Toshio Sawa

Heat recovery characteristics of membrane distillation

A simulation model, using a mass transfer coefficient which has as its driving force a humidity difference and a heat transfer coefficient which has as its driving force an enthalpy difference, is developed for investigating the operating characteristics of membrane distillation. Optimum conditions are calculated by the model and the calculated results are proved by actual experiments. Simulated results show that the higher the feed input temperature and the cooling water input temperature, the smaller are the recycling rate, membrane area, and heat energy. For a system which has a 0.216-m 2 membrane and heat exchange area, optimum conditions of feed input and cooling water input temperature and recycling rate are 90 °C, 50 °C, and 18.5 kg/h, respectively. Furthermore, an actual experiment was performed and its permeate rate and heat input were 1 kg/h and 0.21 kW.h. From the work, the latent heat recovery is seen as about 66 percent and the simulation model reliability is verified.

Development of water re-use treatment system for municipal wastewater

Abstract The move for environmental protection and preservation of clean water sources has given rise to the latest tendency in Japan toward reuse of waste water after advanced treatment. As a means of saving water and securing water sources, water reuse systems are beginning to be installed to treat waste water for reuse as flushing water and cooling water for air conditioners. This study was conducted with the aim of developing a compact high-performance water treatment system that may meet the stated requirement. Most water reuse systems in practical use today are based on biological treatment. With this process, treatment is slow and equipment takes up much space. In addition, it involves a large amount of sludge. In an attempt to overcome these disadvantages, we developed a physico- chemical treatment technique that combines the three elementary techniques of dissolved air flotation, foaming separation, and activated carbon packed electrolyzer, and that permits multifunctional treatment with compact equipment. The activated carbon packed electrolyzer, which forms the core of the new system, is a newly developed technique. The purification mechanism was established, and conditions of application to actual waste water were determined. The electrolyzer was combined with the dissolved air flotation and foaming separation methods, to make up an optimum system. With a 10 m3/d trially manufactured apparatus, a demonstration test on reuse of sewage was performed to study the performance and economics of the system. The desired target was achieved and prospects were gained for commercialization.

Permeate Characteristics of Volatile or Nonvolatile Substances in Membrane Distillation.

直接接触型膜蒸留法を実用化するに際し, 原水中に不純物が含まれている場合を想定し, 不揮発性成分ではその濃縮限界を, また, 揮発性成分では水蒸気透過特性に及ぼす影響を実験から検討した.実験は, 不揮発性成分としてNa2SO4水溶液を, 揮発性成分としてエタノールおよびアンモニア水溶液を用いて膜蒸留実験を行った.その結果, 揮発性不純物の透過機構を解明するとともに, アンモニアについては, 原水のpHを酸性 (6以下) にすることで, 透過水側へのアンモニアの混入がないことを把握した.さらに, 膜前後の熱・物質収支の式に濃度分極を考慮することで, いずれの場合もシミュレ-ションを可能にした.

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.