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Process Synthesis and Intensification (PSI)

Process Synthesis (PS) is an elementary part of conceptual process design, which comprises of the determination of a general flowsheet structure, the selection of unit operations, their combination, and interconnection as well as rough sizing and costing. It is one of the most complex, but also important tasks in the development of new (bio-)chemical processes, since the choices made in this early stage account for about 80% of the final cost of the entire process. In order to generate and navigate the vast space of potential options systematic methods based on mathematical models are required that guide the way to the most promising process flowsheets. In order to obtain reasonable results, these models need to be capable of describing at least the inherent limitations to certain separation technologies. More accurate models allow for the quantification of the process performance in terms of energy requirements, process economics or environmental criteria. By combining process synthesis methods, model discrimination/refinement and experimental design a systematic approach to process synthesis can be obtained, which facilitates a targeted development of new processes with significantly reduced experimental effort and time requirements.


Process Intensification (PI) on the other hand can be summarized as a tool for the targeted improvement of processes by addressing bottlenecks at different scales. At a process-wide scale, hybrid separation processes, can overcome the limitations of simple sequences by utilizing the synergies between the different separation technologies. Performance improvements can also be achieved at an equipment level, taking into account different means for improving energy efficiency, such as thermal coupling or the integration of several distillation columns in form of dividing wall columns. The combination of reaction and separation, as e.g. in reactive distillation or extraction addresses PI on a phase and transport level and can overcome equilibrium limitations of both reaction and separation. At last, the optimal choice of additives, such as solvents or catalysts, as well as the implementation, can improve process performance based on modifications on a molecular level.


In order to bring PS and PI together and identify the most suitable process design in a systematic and directed way, the PSI group performs research in different directions. Experimental as well as model based investigations of different intensified fluid separation processes are performed in order to generate an improved understanding and facilitate the development of suitable models. These are the necessary basis for further consideration in process synthesis, where the work is focused on the development of structured and standardized design workflows as well as optimization-based tools for conceptual process design, taking into account the different elements of PI. The integration between experiments and modelling plays an essential role in the developed design workflows and detailed design calculations and experimental validation are the final element to perform scale-up and reliable sizing.

Currently the PSI group focusses on:

  •          (Enzymatic) reactive absorption/distillation
  •          Organic solvent nanofiltration and membrane-assisted separation processes
  •          Rotating packed beds
  •          Optimization-based synthesis of intensified separation processes


Development of a general design approach for membrane-assisted hybrid processes

Membrane processes provide a tremendous potential for improving separation performance and energy efficiency compared to thermal separation processes. However, the benefits of membrane processes, as e.g. the ability to overcome limitations of other separation techniques introduced by azeotropes or eutectic points, are exploited best when integrated in hybrid processes. In hybrid separations, two or more unit operations, based on different separation phenomena, are combined that lead to more efficient and sustainable processes. Nevertheless, industrial applications are still limited due to a lack of reliable models that allow for an accurate description of the separation performance of the membrane. Process design approaches oftentimes neglect limiting effects, such as pressure drop, or concentration and temperature polarization.

In order to overcome those limitations, a systematic five-step design method evaluating the potential of membrane-assisted hybrid processes is developed at the chair of fluid separations. Current investigations are focused on pervaporation-assisted distillation and organic solvent nanofiltration. Therefore, the developed design approach includes driving force reducing effects as well as means for process intensification, such as energy integration, in order to evaluate the overall potential of the process. Additionally, the correct consideration of the flow pattern, such as co- and counter-current as well as cross-flow, can have a significant impact on the accuracy of the model that will also be considered within this work. The aim is to evaluate the potential of membrane-assisted processes prior to any experimental effort.


Figure 1: Common pervaporation-assisted distillation processes for the separation of binary mixtures

Project: SFB/TR 63 Inprompt – Transferprojekt "Hybride Trennprozesse: Modellierung und Entwurf von membrangestützter Rektifikation"

Contact Person: Bettina Scharzec, Mirko Skiborowski


Optimization-based conceptual design of separation processes in combination with rigorous modeling approaches

Process design in general requires accurate models for dimensioning of separation units, i.e. for the design of distillation columns. At present process engineers commonly use equilibrium (EQ) models for column dimensioning. However, for some separation tasks using the EQ models leads to considerable uncertainties and possible design errors. In these cases so called non-equilibrium (NEQ) models may be used to decrease the uncertainties. NEQ models consider the actual mass and energy transfers within the column and thus, allow correct design and dimensioning of columns even for the separation of highly non-ideal mixtures. However, currently no reliable criteria are available for the selection of the necessary modeling depth – EQ or NEQ model – for a specific separation task. At the laboratory of fluid separations, the need for NEQ modeling for distillation processes is systematically investigated.

