Modelling of an ethanol‐water distillation column assisted by an external heat pump

This paper presents a simulation of an ethanol–water distillation column assisted by a vapour‐compression heat pump. The heat pump is of the external type, i.e., it uses a working fluid (refrigerant) different from that of the column. A simulation model was developed and four different working fluids were studied: R‐11 and R‐114 and, as substitutes, the column own fluids, water and ethanol. Results from the simulation model have shown that considerable reduction in energy consumption can be achieved with the installation of a heat pump. Copyright © 2002 John Wiley & Sons, Ltd.     

Heat pump assisted distillation. VIII: Design of a system for separating ethanol and water

A heat pump assisted distillation system has been designed for the separation of ethanol from 7 per cent aqueous mixtures to produce 93 per cent ethanol by weight. The distillation column has been designed on the basis of conventional design procedures. Valve trays were chosen to provide operational flexibility. R114 was chosen as the working fluid for the mechanical vapour compression heat pump. The heat pump system has been designed to match the heat loads determined for the column. Auxiliary heat exchangers have been provided to aid flexibility and control of column operation.     

Direct numerical simulation of interphase heat and mass transfer in multicomponent vapour–liquid flows

A Volume-of-Fluid methodology for direct numerical simulation of interface dynamics and simultaneous interphase heat and mass transfer in systems with multiple chemical species is presented. This approach is broadly applicable to many industrially important applications, where coupled interphase heat and mass transfer occurs, including distillation. Volume-of-Fluid interface tracking allows investigation of systems with arbitrarily complex interface dynamics. Further, the present method incorporates the full interface species and energy jump conditions for vapour–liquid interphase heat and mass transfer, thus, making it applicable to systems with multiple phase changing species. The model was validated using the ethanol–water system for the cases of wetted-wall vapour–liquid contacting and vapour flow over a smooth, stationary liquid. Good agreement was observed between empirical correlations, experimental data and numerical predictions for vapour and liquid phase mass transfer coefficients. Direct numerical simulation of interphase heat and mass transfer offers the clear advantage of providing detailed information about local heat and mass transfer rates. This local information can be used to develop accurate heat and mass transfer models that may be integrated into large scale process simulation tools and used for equipment design and optimization.     

Performance improvement of bioethanol-fuelled solid oxide fuel cell system by using pervaporation

This work proposes an improvement in performance with respect to the electrical efficiency of a bioethanol-fuelled Solid Oxide Fuel Cell (SOFC) system by replacing a conventional distillation column by a pervaporation unit in the bioethanol purification process. The simulation study indicates that the membrane separation factor has a significant influence on the electrical power and heat energy required to generate a feed of 25 mol% ethanol in water to the reformer. The values of overall electrical efficiency of the SOFC systems with a distillation column and with a pervaporation unit are compared under the thermally self-sufficient condition (Q_net = 0) which offers their maximum electrical efficiency. At the base case, the SOFC system with a pervaporation unit provides an electrical efficiency of 42% compared with 34% achieved from the system with a distillation unit, indicating a significant improvement by using a pervaporation unit. An increase in ethanol recovery can further improve the overall electrical efficiency. The study also reveals that further improvement of the membrane selectivity can slightly enhance the overall efficiency of the SOFC system. Finally, an economic analysis of a bioethanol-fuelled SOFC system with pervaporation is suggested as the basis for further development.     

The impact of heat transfer on Murphree tray efficiency

This work features the experimental determination of heat transfer coefficients and Murphree tray efficiencies on a diabatic (heat-transferring) distillation tray. The present investigation, focussing on the impact of heat transfer on sieve tray performance, is part of a long-term project on heat integrated distillation columns (HIDiC). Heat transfer coefficients and tray efficiencies have been determined experimentally for the methanol/water system in a 150 mm diameter distillation column. The heat-transferring tray was operated in both heating and cooling modes, with heat fluxes up to 50 and 100 kW m⁻², respectively. The experimental data from these diabatic experiments were compared with data obtained from the same column in adiabatic mode and were correlated with the vapour velocity and the heat flux to/from the tray.     

