Guido Sturm

A helicopter view of microwave application to chemical processes: reactions, separations, and equipment concepts

Abstract We present a helicopter view of microwave technology application to various reaction and separation processes, including liquid-phase organic syntheses, gas-solid catalytic reactions, polymerizations, extraction, distillation, crystallization, membrane separation, and adsorbent regeneration/dehydration. The overarching aim is to demonstrate the breadth of potential applications of microwave technology to chemical industry, with particular attention to separations, as this is a less explored microwave application area. In this context, some key findings, opinions, and developments in the relevant literature are summarized. In addition, the present microwave equipment concepts for chemical processes are critically reviewed and new ones are put forward, as we believe that an important milestone in the road from laboratory-scale microwave experimentation to industrial-scale microwave-assisted chemical processing is the design and development of innovative microwave equipment concepts tailored for specific chemical processes.

Complexity and Challenges in Noncontact High Temperature Measurements in Microwave-Assisted Catalytic Reactors

The complexity and challenges in noncontact temperature measurements inside microwave-heated catalytic reactors are presented in this paper. A custom-designed microwave cavity has been used to focus the microwave field on the catalyst and enable monitoring of the temperature field in 2D. A methodology to study the temperature distribution in the catalytic bed by using a thermal camera in combination with a thermocouple for a heterogeneous catalytic reaction (methane dry reforming) under microwave heating has been demonstrated. The effects of various variables that affect the accuracy of temperature recordings are discussed in detail. The necessity of having at least one contact sensor, such as a thermocouple, or some other microwave transparent sensor, is recommended to keep track of the temperature changes occurring in the catalytic bed during the reaction under microwave heating.

Microwave-Driven Plasma Gasification for Biomass Waste Treatment at Miniature Scale

Gasification technology may combine waste treatment with energy generation. Conventional gasification processes are bulky and inflexible. By using an external energy source, in the form of microwave-generated plasma, equipment size may be reduced and flexibility as regards to the feed composition may be increased. This type of gasification may be combined with fuel cell technology to generate electricity for on-site microwave generation. In this paper, we present short gasification experiments with cellulose, as model biomass compound, in air plasma. In order to optimize reaction rates, gasification and plasma generation are combined in the same volume in order to expose the solids to plasma of maximum intensity. The heating value of the fuel gas yield exceeds, up to 84%, the net microwave energy transmitted into the reactor over a range of operating conditions. As the system has not been optimized, in particular regarding residence time, the results give confidence that this concept can eventually be developed into a viable small-scale decentralized gasification technology.

Furfural Synthesis from d-Xylose in the Presence of Sodium Chloride: Microwave versus Conventional Heating

We investigate the existence of specific/nonthermal microwave effects for the dehydration reaction of xylose to furfural in the presence of NaCl. Such effects are reported for sugars dehydration reactions in several literature reports. To this end, we adopted three approaches that compare microwave-assisted experiments with a) conventional heating experiments from the literature; b) simulated conventional heating experiments using microwave-irradiated silicon carbide (SiC) vials; and at c) different power levels but the same temperature by using forced cooling. No significant differences in the reaction kinetics are observed using any of these methods. However, microwave heating still proves advantageous as it requires 30 % less forward power compared to conventional heating (SiC vial) to achieve the same furfural yield at a laboratory scale.

Subtle Microwave-Induced Overheating Effects in an Industrial Demethylation Reaction and Their Direct Use in the Development of an Innovative Microwave Reactor

A systematic study of the conventional and microwave (MW) kinetics of an industrially relevant demethylation reaction is presented. In using industrially relevant reaction conditions the dominant influence of the solvent on the MW energy dissipation is avoided. Below the boiling point, the effect of MWs on the activation energy Ea and k0 is found nonexistent. Interestingly, under reflux conditions, the microwave-heated (MWH) reaction displays very pronounced zero-order kinetics, displaying a much higher reaction rate than observed for the conventionally thermal-heated (CTH) reaction. This is related to a different gas product (methyl bromide, MeBr) removal mechanism, changing from classic nucleation into gaseous bubbles to a facilitated removal through escaping gases/vapors. Additionally, the use of MWs compensates better for the strong heat losses in this reaction, associated with the boiling of HBr/water and the loss of MeBr, than under CTH. Through modeling, MWH was shown to occur inhomogeneously around gas/liquid interfaces, resulting in localized overheating in the very near vicinity of the bubbles, overall increasing the average heating rate in the bubble vicinity vis-à-vis the bulk of the liquid. Based on these observations and findings, a novel continuous reactor concept is proposed in which the escaping MeBr and the generated HBr/water vapors are the main driving forces for circulation. This reactor concept is generic in that it offers a viable and low cost option for the use of very strong acids and the managed removal/quenching of gaseous byproducts.

CHAPTER 4:Microwave Reactor Concepts: From Resonant Cavities to Traveling Fields

Microwave chemistry has been investigated for nearly thirty years with many notable results being published on apparent process enhancement due to microwave exposure. Conclusive proof of beneficial microwave-chemical interactions is lacking though, as are design rules for successful implementation of microwave-chemical processing systems. In this chapter, the main cause for this is asserted to be the current absence both of suitable instrumentation for research, and processing equipment that merges chemistry with electromagnetic aspects. Several concepts are presented to show how these challenges may be addressed.