| Gas-solid dense phase fluidized bed have been widely applied in many fields,such as petrochemical engineering,coalchemical engineering and biochemical engineering.Methanol to olefin is one of the typical technology processes with gas-solid dense phase fluidized bed.The methanol-to-olefin reaction process is a reaction in which the number of gas molecules increases.The change in the number of gas-phase molecules causes the gas line velocity in the reactor to change,which affects the gas-solid flow characteristics in the reactor,which in turn affects the methanol-to-olefin reaction characteristics and product distribution.Therefore,studying the gas-solid flow characteristics of variable gas velocity dense phase fluidized bed can provide the necessary basic data and theoretical basis for the process design of methanol to olefins.In this study,a dense phase fluidized bed experimental unit with a diameter of 200 mm and a height of 4.5 m was built.The variable gas velocity conditions were designed based on the methanol-to-olefin process.The gas-solid flow characteristics of the variable gas velocity dense phase fluidized bed were studied in detail.The characteristics of clusters and bubbles in a variable gas velocity dense-phase fluidized bed were studied in emphasis.Firstly,the gas-solid flow characteristics in the variable gas velocity dense phase fluidized bed are systematically studied.In the axial direction of the reactor,the solids holdup and particle velocity showed an uneven distribution characteristic of "exponential" and "inverted C",respectively.According to the distribution characteristics of the solids holdup along the axial direction of the reactor,it can be divided into three areas from bottom to top: the dense phase areas at the bottom,the intermediate transition areas and the dilute phase areas at the top,the fluctuation range of the solids holdup corresponding to the three areas is 0.150 ～ 0.250,0.050 ～ 0.150 and0.005 ～ 0.050.According to the particle velocity distribution in the axial direction,it can be divided into the bottom acceleration zone and the upper deceleration zone from bottom to top,and the highest particle velocity exists at the junction of the acceleration zone and the deceleration zone,up to 0.80 m / s.In the radial direction of the reactor,the solids holdup and particle velocity showed an uneven distribution characteristic of "sparse center,thick side wall" and "high middle,low side wall".The solids holdup under different operating conditions was between 0.004 and 0.300 in the radial direction,the particle velocity fluctuates radially up to 1.8 m/s.Compared with the gas-solid flow characteristics of the constant gas velocity dense phase fluidized bed,the solids holdup and the particle velocity axis and radial distribution of in the variable gas velocity dense phase bed are more uniform,and the gas-solid contact and mixing characteristics are more superior.Secondly,systematically explored the distribution of bubble characteristics in the variable gas velocity dense phase fluidized bed.The bubble phase time fractions range from 17% to 53%,and the bubble phase time fraction increases overall with the increase of the superficial gas velocity,decreases with the increase of the static bed height,and decreases with the increase of the radial position;The bubble phase solids holdup distribution is similar to the solids holdup distribution,and the bubble phase solids holdup is generally lower than the solids holdup,the difference between the two is between 0.02 ～ 0.15;the bubble size has a maximum at a certain axial height,and the maximum bubble size up to 25 mm,and the maximum bubble size is closely related to the axial and radial position,operating conditions.Compared with the distribution characteristics of bubbles in the constant gas velocity dense-phase fluidized bed,the gas volume in the dense-phase fluidized bed is higher under the condition of variable gas velocity,which is more conducive to the formation and growth of bubbles.Finally,the distribution characteristics of cluster in the variable gas velocity dense phase fluidized bed were studied.The cluster time fraction distribution range is 8% ～30%.Compared with the constant gas velocity condition,the cluster time fraction is not sensitive to the change of the radial and axial position under the condition of variable gas velocity,and is more sensitive to the change of operating conditions;The cluster solids holdup is between 0.08 ～ 0.42,and the axial distribution of the cluster solids holdup is relatively uniform under variable gas velocity conditions,and is less affected by the operating conditions;the cluster size distribution range is 1 ～ 8 mm,and the particles under variable gas velocity conditions the cluster size distribution range is narrow,and the average cluster size is slightly increased;the frequency of cluster is between 2 and 20 Hz,and the frequency of cluster under variable gas velocity conditions is more affected by the operating conditions in the bottom area.In the top zone,the operating conditions have almost no effect on the frequency of cluster;the cluster during time is between 4 and 14 ms,and the distribution characteristics of the cluster during time under variable gas velocity conditions are similar to the solids holdup distribution.The higher the solids holup,the longer the cluster during time;the cluster velocity is between-0.2 ～ 0.3 m/s,and the radial distribution of the cluster velocity axis is more uniform under the condition of variable gas velocity,but the axial height corresponding to the fully developed zone hardly changes,the cluster into the region corresponding to the full development of an axial difference in height is within0.1 m at a constant gas velocity conditions and variable gas velocity.Compared with the distribution characteristics of cluster in constant gas velocity dense-phase fluidized bed,the gas-solid turbulence degree in dense-phase fluidized bed is more intense under variable gas velocity,which is not conducive to the stable existence of cluster and more easily destroyed. |
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