5810
Ind. Eng. Chem. Res. 2004, 43, 5810-5819
Scale-Up Effect of Riser Reactors (1): Axial and Radial Solids Concentration Distribution and Flow Development Aijie Yan and Jesse (Jingxu) Zhu* Powder Technology Research Centre, Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9
The influence of the riser diameter on the axial and radial solids holdup profiles and flow development is studied by measuring local solids holdups with a fiber-optic probe in a 10-m twin-riser system with 0.076- and 0.203-m inner diameters, at solids circulation rates of up to 200 kg/(m2 s) and superficial gas velocities of up to 8 m/s. It was found that the solids concentration increases with increasing riser diameter. Furthermore, the radial profiles of the solids concentration are steeper with larger-diameter risers. For both risers, the flow development in the riser center is nearly instantaneous, with the solids concentration remaining low under all operating conditions. In the wall region, the flow development slows down. Moreover, the flow development is slower in the larger-diameter riser. Increasing the solids flux also slows the flow development. However, increasing the superficial gas velocity makes the flow development faster while also lengthening the fully developed region. 1. Introduction Solids concentrations in circulating fluidized bed (CFB) risers are of great interest not only to scholars in academic areas but also to engineers in industry.1,2 Fluidized catalytic cracking (FCC) units have been considered the most successful application of circulating fluidized beds (CFBs) in the past century. The total worldwide capacity of FCC units has increased to over 16 million barrels per day, whereas the uplift (difference between the values of products and feeds) could be as high as $10 U.S. per barrel across the FCC unit.3 Although the first commercial fluid catalytic cracker was initiated in the 1930s, the application of CFBs is still facing challenges in scale-up. FCC units are used in most refineries all over the world to convert highmolecular-weight gas oils or residuum stocks into lighter hydrocarbon products in riser reactors within a few seconds. Major improvements have been recorded for these processes as knowledge and design methods (e.g., CFD modeling4-6) have been accumulated. Good understanding of the gas and solids flow structures in riser reactors is critical for proper industrial design. The solids distribution is directly related to the solids residence time in the riser and also significantly affect the gas-solids contacting efficiency, heat and mass transfer, and conversion and selectivity of the chemical reaction. To minimize the possibility of expensive errors in scaling-up to commercial operation and to optimize and improve the designs of existing industrial FCC riser reactors, a good understanding of the scale-up obtained from pilot-scale risers, especially knowledge of the solids distribution in CFB risers of significant size, is necessary. Much research has been carried out on CFB riser reactors.7-15 From some related research in the literature,16-23 some conclusions can be drawn about the dependence of the solids concentration on the riser diameter. However, the variation of the solids concen* To whom correspondence should be addressed. Tel.: 1-519661-3807. Fax: 1-519-850-2441. E-mail:
[email protected].
tration with riser diameter cannot be unequivocally defined from these investigations because most research did not isolate the effect of the riser diameter from other influential factors (such as riser height,16,18,24 gas velocity,14 entrance/exit geometry,25,26 and so on). According to the theoretical analysis of Bi and Zhu,8 both increased downcomer-to-riser height ratio and increased solids inventory in the system have the potential to increase the solids holdup in the riser. Xu et al.27 reported that the riser diameter influences the bed density for different particles (FCC and silica sand) by measuring the total pressure drop across the whole riser and the differential pressure gradient (apparent solids concentration) over a section of the bed at a specified bed elevation. The risers had the same bed height of 3.0 m but different inner diameters of 0.066, 0.097, and 0.150 m. They stated that the riser diameter has opposite influences for FCC (Geldart A particles)28 and silica sand (Geldart B particles).28 With increasing riser diameter, these two pressure drops increase for group B particles but decrease for group A particles at a specified gas velocity and solids circulation rate. However, the particle flow in their risers was not fully developed given that the bed height was only 3.0 m. In addition, the measurements were taken at very low gas velocity (
