Fundamentals And Processes For Separation And Purification Of Levulinic Acid

Levulinic acid (LA), which can be produced cost effectively and in high yield from renewable biomass, has the potential to become a new green platform chemical. Systematic studies of the separation and purification of LA have not been reported, and the uptake of carboxylic acids by basic polymeric adsorbents has been employed extensively but the theoretical foundation is imperfect so far. Therefore, the theory and process of the separation and purification of LA were studied in this thesis. The details in this work are summarized as follows.1. The adsorption equilibrium of LA onto three basic polymeric adsorbents with different matrix was investigated. The isotherm models and the thermodynamic properties of the adsorption process were analyzed. It was found that the adsorption capacity of LA on three resins decreased with increasing solution temperature and pH. Toth isotherm model can correlate the adsorption isotherm very well in a broad range of temperature and concentration. The adsorption is an endothermic process, and the isosteric heat of adsorption increases with increasing amount adsorbed.2. The binary-solute competitive adsorption equilibrium experiments of LA and formic acid, the key impurity, were carried out. The results show that formic acid is more strongly adsorbed than LA due to the stronger acidity. Formic acid is adsorbed preferentially and is able to displace LA from the adsorption sites effectively. The IAS model coupled with the isotherm equations derived from single-solute adsorption data was applied to predict the adsorptive behavior of LA/formic acid mixtures. The prediction results were fairly good, especially for the more retained formic acid.3. The adsorption kinetics of LA onto basic polymeric adsorbents was studied by batch adsorption experiments. The pore diffusion model and parallel diffusion model, both taking into account external mass transfer, were used to analyze the uptake kinetic data. Although the pore diffusion model could simulate the kinetic data very well, the correlated apparent pore diffusivities were erroneously large, sometimes even larger than the molecular diffusivity of LA. This is contradictory to thewell-established theory of diffusion in porous media. The parallel pore and surface diffusion model gave a better description of the adsorption kinetics of LA in resin particles. The resulting surface diffusivities decreased with increasing initial LA concentrations, the inverse concentration dependence was correlated reasonably well to the change of isosteric heat of adsorption as amount adsorbed.4. The effect of inlet concentrations, flow rates and resin bed heights on the breakthrough curves of LA in the fixed bed was investigated. Using the fundamental adsorption equilibrium and kinetic parameters obtained from batch experiments, the general rate model was applied to predict the breakthrough data. Results reconfirm that intraparticle diffusion is governed by parallel pore and surface diffusion, and surface diffusivity decreases with increasing amount adsorbed. It can be concluded that the external mass transfer coefficient can be correlated to the Wilson-Geankoplis equation and the axial dispersion coefficient can be estimated by the Chung-Wen equation for short beds, axial dispersion can be neglected when the bed height reaches to 5.5cm.5. On the basis of the above-mentioned results and various experimental attempts, a new process was developed for recovery of LA from sugar hydrolysate. The process consists of three steps: column chromatography of basic polymeric adsorbents, then decoloration by macroreticular adsorptive resins, and finally adsorption separation by fixed-bed of granular activated carbon. In the first step, formic acid, the key impurity in the hydrolysate, was used as displacer to elute LA adsorbed on the resins in order to avoid introducing new impurity. In the third step, granular activated carbon was used to separate two carboxylic acids for the first time. LA was eluted with ethanol-water solution after formic acid was totally desorbed with hot water, LA and formic acid can be separated completely after only one cycle, therefore it exhibits great advantages.