Reactions gas + liquid

Reactions between gases and liquids may involve solids also, either as reactants or as catalysts. Table 17.9 lists a number of examples. The lime/limestone slurry process is the predominant one for removal of S02 from power plant flue gases. In this case it is known that the rate of the reaction is controlled by the rate of mass transfer through the gas film. 

Some gases present in waste gases are recovered by scrubbing with absorbent chemicals that form loose compounds the absorbent then may be recovered for reuse by elevating the temperature or lowering the pressure in a regenerator. Such loose compounds may exert appreciable back pressure in the absorber, which must be taken into account when that equipment is to be sized. 

In all cases, a limiting reactor size may be found on the basis of mass transfer coefficients and zero back pressure, but a size determined this way may be too large in some cases to be economically acceptable. Design procedures for mass transfer equipment are in other chapters of this book. Data for the design of gas-liquid reactors or chemical absorbers may be found in books such as those by Astarita, Savage, and Bisio (Gas Treating with Chemical Solvents, Wiley, New York, 1983) and Kohl and Riesenfeld (Gas Purification, Gulf, Houston, TX, 1979). 

Details on gas-liquid applications in microstructured reactors can be found as well in the book [4] and in the reviews [26-28]. 

Intimate contacting between chemically reacting gas and hquid phases is achieved in a variety of equipment, some examples of 

Several industrially important gases dissolve in liquids and then react chemically in the liquids. For carbon dioxide in water the sequence seems to be 

We see that the values shown agree with the convention in Eq. 13.AS and correspondingly 

The reason for this convention is that when we are thinking about the gas-liquid equilibrium, we treat it by Henry s law, which relates the partial pressure of CO2 or SO2 in the gas to the concentration of the same material dissolved in the liquid, while when we are talking about the ionization reactions we think about the dissolved CO2 or SO2 as the undissociated carbonic or sulfurous acid. 

Example 13.8 Estimate the amount of CO2 dissolved and the concentrations of HCOJ, COj , and H when water is in equilibrium with atmospheric air, which contain 390 ppm of CO2 at 1 atm and 20 °C. Assume that air behaves as an ideal gas and that the liquid and the ions behave as ideal solutions. 

Taking the two ionizations constants from Problem 13.10 and the value of the Henry s law constant from Table A.3 (and ignoring the difference between 20 and 25 °C), we can write 

In a number of important industrial processes, it is necessary to carry out a reaction between a gas and a liquid. Usually the object is to make a particular product, for example, a chlorinated hydrocarbon such as chlorobenzene by the re action of gaseous chlorine with liquid benzene. Sometimes the liquid is simply the reaction medium, perhaps containing a catalyst, and all the reactants and products are gaseous. In other cases the main aim is to separate a constituent such as C02 from a gas mixture although pure water could be used to remove CO2, a solution of caustic soda, potassium carbonate or ethanolamine has the advantages of increasing both the absorption capacity of the liquid and the rate of absorption. The subject of gas-liquid reactor design thus really includes absorption with chemical reaction which is discussed in Volume 2, Chapter 12. 

Reaction between an absorbed solute and a reagent lowers the equilibrium partial pressure of the solute and thus increases the rate of mass transfer. The mass transfer coefficient likewise may be enhanced which contributes further to increased absorption rate. Three modes of contacting gas and liquid phases are possible The gas is dispersed as bubbles in the liquid, the liquid is dispersed as droplets, the two phases are contacted on a thin liquid film deposited over a packing or wall. The choice between these modes is an important practical problem. 

The mathematical relations of gas-liquid reactions are like those for physical absorption but the equilibria and mass transfer coefficients are much more complex because they depend on the chemical nature of the reactant and its remaining concentration at each location in the reactor. Such data are not plentiful or well correlated in print, and the main reliance for particular reactions is on laboratory or pilot plant testing. 

