连续聚合过程中产品牌号过渡的优化

OPTIMAL GRADE TRANSITION IN CONTINUOUS POLYMERIZATION PROCESS

以流化床乙烯气相聚合工艺为例建立了产品牌号切换模型 ,通过同时优化聚合温度、氢气和共聚单体的浓度、催化剂流率和料位高度等 5个操作变量的变化轨迹 ,使产品牌号的切换时间最短 ,过渡料数量最少 .研究表明 ,聚合温度、氢气和共聚单体的浓度对树脂性能指标的控制起主要作用 .为了减少过渡料的数量 ,一般均需降低催化剂流率和料位高度 .通过在目标函数中增加熔流比控制项可以抑制操作变量的剧烈调节 ,达到控制分子量分布的目的 .同时发现 ,产品牌号的正向和反向切换所得到的操作变量的最优轨迹很不相同 。

A model of grade transition is presented for the commercial fluidized bed gas-phase polyethylene production process. For two polyethylene products,the model-based dynamic optimization is used to determine optimal grade changeover policies. The quantity of off-specification product and the time of grade transition can be minimized by optimization of manipulated variable profiles, such as polymerization temperature, the ratio of hydrogen to ethylene, the ratio of co-monomer to ethylene, catalyst feed rate, and bed level. The results show that polymerization temperature, hydrogen and co-monomer are sensitive to the quality of resin. It is always recommended that catalyst feed rate and bed level decrease in order to reduce the grade transition time and the quantity of off-specification product. It is demonstrated that melt index and density are not sufficient to characterize the properties of polymer produced during grade transition. A new performance index, i.e. ratio of melt flow ( MFR ), is included into the objective function, which could restrain the sharp fluctuation of operation variables and narrow the distribution of molecular weight of resin. There are much difference between the transition and reverse transition of grade which indicates the non-linearity of operation.