Homogeneous liquid-phase flow reactors

Example 5-4 A first-order homogeneous (liquid-phase) reaction is carried out in an ideal stirred-tank reactor. The concentration of reactant in the feed is 3.0 g moles/liter and the volumetric flow rate is 60 cm /sec. The density and specific heat of the reaction mixture are constant at I.O g/cm and 1.0 cal/(g)(°C), respectively. The reactor volume is 18 liters. There is no product in the feed stream and the reactor operates adiabatically. The heat and rate of the irreversible reaction are... 

Eldridge J. W., Piret E.L. Gontinuous-flow stirred-tank reactor system I, Design equations for homogeneous liquid phase reactions. Experimental data, Chem. Eng. Prog., 1950 46 290. 

The Boyle method (see A5.13) can be used to estimate the diameter required for a dump system. It will usually be safe to assume liquid-only flow if dumping occurs well before the maximum rate (the original Boyle method). Alternatively, the conservative assumption that a homogeneous two-phase mixture (rather than liquid alone) enters the dump system from the reactor could be made (the modified Boyle method). 

Several reactor types have been described [5, 7, 11, 12, 24-26]. They depend mainly on the type of reaction system that is investigated gas-solid (GS), liquid-solid (LS), gas-liquid-solid (GLS), liquid (L) and gas-liquid (GL) systems. The first three arc intended for solid or immobilized catalysts, whereas the last two refer to homogeneously catalyzed reactions. Unless unavoidable, the presence of two reaction phases (gas and liquid) should be avoided as far as possible for the case of data interpretation and experimentation. Premixing and saturation of the liquid phase with gas can be an alternative in this case. In homogenously catalyzed reactions continuous flow systems arc rarely encountered, since the catalyst also leaves the reactor with the product flow. So, fresh catalyst has to be fed in continuously, unless it has been immobilized somehow. One must be sure that in the analysis samples taken from the reactor contents or product stream that the catalyst docs not further affect the composition. Solid catalysts arc also to be fed continuously in rapidly deactivating systems, as in fluid catalytic cracking (FCC). 

The catalyst must be as homogeneous as possible to get good spectroscopic data. On the other hand, basic engineering rules such as flow patterns through the reactor, heat- and mass-transport properties, dead volume, and catalytic measurements need to be fulfilled. Therefore, preferentially, a thin layer of a catalyst or a sieved catalyst fraction should be applied, especially if the reactions are rapid [31], Moreover, such studies should be performed under realistic conditions (i.e. in gas phase, liquid phase [including catalyst preparation], or even at high pressure). 

The units of space velocity are the reciprocal of time. Usually, the hourly volumetric feed-gas flow rate is calculated at 60 °F (15.6 C) and 1.0 atm (1.01 bar). The volumetric liquid-feed flow rate is calculated at 60 F (15.6 °C). Space velocity depends on the design of the reactor, reactor inlet conditions, catalyst type and diameter, and fractional conversion. Walas [7] has tabulated space velocities for 102 reactions. For exanple, for the homogeneous conversion of benzene to toluene in the gas phase, the hoiuly-volumetric space velocity is 815 h . This means that 815 reactor volumes of benzene at standard conditions will be converted in one hoiu. Although space velocity has limited usefulness, it allows estimating the reaction volume rapidly at specified conditions. Other conditions require additional space velocities. A kinetic model is more useful than space velocities, allowing the calculation of the reaction volume at different operating conditions, but a model requires more time to develop, and frequently time is not available. 

The main problem in the design of tubular reactors for quantitative studies of homogeneous reactions is to confine the reaction sharply to the reactor itself. This requires rapid mixing of the reactants at the entry, and equally excellent quenching of the exiting fluid. Both are easier to achieve for liquid-phase than for gas-phase reactions. Tubular reactors are not suited for gas-liquid reactions because gas sparging would disrupt the flow pattern. 

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