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.     

Advanced energy saving in distillation process with self-heat recuperation technology

In the distillation process, heat is supplied at a feed heater and a reboiler, and an overhead stream is cooled at a condenser. Almost all of the supplied heat at the reboiler in the conventional distillation process is discarded in the overhead condenser. Conventional energy savings of the distillation processes were fundamentally attained as a result of heat recovery duty in the feed heater being maximized by using the heat of the bottom stream, which enabled the utility (steam or hot oil) rate to the feed heater to be reduced. To achieve further energy saving in the distillation process, “self-heat recuperation technology” (SHRT) was adopted. In this technology, two compressors are installed in the overhead vapor line, which consists of the reflux and the overhead product streams. A compressor (compressor-1) treats the reflux stream and the other compressor (compressor-2) treats the overhead stream. The reboiler duty is supplied by the recuperated heat of the discharged stream from compressor-1 and the feed heater duty is supplied by that from compressor-2, by adiabatic compressions. It could be found that the advanced process with SHRT was able to reduce the energy consumption significantly by using the recuperated heat of the overhead vapor.     

Impact of vapor recompression in batch distillation on energy consumption,cost and CO2 emission: Open-loop versus closed-loop operation

This paper aims at analyzing the thermal integration of direct vapor recompression system with a distillation column operated in batch mode. The unsteady state nature of the batch processing makes the use of vapor recompression a challenging task. For a meaningful comparison, it is attempted to run the vapor recompressed batch distillation (VRBD) column at same dynamics with its conventional counterpart. With this operating objective, the manipulation of a few process variables is proposed in open-loop fashion. Along with the energy savings and total annualized cost, the CO₂ emission level is also used for quantitative performance evaluation of the proposed VRBD column. Aiming to produce a constant and high-purity distillate product, a gain-scheduled proportional integral (GSPI) controller that targets to keep the stability margin constant is devised for the VRBD process. Finally, this heat integration mechanism is illustrated by a binary batch distillation example in both open-loop and closed-loop modes. It is investigated that the VRBD shows a slightly higher energy savings (62%) compared to its closed-loop version (59.13%) but at the expense of distillate purity. The heat integration also achieves a significant savings in cost and CO₂ emission.     

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.     

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.     

Process simulations of HI decomposition via reactive distillation in the sulfur–iodine cycle for hydrogen manufacture

Process simulations of HI decomposition via reactive distillation in the Sulfur–Iodine (S–I) cycle have been performed using heat pumps for energy recovery and a recently developed thermodynamic properties model. Several differences from previous flow sheets have been found through manual optimization of reflux ratio, number of stripping and rectifying stages, and pressure of the distillation column for typical inlet conditions to the HIx Section III. In particular, the RD column should have a minimal stripping section, can have as few as 10 total stages, an operating pressure of 12 bar, and a reflux ratio of 0.75, while achieving the production requirements. Though this design has limited improvement in energy requirements because the General Atomics energy recovery system is extremely effective, these results mean there should be a significant reduction in capital costs from prior estimates. In addition, as the inlet flow rate is increased, the input energy requirements decrease because of an increased ratio of H₂O to I₂ in the reboiler, lowering its temperature, and reducing the temperature differences for heat pump operations. The optimal inlet flow is between 126 and 140 mol/mol H₂, with a Section energy requirement of 367 kJ/mol H₂, and an overall process thermal efficiency estimated to be 41.5% relative to the higher heating value of hydrogen. These findings suggest there may be greater flexibility in conditions for the Bunsen reaction section as well as other possibilities for further energy efficiency improvement.     

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.     

