The Role of Recycle in Countercurrent Recycle Distillation Cascades. I. Constant Reflux,Ideal, and Squared-Off Cascades

Abstract

In most textbooks concerned with countercurrent multistage separations, minimum reflux ratio for continuous distillation is usually defined only in terms of a graphical construction on a McCabe—Thiele diagram: it is the recycle ratio (liquid flow rate) associated with the operating line that touches the equilibrium curve at the feed point. However, it is easily shown that minimum recycle ratio depends on local α and composition, as well as product compositions, and thus, it is a stage-composition phenomenon. As a result, for a specified separation, each ideal stage in a continuous distillation cascade has a specific minimum recycle ratio associated with it. For constant α, the minimum recycle ratio increases as the stage compositions depart more from product (distillate or bottoms) compositions. As a result, the textbooks only consider the maximum minimum recycle ratio. This paper presents the results of some theoretical calculations which illustrate how minimum recycle ratio varies with stage α, stage and product compositions, and presents an example of distillation cascade behavior when minimum recycle ratio is approached at a composition other than the feed point. An example is also presented which shows how the separation is effected when the reflux ratio is reduced below the design value in a distillation column containing a fixed number of ideal stages. A brief comparison is also made between constant reflux, ideal, and squared-off cascades in terms of number of stages, total interstage flow, and relative energy requirements for the different designs to illustrate and emphasize the consequences of the stagewise behavior of minimum recycle ratio.     

GRAPHICAL PROCEDURE FOR REACTIVE DISTILLATION SYSTEMS

In this paper we analyze a ternary reactive distillation system, where an equilibrium chemical reaction occurs in the liquid phase. By using a set of transformed variables proposed by Barbosa el al. (1988b) and well known graphical procedures for non-reactive systems; the minimum reflux ratio, minimum number of equilibrium stages, mass and energy balances for a reactive column and flash can be easily obtained. The procedures developed in this paper are applied to ISOBUTYLENE-METHANOL-MTBE system.

The main objective in developing this model is to obtain, as much as possible, rigorous information for the analysis of one reactive distillation column in a Process Simulator with capability of handling this type of new operation.     

A graphical design method for reaction–extraction processes in quaternary systems

This paper proposes a graphical design method for quaternary systems in simultaneous chemical and physical liquid–liquid equilibrium (reaction–separation process). The reactive phase equilibrium data and the stage by stage calculation are represented into two projections diagrams with rectangular coordinates. The graphical method has been applied to reversible reactive systems where all of the components of the reaction have the same stoichiometric coefficient. This graphical method allows estimating the number of reactive theoretical stages, the limits of the solvent to feed ratio (D/F), the extent of reaction and the conversion. Results obtained for seven design problems (involving four different reactive systems) are in good agreement with state of the art simulation software and with literature.     

Investigations on the Synthesis of Methyl Acetate in a Heterogeneous Reactive Distillation Process

The production of methyl acetate in a reactive distillation process – prior art for 15 years – is often used as an example to study the basic phenomena of reactive distillation. The present paper deals with a theoretical and experimental analysis of methyl acetate synthesis in a reactive distillation column. A design method based on the interpretation of reactive distillation line diagrams is used to identify the main process parameters and to provide a foundation for experimental investigation. The significant influence of the reflux ratio on the conversion in the column is shown by mini plant experiments using supported ion exchanger in the form of Raschig rings as a heterogeneous catalyst. These experiments demonstrate the catalytic quality of this packing material. To simulate the reactive distillation column with a simple stage-to-stage method, the separation efficiency of the catalytic rings is investigated. Comparison of experimental and simulation results reveals that a simple model based on the assumption of simultaneous chemical and phase equilibrium describes the experimental data quite well over a wide range of reflux ratios. Furthermore, simulation results show that the conversion depends less on the number of reactive stages than on the use of two feed stages.     

Hydrodynamic considerations on optimal design of a three‐phase airlift bioreactor with high solids loading

The hydrodynamic study of a three‐phase airlift (TPAL) bioreactor with an enlarged gas–liquid dual separator was carried out. Different lengths and diameters of the draft tube were tested to show how the design of the separator zone affects the hydrodynamic performance of the TPAL reactor. Ca‐alginate beads with entrapped yeast biomass at different loadings (0, 7, 14 and 21% v/v) were used in order to mimic the solid phase of conventional high cell density systems, such as those with cells immobilized on carriers or flocculating cells. Important information on multiphase flow and distribution of gas and solid phases in the internal‐loop airlift reactor (ALR) with high solids loading was obtained, which can be used for suggesting optimal hydrodynamic conditions in a TPAL bioreactor with high solids loading. It is finally suggested that the ALR with a dual separator and a downcomer to riser cross‐sectional area ratio (A_D/A_R) ranging from 1.2 to 2.0 can be successfully applied to batch/continuous high cell density systems, where the uniform distribution of solid phase, its efficient separation of particles from the liquid phase, and an improved residence time of air bubbles inside the reactor are desirable. Copyright © 2003 Society of Chemical Industry     

