Process Intensification
Intensifying energy and material transport

A reduction in size undoubtedly remains an important strategy in the process intensification toolkit (micro process technology). However, the hardware, apparatus or any other component is not the primary consideration. The real attention is focused on the functions, for example heat exchange as a unit of operation rather than on the heat exchanger per se. Essentially, the goal is to identify the limitations of conventional material and heat transport systems and then circumvent these limitations.

Better, faster, safer
A number of different strategies are combined under the heading “process intensification”, making a precise definition elusive. The term can be used to describe a preference for multi-purpose systems or small dedicated-lines which can be duplicated so that capacity can be ramped up as needed (numbering up).

Process intensification can also refer to completely new types of reactors and techniques which can substantially improve space/time yield and reduce the costs associated with reactors and pipes. Typical examples from the field of polymerization engineering include micro reactors, rotating-disc reactors, kneader reactors and similar screw-based machines as well as spray polymerization. There are also methods which are based on ionic or catalytic polymerization mechanisms, where polymerization speed is inherently higher. Process integration based on reactive distillation or reactive chromatography also appear to offer significant potential

It is, however, difficult to distinguish between “process intensification”, “process optimization” and “process integration.” How do you differentiate between “micro reactor technology”, “micro process technology”, etc.? Actually, there is a clear distinction. Process intensification is a strategy, and the other two terms belong more in the tool category (hardware). The important thing is to get educts and products to the right place at the right time and to efficiently control the heat and material transport.

“Intensification means revolution rather than evolution,” explained Henrik Hahn who works at Degussa2. He and his team are not trying to alleviate the familiar bottlenecks. Instead, they are aiming for a quantum leap which will significantly shift process efficiency in the direction of the theoretical maximum or that will take the process in an entirely new direction.

Otto Machhammer from BASF sees differences in approach3. Process intensification strategies vary depending on how aggressive the goals are. If a reactor has to be as small as possible, then a micro reactor can be the answer. If the goal is business driven, then the strategy must address the total process or situation including raw materials, energy flows, staffing requirements and logistics issues. The problem will be defined differently, and an interdisciplinary effort is the key to success.

IMM (Institute for Microtechnology in Mainz) uses a more concrete definition. According to IMM, process intensification refers to a process which is

• better (higher yield and/or selectivity)
• faster (enhanced space/time yield)
• safer and better for the environment (green chemistry with no high-risk process systems)
• more cost effective (lower investment costs and/or total operational costs) .

Because process intensification appears to offer so much potential, a “Process Intensification“ group has been established at DECHEMA. The fact that there were 135 founding members from industry and sciences reflects the high level of interest in this technology. The goal is to significantly increase the economic and ecological efficiency of chemical and biotechnology processes. The organization believes that this will require a “new level of effort which goes far beyond the process optimization that we have seen in the past”. Process intensification means holistic process development using totally new process strategies which will lead to quantum improvements.

The Process Intensification group will focus on activities at a national level and act as the German point of contact for international bodies. The organization plans to work closely with process intensification networks in Holland and the UK and with the EFCE “Process Intensification” working party.

A new technological approach will be required which encompasses everything from equipment and a deeper understanding of the process to process automation. Possible strategies include micro process technology, intensification of heat and material transfer, non-traditional methods of energy input, new approaches to process control and a reduction in the number of process steps by integrating reaction and product preparation.

“We need new, unconventional strategies for process intensification - revolution instead of evolution,” declared Martin Strohrmann from BASF. “The engineers cannot solve these problems on their own. We have to delve deeper into the basics and strengthen our networks with universities, professional bodies and partners in the EU.”

Numbering-up better than scaling-up

What actually happens? Dimensions shrink down to the millimeter or micrometer range. Size minimization is accompanied by a very significant intensification of heat and material transfer. The surface to volume ratio, the specific phase boundary, rises in micro-structured equipment to several thousand m²/m³. For example, micro heat exchangers which are no larger than a cube of sugar can handle the energy of an entire one-family home.

The new numbering up approach eliminates the risks which are associated with scaling up, and this gives process intensification a significant advantage. Micro systems are often run in parallel using the optimal parameters which were identified in the lab. Multi-scale systems are an extension of this approach. The German government subsidized research into this strategy in the DEMiS project, and the large-scale European IMPULSE project continues to develop the concept.

