Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.     

Reactive column profile map topology: Continuous distillation column with non-reversible kinetics

In this paper we present a topologically based approach to the analysis and synthesis of reactive distillation columns. We extend the definition of Tapp et al. [Tapp, M., Holland, S., Glasser, D., & Hildebrandt, D. (2004). Column profile maps part A: Derivation and interpretation. Industrial and Engineering Chemistry Research, 43, 364–374] of a column section in non-reactive distillation column to a reactive column section (RCS) in a reactive distillation column. A RCS is defined as a section of a reactive distillation column in which there is no addition or removal of material or energy. We introduce the concept of a reactive column profile map (RCPM) in which the profiles in the RCPM correspond to the liquid composition profiles in the RCS. By looking at the singular points in the RCPM, it is demonstrated that for a single chemical reaction with no net change in the total number of moles, the bifurcation of the singular points depends on both the difference point as introduced by Hauan et al. [Hauan, S., Ciric, A. R., Westerberg, A. W., & Lien, K. M. (2000). Difference points in extractive and reactive cascades I-Basic properties and analysis. Chemical Engineering Science, 55, 3145–3159] as well as the direction of the stoichiometric vector. These two vectors combine to define what we call the reactive difference point composition. We show that there only certain feasible topologies of the RCPM and these depend only on the position of the reactive difference point composition. We look at a simple example where the vapour liquid equilibrium (VLE) is ideal and show that we can classify regions of reactive difference point compositions that result in similar topology of the RCPM. Thus, by understanding the feasible topologies of the RCPM, one is able to identify profiles in the RCPM that are desirable and hence one is able to synthesize a reactive distillation column by combining RCS that correspond to the desired profile in the RCPM.We believe that this tool will help understand how and when reaction could introduce unexpected behaviors and this can be used as a complementary tool to existing methods used for synthesis of reactive distillation columns.     

Droplet coalescence model optimization using a detailed population balance model for RDC extraction column

Single droplet experiments in a small lab scale Rotating Disk Contactor (RDC) for two different liquid–liquid systems were used to evaluate the coalescence parameters necessary for column simulations. Five different coalescence models are studied; the models parameters were obtained by an inverse solution of the population balance model using the extended fixed-pivot technique for the discretization of the droplet internal coordinate. The estimated coalescence parameters by solving the inverse problem were found dependent on the chemical test system. The Coulaloglou and Tavlarides model was found to be the best model to predict the experimental data for both test systems. These parameters were used to study the hydrodynamics and mass transfer behavior of pilot plant RDC extraction column using the simulation tool LLECMOD. This is performed for two different liquid–liquid systems as recommended by the European Federation of Chemical Engineering (EFCE) (butylacetate–acetone–water (b–a–w) and toluene–acetone–water (t–a–w)). The simulated Sauter mean droplet diameter, hold-up values and concentration profiles for organic and aqueous phase were found to be well predicted compared to the experimental data.     

反应/分离集成系统塔板组成线设计法研究

反应/分离系统的集成是对现有工业过程进行强化的主要途径之一。文中在精馏塔板组成线的基础上,推导出了反应精馏塔板组成线基本方程及设计可行性判据,提出了给定进料和一个目标产品组成的塔板组成线反应精馏设计的新策略,并对非理想性较强的MTBE反应体系进行了实例设计计算。结果表明,提出的设计策略能同时获得多组可行设计方案,可有效地对单一和集成型反应精馏塔进行设计,结果与Aspen P lus的模拟结果吻合较好。     

Activity evaluation of a catalytic distillation packing for MTBE production

The octane enhancer methyl tertiary butyl ether (MTBE) can be produced very efficiently from methanol and isobutene in a reactive distillation column where the heterogeneous catalyst also acts as distillation packing. Some mathematical models have been published for the simulation of such a process but they focus on the physical transport processes between the vapour and liquid phases. However, the aim of this paper is to analyze the importance of the internal and external multicomponent mass and heat transfer phenomena on the catalyst under boiling conditions. Therefore, experiments were carried out in a reactive distillation column at different compositions of feed, column pressures and reflux ratios using a Raschig ring shaped acidic ion exchange resin as the catalyst. The temperature and composition of the liquid phase entering and leaving the catalytic column section were measured. These data were used to evaluate the effectiveness factor of the catalyst with a rigorous macrokinetic model. It is shown that the effectiveness factor varies significantly along the column length. Under certain operating conditions, decomposition of MTBE can occur accompanied by boiling processes inside the catalyst macropores.     

