Reactor-separator-recycle processes

Let us consider a reactor/separator/recycle process, where n reactants Aj give m products and intermediates Pk. The network of r reactions can be described by ... 

Since Luyben identified the snowball effect (Luyben, 1994), the sensitivity of reactor-separator-recycle processes to external disturbances has been the subject of several studies (e.g., Wu and Yn, 1996 Skogestad, 2002). Recent work by Bildea and co-workers (Bildea et al., 2000 and Kiss et aL, 2002) has shown that a critical reaction rate can be defined for each reactor-separator-recycle process using the Damkohler number. Da (dimensionless rate of reaction, proportional to the reaction rate constant and the reactor hold-up). When the Damkohler number is below a critical value, Bildea et al. show that the conventional unit-by-unit approach in Figure 20.15 leads to the loss of control. Furthermore, they show that controllability problems associated with exothermic CSTRs and PFRs are resolved often by controlling the total flow rate of the reactor feed stream. 

In most chemical processes reactors are sequenced by systems that separate the desired products out of their outlet reactor streams and recycle the unconverted reactants back to the reactor system. Despite the fact that process synthesis has been developed into a very active research area, very few systematic procedures have been proposed for the synthesis of reactor/separator/recycle systems. The proposed evolutionary approaches are always based upon a large number of heuristic rules to eliminate the wide variety of choices. Many of these heuristics are actually extensions of results obtained by separately studying the synthesis problem of reactor networks or separator systems, and therefore the potential trade-offs resulting from the coupling of the reactors with the separators have not been investigated. 

Floquet et al. (1985) proposed a tree searching algorithm in order to synthesize chemical processes involving reactor/separator/recycle systems interlinked with recycle streams. The reactor network of this approach is restricted to a single isothermal CSTR or PFR unit, and the separation units are considered to be simple distillation columns. The conversion of reactants into products, the temperature of the reactor, as well as the reflux ratio of the distillation columns were treated as parameters. Once the values of the parameters have been specified, the composition of the outlet stream of the reactor can be estimated and application of the tree searching algorithm on the alternative separation tasks provides the less costly distillation sequence. The problem is solved for several values of the parameters and conclusions are drawn for different regions of operation. 

Particularly strong and complex interactions prevail among reaction and separation systems that are generally not at all or not fully exploited as a result of the application of the available synthesis methods for reactor networks and separation systems in isolation. The lack of generality in the synthesis methods is a tribute to the nonlinear process models required to capture the reaction and separation phenomena as well as to the vast number of feasible process design candidates. These complexities even make it difficult to synthesize the decomposed subsystems, which are typically reactor networks, separation systems, reactor-separator-recycle systems, and reactive separation systems. The development of reliable synthesis tools for these sub-systems is still an active research area. 

Vasudevan, S. and Rangaiah, G.P. (2009). Development of guidelines for plantwide control of gas-phase industrial processes, from reactor-separator-recycle results. Ind. Eng. Chem. Res., 50, 297-337. 

Note that the evolution of design between the levels 4 and 5 can generate a number of alternatives, but these should not affect the basic flowsheet structure defined at the reactor/separations/recycle level. In addition, employing complex units and process-intensification techniques can produce more compact flowsheet and cheaper hardware. 

Figure 2.13 illustrates the variation of the economic potential during flowsheet synthesis at different stages as a function of the dominant variable, reactor conversion. EPmin is necessary to ensure the economic viability of the process. At the input/output level EP2 sets the upper limit of the reactor conversion. On the other hand, the lower bound is set at the reactor/separation/recycle level by EP3, which accounts for the cost of reactor and recycles, and eventually of the separations. In this way, the range of optimal conversion can be determined. This problem may be handled conveniently by means of standard optimization capabilities of simulation packages, as demonstrated by the case study of a HDA plant [3]. 

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. 

In the reactor/separation/recycle system, the steady-state values of the reactor-inlet flow rate F, are the intersections of this curve with the horizontal fine representing the net amount of reactant fed in the process. This is given, in a dimensionless form, by F0(l -z4)/(cokV) = (1 — z4)/Da. 

We emphasize that traditional procedures tackle plantwide control and heat integration toward the end of the design. The newly introduced reactor/separation/recycle level (Chapters 2 and 4) allows an early solution to these problems, with the result of avoiding unnecessary loops in the design process. Rigorous design and closed-loop dynamic simulation prove the effectiveness of the approach (Section 9.6). 

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. 

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 Selected schemes for the production of A (F - feed, S - solvent, SR - solvent removed), a) Reactor-separator-recycle system w/ four zones and solvent removal, b) Partially integrated process w/ three zones and side reactors, c) Fully integrated process w/ four zones, distributed reaction and solvent removal, d) Fully integrated scheme w/ three zones and distributed reaction.

