Rate laws continued CSTRs

The CRE approach for modeling chemical reactors is based on mole and energy balances, chemical rate laws, and idealized flow models.2 The latter are usually constructed (Wen and Fan 1975) using some combination of plug-flow reactors (PFRs) and continuous-stirred-tank reactors (CSTRs). (We review both types of reactors below.) The CRE approach thus avoids solving a detailed flow model based on the momentum balance equation. However, this simplification comes at the cost of introducing unknown model parameters to describe the flow rates between various sub-regions inside the reactor. The choice of a particular model is far from unique,3 but can result in very different predictions for product yields with complex chemistry. 

For a closed chemical system with a mass action rate law satisfying detailed balance these kinetic equations have a unique stable (thermodynamic) equilibrium, lim c( )=Cgq. In general, however, we shall be concerned with chemical reactions that are maintained far from chemical equilibrium by flows of reagents intoand out of a continuously stirred tank reactor (CSTR). In this case the chemical kinetic equation (C3.6.1) must be supplemented with flow terms... 

A typical example of dynamic optimization in ch ical engineering is the change between steady states in a continuous-stured tank reactor (CSTR) in which the irreversible reaction A B takes place ([21,22]) (Figure 14.4). The reaction is first order and exothermic and follows Arrhenius rate law. The reactor is equipped with a cooling jacket with refrigerant fluid at constant temperature T . To develop model equations, we formulate mass and energy balances. 

Kinetic models can be used to link the reactor design with its performance. The reaction rate may be expressed by power law functions, by more complex expressions, as Langmuit-Hinselwood-Hougen-Watson (LHHW) correlations for catalytic processes, or by considering user kinetics. There are two ideal models, continuous stirred tank reactor (CSTR) or plug flow (PFR), available in rating mode (reaction volume fixed) or design mode (conversion specified). 

The rate expression for Fiseher-Tropseh (FT) synthesis has been obtained using a 25 wt.% C0/AI2O3 eatalyst in a 1 liter continuously stirred tank reactor (CSTR) operated at 493K, 1.99 MPa (19.7 atm), H2/CO feed ratios of 1.0-2.4 with varying space velocities to produce 14-63% CO eonversion. Adjusting the ratios of inert gas and added water permitted the impact of added water to be made at the same total flow rate and H2 and CO partial pressures. The addition of water at low levels during FT sjmthesis did not impact CO conversion but at higher levels it decreased CO conversion relative to the same conditions without water addition. The catalytic activity recovered after water addition was terminated. The temporary reversible decline in CO conversion when water was added may be due to the kinetic effect of water by inhibition of CO and/or H2 adsorption. The data of this study are fitted fairly well by a simple power law expression of the form ... 

The addition of water at higher levels in FT synthesis decreased the CO conversion but the activity recovered after water addition was terminated. A rate expression has been obtained for a 25 wt.% C0/AI2O3 catalyst operated in a 1 liter continuous stirred tank reactor (CSTR) at 493K, 19.7 atm. (1.99 MPa), over a range of reactant partial pressures. The data of this study are fitted by a simple power law expression of the form ... 

In a completely mixed continuous flow reactor, the composition is thought to be uniform throughout the reactor, and is the same as in the exist stream (cf. Fig. 3.30). Applying the law of conservation of mass (cf. Equ. 2.3) to a CSTR yields Equ. 3.90 for the steady state. The dilution rate D is reciprocal to the mean residence time of a fluid (7). 

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