Synthesis And Process Optimization Of Salicylic Acid

Salicylic acid is an essential intermediate in organic synthesis,which is used widely as the primary raw material for the preparation of industrial products such as specific medicine,preservatives,fungicides and dyes.Kolbe-Schmitt reaction is a common way for industrial preparation of salicylic acid and the existing production process is mainly based on the solid-state melting.However,this production process has difficulties in water removal,poor dispersion of raw materials,long reaction time,the stress in post-treatment of waste liquid and waste residue,and sublimation treatment required to obtain refined salicylic acid.So,it can’t meet the requirements of modern industrial production anymore.The solvent is a suitable reaction medium with good dispersion performance and azeotropic water removal effect.At the same time,the particle interaction between the reaction raw material and the solvent molecule has an important influence on the purity and properties of the product.Therefore,using the solvent method instead of the solid-state melting method for industrial production is of great research significance for improving the synthesis process of salicylic acid.This research takes the traditional process of synthesizing salicylic acid by the Kolbe-Schmitt method as the research object.By examining the dehydration performance and reaction performance of different solvents,the solvents of the Kolbe-Schmitt reaction are systematically screened.And the kinetic control factors of the Kolbe-Schmitt reaction are explored based on the shrinking model.The results are shown as follows:The lower the polarity of the solvent and the dielectric constant,the better the reaction performance.Therefore,n-octanol was selected as the optimal solvent for the salicylic acid synthesis process.When the reaction time exceeds5 h,the reaction process of sodium phenolate in n-octanol is controlled by the surface reaction.At this time,the reaction is mainly affected by pressure.The resulting dynamic equation is:t=-0.0091[1-（1-x）^1/3]+0.0918,and the linear correlation coefficient R² is 0.9964.The synthesis of salicylic acid by the solvent Kolbe-Schmitt method was optimized.First,the salt formation reaction of phenol was investigated by the single factor variable method and orthogonal experiment.The results showed that the best process for synthesizing sodium phenolate was:Under the protection of nitrogen,Na OH was used as the alkali source,the molar ratio of phenol and Na OH was 1:1.05,the reflux time was 3 h,and the reflux temperature was110℃.Under this condition,the yield of phenol salt formation was 98.8%.Subsequently,the carboxylation reaction of sodium phenolate was investigated by a single factor variable method.The results showed that the best process for the synthesis of salicylic acid was:Using sulfuric acid solution as the acidifying reagent,the reaction temperature is 190℃,the reaction time is4 h,the reaction pressure is 3.5 MPa,the stirring speed is 1200 r/min,and the mass ratio of sodium phenolate and solvent is 1:4.Under this condition,the yield of the target product salicylic acid is 46.8%,and the purity is 99.6%.The product can achieve the purity requirement of industrial production（99%）without further purification.The Aspen process simulation software was used to simulate and calculate the each operating unit of the industrial production process of salicylic acid synthesized by the solvent method with an annual output of 30,000 t.In the simulation process,the ELECTRTL model was selected as the thermodynamic method and the RStoic module is used as the reaction stoichiometric reactor.The Aspen default module was used for the other modules.The calculation results show that the purity of the obtained salicylic acid product is 97.6%,and the purity of the refluxing solvent n-octanol is 94.3%.The technological parameters calculated by Aspen are convergent,which can satisfy the requirements of industrial production and have certain guiding significance for industrial production.