Riser reactors

The modern HF alkylation processes are also differentiated primarily by the reactor system that is used. The Phillips process employs a gravity acid circulation system and a riser reactor (19). The UOP process uses a pumped acid circulation system and an exchanger reactor (20). 

Sometimes insufficient differential across the regenerated catalyst slide valve is not due to inadequate pressure buildup upstream of the valve, but rather due to an increase in pressure downstream of the slide valve. Possible causes of this increased backpressure are an excessive pressure drop in the Y or J-bend section, riser, reactor cyclones, reactor overhead vapor line, main fractionator, and/or the main fractionator overhead condensing/cooling system. 

Direct measurement of particle velocity and velocity fluctuations in fluidized beds or riser reactors is necessary for validating multiphase models. Dudukovic [14] and Roy and Dudukovic [28] have used computer-automated radioactive particle tracking (CARPT) to foUow particles in a riser reactor. From their measurements, it was possible to calculate axial and radial solids diffusion as well as the granular temperature from a multiphase KTGF model. Figure 15.10 shows one such measurement... 

While having some advantages over riser reactors, downer reactors also suffer fixrm some serious shortcomings, such as a low solids holdup in the bed, difficulty in even distribution of injected residual on the catalysts, and a high sensitivity to the structure of the inlet [11,12]. Therefore, the development of a new coupled CFB reactor that can fully utilize the advantages of the riser and the downer is of interest. 

Advances in multiphase reactors for fuel industry are discussed in this work. Downer reactors have some advantages over riser reactors, but suffer from some serious shortcomings. The coupled reactors can fully utilize the advantages of the riser and the downer. For fuel industry that involves gas-liquid-solid system, slurry bed reactors especially airlift reactors are preferred due to their performance of excellent heat control and ease of seale up. For high-pressure processes, the spherical reactor is promising due to its special characteristics. 

The FCC process is used worldwide in more than 300 installations, of which about 175 are in North America and 70 in Europe. Figure 9.10 shows the principle of an FCC unit. The preheated heavy feed (flash distillate and residue) is injected at the bottom of the riser reactor and mixed with the catalyst, which comes from the regeneration section. Table 9.5 gives a typical product distribution for the FCC process. Cracking occurs in the entrained-flow riser reactor, where hydrocarbons and catalyst have a typical residence time of a few seconds only. This, however, is long enough for the catalyst to become entirely covered by coke. While the products leave the reactor at the top, the catalyst flows into the regeneration section, where the coke is burned off in air at 1000 K. 

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). 

When a chemical reaction occurs in the system, each of these types of behavior gives rise to a corresponding type of reactor. These range from a fixed-bed reactor (Chapter 21-not a moving-particle reactor), to a fluidized-bed reactor without significant carryover of solid particles, to a fast-fluidized-bed reactor with significant carryover of particles, and ultimately a pneumatic-transport or transport-riser reactor in which solid particles are completely entrained in the rising fluid. The reactors are usually operated commercially with continuous flow of both fluid and solid phases. Kunii and Levenspiel (1991, Chapter 2) illustrate many industrial applications of fluidized beds. 

As practiced today, FCC is a fluidized-bed process with continuous catalyst regeneration which reUes on short contact in a riser reactor between the feed and catalyst, fluidized with an inert gas, followed by disengagement and catalyst regeneration to burn off coke deposits and return the catalyst to near-fresh activity. 

A simplified diagram of a typical FCC unit is shown in Figure 16.9. The reaction chemistry described above is carried out in this process at temperatures in the range of 500-540 °C by contachng the fluidized catalyst in the form of particles in the range of 30-120 xm in diameter with the hot feed injected near the top of a riser reactor followed by rapid disengagement after a short contact time (on the... 

Corella, J. and Frances, E. (1991) Modeling some revampings of the riser reactors of the FCCUs. AIChE Annual conference, Los Angeles, USA. 

Figure 7-4 Slurry reactor (left) for well-mixed gas-solid reactions and fluidized bed reactor (center) for liquid-solid reactions. At the right is shown a riser reactor in which the catalyst is carried with the reactants and separated and returned to the reactor. The slurry reactor is generally mixed and is described by the CSTR model, while the fluidized bed is described by the PFTR or CSTR models.

A variant on the fluidized bed is the riser reactor. In this reactor the flow velocity is so high that the solids are entrained in the flowing fluid and move with nearly the same velocity as the fluid. The solids are then separated trom the effluent gases at the top of the reactor by a cyclone, and the solids are returned to the reactor as shown in Figure 7-4. The FCC reactor is an example where the catalyst is carried into the regenerator, where carbon is burned off and the catalyst is heated before returning to the reactor. 

We will develop the rest of this chapter assuming that the catalyst is in a sohd phase with the reactants and products in a gas or liquid phase. In Chapter 12 we will consider some of the more complex reactor types, called multiphase reactors, where each phase has a specific residence time. Examples are the riser reactor, the moving bed reactor, and the transport bed reactor. 

The two industrial units are relatively small compared to more modern large capacity riser-reactor units. 

Develop the mathematical model of a modern riser reactor FCC unit and construct its static bifurcation diagram numerically. 

Develop a model for an industrial riser reactor FCC unit. Collect the necessary data and develop the MATLAB code needed to design the unit from your model. 

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