Distillation Regions Research Articles

Distillation profiles in ternary heterogeneous mixtures with distillation boundaries

Distillation boundaries are created by saddle azeotropes and divide the composition space into distillation regions. In this study, the behavior of rectification profiles was investigated in the following heterogeneous systems: benzene, tert-butanol, water and ethanol, water, toluene. These systems were chose in order to scrutinize whether distillation regions overlapped. The experiments were performed using Distil (afterwards Conceptual Engineering) software from AspenTech, Inc. For each type of distillation column (staged and packed), a different pair of distillation boundaries is referred (TRB & PDB and SDB & PDB accordingly). Similar to homogeneous mixtures, distillation regions were found to overlap each other in heterogeneous mixtures. As a consequence, their common part was parametrically sensitive.

Design and Control of Heterogeneous Azeotropic Column System for the Separation of Pyridine and Water

The design and control of two types of separation systems using heterogeneous azeotropic distillation have been studied in the literature. One type (e.g., isopropyl alcohol dehydration) is to add a light entrainer (cyclohexane) into the system so that a minimum-boiling ternary azeotrope is formed which can split into two liquid phases in a decanter. This type of system also introduces two additional azeotropes (isopropyl alcohol−cyclohexane and water−cyclohexane), thus dividing the ternary system into three distillation regions. Another type (e.g., acetic acid dehydration) does not contain azeotrope in the original system. However, due to a tangent pinch near the pure water end an entrainer (isobutyl acetate) is added into the system to aid the separation. This type of system has only one binary heterogeneous azeotrope (water−isobutyl acetate); thus there is only one distillation region. In this paper, design and control of a different residue curve map (RCM) type of separation system utilizing heterogeneous azeotropic distillation will be studied. An example of this RCM type is pyridine and water separation using toluene as entrainer. Adding toluene into this system introduces two additional azeotropes, one is minimum-boiling binary heterogeneous azeotrope (water−toluene) and the other one is a binary homogeneous azeotrope (pyridine−toluene). There is no ternary azeotrope for this system. Two alternative design flow sheets are compared in this paper to find the one which is most economical and also meet stringent product purity specifications. A simple overall control strategy of this process has also been developed which requires only one temperature control loop in each column.

Advances in synthesis of azeotropic distillation column sequences: internal secondary recycles

AbstractAn algorithm for automatic generation of sequences of distillation columns and decanters for separation of azeotropic mixtures has been developed (S.K. Wasylkiewicz, 54th Canadian Chemical Engineering Conference, paper No. 243, Calgary, October 2004; S.K. Wasylkiewicz, AIChE Spring National Meeting, paper No. 83e, Atlanta, GA, April 2005) where distillation boundaries can be crossed by moving them with pressure change, by exploring curvatures of distillation boundaries, or by liquid‐liquid splits in decanters. In the first step of the algorithm, open‐loop sequences are generated and primary recycles are automatically detected. Then, mass balances are calculated to finish the sequences. In this paper, we are focused on internal secondary recycles where species present in the sequence feed are introduced as separating agents. This can be a pure component produced somewhere downstream in the sequence or any other intermediate stream. On the basis of a broad knowledge about distillation regions and boundaries for the separated mixture, a preferred distillation region can be identified and a suitable recycle stream can be selected. These types of recycles can simplify tremendously, the whole sequence and reduce significantly the total cost of separation. We present in detail an example based on an industrial case where the internal secondary recycle was efficiently found and calculated during synthesis of column sequences by using the Split Generator in Distil and the mass balance calculation in MS Excel. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Experimental Measurement of the Saddle Node Region in a Distillation Column Profile Map by Using a Batch Apparatus

A simple theoretical method for the evaluation of the separation of mixtures using distillation columns operating at finite reflux, called column profile maps (CPMs), has been developed (Tapp et al., 2004). These CPMs are simply transforms of the residue curve maps (RCMs) and are used for sequencing and synthesis of distillation columns. Thus for example, the methanol, diethyl ether and benzene system has a low boiling azeotrope between methanol and benzene which appears as a saddle point in the RCM. As a result the RCM has two stable nodes and hence two distillation regions divided by a simple distillation boundary. It can be theoretically shown that the transformation of the CPM moves the saddle point that was on the boundary of the mass balance triangle in the RCM into the mass balance triangle of the CPM. Similarly the two stable nodes, corresponding to pure component nodes, in the RCM move out of the mass balance triangle of the CPM. The CPM of this system was experimentally evaluated to verify that a saddle point node does indeed occur inside the mass balance triangle. The experimental technique uses a semi-batch apparatus and measures the boiling liquid concentration in the still as a function of time (Modise et al., 2005). The importance of this is that concentration profiles achieved in the semi-batch still are essentially the same as those of a continuous distillation column section. The experimental measurements showed that there is indeed a saddle point in the CPM.

