重氧水分离级联中氘和氧18浓度分布及其影响

在水精馏法分离同位素的系统中,氘和氧18同时得到富集,情况比较复杂。为使该系统的模拟更加贴近实际情况,提出了一种可同时计算氘和氧18浓度分布及氧16/氧18分离系数的新方法。此方法将不同的水分子划分为四个集总,简化了复杂的同位素交换反应网络。然后利用ASPEN PLUS的反应精馏模块进行计算。对一个五塔级联的水精馏制氧18水系统的模拟计算表明,产品中氘浓度达到了70.7%,氧18浓度达到了98.0%。在最后一个精馏塔内,随着氘含量的增加,氧16/氧18分离系数α_cal从接近α_H216O/H218O变得更接近α_D216O/D218O。     

The steady-state and dynamic simulation of cascade distillation system for the production of oxygen-18 isotope from water

Accurate simulation of water distillation system for oxygen-18(18O) isotope separation is necessary to guide industrial practice, since both deuterium(D) and oxygen-18 isotope get enriched and interfere with each other. In the present work, steady-state and dynamic distillation models are established based on a classic method and a cascade distillation system with 5 towers is introduced to test the models. The theoretical expressions of separation factor αH/Dfor protium/deuterium and separation factor α~(16)O/~(18) O.for oxygen-16/oxygen-18 were derived,with the existence of deuterium and oxygen-18, respectively. The results of the steady-state simulation by the classical method proposed in the present work agreed well with the results of the lumping method. The dynamic process could be divided into 5 stages. Impressively, a peak value of product withdraw was observed before the final steady state, which was resulted from the change of ~(16)O/~(18) O separation factor and isotope distribution. An interesting low concentration zone in the towers of T2–T5 existed at the beginning of the dynamic process and it required industrial evidence.     

Life cycle assessment of oxygen-18 production using cryogenic oxygen distillation

In this study, life cycle assessment of oxygen-18 by using cryogenic distillation of oxygen is performed using SimaPro 8.3 software. Life cycle assessment is performed to understand the environmental profile and hotspots of this process in order to be used in design and development. Simulation of oxygen-18 process is executed by Hysys software, and the required inputs and outputs for inventory of life cycle were acquired. By doing life cycle assessment and considering achieved results after characterization and normalization of inventory data it has been investigated that in the majority of environmental impacts electricity consumption has a huge contribution relative to other parts of the system like liquefied oxygen production from air separation unit, required facilities for air separation and oxygen-18 units, and needed transportation. Also, among 17 impact categories investigated in ReCiPe impact assessment method, fossil depletion, climate change (human health), particulate matter formation, climate change (ecosystem), human toxicity, and metal depletion have the most contribution in entire environmental loads respectively. Furthermore, sensitivity analysis showed that changing life cycle impact assessment method from ReCiPe to IMPACT 2002 + has no significant effect on acquired results and results are confident. In addition, assumption of market for depleted oxygen from heavy isotopes which is withdrawn from top of distillation columns showed some positive effects compared to first case and environmental impacts resulted from liquefied oxygen production (feed) reduced but because of huge contribution of electricity consumption compared to other sections, this positive effect has no remarkable influence on entire environmental loads of product system.     

Flow dynamics in distillation columns packed with dixon rings as used in isotope separation

Packed distillation columns are common in isotope separation. The pressure drop serves as an indication for the hydrodynamic state of the column. Models were formulated for flow and pressure drop dynamics in packed distillation columns. These models were confirmed on columns packed with Dixon rings and operated with water for separation of oxygen isotopes. Liquid holdup displacement is very important in isotope separation practice. Experiments proved that distillation columns packed with Dixon rings exhibit a behaviour close to plug flow.     

Dynamic modeling and collocation‐based model reduction of cryogenic air separation units

High purity distillation columns and multi‐stream heat exchangers (MSHXs) are critical units in cryogenic air separation plants. This article focuses on modeling approaches for the primary section of a super‐staged argon plant. A full‐order stage‐wise model for distillation columns in air separation units (ASUs) that considers key process phenomena is presented, followed by a reduced‐order model using a collocation approach. The extent of model reduction that can be achieved without losing significant prediction accuracy is demonstrated. A novel moving boundary model is proposed to handle MSHXs with phase change. Simulation results demonstrate the capability of the proposed model for tracking the phase change occurrence along the length of the heat exchanger. Dynamic simulation studies of the integrated plant show that the thermal integration between the feed and product streams captured in the primary heat exchanger is critical to accurately capture the behavior of ASUs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1602–1615, 2016     

A fundamental principle and systematic procedures for process intensification in reactive distillation columns

