Plug-flow Operation Research Articles

Kinetic Isoconversion Loop Catalysis: A Reactor Operation Mode To Investigate Slow Catalyst Deactivation Processes, with Ni/Al2O3 for the Dry Reforming of Methane

Deactivation of heterogeneous catalysts is an issue of high importance and extensive study. However, under normal isothermal plug flow operation the experimental conditions within the catalyst bed ...

Oscillatory fluid motion unlocks plug flow operation in helical tube reactors at lower Reynolds numbers (Re ≤ 10)

Flow in helical tubes exhibits plug flow characteristics above a critical flow rate due to the formation of Dean vortices. In this study, we report the novel use of oscillatory flow inside coiled tubes to achieve high degrees of plug flow at lower flow rates. The plug flow enhancement appeared to ‘switch-on’ then ‘switch-off’ as the oscillation intensity was increased, corresponding to ‘oscillatory Dean numbers’ of Deo = 70–200, where secondary instabilities are expected. It is believed that oscillatory motion leads to the periodic formation and unravelling of Dean vortices. For a 5 mm tube diameter, the effects of radius of curvature (12.5–32.5 mm), tube pitch (7.5–12.5 mm), net flow rate (Ren = 10–50), oscillation frequency (2–8 Hz) and oscillation amplitude (1–8 mm) were studied. The prototype helical coil reactors were rapidly manufactured using 3D printing. The optimal conditions for plug flow corresponded to Deo/Ren = 2–8, at Strouhal numbers in the range St = 1–2. Plug flow was observed at net flow Reynolds numbers at least as low as Ren = 10, compared to Ren = 70–300 in the literature for coiled tubes subjected to steady flows.

Oscillatory Flow Reactors (OFRs) for Continuous Manufacturing and Crystallization

Continuous crystallization is an attractive approach for the delivery of consistent particles with specified critical quality attributes (CQAs), which are attracting increased interest for the manufacture of high value materials, including fine chemicals and pharmaceuticals. Oscillatory flow reactors (OFRs) offer a suitable platform to deliver consistent operating conditions under plug-flow operation while maintaining a controlled steady state. This review provides a brief overview of OFR technology before outlining the operating principles and summarizing applications, emphasizing the use for controlled continuous crystallization. While significant progress has been made to date, areas for further development are highlighted that will enhance the range of applications and ease of implementation of OFR technology. These depend on specific applications but include scale down, materials of construction suitable for chemical compatibility, encrustation mitigation, the enhancement of robust operation via auto...

Evaluation of crystallization kinetics of adipic acid in an oscillatory baffled crystallizer

For solution crystallization, nucleation can be characterized by the maximum sub-cooling (or metastable limit), which is known to vary with numerous process parameters. The relationship between the metastable limit and cooling rate is of particular interest, as it can be utilized to derive nucleation kinetic parameters. However, this relationship is open to interpretation. This work presents the application of three such interpretations (Nývlt, Kubota and a population balance based method) to a cooling crystallization of adipic acid in an oscillatory baffled crystallizer, a relatively new type of crystallizer with increased studies and applications in continuous plug flow operation. It also considers the role the device employed to detect nucleation events plays in the derived kinetic parameters. The result of this study shows that although all three interpretations can reasonably predict the maximum sub-cooling over a tested range of cooling rates, the linear assumptions in the Nývlt and Kubota interpretations give increased deviations from the experimental data, in particular for faster cooling rates. In contrast to the two aforementioned models, the population balance based method maintains a minimal deviation across the whole range of cooling rates used. In addition, although the population balance method does not consider the sensitivity of detection tools in its implementation, while the Kubota method does, the sensitivity of nucleation detection is reflected in the derived nucleation rate constants.

Comparison of two different anaerobic feeding strategies to establish a stable aerobic granulated sludge bed

Two different anaerobic feeding strategies were compared to optimize the development and performance of aerobic granules. A stable aerobic granulation of activated sludge was achieved with an anaerobic plug flow operation (PI) and a fast influent step followed by an anaerobic mixing phase (PII). Two lab scale sequencing batch reactors (SBRs) were operated to test the different operation modes. PI with plug flow and a reactor H/D (height/diameter) ratio of 9 achieved a biomass concentration of 20 gTSS/L and an effluent TSS concentration of 0.10 gTSS/L. PII with the mixed anaerobic phase directly after feeding and a reactor H/D ratio of 2 achieved a biomass concentration of 9 gTSS/L and an effluent quality of 0.05 gTSS/L. Furthermore, it is shown that the plug flow regime during anaerobic feeding together with the lower H/D ratio of 2 led to channeling effects, which resulted in lower storage of organic carbon and a general destabilization of the granulation process. Compared to the plug flow regime (PI), the anaerobic mixing (PII) provided lower substrate gradients within the biofilm. However, these disadvantages could be compensated by higher mass transfer coefficients in PII (kL = 0.3 m/d for PI; kL = 86 m/d for PII) during the anaerobic phase.

