Separation/recycle structure

For the designer, understanding the mass balance of the plant is a key requirement that can be fulfilled only when the reactor/separation/recycle structure is analyzed. The main idea is that all chemical species that are introduced in the process (reactants, impurities) or are formed in the reactions (products and byproducts) must find a way to exit the plant or to be transformed into other species [4]. Usually, the separation units take care that the products are removed from the process. This is also valid for byproducts and impurities, although is some cases inclusion of an additional chemical conversion step is necessary [5, 6]. The mass balance of the reactants is more difficult to maintain, because the reactants are not allowed to leave the plant but are recycled to the reaction section. If a certain amount of reactant is fed to the plant but the reactor does not have the capacity of transforming it into products, reactant accumulation occurs and no steady state can be reached. The reaction stoichiometry sets an additional constraint on the mass balance. For example, a reaction of the type A + B —> products requires that the reactants A and B are fed in exactly one-to-one ratio. Any imbalance will result in the accumulation of the reactant in excess, while the other reactant will be depleted. In practice, feeding the reactants in the correct stoichiometric ratio is not trivial, because there are always measurement and control implementation errors. 

The reactor/separation/recycle structure of the plant is presented in Figure 4.9. Hydrogen and toluene, fresh and recycled, are mixed and fed to a tubular reactor. The reaction effluent, containing mainly toluene, hydrogen, benzene and methane is sent to a gas-liquid separator. From the liquid stream, benzene and eventual... 

Figure 4.9 Toluene hydrodealkylation reactor/separation/recycle structure of the plant.

The reactor/separator/recycle structure is decided by considering the physical properties of the species found in the reactor effluent (Table 9.1). The catalyst and the organic phase are immiscible. Therefore, they can be separated by liquid-liquid splitting. The separation of the organic components by distillation seems easy. In a direct sequence, the inert and any light byproduct will be removed in the first column. The second column will separate the reactants, which have adjacent volatilities. Therefore, there will be only one recycle for both reactants. The third column will separate the product from the heavies. The reactor/separation/ recycle structure of the flowsheet is presented in Figure 9.2. 

The case study of the synthesis of vinyl acetate emphasizes the benefits of a systematic design based on the analysis of the reactor/separation/recycles structure. The core of the process is the chemical reactor, whose behavior in recycle depends... 

The case study on Vinyl Acetate Process, developed in Chapter 10, demonstrates the benefit of solving a process design and plantwide control problem based on the analysis of the reactor/separation/recycles structure. In particular, it is demonstrated that the dynamic behavior of the chemical reactor and the recycle policy depend on the mechanism of the catalytic process, as well as on the safety constraints. Because low per pass conversion of both ethylene and acetic acid is needed, the temperature profile in the chemical reactor becomes the most important means for manipulating the reaction rate and hence ensuring the plant flexibility. The inventory of reactants is adapted accordingly by fresh reactant make-up directly in recycles. 

Figure 1 presents the Reactor / Separation / Recycle structure used for performing reactor design and bifurcation analysis. Because of incomplete conversion, both ethylene and acetic acid are recycled. The recycle policy should maintain an ethylene/acetic acid ratio of 3 1, as well as oxygen concentration at reactor inlet bellow 8 vol%. The choice of gaseous inert is a key design decision. Some reports [4-7] consider ethane (impurity in the fresh feed), but this solution is not adopted here. Because CO2 is produced by reaction in large amount, its use as inert in a concentration of 10-30 % vol is the most economical [8]. However, the presence of CO has to be prevented since this is a catalyst poison. 

The case study of vinyl acetate synthesis emphasises the benefits of an integrated process design and plantwide control strategy based on the analysis of the Reactor / Separation / Recycles structure. The core is the chemical reactor, whose behaviour in recycle depends on the kinetics and selectivity of the catalyst, as well as on safety and technological constraints. Moreover, the recycle policy depends on the reaction mechanism of the catalytic reaction. 

Figure 3.3 Reaction-Separation-Recycle structure for HDA proeess...

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