorship of the Ministry of International Trade and Industry in Japan. We studied many decarbonators, i.e, packed tower type, spary tower type, air bubbling type and drop structure type. The packed tower type decarbonator had most reliable performance in these equipments. As the scale-up technique of the packed tower had been completed, we examined the drop structure type decarbonator that was more simple than the others. The apparatus consists of eight step channels, (3m in length, 7m in width and 0.8m in drop distance) At the results of experiments, decarbonation phenomena had been successfully revealed as the mass transfer with chemical reaction of the carbon dioxide. Decarbonation ability depended on acid addition and sea water temperature from the point of equilibrium and drop distance for the structure factor. It became clear for the estimation of decarbonation performance to be able to adapt with mixing model rowing each step channel. On the other hand, it was established for the pH control condition that brine pH could set up in the range from 7.0 to 7.8 by using the method of partial neutralization. Further, during the long time continuous operation using these apparatus, we succeeded in getting the constant performance and preventing the scale deposition.

Decarbonation characteristics of the packed tower and its effects on scale prevention in flash evaporation plants

Abstract Scale prevention is one of the most important problems in the distillation type desalination plants. The pH control by means of decarbonation is well known as the effective method for scale prevention. However, decarbonation characteristics and the optimum conditions of the pH control are not so clear. We revealed the total carbonate ion equilibrium chart in sea water at the temperature range of up to 80°C, and examined decarbonation characteristics by using a packed tower type decarbonator with 0.8m in diameter and 2.5m packed height. At the results of experiments, decarbonation phenomena have been successfully revealed as the mass transfer with chemical reaction of the carbon dioxide. As main resistance of carbon dioxide diffusion in sea water existed in liquid falling films, it was quantitatively analyzed that tower ability (H.T.U) at the partial neutralization zone increased as acid was dosed. We used a scale test devices that was composed in 10-stage flash evaporator with a capacity of lOOm 2 /d, and measured scale deposition under various decarbonation level and the pH of feed sea water. According to the obtained results, factor of the scale deposition depended on brine pH and carbonate ion concetration. We recommend that brine pH is about 7.5 and that carbonate concentration is below lOppm.