Heike Glade

CO2 release in multiple-effect distillers controlled by mass transfer with chemical reaction☆

Abstract For predicting the CO 2 release rates in multiple-effect distillers, the theory of desorption with chemical reaction was applied. The mass transfer processes and the reaction kinetics controlling the CO 2 release were described. In the pH and temperature range found in the evaporator stages of multiple-effect distillers, the alkaline reaction mechanism with the steps CO 2 + OH − ↔ HCO 3 − and HCO 3 − + OH − ↔ CO 3 2− + H 2 O predominates. The reaction CO 2 + OH − ↔ HCO 3 − is relatively slow, and therefore it is the rate-determining step in the reaction mechanism. Information for predicting mass transfer coefficients in water films flowing over horizontal tubes is still limited. Different approaches were shown. The phase interface area for CO 2 release was described as the sum of the surface area of the liquid film on the tubes plus the surface area of the liquid between the tubes, assuming that liquid columns can be expected to occur between the tubes. Depending on the magnitude ofthe mass transfer coefficient, it was found that CO 2 desorption from the evaporating brine either takes place in the transition regime from a slow to a fast reaction or in the fast reaction regime. Mass transfer and chemical reactions simultaneously take place in the boundary layer at the phase interface. Mass transfer is slightly enhanced by the reaction. The chemical rate constant becomes more important and the mass transfer coefficient less important in the correlation for the CO 2 release rate.

Techno-economic analysis of combined concentrating solar power and desalination plant configurations in Israel and Jordan

Abstract Combined concentrating solar power (CSP) and desalination plants represent a realistic future option for the production of electricity and fresh water for countries of the world’s sunbelt. In this paper, parabolic trough power plants for electricity production have been analysed in combination with multi-effect distillation (MED) and ultrafiltration/reverse osmosis (RO) desalination plants for two sites in Israel (Ashdod) and Jordan (Aqaba). Both RO and MED desalination plants were designed for a fresh water production capacity of 24,000 m3/d. The power block of the CSP plant was selected to meet the steam consumption of the MED plant at the design point, which led to a gross electrical power generation capacity of the power block of 42 MWel. Due to the low availability and generally high cost of coastal land, the CSP + RO plant consists of two separate units. It was assumed that the CSP plant is located at an inland location where there is land available. The RO plant is located at the sea, whil...

Scale Formation and Mitigation of Mixed Salts in Horizontal Tube Falling Film Evaporators for Seawater Desalination

Scale formation on heat transfer surfaces is one of the most severe problems in multiple-effect distillers for seawater desalination. A horizontal tube falling film evaporator in pilot-plant scale was used to study crystallization fouling under conditions close to those prevailing in industrial multiple-effect distillers. Experiments were performed with artificial seawater and model solutions based on artificial seawater under various process conditions. In experiments with artificial seawater, the surface of copper–nickel 90/10 tubes was covered with a two-layer scale comprising a thin, flaky magnesium-rich and calcium-free base layer underneath a thick layer of calcium carbonate crystals in the form of aragonite. Analyses indicated Mg(OH)2 (brucite) and iowaite in the thin base layer. The magnesium-rich scale layer was formed even at a low evaporation temperature of 50°C, which promotes the assumption of locally high pH values at the metal-solution interface. A shift of pH to high values in the thin seawater film due to CO2 release and, additionally, cathodic reactions may promote a high degree of supersaturation of Mg(OH)2. Once the tube surface is completely covered with the thin Mg-rich scale layer, it seems that the growth of the Mg-rich layer ceases and aragonite crystals start to precipitate. A decrease in the Mg2+ ion concentration in the solution results in an increase in the mass of calcium carbonate as aragonite in the scale layer. Results suggest that Mg2+ ions retard the calcium carbonate crystallization.

