Studv On Optimization Of Industrial Preparation And In-situ Synthesis Of NaA Zeolite Membranes

Zeolite NaA has strong hydrophilicity,high mechanical strength,uniform pore structure and chemical stability.Due to these advantages,NaA zeolite membranes have been industrialized in the field of organic solvent dehydration by pervaporation(PV).Reported methods for synthesizing NaA zeolite membranes include secondary growth method,microwave method,in-situ synthesis with support modification and multiple in-situ synthesis.However,at present,the industrial application of NaA zeolite membranes still has high preparation costs and few formulas,so the study of membrane preparation technology is still of great significance.This paper studied two aspects of NaA membranes:first,because the wet-rubbing seeding method proposed by our research group has high repeatability,we conducted industrial scale-up research of this method to reduce the production costs;second,the secondary growth method has complex coating steps.In order to solve this problem,we conducted in-depth research of in-situ synthesis,using ceramic plates as supports to explore a more simple preparing method.The main research contents and results are as follows:(1)Optimization preparation of NaA zeolite membranes for industrial applicationIn this work,we used cheap domestic raw materials and macroporous alumina tubes to synthesis NaA membranes by secondary growth method.The effects of alkalinity,water content and raw materials on the properties of the membranes were explored and a broader range of formulation conditions have been identified.The properties of membranes were characterized by SEM,PV and other means.It is found that as the w value of the water content in xNa20:1Al2O3:2SiO2:wH2O increased from 110 to 190,the separation factors of the NaA zeolite membranes first increase and then decrease.Dense NaA membranes can be obtained when the w value is at the range of 140-180,and its separation factor for pervaporation of 90%ethanol/H2O is higher than 8000.When the value of alkalinity x is in the range of 2.4-2.7,NaA membranes with a separation factor higher than 5000 can be prepared.And when x=2.5(slightly higher than the minimum alkalinity),the prepared NaA membrane has better separation performance.In addition,this work use different industrialized aluminum sources,and found that when the alkalinity is kept to a minimum value(which means NaOH is not added),and the water content is 150,high separation performance NaA membranes can be obtained.The above exploration provides a wider range of preparation conditions and raw materials for industrial preparation.The investigation of scale-up revealed that this formula is not affected by the Teflon liner in the autoclave,but different PV devices will have a relatively large impact on separation performance.Due to the existence of concentration polarization,the flux of membrane is low for pervaporation in the tubular membrane module.Finally,the NaA membranes with high separation performance are prepared on the 80 cm large pore alumina tubes by using optimized scale-up preparation conditions.Compared with the commercial NaA zeolite membranes,the flux of the membrane in this work is 2 times higher,and the separation factor is also higher.(2)Research on synthesis conditions of in-situ synthesis of NaA zeolite membranes and its formation process.In this work,we used cheap domestic raw materials and perforated ceramic plates to synthesis NaA membranes by in situ synthesis method in the clear synthesis solution.The synthesis conditions included the alkalinity,the aging time of the synthesis solution,the synthesis time,and the synthesis temperature.Besides,the formation process of the membrane was analysed through different supports and treatment of the supports.The properties of membranes were characterized by XRD,SEM and PV.It is found that alkalinity is a sensitive factor for in-situ synthesis of NaA membranes.Only when the synthesis solution is(39/38.5Na2O:4SiO2:1Al2O3:700H2O),the separation factor of the membrane synthesized at 80? for 4 h could be above 10000.The effect of aging time on the formation of a dense membrane is not obvious,but with the aging time increased(0-1.5 h),the flux of the NaA membranes gradually increased.This may be because long aging time helps the synthesis solution to crystallize and nucleate more quickly during the reaction.The surface of the membrane changed from a gel layer to a NaA crystal membrane layer as the reaction time increased(0-4 h).A NaA membrane with a separation factor higher than 10000 could be prepared after aging 1 h and reacting for 3 h.The synthesis temperature is studied,and it is found that when the reaction temperature is 70?,a NaA membrane with high separation proformance can also be prepared by aging for 1 h and reacting for 4 h.In addition,we found that the NaA zeolite membranes synthesized by in-situ method had a lower flux than the membrane synthesized by the secondary growth method.We also found that the supply method of the synthesis solution has a great influence.When only one side of the carrier contacts the synthesis solution,a dense membrane cannot be obtained.When the synthesis solution is supplied on both sides of the support,a dense membrane with high performance can be obtained;even if the inner membrane is polished,only the outer NaA membrane remains,the separation factor is still higher than 10,000,but the flux has not increased significantly.That is,the presence of the inner membrane has no effect on the high separation factor of the membrane,nor does it significantly decrease the flux.We speculate that the supply of the synthesis solution from the inside of the support and the pressure balance between two sides during the synthesis process is very important for the formation of a high-performance membrane.