1.5.1
Introduction & Importance
The effects of the column exit in CFB risers have been studied extensively in several works, with a comprehensive review provided by Chan et al. (2010). Experimental results have shown that the exit configuration has a huge impact on hydrodynamics and mixing, with abrupt exits having much stronger gas and solids recirculation than smooth exits (Pugsley et al., 1997). Gas and solids recirculation at the riser exit has also been shown to impact on the overall unit hydrodynamics. However, as noted earlier, no previous studies have investigated the effect of the downer exit on the local and overall unit hydrodynamics and performance. Furthermore, since ICFAR’s gas-solids separator is essentially an integrated downer exit and fast separator, there is a need first to understand CFB column exit effects and second to review separator performance characteristics in the literature. Finally, since stripping gas is meant to be used to reduce gas backmixing in ICFAR’s gas-solids separator, and since stripping is normally performed on the solids before they flow to the regenerator of fluid catalytic crackers and fluid cokers, it is also important to understand how stripping performance has been assessed in the literature. Therefore, a broad definition of “separator performance” must necessarily be used in the context of ICFAR’s integrated gas-solids separator.
1.5.2
Performance Characteristics
The column exit effects in CFB risers in the literature have typically focused on four main topics:
i) Retrograde length of influence relative to column height (Harris, Davidson, & Thorpe, 2003b)
ii) Axial and radial mixing of both the gas and solids phases (concentration distributions, recirculation) (Zhou, Grace, Lim, & Brereton, 1995)
iii) Solids reflux and recirculation (Gupta & Berruti, 2000) iv) Solids RTD (Harris, Davidson, & Thorpe, 2003b)
In general, as noted above, abrupt exits were found to have stronger particle refluxing than smooth exits, effectively making them behave as gas-solids separators. Gupta and Berruti (2000) indicated that particle characteristics were also important with regard to exit effects, with Geldart Group A particles leading to less severe exit effects than Geldart Group B particles. However, in spite of the difference between abrupt and smooth riser exits, the impact of exit geometry on the solids RTD was shown to be distinguishable, though limited, by both Rhodes et al. (1991) and by Harris et al. (2003b). Since refluxing has not been shown to be a prominent phenomenon in CFB downer exits, only the solids RTD is expected to be relevant with regard to ICFAR’s gas-solids separator performance among the topics listed above.
Primary gas-solids separator performance has been assessed in cyclones and special separator designs in both CFB riser and downer units. An extensive review of gas-solids separator performance characteristics is provided by Huard (2009). Separators of all types have been assessed mainly according to:
i) Solids collection efficiency ii) Pressure drop
iii) Gas underflow iv) Gas RTD
The solids collection efficiency is always desired to be maximized, but usually comes at the expense of other performance metrics, namely pressure drop. Gas underflow refers to the fraction of the total gas stream that is entrained with the collected solids stream into
the diplegs (in FCC units). Underflow is typically desired to be minimized since product vapors can be degraded into undesirable permanent gases through excessive residence time in the separator and diplegs. Gauthier (1991) and Gauthier et al. (2005) both studied gas underflow in CFB gas-solids separators of two different designs and found that the addition of separator sealing gas (i.e. stripping gas) greatly aided to reduce excessive gas residence time and reaction.
1.5.3
Effect of Particle Size on Separator Performance
The effect of particle size on fluid-solids and fluid-fluid separator performance is likely the single most important consideration affecting the separator design. In most studies on dust cyclones, hydrocyclones, and demisters of various designs, the grade efficiency curve is typically used to characterize a separator’s ability to remove particles of different sizes (e.g. Vaughan, 1988; Hoffmann et al., 1992; Yang et al., 2010). The grade efficiency curve is a plot of the particle collection efficiency plotted for specific ranges of particle size, as shown in Figure 1.4. Most authors have used experimental grade efficiency curves either to calibrate analytical collection efficiency models (e.g. Maynard, 2000) or to determine whether the collection efficiency for a specific separator design and range of particle size will be sufficient for a given application. In general, experimental grade efficiency curves are S-shaped, where particle collection efficiency increases with particle size. This is due to two main effects: the terminal velocity of a particle in either the gravity field or a centrifugal field increases with particle size, assisting in particle collection. Agglomeration and clustering of particles may further complicate the situation and result in non-monotonic behavior of collection efficiency with changing particle size.
Figure 1.4 – Example grade efficiency curve in a cyclone (from Hoffmann et al., 1992) Particle size and particle size distribution (PSD) have also been shown to have a significant effect on the hydrodynamics, mixing, and performance in conventional fluidized bed reactors (Grace & Sun, 1991). Among Geldart Group A PSDs with the same average size, wider PSDs have been shown to result in an expanded dense phase, lower effective viscosity, smaller bubble size, better gas-solids contacting, and higher conversion in tests spanning the bubbling to fast fluidization regimes. However, Zhu et al. (1995) identified a need for research on the effect of particle size in CFB downers. Wang et al. (2005) tested the effect of three different coal particle size distributions (< 280 μm, 280 μm to 450 μm, and 450 μm to 600 μm) and 250 μm silica sand on the pyrolytic conversion of coal to gas and liquid products in a CFB downer. The authors found that the liquid yield, and specifically of desired aliphatics and methylphenols, decreased with increasing particle size. This was attributed to reduced gas-solids heat transfer and increased secondary reactions with the large PSD. However, to the author’s knowledge, no other studies exist in the literature investigating the effect of PSD on downer hydrodynamics and performance.
1.5.4
Motivations
Given that several different performance characteristics have previously been used to assess different parts of CFB units surrounding the exit and gas-solids separator, it is important to recognize how ICFAR’s gas-solids separator can be developed to integrate several of these components into one effective separator. Therefore, a comprehensive
evaluation of all relevant factors under realistic conditions must be considered. The following performance characteristics and other effects are deemed relevant to investigate further both in the literature and in ICFAR’s cone-shaped gas-solids separator:
i) Solids collection efficiency (performed by Huard, 2009) ii) Gas RTD
iii) Solids RTD
iv) Pressure drop (reviewed by Huard, 2009) v) Gas underflow / solids stripping
vi) Particle size
The remainder of this chapter is devoted to a literature review of CFB gas and solids RTD measurement and modelling, of previous stripping gas studies, and of specific objectives for this thesis.