Generalised method for the determination of heterogeneous batch distillation regions

A new, general method for the calculation of residue curves and for the determination of batch distillation regions of heteroazeotropic distillation is suggested. The method proposed, which means the extension of the method of Pham and Doherty, takes into consideration the possibility of the withdrawal of any fraction of either liquid phase from the decanter as distillate. The simplified and rigorous simulation calculations were carried out for the mixtures dichloromethane–acetone (low α ) + water (heavy, selective, heterogeneous entrainer), water–ethylenediamine (maximum azeotrope) + benzene (light, selective, heterogeneous entrainer) and isopropanol–water (minimum azeotrope) + benzene (light, non-selective, heterogeneous entrainer). The new method gives the right sequence of the cuts for all operation modes and strategies of heterogeneous batch distillation.

Separation of Homogeneous Azeotropic Mixtures by Pressure Swing Distillation - Analysis of the Operation Performance

The complex separation of a homogenous azeotropic mixture by pressure swing distillation is analyzed based on a rigorous dynamic model considering the system acetonitrile/water. The model is validated with our own experimental data. Investigations showed that the process can be operated in a very stable and robust manner even when large disturbances set the feed concentration to the other distillation region.

Determination of distillation regions for non‐ideal ternary mixtures

AbstractThis work concerns the determination of distillation boundaries. This is done comprehensively, using a bifurcation study for a single feed column. The approach used is to track all of the pinch points of rectifying and stripping profiles, mapping the branches of solutions and checking for a “jump” of composition profiles at bifurcation points. The main result is a systematic and exact method of selecting appropriate pairs of boundaries for a given type of column, which demarcate distillation regions. The distillation regions overlap. The common area of two distillation regions is parametrically sensitive, and it determines the possibilities of crossing (at a finite reflux) the distillation boundaries defined for a total reflux or reboil ratio. However, such columns may be difficult to start up and control. An algorithm determining the distillation regions, which takes into account properties of distillation boundaries, has been formulated. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Analysis of the possibility of implementing the principle of redistribution of concentration fields for reactive distillation processes

The evolution of the phase portrait for the dynamic system of open evaporation combined with a chemical reaction for a fixed liquid amount was studied as dependent on the rate constants of the reaction. A three-component azeotropic mixture whose vapor-liquid equilibrium diagram is characterized by two distillation regions was considered. The possibility of the open evaporation trajectory to transfer through the separatrix between the distillation regions was shown, as well as the possibility of implementing the principle of redistribution of the concentration fields due to chemical reactions occurring in reactive distillation columns.

Shortcut Design Method for Columns Separating Azeotropic Mixtures

A shortcut method is proposed for the design of columns separating homogeneous azeotropic mixtures. Azeotropes are treated as pseudocomponents, and a C-component system with A azeotropes is treated as an enlarged (C + A)-component system. This enlarged system is divided into compartments, where each compartment behaves like a nonazeotropic distillation region formed by the singular points that appear in it. The compartment boundary is linearly approximated. A procedure is proposed for transforming vapor−liquid equilibrium behavior in terms of pure components into that in terms of singular points, allowing relative volatilities to be characterized in terms of singular points. The classical Fenske−Underwood−Gilliland method can then be used to design columns separating azeotropic mixtures. This method is extremely computationally efficient and can be applied to homogeneous azeotropic mixtures with any number of components. The results of the shortcut design method are useful for initializing rigorous simulations using commercial software.

Feasibility of Continuous Reactive Distillation with Azeotropic Mixtures

By using phase and reaction equilibrium information, the feasibility criteria for single-feed and double-feed reactive distillation systems are derived: When the products are a stable node and an unstable node and a part of the reaction equilibrium curve lies in the same distillation region as the products, both reactive rectifying and reactive stripping sections can feasibly produce pure products in a single-feed column. If one of the products is a saddle and no azeotrope exists between the products, a single-feed column is still feasible, but only with large reaction equilibrium constants. If a saddle product forms a minimum-boiling azeotrope with another product, an entrainer is needed to break this azeotrope in a double-feed column. If both reaction products are saddles, the same double-feed column configuration with an entrainer can produce pure products.

Simple Method for Determining the Location of Distillation Region Boundaries in Quaternary Systems

For ternary systems, the use of residue curve maps as a representation of the phase behavior has greatly facilitated process synthesis. Especially important is the knowledge of distillation boundaries, which cannot be crossed by simple distillation. Here, an attempt is made to extend these procedures to quaternary systems by presenting a method to calculate the exact location of distillation region boundaries, i.e., the surfaces separating different distillation regions in composition space. The procedure is illustrated with a few examples.

