Jens-Uwe Repke

Fluid dynamics in multiphase distillation processes in packed towers

In order to design packed columns for multiphase operation it is necessary to include reliable data on the flow behavior in the model. Since experiments inside packings are very difficult to perform, computational fluid dynamics (CFD) simulations are considered to be a sensible approach as numerical experiments. To obtain reliable results validation is inevitable. We present the validation of two- and three-phase film flow under transient conditions. This application is not abundant in literature. Numerical results are compared with data from own experiments. The usability of a VOF-like code for such applications is shown.

Modeling and Parameter Identification of the Simultaneous Saccharification-Fermentation Process for Ethanol Production

Despite many environmental advantages of using alcohol as a fuel, there are still serious questions about its economical feasibility when compared with oil‐based fuels. The bioethanol industry needs to be more competitive, and therefore, all stages of its production process must be simple, inexpensive, efficient, and “easy” to control. In recent years, there have been significant improvements in process design, such as in the purification technologies for ethanol dehydration (molecular sieves, pressure swing adsorption, pervaporation, etc.) and in genetic modifications of microbial strains. However, a lot of research effort is still required in optimization and control, where the first step is the development of suitable models of the process, which can be used as a simulated plant, as a soft sensor or as part of the control algorithm. Thus, toward developing good, reliable, and simple but highly predictive models that can be used in the future for optimization and process control applications, in this paper an unstructured and a cybernetic model are proposed and compared for the simultaneous saccharification‐fermentation process (SSF) for the production of ethanol from starch by a recombinant Saccharomyces cerevisiae strain. The cybernetic model proposed is a new one that considers the degradation of starch not only into glucose but also into dextrins (reducing sugars) and takes into account the intracellular reactions occurring inside the cells, giving a more detailed description of the process. Furthermore, an identification procedure based on the Metropolis Monte Carlo optimization method coupled with a sensitivity analysis is proposed for the identification of the modelapos;s parameters, employing experimental data reported in the literature.

Experimental Investigations of Three-Phase Distillation in a Packed Column

An experimental study with three different heterogeneous azeotropic mixtures (acetone/toluene/water, 1-propanol/1-butanol/water and ethanol/cyclohexane/water) has been carried out with the object to analyze the separation efficiency of three-phase distillation in a packed column. The experiments were made under total reflux in a 70 mm column packed with Sulzer Optiflow C.36. The results show the influence of the second liquid phase on the mass transfer and with that on the separation efficiency in a packed column.

A new algorithm for global optimization: Molecular-Inspired Parallel Tempering

A novel stochastic algorithm for global optimization, Molecular-Inspired Parallel Tempering (MIPT), is presented. MIPT incorporates some basic features of molecular dynamics simulation into the Parallel Tempering formulation. In MIPT, molecules move in the decision-variable-space as the result of different forces: repulsion, friction and random forces. Two different types of molecules are considered: explorers and refiners. Explorers present lower friction and are subject to repulsion forces causing them to move faster towards low molecular density regions. Refiner molecules achieve better values of the objective function and are subject to larger friction forces restricting their motion to a narrow region around their current position. The efficiency of MIPT is tested in five challenging case studies and compared with other established, well-known optimization methods. The results demonstrate that new MIPT is a competitive and efficient algorithm, reaching the global optimum with 100% success ratio in most cases, without requiring much computational effort.

Modeling and simulation of a hybrid separation process for the carbon dioxide removal of the oxidative coupling of methane process

Abstract The oxidative coupling of methane (OCM) is a promising alternative for the oil-based production of olefins. The aim is to convert methane-containing natural gas catalytically to ethylene and open up a new feedstock for olefins and further organic synthesis products [1], [2]. The whole process is designed modular and built up in a miniplant to investigate different new approaches. For realization in a short time period, but in a more efficient way, the entire process is divided into three units: reaction unit, purification unit and separation unit, which are designed simultaneously. Particular requirements for process conditions on the transitions had to be defined and were done by laboratory screenings and literature study. Due to the novel process design strategy, downstream process conditions affect the design specification for the catalyst and the reaction unit. In the article the purification section is discussed particular and a novel hybrid separation process for the CO2 removal is presented. An efficient and modern carbon dioxide separation process of a membrane and an amine unit was developed. The membrane unit has been modeled with Aspen Custom Modeler® (ACM), and was integrated in the Aspen Plus® process simulation. The amine unit was modeled with a rate-based absorption model, including an electrolyte NRTL approach [3] and concentration-based reaction kinetics [4]. The simulation results of the conventional amine process, the single membrane unit and the improved novel hybrid process are presented in this paper.

