Gerhard Schembecker

Process synthesis for reactive separations

The integration of reaction and separation in one single process unit proved to lead to enormous savings in capital as well as in operating costs. This paper provides a systematic framework to consider reactive separations in the early stages of process development. It highlights a process synthesis procedure guaranteeing that processes like reactive distillation, reactive crystallization, reactive extraction or reactive stripping will not be overlooked during conceptual flowsheet development. First step of this procedure is the analysis of the chemical reaction system. In case a separation of one or more products, by-products or solvents seems to be beneficial for the performance of the reactor the physical separation behavior of the components has to be checked. The chemical reaction, the physical separation and the design on the apparatus need a defined operation window. Only when an overlap of these windows can be identified a reactive separation process is feasible and might lead to advantages in the overall process performance. The paper finally demonstrates that the integration of reaction and separation in one unit does not necessarily lead to economic benefits as the overall performance of the total chemical process has to be considered.

Investigations on the Synthesis of Methyl Acetate in a Heterogeneous Reactive Distillation Process

The production of methyl acetate in a reactive distillation process – prior art for 15 years – is often used as an example to study the basic phenomena of reactive distillation. The present paper deals with a theoretical and experimental analysis of methyl acetate synthesis in a reactive distillation column. A design method based on the interpretation of reactive distillation line diagrams is used to identify the main process parameters and to provide a foundation for experimental investigation. The significant influence of the reflux ratio on the conversion in the column is shown by mini plant experiments using supported ion exchanger in the form of Raschig rings as a heterogeneous catalyst. These experiments demonstrate the catalytic quality of this packing material. To simulate the reactive distillation column with a simple stage-to-stage method, the separation efficiency of the catalytic rings is investigated. Comparison of experimental and simulation results reveals that a simple model based on the assumption of simultaneous chemical and phase equilibrium describes the experimental data quite well over a wide range of reflux ratios. Furthermore, simulation results show that the conversion depends less on the number of reactive stages than on the use of two feed stages.

Influence of physical properties and operating parameters on hydrodynamics in Centrifugal Partition Chromatography

Besides the selection of a suitable biphasic solvent system the separation efficiency in Centrifugal Partition Chromatography (CPC) is mainly influenced by the hydrodynamics in the chambers. The flow pattern, the stationary phase retention and the interfacial area for mass transfer strongly depend on physical properties of the solvent system and operating parameters. In order to measure these parameters we visualized the hydrodynamics in a FCPC-chamber for five different solvent systems with an optical measurement system and calculated the stationary phase retention, interfacial area and the distribution of mobile phase thickness in the chamber. Although inclined chambers were used we found that the Coriolis force always deflected the mobile phase towards the chamber wall reducing the interfacial area. This effect increased for systems with low density difference. We also have shown that the stability of phase systems (stationary phase retention) and its tendency to disperse increased for smaller values of the ratio of interfacial tension and density difference. But also the viscosity ratio and the flow pattern itself had a significant effect on retention and dispersion of the mobile phase. As a result operating parameters should be chosen carefully with respect to physical properties for a CPC system. In order to reduce the effect of the Coriolis force CPC devices with greater rotor radius are desirable.

Selection of operating parameters on the basis of hydrodynamics in centrifugal partition chromatography for the purification of nybomycin derivatives.

The selection of solvent systems in centrifugal partition chromatography (CPC) is the most critical point in setting up a separation. Therefore, lots of research was done on the topic in the last decades. But the selection of suitable operating parameters (mobile phase flow rate, rotational speed and mode of operation) with respect to hydrodynamics and pressure drop limit in CPC is still mainly driven by experience of the chromatographer. In this work we used hydrodynamic analysis for the prediction of most suitable operating parameters. After selection of different solvent systems with respect to partition coefficients for the target compound the hydrodynamics were visualized. Based on flow pattern and retention the operating parameters were selected for the purification runs of nybomycin derivatives that were carried out with a 200 ml FCPC(®) rotor. The results have proven that the selection of optimized operating parameters by analysis of hydrodynamics only is possible. As the hydrodynamics are predictable by the physical properties of the solvent system the optimized operating parameters can be estimated, too. Additionally, we found that dispersion and especially retention are improved if the less viscous phase is mobile.

READPERT — development, selection and design of chemical reactors

Abstract A new type of computer-based consulting system for the choice of chemical reactors during the first step of process design is presented. Because neither expert systems nor numerical calculations alone are able to solve the problem of proper reactor choice, a new type of computer program has been developed, the heuristic—numeric consulting system readpert (Reaktor Entwicklungs-Auswahl und Design Expertensystem, i.e. Reactor Development, Selection and Design Expert System). On the one hand, it uses expert system technology and knowledge representation forms, such as rules and frames, and on the other hand numerical calculations, as a part of a wide range of chemical engineering tools. In this way advice for the design of a chemical reactor can be given even at an early stage of process design. readpert can be used as a stand-alone system as well as part of prosyn (Process Synthesis), a range of cooperating distributed heuristic—numeric systems, for computed-aided process synthesis.