Feasible products in batch reactive distillation

AbstractA systematic feasibility evaluation for batch reactive distillation systems is addressed. In each batch configuration, feasibility criteria are obtained for ternary reactive systems by using residual curve maps (RCMs) and reaction equilibrium manifolds. If one of the reaction products is an unstable node (UN) and is reachable from all distillation regions, a rectifier can be used for producing pure products. Even if this unstable node is only reachable from one specific distillation region, a rectifier can still be used when a part of the reaction equilibrium curve lies within the same distillation region as the product. Symmetric results are conserved for a batch stripper with one stable node (SN) product. Finally, pure products are produced in middle‐vessel columns when the UN and SN products share the same distillation region with a part of the reaction equilibrium curve. Although these criteria are obtained for ternary systems, they can be extended to multicomponent reactive systems.

Operation of Homogeneous Azeotropic Distillation Column Sequences

A self-optimizing control concept is provided for sequences of three homogeneous azeotropic distillation columns with two recycles separating a ternary feed of a 020 mixture that has two simple distillation regions that are separated by a residue curve boundary. Studying a case with reduced complexity (columns of infinite length operated at infinite reflux), one of the two recycles (the entrainer recycle) is identified as the key manipulated variable for ensuring optimal operation of the sequence. Different setups of the scheme have different optimal feed regions. The theoretical results are illustrated with rigorous simulations of a 020 mixture: methanol (which acts as an entrainer), 2-propanol, and water. Dynamic simulations confirm that the process operates robustly by using the (L/D)QR scheme for the individual columns and one additional control loop that controls the methanol (entrainer) composition of the distillate by manipulation of the overall feed of one column. Dynamic simulations also confirm that column profiles can cross residue curve boundaries, distillation boundaries, and even azeotropes if the feed composition is changed such that a column profile is only feasible in the other distillation region.

Co-solvent selection and recovery

The selection and recovery of two or more solvents (i.e. co-solvents) are studied because of their importance in environmentally responsible processing. A particularly challenging problem arises when the contaminants (or solutes) are comprised of a mixture of volatile and nonvolatile compounds. The nonvolatile components present difficulties in solvent selection while the volatile components give rise to solvent recovery challenges. Traditional approaches such as the Kauri–Butanol test and predictions based on solubility parameters from regular solution theory can result in poor solvent selection when some of the contaminants are nonvolatile. A simple, graphical and rigorous procedure based on Gibbs energies of mixing for all binary mixtures of solvents and contaminants is proposed for choosing solvent/contaminant pairs. This selection procedure is based on the choice of solvent resulting in the lowest solvent/contaminant Gibbs free energy of mixing for a given contaminant. Nonvolatile contaminants are often easily separated in a single-stage flash vessel. Volatile contaminants, on the other hand, can form homogeneous or heterogeneous azeotropes with commonly used solvents, give rise to distillation boundaries and result in a challenging co-solvent recovery problem. A systematic procedure for the synthesis, design and economic analysis for co-solvent recovery is presented. Residue curve maps are used to identify distillation boundaries and to generate a conceptual process flow sheet for co-solvent recovery. Equipment sizes for the separators and auxiliaries (i.e. condensers, reboilers, decanters, etc.) are determined and capital investment, installation and operating costs are calculated. The removal of oil–water emulsions from the surface of machined metal parts using n-propyl bromide (NPB) and isopropyl alcohol (IPA) in a closed-loop degreaser is used to illustrate the proposed methodology of co-solvent selection and recovery. Analysis shows that the Kauri–Butanol test and the use of solubility parameters can result in poor solvent selection. In contrast, the proposed methodology shows that NPB is competitive with banned solvents like chlorodifluoro, fluorodichloroethane and trichloroethane and highly regulated solvents such as perchloroethylene, trichloroethylene and is a good solvent for the removal of machining oils while IPA is a good solvent for removing water. Separation of mixtures of NPB/IPA/water is difficult because of the presence of three binary and one ternary azeotrope that create three distinct distillation regions as well as regions of vapor–liquid–liquid behavior. It is shown that these distillation boundaries preclude direct water separation and that a two-distillation sequence with decantation is required to cross boundaries and recover both solvents. A variety of geometric illustrations are presented to elucidate key concepts.

Effect of metal oxide on light oil combustion

Abstract Simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) were applied to light crude oil combustion in the presence and absence of metal oxide. In crude oil-limestone mixture, three main transitional stages are detected. These are distillation, low-temperature oxidation (HTO) and high temperature oxidation (HTO) regions respectively. In the case of experiments with Fe(III)-chloride at different amounts, the shape of TG-DTA curve is changed considerably. Kinetic parameters of the samples are determined using ASTM method. Reduction in activation energy is considered to be an indication of the catalytic activity of the additive.

