Distillation Boundaries Research Articles

Proposal for the introduction of the concept of multiplicity of solutions in steady-state distillation problems in the syllabus of separation processes

The existence of multiplicity in steady-state separation problems based on the use of equilibrium stage cascades has been known since the second half of the 20th century. Nevertheless, there are few textbooks -among those usually accessible to Chemical Engineering students- explaining this behavior and even when it is considered it is not deeply treated. In this work, we propose a distillation problem whose resolution allows the instructor the opportunity to introduce some specific characteristics and behavior that may otherwise pass unnoticed. Thus, the simulation of a column for the heterogeneous azeotropic distillation of ethanol using benzene as an entrainer is suggested, and the results obtained do not seem to be compatible with the distillation boundaries of the ternary system. The apparent contradictions can be explained by means of the representation of mass balances on the residue curve map, and allow the students to actually understand the requirements for the feasibility of the separation. The obtainment of two different solutions depending on the initial composition profiles used for starting the calculations allows the instructor to introduce the steady-state multiplicity concept. Errors associated with bad checking of the thermodynamic model are also pointed out.

Design and control of pressure‐swing distillation for separating ternary systems with three binary minimum azeotropes

The separation of ternary nonideal systems with multi‐azeotrope is very important because they are often found in the waste of chemical and pharmaceutical industries, which is much more difficult due to the formation of multi‐azeotrope and distillation boundary. We propose a systematic procedure for design and control of a triple‐column pressure‐swing distillation for separating ternary systems with three binary minimum azeotropes. This procedure involves thermodynamic insights, a two‐step optimization method, and effective control strategy. The separation of tetrahydrofuran (THF)/ethanol/water is used to illustrate the capability of the proposed procedure. It is found that the pressure limits in columns can be determined through the analysis of residue curve maps, distillation boundary, and isovolatility curves. The optimal triple‐column pressure‐swing distillation is generated with the minimum total annual cost (TAC) of $2.181 × 106 in sequence A. The operating conditions are well controlled approaching their desired specifications in an acceptable time when disturbances occur. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1281–1293, 2019

ENTRAINER SELECTION FOR SEPARATION OF AZEOTROPIC MIXTURES BY DISTILLATION METHODS

Azeotropic or close – boiling mixtures often preclude conventional distillation as a method of separation. Instead, extractive or azeotropic distillations are commonly used to separate azeotropic or close – boiling mixtures. For the design of those separation units, selecting suitable entrainers (solvents) is a key step. The traditional method for solving this problem is to use experimentation which is time – consuming and expensive. Currently available selection criteria are inadequate. They contradict one another and often lead to incorrect conclusions. Indeed, for a minimum boiling azeotrope, the existing entrainer selection rules state that one should use a high boiling component that introduces no additional azeotrope (Benedict & Rubin, 1945), an intermediate boiling component that introduces no additional azeotrope (Hoffman, 1964), a component which introduces no distillation boundary between the azeotropic constituents (Doherty & Caldarola, 1985), and either a low boiling component that introduces no additional azeotrope or a component that introduces new minimum boiling azeotrope (Stichlmaric, Fair & Bravo, 1989).In this work, Aspen Plus simulator was used to propose an entrainer selection procedure based on the criteria: 1) A good entrainer is a component that eliminates the azeotrope easily (i.e. even when it’s concentration is small). 2) A component that yields high relative volatilities αAB between the two azeotrope constituents.

Relevance of regulatory constraints in designing pharmaceutical manufacturing processes: A case study on waste solvent recovery

This work deals with the relevance of regulatory constraints on the outcome of process design in pharmaceutical manufacturing with a case study on waste solvent recovery. The role of the investigated process was to separate and purify tetrahydrofuran from an azeotropic mixture with water and methanol. As the technologies to overcome the distillation boundary, zeolite membrane, pressure swing, azeotropic distillation, and entrainer processes were considered as alternatives, and were modeled and evaluated with regard to economy, environmental impact, and environmental, health, and safety hazards. The target concentration of recovered solvent, which cannot be altered because of regulations, was imagined to be modifiable, and two design problems, initial and extended, were formulated. A type of pressure swing process that was found to be optimal in the extended problem was equal to or better than any of the optimal alternatives in the initial design problem. Remarkably, the net present value of this alternative was about 17% larger than the maximum in the initial design problem. These results confirmed quantitatively that the way in which regulatory constraints are taken into account makes a difference in the outcome and that the appropriate formulation of a design problem is critical for pharmaceutical manufacturing processes.