Effect of Changes in Operating Variables on Stage-wise and Cumulative Separation in Binary Multistage Distillation

Abstract

The progressive separation that occurs stage-wise within a multistage distillation column is characterized by the cumulative extent of separation, ζ _N ; while the contribution of individual stages, δ _N , to the overall separation is given by the difference between ζ _N for successive stages. These indexes permit the “goodness” of separation for individual stages and for the entire column to be compared on an equivalent basis. This paper examines the effects of changing operating variables of reflux ratio, feed rate, feed composition, and feed stage location on the separation obtained in a distillation column containing a fixed number of ideal stages, and how the single-stage contribution changes when these variables are altered from the design value. The calculations show that the reflux ratio (R) is probably the most important variable in determining how well a column makes a separation. Separation declines rapidly as R is reduced below the design value, as the feed rate is increased at constant boil-up rate, and as the feed concentration drops below the design value. Changing the feed stage location of ±5 stages in a 50-stage column has a minimal effect on separation at all feed compositions. δ _N clearly shows how the contribution of individual stages changes when operating variables are varied from the design values.     

流化床中气固均匀分布的失稳现象

流化床因其均匀且剧烈的气固相互作用保证了其优异的流动和传递性能,因而广泛应用于化学工业中。因此,构建定量计算气固均匀分布的失稳临界点既是重要的学术问题又具有工程意义。本文分别使用气相和固体颗粒相的质量分数表示气固分布状态;引入颗粒床层压力载荷（Φ _T）描述分布器输入的规则负熵和固体颗粒床层自身混沌熵产生之间相互作用;由于密相颗粒床层远离平衡态且具有强非线性耗散项,因此需基于普利高津最小超量熵增原理给出气固密相流在并联系统均布状态的失稳临界点（Φ _Tc）：分布器和固体颗粒床层总熵增在气固均布和气固非均布情况下相等;由于并联系统的对称性,可将N单元路径并联系统气固均布稳定性分析简化为判断单元路径压降二阶导数正负;在此基础上讨论了操作参数、固体颗粒性质和分布器结构参数对气固密相床层均布稳定性的影响。此外,通过气体示踪和压力脉动频谱分析在直径为300mm冷模实验验证了颗粒床层压力载荷（Φ _T）对密相气固均布稳定性的影响;同时应用该方法论计算了工业流化床反应器临界床层高度、临界表观气速以及分布器临界阻力系数,指导了操作工况的调整和分布器结构设计,对比分析了改造前后的反应情况。     

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).     

Preparation of microporous PVDF membrane via tips method using binary diluent of DPK and PG

Microporous polyvinylidene fluoride (PVDF) membrane was prepared via thermally induced phase separation (TIPS) method using a binary diluent of diphenyl ketone (DPK) and 1,2‐propylene glycol (PG). The phase diagram for the PVDF/binary diluent of DPK and PG system was measured in the range of the PG/DPK mass ratio changing from 0 to 2/3. Then the effects of the PG/DPK mass ratio and the PVDF concentration on membrane cross‐section structures and tensile strength were also investigated. The results showed that the addition of PG brought about a shift of the cloud point curve to a higher temperature and the extension of the liquid–liquid phase separation region to a higher polymer concentration. Therefore a bicontinuous cross‐section structure was obtained when the PG/DPK mass ratio was 3/7 and the polymer concentration was 30 wt %. As an increase of the PG/DPK mass ratio, the tensile strength increased gradually at a fixed PVDF concentration. Moreover, for the same PG/DPK mass ratio, the cross‐section microstructure changed from a bicontinuous or a cellular structure to a spherulitic structure, and the tensile strength increased drastically as the polymer concentration increased from 20 to 50 wt %. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

立板式隔板塔分离直馏汽油工艺

对利用一种新型立板式隔板精馏塔切割直馏汽油工艺进行了研究。首先,利用模拟软件HYSYS对该分离过程进行模拟,得到较优的工艺条件。在此基础上,利用立板式隔板塔实验装置,考察了塔顶总回流量、液体分配比等操作参数对产品的影响及装置的操作稳定性。结果表明,塔顶总回流量的增大有助于产品的分离,且液体分配比的选择范围更广。液体分配比的改变对中间侧线产品质量的影响较大,是操作的关键变量。     

Anaerobic digestion of liquid dairy manure using a sequential continuous‐stirred tank reactor system

