Study On Deep Desulfurization Of Diesel Fuel By Hi-efficient Oxidation Systems

The deep desulfurization of diesel fuel has become an urgent task for the oil refinery at present due tothe more stringent standard limiting the sulfur level in diesel fuel. Conventional hydrodesulfurization(HDS) process must be carried out under severe operating conditions to achieve the lower sulfur content indiesel fuel. HDS process is confronted with a technical and economic problem. Therefore, development ofthe economical and effective desulfurization process has been paid a great attention at home and abroad.Oxidation desulfurization(ODS) can be carried out under very mild conditions.The process can achievea high desulfurization efficiency and capital investment is low. Consequently, ODS has become animportant research subject. The paper uses the model compounds and diesel fuel as objects. Oxidationdesulfurization performance and factors of H₂O₂/HCOOH, KMnO₄/HCl and Fe(â…¥) system areinvestigated respectively. The oxidation reaction mechanism and the reaction kinetics are studied. Theinnovation research results are acquired.The following are the three parts of the paper.1．H₂O₂/HCOOH system for deep oxidation desulfurizationOxidation desulfurization of diesel fuel can be mostly conducted by using H₂O₂ as oxidizing agentsand employing organic acid, heteropolyacids or inorganic solid catalysts. A large amount of H₂O₂ isconsumed. The effect of oxidation desulfurization is enhanced. The amount of consumed H202 is reduced.Phase transfer catalyst and active carbon are used to catalyze oxidation desulfurization in H₂O₂/HCOOHsystem. This is to enhance effect of oxidation desulfurization and to reduce the H₂O₂ amounts andoperational costs.(1) Oxidation desulfurization of model compounds (such as benzothiophene (BT) anddibenzothiophene (DBT)) and diesel fuel in H₂O₂/HCOOH system are investigated. The results indicatethat addition of phase transfer catalyst to H₂O₂/HCOOH system can increase the effect of oxidationdesulfurization and reduce the amount of consumed H₂O₂. The time of addition of phase transfer catalysthas a great affect on catalytic oxidation desulfurization. For the addition of phase transfer catalyst at initialstage of reaction, removal rate of BT and DBT appears in a lowing, phase transfer catalyst does not showactivity. Removal rate of BT and DBT reach a maximum when phasetransfer catalyst is added at reaction35min.Addtion of phase transfer catalyst after 35min results in a decrease of removal rate of BT and DBT.(2) For oxidation of sulfur compounds in synthesized oil and diesel fuel, catalysis of active carbonshows mainly as follows: (â…°) The hydroxyl radicals (OH. and HO₂·), H₂O₂ decomposing catalyzed byactive carbon, are stabilized through conjugating on active carbon. Thus, sulfur compounds are oxidizedefficiently. (â…±) Active carbon plays a role of phase transfer catalyst because active carbon can adsorbstrongly H₂O₂ and disperse into oil phase, supplying with a bigger contact area. (3) Active carbon is modified with (NH₄)₂S₂O₈ and concentrated H₂SO₄ respectively. Compared tounmodified active carbon, oxidation desulfurization of synthesized oil and diesel fuel is enhanced markedlyusing modified active carbon as catalyst. Active carbon is characterized through nitrogen adsorption andBoehm titration and so on. The results show that surface chemistry and structure properties of modifiedactive carbon were changed obviously. After active carbon was modified, the specific surface wasdecreased slightly. The middle pores were increased. The content of acidic groups were increased markedly.The content of acidic groups such as carboxyl on active carbon modified with (NH₄)₂S₂O₈ was highestamong characterized active carbons. The increasing of the acidic groups and the middle pores on activecarbon displayed to advantage for oxidation removal of sulfur compounds.2. KMnO₄/HCl system for deep oxidation flesulfurizationThe KMnO₄/HCl system, a low-cost strong oxidation system, is made up of potassium permanganateand hydrochloric acid. The activity of oxidation desulfurization of KMnO₄ can be enhanced with adjustingpH of reaction medium, adding phase transfer catalyst and exposing to infrared radiation. The conditions ofoxidation desulfurization are optimized.