Selectivity Engineering with Hybrid Reactive Distillation Column: Mixtures Containing Inerts and Multi-Azeotropes

Reactive Distillation (RD) which combines reaction and distillation can be effectively used to get desired selectivities in multi-reaction systems. In case of azeotropic systems, nonideal vapor-liquid equilibrium and distillation boundaries are responsible for the contraction of the feasible reactive stage composition region required for the design of hybrid RD column. In the present study, we show that if the mixture contains inert as well then this feasible reactive stage composition region reduced further and thereby decreases the feasible design options of desired selectivity in multireaction systems. Therefore, an attempt is made here, to developed a conceptual design algorithm which is based on combined graphical-simulation approach and attainable region concepts. The developed methodology successfully finds at least one feasible design of hybrid RD column to give desired selectivity. Method gives excellent starting values for rigorous simulations suited to industrial applications.

Conceptual Design of a Novel Process for the Production of OME Fuels

Poly(oxymethylene) dimethyl ethers (OME) are oxygenates of the general structure H3C-O-(CH2O)n-CH3 with n = 2. OME are synthetic diesel fuels, which strongly reduce the soot formation and indirectly also the NOx formation in diesel engines. This work presents the conceptual design of a novel OME process, which employs aqueous solutions of formaldehyde and methanol as feedstock. Process-relevant data on physico-chemical properties are determined experimentally and models for their description are developed. This includes describing the chemical equilibrium and the vapor-liquid-liquid equilibrium in the system (formaldehyde + water + methanol + methylal + OME). The models are used for determining thermodynamic limits of the process like chemical equilibria and distillation boundaries. Based on that knowledge the novel OME process is designed. It consists of a reactor, two distillation columns, and a pervaporation unit. The first column is a reactive distillation in which OME with n = 3 are separated from a complex reactive multicomponent mixture with more than 30 components. All critical units, including the reactive distillation and the pervaporation are tested in lab-scale experiments. The column profiles of the distillation experiments are well described by simulations using the equilibrium-stage model.

Basic Principles of a Hybrid Distillation and Pervaporation Unit in a Single Column: an Analytic Tool

Distillation-Pervaporation Hybrid Systems (DPHS) are a relatively novel technology for separating multicomponent azeotropic mixtures, due to pervaporation is not limited by the liquid-vapor (LV) equilibria. However, conventional Distillation-Pervaporation Hybrid Systems, where the pervaporation unit is externally connected to the distillation column, have several drawbacks in the pervaporation unit, for instance high resistance to heat and mass transfer and an inter stage heating system is required between the pervaporation modules. Distillation-Pervaporation in a Single Unit (DPSU) is a novel concept where the pervaporation membrane is located inside the distillation column working as a packing bed in a section of the column. Due to it is a new concept, there is little data about its separation behavior. However, DPSU is able to overcome distillation boundaries. In this work, a simplified model is used to evaluate the separation of ternary homogenous azeotropic mixtures and thermodynamic conditions in a DPSU column. A pervaporation sections located in rectifying and stripping sections were studied. Two topological types, based on Serafimov’s topological classification, were selected as case studies: acetone-isopropanol-water (1.0-2) and ethyl acetate-ethanol-methanol (2.0-2b) mixtures. The effects of the DPSU column on the mixture topology, such a perturbation of stationary points, are studied. It is possible to overcome distillation boundaries using a selective membrane but also new stationary points in middle of the composition space appear.

