Fluid Flow and Mass Transfer

As the polymer molecules form and dissociate from the catalyst, they remain ia solution. The viscosity of the solution increases with increasing polymer concentration. The practical upper limit of solution viscosity is dictated by considerations of heat transfer, mass transfer, and fluid flow. At a mbber soflds concentration of 8—10%, a further increase in the solution viscosity becomes impractical, and the polymerisation is stopped hy killing the catalyst. This is usually done by vigorously stirring the solution with water. If this is not done quickly, the unkilled catalyst continues to react, leading to uncontrolled side reactions, resulting in an increase in Mooney viscosity called Mooney Jumping. 

Modi, V. Analysis of Mass Transfer and Fluid Flow for Electrochemical Processes 32... 

Finite-time thermodynamics is an extension to traditional thermodynamics in order to obtain more realistic limits to the performance of real processes, and to deal with processes or devices with finitetime characteristics. Finite-time thermodynamics is a method for the modeling and optimization of real devices that owe their thermodynamic imperfection to heat transfer, mass transfer, and fluid flow irreversibility. 

Lu, Y., Schaefer, L. and Li, P. (2005) Numerical study of a flat tube high power density solid oxide fuel cell. Part I Heat/mass transfer and fluid flow, Journal of Power Sources 140, 331-339. 

Resistance and capacitance in combination are the most common in industrial processes involving heat transfer, mass transfer, and fluid flow operations. The combined effect of supplying a capacity through a resistance is time retardation (time constant). Combining a capacitance-type process element (tank) with a resistance-type process component (valve) results in a single-time-constant (t) process. If the tank was initially empty and then an inflow was started at a constant rate of m, the level in the tank would rise as shown in Figure 2.23 and would eventually rise to the steady-state height of c = Rm in the tank. 

The parameters defined in this chapter are divided into model parameters and evaluation parameters. Model parameters are porosity, voidage and axial dispersion coefficient, type and parameters of the isotherm as well as mass transfer and diffusion coefficient. All of them are decisive for the mass transfer and fluid flow within the column. They are needed for process simulation and optimisation. Therefore their values have to be valid over the whole operation range of the chromatographic process. Experimental as well as theoretical methods for determining these parameters are explained and discussed in Chapter 6. 

The problems solved in Chapters 5 and 6 are simple problems with many numerical parameters specified. You may have wondered where those numbers came from. In a real case, of course, you will have to make design choices and discover their impact. In chemical engineering, as in real life, these choices have consequences. Thus, you must make mass and energy balances that take into account the thermodynamics of chemical reaction equilibria and vapor-liquid equilibria as well as heat transfer, mass transfer, and fluid flow. To do this properly requires lots of data, and the process simulators provide excellent databases. Chapters 2-4 discussed some of the ways in which thermodynamic properties are calculated. This chapter uses Aspen Plus exclusively. You will have to make choices of thermodynamic models and operating parameters, but this will help you learn the field of chemical engineering. When you complete this chapter, you may not be a certified expert in using Aspen Plus , but you will be capable of actually simulating a process that could make money. 

You saw how the equations governing energy transfer, mass transfer, and fluid flow were similar, and examples were given for one-drmensional problems. Examples included heat conduction, both steady and transient, reaction and diffusion in a catalyst pellet, flow in pipes and between flat plates of Newtonian or non-Newtonian fluids. The last two examples illustrated an adsorption column, in one case with a linear isotherm and slow mass transfer and in the other case with a nonlinear isotherm and fast mass transfer. Specific techniques you demonstrated included parametric solutions when the solution was desired for several values of one parameter, and the use of artificial diffusion to smooth time-dependent solutions which had steep fronts and large gradients. 

In industrial electrolytic processes, including metal electrodeposition and preparation reactions, mass transfer and fluid flow are usually of central importance, especially in scaleup from laboratory-scale experimentation. In the final chapter of this volume. West and co-authors give the essential aspects of computer analysis and modeling of such processes in terms of fluid dynamics and mass transfer. 

The heat and mass transfer and fluid flow phenomena in the planar micro-SOFC is described by the CFD model. Due to the low gas velocity and small size of the SOFC, the Reynolds number in the micro-channel is usually much lower than 100 (Yuan et al., 2003). Thus, the gas flow in an SOFC is typically laminar. From a heat transfer analysis, it is found that the local thermal equilibrium assumption is valid for the porous electrodes of an SOFC (Zheng et al., 2013). The governing equations for the CFD model include mass conservation, momentum conservation, energy conservation, and species conservation (Wang, 2004) ... 

It is well-known that many industrial production processes involving heat transfer, mass transfer and fluid flow can be described by a few dimensionless numbers, and therefore dimensional analysis has been widely applied to investigate these processes. 

By considering the analogy between mass transfer and fluid flow, the turbulent mass diffusivity Dt may be analogous to the turbulent diffusivity (eddy diffusivity)... 

Chemical reaction is accompanied with heat effect, and the model equation sets should involve the heat transfer besides the mass transfer and fluid flow. 

---