Epoxidation Of Propylene Catalyzed By TPAOH Treated Titanium Silicalite-1 And Modeling Of The Separation Process

Propylene oxide is an important chemical raw material, which is the third largest acrylic derivatives, second only to polypropylene and acrylonitrile. Currently,propylene oxide is mainly produced through chlorohydrin process and hydroperoxide oxidation process, which have obvious disadvantages. The process of epoxidzing propylenen with hydrogen peroxide catalyzed by titanium silicalite-1(TS-1) has been investigated to some extent and proved to be an environmentally-friendly method with mild reaction conditions, fast reaction rate, high selectivity, etc.In this work,TS-1 zeolite was hydrothermally synthesized, and then shaped to SiO2-supported TS-1 catalyst and studied in the epoxidation of liquid phase propylene in a fixed-bed reactor. Firstly, the extrudsted strips(TS-1/SiO2) were treated by dilute tetrapropylammonium hydroxide(TPAOH) aqueous solution and the effects of TPAOH concentration, treatment temperature, and treatment time on the catalytic properties, expecially the catalytic stability in the epoxidation reaction were investigated, and then the catalysts were characterized by several characterization techniques. Secondly, the process for separating the products of the propylene epoxidation was estabilished. Since propylene oxide and methanol form an azeotrope,the mixture of propylene oxide and methanol cannot be separated through a simple distillation process. So an extractive distillation was proposed for separation propylene oxide and methanol. Rigorous steady state simulations were carried out with the chemical process simulation software Aspen Plus. To minimize the total annual cost of the process, sequential iterative optimization procedure was used in the products separation process.The experimental results showed that, the catalystic stability was appreciably improved by treating TS-1/SiO2 with TPAOH aqueous solution. The optimal treatment conditions were as follows: TPAOH concentration 0.008 mol/L, treatment temperature 170 oC and treatment time 96 h.The characterization results showed that, the amorphous SiO2 in TS-1/SiO2 catalysts was transformed into the framework Si and many irregular hollows were generated in TS-1/SiO2 catalysts after the TPAOH treatment. The generated hollows make it easier for the reactants and products to diffuse out of the zeolite channels,which might be the reason for the catalystic stability improvement in the epoxidation of propylene.For a 200000 t/a propylene oxide plant, a three-column separation process,including a pre-distillation column(C1), an extractive distillation column(C2) and a methanol distillation column(C3), was estabilished to separate the products. The products were first separated by C1, then pure propylene oxide was obtained from C2 through extractive distillation, and the solvent, methanol, was recycled after a simple distillation process through C3. The effects of the theortical plates and the feed stage on the total annual cost(TAC) were investigated for C1 and C3, and the effects of the theoretical plate number, the feed stage, the extraction agent feed stage and the amount of the extraction agent on TAC were investigated for C2. The optimal parameters in the simulation were obtained.