A note on an extended short-cut method for the design of multicomponent reactive distillation columns

Until now, few studies have proposed analytical short-cut methods for reliably designing multicomponent reactive distillation columns. Therefore, in this study we have improved and extended a design methodology for the design of RD columns of multicomponent systems. We have developed a graphical design method, based on distillation lines and tray-by-tray calculations defined in terms of reaction-invariant composition variables, to determine RD design parameters such as the number of theoretical stages, operating reflux ratio, the feed tray location and the top or bottom flow. In this note, we report our extended and improved method, which is analytical and useful for reliably determining the design parameters of multicomponent RD systems. We study the synthesis of TAME with inert components (with different feed thermal conditions) as case of study to show the effectiveness of the proposed strategy. Results obtained with our strategy show a significant agreement with those obtained using a rigorous model of commercial simulator AspenONE Aspen Plus^®.     

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.     

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).     

A synthesis method for multicomponent distillation sequences with fewer columns

An easy‐to‐use matrix‐based method for the systematic synthesis of distillation configurations using less than n‐1 columns to separate any zeotropic n‐component feed into n product streams is described. The method is easily extended to obtain additional thermally coupled configurations. The only information needed to generate the configurations is the number of components in the feed, or equivalently, the number of distinct composition final product streams. We have successfully enumerated configurations for feeds containing up to eight components. This has resulted in a large number of hitherto unknown configurations even for four‐component separations. Some of the novel configurations generated using the method have substantially lower heat duty than the previously known fewer column configurations for a four‐component feed separation. Therefore, it is essential to include these novel configurations in the search space to find the optimal distillation configuration with fewer columns for a given application. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2479–2494, 2012     

Analysis of packed distillation columns with a rate-based model

Multicomponent packed column distillation is simulated using a rate-based model and the simulation results are compared with the experimental results obtained from a 0.2 m diameter pilot-scale packed column. The simulation algorithm used is previously proposed by the authors, which based on an equation-tearing method for (6c+7) equations of one packing segment and the whole column is solved by an iterative segmentwise calculation with the overall normalized θ method for acceleration. The performance of two packings is examined by simulating the pilot-scale column experiments using the published correlations for estimating liquid and vapor phase mass transfer coefficients and an effective interfacial area.     

Natural gas sweetening using low temperature distillation: simulation and configuration

ABSTRACT

Hypothesis: Separation of CO₂ from a stream of natural gas using low-temperature distillation process is complicated not only because of the existence of ethane (C₂) and CO₂ azeotrope but owing to the freezing of CO₂. A modified process based on an existing Ryan–Holmes process in the form of a low-temperature distillation is presented here for the separation of sour gases. In addition, solvents were employed to prevent CO₂ freezing and to avoid formation of the C₂-CO₂ azeotrope.

Simulation: Simulation calculations were carried out using a natural gas stream containing CO₂ and H₂S. The process was designed and simulated using Aspen HYSYS 9.

Findings: A low-temperature process for the separation of a feed stream containing up to 30 mole% CO₂ and 2.5 mole% H₂S was achieved, which lowered the levels of these gases to within 50 and 4 ppm, respectively. These values conform to liquefied natural gas specifications. The compositions of the product streams, the required solvent circulation flow rates, and the total energy requirements were estimated. The reported process successfully separated H₂S and produced CO₂ at a pressure of 25.0 bar and a temperature of ?13°C, thereby ensuring its suitability for application in enhanced oil recovery. This described process also delivered the desired natural gas product at 35 bar and ?90°C ready for liquefaction and further cooling.     

A formulation methodology for multicomponent distillation sequences based on stochastic optimization

Based on stochastic optimization strategy, a formulation methodology is proposed for synthesizing distillation column sequences, allowing more than one middle component as the distributing components between a pair of key components in the non-sharp split. In order to represent and manipulate the distillation configuration structures, a new coding procedure is proposed in the form of one-dimensional array. Theoretically, an array can represent any kind of split (non-sharp and sharp).With the application of a binary sort tree approach, a robust flow sheet encoding and decoding procedure is developed so that the problem formulation and solution becomes tractable. In this paper, the synthesis problem is formulated as a mixed-integer nonlinear programming (MINLP) problem and an improved simulated annealing approach is adopted to solve the optimization problem. Besides, a shortcut method is applied to the evaluation of all required design parameters as well as the total function.     

BATCH-DIST: A comprehensive package for simulation, design, optimization and optimal control of multicomponent, multifraction batch distillation columns

BATCH-DIST is a general-purpose simulation package for the design, simulation and optimization of multicomponent, multifraction batch distillation columns operating under different modes (constant reflux, variable reflux and optimal reflux policy). The package includes simulation models of varying degrees of complexity and rigor; efficient but simplified models (based on short-cut methods) for preliminary design and rapid analysis of column behavior, and rigorous models (based on solution of transient heat and mass balance differential equations) for verification and detailed column design. Besides simulation and design, BATCH-DIST can also accomplish optimization and optimal control of columns. Coded in Fortran 77, the package is flexible and user-friendly. BATCH-DIST has been extensively tested with benchmark cases involving binary and multicomponent systems, with nonideal behavior and in columns with appreciable holdup effects. Such test cases have clearly demonstrated that predictions of the simplified models in the package compare well with those of the rigorous models. This powerful and comprehensive package is expected to be computationally more efficient than existing packages.     

