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Coagulation and Flocculation
Published in Subhash Verma, Varinder S. Kanwar, Siby John, Environmental Engineering, 2022
Subhash Verma, Varinder S. Kanwar, Siby John
The concentration of the reactant decreases as the liquid moves along the direction of flow, remaining within the imaginary plug of water moving through the basin (Figure 6.2). Due to short-circuiting and intermixing, it may be difficult to achieve ideal plug flow. Flocculators in water treatment systems are designed as plug flow reactors. Ct(Zeroorder)=Ct−ktort=1k(Ct−C0)
Refinery Reactors
Published in James G. Speight, Refinery Feedstocks, 2020
The main advantage of packed beds is the flow pattern. Conditions approaching a plug flow are advantageous for most reaction kinetics. Diffusion resistance in catalyst particles may sometimes reduce the reaction rates, but for strongly exothermic reactions, effectiveness factors higher than unity can be obtained. Hot spots appear in highly exothermic reactions and these can have negative effects on the chemical stability and physical sustainability of the catalyst. If the catalyst in a packed bed is poisoned, it must be replaced, which is a cumbersome procedure. A packed bed is sometimes favorable because the catalyst poison is accumulated in the first part of the bed and deactivation can be predicted in advance. In the hydrogenation of sulfur-containing aromatic compounds over nickel catalysts in a packed bed, the sulfur is adsorbed as a multimolecular layer on the catalyst at the inlet of the reactor. However, this layer works as a catalyst poison trap. For bubble flow, the solid particles are evenly distributed in the reactor. This flow pattern resembles fluidized beds where only a liquid phase and a solid catalyst phase exist. At high gas velocities, a flow pattern called aggregative fluidization develops. In aggregative fluidization, the solid particles are unevenly distributed, and the conditions resemble those of a fluidized bed with a gas phase and a solid catalyst phase. Between these extreme flow areas, there exists a slug flow domain, which has the characteristics typical of both extreme cases. An uneven distribution of gas bubbles is characteristic of a slug flow.
Catalytic Three-Phase Reactors
Published in Salmi Tapio, Mikkola Jyri-Pekka, Wärnå Johan, Chemical Reaction Engineering and Reactor Technology, 2019
Salmi Tapio, Mikkola Jyri-Pekka, Wärnå Johan
The main advantage with packed beds is the flow pattern. Conditions approaching a plug flow are advantageous for most reaction kinetics. Diffusion resistance in catalyst particles may sometimes reduce the reaction rates, but for strongly exothermic reactions, effectiveness factors higher than unity (1) can be obtained. Hot spots appear in highly exothermic reactions, and these can have negative effects on the chemical stability and physical sustainability of the catalyst. If the catalyst in a packed bed is poisoned, it must be replaced, which is a cumbersome procedure. A packed bed is sometimes favorable because the catalyst poison is accumulated in the first part of the bed and deactivation can be predicted in advance. In the hydrogenation of sulfur-containing aromatic compounds over nickel catalysts in a packed bed, the sulfur is adsorbed as a multimolecular layer on the catalyst at the inlet of the reactor. However, this layer works as a catalyst poison trap.
Single-step full-state feedback control design for nonlinear hyperbolic PDEs
Published in International Journal of Control, 2019
Qingqing Xu, Ilyasse Aksikas, Stevan Dubljevic
In this paper, we seek a novel extension of a single-step design method that achieves simultaneous coordinate transformation and closed-loop desired target dynamics assignment for the broad class of first-order nonlinear hyperbolic PDEs systems and second-order hyperbolic PDEs systems. First, a scalar hyperbolic PDE system which describes the dynamics of a tubular reactor is explored to develop a solution of the associated first-order quasi-linear PDE. Specifically, we consider the class of transport-reaction systems described by the first-order hyperbolic PDEs, which are physical relevant models of industrial exothermic plug-flow reactors. In practice, reactor design and operation often involve a trade-off between conflicting costs and in particular conversion and energy costs. For industrially important exothermic plug-flow reactors, the aim is to maximise reactant conversion, while minimising side products and compression power. In contrast to optimising a finite number of parameters, the optimal solution for conflicting conversion and energy costs is derived from optimal heat exchanger temperature, which has been accomplished by Smets, Dochain and Van Impe (Logist, Van Erdeghem, Smets, & Van Impe, 2009; Smets, Dochain, & Van Impe, 2002). The optimal solution with respect to a defined cost function is the steady state of the temperature and reactant concentration in an exothermic plug-flow reactor. However, the optimal temperature and reactant concentration profiles of interest are unstable steady states. Therefore, we utilise the single-step full-state feedback control design with a nonlinear coordinate transformation that achieves desired stabilisation of the closed-loop system dynamics.