Model-based conceptual design and optimization tool support for the early stage development of chemical processes under uncertainty

Abstract Despite many efforts to formalize the early phase of conceptual process design, no tool-supported systematic method for the conceptual design of chemical processes has been developed to date. This contribution presents a methodology and a software tool for the early conceptual design phase, which is characterized by limited and uncertain information on the available process units. The proposed methodology supports the graphical modeling of hierarchical superstructures which are subsequently optimized by a hybrid evolutionary algorithm considering uncertainties in model parameters.

Investigation of structural changes of β-casein and lysozyme at the gas–liquid interface during foam fractionation

Purification and separation of proteins play a major role in biotechnology. Nowadays, alternatives to multistep operations suffering from low product yields and high costs are investigated closely amidst which one of the promising options is foam fractionation. The molecular behavior at the gas-liquid interface plays an important role in the formation and stabilization of enriched foam. This study for the first time correlates the physico-chemical parameters to the molecular structure in view of protein enrichment during foam fractionation of the two relatively different proteins lysozyme and β-casein employing biophysical techniques such as circular dichroism (CD) spectroscopy and infrared reflection absorption spectroscopy (IRRAS). In case of lysozyme, high enrichment was achieved at pH<pI in contrast to current opinion. This is due to partial unfolding and aggregation of the lysozyme molecules under favorable foaming conditions that resulted with high enrichment of foamed protein. Under these favorable conditions, CD spectra and IRRA spectra show that the unfolding of lysozyme is partially irreversible. However, the unfavorable foaming conditions, giving low enrichment, promote only minor structural changes and these changes are fully reversible. In case of β-casein, no pronounced unfolding can be observed using CD spectroscopy and IRRAS. The β-casein molecules adsorb and purely reorient at the gas-liquid interface, depending on favorable or unfavorable conditions.

Modeling induced nucleation processes during batch cooling crystallization: A sequential parameter determination procedure

Abstract An existing model is extended to simulate batch cooling crystallizations with induced nucleation processes like ultrasound or gassing. All important phenomena such as nucleation, growth, agglomeration and breakage are taken into account. A differentiation between ultrasound and gassing is necessary. Induced nucleation processes also require a modification of crystal growth mechanism. In general, the model parameters required for the kinetics are determined simultaneously by fitting them to experimental data. Mostly a correlation model results without physical basis. Here, the model parameters are sequentially determined by decoupling the mechanisms, saving effort and time, and make it possible to reduce the number of parameters also. The model and the model parameter determination procedure are validated using three different organic solute/solvent systems. The number of simulation runs for parameter fitting was reduced to less than 100 instead of simultaneous parameter determination, which requires several hundreds of thousands runs, resulting in physically reasonable solutions.

Multiphase flow modeling in centrifugal partition chromatography

The separation efficiency in Centrifugal Partition Chromatography (CPC) depends on selection of a suitable biphasic solvent system (distribution ratio, selectivity factor, sample solubility) and is influenced by hydrodynamics in the chambers. Especially the stationary phase retention, the interfacial area for mass transfer and the flow pattern (backmixing) are important parameters. Their relationship with physical properties, operating parameters and chamber geometry is not completely understood and predictions are hardly possible. Experimental flow visualization is expensive and two-dimensional only. Therefore we simulated the flow pattern using a volume-of-fluid (VOF) method, which was implemented in OpenFOAM®. For the three-dimensional simulation of a rotating FCPC®-chamber, gravitational centrifugal and Coriolis forces were added to the conservation equation. For experimental validation the flow pattern of different solvent systems was visualized with an optical measurement system. The amount of mobile phase in a chamber was calculated from gray scale values of videos recorded by an image processing routine in ImageJ®. To visualize the flow of the stationary phase polyethylene particles were used to perform a qualitative particle image velocimetry (PIV) analysis. We found a good agreement between flow patterns and velocity profiles of experiments and simulations. By using the model we found that increasing the chamber depth leads to higher specific interfacial area. Additionally a circular flow in the stationary phase was identified that lowers the interfacial area because it pushes the jet of mobile phase to the chamber wall. The Coriolis force alone gives the impulse for this behavior. As a result the model is easier to handle than experiments and allows 3D prediction of hydrodynamics in the chamber. Additionally it can be used for optimizing geometry and operating parameters for given physical properties of solvent systems.

Investigation, comparison and design of chambers used in centrifugal partition chromatography on the basis of flow pattern and separation experiments.

In centrifugal partition chromatography (CPC) the separation efficiency is mainly influenced by the hydrodynamic of mobile and stationary phase in the chambers. Thus, the hydrodynamic has to be investigated and understood in order to enhance a CPC separation run. Different chamber geometries have been developed in the past and the influence of several phase systems and CPC operating conditions were investigated for these chambers. However, a direct comparison between the different chamber types has not been performed yet. In order to investigate the direct influence of the chamber design on the hydrodynamic, several chamber designs - partially similar in geometry to commercial available designs - are investigated under standardized conditions in the present study. The results show the influence of geometrical aspects of the chamber design on the hydrodynamic and therewith, on the separation efficiency. As a conclusion of the present study, some ideas for an optimal chamber design for laboratory and industrial purpose are proposed.