Investigating azeotropic separation of hydrochloric acid for optimizing the copper-chlorine thermochemical cycle

In this paper, an atmospheric-pressure distillation system is designed and constructed for partial to separation of hydrochloric acid and water. The system concentrates HCl(aq) between the electrolyzer and hydrolysis processes of the Copper–Chlorine (Cu–Cl) cycle for hydrogen production. The motivation behind this study is to investigate azeotropic separation of HCl(aq), as needed for integration of unit operations in the Cu–Cl cycle. The separation is only partial, as the mixture is unable to cross the azeotrope with only a single pressure. The distillation system consists primarily of one packed distillation column, which employs heating tapes and thermocouples to achieve a desired axial temperature profile. The column can be operated in batch or continuous mode. The distillate is H 2O(l) and the bottoms is HCl(aq) near the azeotropic concentration; feed concentrations are less than azeotrope. Thus, the degree of separation is determined to be independent of the feed concentration. The bottoms concentration varies from experiment to experiment, but does so independently of feed concentration, likely the result of corrosion impurities affecting the calculation of its concentration. It is found that HCl(aq) can be concentrated up to approximately 0.1068 mol/mol from an initial concentration of 0.0191 mol/mol. A simulation of pressure-swing distillation (PSD) is also performed, but due to safety constraints (a column operating at 10 atm must be certified to CSA B51), a single-pressure (single-column) distillation is physically performed. A single-pressure column is beneficial to the Cu–Cl cycle because it partially recycles HCl, which reduces the cost of the cycle, and still provides valuable results for analysis. The maximum HCl concentration achieved experimentally is 0.1068 mol/mol and the maximum HCl concentration determined from simulation is 0.11 mol/mol (the azeotropic concentration). The novelty of this research is that the experimental column built to study HCl partial separation is designed to be simple yet safe to integrate within the Cu–Cl cycle for hydrogen production.     

Techno-economic comparison of energy usage between azeotropic distillation and hybrid system for water–ethanol separation

Conventional azeotropic distillation, consuming very high energy, is mostly used to produce high purity ethanol for renewable energy usage. In this study, the techno-economic comparison between azeotropic distillation (distillation followed by practical azeotropic distillation) and hybrid system (distillation followed by pervaporation system) for producing high purity of ethanol is demonstrated using the Pro II by Provision version 8.0. In the hybrid system, NaA zeolite membrane is used to separate the water from ethanol–water mixture. It is found that the hybrid system is the most effective technique for producing more than 99.4%wt of ethanol with an energy consumption of 52.4% less than the azeotropic distillation.     

Ammonia–water absorption refrigeration systems with flooded evaporators

The harmful effects of water accumulation in the evaporator in ammonia–water absorption refrigeration systems (AARS) with flooded evaporators are a crucial issue. In this paper, the effects of the ammonia purification and the liquid entrainment and blow-down from the evaporator in AARS are analyzed. A mathematical model based on a single stage system with complete condensation has been developed. The ammonia purification is evaluated by means of the Murphree efficiencies of the stripping and rectifying sections of the distillation column. The entrainment and blow-down are taking into account considering the corresponding flow rates as a fraction of the dry vapour at the evaporator outlet. The influence of the distillation column components efficiency on the attainable distillate concentration and the effects of the distillate concentration and the liquid entrainment and blow-down on the system operating conditions and performance are analyzed and quantified. If no liquid entrainment or blow-down is considered, very high efficiencies in the distillation column are required. Small values of liquid entrainment or blow-down fractions increase significantly the operating range of the absorption system. If high values of the blow-down fraction are required, then a heat exchanger should be added to the system in order to recover the refrigeration capacity of the blow-down by additional subcooling of the liquid from the condenser. For a fixed value of the distillation column efficiency, an optimum value of the liquid blow-down fraction exists. Moreover, an optimum combination of generation temperature, reflux ratio and blow-down fraction can be found, which should be considered in designing and controlling an AARS.     

Exergy simulation and optimization of adiabatic and diabatic binary distillation

A detailed exergy analysis of a distillation system has been conducted. This analysis is divided in four parts: (1) adiabatic rectification column; (2) adiabatic stripping column; (3) diabatic rectification column; and (4) diabatic stripping column. The results of all cases are presented in this paper. This analysis concerns the mixture water–ethanol working at 1 bar. The main objective is to determine the distribution of exergy losses inside the column and the optimal distribution of heat to be transferred inside the column in order to produce the minimum overall exergy losses.     

