基于热重分析测定物质的蒸气压

浙江大学学报(理学版第35卷第1期Journal of Zhejiang University(Science EditionVol 35 No. 12008年1月http://www.journalszju.educn/sciJan.2008基于热重分析测定物质的蒸气压郑青,雷群芳,方文军浙江大学化学系,浙江杭州310027)摘要:根据自由蒸发过程的 Langmuir关系,将热重分析(TGA)用于正十六炕、正十七烷和正十八炕等3个正构烷烃的蒸气压测定,分別采用常数法和比较法计算蒸气压,其结果与文献数据符合较好.结果表明,只要选择合适的参考物质,TGA可以成功地应用于未知液体物质蒸气压的测定,同时,根据从TGA实验得到的蒸发速率和蒸气压数据,计算了蒸发过程的表观活化能和平均蒸发焓,发现活化能教值仅稍大于蒸发焓,证实TGA实验中的蒸发过程接近非活化自由燕发过程,TGA是一种快速实用的蒸气压测量新方法,尤其适用于较低压力范国的蒸气压测定,是传统实验方法的很好补充关键词:热重分析;正构烷烃;蒸气压;活化能;蒸发焓中图分类号:O642.4文献标识码:A文章编号:1008-9497(2008)01-059-04ZHENG Qing, LEI Qun-fang, FANG Wen-jun( Department of Chemistry, Zhejiang University, Hangzhou310027, China)Measurements of vapor pressures of alkanes by thermogravimetry. Journal of Zhejiang University( Science Edition)2008,35(1):59~62Abstract: Thermogravimetric analysis (TGA) was extended to the measurements on vapor pressures of three n-alkanes, hexadecane, heptadecane and octadecane, on the basis of the Langmuir equation for zero-order vaporizationprocess. The vapor pressures were calculated by two different methods, a constant-parameter one and a comparativeone, from the tGa data. These results are in good agreement with the reference data calculated from the antoine equation. It follows that TGa can be successfully extended to the vapor pressure measurements of some liquids proided an appropriate reference sample is used. Furthermore, the apparent energies of activation for the vaporizationprocess and the average enthalpies of vaporization are obtained through the vaporization rates and the vapor pressuredata. It was found that the values of the energy of activation are only slightly larger than those of the enthalpies of vapori-zation and it indicated that the vaporization process during the tga is nearly a nonr activated one. The experimental resultsshowed that the tga method is a rapid and practical one for constructing vapor pressure curves especially at low pressurerange. TGa can serve as a new remedy for the traditional methods of vapor pressure measurements.Key words: thermogravimetric analysis(TGA); alkane; vapor pressure; energy of activation; enthalpy of vaporization蒸气压数据常用于相平衡计算,由它还可以获概括为动态法和静态法两大类,应用中需考虑测定得蒸发焓、升华焓等重要热力学函数;蒸气压也是评对象和测压范围来选择-.一般地静态法多用于测价物质稳定性的重要指标,比如先进的航天航空燃定中、高压蒸气压,测压下限约为1kPa,等压计法料的蒸气压就需要严格调控.通常,蒸气压数值范围 ASTMD287986)是典型的静态法;沸点计法则是宽,受物质状态、温度等影响大,因此针对不同的研典型的动态法,可用于相对低些的蒸气压测定,但最好究对象和具体要求,已经形成了很多测定方法,如沸不低中国煤化工是为0.05Pa;努森扭点计法、等压计法、流逸法、怒森隙透法等,总体上可矩隙CNMHG隙透法的测压下限收稿日期:2006-12-18基金项目:国家自然科学基金资助项目(20573096);浙江省自然科学基金资助项目(Y404329)作鹗或据(132)::士明究生,主要从事物理化学研究浙江大学学报(理学版)第35卷分别为105、10-6、10-8Pa,是较好的测定低微蒸气压的方法.对于多组分体系来说,拟静态法是一类较好实验部分的实验方法,它是吸收了动态法和静态法的优点而建立起来的,在流体相平衡研究和燃料开发中应用较1.1试样多47.近年来把热重分析(TGA用于蒸气压测定得正十六烷、正十七烷和正十八烷(简写为C16到快速发展,热重分析仪具有自动化程度高操作方C2和C8)均为进口试剂( Sigma-Aldrich,美国),纯便测量速度快,用样量少等优点弥补了传统动态法度大于99%.文中计算需用的相关数据列于表1,其和静态法的一些不足,因此受到关注.