Reactive crystallisation process for magnesium recovery from concentrated brines

AbstractSeawater brines, generated either by natural or anthropic processes, often cause significant environmental issues related to their disposal. A clear example is the case of brines from desalination plants, which can have severe environmental impacts on the receiving water body. On the other side, brines can represent a rich and appealing source of raw materials, especially when they are very concentrated, as it happens with bitterns (i.e. exhausted brines) produced in saltworks. In particular, magnesium concentration can reach values up to 30–40 kg/m3 of brine, which is 20–30 times that of typical seawater. An experimental campaign has been carried out in the present work for assessing the potentials for magnesium recovery from concentrated brines. Real brines were collected from the final basins of the saltworks operating in the district of Trapani (Sicily, Italy). Experiments were performed both in a semi-batch and in a continuous 5 liters crystalliser operated by a reactive precipitation process...

Wetting behaviour of different tube materials and its influence on scale formation in multiple-effect distillers

AbstractMultiple-effect distillation (MED) plants with horizontal tube falling film evaporators for sea water desalination exhibit relatively high heat transfer coefficients achieved by film flow under clean surface conditions. However, they are susceptible to heat transfer deterioration and scale formation accompanied by film breakdown. Thus, maintaining all heated tubes in a fully wetted state is one of the key issues to be considered in designing falling film evaporators. Surface tension, hydrophilicity, hydrophobicity and wettability of materials play an important role in scale formation. The wetting rate, that is the brine mass flow rate on one tube per unit tube length, is one of the most important parameters in the design and operation of MED plants. It has an impact on heat transfer, tube bundle design, scale formation, the capital and operational costs of the plant and its operational flexibility. Thus, it is very important to acquire a better understanding of the wetting behaviour and the impact...

Impact of Tube Surface Properties on Crystallization Fouling in Falling Film Evaporators for Seawater Desalination

ABSTRACT Crystallization fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators for seawater desalination. The choice of tube material affects the wettability, the adhesion forces between surface and deposit, and the induction time of crystallization fouling. The effects of surface properties on crystallization fouling from seawater have been investigated in a horizontal tube falling film evaporator in pilot plant scale. Experiments were performed with artificial seawater and various tube materials. The tube surfaces were characterized by measuring surface roughness and contact angles and by determining surface free energies. The tube materials show qualitative and quantitative differences with respect to scale formation. The interfacial defect model was applied to the system. Spreading coefficients of CaCO3 scale on the aluminum alloys 5052 and 6060 and stainless steel grade 1.4565 were calculated to be higher than those on copper–nickel 90/10 and aluminum brass, but the quantities of CaCO3 scale measured on the tube surfaces were much lower compared to CuNi 90/10 and aluminum brass. The application of advanced approaches such as the interfacial defect model depends on the precise knowledge of interfacial free energies, which are very difficult to find. However, results suggest that more similar values of the interfacial free energies of heat transfer surface and deposit lead to increased scale formation.

Polymer Composite Heat Exchangers

The innovative development of polymer composite tubes exhibiting high thermal conductivities for use in heat exchangers, various aspects for implementing these tubes and designing polymer composite heat exchangers are presented. Polymer composite grades based on polypropylene and polyphenylene sulphide filled with graphite have been developed by Technoform Kunststoffprofile GmbH (Lohfelden, Germany). A special extrusion process allows high filler loadings of up to 60 vol% and the orientation of filler particles in the polymer matrix to reach enhanced thermal conductivities in the radial direction. Thermal and mechanical properties, chemical resistance to various acids and alkaline media, lifetime behaviour, heat transfer performance and fouling behaviour were studied for composites with 50 vol% graphite. The extruded polymer composite tubes have a thermal conductivity of about 13–20 W/(m K) which is around 50–100 times higher than that of standard polymers and is comparable to stainless steel grades. The excellent chemical resistance, low weight, good processability as well as highly promising initial test rig results in fouling studies compared to corrosion-resistant metals open up cost-efficient opportunities for heat exchangers in various industries such as chemical, petrochemical, oil and gas, food and beverage, and seawater desalination industries. Important design aspects such as various possibilities of mounting the polymer composite tubes in the tube plates and the maximum unsupported tube span for avoiding tube failures by flow-induced vibrations are discussed. A potential enhancement of heat transfer by shaping polymer composite tubes other than circular plain tubes and the construction of fully polymer-based heat exchangers will be in the focus of future development.