SIMULATION OF FCC REACTOR

Abstract

A study of kinetic model for fluidized catalytic cracking (FCC) riser reactor is presented in the present work. Both the four-lump (Lee et. al. 1989) and ten-lump (Jacob et. al. 1976) kinetic model schemes have being studied to predict the effect of various process parameters, like space velocity, composition of feed, temperature and catalyst to oil ratio, on the performance of the riser reactor. The four-lump model is capable of predicting the deposition of coke on the catalyst, isolated from the light gases yield and gasoline yield. Where as ten-lump kinetic scheme involves lumped species consisting of paraffins, naphthenes, aromatic rings, and aromatic substituent groups in light and heavy fuel oil fractions beside gasoline and C-lump (it includes coke and light gases). The kinetic model also incorporates aromatic ring adsorption, nitrogen poisoning and time-dependent catalyst decay. With the use of ten-lump model, the HFO and LFO compositions can be traced as the conversion proceeds. It is observed that naphthenic feedstock gives highest yield of gasoline, but the yield of coke is also high. The paraffinic feedstock is the next in yield of gasoline with least yield of coke. The feed is considered as a single lump in four-lump kinetic model, and as the rate constants are not a function of feed stock composition, hence the feed stock composition does not affect the performance of riser reactor. Also, the rate constants for ten-lump kinetic schemes are not a function of feed stock composition. But the nature of the feed stock is represented by eight of the ten lumps of the kinetic scheme, hence use of the ten-lump kinetic scheme reflects the effect of change in feed stock properties on the performance of the riser reactor. In this work, the predictive capabilities of both the models are being verified for wide ranges of charge stocks and process conditions. By the use of four-lump model, it is found that at higher reactor temperature i.e. 821.5 K, the conversion of gas oil is high i.e. 95 mass percent. The production of light gases is also high at high temperature. However, at higher temperature the gasoline yield is 54 mass percent, which is less than the yield given at lower temperature. Similar results are found iii from the ten-lump model. So there must be an optimum point to consider gas oil conversion and gasoline production simultaneously. The next important parameter is the space velocity of reactant. Space velocity between 3- 5s_Igives the maximum yield of gasoline. Slight increase in space velocity reduces the gasoline yield sharply and increases light gases production. Low space velocity in the given range increases the contact time and favour gas oil conversion and gasoline yield, Another important factor is the ratio of catalyst circulation to gas oil mass flow rate. At high C/O ratios, the gas oil conversion as well as gasoline yield is more.