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脱硫
原油中含有硫，它从最简单的混合物H2S到复杂的环状结构等多种形式与原油混合在一起。原油蒸馏过程中，分解较高沸点的硫化物产生H2S,并进入石油液化气里。由于H2S有毒性和腐蚀性，故石油液化气里的H2S必须除去。用一种胺溶液（例如二乙醇胺）进行逆流洗涤除去H2S；然后在一个单独的容器里加热吸收H2S的胺溶液脱除H2S回收硫，并将胺溶剂再生循环用作逆流洗涤。硫醇可以看作是H2S的衍生物，硫化氢中一个氢原子被一个碳氢团替代了。硫醇也有H2S的臭味和腐蚀性这样一些令人不愉快的性质。那些沸点在80℃以下的硫醇极易溶解于碱性溶液，不过温度在80℃以上时，其可溶性就迅速降低。因此，在生产石油液化气和轻质汽油时采用苛性钠溶液进行逆流洗涤以除去硫醇。
UOP Merox 硫醇氧化法使用苛性钠萃取硫醇，然后硫醇用空气氧化后生成二硫化物，苛性钠再生后循环使用。氧化这一步由一种溶于苛性钠的金属络合物作催化剂。
这个流程可以用以下方程表示：
4C2H5SH+4NaOH= 4C2H5SNa+2H2O +2H2O
+O2
2C2H5S—SC2H5+4NaOH
二硫化物不能溶于苛性钠，而形成一层可以除去的油层。
沸点在80℃和250℃之间馏分里的硫醇不能萃取，但是在Merox溶液里能用空气氧化生成二硫化物。这些二硫化物无腐蚀作用而且几乎没有气味；它们依然溶于油中，所以并未达到脱硫的目的，不过，产品已经“去臭”了。另一种氧化硫醇的工艺是使用氯化铜作催化剂。上述两种工艺均可用来加工喷气式发动机燃料。
随着原油中蒸馏出来的馏分沸点的增高，会发现含硫量也在增多。生产柴油和家庭供暖系统燃料油的沸点在250℃到350℃之间，在这个沸点范围内，中东产的原油含硫量大约为1%重量。这种油料燃烧时，硫就氧化成SO2，而SO2又极易氧化成硫酸，会引起大气污染和金属腐蚀。由于硫主要与碳和氢结合，其结构比简单硫醇复杂得多，它就不能用上面介绍的方法处理。这些复杂的化合物必须经过裂解才能得到硫，其方法如下：在高温（320℃至420℃之间）及高压（25至70巴之间）条件下，使原油连同氢气一起通过以铝矾土为载体的氧化钴和氧化钼的催化剂（制成小球或压制物）。当氢气与进料的比例超过所需要的比例几倍的时候，这一反应就比较容易，催化剂的寿命也较长。在这些条件下，硫化物就分解；硫与氢气化合生成为H2S。几乎所有的硫化物均可用这一方法分解而不影响所余的碳氢化合物。
这种脱硫工艺，也称为“加氢精制”，在处理各种结构的硫化物方面是有效的，而且也可用来处理原油的任一馏分。
原则上，处理各种原料的设备都基本相似，它将包括完成下列各步骤：
1． 在合适的温度和压力下，为反应塔提供进料和氢气。
2． 冷却反应塔出口产品使油品冷凝，并让过量的氢气分离出来以供反应塔循环使用。
3． 除去H2S和少量的（2~3%）在反应过程中产生的低沸点碳氢化合物。
用泵输送原料并使之增高到所要求的压力后，通过管道送进加热炉，原料在加热炉加热到所要求的温度，然后与氢气混合在一起进入反应塔。出反应塔的高温产品，一部分由热交换器中新鲜的原料冷却以节省燃料，一部分由另一个热交换器里的水冷却。过量的氢气在一个卧罐里从冷凝油中分离出来，然后由一台压缩机将它再循环回到反应塔里与新加进的氢气一起补充反应中消耗掉的部分。卧罐中出来的液体被送入一个蒸馏塔（汽提塔）；在蒸馏塔里除去H2S和低沸点的裂解产品，并在塔底取得脱去硫的产品油。
