Distillation Boundaries Research Articles

Experimental demonstration of the production of poly(oxymethylene) dimethyl ethers from methanolic formaldehyde solutions in a closed-loop mini-plant

Poly(oxymethylene) dimethyl ethers of chain length 3–5 ( ▪ ) are discussed as synthetic diesel fuels due to their potential to significantly reduce soot emissions while presenting physicochemical properties similar to conventional diesel fuels. A recently developed process to produce ▪ directly from methanol and aqueous formaldehyde is a promising way to avoid expensive intermediates such as trioxane, methylal, or dimethyl ether. The process consists of a reactor, a distillation sequence, and a membrane unit. The first distillation column is particularly challenging due to the reactive character of the separation, the high number of components present, and the limited solubility of formaldehyde. Up to now, the feasibility of this separation has yet to be demonstrated. This work presents closed-loop experiments in a demonstration plant erected at the Campus Straubing of the Technical University of Munich with closing recycle. In the distillation step, ▪ with very small impurities of around 800 ppm of formaldehyde was obtained. The experimental results were compared with simulations based on a reactive equilibrium stage model. The simulations of the temperature and composition profiles for the majority of the components are in line with the experiments. Although the membrane separation exhibited weaker water selectivity than in previous tests, it was sufficient to overcome distillation boundaries. Additionally, this study assesses the potential for solid precipitation, explores trade-offs in product quality, and discusses overall mass balances of the process, demonstrating the overall feasibility of the process.

Energy-efficient separation of a ternary azeotropic mixture via azeotropic reflux-induced sequences based on the intensified pressure-swing distillation

The recovery and separation of chloroform, acetonitrile and ethanol from waste water involving two azeotropes and a distillation boundary has emerged as a pivotal concern in the pursuit of environmental protection and resource conservation. Herein, the energy-efficient separation design of the ternary azeotropic mixture based on a triple-column pressure-swing distillation (TCPSD) process with five azeotropic reflux induced sequences is comprehensively investigated. The suitable operating pressure range of the distillation columns and the potential optimal distillation sequence is investigated based on the thermodynamic intrinsic probe. According to the optimal distillation sequence, further process intensification strategies are proposed to improve the thermodynamic efficiency encompassing the heat integration pressure-swing distillation (HI-PSD), heat pump assisted pressure-swing distillation (HP-PSD), and a combination of heat pump assisted and heat integration pressure-swing distillation (HPHI-PSD). It unveils the economic preeminence of the HI-PSD process, characterized by the lowest total annual cost (TAC) in a shorter payback period (3 to 7 years). Conversely, both the HP-PSD and HPHI-PSD processes exhibit superior environmental performance on CO2 emissions of 111.46 kg/h and 119.78 kg/h respectively, with the HP-PSD demonstrating prolonged economic viability and the HPHI-PSD showcasing heightened thermodynamic efficiency of a 65.12 % improvement. This study potentially provides a theoretical basis for devising industrial separation schemes for analogous pressure-sensitive ternary azeotropic systems.

Sustainable and efficient separation of ternary multi-azeotropic mixture butanone/ethanol/water based on the intensified reactive extractive distillation: process design, multi-objective optimization, and multi-criteria decision-making

It is of great significance and necessity to develop a sustainable and efficient separation scheme of butanone (MEK), ethanol (EtOH), and water, which are usually formed during the cinepazide maleate synthesis process. To the best of our knowledge, the systematic separation of this ternary multi-azeotropic mixture remains comparatively less studied, compounded by the significant challenges posed by its intricate composition comprising four azeotropes and three distillation boundaries. Therefore, a systematic approach by integrating the process design, multi-objective optimization, and multi-criteria decision-making (MCDM) methods was proposed to achieve the sustainable separation. Initially, three separation schemes, i.e., triple-column extractive distillation (TCED), double-column reactive extractive distillation (DCRED), and side stream intensified double-column reactive extractive distillation (SS-DCRED), were mainly explored via the thermodynamic and kinetic analysis. Subsequently, the multi-objective particle swarm optimization (MOPSO) algorithm was employed to optimize the process based on three objectives namely the total annual cost (TAC), CO2 emissions, and process route index (PRI). Finally, an efficient MCDM method combining CRITIC and TOPSIS was employed to rank the Pareto front solutions of the corresponding process design. Based on the comparison results, the optimal TCED process exhibits the lowest PRI with a decrease of 50.61% and 55.06% compared to that of the SS-DCRED and the DCRED, respectively. It also shows the best performance simultaneously considering economic, environmental, and safety criteria. Of note, the SS-DCRED separation process exhibited the lowest TAC, with a decrease of 60.67% and 11.45% compared to that of the TCED and the DCRED, respectively.

