Study On The Recycle Process And Mechanism Of N-methylmorpholine-N-oxide(NMMO) As A Solvent For The Lyocell Fibers Production

In this thesis, the recycle process and mechanism of N-methylmorpholine-N-oxide （NMMO） as a solvent for the lyocell fibers production have been studied. In order to select the optimum recycle process parameter and use NMMO repeatedly, a new recycle process has been developed and the mechanism of the recovery of NMMO has also been investigated.First of all, the effects of process parameters on the degradation of NMMO and cellulose in the system NMMO·H₂O /cellulose were studied; The quantitative determination of NMMO, N-methylmorpholine （NMM） and morpholine （M） was investigated. The quantity of main NMMO degradation products, i.e. NMM, M, under different conditions were discussed. A high performance liquid chromatography （HPLC） method for the determination and quantification of NMMO, NMM and M was also developed. A study on composition, structure and the sizes of the deformable floccule particles in the coagulation bath has been carried out; The influences of water quality on the properties of fiber and the sizes of the deformable floccule particles were discussed. The preparation mechanism and process of the ZrO₂ ceramic membrane, and the effects of applying the ZrO₂ ceramic membrane on NMMO recovery in lyocell fiber process were further investigated. The effects of operating conditions on removing the colored impurity, metal ions and morpholine （M） by ion exchange resin were investigated and the mechanism of this process was also studied. In addition, the processing conditions and mechanism of the discoloring oxidation reaction and how to oxidize the N-methylmorpholine to N-methylmorpholine-N-oxide were also discussed. The main results obtained are as follows:1. The study of the degradation and stabilization in cellulose/NMMO/H₂O mixtures indicates that the acid environment and the elevated temperature increases the instability of NMMO, the degradations of NMMO and cellulose increase with heating time, the decompositions of NMMO and cellulose are further accelerated by metal ions. If the concentration of metal ions reaches 600 ppm in the system of NMMO·H₂O /cellulose, more than 30% of NMMO will be decomposed, while cellulose is degraded severely. It is shown that the thermal instability of NMMO/cellulose system in the presence of metal ions would be increased remarkably, even an uncontrollable decomposition process could occur. Propyl gallate could stabilize the NMMO/cellulose system better than hydroquinone and ellagic acid. The mechanism of stabilization using propyl gallate is that it acts as a free radical scavenger agent and inhibits the degradation of cellulose by preventing free radicals to attack cellulose. On the other hand, propyl gallate can be oxidated into highly conjugated bis（ortho-quinone） which is deeply colored. It represents one major reason for the discoloration of concentrating NMMO in the presence of propyl gallate. The optimum concentration of propyl gallate is 0.4wt% in order to stabilize NMMO/cellulose system, propyl gallate in combination with NaOH can not only counteract the degradation of NMMO and cellulose but also considerably increase the thermal stability of NMMO-cellulose-water solution.2. A HPLC method for the separation and quantitative determination of NMMO,NMM and M from the coagulation bath has been established. The separation is carried out on a Xterra^TM C₁₈ column. A standard method is employed to optimize the mobile phase, the mobile phase composition is V （0.02mol/L sodium carbonate and sodium bicarbonate buffer containing 0.55% triethylamine, pH10.54） :V （acetonitrile） =97:3. The detection is performed at 218nm and the injection volume is 10μl. A baseline separation is achieved within 4 min and the retention times for NMMO, M and NMM are 1.231 min, 1.924 min and 2.726 min respectively. NMMO,NMM and M show good linearity in the ranges of 50 mg/L to 300 mg/L, 20 mg/L to300 mg/L and 20 mg/L to300 mg/L respectively. The average recoveries is 96.85±0.48% for NMMO, 98.41±0.45% for NMM, and 98.61+0.40% for M. The detection limits for NMM0,NMM and M are 50 mg/L, 20mg/L and 20mg/L, RSD is less than 3.6%, 3.4% and 3.1% respectively. HPLC method is rapid and accurate, and suitable for control of the recovery of NMMO.3. The composition and structure of the deformable