Study On The Improved Technique Of Extraction Water Recovery Process During The Production Of Nylon 6

Nylon 6,also known as polycaprolactam,is a polymer with caprolactam as the monomer.It has the characteristics of high strength,high toughness and high value.It plays an important role in chemical production.In this paper,through the study on the recovery of monomer from the extracted water of caprolactam polyamide nylon 6,combined with the actual operation status of the enterprise,the start-up and stable operation of the monomer recovery system(including three-way evaporation system and concentration and filtration system)of the nylon 6 chip polymerization unit with an annual output of 35,000 tons were fully analyzed and discussed.The main research contents and conclusions are as follows:(1)The improved design of nylon 6 chip polymerization unit was carried out.The two-stage polymerization method was widely used at present.The recovery of extractive water in the main reaction process of converting raw caprolactam into linear polymer with clear water as ring-opening agent under certain temperature and pressure was discussed in detail.The 8~10%liquid containing caprolactam in the extracting water collecting tank is pumped to the evaporation recovery system.After the caprolactam monomer is recovered and concentrated in the system,the concentration of the concentrated liquid can reach more than 70%,but it contains more oligomer impurities.If these oligomers are not removed,they will directly affect the quality of the third-line production products.With the use of the filter,the filter residue can be effectively removed.(2)Experiments show that the savings of heating steam decrease with the increase of efficiency.For example,from single-effect to double-effect,steam can be saved about 48%,but the four-effect can be increased to five-effect,saving only 10%,while the cost of equipment increases with the increase of efficiency.When the efficiency increases to a certain extent,the savings in steam costs will not make up for the increased equipment costs due to the increase in equipment.Therefore,taking into account various factors in chemical production,multi-effect evaporation is generally controlled at 2~3 effects.(3)The concentrating and filtering technology of three-effect evaporation system in monomer recovery system is introduced,and the process parameters of the system are designed and calculated.In addition,the higher the water temperature,the longer the residence time and the larger the water bath ratio,the better the extraction of unreacted monomers and oligomers.The three-effect evaporation system is counter-current method,with forced circulation of bottom liquid and pump.After three-effect evaporation,the monomer water was replaced by traditional distillation method by condensation method.The concentrated solution was filtered out in the automatic filtration system,and the low polymer residues were filtered out,and then re-entered the polymerization system for reuse.According to practical experience,the parameters of water capacity,single water concentration,temperature and pressure are given.The centralized temperature,pressure,heat transfer area and volume of equipment at the inlet and outlet of each system can be obtained,and the process flow chart and equipment work are given.(4)Based on the existing parameters of process equipment of Luxi Chemical Group Company,the concentrating filter was improved,and the filter grille,filter tank,Carousel oxidation ditch and vertical secondary sedimentation tank were designed and calculated.The design of oxidation ditch adopts the design method of delayed aeration tank,which derives the volume of aeration tank from the sludge volume W_V=0,and then arranges it into Carousel type according to the oxidation ditch process.(5)The monomer recovery system is designed,and the stability of the driving data and operation data collected in the field is analyzed.The effects of pressure,material level and concentration fluctuations on the system were studied in detail,and the data were optimized.The process was effectively improved.The concentration of three-effect evaporation concentrate should be controlled at 70%~75%.On the one hand,it can lighten the burden of pre-polycondensation dehydration after pressure filtration system.On the other hand,it can prevent flooding caused by boiling intensified in the two-effect because of low concentration,at the same time,it can prevent blockage of reboiler and outlet pipeline due to excessive concentration.Based on the comparison of experimental data and design data,an improved scheme is proposed for the system:in view of the instability of the three-effect evaporation system,the pump forced circulation mode is adopted instead of the current thermodynamic circulation mode to maintain the stability of liquid level.For the scale formation of the concentration system,the vertical concentration kettle is adopted,and the vertebral structure is adopted at the bottom to avoid the phenomenon of residue accumulation.For the current automatic filtering system,reduce the risk of artificial exercise.