Cracking And Mechanism Of Hydrogenation In Heavy Oil Model Compounds By Liquid Phase Discharge Plasma

The efficient and rational utilization of heavy oil is an important part of carbon neutrality and carbon emissions,and it has always been a hot spot and difficult point of research at home and abroad.Through the process of cracking and hydrogenation,heavy oil can be effectively converted into high value-added oil products.However,due to its high operating temperature,high pressure and long cycle time,there are some limitations such as high cost,high energy consumption and easy coking.And the carbon deposits further lead to catalyst inactivation and bed blockage,which will greatly affect the upgrading of heavy oil.Therefore,it is of great significance and research value to develop a catalyst-free technology for rapid cracking and simultaneous hydrogenation of heavy oil.The pulsed discharge plasma in liquid（PDPL）is one of the advanced technologies for preparing clean energy,which can be obtained by collisions between high-energy electrons and their surrounding molecules at ambient condition.Compared with conventional processes,PDPL is capable of handling a wide range of feedstocks and has the characteristics of fast reaction,low energy consumption and catalyst-free,which can overcome the deficiencies.However,there are many problems in upgrading heavy oil or its compounds by using PDPL technology.In terms of these problems,this thesis focuses on the key problem that exists in plasma-enabled hydrogenation,which mainly solve how to achieve efficient cracking and hydrogenation in heavy oil including the production of light liquid products and the improvement of calorific value in liquids.The specific findings and results are as follows:（1）In this thesis,PDPL technology was used to study heavy oil model compounds.However,it was found that the liquid-phase discharge was difficult to initiate in liquid oil.A method to initiate discharge by adding working liquid or gas to the non-uniform electric field region was used,and the effect of the addition of working substance and its phase state on the plasma generation was examined.The results show that the continuous transportantion of the working liquid or gas to the reactor from the needle-tip strong field or its co-addition to the reactor with the feedstock can achieve direct discharge in liquid oil at a relatively lower voltage.According to the U-I characteristics,the discharge voltage was greatly reduced from40 k V to 5 k V after the introduction of working liquid（deionized water）into the liquid heavy oil（n-hexadecane）,and the introduction of liquid water into a strong field with a non-uniform electric field has the characteristics of reducing the initial voltage,and allowing more feedstocks to participate in the discharge,and making the discharge process more stable.（2）The cracking characteristic of n-hexadecane using liquid-phase pulsed discharge plasma was investigated to explore the effects of parameters（including discharge parameters,electrode distance,the flow rate of water,and discharge time）on the spatial and temporal distribution of the plasma and the reaction process after the introduction of the working liquid under the action of a non-uniform electric field,and the partition control of the strong and weak field regions in the non-uniform electric field was discussed.The results show that,by studying and optimizing the effects of single discharge energy,electrode distance,the flow rate of water and discharge time,it was found that the highest cracking rate of light products reaches 81.32%when the single discharge energy was 2.06 m J,the electrode distance and the flow rate of water were 5.5 mm and 0.55 m L/min,respectively,and the discharge time was 60s.Considering the intermediates and products in liquid or gas phase of cracking process,it was obtained that under the lower input energy,n-hexadecane first generates alkyl radicals（·R）through random C-C bonds breaking in the strong field region in the non-uniform electric field,and these alkyl radicals eventually form stable light hydrocarbons with the active species（·OH,·H,·O）provided by water.When the input energy was increased to break the C-H bond of n-hexadecane,the cetyl radical(·C₁₆H₃₃)might be obtained.Theβ-break of cetyl radicals would generate smaller alkyl radicals（·R）,and these smaller alkyl radicals can be recombined to generate stable light hydrocarbons through hydrogenation,or produce light liquid products by C-C bonds andβ-scission reaction.（3）In-situ hydrogenation of heavy oil model substances was achieved using a non-uniform electric field and partition control of the hydrogen-rich working material.The color change,the calorific value and the H/C of the liquid products were analyzed before and after the plasma treatment.The results showed that both the color and chemical composition of the liquid products of three heavy oil model compounds including n-hexadecane,decalin and o-xylene,changed after plasma hydrogenation,and the calorific value and H/C were enhanced under the plasma action,which indicated that additional hydrogen was added to the products by plasma.The optimal results for the hydrogenation of three model compounds are as follows:in n-hexadecane,the increment of apparent calorific value and the growth rate of converted part increased by 276.56 J/g and 8.55%,respectively,and the H/C_ratioincreased from 1.95 to 2.09;in decalin,the increment of apparent calorific value and the growth rate of converted part increased by 354.30 J/g and 14.81%,respectively,and the H/C_ratioincreased from 1.82 to 1.84;in o-xylene,the increment of apparent calorific value and the growth rate of converted part increased by 275.86 J/g and 6.98%,and the H/C_ratioincreased from 1.23 to1.33,respectively.（4）The evaluation and study of the effect of in-situ hydrogenation of alcohols was carried out,and the reaction path was analyzed in depth.The results show that the addition of alcohols into n-hexadecane can significantly increase the calorific value of the liquid and achieve in-situ hydrogenation by liquid-phase discharge plasma.When the working substance is 75%methanol aqueous solution,the apparent calorific value reaches the maximum value of874.84 J/g,and its total energy conversion efficiency is as high as 93.07%.It is proved that the method can achieve efficient hydrogenation and reforming with low energy consumption and has potential application prospects.Based on the analysis of products in gas or liquid phase and intermediates,a series of reactions that may be triggered by the collision of high-energy electrons with surrounding n-hexadecane molecules and methanol water molecules in a non-uniform electric field are discussed.This study provides strong evidence for further improvement of the hydrogenation performance of the plasma.This study points out the direction for the design of complete sets of equipment for efficient utilization of heavy oil by plasma,and provides a theoretical basis for the improvement of heavy oil upgrading and hydrogenation performance.