Residence time distribution product yield

Recycling to monomers, fuel oils or other valuable chemicals from the waste polymers has been attractive and sometimes the system has been commercially operated [1-4]. It has been understood that, in the thermal decomposition of polymers, the residence time distribution (RTD) of the vapor phase in the reactor has been one of the major factors in determining the products distribution and yield, since the products are usually generated as a vapor phase at a high temperature. The RTD of the vapor phase becomes more important in fluidized bed reactors where the residence time of the vapor phase is usually very short. The residence time of the vapor or gas phase has been controlled by generating a swirling flow motion in the reactor [5-8]. 

The effects of hydrocarbon partial pressure and residence time distribution are easily incorporated into the reaction model, when the reaction kinetics are mathematically described. Figures 3 and 4 show the examples of simulated results by use of the model. They clearly show that the effect of hydrocarbon partial pressure on the product distribution is larger than generally recognized and low pressure is preferable to obtain high liquid yield and that residence time distribution should be controlled as narrow as possible to reduce coke precursor(Ql) in the residual component. 

Extreme control of the hydrodynamics and the residence time distribution has been shown to lead to higher yields in reaction systems on various occasions. The work by Hoenicke et al. [46, 47] on partial hydrogenation of various aromatics is classic in this area. In the system studied here, the hydrogenation ofcyclododeca-triene to the mono-olefin would open up a new route to the synthesis of Nylon-12. In practice, however, the hydrogenation of the mono-olefin to cyclododecane is far faster, and the reaction is hard to stop at the intermediate product. 

To avoid mass and heat transfer resistances in practice, the characteristic transfer time should be roughly 1 order of magnitude smaller compared to the characteristic reaction time. As the mass and heat transfer performance in microstructured reactors (MSR) is up to 2 orders of magnitude higher compared to conventional tubular reactors, the reactor performance can be considerably increased leading to the desired intensification of the process. In addition, consecutive reactions can be efficiently suppressed because of a strict control of residence time and narrow residence time distribution (discussed in Chapter 3). Elimination of transport resistances allows the reaction to achieve its chemical potential in the optimal temperature and concentration window. Therefore, fast reactions carried out in MSR show higher product selectivity and yield. 

In this chapter, residence time distribution (RTD) of ideal and nonideal reactors along with the method of determination are described in detail. The influence of nonideality and RTD on the reactor performance, the target product yield, and selectivity, including complex reactions, is presented. 

The residence time distribution (RTD) is a probability distribution function used to characterize the time of contact and contacting pattern (such as for plug-flow or complete backmixing) within the reactors. Excessive retention of some elements and shortdrcuiting of others due to backmixing and other dispersive phenomena lead to a broad distribution in the residence times of individual molecules in the reactor. This tends to decrease conversion and exerts a negative influence on product selectivity/yield. The RTD depends on the flow regime and is characterized by Reynolds (Re) and Schmidt (Sc) numbers. 

In an effort to explain the second impeller effect, residence time data was collected for the one and two impeller reactors. Step testing was used and thus the response was in the form of the residence time distribution function F(t). For each system the residence time data was replicated twice. Figure 4 shows the average results for the respective reactors. While there is clearly a difference in F(t) behavior, there is no direct indication as to why the one impeller reactor yielded about half the productivity as the two impeller reactor. 

How much mixing is enough, and when could overmixing be damaging to yield or quality These critical issues depend on the process and the sensitivity of selectivity, physical attributes, separations, and/or product stability to mixing intensity and time. The nonideality of residence time distribution effects combined with local mixing issues can have a profound effect on continuous processes. 

The temperature profile is the most important aspect of operational control for pyrolysis processes. Material flow rates, both solid and gas phase, together with the reactor temperature control the key parameters of heating rate, highest process temperatures, residence time of solids and contact time between solid and gas phases. These factors affect the product distribution and the product properties. Solid residence time is another important factor in the bio-oil yields. A short residence time enhances biooil yields, while a longer residence time increases char production (Antal and Gronli, 2003). 

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