FIBONACCI SEARCH FOR OPTIMAL FEED LOCATION

Abstract

A Fibonacci search technique is used in conjunction with a rigorous mullicomponenl distillaiion computer module to find the optimal feed location within a section of a distillation column. The function lo be minimized can be one of the following:

?key component ratio difference

?reflux ratio or reboiler ratio

?condenser duty or reboiler duty

This technique has been used successfully in the relocation of feed stages of many existing distillation columns and thus saved energy consumption. We shall describe this technique with a sample problem.     

Separation of Heavy Water by Vapor-Phase Thermal Diffusion Coupled with Distillation and Condensation

Abstract

A study on the enrichment of heavy water in a vapor-phase thermal-diffusion column has been conducted. With the combination of the effects of distillation, vapor-phase thermal diffusion, and partial condensation, considerable improvement in the degree of enrichment has been achieved in a vapor-phase column rather than in a liquid-phase column. It was also found that even the part of enrichment contributed only by vapor-phase thermal-diffusion effect is much higher than that obtained by liquid-phase thermal diffusion.     

Comparison of Energy Usage for the Vacuum Separation of Acetic Acid/Acetic Anhydride Using an Internally Heat Integrated Distillation Column (HIDiC)

Abstract

Energy savings for an internally heat-integrated distillation column (HIDiC) and a vapor recompression column for the vacuum separation of acetic acid/acetic anhydride was theoretically analyzed and compared to the simulation of a reference column configuration of the Eastman Chemical Company using ASPEN Plus. In these simulations, the design and operating variables were defined and optimized to minimize total energy used. The effects of design variables such as quantity and location of the heat integration stages, reflux ratio, and rectifying section absolute pressure on energy consumption and product purity revealed that one HIDiC configuration had 62% energy savings over the reference column. The distillation column using vapor recompression was evaluated as a benchmark for comparing the HIDiC configurations and the reference column. The VRC column simulation predicted both increased product purity and an energy savings of 91% over the reference unit.     

Heterogeneous extractive batch distillation of chloroform-methanol-water: Feasibility and experiments

A novel heterogeneous extractive distillation process is considered for separating the azeotropic mixture chloroform-methanol in a batch rectifying column, including for the first time an experimental validation of the process. Heterogeneous heavy entrainer water is selected inducing an unstable ternary heteroazeotrope and a saddle binary heteroazeotrope with chloroform (ternary diagram class 2.1-2b). Unlike the well-known heterogeneous azeotropic distillation process and thanks to continuous water feeding at the column top, the saddle binary heteroazeotrope chloroform-water is obtained at the column top, condensed and further split into the liquid-liquid decanter where the chloroform-rich phase is drawn as distillate. First, feasibility analysis is carried out by using a simplified differential model in the extractive section for determining the proper range of the entrainer flowrate and the reflux ratio. The operating conditions and reflux policy are validated by rigorous simulation with ProSim Batch Column^® where technical features of a bench scale distillation column have been described. Six reproducible experiments are run in the bench scale column matching the simulated operating conditions with two sequentially increasing reflux ratio values. Simulation and experiments agree well. With an average molar purity higher than 99%, more than 85% of recovery yield was obtained for chloroform and methanol.     

A Simple Analytic Formula for the Number of Theoretical Plates in Distillation

Abstract

The Lewis equation for the number of theoretical plates in a distillation column has been simplified. The simplification is not at the expense of accuracy. On the contrary, it is shown that the simple equation leads almost exclusively to more accurate results.     

Analysis of Ternary Distillation Column Sequences

Abstract

Eight complex distillation column configurations for a ternary feed mixture are modeled and studied. A modified complex method is used to minimize the cost of the configurations. Optimum regions for each configuration depending on the feed composition are derived. The result provides guidelines for the synthesis of distillation column configurations.     