Anhydrous ethanol: A renewable source of energy

Anhydrous ethanol is one of the biofuels produced today and it is a subset of renewable energy. It is considered to be an excellent alternative clean-burning fuel to gasoline. Anhydrous ethanol is commercially produced by either catalytic hydration of ethylene or fermentation of biomass. Any biological material that has sugar, starch or cellulose can be used as biomass for producing anhydrous ethanol. Since ethanol–water solution forms a minimum-boiling azeotrope of composition of 89.4 mol% ethanol and 10.6 mol% water at 78.2 °C and standard atmospheric pressure, the dilute ethanol–water solutions produced by fermentation process can be continuously rectified to give at best solutions containing 89.4 mol% ethanol at standard atmospheric pressure. Therefore, special process for removal of the remaining water is required for manufacture of anhydrous ethanol. Various processes for producing anhydrous ethanol have been used/suggested. These include: (i) chemical dehydration process, (ii) dehydration by vacuum distillation process, (iii) azeotropic distillation process, (iv) extractive distillation processes, (v) membrane processes, (vi) adsorption processes and (vii) diffusion distillation process. These processes of manufacturing anhydrous ethanol have been improved continuously due to the increasingly strict requirements for quantity and quality of this product. The literature available on these processes is reviewed. These processes are also compared on the basis of energy requirements.     

Thermodynamic model of pressure swing distillation for a high pressure process of hydrochloric/water separation and hydrogen production

Recycling of concentrated HCl within the CuCl cycle for hydrogen production is necessary to achieve lower operating costs and higher efficiency. HCl and water molecules have an attraction that leads to a maximum boiling azeotrope. Thus, HCl cannot be separated from water by using a conventional distillation column. In this paper, a novel thermodynamic model and simulations in Aspen Plus are presented for a pressure swing distillation high-pressure process to separate the HCl-water azeotropic binary mixture into a concentrated HCl stream. Furthermore, a heat transfer analysis is presented to predict the packing column height. All of the stream properties and compositions in the binary azeotropic mixture, thermodynamic analysis and steady-state simulation of a high pressure distillation column are examined with Aspen Plus. The present results indicate that the high-pressure distillation system enhances the mole fraction of HCl (aq) from 0.11 up to 0.21 and the increase of reflux ratio and feed temperature assist it to be higher. The increment of 76% and 42% for the condensation and re-boiler heat duties with the growth of reflux ratio indicate the significant impact of this parameter on the high pressure distillation column. Furthermore, at the specific system operating condition, the results from the heat transfer model suggest that the distillation column with a height of 2 m would be suitable for practical operation.     

Simultaneous heat and mass transfer of a packed distillation column for ammonia–water absorption refrigeration systems

This paper presents a study on the NH₃–H₂O distillation process using a packed column with liquid reflux from the condenser in an absorption refrigeration system. A differential mathematical model has been developed on the basis of mass and energy balances and the heat and mass transfer equations. A net molar flux between the liquid and vapour phases has been considered in the mass transfer equation, which obviates the need to assume equimolar counter-diffusion. The model equations have been solved using the finite-difference method. Results obtained for a specific application are shown, including parameter distributions along the column length. The influence of rectifying and stripping lengths, mass and heat transfer coefficients and volumetric heat rejection from the column, on the distillate ammonia concentration has been analysed.     

Effect of fuel temperature on in-nozzle cavitation and spray formation of liquid hydrocarbons and alcohols from a real-size optical injector for direct-injection spark-ignition engines

