Distillation Example Research Articles

Controllability assessment and intentional operation of extractive distillation configurations with preconcentration

There has always been an opaque link between economic and control performance in process intensification. In fact, whether the excessive interactions produced by intensification are detrimental or beneficial hinged upon the specific circumstances. This gives rise to a systematic approach to controllability analysis, which is discussed on two extractive distillation examples with preconcentration. The detailed frequency domain dependent open-loop controllability analyses are first carried out to investigate set-point tracking and load rejection performance. The closed-loop validation in time domain is then performed. The results show that the remarks of the open-loop analysis are correct in determining the control structure and are ambiguous in loads rejection. Further, a surrogate-based intentional control strategy is applied, faster in response and greater in robustness than the conventional fixed sensitive tray temperature strategy, well similar in energy consumption, under simultaneous disturbances in multiple operation conditions and product specifications.

Взаимное влияние гидродинамических условий и фазовых переходов при кипении многокомпонентных вязких жидкостей

The paper presents results on the intensity of heat and mass transfer during boiling of viscous liquids in a heated channel on the example of distillation of vegetable oil miscella. For the first time, local characteristics of heat transfer along the channel length were obtained for miscella containing linoleic sunflower oil. The values of the heat transfer coefficients are 100–250 W/m2·K. The mechanism of mass and energy transfer in the active working zone of the heated channel is described. The choice of the channel length with a diameter of 30 mm depending on the input mass concentration of vegetable oil in the misсella is justified: up to 60–70% no more than 1500–2000 mm, above 60–70% up to 4000–6000 mm. An experimental study of the formation in miscella of an oil-rich layer near heated surface is carried out. We think the thickness may be 0.3–0.4 microns. Dependences are obtained and a simple formula for calculating the thickness of this layer is proposed. The relationship between the hydrodynamic environment and the intensity of heat exchange along the length of the channel is established. The best mode is the bubble mode of the two-phase flow.

From modeling to virtual laboratory development of a continuous binary distillation column for engineering education using MATLAB and LabVIEW

AbstractThe utilization of computers in all aspects and scales of chemical engineering, from fundamental understanding to solution of large‐scale optimizations, is spreading quickly, hand‐in‐hand with access to data science and information technology. It is straightforward to involve computers and virtual laboratory tools into engineering education. This study presents the development of an interactive virtual laboratory method for a continuous binary distillation process. The three main steps, namely, mathematical modeling, model calibration, and graphical user interface (GUI) development are described and discussed in this paper. The model takes into account the specific needs of the simulator (fast run time) and limitations from the measurement system (calibration considerations). Then, it is validated based on temperature and concentration data from a lab/pilot scale continuous rectification column. The separation of an ethanol–water mixture is considered, the classroom example of distillation, which is, on the other hand, an azeotrope‐forming nonideal mixture. The objective of this paper, beyond the virtual laboratory coding description, is to provide useful guidelines for a user‐friendly and yet realistic simulator development for commonly available experimental devices in chemical engineering education. To fulfill this aim, MATLAB and LabVIEW, two routinely used programming languages by chemical and process engineers, are used.

Pressure selection for non-reactive and reactive pressure-swing distillation

Pressure-swing distillation is the only enhanced distillation technique that does not rely on using an agent to promote the separation of azeotropes. This technology exploits the variation of the azeotropic composition or the disappearance of the azeotrope itself with pressure. A heuristic based on the azeotrope pressure-sensitiveness is commonly considered to evaluate the process feasibility. However, column operating pressures are chosen rather arbitrarily. Therefore, the operating pressure selection is addressed in the present study. When the reboilers and condenser duties are provided by the services available, the temperatures of the cooling and heating services available should be considered to adjust the columns pressures. The low-pressure distillate temperature must be the suitable to be cooled with refrigeration water and the high-pressure bottoms temperature must be the suitable to be heated with the available steam hot services; these boiling point temperatures depend on the column pressure. However, an analysis of the literature available shows that the low pressure is usually fixed at atmospheric pressure for sake of simplicity and the different steam qualities costs are disregarded. Two case studies are presented for illustrative purpose: binary mixture ethanol/ethyl acetate and methyl acetate transesterification with isobutanol as example of reactive pressure swing distillation.