Performance evaluation of the conservation element and solution element method in SMB process simulation

The computational performance of the space–time conservation element and solution element method (CESE) is evaluated in solving simulated moving bed (SMB) chromatographic models, compared with a 2nd-order upwind discretization method (UDS2) and 4th-order orthogonal collocation finite element method (OCFE4) of ASPEN chromatography (ASPEN Tech, USA). A novel finite volume discretization is proposed for the CE/SE method to treat the exit boundary condition in the adsorption column close to plug-flow operation. A single column model and a linear/equilibrium SMB model which have the analytic solutions are used for the computational performance assessment of the numerical methods. For the two examples, CESE shows the superiority over UDS2 and OCFE4 in terms of computational efficiency and accuracy. Other five practical SMB examples which have no analytic solution are used to examine computational features of the three numerical methods. Using UDS2, it shows a fast calculation but the sufficient number of mesh points is required to enhance accuracy. The accuracy of CESE is comparable with OCFE4. Since CESE enforces both local and global flux conservation in space and time and is combined with the finite volume discretization of the exit boundary condition, its numerical solution of SMB adsorption models is accurate and computationally efficient.

Comparative performance of fixed-film biological filters: Application of reactor theory

Nitrification is classified as a two-step consecutive reaction where R1 represents the rate of formation of the intermediate product NO2-N and R2 represents the rate of formation of the final product NO3-N. The relative rates of R1 and R2 are influenced by reactor type characterized hydraulically as plug-flow, plug-flow with dispersion and mixed-flow. We develop substrate conversion models for fixed-film biofilters operating in the first-order kinetic regime based on application of chemical reactor theory. Reactor type, inlet conditions and the biofilm kinetic constants Ki (h−1) are used to predict changes in NH4-N, NO2-N, NO3-N and BOD5. The inhibiting effects of the latter on R1 and R2 were established based on the γ relation, e.g.:γ=KiKi,max=1−[BOD5][BOD5,max]Nwhere BOD5,max is the concentration that causes nitrification to cease and N is a variable relating Ki to increasing BOD5. Conversion models were incorporated in spreadsheet programs that provided steady-state concentrations of nitrogen and BOD5 at several points in a recirculating aquaculture system operating with input values for fish feed rate, reactor volume, microscreen performance, make-up and recirculating flow rates. When rate constants are standardized, spreadsheet use demonstrates plug-flow reactors provide higher rates of R1 and R2 than mixed-flow reactors thereby reducing volume requirements for target concentrations of NH4-N and NO2-N. The benefit provided by the plug-flow reactor varies with hydraulic residence time t as well as the effective vessel dispersion number, D/μL. Both reactor types are capable of providing net increases in NO2-N during treatment but the rate of decrease in the mixed-flow case falls well behind that predicted for plug-flow operation. We show the potential for a positive net change in NO2-N increases with decreases in the dimensionless ratios K2,(R2)/K1,(R1) and [NO2-N]/[NH4-N] and when the product K1,(R1)t provides low to moderate NH4-N conversions. Maintaining high levels of the latter reduces the effective reactor utilization rate (%) defined here as (RNavg/RNmax)100 where RNavg is the mean reactive nitrogen concentration ([NH4-N] + [NO2-N]) within the reactor, and RNmax represents the feed concentration of the same. Low utilization rates provide a hedge against unexpected increases in substrate loading and reduce water pumping requirements but force use of elevated reactor volumes. Further γ effects on R1 and R2 can be reduced through use of a tanks-in-series versus a single mixed-flow reactor configuration and by improving the solids removal efficiency of microscreen treatment.

Polymerisation of methyl methacrylate in a pilot-scale tubular reactor: modelling and experimental studies

A pilot-scale tubular reactor fitted with in-line static mixers is experimentally and theoretically evaluated for the polymerisation of methyl methacrylate (MMA). A non-isothermal and non-adiabatic axially dispersed plug-flow model is used to describe the flow characteristics of the reactor. The model is applied to the polymerisation of a concentrated MMA solution (up to 72% (v/v)). Key model parameters were attained through independent bench and pilot-scale experiments. Measured monomer conversions and polymer molecular weight were accurately predicted by model simulation. The presence of static mixers is shown to give near-ideal plug-flow operation for the experimental conditions of this study. Furthermore, an approximately four-fold increase in overall heat transfer coefficient is indicated due to the radial mixing incited by the mixers. Studies also demonstrated the importance of inhibitor kinetics on the dynamic and steady-state performance of the reactor.