High-pressure multi-hole injectors for direct-injection spark-ignition engines offer some great benefits in terms of fuel atomisation, as well as flexibility in fuel targeting by selection of the number and angle of the nozzle holes. The flow through the internal passages of injectors is known to influence the characteristics of spray formation. In particular, understanding how in-nozzle cavitation phenomena can be used to improve atomisation is essential for improving mixture preparation quality under homogeneous or stratified engine operating conditions. However, no data exist for injector body temperatures representative of real engine operation, especially at low-load conditions with early injection strategies that can also lead to phase change due to fuel flash-boiling upon injection. This challenge is further complicated by the predicted fuel stocks which will include a significant bio-derived component presenting the requirement to manage fuel flexibility. The physical/chemical properties of bio-components, like various types of alcohols, can differ markedly from gasoline and it is important to study their effects. This work outlines results from an experimental imaging investigation into the effects of fuel properties, temperature and pressure conditions on the extent of cavitation, flash-boiling and, subsequently, spray formation. This was achieved by the use of real-size transparent nozzles, replica of an injector from a modern direct-injection spark-ignition combustion system. Gasoline, iso-octane, n-pentane, ethanol and butanol were used at 20, 50 and 90 °C injector body temperatures for ambient pressures of 0.5 bar and 1.0 bar in order to simulate early homogeneous injection strategies for part-load and wide open throttle engine operation. The fuel matrix also included a blend of 10% ethanol with 90% gasoline (E10) because the vapour pressure of E10 is higher than the vapour pressure of either ethanol or gasoline and the distillation curve of E10 reflects strongly this effect. Therefore, the distillation curves of the fuels, the vapour pressures, as well as density, viscosity and surface tension were obtained and the Reynolds, Weber, Ohnesorge and Cavitation numbers were considered in the analysis. The in-nozzle flow regime and spray formation was found to be sensitive to the fuel temperature and gas pressure as a result of the vapour pressure and temperature relationships.     

Thermodynamically equivalent distillation schemes to the Petlyuk column for ternary mixtures

Thermally coupled distillation sequences for ternary separations have been shown to provide significant energy savings with respect to the conventional direct and indirect distillation sequences. Although the Petlyuk column is generally more efficient than the other thermally coupled schemes, its structure creates potential operating problems because of the bi-directional vapour interconnecting streams. In this paper, second law calculations were performed for the Petlyuk column and six alternative distillation schemes that show unidirectional flows; in principle, such alternative configurations are expected to present better operational properties than those of the original Petlyuk column. The results indicate that the proposed distillation arrangements have values of thermodynamic efficiency very close to that of the Petlyuk column. This result is important because let us establish that the alternative distillation sequences: (i) are thermodynamically equivalent to the Petlyuk column, (ii) could be more easily implemented in practice, and (iii) also achieve significant energy savings.     

Single stage and double absorption heat transformers used to recover energy in a distillation column of butane and pentane

This paper presents the theoretical analysis of the use of single stage and double absorption heat transformers operating with the water–lithium bromide mixture coupled to a butane and pentane distillation column in a Mexican refinery. A mathematical model of the heat transformers was developed in FORTRAN and integrated as a user model to the Aspen Plus simulation code. Both components coupled to the column were modelled on steady-state conditions. The results show that it is theoretically possible to reduce the energy consumed in the reboiler between 26 and 43% by the use of single stage heat transformer at specific conditions, and between 28 and 33% with double absorption heat transformers for a wide range of operating conditions. Copyright © 2003 John Wiley & Sons, Ltd.     

Comparative study of fuel gas production for SOFC from steam and supercritical-water reforming of bioethanol

Theoretical study of fuel gas (H₂ + CO) production for SOFC from bioethanol was carried out to compare performances between two reforming technologies, including steam reforming (SR) and supercritical-water reforming (SCWR). It demonstrates that the fuel gas productions are comparable among the two reforming systems; however, SCWR requires the operation at much higher temperature and pressure than SR. The maximum hydrogen yield can be obtained at 850 K, atmospheric pressure, ethanol to water molar feed ratio of 1:20 for SR system and at 1300 K, 22.1 MPa, and ethanol to water feed ratio of 1:20 for SCWR. The use of a distillation column to purify the bioethanol feed was proven to improve the fuel conversion efficiency of both systems. The analysis reveals that SCWR is a promising system for fuel production for SOFC when a gas turbine is incorporated to the system for energy recovery. Further, it is not necessary to distil bioethanol to obtain too high ethanol recovery (i.e. >90%) as higher energy consumption at the distillation column could lead to lower overall thermal efficiency.     

