Experimental Study On Catalytic Cracking Of Waste Plastics To Produce Hydrocarbons

With the increasing pollution of the natural ecological environment,the country has paid more and more attention to ecological development and the development and utilization of green energy at the strategic level.The recycling and utilization of biomass waste has become a research hots pot in the field of environmental and ecological protection.To promote green development and reduce agricultural pollution,in order to achieve the goal of"controlling agricultural film pollution in agriculture and improving the efficiency of agricultural film recycling and utilization"proposed in the Outline of the 2035 Vision Goals,it is very important to recycle and reuse waste plastic from agricultural production as a resource.This paper conducts an experimental study on the catalytic cracking of polyolefin waste plastics to produce hydrocarbons using HZSM-5 and NaY molecular sieves as raw materials.The research content is as follows:（1）Using NaY molecular sieve and HZSM-5 molecular sieve as catalysts,and using low density polyethylene（LDPE）,polypropylene（PP）and polystyrene（PS）as raw materials,the catalytic cracking reaction tests were carried out.The effects of catalyst type,temperature,feedstock and catalyst mass ratio on catalytic cracking liquid product yield,liquid acid value,liquid density,liquid kinematic viscosity,catalyst coking rate and combustible gas yield were mainly studied.The test determined that the optimum reaction temperature of LDPE,PP and PS was 400℃,420℃and 420℃respectively,and the optimum catalyst type was NaY molecular sieve.The yield of combustible gas generated was small,which was conducive to the formation of liquid products.Under the condition that the plastic raw material is completely reacted and the liquid product yield is high,the optimum mass ratio of raw material to catalyst is 1:2.Through GC-MS comparative analysis of liquid products obtained from three kinds of mono-component polyolefin raw materials under the optimum reaction conditions,it can be seen that when the reaction temperature of low density polyethylene is 400℃and the reaction temperature of polypropylene is 420℃,and the quality of raw materials and catalyst is 1:2,the main components of liquid products are hydrocarbons,of which the relative content of alkanes is the highest,55.18%and 57.28%,respectively,The catalytic cracking of LDPE and PE raw materials is more likely to produce alkanes.When the reaction temperature of polystyrene is 420℃,the quality of NaY catalyst,raw materials and catalyst is 1:2,the main components of liquid products are also hydrocarbons,but the relative content of aromatic hydrocarbons in hydrocarbon products is up to 76.67%.The catalytic cracking of polystyrene raw material is more likely to produce aromatic hydrocarbons and the relative content is relatively high.（2）With three kinds of mixed agricultural waste plastics as raw materials,when the optimal reaction temperature is 440℃,the optimal catalytic reaction time is 40min,and the optimal mixing ratio of raw materials（agricultural film,woven bag,foam board）is 2:1:2,the product yield is 71.4%,the lowest acid value is 0.75mg KOH·g^-1,the kinematic viscosity is 0.78 mm2·s^-1and the density is 0.79 g·cm^-3.According to the GC-MS analysis of hydrocarbons in the liquid products accounts for98.89%,and a small amount of alcohol,naphthalene and other substances are also contained.The highest relative content is 47.75%,and the highest component is ethylbenzene,with a relative content of 16.35%.The relative content of alkanes is45.85%.The highest content of heptane is 11.85%.The relative content of alkene is5.29%,and the highest content of hexene is 3.74%.The test results show that the liquid products prepared by catalytic cracking after mixing the raw materials at 1:2:2are mainly hydrocarbons,and the proportion of aromatic hydrocarbons and alkanes is relatively large,both of which reach 93.6%.The products obtained from catalytic cracking of polyolefin mixed waste plastics include not only saturated hydrocarbon compounds,but also products of other functional groups in the liquid products,such as alcohol,naphthalene and other products with high added value,so as to better realize the recycling and effective utilization of polyolefin agricultural waste plastics.（3）Taking the catalytic cracking test factors of mixed waste plastics as the analysis conditions,Based on the test factors of mixed waste plastics,the echo surface analysis test with 3 factors and 3 levels was designed by Box-Behnken method.Analyze the significance of test factors,and carry out multiple regression fitting of the data to obtain the fitting regression equation between Chemical reaction temperature of A,the reaction time of B,the mixing ratio of C raw materials and the response value of liquid product yield Y.The order of significance of experimental factors on liquid product yield is from high to low:raw material mixing ratio（C）>B2>A2>AC>BC>reaction temperature（A）>AB>C2>catalytic reaction time（B）.According to the response surface analysis,the reaction temperature in the test factors is 440℃,the catalytic reaction time is 40 min,and the raw material mixing ratio is 2:1:2,which reaches the local optimal solution,that is,the maximum liquid product yield is 69.81%.The catalytic cracking reaction of mono-component polyolefin raw materials follows the free radical chain reaction mechanism.The condensation reaction of small molecular free radicals generates olefins,and the bond of large molecular free radicals breaks directly to form alkanes.The catalytic cracking of mixed waste plastics follows the carbocation reaction mechanism.The mixed waste plastic raw materials mainly undergo alkane isomerization,cyclization,aromatization,hydrogen transfer and other reactions.As a result,liquid products contain more hydrocarbons and aromatic hydrocarbons.The study of pyrolysis kinetic equation shows that the activation energy Ea of pyrolysis kinetic reaction calculated the data shows that the order of activation energy of each single component raw material from high to low is:low density polyethylene(2.01k J·mol^-1)>polypropylene(1.91k J·mol^-1)>polystyrene(1.82 k J·mol^-1).The reaction activation energy E_bequal to 2.11 k J·mol^-1of the mixed waste plastics is lower than the linear superposition value of the activation energy E_aof the three single components.The composition proves that there is a mutual promotion between the components during the mixed cracking of waste plastics.In total,the research results of this paper can provide research study of catalytic cracking of single component polyolefin waste plastics and mixed agricultural waste plastics to study Physical and chemical properties of hydrocarbons and liquid products.