Moreover, optimization-based methods for the design of energy efficient distillation processes are developed. Conventional distillation processes suffer from low energy efficiency. The energy efficiency can be significantly improved by using alternative process variants such as heat-pump assisted and thermally coupled distillation columns. The developed design methods are based on rigorous models and can consider different process configurations, e.g. dividing wall columns. These methods are extended step by step and are applied for the design of difficult separation tasks such as extractive or azeotropic distillation processes.


Figure 2: Scheme of the design and modeling aspects of a simple distillation column

Project: SFB/TR 63 Inprompt – Transferprojekt "Hybride Trennprozesse: Modellierung und Entwurf von membrangestützter Rektifikation"

Contact Person: Thomas Waltermann, Mirko Skiborowski


Shortcut model for predictions in organic solvent nanofiltration

Membrane processes have been attracting more and more attention in industry. Because of different interactions between components and the membrane material some components can permeate through the membrane preferably which leads to a selective separation. A new but promising technology is organic solvent nanofiltration which can be operated in liquid state without phase transitions. Organic solvent Nanofiltration can be employed for the retention of different molecules like catalysts or pharmaceutical products, since the operating temperatures are not necessarily high.

Due to complex interactions between membrane, solvents and solutes varying for each membrane type and chemical system, a high number of experiments needs to be performed in order to identify suitable membranes for a given separation task or even for feasibility studies. In order to reduce the experimental effort, a new approach for predictions of the separation behavior of organic solvent nanofiltration membranes is developed. Based on a smaller number of experimental measurements a new shortcut model is developed following the procedure in the graphic below. For identifying the new model structure a data driven approach is used.


Figure 3: Approach for the prediction of the separation behavior of organic solvent nanofiltration membranes

Project: ESIMEM "Energy efficient Separation in the chemical and pharmaceutical Industry using Membrane processes"

Contact Person: Rebecca Goebel, Mirko Skiborowski


Simulation and optimization of in situ product recovery processes

In-situ product recovery or removal (ISPR) is a group of techniques which utilize selective removal of reaction products from the reaction site in the separation unit integrated with the reactor. Most often, it is carried out in one of two manners – by removing either the desired product or by-product of the reaction. In-situ separation processes provide significant potential for process intensification. Continuous removal of one of the products enables overcoming equilibrium limitations of the system, like conversion limiting chemical equilibrium or separation limiting presence of the azeotropes and distillation boundaries.

Within this project, pervaporation-based membrane reactors (PVMRs) and their application for enzymatic esterification and transesterification are investigated. (Trans)esterification reactions suffer from low conversion due to chemical equilibrium. Pervaporation integrated with the reactor facilitate continuous, low-temperature and gentle separation, therefore enabling use of ISPR concept and change from non-organic to enzymatic catalysis. Use of the enzymes increases the selectivity of reaction and environmental sustainability of the process. Additionally, to the continuous product removal, the low temperature of the process further shifts the equilibrium toward the product side. The goal of the project is to develop an optimization-based framework for evaluation and design of the reactive separation processes. The framework will simplify the assessment of the process and choosing the most promising configurations and also accelerate the design of integrated enzymatic reactive pervaporation processes.


Figure 4: Sketch of the enzymatic pervaporation membrane reactor

Contact Person: Jerzy Pela, Mirko Skiborowski


Distillation in Rotating Packed Beds

Rotating packed beds (RPB) provide the opportunity for mass transfer processes in high gravity fields as part of the HIGEE-technology (high “g” for high gravity). This intensification step reduces the required equipment volume and changes operating limits, such as flooding velocities or maximum pressure drops. Although increased mass transfer rates, high loading points, intense shear forces and high micromixing effects in RPBs result in significantly decreased equipment volumes, the acceptance of this technology in industry is still very low.

