低温多效蒸发海水淡化系统热力分析

建立了喷射器低温多效蒸发海水淡化系统的数学模型,计算分析了各种温度损失随温度的变化,并研究了顶值盐水温度、蒸发器效数和动力蒸汽等参数对系统的造水比和生产单位质量淡水所需传热面积的影响。结果表明各种温度损失在末效蒸发器内显著增加;喷射器低温多效蒸发系统的热力特性明显优于多效蒸发系统;通过增加顶值盐水温度、蒸发器效数和动力蒸汽温度,可以实现系统的优化运行。     

Technical comparison between TVC/MEB and MSF

Recently, large numbers of high capacity TVC/MEB (multi-effect thermal vapor compression combined withconventinalmulti effect boiling) desalting units have been built or ordered in the UAE and elsewhere. An in-depth comparison between the predominantly used MSF and the TVC/MEB is conducted in this paper to show the reasons of this trend to used the newly used TVC/MEB units. The TVC/MEB units operate at lower top brine temperature TBT to limit the risk of scale formation and corrosion. This decreases the difference between TBT and last effect (stage) temperature Tn, and this tends toincrease the heat transfer surface area of the TVC/MEB compared to the MSF system. However, the high heat transfer rates in film boiling in TVC/MEB evaporators keep the heat transfer area the same in both systems. Moreover, the TVC/MEB system has more merits copared to the MSF such as: it has better response to the variation of steam supply, less footprint area, less pumping energy, and the ability to operate at different modes than the design.     

Comparison of Exergy Losses for Reformers Involved in Hydrogen and Synthesis Gas Production

A user‐coded, customized methodology for exergy balance and analysis of chemical and thermal processes is developed with the Aspen HYSYS process simulator, to assess and explain exergy loss in steam methane reforming. Exergy analysis is presented for four configurations of primary and secondary reformers to produce synthesis gas: (1) an autothermal reformer (ATR) alone, (2) a top‐fired reformer (TFR) alone, (3) ATR‐TFR configured in parallel, and (4) ATR‐TFR configured in series. The same states and feed conditions, i.e., mass flow rate, temperature, pressure, and feed gas composition, are applied to the four reformer configurations considered, with the single ATR showing the lowest exergy loss of about 0.43 W kg^?1 of dry productivity.     

Steady-State Analysis of the Multiple Effect Evaporation Desalination Process

Mathematical modeling of the multiple effect evaporation (MEE) desalination process has been carried out to determine the effects of the important design and operating variables on the parameters controlling the cost of producing fresh water. The model assumes the practical case of constant heat transfer areas for both the evaporators and feed preheaters in all effects. In addition, the model considered the impact of the vapor leak in the venting system, the variation in thermodynamic losses from one effect to another, the dependence of the physical properties of water on salinity and temperature, and the influence of noncondensable gases on the heat transfer coefficients in the evaporators and the feed preheaters. The modified fixed-point iterative procedure is used to solve the large number of highly nonlinear equations describing the MEE desalting system. The algorithm consists of 10 calculation blocks and 6 logical blocks. The algorithm is implemented using L-A-S computer aided language. Results show that the heat transfer coefficients increase with the boiling temperature. Also, the heat transfer coefficient in the evaporator is always higher than that in the feed preheater at the same boiling temperature. The plant thermal performance ratio is nearly independent of the top brine temperature and strongly related to the number of effects. The specific heat transfer area increases by raising the number of effects and reducing the top brine temperature. The effect of the top brine temperature on the specific heat transfer area is more pronounced with a larger number of effects. The required specific heat transfer areas at a top brine temperature of 100°C are 30.3% and 26% of that required at 60°C when the number of effects are 6 and 12, respectively. The specific flow rate of cooling water is nearly constant at different values of top brine temperature and tapers off at a high rate as the number of effects is increased. Two correlations are developed to relate the heat transfer coefficients in the preheater and the evaporator to the boiling temperature. Design correlations are also developed to describe variations in the plant thermal performance, the specific heat transfer area, and the specific flow rate of cooling water in terms of the top brine temperature and the number of effects.     

