| Aldehydes are often produced by many industrial processes,such as Fischer Tropsch synthesis,carbonylation synthesis and the production of bio-oils.These aldehydes are often present in a few percent concentrations in target products such as hydrocarbons,limiting the purity of target products.The mixture consisted of heptanal and undecane was selected in this paper.Using sodium bisulfite aqueous solution as extractant,heptanal was separated by reactive extraction from undecane.Firstly,the homogeneous reaction processes between heptanal and sodium bisulfite were monitored by means of ReactIR at 283.15～298.15K.Through the calculation and fitting of the experimental data,the rate constants and equilibrium constants of the reaction at different temperatures were obtained,and the kinetic equation of the reaction was dctermined.The results show that the rate of addition reaction between heptanal and sodium bisulfite is first order for heptanal,sodium bisulfite and sodium α-hydroxyl heptyl sulfonate,and the activation energy of the forward reaction is 34.68 kJ·mol-1,and the pre-exponential factor of the forward reaction is 1.369×107 L·mol-1·min-1,the activation energy of the reverse reaction is 94.69 kJ·mol-1,and the pre-exponential factor of the reverse reaction is 2.500×1015 min-1.The reaction between heptanal and sodium bisulfite is an exothermic process,and the reaction heat is-60.01 kJ·mol-1,and the reaction rate of heptanal increases while the equilibrium conversion decreases with the increase of the temperature.In the sodium bisulfite aqueous solution,heptanal also reacts with water,and the hydration reaction of heptanal is an exothermic process,and the reaction heat is-20.12 kJ·mol-1,and the proportion of hydrated heptanal decreases while the proportion of free heptanal increases with the increase of the temperature.Secondly,the distribution coefficients of heptanal between undecane and water were determined by phase equilibrium experiments.The results show that the distribution coefficient of heptanal can be regarded as a constant only within a specific concentration range,and the distribution coefficient of low heptanal concentration is obviously higher than the distribution coefficient of high heptanal concentration.With the increase of the temperature or the concentration of inorganic salts,the distribution coefficient of heptanal decreases.Surfactant can increase the distribution coefficient of heptanal.Using sodium bisulfite aqueous solution as extractant,the reactive extraction experiments were carried out.The results show that when the concentration of heptanal,the molar ratio between sodium bisulfite and heptanal,the stirring speed,the volume fraction of water phase or the temperature increase,the reactive extraction rate increases.The higher concentration of heptanal,or the larger molar ratio between sodium bisulfite and heptanal,or the smaller volume fraction of water phase or the lower temperature can improve the equilibrium conversion of heptanal.According to the mass transfer coefficients of the reactive extraction systems under the initial condition and the rate constants obtained from the study of intrinsic kinetics,the Ha numbers were estimated.The results show that under the study conditions,the reactive extraction is a mixed control process,that means the effects of chemical reaction and mass transfer cannot be ignored during the reactive extraction.According to the solubility of the reactants in two phases,that is,heptanal is soluble in water while sodium bisulfite is not soluble in undecane,it can be judged that the reaction occurs both within the film and in the main body of the water phase.Finally,using 0.1838 mol·L-1 sodium bisulfite aqueous solution as extractant,under the conditions of 303.15K and water phase volume fraction of 0.5,the heptanal was separated by different types of multistage reactive extraction from undecane.The results show that the multistage reactive extraction can improve the conversion of heptanal compared with single stage reactive extraction.The mass fraction of heptanal could be reduced from 1.0%to 393 ppm,34 ppm and 245 ppm,by three-stage cross-current nonequilibrium extraction with residence time of two hours,three-stage cross-current equilibrium extraction with residence time of five hours and three-stage counter-current equilibrium extraction with residence time of five hours,respectively.Among these multistage extractions,the three-stage cross-current equilibrium extraction can reduce the concentration of heptanal to a lower level and the three-stage counter-current equilibrium extraction consums the least amount of extractant.Because of the long residence time and the ease of stratification,the extraction equipment of reactive extraction should adopt the multistage mixer-settler extractor or the cascade stirred tank reactor. |