流程模拟技术在镇海炼化2号常减压装置上的应用

镇海炼化600×104t/a常减压装置为研究对象,采用Aspen Plus流程模拟软件,建立了与实际工况相吻合的常减压装置稳态流程模拟模型,通过对初馏塔、常压塔、一级减压塔和二级减压塔的模拟,了解各操作参数对装置性能的影响;通过蒸馏塔模型中的气液相负荷分布和温度梯度的分布情况,加深对蒸馏操作的理解。随着重油加工工艺技术的发展,炼厂能够加工更加劣质的渣油,因此常减压装置轻油收率和总拔出率的提高,对提高原油的利用率及炼厂的经济效益极为重要。为此,重点对影响常减压装置轻油收率的关键操作参数进行灵敏度分析,优化操作,实现提高常减压装置轻油收率的目的。经过流程模拟优化后,常减压装置一级减三线蜡油350℃馏出量降低2.3mL/100mL,以一级减三线平均流量为90t/h,以及柴油和蜡油差价300元/t计算,全年可增加装置效益536.544万元。     

Heat pump assisted distillation. III: Experimental studies using an external heat pump

An experimental study has been carried out on a continuously operated pilot fractional distillation column equipped with an external heat pump. The distillation column was a 15 cm diameter glass unit containing eleven single bubble cap plates. A methanol-water mixture was fed to the column and the heat pump working fluid was R114. The actual coefficient of performance (COP)_A of the heat pump increased with an increase in the mass flow rate of the working fluid. A maximum (COP)_A value of 4–3 was obtained with a gross temperature lift of 41–3°C. The performance of two reciprocating compressors was compared. The experiments have shown that continuous heat pump assisted distillation using an external working fluid can greatly reduce the energy used in a distillation process. No control problems were encountered in the experiments.     

Thermodynamic assessment of solid oxide fuel cell system integrated with bioethanol purification unit

A solid oxide fuel cell system integrated with a distillation column (SOFC–DIS) has been proposed in this article. The integrated SOFC system consists of a distillation column, an EtOH/H₂O heater, an air heater, an anode preheater, a reformer, an SOFC stack and an afterburner. Bioethanol with 5 mol% ethanol was purified in a distillation column to obtain a desired concentration necessary for SOFC operation. The SOFC stack was operated under isothermal conditions. The heat generated from the stack and the afterburner was supplied to the reformer and three heaters. The net remaining heat from the SOFC system (Q_SOFC,Net) was then provided to the reboiler of the distillation column. The effects of fuel utilization and operating voltage on the net energy (Q_Net), which equals Q_SOFC,Net minus the distillation energy (Q_D), were examined. It was found that the system could become more energy sufficient when operating at lower fuel utilization or lower voltage but at the expense of less electricity produced. Moreover, it was found that there were some operating conditions, which yielded Q_Net of zero. At this point, the integrated system provides the maximum electrical power without requiring an additional heat source. The effects of ethanol concentration and ethanol recovery on the electrical performance at zero Q_Net for different fuel utilizations were investigated. With the appropriate operating conditions (e.g. C_EtOH = 41%, U_f = 80% and EtOH recovery = 80%), the overall electrical efficiency and power density are 33.3% (LHV) and 0.32 W cm⁻², respectively.     

Experimental determination of the behaviour of wet porous beds in which natural convection occurs

A theory developed previously has shown that a similarity relationship can be formed between natural convection heat transfer in a porous bed with a single fluid, and a similar system where the particles are wet and the interstitial mixture contains vapour and a second non-condensing gas. In this paper, data from a bed of glass spheres, bounded by horizontal, isothermal plates and with several gas and vapour mixtures, is reported. The enhancement in heat transfer rates forecast by the theory for large increases in gas to vapour molecular weight differences is shown by comparing water/air and water/Freon 12. The decrease in heat transfer rates forecast by the theory when the gas molecular weight is less than that of the vapour, is shown by comparing ethanol/air and ethanol/Freon 12. The validity of the similarity relationship is established by comparing these data plus a set from a different apparatus, with data for a water saturated bed of glass spheres. Heat transfer rates from the ethanol/air experiment are higher than predicted from models of heat transfer in stagnant packed beds. It appears that the system may be convecting in spite of an unfavourable density gradient.