A real column design exergy optimization of a cryogenic air separation unit

Distillation columns are one of the main methods used for separating air components. Their inconvenient is their high energy consumption. The distillation process is simulated in three types of columns and the exergy losses in the different parts calculated. A sensitivity analysis is realized in order to optimize the geometric and the operational parameters of each type of column. A comparative exergy analysis between the distillation columns considered for cryogenic air separation shows that the exergy efficiency of a double diabatic column, with heat transfer all through the length of the column, is 23% higher than that of the conventional adiabatic double columns.     

Using thermally coupled reactive distillation columns in biodiesel production

Production of methyl dodecanoate (biodiesel) using lauric acid and methanol with a solid acid catalyst of sulfated zirconia is studied by using two distillation sequences. In the first sequence, the methanol recovery column follows the reactive distillation column. In the second sequence, the reactive distillation and methanol recovery columns are thermally coupled. Thermally coupled distillation sequences may consume less energy by allowing interconnecting vapor and liquid streams between the two columns to eliminate reboiler or condenser or both. Here we study the thermally coupled side-stripper reactive distillation and eliminate the condenser of the reactive distillation column. Both the sequences are optimized by using the thermal and hydraulic analyses of the Column Targeting Tools of Aspen Plus simulator. Comparisons of the optimized sequences show that in the thermally coupled sequence, the energy consumption is reduced by 13.1% in the reactive distillation column and 50.0% in the methanol recovery column. The total exergy losses for the columns are reduced by 281.35 kW corresponding to 21.7% available energy saving in the thermally coupled sequence. In addition, the composition profiles indicate that the thermally coupled reactive distillation column operates with the lower concentration of water in the reaction zone which reduces catalytic deactivation.     

Planning and optimization of dynamic plant operation

Dynamic optimization is currently gaining attention in the design, synthesis and operation of industrial processes. Recent developments in the optimization algorithms, in the commercial tools and in the computation power of computer systems make it possible for process engineers to optimize the dynamic plant operations. Planning a plant operation can also be made in the scope of optimization. Dynamic optimization is usually used for developing optimal operation policies for new operational situations such as startup procedures, load changes, product changeovers. This paper attempts to give an overview to the methods, available tools and their properties for the planning and dynamic optimization of chemical processes. We will report our experiences in process optimization and show the results achieved in several practical applications: startup for distillation columns with and without sidestreams; heat-integrated distillation columns; multiple-fraction reactive batch distillation. The realization of the developed optimal policies to the real plants shows the benefit and potential of the optimization.     

Synthesis and heat integration of thermally coupled complex distillation system

Based on stochastic optimization strategy and pinch technique, a method is proposed for optimal synthesis and heat integration of thermally coupled complex distillation column systems comprising simple columns, complex columns with side rectifier and/or side stripper as well as partially or fully thermally coupled (Petlyuk) columns with pre‐fractionators. Three example problems for five‐component mixtures separation have been solved and the optimized parameters and economic benefits of different optimal schemes have been analyzed and compared. The results demonstrate that the annual total cost in energy consumption and investment can be effectively reduced by using the heat‐integrated complex distillation configuration. The solutions of example problems also demonstrate that the proposed method is efficient for the heat‐integrated complex distillation synthesis problem. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental study of the operating characteristics of a heat pump assisted distillation system using R11 as the external working fluid

Heat pump assisted distillation with an external working fluid is one of the most obvious methods to reduce the energy consumption in a distillation process. The heat pump working fluid extracts heat from the top of the column, increases the temperature of the recovered heat and recycles it to provide the heat input to the reboiler.The interaction between the external parameters and the internal parameters for a specially designed heat pump assisted distillation system has been studied experimentally. The external parameters were mass flow rate, temperature and concentration of the feed, the concentration of the top and bottom products and the mass flow rate of the working fluid. The effects of the variations of these external parameters on the internal parameters such as the energy (steam) consumption, the actual coefficient of performance and the temperatures at the top and bottom of the column, together with the condensation and evaporation temperatures, are presented.     

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.     

Co- and counter-current mass transfer in bubble column

Design and scale-up has gained considerable attention in recent years because of complex hydrodynamics and its influence on bubble column reactor performance. The concentration difference is important variable while characterizing bubble column reactor efficiency and this is a function of hydrodynamics prevailing inside the column. The efficiency of bubble column reactor is a function of physical properties of phases, geometry of column and operating conditions. Literature lacks on the simulation work on variation of concentration of solute for mass transfer rate and mass transfer efficiency in the complex system of bubble column device. In the present work a mechanistic model is formulated to predict the mass transfer efficiency of column and its dependency on various physical parameters, operating condition and column geometry. In this work the mass transfer efficiency has been analyzed based on a mechanistic model in the case of both co-current and counter current operations in bubble column reactor. The concentration variation of the phases obtained by simulation of model may be useful for further understanding the mass transfer phenomena in bubble column reactor.     

