Process Intensification

M icro-chem ical system s can be considered as a lim iting case o f the Process Intensification (PI) approach to chem ical plant design. In general, PI is defined as the developm ent o f novel apparatus and techniques that can b rin g dram atic im provem ents in m anufacturing and processing, substantially decreasing equipm ent size, production/capacity ratio, energy consum ption or w aste production and ultim ately resulting in cheaper, sustainable technologies (S tankiew icz and M oulijn, 2000). PI techniques w ould, therefore, include high pressure, tem perature or concentration operations to im prove m ass transfer and reaction rates and utilisation o f the fluid dynam ic environm ent w ithin reactors so that perform ance is dictated by intrinsic reaction kinetics and not by heat and m ass transfer (G reen, 1998, G reen, et al, 1999). Process intensifying equipm ent includes m icroengineered reactors.

G as-liquid catalytic reactions have been carried out in m icro-packed beds o f catalytic particles w ith size 50-75 pm (see Figure 2.5, Losey, et al, 1999, 2000). To im prove gas-liquid contacting a m ultilam ination m ixer w as used at the reaction channel inlet. O xidation o f benzaldehyde w as safely operated at tem peratures as high as 140 °C w ith pure oxygen and organic solvent due to the small reaction volum e (4 pi).

Figure 2.5; G as-liquid m icro-packed bed with catalyst restrainer (L o sey , et al, 2 0 0 1 )

Hydrogenation o f cyclohexene was used to characterise the mass transfer coefficient. Values o f the mass transfer coefficient {kid) were determined to be 5-15 s '\ which is two orders o f magnitude larger than those reported for traditional multi-phase fixed bed reactors. The greatly improved mass transfer in the microreactor can be attributed to the large external particle surface area and the well-controlled distribution o f gas and liquid over small catalyst particles, hence the high gas-liquid interfacial area generated by the microreactor.

Hessel, et al (1999) used direct contact o f the gas and liquid phases, which were either introduced together in microchannels (microbubble columns) or were flowing in separate layers in falling film reactors. The falling film reactor consisted o f two parts: a gas-liquid reaction chamber with a platelet comprising a large number o f microchannels (100 x 300 pm cross-section) to generate a thin liquid film o f several tens o f microns thickness by means o f gravity, and a heat exchanger located on the

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back o f the platelet. The microbubble column consisted o f a static mixer and a reaction channel array with heat exchangers. The flow pattern in channels was found to be Taylor or bubble train flow at low gas superficial velocities and annular flow at higher gas superficial velocities. Specific interfacial areas obtained were up to 15000

m^/w? and 27000 m^/m^ for the microbubble column and the falling film reactor

respectively. These represent a large increase compared to conventional bubble columns (50-600 rr^lvci) or even impinging jets for intensive gas-liquid contacting (2700 m^/m^). The microbubble column gave much higher conversions (in some cases 100 %) in a model CO2 absorption reaction compared to other gas-liquid dispersing micromixers (for the same residence times). These reactors were also employed for the direct fluorination o f toluene dissolved in acetonitrile or methanol

(Hessel, et al, 1999, Jâhnish, et al, 2000). Both reactors were able to operate at high hydrocarbon concentrations and high fluorine content in a nitrogen carrier (up to 50 vol% o f fluorine) at a temperature o f about -17 °C. Yields o f up to 28 % o f monofluorinated ortho and para products for toluene conversions o f 76 % were obtained, which is comparable with the industrially applied Schiemann process. The space-time yield o f the microreaetors, based on the channel volume, was several orders o f magnitude higher than those o f a laboratory bubble column. Direct fluorination was also conducted in a microreactor developed by Chambers and Spink (1999). The microreactor was fabricated on a block o f nickel (or copper) with a groove as reaction channel and channels beneath the groove as a heat exchanger. The gas-liquid mixing was carried out in the reaction channel through annular flow, offering the advantage o f very large surface to volume ratio for the liquid phase. Various selective fluorinations were tested in this microreactor, e.g. sulfur pentafluoride deriatives were obtained from the direct fluorination o f di(m-nitophenyl) disulphide with 44-75 % yield.

Another approach for bringing two phases in contact was proposed by Robins, et al (1997). Open microchannels etched in silicon and glass wafers are brought together, so that slits with width as small as 5 pm are formed after careful alignment and bonding. When two different fluids (gas and liquid or organic and inorganic phases) flow in the two channels, an interface is formed at the slit which allows mass transfer from one channel to the other while keeping the two phases separated. Alternatively the interface can be pinned by using metal micromeshes or laser-machined polymer membranes to keep the phases separate (Turner, et al, 2000, Martin, et al, 1999). Fast mass transfer between phases can be achieved as demonstrated by acid neutralisation.