Effect of Organic Solvents in Separation Section of Levulinic Acid Production: Synthesis of Distillation Sequences

This work highlights the system specific characteristics encountered when considering a distillation-based separation strategy for levulinic acid (LA) and the by-product formic acid (FA) in the presence of water and water-miscible sulfolane or ?-valerolactone (GVL) solvents. Simulation using Aspen Plus® was applied to identify the presence of distillation boundary surfaces and tangent pinches resulting from the non-ideal behaviour of the mixtures in question. In the applied feed composition regions, the separation performance of the GVL solvent system in particular is limited by these restricting conditions. Minimum energy requirements for the separation of valuable components are presented as the conceptual design performance indicators, allowing the benchmarking of separation strategies. Sidedraw column configurations offer advantages in enriching the dilute valuable compounds from the feed and performing difficult separations at lower solvent quantities. This is especially advantageous for the sulfolane system where both LA and FA are intermediate-boiling.

Tools for the Design of Hybrid Distillation–Pervaporation Columns in a Single Unit: Hybrid Rectifying–Pervaporation Section

Distillation–pervaporation in a single unit is a novel concept where the pervaporation membrane is located inside the distillation column as a packing bed. In this work, a simplified model, based on phase equilibrium and membrane selectivity, is derived for a DPSU column. The model provides the separation trajectories inside the column, where distillation and pervaporation mechanisms simultaneously occur. A Serafimov’s topological classification (1.0–2), specifically acetone–isopropanol–water, is evaluated. The results show that a feasible separation is achieved with product compositions separated by a distillation boundary. The stationary points are modified as compared to conventional distillation overcoming the distillation boundaries. General aspects of a DPSU column are identified to provide fundamentals in the DPSU column design.

Effects of the ease of separation index (ESI) and feed composition on the selection of distillation configuration

AbstractDistillation process consumes the energy in chemical industry. To reduce the consumption, divided wall column (DWC) has been known to be suitable for multicomponent separation and be able to save both energy and capital cost compared to conventional distillation column. However, due to more operation parameters than traditional distillation, design method, control and operation of the DWC are more complicated. In this paper, firstly, a conceptual design of a DWC system is proposed. The method based on shortcut methods of Fenske, Underwood, Gilliland and Kirbride, which can rapidly determine structural and operational parameters of the DWC. Secondly, to compare the energy of use between DWC and conventional column, three ternary mixtures are studied with different values of the ease of separation index (ESI) and feed compositions. The results show that the energy consumption of the DWC system is more efficient than the traditional distillation column but it is not always the best arrangement in the multicomponent separation process. The selecting depended on both ESI value and feed composition of a mixture. Based on the comparison, a distillation boundary of ternary diagram is drawn to make a proper choice for the best configuration.

Pressure-Swing Dividing Wall Column with Multiple Binary Azeotropes: Improving Energy Efficiency and Cost Savings through Vapor Recompression

This work introduces a novel thermal integration scheme for a pressure-swing distillation train used to separate a mixture of methanol/benzene/acetonitrile. This mixture typically forms three azeotropes as shown through developing its residue curve map with two distillation boundaries at atmospheric pressure. This ternary mixture is traditionally separated via a triple column pressure-swing distillation (TCPSD) scheme, in which, all columns operate at different pressures. Here, the first two columns of the TCPSD are proposed to be replaced by a single column having a dividing wall that allows heat transfer through it. Thus, the resulting scheme is called pressure-swing dividing wall column (PSDWC). Aiming to improve its performance, further advancement is subsequently made by developing the heat intensified PSDWC (HiPSDWC) and vapor recompressed PSDWC (VRPSDWC). The performance of these proposed schemes is evaluated in the context of energy saving and total annual cost (TAC). Among these configurations, i...

Control of heat-integrated extractive distillation processes

Pressure in many distillation columns is set such that cooling water can be used in the condenser. Pressure selection is more involved in some columns such as reactive distillation in which there is a trade-off between temperatures favorable for reaction kinetics and temperatures favorable for vapor-liquid equilibrium. In azeotropic systems, pressure selection is critical in achieving the desired separation by consideration of distillation boundaries and isovolatility curves. A recent paper presented a striking example of pressure selection in extractive distillation. Operation at 1 atm required a solvent-to-feed (S/F) ratio of 3.52 while operating at 10 atm cut the S/F to only 0.717.The purpose of this paper is to explore the dynamic controllability of this low S/F extractive distillation system. Results show that the control structure must be modified from the conventional configuration used in most extractive distillation processes.