In the field of reactor technology, the traditional approach was to adapt the chemical process to the equipment. For example, the walls were used for temperature control, but this becomes increasingly difficult as equipment dimensions continue to increase. Now the equipment can be adapted to the chemical process, so that the full potential of a chemical reaction can be exploited. Process intensification will be the solution of choice for reactions which involve intensive mixing or which are very endothermic or exothermic. The chemical reactions can take place without any limitations on heat and material transport.

Last, but not least, safety aspects are not insignificant, because smaller volumes of reactants are easier to handle and control

No alternative to an interdisciplinary approach

The “Multi-Phase Flow, Material Transport Process and Reactor Development” working group at the University of Bremen’s Environmental Engineering Institute shows very clearly that “process intensification” is an interdisciplinary effort which encompasses a whole range of technology and research fields. The working group is currently working on the development of complete, fully functional micro reactor systems which are designed to replace entire processes.

The working group’s partners use micro metallic powder injection molding (Fraunhofer IFAM, Bremen) to produce components. The also supply silicon etch processes (IMTEK Microsystems Technology Institute, University of Freiburg; IMSAS, University of Bremen), develop µ-MSR technology (BIAS Bremen Institute for Applied Radiation Technology) and perform system integration (Schulz-Systemtechnik, Visbek).

In its micro powder molding injection process (µ-MIM), Fraunhofer IFAM uses very fine particles (< 5 µm) to mold complex shapes. The powder is mixed with a special binder system and then injected into a tool or mold insert. Techniques such as silicon etching, micro machining, micro erosion, laser machining and LIGA are used to make the reusable mold inserts.

IFAM has used µ-MIM on a number of different materials including stainless steel, iron, hard alloy, copper and tungsten-copper. Small structures (10 µm) with an aspect ratio of 16 (height to width) can be produced. Additional development goals include:

• large-surface forming of micro structures,
• volume production of micro components,
• achievement of closer tolerances in component dimensions,
• reduced surface roughness,
• development of further materials for use in micro system technology.

Micro-scale mixers, heat exchangers, pumps and reactors are already available at IMTEK. The micro devices, which have flow channels in the low µm to mm range, are examples of a significant scale down which influences material characteristics and the transport processes. Miniaturization has a number of advantages including:

• high gradients for pulse, heat and material exchange,
• good control of process parameters due to smaller volumes and shorter paths,
• high integration of process units with each other and with instrumentation components.

Process Intensification “Project House”

Degussa will invest € 15 million over the course of three years in its process intensification project to conduct research into new process strategies and reactor designs. The team will be looking to develop process strategies in three areas: “highly active catalysts”, “functional materials” and “disperse systems”.

The fourth area, “Chemical ExplorENG” (Exploring Chemical Engineering), acts at the glue which holds the project together. The focus here is on development of modular systems which are used to produce special chemicals. Various modules can be built simultaneously and “plugged together” on site. This reduces the time is takes to get a line up and running, and it reduces time to market. Modular design can also save money when capacity needs to be ramped up. This is a big advantage when demand is low at the time of product introduction.

The “highly active catalysts” group plans to use micro process technology and a new reactor design to significantly improve gas synthesis processes which make use of heterogeneous catalysts. The project team will benefit from the results of the “Demonstration Project to Evaluate Microreaction Technology in Industrial Systems” (DEMiS) which was subsidized by the German government and which Degussa has now brought to a successful conclusion together with Uhde GmbH and participating universities. More active catalysts and new catalyst preparation techniques will be needed to exploit the full potential of microprocess technology.

The goal of “functional materials” research is to find new ways of encapsulating solids, polymerizing water-insoluble monomers and producing ultra-fine organic particles. The team is looking at products like adhesives that can be activated and impact modifiers. The team intends to use mini emulsions, which have super fine droplets with a narrow size distribution and diameters in the 110 – 100 nm range, as the vehicle. This type of nano droplet reactor can only be produced by using high specific energy input during the emulsification process. A number of current scientific articles contain an impressive description of how mini emulsions can be produced in a laboratory, but no one has succeeded in implementing the process in a production-scale system.