Energy savings in distillation plants by using vapor thermo-compression

A suggestion for more efficient utilization of the available energy in single purpose distillation desalination units is presented. It includes the introduction of a vapor thermo-compression multi-effect unit to replace the pressure reducing valves and/or the superheaters usually used at the brine heaters. Furthermore, using the VTC/VTE configurations would allow raising the top effect temperature up to 136°C.The combined system including the VTC/VTE is analyzed and some practical cases are investigated. Thermodynamic analysis showed an increase in the fresh water production by up to 100% with extra heat input of only about 19% of the original plant heat requirements. In the analysis, subatmospheric effects were not used in order to avoid the interference between the jet booster of the vapor compression and the steam jet ejector required to evacuate the non-condensable gases.A flow sheet for the combined system is presented and the operating flexibility is pointed out. Such a system can operate efficiently with favorable response to heat input and demand changes.     

单效热压缩蒸馏海水淡化热力过程数值模拟

建立了单效热压缩蒸馏海水淡化系统的热力学模型。分析了蒸发温度、压缩比和主动蒸汽压力等主要设计参数对系统造水比、单位造水换热面积和单位造水冷却水量的影响。结果表明,降低蒸发温度、减小压缩比和提高主动蒸汽压力将提高系统造水比,而单位产水换热面积将随蒸发温度和压缩比的减小而增加;提高蒸发温度和减小压缩比可以减小单位产水冷却水量;主动蒸汽压力对单位产水面积和冷却水量影响较小。研究成果将对系统优化设计提供有益的参考。     

带热压缩串联低温多效海水淡化系统热力性能

建立了带热压缩的6效串联低温多效蒸发海水淡化系统数学模型,并利用MATLAB语言编制程序进行了求解.针对5种不同的蒸汽喷射器(TVC)位置的流程,分析了不同流程对系统热力性能的影响.同时,针对TVC位置不变的流程,分析了加热蒸汽温度的变化对系统热力性能的影响.计算结果表明:在文中的计算范围内,相比其他4种不同TVC位置...     

机械蒸汽再压缩硫酸铵废水处理系统的分析

为全面反应机械蒸汽再压缩硫酸铵废水处理系统的用能情况,提高系统的效率,采用以热力学第二定律为基础的分析方法,对系统进行了分析。将硫酸铵废水视为实际溶液,建立了实际物流的分析模型,运用工厂实测数据对系统进行了分析计算,研究了参量:压缩比、蒸发温度和一效排出浓度对系统效率的影响并根据测算结果提出了改进建议。分析结果表明,该系统总的效率约为12.04%;各设备分析表明,换热器及压缩机是主要的损失部位,二者的损失约占总数的79.5%;参量分析表明,系统的效率随着压缩比的增大而减小,随着蒸发温度的升高而增大,在其他条件允许的情况下,应尽量采用压缩比小的压缩机和维持高的蒸发温度;在文中条件下,当一效排出质量分数为32%时系统的效率最大,应尽可能地维持在该质量分数下排料;消除进入加热器蒸汽的过热可以提高系统的效率。     

蒸汽喷射压缩器的变工况特性模拟与分析

采用改进热力学,建立了蒸汽喷射压缩器变工况特性数值计算模型。结合水蒸气物性程序,对热压缩海水淡化系统中的蒸汽喷射压缩器的变工况性能进行了计算,分析了工作蒸汽压力、引射压力、混合蒸汽压力的变化对引射系数的影响。计算结果表明,当压缩蒸汽的压力高于设计值时,引射系数随压缩压力的增大而减小,而当压缩低于设计值时,引射系数保持不变;提高引射蒸汽压力会引起引射系数的增大;主动蒸汽压力偏离设计值时,主动蒸汽压力的降低（低于设计值）或提高均会引起喷射器性能的降低。     