Surrogate Models for Distillation Boundaries in Azeotropic Multicomponent Mixtures

Surrogate Models for Distillation Boundaries in Azeotropic Multicomponent Mixtures

Methylpyrrolidonium Trifluoroacetate Protic Ionic Liquid: Distillation Boundary in the Ethyl Trifluoroacetate–Trifluoroacetic Acid–N-Methyl-2-pyrrolidone System

Methylpyrrolidonium Trifluoroacetate Protic Ionic Liquid: Distillation Boundary in the Ethyl Trifluoroacetate–Trifluoroacetic Acid–<i>N</i>-Methyl-2-pyrrolidone System

Process design and multi-objective optimization for separation of different feed composition of acetonitrile / ethanol / water with extractive distillation by varying pressure / pervaporation

In order to separate and purify the reaction raw material ethanol (EtOH) and product acetonitrile (ACN) from the azeotrope of ACN / EtOH / water, an efficient extractive distillation by varying pressure process (EDP) was designed to separate them. Because the feed composition in industrial production was constantly changing, processes of different feed compositions was simulated. Taking into account both economic and environmental factors, the non-dominated sorting genetic algorithm comprehensively optimizes the parameters. The azeotrope was pressure sensitive and the pressure parameter was also optimized in order to improve the efficiency of process energy utilization. Heat exchange network optimization technology was used to fully utilize excess heat in the process. The distillation coupled pervaporation technology has also been designed to explore emerging technologies. Among the three different coupling processes, the performance of EDP-pervaporation (EDPV) was better. Compared with EDP, EDPV under different schemes reduce the total annual cost (TAC) and gas emissions by more than 15% and 21 %, respectively. It was found that the energy consumption was related to the normal distance from the feed composition to the distillation boundary and isovolatility curve.

Insight on the sustainable design and multi-objective optimization for separating the ternary azeotropic mixture of toluene/n-butanol/water by natural decanting coupled with pressure swing distillation

The effective recovery of n-butanol (BuOH) and toluene from wastewater via sustainable separation process is critical to promote environmental protection and resource utilization. In this work, we propose a comprehensive methodology to develop a natural decanting coupled with pressure-swing distillation (NDPSD) for the azeotropic separation of BuOH/toluene/water system. Firstly, the thermodynamic topologic insights are employed to analyze the feasibility of the NDPSD. Secondly, conceptual design involving the understanding of mass balance lines, the liquid–liquid envelope, and distillation boundaries were applied to develop initial design of the NDPSD with two different separation sequences (i.e., configurations). Then, both initial designs are optimized to obtain the optimum column configurations via an improved non-sorting genetic algorithm, with the total capital and operating cost as dual objective function. Finally, both optimized schemes are heat-integrated for the purpose of energy conservation. The performance of all the developed configurations were examined using three indicators, i.e., economic, environmental, and energy efficiency, and our results revealed that the heat-integrated configuration B with separation sequence of water → BuOH → toluene provides the best economic, environmental, and thermodynamic efficiency.

Application of dividing wall column in azeotropic distillation with intermediate boiling-point heteroazeotrope: Simulation and optimization

Dividing wall column (DWC), as a proven process intensification technology, has been used widely in separation of ideal multi-components, and some azeotrope systems. For heterogeneous azeotrope with the potential possibility of crossing distillation boundary by liquid-liquid splitting, it is advantageous to integrate liquid-liquid decanter with distillation column, especially with DWC to further simplify flowsheets and reduce energy consumption. In this work, existing heterogeneous azeotrope separation processes are described, and an innovative DWC-based process (MHADWC) is proposed for intermediate boiling-point heteroazeotropic systems. Then an industrial case of MIPK system, is used to evaluate mentioned configurations by rigorous simulation and optimization. These configurations are compared and analyzed in detail on energy consumption, exergy analysis and equipment invest. The results show that, from the perspective of process energy consumption, MHADWC process has the most advantages, because it saves 37% of energy consumption; if both equipment costs and exergy recovery are taken into account, HADWC2 has a higher thermodynamic efficiency (43.5%) at its TAC and can be used as a preferred target. And in this work, purification of an intermediate boiling-point heteroazeotrope is the main difference of these processes compared with the well-known heterogeneous azeotropic distillation process where the top vapor product is the lowest boiling point of the ternary system.

