Conference Paper

Chances in modern plasma scale-up: seen holistically.

Authors:
  • Dow Benelux B.V.
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Abstract

Process-design intensification: Micro Process Technology has given strong push to continuous chemical manufacture via faci¬litating heat and mass transfer; named transport intensification = majorly developed during 1995-2005 and on-going. The next big step was to develop a tailored process chemistry in flow under highly intensified conditions – which is one essence among the developing field of Flow Chemistry = majorly developed during 2005-2014 and on-going. This has been coined Novel Process Windows [1-3] and has two research pillars, – the exploration of unusual and typically harsh process conditions (chemical intensification) and, in a more holistic picture, a completely new and often simpler process design (process-design intensification). New process designs in pharma/fine chem = plug-in strategy Starting from such new reaction designs, there is exactly now the big chance to develop new process designs in flow = this is considered to be an utmost task in the next 10 years. In the past flow and other intensified plants were mainly made by retrofit For example, the exchange of a batch reactor versus the new intensified flow reactor was the major step. Engineering was used “just” to enable the Green and Flow Chemistry benefits. Thus, PI process design for pharma and fine chemistry applications is still mostly done in a plug-in (retrofit) manner and has been demonstrated many times by industries in the last years. New process designs in bulk-chem/energy = plug-in strategy: Yet, there is also an own intensification momentum in Green Engineering apart from provi¬ding the mentioned service. PI process design for specialty and bulk chemistry is supposed to change virtually all components (‘holistic process design’). Such end-to-end concepts have the advantage to exert major impact on CAPEX/OPEX costs, sustainability, and energy con¬sumption. This is demonstrated at process designs of superficial (400 t/a) direct adipic acid process standing for the fine- and bulk-chemical market. It will be outlined as well for the on-going process design developments for a plasma-based nitrogen fixation plant as opposed to the classical Haber-Bosch process. Beyond the single plasma plant itself, it comes out that an end-to-end vision on a ChemPark-Verbund scale and even more end-to-end on a (national) energy grid scale is mandatory as well which naturally is difficult to be implemented in early laboratory measurements (yet finally needed) This holistically guided work is done in the frame of the EU Project MAPSYN, which has the mission to explore alternative energies (plasma, MW, US) for chemical process industries. The development partner is Evonik who run commercial plasma processes. Features on res¬pective plasma catalysis [4], knowledge gaps in processing [5], and energy considerations [6] have already been given in literature by us. First own results with a newly designed plasma setup will be presented, characterizing plasma formation in catalyst-loaded and –free DBD and GlidArc minireactors Modular, compact, intensified plants at movale container-scale = Factories of Tomorrow: On top of that, the embedment of flow and other intensified (plasma) processing into compact, mobile and modular chemical production platforms (‘Future Factories’; container) such as Evonik’s Evotrainer is discussed. A recent cash-flow analysis gives evidence on net-present value and financial risk-assessment for the pharma, fine-chemical and bulk-chemical markets. Distributed production / future factories are topics of relevance as well for energy/biofuel gener¬ation. The EU-Large-Scale project BIOGO (www.biogo.eu) resear¬ches and develops advanced nanocatalysts, which are allied with advanced reactor concepts to realise a modular, highly efficient, integrated process for the production of fuels from renewable bio-oils and biogas. Acknowledgments: We kindly acknowledge support by the ERC Advanced Grant on "Novel Process Windows” (grant no. 267443) and the EU FP7-NMP MAPSYN project (grant no. CP-IP 309376). References: [1] Hessel, V.; Chem. Eng. Technol. 2009, 32 (11)1655. [2] Hessel, V.; Cortese, B.; de Croon, M.H..J.M. de; Chem. Eng. Sci. 2011, 66 (7), 1426. [3] Hessel, V.; Kralisch, D.; Kockmann, N.; Noel, T.; Wang, Q.; ChemSusChem 2013, 6, 5 (2013) 746 [4] Hessel, V.; Anastasopoulou, A.; Wang, Q.; Kolb, G.A.; Lang, J. Catalysis Today, 2013, 211, 9. [5] Hessel, V.; Cravotto, G.; Fitzpatrick, P.; Patil, B.; Lang, J.; Bonrath, W. Chemical Engineering and Processing: Process Intensification, 2013, 71, 19. [6] Hessel, V.; Anastasopoulou, A.; Wang, Q.; Lang, J. Processes, 2014, Submitted.