Resistance to transfer of mass between phases is assumed to be confined to that of fluid films between the phases. Let D = diffusivity 

Pi = f(Cj) or Pi = HC, equilibrium at the interface a = interfacial area per unit volume zg and zL, film thicknesses 

The steady rates of solute transfer across the films are r  

These processes are carried out in a variety of equipment ranging from a bubbling absorber to a packed tower or plate column. The design of the adsorber itself requires models characterizing the operation of the process equipment and this is discussed in Chapter 14. The present chapter is concerned only with the rate of reaction between a component of a gas and a component of a liquid—it considers only a point in the reactor where the partial pressure of the reactant A in the gas phase is and the concentration of A in the liquid is C, that of B, Cg. Setting up rate equations for such a heterogeneous reaction will again require consideration of mass and eventually heat transfer rates in addition to the true chemical kinetics. Therefore we first discuss models for transport from a gas to a liquid phase. 

Four blade pitched paddle agitator Vessel fittings omitted for clarity 

If equivalent performance is required on the transfer of a gas-liquid reaction to a different item of equipment, it is necessary to maintain the k a constant. Failure to do so is a frequent cause of difficulty in chemical process development. 

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Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

In terms of cost and versatility, the stirred batch reactor is the unit of choice for homogeneous or slurry reactions and even gas/liquid reactions when provision is made for recirculation of the gas. They are especially suited to reactions with half-lives in excess of 10 min. Sam-... 

A necessary prerequisite to understanding the subject of absorption with chemical reaction is the development of a thorough understanding of the principles involved in physical absorption, as discussed earlier in this section and in Section 5. There are a number of excellent references the subject, such as the book by Danckwerts Gas-Liquid Reactions, McGraw-Hill, New York, 1970) and Astarita et al. Gas Treating with Chemical Solvents, Wiley, New York, 1983). 

TABLE 23-9 Mass-Transfer Coefficients/ Interfacial Areas and Liquid Holdup in Gas/Liquid Reactions... 

In order to allow integration of countercurrent relations like Eq. (23-93), point values of the mass-transfer coefficients and eqiiilibrium data are needed, over ranges of partial pressure and liquid-phase compositions. The same data are needed for the design of stirred tank performance. Then the conditions vary with time instead of position. Because of limited solubihty, gas/liquid reactions in stirred tanks usually are operated in semibatch fashion, with the liquid phase charged at once, then the gas phase introduced gradually over a period of time. CSTR operation rarely is feasible with such systems. 

Two lists of gas/liquid reactions of industrial importance have been compiled recently. The literature survey by Danckwerts (Gas-Liquid Reactions, McGraw-Hill, 1970) cites 40 different systems. A supplementary list by Doraiswamy and Sharma (Heterogeneous Reactions Fluid-Lluid-Solid Reactions, Wiley, 1984) cites another 50 items, and indicates the most suitable land of reactor to be used for each. Estimates of values of parameters that may be expec ted of some types of gas/liquid reac tors are in Tables 23-9 and 23-10. 

Effect of reactor design on size and productivity for a gas-liquid reaction... 

Tubular reactors often offer the greatest potential for inventory reduction. They are usually simple, have no moving parts, and a minimum number of joints and connections that can leak. Mass transfer is often the rate-limiting step in gas-liquid reactions. Novel reactor designs that increase mass transfer can reduce reactor size and may also improve process yields. 

There is an increasing number of industrially important gas-liquid reaction precipitation systems (see Kirk-Othmer, 1993), including the following ... 

Danckwerts, P.V., 1970. Gas-liquid reactions. New York McGraw-Hill. 

Rigopoulos, Stelios and Alan G. Jones, 2001. Dynamic Modelling of a Bubble Column for Particle Fonuation via a Gas-Liquid Reaction. Chemical Engineering Science (in press). 

Yagi, H., 1986. Kinetics of solid production accompanying gas-liquid reaction. Proceedings of World Congress III Chemical Engineering, Tokyo, 4, 20-23. 