2.6.7 Extraterrestrial Processing

Utilisation o f micro/nano-technologies in the space sector is very promising. Lightweight, compact chemical systems can be employed to reduce total resources required yet provide higher system performance per unit cost and mass. Research is being conducted at the Pacific Northwest National Laboratory (PNNL) for the NASA In-Situ Resource Utilization (ISRU) program planned for future missions to Mars. One aspect o f this program is In-situ Propellant production (ISSP) and involves reacting carbon dioxide from the Martian environment with stored hydrogen from Earth to produce propellants and oxygen for the return trip so that the required launch mass from Earth is reduced. For this purpose a microchemical plant is being designed which includes the collection and pressurisation o f atmospheric carbon dioxide, conversion reactors, chemical separators, heat exchangers and cryogenic storage (TeGrotenhuis, et al, 2000). Due to negligible convective heat transfer within the low- pressure environment o f space, high energy efficiency is achieved through extensive recuperation and energy cascading from hot to cold unit operations to minimise thermal and electrical wastage. NASA plans to launch such microchemical plants to Mars in 2011, followed by human missions in 2013 (Wegeng and Drost, 1998). Other applications o f microreaetors in space include micro-fuel cells, compact clean-up units for waste treatment, portable heating and cooling systems. Mining and chemical processing o f raw materials on the Moon, Mars, M ars’ moons, asteroids, etc. is also being considered.

However, there are significant challenges to extraterrestrial chemical processing plants. Energy management is one o f the main concerns for space-based processes. The systems should be able to operate very efficiently and reliably for long periods o f

time. Such developments require high cost for adaptation to space use due to the reduced gravity and pressure environment. These advancements though would result in a significant reduction in payloads and spacecraft size (de Aragon, 1998).

2.6.8 Scale-out

Repetition o f basic units o f microengineered reactors provides the ability to increase production throughput by scaling out (or numbering up) instead o f scaling up (Lerou, et al, 1996). Hence, instead o f following the usual laboratory-pilot plant-commercial scale reactor route, the microreactor architecture can first be optimised and then production increased by the use o f a cascade o f similar reactors. This approach guarantees that the critical features o f the basic unit are kept constant thus minimising the risk o f different performance at the large scale. The development time o f a process from laboratory to production would also be reduced and phased increase in production can be achieved by progressively adding parallel reaction modules.

However, the concept o f scale-out is not without its challenges. Microengineered reactors m ust achieve uniform flow distribution so that the flowrates in each unit are identical. This is accomplished in conventional catalytic multi-tubular reactors, but the different manufacturing methods and tolerances in microengineered reactors may require different approaches. Inlet regions in the reactors or their channel geometry must be designed to guarantee flow equipartition between channels (Ehrfeld, et al, 2000b, Commenge, et al, 2000). Where small channels and orifices are present, plugging by particulates can be a problem as this can cause flow maldistribution and eventually even reactor failure. Hence, filtration is required. Particles can also form

inside the reactors and in this case appropriate reactor design and surface engineering will be needed to avoid fouling.

For reliable operation o f scaled-out systems, heat management systems need to achieve the same local thermal environment accomplish in each sub-unit. As such, the development o f simple and robust reactor instrumentation and control is imperative. MIT and Dupont’s approach is to integrate feed gas valves, flowmeters, power supply and control circuitry (Quiram, et al, 2000b). These units along with electrical and fluidic interconnects will be packaged in a multiple microreactor test station for gas phase systems with a small footprint.

Despite the above problems, a few examples o f the scale-out concept exist. Axiva followed this approach for an acrylate polymerisation system where a micromixer was demonstrated to aid in avoiding the production o f high molecular weight compounds. On the laboratory scale, a throughput o f 6.6. kg/h was achieved with one mixer array containing ten mixing units. For the industrial scale, 32 such micromixers can be combined in an assembly enabling an acrylate capacity o f 2000 tons/year (Ehrfeld, et al, 2000). Merck used 5 minireactors for a ketone reduction production unit after successful trials o f pilot-scale minireactors (as detailed in Section 2.6.4). This involved the automation o f the entire process and the proper splitting o f the feed streams to the individual minireactors (Krummradt, et al, 2000).