Overall coking model

Overall Coking Model. From the tracer experiments, quenching and toluene solubility experiments, rates and product selectivities, the following model appears to describe the gross behavior of residue in a dealyed coking process. 

Using a "home made" aneroid calorimeter, we have measured rates of production of heat and thence rates of oxidation of Athabasca bitumen under nearly isothermal conditions in the temperature range 155-320°C. Results of these kinetic measurements, supported by chemical analyses, mass balances, and fuel-energy relationships, indicate that there are two principal classes of oxidation reactions in the specified temperature region. At temperatures much lc er than 285°C, the principal reactions of oxygen with Athabasca bitumen lead to deposition of "fuel" or coke. At temperatures much higher than 285°C, the principal oxidation reactions lead to formation of carbon oxides and water. We have fitted an overall mathematical model (related to the factorial design of the experiments) to the kinetic results, and have also developed a "two reaction chemical model". 

Although the deactivation of Industrial catalysts is often due to two or more different causes, the modeling of simultaneous deactivation phenomena has not been widely studied (refs. 1, 2). The occurrence of two different deactivation processes not only adds another level of complexity to the determination of the intrinsic kinetic behavior but also complicates the interpretation of the experimental results. In our previous studies regarding the thloresistance of naphtha reforming catalysts (refs. 3, 4) we have shown that the activity decay caused by the presence of sulfur compounds in the feed is often accompanied by coking. In this situation, the thioresistance cannot be obtained in a simple way from the deactivation curves. The characteristics of the sulfur poisoning have to be deduced from the overall deactivation rate. 

Although not shown here, with Eq. (8), and Eq.(12) we have been able to correctly predict the reactor coke profile based on model-predicted C5N concentration profile. This is detailed elsewhere [15]. We thus complete the circle by providing an overall kinetics package that can predict not only the time evolution of the reforming products but also the coke profile along the catalyst bed. 

The effect of quinoline and phenanthrene additions to a n-hexadecane feedstock has been determined for a model four-component FCC catalyst by means of a MAT reactor with analysis of all products and characterisation of the coke produced. Both additions lead to an overall loss in conversion quinoline is considered to act as a poison while phenanthrene participates strongly in coke formation and the resultant coke becomes more aromatic in nature. The cracking propensity and associated coke formation have been measured for a series of FCC catalysts with differing compositions. Increasing amounts of zeolite in a matrix lead to increasing extents of conversion but with little effect on the extent of coke production. However, a pure zeolite gave a very high coke content. 

Description by kinetic models. The development of the kinetic model that describes the performance of the regenerated catalyst has been given in previous work. A short summary is given below. The different reactants and products are lumped into five hydrocarbon groups. The kinetic scheme is shown in Figure 5. The lumps all react according to first order kinetics. The overall activity of the system is dominated by two different time scales the formation of coke takes place on a time scale of milliseconds and the formation of the other products takes place on a time scale of seconds. 

In situ poisoning experiments were carried out on MgY zeolite by doping the toluene/methanol mbdure with either an acid (acetic acid) or a lase (3,5-dimethyl pyridine). Fresh catalysts were initially tested for about 3 h using a pure toluene/methanol mixture before introducing the doped feed. The activity was defined as a = ro/ro, where r and r(t) are the reaction rates at zero time and time t, respectively. The MgY activity diminished with time on stream when using undoped reactants because of the formation of carbonaceous deposits. Hence, when a teic confound is added into the feed, a simultaneous deactivation process by coke and poison takes place. The activity decay caused by 3,5-dimethyl pyridine (3,5-DMP) alone can not be obtained directly fiom the experimental data. To estimate the poison intrinsic effect, it can be assumed that both effects are additive, which implies that the overall deactivation rate is a simple sum of each individual rates (hypothesis of independence) [19]. According to mechanistic deactivation models [20], the overall deactivation rate is expressed as follows ... 

The definition of the conversion of gas oil was chosen to be consistent with the kinetic models used. On this basis the unconverted gas oil was taken as the fraction of products boiling above 220°C which in turn corresponds to compounds eluting from the chromatographic column after n-Ci2. Then, the GC-conversion of gas oil was defined as one hundred minus the weight percent of unconverted gas oil. This value was then corrected to account for the yield of coke to define the overall conversion. 

The basic-state solution can also be applied to describe the combustion zone of a countercurrent packed-bed oil-shale retort, and possibly also to describe the combustion of coke deposited on cracking catalyst particles during catalyst reactivation. The model grossly oversimplifies the combustion kinetics, but it is felt adequate to describe the overall behavior of the process and to show the primary effect of process parameters on the system. 

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