Separation of n-heptane/isobutanol via eco-efficient vapor recompression-assisted distillation: process optimization and control strategy

In this study, vapor recompression and heat integration assisted distillation arrangements with either the low or high pressure in the reflux drum are proposed to reduce and/or eliminate the application of the costly refrigerant for the separation of n-heptane and isobutanol mixture. The high-pressure arrangement with vapor recompression and heat integration is the most attractive among these four intensified configurations since it can reduce total annual cost by 18.10%, CO₂ emissions by 75.01% based on natural gas (78.78% based on heavy oil fuel), and second-law efficiency by 61.20% compared to a conventional refrigerated distillation system. Furthermore, exergy destruction in each component is calculated for the heat integration configurations and is shown in pie diagrams. The results demonstrate that the high-pressure configuration presents unique advantages in terms of thermodynamic efficiency compared to the low-pressure case. In addition, dynamic control investigation is performed for the economically efficient arrangement and good product compositions are well controlled through a dual-point temperature control strategy with almost negligible product offsets and quick process responses when addressing 20% step changes in production rate and feed composition. Note that there are no composition measurement loops in our developed control schemes.     

低温多效蒸发海水淡化系统性能估算方法及公式

Low temperature multi-effect evaporation (LT-MEE) seawater desalination systems with top brine temperature lower than 70℃ have attracted attention in recent years.A method for performance estimation of LT-MEE is proposed, and a formula is built for a typical LT-MEE configuration.Excluding complex properties of working fluids and details of heat and mass transfer processes, this formula involves only simple algebraic operations, thus provides a convenient way to evaluate the water production of LT-MEE for different motive heat source conditions and design parameters.Comparing the formula prediction with the data from both rigorous models and references, it indicates that the formula can give good results in wide parameter ranges.Although this method is proposed based on a typical LT-MEE configuration, it is also applicable to other LT-MEE configurations and thermal desalination systems.     

Simulation of a forward feed multiple effect desalination plant with vertical tube evaporators

In recent years, vertical tube falling film evaporators have been widely used in desalination industries. In this paper mathematical modeling of a multiple effect evaporators (MEE) system has been carried out for brackish water desalination. The system includes a set of forward feed vertical tube evaporators with thermal vapor compression (TVC) and a condenser. Modeling has led to calculation of several parameters such as overall heat transfer coefficients, entrainment ratio and recovery of the process which is restricted by scale formation. A scaling prediction chemical model has been employed to calculate the allowable rate of recovery for prevention of scale formation. Physical properties of streams have been assumed as functions of temperature and salinity. A code has been developed for simulation of the process based on mass and energy balance equations. Results showed maximum allowable recovery of 74% for the treated brackish water sample with total dissolved solid (TDS) of 14,761 ppm is achievable. Dealing with mentioned sample under specified set of conditions, it was concluded that changing the number of effects from 3 to 8, enhances gained output ratio (GOR) value from 3.8 to 7.5. However, specific heat transfer surface is increased from 215 to 1052.     

基于热力蒸汽压缩蒸发的油田污水淡化系统及分析

针对油田污水污染物成分复杂、污染性强不适合膜法脱盐的特点和淡水消耗与余热资源共存的实际,提出用热力蒸汽压缩（TVC）蒸发技术对油田污水进行集中脱盐处理的技术方案。建立了基于TVC的油田污水脱盐系统的工艺流程设计计算模型,系统分析了蒸发温度等主要运行参数的影响。结果表明：系统性能系数随蒸发温度的降低而增大,降低蒸发温度有助于减少主蒸汽消耗,但所付出的代价是总传热面积和污水泵流量的增大;TVC系统的热力学完善度相对较低,与多效蒸发系统联合使用是改善系统性能的基本途径之一。尽管如此,对于油田生产来说,当存在合适的余热资源时,简单的单效TVC系统仍不失为合理的技术方案。     

