PetroleumScienceinEnglishChapterEightPetrochemicalIndustryCrudeoilisarawmaterial,likeironore,whichmustbeprocessedbeforeitcanbeused.Infact,thereareseveraldifferentprocessesthroughwhichoilisprocessed,buttogethertheyarecalledrefining（炼制）.Therefiningprocessseparatesthedifferentfractionsofmolecules（分子组分）inthecrudeoilmixtureintousableproducts.Italsoremovestheimpurities（杂质）fromtheoil.Methodsusedinrefiningprocesses.PhysicalseparationprocessesThemaintechniqueis:Distillation-separation（蒸馏）accordingtomolecularsize,makinguseofthedifferentinboilingpoint.Othertechniquesare:Absorption（吸收）Solventextraction（溶剂提取）Crystallization（结晶）Adsorption（吸附）Methodsusedinrefiningprocesses.TherearethreemaincategoriesofChemicalConversionProcessesReductionofmolecularsize(cracking)(裂化）---inwhichfueloilcomponentsareconvertedintolighter,distillateproducts（馏出产品）suchasgasoil（粗柴油）andgasoline.Changeofmolecularstructurewithoutsizechange---e.g.catalyticreforming（催化重整）andisomerisation（异构化）.Increaseinmolecularsize---e.g.polymerisation（叠合）andalkylation（烷基化）Distillation:Thefirststepofrefiningiscalleddistillation.Thetheoryofdistillationisveryeasy.Mostliquidsboil（沸腾）whentheyreachacertaintemperature.Whencrudeoilboils,itsendsout（释放出）vapor（蒸汽）andcondenses（冷凝）.Allthedifferenthydrocarbons（烃）incrudeoilareboilingatdifferenttemperatures,andtheirvaporsallmakeseparate（不同的）liquids.So,inordertoseparate（分离）thehydrocarbons,thescientistsdistill（蒸馏）thecrude.Whenthedifferenthydrocarbonsboilatdifferenttemperatures,theirvaporscondenseintoitsfractions（馏分）andformsdifferentliquids.Theoilisdistilled（蒸馏）inaverylargesteeltower（钢塔）,thetechnicalname（技术名称）ofwhichisthe“fractionatingtower”（分馏塔）.Thetoweristhirtytofiftymeterstallanditsdiameter（直径）isonetothreemeters.Itisdividedinto“chambers”（室）,eachofwhichcontainsalayeroftrays（塔盘）.Thereareholesinthetrays.Theoilisheatedtoahightemperatureincoilofpipe（盘管）thatpassoverafurnace（热炉）.Thentheoilispipedintoafractionatingtowerandallbuttheheaviestmolecules（重分子）changeintoavapororagas.Thevapors