Investigation On Chlorine Distribution And Formation Condition Of NH₄Cl In Diesel Hydrogenation Unit

The organic chlorides and organic nitrogen compounds in the raw material of diesel hydrogenation unit can be hydrogenated and produce ammonia and hydrogen chloride respectively in the process of diesel hydrogenation.The ammonia and hydrogen chloride can continue to react in the diesel hydrogenation unit and form the gas of ammonium chloride,which can become crystallization when the temperature decrease in the heat exchangers and pipes.These crystallization would cause blockage of pipe,which seriously affect the long period operation of the unit and would bring huge economic losses to the enterprise.In order to avoid the occurrence of the above situation,this paper mainly investigates the chlorides distribution of diesel hydrogenation unit and the formation condition of NH₄Cl.Therefore,it could provide some guidance for the development of preventing NH₄Cl salted technology in the hydrogenation unit,and also provide technical support for the safe and stable operation of hydrogenation unit.The KY-200 type of micro coulomb titration instrument and RPP-200 C type of salt content determintor are used to measure the content of organic chlorine and inorganic chlorine in the feedstock of the diesel hydrogenation unit accurately.The chlorine distribution of crude oil and raw material of diesel hydrogenation unit are determined systematically.The analysis results demonstrate that the super-heavy oil has the highest content of chloride,i.e 180.593mg/L,and its content of organic chloride is up to 133.543mg/L.The crude oil for visbreaking unit,west and south distillation unit has relatively higher content of chloride,the total chlorine content of crude oil in visbreaking unit,west distillation unit and south distillation uint is about 170mg/L,70mg/L and 65mg/L,respectively.All of them become the main source of chlorine in the subsequent processing process,such as diesel hydrogenation.For the feedstock of the diesel hydrogenation unit,i.e diesel fraction,its inorganic chlorides mainly come from visbreaking diesel,since the content of inorganic chloride in visbreaking diesel is 200mg/L,but its organic chlorides mainly come from the diesel fraction of west distillation unit,due to its content of organic chlorine is about 30mg/L.Reference to the measuring principle of salt content in crude oil,the dechlorination experiments for the diesel fraction of visbreaking unit are carried out by water wash.According to the results of the experiments,the dechlorination device for the visbreaking diesel by water wash is set up,and the proper operating conditions are as follows,the temperature is 60? and the volumetric ratio of water to oil is 0.2.The content of inorganic chlorine in visbreaking diesel is decreased significantly by the water washing dechlorination process,the removal rate of inorganic chloride and total chlorine is 98.4% and 95.9%,respectively.Based on the feature of diesel hydrogenation unit with the capacity of 120�104 t/a,17 sampling points are specified in the unit,among of them,9 oil sampling points,5 water sampling points and 3 gas sampling points.The content of chloride in each sample is measured precisely,and the distribution of chlorides in the diesel hydrogenation unit is determined.The analysis results illustrate that the relative error of flowrate of total chlorine for input and output of the unit is only 3.81%,and the total flowrate of chlorine into and out of the unit is about 1.2kg/h.Since the continuous hydrogenation conversion of organic chlorides in feedstock in the hydrogenation process,the flowrate of chlorine in recycle hydrogen is up to 14.23kg/h,which is the main reason for the blocking of heat exchanger in the diesel hydrogenation unit.The organic chlorides and organic nitrides with the boiling point in the range of 200?³00?,which may exist in the feedstock of diesel hydrogenation uint,are selected as the model organic chloride and organic nitride,i.e.three kinds of typical organic chlorides,perchloropropene,1,2,4-trichlorobenzene and 4-chloro-2-aminotoluene,and two types of organic nitrides,quinolone and indole.Their hydrogenation reactions are respectively analyzed by thermodynamic method.The analysis results indicate that these reactions could spontaneously generate HCl and NH₃ respectively under the conditions of diesel hydrogenation.Reference to the temperature and pressure condition at the ammonium chlorides salted location in the diesel hydrogenation unit,and the measured concentration of hydrogen chloride and ammonia,the thermodynamic analysis on the reaction of producing ammonium chloride is completed.It is found that the temperature of spontaneous formation of NH₄Cl is 149? under atmosphere pressure.And with the increasing of system pressure,the temperature of spontaneous formation of NH₄Cl would gradually rised.Under the 11 MPa condition of the diesel hydrogenation unit,the temperature at which NH₄Cl could be spontaneously formed is up to 234?,and higher than the stream temperature in the tube of heat exchanger.Therefore,a large amount of NH₄Cl crystallization are produced in the tube of heat exchanger.The effect of concentration of hydrogen chloride in recycle hydrogen on the formation reaction of NH₄Cl is investigated.When the partial pressure ratio of hydrogen chloride in recycle hydrogen decreases from 102.82�10-6 to 4�10-6,the temperature of spontaneous formation NH₄Cl would fall to 101?,which is lower than the stream temperature in the tube of heat exchanger,thus NH₄Cl crystallization could not be generated in the heat exchangers.Moreover,the concentration and formation rate of NH₄Cl in the upgrading oil by hydrogenation is estimated as 125.78mg/L and 21.34kg/h,respectively.Finally,some suggestions are proposed to prevent NH₄Cl salted in the diesel hydrogenation unit by this paper.