Solar barometric distillation for seawater desalting Part II: Analyses of one-stage and two-stage distillation technologies

The report concerns design aspects for the recently proposed solar barometric distillation technology for seawater desalting (SW–SBD) with an underground barometric layout. Two types of SW–SBD desalting plants are analysed. The first plant utilizes a single-stage distillation process with one distillation heat exchanger (operative condensation temperature 50°C). The second plant utilizes a two-stage distillation process with two distillation heat exchangers connected in series (operative condensation temperatures 40 and 60°C). Vacuum solar collectors of simple design and construction, utilizing glass or suitable glass/polymer blends as transparent material, are proposed for the SW–SBD plants. The present analyses suggest that SW–SBD desalting technology may have a promising technico-economic potential. Field research on SW–SBD prototype plants is necessary to bring SW–SBD desalting technology to its full technological development.     

First experimental results of a new hybrid solar/gas multi-effect distillation system: the AQUASOL project

From 2002 to 2006, a combined R&D project named AQUASOL has been carried out at the facilities of the Plataforma Solar de Almería (Spain). Main objective of this project has been the development of a hybrid solargas desalination system based on multi-effect distillation process that meets at the same time the requirements of low-cost, high efficiency and zero discharge.

The final AQUASOL plant, implemented at the Plataforma Solar de Almería for its evaluation under real meteorological conditions, is composed of: (i) a 14-cell forward-feed vertically-stacked MED unit, (ii) a 500m2 stationary CPC (compound parabolic concentrator) solar collector field, (iii) a 24 m3 thermal storage system based on water, (iv) a new advanced prototype of double-effect absorption (LiBr-H2O) heat pump, (v) a smoketube gas boiler to guarantee 24-h operation

This paper shows the first experimental results obtained during the test campaign of the project. The performance ratio reached by the distillation plant in different operational modes is evaluated, as well as the issues related with the operation of the subsystems that compose the AQUASOL desalination system.     

The seawater greenhouse in the United Arab Emirates: Thermal modelling and evaluation of design options

The Seawater Greenhouse combines a solar desalination system with an environment for cultivating crops in which transpiration is minimised. Results from the prototype greenhouse in the United Arab Emirates (UAE) are used to calibrate a computational fluid dynamic (CFD) model. With the UAE design taken as a baseline, the model is used to evaluate three proposed options for improving performance. These differ with respect to how the plants are shaded and how air flows through the greenhouse. In the baseline design, a semi-opaque plastic sheet is used to provide shade. The first option uses a similar sheet; however the air flows in a C-shaped path, travelling in opposite directions above and below the sheet. The second option uses a perforated sheet through which air is drawn. In the third option, an array of plastic pipes carrying seawater provides shade. The warmed seawater from the pipes is fed into the back evaporator to boost fresh water production. We present results for freshwater production, evapotranspiration and temperatures inside the greenhouse, covering a range of ventilation airflows. The first option gives only marginal improvements in freshwater production and cooling. In contrast, the second option significantly increases water production. Air temperature in the greenhouse increases, whereas the mean radiant temperature decreases. The third option gives the greatest increase in water production in addition to slightly lower air temperature in the greenhouse and significantly lower mean radiant temperature. Evapotranspiration varied little over all the cases studied. The reasons for these findings and their implications for the design of the system are discussed.     

Comparison of classical solar chimney power system and combined solar chimney system for power generation and seawater desalination

An alternative method of heat and moisture extraction from seawater under the collector of a solar chimney system for power generation and seawater desalination is investigated with the aim of estimating the output of power and fresh water when used in seawater desalination using one-dimensional compressible flow model. It is found that the temperature and velocity of the airflow inside the chimney in the combined plant is less than that inside the chimney in the classic plant due to the release of vapor latent heat as the air rises up the chimney. Additionally, the power output from air turbine generators and water generators in the combined plant is less than that of the classic plant. Furthermore, a revenue analysis based on the price of fresh water and electric power in Dalian, China shows that the chimney less than 445 m high for the proposed combined solar chimney power plant having a collector 3000 m in radius is more economical than for the classic plant. The critical chimney height is found to depend on the local price of fresh water and electricity.     

10t/d船用板式蒸馏海水淡化装置性能测试与模拟计算

建立了一套船用板式蒸馏海水淡化装置试验测试平台,对10 t/d淡化装置进行了性能测试,在测试所采用的海水及热水流量下,热水温度由59℃升高至83.77℃,淡水产量由400 L/h提高至550 L/h,进一步提高热水温度,淡水产量提高趋于平缓,操作时热水温度不宜超过85℃;随热水流量的增大,淡水产量呈现先升高后降低趋势;海水入口温度升高造成冷凝器中传热温差降低,真空度降低,产水量下降;海水流量是制约海水淡化装置真空度的关键因素,作为真空喷射泵的驱动水,当海水流量低于一定值时,淡化机真空度急剧下降,几乎不能出水。对板式蒸馏海水淡化装置进行了理论模拟,模拟结果与试验测试数据吻合良好。     

