Influence of the reactive distillation column configuration on its performance: A computational study

Comparison of the performance of a reactive distillation column with three different hardware configurations is presented. As a reaction system the methyl tertiary-butyl ether (MTBE) synthesis has been chosen. The sieve tray columns with catalyst (encased inside wire gauze envelopes) placed along the liquid flow path differ in the number of reactive trays. The column simulations have been performed using the nonequilibrium model. The steady state behaviour of the three different hardware configurations was studied regarding the three input parameters; feed flow rate of methanol, feed flow rate of butenes, and reflux ratio. It has been shown that by varying the location of the methanol feed stage, the columns exhibit significantly different solution diagrams using the butenes feed flow rate as a continuation parameter. Using dynamic simulations, different perturbations of the manipulated variables were found to cause transitions between multiple steady states and these were also investigated. The major objective of this paper is to demonstrate the importance of the hardware choice in the performance of a reactive distillation column e.g. during the start-up or if occasional variations of the operating parameters occur. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

Theoretical study on transesterification in a combined process consisting of a reactive distillation column and a pervaporation unit

Steady state analysis of a combined hybrid process consisting of a reactive distillation column, pervaporation unit, and a distillation column is presented. This process configuration was first presented by Steinigeweg and Gmehling (2004) for the transesterification of methyl acetate and butanol to butyl acetate and methanol. This system is characteristic for its low reaction rate and complex phase equilibrium. Steinigeweg and Gmehling (2004) have shown that the combination of reactive distillation and pervaporation is favourable since conversions close to 100 % can be reached with a reasonable size of the reactive section in the reactive distillation column. The aim of this paper is to show that although high conversion can be achieved, very complicated steady state behaviour must be expected. The presented analysis is based on mathematical modelling of a process unit, where the steady-state analysis, including continuation and bifurcation analyses, was used. Multiple steady states were predicted for the studied system; three steady states with conversions higher than 98 %. However, not all predicted steady states met the maximal allowed temperature condition in the reactive section (catalyst maximal operation temperature of 393 K). The presence of multiple steady states reduces the operability and controllability of the reactive distillation column during its start-up and during the occurrence of any variation of operating parameters because the system can be shifted from one steady state to another one (concurrent exceeding the maximal allowed temperature) with unwanted consequences, e.g. production loss. Therefore, design and subsequent operation of such a complicated system is an ambitious task requiring knowledge of any possible system behaviour.     

Relationship between fine structure of polyoxymethylene copolymer plates and methanol transport properties

An experimental study was made of diffusion behavior of methanol through three kinds of injection‐molded plates of a polyoxymethylene (POM) copolymer with different molecular weights M at 60 °C. Fine structure of the three sample plates was also examined by wide‐angle X‐ray diffraction and small‐angle X‐ray scattering, and moreover, their dynamic properties were investigated by the dynamic mechanical analysis (DMA). It is shown that the diffusion behavior may be well explained by the one‐dimensional Fick diffusion equation with a constant diffusion coefficient, and that the steady‐state transport rate increases with increasing M. As for fine structure, the crystallinity decreases slightly, and the preferential orientation and the long period increase, with increasing M. The long period of the lamellar stacking structure increases with increasing M, and it also increases with methanol transport. In DMA, the loss tangent tan δ becomes higher after the methanol transport in the wide range of temperature around the glass transition one. These results indicate that amorphous regions serve as channels for methanol molecules in the lamellar stacking structure, leading to the conclusion that the dependence of the steady‐state transport rate on M arises from the factors of crystallinity and long period. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1234–1242, 2007     

Solubility and preferential solvation of phenacetin in methanol + water mixtures at 298.15 K

The mole fraction solubility of phenacetin (PNC) in methanol + water binary solvent mixtures at 298.15 K was determined along with density of the saturated solutions. All these solubility values were correlated with the Jouyban–Acree model. Preferential solvation parameters of PNC by methanol (δx_1,3) were derived from their thermodynamic solution properties using the inverse Kirkwood–Buff integrals (IKBI) method. δx_1,3 values are negative in water-rich mixtures but positive in methanol mole fraction of >0.32. It is conjecturable that in the former case the hydrophobic hydration around non-polar groups of PNC plays a relevant role in the solvation. The higher solvation by methanol in mixtures of similar cosolvent compositions and methanol-rich mixtures could be explained in terms of the higher basic behaviour of methanol.     

