Disperse phase contact

Direct gas-liquid (dispersed phase) contact forming a gas-organic complex . 

Dispersed phase contacting that is obtained when one of the fluid phases is dispersed into the other phase. 

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... 

Two classes of gas-liquid microchannel reactors were developed in the past years -continuous-phase contacting falling film, overlapping charmel, mesh, and annular flow approaches, and dispersed-phase contacting by Taylor flow reactors, micromixers for bubble and foam formation, and miniaturized packed bed microreactors, which follow classical trickle-bed operation at smaller scale. Recently integration of operations inside a microdevice has been studied and led to the development of membrane microreactors. 

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. 

Minimum allowable capacity of a column is determined by the need for effective dispersion and contacting of the phases. The types of plates differ in their ability to permit Tow flows of gas and liquid. A cross-flow sieve plate can operate at reduced gas flow down to a point where liquid drains through the perforations and gas dispersion is inadequate for good efficiency. Valve plates can be operated at veiy... 

Operating holdup contributes effectively to mass-transfer rate, since it provides residence time for phase contact and surface regeneration via agglomeration and dispersion. Static holdup is hmited in its contribution to mass-transfer rates, as indicated by Thoenes and Kramers [Chem. Eng. ScL, 8, 271 (1958)]. In laminar regions holdup in general has a negative effecl on the efficiency of separation. 

Types of Gas-in-Liquid Dispersions Two types of dispersions exist. In one, gas bubbles produce an unstable dispersion which separates readily under the influence of gravity once the mixture has been removed from the influence of the dispersing force. Gas-hquid contacting means such as bubble towers and gas-dispersing agitators are typical examples of equipment producing such dispersions. More difficulties may result in separation when the gas is dispersed in the form of bubbles only a few micrometers in size. An example is the evolution of gas from a hquid in which it has been dissolved or released through chemical reaction such as electrolysis. Coalescence of the dispersed phase can be helpful in such circumstances. 

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. 

Equations describing the transfer rate in gas-liquid dispersions have been derived and solved, based on the film-, penetration-, film-penetration-, and more advanced models for the cases of absorption with and without simultaneous chemical reaction. Some of the models reviewed in the following paragraphs were derived specifically for gas-liquid dispersion, whereas others were derived for more general cases of two-phase contact. 

Some of the additional mathematical models mentioned below were derived for the characterization of swarms of bubbles whereas others were derived for the specific case of a single bubble or the general case of two-phase contact. Most models for the bubble-liquid contact are limited to the case of a single bubble, and consequently their direct applicability to gas-liquid dispersions is very restricted. 

Most methods of separating molecules in solution use direct contact of immiscible fluids or a sohd and a fluid. These methods are helped by dispersion of one phase in the other, fluid phase, but they are hindered by the necessity for separating the dispersed phase. Fixed-bed adsorption processes overcome the hindrance by immobilizing the solid adsorbent, but at the cost of cyclic batch operation. Membrane processes trade direct contact for permanent separation of the two phases and offer possibilities for high selectivity. 

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. 

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... 

A high internal phase liquid-liquid emulsion (HIPE) is one where the internal or dispersed phase droplets occupy >74% of the total volume of the emulsion. At this point the droplets contact each other and beyond this volume % the droplets are forced into distorted polyhedra. If for example styrene and divinylbenzene are employed as the continuous phase and water droplets dispersed in this oil phase using a suitable surfactant to form a HIPE, the comonomers can be polymerized to form a poly(styrene-divinylbenzene) polyHIPE. Typically the water droplets are... 

It is also possible to generate microcapsules through interfacial polymerization using only one monomer to form the shell. In this class of encapsulations, polymerization must be performed with a surface-active catalyst, a temperature increase, or some other surface chemistry. Herbert Scher of Zeneca Ag Products (formerly Stauffer Chemical Company) developed an excellent example of the latter class of shell formation (Scher 1981 Scher et al. 1998). He used monomers featuring isocyanate groups, like poly(methylene)-poly(phenylisocyanate) (PMPPI), where the isocyanate reacts with water to reveal a free primary amine. Dissolved in the oil-dispersed phase of an oil-in-water emulsion, this monomer contacts water only at the phase boundary. The primary amine can then react with isocyanates to form a polyurea shell. Scher used this technique to encapsulate pesticides, which in their free state would be too volatile or toxic, and to control the rate of pesticide release. 

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