Modelling and design of thin-film slurry photocatalytic reactors for water purification

Photocatalytic oxidation processes are highly effective clean technologies for the degradation and mineralization of a wide variety of priority pollutants in water and wastewater. However, the application of heterogeneous photocatalysis for wastewater treatment on an industrial scale has been impeded by a lack of mathematical models that can be readily applied to reactor design and scale-up. As a results current photocatalytic reactors in research and development have been designed by empirical or semi-empirical methods only. In this paper, a simple and generic mathematical model for steady-state, continuous flow, thin-film, slurry (TFS) photocatalytic reactors for water purification using solar and UV lamps is presented. The model developed is applicable to TFS flat plate and annular photoreactors of (a) falling film design or (b) double-skin design, operating with three ideal flow conditions: (1) falling film laminar flow, (2) plug flow and (3) slit flow. The model is expressed in dimensionless form and scale-up of TFS photocatalytic reactors can be carried out by dimensional analysis. In addition, the model parameters can be estimated easily from real systems and model solutions can be obtained with little computational effort. Comparison of a number of ideal flow systems shows that both falling film laminar flow and plug flow operation modes give higher performance than the slit flow system. Slit flow operation mode results in lower conversions due to the non-correspondence of fluid-residence time and the transversal radiation field. The effect of optical thickness, on reactor performance and the evolution of radial profiles of a model pollutant with photoreactor length are presented for each of the operation modes. The falling film laminar flow system was found to be more efficient than the plug flow system when the reactor conversion is above 80%. For lower reactor conversion the plug flow system was found to be marginally more efficient than the falling film laminar flow system. A methodology for the optimal geometrical design of a highly efficient configuration of TFS photocatalytic reactors is also presented. The mathematical models presented may be used as a tool for the design, scale-up and optimization of these types of photocatalytic reactors.

Membrane aeration bioreactors for wastewater treatment: completely mixed and plug-flow operation

A laboratory scale membrane aeration bioreactor (MABR) with a void volume of 1.35 l was tested for its ability to treat synthetic wastewater. The MABR process couples aerobic biological wastewater treatment with the bubbleless mass transfer of pure oxygen through hollow fibre membranes. The biofilm that was formed on the surface of the hollow fibres utilised oxygen transferred from the fibre lumen for the degradation of organics transferred from the synthetic wastewater flowing over the biofilm. When operated as a completely mixed reactor at an organic loading rate of 24.5 kg chemical oxygen demand (COD)/m 3/day and a hydraulic retention time of 34 min, an 89% COD removal efficiency was achieved. During plug flow operation, 86% COD removal efficiency was obtained at an organic loading rate of 5.8 kg COD/m 3/day and a hydraulic retention time of 47 min. Regular membrane cleaning to control biomass growth was necessary to prevent channeling. The ability of the reactor to operate under plug flow conditions demonstrated that the biofilm plays a significant role in oxygen mass transfer.

Enhancement of catalytic reaction by pressure swing adsorption

A theoretical study of an adsorptive reactor which combines multibed pressure swing adsorption and chemical reaction is presented; such a reactor is referred to as a pressure swing reactor, or PSR. Studies have concentrated on an asymptotic case in which there is the ideal propagation of concentration waves within the reactor beds; the method of characteristics was employed in the solution of the governing PSR equations. The studies assessed the effects of operating conditions, and cycle configurations, on the PSR performance. Calculations indicate enhanced reactant conversion when compared to conventional steady state plug flow operation. In particular, for some reversible reactions, substantial improvements over equilibrium yields have been calculated. For example, for the dissociation reaction 2A ⇔ B + C, and where B is the only adsorbing component, approximately two-fold improvements over the equilibrium yield of product B have been predicted. Such reaction enhancement can be attributed to the limitation of the backward reaction, which results from the separation of the product species B and C. In addition to the method of characteristics, a cells-in-series method for the asymptotic case has been developed, and found to yield calculations consistent with the method of characteristics solutions. In a third numerical approach, the spatial discretisation technique of orthogonal collocation on finite elements was applied to the governing PSR equations, and the resulting system of ordinary differential equations solved by a standard integration algorithm. In this case, many of the simplifying model assumptions were relaxed, allowing, for example, the simulation of a non-isothermal PSR with finite mass transfer rates. One practical significance of reaction enhancement by pressure swing adsorption is a lower temperature of operation than in a conventional reactor. This would lead to savings in the energy requirements of the reactor, and limit the rate and degree of catalyst deactivation due to coke deposition or sintering.