Concerning the level of integration, classical flowsheet-integrated processes as reactor-separator-recycle systems (Fig. 3a) and the use of side reactors ( Hashimoto process . 

Hence, at the level Reactor-Separators-Recycles the material balance can be brought in a narrow optimal region. On this basis can be started the process integration steps regarding the optimal management of energy, mass separation agents, process water, waste minimisation, etc. 

Fig. 13.18 depicts reaction systems involving material recycles that are common in the industrial practice. The plant receives the feed Fq of concentration cq. The reactor effluent (F, C2) is firstly processed by a separation section, and only afterwards recycled. Fixed composition of product (4) and recycle (3) streams is achieved by local control of the separation units. Hence, the composition at reactor inlet is not directly dependent on the reactor effluent. The temperature at the reactor inlet has a constant value by an upstream heat exchanger. This excludes the energy feedback. Such plant will be designated here by Reactor-Separator-Recycle system. 

What was the extent of reaction for this system What would the extent of reaction be if there was no separation/recycle process after (assume that the mass percent of hydrogen leaving the reactor is the same) What limits how effective this process can be ... 

The three solution branch dominating the nonlinear behavior of the process can also be observed in a conventional process with a reactor-separator recycle as recently reported by Blagov et al. [12]. This flowsheet is analogous to a column with one reactive and one non-reactive column section with a single product stream, the multiplicity behavior of which has been studied Cerafimov and coworkers [49, 83]. In these papers it is shown that this type of output multiplicity is a generic phenomenon for reaction systems with competing irreversible reactions and a similar distribution of volatUities between reactants and products. 

Consider using a side stream when the column is used to recycle the reactants back to the reactor from a liquid-separation system in a reaction-separation-recycle process... 

Reactor design for complete conversion may be impossible thermodynamically or undesirable because of reduced yields when byproducts are formed. In such cases, an economic alternative is to design a combined reactor-separator-recycle system, as illustrated in the simple example in Figure 20.4 and discussed in Chapter 8. Here, the reaction A B is carried out in a CSTR, whose liquid feed is a stream containing pure A. In the event that B is sufficiently more volatile than A, the separation can be performed using a flash vessel and unreacted A is recycled to the reactor. As will be seen in the plantwide control examples at the end of this chapter and in the quantitative analysis in Chapter 21, the presence of the recycle comphcates control of the process and requires special attention. ... 

Process synthesis is introduced mostly using heuristics in Part One (Chapters 3 and 5), whereas Part Two provides more detailed algorithmic methods for chemical reactor network synthesis, separation train synthesis, the synthesis of reactor-separator-recycle networks, heat and power integration, mass integration, and the optimal design and sequencing of batch processes. 

The presence of at least one chemical reactor and one or more separation sections for llie separation of the effluent mixture leaving the reactor(s) characterizes many chemical processes. In almost all cases, one or more of the streams leaving the separation section(s) are recycled to the reactor. In Chapter 6, the design of reactors and reactor networks was considered without regard for the separation section(s) and possible recycle itere ftoa Chapter 7 was concerned with the design of separation sections in the absence of any conad-eration of the reactor section. Chapter 5, which dealt with the synthesis of the entire process, included a few examples of the interaction between the reactor and separation sections. This chapter extends that introduction to give a detailed treatment of reactor-separator-recycle networks. 

Recycles are an essential part of most processes involving chemical reactions because it is usually difficult to achieve near-equilibrium conversion in a single pass of reactants through a reactor. It may also be the case that equilibrium conversion is very low, e.g. in ammonia or methanol synthesis (see section 12.4). The overall conversion for the reactor-separator-recycle system can be much closer to 100%. The following example illustrates this. 

Fig. 9. Reactor-separator system. Process sensitivity function 5p before (dashed) and after (solid) addition of delay tank in recycle path.

Let us consider the A + B — P reaction, taking place in CSTR / Separation / Recycle process. When the reactants A and B are lighter and heavier, respectively, than the product P, the flowsheet involves two distillation columns and two recycle streams. The control structure includes loops for reactor level and temperature, as well as for distillation columns top and bottom purity (Fig. 5a). 

Further, we will briefly comment other interesting works in this area. Pushpavanam Kienle [16] studied the reaction A- P in a non-isothermal CSTR / Separation / Recycle process. Assuming infinite activation energy and equal eoolant and reactor-inlet temperatures, they reported state multiplicity, isolated solution branches and instability, for both conventional and fixed-recycle control structures. In addition, the conventional structure showed regions of unfeasibility. The authors claimed the superiority of the fixed-recycle control structure over the fixed-fresh flow rate control. 

In this case, because there are no raw materials losses in the separation and recycle system, the only yield loss is in the reactor, and the process yield equals the reactor selectivity. 

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