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重油催化裂化进料喷嘴雾化特性分析与结构优化

作者:润色论文网  来源:www.runselw.com  发布时间:2019/10/31 9:46:28  

摘要:催化裂化(FCC)是石油炼制工艺的主要手段之一,随着原料油的重质与劣质化,出现裂化产品低劣、转化率较低以及中间副产品增多,作为催化裂化的关键设备,进料喷嘴的雾化效果将直接影响轻质油产率和产品分布。通过模拟进料喷嘴的雾化性能,得到雾化场中速度分布,雾化液滴粒径分布等结果,为优化和设计更高效的进料雾化喷嘴提供借鉴和指导意义。

针对催化裂化进料喷嘴结构特征及雾化特点,建立欧拉-拉格朗日气液两相流模型,采用CFD方法的离散相模型,对进料喷嘴的外部雾化场进行数值模拟,研究了不同操作工况(进气压力、进液流量)以及不同进料工质物性下(工质黏度、表面张力)的雾化效果,数值模拟结果表明:

(1)根据喷嘴出口气相流动状况,在喷嘴出口喉缩段,气相速度达到最大,最大速度可达到557m/s,随着距喷口轴向距离的增加,速度不断减小,在出口径向距离5mm的范围内,湍流强度最大,这段范围也是雾化的关键段,径向距离越大,速度越小,湍流强度逐渐减小。

(2)以重油为进液工质得到雾化液滴粒径分布,轴向距离50mm以后,粒径突变较为明显,径向距离R=5mm范围内,液滴的整体粒径相对较大,随着轴向距离的增大,雾化场速度逐渐降低,湍流强度减小,雾化粒径趋于增大。在进气压力为0.03MPa~0.09MPa范围内,水的速度及雾化粒径的变化较为明显,平均粒径降低了约77.6%,继续增大压力,气液间掺混越激烈,雾化粒径的整体分布更细小,径向位置的粒径变化幅度减小,雾化更均匀。

(3)同一操作工况下,黏度较小的液体工质流动性能较好,受气动力的剪切及挤压作用,在雾矩中心处容易发生二次雾化,较高黏度工质雾化液滴速度及粒径分布变化趋势较为明显。不改变喷嘴出口结构的情况下,水与石蜡基基础油的雾化锥角相差10o左右,工质黏度越大,喷雾不易扩散,雾化锥角相对较小。工质表面张力越小,其雾化粒度越细,雾化效果相对较好。

根据结构尺寸设计进料喷嘴实物模型,搭建实验装置进行冷态雾化实验,研究进气压力和进液流量对雾化液滴分布产生的影响。通过对原结构雾化效果的分析,正交试验分析了出口喉缩段直径d、长度l以及出口直径D对雾化场的影响,实验结果及优化分析表明:

(1)同一进气压力下黏度较高的石蜡基基础油的雾化平均粒径比较大,约为水的1.7倍,基础油的雾化效果受进气压力的影响比较明显,进液流量越大,雾化粒径整体区域增大,但基础油的雾化粒度更为分散。

(2)取粒径在50 μm~150 μm区间内,被雾化的水的小粒径液滴所占的体积分数明显多于石蜡基基础油,比粒径约50μm的基础油液滴所占体积比提高近3.7%。随着进气压力的不断增加,基础油雾化液滴的体积分数分布逐渐趋于均匀,与水的平均粒径差距逐渐缩小。

(3)实验值与模拟值吻合效果较好,误差在合理范围内,数值模拟可以预测雾化粒径的分布。

(4)出口喉缩段直径d对气液混合物的速度及湍流程度影响较大,直径越大,粒径的分布相对更均匀。改进后的实验结果显示,射流雾化锥角增大了18.4%,相比原结构,射流雾化在空间分布的更广,有利于在雾化充分的进行。

Fluid catalytic cracking (FCC) is one ofthe main means of petroleum refining process, with the densification andinferior quality of crude oil, inferior cracking products, lower conversionrate and more intermediate by-products arise, As the key equipment of catalyticcracking unit, the yield and products distribution of light oil will bedirectly affected by the atomization effect of feed nozzle. The velocity and dropletsize distribution of the atomization field have been concluded by numericalsimulation of atomization effect of feed nozzle. It provides reference andguidance for optimizing and designing more efficient atomizing nozzles.

According to the structure and atomizationcharacteristics of catalytic cracking feed nozzle, the gas-liquid two-phaseflow model of Euler-Lagrange was established, the discrete phase model of CFDis adopted to simulate the external atomization field of feed nozzle. Studyanalyzed the atomization effect under the different operating conditions (inletpressure, inlet flow) and different working fluid properties (working viscosityand surface tension). The numerical simulation results show that:

(1) According to gas phase flow pattern ofnozzle exit, at the throat contraction section of nozzle exit, the gas phasevelocity obtains maximum value, the maximum speed can reach 557m/s. With theincrease of axial distance from the nozzle, the velocity decreases, within the5mm of radial distance of outlet, the turbulence intensity is the largest, thisrange is also key part of atomization, the larger the radial distance, thesmaller the velocity, and the turbulence intensity decreases gradually.

(2) The size distribution of atomizeddroplets was obtained from working fluid of heavy oil. After the axial distanceis 50mm, droplet size was obvious varied, from the range of radial distanceR=5mm, the overall droplet size is relatively large, with the increase of axialdistance, the velocity of atomization field decreases gradually, the turbulenceintensity decreases and the atomized particle size tends to increase. Duringthe inlet pressure is 0.03MPa~0.09MPa, water velocity and atomized particlesize changed obviously, the average particle size was reduced about 77.6%, whencontinue to increase the pressure, the mixture of air and liquid becomes moreintensely, the overall distribution of atomized particle size is smaller, thedroplet size of the radial position decreases, and the atomization becomes moreuniform.

(3) Under the same operating condition, theliquid with small viscosity has better flow performance, due to the shear andextruding action of the gas power, secondary atomization is easily to occur atthe center of the fog moment, the change trend of droplet velocity and particlesize distribution of larger working fluid was obvious. Without changing thenozzle exit structure, the difference of spray cone angle between water andparaffin base oil is about 10o, the higher the viscosity, the less diffused andthe relatively small spray angle. The smaller the surface tension of workingfluid, the finer the atomization droplet, and the better atomization effect.

According to the structure dimension, theentity model of feed nozzle was designed, and set up the experiment device toconduct the experiment of cold condition, the influence of inlet pressure andflow rate on droplet distribution was studied.

关键词:FCC进料喷嘴;雾化性能;数值模拟;实验研究;结构优化

FCC feed nozzle; atomization performance;numerical simulation; experimental study; structure optimization

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