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Thesis
Nitrogen, which constitutes 78% of the Earth’s atmosphere, is crucial not only for the growth of plants and living beings but also for the production of industrially important chemicals. Centralized chemical N2 fixation by “Haber-Bosch process” is one of the most important chemical processes; it sustains over 40% of the global population by producing > 130 million tons of NH3 per year. However, it consumes ~1-2% of the world’s total energy and emits 300 million tons of CO2. Plasma assisted N2 fixation, via nitric oxide or NH3 production, is attractive alternative due to inherent non-equilibrium conditions, lower energy demand and the prospect to use an alternative energy sources. This PhD thesis is the part of the EU funded project “MAPSYN”, which aims at investigation of plasma assisted N2 fixation reactions by employing nonthermal plasma with and without catalysis. Achieving higher energy efficiency and product concentration are the key motives of this research work. The focus of this thesis is to provide insights into the reactor concepts and its operational regimes to enable development of demonstration unit at industrial partner’s site at Evonik Industries, Germany. Plasma assisted nitric oxide and NH3 synthesis reactions were studied in gliding arc (GA) and dielectric barrier discharge (DBD) reactor, without and with catalyst respectively. Nitric oxide synthesis (chapter 2, 3 and 4): The production of nitric oxide (NOx =NO+NO2) was investigated in 2 different prototypes of GA reactor (chapter 2 and 3) and a catalytic DBD reactor (chapter 4). First electrical characterization of the GA reactor was performed to ascertain the optimum operating regime for the reactor and the accompanying power source in chapter 2. Operation of GA reactor in kHz frequency range with power delivered in microsecond pulses was found to be optimum. In the next step performance of the GA reactor was explore for range of process conditions such as feed mixture (air, air+O2, and N2+O2), O2 % in feed, feed flowrates, argon addition, and feed preheating at varying specific energy input aiming at higher NOx production. It is reported in chapter 2. Based on the improved understanding of the GA reactor, it was further intensified by developing an advanced version, which had thinner electrodes (to avoid feed bypassing) and was more flexible to change the electrode material and electrode discharge gap as reported in chapter 3. A systematic study was undertaken to establish the effect of electrode material and discharge gap on the GA dynamics and the NOx formation, aiming to increase the NOx concentration and energy efficiency. Moreover, the performance of GA reactor in terms of V-I signal and NOx formation was correlated with gliding arc formation and propagation as revealed using high-speed imaging and optical emission spectroscopy. Air+O2 gave NOx concentration very similar to N2+O2, so air or air+O2 can be used as a feed in container plant. Oxygen content of 40-48% was found as the optimum to maximize NOx production. Moreover, lower flowrates produced higher concentrations of NOx due to associated higher GA processing time. The high-speed photography revealed that the average number of GA cycles and GA velocity increases with the gas flowrate, emphasizing that the gas flowrate has major impact on the GA dynamics and it eventually determines the reaction kinetics. The highest concentration of NOx realized was 2 vol% for 1 L/min, while the lowest energy consumption of 2.8 MJ/mol was achieved for 6 L/min. The NOx production was investigated in a DBD reactor by packing different catalyst supports and the metal oxide catalysts as reported in chapter 4. The support materials and their particle sizes both had a significant effect on the concentration of NOx. This was attributed to different surface areas, relative dielectric constants and particles shapes. The γ-Al2O3 with smallest particles size of 250-160 μm, gave the highest concentration of NOx. The NOx concentration of 5700 ppm was reached at the highest residence time of 0.4 sec investigated and an N2/O2 feed ratio of 1 was found to be the most optimum for NOx production. A 5 % WO3/ γ-Al2O3 catalyst increased the NOx concentration further by about 10 % compared to γ-Al2O3. This study showed that O2 activation plays a minor role in plasma catalytic N2 fixation with the main role ascribed to the generation of microdischarges on sharp edges of large-surface area plasma catalysts. Ammonia synthesis (chapter 5 and 6): Plasma assisted NH3 synthesis was studied in a DBD reactor packed with various catalyst supports (chapter 5) and catalysts (metal oxides and Pt group metals in chapter 6). The commonly used catalyst supports, γ-Al2O3, α-Al2O3, TiO2, MgO, CaO, quartz wool, and BaTiO3, were explore for the synergetic effect between plasma and these materials. All the catalyst supports had substantial effect on the NH3 production, very similar to NOx synthesis studies. The quartz wool followed by γ-Al2O3 produce the highest amount of ammonia, 2900 and 2700 ppm respectively, and the particles with average diameter of 200 μm yielded 64% higher concentration of NH3 than 1300 μm diameter particles. A feed flow ratio > 2 gave higher concentrations of NH3 and improved energy efficiency than the stoichiometric feed ratio of 0.33. The feed flowrate had a negligible influence, however specific energy input per unit volume showed greater impact on the NH3 production. At 0.4 L/min, 3505 ppm of NH3 was produce with an energy efficiency of 1.23 g NH3/kWh and per pass N2 conversion of 0.26%. Chapter-6 reports the screening of 16 transition metal oxides supported on γ-Al2O3 for plasma assisted ammonia synthesis. Moreover, influence of the feed ratio (N2/H2), specific energy input and temperature is also investigated for all these catalysts. All the catalysts found to have substantial effect on the ammonia production. The 2 % Rh from Pt-group and 5 % NiO from transition metal oxides produced the highest concentration of ammonia. An optimum feed flow ratio found to be between 1 and 2, depending on the supported metals. With 2 % Rh at 0.1 L/min, 1.43 vol % of ammonia could be produced with an energy efficiency of 0.94 g/kWh and per pass hydrogen conversion as high as 6.4 % was realized. Finally, a mechanism for plasma assisted catalytic ammonia formation have been proposed and supported with the optical emission data. In conclusion, this thesis has demonstrated that the N2 could be fixed in the form of NOx and NH3 in sizable amounts at substantially lower temperatures and at atmospheric pressure by employing non-thermal plasma reactors such as GA and catalytic DBD reactor. Furthermore, this work developed in-depth understanding of plasma assisted N2 fixation by clearly elucidating the interplay between plasma-catalyst and the reactor’s geometrical and operational parameters via NOx and NH3 synthesis. These promising results could be further exploited for fertilizer production via “Fertilizing with the Wind” concept for stranded places e.g. African countries or hydroponics in horticulture.
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