Investigation on energy-efficient heterogeneous pressure-swing azeotropic distillation for recovery of cyclohexane and tert-butanol from industrial effluent

This paper highlights the separation of cyclohexane/tert-butanol from industrial effluent by heterogeneous pressure-swing azeotropic distillation based on the properties of pressure-sensitive distillation boundaries and natural liquid–liquid separation. There is no excess component in the separating process. The optimal parameters of the conventional process are obtained by using the simulated annealing algorithm. Additional heat pump and heat integration strategies are introduced into the process with the optimal operation parameter to save more energy. Evaluation indicators, such as total annual cost (TAC), total energy consumption (TEC), gas emission, and exergy destruction, are used to assess the potential of economic and environment friendly processes. The heat pump assisted HADPSD process employing auxiliary reboilers and heat integration is the optimal energy-saving process, exhibiting decreases of 41.98%, 55.32%, 52.30%, and 55.26% for TAC, TEC, gas emission, and exergy destruction compared to conventional HADPSD.

Design and control of fraction cutting for the separation of mixed alcohols from the Fischer–Tropsch aqueous by-products

Since the minimum-boiling azeotropes of C2–C8 alcohols with water and high-water content (up to 95% (mass)) in the Fischer–Tropsch aqueous by-products, the separation is energy-intensive and challenging. The energy-saving strategy for the complete separation of the Fischer–Tropsch aqueous by-products has received massive attention in recent decades. In this study, a stripper-sidestream decanter process is proposed by exploiting homogeneous azeotropes (C2–C3 alcohols–water) and heterogeneous azeotropes (C4–C8 alcohols–water). The introduction of the stripping column for pre-dehydration avoids the re-vaporization of the mixture, and energy carried by the overhead vapor is conserved instead of being removed in a condenser. The precise fraction cutting of C1–C3 alcohol–water mixture, C4–C8 alcohols, and water is realized by the sidestream distillation column. The C4–C8 alcohols rich mixture withdrawn from the sidestream flows into the decanter to break the distillation boundary, where the organic phase returns to the sidestream distillation column to obtain the dehydrated C4–C8 alcohols, and the aqueous phase enters the stripping column. Steady-state optimization based on total annual cost (TAC) minimization shows that the stripper-sidestream decanter process reduces TAC by 17.00% and saves energy by 21.27% compared with the conventional three-column distillation process. Further, a control structure of the process is established, and dynamic simulations show that the control structure combining a differential controller with a low-selector exhibits robust control. This study provides a novel design scheme and deepens the insights into the efficient separation of aqueous by-products of the Fischer–Tropsch synthesis.

Towards energy saving and carbon reduction of pressure-swing distillation for separating the ternary azeotropic mixtures by thermodynamic insights and process intensification

The recovery and separation of organic solvents such as tetrahydrofuran (THF), ethanol (EtOH), and methanol (MeOH) are important to protect the environment and save resources. Such ternary mixture has two azeotropes and one distillation boundary that makes separation difficult to be carried out using ordinary distillation technique. In this work, we developed a systematic approach for heat-integration (HI) and vapor recompression heat pump (VRHP) assisted triple-column pressure-swing distillation (TCPSD) for the recovery and separation of these valuable compounds. Firstly, the thermodynamics topological analysis is introduced to evaluate the feasibility of TCPSD for separating the ternary azeotropic mixture THF/EtOH/MeOH. Different separation sequence is determined via the combination of residue curve, component balance line, isovolatility curve, and distillation boundary. Subsequently, the potential application of HI and VRHP technologies is additionally explored for further minimizing the energy consumption and it is found that both process intensification technologies provide significant improvement in terms of economic, environmental impact, and energy efficiency. Relative to the traditional TCPSD, the application of VRHP technology to the TCPSD sequence B, in particular, provides 43.5% and 76.4% reduction in TAC and CO2 emission, respectively, while it improves the energy efficiency 100.4%.

