Efficient generalized shortcut distillation model with improved accuracy for superstructure-based process synthesis

We propose a general shortcut distillation column model that can address a wide range of types of separations including nonsharp/sloppy splits. The model can automatically identify key components and estimate the energy requirement of a desired separation in the presence, potentially, of components with zero flow rates. To obtain more practical column height and vapor flow rates, we augment the model with constraints inspired by the Fenske equation. We also propose valid constraints to enhance the computational performance of the proposed model. Due to its flexibility and computational efficiency, the proposed model can be readily used for superstructure-based process synthesis.     

Evolutionary thermodynamic synthesis of zeotropic distillation sequences

The objective of this paper is to present a new evolutionary strategy for the synthesis of simple and complex distillation sequences. The initial flowsheet is the reversible multicomponent distillation scheme, which includes as the thermodynamically optimum superstructure many of the possible separation configurations. Assumptions with respect to ideal boiling behaviour or constant molar overflow are not required, but azeotropic systems are excluded. The computation time requirement is extremely low, because only flash-type calculations and no rigorous column simulations are involved. In this approach columns and heat exchangers are successively removed from the initial flowsheet by using thermodynamic information on splits. In this way not only simple sequences, but also sidestream withdrawals, sidestream strippers and sidestream enrichers can be created. The evolutionary strategy and the underlying modification criteria will be illustrated by several examples. Some comments on common heuristic rules and heat-integration possibilities are made in conclusion.     

Efficient structure synthesis of reactive distillation processes using the IDEAS approach

A new synthesis method for reactive distillation processes is proposed. At each stage of a column, vapor–liquid equilibrium (VLE) is assumed and kinetically controlled reaction in liquid phase is considered. First, the liquid composition space is divided into small subspaces. Then, for each subspace a representative liquid composition is decided and assigned to a module corresponding to a stage of a distillation column. Then, after the calculation of the VLE and the reaction rate, the distribution network (superstructure) connecting all modules by vapor and liquid flow paths is constructed. The feature of the proposed model is that all constraints are linear to the optimization variables: the liquid and vapor flow rate and the liquid hold-up. The developed method was applied to the metathesis reaction of 2-pentene, and a completely new process structure was obtained. The effectiveness of the implied structure was confirmed through a comparison with conventional structures.     

MINLP synthesis of reactor networks in overall process schemes based on a concept of time-dependent economic regions

The design and optimization of reactor networks using the conventional concept of attainable region is based on technological (conversion, selectivity, etc.), rather than, economical criteria (cost, profit, etc.). The solution from the economical point of view when the operating and investment costs cannot be neglected may not be optimal, not even in regard to the structure. In order to circumvent this deficiency, the conventional Concentration Attainable Region (CAR) is transformed into an Economic Region (ER) using economic optimization criteria. A novel concept for ER construction is proposed for multi-D problems. One-parametric NLP or MINLP optimizations with reactor volume as a varying parameter are performed to construct trajectories in the ER. In this way the ER is aided by mathematical programming for 3D or more D problems in order to handle dimensions higher than 2 as degrees of freedom for the economic objective function. ER is attainable (EAR) only when the economic objective function is linear. The more the objective function is discontinuous, discrete, nonlinear and non-convex and the more its cost coefficients vary with time, the less the principles of AR can be applied to ER. However, economically optimal reactor systems always lie at the borders of ERs. Two important insights have been gained during the construction of different time-dependent ERs, which have been used in upgrading the MINLP approach to the synthesis of reactor networks in overall process schemes: (i) A reactor network superstructure has been reestablished based on economical optimization criteria and the principles of ER. (ii) Over a longer period, the performing of a stochastic multi-period MINLP synthesis is recommended, in order to consider the time variability and uncertainties of economic parameters. A special multi-period strategy is proposed to upgrade the efficiency of the MINLP synthesis.     

