Chemical reaction in the liquid phase

Extrapolation of KgO data for absorption and stripping to conditions other than those for which the origin measurements were made can be extremely risky, especially in systems involving chemical reactions in the liquid phase. One therefore would be wise to restrict the use of overall volumetric mass-transfer-coefficient data to conditions not too far removed from those employed in the actual tests. The most reh-able data for this purpose would be those obtained from an operating commercial unit of similar design. 

The absorption of reactants (or desorption of products) in trickle-bed operation is a process step identical to that occurring in a packed-bed absorption process unaccompanied by chemical reaction in the liquid phase. The information on mass-transfer rates in such systems that is available in standard texts (N2, S6) is applicable to calculations regarding trickle beds. This information will not be reviewed in this paper, but it should be noted that it has been obtained almost exclusively for the more efficient types of packing material usually employed in absorption columns, such as rings, saddles, and spirals, and that there is an apparent lack of similar information for the particles of the shapes normally used in gas-liquid-particle operations, such as spheres and cylinders. 

Increase in interfacial area. The total surface area for diffusion is increased because the bubble diameter is smaller than for the free-bubbling case at the same gas flow rate hence there is a resultant increase in the overall absorption rate. The overall absorption rate will also increase when the diffusion is accompanied by simultaneous chemical reaction in the liquid phase, but the increase in surface area only has an appreciable effect when the chemical reaction rate is high the absorption rate for this case is then controlled by physical diffusion rather than by the chemical reaction rate (G6). 

In a gas the combustion rate falls with the combustion temperature—if the lower combustion temperature occurs for a given initial state as a result of incomplete combustion. The combustion rate, however, again increases when the combustion temperature does not exceed TB so that the reaction is limited to the liquid phase the reasons for the increase are indicated above. However, it is better here to speak not of the combustion rate, but of the propagation rate of the heating wave of the liquid due to reaction in the liquid phase. The maximum temperature achievable in a liquid is limited by the quantity TB, just as the combustion temperature, in the strict sense, is limited by the quantity Tc. Calculation of the velocity of the heating wave in a liquid is not difficult [see formulas (3.32), (3.34)] if the kinetics of the chemical reaction in the liquid phase are known. 

In the absence of a chemical reaction in the liquid phase, the density nj can be expressed approximately as... 

Examples of applications will be presented later in this contribution. However, first we will discuss transition state theory for liquid-phase reactions and parametrization of continuum models for reactive problems, because these theoretical constructs are required for applications to chemical reactions in the liquid phase. 

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. 

From Eqn. (14) it follows that with an exothermic reaction - and this is the case for most reactions in reactive absorption processes - decreases with increasing temperature. The electrolyte solution chemistry involves a variety of chemical reactions in the liquid phase, for example, complete dissociation of strong electrolytes, partial dissociation of weak electrolytes, reactions among ionic species, and complex ion formation. These reactions occur very rapidly, and hence, chemical equilibrium conditions are often assumed. Therefore, for electrolyte systems, chemical equilibrium calculations are of special importance. Concentration or activity-based reaction equilibrium constants as functions of temperature can be found in the literature [50]. 

Figure 16 also demonstrates the complexity of a chemical reaction in the liquid phase. A photochemical reaction such as those discussed in this section can easily span several orders of magnitude in time from its initiation to completion, as results of the intricate dynamical processes in... 

Conversely, if chemical reaction in the liquid phase limits the overall reaction rate, a large value of p suggests minimizing the gas-liquid interface and maximizing both liquid volume and turbulent mixing. The best contactor would disperse discrete bubbles in a continuous liquid phase. Both the bubble column... 

Although the penetration theory better describes the gas-liquid mass transfer than the film theory, its advantage is only significant with physical absorption. For gas absorption with fast chemical reactions in the liquid phase, the entire mass transfer process is gas-phase-controlled rendering the penetration theory inapplicable.f ... 

The term homogenization is used, if a uniform liquid phase has to be realized, e.g. a molecularly homogeneous mixture of several miscible liquids or equalization of concentration and temperature differences during a chemical reaction in the liquid phase. (The same term is used in the food industry for a completely different operation, namely for L/L (liquid/liquid) dispersion under extreme shear conditions e.g. the homogenization of milk). 

A signihcant number of commercial absorption processes involve a chemical reaction in the liquid phase. The effect of the reaction is to speed up the rate of mass transfer and, to some extent, make the determination of transfer units or stages simpler. 

A large number of chemical reactions in the liquid phase have been studied by means of such methodologies [2-7]. 

The polymerisation of PO and EO, initiated by polyfunctional starters, to make short chain polyether polyols is a reaction that is strongly dependent on diffusion. The consumption rate of PO or EO is given by two simultaneous factors the rate of the chemical reaction in the liquid phase and the efficiency of the monomer mass transfer from the gaseous phase to liquid phase (see details in section 4.1.5). The PO (or EO) consumption rate, considering the mass transfer, is described by equation 13.27 [45-50] ... 