Transesterification of methyl myristate in a continuous reactive distillation column: simulation and experiment

The transesterification of methyl myristate with isopropanol to methanol and isopropyl myristate is presented. This reactive system is suitable for reactive distillation and is used for the investigation carried out in this paper. A simulation model for the steady state and dynamic simulation of the reactive distillation processes was developed. For the validation of this model experiments were carried out. Based on dimensioning calculations, a 6 meter high reactive distillation column (0.1 m i.d.) with 22 bubble cap trays and a total liquid holdup of about 10 litres was constructed and built up. The stationary results obtained from the column show a good comparison between the simulated and the experimental concentration and temperature profiles. The experimentally determined dynamic time constants and sensitivities of the temperature profile of the column could be reproduced very well by the simulation tool developed.     

Analysis of packed distillation columns with a rate-based model

Multicomponent packed column distillation is simulated using a rate-based model and the simulation results are compared with the experimental results obtained from a 0.2 m diameter pilot-scale packed column. The simulation algorithm used is previously proposed by the authors, which based on an equation-tearing method for (6c+7) equations of one packing segment and the whole column is solved by an iterative segmentwise calculation with the overall normalized θ method for acceleration. The performance of two packings is examined by simulating the pilot-scale column experiments using the published correlations for estimating liquid and vapor phase mass transfer coefficients and an effective interfacial area.     

Investigation of ethyl acetate reactive distillation process

In this paper, an extensive study of ethyl acetate synthesis by homogeneously catalyzed reactive distillation is presented. Reactive distillation is a promising operation whereby reaction and separation take place within a single distillation column. The synergistic effect of this combination has the potential to increase conversion, improve selectivity and facilitate separation tasks. The feasibility of ethyl acetate synthesis is examined using the reactive distillation lines diagram. A completely rate-based simulator DESIGNER developed within the framework of a large European research project is used in order to predict concentrations, temperatures and other important process variables. In order to validate theoretical predictions, a set of reactive distillation experiments is performed in a glass tray column with 80 bubble cap trays. The concentrations and temperature profiles computed by DESIGNER agree well with the experimental data.     

Production of ethyltert-butyl ether fromtert-butyl alcohol and ethanol catalyzed byβ-zeolite in reactive distillation

The synthesis of ethyl tert-butyl ether (ETBE) from a liquid phase reaction between tert-butyl alcohol (TBA) and ethanol (EtOH) in reactive distillation has been studied.β-Zeolite catalysts with three compositions (Si/Al ratio=13, 36 and 55) were compared by testing the reaction in a semi-batch reactor. Although they showed almost the same performance, the one with Si/Al ratio of 55 was selected for the kinetic and reactive distillation studies because it is commercially available and present in a ready-to-use form. The kinetic parameters of the reaction determined by fitting parameters with the experimental results at temperature in the range of 343–363 K were used in an ASPEN PLUS simulator. Experimental results of the reactive distillation at a standard condition were used to validate a rigorous reactive distillation model of the ASPEN PLUS used in a simulation study. The effects of various operating parameters such as condenser temperature, feed molar flow rate, reflux ratio, heat duty and mole ratio of H₂O : EtOH on the reactive distillation performance were then investigated via simulation using the ASPEN PLUS program. The results were compared between two reactive distillation columns: one packed withβ-zeolite and the other with conventional Amberlyst-15. It was found that the effect of various operating parameters for both types of catalysts follows the same trend; however, the column packed withΒ-zeolite outperforms that with Amberlyst-15 catalyst due to the higher selectivity of the catalyst.     