基于Aspen Plus的原油蒸馏装置流程模拟及优化

常减压蒸馏是原油加工过程中的第一道工序,常减压蒸馏装置运行的优化程度对炼厂的下游加工流程及经济效益产生重要影响。以荆门石化350×104t/a常减压装置为对象,采用Aspen Plus流程模拟软件,建立了与装置实际工况相符合的三塔全流程稳态模型,利用此模型,对常减压蒸馏装置的初馏塔、常压塔和减压塔进行灵敏度分析,以最佳轻油收率和减压渣油收率为优化目标,对加热炉出口温度、塔底汽提蒸汽量、各塔中段回流量、常压渣油350℃馏出量及减压渣油500℃馏出量,以及各参数之间的关系进行研究,并以计算数据为指导,对装置操作进行如下优化:将常压炉出口温度控制在360℃,将常压塔底汽提蒸汽量控制在2.7t/h,并对常压塔各中段回流量进行调整。经过调整优化,装置每年由于燃料油消耗下降而增加的直接经济效益达299.44万元。     

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.     

Simulation and energy optimization of the stabilizer tower of a pyrolysis gasoline hydrogenation unit

Energy optimization of second distillation tower of a pyrolysis gasoline hydrogenation unit has been studied by the thermal cycle of vapor recompression method. The mentioned cycle is connected to the second distillation tower of the stabilizer of pyrolysis gasoline, and the results are found promising. The composite pinch curve for both the current and the optimized methods are shown. Moreover, an increase in the heat transfer rate in heat exchanger E-1014 causes energy recovery in reboiler. According to simulation results, by vapor recompression to 1970 kPa and using this heat source for thermal integration, condenser and reboiler’s energies are decreased by 56.93 and 30.4 percentage, respectively.     

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.     

Optimization of ethanol fermentation process design

The ethanol fermentation process using molasses as the feedstock has been studied. The process alternatives, compared to the original fermentation process described in the literature, have been modeled using single and double distillation columns. Short-cut and rigorous models for the distillation column were compared and their influence on the process economics indicate the rigorous model to be more exact, the short-cut model to be faster and more convenient for comparative analysis of finding the optimal process structure. The process with the single distillation column was shown to be the optimal variant requiring the lowest equipment and utilities cost. Estimated net present worth value for the single column was 29.6 MUSD and for the double columns 27.1 MUSD. A heat pump use was found to be a possible option for heat integration. The span of temperatures in distillation columns was confirmed to be the decisive factor for the economy of heat pump use. The pay back time was more than 5 years. Thermal integration reduced the total annual costs by 27% as compared to the basic process scheme.     

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.     

酸洗废液间歇式精馏过程的热力性能分析

工业生产过程中产生大量的各种工业废酸,其中酸洗废液是重要的工业废酸之一,主要来源于电路板刻蚀、管材酸洗以及金属表面处理等行业的酸洗过程,其中含少量铁盐的低浓度废酸可以通过回收利用达到保护环境和节约资源的双重目的。针对低浓度废盐酸的资源化利用问题,本文通过搭建精馏塔实验平台,以蒸汽为供热源,进行了低浓度废盐酸间歇精馏操作实验。实验测定了HCl-H₂O体系的相对挥发度和气、液相平衡数据,研究了不同时间下塔釜、塔顶温度场和浓度场的分布情况以及不同的初始溶液浓度、回流比等操作参数对间歇精馏过程的影响,并对整个系统进行了有效能分析,探究能量损失的主要原因。研究结果表明：当实验达到稳定运行状态时,随着加热时间的延长,塔釜酸液浓度缓慢上升接近某一个特定值,测得其质量分数为18.2%,塔顶馏出液pH维持在5.5左右,验证了该精馏塔装置浓缩低浓度稀盐酸的可行性。该系统有效能的效率为43.94%,需要进一步对系统进行能量优化和系统改进,以提高其能量效率。     

Simulation,exergy analysis and application of diabatic distillation to a tertiary amyl methyl ether production unit of a crude oil refinery

