Generation Mechanism And Control Method Of The Impurity Substance Of C4 Separation Recycling Solvent

During these years, the comprehensive utilization of C₄ fractions has attracted much attention of the chemical industry. In most applications, the sepearation of C₄ fractions is required in the first step. The aim for the C₄ separation is to separate butene from butane for their individual application. Generally, extractive distillation processes is used in the devices of C₄ separation. Among various extractive distillation processes, the process using mixed solvent of NFM and MEK to seperate C₄ butene from butane is the most widely used one.However, it is found that some impurities are gradually generated in the recycled solvent during the application of this process. The analysis result shows that the impurities include methanol, MTBE, C8 hydrocarbons and other polymers. The mechanism for the generation of these impurities is very clear and it is easy to remove them. However, there exist unknown impurities in some production apparatus. The impurities are accumulated very fast in the solvent of a few enterprises. The accumulation of the impurities resutls in the reduction of selectivity and separation efficiency of solvent. There is no related report about the impurities. Based on this background, the unknown impurities in this process was analyzed by spectral method to confirme their structure. Also, the impurities were synthized under the simulated industrial conditions and the gneration mechanism of impurities was studied. The factors for influencing the generation rate of impurities wer studied. Based on the obtained results, the method for controlling the production rate of impurities was proposed.（1） Structure identification of the unknown impurities in recycled solvent: the poor solvent from industry was treated by distillation to increase the purity of impurities to98.5%. Then, the molecular structure of impurity was analyzed by IR, 1HNMR, 13 CNMR and GC-MS. The unknown impurity was ultimately identified as 4-sec-butylmorpholine with chemical formula of C8H17 NO.（2） The generation mechanism of 4-sec-butylmorpholine in recycled solvent:4-sec-butylmorpholine was synthesized without catalyst under simulated industrial conditions according to the C₄ separation process and literature. It was shown that4-sec-butylmorpholine in recycled solvent was producted through the reactions of NFM,water and MEK. The mechanism is as following: N-formyl morpholine is hydrolyzed into morpholine and formic acid at high temperature. Then, reactions occur between morpholine and MEK in acidic environment to form enamine. This enamine is chemically unstable, which is apt to react with formic acid, forming 4-sec-butyl morpholine.（3） Controlling method about the generation rate of impurities: Under simulated industrial conditions, the influence of solvent water content, temperature and solvent p H on the production of 4-sec-butyl morpholine was studied. It was demonstrated that the content of 4-sec-butyl morpholine in the solvent increases with the increase of the water content. The production rate of 4-sec-butyl morpholine increases with the reaction temperature. However, the production rate of 4-sec-butyl morpholine decreases with the increase of p H. Based on these results, the generation of 4-sec-butyl morpholine was controlled by adding feedstock buffer tank, reflux tank with liquid package, recycled solvent drying device, intermediate the stripper reboiler.