Examples of Gas-Liquid Reactions

A number of examples of gas-liquid reactions are listed in Table 14.7. The more recent trend has been to use homogeneous catalysts dissolved in the liquid phase. 

Examples of gas-liquid reactions (including some using homogeneous 

Ethylation of aniline to 2,6-diethylaniline, a herbicide (Dual ) intermediate, in the presence of Al-trianilide 

Reaction between ketene and acetic acid to make acetic anhydride 

Reaction between ethylene and chlorine to give diehloroethane 

Especially fast reactions benefit from the excellent mass transfer characteristics of microstructured devices. In addition, heat management for highly exothermic reactions is greatly facilitated because of efficient removal of heat produced during the reaction. Selective examples of different gas-liquid reactions that have been studied in the microstructured reactors are listed in Table 7.14. 

Selective fluorination of 4-nitrotoluene, 1,3-dicarbonyl, and heterocyclic compounds [76] 

Oxidation of alcohols and Baeyer-Villinger oxidation of ketones using elemental fluorine [80] 

Sulfonation of toluene with gaseous sulfur trioxide [81] Asymmetric hydrogenation of Z-methylacetamidocinnamate (mac) with rhodium chiral diphosphine complexes [82] 

Microstructured falling film and micro bubble column reactor Single micro channel operating in annular flow regime Microstructured falling film reactor Microstructured falling film reactor 

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Heterogeneous fluid-fluid reactions are made to take place for one of three reasons. First, the product of reaction may be a desired material. Such reactions are numerous and can be found in practically all areas of the chemical industry where organic syntheses are employed. An example of liquid-liquid reactions is the nitration of organics with a mixture of nitric and sulfuric acids to form materials such as nitroglycerin. The chlorination of liquid benzene and other hydrocarbons with gaseous chlorine is an example of gas-liquid reactions. In the inorganic field we have the manufacture of sodium amide, a solid, from gaseous ammonia and liquid sodium ... 

The absorption of SO3 and the removal of CO2 by a chemical absorbent are also examples of gas-liquid reactions. Since the principal aim is to absorb the acid gas, it is generally stated that these are examples of absorption with reaction. However, it should be remembered that the reaction step, which comes after the physical dissolving of the gas at the surface, can control the overall rate of the process. Roughly speaking, the liquid will, in these cases, be saturated with the free gas and more is not absorbed until some is... 

Let us take the example of gas liquid reactions. Most engineering calculations are based on the film model of Whitman,which assumes that interphase... 

The discussion is centered around gas-liquid reactors. If the dissolved gas content exceeds the amount needed for the reaction, the liquid may be first saturated with gas and then sent through a stirred tank or tubular reactor as a single phase. If the residence times for the liquid and gas are comparable, both gas and liquid may be pumped in and out of the reactor together. If the gas has limited solubility, it is bubbled through the reactor and the residence time for gas is much smaller. Figure 19-29 provides examples of gas-liquid reactors for specific processes. 

Table 1-1 Examples of gas-liquid-solid-reaction systems where all three phases are either reactants or products...

Table 1-3 Examples of gas -liquid solid-reaction systems where only two phases take active parts in the reaction. The third phase is inert...

Many examples of gas-liquid contacting operations are found in the process industries, often involving gas incorporation or absorption into liquid, perhaps with chemical reaction in the liquid, washing or humidifying a gas stream, removal of gas from liquid, and so forth. 

Alkylation of mixed cresols using isobutylene, co-oxidation of cyclohexane and acetaldehyde, and chlorination of mixed xylenols are examples of gas-liquid organic reactions in which absorption of a gas is followed by reaction with two reactants in the liquid. This class of reactions can be represented as... 

The first three chapters of this part dealt with two-phase reactions. Although catalysts are not generally present in these systems, they can be used in dissolved form in the liquid phase. This, however, does not increase the number of phases. On the other hand, there are innumerable instances of gas-liquid reactions in which the catalyst is present in solid form. A popular example of this is the slurry reactor so extensively employed in reactions such as hydrogenation and oxidation. There are also situations where the solid is a reactant or where a phase-transfer catalyst is immobilized on a solid support that gives rise to a third phase (see Chapter 20). A broad classification of three-phase reactions and reactors is presented in Table 17.1 (not all of which are considered here). This is not a complete classification, but it includes most of the important (and potentially important) types of reactions and reactors. 

Table 17.8 Examples of gas-liquid-solid catalytic reactions in organic synthesis (from Mills and Chaudhari, 1997)... 

Fluid-fluid systems are widely used in chemical, petroleum, pharmaceutical, hydrometaflurgical, and food industries. Commercially important examples of gas-liquid mass transfer with or without reaction include gas purification, oxidation, halogenations, hydrogenation, and hydroformylation to name but a few. Important liquid-liquid reactions include nitration, phase transfer catalysis (PTC), cyclization, emulsion polymerization, homogenous catalyst screening, enzymatic reactions, extraction, precipitation, crystallization, and cell separation. 