Synthesis of dividing-wall columns (DWC) for multicomponent distillations—A systematic approach

Dividing-wall columns (DWC) are intensified distillation systems for multicomponent separations. They have the potential to save significantly both energy and capital costs than conventional simple column configurations. In this paper, it is shown that the DWC columns can be systematically generated from the conventional simple column configurations. Because of the simple column sequences with sharp splits are the simple and widely studied conventional schemes for multicomponent distillation, the purpose of this work is to formulate a procedure for systematic synthesis of DWC columns for such simple conventional schemes. A four-step procedure is formulated which systematically generates all the possible DWC columns from the simple column sequences. First, the subspace of the original thermally coupled configurations corresponding to the simple column configurations is generated. Then, the subspace of the thermodynamically equivalent structures corresponding to the original thermally coupled configurations is produced. Finally, the subspace of the DWC columns corresponding to the thermodynamically equivalent structures is achieved. An example of quaternary distillation is used to illustrate the synthesis procedure which is applicable to a mixture with any number of components.     

Understanding the dynamic behaviour of middle-vessel continuous distillation columns

The fundamental dynamic behaviour of a middle-vessel continuous distillation column is studied in this paper. The interaction between the middle vessel (MV) design and control parameters is identified by means of a linear analysis. Transfer functions relating the dynamics of the product composition responses to changes in the disturbance inputs and manipulated inputs are derived. The role of the MV holdup and MV level controller gain in altering the product composition responses is identified. It is shown that the MV level controller gain can significantly affect the control performance when feed flow disturbances must be compensated for. However, it has a much lower impact in the control performance when feed composition disturbances need to be rejected; in this case, exploiting the “buffering” effect of the MV is more important for control. The analysis is carried out in detail for a LV control configuration, and is subsequently extended to an “on-demand” DB configuration. Finally, simple guidelines for the choice of the MV level controller gain and MV holdup are provided.     

Global optimization of multicomponent distillation configurations: 1. Need for a reliable global optimization algorithm

Nonazeotropic multicomponent mixtures are often separated into products by distillation configurations containing multiple distillation columns. One method of calculating the minimum vapor duty of a configuration is to sequentially calculate the minimum vapor duty of each mixture as it is split into two streams within a given column starting from the feed column. The other method simultaneously manipulates all the splits to yield the overall minimum vapor duty of the entire configuration. Of these two methods, the sequential minimization is attractive as it can be analytically solved. However, through extensive computations, we find that the sequential minimization method is not a valid substitute for the simultaneous minimization method. As the number of components in the feed increases, the fraction of the basic configurations for which sequential method yields a reasonable estimate decreases rapidly, thereby emphasizing the need for a more robust and reliable global optimization algorithm. © 2012 American Institute of Chemical Engineers AIChE J, 59: 971–981, 2013     

A matrix method for multicomponent distillation sequences

We describe a simple‐to‐use “matrix” method for obtaining all the basic distillation configurations and additional thermally coupled configurations that separate a zeotropic multicomponent feed into essentially pure product streams. This provides an opportunity to rank‐list the configurations for a given application subject to criteria of interest. The only information needed to generate the configurations is the number of components in the feed. We have successfully enumerated all the configurations for feeds containing up to eight components. The method can also be used to generate nondistillation and hybrid separation configurations, and even easy‐to‐retrofit configurations. We illustrate the use of this method by applying it to the highly energy‐intensive problem of petroleum crude distillation. We have identified more than 70 new configurations that could potentially have lower heat duty than the existing configuration. A significant number of these could reduce the heat demand by nearly 50%. © 2009 American Institute of Chemical Engineers AIChE J, 56: 1759–1775, 2010     

Design and control of dual condensers in distillation columns

Most distillation columns use water-cooled condensers cooling water that is inexpensive. With cooling water supplied at 305 K, a reflux-drum temperature of 322 K is normally used for column design. This temperature and the distillate composition set the require column pressure.This paper demonstrates that the use of refrigeration in a second condenser in series with the primary water-cooled condenser has economical advantages in some separations. The dual-condenser process can be less expensive than the single-condenser process in cases in which a lighter-than-light-key component is present in the feed and in which the separation between the key components is difficult (low relative volatility systems). Using a small refrigerated condenser permits the column to operate at a lower pressure, which reduces reboiler duty enough to compensate for the small amount of refrigeration required.The dynamic control of the dual-condenser process is also studied, and an effective control structure is developed.     

Rapid and reliable tuning of dynamic distillation column models on experimental data

A systematic procedure is presented to assist the calibration of tray distillation column models over plant data. Practical suggestions are provided that contribute to reduce the time needed to check the closure of the experimental material and energy balances. Typical modeling inaccuracies that may have a strong impact in the model performance are highlighted. A set of "cold" and "hot" test runs is suggested to minimize the time that is needed to reliably tune the model in view of using it for control system design and testing. The procedure is discussed with reference to the experimental results obtained in a pilot-plant column.     