内部热耦合精馏塔的操作性能与模拟

对同轴式内部热耦合精馏塔的操作性能和节能效果进行了研究,考察了全回流操作条件下,压缩比对回流量、冷凝器负荷和再沸器负荷的影响。结果表明,随着压缩比的增大,回流量、冷凝器负荷和再沸器负荷均降低。通过实验数据计算得到了该塔的理论板数和两塔间的传热量,精馏段为9块理论板,提馏段为4块理论板,当压缩比为2.2:1时,两塔间传热量为9.98kW。连续操作条件下,对内部热耦合精馏塔的节能效果进行了分析,通过与常规精馏塔的比较,内部热耦合精馏塔可节约52.3%的冷量,输入的再沸器和压缩机总负荷可节约20.34%。另外,基于实验数据,对该内部热耦合精馏塔进行了动态模拟,经连续操作下的实验验证,该内部热耦合精馏塔可在2h后达到稳定操作。     

Optimal operation of a batch packed distillation column

Optimal policies for a batch packed distillation unit are examined for the cases of variable reflux ratio and variable boilup rates. This work was to act as a follow-up to Converse's work⁽¹⁾ for a batch plate distillation column with variable reflux policy. For the cases studied, it would seem that the boilup rate optimal policy yields better performance for the still than the reflux ratio optimal policy. The results of the present analysis and those of other authors seem to indicate that more effort should in the future be devoted to new modes of operation rather than to various refinements of the model used in the present and past studies. The present exploratory work seems to support the need for further studies using a packed rectifying section with variable boilup policy along with a novel mode of operation such as that considered by Mayur et al,⁽⁶⁾.     

Experimental model validation for n-propyl propionate synthesis in a reactive distillation column coupled with a liquid–liquid phase separator

In the synthesis of some organic esters, reactive distillation coupled with a liquid–liquid phase separator is often used to increase the product purity or to recover the reactants. In this article, we present a comprehensive experimental and theoretical study on the heterogeneously catalysed synthesis of n-propyl propionate by reactive distillation and a subsequent liquid–liquid phase separator. The experiments were performed in a pilot-scale reactive distillation column. Data-reconciliation tests proved that the experimental results obtained comprise a complete, reliable set of composition and temperature profiles along the pilot-scale reactive distillation column and can be used for further model validation. A nonequilibrium-stage model was applied to predict the experimental results. Simulation studies demonstrated that the composition and temperature profiles in the rectifying section of the column were highly sensitive to the composition of the reflux stream entering the column. Deviations between the experimental and predicted composition profiles in the rectifying section were identified. An explanation for the deviations is given in this article.     

Reactive column profile map topology: Continuous distillation column with non-reversible kinetics

In this paper we present a topologically based approach to the analysis and synthesis of reactive distillation columns. We extend the definition of Tapp et al. [Tapp, M., Holland, S., Glasser, D., & Hildebrandt, D. (2004). Column profile maps part A: Derivation and interpretation. Industrial and Engineering Chemistry Research, 43, 364–374] of a column section in non-reactive distillation column to a reactive column section (RCS) in a reactive distillation column. A RCS is defined as a section of a reactive distillation column in which there is no addition or removal of material or energy. We introduce the concept of a reactive column profile map (RCPM) in which the profiles in the RCPM correspond to the liquid composition profiles in the RCS. By looking at the singular points in the RCPM, it is demonstrated that for a single chemical reaction with no net change in the total number of moles, the bifurcation of the singular points depends on both the difference point as introduced by Hauan et al. [Hauan, S., Ciric, A. R., Westerberg, A. W., & Lien, K. M. (2000). Difference points in extractive and reactive cascades I-Basic properties and analysis. Chemical Engineering Science, 55, 3145–3159] as well as the direction of the stoichiometric vector. These two vectors combine to define what we call the reactive difference point composition. We show that there only certain feasible topologies of the RCPM and these depend only on the position of the reactive difference point composition. We look at a simple example where the vapour liquid equilibrium (VLE) is ideal and show that we can classify regions of reactive difference point compositions that result in similar topology of the RCPM. Thus, by understanding the feasible topologies of the RCPM, one is able to identify profiles in the RCPM that are desirable and hence one is able to synthesize a reactive distillation column by combining RCS that correspond to the desired profile in the RCPM.We believe that this tool will help understand how and when reaction could introduce unexpected behaviors and this can be used as a complementary tool to existing methods used for synthesis of reactive distillation columns.     

A review on process intensification in internally heat-integrated distillation columns

Internally heat-integrated distillation column (HIDiC) is the most radical approach of a heat pump design, making efficient use of internal heat-integration: the rectifying section of a distillation column operating at a higher pressure becomes the heat source, while the stripping part of the column acts as a heat sink. Remarkably, a HIDIC can bring up to 70% energy savings compared to conventional distillation columns. This is highly appealing regarding the fact that distillation is one of the most energy intensive operations in the chemical process industry accounting for over 40% of the energy usage. This review paper describes the latest developments concerning this promising but difficult to implement process intensification technology, covering all the major aspects related to the working principle, thermodynamic analysis, potential energy savings, various design configurations and construction options (ranging from inter-coupled or concentric columns, shell and tube and plate–fin heat exchanger columns to SuperHIDiC), design optimization, process control and operation issues, as well as pilot-scale and potential industrial applications. Further advancement, i.e., development of HIDiC technology for multi-component mixture separations is an extremely challenging research topic, especially when HIDiC becomes associated with other technologies such as dividing-wall column (DWC) or reactive distillation (RD).     

