Mass transfer separation processes

Mass transfer separation processes, e.g., distillation, gas absorption, etc., are normally treated in terms of stagewise or differential procedures. In a stagewise procedure, concentration changes are taken to occur in distinct jumps, as, for... 

In the following, the principles of mass-transfer separation processes will be outlined first. Details of mass-transfer calculations will be introduced next and examples will be given of both equilibrium-stage processes and diffusional rate processes. The chapter will then conclude with a detailed discussion of the two single most applied mass-transfer processes in the chemical industries, namely distillation and absorption. 

Our primary interests here are directed toward vertical-flow contacting devices, generally referred to as bubble-column reactors. These can be empty, with stage-wise placing of porous plates for bubble redispersion, or filled with a commercial packing such as Raschig rings. These two types have received full attention in work directed toward mass-transfer/separation processes, and will not be considered further here. What we will examine now is the case in which there are discrete... 

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). 

One temporal concept to be borne in mind in this context is whether the (bio)chemical reactions and mass transfer separations taking place at the active microzone (one or both of which, by definition, take place simultaneously with detection) are simultaneous or sequential relative to each other. Whether such processes take place at the same or a different time has a marked effect on the sensor performance and type of transient signal obtained. 

Mass transfer-limited processes favor SRs over monoliths as far as the overall process rates are concerned. Moreover, SRs are more versatile and less sensitive to gas flow rates. However, the productivity per unit volume is not necessarily higher for SRs because of the low concentration of catalyst in such reactors. There is also no simple answer to the selectivity problem, and again each process should be compared in detail for both reactors. For a kinetic regime, monoliths can be more advantageous due to their easier operation. The catalyst does not disintegrate due to the stirrer action, and catalyst separation is avoided. Catalysts are often pyrophoric materials, handling of which is usually a hazardous operation. The benefits of MRs can be achieved only for stable catalysts. For quickly deactivating catalysts, SRs are easier to operate, since replacement of decayed catalysts is simpler. 

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. 

Separation processes rely on various mechanisms, implemented via a unit operation, to perform the separation. The mechanism is chosen to exploit some property difference between the components. They fall into two basic categories the partitioning of the feed stream between phases and the relative motion of various chemical species within a single phase. These two categories are often referred to as equilibrium and mass transfer rate processes, respectively. Separation processes can often be analyzed with either equilibrium or mass transfer models. However, one of these two mechanisms will be the limiting, or controlling, factor in the separation and is, therefore, the design mechanism. 

Phase equilibrium information characterizes partitioning between phases for a system and is important for describing separation processes. For equilibrium-limited processes, these values dictate the limits for separation in a single stage. For mass transfer-limited processes, the partitioning between phases is an important parameter in the analysis. The data can be presented in tabular form. But this approach is restricted in application, since an analysis typically requires phase equilibrium values that are not explicitly listed in the table. So, graphical representation and computational methods are usually more useful. 

Gas chromatography is a separation technique based on the fact that different components in the mixture exhibit different average residence times due to interactions with the porons packing material. These interactions can be classified as intrapellet diffusion and the column operates similar to a packed catalytic tubular reactor. The important mass transfer mechanisms are convection and diffusion. Hence, it is important to calculate the mass transfer Peclet number that represents an order-of-magnitude ratio of these two mass transfer rate processes. Intrapellet diffusion governs residence times, and interpellet axial dispersion affects the degree to which the output curve is broadened. For axial dispersion in packed columns and packed catalytic tubular reactors. 

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. 

The importance of these operations is profound. There is scarcely any chemical process which does not require a preliminary purification of raw materials or final separation of products from by-products, and for these the mass-transfer operations are usually used. One can perhaps most readily develop an immediate appreciation of the part these separations play in a processing plant by observing the large number of towers which bristle from a modern petroleum refinery, in each of which a mass-transfer separation operation takes place. Frequently the major part of the cost of a process is that for the separations. These separation or purification costs depend directly upon the ratio of final to initial concentration of the separated substances, and if this ratio is large, the product costs are large. Thus, sulfuric acid is a relatively low-priced product in part because sulfur is found naturally in a relatively pure state, whereas pure uranium is expensive because of the low concentration in which it is found in nature. 

Dyna.micPerforma.nce, Most models do not attempt to separate the equiUbrium behavior from the mass-transfer behavior. Rather they treat adsorption as one dynamic process with an overall dynamic response of the adsorbent bed to the feed stream. Although numerical solutions can be attempted for the rigorous partial differential equations, simplifying assumptions are often made to yield more manageable calculating techniques. 

Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

Chemistry. Chemical separation is achieved by countercurrent Hquid— Hquid extraction and involves the mass transfer of solutes between an aqueous phase and an immiscible organic phase. In the PUREX process, the organic phase is typically a mixture of 30% by volume tri- -butyl phosphate (solvent) and a normal paraffin hydrocarbon (diluent). The latter is typically dodecane or a high grade kerosene (20). A number of other solvent or diluent systems have been investigated, but none has proved to be a substantial improvement (21). 

The process of flushing typically consists of the foUowing sequence phase transfer separation of aqueous phase vacuum dehydration of water trapped in the dispersed phase dispersion of the pigment in the oil phase by continued appHcation of shear thinning the heavy mass by addition of one or more vehicles to reduce the viscosity of dispersion and standardization of the finished dispersion to adjust the color and rheological properties to match the quaHty to the previously estabHshed standard. 

Kirwan, Mass Transfer Principles in Rousseau, Handbook of Separation Process Technology, Wiley, 1987. 

Transfer of material between phases is important in most separation processes in which two phases are involved. When one phase is pure, mass transfer in the pure phase is not involved. For example, when a pure liqmd is being evaporated into a gas, only the gas-phase mass transfer need be calculated. Occasionally, mass transfer in one of the two phases may be neglec ted even though pure components are not involved. This will be the case when the resistance to mass transfer is much larger in one phase than in the other. Understanding the nature and magnitudes of these resistances is one of the keys to performing reliable mass transfer. In this section, mass transfer between gas and liquid phases will be discussed. The principles are easily applied to the other phases. 

The separation of components by liquid-liquid extraction depends primarily on the thermodynamic equilibrium partition of those components between the two liquid phases. Knowledge of these partition relationships is essential for selecting the ratio or extraction solvent to feed that enters an extraction process and for evaluating the mass-transfer rates or theoretical stage efficiencies achieved in process equipment. Since two liquid phases that are immiscible are used, the thermodynamic equilibrium involves considerable evaluation of nonideal solutions. In the simplest case a feed solvent F contains a solute that is to be transferred into an extraction solvent S. 

The reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

Dry Scrubbing Diy scruhhing is an umbrella term used to associate several different unit operations and types of hardware that can be used in combinations to accomplish the unit process of dry scrubbing. They all utilize scrubbing, in which mass transfer takes place between the gas phase and an active liquidlike surface, and they all discharge the resulting products separately as a gas and a sohdlike diy product for reuse or disposal. 

Calculations. To check a design for possible fogging, a procedure is presented that rightly considers mass transfer and heat transfer as two separate processes. 

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