A fundamental principle is developed for process intensification through internal mass and energy integration in reactive distillation columns and three systematic procedures are devised for process synthesis and design. For reactive distillation columns involving reactions with highly thermal effect, process intensification can be achieved with an exclusive consideration of internal energy integration between the reaction operation and separation operation involved. However, in the case of a highly endothermic reaction with an extremely low reaction rate and/or small chemical equilibrium constant, internal mass integration has also to be considered between the reactive section and stripping section. For reactive distillation columns involving reactions with negligibly or no thermal effect, process intensification can be performed with an exclusive consideration of internal mass integration. For reactive distillation columns involving reactions with moderately thermal effect, process intensification must be conducted with a careful trade-off between internal mass and energy integration. Five hypothetical and two real reactive distillation systems are employed to evaluate the principle and procedures proposed. It is demonstrated that intensifying internal mass and energy integration is really effective for process intensification. Not only can the thermodynamic efficiency be improved substantially, but also the capital investment can be further reduced.     

Multistage Process of Deuterium and Heavy Oxygen Enrichment by Membrane Distillation

Abstract

Deuterium and oxygen-18 isotope enrichment in water by membrane distillation were studied in a 4-stage cascade. Two configurations of membrane distillation (MD) employing PTFE-flat-sheet membranes were investigated, including direct contact MD and air gap MD. The first, direct contact MD is more efficient. It is characterized by high distillate flow rate. The temperature polarization coefficients were higher for direct contact MD. H/D and ¹⁶O/¹⁸O separation factors were determined in the 4-stage cascade.     

A Generalized Computer Model of the Transient Behavior of Multicomponent Isotope Separation Cascades

Abstract

The time dependent performance of large separation systems is a major consideration in the enrichment of the isotopes of elements that have a direct role in nuclear fuel cycles. The transient behavior of multicomponent separation cascades is described by a set of nonlinear partial differential equations that are similar in form for chemical exchange, distillation, gaseous diffusion, thermal diffusion, and other countercurrent processes. The Mound computer model is set up to solve the differential equations by a fast, implicit forward difference technique. Systems of up to 10 components can be handled with a wide variety of multiple input and output streams. With modifications, the program can be used to model systems of two or more cascades. It has been applied to the separation of the isotopes of uranium, the noble gases, carbon, oxygen, nitrogen, chlorine, sulfur and calcium. A neon isotope separation problem is given as an example of the precision with which performance can be predicted for multicomponent systems.     

Intensified process for aromatics separation powered by Kaibel and dividing-wall columns

Process intensification in distillation led to major developments, such as reactive distillation, heat-integrated distillation, cyclic distillation, as well as Kaibel and dividing-wall column. Still, the separation of aromatics at industrial scale is carried out typically in a series of conventional distillation columns, with severe penalties on the associated plant footprint, investment and operating costs. To solve this problem, this study investigates novel separation alternatives powered by dividing-wall column (DWC) and Kaibel distillation column. The new sequences using process intensification are able to separate five products (lights, benzene, toluene, xylene and heavies) at high purity levels, in only two distillation columns.AspenTech Aspen Plus^® was used as a computer aided process engineering tool to perform the rigorous simulation and optimization of the new separation alternatives, applied to a simplified industrial case study. In order to allow a fair comparison, all design alternatives were optimized using the sequential quadratic programming (SQP) method.Notably, the novel design with two consecutive DWC units reduces the energy demand by 14%, while the alternative combining a conventional stripper with a Kaibel column leads to over 17% energy savings as compared to the conventional direct distillation sequence. Moreover, the new separation schemes require less equipment and a reduced plant footprint.     

Short-cut methods for the optimal design of simple and complex distillation columns

New short-cut methods providing optimal design parameters for distillation columns with simple and complex configurations including two-feed and one-feed-one-side-stream columns are presented. The methods assume constant relative volatilities and constant molar flow rates within each distillation section. The design equations are based on the Underwood equations for the calculation of minimum reflux (reboil) ratio, the analytical formulations of the distillation line, the Eigenfunction and the number of theoretical stages for each mass transfer section of the column. Furthermore, the geometrical properties of a given separation are considered. Optimization algorithms based on the minimization of the total number of theoretical stages of the column with taking into account the mass balance at each feed section have been elaborated. In comparison to the boundary value method the new short-cut methods require a minimum number of specifications; they do not need any graphical support, and provide a lower total number of theoretical stages particularly for complex configurations. The new short-cut methods have been extended to the design of columns separating azeotropic mixtures by approximating the latter by appropriate pseudo-ideal mixtures. Several separation examples for azeotropic mixtures, including different types of splits as well as columns with simple and complex configurations were tested and show a very good agreement with the simulation results obtained with Radfrac (Aspenplus).     