Heat and mass transfer in vacuum membrane distillation

In the membrane distillation (MD) literature, the heat transfer coefficients of the boundary layers are usually estimated from well known heat transfer empirical correlations developed for non-porous and rigid heat exchangers. A difference between the mechanism of heat transfer in MD systems, which is coupled with transmembrane mass transfer, and the mechanism of heat transfer in “pure” heat exchangers is expected to exist. Vacuum membrane distillation has been experimentally studied in a capillary membrane module and the heat transfer coefficients have been evaluated in both the lumen and the shell side of the membrane module. A critical review of the most frequently used heat transfer empirical correlations in MD systems is presented. Finally, the experimental results obtained in this paper are compared to those of literature, in order to test their applicability in membrane distillation systems.     

Azeotropic pressure swing distillation of hydrochloric-water for hydrogen production in the CuCl cycle: Thermodynamic and design methods

A pressure swing distillation (PSD) process is designed and analyzed in this paper for the maximum boiling azeotrope separation of an HCl-water binary mixture to recycle concentrated HCl (aq) within the CuCl cycle of thermochemical hydrogen production. Aspen Plus simulation and EES software are used to evaluate the characteristics of the PSD apparatus in terms of flow streams, thermodynamic properties and compositions in the binary azeotropic mixture. A heat transfer and mass transfer analysis (with the McCabe-Thiele method) are also used to predict the height of the packed bed distillation column. Results indicate that both analyses predict the same values for the low and high pressure packing column height of 1.7 m and 2 m, respectively. Due to the component's volatility changes through the azeotropic transition at the HCl-water separation, the minimum and maximum concentration of the HCl (aq) would be at the distillate ports of low and high pressure columns, respectively. Moreover, to break the azeotropic point of HCl (aq) in the PSD system, the minimum required low pressure feed concentration at the operating line slopes of 0.2, 0.4, 0.6 and 0.8 should be 0.086, 0.092, 0.097 and 0.103, respectively. From the results, the re-boiler and condenser heat duties at the high pressure distillation column are more affected with the change in the slope of the operating line, compared to the low pressure side.     

A study on membrane distillation by a solar thermal‐driven system

Membrane distillation (MD) is a membrane separation process that has long been investigated in small scale laboratory studies and has the potential to become a viable tool for water desalination. MD is a separation process that combines simultaneous mass and heat transfer through a hydrophobic microporous membrane. A solar collector is used in direct contact membrane distillation (DCMD) to heat seawater as a temperature driving force in heat transfer to establish seawater desalting systems. The effect of the temperature difference makes the brine vaporize in the hot fluid side and condense in the cold fluid side. The optimal operating parameters on the pure water production rate will also be examined in this study. The purposes of this study are to develop the theoretical heat and mass transfer formulations, simulate heat transfer rate of solar collector with internal fins in membrane distillation, and investigate the mass‐transfer efficiency improvement in membrane distillation with the brine flow rate, solar collector efficiency, and temperature difference between both sides of membrane as parameters. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(7): 417–428, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20172     

复式断面共轭水深范围的判定

复式断面因共轭水深范围的不同会有多个水跃方程,为选用正确的水跃方程一般均需进行试算,这个过程异常繁琐。在深入分析复式断面跃前水深与跃后水深二者之间共轭关系的基础上,提出一种直接判定共轭水深取值范围的可靠方法,从而不必反复试算即可直接选用正确的水跃方程。尤其是当大量不同水力参数条件下的共轭水深需要计算时,更可大幅减少试算工作量。     

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.     

Optimal design of distillation column using three dimensional exergy analysis curves

This paper presents two main contributions. Firstly, a new exergy graphical method is proposed for optimal design of distillation column with minimum exergy lost. The method is applicable to both grass-root and retrofit cases, respectively. The effect of design and operating parameters of a distillation column on the exergy lost is graphically visualized by three dimensional exergy analysis curves. The curve shows the correlations between exergy lost, design and operating parameters of a distillation column. This technique can be used as an effective method to reduce the simulation effort to search for the optimum design and operating parameters of a distillation column at minimum exergy lost. Besides, visualization also enhances the engineers’ understanding of the column performance. The other contribution is a four-level idealization concept, which is based on three dimensional graphical exergy analysis curves. The concept defines the effect of transport rate and configuration on exergy lost of distillation column. The effectiveness of the method has been demonstrated on a xylene column, which suggested that an implementation of feed pre-heater yields a significant reduction in exergy lost by up to 15.5%.     

Exergy analysis proves viability of process modifications

Stork Comprimo has developed a commercially available exergy analysis program. With this program several exergy analyses were carried out. Most recently an exergy analysis of a reaction and distillation section within a refinery has been performed. In the reaction section endothermic reactions take place after which the product stream is cooled in a heat exchanger network whereafter the reaction products are separated in a distillation section. From the exergy analysis it can easily be notified that the main part of the losses occurs in the furnaces and distillation columns. A closer look at one of the distillation columns with the largest exergy losses shows that the main part of the losses occurs in the reboiler heated with a furnace. To reduce the exergy losses in this distillation column, Stork Comprimo proposed several process modifications. These modifications are: (1) decreasing operating pressure (lower operating temperature), (2) HP steam reboiling instead of a furnace, (3) splitting feed streams, and (4) recompressing overhead. With these process modifications total exergy losses may be reduced by 70% that directly results in a primary fuel reduction of almost 40% for this column! Splitting of the feed streams has been implemented now and results in an energy saving of 10% and a more stable operation of the column.     

