In gas-liquid reactions

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

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. 

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 Hatta criterion compares the rates of the mass transfer (diffusion) process and that of the chemical reaction. In gas-liquid reactions, a further complication arises because the chemical reaction can lead to an increase of the rate of mass transfer. Intuition provides an explanation for this. Some of the reaction will proceed within the liquid boundary layer, and consequently some hydrogen will be consumed already within the boundary layer. As a result, the molar transfer rate JH with reaction will be higher than that without reaction. One can now feel the impact of the rate of reaction not only on the transfer rate but also, as a second-order effect, on the enhancement of the transfer rate. In the case of a slow reaction (see case 2 in Fig. 45.2), the enhancement is negligible. For a faster reaction, however, a large part of the conversion occurs in the boundary layer, and this results in an overall increase of mass transfer (cases 3 and 4 in Fig. 45.2). 

This type of high pressure NMR flow cell is specially convenient for mechanistic studies in gas-liquid reactions [61-69]. 

The temperature range used is determined mainly by the catalyst used, and whether formation of side-products will occur. Each catalyst has a specific ignition temperature at which it becomes active for the desired reaction. This temperature has to be exceeded, otherwise no catalytic reaction will occur. Above this temperature, the reaction rate increases only slowly at increasing temperature ( cf. the Arrhenius function). In general, the reaction rate is much more temperature-sensitive than is the mass-transfer rate. Thus, in reactions where the mass-transfer determines the reaction rate, as in gas - liquid reactions, a temperature rise above the ignition temperature has only a minor effect on the reaction rate. 

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. 

The problem of heat effects in gas liquid reactions was first analyzed by Danckwerts.38,41 He showed that for the absorption of C02 in amine solutions, the heat- effects are negligible. Carberry25 showed that, in many gas-liquid reactions, heat effects are small because of the low activation energy for the reactions. 

Various absorbers used for the measurements of absorption rates in gas-liquid reaction processes can also be used to make similar measurements for the gas-liquid-solid reaction processes. Commonly-used absorbers are the laminar-jet absorber (Fig. 5-19). the wetted-wall column absorber (Fig. 5-20), the rotary-drum absorber, the disk column absorber (Fig. 5-21), the single-pellet absorber (Fig. 5-22). and the gradientless contactor (Fig. 5-23). The key features of these absorbers... 

Since there is no radial bulk transport of fluid between the monolith channels, each channel acts basically as a separate reactor. This may be a disadvantage for exothermic reactions. The radial heat transfer occurs only by conduction through the solid walls. Ceramic monoliths are operated at nearly adiabatic conditions due to their low thermal conductivities. However, in gas-liquid reactions, due to the high heat capacity of the liquid, an external heat exchanger will be sufficient to control the reactor temperature. Also, metallic monoliths with high heal conduction in the solid material can exhibit higher radial heat transfer. 

High surface area to volume ratios are another distinctive feature of microscale reaction systems. Interfacial areas per unit volume in falling film microreactors have been reported to be as high as 25000 m /m. By comparison, interfacial areas in bubble columns are typically of the order of 1-200 m /m. As a consequence, surface effects that are often neglected in the macroscale become dominant. This makes a tremendous difference in gas-liquid reactions, where mass transfer from the gas to the liquid often limits the rate. 

In gas-liquid reactions carried out in agitated tanks, generally the gas-side resistance is small compared with the liquid-side resistance, and hence the following discussion is limited to only processes where the liquid-side resistance controls. If a pure gas is used, there is no gas-side resistance. 

FIGURE 11.13 The different operating regimes in gas-liquid reactions. 

Obviously, many steps are involved, and any step can be the rate-determining one. In addition, if the reaction is highly endothermic or exothermic (typical of oxidation, hydrogenation reactions), then heat has to be supplied or removed from the reactor. Sometimes the rate of heat transfer may control the overall rate of the reaction. In gas-liquid reactions catalyzed by solid particles, the suspension of catalyst particles can sometimes control the overall rate of reaction. As a first step in the process design portfolio, the rate-controlling step has to be determined, as described below. 

The alteration of selectivity due to transport in gas/liquid reactions is less commonly encountered than for gas/solid reactions. The reason for this is basically that the reaction essentially must occur completely in the film for transport to alter selectivity. If the reaction occurs predominately in the (mixed) bulk phase, concentrations of reactants and products are observable and the selectivity is simply determined by the reaction kinetics in the homogeneous phase. 

As in gas-liquid reactions, here also we consider various regimes and their combinations but focus on their qualitative behavior. For the more important systems, we also summarize in tabular form the conditions to be satisfied for the different regimes and the corresponding rate equations. 

This is analogous to that of a fast reaction in gas-liquid reactions discussed in Chapter 14, but is not exactly identical. Referring to Figure 15.3, there is a finite flux of B at the interface toward phase 1. No such flux exists in gas-liquid reactions except where B desorbs from phase 2 and diffuses into phase 1, a rather rare occurrence. This flux of B leads to a situation where the interfacial concentration of B is different from the bulk concentration, that is. 

Ning Yang, Mesosade Transport Phenomena and Mechanisms in Gas—Liquid Reaction Systems Harry E. A. Van den Akker, Mesosade Flow Structures and Fluid—Particle Interactions... 

The fading film MSR is one of the most commonly used devices for gas-liquid reactions (examples are given in gas-liquid reactions section). The liquid flows downward because of gravity in the form of film and gas flows through the open space that lies in the top cover of the housing. The falling film contactor consists in general of a stainless steel plate with open channels, typically 300 pm deep, separated by about 100 pm thick walls. The role of open microchannels is to prevent the breakup of the liquid film. 

Gas absorption and any associated chemical reaction is always accompanied by the simultaneous release of heat of solution and heat of reaction. The micro-scale phenomena taking place close to the interface therefore involve the generation and diffusion of heat as well as the diffusion and reaction of material species. In developing a fundamental appreciation of simultaneous mass and heat transfer in gas-liquid reactions it is important for the heat effects to be incorporated into the analysis of diffusion and reaction because the rates and pathways of chemical reactions are usually enormously sensitive to temperature. In particular, for the case of gas-liquid reactor performance, if the heat effects are such that the mass transfer with reaction zone adjacent to the interface is at a temperature significantly different from the bulk, the yield and the selectivity performance will be erroneously interpreted if reaction is assumed to take place at the bulk liquid temperature. In consequence, the basic conceptual design of a commercial gas-liquid reactor could incorporate fallacious reasoning leading to inefficient operation at sub-optimal yield. 

The hydrodynamic characteristics of three-phase reactors, such as pressure drop and residence time distribution, can be determined from those for fluid-solid and fluid-fluid reactors. The difference between the gas-liquid and gas-liquid-solid systems is that due to the reaction at the surface of the catalyst, there is always a concentration gradient in the liquid phase in the latter case. Unlike in gas-liquid reactions, it is always important to saturate the liquid film with the gaseous... 

Molecular size asymmetry is always present in gas-liquid reactions performed under supercritical media. It is important to have prior knowledge of the potential type of phase behaviour that a system can present. In general, type l AlnQ phase behaviour should be avoided because it presents liquid-liquid immiscibility even at extremely high pressures. In contrast, if a type 2 phase behaviour is found, the region of partial hquid miscibility can be avoided by increasing the pressure to reach an homogenous region. Peters et present... 

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