Steady-state design of thermally coupled reactive distillation process for the synthesis of diphenyl carbonate

Diphenyl carbonate, a precursor in the production of polycarbonate, is traditionally synthesized by the transesterification reaction of dimethyl carbonate and phenol. In this study, phenyl acetate was used instead of phenol to react with dimethyl carbonate and yield diphenyl carbonate, due to its higher reaction conversion and the absence of side reactions and azeotropes. A plant-wide process with a reactive distillation (RD) column and a separation column was optimized by minimizing the total annual cost. The performance of the thermal coupling between these two columns was also investigated. RD with thermal coupling was demonstrated to provide better energy efficiency than conventional RD. The remixing phenomenon associated with thermodynamic inefficiency in conventional distillation sequences could be greatly reduced by implementing thermal coupling between columns. Reactant concentrations that were closer to stoichiometric balance in the reaction zone were given for the thermally coupled RD column.     

外部热耦合技术在BTX分离中的应用

外部热耦合式复合精馏塔将热量从高压塔的精馏段传向低压塔的提馏段,降低了分离操作的不可逆性,从而提高系统的热力学效率,降低系统所需的能耗。目前国内苯/甲苯/二甲苯（BTX）的分离技术仍然与国外先进水平相差很远,主要表现在系统的能耗高、工艺落后等方面。本文将外部热耦合技术应用到苯/甲苯/二甲苯的分离过程中,并用M athem atica软件对系统的设备投资和操作费用进行了仿真计算。结果表明,使用外部热耦合结构,系统节能幅度可达到37.94%,年度总费用降低达18.84%。     

Energy‐efficient complex distillation sequences: Control properties

Four thermally coupled distillation systems were designed for the separation of five‐component mixtures (the light‐ends separation section of a crude distillation plant); their steady‐state design was obtained by starting from a conventional distillation sequence and then optimizing for minimum energy consumption. The thermally coupled distillation systems were compared to sequence based on conventional columns design. Comparison was based on controllability properties under open and closed loop operation, following the dynamic behaviour after common industrial operating disturbances. Simulation results were analyzed by the singular value decomposition technique and with the performance examination of elimination of feed disturbances using PI controllers. It was found that thermally coupled distillation systems are controllable and, sometimes, they exhibit dynamic responses that are easier to manage than in the case of conventional distillation sequences; this result is innovative in the study of this kind of systems.     

A Chemical Exchange System for Isotopic Feed to a Nitrogen and Oxygen Isotope Separation Plant

Abstract

A process has been developed to provide isotopic feed to a nitric oxide isotope distillation plant. Central to the process is the isotopic chemical exchange of NO and nitric acid in countercurrent flows in a 3-in. diameter packed column. An isotopically depleted stream of NO is reenriched to natural isotopic abundances by the exchange and is recycled as feed back to the distillation columns. Makeup NO is generated in another column from sulfur dioxide and nitric acid. Multistage gas purifiers reduce condensible impurities in the nitric oxide below 10 ppm. The process operates unattended at flow rates of 0.5 to 2 mol/min. The new NO recycle-enrichment and generation processes have successfully provided the feedstock for the NO isotope separation columns for over 6 years.     

外部热耦合复合精馏塔的经济性与可控性评价

外部热耦合复合精馏塔系统是一种新型的精馏塔系统,通过操作在不同压力下的两个精馏塔的精馏段和提馏段之间的热传递来提高热力学效率。根据精馏段和提馏段热耦合的相对位置不同,外部热耦合复合精馏塔系统可分为对称型和非对称型两种结构。为便于设计和实现,可用外部换热器替代外部热耦合得到简化的结构。本文以乙烯乙烷物系分离过程为对象,通过对外部热耦合复合精馏塔系统建立了静动态模型,采用四点控制的方法,对三种结构的外部热耦合复合精馏塔的经济性和可控性两方面做了分析,证明了非对称型优于对称型外部热耦合复合精馏塔。同时,对使用外部换热器简化外部热耦合结构的方法提供了理论依据和参考。     

Energy saving and thermodynamic efficiency of a double-effect distillation column using internal heat integration

The existing internally heat-integrated distillation column with the problem of utilizing a compressor is modified to propose a new heat-integrated distillation column without the compressor. Two identical columns of a conventional binary distillation are implemented to the heat integration. The energy used in the reboiler is recovered by the internal heat integration between the stripping section of one of the columns at lower pressure and the rectifying section of the other higher pressure column. The heat integration is similar to double-effect distillation, but internal heat integration requires less pressure elevation. The performance of energy saving and thermal efficiency improvement of the proposed system is evaluated with the two examples of the benzene-toluene and methanol-ethanol processes. The performance comparison indicates that the proposed system requires 17.4% less of reboiler duty for the benzene-toluene process and 15.8% less of heating duty for the methanol-ethanol process. The thermal efficiencies are 16.3% and 23.8% for the benzene-toluene and methanol-ethanol processes, respectively. Elimination of the compressor makes the column operation easy and the separate reboilers and condensers for the two columns in the proposed system provide flexible control, when the controllability of the proposed system is compared with that of the existing internally heat-integrated distillation column.     