BACKGROUND: Anaerobic digestion of liquid manure from flush dairy operations is an alternative to manure management practices with environmental benefits and producing biogas. A unique sequential reactor system was used to study the anaerobic digestion (AD) of flush dairy manure with the aim of determining (1) the effects of organic strength of the influent on AD performance; (2) whether phase separation is beneficial to treatment and AD design; and (3) whether there is any difference between completely mixed and plug‐flow approaches to implementing the AD process. RESULTS: The influent with high organic strength resulted in higher levels of biogas production and chemical oxygen demand (COD)/solids removal. When the manure stream is used as influent for the anaerobic digestion, the phenomenon of two‐stage anaerobic digestion process does not exist because the stream contained high levels of alkalinity and volatile fatty acids. The biogas production and COD/solids destruction achieved by plug‐flow and completely mixed reactor were very similar when treating the liquid manure stream. CONCLUSION: The organic strength of the influent significantly influenced AD performance. The plug‐flow reactor and the completely mixed reactor had a similar AD performance. For the type of manure and organic loading rate investigated, the results give indications for designing an AD process in flush dairy operations. Copyright © 2007 Society of Chemical Industry     

The Extent of Separation in Continuous Multistage Binary Distillation

Abstract

Rony's extent of seapration has been applied to binary distillation, analyzing both single equilibrium stages and the cumulative separation obtained in a multistage column both at finite and total reflux. Both separation indices depend on relative volatility, reflux ratio, and composition. The cumulative extent of separation clearly shows the influence of reflux ratio on the separation obtained in a continuous distillation column. Small variations in the single-stage extent of separation appear to have a pronounced effect on the cumulation extent of separation and the number of stages required to make a given separation.     

Liquid velocity in a high‐solids‐loading three‐phase external‐loop airlift reactor

BACKGROUND: Airlift reactors are of interest for many different processes, especially for three‐phase systems. In this study the behavior of a high‐loading three‐phase external‐loop airlift reactor was examined. In particular, the effect of parameters such as airflow rate (riser superficial gas velocities between 0.003 and 0.017 m s^?1), solids loading (up to 50%, v/v) on liquid circulation velocity in the air‐water‐alginate beads system as a crucial hydrodynamic parameter was studied. RESULTS: It was observed that increase of the airflow rate resulted in increase of the liquid velocity in the system. The same result but less pronounced was observed by introducing small amounts of solid particles up to 7.5% v/v. However, further introduction of solids caused decrease of the liquid velocity. Laminar regime for the liquid circulation was observed for low gas velocities. Minimum gas velocities for recirculation initiation in the reactor were determined for all solid loadings and linear dependence on the solid content was found. Gas holdups for the three‐phase system were larger than for the two‐phase system in all experiments. A simple model for predicting the liquid circulation velocity in the three‐phase system with high solid loading of low‐density particles was developed. This model is based on the viscosity of integrated medium (solid + liquid) which is a new aspect to analyze this phenomenon. CONCLUSIONS: The developed model shows very good agreement with the experimental results for all solid loadings. It also includes the influence of reactor geometry on the liquid circulation velocity thus enabling optimization. Copyright © 2012 Society of Chemical Industry     

NOVEL ENERGY-SAVING METHOD OF RECTIFICATION

The conventional rectification method to separate and purify chemical mixtures has the disadvantage of being an energy-intensive unit operation. The innovative method proposed in this article employs vapor and liquid recycles to the trays that in turn reduce the reflux rate and vapor boil-up rate, resulting in energy reduction without compromising the separations.

These findings were supported with examples of ethanol-water and butane- pentane system separations. With ethanol-water system, the experiments were conducted without overhead liquid recycle and using 16 and 5 vapor recycles. The energy saved was found to be 25-35% for 16 recycles and 20-30% for 5 recycles. Butane-pentane system was chosen as an example of industrial use of the proposed method. A liquid overhead reflux ratio of 0.5 (L/D) was used with vapor recycles, compared to a reflux ratio of 1.25 for the conventional separation without vapor recycles. This resulted in 30-35% saving in energy while giving the same productivity of the column. Butane-pentane separation was also simulated using a commercial process simulator (ASPEN^©PLUS).     

