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Delft University of Technology takes Perfect chemical reactors one step further...

The project presents a pioneering step towards transferring some fundamental concepts of chemical physics and molecular reaction dynamics into the engineering science of chemical reactors. It aims at the development of structured reactors, in which reaction efficiency will be drastically increased by local control of alignment, orientation and activation of molecules. In the project we pursue the use of alternative forms and transfer mechanisms of energy (such as laser, electric or microwave fields) in a controlled milli- or microreactor environment. We will build on the Nobel Prize-awarded fundamental works in the area of the reaction dynamics and molecular reaction control (Herschbach, Lee and Polanyi, 1986), which were never considered in the chemical engineering field thus far.

The control of chemical reaction pathways at molecular level presents undoubtedly the most important scientific challenge on the way to fully sustainable, thermodynamically-efficient chemical processes. The most obvious advantages of enhanced molecular reaction control are i) higher reaction rates leading to low-temperature processes and smaller equipment, ii) better selectivities leading to minimization or elimination of waste, iii) reduction of separation operations, which are responsible for circa 40% of energy consumption in chemical and related industries, and iv) the possibility for tailored manufacturing of new, advanced products. An excellent example of such enhanced molecular control can be seen in the fundamental work by the group of Richard Zare at Stanford, where the application of a laser field for the excitation and “stretching” of the C-H bond in methane molecule during its chlorination introduced the so-called “stripping” collisions increasing the reaction rate by a factor of more than 100 (Kandel and Zare, 1998). This clearly proves that a targeted introduction of an alternative energy form can improve the reaction performance dramatically.  It can enable getting far beyond the limits of the “conventional inherent kinetics” which is based on the macroscopic temperature, pressure and concentrations.

Factors responsible for the effectiveness of a reaction include: number/frequency of molecular collisions, geometry of approach, mutual orientation of molecules at the moment of collisions and their energy.  Unfortunately, current chemical reactors offer a very limited degree of control of molecular-level events. In order to bring more molecules at the energy levels exceeding the activation energy threshold conductive heating is conventionally applied.  However, conductive heating offers only a macroscopic control upon the process and is thermodynamically inefficient. It is non-selective in nature, which means that non-reacting (bulk) molecules heat up together with the reacting ones. Also, other elements of the reactor are unnecessarily heated up. Secondly, the conductive heating generates temperature gradients, which creates a broad Maxwell-Boltzmann distribution of molecular energy levels.  I illustrate that problem with a simplified example shown in Figure 1. 




In a conventional system with temperature gradients, the energy of molecules is distributed.  In case of a parallel reaction scheme of the type shown in the figure, where P is the required product, a part of molecules (A) has energy insufficient to pass the transition state P*.  Another part (B) has sufficient energy to get over the threshold and form product P.  A large portion of these molecules has in fact more energy than it is needed to form P.  Finally, there are molecules in the pool that possess enough energy to generate also the transition state W* which eventually leads to the formation of the unwanted waste product W.  Ideally, one should provide all molecules with a narrowly distributed amount of energy, just exceeding the potential energy level of P*, as it is illustrated by the dashed curve D.

It is clear that in order to meet the future needs of the sustainable world, a new generation of chemical reactors, which I call here “perfect reactors”, must emerge. A groundbreaking solution in those reactors will consist in creating a reaction environment, in which the geometry of molecular collisions is controlled while energy is transferred selectively from the source to the required molecules in the required form, in the required amount, at the required moment, and at the required position (Fig. 2). Creating such “perfect” reaction environment will in turn require several basic functions to be integrated in the reactor, including:
-removal of molecules not participating in the reaction
-equalizing molecular trajectories and velocities, minimization of random motions
-spatial orientation of molecules
-controlled activation of the molecules
-control of energy distribution among the reaction products
- instantaneous removal of the reaction products




The enhanced control of molecular collisions in perfect reactors addresses directly the first of the four generic principles of Process Intensification: maximize the effectiveness of intra- and intermolecular events (Van Gerven and Stankiewicz, 2009). However, perfect chemical reactors need to address the other three principles as well:

-They need to provide each molecule with the same processing experience since processes in which all molecules undergo the same history, deliver ideally uniform products with minimum waste.  This means reduction of the macroscopic residence time distribution, dead zones, bypassing, and temperature gradients on one hand and enhancement of meso- and micromixing on the other hand. It can be easily shown that most of the reactor concepts developed thus far come short of this principle.
-They need to optimize the driving forces at every scale and maximize the specific interfacial areas to which those forces apply. This enables optimum transport rates across interfaces. 
- They need to maximize the synergistic effects from partial processes. An example of such synergistic effects can be seen in reactive separations, where the reaction equilibrium is shifted by removing the products in-situ from the reaction environment.

