Azeotropic Distillation Column Research Articles

Energy-saving design and optimization of pressure-swing-assisted ternary heterogenous azeotropic distillations

A huge amount of energy is always consumed to separate the ternary azeotropic mixtures by distillations. The heterogeneous azeotropic distillation and the pressure-swing distillation are two kinds of effective technologies to separate heterogeneous azeotropes without entrainer addition. To give better play to the synergistic energy-saving effect of these two processes, a novel pressure-swing assisted ternary heterogeneous azeotropic distillation process is proposed firstly. In this process, the ternary heterogeneous azeotrope is decanted into two liquid phases before refluxed into the azeotropic distillation column to avoid the aqueous phase remixing, and three columns’ pressures are modified to decrease the flowrates of the recycle streams. Then the dividing wall column and heat integration technologies are introduced to further reduce its energy consumption, and the pressure-swing assisted ternary heterogeneous azeotropic dividing wall column and its heat integration structure are achieved. Genetic Algorithm procedure is employed to optimize the proposed processes. The design results show that the proposed processes have higher energy efficiencies and lower CO2 emissions, compared with the published ternary heterogeneous azeotropic distillation process.

Conceptual process design and process optimization of triple-column pressure-swing distillation for the separation of ternary mixtures embedding two azeotropes with different feed composition via thermodynamic insights and genetic algorithm

In this article, we take the tetrahydrofuran (THF)/ethanol (ETOH)/methanol (MeOH) as an example to investigate the process design of the triple-column pressure-swing distillation (TCPSD) for the separation of Serafimov’s class 2.0-2b mixtures with different feed composition. Two basic topology structures (TSI, TSII) and two separation sequences (SSI, SSII) are investigated, then, process optimization with total annual cost (TAC) as objective function is conducted via genetic algorithms. Heat integration (HI) is also applied to TCPSD. From a new perspective, the separation of THF/ETOH/MeOH can be regarded as the azeotropic distillation for THF/ETOH with MeOH as azeotropic agents based on thermodynamic features of Serafimov’s class 2.0-2b mixtures. The feasible topology structure and separation sequence of TCPSD for Serafimov’s class 2.0-2b mixtures depend on the feed composition and operating pressure. The feed composition is divided into three regions according to the concentration of methanol in azeotropic distillation column. The two critical points are azeotropic points at allowed minimum and maximum operating pressure. Especially, when feed composition is below azeotropic point at allowed minimum operating pressure, TSI is not feasible due to less MeOH as azeotropic agents. Furthermore, TCPSD combining pure azeotropic agents (ATPSD) can be developed based on this new insight. Then, two feed compositions are used to illustrate the detailed conceptual design of three topology structures as well as corresponding two separation sequences and their economic performance. The results show that when feed composition is 0.4ETOH/0.25THF/0.35MeOH, TSI and TSII are feasible, the best economical configurations without and with heat integration is TSII-SSII and TSII-SSII-HI. When feed composition is 0.4ETOH/0.4THF/0.2MeOH, TSII and ATPSD are feasible, the best economical configurations without and with heat integration is ATPSD-SSI and ATPSD-SSI-HI. Furthermore, ATPSD-SSI and ATPSD-SSI-HI have 18.28% and 19.38% TAC reduction than TSII-SSI and TSII-SSI-HI, respectively. In summary, feasible and the best economic topology structure and separation sequence of TCPSD for Serafimov’s class 2.0-2b mixtures depend on the feed composition and operating pressure. In addition, the proposed ATPSD shows good economic performance for Serafimov’s class 2.0-2b mixtures.

Heterogeneous phase diagram and residue curve map for ethanol-water-ethyl acetate and ethanol-water-ethyl acrylate system

Heterogeneous phase diagram and residue curve map data are simulated using NRTL equation which are useful for design of heterogeneous Azeotropic distillation column. Simulated data are compared with experimental data.