Quantitative comparison of dynamic controllability between a reactive distillation column and a conventional multi-unit process

A comparison of the steady-state economic optimum designs of two alternative chemical processes was presented in a previous paper [Kaymak, D. B., & Luyben, W. L. (2004). A quantitative comparison of reactive distillation with conventional multi-unit reactor/column/recycle systems for different chemical equilibrium constants. Industrial & Engineering Chemistry Research, 43, 2493–2507]. A generic exothermic reversible reaction A + B ↔ C + D occurs in both flowsheets, which consist of a conventional multi-unit reactor/separator/recycle structure and a reactive distillation column. Results showed that the reactive distillation process is significantly less expensive than the conventional process for a wide range of the chemical equilibrium constant when there is no mismatch between the temperature favorable for reaction and the temperature favorable for vapor–liquid separation.

A reactive distillation column has fewer control degrees of freedom than a conventional multi-unit system. Therefore a reactive distillation column may have worse dynamic response than a conventional process. The purpose of this paper is to compare the dynamic controllability of these two alternative processes.

Three different chemical equilibrium constants are considered. Several control structures are developed for each flowsheet, and their effectiveness is evaluated. Disturbances in production rate and fresh feed compositions are considered.

The conventional multi-unit process provides significantly better control. The operability region is much larger, there is less variability in product quality and the dynamic responses are faster than those of the reactive column. Thus, these results demonstrate that there is a significant trade-off in this system between optimum economic steady-state design and dynamic controllability.     

Simulation of continuous packed bed reactive distillation column for the esterification process using activity based kinetic model

A mathematical model for the continuous packed bed reactive distillation process of esterification of acetic acid with methanol is developed. The kinetic rate equation, which plays a major role for the performance of reactive distillation and it is the part of model, is required for the liquid phase reversible esterification reaction. The mineral sulphuric acid is used as the catalyst. The kinetic experiments are carried out under different temperatures in the range of 305.15 to 333.15 K and catalyst concentrations in the range of 0.1267 mole H⁺/lit to 0.6537 mole H⁺/lit. From that experimental data the kinetic model is developed and the same is used for the simulation of reactive distillation process. Equilibrium stage model, in which the vapour and the liquid leaving a stage are assumed to be in equilibrium with each other, has been used for the simulation of reactive distillation process by incorporating our kinetic model. Conversion of acetic acid as function of reflux ratio and reboiler ratio has been predicted. The liquid composition and temperature profiles versus stage number have been also predicted. Finally, the optimum operating conditions obtained from the simulation results for high pure methyl acetate by reactive distillation process.     

反应精馏过程中的多稳态分析

Reactive distillation processes for synthesis of ethylene glycol （EG） and ethyl tert-butyl ether （ETBE） were modeled with the simulation package ASPEN PLUS. The input multiplicity and output multiplicity were discussed with the method of sensitivity analysis for both cases. In EG production process, steady state multiplicities were studied in terms of effective liquid holdup volume and boil-up ratio. In ETBE synthesis process, the user kinetic subroutine was supplied into ASPEN PLUS firstly, and then the composition, temperature and reaction-rate profiles within the reactive distillation column were presented in detail. A set of stable solution branches based on distinct initial guesses for a range of boil-up ratio were found in EG synthesis. Input multiplicities were observed for a range of reboiler duty at several values of reflux ratio for ETBE synthesis process. These results can be used to avoid excessive energy consumption and achieve optimum design of reactive distillation column.     