This paper presents the results of a detailed exergy analysis of a tertiary amyl methyl ether (TAME) unit of a crude oil refinery and the application of diabatic distillation to the depentanizer tower of the unit. Diabatic distillation is a separation process in which heat is not only supplied to the reboiler and extracted from the condenser [as in a conventional (adiabatic) distillation column], but is also transferred inside the column. The process enables operation to approach equilibrium conditions, thus reducing exergy losses and increasing exergy effectiveness. In a TAME unit of a refinery, isoamylenes are converted to TAME. Before transforming the isoamylenes in the reactors, it is necessary to recover them from a catalytic gasoline stream by a depentanization process. The exergy losses of this depentanization process represent about 70% of the total exergy losses of the unit. The results of the exergy analysis of the TAME unit are presented and a detailed exergy analysis of the conventional adiabatic depentanizer column is conducted for comparison purposes. Then, the application of diabatic distillation to the system is evaluated by using cooling water circulating in series from tray to tray in the rectification section and by making the steam emanating from the reboiler circulate in series from tray to tray in the stripping section. The results in terms of the reduction of exergy losses, heating and cooling media flow rates, and cost effectiveness of the diabatic option for the depentanizer section of the plant are compared to the original adiabatic system, and the effect of the diabatization on the overall exergy performance parameters of the depentanizer section and on the whole TAME unit, are presented in this paper.     

Retrofit of distillation columns in biodiesel production plants

Column grand composite curves and the exergy loss profiles produced by the Column-Targeting Tool of the Aspen Plus simulator are used to assess the performance of the existing distillation columns, and reduce the costs of operation by appropriate retrofits in a biodiesel production plant. Effectiveness of the retrofits is assessed by means of thermodynamics and economic improvements. We have considered a biodiesel plant utilizing three distillation columns to purify biodiesel (fatty acid methyl ester) and byproduct glycerol as well as reduce the waste. The assessments of the base case simulation have indicated the need for modifications for the distillation columns. For column T202, the retrofits consisting of a feed preheating and reflux ratio modification have reduced the total exergy loss by 47%, while T301 and T302 columns exergy losses decreased by 61% and 52%, respectively. After the retrofits, the overall exergy loss for the three columns has decreased from 7491.86 kW to 3627.97 kW. The retrofits required a fixed capital cost of approximately $239,900 and saved approximately $1,900,000/year worth of electricity. The retrofits have reduced the consumption of energy considerably, and leaded to a more environmentally friendly operation for the biodiesel plant considered.     

Design of a thermally integrated bioethanol-fueled solid oxide fuel cell system integrated with a distillation column

Solid oxide fuel cell systems integrated with a distillation column (SOFC-DIS) have been investigated in this study. The MER (maximum energy recovery) network for SOFC-DIS system under the base conditions (C_EtOH = 25%, EtOH recovery = 80%, V = 0.7 V, fuel utilization = 80%, T_SOFC = 1200 K) yields Q_Cmin = 73.4 and Q_Hmin = 0 kW. To enhance the performance of SOFC-DIS, utilization of internal useful heat sources from within the system (e.g. condenser duty and hot water from the bottom of the distillation column) and a cathode recirculation have been considered in this study. The utilization of condenser duty for preheating the incoming bioethanol and cathode recirculation for SOFC-DIS system were chosen and implemented to the SOFC-DIS (CondBio-CathRec). Different MER designs were investigated. The obtained MER network of CondBio-CathRec configuration shows the lower minimum cold utility (Q_Cmin) of 55.9 kW and total cost index than that of the base case. A heat exchanger loop and utility path were also investigated. It was found that eliminate the high temperature distillate heat exchanger can lower the total cost index. The recommended network is that the hot effluent gas is heat exchanged with the anode heat exchanger, the external reformer, the air heat exchanger, the distillate heat exchanger and the reboiler, respectively. The corresponding performances of this design are 40.8%, 54.3%, 0.221 W cm⁻² for overall electrical efficiency, Combine Heat and Power (CHP) efficiency and power density, respectively. The effect of operating conditions on composite curves on the design of heat exchanger network was investigated. The obtained composite curves can be divided into two groups: the threshold case and the pinch case. It was found that the pinch case which T_SOFC = 1173 K yields higher total cost index than the CondBio-CathRec at the base conditions. It was also found that the pinch case can become a threshold case by adjusting split fraction or operating at lower fuel utilization. The total cost index of the threshold cases is lower than that of the pinch case. Moreover, it was found that some conditions can give lower total cost index than that of the CondBio-CathRec at the base conditions.     

