Mass, separation transfer

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

HETP is another quantity that is used to express the efficiency of a device for carrying out a separation, particularly in which mass is transferred by a stage-wise action rather than a differential contact. For example, in a tray column, the HETP value is the tray spacing divided by the fractional overall tray efficiency. 

Another difficulty is that all these subsystems have to function at once, i.e., the development, characterization, and optimization of the individual subsystems is limited, and there is no way of a mechanical fine tuning. This means that the total system has to be designed on very reliable simulations and pattern transfer, which comprise not only the geometrical features for potentials and trajectories of electrons and ions, but also the behavior of the mass separator or pressure regimes in different areas of the system. Such an approach may appear rather adventurous and risky at least it may and will take much more time to generate a working system than the demonstration of a single subsystem implemented into a standard mass spectrometer environment. 

Table 1 is a compilation of the more common industrial separation operations based on inter-phase mass transfer between two phases, either created by an energy-separating agent or added as a mass-separating agent. A more comprehensive table is given by Seader and Henley.1 In the following, the operations listed in Table 1 will be outlined briefly. The first two methods, distillation and absorption, will be discussed in more detail later. 

The removal of one of more selected components from a mixture of gases by absorption into a suitable solvent (Mass Separating Agent, MSA) is the second major operation of chemical engineering after distillation. Absorption is based on interface mass transfer controlled largely by rates of diffusion. It is worth noting that absorption followed by a chemical reaction in the liquid phase is often used to get more removal of a solute from a gas mixture. 

The task of MEN is to transfer certain species (often pollutants) from a set of rich streams (contain contaminants to be removed) to a set of lean streams (often Mass Separating Agents, MSAs). By specifying a minimum composition difference, e, the mass transfer pinch can be located, which is the thermodynamic bottleneck for mass transfer between process streams. 

Chemical engineering processes involve the transport and transfer of momentum, energy, and mass. Momentum transfer is another word for fluid flow, and most chemical processes involve pumps and compressors, and perhaps centrifuges and cyclone separators. Energy transfer is used to heat reacting streams, cool products, and run distillation columns. Mass transfer involves the separation of a mixture of chemicals into separate streams, possibly nearly pure streams of one component. These subjects were unified in 1960 in the first edition of the classic book. Transport Phenomena (Bird et al., 2002). This chapter shows how to solve transport problems that are one-dimensional that is, the solution is a function of one spatial dimension. Chapters 10 and 11 treat two- and three-dimensional problems. The one-dimensional problems lead to differential equations, which are solved using the computer. 

An insulated vertical cylinder is fitted with two frictionless pistons forming compartments A and B. The upper piston is insulated and the lower piston, of negligible mass, separates the two compartments and can transfer heat. Compartment A contains moles of an ideal gas at initial temperature Tk, and compartment B contains moles of the same ideal gas at initial temperature Tk,. As heat is transferred across the lower piston, show that the maximum total entropy change is reached when the temperatures in both compartments are equal. Calculate the maximum total entropy change if = 60 kmol, = 25 kmol, = 100°C, and Tgi = 150°C. Assume CpQ is constant, equal to (7/2)7 . 

Extraction is a process in which one or more solutes are removed from a liquid by transferring the solute(s) into a second liquid phase (the mass-separating agent, or MSA), for which the solutes have a higher affinity. Just as in other separations involving an MSA, the two phases are brought into intimate contact with each other, then separated. Extraction depends on differenees in both solute solubility and density of the two phases. The solubility difference of the solute between the two liquid phases makes separation possible, while the difference in density allows the two liquid phases to be separated from each... 

Stripping is very similar in concept to absorption, but mass transfer occurs in the opposite direction it is the transfer of a component from a liquid stream into a gas stream mass-separating agent. The mass balances and the operating line are derived in a similar fashion to those for absorption. Referring to Figure 6.6, the operating line is... 

Hollow-fiber membrane modules are the mass-transfer equivalent of shell-and-tube heat exchangers. As fluids flow through the shell and lumen, mass is transferred from one stream to the other across the fiber wall. In contrast to heat exchangers, though, mass transfer may involve a combination of diffusion and convection, depending on the nature of the membrane. These modules are used for a wide range of membrane processes, including gas separation, reverse osmosis, filtration, and dialysis. 

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