Evaporative liquid jets in gas–liquid–solid flow system

Some aspects of the fundamental characteristics of evaporative liquid jets in gas–liquid–solid flows are studied and some pertinent literature is reviewed. Specifically, two conditions for the solids concentration in the flow are considered, including the dilute phase condition as in pneumatic convey and the dense phase condition as in bubbling or turbulent fluidized beds. Comparisons of the fundamental behavior are made of the gas–solid flow with dispersed non-evaporative as well as with evaporative liquids.For dilute phase conditions, experiments and analyses are conducted to examine the individual phase motion and boundaries of the evaporative region and the jet. Effects of the solids loading and heat capacity, system temperature, gas flow velocity and liquid injection angle on the jet behavior in gas and gas–solid flows are discussed. For dense phase conditions, experiments are conducted to examine the minimum fluidization velocity and solids distribution across the bed under various gases and liquid flow velocities. The electric capacitance tomography is developed for the first time for three-phase real time imaging of the dense gas–solid flow with evaporative liquid jets. The images reflect significantly varied bubbling phenomenon compared to those in gas–solid fluidized beds without evaporative liquid jets.     

萃取法从盐湖卤水提取铷实验研究

研究了t-BAMBP［4-叔丁基-2-（α-甲基苄基）苯酚］/磺化煤油萃取体系,从提铯后的母液中,萃取分离钾铷的过程。考察了萃取时间、萃取剂浓度、萃取相比等萃取条件、水洗条件和反萃取条件对铷钾分离的影响。确定了适宜的工艺条件为：t-BAMBP浓度为0.7 mol/L,相比O/A=3∶1,萃取时间为5 min;以0.1 mol/L氯化钠溶液为洗涤剂,洗涤相比O/A=4∶1;以0.5 mol/L 氯化氢溶液为反萃剂,反萃相比O/A=5∶1。经过5级逆流反萃,铷的反萃率达到95.6%以上,铷钾的分离系数较高,实现了铷钾分离。     

Single-Stage Contribution to the Overall Separation in Binary Multistage Distillation

Abstract

The contributions of individual stages to the overall separation obtained in a multistage distillation column are calculated by taking the difference between the cumulative extent of separation for successive stages. The calculations clearly show how the reflux ratio effects the number of stages required for the separation, the contribution of individual stages to the overall separation, and how the stage contributions are distributed as the number of stages required to make the separation decreases with increasing reflux ratio. The individual stage contribution is closely related to the stage-wise derivative of the cumulative extent of separation.     

外循环对IC反应器运行效果的影响

为改善已形成颗粒污泥的IC反应器运行性能,增设外循环装置,并在回流比分别为0、1.0、2.0、3.0、4.0的情况下,进行了系统运行稳定性研究。与无外循环的情况相比,在设定的回流比范围内,附加外循环不会破坏反应器内部厌氧条件,反应器运行稳定;在进水COD为6000 mg·L^-1左右,系统HRT约为10 h,容积负荷为14 kg·m^-3·d^-1的情况下,系统污泥MLSS有所增加,COD去除率随回流比的增大而增加,最高可达97.3%,出水COD低至160 mg·L^-1;随回流比的增大,产气总量逐渐增加且最终可达171.2 L·d^-1,甲烷产量先增加后趋于稳定,在回流比为2.0时可达到91.7 L·d^-1;增加外循环运行一段时间后,厌氧颗粒污泥生物相更为丰富,产甲烷优势菌由甲烷杆菌转变为甲烷八叠球菌。结合能耗和去除效率等考虑,回流比为2.0时最佳。     

萃取精馏分离乙酸乙酯和正己烷的过程模拟

利用Aspen Plus化工流程模拟软件,通过萃取精馏技术以N-甲基吡咯烷酮(NMP)为萃取剂,对制药过程产生的乙酸乙酯和正己烷混合液进行了模拟研究,分析了萃取精馏塔理论板数、进料位置、回流比及溶剂比对分离效果的影响,确定最佳工艺方案,为分离过程的优化操作和设计提供依据。     

Reaction induced phase separation in mixtures of multifunctional polybutadiene and epoxy

The phase behavior and separation dynamics have been investigated in blends of diglycidyl ether of bisphenol A (DGEBA), curing agent methylene dianiline (MDA), and a reactive liquid rubber (R45EPI) through application of differential scanning calorimetry (DSC), one- and two-dimensional light scattering, and optical microscopy. DSC analysis indicates that the system consists of three reactions: the self-condensations of DGEBA and R45EPI, as well as a cross-reaction between the two constituents. Observation of the dynamics of the 50/25.4/50 DGEBA/MDA/R45EPI system reveals that an initial phase separation is governed by the dominant self-curing reaction of DGEBA, followed by a phase dissolution characterized by a broadening of the interfacial regions catalyzed by a cross-reaction between the two species. A subsequent phase separation occurs at late stages since the copolymerization reaction does not proceed to completion. On the other hand, by changing the ratio of the beginning constituents to 70/25.4/30 DGEBA/MDA/R45EPI, the dissolution phenomena is not observed resulting from an even more dominant DGEBA/MDA condensation reaction. It is demonstrated that alterations in the initial compositional ratio greatly affect the phase separation dynamics of the system.