A combination of pressure-swing and extractive distillation for separating complex binary azeotropic system

Methyl acetate/methanol/water mixture forms more than one different azeotrope, whereas its triangular diagram presents a distillation boundary at atmospheric pressure. The two different simulation processes of the double column pressure-swing with extractive distillation (DCPSED) and triple column pressure-swing with extractive distillation (TCPSED) were proposed to separate the complex ternary system. Furthermore, the heat integration and mechanical vapor compression (MVR) heat pump distillations were also tested (through simulation) to separate the complex ternary system to save energy. The feasibility of the processes was confirmed using vapor-liquid equilibrium curve, while rigorous steady-state simulations were implemented using Aspen Plus (version 7.1). With the goal of achieving minimum energy consumption and minimum total annual cost (TAC), the purities of methyl acetate, methanol and water were used as the constraint variables. The results show that water was a better solvent for this separation system. Compared with the TCPSED process, the energy consumption and TAC for DCPSED process could be reduced by 44.83%, and 47.74%, respectively. Compared with the DCPSED and DCPSED with heat integration processes, DCPSED with MVR process could reduce the energy consumption by 73.34% and 59.52%, while TAC was decreased by 37.64% and 11.77%, respectively. The simulation results indicated that DCPSED with MVR process has great advantage over other processes used to separate methyl acetate/methanol/water ternary mixture.

Effect of multi-recycle streams on triple-column pressure-swing distillation optimization

Different structures of multi-recycle triple-column pressure-swing distillation (TCPSD) are synthesized and explored. Acetonitrile/methanol/benzene ternary mixture is set as a case to identify the applicability of different multi-recycle TCPSD flowsheets. Four multi-recycle flowsheets are analyzed and compared with convention TCPSD process and rigorous steady-state simulations are implemented using Aspen Plus. Operating parameters are optimized by using the sequential iterative optimization procedure when the split fraction changes with the objective function of minimum total annual cost. Two structures are found that distillate compositions surpass the distillation boundaries at finite reflux ratios. However, the results show that different multi-recycle TCPSD processes are feasible.

Conceptual Design of a Novel Process for the Production of Poly(oxymethylene) Dimethyl Ethers from Formaldehyde and Methanol

Poly(oxymethylene) dimethyl ethers (OME) are environmentally benign alternative fuels. This work presents the conceptual design of a novel OME process which employs aqueous solutions of formaldehyde and methanol as feedstock. In this process, OME of the desired chain lengths n = 3–5 and water are separated from the reactive mixture (formaldehyde + water + methanol + methylal + OME). Thermodynamic limits are identified by studying distillation boundaries and chemical equilibria. By that it is shown that OME of chain lengths n = 3–5 can be separated from the reactor outlet by distillation. The separation of water is carried out using either an adsorption or a membrane process. Adsorption isotherms of water on Zeolite 3A are determined experimentally. The OME process is simulated and optimized using a reduced process model accounting for the mass balances and the thermodynamic limits. Favorable operating points of the process are identified using multi-objective optimization.

The production zone method: A non-ideal shortcut method for the design of distillation columns

Abstract Graphical shortcut methods are useful tools for the design of distillation columns. The proposed nonideal shortcut method includes a graphical representation and is based on the concept of operation leaves. This new method uses a production segment rather than a completely specified product, which eliminates any sensitivity to the composition of the minor product. Concerning phase equilibria, no restrictive assumptions are made. The study aimed (1) to determine whether a specified separation respects the mass balance and thermodynamic feasibility and (2) to find the minimum reflux ratio for a preliminary design of the column. Designs obtained with this new method for ideal, non-ideal, and azeotropic mixtures give purity and recovery rates close to the specifications, which might be impossible to obtain with a conventional ideal shortcut like the well-known Fenske–Underwood–Gilliland shortcut method. The distillation boundaries of azeotropic mixtures are taken into account thanks to a non-ideal thermodynamic model applied to the calculation, which is not the case with a conventional ideal shortcut. The paper examines the following mixtures: an ideal mixture of ethanol, n -propanol, and n -butanol; a non-ideal mixture of acetone, water, and acetic acid; and an azeotropic mixture of acetone, isopropanol, and water.

