Study On Pervaporation Dehydration Of Polyoxymethylene Dimethyl Ethers Products And Inhibition Of Formaldehyde

As a new type of clean diesel fuel additive,polyoxymethylene dimethyl ethers(PODEn)have the advantages of high cetane number,high oxygen content and can be reconciled with diesel oil in a certain proportion.However,In the process of synthesizing polyoxymethylene dimethyl ethers with high concentration formaldehyde solution and methanol,there are shortcomings that the high concentration of formaldehyde is easy to self-polymerize and the water that produced in the reaction is difficult to be removed.In this paper,?-cyclodextrin,which has internal hydrophobic and outer hydrophilic structure,was added to the high concentration formaldehyde solution to prevent the self-polymerization of formaldehyde molecules.The NaA zeolite membrane was used for pervaporation to remove the water which is produced in the process of synthesizing polyoxymethylene dimethyl ethers.In the experiment with high concentration of formaldehyde in the addition of?-cyclodextrin,the effects of formaldehyde concentration,?-cyclodextrin addition,constant temperature and centrifugal conditions(centrifugal time and centrifugation rate)were investigated.It was found that the higher the concentration of formaldehyde,the lower the amount of?-cyclodextrin,the lower the constant temperature,the easier the polymerization of formaldehyde solution.The centrifugal conditions(centrifugal time and centrifugal velocity)have some effect on the determination of formaldehyde polymerization.In the experiment of removal of water from polyoxymethylene dimethyl ethers by pervaporation of NaA zeolite membrane,the effects of operating temperature,feed rate,membrane pressure,and water content in the feed solution on water enrichment factor and membrane flux were investigated by using a single variable method to carry out the pervaporation experiment with water content 4.94 wt%in polyoxymethylene dimethyl ethers solution.The optimum operating conditions which were optimized by Design Expert software were as follows:operating temperature 80?,feed flow rate 0.8 L/min,membrane pressure300 Pa.The water enrichment factor was 18.26,and membrane flux was 884.27 g/(m~2·h).In the best optimal operating conditions(operating temperature 80?,feed flow rate 0.8L/min,membrane pressure 300 Pa),NaA zeolite membrane pervaporation stability tests were carried out with water content 6.95 wt%in polyoxymethylene dimethyl ethers solution.The effects of water content,water enrichment factor and membrane flux on the time in membrane tube series or parallel were investigated.It was found that the water content,water enrichment factor and membrane flux changed with time in the same way however the membrane tubes were connected in series or parallel way:Membrane flux and water content with time prolonging were decreased firstly and then gradually;The water enrichment factor increased with time.Compared with the series of cases,the feed water content,water enrichment factor and membrane flux of the membrane tube in parallel due to the larger effective membrane area,can be faster to achieve balance.