The aim of this research is to expand the state of knowledge about RPBs by characterizing this technology. In order to identify possible fields of application, experimental and theoretical investigations of absorption and distillation processes are performed. To achieve this goal, two single stage modified types of RPBs are manufactured with a cooperation partner. Furthermore one three stages containing pilot-scale RPB is already available in the laboratory. The hydrodynamic behaviour and separation efficiency of the different RPBs is experimentally investigated. On this basis, an already developed model of adequate complexity is used and enhanced to localize possible fields of application of RPBs for different unit-operations such as distillation, absorption or reactive distillation.

Distillation in Rotating Packed Beds_DS

Figure 5: Scheme of a rotating packed bed.

Contact Person: Hina Qammar, Mirko Skiborowski


Design of membrane-assisted reaction-separation processes for homogeneously catalyzed reactions

Homogeneous catalysis offers several advantages for chemical processes due to high conversion and selectivity as well as mild reaction conditions. However, separation of the homogeneous catalyst is often challenging and catalyst loss can result in high operating cost. This limitation has been overcome in previous work in the hydroformylation of 1-dodecene using a combination of organic solvent nanofiltration (OSN) and thermomorphic multiphase systems (TMS). In this combination, a TMS consisting of at least two solvents, which are homogeneous at high temperature in the reactor, forms a two-phase system at lower temperature. This system is first separated by a decanter into a catalyst-rich polar phase and a product-rich apolar phase, which is further processed by OSN to recover remaining catalyst and recycle it back to the reactor.


Figure 6: Water separation from TMS 

Yet, reactions that are more complex, such as the hydroaminomethylation, pose additional challenges for process designs using TMS. In this case, the co-product water would accumulate in the polar catalyst phase and might have a negative impact on reaction performance or phase separation. Thus, the water content has to be controlled. Several different membrane processes are investigated for this separation of water from organic solvents. The project focuses on the experimental investigation and modeling of these membrane processes. The developed process concepts are first investigated in laboratory scale and later on applied in a continuously operated miniplant in cooperation with the Laboratory of Industrial Chemistry.

Project: SFB/TR63 InPROMPT – Subproject D3 Development and testing of integrated reaction and catalyst separation for the homogeneously catalyzed reductive amination and hydroaminomethylation of long-chain alkenes in a miniplant.

Contact person: Stefan Schlüter, Mirko Skiborowski


Designing of tailored packings for RPBs based on computational fluid dynamics and additive manufacturing

The focus of the research is the computer-aided designing of tailor-made packings for rotating packed beds (RPBs). The centrifugal contactors are an example of innovative HiGee technology utilizing centrifugal forces to enhance mass transfer in chemical engineering processes e.g. gas absorption and organic solvents distillation. However, to better exploit capabilities of the RPBs for various processes, custom-made shapes of packings must be designed.

In the research, different concepts of internals are investigated by means of computational fluid dynamics (CFD) methods. CFD simulations enable pre-screening of velocity field distribution of the gas phase and the pressure drop developed in conceptual packings with different inner structures. Based on the CFD predictions of hydrodynamics, the most promising prototypes of packings are selected and produced with the SLA 3D printer. The printed single-block packings are tested in the laboratory RPB unit for various operational parameters. Furthermore, mass transfer experiments with chemical absorption are conducted to estimate effective interfacial area of the designed internals. At the final stage, the chosen optimized packings are considered to be made with the use of the metal 3D printing technique to provide a durable, highly efficient product intended for commercial use in industrial applications.





Contact: Kamil Kamiński, Mirko Skiborowski

Design of membrane-assisted reactive distillation processes

Further information


Renewable resources show a great potential for the application in future processes. Promising and cheap feedstocks for the production of biodiesel are low-quality oils such as waste cooking oils. The drawback of these low-quality oils is the high amount of free fatty acids which lead to undesired side reactions in the transesterification of biodiesel. Therefore, a pre-treatment is needed to minimize the amount of free fatty acids in the waste cooking oil. A possible technology is reactive distillation (RD) but its operating window is restricted by reaction and separation. To enable the use of new feedstocks and enlarge the operating window of RD an integrated process incorporating organic solvent nanofiltration (OSN) and RD is proposed. OSN might allow an energy efficient separation of FFA and triglycerides.

