Supercritical route for green materials
Professor Tadafumi Adschiri, Tohoku University, Japan
Green materials processing is a philosophy of chemical research and engineering to encourage the design of products and processes that minimize the use and generation of hazardous substances, which involves 1) contribution of products to minimize environmental problems (CO2 emission, environmental cleaning catalyst etc.), 2) recycle of materials to resources, 3) holistic life cycle assessment of the materials, and 4) combined multiple technological and operational systems for reduction of energy and resources. Supercritical fluids technology is expected to contribute for new materials synthesis with the green sustainable chemistry route, especially for nanomaterials.
So far, variety of materials have been developed, including ceramics, metals and polymers, but recent needs in the industries are of multi-functions of ceramics/metals and polymers. For fabricating multi-functional materials, we proposed a new method to synthesize organic modified nanoparticles (NPs) in supercritical water. Since the organic molecules and metal salt aqueous solutions are miscible under the supercritical state, and water molecule works as an acid/base catalyst for the reactions, organic-inorganic conjugate nanoparticles can be synthesized under the condition. This synthesis method can control the exposed surface of NPs, which shows high catalytic activity of nano-catalysis; This promotes the bitumen or biomass waste decomposition (endothermic reaction) at lower temperature without coke formation. This gives rise to recover the waste heat and the waste treatment problems at the same time, namely solve the energy (CO2) problems.
Supercritical water oxidation (SCWO) for the destruction of hazardous wastes – better than incineration
Dr Bushra Al-Duri, University of Birmingham
Supercritical water oxidation (SCWO) is based on the complete miscibility of supercritical water (SCW) with organics including hydrocarbons; biopolymers; and all gases, rendering SCW a superior reaction medium for the destruction of a diverse range of chemically stable wastes existent in municipal, clinical and industrial aqueous effluents, otherwise treated by incineration. The process is highly exothermic, generating high-grade heat, which is recoverable for other uses like production of electricity, after energy integration within the system. This contribution presents the main activities and studies at the University of Birmingham – UK. The scope of work is to achieve high performance, while using ‘simple’ reactor design, via improving the reaction kinetics. Laboratory investigations focus on the destruction of aliphatic and aromatic Ncontaining compounds, due to nitrogen abundance in waste and its interesting chemistry.
To this end, investigations have been carried out in a continuous (plug flow) reactor bench scale rig. Two approaches have been adopted: (i) ‘Split-oxidant’ system, where oxidant is split and fed via two inlets at different ratios, and (ii) the cofuel approach where iso-propyl alcohol is premixed with the feedstock. Both approaches gave positive outcomes. Theoretically, kinetic investigations have been conducted on the aliphatic, aromatic and real waste with success. Also, detailed simulation of SCW heat transfer thermodynamic properties has been investigated using COMSOL, PROII and Matlab. Based on a PATENT (in application), a £0.5m industrially funded project of SCWO process prototype will start next month, including detailed consideration of the various process aspects, with a view of commercialization.
Synthesis of nanostructured materials using supercritical CO2
Professor Albertina Cabanas, Universidad Complutense de Madrid, Spain
Supercritical CO2 (scCO2) is emerging as an excellent medium to prepared and/or modify nanostructured materials. Beside the environmental benefits, its high diffusivity and low viscosity and surface tension favour the penetration of precursors dissolved in scCO2 into nanostructures such as nanopores and the impregnation of high surface area materials, while preserving the support structure.
In particular, scCO2 can be used as the solvent and/or reaction media to deposit metals or metal oxides on different supports. Depending on the methodology employed and the experimental conditions nanoparticles, nanowires or thin films can be obtained. The preparation of these nanostructured metal-composite materials has numerous applications in catalysis, microelectronics, gas separation, hydrogen storage, sensors, fuel cells. In this presentation, they give examples of the deposition of Pd, Pt, Ru and Ni nanoparticles into mesoporous silica, carbon and graphene sheets using scCO2. The materials prepared have been tested as heterogeneous catalysts in hydrogenation reactions showing advantages over commercial catalyst.
The surface modification of highly porous materials can be also carried out in scCO2 by reaction of soluble silanes. Using this approach, they have introduced amine and thiol groups on the surface of mesoporous silica. The amine modified materials are proposed as CO2 sorbents in carbon capture processes. The thiol modified materials can serve as metal adsorbents. These surface modified supports can be also used in metal deposition experiments. In comparison to the conventional process using toluene, the surface modification in scCO2 is faster and yields, in some cases, larger silane loads.
Industrial scale production of nanomaterials using continuous hydrothermal synthesis
Professor Ed Lester, University of Nottingham
Work is currently underway to build a full scale continuous hydrothermal synthesis plant in the UK. The plant uses a counter current reactor, designed at Nottingham. The plant will be capable of producing up to 2000 tons/year (dry weight equivalent) of a range of nanomaterials, from metals, metal oxides, hydroxides, carbonates and sulphides, as well as some more unusual materials such as metal organic frameworks. Specific questions need to be answered in the development of this plant and the presentation will cover each area in some detail.
Life Cycle Analysis – how does this process compare with other processes for nanomaterial production?
Formulation – can the nanomaterials be formulated continuously during production to facilitate product collection?
Waste treatment – what is present in the waste water post production? The precursors tend to be simple inorganic (e.g. nitrates) or organic (e.g. formates) salts, so what happens to the counter ion?
Recycling – can this waste water be recycled and/or can it be discharged without chemical pretreatment? The weight loading from the outlet tends to be around 1% wt/wt or less so can the products can be collected easily and effectively to produce a clean waste stream?
Reactor Performance – what are the fundamental limitations of the counter current reactor system in terms of mixing and fluid dynamics? As the Reynolds numbers increase with increasing flow rate, what happens to the mixing regime around the nozzle outlet?
Scale up design and sustainability– What does the plant look like and how can the energy demands be reduced through heat integration?
Scale out – can a system be built that allows multiple reactors to operate simultaneously? Does this allow different products at the same time?
To the authors knowledge this will be the largest continuous hydrothermal synthesis plant in the world, and should be completed by mid 2015. The plant itself should act as a demonstration facility for production at large scale and should hopefully help to provide a roadmap for future plants.