Synthesis of heat‐integrated pressure‐swing azeotropic distillation using a graphical pinch analysis

AbstractA new graphical method for the fundamental design of pressure‐swing distillation (PSD) and valid heat integration of the designed process is demonstrated based on a pinch analysis. As the minimum pressure constraint, the pinch pressure is determined by this graphical method to secure the necessary temperature driving force and circumvent the distillation boundaries. For a feasibility evaluation of heat‐integrated PSD (HIPSD), the suggested approach requires visualization of the pinch temperature contours and the feasible composition region of an initial feed according to the pressure change and dynamic properties of ternary azeotropic systems. We thus carried out an in‐depth examination by conducting case studies to show the feasible integration into HIPSD. The new insight is that the HIPSD system can have different pinch pressures depending on the column sequence or the separation order of pure components. In addition, the analysis verifies that energy‐efficient HIPSD can be generated by considering pinch constraints.

CFD-aided design of internally heat-integrated pressure-swing distillation for ternary azeotropic separation constrained by pinch pressure

• Novel HIPSD for separating an azeotropic ternary mixture is proposed. • An extended pinch-based method to determine the pressure of HIPSD is introduced. • HIPSD alternatives are generated for the given initial feed composition. • Practical heat transfer rates in HIPSD are determined by the CFD method. • Significant energy savings in HIPSD are achieved for ternary azeotropic separation. This study addresses computational fluid dynamics (CFD) for designing internally heat-integrated pressure-swing distillation (HIPSD) with improved energy efficiency in azeotropic distillation. An extended concept of pinch pressure is applied to determine the operating pressure of the HIPSD in a double annular column configuration for the circumvention of a distillation boundary and adequate heat transfer. For the separation of a highly azeotropic ternary mixture of butyl acetate, butanol, and water, the combination of a single unit of HIPSD and a decanter is employed. This azeotropic mixture is separated in different design alternatives for the given initial feed compositions. In each sequence, the heat transfer rate inside the HIPSD was calculated by the CFD method, and the total utility consumption accordingly decreased by 9.72% and 15.44%. The reboiler and condenser duty in the HIPSD were reduced by up to 48.65%. The separation efficiency in the condenser of the high-pressure column was improved enough to reach a zero reflux by the internal heat integration.

Pressure-swing distillation process for separating ternary azeotropic mixture of acidic aqueous solution

The separation of ternary mixture of acidic aqueous solution is difficult due to presence of both binary and ternary azeotropes that generate distillation boundaries. In this work, a pressure-swing distillation process for separating the mixture was investigated. Three variants of pressure swing distillation process were therefore proposed. The operating pressure of each distillation column was chosen on the basis of the location of the feed composition in the distillation regions. The sensitivity analysis of the operating parameters of each separation process was carried out to determine the best operating conditions to reach higher purity for the three products (i.e., 99.9 mol.%). It has been found that formic acid and water present in the ternary mixture with high content can be separated twice as a distillate product with total molar recovery ratio of about 90%. In addition, the energy consumed to separate water was higher than that required to recover formic acid.

Design and control of an alternative intensified process configuration for separation of butanol-butyl acetate-methyl isobutyl ketone system

Butanol/butyl acetate/methyl isobutyl ketone mixture forms two binary azeotropes at atmospheric pressure with a distillation boundary on the residue curve map. Conventionally, this mixture is separated by a process configuration including five distillation columns. Alternatively, this mixture can be separated by a modified process configuration including three distillation columns and two recycle streams, when an extra amount of MIBK is added into the system. However, this modified configuration increases both the capital and energy costs because of the recycled MIBK in the process. Thus, an alternative intensified process configuration is proposed in this study to separate this ternary system without adding any azeotropic agent. The effect of pressure on the azeotropic compositions is used to develop the proposed triple-column pressure swing distillation process. The economic evaluation shows that the proposed configuration reduces both the capital and energy costs significantly compared with the triple-column pressure swing distillation configuration given in the literature. In addition to the steady-state design, a plantwide control structure is developed and tested for robustness. The dynamic simulation results show that an effective base-level regulatory control is achieved with a control structure including composition controllers.

