Recycle, 161 Steady state approximation

First, a feedstock is cracked in DCR. Sufficient HCO or bottoms oil is collected and blended back with the original feed. Then the blend is fed into the ACE unit to be cracked again. Becanse of the low-recycle ratio, this two-pass cracking is expected to be close to the steady-state recycling operation. This steady-state approximation will be discnssed fnrther in the data processing section below. 

As the recycled fuel composition approaches steady state after approximately four cycles (1), the heat and radiation associated with and Pu require more elaborate conversion and fuel fabrication facihties than are needed for virgin fuel. The storage, solidification, packaging, shipping, and disposal considerations associated with wastes that result from this approach are primarily concerned with the relatively short-Hved fission products. The transuranic... 

The relationship between the peroxy radical concentration and the ozone photolysis rate constant for these higher NO conditions can be again approximated using steady-state analysis (Penkett et al., 1997 Carpenter et al., 1997). While OH is recycled in its reactions with CO and CH4 via H02, it is permanently removed at higher NOx concentrations by the reaction of OH with N02, forming nitric acid ... 

The Streamline flow cell was connected to a glass-lined 500-gal vessel via a recycle loop as shown in Figure 22.1. A Yamada pump (1/2" pump with Kynar internals) was used to circulate the reaction solution from the vessel through Mettler Toledo s Streamline flow cell and back to the vessel via the above surface line. The process lines (3/8" JIC) composing the recycle loop were partially insulated, but not heat traced, resulting in a temperature drop between the vessel and the return line. Although the temperature at the probe was approximately 10°C lower than the batch temperature, the probe temperature did not fluctuate substantially once the temperature in the vessel reached a steady state. This temperature difference did not adversely affect the reaction yield, cycle time, or method development. 

A case of practical interest is a chemical reactor coupled with a separation section, from which the unconverted reactants are recovered and recycled. Let s consider the simplest situation, an irreversible reaction A—>B taking place in a CSTR coupled to a distillation column (Fig. 13.5). Here we present results obtained by steady state and dynamic simulation with ASPEN Plus and ASPEN Dynamics. The reader is encouraged to reproduce this example with his/her favourite simulator. The species A and B may be defined as standard components with adapted properties. In this case, we may take as basis the properties of n-propanol and iso-propanol, and assume ideal phase equilibrium. The relative volatility B/A increases at lower pressures, being approximately 1.8 at 0.5 atm. We consider the following data nominal throughput of 100 kmol/hr of pure A, reactor volume 2620 1, and reaction constant =10 s". For stand-alone operation the reaction time and conversion are r= 0.106 hr and = 0.36. 

A.6. Perform structural analysis based on a steady-state model and evaluate the possibilities for decomposition of the control problem. To simply this step, we assume that the pressure and temperature control loops are essentially decoupled from the plant holdups (integrating modes), the compositions, and the liquid flows. If this assumption is approximately valid, we can analyze a core plant model ( core model ) that comprises the reactor, flash unit, and recycle tank—all assumed to operate isothermally and isobarically (see Fig. H.5). Thus, the approximate plant model consists only of material balances but includes the key flows, levels, and compositions. This type of approach, in which temperatures and pressures are assumed to remain constant at their nominal values, was employed by Robinson et al. (2001) in their analysis of a similar plant. 

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