Industrial experience in the scale-up of reactive distillation with examples from C4-chemistry

The objective of this paper is to illustrate process design from the industrial point of view for a heterogeneous reactive distillation exemplified by the decomposition of MTBE. Based on thermodynamics a plausible column concept is suggested. Open questions concerning scale-up of structured catalytic packings are discussed on the basis of experiments in the lab and pilot plant scale. Lab scale experiments were modelled satisfactorily with an equilibrium stage approach. In order to perform the scale-up from lab to pilot scale with the equilibrium stage approach the reaction rate constant had to be reduced significantly. Incomplete catalyst wetting due to maldistribution effects or mass transfer phenomena might be possible reasons.     

Liquid flow and phase holdup—measurement and CFD modeling for two-and three-phase bubble columns

Bubble columns are an important class of contacting devices in chemical industry and biotechnology. Their simple setup makes them ideal reactors for two- and three-phase operations such as fermentations or heterogeneous catalysis. Still, design and operation of these reactors is subject to widely empirical scale-up strategies. With recent advances in the development of measurement techniques, a more detailed approach to the development of optimized reactors for specific operations should become possible. This report is based on detailed measurements of local dispersed phase holdups in a pilot plant-sized bubble column operated at high superficial gas velocities and solid holdups. It deals with the influence of superficial gas velocity, solid loading and sparger geometry on measured and computed liquid flow velocities and holdup distributions. Liquid velocity measurements have been performed using the electrodiffusion method, modeling calculations have been carried out using the computational fluid dynamics (CFD) code CFX-4.3. Measurement results presented here give an insight into the development of liquid circulation and fluctuating velocity distribution depending on superficial gas velocity, solid loading and sparger geometry. CFD results implementing a multi-fluid model with k-ε turbulence and special momentum exchange terms for direct gas-solid interactions show that, even on standard PC workstations, this kind of computations can deliver qualitatively reasonable agreement with measurements.     

Dispersion coefficient and settling velocity of the solids in agitated slurry reactors stirred with multiple rushton turbines

The feature of solids distribution in tanks stirred with multiple Rushton turbines was investigated. Both transient and steady-state experiments were performed in tanks of two scales with a variety of suspensions. The data were analysed with the axial sedimentation-dispersion model. The axial dispersion coefficient of the solid phase was found not to differ from that of the liquid by more than 20%. The effective particle settling velocity in the stirred medium was then determined. It is confirmed that this parameter is different from the terminal settling velocity. Their ratio exhibits the same dependence on Kolmogoroff microscale and particle size as obtained previously with an indirect, approximate approach.     

Stationary and traveling hot spots in the catalytic combustion of hydrogen in monoliths

In the course of catalytic combustion of hydrogen (1-5% H₂ in air) in monolith reactors, strongly localized stationary and traveling hot spots arise in response to a sudden and persistent rise of gas flow velocity. Such hot spots may occur, e.g. in a catalytic converter following the acceleration of a car or in a catalytic combustor as a result of a load increase. This phenomenon is illustrated by simulations using a two-phase reactor model. The temperature overshoot of the adiabatic limit is typically of the order of the adiabatic temperature rise itself.The following mechanism underlies this behavior. Light fuel is supplied to the catalytic wall by fast diffusion (in the direction perpendicular to flow), while the heat released by reaction is removed from the wall by the slower, mixture-averaged heat conduction. This leads to accumulation of heat at the catalytic surface that eventually saturates at high temperatures. The hot spots may exhibit intricate dynamics, propagating downstream or upstream, or they may remain stationary. The direction of propagation depends on the relative strength of convective downstream and conductive upstream contributions to the overall displacement of reaction fronts. Generally, the hot spot tends to drift downstream at low flow velocities, remain stationary at intermediate flow velocities, and drift upstream at high flow velocities.     

Symmetry properties and bifurcation analysis of a class of periodically forced chemical reactors

In this work, we discuss how periodic forcing may induce symmetry properties into mathematical models of chemical reactors. We define a class of reactors subjected to discontinuous periodic forcing, and show that all the reactors belonging to this class have spatio-temporal symmetry. This symmetry and its influence on the possible bifurcation scenarios are discussed. The bifurcation analysis is carried out with suitable discrete systems that exploit a property of the Poincaré map. In fact, it is shown that the spatio-temporal symmetry induced by the forcing makes the Poincaré map of the continuous system an iterate of another map. On this basis, a technique to implement parameter continuation methods is proposed. With such a technique, it is also possible to characterize symmetric and nonsymmetric regimes and unstable limit sets otherwise undetected with “bruteforce” approaches. Examples for reverseflow reactors and networks of n-reactors with periodically switched feed and discharge positions are presented.     