The “disperse systems” team is exploring ways to reduce process times and thereby increase process efficiency. Research is focused on alternative process paths for the production of color paste and on new reactor designs for intensive fermentation. This could expand the operating range of the traditional continuous stirred tank reactor. “The production cultures which are used in fermentation have been continuously improved over the years. The Project House can help capitalize on the progress which the company has made in its culture development activities by improving our reactor technology,” explained Project House Manager Henrik Hahn. One of the team’s major priorities is to develop reactors which improve oxygen input, because this is a major limiting factor during fermentation.

Current practical examples

Example I: On September 27th 2005, six months after the cooperation agreement was signed with IMM Institut für Mikrotechnik Mainz GmbH, the pilot production phase was successfully completed on a micro reactor system which is used to produce nitroglycerin at the Xi'an Chemical Industrial Group (HAC) in China. The line is designed for continuous production of nitroglycerin, and it has a throughput of about 15 kg/h. The nitroglycerin is used exclusively as medication to treat acute angina pectoris attacks. The product has to meet very stringent quality requirements, and it is produced under GMP conditions. The micro reactor has three main parts: a unit to produce nitrating acid from fuming nitric acid and sulphuric acid, the actual micro reactor and the subsystems which are used for phase separation, purification and drying of the synthesized nitroglycerin. The nitrating acid is produced continuously just prior to being fed into the micro reactor, and it reacts directly with the glycerin. The two reactants are mixed continuously, and mixing only takes milliseconds to complete. A large surface-to-volume ratio ensures that reaction heat is dissipated immediately. The low reaction volumes also reduce the potential risks.

The project demonstrates the “classic” advantages of process intensification: higher yield, improved product quality, enhanced safety and lower environmental risk.

Example II: A high-performance reactor, which was developed at the Karlsruhe Research Center, has proven its suitability for chemical production in an industrial environment. DSM Fine Chemicals GmbH in Linz, Austria used the new system to produce more than 300 tons of a high-grade product for the plastics industry in 10 weeks. The yield increased significantly compared to traditional methods. Raw material consumption and waste volumes were reduced, and the micro reactor also enhances process reliability.

The core element in the new production system is a “micro reactor” made of special nickel alloy. The reactor, which is 65 cm long and weighs 290 kg, has a throughput capacity of 1700 kg of liquid chemical per hour. Klaus Schubert, Director of the Institute for Micro Process Technology at the Karlsruhe Research Center, explained that “the term micro refers to what goes on inside the reactor. Chemical substances are brought together in micro blenders and then react in tens of thousands of micro channels. The heat created by the reactions is dissipated via micro channels in a matter of seconds. The micro reactor can handle several hundred kW of heat energy.”

The micro reactor at DSM replaces a core reaction step which previously had taken place in a large continuous stirred tank reactor where several thousand tons of toxic, corrosive chemicals were mixed. “The Karlsruhe micro reactor has substantially increased yield compared to our previous process which was purely based on continuous stirred tank reactor production”, commented Peter Pöchlauer, Project Manager at DSM Fine Chemicals GmbH in Linz, Austria. “We have been able to reduce raw material consumption and the volume of waste, and this improves our efficiency and reduces the impact on the environment. The micro reactor has also increased process reliability.”

Example III: The IMRET series of conferences (International Conference on Microreaction Technology) reflects the rapid pace of development in the field. The conferences, which alternate between Europe and the US, were initiated in 1997 by the micro reaction working group at DECHEMA. Some interesting applications which show progress towards industrial-scale production were described at IMRET 8 which took place on April 11th – 14th 2005 in Atlanta and in a report on micro process technology in the US which was published by VDI/VDE Innovation + Technik GmbH . Examples include hydrogenation for the production of pharmaceutical intermediates by Bristol Myers Squibb (USA), free radical polymerization at the University of Kyoto (Japan) and the use of hydrogen peroxide to epoxidize propylene, which is a sample application of a gas-phase reaction in an Uhde DEMiS reactor. These are examples of systems for industrial production which go beyond component development and characterization. The system at the University of Kyoto is 3.5 m x 0.9 m and has an annual production capacity of 5 – 10 tons.

1. www.pinetwork.org
2. PROCESS, Vogel Industrie Medien, September 2005, S. 10-13
3. Chemie-Ingenieur-Technik, Wiley-VCH Verlag, 11/2005, S. 3