Solubility and preferential solvation of benzocaine in {methanol (1) + water (2)} mixtures at 298.15 K

The equilibrium solubility of benzocaine (BZC) in several {methanol (1) + water (2)} mixtures at 298.15 K was determined. Solubility values are expressed in mole fraction and molarity and were calculated with the Jouyban–Acree model. Preferential solvation parameters of BZC by methanol (δx_1,3) were derived from their thermodynamic solution properties using the inverse Kirkwood–Buff integrals method. δx_1,3 values are negative in water-rich mixtures (0.00 < x₁ < 0.32) but positive in the other mixtures (0.32 < x₁ < 1.00). To explain the preferential solvation by water in the former case, it is conjecturable that the hydrophobic hydration around non-polar groups of BZC plays a relevant role in the solvation. Moreover, the higher solvation by methanol in mixtures of similar cosolvent compositions and methanol-rich mixtures could be explained in terms of the higher basic behaviour of methanol regarding water.     

HAZOP study of a fixed bed reactor for MTBE synthesis using a dynamic approach

A methodology for hazard investigation based on the integration of a mathematical model approach into hazard and operability analysis is presented. This approach is based on mathematical modelling of a process unit where both steady-state analysis, including analysis of the steady states multiplicity and stability, and dynamic simulation are used. The dynamic simulation serves for the investigation of consequences of failures of the main controlled parameters, i.e. inlet temperature, feed temperature and feed composition. This simulation is also very useful for the determination of the influence of failure duration on the reactor behaviour. On the other hand, the steady state simulation can predict the reactor behaviour in a wide range of failure magnitude and determine the parametric zones, where shifting from one steady state to another one may occur. A fixed bed reactor for methyl tertiary-butyl ether synthesis was chosen to identify potential hazard and operational problems of a real process. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

Numerical Simulation of the Effect of Solvent Viscosity on the Motions of a β‐Peptide Heptamer

This report examines the effect of a decrease in solvent viscosity on the simulated folding behaviour of a β‐peptide heptamer in methanol. Simulations of the molecular dynamics of the heptamer H‐β³‐HVal‐β³‐HAla‐β³‐HLeu‐(S,S)‐β³‐HAla(αMe)‐β³‐HVal‐β³‐HAla‐β³‐HLeu‐OH in methanol, with an explicit representation of the methanol molecules, were performed for 80 ns at various solvent viscosities. The simulations indicate that at a solvent viscosity of one third of that of methanol, only the dynamic aspects of the folding process are altered, and that the rate of folding is increased. At a viscosity of one tenth of that of methanol, insufficient statistics are obtained within the 80 ns period. We suggest that 80 ns is an insufficient time to reach conformational equilibrium at very low viscosity because the dependence of the folding rate of a β‐peptide on solvent viscosity has two regimes; a result that was observed in another computational study for α‐peptides.     

从多重定态经复杂奇点向极限环型化学振荡的跃迁

在文Ⅰ~[1]的相平面分析的基础上, 利用线性稳定性分析和数值模拟的方法进一步分析了Schlögl模型在CSTR中的行为, 发现在适当条件下可以发生从定态向振荡态的不连续的突然跃迁现象。认为这种突然跃迁现象是通过鞍点-结点型复杂奇点的形成和消失而实现的。分析了这种鞍点-结点型跃迁与亚临界Hopf分支现象的差别, 并讨论了这种跃迁现象和实验上发现的某些突变型化学振荡现象之间的联系。     

Effect of immobilization on the main dynamic characteristics of the enzymatic oxidation of methane to methanol by bacteria <Emphasis Type="Italic">Methylosinus sporium</Emphasis> B-2121

The main dynamic characteristics of biochemical methanol formation by the oxidation of methane using a biocatalyst were studied. The biocatalyst is based on cells of bacteria Methylosinus sporium B-2121, both suspended in a medium and immobilized in the poly(vinyl alcohol) cryogel. The change in the methane concentration and the biocatalyst amount affects the productivity of the system, the maximal concentration of methanol in the cultural liquid, and the rate of methanol accumulation. The most part of the dynamic characteristics are described by extremal curves. The experimental conditions were optimized prior to experiments. The use of the immobilized biocatalyst makes it possible to enhance the productivity of the process more than fivefold compared to that of the free cells and to achieve the highest methanol concentration in the medium: 62±2 mg L⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1603–1606, August, 2008.     

