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Gas-Liquid-Solid Reactors
Published in James J. Carberry, Arvind Varma, Chemical Reaction and Reactor Engineering, 2020
Yatish T. Shah, Man Mohan Sharma
Two main categories of three-phase reactors are fixed-bed reactors, which are normally used when catalyst particles are larger than 10−3 m, and slurry reactors, wherein the solid phase is in suspension. The most common type of fixed-bed reactor used in industry is the trickle-bed reactor, wherein liquid trickles over a fixed bed of catalyst while gas flows co-currently downward as a continuous phase. Fixed-bed reactors are often operated in the bubble flow regime, wherein both gas and liquid flow cocurrently upward and gas is the dispersed phase. In some processes, such as for hydrodemetallization of high-metal-content heavy crude, the catalyst needs to be replaced frequently, and a moving fixed-bed reactor is used. In such reactors, a pulsating flow regime may prevail in the bottom part of the reactor. As pointed out by Østergaard (1968), in true slurry reactors, solid particles are very fine (generally less than 10−3 m) and gas flows mainly in and out of the reactor. In many cases, slurry reactors can be mechanically agitated. If solid particles are large such that they form a discrete phase within the reactor and if the liquid-solid mixture flows in and out of the reactor, the reactor is often called a three-phase fluidizedbed reactor. In the present chapter we concentrate on slurry reactors (with fine catalyst particles) with some allowance for slurry flow in and out of the reactor. Several types of fixed-bed and slurry reactors used in industrial practice are illustrated in Fig. 1.
Refinery Reactors
Published in James G. Speight, Refinery Feedstocks, 2020
The flow properties are of utmost importance for packed beds used in three-phase reactions. The most common operation policy is to allow the liquid to flow downward in the reactor. The gas phase can flow upwards or downwards, in a concurrent or countercurrent flow. The name of this reactor – the trickle-bed reactor – is indicative of flow conditions in the reactor, as the liquid flows downward in a laminar flow wetting the catalyst particles efficiently (trickling flow). It is also possible to allow both the gas and the liquid to flow upward in the reactor. In this case, no trickling flow can develop, and the reactor is called a packed-bed reactor or a fixed-bed reactor.
Chemical Reaction Kinetics, Reactor/Bioreactor Analysis and Stoichiometry of Bioprocesses
Published in Debabrata Das, Soumya Pandit, Industrial Biotechnology, 2021
The trickle-bed reactor is largely used in the wastewater treatment process. A major problem with this process is channelling due to growth of microorganisms. Since the particles are in contact with each other, as the time increases the microorganisms grow on the surface of the solid matrix leading to loss of porosity of the bed. As the porosity of the bed is lost then there will be a channelling effect. The performance of the reactor will be drastically reduced due to the channelling effect. This is the major problem with the packed-bed reactor.
Catalytic wet air oxidation of phenol: Review of the reaction mechanism, kinetics, and CFD modeling
Published in Critical Reviews in Environmental Science and Technology, 2021
Tladi J. Makatsa, Jeffrey Baloyi, Thabang Ntho, Cornelius M. Masuku
Where, FGL, FGS, FLS are gas-liquid, gas-solid and liquid-solid momentum exchange terms. Ranade et al. used a model developed by Attou and Ferschneider (1999) to simulate the flow regime where the liquid flow was in the form of droplets. Mousazadeh (2013) used CFD to predict the formation of hot spots in a trickle bed reactor. A hot spot was observed when there was a local blockage preventing the fluid from flowing. Furthermore, there was a temperature difference of 153 °C between the hot spot and the surrounding area. It was concluded that the hot spots were formed when liquid cannot convect in the radial or axial direction. Lopes and Quinta-Ferreira (2007) developed a computational fluid dynamics model of a trickle bed reactor operated between the temperature of 170 °C – 200 °C and pressures of 10 – 30 bar. These authors used FLUENT 6.1 and Euler–Euler multiphase flow approach to model the behavior of the fluid inside the reactor. Furthermore, the researchers studied the influence of gas and liquid flow rate within the trickle flow regime ranging between (gas: 0.10 – 0.70 and liquid: 0.5 – 5 kg/m2s). In order to validate their findings for pressure drop and liquid holdup, a spherical catalyst of a 2 mm diameter was used as a reactor packing. In addition, they mapped both gas and liquid flow; and found maximum velocities to be 0.5 and 0.005 cm/s respectively. Their results showed that the reactor was operated within a trickle flow regime. According to their findings, an increase in liquid mass flux resulted in an increase in liquid holdup whereas an increase in pressure resulted in a significant decrease of the liquid holdup. The increase in liquid mass flux improves interaction between gas and liquid which causes turbulence and thickens the liquid film. These changes resulted in an increase in liquid side shear stress due to high-pressure drop and resistance became more pronounced in comparison to the driving force. Their results were in agreement with (Kuzeljevic, 2010; Mousazadeh, 2013; Beni & Khosravi-Nikou, 2015). The researchers concluded that a change in reactor pressure is more pronounced on pressure drop than liquid holdup. Similarly, Beni and Khosravi-Nikou (2015) modeled hydrodynamics of the trickle bed reactor and used 300 spherical particles arranged in a hexagonal pattern with maximum space between them, not exceeding 3% of particle diameter. They simulated only 12 layers due to computational limitation and investigated the effect of pressure on hydrodynamics parameters at lower pressures ranging between (0.1, 0.5 & 1 MPa). They also varied gas and liquid superficial velocities between (0.086 & 0.25 m/sec) and (<0.005–0.03 m/s) respectively. Regardless of mild pressure they also reached the same conclusion as (Lopes & Quinta-Ferreira, 2007).