Liquid phase component transport limited

Liquid phase component transport limited reactions. An industrially inportant reaction of this type is hydrotreating to remove sulfur, nitrogen and metals from gas oils and residual oils. These feeds are commonly liquid under reactor conditions. 

Gas phase component transport limited reactions. There are several studies in the literature representative of reactions limited by transport of the gas phase component. In these studies, the gas phase component must first dissolve in the liquid to be then transported to a catalytic site by diffusion into the catalyst pores. Alternatively, the gas phase reactant can contact the catalyst particle outer surface directly without first dissolving in the moving liquid stream. Because the direct contact... 

The next more complicated treatment of liquid water is to have a way in which to model also its transport without going to a two-phase model. The models of this sort assume that the liquid water exists as droplets that are carried along in the gas stream. - - Thus, while evaporation and condensation occur, a separate liquid phase does not have to be modeled. Instead, the liquid is assumed to be a component of the gas, and usually one that has a negligible effect on the gas-phase flow and velocity. There is a change in the gas-phase volume fraction due to the water, however. This type of model allows for the existence and location of liquid water to be noted, and to a limited extent the change in the water pressure or concentration. 

Nevertheless, there are some problems limiting the practical apphcation of SLMs. The main problem is the stability of the liquid membrane, caused by leakage and/or losses of membrane phase components during transport process. However, by proper choice of the porous polymeric support, using organic solvents used as a membrane phase and membrane phase components, the instability can be significantly reduced. 

Pick s laws also describe diffusion in solid phases. In solids transport properties can be considerably different than in liquid phases. Only one component can mobile diffuse in the matrix of the second component. At higher temperatures the diffusion coefficient can be more similar in size than in liquid phases, but the diffusion coefficient at room temperature can be orders of magnitudes smaller, e.g., D < 10 ° cm s k To overcome the time limitation one must make the diffusion length smaller. Ultra-thin layers or nanoparticles provide such small dimensions. Under such conditions the diffusion is not semi-infinite but has a restricted extension. This has to be considered in the boundary conditions. 

Liquid-phase olefin separation was performed in perstraction mode at 25°C (Sungpet et al. 2001). For this to occur, it is postulated that simultaneous complex-ation of olefin-silver (I) ion-poly(pyrrole) is a necessary condition. Althongh silver is believed to be able to facilitate the transportation of certain components in this research, the researchers did not explain in detail the conditions or limitations for this facilitated transportation to occur. The researchers did not clearly state whether this facilitated transportation can only be achieved by silver (I) ions and not by any other metal ion with the same charges. Thus, there is room for future investigation on the properties of the different types of incorporated metal ions with the same charges, to facilitate the separation processes. 

However, the use of activated carbons also leads to severe problems. First of all, the presence of micropores can cause (appreciable) transport limitations. To minimize transport limitations small support bodies, just sufficiently large to be separated fi-om the liquid, can be used within a vigorously agitated liquid. Prevention of attrition to bodies of a size, that separation from the liquid cannot be performed efficiently, asks for catalysts of a high mechanical strength. Most active carbons, do not exhibit a high mechanical strength. Attrition can therefore lead to the fonnation of fines and thus to the loss of active components. Moreover, fines may severely hamper the separation of the catalyst fi om the liquid phase. 

In the analysis of the performance of these reactors, we have to distinguish between those vfliich are limited by the transport of a liquid phase reacting conponent and those which are limited by a reacting gas phase component. The characteristics of these two types of reactors are somevdiat different as demonstrated by the examples below. Table 1 gives a listing of selected examples from the literature for each of these reactor types. 

Rate processes, on the other hand, are limited by the rate of mass transfer of individual components from one phase into another under the influence of physical shmuli. Concentrahon gradients are the most common stimuli, but temperature, pressure, or external force fields can also cause mass transfer. One mass-transfer-based process is gas absorption, a process by which a vapor is removed from its mixture with an inert gas by means of a liquid in which it is soluble. Desorption, or stripping, on the other hand, is the removal of a volatile gas from a Hquid by means of a gas in which it is soluble. Adsorption consists of the removal of a species from a fluid stream by means of a solid adsorbent with which it has a higher affinity. Ion exchange is similar to adsorption, except that the species removed from solution is replaced with a species from the solid resin matrix so that electroneutrality is maintained. Lastly, membrane separations are based upon differences in permeability (transport through the membrane) due to size and chemical selectivity for the membrane material between components of a feed stream. 

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