蒸汽喷射压缩器的变工况特性分析

蒸汽喷射压缩器工作特性对系统性能产生很大的影响。文中采用气体动力函数法,建立了蒸汽喷射压缩器变工况特性数值计算模型,对热压缩海水淡化系统中的蒸汽喷射压缩器的变工况性能进行了计算,分析了工作蒸汽压力、引射压力、混合蒸汽压力对喷射系数的影响。计算结果表明,当压缩蒸汽的压力低于一定值时,喷射系数保持不变;喷射压缩器特性对蒸发压力的变化最为敏感,引射压力的微小变化将导致喷射器性能的明显变化;而提高工作蒸汽压力,在一定范围内可以提高喷射器的性能,但超过一定数值后反而会引起喷射器性能的降低。与试验结果对比表明,采用该方法能够正确地预测喷射压缩器的工作性能。     

Exergy analysis of a reverse osmosis desalination plant in California

The exergy analysis of a 7250 m³/d reverse osmosis (RO) desalination plant in California was conducted by using actual plant operation data, and an alternative design was investigated to improve its performance. The RO plant is described in detail, and the exergies across the major components of the plant are calculated and illustrated using exergy flow diagrams in an attempt to assess the exergy destruction distribution. The primary locations of exergy destruction were the membrane modules in which the saline water is separated into the brine and the permeate, and the throttling valves where the pressure of liquid is reduced, pressure drops through various process components, and the mixing chamber where the permeate and blend are mixed. The largest exergy destruction occurred in the membrane modules, and this amounted to 74.07% of the total exergy input. The smallest exergy destruction occurred in the mixing chamber. The mixing accounted for 0.67% of the total exergy input and presents a relatively small fraction. The second law of efficiency of the plant was calculated to be 4.3%, which seems to be low. The analysis of the alternative design was based on the exergy analysis. It is shown that the second law of efficiency can be increased to 4.9% by introducing a pressure exchanger with two throttling valves on the brine stream, and this saved 19.8 kW electricity by reducing the pumping power of the incoming saline water.     

Improved modelling and simulation of once-through and reverse multi-stage flash desalination configurations

An improved model for multistage flash (MSF) structures is developed and used to assess the performance of a novel MSF configuration, termed as MSF reversal (RV-MSF) and consisting of reversing the brine circulation stream. The improved model determines the temperature distribution within the stages using heat balance equations while the simplified one, which is commonly used, is based on pre-specified, constant, and equal temperature distribution throughout the stages. The performance of the RV-MSF is investigated and compared with conventional MSF once-through (OT-MSF) with and without brine mixing using simplified and full models. It is found that the simplified model overestimates the required heat transfer specific area for both MSF configurations. Moreover, it underestimates the cooling water and energy requirements for the reversal configuration. Hence, the simplified model may be good for quick analysis but leads to inaccurate design specifications and economic analysis. When brine mixing is utilized, the simplified model still provides erroneous estimates of the heat transfer area for both MSF configurations. Nevertheless, for OT-MSF structure, the simple model can provide comparable predictions with that of the improved model in terms of recovery ratio, performance ratio, and specific energy consumption. For the RV-MSF structure, a mismatch in the two model predictions of surface area, cooling water, and energy requirements is observed. Furthermore, the temperature drop in the cooling system for RV-MSF has a significant influence on the specific surface area and cooling water requirements especially at low values. The different behaviour of the simplified model between the OT-MSF and RV-MSF configurations is attributed to the fact that brine recycling does not affect the feed temperature or the temperature distribution in the system.     

