Reactant nonvolatile liquid

Liquid reactant (nonvolatile component) liquid-phase mass balance ... 

A summary of reactor models used by various authors to interpret trickle-bed reactor data mainly from liquid-limiting petroleum hydrodesulfurization reactions (19-21) is given in Table I of reference (37). These models are based upon i) plug-flow of the liquid-phase, ii) the apparent rate of reaction is controlled by either internal diffusion or intrinsic kinetics, iii) the reactor operates isothermally, and iv) the intrinsic reaction rate is first-order with respect to the nonvolatile liquid-limiting reactant. Model 4 in this table accounts for both incomplete external and internal catalyst wetting by introduction of the effectiveness factor r)Tg developed especially for this situation (37 ). 

For a volatile liquid reactant or a volatile product, these steps are essentially reversed. For a nonvolatile liquid reactant or product, only the reaction and diffusion in the liquid take place. Figure 7-15 describes the absorbing gas concentration profiles in a gas-liquid system. 

This applies to other multiphase reactors, not just trickle-beds. When the reactions in trickle-beds involve nonvolatile liquid reactants which limit the reaction rate, then the interpretation of liquid-phase tracer data in a TBR can be approached based on Case 2 discussed in Section 6.1.1. Clearly, scale-up is possible under conditions discussed in that section. In case of volatile liquid reactants or rates governed by both gas and liquid reactants the trickle-bed must be considered as a system with two flowing phases discussed in Section 6.1.2 and scale-up is difficult. 

Most of the previously used expressions to account for incomplete catalyst wetting in trickle-beds are summarized in Table I. All of these, with the exception of the last one, are based on the assumptions of a) plug flow of liquid, b) no external mass transfer limitations, c) isothermal conditions, d) first order irreversible reaction with respect to the liquid reactant, e) nonvolatile liquid reactant, f) no noncatalytic homogeneous liquid phase reaction. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

A reaction between organic compounds is carried out in the liquid phase in a stirred-tank reactor in the presence of excess formaldehyde. The organic reactants are nonvolatile in comparison with the formaldehyde. The reactor is vented to atmosphere via an absorber to scrub any organic material carried from the reactor. The absorber is fed with freshwater and the water from the absorber rejected to effluent. The major contaminant in the aqueous waste from the absorber is formaldehyde. 

The rate expressions developed in this section for gas-liquid systems, represented by reaction 9.2-1, are all based on the two-film model. Since liquid-phase reactant B is assumed to be nonvolatile, for reaction to occur, the gas-phase reactant A must enter the liquid phase by mass transfer (see Figure 9.4). Reaction between A and B then takes place at some location within the liquid phase. At a given point, as represented in Figure 9.4, there are two possible locations the liquid film and the bulk liquid. If the rate of mass transfer of A is relatively fast compared with the rate of reaction, then A reaches the bulk liquid before reacting with B. Conversely, for a relatively fast rate of reaction ( instantaneous in the extreme), A reacts with B in the liquid film before it reaches the bulk liquid. Since the intermediate situation is also possible, we may initially classify the kinetics into three regimes ... 

Three-phase reactors are generally needed in cases where there are both volatile and nonvolatile reactants, or when a liquid solvent is necessary with all reactants in the gas-phase (Smith, 1981). Some examples are... 

The gas phase is dilute, i.e. it contains the reactant A and inerts in great excess. The other reactants and products are nonvolatile and are present only in liquid phase. In this case, the expansion could be taken as zero. 

The employment of three-phase reactors is mostly desirable when there are some reactants that are too volatile to liquefy, whereas some others are too nonvolatile to vaporize. Hence, the situation where a gaseous component reacts with another reactant in the liquid-phase is of great interest. The following reaction represents this case (Smith, 1981) ... 

A trickle bed is a continuous three-phase reactor. Three phases are normally needed when one reactant is too volatile to force into the liquid phase or too nonvolatile to vaporize. Operation of a trickle bed is limited to cocurrent downflow to allow the vapor to force the liquid down the column. This contacting pattern gives good interaction between the gaseous and liquid reactants on the catalyst surface. 

If the reaction between the absorbed species, and the nonvolatile reactant is reversible, the term instantaneous reaction is.synonymous with equilibrium reaction. Both forward and backward reactions in this case are so fast that, at all times, concentrations of the various reacting species in the liquid are in equilibrium. The absorption rate in this situation would be independent of the reaction and solely determined by the diffusion of various reacting species. 

Two reactants (one gas, one liquid) generating a nonvolatile product If the reaction involvesa gaseous and a liquid reactant,... 

The above equations assume that the liquid-phase reactant C, the product of the reaction, and the solvent are nonvolatile. The effective interfacial area for mass transfer (nL) and the fractional gas holdup (ii0o) arc independent of the position of the column. The Peclet number takes into account any variations of concentration and velocity in the radial direction. We assume that Peclet numbers for both species A and C in the liquid phase are equal. For constant, 4 , Eq. (4-73) assumes that the gas-phase concentration of species A remains essentially constant throughout the reactor. This assumption is reasonable in many instances. If the gas-phase concentration does vary, a mass balance for species A in the gas phase is needed. If the gas phase is assumed to move in plug flow, a relevant equation would be... 

These kinetic expressions can be useful in many situations, since they capture two key aspects of heterogeneous catalysis the rate of the reaction, and the saturation of the surface by the reactants. The values assigned to the various kinetic and adsorption parameters in this work produce rates that agree well with those reported in the literature. The liquid-phase components were considered nonvolatile. The saturation concentration of H2 was evaluated using Henry s law. All physical parameters were treated as constants. The catalyst properties were representable for a supported noble metal hydrogenation catalyst. 

Ionic liquids such as quaternary ammonium and phosphonium salts have also attracted attention as a means of immobilizing the rhodium catalyst. Processes for carbonylation of methanol with either gas-phase [109] or liquid-phase [110] reactants using [Rh(CO)2I2] dissolved in an ionic liquid have been claimed. Ionic liquids are nonvolatile, which aids in product separation, and the anionic rhodium complex is highly soluble in them. However, the ionic liquids are relatively expensive and their high viscosities can create mass-transfer limitations associated with slow diffusion of reactants. 

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