A new method for determining coupled heat and mass transfer coefficients between air and liquid desiccant

This paper presented the characteristic of liquid desiccant dehumidification based on NTU–Le model. The results showed that the Lewis number Le had little effect on air outlet humidity ratio during desiccant solution dehumidification process. A new method called h_D–Le separative evaluation method was developed for determining coupled heat and mass transfer coefficients between air and liquid desiccant, through which the heat and mass transfer coefficients between air and liquid desiccant were calculated to obtain from experimental inlet and outlet parameters of air and desiccant solution. The effects of the air volume flow rate, temperature, humidity ratio and the solution concentration, temperature on the Lewis number, heat and mass transfer coefficient were analyzed according to experimental data and the h_D–Le separative evaluation method. Based on the computation results, it was concluded that the Lewis number greatly depended on the operation parameters and conditions of the air and desiccant. In addition, the correlations of the heat and mass transfer coefficients were developed. The additional 74 groups of experiments validated the developed correlations by comparison of air/solution parameters change with the calculation data.     

空气隙膜蒸馏系统对流换热实验研究

空气隙膜蒸馏是一种新型的膜分离技术,可用于海水、苦咸水的淡化处理,尤其是在超纯水制备方面具有极强的竞争力.文中的空气隙膜蒸馏实验装置采用旋转切向入流的方式来强化膜蒸馏的传热传质过程,针对这种新型的入流方式,结合实验数据,给出了料液进口温度对于膜通量的影响,给出了计算空气隙膜蒸馏系统中热容腔内对流换热传热系数的经验公式.     

Evaluation of estimation method of ground properties for the ground source heat pump system

Technology directed at geothermal energy, one of our renewable energy sources, to heat and air-condition buildings has become very attractive in recent years following the significant developments in ground-source heat pump (GSHP) systems. In general, although the energy efficiency of GSHP systems is far superior to conventional air-source heat pump (ASHP) systems, GSHP system is still expensive. Therefore, GSHP system employs the foundation pile of buildings as heat exchanger is introduced in order to reduce the initial cost. When designing a GSHP system (especially in case of the energy pile system), it is necessary to accurately predict the heat extraction and injection rates of the heat exchanger. The thermal and hydraulic properties of the ground are very important to accurately predict heat transfer between the ground heat exchanger and the ground. In particular, those are the most important design parameters because energy pile system is installed only a few tens of meters deep. In this paper, an estimation method is suggested in order to determine the thermal and hydraulic properties of the ground for design the heat exchanger of energy pile system base on geotechnical investigation for the design the building's foundations. The use of results from generally applied geotechnical site investigation methods to estimate ground thermal and hydraulic properties was evaluated.     

Discharge characteristics of latent heat storage columns packed with cross-linked plastic particles

This investigation deals with experimental and numerical analyses carried out to investigate the discharge characteristics of latent heat storage columns, using cross-linked cylindrical plastic particles as a phase-change type of heat storage material and ethylene glycol as a heat transfer medium. In the experiment, the transient response of the outlet temperature of the heat transfer medium was measured under conditions of varying the initial temperature in the column, the inlet temperature of the heat transfer medium, the mass flow rate of the heat transfer medium, and the mass of the heat storage material packed in the column. In the numerical analysis, the transient temperature distributions in the column was calculated by using an empirical formula for estimating the heat transfer coefficient for a fixed bed, which was recommended in the authors' previous paper. The experimental results were found to be in fair agreement with the numerical results. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(3): 193–206, 1997     

On the scale effect and scale-up in the column apparatuses. 3. Circulation zones

A diffusion type of model is proposed for modeling of the mass transfer with chemical reaction in the column apparatuses in the cases of circulation zones. The presence of rising and descending flows (the change of the velocity direction) leads to using three coordinate systems. An iterative algorithm for the concentration distribution calculation is proposed. The influence of the zones breadths on the mass transfer efficiency in the column is investigated.     

循环流化床烟气脱硫效率的研究

基于对气固传质、气固反应模型、浆滴蒸发等理论,从物料循环对热力学参数、传质过程和化学参数的影响几个方面对物料循环的作用过程进行研究。通过对循环流化床烟气脱硫物料循环过程的研究与分析,提出物料循环对脱硫效率的贡献由新鲜吸收剂脱硫效率的提高和循环物料中未完全反应的吸收剂参与脱硫反应产生的脱硫效率两个部分组成,还提出了物料循环对热力学、传质以及化学等参数影响的模拟计算方法。实验验证了方法的可行性与有效性。图3参12