Theory and computational methods for studies of nonlinear phenomena in laser spectroscopy. I. General formalism

A general formalism will be outlined, which uses steady-state wave functions for the study of nonlinear phenomena occurring in molecular systems interacting with intense electromagnetic fields. The steady-state approach has the advantage of being free from the secular divergencies and normalization terms which appear in a perturbation expansion of the time-dependent wave function. A physical interpretation of the steady states will be given by considering the interaction between the molecule and the quantized electromagnetic field. The steady states appear as states of the combined system molecule plus electromagnetic field, with eigenvalues corresponding to the energy levels of the combined system in a semiclassical approximation. Evolution operators will be introduced and used to derive formulas for n-photon transition probabilities between molecular states both in the ordinary configuration Hilbert space and in the composite Hilbert space spanned by the steadystate wave functions.     

Solubility and preferential solvation of some non-steroidal anti-inflammatory drugs in methanol + water mixtures at 298.15 K

The equilibrium solubilities of naproxen (NAP), ketoprofen (KTP), and ibuprofen (IBP) in methanol + water binary mixtures at 298.15 K were determined and the preferential solvation parameters were derived by means of the inverse Kirkwood–Buff integrals (IKBI) method. These drugs are very sensitive to specific solvation effects. The preferential solvation parameters by methanol δx_1,3 are negative in water-rich mixtures but positive in compositions from 0.32 in mole fraction of methanol to pure methanol. It is conjecturable that in the former case the hydrophobic hydration around aromatic rings and/or methyl groups plays a relevant role in the solvation. The higher solvation by methanol in mixtures of similar co-solvent compositions and in methanol-rich mixtures could be explained in terms of the higher basic behaviour of this co-solvent interacting with the hydroxyl group of the drugs. Moreover, drug solubilities were correlated by using the modified nearly ideal binary solvent/Redlich–Kister model obtaining average percentage deviations (APDs) lower than 9.0%.     

Mode-Coupling Theory for Glass Transition of Active-Passive Binary Mixture?

Collective behaviours of active particle systems have gained great research attentions in recent years. Here we present a mode-coupling theory (MCT) framework to study the glass transition of a mixture system of active and passive Brownian particles. The starting point is an effective Smoluchowski equation, which governs the dynamics of the probability distribution function in the position phase space. With the assumption of the existence of a nonequilibrium steady state, we are able to obtain dynamic equations for the intermediate scattering functions (ISFs), wherein an irreducible memory function is introduced which in turn can be written as functions of the ISFs based on standard mode-coupling approximations. The effect of particle activity is included through an effective diffusion coefficient which can be obtained via short time simulations. By calculating the long-time limit of the ISF, the Debye-Waller (DW) factor, one can determine the critical packing fraction η_c of glass transition. We find that for active-passive (AP) mixtures with the same particle sizes, η_c increases as the partial fraction of active particle x_A increases, which is in agreement with previous simulation works. For system with different active/passive particle sizes, we find an interesting reentrance behaviour of glass transition, i.e., η_c shows a non-monotonic dependence on x_A. In addition, such a reentrance behaviour would disappear if the particle activity is large enough. Our results thus provide a useful theoretical scheme to study glass transition behaviour of active-passive mixture systems in a promising way.     

Tricoordinate Coinage Metal Complexes with a Redox-Active Tris-(Ferrocenyl)triazine Backbone Feature Triazine–Metal Interactions

2,4,6-Tris(1-diphenylphosphanyl-1’-ferrocenylene)-1,3,5-triazine ( 1 ) coordinates all three coinage metal(I) ions in a 1:1 tridentate coordination mode. The C₃-symmetric coordination in both solid state and solution is stabilised by an uncommon cation–π interaction between the triazine core and the metal cation. Intramolecular dynamic behaviour was observed by variable-temperature NMR spectroscopy. The borane adduct of 1 , 1BH₃ , displays four accessible oxidation states, suggesting complexes of 1 to be intriguing candidates for redox-switchable catalysis. Complexes 1Cu , 1Ag , and 1Au display a more complicated electrochemical behaviour, and the electrochemical mechanism was studied by temperature-resolved UV/Vis spectroelectrochemistry and chemical oxidation.     

基于TiO2-Y2O3粉体催化发光甲醇气体传感器的研究

发现当甲醇气体通过TiO₂-Y₂O₃ (质量比为3∶1)粉体表面时, 可被空气中的O₂催化氧化产生强烈的化学发光, 基于此研制了一种新型的甲醇气体传感器. 此传感器对甲醇的检测具有较高灵敏度和较强的选择性. 在波长490 nm处进行定量分析, 催化发光强度与甲醇浓度在一定范围内呈良好的线性关系, 其线性范围为25.74～12870 mg/m³ (r＝0.9995, n＝8); 检出限为8.58 mg/m³(信噪比＝3). 外来物质如正己烷、三氯甲烷、苯、无水乙醇、甲醛、丙酮、氨、甲苯与甲醇共存时, 除了丙酮、乙醇和氨分别引起干扰外, 苯与其它气体不干扰测定. 该传感器工作时间可持续80 h以上, 是一种长寿命的、性能稳定的气体传感器, 并成功地实现了对甲醇气体的实时在线检测.     