Energy saving in acetic acid process using an azeotropic distillation column with a side stripper

ABSTRACTDilute acetic acid is obtained primarily from fermentation and synthesis processes and cannot be produced by simple distillation due to relatively low volatility of acetic acid compared to water. Instead, an azeotropic distillation is applied to increase the concentration of dilute acetic acid. When acetic acid is extracted from a dilute aqueous solution using a solvent mixture of ester and alcohol, its recovery requires an energy-intensive azeotropic distillation. In the water stripping process that follows azeotropic distillation, two distillation columns handle the acetic acid and water mixture in similar composition. Therefore, the two columns can be combined as a side stripper connected to the azeotropic distillation column. The energy-saving effect is examined with the HYSYS (Aspentech Corporation) evaluation of the process. Compared to the conventional process, the modified process suggests 39% reduction in heating duty and 24% coolant savings. The economic analysis shows 32% decrease in investment and 36% utility savings. Based on heat utilization analysis, the thermodynamic efficiency is enhanced by 11%.

Separation of acetone/isopropyl ether/water ternary mixture via hybrid azeotropic-extractive distillation

An industrial ternary mixture of acetone/isopropyl ether/water forms two binary azeotropes and a ternary azeotrope at atmospheric pressure and conventional distillation processes cannot achieve efficient separation. A hybrid azeotropic-extractive distillation (HAED) process is proposed for the separation of acetone/isopropyl ether/water. In this process, isopropyl ether is chosen as a light entrainer aimed to form a binary azeotrope with acetone, so that water can be separated from the bottom of azeotropic distillation column (ADC) at 0.3 atm. Then extractive distillation for the separation of binary azeotrope of acetone/isopropyl ether is investigated with ethylene glycol (EG) as an entrainer. On the basis of minimum total annual cost (TAC), a sequential iterative optimization procedure is performed to optimize the economy of two processes with two different feeding ways of light entrainer. The results demonstrate that both processes can make three products purities achieve 99.5 wt% with three columns. Additionally, compared with the process in which light entrainer is blend with the fresh feed, the process in which the light entrainer is added in the upper of the ADC can reduce the TAC and energy consumption by 9.3 % and 9.21 %.

Conceptual design for the recovery of 1,3-Butadiene and methyl ethyl ketone via a 2,3-Butanediol-dehydration process

This paper proposes a process scheme for the recovery of 1,3-Butadiene (BD) and methyl ethyl ketone (MEK) from the dehydration products of 2,3-Butanediol (BDO). BDO is produced via the bio-fermentation of the industrial-gas wastes from the steel industry and various biomasses. The suggested process is environmentally beneficial because it can extend the feedstock rather than petroleum, and it replaces the use of the industrial-plant gas wastes. The proposed process is composed of a catalytic-dehydration reactor, a quencher, a BD-purification unit, a decanter, a water-removal column, and an MEK-purification unit. Based on the proposed scheme, the BD- and MEK-recovery rates are approximately 94% and 98%, respectively. Most of the units in the process are well developed, except for the azeotropic distillation column (ADC) that is used in the MEK-purification unit. A pilot test is also performed, and the results are compared with those of a simulation for an analysis of the viability of an ADC. Through the use of the experimental ADC, a mixture of MEK and 2-methylpropanal (2-MPA) is obtained via the removal of water. The results of the pilot test show that a water trace, MEK 84.5%, and 2-MPA 15.5% are separated to the bottom stream, which is in sound agreement with the simulation data. Furthermore, the simulated temperature profile is consistent with the experimental data with an error range of only ±2%.

