催化裂解多产丙烯过程热力学分析

THERMODYNAMIC ANALYSIS OF PROPYLENE-ENHANCING FCC PROCESS

近年来催化裂解多产丙烯工艺取得了显著进展,但现有工艺的丙烯单程收率都难以超过30%。丙烯单程收率有可能存在热力学平衡限制,为此采用Gibbs自由能最小化方法对这一过程进行了热力学分析。结果表明,虽然原料转化率不受热力学平衡的限制,但丙烯收率受到热力学平衡的限制;与低碳烷烃相比,烯烃和长碳链烷烃都更适合作为催化裂解多产丙烯的原料;低压有助于丙烯平衡收率的提高,并且在每一固定压力下都存在最优的反应温度,使丙烯平衡收率达到极大值;与热裂解相比,采用Y和ZSM-5分子筛的混合物为催化剂,虽然改变了产物分布,但并没有显著提高丙烯的平衡收率,而采用对产物有择形作用的小孔分子筛为催化剂,则可明显提高丙烯的平衡收率。

Although the propylene-enhancing FCC process has been made great progress in recent years, the single-pass yield of propylene can hardly exceed 30%. The propylene yield may be limited by the thermodynamic equilibrium in this process. The thermodynamic analysis of the propylene-enhancing FCC process was carried out by the method of minimum Gibbs free energy. The analysis illustrated that the propylene yield was limited by the thermodynamic equilibrium, while the feed conversion wasn＇t limited. Compared with the low carbon paraffin, the olefin and high carbon paraffin were both more preferable feeds for the propylene-enhancing FCC process. Decreasing the pressure was favorable to increase the equilibrium yield of propylene, and there existed an optimum temperature to obtain the maximum propylene yield at any given pressure. Compared with the thermal cracking, the catalytic cracking with the mixture of ZSM-5 and Y zeolite as catalyst showed less influence in the equilibrium yield of propylene, though the product distribution was changed. When the product-shape-selective zeolite was applied in the catalytic cracking process, the equilibrium yield of propylene was increased distinctly.