floccule particles in coagulation bath and the effect of water quality on fiber properties are investigated, and the sizes of the deformable floccule particles are also determined. The deformable floccule particles in the coagulation bath show a main characteristic of cellulose from IR spectrum. The deformable particles average size in coagulation bath is bigger than 1μm whatever the coagulation bath is tap water or distilled water. Compared with the distilled water, the whiteness, tenacity and elongation at break of the fiber made from using tap water as coagulation bath decrease 15.5%, 9.5% and 7.4% respectively.4. The preparation and application of ZrO₂ ceramic membrane in NMMO recycle process. The results show that ZrO₂ powder with size bigger than 5μm and too high or too low concentration of ZrO₂ powder are not suitable to for membrane. The best concentration for ZrO₂ membrane is 0.7-0.8g/l. The permeation flux increases with the cross-flow velocity, and the optimum cross-flow velocity for forming membrane is 0.2m/s. It is found permeation flux will be stable 30 minutes after forming membrane, so the time of forming dynamic membrane should be 30 minutes. The deformable particle sizes are big if the coagulation bath is filtrated through a membrane by using the ZrO₂ powder whose average size is 4.98μm. On the other hand, if the coagulation bath is filtrated through a membrane made by ZrO₂ powder with average size of 1.90μm or 1.15μm, the deformable particle sizes are small and can meet the demand of the recycle process. In addition, with the applied pressure increasing, the permeation flux increases rapidly at first, increases slightly and finally the applied pressure reaches 150kPa.5. The anion exchange resin X can not only remove the deformable floccule particles in the coagulation bath but also decolorize the coagulation bath, while the cation exchange resin Y can selectively remove morpholine（M） as NMMO degradation product and eliminate metal ions like iron, copper etc, at the same time the cation exchange resin Y has a good fatigue resistance. The regeneration process of the aqueous solutions of NMMO is developed, that is, the coagulation bath to be recycled is treated by the anion exchange resin X firstly, then is treated by the cation exchange resin, the above process discolors the coagulation bath, removes morpholine（M） and eliminates metal ions, and is easy to operate, can be used repeatedly. The recycled NMMO aqueous solution can completely meets the demand of the industrial production for lyocell fibers. In addition, the properties of fiber which is made by recycled NMMO are almost equal to the fibers which are made by virgin NMMO.6. The factors which influence the discoloring oxidation reaction of the coagulation bath are investigated. The optimum processing condition for the discoloring oxidation reaction is obtained. The results indicate that the regenerating temperature strongly affects the rate and extent of the oxidation reaction and it is found that the optimum regenerating temperature is 70℃. The amount of H₂O₂ mainly affects the remains of H₂O₂ besides influencing the rate and extent of the discoloring oxidation reaction. On the other hand, the NMMO concentration of the coagulation bath is not the main factor which influences the discoloring oxidation reaction. Finally the optimum processing conditions of the discoloring oxidation reaction are that, the regenerating temperature is 70℃, hydrogen peroxide added is 1.0% by weight of the total solution, the pH value of the coagulation bath is 8, and the NMMO concentration of the coagulation bath is 12.2%. The final processing condition of oxidizing N-methylmorpholine to N-methylmorpholine-N-oxide are that, the pH value of the coagulation bath is 8, the oxidation reaction temperature is 70℃, the oxidation reaction time is 5 hours, and hydrogen peroxide employed is 1.2 mole per mole of N-methylmorpholine. In addition, approximately 85% N-methylmorpholine can be converted into N-methylmorpholine-N-oxide.7. The final recycle process of solvent NMMO for the lyocell fibers production has been established:Coagulation bath（concentration of NMMO is 10%（w/w））â†'rough filtratedâ†'fine filtrated （processed by ZrO₂ ceramic membrane）â†'treated with anion exchange resinâ†'treated with cation exchange resinâ†'oxidized by H₂O₂â†'concentrated into 50% NMMO aqueous solution （w/w） .