Contacting with Disperse Phases

The different flow patterns largely resemble those known from flow in other continuous flow conduits as pipes, tubes, capillaries, and monoliths [29-40]. Bubbly flow, slug flow (Taylor flow), annular, and churn flow are found and a few more intermediate regimes between the ones mentioned. These comprise different gas-liquid configurations such as segmented flow (bubble-train), gas core with encompassing stable thin liquid film, which wets the channel wall, and dynamic wavy liquid films. In case of high gas contents, spray is created with small droplets in 

The feed of dispersive systems in many parallel microchannels is not trivial and mixed flow patterns and even drying of the channels were reported for the first-hour devices [66,67]. Distributor design solutions for phase equipartition were proposed for some devices, for example, for mini-packed reactors [68-70]. 

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Multiphase catalytic reactors are employed in nearly 80% of industrial processes with annual global sales of about 1.5 trillion, contributing around 35% of the world s GDP [17]. Microreactors for multiphase reactions are classified based on the contact principles of gas and liquid phases continuous-phase contacting and dispersed-phase contacting [18]. In the former type, the two phases are kept in continuous contact with each other by creating an interface. In the latter case, one fluid phase is dispersed into another fluid phase. In addition, micro trickle bed operation is reported following the path of classical chemical engineering. The study of mass and heat transfer in two-phase flow in micro trickle bed reactors still remains as a less... 

Coalescence depends on the collision rate, which increases with dispersed phase concentration. To quantify this process, it is convenient to define a collision frequency (d, d ), between drops of diameter d and d, which is independent of concentration. The collision frequency depends on agitation rate and drop size. As shown in Figures 12-14 and 12-17, the collision of two drops does not ensure coalescence. As the drops approach each other, a film of continuous phase fluid keeps them apart. Coalescence depends on the rupture of this film. It must drain to a critical thickness before coalescence can occur. The critical drainage time is the time it takes for the film to thin sufficiently that rupture occurs or in other words, coalescence occurs only if the collision interval, referred to as the contact time, exceeds Ihe critical film drainage time. The probability that this will occur is called the coalescence efficiency, k(d,d0. It depends on a different set of hydro-dynamic factors as well as drop size and physicochemical variables. Because collision frequency and coalescence efficiency depend on different factors, then-contributions to coalescence are treated separately. As a result, the coalescence frequency F(d, d ) between two drops of diameter d and d is defined as... 

Classical cocurrent separators in two-phase systems generally use dispersive contacting with one phase dispersed as drops (or bubbles). Porous membrane based systems can avoid such dispersive contacting, as we have... 

For dense flows at low shear rates the total solid phase kinetic and collisional pressure tensor closure derived from kinetic theory was extended by an semi-empirical frictional pressure tensor in order to consider the impact of the long term particle-particle interactions such as sliding or rolling contacts. The dispersed phase pressure was thus calculated from (4.151), closed with (4.158) and (4.155) ... 

Many substances used in modem processing industries occur in a mixture of components dispersed through a soHd material. To separate the desired solute constituent or to remove an unwanted component from the soHd phase, the soHd is contacted with a Hquid phase in the process called Hquid—soHd extraction, or simply leaching. In leaching, when an undesirable component is removed from a soHd with water, the process is called washing. 

Water-in-OilEmulsions. A water-in-od or invert emulsion consists of a continuous od phase which surrounds finely divided water droplets that are uniformly dispersed throughout the mixture. The invert emulsion ensures that the od is in constant contact with the hydrauHc system s moving parts, so as to minimise wear. 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only aline of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended soHd, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase iacreases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because iaterfaces can only exist between two phases, analysis of phenomena ia the L—L—S—G system breaks down iato a series of analyses, ie, surfactant solution to the emulsion, soHd, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed soHd. FiaaHy, the dispersed phases are ia equiUbrium with each other through their common equiUbrium with the surfactant solution. 

Down spouts (or up spouts) are best set flush with the plate from which they lead, with no weir as in gas-hquid contact. The velocity of the continuous phase in the down spout V, which sets the down-spout cross section, should be set at a value lower than the terminal velocity of some arbitrarily small droplet of dispersed phase, say, 0.08 or 0.16 cm i M or Mfi in) in diameter otherwise, recirculation of entrained dispersed phase around a plate will result in flooding. The down spouts should extend beyond the accumulated layer of dispersed phase on the plate. 

