Studies on the sulfonated carbon nanotubes catalyst and membrane reactor for biodiesel production
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Date
2015-01-01
Authors
Shuit Siew Hoong
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Abstract
This study focused on the synthesis of biodiesel using sulfonated multi-walled carbon nanotubes (s-MWCNTs) as catalyst and pervaporation membrane reactor as the conversion technology. First, s-MWCNTs were synthesized and utilized as catalysts to transform palm fatty acid distillate (PFAD) into biodiesel. The biodiesel yields achieved by the s-MWCNTs prepared via thermal treatment with concentrated sulfuric acid, the in situ polymerization of acetic anhydride and sulfuric acid, the thermal decomposition of ammonium sulfate ((NH4)2SO4) and the in situ polymerization of poly(sodium4-styrenesulfonate) were 78.1 %, 85.8 %, 88.0 % and 93.4 %, respectively. Sulfonation via the thermal decomposition of (NH4)2SO4 was the most suitable method to prepare s-MWCNTs because it is a facile and acid-free method. Next, the effects of the concentration of (NH4)2SO4 solution and the ultrasonication period of MWCNTs in the (NH4)2SO4 solution were studied and optimized. The results showed that the best performance of the s-MWCNTs was obtained by ultrasonicating the purified MWCNTs in a 10 wt% (NH4)2SO4 solution for 10 min and heating at 235 °C for 30 min. s-MWCNTs prepared by this method demonstrated good thermal stability, good dispersibility in methanol and high Brunauer-Emmett-Teller (BET) surface area coupled with a large pore width. Then, the optimized s-MWCNTs were subjected to process parameters study, kinetic study, catalyst reusability and regeneration study to reveal the potential of s-MWCNTs as a catalyst for biodiesel production. The process parameters studied included the methanol-to-PFAD ratio (8 – 30), catalyst loading (1 – 3 wt %), reaction temperature (80 – 200 ºC) and reaction time (1 – 5 h). A high FAME yield of 93.5 % was obtained at a methanol-to-PFAD ratio of 20, catalyst loading of 3 wt %, reaction temperature of 170 ºC and reaction time of 2 h. The s-MWCNTs exhibited good catalytic activity, with a FAME yield higher than 75 % even after 5 repeated runs. The regeneration of the spent s-MWCNTs (after 5 runs) with sulfuric acid was able to restore the catalytic activity to its original level. A pseudo-homogeneous kinetic model for the esterification of PFAD with methanol using s-MWCNTs as a catalyst was then developed based on the experimental results. The pre-exponential factor, molar heat and activation energy for the esterification were found to be 1.9 × 102 L mol-1min-1, 84.1 kJ mol-1 and 45.8 kJ mol-1, respectively. Then, the polyimide, copoly(1,5-naphthalene/3,5-benzoicacid-2,2’-bis(3,4-dicarboxyphenyl) hexafluoropropanedimide (6FDA-NDA/DABA) was synthesized and modified via thermal cross-linking to serve as a membrane in membrane reactor. At 10 h of reaction time, the thermally cross-linked 6FDA-NDA/DABA polyimide membrane was able to remove 94.8 % of the generated water from the reaction mixture. The high removal percentage of water by the polyimide membrane has triggered a 17.9 % increment of FAME yield achieved by pervaporation membrane reactor as compared to the FAME yield achieved by the batch reactor under the same reaction conditions. The thermally cross-linked polyimide membrane was a hydrophilic membrane which demonstrated negligible swelling degree in the reaction mixture and high thermal stability under high reaction temperature and pressure.