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.     

Separation of 2-Propanol–Water Mixture with Capillary Porous Plates

ABSTRACT

Distillation of the 2-propanol–water mixture of different compositions was experimentally studied in a continuous distillation column equipped with polar and nonpolar porous plates as well as normal sieve plates. The results showed that while no separation was achieved in a distillation column with porous carbon plates or with conventional stages, the azeotropic. point of this system was broken in a distillation column with polar sintered stainless steel porous plates. A distillate of about 80.0 mol% 2-propanol was obtained for a feed of azeotropic composition, i.e., 68.0 mol% 2-propanol. These results showed that the main factors affecting the separation efficiency in a given porous plate are the polarization of the pure liquids and porous plates as well as the polarization difference between the mixture components.     

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.     

Logic hybrid simulation-optimization algorithm for distillation design

In this paper, we propose a novel algorithm for the rigorous design of distillation columns that integrates a process simulator in a generalized disjunctive programming formulation. The optimal distillation column, or column sequence, is obtained by selecting, for each column section, among a set of column sections with different number of theoretical trays. The selection of thermodynamic models, properties estimation, etc. is all in the simulation environment. All the numerical issues related to the convergence of distillation columns (or column sections) are also maintained in the simulation environment. The model is formulated as a Generalized Disjunctive Programming (GDP) problem and solved using the logic based outer approximation algorithm without MINLP reformulation. Some examples involving from a single column to thermally coupled sequence or extractive distillation shows the performance of the new algorithm.     

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.     

Studies of Controlled Cyclic Distillation: I. Computer Simulations and the Analogy with Conventional Operation

Abstract

Computer simulations of a controlled cycling rectification still were run to determine the theoretical effects of local point efficiency, slopes of the equilibrium and operating lines, amount of liquid dropped during the liquid-flow period, and other parameters on over-all column and individual plate efficiencies. It is shown that, although the Murphree vapor efficiency of a plate at any instant of time is assumed to be constant and equal to the local point efficiency, the effective plate efficiency based on the resulting liquid-phase plate compositions is usually significantly greater than the point efficiency. This leads to the improved separating ability achieved in controlled cyclic operation. The existence of a precise analogy between controlled cyclic distillation and conventional distillation with liquid-phase concentration gradients across the plates of the column is shown. In essence, this analogy reduces to the substitution of time as the independent variable in the former case for distance in the latter case.     

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.     

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。     

Retrofit of Distillation Columns Using Thermodynamic Analysis

Abstract

Thermodynamic analysis provides the column grand composite curves and exergy loss profiles, which are becoming readily available for a converged distillation column simulation. For example, the Aspen Plus simulator performs the thermodynamic analysis through its Column–Targeting tool for rigorous column calculations. This study uses the column grand composite curves and the exergy loss profiles obtained from Aspen Plus to assess the performance of the existing distillation columns, and reduce the costs of operation by appropriate retrofits in a methanol plant. Effectiveness of the retrofits is also assessed by means of thermodynamics and economics. The methanol plant utilizes two distillation columns to purify the methanol in its separation Section. The first column operates with 51 stages, has a side heat stream to the last stage, a partial condenser at the top and a side condenser at stage 2, and no reboiler. The second column operates with 95 stages, has a side heat stream to stage 95, a total condenser, and high reflux ratio. Despite the heat integration of the columns with the other Sections and a side condenser in column 1, the assessment of converged base case simulations have indicated the need for more profitable operations, and the required retrofits are suggested. For the first column, the retrofits consisting of a feed preheating and a second side condenser at stage 4 have reduced the total exergy loss by 21.5%. For the second column, the retrofits of two side reboilers at stages 87 and 92 have reduced the total exergy loss by 41.3%. After the retrofits, the thermodynamic efficiency has increased to 55.4% from 50.6% for the first column, while it has increased to 6.7% from 4.0% for the second. The suggested retrofits have reduced the exergy losses and hence the cost of energy considerably, and proved to be more profitable despite the fixed capital costs of retrofits for the distillation columns of the methanol plant.