New Reaction Engineering Concepts for Selective Oxidation Reactions

In order to enhance product yields in selective oxidation reactions, numerous reaction engineering concepts are being studied worldwide. Periodic operation has been investigated for decades, yet its application is limited to a few examples, such as the butane oxidation after DuPont or reverse-flow reactors for VOC removal. The use of microchannel reactors is a younger field, but it has already yielded promising results for process optimization. Catalytic wall reactors have proved to be a helpful tool for kinetic studies. On the laboratory scale, membrane reactors have displayed favorable behavior in selective oxidation. The Na vapor-catalyzed dehydrogenation of methanol to formaldehyde is a final example of an unusual new concept for selective oxidation.     

甲苯氯化连串反应的RD和SRC过程分析与比较

侧反应器与精馏塔集成(SRC)是塔内耦合反应精馏(RD)的新拓展,同时适用于反应与精馏处于相同工况和不同工况两种情形。以甲苯氯化连串反应为研究对象,分别建立了RD和SRC过程的数学模型,在相同总塔板数和塔釜上升蒸气量条件下,计算了相同工况下(反应与精馏压力均为101.3 kPa),RD与SRC过程的气/液流率、温度、组成分布,发现两者性能具有一致性。在此基础上,以产品氯化苄的生产成本为目标函数、标志系统产能的氯气进料流率和分配比例为优化变量,对甲苯氯化连串反应的相同工况RD与SRC过程、不同工况SRC过程(反应压力为101.3 kPa、精馏压力为10 kPa)进行优化,结果表明,相同压力工况下,优化得到的RD与SRC的装置产能与产品质量一致;但在不同压力工况下,SRC的产能明显提升、产品质量也有所提高,反映出不同工况SRC明显的优势。     

GAS-LIQUID REACTORS WITH VOLATILE LIQUID REACTANTS: APPROXIMATE SOLUTION FOR CONSECUTIVE BIMOLECULAR REACTIONS OF GENERAL ORDER

An approximate solution of the film model in presence of volatile liquid reactants is presented for the case of a general van de Vusse-like reaction scheme and general power-law reaction rates. The solution is obtained in a closed-form and simply requires the numerical solution of a system of nonlinear equations. The developed approximate solution is valid for any reaction regime and any number of reactions taking place within the liquid him. The application to a CSTR is discussed; this also serves as a test of the accuracy of the approximate solution carried out by comparison with the complete film model solved via the orthogonal collocation method. The numerical studies show that the approximate solution can be used with confidence for the usual encountered rate laws and that the volatility of the liquid reactants affects the performance of the CSTR mostly at the intermediate value of the Halt a number.     

ACOUSTIC STUDIES OF INTERFACIAL EFFECTS IN AIRLIFT REACTORS

Rational design of airlift reactors for the culture of plant and animal cells is impeded by a lack of understanding of the causes of loss of viability. Some recent speculations in the literature suggest that gas-liquid interfacial phenomena have played prominent roles in observed instances of loss of viability in both insect and animal cell cultures. The important interfacial events may include bubble formation and detachment at the sparger, bubble coalescence and breakage, and bubble disengagement at the free surface-entailing film thinning, film rupture, and film droplet ejection.

We have carried out an experimental investigation of interfacial phenomena using macrovideography and acoustic signals recorded at both the sparger and the free surface. Data have been collected for several liquid media including distilled water, distilled water with electrolyte (NaCl), and aqueous solutions of glycerol (μ = 3 to 9.5 cP). The studies were conducted in a 3-liter, acrylic plastic, split-column airlift reactor, and air was introduced through interchangeable sieve plates (with hole diameters of 1, 2, and 3 mm). Time-series data collected from the microphone were processed for mean and root-mean-square values, and Fourier transforms were computed to identify important signal energies. A sequence of experiments was also conducted that was designed to relate specific interfacial phenomena to the frequencies of the noises produced.