Trickle Bed Reactors: Reactor Engineering & Applications
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This book provides a hybrid methodology for engineering of trickle bed reactors by integrating conventional reaction engineering models with state-of-the-art computational flow models. The content may be used in several ways and at various stages in the engineering process: it may be used as a basic resource for making appropriate reactor engineering decisions in practice; as study material for a course on reactor design, operation, or optimization of trickle bed reactors; or in solving practical reactor engineering problems.
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Trickle Bed Reactors Reactor Engineering by Vivek Ranade
Koleksiku Bantuan Penelusuran Buku Lanjutan. Access Online via Elsevier Amazon. Vivek V. Ranade , Raghunath Chaudhari , Prashant R.
Elsevier , 18 Mar - halaman. Facilitates development of high fidelity models for industrial applications Facilitates selection and application of appropriate models Guides development and application of computational models to trickle beds. In other models, appearance of pulse flow is related to the instability occurring in the liquid film due to the shear exerted by the gas phase Grosser et al. For fixed liquid mass flux, shear exerted by the gas phase on the liquid film drives the excess liquid in the film along the solid surface.
Accumulated excess liquid generates blockage to the gas flow passage which eventually leads to pulse formation. Though measurable size of disturbance pulse is considered as the inception of pulse flow regime, particle-scale processes suggest that actual inception starts much earlier. Several experimental methods have been used to detect the transition from trickle to pulse flow regime. Most of the earlier studies were based on visual observations. Experimental studies have identified several key system parameters which influence regime transition. Some of these are particle diameter and its shape factor dp and 4 , porosity of the bed 3 and properties of gaseliquid phases viscosity, density, surface tension, contact angle , column diameter, and operating parameters like pressure, temperature, and gas or liquid flow rates.
Inception of pulse flow regime is usually calculated based on critical liquid velocity for transition at the given gas flow rate. The fluid properties can be grouped together in the form of following dimensionless number which can be related to the flow regime transition Baker, ; Bansal et al.
Bansal et al. Few attempts have been made to develop models and correlations based on physical understanding of the transition.
Trickle Bed Reactors by Vivek V. Ranade (ebook)
Another criterion is proposed by Holub et al. A phenomenological criterion is developed on the basis that instability of waves on the surface of the liquid film appears due to high shear exerted by the gas phase. This semi-empirical model requires knowledge of liquid phase pressure drop and Ergun constant E1 and is suitable for estimation of the flow regime boundary for the reactors operated at or near atmospheric pressure.
Grosser et al. Though this approach uses fundamental equations of hydrodynamics, some terms appearing in the equations capillary pressure, relative permeability, static liquid holdup, interphase drag, etc. Attou and Ferschneider have developed models to reduce the empiricism associated with these parameters. Influence of various system parameters bed geometry, operating parameters, and fluid properties on the transition of regimes is briefly discussed in the following.
In smaller diameter trickle beds, inception of pulse flow regime is observed earlier than the larger diameter columns. Reactor diameter plays a significant role in the formation of blockage in the form of liquid-rich zone. It is usually difficult to operate larger diameter trickle bed reactors in a pulse flow regime. For larger diameter particles, capillary forces are less dominant than gravitational forces. Therefore the liquid holdup in the bed is substantially lower for the larger-sized particles.
The transition to pulse flow therefore gets delayed for larger-sized particles Gunjal et al. Similar phenomenon is observed in higher porosity beds Chou et al. Influence of gaseliquid throughput on the trickle to pulse flow regime boundary is shown in Fig. At higher gas or liquid throughputs, inertial forces attenuate waves on the liquid film which are responsible for destabilization of the trickle flow pattern.
Gas and liquid phase properties also have significant effect on transition boundary. Effect of capillary force can be significantly reduced by decrease in surface tension of the liquid.
Experiments carried out by Charpentier and Favier , Chou et al. Cyclohexane with lower surface tension 3 times lower than water shows early inception of the pulse flow regime see Fig. Higher liquid phase viscosity leads to increase in liquid holdup, and therefore earlier inception of the pulse flow regime. Most of the industrial trickle bed reactors are operated at high pressures. It is therefore important to understand Chapter 2 39 Hydrodynamics and Flow Regimes the influence of operating pressure or gas density on the regime boundary.
Al-Dahhan and Dudukovic have shown that effect of operating pressure is negligible as long as gas density is below 2. However, effect of operating pressure is significant when gas density exceeds 2.
Attou and Ferschneider have attributed effect of increase in gas density to decrease in inertial forces; therefore transition boundary occurs at higher gas and liquid velocities. Despite years of experimental and modeling efforts, estimation of trickle to pulse flow regime transition is still not very accurate. Though macroscopic models have shown relatively better agreement with the experimental data, experimental data also indicate the importance of pore-scale processes in the inception of regime transition.
Therefore, understanding physical phenomenon associated with inception of pulse flow is still a challenge and further efforts based on multiscale modeling approach may lead to improvements in estimation of regime transition. In the mean time, equations and regime maps included here may be used for estimating the regimes and transition boundaries. Estimation of key hydrodynamic parameters is discussed in the following section. It is one of the key interaction indices for the overall system and therefore is often used as correlating parameter for prediction of other design parameters such as gaseliquid, liquidesolid mass transfer coefficient, wetting efficiency, and heat transfer coefficient.
Two-phase pressure drop along the length of the bed is a function of 1 the reactor hardware such as column diameter, particle size and shape, and internals; 2 operating parameters such as gaseliquid flow rates flow regime ; and 3 fluid properties like density and viscosity of flowing fluid, surface tension, and surface characteristics.
Operating pressure and temperature indirectly affect the pressure drop through fluid properties. Reactor column diameter D has relatively lower influence on pressure drop as compared to the particle diameter dp. Chapter 2 41 Hydrodynamics and Flow Regimes columns, uniform distribution of liquid phase is rather difficult. Liquid maldistribution across the bed cross-section may lead to lower interaction among the phases and therefore lower pressure drop.
Trickle bed reactors are often operated at low liquid flow rates which cause incomplete wetting of particles. Pressure drop for incompletely wetted particles is often less than completely wetted particles. It is highest for uniformly distributed liquid and completely wetted particles.
However, at lower liquid flow rates, measured pressure drop values often show large variation due to non-uniform liquid spreading and wetting. Particle size and shape also affect the bed pressure drop considerably. Pressure drop is less sensitive to shape factor as compared to the porosity of the bed. Denser beds lower porosity lead to higher pressure drop. Pressure drop increases with decrease in size of the particle. Fluid has to follow more tortuous path in the bed with smaller-sized particles. The commercial trickle beds therefore generally use particles in the range of 1 mm to 3 mm to strike appropriate balance of pressure drop and catalyst utilization.
A sample of experimental results indicating pressure drop in trickle beds as a function of liquid velocity for two different particle sizes is shown in Fig.
Trickle Bed Reactors
Typical variation of pressure drop with liquid flow rate at a constant gas flow rate is shown in Fig. Pressure drop variation with liquid flow rate shows hysteresis behavior in trickle flow regime see Figs. Pressure drop for non-prewetted bed is represented by the lower curve due to low interaction between gas and liquid phases. This effect is negligible for trickle flow operation with prewetted bed or in pulse flow regime. Reactor hardware reactor diameter, particle size and shape, etc.
Hence most of the correlations were expressed in terms of Reynolds number of gas and liquid phases. The subscripts L and G denote liquid and gas phases, respectively. The above correlation is valid for trickle as well as pulse flow regimes operated at atmospheric pressure and temperature conditions.