Simulation of riser reactors

It should be noted that, Model G and Model M predict quite different values of slip velocities, though their resolved structure may look similar, as shown by insets of Figure 5. Extending this seeming inconsistency to larger scales, we may expect that, simulations of real reactors with Model G and Model M may predict different solids flux even with similar impression of structures. Then, it is natural to question, which solution of these two models coincides with the reality. To answer this question, simulations of CFB risers are needed to test which will agree with experiments. 

Figure 2.3.2 (Kraemer and deLasa 1988) shows this reactor. DeLasa suggested for Riser Simulator a Fluidized Recycle reactor that is essentially an upside down Berty reactor. Kraemer and DeLasa (1988) also described a method to simulate the riser of a fluid catalyst cracking unit in this reactor.

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

Gas-particle flows in fluidized beds and riser reactors are inherently unstable and they manifest inhomogeneous structures over a wide range of length and time scales. There is a substantial body of literature where researchers have sought to capture these fluctuations through numerical simulation of microscopic TFM equations, and it is now clear that TFMs for such flows do reveal unstable modes whose length scale is as small as ten particle diameters (e.g., see Agrawal et al., 2001 Andrews et al., 2005). 

Several different reactor types were used for catalyst evaluation, including a DCR pilot riser [3] an ACE fixed fluidized bed (FFB) reactor [7], a Riser simulator [4,9], and a specially designed extended residence time circulating pilot unit. The reaction conditions of each of the reactors will be reported in the sections dealing with the specific reactor type. Different grades of Brazilian Campos Basin derived VGOs were used in the experiments. Feed properties are presented in Table 2.1. 

Notwithstanding the possibility of doing detailed simulations with bench or pilot scale riser reactors, the traditional Micro Activity Test (MAT) remains the main tool for basic FCC research and catalyst and feedstock evaluation and monitoring. 

A typical pressure profile obtained from the two transducers is presented in the Figure 2. Curve I along with points A, B and C illustrates the characteristic pressure prorile observed during the operation of the reactor. Meanwhile, curve II depicts the pressure profile inside the vacuum chamber. Point A of curve I indicates the pressure condition inside the Riser Simulator just prior to the hydrocarbon injection. Point B gives the Riser Simulator pressure at the end of the reaction period (just before evacuation commences) and Point C represents the equilibrium pressure once the pressures between the vacuum chamber and the Riser Simulator have stabilized. 

Theologos and Markatos (1992) used the PHOENICS program to model the flow and heat transfer in fluidized catalytic cracking (FCC) riser-type reactors. They did not account for collisional particle-particle and particle-wall interactions and therefore it seems unlikely that this type of simulation will produce the correct flow structure in the riser reactor. Nevertheless it is one of the first attempts to integrate multiphase hydrodynamics and heat transfer. 

Recentlyr Schockaert and Proment [ref. 41] simulated the catalytic cracking of gasoil in both fluidized bed or riser reactors, connected with a fluidized bed regenerator. The kinetic model for the cracking was based upon the lO- lump loodel of Mobil [ ref 42 ]. Only one deactivation function was used for all the coking reactions and it was exponential in the coke content ... 

This paper presents a deactivation model derived from experimental data. A version of the classical microactivity test (MAT) is employed because it is used world-wide to simulate the cracking process, although the contact time is larger than in a riser reactor. 

DPMs can also be used to understand the influence of particle properties on fluidization behavior. It has been demonstrated that ideal particles with restitution coefficient of unity and zero coefficient of friction, lead to entirely different fluidization behavior than that observed with non-ideal particles. Simulation results of gas-solid flow in a riser reactor reported by Hoomans (2000) for ideal and nonideal particles are shown in Fig. 12.8. The well-known core-annulus flow structure can be observed only in the simulation with non-ideal particles. These comments are also applicable to simulations of bubbling beds. With ideal collision parameters, bubbling was not observed, contrary to the experimental evidence. Simulations with soft-sphere models with ideal particles also indicate that no bubbling is observed for fluidization of ideal particles (Hoomans, 2000). Apart from the particle characteristics, particle size distribution may also affect simulation results. For example, results of bubble formation simulations of Hoomans (2000) indicate that accounting... 

In light of these comments, some recent work (Kuipers et al., 1998 Dasgupta etal., 1998 Mathiesen etal., 1999,2000 Neri and Gidaspow, 2000) on the application of granular models to simulating gas-solid flows in riser reactors is briefly reviewed... 

The list is merely suggestive. Complexity of reactive flows may greatly expand the list of issues on which further research is required. Another area which deserves mention here is modeling of inherently unsteady flows. Most flows in engineering equipment are unsteady (gas-liquid flow in a bubble column reactor, gas-solid flow in a riser reactor and so on). However, for most engineering purposes, all the details of these unsteady flows are not required to be known. Further work is necessary to evolve adequate representation of such flows within the CFD framework without resorting to full, unsteady simulations. This development is especially necessary to simulate inherently unsteady flows in large industrial reactors where full, unsteady simulations may require unaffordable resources (and therefore, may not be cost effective). Different reactor types and different classes of multiphase flows will have different research requirements based on current and future applications under consideration. 

N V Dewachtere, F San tael la, G F Froment. Application of a single-event kinetic model in the simulation of an industrial riser reactor for the catalytic cracking of vacuum gas oil. Chem. Eng. Sc., 54, 3653-3660, 1999. 

Theologos et al. [1997] used the Eulerian-Eulerian gas-solid flow model and the 10-lump kinetic model of Jacob et al. [1976] to carry out 3D simulations of FCC riser reactors. Extensions of the flow model to account for the details of the feed atomization were studied by Theologos et al. [1999] and by Simonin and co-workers [Saulnier et al., 2005]. 

Simulation of a circulating fluidized bed riser reactor for the gasification of biomass (synthesis gas production). 

In the riser model, detailed hydraulics and heat effects are eaptured by linking flow equations to kineties. The riser model can be eonfigured vertieally, horizontally, or at any angle of inclination. Multiple risers ean be used. The individual risers can process different feeds or the same feed. In essenee, any commercial riser/reactor configuration can be simulated with the modular components of AFCC. 

ABSTRACT. The present contribution reviews the state-of-the-art on various aspects of catalytic cracking chemistry, catalyst formulation, catalyst preparation and FCC reactor engineering. Special consideration is given to the matters that relates to kinetic modelling. A detailed discussion is also presented on the characteristics and performance of a novel unit named Riser Simulator of particular value for FCC catalyst testing and kinetic modelling. 

---