Synthesis and design of heterogeneous separation systems with recycle streams

Geometric methods are one of the most valuable tools for distillation process synthesis and design. The graphical representation of azeotropes, residue curves, distillation regions, and distillation boundaries provide designers with intuition, a wealth of knowledge about the entire composition space and quantitative results. In this paper we show how residue curve maps and geometric methods can be used in the synthesis and design of sequences of complex heterogeneous azeotropic distillation columns. A new algorithm for the design of heterogeneous azeotropic distillation column with top decanter is presented and applied in two design case studies of column sequences with recycle streams and top decanters. In both the cases, a feasible structure has been quickly found based on information obtained by examining the entire composition space.

Sectional and overall reachability for systems with S-shaped distillation lines

Total and minimum reflux bounds serve as limiting case scenarios for determining feasibility in azeotropic distillation columns. Specifically, all reachable feed compositions in a staged rectifying or stripping section have been reported as the union of points enclosed by the distillation and pinch-point curves through a desired product. By eigenvalue considerations of pinch bifurcations and geometric analysis of fundamental region structures, the region of all reachable compositions has seemingly been identified. Additional insights are needed to fully characterize the feasible compositions in a subset of azeotropic mixtures. This article describes how sectional profiles can actually cross pinch-point curves and generate a region of additional feasibility outside the one previously reported. It also demonstrates how a substantially larger region between the distillation line and pinch-point curve after the intersection is only accessible by composition profiles going through the extended region. This extended reachable region is explained by evaluating the conditions for a fixed point in composition space. The effect on sectional profiles is illustrated for both an artificially parametric VLE manifold and a nonideal azeotropic system with more than one simple distillation region. Sectional profiles extended beyond the traditional bounds affect coupled sections in a distillation cascade.

On the Integration of Reaction and Separation in a Batch Extractive Distillation Column with a Middle Vessel

In this work, the integration of reaction and separation in a batch extractive distillation column with a middle vessel/reactor is analyzed for azeotrope-forming mixtures, This equipment configuration has the potential to promote the complete conversion of reactants; therefore, the main process characteristics are investigated. A mixture showing several azeotropes and involving an esterification reaction was selected as an academic example. The first part of the paper deals with the phase-equilibrium analysis of the mixture. The nodes (pure components and azeotropes) and the distillation regions of the multicomponent mixture are obtained. The analysis of the topology of the residue-curve map is used to select one of the reagents as entrainer. Feasibility of the combined operation is studied based on the phase-equilibrium analysis and the investigation of the feasible cuts at infinite separation power. The second part of the contribution focuses on the different steps of the process. The influence of operating and process parameters on the operation performance is studied with the aid of a process simulator. Physical explanations are given for the results. Results show the advantages of integrating reaction and separation to enhance both reagents conversion and product separation.

Topological analysis of vapor–liquid equilibrium diagrams for distillation process design

Residues curve and distillation line maps have become an important tool for conceptual distillation process design. These maps are based solely on thermodynamic information on the vapor–liquid equilibrium and are called vapor–liquid equilibrium (VLE) diagrams here. They give, among other, information on the composition regions, in which products of a distillation process can be found. The entire composition simplex can be divided into distinct distillation regions separated by distillation boundaries, which usually cannot be passed in distillative separations. Up to now, VLE-diagrams are mostly used for the well-studied case of ternary systems, for which analysis and visualization is straightforward. However, in many applications, systems containing more than three components have to be considered and, hence, multicomponent VLE-diagrams are needed. In the present work, two approaches are suggested for automated synthesis of VLE-diagrams, which can be applied to any system of interest without limitations to its complexity. The first method is hierarchical and based on a sequential consideration of all constituent subsystems of the system of interest and combining the results to synthezise the VLE-diagram. The second approach is evolutional, because the multicomponent system is considered as it is, but the structure of the diagram is changed from the simplest state of a zeotropic system to the end state with all azeotropes. Both approaches are in principle combinatorial, but the first one needs consistency criteria to discriminate the proposed diagrams, whereas the second one produces always consistent diagrams.

Synthesis of distillation-based processes for non-ideal mixtures

Our understanding of how to design distillation-based processes to separate mixtures displaying azeotropic behavior has grown enormously in the past quarter century. We elegantly sketch distillation column behavior on a composition diagram where we display VLE and column behavior using residue and distillation curves. In the presence of azeotropes, these curves partition composition space into distillation regions that trap the performance of individual columns. We can view liquid–liquid behavior as a tearing of composition space, a tear that always spawns from a minimum binary azeotrope. A material balance across a column section leads to a difference point, which we can use to understand column tray-by-tray and limiting behavior for ordinary, extractive and even reactive distillation. We demonstrate how to synthesize alternative separation processes, concentrating on those that produce pure products. We use boundary curvature, solvent addition, extractive agents, liquid–liquid behavior and strategically placed reaction to step across distillation boundaries. We show that these processes always contain recycles to gain feasibility but also to have economically attractive processes.