Targeted waveform enhanced plasma microreactor: Engineering Chemistry at the Interface of Microbubbles

Lead Research Organisation: University of Sheffield
Department Name: Chemical & Biological Engineering


One class of electrochemical reaction are reactions in the plasma state. The PI and his team have been pioneering plasma microreactors that feed directly into microbubbles for the last decade. With the output of the plasma reactor entering the microbubble directly, the maximum activation is retained in the bubble, which then mediates the formation of active species on the microbubble interface. Recently, this approach has been used to catalyse the esterification reaction of free fatty acids to form esters (particularly biodiesel).

More than the effectiveness of the plasma activated microbubble reaction, microbubble processing is not limited by surface area of "electrode" in quite the same way. The grand aim of this proposal is to create heterogeneous catalysis capability by tuning the plasma activated species on the gas-liquid interface of microbubbles. Conventional electrochemistry has severe issues around upscaling. Plasma microreactors, particularly those that feed into liquid media as injected microbubbles, are a class of electrochemical reactors that can potentially upscale readily. Microbubbles can have hectares of gas-liquid interface per cubic metre of liquid reactant volume, so if the (plasma)electrochemical reaction can be catalysed on the gas-liquid interface, high throughput reaction rates can be achieved in large volume, continuous flow reactors. Already achieved in pilot plant studies of anaerobic digestion is a bubble surface area flux of 0.15 hectares/sec! If even a fraction of this surface area flux is effective at mediating plasma chemical transformations, the rate of transformation processes should far exceed conventional heterogeneous reactions.

This project aims to optimise how the formation of plasma-activated species is coupled to the transient operation of the plasma electronics that create the excited species that eventually react at microbubble gas-liquid interfaces. Preliminary studies show that the composition of an excited air plasma, for instance, can dramatically change with the contacting time in the reactor and the electric field applied. They also suggest that how that electric field is applied in space and time dramatically affects the chemical composition of the plasma, and consequently what chemical reactions dominate the microbubble mediated gas-liquid chemistry. The purpose of this proposal is to characterise this coupling between the time-varying plasma electronics output, as implemented with tuneable electrical engineering design, and the induced chemistry of the plasma and microbubble mediated reaction. The characterisation will be captured in computer models that permit inversion; from the desired chemical outputs, the optimum plasma electronics design, control and operating mode ("the waveform") will be predicted.

In the UK plasma chemistry research is vibrant but the work is mainly centred on nuclear science, capactively coupled plasmas with applications to surface treatment (i.e. EP/K018388/1) and medical applications. Globally, several research groups are investigating tailored waveform plasmas more generally but not with specific application to chemical generation on an industrial scale. The proposed closed-loop control of tailored waveform plasma microbubble reactors offers new possibilities to increase efficiency, throughput and scale-up. This, therefore, complements the contributions from these research groups (both national and international) and so will stimulate new research and commercial opportunities. By bringing together experts from the interface of chemical engineering, electrical engineering and mathematics who, together with some eight project partners providing £160k of support, can drive a blue-skies approach to targeted waveform control of plasma reactions (using novel chemical modelling and waveform generator design) while blazing a trail for industrial adaptation to a game-changing approach to chemical production.

Planned Impact

We start from two premises that a sustainable future for bio/processing for food, fuel and commodity chemicals requires (i) inventive problem solving and innovation for much more efficient and effective transformations than conventional technology; (ii) *scalable* electrochemical processing in some guise will be essential to provide at least the Gibbs free energy of reaction for endergonic reactions, replacing fossil fuels, and potentially for the *activation energy* or catalysis organising principle that reduces activation energy.

With limited proof of concept already achieved with *untuned* plasma sources, we expect that the near term impact will result from three classes of applications:
1. Chemical processing industries where novel and innovative heterogeneous catalysis, potentially with electricity providing the Gibbs free energy of reaction, as well as catalytic impetus and in situ product removal, are needed for disruptive change in processing.
2. Bio/chemical producers in industrial biotech, where in situ disinfection in fermenters (potentially preferential in attacking targeted contamination) and pretreatment of feedstocks for breakdown, say lignocellulosic or wet food waste, as simultaneous disinfection are needed for sustainable processing.
3. Integrated water purification or wastewater treatment, potentially for novel cleaning devices, and distributed (end of tap?) or municipal water purification systems.

Our impact strategy includes the following activities and features to set in motion:

1. Maintain regular briefings with at least twelve companies that have expressed an interest in untuned plasma-activated microbubbles for the general targets above.
2. Incubate more interactions from the agricultural and sustainable biomass utilisation sectors through the Anaerobic Digester and Biogas Association (named finalist with another microbubble intensification process for "project of the year", one of 3 from 200 nominees) and the trade conference on bioethanol/ethanol and biofuels (biannually) organised by FO Licht where the PI's InnovateUK/EPSRC "flagship" project on intensification of bioethanol processing is commissioning a pilot plant.
3. Disseminate bio/processing related innovations through the appropriate NIBBs when awarded - PI is Co-I of one such shortlisted NIBB proposal on waste gas utilisation. Disseminate chemical applications through c1net and CDUK where the PI is a founder member.
4. Hold a "mid-term" consortium building exercise - a workshop aimed at potential end users, development partners, supply chain members and academics - which will serve a "pathfinding" role for the potential different impact directions. The last such invited consortium building exercise held by the PI resulted in InnovateUK (£2.2m, bioethanol) and ERA-IB (EUR2.3m, biobutanol) consortium grants in 2016.
5. From expressions of interest and follow up planning discussions with participants of the mid-term consortium building exercise, agree two "Business Interaction Vouchers" of ~£10k each for small feasibility studies which require similar levels of partner input.
6. Engage in dissemination activities with a variety of target audiences - general public, academic conferences and journal articles, and trade conferences and journal articles. The PI has a standing offer from the editor of The Chemical Engineer (premier trade journal) for feature articles (used once in 2012) on microbubble advances and had an invited article for the International Journal of Sugar (2011).


10 25 50