DISTILLATION OF BINARY TWO-PHASE MIXTURES WITH VARIOUS PHYSICO-CHEMICAL PROPERTIES: NORMAL MODE AND DEVIATIONS FROM IT

In this article, the normal continuous mode of distillation is considered using an example of binary two-phase mixture distillation. In practice, there are often deviations from the normal mode. It has been proved that the deviation leads to an increase in energy consumption for the ongoing process. In the industry, columns separating binary mixtures are normally the finishing apparatuses in the flow-sheet separation of multicomponent mixtures, which are obtained in the reactor as a result of main reactions as well as by-reactions. The distillation of binary mixtures is relatively simpler than that of multicomponent mixtures. In this regard, the fundamental parts of monographs especially in the thirties-forties of the last century started with the study of binary mixtures, although multicomponent mixtures were the main focus. The aim of analyzing this complex and the highly energy-intensive process is to facilitate the choice of a mathematical model for the process and the determination on its basis variance. Variance is a set of independent variables that allow calculation of a process only after taking into account the number of independent equations related to these variables. A case of deviation from the normal distillation regime where an under-heated liquid enters the column feed has been revised. This leads to an increase in energy consumption during distillation. Key indicators of the normal regime relative to the level of feed and the temperature of the liquid are shown in the text.

A Derivative Method for Minimising Total Cost in Heat Exchanger Networks through Optimal Area Allocation

This paper presents a novel Cost Derivative Method (CDM) for finding the optimal area allocation for a defined Heat Exchanger Network (HEN) structure and stream data, without any stream splits to achieve minimum total cost. Using the Pinch Design Method (PDM) to determine the HEN structure, the approach attempts to add, remove and shift area to exchangers where economic benefits are returned. From the derivation of the method, it is found that the slope of the e-NTU relationship for the specific heat exchanger type, in combination with the difference in exchanger inlet temperatures and the overall heat transfer coefficient, are critical to calculating the extra overall duty each incremental area element returns. The approach is able to account for differences in film coefficients, heat exchanger types, flow arrangements, exchanger cost functions, and utility pricing. Incorporated into the method is the newly defined “utility cost savings flow-on” factor, θ, which evaluates downstream effects on utility use and cost that are caused by changing the area of one exchanger. To illustrate the method, the CDM is applied to the distillation example of Gundersen (2000). After applying the new CDM, the total annual cost was reduced by 7.1 % mainly due to 24 % less HEN area for similar heat recovery. Area reduction resulted from one exchanger having a minimum approach temperature (ΔTmin) of 7.7 °C while the other recovery exchangers had larger ΔTmin values. The optimum ΔTmin for the PDM was 12.5 °C. The CDM solution was found to give a comparable minimum total area and cost to two recently published programming HEN synthesis solutions for the same problem without requiring the increased network complexity through multiple stream splits.

Impact of vapor recompression in batch distillation on energy consumption, cost and CO2 emission: Open-loop versus closed-loop operation

This paper aims at analyzing the thermal integration of direct vapor recompression system with a distillation column operated in batch mode. The unsteady state nature of the batch processing makes the use of vapor recompression a challenging task. For a meaningful comparison, it is attempted to run the vapor recompressed batch distillation (VRBD) column at same dynamics with its conventional counterpart. With this operating objective, the manipulation of a few process variables is proposed in open-loop fashion. Along with the energy savings and total annualized cost, the CO2 emission level is also used for quantitative performance evaluation of the proposed VRBD column. Aiming to produce a constant and high-purity distillate product, a gain-scheduled proportional integral (GSPI) controller that targets to keep the stability margin constant is devised for the VRBD process. Finally, this heat integration mechanism is illustrated by a binary batch distillation example in both open-loop and closed-loop modes. It is investigated that the VRBD shows a slightly higher energy savings (62%) compared to its closed-loop version (59.13%) but at the expense of distillate purity. The heat integration also achieves a significant savings in cost and CO2 emission.

Modularisation of Microstructured Separation Devices as a Method of Scale‐up using the Example of Distillation

AbstractMikrostrukturierte Apparate werden seit geraumer Zeit als ein Ansatz zur Prozessintensivierung im Bereich der Reaktionstechnik diskutiert. Immer öfter wird in diesem Zusammenhang als Hemmnis für eine erfolgreiche Implementierung der Mikroverfahrenstechnik in Labor und Technik das Fehlen von Komponenten aus dem Bereich der Trenntechnik genannt. Dieser Beitrag stellt anhand von theoretischen Überlegungen am Beispiel der Destillation die Potenziale der Mikrotrenntechnik dar.

Synthesis of dividing-wall columns (DWC) for multicomponent distillations—A systematic approach

Dividing-wall columns (DWC) are intensified distillation systems for multicomponent separations. They have the potential to save significantly both energy and capital costs than conventional simple column configurations. In this paper, it is shown that the DWC columns can be systematically generated from the conventional simple column configurations. Because of the simple column sequences with sharp splits are the simple and widely studied conventional schemes for multicomponent distillation, the purpose of this work is to formulate a procedure for systematic synthesis of DWC columns for such simple conventional schemes. A four-step procedure is formulated which systematically generates all the possible DWC columns from the simple column sequences. First, the subspace of the original thermally coupled configurations corresponding to the simple column configurations is generated. Then, the subspace of the thermodynamically equivalent structures corresponding to the original thermally coupled configurations is produced. Finally, the subspace of the DWC columns corresponding to the thermodynamically equivalent structures is achieved. An example of quaternary distillation is used to illustrate the synthesis procedure which is applicable to a mixture with any number of components.

