Riser simulator reactor

FIGURE 2.11 Riser simulator reactor schematics, showing reaction chamber at left and peripherals at right, including a preheated vacuum cylinder linked to a gas chromatograph to drain, quantify and identify reaction products at the end of a rnn. 

Figure 3.13 Scheme of a riser simulator reactor for plastic conversion [83]. (Reproduced by permission of Javier Bilbao)... 

G. de la Puente et al. [48] LDPE Riser simulator reactor 6.5 EquiUbrium Engelhard FCC catalyst ASA... 

ABSTRACT. Characteristics and fluid dynamics of gas phase recirculation in a novel Riser Simulator Reactor have been investigated using constant temperature hot wire anemometry. In situ concentration and velocity measurements enabled to evaluate the mixing time and the inner recirculation ratio of the gas phase. In addition, fibre optic techniques allowed to characterize the degree of fluidization of the catalyst particles and the effect of gas phase density changes. By combining the anemometry and the fibre optic techniques, mixing patterns in the Riser Simulator have been evaluated. The importance of the study can be realized in the context of the potential use of the Riser Simulator for gas-solid reaction kinetics. 

The Riser Simulator Reactor is an internal recycle fluidized batch reactor. This patented novel device (de Lasa 1989 de Lasa, 1991), has been developed and successfully tested for the estimation of kinetic parameters of catalytic cracking of heavy oils (Kraemer and de Lasa, 1988). The details of the unit are given in the section entitled "Novel Techniques For FCC Catalyst Selection and Kinetic Modelling" by de Lasa and Kraemer in this NATO-ASI Proceedings. 

Figure 1. Schematics of the experimental system fibre optic system and hot wire anemometer are described as connected to the Riser Simulator Reactor.

The gas volumetric flow rate inside the annulus can also be calculated by multiplying the measured velocity by the cross sectional area of the annulus, which is 4.38 cm (refer to the column volumetric flow in Table 1), and assuming a flat velocity profile along the annulus section. For instance, knowing the volume of the lower chamber, which is approximately 20 cm then the gas can be recirculated through the lower chamber 1000 / 20 = 50 times in one second at the impeller speed of 7350 RPM (70%). It has to be mentioned that the reactor is normally operated at 7875 RPM (75%). Then a recycle rate above 50 in one second could be expected in this novel Riser Simulator Reactor. 

TABLE 1. Gas mixing results from the Riser Simulator Reactor... 

To further evaluate the fluid-solid mixing characteristics taking place in the catalyst bed a fibre optic probe technique was used. The fibre optic probe was inserted from the side of the reactor through the basket where the probe contacts with catalyst particles. A schematic of the system is shown in Figure 1. It has to be noted that two models, cold and hot model, were constructed to apply the fibre optic technique (Kraemer, 1990). The cold model was made of plexiglass material and had the same internal dimensions as the hot model or the Riser Simulator Reactor made of inconel. 

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.

The riser is a vertical pipe. It usually has s 4- to 5-inch (10 to 1" cm) thick refractory lining for insulation and abrasion resistance. Typical risers are 2 to 6 feet (60 to 180 cm) in diameter and 75 to 120 feet (25 to 30 meters) long. The ideal riser simulates a plug flow reactor, w here the catalyst and the vapor travel the length of the riser with minimum back mixing. 

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. 

In the Riser Simulator, an impeller rotating at very high speed on the top of the reaction chamber keeps the catalyst fluidized between two metal porous plates, inducing the internal circulation of the reacting mixture in an upward direction through the chamber. When the reactor is at the desired experimental conditions, the reactant is fed through an injection port, and immediately after the set reaction time is attained, products are evacuated and analyzed by gas chromatography. Of particular importance to the experiments performed was the ability to extend reaction... 

Tables 2.1 and 2.4 show the VGO-C feed quality properties and the test conditions of the Riser Simulator experiments. The three catalysts tested were the same ones used in the FFB reactor experiments. Temperature for the LZM catalyst was lower than for the other catalysts to reproduce typical conditions used for mid-distillate maximization in commercial units.

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. 

Figure lb. Riser Simulator Components reactor, vacuum box, glass chamber... 

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. 

Theologos, K.N., Nikou, I.D., Lygeros, A.I. and Markatos, N.C. (1997), Simulation and design of fluid catalytic cracking riser-type reactors, AIChE J., 43, 486. 

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. 

The Riser Simulator interconnected with a number of valves, as described in Figure 7, allows the reactor to be operated in a continuous or a discontinuous mode. Valve V3 is used to switch flow from argon (used as an inert gas in the reactor) to air which is used for regeneration of the catalyst between injections of oil. The steps involved for a cracking run can be divided into five parts ... 

Since the Riser Simulator operates as a batch reactor, then the change in the number of moles of gas oil with time is equal to the cracking rate of the oil. This can be expressed in terms of molar concentrations (or equivalently as partial pressures) as follows ... 

The results described in this review demonstrate the importance of novel tools for kinetic modelling and catalyst development. In this context it is proposed to adopt the Riser Simulator to effectively represent the catalytic cracking reactions which take place in a FCC riser reactor. The technique allows for similar conditions to be used such as temperature, catalyst to oil ratio, partial pressure of hydrocarbons, solids loading and reaction time as those used in commercial units. Moreover, the contacting of the hydrocarbons with the catalyst is more representative of riser cracking where the catalyst moves in time with the vapours, than that achieved in the MAT test. 

Thus, at the operational speed of 7875 RPM, the catalyst was intensively fluidized and gas was completely well mixed in the range of reaction times (1-10 seconds) expected for novel downflow reactors or riser units. In summary, this study proves the excellent ability of the Riser Simulator for catalyst testing and kinetic modelling of catalytic reactions to be conducted under short contact times. 

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). 

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. 

The test conditions for this Microscale Simulation Test (MST) correspond to the low vapor contact times as applied in today s FCC riser technology. An effective feed preheat and feed dispersion is ensured, while the isothermal reactor bed is set to the dominating kinetic temperature in the riser, being approximately the feed catalyst mix temperature. The MST conditions enable the testing of high Conradson Carbon residue feedstocks. 

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. 

The Ketjen-MAT conditions seem to simulate a FCC bed reactor, but not a riser reactor. 

Using equilibrium catalyst from commercial FCC units, we modified the MAT reactor conditions in order to meet the simulation criteria. This work was complemented with ARGO pilot riser plant tests, exploring the influence of the main process parameters such as residence time, mixing, reactor temperature and temperture profile. 

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 ... 

---