它在药物、香料、中Tb为常压沸点,M为相对分子质量,蒸气压染料等固体物质的低蒸气压测定方面具有独特优p(10°Pa)与温度T(K)的关系用 Antoine方程表势1.本文则主要以具有较高沸点的正构烷烃为对示:logp=AA,B,C为 Antoine常数象,以探讨热重法在测定液体物质蒸气压中的应用表1正十六、十七和十八炕的基本性质Table 1 Fundamental properties of n-hexadecane, n-heptadecane and n-octadecaneAntoine constantsAlkatABTemp. range/K554.0226.444.173121845.672-117.054463.2~559.8575.0240.473.927301718.004-138.126436.6~576.0589.3254.494.332092068.963-111.927447.6~5901.2仪器与操作表2不同挥发率时的温度Tv以及峰温T和采用德国产 NETZSCHSTA409PG/PC热重分析终温仪用经过灼烧恒重的刚玉坩埚盛放试样,其截面积为able2 Temperatures(Tv) at different volatile ratios2.83×10-5m2.试样用量为10~11mg,用Metr十万(V), and peak(T )and final(T) temperatures分之一天平准确称量以空坩埚作参比,用高纯氮气为Temperature/℃载气,流量为30mL·min-1,控制升温速度为10℃186.1min-1,记录温度范围为室温至400℃,热重(TG)曲线216.1228.6及其微分(DTG)数据由仪器软件自动给出V=0.4223.4236.5247.5V=0.5230.3242.8253.82结果与讨论V=0.6235.3259.52.1 TG-DTG曲线243.7267.9图1给出实验测得的3种物质的 TG-DTG曲线247.1259.5272.1246.5很明显,在各自沸点温度之前(530~550K),试样已经268.9258.6271.3284.1完全蒸发,此后基线平直;DTG曲线只有一个锐利的峰,而且在峰值之前变化相对缓和些,峰值之后迅速变2蒸发过程的活化能化,说明蒸发过程没有发生裂解结焦等现象,并且符合零级蒸发动力学特征表2列出了它们在不同挥发类似于化学反应动力学过程,蒸发过程也可用Arrhenius方程表示{101率V时的温度Tv,以及峰温T和终温T其中,k为实验温度T时的蒸发速率常数kg·s-1·m-2,ko为指前因子,Ea为蒸发过程的表观活化能小 kL. mol1R为气体常数,J·mol-·K如中国煤化工率过程,则kp可以从CNMH(发表面积直接计算图1 TG-DTG曲线TG-DTG curves for the n-alkanes其中,A为蒸发表面积,m为质量,t为时间.在实验1郑青,等:基于热重分析测定物质的蒸气压61温度范围内,如果E可视为常数,由式(1)得Im,p和ν的关系示于图3.可见,p和y确实存在良好的对1/T呈直线关系.TGA实验得到的相应结果示直线关系,关联实验数据,可得常数k=3.8653×于图2,于是从直线斜率可计算得到蒸发过程的表105Pa·kgs·mol.5·s1·m2·K05,进一步计算观活化能,其数值列于表3得蒸发系数a=1.87×10.需要指出的是,实验条件下的a数值远小于1,这主要是因为实验在载气存在下的流动体系中进行,与真空状态相差很远,文献12中也有类似的结果报道1.8192.02.1222.3242.5262.71000/T/K图2蒸发过程的Ink,与1/T的关系Fig. 2 Plots of Ink a versus 1/t during the0.06008vaporization processw/kg:mor∵s:m:K"表3从TGA数据关联得到的蒸发动力学参数和图3蒸气压p和变量ν的关系蒸发焓3 Relationship between vapor pressure p and variable vTable 3 Correlation results for the parameters of(2)比较法:根据式(4),可以得到vaporization kinetics and the enthalpy of vaporization(dm/dt)from TGA dataPsm T"PRef, T-(dm/dt).TMS.(5)n如mw:my8:-可见,只要比较两者在相同温度下的蒸发速率,便可式中,下标Sam表示未知试样,Ref表示参考物质Alkane range/K pointsC16397~5205010.08769.725.53964.4从参考物质的蒸气压计算出未知物质的蒸气压C1409~532509.90170.825.66566C1402~542579.63771.625,76768.9由上述两种方法得到的3种物质的蒸气压实验值,及其与文献7数据计算值的比较示于图4.可2.3蒸气压和蒸发焓见,两者符合较好,说明TGA可以用于测定液体物对于自由蒸发过程,蒸发速率可以用 Langmuir质的蒸气压以后将进一步扩充体系范围方程[8-10表示:400001 dMAat=aL2Rr」(3)△ Comparative式中,a是蒸发系数,在真空条件下,数值为112.重排式(3),可以得到20000(2R)I(T1 d=ky,(4)其中,k=(2πR)1/a是与温度和物质无关的常数,y=[(-dm/dt)](T/M)2则取决于温度和物质因此,只要在相同条件下对参考物质和未知物质作热重分析,就可以从参考物质的蒸气压求出未知物溜定的蒸压值与文献值的比较质的蒸气压根据具体的计算过程,则有常数法和比中国煤化工ssures from TGA较法等处理方法11,这里分别用这两种方法计算,CNMHGdata以作比较另外,蒸气压p随温度T的变化可用 Clausius(1)常数法:由参考物质的p和v求出各实验温 Clapeyron方程表示度下的k,然后求算术平均作为常数k;或者以p对Inp(6)作真秀分数级嘉求出常数,本文讨论的3种物质的浙江大学学报(理学版)第35卷式中,△H是摩尔蒸发焓.假定△H灬与温度无关,定[.石油化工,1997,26(9):626-631积分式(6),有:NIE Li, LEI Qun-fang, ZONG Han-xing, et alIn p=Inz-△H、Measurement of the saturated vapor pressure of multcomponent systems [J]. 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