Analysis of a hybrid distillation-pervaporation column in a single unit: Intermediate membrane section in the rectifying and stripping section

Distillation-pervaporation in a single unit (DPSU) column can perform separations that are not possible in conventional distillation by overcoming distillation boundaries. Unlike conventional hybrid distillation-pervaporation columns, in a DPSU system the pervaporation membrane is located inside the column. The separation by distillation and pervaporation is carried out simultaneously inside the same column section. In a previous work, a simplified model was used to design and analyze distillation-pervaporation in a single unit (DPSU) systems with a hybrid rectifying-pervaporation section, where the membrane constitutes the whole section. In this study, this simplified model is applied to DPSU columns where the membrane partially constitutes the rectifying or the stripping sections, including the model derivation of the stripping section and the operation leaves. The simplified model is applied for the separation of two mixtures with different Serafimov's topology classifications: acetone-isopropanol-water (topology type 1.0-2) and ethyl acetate-ethanol-methanol (topology type 2.0-2b). Thermodynamic limitations are identified for the separation of the ethyl acetate-ethanol-methanol mixture. Multiple operation leaves are produced depending on the liquid composition at the beginning of the membrane section, hindering the conditions that help to overcome the distillation boundary through a DPSU column. For some conditions, a section that is partially constituted by a membrane performs better than if the membrane constitutes the whole section.     

Dividing wall columns: Fundamentals and recent advances

Different distillation sequences for the separation of near-ideal multicomponent mixtures have been proposed in the past. These sequences included both conventional and thermally coupled distillations. Investigations of these sequences based on thermodynamics and steady-state simulations aimed for identifying the economic and energetic favourable configuration. Dividing wall columns have shown to be superior to conventional distillation sequences in certain cases. For this reason dividing wall columns gained increasing application in the last years. More than 90 applications in production scale are known. The advantages are obvious. Depending on the case considered the energy and investment costs are reduced up to 30% compared to conventional technologies. The footprint is significantly smaller. Also advantageous is the higher flexibility of these systems in comparison to conventional column sequences. For temperature-sensitive products the thermal stress is reduced since the product is reboiled only once. Especially for high price products the product quality can be raised by simultaneously increasing the separation yield.     

Enhancing mass and energy integration by external recycle in reactive distillation columns

The synthesis and design of reactive distillation columns separating reacting mixtures with the most unfavorable relative volatilities (i.e., the reactants are the heaviest and lightest components with the products being the intermediate ones) are described. The unfavorable thermodynamics poses great difficulties in combining the reaction operation and the separation operation involved and limits severely the potential of reactive distillation columns in the reduction of capital investment (CI) and operating cost. To remove the limitation, we propose two strategies for facilitating the synthesis and design of this kind of reactive distillation columns in this article. One is to arrange prudentially the reactive section so as to strengthen internal energy integration between the reaction operation and the separation operation involved; that is, while the reactive section should be placed at the bottom of the reactive distillation columns separating exothermic reactions, it should be at the top of the reactive distillation columns separating endothermic reactions. The other is to introduce an external recycle flow between the two ends of the reactive distillation columns to reinforce internal mass integration and internal energy integration between the reaction operation and the separation operation involved; that is, whereas the external recycle flow should be directed from the top to bottom of the reactive distillation columns separating exothermic reactions, it should be from the bottom to top of the reactive distillation columns separating endothermic reactions. Separation of four hypothetical ideal (i.e., two quaternary and two ternary systems, respectively) and two real nonideal (i.e., two quaternary systems) reacting mixtures is chosen to evaluate the proposed strategies. The results show that they can considerably lower energy requirement besides a further reduction in CI. © 2012 American Institute of Chemical Engineers AIChE J, 59: 2015–2032, 2013     

A simulation and parametric study of four existing multi-component distillation columns

The Θ-method and block-relaxation method of solving The non-linear set of algebraic equations which describe the leady-state behavior of standard distillation columns are compared. The results from the mathematical models are compared with actual plant performance of a depropanizer, debutanizer, de-ethanizer, and ethane-ethylene splitter. Column section efficiencies are tabulated. A parametric study of the variation of feed condition, reflux condition and flow, and feed tray location was made on these columns and the responses of the key component concentrations have been mapped. Such a study permits improvement of current plant operation. Its use for design of more efficient columns and arranangements is indicated.