Heterogeneous azeotropic distillation for the separation of n-propanol + water mixture using n-propyl acetate as entrainer

Heterogeneous azeotropic distillation is a widely used process in the chemical process industries for the separation of binary azeotropic mixtures into their constituent pure components. It involves the addition of an entrainer which is partially miscible and forms a heterogeneous azeotrope preferably with one of the original components. For the design of heterogeneous azeotropic distillation columns, a detailed study of the heterogeneous region is crucial and this requires experimental vapor-liquid-liquid equilibrium (VLLE) data. The present study focuses on the study of n-propyl acetate as entrainer to effect the separation of industrially available azeotropic mixture n-propanol-water. A new and simple still was constructed to measure the VLLE data for the ternary system. The isobaric VLE data for the two binary systems n-propanol - water and n-propanol - n-propyl acetate and isobaric VLLE data for the ternary system n-propanol - water - n-propyl acetate were measured at atmospheric pressure. The experimental results have been compared with various thermodynamic models such as NRTL, UNIQUAC, UNIFAC (VLE) and UNIFAC (LLE). The set of parameters obtained from the above models has been used to construct the residue curve map (RCM) for the ternary system and the effect of n-propyl acetate to accomplish the separation of the azeotropic mixture has been discussed.

Pressure-Swing Distillation for Separating Pressure-Insensitive Minimum Boiling Azeotrope Methanol/Toluene via Introducing a Light Entrainer: Design and Control

In this paper, pressure-swing distillation (PSD) is applied to separate pressure-insensitive minimum boiling azeotrope methanol/toluene via introducing chloroform as a light pressure-swing entrainer. The feasibility of this process is verified by analyzing the residue curve maps. Both partially and fully heat-integrated PSD processes are implemented. The steady-state designs of these processes are optimized based on total annual cost (TAC). In this extended PSD process, the high-pressure column (HPC) serves as the homogeneous azeotropic distillation column, while the low-pressure column (LPC) serves as the entrainer recovery column. It is found that the HPC has large temperature slope in the stripper section while with relatively flat trend above. This unique phenomenon makes it beneficial to reduce the energy consumption by preheating the feed and recycling stream using low-grade heat. Both conventional and partially heat-integrated extractive distillation processes with aniline as the entrainer are also...

Application of Vapor Recompression to Heterogeneous Azeotropic Dividing-Wall Distillation Columns

Although heterogeneous azeotropic dividing-wall distillation columns (HADWDCs) can achieve considerable reductions in capital investment and operating cost as compared to conventional heterogeneous azeotropic distillation systems, their steady-state performance is likely to be greatly enhanced by the application of vapor recompression heat pump (VRHP) technology owing to their unique process configuration of one top condenser and two bottom reboilers and favorable steady-state behavior of relatively small top-to-bottom temperature elevations. In the current work, the feasibility and effectiveness of reinforcing the HADWDC with the VRHP (HADWDC-VRHP) are examined. A systematic procedure is developed to effectively determine the optimum combination of the VRHP and the HADWDC in order to minimize the total annual cost. Two example systems for the separation of isopropyl alcohol and water and pyridine and water with cyclohexane and toluene as entrainers, respectively, are studied to evaluate the steady-state ...

Ethanol dehydration via azeotropic distillation with gasoline fraction mixtures as entrainers: A pilot-scale study with industrially produced bioethanol and naphta

Various hydrocarbons (n-hexane, cyclohexane, toluene, isooctane) and mixtures of them (binary, ternary or quaternary), as well as two different types of industrially produced naphtha (one obtained by direct distillation and the other from a catalytic cracking process), have been tested as candidate entrainers to dehydrate ethanol. The tests were carried out in an azeotropic distillation column on a semi pilot plant. The results show that it is possible to dehydrate bioethanol using naphtha as entrainer, obtaining as a result a fuel blend with negligible water content and ready for immediate use in motor vehicles.