Concentrating solar power for seawater desalination in the Middle East and North Africa

The paper presents a long-term scenario for the demand of freshwater in the Middle East and North Africa (MENA) and shows how it may be covered by a better use of the existing renewable water sources and by sea water desalination powered with solar energy. Growth of population and economy, increasing urbanization and industrialization, and the rather limited natural resources of potable water in MENA are leading to serious deficits of freshwater in many parts of MENA. Modern infrastructure for water distribution, enhanced efficiency of use and better water management are to be established as soon as possible. However, even the change to best practice would leave considerable deficits, which are poorly covered by over-exploiting groundwater resources. Increased use of desalted seawater is therefore unavoidable in order to maintain a reasonable level of water supply. The desalination of seawater based on fossil fuels is neither sustainable nor economically feasible in a long-term perspective, as fuels are increasingly becoming expensive and scarce. Concentrating solar power (CSP) offers a sustainable alternative to fossil fuels for large scale seawater desalination. CSP can help to solve the problem, but market introduction must start immediately in order to achieve the necessary freshwater production rates in time.     

Reactive distillation for toluene disproportionation: a technical and economic evaluation

The potential of reactive distillation for xylenes manufacture via the toluene disproportionation reaction is considered in detail. The conventional process is carried out in the vapour phase over a fixed bed catalyst at elevated temperature and pressure. Process design and flowsheeting were carried out for both the conventional and reactive distillation processes using the Aspen Plus simulation package, specifically using the facility for evaluation of simultaneous reaction and physical equilibrium within a distillation column calculation block. Using the flowsheeting based process and equipment designs, a detailed comparative cost assessment and economic analysis of the two processes was performed. The economic evaluation is equivocal, and the reactive distillation process does not appear to offer significant benefits. While the reactive distillation approach does allow significant flowsheet simplification, the design compromises required to enable simultaneous reaction and distillation, particularly relating to design pressure, negate the inherent process benefits. Further, changes in reaction selectivity between the two reaction environments influenced not only the process product slate but also forced process design features to suppress an unwanted (heavy) by-product.     

Reducing energy consumption and CO2 emissions in extractive distillation: Part II. Dynamic behavior

The structure of thermally coupled distillation systems offers several control challenges arising from the transfer of vapor (or liquid) streams between columns. In particular, the presence of recycle streams for coupled schemes has led to the notion that control problems might be expected during the operation of these systems, in contrast to the rather well-known behavior of conventional distillation sequences. In this work, we analyze the control properties of thermally coupled extractive distillation schemes studied previously (Gutiérrez-Guerra, R., Segovia-Hernández, J.G. and Hernández, S., 2009, Reducing energy consumption and CO₂ emissions in extractive distillation. Chem Eng Res Des, 87: 145–152). Control properties are analyzed with the application of the singular value decomposition technique and a closed-loop analysis. The results showed that the energy savings predicted in the complex extractive distillation sequence can be achieved along with good dynamic behavior and reductions in greenhouse gas emissions.     

Methodology for the design and evaluation of distillation systems: Exergy analysis,economic features and GHG emissions

This work presents a process design methodology that evaluates the distillation systems based on exergetic, economic, and greenhouse gas (GHG) emission aspects. The aim of the methodology is to determine how these three features should be applied in process design to obtain information about the accuracy of the design alternatives. The methodology is tested and demonstrated on three different energy‐integrated distillation systems: the direct sequence with backward heat‐integration (DQB), fully thermally coupled distillation column (FTCDC), and sloppy distillation system with forward heat‐integration (SQF). The average relative emission saving is the highest for the DQB scheme and this sequence shows the most flexible range of use. The case studies prove the accuracy of our evaluation methodology. On the other hand, it highlights and demonstrates that the exergy analysis can predict the results of the economic study and the environmental evaluation to make the decisions, associated with process design, much simpler. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Base case process development for energy efficiency improvement, application to a Kraft pulping mill. Part I: Definition and characterization

The development of a base-case process is a fundamental step in an energy efficiency study to obtain reliable results. However, this step is often overlooked and there are no clear guidelines for the systematic development of the base-case. A methodology has been proposed to properly define and evaluate the complete process for a subsequent in-depth energy analysis. It consists of two stages: definition and characterization of the process, and benchmarking analysis. In this paper, the first stage is presented. The base-case should encompass the process and the utilities systems, i.e., steam and water, as they are the driving forces of the chemical transformations. A four-pronged procedure is proposed to properly define and characterize a process and its utilities: data gathering, master diagram construction, utilities systems analysis, and simulation. The main objective is to build a computer simulation model to provide detailed information on production, distribution, utilization and post-utilization treatment of steam and water. Process inefficiencies are also identified, such as the low condensate recovery or the presence of non-isothermal mixing points. The procedure has been applied to an operating Kraft pulping mill in Eastern Canada.