Plantwide Control for Maximum Throughput Operation of an Ester Purification Process

Plantwide control for maximum throughput operation of an ester purification process consisting of a liquid–liquid extractor (LLX), a distillation column, and material recycle is evaluated. The extractor uses water solvent and is necessary to cross a distillation boundary making pure ester product recovery feasible. Extraction capacity is the bottleneck constraint. A steady state analysis reveals the occurrence of steady state multiplicity induced infeasibility for too high fresh feed rates and for too low water feed rates. To avoid infeasibility due to the multiplicity behavior, we recommend using the extractor feed rate as the throughput manipulator (TPM) with the fresh feed rate adjusted as a makeup stream (CS1). The other control loop pairings are conventional. Rigorous closed loop dynamic results show that CS1 is significantly more robust than the conventional control structure (CS2) with the fresh feed rate as the TPM. In particular, CS1 achieves more than 25% higher maximum throughput compared to CS...

Separation of acetonitrile/methanol/benzene ternary azeotrope via triple column pressure-swing distillation

Abstract Acetonitrile/methanol/benzene mixture forms more than one different azeotropes and its triangular diagram presents several distillation boundaries at atmospheric pressure. A process named as triple column pressure-swing distillation (TCPSD) was proposed to separate this complex ternary system. The feasibility of the process was confirmed using residue curve maps and rigorous steady-state simulations were implemented on Aspen Plus. On basis of minimum total annual cost, several operating parameters were optimized by pressure-swing optimization software using the sequential iterative optimization procedure and the economics of TCPSD were compared with four different separation configurations. The results demonstrated that the A-M-B separation configuration was the most optimal column sequence in global optimization to separate acetonitrile/methanol/benzene azeotropic mixture using TCPSD. TCPSD may arouse the interest of researchers in various fields and can assist engineers to select the optimal separating process.

Enhanced recovery of PGME and PGMEA from waste photoresistor thinners by heterogeneous azeotropic dividing-wall column

Propylene glycol monomethyl ether (PGME) and propylene glycol monomethyl ether acetate (PGMEA) are representative photoresistor thinners used extensively and generated as waste during the display and semiconductor material manufacturing processes. Although the waste thinner is normally retrieved by distillation, the azeotropes of these two thinner components with water limit the distillation performance. In this paper, an extensive design study of enhanced distillation processes was carried out to determine a favorable path for waste thinner recovery. Appropriate thermodynamic models for the design of a waste thinner recovery process were obtained through the regression and validation of experimental vapor-liquid-liquid equilibrium data. An optimal direct sequence using three conventional distillation columns with a decanter was introduced as a base design to overcome the distillation boundary by azeotropes. Several advanced distillation configurations were examined to further improve the energy efficiency of the conventional recovery process. A novel heterogeneous azeotropic dividing-wall column was developed based on process intensification and integration. The proposed enhanced recovery process reduced the energy requirement for waste thinner recovery significantly by 33.1%. The advanced distillation configuration can be an attractive option for improving the economic and environmental efficiency of the commercial waste thinner recovery and recycling processes.

Design and simulation of divided wall column: Experimental validation and sensitivity analysis

This article deals with design and simulation of divided wall column. Design parameters are provided to the rigorous simulation in the ProSimPlus® software. The results show that the procedure can determine parameters quickly in the case studies and can give a good initialization for rigorous simulation. Secondly, a pilot plant has been design, built and operated in our laboratory. The pilot plant will provide necessary experimental evidence to validate the previous procedure. Ternary mixture and four-component mixture of alcohols have been used in our pilot plant in steady state conditions. The results show that the composition of products, composition and temperature profile along the column are in very good agreement with simulation results. Finally, in order to determine the optimal parameters of divided wall columns, the effects of the structural parameters of the divided wall column such as the height of the wall, the vertical position of the wall and number of stages of each section are analyzed. Ternary diagram is used as an indicator both in showing what the most economical configuration is and in showing the distillation boundary.