Thermovapor compression desalters: energy and availability - Analysis of single- and multi-effect systems

Sidem (France) has made some recent advancements in thermal vapor compression (TVC) systems [1,2] by installing four units, each having four effects, 1 migd in capacity with a gain ratio of 8, and another four units of 12 effects each, with 2 migd capacity per unit and a gain ratio close to 17. This increased the interest in the system particularly for plants of low and medium capacity. The new system is characterized by the following: (1) the compression of most of the generated vapor and its usage as a heating medium drastically reduce the primary heat source (boiler) and heat sink (i.e., cooling water and condenser), as compared to conventional single-effect desalting systems; (2) low energy consumption; (3) simple water pretreatment as compared with reverse osmosis (RO) systems, which are the main competitors of mechanical and thermal vapor compression systems in small and medium capacities; (4) low capital and operating costs; and (5) recently developed reliable thermocompressors. Since very little is known about the principles and design of the system, a parametric analysis, using the first and second laws of thermodynamics, was conducted for the TVC system components, e.g., steam ejector, evaporator, condenser, as well as the system as a whole. The analysis pinpoints the deficiencies in the system and the methods of overcoming these deficiencies.     

30t·d-1低温多效海水淡化装置中试试验

引言缺水问题已经是一个世界性问题,与跨流域调水、蓄水、开采地下水等解决水资源短缺的传统措施相比,海水淡化是一种可持续、长久解决水资源缺乏的方式。我国是一个海洋大国,在11个沿     

Once-through and brine recirculation MSF designs — a comparative study

This study investigated the feasibility of the once-through (OT) MSF design for constructing large-capacity desalting plants rather than the conventional brine recycle (BR) MSF configuration. This was to explore the possibility of improving MSF process economics through the application of a simpler process design. It is known that the OT design is characterized by its simplicity and elimination of the brine recirculation pump as well as the rejection section. These features are expected to reduce capital and maintenance costs. The current study is based on comparing the two MSF plant configurations, OT and BR, from the standpoint of minimum heat transfer area, which is a major element in capital investment. Alternative tube layouts, long-tube (LT) and cross-tube (CT) arrangements, were considered. Design calculations were based on a plant capacity of 20 MGD, a gained output ratio of 10, a top brine temperature of 120°C, a feed temperature and concentration of 35°C and 48,000 ppm, respectively. The total number of stages was varied stepwise between 20 and 40. A rigorous mathematical model was developed to solve the optimization problem taking into consideration the nonlinearity of the thermo-physical properties of seawater and steam. The Solver tool of Microsoft Excel was used to determine the optimal solutions. Substantial savings in the heat transfer area can be realized through the application of the LT-OT design when the number of stages goes up to 40. However, comparison of the different MSF designs with the optimal number of stages and minimum heat transfer area tells us that the use of the OT designs is not likely to save more than 1% in heat transfer area relative to the conventional CT-BR configuration. Findings related to the specific chemical consumption are not in favor of OT plants where the consumption ratio varies between 1.7 and 1.9 relative to the CT-BR plants. Of course, excessive chemical consumption penalizes the operating cost of the OT plants. However, the increase in operating costs has to be weighed against the savings in capital cost and lower power consumption due to the elimination of the recycle pump and the heat rejection section. This will determine the use of the OT configuration for building MSF plants in the future. In this event, a detailed cost analysis will be needed.     

A new exergy method for process analysis and optimization

A two level idealization concept involving the definition of intrinsic and extrinsic exergy destruction is incorporated into exergy analysis, exergoeconomic analysis and exergoeconomic optimization. The intrinsic exergy destruction is caused by the configuration constraints, whereas the extrinsic exergy destruction is caused by the transport rate limitations. For exergy analysis, intrinsic and extrinsic exergy destructions can be quantified for each process operation. For exergoeconomic analysis, the monetary costs associated with these exergy destructions can be determined. For exergoeconomic optimization, the variables associated with process configuration and transport rate limitations can be optimized independently. Methods for analysis as well as optimization are described and demonstrated by two case studies, an ethylene product recovery and separation plant and a benzene-toluene distillation column. Improvements demonstrated from these case studies are significant.