Heterogeneous Azeotropic Distillation in Packed Columns: Experimental Results

Heterogeneous azeotropic distillation in packed columns is often very difficult because three phases, i.e., two liquid phases and one gas phase, can occur. In this type of distillation, also known as three-phase distillation, the second liquid phase can change the mass transfer efficiency of the packing in a significant way. The present paper focuses on simple distillation experiments with the mixture acetone/water/toluene. The evaluation of these experiments helps better understand the effects of a second liquid phase on the mass transfer in a packed distillation column.

A nonequilibrium model for three-phase distillation in a packed column: Modelling and experiments

Abstract A nonequilibrium model (Figure 1a) is developed for the simulation of a three-phase distillation process in a column equipped with a structured packing. The model is taking into account the mass transfer resistance between all three existing phases. Furthermore the convective and conductive part of the heat transfer rate is calculated, which is crucial for the precision of the model. The development of the model was strongly connected with experimental investigations of a packed column. When a heterogeneous azeotropic mixture is separated by distillation in a packed column, specific steady states can occur, which cannot be described by the use of an equilibrium model (Figure 1b). The observed remarkable column behaviour is caused by the fluiddynamic situation of the two immiscible liquid phases on the structured packing surface. The simulation results are in good agreement with the experimental data of the packed column.

Sustainable Design of Different Seawater Reverse Osmosis Desalination Pretreatment Processes

Abstract Sustainable development is usually presented using the three pillar model: economy, environment and society. This research uses life cycle based assessments in conjunction with process system engineering methods for the design and analysis of sustainable processes with the example of pretreatment of seawater membrane desalination. At the first step, the sustainability potential of several state of the art process alternatives is analyzed (namely granular media- and membrane-based filtration). Conducting a broad environmental life cycle assessment (LCA, using GaBi4® software) together with a life cycle costs assessment (LCC, according to SETAC guidelines) forms the so called eco-efficiency analysis (based on a definition by Germanys institute for applied Ecology) which together with a qualitative process performance assessment gives a good idea about the sustainability potential of the different process alternatives. The results show that the current trend of using a membrane based pre-treatment is proven here to not always be a more sustainable process solution.

CFD Study on Liquid Flow Behavior on Inclined Flat Plate Focusing on Effect of Flow Rate

Abstract The factors affecting the separation efficiencies of structured packed columns are multi-scale, including the macroscopic variables (eg feed rate, pressure and temperature) and the microscopic liquid flow behavior and distribution. Especially in the situation in which the second liquid phase is inevitable within distillation columns, the microscopic liquid flow behavior could lead to the uncertainty of the separation efficiencies. This work mainly focuses on the investigation of the local flow behavior of a liquid phase flowing on an inclined steel plate at different situations. On one hand, a three-dimensional CFD model based on the VOF model is developed with the commercial CFD package, FLUENT 6.3. On the other hand, the results of the experimental study are adopted to validate partially the model. Within the developed model, the influences of the liquid and gas flow rates are studied with respect to the specific wetted area, the thickness profile of film and rivulet flows as well as the average velocity of the liquid phase.

A toolbox using the stochastic optimization algorithm MIPT and ChemCAD for the systematic process retrofit of complex chemical processes

Abstract Global optimization techniques using powerful algorithms have led to a wide range of applications to increase the efficiency of chemical processes. Nevertheless, the performance for optimization of process models is limited by a certain complexity, especially accounting for existing processes (retrofit). Due to the great combinatorial diversity of possible alternatives a systematic approach is essential. The local integration of modifications in the overall process leads to changes in internal streams. Therefore new operating points have to be found and resulting effects on the plant performance have to be evaluated. An optimization framework for the purpose of retrofitting using a rigorous process simulation tool is proposed to fulfill this task. Here, flowsheet simulation software packages are offering a high performance for the prediction of new operation points for following units, and for units affected by recycle streams. An optimization approach using flowsheet simulation software and the stochastic optimization algorithm Molecular-Inspired Parallel Tempering (MIPT) implemented in the programming software Matlab™ is presented. Both of these programs are linked via OPC (OLE for process control), a standard communication platform. The toolbox provides a quick evaluation of the process by searching for the global optimum. The MIPT algorithm is suitable for large optimization problems and can handle constraints and infeasibilities. The usage of a rigorous process simulator is providing a high accuracy of the thermodynamic results which is necessary to evaluate the influence of the new process design. Furthermore, a simulation model of the industrial plant can directly be used for the optimization. A complex multicomponent separation process with recycle streams is used to demonstrate the advantages of the proposed toolbox. To simplify the user input a graphical user interface was programmed. The results of a sensitivity analysis and the optimization for different feed compositions are presented.