反应精馏隔壁塔内合成乙酸甲酯的模拟

提出了一种应用反应精馏隔壁塔合成乙酸甲酯的新工艺流程,采用反应精馏隔壁塔替代常规反应精馏流程中的反应精馏塔及甲醇回收塔。利用Aspen Plus模拟软件,对反应精馏隔壁塔及常规流程进行了模拟,比较分析了两种流程塔内液相组成分布,并分析了塔顶回流比与气相分配比对反应精馏隔壁塔的影响。结果显示新流程可以节能11.9%,并能降低设备投资费用和操作费用。     

Pilot plant synthesis of n-propyl propionate via reactive distillation with decanter separator for reactant recovery. Experimental model validation and simulation studies

Successful process intensification (PI) stories applied broadly are the reactive distillation (RD) processes used in esterification syntheses. One application is developed here with the heterogeneously catalyzed synthesis of n  -propyl propionate (ProPro) from 1-propanol (ProOH) and propionic acid (ProAc). In this investigation, conventional RD of ProPro was further improved. With the objective to recover product and reactant, an experimental column set-up was equipped with a decanter on top enabling to separate the distillate product into two main streams. The aqueous phase was discharged and part of the organic phase was refluxed back to the column. Experimental results comprising temperature and composition column profiles were obtained in a pilot-scale column (DN-50), equipped with structured packings (Sulzer BX and Katapak-SP 11 with Amberlyst 46™ for the reactive part). For simulation studies a non-equilibrium stage model (NEQ model) was applied which shows satisfactory agreement with the performed experiments. Further theoretical investigations of relevant operating parameters (total feed, molar feed ratio, reflux ratio and heat duty) and their effect on the overall process performance were realized. Studies with the given column configuration showed that product purity in the bottom stream could be increased to wProPro,bottom=91.0%$w_{\text{ProPro,bottom}}=91.0\%$ and maximum ProAc conversions to X_ProAc = 94.5%.     

酯化反应蒸馏过程的模型化及模拟研究

In this paper,a generalized model of the reactive distillation processes was developed via rate-based approach. The homotopy-continuation method was employed to solve the complicated nonlinear model equations efficiently. The simulation on the reactive distillation processes was carried out with the profiles of stage temperature,composition and flow rate for both vapor and liquid phases obtained. Based on careful analysis of the simulation results, the pitfalls in experimental design were detected. Finally, a software package for the simulation of reactive distillation processes was developed.     

用新离子交换树脂催化剂优化连续催化精馏塔中乙基叔丁基醚的合成(英文)

Liquid phase synthesis of one of the important fuel oxygenate, ethyl tert-butyl ether （ETBE）, from etha-nol and tert-butyl alcohol （TBA） has been studied in catalytic distillation column （CDC） using ion exchange resin catalyst CT-145H. A packed CDC of 1.2 m height and 50 mm diameter with indigenously developed reactive sec-tion packing was used to generate experimental data. Effect of different key variables on product purity in distillate, was investigated to find the optimum operating conditions for ETBE synthesis. The optimum conditions for 0.2 kg·s-1 of ethanol feed were found：reboiler duty of 375 W, molar feed ratio of 1︰1.3 of reactants, and reflux ratio of 7. Concentration profiles for each component along each column section at optimum conditions were also drawn. Neither output nor input multiplicity was observed at experimental conditions.     

Vapour- and liquid-side volumetric mass transfer coefficients measured in distillation column. Comparison with data calculated from absorption correlations

Volumetric mass transfer coefficients in liquid and vapour phases in distillation column were measured by the method consisting of a fitting of the concentration profile of liquid phase along the column obtained by the integration of a differential model to the experimental one. The mathematical model of distillation process includes mass and energy balances and the heat and mass transfer equations. The film model flux expressions with the convective transport contributions have been considered in the transfer equations. Vapour and liquid phases are supposed to be at their saturated temperatures along the column. Effect of changes of phase flows and physical properties of phases on the mass transfer coefficients along the column and non-ideal thermodynamic behaviour of the liquid phase have been taken into account. The concentration profiles of liquid phase are measured in the binary distillation of the ethanol-water and methanol-ethanol systems at total reflux on metal Pall Rings and Intalox saddles 25 mm in the column with diameter of 150 mm. The distillation mass transfer coefficients obtained by the fitting procedure are compared with those calculated from absorption data using Onda's, Billet's and Linek's correlations. The distillation heat transfer coefficients calculated from the model assuming saturated temperatures in both phases are compared with those calculated from the Chilton-Colburn and penetration model analogy between mass and heat transfer. The results have confirmed an agreement neither between distillation and from absorption correlations calculated mass transfer coefficients nor between analogy and from enthalpy balance calculated heat transfer coefficients. Also the concentration profiles obtained by the integration of the differential model of the distillation column using the coefficients from absorption correlation have differed from the experimental profiles considerably.     

