Mass transfer determination

As will be explained later, the first two factors influence mainly the size of extractors and separators, while mass-transfer determines the CO2 circulation system, and consequently the energy consumption and the size of heat exchangers and the piping system. 

The temperature range used is determined mainly by the catalyst used, and whether formation of side-products will occur. Each catalyst has a specific ignition temperature at which it becomes active for the desired reaction. This temperature has to be exceeded, otherwise no catalytic reaction will occur. Above this temperature, the reaction rate increases only slowly at increasing temperature ( cf. the Arrhenius function). In general, the reaction rate is much more temperature-sensitive than is the mass-transfer rate. Thus, in reactions where the mass-transfer determines the reaction rate, as in gas - liquid reactions, a temperature rise above the ignition temperature has only a minor effect on the reaction rate. 

Basically, using these technologies one would like to move forward to the theoretical optimum of a chemical process, which is that there are no other limitations than chemical kinetics. Normally a chemical process is influenced by more than just kinetics hydrodynamics (mixing), heat transfer, and mass transfer determine the quality of the process. Process intensification focuses on removing these three limitations to reaching the goal of kinetically limited processes. This is schematically depicted in Figure 2. 

Macanas, J. and Munoz, M. (2005) Mass transfer determining parameter in facilitated transport through di-(2-ethylhexyl) dithiophosphoric acid activated composite membranes. Analytica Chimica Acta, 534, 101. 

The principles of mass transfer determine the rate at which the equilibrium is established, that is, the rate at which the solute is transferred into the solvent. 

Figure 8. Mass transfer determination of evaporation dishes...

Thus, corrected for stoichiometry, the key component with the lowest rate of mass transfer determines the rate of conversion, and the component in excess at the catalyst interface has no influence at all on the rate in this case. 

Consider a circular pipe of inner diameter D = 0.015 m whose inner surface is I covered with a layer of liquid water as a result of condensation (Fig. 14-49). In I order to dry the pipe, air at 300 K and 1 atm is forced to flov/ through if with an average velocity of 1.2 m/s. Using the analogy between heat and mass transfer, determine the mass transfer coefficient inside the pipe for fully developed flov/. 

Along with kinetics, the engineering concepts required to evaluate the conversion in the product from a reactor are of two types the principles of conservation of mass and energy and rate equations for the physical processes. The conservation of mass and the rate of mass transfer determine the composition as a function of position in a continuous reactor and as a function of time in a batch reactor. Similarly, the conservation of energy and the rate of energy transfer determine the temperature as a function of position or time. The application of these principles is discussed in subsequent chapters, starting with Chap. 3 where mathematical formulations are considered for various types of reactors. 

R. Abdallah, P. Magnico, B. Fumey, C. De Bellefon, CFD and kinetic methods for mass transfer determination in a mesh microreactor, AIChE J. 52 (2006) 2230. 

The equation systems were solved numerically by a computer programm. Fig. 18 shows one example for mass transfer determined conditions. 

Usually, the feed streams are known and the desired product streams are estimated from other process requirements. Equilibrium laws are used to decide whether it is feasible to use an operation like absorption, or whether an alternative method is needed. If the equilibrium calculations establish the feasibility of the operation, then the rates of heat, momentum, and mass transfer determine the design parameters and the extent to which true equilibrium is approached in the equipment. 

For the system of kinetic regime, the conversion remains unchanged in experiments that have the same ratio (mcatlvo)- If the external mass transfer determines the reaction rate, an increase in volumetric flow tends to decrease the boundary layer between gas and catalyst surface, increasing the concentration of gaseons species on the surface and consequently the reaction rate. 

C. De Bellefon, CFD and kinetic methods for mass transfer determination in a... 

Preparatory work for the steps in the scaling up of the membrane reactors has been presented in the previous sections. Now, to maintain the similarity of the membrane reactors between the laboratory and pilot plant, dimensional analysis with a number of dimensionless numbers is introduced in the scaling-up process. Traditionally, the scaling-up of hydrodynamic systems is performed with the aid of dimensionless parameters, which must be kept equal at all scales to be hydrodynamically similar. Dimensional analysis allows one to reduce the number of variables that have to be taken into accoimt for mass transfer determination. For mass transfer under forced convection, there are at least three dimensionless groups the Sherwood number, Sh, which contains the mass transfer coefficient the Reynolds number. Re, which contains the flow velocity and defines the flow condition (laminar/turbulent) and the Schmidt number, Sc, which characterizes the diffusive and viscous properties of the respective fluid and describes the relative extension of the fluid-dynamic and concentration boundary layer. The dependence of Sh on Re, Sc, the characteristic length, Dq/L, and D /L can be described in the form of the power series as shown in Eqn (14.38), in which Dc/a is the gap between cathode and anode Dw/C is gap between reactor wall and cathode, and L is the length of the electrode (Pak. Chung, Ju, 2001) ... 

C.J.N. (2010) Cathode potential and mass transfer determine performance of oxygen reducing biocathodes in microbial fuel cells. Environ. Sci. Technol,... 

Figure 4.5.26 shows the radial concentration distribution in a porous spherical particle with diameter 2tp according to Eq. (4.5.115) for two values of the Thiele modulus ( reversible fot the example of a gas phase free of B (cB,g = 0) and Defr/O tp) = 0.05 and K = l. Note that in the case of high values of ( reversible (S>5 in Figure 4.5.26), the external mass transfer determines the effective reaction rate, that is, the equilibrium concentrations are almost reached within the porous particle (for the example of Figure 4.5.26, K<, = 1 and Ca,equilibrium = Cb,equilibrium = 0.5c J, and the concentrations vary strongly in the boundary layer, for example,...

The design calculations considered in the preceding chapters were based on theoretical plates. In order to complete the design, it is necessary to have the relationship between these idealized values and the actual performance of the contacting device. The vapor and liquid brought into contact with each other in the tower are not at equilibrium, and the rate of mass transfer determines the effectiveness of the unit. This chapter will consider the methods of predicting the effectiveness of the vapor-liquid contact for the various types of units. 

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