Oscillations and waves in the Belousov-Zhabotinskii reaction in a finite medium

Travelling reaction-diffusion waves are considered in a finite medium. The model considered is a simplified model of the Belousov-Zhabotinskii reaction, described mathematically by the two-variable Oregonator. A one-dimensional problem consisting of three regions is considered. Regions I (x 0) and III (x 1) act as reservoirs with fixed concentrations of the reactant X (the autocatalyst, hypobromous acid (HBrO₂)), where the concentration of X in regions I and III may be different. Region II represents a catalyst-loaded membrane, within which species X can diffuse while species Z (the oxidized form of the metal-ion catalyst (Ce(IV))) is fixed spatially. The large time behaviour of the system is considered and both stable steady states and periodic spatio-temporal structures are seen.     

Modeling the dynamic response of an activated sludge process

Previously collected data describing aerated synthetic waste water, treated in a continuous stirred-tank reactor, are analyzed to understand better the dynamic response to step changes in the dilution rate,D. Comparing a change inD between steady states leads to hysteresis trajectories on both the graph of specific growth rate, μ, vs limiting substrate level (S) and the graph of (S) vs the cell level (X). Qualitative differences between the three different monitored cases will be compared to simulations of simple models at various dilution level changes in order to gain understanding of the dynamics of the process.

     

Kinetic model ofCO oxidation on a nonuniform surface analyzed with regard toCOads spillover, III. Homotopic method. Effect of the reaction mixture composition

Homotopic method has been used to analyze the kinetic model of three-stageCO oxidation on two nonuniform surface patches conjugated byCO _ads spillover. The diagrams of steady states depending onP(CO) at various temperatures have been built.CO _ads spillover from one patch to another changes substantially the bifurcation picture of the steady states.     

Behaviour of Salbutamol on Diol Normal-Phase Column

Summary The behaviour of salbutamol, a sympathomimetic amine-and catecholamine-like substance, was studied on Diol normal-phase column. The possibility for controlled retention of salbutamol using methanol containing mobile phase was proved. The mobile phase consisted of buffer (0.05 M H₃PO₄, pH 5.0 with TEA) – methanol (15:85 v/v). The effects of organic solvent, pH, ion power of the buffer and the length of amine alkyl chain in buffer were studied. The retention of salbutamol increased significantly with the increase of methanol content above 80%. At pH values between 3.5 and 7.5 the retention of salbutamol varied from 2.16 to 2.36 only. The limit of quantitation was 0.30 ng · mL^–1. The investigations confirm the H-bonding retention mechanism.     

Macroscopic solvation of molecules in excited states: An MCSCF model including solvent polarization effects-I

An MCSCF model including the effects of solvent polarization is developed. The model is applied within the limitations of INDO approximations to look into the dominant effects of solvent polarization on the electronic structure in the excited states of a model system (e.g.nπ ^* states of H₂CO). Important features of macroscopic solvation-induced reorganization of electron density and some consequence thereof are noted.     

Steady state and dynamic simulation of a hybrid reactive separation process

Modelling and simulation of hybrid reactive separation system in steady state and in dynamic regime was carried out. The investigated hybrid process consisted of a reactive distillation column and a pervaporation membrane located in the distillate stream to remove water from the process. Heterogeneously catalysed esterification of propionic acid with propan-1-ol to propyl propionate and water was chosen as the model chemical reaction. Esterification reactions are a typical example of equilibrium-limited reactions producing water as a by-product. Using just a pervaporation membrane brings the biggest benefit in increasing the yield of one of the reactants due to the removal of water. To study reactive separation processes, a model of the hybrid system in steady state and in dynamic regime was developed. Steady state behaviour of the model was studied for different hybrid system configurations. The effect of catalyst amount doubling was also investigated. Dynamic behaviour of the system during the step changes of propionic acid feed flow rate and during the membrane module failure was investigated. For this reason, the conversion of propionic acid, purity of the product stream, mole fraction of water, and the temperature in three different parts of the reactive distillation column were monitored.