Impact of reactor configuration and relative operating conditions on volatile fatty acids production from organic waste
Published in Environmental Technology Reviews, 2022
Elena Rossi, Isabella Pecorini, Antonio Panico, Renato Iannelli
Reactor configurations, in particular shape, geometry, feeding and mixing mode, play a crucial role in VFAs yield and composition mainly for two reasons: (i) they affect the hydraulic conditions inside the reactor; (ii) they influence the microbial community composition. This latter aspect is a direct consequence of selection pressures such as solids retention time (SRT), that can result in the wash out of microorganisms with slower growth rates and/or enrichment of those that grow faster [42]. Whereas the hydraulic conditions are essential to [43]: (i) ensure the contact between microorganisms and substrates (e.g. organic solids as well as VFAs), thus regulating the microbial kinetics (e.g. VFAs production as well as consumption rate); (ii) enable the removal of metabolites (e.g. VFAs and gas) that, in specific cases, can be even toxic to microorganisms; (iii) avoid the settlement of inert materials. Due to the relevance of this aspect, reactors are commonly equipped with artificial mixing systems that enhance the effect of the natural dispersive, diffusive and adventive transport. Commercial mixing systems can be grouped in two main categories: mechanical systems and hydraulic or gas recirculation systems [44–46]. According to the hydraulic conditions, reactors can be divided in three main categories: (i) static, (ii) completely mixed and (iii) plug-flow reactors. Moreover, reactors can be fed in batch mode or, alternatively, in continuous mode. Batch and continuous modes are the extreme of a wide interval, including, fed batch, semi-batch, repeated batch, semi-continuous, etc. Finally, reactors can operate in a single or two stages with the aim to optimize the control of the different steps that compose the AD process (i.e. hydrolysis, acidogenesis, acetogenesis and methanogenesis) [47].
Auto-Ignition and Numerical Analysis on High-Pressure Combustion of Premixed Methane-Air mixtures in Highly Preheated and Diluted Environment
Published in Combustion Science and Technology, 2022
Subrat Garnayak, Ayman M Elbaz, Olawole Kuti, Sukanta Kumar Dash, William L Roberts, V. Mahendra Reddy
In the present study, numerical simulation of a steady, one-dimensional, constant pressure laminar premixed flame in highly preheated and diluted conditions was conducted using the adiabatic plug flow reactor model (PFR) module Ansys Chemkin Pro and GRI 3.0 chemistry. The plug flow reactor model is computationally efficient since it solves first-order ordinary differential equations (ODEs) of continuity, momentum, energy, and species without requiring transport properties. The fundamental assumption of the plug flow reactor model is that the fluid is perfectly mixed in the radial direction, while there is no mixing in the axial direction. This work considers the domain of length 1.4 m and diameter of 0.01 m, in which the combustion of laminar premixed CH4/O2/N2 mixture in a diluted and at high-pressure condition is analyzed. The reactor’s dimensions were taken from the experimental tubular reactor used by Sabia et al. (2013). A stoichiometric premixed mixture (ϕ = 1) of CH4 and oxidizer (O2 and N2) was considered, with different O2 dilution levels. Reactants were supplied to the inlet at different reactant temperatures ranging from 1100 to 1500 K. The percentage of O2 in the air was varied from 21% to 3%. The pressure of the reactor was varied from 1 to 10 atm. The calculated jet Reynolds number of the mixture for all cases covered was ≈ 1750. The effect of all these parameters (%O2, combustor pressure, inlet temperature) on achieving distributed combustion is investigated in this work. At the combustor inlet, the mass flow inlet boundary condition was applied with a constant mixture mass flow rate of 0.00061 kg/s for all the cases here. The mixture’s mass flow rate was calculated by considering the inlet velocity of 30 m/s as the base case for the reactant temperature of 1300 K, and at a pressure of 1 atm with a 21% O2 level. The operating conditions considered in the present work (as well as the corresponding flow velocity under stoichiometric conditions for all cases) are supplied in Table S1 of the supplementary material. Similarly, the mole fractions of the mixture species (CH4, O2, and N2) provided at the inlet with different dilution levels for ϕ = 1 are listed in Table 1.