Heat, and Mass Transfer

Another concept sometimes used as a basis for comparison and correlation of mass transfer data in columns is the Clulton-Colbum analogy (35). This semi-empirical relationship was developed for correlating mass- and heat-transfer data in pipes and is based on the turbulent boundary layer model... 

K. H. Reichert and R. Michael, "Polymerization iu Bubble Columns, Problems of Mass and Heat Transfer at High SoHd Contents," Inst. Chem. 

Transfer Coefficient. The design method described depends for its utiHty on the avadabiHty of mass- and heat-transfer coefficients. Typically, ky-a and /i -a are needed. These must be obtained from the standard correlations for mass and heat transfer, from data reported in the Hterature (23—30),... 

Ryan et al. [Chem. Eng. Progr, 90(8X 83 (1994)] showthat separate mass and heat transfer-rate modeling of an HCl absorber predicts 2 percent fog in the vapor. The impact is equivalent to lowering the stage efficiency to 20 percent. 

Local equilibrium theory Shows wave character—simple waves and shocks Usually indicates best possible performance Better understanding Mass and heat transfer very rapid Dispersion usually neglected If nonisothermal, then adiabatic... 

Transport Transport units can be scaled up on the principles of pneumatic conveying. Mass and heat transfer can be predicted on both the shp velocity during acceleration and the shp velocity at full acceleration. The slip velocity is increased as the sohds concentration is increased. 

Processes in which solids play a rate-determining role have as their principal kinetic factors the existence of chemical potential gradients, and diffusive mass and heat transfer in materials with rigid structures. The atomic structures of the phases involved in any process and their thermodynamic stabilities have important effects on drese properties, since they result from tire distribution of electrons and ions during tire process. In metallic phases it is the diffusive and thermal capacities of the ion cores which are prevalent, the electrons determining the thermal conduction, whereas it is the ionic charge and the valencies of tire species involved in iron-metallic systems which are important in the diffusive and the electronic behaviour of these solids, especially in the case of variable valency ions, while the ions determine the rate of heat conduction. 

Chemical reaction engineering is part of chemical engineering in general. It aims at controlling the chemical conversion on a technical scale and will ultimately lead to appropriate and successful reactor design. An important part is played by various factors, such as flow phenomena, mass and heat transfer, and reaction kinetics. It will be clear that in the first place it is necessary to know these factors separately. 

Chemical reactions obey the rules of chemical kinetics (see Chapter 2) and chemical thermodynamics, if they occur slowly and do not exhibit a significant heat of reaction in the homogeneous system (microkinetics). Thermodynamics, as reviewed in Chapter 3, has an essential role in the scale-up of reactors. It shows the form that rate equations must take in the limiting case where a reaction has attained equilibrium. Consistency is required thermodynamically before a rate equation achieves success over tlie entire range of conversion. Generally, chemical reactions do not depend on the theory of similarity rules. However, most industrial reactions occur under heterogeneous systems (e.g., liquid/solid, gas/solid, liquid/gas, and liquid/liquid), thereby generating enormous heat of reaction. Therefore, mass and heat transfer processes (macrokinetics) that are scale-dependent often accompany the chemical reaction. The path of such chemical reactions will be... 

U we Eissume that the outlet air is saturated, the air state change process is as presented in Fig. 4.22. The exact determination of the air humidity at the end of the process would demand separate mass and heat transfer examinations. 

We can apply this result to determine the analogy between mass and heat transfer factors. Mass flow density /a (mol/m s) can be given as... 

The essential feature of a Jluidized-bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat transfer rates and good mixing. Heat transfer coefficients in the order of 200 W/m-°C between jackets and internal coils are typically obtained. The solids may be a catalyst, a reactant (in some fluidized combustion processes), or an inert powder added to promote heat transfer. 

Superior surface utilization in mass and heat transfer, allowng shorter packed bed heights. Turn-down performance is superior over 2-in. and 3) -in. Pall rings. 

The cooling tower cools hot water tvith cool air by countercurrent (or cross-current) fiow of the tw o fluids past each other in a tower filled with packing. This involves both mass and heat transfer. The water surface that exists on the tower packing is covered with an air film assumed to be saturated at the water temperature. The heat is transferred between this film and the main body of air by diffusion and convection. Detailed presentations of the development of cooling tower theory are given in References 39 and 46. 

Probe/Insirumentalion Developments The principles of good practice in the design, construction and location of corrosion probes have been reviewed. Specific probe designs which acknowledge hydrodynamic influences and the combined effects of mass and heat transfer have been developed. 

Calderbank and Moo-Young (C4) found that mass- and heat-transfer coefficients are largely unaffected by the mechanical power dissipated in the system. According to Eq. (197), this may be the case when the product e3/8 yja remains constant. 

Middleman, S. An Introduction to Mass and Heat Transfer (Wiley, 1997). 

It appears that the complete model for both mass and heat transfer contains four adjustable constants, Dr, Er, K and Xr, but Er and Xr are constrained by the usual relationship between thermal diffusivity and thermal conductivity... 

Most of the actual reactions involve a three-phase process gas, liquid, and solid catalysts are present. Internal and external mass transfer limitations in porous catalyst layers play a central role in three-phase processes. The governing phenomena are well known since the days of Thiele [43] and Frank-Kamenetskii [44], but transport phenomena coupled to chemical reactions are not frequently used for complex organic systems, but simple - often too simple - tests based on the use of first-order Thiele modulus and Biot number are used. Instead, complete numerical simulations are preferable to reveal the role of mass and heat transfer at the phase boundaries and inside the porous catalyst particles. 

The following are some of the reasons that microreactors can be be used (i) reduced mass and heat transfer limitations, (ii) high area to volume ratio, (iii) safer operation, and (iv) ease of seating up by numbering out. The advantages of scaling down zeolite membranes are that it could be easier to create defect-free membranes and... 

Cavitations generate several effects. On one hand, both stable and transient cavitations generate turbulence and liquid circulation - acoustic streaming - in the proximity of the microbubble. This phenomenon enhances mass and heat transfer and improves (micro)mixing as well. In membrane systems, increase of fiux through the membrane and reduction of fouling has been observed [56]. 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

Contrary to RPBRs, in SDRs, intensified heat transfer presents the most important advantage. Liquid reactant(s) are fed on the surface of a fast rotating disk near its center and flow outward. Temperature control takes place via a cooling medium fed under the reaction surface. The rotating surface of the disc enables to generate a highly sheared liquid film. The film fiow over the surface is intrinsically unstable and an array of spiral ripples is formed. This provides an additional improvement in the mass and heat transfer performance of the device. 

The mass and heat transfer performance of the SDR is indeed impressive. Aoune and Ramshaw [91] found local heat transfer coefficients ranging from about 10 000 to about 30 000 W m K and local mass transfer coefficients between about 4E-04... 

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