Thermal crackingDistillate fuels and heavy oils (heated/ were heated) under pressure in large drums until they (cracked/cracking) into (small/ smaller) molecules with (worse/ better) antiknock characteristics. However, this method(produced/ produce) large amounts of solid, unwanted coke. This early process (has evolved/ has beenevolved) into the following applications of thermal cracking: visbreaking, steam cracking, and coking.Visbreaking process. Visbreaking, a mild form of thermal cracking, significantly (lowers/ lower) the viscosity ofheavy crude-oil residue without (being affected/ affecting) the boiling point range. Residual from the2atmospheric distillation tower (is heated/ heated) (800-950oF) at atmospheric pressure and mildly cracked in aheater. It is then (quenched/quench) with cool gas oil to (control/ controlling) overcracking, and (flashed/ flash)in a distillation tower. Visbreaking is used to (reduce/reducing) the pour point of waxy residues and(reduce/reduced) the viscosity of residues used to blend with lighter fuel oils. Middle distillates may also (areproduced/ be produced), depending on the product demand. The thermally (cracked/cracks) residue tar, which(accumulates/ accumulate) in the bottom of the fractionation tower, is vacuum (flashed/flashing) in a stripperand the distillate recycled.Steam cracking process. Steam cracking is a petrochemical process sometimes used in refineries to produceolefinic raw materials (eg. ethylene) from various feedstocks for petrochemical manufacture. The feedstocks(range/ ranged) from ethane to vacuum gas oil, with heavier feeds giving higher yields of by-products such asnaphtha. The most common feeds are ethane, butane, and naphtha. Steam cracking (is carried/ carries) out attemperatures of 1500-1600oF, and at pressures slightly above atmospheric. Naphtha (producing/ produced)from steam cracking (contains/contain) benzene, which is extracted prior to hydrotreating. Residual fromsteam cracking is sometimes (blended/blending) into heavy fuels.Coking processes. Coking is a severe method of thermal cracking (using/ used) to upgrade heavy residuals intolighter products or distillates. Coking (produces/ produced) straight-run gasoline (coker naphtha) and variousmiddle-distillate fractions used as catalytic cracking feedstock. The process so completely (reduces/reducing)hydrogen that the residue is a form of carbon called “coke”. The two most common processes are delayedcoking and continuous (contact or fluid) coking. Three typical types of coke (are obtained/ obtain) (spongecoke, honey comb coke, and needle coke) depending upon the reaction mechanism, time, temperature, andthe crude feedstock.
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