Dynamic Simulation of Continuous-Contact Separation Processes with the Moment Transformation Method

A model for the simulation of continuous-contact separation processes, based on heat and energy balances, multicomponent mass transfer, and chemical engineering correlations for mass and heat transfer coefficients and liquid holdup is developed and solved with the moment transformation method. Polynomials are used to approximate the concentration and flow rate profiles within the packing. The moment transformation method is used to transform the partial differential equations into a system of differential-algebraic equations that is solved with Newton’s method. The model can be used for both steady-state and dynamic simulation, and it can also be used for systems with axial dispersion. The features of the model are demonstrated with an example of ternary distillation.

A new approach to entropy production minimization in diabatic distillation column with trays

Previous approach to direct numerical minimization of entropy production in diabatic distillation column in order to determine heat quantity to be exchanged at trays was based on temperatures on trays as control variables and it was applied only to simple binary columns. Also, previously developed theoretical models for determining optimal exchanged heat profile were determined only at such columns and while they were approximated they produced worse results than numerical minimum of entropy production. In this paper, as control variables for minimization, exchanged heat on the trays is used. It enables application to complex multicomponent diabatic columns. Ishii-Otto global method, based on model linearization and iterative solution by Newton-Raphson technique, is applied for solving column mathematical model. Needed thermodynamical properties for ideal systems are calculated using Lewis-Randall ideal solution model, and for non-ideal slightly polar systems they are calculated using Soave equation of state. Five direct methods are used for numerical optimization. Applied approach is successfully demonstrated at frequently used example of distillation of benzene and toluol mixture by using for these purposes specially written program. Simplex method appeared to be the most convenient optimization method for the considered problem.

A Coupled Strategy for the Solution of NLP and MINLP Optimization Problems: Benefits and Pitfalls

This paper presents a strategy for the solution of nonlinear programming (NLP) and mixed-integer nonlinear programming (MINLP) problems, based on the coupling of the equation-oriented simulator ASCEND IV with the stochastic optimizers MSGA and MSIMPSA. Both NLP and MINLP formulations of a reactive distillation example were explored. The results show that the connection between the two software blocks was successfully established. Despite the highly nonlinear behavior of the proposed example, the results presented generally agree with those previously obtained for the same case study using other approaches, namely, SIMOP, which is a FORTRAN 77-based NLP simulation tool developed for optimization problems. Increasing the column dimension or switching the vapor−liquid equilibrium (VLE) description to a nonideal situation, which produces nonlinear models with a much larger number of equations that must be solved simultaneously, show that the results deteriorate. This is attributed to the lack of user control over the subsystems’ structures (viz, tearing variables). This problem was partially circumvented through the application of a more elaborate scheme initialization for the column variables.

Robust control design for multivariable plants with time-delays

Abstract This paper proposes an extension of CRONE control and design for unstable, multiple time-delay multivariable plants with right half-plane (RHP) zeros. The CRONE control approach developed for multivariable square plants is based on third generation scalar CRONE methodology. The aim is to find a decoupling and stabilizing controller for the open-loop transfer matrix. Fractional order transfer functions are used to define all the components of the diagonal open-loop transfer matrix, β 0 i . Optimization gives the best fractional open-loop transfer matrix, β . Finally, frequency-domain system identification is used to find a robust controller K = G −1 β . When the plant is inversed, time-delays and RHP poles or zeros can appear on the denominator of the transfer matrix, G −1 . To achieve a stable controller some time-delays, RHP zeros and unstable poles must be included within the diagonal transfer function of the open-loop transfer matrix. To assess the proposed design, the CRONE control approach is applied to a distillation example.

A two‐level distillation design method

AbstractRecently, Lucia et al. have used a distillation line method to develop the concept of shortest stripping line distance approach to minimum energy designs of distillation columns and multiunit processes. It is well known that distillation line methods can be very sensitive to specified product compositions. A two‐level distillation design procedure is proposed for finding portfolios of minimum energy designs when specifications are given in terms of key component recoveries. Thus, product compositions are not specified but calculated. It is shown that the proposed two‐level design procedure is flexible and can find minimum energy designs for both zeotropic and azeotropic distillations. It is also shown that the two‐level design method encompasses Underwood's solution but can find minimum energy designs when Underwood's method fails. Numerical results for several distillation examples involving ternary and quaternary mixtures are presented to support these claims, and geometric illustrations are used to elucidate key points. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Non-pinched, minimum energy distillation designs