Yagi, H., Nagashima, S. and Hikita, H., 1988. Semibatch precipitation accompanying gas-liquid reaction. Chemical Engineering Fundamentals, 65, 109-119. 

There are two basic types of packed-bed reactors those in which the solid is a reactant and those in which the solid is a catalyst. Many e.xaniples of the first type can be found in the extractive metallurgical industries. In the chemical process industries, the designer normally meets the second type, catalytic reactors. Industrial packed-bed catalylic reactors range in size from units with small tubes (a few centimeters in diameter) to large-diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat transfer rates in large-diameter packed beds are poor and where high heat transfer rates are required, Jluidized beds should be considered. ... 

Operations such as blending, solids-suspension, dissolving, heat transfer and liquid-liquid extraction are typical of systems requiring high flow relative to turbulence, while gas-liquid reactions and some liquid-liquid contacting require high turbulence relative to flow. The case of (1) 100% of suspension—requires head to keep particles suspended and (2) 100% uniformity of distribution of particles—requires head for suspension plus flow for dis-tiibution. 

Henry s law is a reasonable approximation in the absence of gas-liquid reactions when total pressure is low to moderate. Deviations are usually manifested in the form of H, dependence on P and phase compositions. 

The carbon source affects oxygen demand. In penicillin production, oxygen demand for glucose is 4.9 mol 1 1 h-1. The lactose concentration is 6.7 mol 1 1 h 1, sucrose is 13.4 mol l-1 h. The yield of oxygen per mole of carbon source for CH4 is YQjC = 1.34, T0j/C for Paraffins = 1, and Y(> /c for hydrocarbon (CH20)n = 0.4. The mass transfer coefficient k,a is for gas-liquid reactions, and the film thickness where the mass transfer takes place is 8... 

Danckwerts, P.V. Gas-liquid Reactions (McGraw-Hill, New York, 1970). 

Chapter 11 treats reactors where mass and component balances are needed for at least two phases and where there is interphase mass transfer. Most examples have two fluid phases, typically gas-liquid. Reaction is usually confined to one phase, although the general formulation allows reaction in any phase. A third phase, when present, is usually solid and usually catalytic. The solid phase may be either mobile or stationary. Some example systems are shown in Table 11.1. 

Stirred tanks are often used for gas-liquid reactions. The usual geometry is for the liquid to enter at the top of the reactor and to leave at the bottom. The gas enters through a sparge ring underneath the impeller and leaves through the vapor space at the top of the reactor. A simple but effective way of modeling this and many similar situations is to assume perfect mixing within each phase. 

The diffusivity in gases is about 4 orders of magnitude higher than that in liquids, and in gas-liquid reactions the mass transfer resistance is almost exclusively on the liquid side. High solubility of the gas-phase component in the liquid or very fast chemical reaction at the interface can change that somewhat. The Sh-number does not change very much with reactor design, and the gas-liquid contact area determines the mass transfer rate, that is, bubble size and gas holdup will determine reactor efficiency. 

The rates of gas—liquid reactions are surface area dependent. Hence in the spontaneous combustion of oil impregnating fibrous thermal insulation on hot equipment, oxidation is facilitated by the large exposed surface area and, since the dissipation of heat is restricted, the temperature can rise until the oil ignites spontaneously. 

Many liquid phase or heterogeneous solid—liquid or gas—liquid reactions result in gaseous products or byproducts. These products may be toxic (refer to Table 4.1) or flammable (refer to Table 5.1), or result in overpressurization of any sealed container or vessel. Unless pressure relief is provided, relatively small volumes of reactants — the presence of which may not be expected — may generate sufficient gas pressure to rupture a container. The causes of pressure build-up may be ... 

Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The three-phase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gas-liquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. 

Danckwerts.P.V., Gas-Liquid Reactions, Me. Graw Hill Book Co., New York, 1970. 

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