Control of a Fluid Catalytic Cracker

In this chapter, control of a fluid catalytic cracker will be discussed. A simulator, based on an available model as discussed in the literature, has been developed, which allows the reader to simulate various control schemes and to see the effect of interaction between control loops. Also, the selection of control loops is discussed for this case study. In this particular case study, there are more controlled variables than manipulated variables, which creates the need to develop cascade control structures. Some problems that could occur are highlighted. 

Process Dynamics and Control Modeling for Control and Prediction. Brian Roffel and Ben Betlem. 2006 John Wiley Sons Ltd. 

As a result of the cracking reactions, coke is deposited on the catalyst, consequently the catalyst is poisoned and has to be regenerated. This exothermic regeneration process is carried out by circulating it to a fluidized bed regenerator, where under excess oxygen, the coke is burned off the catalyst at a temperature and pressure of about 1272 T (690 °C) and 34 psia (2.3 bara) respectively. The process conditions should ensure that nearly all carbon monoxide produced in the bed is converted to carbon dioxide. The carbon monoxide concentration in the stack gas should meet the following constraint Xco 10 mol/mol. 

Even though McFarlane et al. (1993) do not provide a model for the fractionator, the model for the fluid catalytic cracker is a corrrplete model, accrtrate enough to show some of the dynamic effects and control interactions that are present. 

The reactor model corrsists of various sub-models, such as o coke and wet gas yield models o reactor mass balances o reactor riser energy balance o reactor riser pressure balance o reactor and main fractionator pressure balances 

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Steady State Optimizing Control of a Fluid Catalytic Cracker. The process model used in this example can be found in (21) while the design parameters are given in (20). The important constraints are T e reactor temperature 930°F, T = regenerator... 

The application of the design methods developed in the previous sections has been demonstrated on a CSTR, a distillation column, fluid catalytic cracking units and a gasoline polymerization plant (20). Here, we will discuss optimizing control of the fluid catalytic cracker. 

Iscol, L. The dynamics and stability of a fluid catalytic cracker. Joint Automatic Control Conference, 1970, pp. 602-607. 

Fig. 35.2. Basic control scheme of a fluid catalytic cracker.

Of the many factors which influence product yields in a fluid catalytic cracker, the feed stock quality and the catalyst composition are of particular interest as they can be controlled only to a limited extent by the refiner. In the past decade there has been a trend towards using heavier feedstocks in the FCC-unit. This trend is expected to continue in the foreseeable future. It is therefore important to study how molecular types, characteristic not only of heavy petroleum oil but also of e.g. coal liquid, shale oil and biomass oil, respond to cracking over catalysts of different compositions. 

Schuldt, S. B. and Smith, F. B., "An Application of Quadratic Performance Synthesis Techniques to a Fluid Catalytic Cracker," Proc. Joint Automatic Control Conference, 1971, 270. 

Model predictive control (MPC) has been widely accepted by an academic and industrial world [1,2]. Early use of MPC to control a fluid catalytic cracker in 1979 [3] resulted in many more investigations on this topic by a large number of researchers [3,4, 5]. 

Arbel, A., Huang, Z., Rinard, I.H., Shinnar, R., and Sapre, A.V. (1995) Dynamics and control of fluid catalytic crackers 1. Modelling of the current generation of FCC s. Industrial Engineering Chemistry Research, 34, 1228. 

In a typical fluid catalytic cracker, catalyst particles are continuously circulated from one portion of the operation to another. Figure 9 shows a schematic flow diagram of a typical unit W. Hot gas oil feed (500 -700°F) is mixed with 1250 F catalyst at the base of the riser in which the oil and catalyst residence times (from a few seconds to 1 min.) and the ratio of catalyst to the amount of oil is controlled to obtain the desired level of conversion for the product slate demand. The products are then removed from the separator while the catalyst drops back into the stripper. In the stripper adsorbed liquid hydrocarbons are steam stripped from the catalyst particles before the catalyst particles are transferred to the regenerator. 

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