Design and control of an improved acrylic acid process

The acrylic acid (AA) process involves the partial oxidation of a flammable and explosive gas medium (propylene), so considerable attention is paid to the concentration of the reactants. Water and air is added to dilute the reactant and enhance the thermal stability of the reaction. Different compositions of water and air can lead to different methods to separate AA product. A modified AA process is developed based on the AA process proposed by Turton, and consists of a tubular reactor, an absorber and only two distillation columns, one of which is an azeotropic distillation column. This process is characterized by shortening and simplifying AA refining process so that equipment investment cost can be reduced. A plantwide control structure featured with cascade control provides effective control for the multiunit process and insures safe operation of the two distillation columns. Dynamic results also reveal that it is useful to apply temperature/temperature cascade control and composition/temperature cascade control to the distillation columns with maximum temperature limitations.

Effect of Using Adjusted Parameters, Local and Global Optimums, for Phase Equilibrium Prediction on the Synthesis of Azeotropic Distillation Columns

We analyze the effect of using adjusted parameters, corresponding to local and global optimums, in the NRTL thermodynamic model on the complete process synthesis (design, optimization and control) of homogeneous azeotropic distillation columns. The adjusted parameters that correspond to a global optimum were obtained with simulated annealing technique, while the adjusted parameters that correspond to a local optimum were taken from the Dechema Collection. Both sets of parameters were used to design a conventional sequence, a side-stream column, and a Petlyuk sequence. These designs were used as the initial solution to a multiobjective genetic algorithm with constraints handling, coupled to a processes simulator, where the number of stages and heat duty of each column were considered as objectives; as a result, a set of optimal designs, called the Pareto front, was obtained. Then, we chose some designs to analyze their theoretical control properties and the dynamic performance. Results show remarkable differences in structure, energy consumption, control properties, and dynamic performance of these schemes, depending on the use of adjusted parameters. The results show the importance of using the best adjusted parameters available, which in our case correspond to global optimums obtained with the simulated annealing technique.

Design of decanter in heterogeneous azeotropic distillation column by minimizing Gibbs free energy

ABSTRACTIn this paper, liquid–liquid equilibrium in decanter of heterogeneous azeotropic distillation is solved by minimizing Gibbs free energy, by using global optimization technique. The azeotropic distillation column is modeled by solving material balance, equilibrium and summation equations, and repulsive particle swarm optimization (RPSO), a stochastic global optimization formalism, is employed to predict stable steady state solution in decanter. To verify the performance of the RPSO algorithm, it is compared with equation solving method for liquid–liquid equilibria by considering two azeotropic systems, namely (1) benzene–ethanol–water and (2) furfural–water. The proposed methodology shows feasibility of the RPSO algorithm in predicting liquid–liquid equilibrium in decanter of heterogeneous azeotropic distillation. © 2013 Curtin University of Technology and John Wiley & Sons, Ltd.

Similarity Analysis of Sequential Alarms in Plant Operation Data by using Levenshtein Distance

Sequential alarms are a collection of many alarms that occur almost simultaneously with specific time lags within a short period of time. They reduce the ability of operators to cope with abnormalities at plants because critical alarms are buried under many unnecessary ones. The grouping of correlated sequential alarms according to the degree of similarity helps to reduce them more effectively than by analyzing individual sequential alarms. We propose a new method of analyzing similarities between sequential alarms by using the Levenshtein distance, which is a string metric for measuring the difference between two sequences defined as the minimum number of edit operations such as insertion, deletion, and substitution of a single character needed to transform one string into another. The proposed method first converts the plant operation data, which consist of the occurrence times and tag names of alarms, into sequential alarm data. Then, the similarities between all combinations of any two sequential alarms in the plant operation data are evaluated using the Levenshtein distance. The proposed method was applied to the simulation data of an azeotropic distillation column and the results revealed our method was able to correctly identify similarities between correlated sequential alarms, even when a conventional method of analysis failed due to deletions, substitutions, and transpositions in the alarm sequence.