A systematic investigation of this integrated process is performed. OSN and RD experiments are performed to provide necessary data for modelling. Within process analysis, feasible operating windows for different process configurations are identified considering conversion of waste cooking oil and product purities as well as energy efficiency.



Contact Person:Kathrin Werth, Mirko Skiborowski

Process intensification for CO2 capture using improved contacting equipment and novel materials

Further information


Mitigation of CO2 emissions is generally regarded as vital requirement to prevent global climate change. The application of mature CO2 capture technologies is currently very limited. Part of the cause for this lack of investment is the high capital and specifically the high operational costs of existing CO2 capture, purification and compression technologies. The negligible economical value of CO2 as a compound due to currently very limited industrial demand further contributes to this lack of investment. In order to decrease the costs associated with CO2 capture processes, different measures of process intensification are applied and investigated in this project. As a first measure intensified contacting technologies specifically membrane contactors and rotating packed beds are investigated to achieve process intensification on a technology level. These technologies are expected to offer substantially more compact and flexible processes due to significantly enlarged and stabilized volume-specific interfacial area, potentially resulting in capital cost savings. In order to enable more energy-efficient CO2 separation, the application of novel absorbent materials is introduced as a second measure for realizing process intensification, providing the potential of significant savings in operating costs. A very promising candidate of novel absorbent material is the enzyme carbonic anhydrase, which increases the absorption rate of bicarbonate forming solvents drastically. This facilitates the use of kinetically limited but thermodynamically favourable absorbents that provide the potential for improvements in energy-efficiency. Synergies from combining both approaches of process intensification are expected to further improve process performance and are therefore investigated as well in order to identify the most energy-efficient CO2 capture processes. As a major outcome of this project a portfolio of innovative, intensified and efficient processes is created based on the different measures of process intensification investigated experimentally and theoretically. This portfolio is a contribution to future development of energy-efficient and intensified CO2 capture processes.


Contact Person: Mathias Leimbrink, Mirko Skiborowski


Process Synthesis based on a superstructure of phenomena building blocks

Further information


The aim of this project is the development of a method for phenomena-based process synthesis, which automatically generates flowsheet variants by means of superstructure optimization. By composing processes of phenomena building blocks (PBB) instead of predefined unit operations, the creativity during process synthesis is maximized and process intensification principles such as integrated or hybrid separation processes can be considered even beyond already existing equipment. The optimization-based method generates promising phenomena-based flowsheet variants which can further be interpreted and translated into equipment by using databases or proposing new equipment that implements the phenomena-based design. A scheme of the generic superstructure, which is used for automatically generating flowsheet variants is shown in the following figure:


Contact Person: Hanns Kuhlmann, Mirko Skiborowski


Hydroformylation and hydroesterification of petrochemicals and oleochemicals in thermomorphic solvent systems in a continuously operated miniplant with highly efficient catalyst recycling

Further information


State of the art

 A continuously operated miniplant for the hydroformylation of 1-dodecene was designed, built and tested. To realize an efficient catalyst recycling, a solvent-system with a highly temperature dependent miscibility gap is used. The miniplant contains a continuously stirred tank reactor (CSTR) where the homogeneous catalyzed reaction takes place and a liquid/liquid phase separator in which the product-phase and the catalyst-phase are separated from each other. After phase-separation the catalyst is fed back into the reactor and the product is taken out of the process. It was possible to realize a steady state operation of this miniplant for 200 h with a yield of 63 % of the main product.

Research goals

During the following research the catalyzed recycling has to be improved further. Therefore, the influence of an organic solvent nanofiltration membrane on the catalyst behavior will be investigated, to expand the miniplant with a nanofiltration-unit and increase the effectiveness of the process. Furthermore the non-polar solvent has to be separated from the product with a distillation unit and reused in the reaction-step. Also an optimal designed reactor (project B1) has to be tested in the miniplant and compared to the CSTR, in order increase the yield. Further scientific investigations on hydroformylation and hydroesterification reactions in the miniplant lead to the processing of renewable materials.


Figure 9: Process concept

Contact Person: Jens Dreimann, Mirko Skiborowski

Sub content

Contact Person:


Dr.-Ing. Mirko Skiborowski