Pressure relief of nonideal mixtures: Composition trajectories and residue curve maps

AbstractDuring the course of a pressure relief discharge from a vessel containing a multicomponent liquid mixture, composition changes occur that affect the properties of the vessel contents. In this work, a model of the dynamic relief process for ternary, nonideal, homogeneous mixtures is developed, under the assumption of vapor‐only venting. Opening/reclosing of the relief valve introduces state‐events, which require reinitialization of the model at each state transition. The relationship between the pressure relief model and the concept of residue curves, which describe simple‐distillation processes, is demonstrated. It is known that the presence of azeotropes and distillation boundaries in mixtures restricts the composition trajectories of simple‐distillation processes, as well as continuous distillation columns at total reflux. In this work, the residue‐curve analogy is extended to vapor‐only pressure‐relief, where vapor composition changes directly affect the operation of the pressure‐relief device. Examples of dynamic relief processes are developed for ternary mixtures with varying nonideality.

Design and control of an energy-efficient process for the separation of benzene/isopropanol/water ternary mixture

In a recent publication, a triple-column side-stream extractive distillation system was proposed for the separation of benzene/isopropanol/water ternary mixture using ethylene glycol as a heavy entrainer. However, the above design needs to introduce a foreign component into the separation task. In this paper, the newly proposed design principle is to make use of large liquid–liquid envelope together with pressure-sensitive distillation boundary of this ternary system for the development of an energy-efficient design consisting of a decanter and three columns operating at different pressures. In order to give reliable simulation results, a new set of UNIQUAC model parameters for this ternary system is obtained in this paper by regression from liquid-liquid and vapor–liquid equilibria data in open literatures. Simulation investigations reveal that significant savings of 63.4% in the total operating cost and 51.0% in the total annual cost can be realized by our proposed heat-integrated design flowsheet. Furthermore, overall control strategy of the proposed design has also been developed using closed-loop and open-loop sensitivity analyses for the determinations of crucial tray-temperature control points and also to make decision on holding which flow ratio(s) in the flowsheet to be fixed. Dynamic simulations demonstrate that the proposed overall control strategy is capable of holding three products at high-purity despite feed flow rate and feed composition disturbances. No online composition analyzer is required in the proposed control strategy.

Hybrid distillation-pervaporation in a single unit: Experimental proof of concept in a batch operation

In a Distillation-Pervaporation in a Single Unit (DPSU) system, the pervaporation unit is located inside a distillation column. Therefore, distillation and pervaporation mechanisms are carried out simultaneously in the same column section. Previously, it was theoretically demonstrated that a DPSU column is able to overcome azeotropic conditions by displacing the liquid composition due to the pervaporation. In this work, this behaviour is experimentally verified for a DPSU system in a batch operation for the separation of ethanol – isopropanol – water mixture, using a silica membrane selective to water. Several feed compositions and permeate pressures were evaluated to identify their effects on the DPSU separation performance. The distillation boundary was experimentally overcome by the DPSU system. The separation order of isopropanol and water was inverted in the DPSU system as compared to conventional distillation. Water became a saddle point for the entire composition space, allowing crossing the distillation boundary with a continuous composition profile of the residue liquid. This study experimentally proves the validity of the DPSU approach for azeotropic mixture separations.

Conceptual Design of Distillation-Pervaporation in Single Unit Columns Using Residue Curve Map Methodology

A short-cut design method based on Residue Curve Maps (RCMs) is applied to the conceptual design of Distillation-Pervaporation in a Single Unit (DPSU) columns. Unlike to a conventional hybrid distillation-pervaporation column, in a DPSU system the pervaporation module is located inside the column. The distillation and pervaporation mechanisms are fully coupled, allowing overcoming azeotropic conditions of non-ideal mixtures inside the distillation column. The simplified design method is applied to four non-ideal mixtures with different Serafimov’s topological classification: acetone-isopropanol-water (1.0-2), ethyl acetate-ethanol-methanol (2.0-2b), acetone-methyl acetate-methanol (3.1-2) and acetone-methanol-chloroform (3.1-4). The results show that it is possible to achieve a feasible separation through a DPSU column with product compositions split by distillation boundaries. Besides, the regions of a feasible separation with a DPSU column are identified. Thermodynamic limitations for a DPSU column are observed when the azeotropes generate isovolatility lines, producing different separation paths for a specified membrane selectivity. DPSU columns promise to strongly reduce the number of units and associated energy consumption in distillation trains for separating complex mixtures.

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

AbstractDistillation‐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.