When a gas comes in contact with a solid surface, under suitable conditions of temperature and pressure, the concentration of the gas (the adsorbate) is always found to be greater near the surface (the adsorbent) than in the bulk of the gas phase. This process is known as adsorption. In all solids, the surface atoms are influenced by unbalanced attractive forces normal to the surface plane adsorption of gas molecules at the interface partially restores the balance of forces. Adsorption is spontaneous and is accompanied by a decrease in the free energy of the system. In the gas phase the adsorbate has three degrees of freedom in the adsorbed phase it has only two. This decrease in entropy means that the adsorption process is always exothermic. Adsorption may be either physical or chemical in nature. In the former, the process is dominated by molecular interaction forces, e.g., van der Waals and dispersion forces. The formation of the physically adsorbed layer is analogous to the condensation of a vapor into a liquid in fret, the heat of adsorption for this process is similar to that of liquefoction. 

When two immiscible liquids are stirred together, one phase becomes dispersed as tiny droplets in the second liquid which forms a continuous phase. Liquid-liquid extraction, a process using successive mixing and settling stages (Volume 2, Chapter 13) is one important example of this type of mixing. The liquids are brought into contact with... 

In these laboratory studies the active catalyst phase (Pt) is highly dispersed on an electronically conductive support (C, Au) in contact with the electrolyte. 

This precipitation process can be carried out rather cleverly on the surface of a reverse phase. If the protein solution is brought into contact with a reversed phase, and the protein has dispersive groups that allow dispersive interactions with the bonded phase, a layer of protein will be adsorbed onto the surface. This is similar to the adsorption of a long chain alcohol on the surface of a reverse phase according to the Langmuir Adsorption Isotherm which has been discussed in an earlier chapter. Now the surface will be covered by a relatively small amount of protein. If, however, the salt concentration is now increased, then the protein already on the surface acts as deposition or seeding sites for the rest of the protein. Removal of the reverse phase will separate the protein from the bulk matrix and the original protein can be recovered from the reverse phase by a separate procedure. 

The first example will be the separation of a ferredoxin mixture using a bonded phase that contains aromatic nuclei as well as aliphatic chains. The stationary phase will thus, exhibit polar interaction from induced dipoles if the aromatic ring comes into contact with a strong dipoles of the solute and, at the same time, exhibit dispersive interactions between the aliphatic chains and any dispersive centers of the solute molecule. An example of the separation obtained is shown in figure 16. 

The interfacial area AtV usually excludes the contact area between the vapor space and the liquid at the top of the reactor. The justification for this is that most gas-liquid reactors have gas bubbles as a dispersed phase. This gives a much larger interfacial area than the nominal contact area at the top of the reactor. There are exceptions—e.g., polyester reactors where by-product water is removed only through the nominal interface at the top of the reactor— but these are old and inefficient designs. This nominal area scales as while the contact area with a dispersed phase can scale as S. 

Figure 2.63 Droplet formation in a micro mixer for a wall contact angle of 40° (left) and 90° (right), with silicone oil being the continuous and water the disperse phase.

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

The formation of nanocomposites can be done using different arrangements, for example, the dispersion of a semiconductor in a continuous matrix, the formation of stacked layers, core-shell geometries, or simply physically contacted, with consequences for the energy transfer between the phases (Figure 4.5) [76]. 

In the liquid acid-catalyzed processes, the hydrocarbon phase and the acid phase are only slightly soluble in each other in the two-phase stirred reactor, the hydrocarbon phase is dispersed as droplets in the continuous acid phase. The reaction takes place at or close to the interface between the hydrocarbon and the acid phase. The overall reaction rate depends on the area of the interface. Larger interfacial areas promote more rapid alkylation reactions and generally result in higher quality products. The alkene is transported through the hydrocarbon phase to the interface, and, upon contact with the acid, forms an acid-soluble ester, which slowly decomposes in the acid phase to give a solvated... 

In the empty tube, bubble and droplet sizes are clearly smaller and hence specific surface areas at the G/L- and L/L-interphase are higher than with the static mixers. Obviously, contact of the dispersed phases with the mixer plates supports the coagulation of bubbles and droplets. However, the overall reaction... 

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