The modified dynamic model of a riser type fluid catalytic cracking unit

Abstract

The dynamic model developed by Ali and Rohani (1997) to describe the transient behavior of a fluid catalytic cracking (FCC) unit is modified (i) to incorporate the effect of volumetric expansion of the feed and product gases flowing in the riser reactor (ii) to consider the enhancement of mass and heat transfer coefficients due to high turbulence in the rcgenerator and (iii) to model the reactor and stripper as a continuous stirred tank. The modified model is validated using steady-state plant data from an industrial unit (Consumer's Co-operative Refineries Ltd., Regina, SK) and the results are found to be in good agreement.Le modele dynamique mis au point par Ali et Rohani (1997) Elnashaie and Elshishini, 1993; McFarlene et al., 1993;Zheng, 1994;Arbel et al., 1995; Ali and Rohani, 1997). We would limit the discussion to only three important models presented previously. McFarlene et al. (1993) published a dynamic model to describe a Model IV FCC Unit. Their model consists of a combination of empirical and semi-empirical correlations with limited predictive capability. Moreover, the modem refineries use the riser type FCC units and Model IV units are considered to be obsolete. The McFarlene et al. (1993) model is particularly valuable as it presents a detailed and realistic description of the fluid dynamic behavior of the regenerator, but the model lacks a vivid description of the cracking reactions.Arbel et ai. (1995) presented a dynamic model of the FCC unit and studied the stability and existence of multiple steady-states in the FCC operation. In this model, the riser is described as an ideal plug flow reactor under quasi-steadystate and adiabatic conditions with no slip velocity between the solids and the gases. They have used the ten-lumped kinetic model of Jacob et al. (1976) to describe the cracking kinetics. The stripper has been modeled as a well mixed stirred tank with a linear stripping function and a constant temperature drop. The regenerator model consists of dense and dilute phases. The dense bed is firther divided into emulsion phase, which is modeled as a CSTR, and bubble phase, which is modeled as three CSTRs in series. This *Author to whom all correspondence should be addressed. E-mail address: rohani@engr.usask.ca model also incorporates the effect of entrainment of the catalyst in the dilute phase. The combustion reactions take place in the dilute phase or freeboard region. The dilute phase is also modeled as a series of three CSTRs.Ali and Rohani (1997) presented a simple dynamic model without using any partial differential equations to describe the modem FCC units. The model is divided into two parts, the rise reactor and the regenerator. The riser is modeled as an ideal plug flow reactor under quasi-steady-state and adiabatic conditions with a constant superficial gas velocity and zero slip velocity between the gas and solid phase. They used a four-lumped kinetic scheme (Lee et at., 1989) to describe the cracking reactions. The regenerator is modcled as a combination of a dense ...

Modelling and Simulation of Fluid Catalytic Cracking Unit

“…Malay et al [6] modified the dynamic model that developed by Ali and Rohani to describe the transient behaviour of a FCC unit. The results were found to be in good agreement with the industrial unit.…”

Model Predictive Control

“…A good compromise is the reliable model of moderate complexity which has been proposed by Rohani and co-workers (Ali et al, 1997;Ali, 1995;Malay, 1998). In their studies, a four-lump reaction scheme (Lee et al, 1989) is used for the riser reactions, the bubbling bed Kunii-Levenspiel for the regenerator with ordinary differential equations for the dense phase and spatial effects for the gaseous species in the bubble phase; the pressure effects have been incorporated.…”

Design of Robust PI Controller with Decoupler for a Fluid Catalytic Cracking Unit

“…1,2 A four-lump parameter is used to model the riser, regenerator, and fractionator. 3 The effectiveness of the modeling is enhanced by incorporating the effects of the volumetric expansion of feed and product gas from the riser reactor and turbulence. There are many factors such as coke deposition rate, 4 the hydrodynamics of the systems, 5 and the concentrations of CO and CO 2 in the regenerator 6 that affect the performance of the model that should be considered during the system modeling.…”