Online switching of entrainers for acetic acid dehydration by heterogeneous azeotropic distillation

Traditionally, a foreign chemical such as isobutyl acetate (IBA) is used as an entrainer in a heterogeneous azeotropic distillation (HAD) column to separate acetic acid (HAc) and water. However, the loss of this entrainer may contaminate the process of manufacturing terephthalic acid because of the recycle of product streams to an oxidation reactor and other parts of the process. In the study, use of p-xylene (PX), a component indigenous to the terephthalic acid process and already present in the feed to the HAc dehydration column, as the entrainer for the separation of HAc and water is investigated. Optimization results show that using the HAD process with PX as the entrainer not only overcomes the shortcomings of IBA as the entrainer but also requires less energy consumption. Moreover, we demonstrate that it is possible to dynamically transform the HAD column with IBA as the entrainer into the HAD column with PX as the entrainer, without the need of shutdown and startup. The key to successful implementation of such a dynamic transition is to maintain a proper PX inventory inside the HAD column. While simple sidedraw flow control can be used, it will create PX imbalance if there is a flow measurement bias. A temperature control scheme was found to accelerate such a transition and maintain HAc purities at their designed values.

Environmental Performance Assessment for Heterogeneous Azeotropic Distillation Partitioned Distillation Column

A heterogeneous azeotropic distillation partitioned distillation column (HADPDC) for alcohol dehydration was investigated compared to a conventional heterogeneous azeotropic distillation sequence (CHADS) from environmental performance perspective by using the software Aspen Plus. HADPDC can eliminate the condenser of the second column and decrease the degree of back-mixing. The optimal design of HADPDC guaranteed the minimum energy consumption, which is related to the minimum CO2 emissions.

Dynamic Simulation and Analysis of Industrial Purified Terephthalic Acid Solvent Dehydration Process

Dynamic model for dehydration process of industrial purified terephthalic acid solvent is investigated to understand and characterize the process. A temperature differential expression is presented, which ensures the equation to convergence and short computation time. The model is used to study the dynamic behavior of an azeotropic distillation column separating acetic acid and water using n-butyl acetate as the entrainer. Responses of the column to feed flow and aqueous reflux flow are simulated. The movement of temperature front is also simulated. The comparison between simulation and industrial values shows that the model and algorithm are effective. On the basis of simulation and analysis, control strategy, online optimization and so on can be implemented effectively in dehydration process of purified terephthalic acid solvent.

Effect of Entrainer Loss on Plant-Wide Design and Control of an Isopropanol Dehydration Process

Azeotropic distillation is an important technology for separating azeotropes or minimizing the energy expenditure of separating close boiling mixtures. In heterogeneous azeotropic distillation, an entrainer is usually added, which is recovered and reused in the subsequent stripping process. Depending on the design, there may be various degrees of entrainer loss. In this work, the plant-wide design and control of an isopropanol dehydration system using cyclohexane as an entrainer via heterogeneous azeotropic distillation is conducted. When there is negligible entrainer loss, a special case of input multiplicity is found, in which a set of design specifications can be achieved by an infinite but bounded set of operating conditions. The optimal condition with the minimum reboiler duty in this bounded set is achieved when the organic recycle flow is the smallest. Due to the existence of these multiple steady states, the entrainer inventory control is self-regulatory over long time operations if a PI control is used for the level of the organic phase in the decanter. If large offsets are allowed in the entrainer inventory control, the system may exhibit a long period of pseudosteady state followed by catastrophic failure. Alternatively, one can allow a small amount of entrainer loss to the bottom of the heterogeneous azeotropic distillation column, and provide a small amount of entrainer makeup. In this case, a control loop is added to guard against process drift due to loss of entrainer. However, because continuous maintenance of entrainer inventory is difficult, an on-off control scheme of entrainer makeup feed is used.

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.

Method for the Design of Azeotropic Distillation Columns

A tray-by-tray method for the design of azeotropic distillation columns is presented. The method uses distillation lines, solves the material balances (in the form of algebraic equations), and detects a feed tray location that minimizes the total number of stages. Several cases of study show that the placement of the feed, as suggested by this method, can provide better designs than those obtained using the boundary value method (BVM).