Mass-transfer operations, definition

A more classical definition of the vertical axis of this chart can be found in either Mass Transfer Operations by R. E. Treybal or Separation Processes by C. J. King, - but be warned, neither of these books is for the faint of heart, especially in the sections where absorption or stripping are concerned. So if you choose to look up these references, please don t get despondent as some of the verbiage (or in places, lack of verbiage) would break the heart of the most aspiring and ardent student. 

In general, it is fair to state that one of the major difficulties in interpreting, and consequently in establishing definitive tests of, corrosion phenomena in fused metal or salt environments is the large influence of very small, and therefore not easily controlled, variations in solubility, impurity concentration, temperature gradient, etc. . For example, the solubility of iron in liquid mercury is of the order of 5 x 10 at 649°C, and static tests show iron and steel to be practically unaltered by exposure to mercury. Nevertheless, in mercury boiler service, severe operating difficulties were encountered owing to the mass transfer of iron from the hot to the cold portions of the unit. Another minute variation was found substantially to alleviate the problem the presence of 10 ppm of titanium in the mercury reduced the rate of attack to an inappreciable value at 650°C as little as 1 ppm of titanium was similarly effective at 454°C . 

Equating the two representations of the wall mass flux (Eqs. 5.31 and 5.32) provides the operational definition for the mass-transfer coefficient,... 

Process intensification can be considered to be the use of measures to increase the volume-specific rates of reaction, heat transfer, and mass transfer and thus to enable the chemical system or catalyst to realize its full potential (2). Catalysis itself is an example of process intensification in its broadest sense. The use of special reaction media, such as ionic liquids or supercritical fluids, high-density energy sources, such as microwaves or ultrasonics, the exploitation of centrifugal fields, the use of microstructured reactors with very high specific surface areas, and the periodic reactor operation all fall under this definition of process intensification, and the list given is by no means exhaustive. 

Unfortunately, simultaneous analytical solution of the mass transfer and kinetic equations of an electrochemical cell is usually complex. Thus, the cell is usually operated with definitive hydrodynamic characteristics. Operational techniques, relating to controlling either the potential or the current, have been developed to simplify the analysis of the electrochemical cell. Description of these operational techniques and their corresponding mathematical analyses are well discussed elsewhere. 

The HTU is determined from experimental data or empirical correlations, and the NTU is related to equilibrium and operating data. Since both HTU and NTU depend on the particular definition of mass transfer coefficients and compositions in the mass transfer equations, it is important to use compatible pairs of HTU and NTU. For instance. Equations 15.24 and 15.25 are rewritten as... 

The just-suspended state is defined as the condition where no particle remains on the bottom of the vessel (or upper surface of the liquid) for longer than 1 to 2 s. At just-suspended conditions, all solids are in motion, but their concentration in the vessel is not uniform. There is no solid buildup in comers or behind baffles. This condition is ideal for many mass- and heat-transfer operations, including chemical reactions and dissolution of solids. At jnst-snspended conditions, the slip velocity is high, and this leads to good mass/heat-transfer rates. The precise definition of the just-suspended condition coupled with the ability to observe movement using glass or transparent tank bottoms has enabled consistent data to be collected. These data have helped with the development of reliable, semi-empirical models for predicting the just-suspended speed. Complete suspension refers to nearly complete nniformity. Power requirement for the just-suspended condition is mnch lower than for complete snspension. 

The experimental time ranges discussed here relate to practical values of electrode radius and diffusion coefficient and are all readily accessible with standard commercial electrochemical instrumentation. A distinguishing feature of a UME is the ability to operate in different mass-transfer regimes. Indeed, we used, in essence, the ability to approach or to achieve the steady-state as the basis for our operational definition of a UME in the opening paragraphs of this section. 

The unit operation of erystallization is governed by some very complex interacting variables. It is a simultaneous heat and mass transfer process with a strong dependence on fluid and particle mechanics. It takes place in a multiphase, multicomponent system. It is concerned with particulate solids whose size and size distribution, both incapable of unique definition, vary with time. The solids are suspended in a solution which can fluctuate between a so-called metastable equilibrium and a labile state, and the solution composition can also vary with time. The nucleation and growth kinetics, the governing processes in this operation, can often be profoundly influenced by mere traces of impurity in the system a few parts per million may alter the crystalline product beyond all recognition. 

Important characteristics of Mode I of operation closely relate to the bubble flow patterns which are coalesced bubble flow, dispersed bubble flow, slug flow, and transitional flow. The exact definition of these regimes is rather subjective and is frequently the result of visual observations. These flow patterns determine many of the properties of cocurrent three phase fluidized beds such as porosity, bubble characteristics, mixing, heat and mass transfer. The specific values of these properties, their changes and their interdependence with respect to the flow patterns is covered in the following sections of this review. 

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