折流式外取热器内传热强化特性

为解决流化催化裂化(FCC)外取热器的取热负荷不足问题，提出了一种新型折流式外取热器，即在工业常用的下流式外取热器的基础上，沿催化剂流动方向增设若干折流板。通过大型冷模实验分别测量了下流式和折流式外取热器密相区和稀相区的传热系数，结果表明：在密相区和稀相区，折流式外取热器的传热系数均大于下流式外取热器，其中密相区和稀相区的传热系数的相对增长量分别为26.9%~267.9%和31.6%~152.5%。在折流式外取热器的密相区，传热系数得到强化，主要是因为颗粒径向流动的增加、密相床层的轴向混合的增强和气泡尺寸的变小；在稀相区，主要是由于颗粒流沿折流板以“折流”形式流动，增长了颗粒的过流路径，同时折流板上的开孔使催化剂分布更加均匀，提高了颗粒与换热管的接触分率。由于下流式和折流式外取热器内密相区传热系数均远大于稀相区，因此密相区为主要换热区域。建立了2种外取热器的密相区传热系数关联式，其计算值与预测值间相对误差小于20%。

Heat Transfer Enhancement Characteristics in Baffle External Catalyst Cooler

A new type of baffle catalyst cooler was proposed tosolve the problem of low cooling capacity in fluid catalytic cracking (FCC)external catalyst cooler through installing several baffles at the downflowcatalyst cooler in current commercial design. A large scale cold modelexperimental device was built to measure heat transfer coefficients in bothdense and dilute regions of the two coolers. Experimental results show thatheat transfer coefficients in the baffle catalyst cooler are higher than thosein the downflow catalyst cooler, either in dense or dilute regions. Heattransfer coefficients relatively increase 26.9%-267.9% and 31.6%-152.5% indense and dilute region, respectively. In dense region, the intensificationeffect mainly results to more particle radial movement, enhance particle axialmixture and smaller bubble size. In dilute region, particles from top inletflow along the baffle and this could extend particle flow path. Especially, thepores in baffles distribute particles more uniformly in the bed, thus couldincrease the particle contact time fraction with tube surface. Mathematicalcorrelations are proposed to predict heat transfer coefficients in denseregions of the two coolers with relative errors less than 20%.