APPLICATION OF MULTIVARIABLE GENERALIZED PREDICTIVE CONTROL TO A PACKED DISTILLATION COLUMN

The present work deals with the application of Multivariate Generalized Predictive Control (MGPC) systems to a packed distillation column. The steady-state and dynamic behaviour of the system have been simulated using two film plug flow model. The model solutions have been obtained employing orthogonal collocation on finite element. The Jacobi polynomials within the finite element procedure was tested to determine the phase flow rates, the liquid and vapour composition profiles and the temperature profiles. All the theoretical results were compared with experimental data obtained from a pilot-plant packed distillation column distilling methanol-water mixture. Decoupling and MGPC control of overhead and/or bottom compositions were examined. Perturbation in feed composition and, reflux ratio and the reboiler heat duty were utilized as the disturbance and the manipulated variables respectively. Performance of these systems was tested by using an integral square of error (ISE and IAE) criterion.     

DYNAMIC BEHAVIOR OF REACTIVE DISTILLATION COLUMNS. EQUILIBRIUM SYSTEMS

This paper deals with the dynamic behavior of simultaneous reaction-separation systems which operate at or near the chemical equilibrium in the liquid phase. The process under study comprises a whole set of “instantaneous”, and very fast reversible reactions where the difference in volatilities favors both the progress of reaction and product separation. The main aim of our study is to gain a deeper understanding of the dynamic behavior of distillation columns by using a model that is simple and efficient, yet informative. This kind of model is outstanding for synthesis and design of control schemes which require a careful modelling and understanding of process response to different changes in the environment. We use a suitable transformation of variables (after Barbosa and Doherty, 1988b) in order to define a new set of state variables; as a result, the balance equations become identical to those for conventional distillation. Also, an efficient physicochemical algorithm that can handle both the original and new state variables is used. Thus, a composition-holdup dynamic model is simulated in the “transformed field” using a stage-by-stage approach. To further reduce computational time, the transformed problem has also been solved by means of a reduction procedure based on approximating by orthogonal polynomials the transformed composition and flow profiles in the column. The performance of the two proposed methods are compared by using the top section of a quaternary reactive column. The results obtained shown that reactive distillation dynamics has certain peculiarities derived from superimposing reaction and separation phenomena.     

Synthesis of dimethyl ether (DME) by catalytic distillation

The intrinsic kinetics of liquid phase catalytic dehydration of methanol to dimethyl ether over a macroporous sulphonic acid ion exchange resin was determined in a fixed-bed micro-reactor in the temperature range of 391–423 K and pressures up to 2.0 MPa. The kinetic model based on Eley–Rideal mechanism, as well as the power-rate law model, was adopted for fitting the experimental data. However, the Langmuir–Hinshelwood mechanism is not feasible for describing the dehydration reaction of methanol, as deduced from the macroscopic kinetic data and/or no dependence of methanol conversion on initial methanol concentration in the absence of water at the inlet using acetone as inert solvent. Moreover, an improved process consisting of the combination of a fixed-bed reactor and a catalytic distillation column for the synthesis of DME (Process A) was proposed, and a mathematical model was established, into which the intrinsic kinetics obtained in this work was incorporated. The comparison of operating performance among the improved process, Process B consisting of a fixed-bed reactor and two ordinary distillation columns, and Process C consisting of a catalytic distillation column and an ordinary distillation column was also made. It was found that the improved process is more promising than others in energy consumption, production capacity and column number under the same product purity, and is easy to be implemented based on Process B that is currently used in the actual industrial plants with a long catalyst lifetime.