Shortcut assessment of alternative distillation sequence schemes for process intensification

Finding good process schemes is a difficult task due to its search among multiple alternatives. In literature, several methods are successfully implemented as computer tools, but their use is limited to their developers. There are several useful rules of thumb and heuristics providing useful guidelines, but sometimes these are contradictory. Some authors tried to rank the various alternatives, defining heuristic equations to provide a quantitative parameter to choose among possible solutions. In this paper, mathematical model of distillation columns is simplified, assuming infinite number of stages. The resulting proposed equation has a great similarity to one of the previous heuristic ones. A simple equation not relying on heuristics and easy to use in calculation is provided to evaluate the distillation sequence energy efficiency (DSE) for each alternative. This allows to quantify the advantages derived from process intensification for a given feed composition. The proposed equation is dimensionless, as the Carnot efficiency is used instead of the temperature difference between distillate and bottoms. On the other hand, intermediate results provide also useful information. For instance, the Carnot efficiency of each column indicates when a heat pump or enhanced distillation would be useful. The recovery efficiency for each particular compound allows an easy comparison between alternatives, considering variations on the feed composition. The new equation is verified comparing its results with cases already solved in literature using different methods. The results show that all the methods in literature are able to provide the best sequence, except the heuristics-based ones that are not providing an overall sequence evaluation. The novelty of the proposed DSE method resides in its ease of application, compared to nowadays available methods, and requires only the feed composition and products boiling points.     

A new approach to thermal integration of distillation sequences

This paper describes a new approach for synthesizing thermally integrated distillation sequences. There are two key developments in this research. First, the overall flowsheet is optimized simultaneously as opposed to the traditional technique of optimizing each tower sequentially followed by heat integration. Second, maximum energy recovery techniques are used to estimate the total energy requirements of the thermally integrated network, eliminating the need to synthesize a heat exchanger network on each function evaluation in the optimization routine. A frequently reported five component light alkane system (Rathore's Problem) was used to illustrate this approach. Optimized process flowsheets, energy savings and a comparison with literature values are presented.     

Stochastic GP synthesis of heat integrated nonsharp distillation sequences

Genetic programming (GP) is used to solve the synthesis problem of heat integration nonsharp distillation sequences and the optimization objective of it is to seek the optimal heat integration nonsharp flow based on minimizing the annually total cost. Meanwhile, the major technological parameters for these important equipments are given. Combining with the domain knowledge of chemical engineering, some evolutionary factors are improved, and a set of special encoding method and heat integration strategy is proposed to deal with this kind of problem. The system structural variable is optimized by GP and the continuous variable is optimized by the complex algorithm simultaneously. Because GP has the automatically searching function, the optimal heat integration solution can be found automatically without any superstructures of nonsharp distillation sequences. Three illustrating examples are presented to demonstrate the effective computational strategies.     

Simultaneous synthesis of distillation trains and heat exchanger networks

A new mathematical formulation for the optimal synthesis of heat-integrated distillation systems that accounts for constrained heat matches between columns and the rest of the process is presented. The definition of temperature intervals commonly used by heat exchanger network synthesis procedures is embedded in the proposed formulation that can be regarded as a generalization of the network flow model of Cerdá and Westerberg (1983, Chem. Engng Sci.38, 1723) under conditions of variable stream temperatures. The minimization of the total flow-sheet utility cost has been chosen as the design target.The problem has been represented through a mixed-integer linear program. By performing the search with a standard branch-and-bound technique the global optimality of the solution found is guaranteed. Three case studies involving the separation of a ternary mixture by distillation have been solved. The consideration of the rest of the process produces in two of them new distillation train designs costing less overall.     

IDEAS approach to process network synthesis: minimum utility cost for complex distillation networks

This work aims at quantifying the minimum utility cost necessary to complete a given separation task using vapor-liquid equilibrium (distillation) operations. The Infinite DimEnsionAl State-space (IDEAS) process is introduced as a new paradigm for this process synthesis activity. The IDEAS conceptual framework includes all possible process configurations, and yields mathematical programs that are convex (linear), thereby guaranteeing the global optimality of the obtained solution.A case study, on nitrogen/oxygen mixture separation, is employed to illustrate the power of the proposed approach. The obtained IDEAS designs are compared with the corresponding McCabe-Thiele (conventional) minimum utility designs, and are shown to be as much as 50% more thermodynamically and economically efficient.     