沸点超过350℃的许多原油用作生产发电厂、船只和大型工厂所需的重燃料油。绝大多数的中东原油含硫量为2.5%~4%（重量），燃烧这种原油会释放出H2S，所以必须建造很高的烟囱（美国以建造了若干个高度在500到600英尺，其中一个高达800影驰的烟囱），这样H2S会飘散在高空，可以避免局部地区的污染，但会造成大气严重污染。理想的解决方法是脱去原油各个馏分的硫。遗憾的是尽管沸点高达550℃左右的蒸馏物的脱硫相对说来容易达到，但是，处理重质原油的残渣油尚有许多困难。脱硫工艺的困难随沸点的增高而增大，而且，含硫分子的比例也会增高（可能到达50%），也就是说，呈现出来的高比例分子必须分解。油中微量金属往往会使最有效的脱硫催化剂失去活性，因此，必须使用高压（高达170巴）。所有这些因素结果都会造成燃料油脱硫的成本增高。再者，近来世界范围的原油供应形势的发展非常强调对能源的保护。因此，只有在不能采用别的方法减少污染时，才能最后采用染料精加工工艺。
在广泛采用脱硫工艺的炼厂里，要达到日产100吨的H2S是容易的。尽管H2S燃烧能生产SO2并从高烟囱里排放掉，但这样处理并不理想，因为这样造成环境污染。因此，需要安装一个装置来回收硫。H2S在氧气供应不足的情况下燃烧生成SO2，大约只有三分之一的H2S的燃烧，结果形成三分之二的H2S和三分之一的SO2的混合物；H2S和SO2将化合成硫和水：
2H2S+SO2=3S+2H2O
硫收集起来后通常出售给化学公司主要用于生产硫酸。
热裂化
碳氢化合物加热到450℃以上就开始分解，结果大的分子分裂或“裂化”成较小的分子。石蜡最容易裂化，其次是石脑油，而芳香烃特别难于裂化。过去有个时期曾广泛采用热裂化来改进石脑油中的辛烷值，或者以重馏分来生产汽油和柴油。石脑油热裂化所生产的汽油质量不够高，故不能用作现代的车用汽油，所以上述工艺已被淘汰。重油裂化产品要求完全加氢以满足现代质量要求。尽管这一加氢工艺目前极少采用。不过，如果需要把重馏分（温度高达550℃）变成粗柴油，那么人们可能还会对这一工艺感兴趣的。当今通用的一种热裂化工艺称为“减粘裂化”，即热裂化粘性原油残渣油，把既复杂有大的分子裂化成较小的分子，从而降低其粘度。那么，在不必使用粗柴油或煤油与粘性残渣油混合的情况下，就可以生产出一种令人满意的燃料油。
当今采用的另一种热裂化工艺称为“延迟焦化”，这一工艺通常用来处理低硫原油的常压或真空蒸馏残渣油，以生产电极焦（主要用于制铝工业）。残渣油加热到500℃左右，进入一个大焦化塔的底部，在塔底部高沸点原料进行裂化反应，焦化塔里形成的低沸点原料在高温下气化，经塔顶部进入分馏系统在这里分馏成石油气、汽油和粗柴油，而塔中留下一堆多孔的焦。焦化塔结满焦后，把进料送入另一个焦化塔，将结满焦的塔通入蒸汽、以除去焦炭。这时，另一塔内的进料裂化结焦又满了。正如在裂化工艺中那样，液态产品要进行加氢处理以便当作石脑油或柴油之用。
催化裂化
热裂化重馏分生产汽油不太适宜，它能产生大量的石油气和燃料油，并伴有汽油，汽油的质量亦不是太好。大约三十年前就发现：漂白土（硅藻）以及相似的材料能起到裂化催化剂的作用，而且还能生产出高辛烷值的汽油（催化裂化）。遗憾的是，漂白土很容易被焦炭覆盖失去催化剂的作用。不过焦炭烧掉后，催化剂又回复其先前所具有的活性。因此，为了使这一工艺流程能持续进行，就必须设计出能大规模地、持续不断地，交替使用的再生催化剂的方法。
达到这一目的最成功的方法之一就是采用流化技术。当空气往上吹经过细粉末层时，粉末的状态取决于空气的速度。如果速度高（大约10米/秒以上，粉末就似一片飞尘往上吹；可是速度中等时（大约1米/秒），粉尘就被空气带动四处转动，粉末层就起到流体的作用，而且能流动，像流体一样保持水平状态，等等。通过利用这一特性，粉末状的催化剂就能在反应和再生两个阶段之间持续流通。
反应塔和再生塔都分别进行设计，以便上升气流的速度大得足以输送催化剂。原料油（正常沸点在350℃到550℃之间）与热的（沸点在620℃与740℃之间）再生催化剂相遇，再生催化剂上没有焦炭。汽化了的油同催化剂一道经输送管进入反应塔，在反应塔里催化剂形成流化层。催化剂遇到油，裂化反应就开始就在反应塔里温度在480℃至540℃之间完成这一反应，之后催化剂被积焦。这种失效的催化剂用蒸汽解吸，除去夹带在催化剂中的碳氢化合物，并使催化剂回到再生塔里，在这里空气把催化剂表面的焦炭烧掉。石油产品经旋风分离器分离催化剂后离开反应塔以减少催化剂的流失，然后分离成燃料气、C3/C4、汽油和粗柴油。汽油的辛烷值可高达95。