The challenges depicted in Figure 2 are obviously not new to the science and have been addressed by numerous fundamental research works in the field of chemical physics, using various forms of energy to align, orient and excite chemical molecules. An example of such fundamental research approach is shown in Figure 3, where carbonyl sulphide molecules are first aligned and oriented in an electric field and then dissociated by a laser beam. However, no attempts of developing reactor concepts directly addressing the above challenges have been made so far.



Fig. 3. (A) - Orientation of a molecular beam of carbonyl sulphide molecules moving along the z-axis by a hexapole electric field (left) followed by their dissociation by a laser beam acting along the x-axis (from Rakitzis, et al, 2004); (B) -  Probability plot of the molecular orientation of the OCS molecule; dotted arrows are proportional to the orientation probability of the OCS dipole moment along each direction.

The main objective and the ambition of the project is to make a groundbreaking step towards the perfect chemical reactors. The project addresses two basic challenges depicted in Figure 2 and focuses on engineering the enhanced control of molecular alignment, orientation and activation in spatially structured reactors consisting of arrays of milli- or microchannels, using different forms of electric or electromagnetic fields or combinations thereof.

More specifically, new concepts of reactors will be developed, in which the molecular alignment and orientation are controlled by the laser or by the electric field, or combination thereof, while the molecules are activated by
-the laser field,
-the light generated via in-situ nano-illumination of the catalyst, or
- the locally applied microwave field.

The methodology put forward in the project is entirely novel and consists in simultaneous and multi-scale application of the selected concepts of Process Intensification in four domains: spatial, thermodynamic, functional and temporal (Van Gerven and Stankiewicz, 2009). Table 1 summarizes those concepts and presents the corresponding argumentation.

Table 1. Concepts of Process Intensification relevant for development of perfect chemical reactors addressed by the present proposal





The project consists of two closely related phases. The first phase, which comprises two 2-year postdoctoral research activities, focuses on the control of molecular alignment, orientation and activation in milli- or micro-channels using laser and electric field as means for controlling molecules. Both research activities are expected to generate fundamental knowledge concerning the behaviour of the molecules moving in confined regular channels subjected to external fields. The second phase, which comprises three 4-year PhD research activities, focuses on the development of structured-reactor concepts with molecular activation control by means of the laser field, the in-situ nano-illumination of the catalysts and the locally applied microwave field, respectively.

In the project we focus on three molecules: H2O, CO2 and CH4. The reasons for this choice are fivefold:
-these molecules are simple and their properties are well described;
-they represent three out of four classes of molecules with regard to the rotational behaviour: linear (CO2); spherical tops (CH4) and asymmetric tops (H2O);
-the orientation and excitation of these molecules can be manipulated by external energy fields (e.g. Metz, et al., 1993; Kandel and Zare, 1998);
-reactions of these molecules are either monomolecular or bimolecular and present very good models for the experimental studies intended in this project;
- reactions of these molecules, which deliver hydrogen or synthesis gas, are of paramount importance for solving the sustainability issues concerning clean fuels and CO2 management; they include methane steam and dry reforming, water splitting and carbon dioxide splitting.

References
Herschbach, D. R., 1986, Nobel Prize Lecture.
Kandel, S.A., Zare, R. N., 1998, J. Chem. Phys, 109, 9719.
Lee, Y. T., 1986, Nobel Prize Lecture.
Metz, R. B., Thoemke, J. D., Pfeiffer, J. M., Crim, F. F., 1993, J. Chem. Phys., 99, 1744.
Polanyi, J. C., 1986, Nobel Prize Lecture.
Rakitzis, T. P., Van den Brom, A. L., Janssen, M. H., 2004, Science, 303, 1852.
Van Gerven, T., Stankiewicz, A., 2009, Ind. Eng. Chem. Res., 48, 2465.

Author: Prof. A. Stankiewicz
Published: 2012-02-14