The condensation of water vapor and its precipitation from the atmosphere in the form of rain, snow, sleet, or hail are important not only for the water cycle, but also because they bring to the earth surface other atmospheric constituents, primarily those substances that have a pronounced affinity toward water in the condensed state. Cloud and precipitation elements may incorporate both aerosol particles and gases. The uptake mechanisms are discussed in this chapter, together with the inorganic composition of cloud and rain water that they determine. These processes are, in principle, well understood. Another subject requiring discussion is the occurrence of chemical reactions in the liquid phase of clouds. The oxidation of S02 dissolved in cloud water is considered especially important. As a result of laboratory studies, the conversion of S02 to sulfate is now known to proceed by several reaction pathways in aqueous solution. 

To ensure a net flux of A from the gas phase to the liquid phase, the bulk liquid-phase concentration of A must not be in equilibrium with the bulk gas-phase partial pressure pAa and cAl < HApAc. (If the reverse is true, then the net flux of A is from the liquid to the gas.) The absence of equilibrium can be a result of aqueous-phase chemical reaction of A or simply the fact that water is undersaturated relative to the atmospheric concentration of the gas. (We do not consider here the enhancement of deposition that arises from chemical reaction in the liquid phase.)... 

Kinetic subregime The chemical reaction in the liquid phase is very slow and dilfusional transport effects are not important. Then... 

No Chemical Reaction in the Liquid Phase. This is the other limiting case where one calculates the time dependence of the radius of the spherical solid pellet when the Damkohler number approaches zero. Hence, species A dissolves into stagnant liquid B and the mass transfer boundary layer thickness grows with the square root of time according to the classic penetration theory. In other words,... 

The next objective is to identify a time constant for each important mass transfer rate process and solve the system of equations for the two-phase CSTR in terms of these time constants. This approach allows one to develop generic solutions in dimensionless form. For example, six time constants can be defined for (1) convection in the liquid phase (r), (2) chemical reaction in the liquid phase (X), and (3-6) interphase mass transfer for each component (0y, j = B,C1,M,H). Obviously, these six time constants produce five dimensionless ratios. Remember that time constants represent order-of-magnitude estimates of the time scales of mass transfer rate processes. The time constant for convective mass transfer in the liquid phase is equivalent to the liquid s residence time ... 

The sequence of equations presented below is required to solve the isothermal gas-liquid CSTR problem for the chlorination of benzene in the liquid phase at 55°C. After some simplifying assumptions, the problem reduces to the solution of nine equations with nine unknowns. Some of the equations are nonlinear because the chemical kinetics are second-order in the liquid phase and involve the molar densities of the two reactants, benzene and chlorine. The problem is solved in dimensionless form with the aid of five time constant ratios that are generated by six mass transfer rate processes (1) convective mass transfer through the reactor, (2) molecular transport in the liquid phase across the gas-liquid interface for each of the four components, and (3) second-order chemical reaction in the liquid phase. 

Time constant ratio (r/A.) = p for convective mass transfer through the reactor (residence time r) relative to the time constant for second-order irreversible chemical reaction in the liquid phase k, where x is incremented as an important design variable. 

Design a two-phase gas-liquid CSTR for the chlorination of benzene at 55°C by calculating the total volume that corresponds to an operating point where r/X = 500 on the horizontal axis of the CSTR performance curve in Figure 24-1. The time constant for convective mass transfer in the liquid phase is r. The time constant for second-order irreversible chemical reaction in the liquid phase is If the liquid benzene feed stream is diluted with an inert, then 7 increases. The liquid-phase volumetric flow rate is 5 gal/min. The inlet molar flow rate ratio of chlorine gas to liquid benzene... 

Evaluation studies in industry have shown that microreactors can significantly contribute to improved chemical production. In principle, a variety of chemical reactions in the liquid phase or the gas phase can be carried out readily for the production of fine and specialty chemicals and active pharmaceutical ingredients. Nevertheless, it has to be said that so far no widespread use of microreactors has occurred. The main reasons for this is the lack of microstructured unit operations that can be connected easily with the reactor modules. In only a few examples has purification been implemented directly in the microreactor, as mentioned earlier... 

Now consider a two phase CSTR consisting of an isobaric flash separator with a simultaneous chemical reaction in the liquid phase, as shown in Fig. 6.1. 

First of all, three special cases of vapor-hquid equilibrium of binary mixtures are presented qualitatively in Fig. 5.1-2. Considered are ideal mixtures (case A ), mixtures with total miscibiUty gap in the hquid phase (case B), and mixtures with irreversible chemical reaction in the liquid phase (case C). By converrtion, the symbols X and y denote the molar fraction of the low-boiling component a in the hq-ttid and the gas phase, respectively. 

Big. 5.1-2 Vapor-liquid equilibrium of three binary mixtures (A) ideal system, (B) system with a total miscibility gap in the liquid phase, and (C) system with irreversible chemical reaction in the liquid phase... 

Mixtures with Irreversible Chemical Reaction in the Liquid Phase (Case C)... 

A number of different methods for the preparation of supported metal (oxide) catalysts are dealt with in this book. In this chapter we discuss deposition precipitation as a generic method to emplace metals, metal oxides, metal sulfides, or metal hydroxides as small particles onto an existing support material. The deposition of the metal (compound) is brought about by a chemical reaction in the liquid phase. This chemical reaction leads to formation of a metal compound with low solubility in the solvent in question. The precipitation that follows is steered to take place exclusively at the surface of a suspended support material. The precipitation of metal hydroxides from an aqueous solution, such as... 

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