合成氯硅烷精馏工艺改造

针对多晶硅生产中合成氯硅烷精馏工艺存在的问题,对其工艺进行了改造,采用Aspen Plus软件对改造后的工艺进行了模拟计算,并结合生产实际确定了较优的操作条件。与原工艺相比,工艺流程简单,设备数量减少,塔釜再沸器能耗减少23.56%,塔顶冷凝器能耗减少24.68%,合成精制三氯氢硅及多晶硅产品的合格率分别由改造前的92.1%和86.4%提高至97.5%和92.1%。     

Fuzzy cognitive approach of a molecular distillation process

Takagi-Sugeno fuzzy model is presented as an alternative modeling tool for the molecular distillation process of heavy liquid petroleum residues. A nonlinear phenomenological model formed by partial differential equations involving temperature and composition is regarded as a virtual working plant for the generation of data required to build the fuzzy models. Furthermore, experimental data from the molecular distillation process of an atmospheric residue 673.15 K upward (673.15 K+) at different operating conditions were used to develop the fuzzy representations. In this work, the system was simulated through the development of software in Fortran 90/95 and the numerical solution by using the finite-difference method. In the fuzzy approaches, the distillation temperature and the feed flow rate are the input variables, while the liquid interface temperature, the film thickness, the concentration profiles, and the distillate flow rate were considered as the output responses. The fuzzy models obtained were compared with the results generated from the phenomenological model, showing an excellent agreement.     

Control of a reactive batch distillation process using an iterative learning technique

Quadratic criterion-based iterative learning control (QILC) was applied to a numerical reactive batch distillation process, in which methacrylic anhydride (MAN) is produced through the reaction of methacrylic acid with acetic anhydride. The role of distillation is to shift the equilibrium conversion toward the direction of the product by removing acetic acid (AcH), a by-product of the reaction. Two temperatures at both ends of the column were controlled by individual control loops. A nonlinear PID controller manipulating the reflux ratio was employed to regulate the top temperature at the boiling point of AcH. A constrained QILC was used for the tracking of the reactor temperature. A time-varying reference trajectory for the reactor temperature that satisfies the target conversion and purity of MAN was obtained through repeated simulations and confrimation experiments in the pilot plant. The QILC achieved satisfactory tracking in several batch runs with gentle control movements, while the PID control as a substitute of the QILC in a comparative study exhibited unacceptable performance.     

Synthesis of ternary distillation process structures featuring minimum utility cost using the IDEAS approach

A synthesis method for ternary distillation process structures is proposed on the basis of the infinite‐dimensional state‐space (IDEAS) approach. The proposed synthesis procedure consists of two steps. At the first step, the utility cost is minimized. The result of the first step contains many tiny flows among the modules because the number of flows is not included in the objective function. Then, at the second step, an evolutionary procedure for process simplification is executed. In this step, the weighted sum of flow rates is minimized recursively while updating the weights at each iteration. The practical process structure is finally determined from the result of the second simplification step. The developed synthesis procedure was applied to the separation problem of a ternary mixture consisting of benzene, toluene, and o‐xylene. It demonstrated that the proposed procedure provides a process whose liquid composition profile is quite similar to that of a Petlyuk column. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1285–1294, 2018     

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     

用于热集成精馏序列综合的改进模拟退火算法

提出一个用于多组分热集成精馏系统综合问题的改进模拟退火算法.将优化问题表示为混合整数非线性规划（MINLP）问题,将算法设计同研究问题的特征相结合,运用本文方法可在同一退火策略下实现流程结构和操作参数的同步优化,并能达到计算精度和计算速度的更好均衡.通过对大规模算例（产品数≥10）的计算表明,该方法对求解此类问题非常有效.     

An algorithmic procedure for the synthesis of distillation sequences with bypass

When a separation system is to produce multicomponent products, it is frequently not necessary to separate a feed mixture completely into pure components. Instead, column bypasses can reduce both capital and operating costs by reducing mass load on the columns.