Energy and environmental sustainability assessment of a crude oil refinery by thermodynamic analysis

This study presents the assessment of energy and environmental sustainability metrics for a crude oil refinery consisting of three distillation columns. The assessments of the current operation and the retrofits for possible improvements are suggested by the thermodynamic analysis and energy analyzer. The main objective is to explore the scope of reducing the thermal energy consumption and CO₂ emissions for a more sustainable refinery operation. Thermodynamic analysis is carried out by using the thermal analysis capability of ‘column targeting tool’ to address the ‘energy intensity metrics’ and the ‘energy analyzer’ to design and improve the performance of the heat exchanger network system for process heat integration. Environmental pollution impact metrics are estimated from the ‘carbon tracking’ options with a selected CO₂ emission data source of US‐EPA‐Rule‐E9‐5711 and using crude oil as a primary fuel source for the hot utilities. The results indicate that column targeting tool, energy analyzer, and carbon tracking can estimate the energy and environmental sustainability metrics of an existing design and determine the scope of considerable improvements for reducing the costs of thermal energy required and emissions of carbon dioxide in a crude oil refinery operation. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production

As a critical subsection in the sulfur-iodine (SI) thermochemical cycle, HI concentration and separation must cope with the pseudo-azeotropy of HIx (HI-I₂-H₂O) and excess iodine in HIx solution. Although electro-electrodialysis (EED) coupled with conventional distillation is a validated method of HI separation from HIx solution in the SI process, the iodine deposition, resulting from changes in temperature and HI molality in HIx solution, can lead clogged flow channels of the EED anode and other tubes. A precipitator can address this problem by recovering excess iodine from HIx solution after the HIx purification column. The energy duty and required input flow rate per mole HI was investigated in this study using a process flowsheet simulation. A decrease in iodine concentration in the streams to EED could reduce cell duty effectively. An increase in HI molality in the EED cathode outlet resulted in an increase in EED duty; however, the amplitude was slight. The iodine molar concentration in the feed of the distillation column exhibited an appreciable influence on the distillation duty. However, with an increase in distillation column pressure, the effect of diminished iodine in feed on the HI distillate duty continued to decline. To assess the utilization of an iodine precipitator in the HI separation subsection, the energy demands and required input flow rates of three different flowsheets were calculated using Aspen Plus and Microsoft Office Excel. Results indicated that the flowsheet that only recovered iodine in the stream to the EED anode chamber exhibited the least HI separation duty and the lowest required input flow rate.     

Residue curves map analysis for tert-amyl methyl ether synthesis by reactive distillation in kinetically controlled conditions with energy-saving evaluation

Distillation is a largely used separation operation, but at the same time it is intensively energy consuming. Distillation columns need huge amount of energy due to evaporation steps involved; more than half of the process heat distributed to plant operations ends up in the reboilers of distillation columns. ∞/∞ analysis is a framework that allows studying distillation systems, checking the feasibility, detecting multiple steady states and getting the influence of the distillate flow rate and recycle streams on purities in early stage of process design. Also, ∞/∞ analysis is useful to evaluate new alternatives for existing processes and to propose energy-efficient alternatives by process integration. Process intensification is an issue of constant interest, providing strategies to develop more effective and cheaper technologies with lower environmental impact. The originality of this paper lies in the applicability of this framework to kinetically controlled reactive distillation including hybrid systems. TAME synthesis is used as an illustrative example. Energy savings obtained by using reactive distillation are evaluated comparing with the traditional system consisting of reactor and distillation columns. The solution proposed, subsequent to the validation by rigorous simulation, offers a 20% decrease in the number of stages and a reboiler energy saving of 10%.