Elsevier

Chemical Engineering Science

Volume 157, 10 January 2017, Pages 209-220
Chemical Engineering Science

Effects of asymmetric feeding on gas–solid–liquid transport and catalytic cracking reaction in the feed zone of riser

https://doi.org/10.1016/j.ces.2016.04.050Get rights and content

Highlights

  • A theoretical framework with an inertia-based multi-spray interaction is developed.

  • Spray model with individual spray properties and feeding operations is developed.

  • A model to calculate radial transport of both gas and solid phases is derived.

  • The inlet condition of catalysts asymmetric feeding is correlated by CFD simulation.

  • Significant difference can be obtained by adjusting asymmetric feedings.

Abstract

Asymmetric feeding of liquid reactants and catalytic solids are common in the operation of fluid catalytic cracking (FCC) riser reactors. The asymmetric feeding of solids can be caused by the designed-geometry of J-bend right before the riser inlet or unbalanced operation of wall-steam feeding. In order to ensure uniform distributions in local catalytic to oil ratio (CTO) and temperature, a possible measure of matching the asymmetric feed of catalysts is to introduce liquid jets in an accordingly asymmetric feeding. The asymmetric feeding will cause complicated three-phase interactions, in terms of the transport and phase-change characteristics of each interacting phase, in the feed zone of a riser reactor. This study has developed a theoretical framework with an inertia-based multi-spray interaction, which considers not only a complex geometric intervene of mutual-penetration sprays but also the individual spray properties and feeding operations. Based on the fact that the traverse gradient of pressure is insignificant compared to that of axial gradient in a riser flow, a sub-model is also established to redistribute the gas and solid phases within the cross-section of concern. Due to the lack of experiment data or available information on the asymmetric feeding of catalysts from J-bend, we have also conducted a CFD study of a J-bend dense gas–solid flow to obtain the asymmetric distribution catalysts feeding to the feed-zone model. Our modeling examples suggest that the reaction and flow characteristics can be altered noticeably by adjusting asymmetric feedings of liquid reactants, such as changes in droplet penetration, spray coverage and local CTO. In addition, the result of over-penetration of droplets onto the feed zone wall shows the need for a various-droplet size/flowrate model rather than mono-sized/constant-flowrate model of liquid phase. This can also assist in better design or selection of the characteristics for the spray nozzle.

Introduction

The feed zone of a riser reactor refers to a very important region, in which the liquid feed is injected into hot solid catalysts pneumatically transported by steam from the riser inlet, as shown in Fig. 1. The solids are typically fed from a J-bend bridging the riser inlet and catalysts regenerator. Upon contact with hot catalytic solids, the liquid feed quickly vaporizes and the resultant vapor then cracks into lighter components via catalytic cracking mechanisms. Hence, within the feed zone, the transport phenomena involve not only gas–solid–liquid flows but also both physical phase change and chemical reaction. The transport is further complicated by an intervened structure of mutual penetrating spray jets and non-uniform inlet gas–solids flows. Due to the bending geometry adjacent to riser inlet (such as an elbow section of J-bend), the inlet flow of steam-assisted catalysts becomes asymmetric. To best use the catalysts and the associated thermal capacity, in terms of obtaining the uniform distributions of catalysts-to-oil (vapor) ratio (CTO) and temperature over the cross-section after the feed-zone, an asymmetric feeding of sprays may be arranged accordingly. Hence, this study is focused on the development of a model to predict the intervened spray structure of asymmetry feeding of liquids and resulted phase transport and reaction distributions inside the feed zone.

There are few feed-zone models that investigate the detailed inter-spray transport, three-phase interactions and reactions. Most prior models of flow transport and reactions in fluid catalytic cracking (FCC) risers consider only two-phase (gas–solid) flows, with assumptions of “instant vaporization” (hence no acceleration of solids) and no reaction within the feed zone (e.g., Weekman and Nace, 1970; Lee et al., 1989; Christensen et al., 1999; Han, Chung, 2001; Van Wachem et al., 2001; Das et al., 2004; Gupta et al., 2007). To investigate the fundamental characteristics of evaporative liquid sprays in gas-liquid-solid three-phase flow systems, some basic experimental studies have been conducted typically using liquid nitrogen sprays injected into FCC flows at room temperature without any cracking reactions (e.g., Skouby, 1998; Zhu et al., 2000; Fan et al., 2001). For cracking reaction, our previous study (Patel et al., 2012, Patel et al., 2013) established a framework of feed zone modeling with the effect of liquid spray injection, which combines a Lagrangian description of the spray behavior with Eulerian modeling of transport-kinetics coupling in the FCC riser. The current study further improves the model by including new features, such as asymmetric feeding of liquid reactants, non-uniform inlet flow of catalytic solids, multi-spray interaction, and also the potential ability to study the optimization of droplet penetration for design and choosing the characteristics of the spray nozzle.

This paper presents a model concerning mutual-overlapped but individually-defined spray feeds from fan-shaped nozzles. Mathematically, governing equations of feed zone model consist of a set of highly-coupled first-order ordinary differential equations (ODEs) that represent, respectively, the inertia-dominated transport of sprays, the convective riser transport of gas and solids, and the kinetic reactions of catalytic cracking. The multi-spray interaction follows a sequence among interacting sprays along their penetration depths. It is noted that the traverse gradient of pressure is insignificant compared to that of axial gradient in a riser flow. Hence, based on the near-equilibrium of pressure over a cross-section of riser flow, a sub-model is established to redistribute the gas and solid phases within the cross-section of concern. Due to the lack of experiment data or available information on the asymmetric feeding of catalysts from J-bend, we have also conducted a CFD study of a J-bend dense gas–solids flow to obtain the asymmetric distribution catalysts feeding to the feed-zone model.

Section snippets

Modeling logics and cascade sequences

The feed zone model is composed of a set of sub-models, including a single spray trajectory model, a convection gas–solid riser flow model, a kinetic reaction model of catalyst cracking, a mutual-penetrating spray structure model, and a model of pressure-induced gas–solid redistribution within a cross-section. The interaction among those sub-models follows a mechanistic-logical sequence, as shown in Fig. 2. Based on the inlet condition of gas–solid flows from J-bend, we start our modeling by

Case condition

As an example of case study, the above proposed model is applied to investigate the feed zone transport and reaction, with a six-nozzle spray feeding. A typical set of operation parameters and properties in feed zone is listed in Table 2.

For the case study of CFD simulation, the packing limit is set as 0.52 since the catalysts in simulation are mono-sized. No steam lubrication is included in the J-bend flow simulation. The CFD results of velocity of solid phase at the outlet of the J-bend are

Conclusion

In this study, a theoretical framework has been established to quantify the parametric effect of asymmetric feeding on catalytic reaction and gas–solid–liquid three-phase flow in the feed zone. A sub-model is also established to redistribute the gas and solid phases within the cross-section of concern, which is induced by and coupled with the intervened spray penetration and vaporization. A CFD simulation is conducted for a J-bend dense gas–solid flow, which is aimed to obtain the asymmetric

Cited by (2)

1

Current address: Department of Chemical and Biomolecular Engineering, Ohio State University, Columbus, OH 43210, United States.

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