This paper presents an algorithmic procedure for the synthesis of a sequence of simple, sharp split distillation columns with both bypasses and mixers that minimizes annualized costs. This procedure can solve problems in which a single feed is to be separated into two or more multicomponent products. We use models that are linear except for stream splitters. We present an analysis of the problem structure that suggest a decomposition which allows significant reductions both in the problem search space and nonlinearities caused by the splitters. The decomposition permits one to solve any three component problem for its global optimum as two linear programs. Four and five component problems require six and twenty-four nonlinear programs, respectively, each of which models a structurally different flowsheet for the process. For each flowsheet, one can establish a lower bound for the corresponding nonlinear program by using a relaxation that allows the splitters to be treated linearly: giving a linear program which is readily solved.

For two example problems, we first produced lower bounds for each structural alternative (a linear program for each). Solving the nonlinear program for the most promising structure eliminated the need for solving the nonlinear program for all remaining alternatives. Upper and lower bounds within 1% of each other strongly imply the solutions were globally optimal.     

Heat-integrated reactive distillation process for synthesis of fatty esters

Catalytic reactive distillation (RD) offers novel opportunities for manufacturing fatty acid alkyl esters involved in specialty chemicals and at a larger scale in biodiesel. The integration of reaction and separation into one RD unit, corroborated with the use of a heterogeneous catalyst, provides major benefits such as low capital investment and operating costs. This work presents a novel heat-integrated process based on reactive distillation that aims to reduce furthermore the energy requirements for biodiesel production, leading to competitive operating costs. Despite the high degree of integration, the process is well controllable using an efficient control structure proposed in this work. Rigorous simulations embedding experimental results were performed using computer aided process engineering tools, such as AspenTech Aspen Plus and Aspen Dynamics. The RD column was simulated using the rigorous RADFRAC unit with RateSep (rate-based) model, and explicitly considering three phase balances. Steady-state and dynamic simulation results are given for a plant producing 10 ktpy fatty acid methyl esters (FAME) from methanol and waste vegetable oil with high free fatty acids (FFA) content, using sulfated zirconia as green catalyst. The heat-integrated RD process eliminates all conventional catalyst related operations, efficiently uses the raw materials and the reactor volume offering complete conversion of the fatty acids and allowing significant energy savings. Remarkably, compared to previously reported RD processes, the energy requirements of this process are about 45% lower - only 108.8 kW h/ton biodiesel - while the capital investment cost remains the same as no additional equipment is required.     

Evolutionary synthesis method of energy integrated distillation separation process

A two-stage evolutionary method is developed to synthesize an optimal energy integrated distillation separation process: the search of the separation sequence, and the introduction of energy integration to the given separation sequence. For the former an evolutionary procedure first generating initial separation sequences and then evolving them is developed. For the latter an evolutionary procedure, first indicating the direction of the evolution by the thermodynamic analysis and then estimating it based on the objective function, is developed. An example problem of a five-component separation demonstrates the effectiveness of this proposed method.     

Generalized short-cut distillation column modeling for superstructure-based process synthesis

We introduce a generalized Underwood method to calculate the minimum vapor flowrate for distillation in superstructure-based process synthesis. Our method addresses two important limitations of current methods, namely, separation tasks where (1) the flow rates of some components may be zero, and (2) the key components may be unknown prior to optimization. This is achieved through the introduction of binary variables and mixed-integer constraints to implicitly determine the active roots and the root loci. In addition, we develop strategies tailored to improve the numerical stability and computational performance when using the proposed method.     

A unified graphical formulation for synthesis of basic and subcolumn distillation sequences

In contrast to the different approaches currently adopted for generating basic and side-split subcolumn distillation sequence for separating zeotropic multicomponent feed mixture, we present a unified graphical method applicable towards both basic and side-split subcolumn distillation sequence. For a given number of components in the feed mixture, we enforce constraints on a base graph to eliminate violations of conservation principles and to preclude distillation sequences that demand higher heat duty in all appraised practical scenarios. A compact set of algebraic constraints is transfixed using the graph counterpart for generating basic-only distillation configurations. These algebraic constraints utilize binary variables to quantify existence of submixture streams and this considerably reduces the number of variables in generating distillation sequences. We also suggest extension of the formulation to enable the exploration of thermally coupled configurations.