Non-pinched, minimum energy solutions are important class of distillation designs that offer the potential advantage of a better trade-off between capital investment and operating costs. In this paper, two important tasks associated with non-pinched distillation designs are studied. Thus the novel contributions of this work to the literature are (1) A comprehensive methodology for finding non-pinched minimum energy designs. (2) Understanding of the reasons for the existence of non-pinched distillation designs. It is shown that the recent shortest stripping line distance approach of Lucia et al. [Lucia, A., Amale, A. and Taylor, R., 2007, Distillation pinch points and more. Comput Chem Eng, available on-line] is capable of systematically and reliably finding non-pinched, minimum energy distillation designs. In addition, we provide an understanding of the reasons behind the existence of non-pinched designs, which include trajectories that follow unstable branches of a pinch point curve in azeotropic systems, the inherent looping structure of trajectories in hydrocarbon separations, and the presence of ancillary constraints in multi-unit processes like extraction/distillation. Several distillation examples are studied and many numerical results and geometric illustrations are presented that show the shortest stripping line distance methodology is indeed a powerful and systematic tool for computing non-pinched, minimum energy designs and that support the underlying reason we provide for the existence of non-pinched designs.

Water desalination using ceramic membrane distillation

In order to avoid hydrophilic behaviour of ceramic membranes prepared with metal oxides, alumina and zirconia membranes with 200 and 50 nm pore diameters were grafted with fluoroalkylsilanes. The grafting was controlled by TGA, and the hydrophobicity was characterised by water contact angle and permeability measurements. Results show that surfaces have hydrophobic behaviour with high values of contact angle. Grafted membranes were characterised by measuring the outlet pressure. The hydrophobic membranes were tested in a membrane distillation process. NaCl solution was used as the salt solution. This is the first example of membrane distillation using ceramic membranes.

Solving distillation problems by terrain methods

This paper clearly shows that the recently proposed terrain methodology of Lucia and Yang (2003) can be used to solve steady-state distillation problems in a reliable and efficient manner. Numerical results are presented that show that terrain methods are superior to Newton’s method and homotopy-continuation for distillation examples with multiple solutions.

Recent advances in optimization-based simultaneous process and control design

In this work, we first discuss recent advances towards the integration of process design, process control and process operability from the open literature and then we focus on techniques towards this endeavor that were developed within our group at Imperial College. While most of the approaches employ controllability measures to achieve this goal, our developments can be classified as a simultaneous process and control design methodology. Based on novel mixed integer dynamic optimization algorithms, our strategy features high fidelity process dynamic models, conventional PI control schemes, explicit consideration of structural process and control design aspects (such as number of trays, pairing of manipulated and controlled variables) through the introduction of 0–1 variables, and explicit consideration of time-varying disturbances and time-invariant uncertainties. The application of this strategy to a typical distillation system is discussed. In the second part of this chapter we present an extension of the process and control design framework that incorporates advanced model-based predictive controllers. Parametric programming is used for the controller derivation giving rise to a closed-form controller structure and removing the need for solving an optimization problem on-line. The resulting parametric controller is readily incorporated in the design optimization framework bringing about significant economic and operability benefits. The key features and advantages of this approach are highlighted via a simple binary distillation example.

An MPEC formulation for dynamic optimization of distillation operations

We consider the dynamic optimization of chemical processes with changes in the number of equilibrium phases. Recent work has shown that transitions in the number of phases can be modeled as a mathematical program with equilibrium constraints (MPEC). This study generalizes the MPEC to consider dynamic characteristics. In particular, we describe a simultaneous discretization and solution strategy for dynamic optimization problems with complementarity constraints. These discretized problems are then solved with IPOPT-C, a recently developed barrier method for MPEC problems. Our approach is applied to two distillation examples. In the first, we consider the optimal startup of a binary batch distillation problem. In the second, we consider the dynamic operation of a cryogenic column for the separation of natural gas liquids. Both cases demonstrate the effectiveness of the approach on large scale MPEC problems.

An MPEC formulation for dynamic optimization of distillation operations

We consider the dynamic optimization of chemical processes with changes in the number of equilibrium phases. Recent work has shown that transitions in the number of phases can be modeled as a mathematical program with equilibrium constraints (MPEC). This study generalizes the MPEC to consider dynamic characteristics. In particular, we describe a simultaneous discretization and solution strategy for dynamic optimization problems with complementarity constraints. These discretized problems are then solved with IPOPT-C, a recently developed barrier method for MPEC problems. Our approach is applied to two distillation examples. In the first, we consider the optimal startup of a binary batch distillation problem. In the second, we consider the dynamic operation of a cryogenic column for the separation of natural gas liquids. Both cases demonstrate the effectiveness of the approach on large scale MPEC problems.