Table 1-2 (continued) - Examples of gas-liquid-solid-reaction systems where the gas and LIQUID are either REACTANTS OR PRODUCTS AND THE SOLID IS A CATALYST ( TyPE II) ( l)... 

Table 4.4 Examples of gas-liquid oxidation reactions investigated by using catalytic membrane reactors... 

Table 15.8 Examples of gas-liquid-liquid reactions studied in MSRs.

We can envision a mechanism of one or more steps for each of these unit operations and we can write a rate equation for each step. We can then relate each of these individual rate equations to an overall rate constant. For a mechanism with two or more steps in series, one step will be slower than the other steps we say this slow step is the rate controlling step. For example, a gas—liquid reaction in a laboratory-sized reactor is either heat transfer controlled or reaction rate controlled. If we cannot supply heat fast enough to maintain the reaction or if we cannot remove heat fast enough to control the reaction, we say the reaction is heat transfer controlled. If, on the other hand, we can supply or remove heat faster than required by the reaction, then we say the reaction is reaction rate controlled. In general, laboratory-sized batch and semibatch reactors have large heat transfer surface area to reaction volume ratios therefore, transferring heat to... 

Recall that there are a number of reactions where homogeneous catalysis involves two phases, liquid and gas, for example, hydrogenation, oxidation, carbonylation, and hydroformylation. The role of diffusion becomes important in such cases. In Chapter 6, we considered the role of diffusion in solid catalyzed fluid-phase reactions and gas-liquid reactions. The treatment of gas-liquid reactions makes use of an enhancement factor to express the enhancement in the rate of absorption due to reaction. A catalyst may or may not be present. If there is no catalyst, we have a simple noncatalytic gas-liquid heterogeneous reaction in which the reaction rate is expressed by simple power law kinetics. On the other hand, when a dissolved catalyst is present, as in the case of homogeneous catalysis, the rate equations acquire a hyperbolic form (similar to LHHW models discussed in Chapters 5 and 6). Therefore, the mathematical analysis of such reactions becomes more complex. 

Here, we will only examine the basic kinetic equations of gas-liquid reactions on solid catalysts, taking as example the simple reaction of a gaseous reactant A that reacts after dissolution in the liquid phase with a liquid reactant B (va = Vb = —1) to a liquid product P ... 

Figure 4.10.11 shows examples of gas-liquid reactors. Gas is usually dispersed in the liquid by a bubble column, tray column, or a stirring reactor with pressurized gas. Liquid is dispersed in the gas by means of a jet type washer or a spray tower. Liquid in the form of a thin film is exposed to a gas by a falling film reactor or a tricHe reactor with filter elements. Details of the interplay of chemical reaction and mass transfer are given in Section 4.4. 

In the example a gas-liquid reaction with particulate solids (e.g., a catalyst) operating in regime 11 in a stirred reactor with a Rushton turbine is to be scaled up. The primary process requirement is for the same degree of reaction conversion at each scale, which means the same number of moles of gas transferred per mole of liquid fed ... 

Mass Tranter and Chemical Reactions 37 Table 2.5 Examples of gas-liquid and liquid reactions carried out in the MSR. 

Table 2.6 Examples of gas—liquid-solid reactions studied in microstructured reactors.

Heterogeneous reactions of industrial significance occur between all combinations of gas, liquid, and solid phases. The solids may be inert or reac tive or catalysts in granular form. Some noncatalytic examples are listed in Table 7-11, and processes with solid catalysts are listed under Catalysis in Sec. 23. Equipment and operating conditions of heterogeneous processes are covered at some length in Sec. 23 only some highlights will be pointed out here. 

The calcium bisulfite acid used in the manufacture of sulfite cellulose is the product of reaction between gaseous sulfur dioxide, liquid water, and limestone. The reaction is normally carried out in trickle-bed reactors by the so-called Jenssen tower operation (E3). The use of gas-liquid fluidized beds has been suggested for this purpose (V7). The process is an example of a noncatalytic process involving three phases. 

As a final example of the application of gas-liquid-particle operation to a process involving a gaseous reactant and a solid catalyst, the possibility of polymerizing ethylene in, for example, a slurry operation employing a metal or metal oxide catalyst can be cited. It has been suggested that the good control of reaction conditions obtained in a slurry-type operation may be of importance in the production of certain types of polyethylene (Rl). 

Processes in which two phases react and result in the formation of a third form an important group of gas-liquid-particle processes. In the production of acetylene, a gaseous phase is formed by reaction between a liquid and a particle phase water and carbide. In the production of gas hydrates in desalination processes, a particle phase is formed by reaction between a liquid and a gaseous phase sea water and, for example, propane. In the melting of gas-hydrate or ice crystals a liquid phase is formed when gaseous and particle phases are brought in contact. 

A more general model of gas-liquid-particle processes than those that have so far appeared in the literature would, it seems, be of considerable interest as a basis for comparing the reaction-engineering properties of the several types of gas-liquid-particle operations, and as a means for analyzing operations with finite liquid flow (for example, trickle-flow operation and gas-liquid fluidization). 

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