(1) Oxidation of DBT and BT with KMnO₄/HCl system is investigated. The results indicate that pH ofthe system, amount of oxidizing agent and reaction temperature have a great effect for the activity ofoxidation desulfurization. Oxidation desulfurization takes place easily with addition of phase transfercatalyst. At the same time, amount of oxidizing agent decreases.(2) Kinetics of oxidative reaction is studied. Oxidation of DBT can be treated as a first-order reaction.The apparent activation energy and exponential factor is 55.23kJ/mol and 4.88×10⁸min^-1 respectively. Thisshows that oxidation desulfurization takes place easily with KMnO₄.(3) Based on the study for oxidation of model compounds, oxidation of diesel fuel with KMnO₄/HClsystem was investigated. The results show that organic sulfur compounds are removed efficiently fromdiesel fuel using KMnO₄/HCl system under condition of reaction temperature 25℃, V(oil) : V(HCl)=3: 4,reaction time 60min, n(KMnO₄): n(S)=6: 1, amount of phase transfer catalyst 0.006mol/L(oil) andpH=0.2. Sulfur removal rate of oxidized diesel fuel is 97.4ï¼….(4) The activity of oxidation desulfurization of diesel fuel in KMnO₄/HCl system increases byexposing to infrared radiation. Oxidized diesel fuel was washed with water by avoiding extraction. Sulfurremoval rate of oxidized diesel fuel reaches 89.2ï¼…. Therefore, extraction solvent and recovery cost aresaved. Equipment investment is saved due to simplification of the process. As a result, cost for producingdiesel fuel with ultra-low sulfur content decreases. At the same time, the amount of consumed oxidizingagent reduces and reaction time is shortened became of infrared radiation.3. Fe(â…¥) system for deep oxidation desulfurizationFe(â…¥) compound is a powerful oxidizing agent with transferring of 3 electrons. Its redox potential ishigher than KMnO₄, O₃ and Cl₂. Most organic substance can be oxidized with Fe(â…¥) compound. Oxidation activity of Fe (â…¥) compound can be adjusted by modifying their structure and pH value ofmedia. The reduction of Fe(â…¥) results in a relatively non-toxic by-product Fe(â…¢) compound, whichsuggests that Fe(â…¥) compound is an environmentally friendly oxidant. Performance of oxidationdesulfurization of Fe(â…¥) compound is investigated.(1) Influence factors for preparation of Fe(â…¥) desulfurizer are studied in detail. Preparation conditionof Fe(â…¥) desulfurizer are optimized.(2) Oxidation of BT and DBT in model oil by K₂FeO₄ in water-phase, in organic acid and in thepresence of phase-transfer catalysts is investigated, respectively. The results show that the activity ofoxidation of BT and DBT is low because K₂FeO₄ reacts rapidly with water to form brown Fe (OH)₃ to loseability of oxidation of organic sulfur compounds. The activity of oxidation of the BT and DBT increasesmarkedly in acetic acid. Conversions of the DBT and BT increase from 15.2ï¼…and 13.7% to 60.3ï¼…and54.2ï¼…, respectively.(3) The addition of the solid catalyst to the acetic acid medium promotes very remarkably oxidation oforganic sulfur compounds. Amount of oxidizing agent reduces. The reaction temperature in the highactivity was decreased from 35℃to 20℃. DBT and BT removal rates reach 98.4ï¼…and 70.1ï¼…respectivelyunder reaction conditions of room temperature, atmospheric pressure, V (acetic acid) : V (oil =0.8, n(FeO₄^2-): n(S)=1:1 and n(KM): n (FeO₄^2-)=1: 1.(4) Reaction kinetics of oxidation of BT and DBT is studied. The kinetics equation of oxidations ofBT and DBT are respectively as follow:r_BT=4.285×10⁸exp(-51.12×10³ï¼RT)×C_BTr_DBT=4.693×10⁶ exp(-33.49×10³ï¼RT)×C_DBTApparent activation energies of oxidation of BT and DBT are 51.12 kJ/mol and 33.49kJ/molrespectively, k_0(BT)is 4.285×10⁸min^-1, k_0(DBT)is 4.693×10⁶min^-1. Activation energy of oxidationdesulfurization is lower than that of hydrodesulfurization. This indicates that oxidation of sulfurcompounds such as thiophene takes place more easily than the hydrogenation.(5) Based on the study for oxidation of model compounds, oxidation of diesel fuel with Fe(â…¥) system,consisting of Fe(â…¥) compound, catalyst KM and acetic acid, is investigated. The conditions for oxidationdesulfurization of diesel fuel are optimized. Sulfur removal rate and yield of diesel fuel reach respectively96.7ï¼…and 93.2ï¼…under reaction conditions of atmospheric pressure, reaction temperature17～25℃,reaction time 30min, V(acetic acid): V(oil) =0.8, n(FeO₄^2-): n (S)=2:1 and n(KM) : n (FeO₄^2-)=0.5: 1.