Experimental Study On Diameter-Changing Riser Of CFB

Although traditional riser has been widely applied in industrial production, there are still some disadvantages, such as low solids holdup, easy to back mixing and difficult to control residence time. Nowadays, because of the demand of reducing olefins in gasoline and producing lighter olefins, a new series of catalytic cracking processes were developed. Diameter-changing risers were applied wildly in many of these processes. The understanding of CFB riser, particularly the dynamic of gas-solid flow in diameter-changing riser, is the foundation to research and develop new-style of riser reactor. To this end, a new set of circulating fluidized bed riser equipment with 50mm internal diameter and 8.3m height was designed. And the installation and commissioning work were completed in the laboratory.Based on the measurements of local solids holdup in the 12 axial levels of the riser, a series of experiments were carried out under the operation conditions of superficial gas velocity Ug=3.8~4.5m/s and solid circulation rate Gs=25~54 kg/(m2·s) (The solid circulation rate could reach 73 kg/(m2·s) after modification). The impact of operation conditions on the radial and axial distribution of particle concentration in diameter-changing riser was investigated. The impact of structure parameters, including the diameter of the diameter-expanding section (respectively 84mm, 95mm and 120mm), the cone angleβat the top of the diameter-expanding section (respectively 30°, 50°and 70°) and the cone angleαat the bottom of the diameter-expanding section (respectively 50°, 70°and 90°).The experimental results showed that diameter-changing riser could significantly increased solids holdup in the diameter-expanding section of the rise, while the upper part of the riser didn't vary greatly. Therefore, using diameter-changing riser could adjust residence time and density flexibly. The axial distribution of solids holdup in the whole riser shows a dense phase in the bottom and a dilute phase in the upper part. The solids holdup in the upper part of the diameter-expanding section is sometimes lower than that of the upper part of the riser with smaller diameter. Thus, under certain operating condition, "increasing concentration" effect is only limited to a certain height range. With the increase of Gs or decrease of Ug, the local solids holdup in whole riser and the radial nonuniformity of solids holdup increase mainly because of the holdup increase near the wall region. When the particle circulation rate is relative high, there is still core-annular structure in the diameter-changing riser.The impact of structure parameters on the radial and axial distribution of solids holdup can be summarized as follows: With the increase of diameter of diameter-expanding section, the solids holdup increased correspondingly, especially in the lower part. But the change in the following small diameter riser section was relatively small. With the increase of cone angleβat the bottom of the diameter-expanding section, the solids holdup increased. While back mixing and sliding of catalyst become serious and radial nonuniformity of solids holdup increased. For the smaller value ofβ, it could increase solids holdup in the diameter-expanding section and improved gas-solid distribution. The impact of cone angleαat the top of the diameter-expanding section was unnoticeable because of the obvious decrease of solids holdup at the top part of diameter-expanding section. After modification, the solids holdup at the top of the diameter-expanding section and radial nonuniformity of solids holdup increased when increased cone angleα, which is similar to the impact of cone angleβat the bottom of the diameter-expanding section.