Recycle reactor conversion

Figure 4.9 shows a plot of Eq. (4.12). As the purge fraction a is increased, the flow rate of purge increases, but the concentration of methane in the purge and recycle decreases. This variation (along with reactor conversion) is an important degree of freedom in the optimization of reaction and separation systems, as we shall see later. 

As the reactor conversion increases, the reactor volume increases and hence reactor capital cost increases. At the same time, the amount of unconverted feed needing to be separated decreases and hence the cost of recycling unconverted feed decreases, as shown in Fig. 8.1. Combining the reactor and recycle costs into a total cost indicates that there is an optimal reactor conversion. 

Now there are two global variables in the optimization. These are reactor conversion (as before) but now also the concentration of IMPURITY in the recycle. For each setting of the IMPURITY concentration in the recycle, a set of tradeoffs can be produced analogous to those shown in Figs. 8.3 and 8.4. 

However, the concentration of impurity in the recycle is varied as shown in Fig. 8.5, so each component cost shows a family of curves when plotted against reactor conversion. Reactor cost (capital only) increases as before with increasing conversion (see Fig. 8.5a). Separation and recycle costs decrease as before (see Fig. 8.56). Figure 8.5c shows the cost of the heat exchanger network and utilities to again decrease with increasing conversion. In Fig. 8.5d, the purge... 

Figure 8.5 Cost tradeoffs for processes with a purge when reactor conversion and recycle inert concentration are allowed to vary. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

Figure 8.6 shows the component costs combined to give a total cost which varies with both reactor conversion and recycle inert concentration. Each setting of the recycle inert concentration shows a cost profile with an optimal reactor conversion. As the recycle inert concentration is increased, the total cost initially decreases but then... 

The overall inventory. In the preceding chapter, the optimization of reactor conversion was considered. As the conversion increased, the size (and cost) of the reactor increased, but that of separation, recycle, and heat exchanger network systems decreased. The same also tends to occur with the inventory of material in these systems. The inventory in the reactor increases with increasing conversion, but the inventory in the other systems decreases. Thus, in some processes, it is possible to optimize for minimum overall inventory. In the same way as reactor conversion can be varied to minimize the overall inventory, the recycle inert concentration also can be varied. 

It might be possible to reduce the inventory significantly by changing reactor conversion and recycle inert concentration without a large cost penalty if the cost optimization profiles are fairly flat. 

Increasing reactor conversion when separation and recycle of unreacted feed is difficult. 

Economic tradeoffs. Interactions between the reactor and the rest of the process are extremely important. Reactor conversion is the most significant optimization variable because it tends to influence most operations through the process. Also, when inerts are present in the recycle, the concentration of inerts is another important optimization variable, again influencing operations throughout the process. ... 

The per pass ethylene conversion in the primary reactors is maintained at 20—30% in order to ensure catalyst selectivities of 70—80%. Vapor-phase oxidation inhibitors such as ethylene dichloride or vinyl chloride or other halogenated compounds are added to the inlet of the reactors in ppm concentrations to retard carbon dioxide formation (107,120,121). The process stream exiting the reactor may contain 1—3 mol % ethylene oxide. This hot effluent gas is then cooled ia a shell-and-tube heat exchanger to around 35—40°C by usiag the cold recycle reactor feed stream gas from the primary absorber. The cooled cmde product gas is then compressed ia a centrifugal blower before entering the primary absorber. 

The differential reactor is the second from the left. To the right, various ways are shown to prepare feed for the differential reactor. These feeding methods finally lead to the recycle reactor concept. A basic misunderstanding about the differential reactor is widespread. This is the belief that a differential reactor is a short reactor fed with various large quantities of feed to generate various small conversions. In reality, such a system is a short integral reactor used to extrapolate to initial rates. This method is similar to that used in batch reactor experiments to estimate... 

By maintaining the first-stage reactor just beyond the phase inversion point, the dispersed rubber phase is relatively rich in dissolved styrene. As polymerization subsequently proceeds in the LFR s, the dissolved styrene will react to form either a graft copolymer with the rubber or a homopolymer. The latter will remain within the rubber droplet as a separate occluded phase. Achieving the first-stage reactor conversion and temperature by recycling a portion of the hot second reactor effluent may permit simplification of the first reactor temperature control system. 

Simulation studies are also conducted for a dispersed PFR and a recycle reactor at 260 °C, 500 psig and feed with DCPD=0.32 mol/min, CPD=0.96mol/min and ethylene=3.2mol/min. Peclet number (Pe) or the recycle ratio is selected as a variable parameter for the dispersed PFR or for the recycle reactor, respectively. Conversion approaches to that of PFR over Pe=50 as can be seen in Fig.4. It is also worth mentioning that the reactor performance is improved with recycle if the residence time is low. 

Once the structure of the recycle and separation has been established, some important degrees of freedom can be optimized that can have a very significant effect on the overall process economics. Start by considering the optimization of reactor conversion. 

If the reactor conversion is changed so as to optimize its value, then not only is the reactor affected in size and performance but also the separation system, since it now has a different separation task. The size of the recycle will also change. If the recycle requires a compressor, then the capital and operating costs of the recycle compressor will change. In addition, the heating and cooling duties associated with the reactor and the separation and recycle system change. 

Obviously, the use of purges is not restricted to dealing with impurities. Purges can also be used to deal with byproducts. As with the optimization of reactor conversion, changes in the recycle concentration of impurity might change the most appropriate separation sequence. 

Figure 13.23 Optimization of reactor conversion and recycle impurity concentration using a univariate search.

As the reactor conversion and recycle impurity changes in the optimization, the most appropriate separation... 

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