(187d) Numerical Evaluation of the Process Intensification Potential of Rotating Fluidized Beds in a Static Geometry: Application to Fluid Catalytic Cracking

1. Introduction

The operating conditions in conventional, that is, gravitational fluidized bed reactors are limited by the use of earth gravity. Internal mass and heat transfer limitations can be encountered, the van der Waals forces becoming too important when trying to fluidize smaller size particles. External mass and heat transfer limitations can be encountered and are related to limitations on the gas-solid slip velocities. With heterogeneous (catalytic) reactions, as well external mass or heat transfer limitations as internal mass transfer limitations may impose limitations on reaction rates and on the activity of the catalysts that can be used.

Fluidization in a centrifugal field may allow overcoming the above mentioned limitations and may, as such, allow fluidized bed process intensification and in particular the use of more active catalysts [1,2]. Current technology for fluidization in a centrifugal field is based on a rotating fluidization chamber [1,2] and its industrial application faces challenges with respect to sealing, mechanical vibrations and continuous operation. Recently, a novel route for fluidization in a centrifugal field was presented, the rotating fluidized bed in a static geometry [3-5], facilitating industrial process application and offering additional specific advantages.

2. Concept

In rotating fluidized beds in a static geometry, the centrifugal field is generated by injecting the fluidization gas tangentially in the fluidization chamber via multiple gas inlet slots in its outer cylindrical wall [3,4,5] (Figure 1). A combined tangential-radial fluidization of the particle bed can be obtained by forcing the fluidization gas to leave the fluidization chamber via a central chimney. The fluidization gas flow rate influencing both the centrifugal force and the counteracting radial gas-solid drag force in a similar way renders rotating fluidized beds in a static geometry extremely flexible with respect to the fluidization gas flow rate and the gas-solid contact time. In particular, dense operation at high fluidization gas velocities is possible, allowing fast and highly endothermic or exothermic reactions to be carried out in a relatively small volume rotating fluidized bed in a static geometry.

Figure 1. Rotating fluidized bed in a static geometry and rotating chimney concepts.

Rotating fluidized beds in a static geometry furthermore allow angular multi-zone operation, the fluidization gases injected via successive gas inlet slots hardly being mixed in the rotating particle bed. Such multi-zone operation opens perspectives for carrying out catalytic reactions and catalyst regeneration within the same reactor vessel.

3. Numerical evaluation of the fluid catalytic cracking process intensification potential

The potential process intensification of the fluid catalytic cracking (FCC) process using a rotating fluidized bed in a static geometry is numerically evaluated. A comparison with the performance of a riser type reactor as described in Froment and Bischoff [6] is made.

3.1.      Hydrodynamic model

For the riser reactor simulations, a 1D plug flow model with slip between the gas and solids is used [6].

For the rotating fluidized bed in a static geometry, experimental observations and computational fluid dynamics (CFD) type simulations show an essentially cross-flow type flow pattern for the gas and solids - radial for the gas and angular for the solids [7]. Furthermore, plug flow can be assumed [7]. The relations between the gas and solids phase velocities and between the gas phase velocity and the solids volume fraction (or bed density) is taken from experimental observations [7]. The hydrodynamic model for the rotating fluidized bed in a static geometry is schematically shown in Figure 2.

Figure 2. Geometry and hydrodynamic model for a rotating fluidized bed in a static geometry.

3.2.      Kinetic model

The 10-lump model proposed by Jacobs et al. (see [6]) is adopted. A distinction is made between heavy paraffins, heavy naphtenic molecules, heavy aromatic substituent groups, heavy carbon atoms among aromatic rings, light paraffins, light naphtenic molecules, light aromatic substituent groups, light carbon atoms among aromatic rings, gasoline, and a lump of coke and light gases (C1-C4).

3.3       Simulation cases

First, identical catalyst is assumed to evaluate the intrinsic process intensification by rotating fluidized beds in a static geometry (Figure 3). Particular attention is paid to the cat-to-oil-ratios that can be achieved. Next, the possibility of using more active catalyst is investigated. Multi-zone aspects are also discussed.

Figure 3. Fluid catalytic cracking process intensification by a rotating fluidized bed in a static geometry. Comparison with conventional riser technology using identical catalyst. Intensification factor defined as the ratio of the reactor volumes required to convert a given flow rate to a given gasoline yield.

References

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2.       Watano, S., Nakamura, H., Hamada, K., Wakamatsu, Y., Tanabe, Y., Dave, R.N., Pfeffer, R. Powder Technol., 141, p. 172-176, (2004).

3.       de Broqueville, Axel : Belgian Patent 2004/0186, Internat. Classif. : B01J C08F B01F; publication number : 1015976A3.

4.       De Wilde, J., de Broqueville, A. AIChE J., 53(4), p. 793-810, (2007).

5.       De Wilde, J., de Broqueville, A. Powder Technol., 183:(3), p. 426-435, (2008).

6.       Froment, G.F., Bischoff, K.B., Chemical Reactor Analysis and Design, Wiley, (1990).

7.       Staudt, N., De Wilde, J., UCL report (2008).