高密度下行床反应器的流体力学特性

在一套内径为80 mm，高5.6 m的新型下行循环流化床内，以硅胶、FCC催化剂以及玻璃珠等颗粒为实验物料，在颗粒循环流率最高达600 kg•m^-2•s^-1，床层颗粒平均浓度达14%的条件下，进行了低气速、高浓度下行床内气固流动特性的研究.实验结果表明：高浓度下颗粒浓度的波动特性与低密度的有所差异.在低浓度操作条件下，颗粒浓度的概率分布曲线为单峰，而在高浓度下，概率密度分布曲线近似为水平直线;床层颗粒浓度随固体颗粒循环流率的增加而提高，颗粒直径及密度小的物料容易达到高的床层浓度，密度大而流动性好的物料容易达到高的颗粒循环流率;在低密度操作条件下，下行床内气固沿轴向流动过程可分为两个区域：加速区以及恒速区;而在高浓度操作条件下，可分为3个区域：加速区、恒速区以及出口受限区.

Hydrodynamics in high-density downer reactor

Experiments were carried out in a special circulating fluidized bed of 5.6 m in height, 0.08 m in ID downcomer.Glass beads(ρ_p=2480 kg•m^-3,d_p=131 μm),FCC catalyst particles (ρ_p=992 kg•m^-3,d_p=82 μm)and two kinds of silica gel (ρ_p=750 kg•m^-3,d_p=128 μm and 572 μm) were used.The maximum of the solids flux was 600 kg•m^-2•s^-1 and a solids holdup as high as 0.14 was achieved.The results showed that the characteristics of the transient solids concentration at high-density operation were different from those at low-density operation. Under low-density operation, the probability density distribution (PDD)curves had one peak,but under high-density operation,there was a horizontal line around the top of the PDD curves. Solids concentration increased with increasing solids flux.Smaller and lighter particles could achieve higher solids concentration, while the good fluidity and heavier particles could achieve high solids flux.The pressure gradient was positive in the overall downer and the length of particles acceleration zone was about 1.5—2.0 m.Depending on the structure of the exit, the downer could be divided into two regions：particle acceleration region and developed region under low-density operation or three regions：particle acceleration region, developed region, and exit region with high-density operation.