Designing four-product dividing wall columns for separation of a multicomponent aromatics mixture

Preliminary evaluations using a simple but reliable short-cut method indicated that a 15 component aromatics mixture can be separated very efficiently into four fractions according to the given product specifications employing either a single or a multiple partition wall dividing wall column (DWC). The obtained results have been used to initiate rigorous simulations, to determine the number of stages required in different sections, as well as to obtain internal flows of vapour and liquid necessary for dimensioning and adequate cost estimation for two design alternatives. Based on the comparison of total annualised costs it appears that a multi-partition wall configuration that maximizes energy efficiency is a more attractive option for implementation in aromatics processing plants than more practical single partition wall configuration.     

A boundary value design method for complex demethaniser distillation columns

Typically the number of design options for demethaniser flowsheets, that recover methane from a gas mixture, is large. Repetitive simulations, to evaluate the economic viability of alternatives, do not usually enable a thorough exploration of the variable space for the purposes of process synthesis. More comprehensive process optimisation is facilitated by shortcut design models and a suitable optimisation framework. These optimisation results, applying shortcut models, are useful at the initial design stage, when the range of flowsheet options and operating conditions to be explored is relatively wide.A demethaniser column has many degrees of freedom, including the operating pressure, the location and the order of feeds, the number and duty of side reboilers and the flow rate of the external reflux stream. The complexity of the demethaniser column precludes the use of the Fenske-Underwood-Gilliland shortcut design method. An appropriate design model for the demethaniser is presented for application within an optimisation framework for process synthesis and evaluation. The column design model is computationally relatively undemanding, yet accurate, so should allow evaluation of both energy demand and equipment requirements.The design model presented is a semi-rigorous boundary value method for the design of complex demethaniser columns separating multicomponent mixtures. The method has been implemented within MATLAB and linked to HYSYS for prediction of physical and thermodynamic properties. Industrially relevant examples demonstrate that the results of the proposed design methodology are in good agreement with those of rigorous simulation.     

A comparative study of controlling the externally heat-integrated double distillation columns (EHIDDiC)

The externally heat-integrated double distillation columns (EHIDDiC) is a newly proposed scheme featuring complete heat integration between the rectifying section of a high pressure distillation column (HPDC) and the stripping section of a low pressure distillation column (LPDC). In terms of its structural characteristics, three decentralized control systems are devised, which avoid using the pressure difference between the HPDC and LPDC as a manipulated variable and ease consequently the interaction between the control loops involved. While the first one attempts to control the composition of the blended top products of the HPDC and LPDC, the second one the composition of their blended bottom products, thereby simplifying the control structure from 4 × 4 to 3 × 3 system. The third one focuses on the simplified EHIDDiC with only three heat exchangers between the HPDC and LPDC (S-EHIDDiC) and their heat duties are employed as a combined manipulated variable. These control systems are evaluated in terms of the separation of a binary mixture of benzene and toluene and it is found that they outperform exclusively the conventional control system with the pressure difference as a manipulated variable. Both the top-mixed and bottom-mixed control systems appear to be superior to the one for the S-EHIDDiC and conventional double-effect distillation column, implying the advantages of the simplified design of decentralized control systems. The obtained results are considered to be of general significance and can be used to guide the design and operation of the EHIDDiC (S-EHIDDiC).     

规整填料塔多元蒸馏过程混合型模型的模拟

填料塔精馏过程的模拟设计大多采用平衡级模型,但由于单一板效率值的难以确定,很多场合模拟很不成功;而新提出的非平衡级模型认为塔内传质传热均处于非平衡状态,由于方程数和经验性参数多,模型求解非常困难。文章针对规整填料的特点,建立了规整填料塔蒸馏过程的一种混合型模型,模型的主要特征是认为气液二相传质处于不平衡状态,而传热处于平衡状态。模型建立在实际填料基础上,既舍去传统的平衡级模型不确定性,又省略了非平衡级模型中复杂的经验性传热系数和液相传质系数的计算。模型计算值和实验值符合较好,也证实混合型模型既反映实际,又使模型求解变得相对容易。     

Minimum reflux calculation for multicomponent distillation in multi-feed,multi-product columns: Mathematical model

Multi-feed, multi-product distillation columns are ubiquitous in multicomponent distillation systems. The minimum reflux ratio of a distillation column is directly related to its energy consumption and capital cost. Thus, it is a key parameter for distillation systems design, operation, and comparison. In this series, we present the first accurate shortcut based algorithmic method to determine the minimum reflux condition for any general multi-feed, multi-product (MFMP) distillation column separating any ideal multicomponent mixture. The classic McCabe-Thiele or Underwood method is a special case of this general approach. Compared with existing techniques, this method does not involve any rigorous tray-by-tray calculation, nor does it require guessing of key components. In this first part of the series, we present the mathematical model for a general MFMP column, derive constraints for feasible separation and minimum reflux condition, discuss their geometric interpretations, and present an illustrative example to demonstrate the effectiveness of our approach.