由于水蒸汽减活作用以及进料中的金属积聚（进料中钒和镍的含量高达1ppm），催化剂会失去活性。通过不断加进新鲜催化剂和回收平衡催化剂以维持总量不变来保持催化剂的活性。
除了直馏粗柴油和减压粗柴油以及焦化粗柴油外，假若金属含量十分低的话，输送到催化裂化装置的进料还可以包括常压残渣油。凯洛克重油裂化（HOC）工艺就是为金属含量高的常压残渣油设计的工艺。
目前，在生产与分子筛（沸石）混合的、并具有很高活性的催化剂方面，已有很大改进。业已发现，蒸汽在反应塔里长时间阻滞会引起副反应，这就会降低生产汽油的选择性，并产生更多的气体和焦炭。新设计省略了反应塔的流化层，而在输送管道里进行反应；油和催化剂迅速在输送管道的末端分离（例如，在旋风分离器内）。催化剂在输送到再生塔之前，像先前一样进入汽提塔，除去滞留的碳氢化合物。这就称为短接触时间（SCT）裂化，这一工艺大大提高了生产流程中可得的汽油产量。
加氢裂化
当碳氢化合物所含的碳原子数目增加时，氢原子数目与每个碳原子的比例就下降。譬如，甲烷（CH4）为4；戊烷（C5H12）为2.4；癸烷（C10H22）为2.2。如果想要从高沸点碳氢化合物（譬如含有20个碳原子）中生产出低沸点碳氢化合物（譬如含有5到10个碳原子的汽油），就必须找到增大氢与碳之比的一些方法。在热裂化过程中，生产出烯烃（其氢与碳的比例比链烷烃低）；在催化裂化过程中，生产出烯烃，同时碳被催化剂的沉积作用除去。另一种方法是加氢，它是在加氢裂化过程中通过在氢气中进行高压裂化完成的。
由于这一工艺灵活性很大，而且能从石蜡馏分中高产汽油或粗柴油；或者从粗柴油里高产汽油。能在压力为150巴至170巴、温度为430℃左右下进行操作。能够承受这些苛刻条件的反应塔可能需要150至200毫米厚，而且在设计和制造过程中需要考虑许多工程上的难题。加氢裂化对氢的需求量很大，加工每立方米的石油需要高达300立方米左右的氢气，这一需求量大大超过了催化重整装置内得到的氢气，所以必须建造一座大型的制氢工厂以供应加氢裂化装置。
设计加氢裂化装置生产粗柴油时采用一座反应塔；基本流程看上去与一个加氢精制装置十分相似。不过，要求生产汽油时，要使用两个装有不同催化剂的反应塔。没有参加反映的进料回到反应塔里，以便将进料完全变成低沸点的产品。应注意的是：这一工艺所要求的苛刻操作条件需要重型设备以承受高温高压，如：大型气体压缩机和泵，以及一个制氢装置，因此基本投资很高。
催化重整
目前，催化重整是生产车用汽油的最重要工艺之一，本工艺采用约70℃~190℃沸点范围内的直馏原料作进料，并将辛烷值从大约40提高到95~100。
发生的主要反应是：环烷脱氢生成芳烃
CH2 CH3 CH2
CH2 CH CH2 CH CH3
+3H2CH2 CH2 CH2 CH
CH2 CH2
a/石蜡异构化
n己烷 2甲基戊烷b/环烷异构化
CH3 CH2
CH2 CH CH2 CH2
CH2 CH2 CH2 CH2
CH2 CH2
然后，环己烷经脱氢反应（1）生成苯。
脱氢环化
n己烷 甲基环己烷+H2正构烷烃能通过环化作用生成环烷烃并伴生出氢气，然后环烷烃由反应（1）脱氢生成一种芳香烃。
加氢裂化
C10H22 +H2 C6H14+C4H10加氢裂化包含了把一个碳链断裂成两个较小的分子。由于把氢加到了裂化后产生的烯烃双键上，所以生产出了烷烃
上述四个反应的结果都使辛烷值增加，像在头三个反应中生成的芳香烃的辛烷值与相应的链烷和环烷的辛烷值相比要大得多，而在第四个反应中，辛烷值的长链烷烃被断裂。
原先研制的催化剂是由高纯度矾土和铂（大约0.3~o.75%重量）组成。在过去几年中，已经研制出新的催化剂，仍然把铂作为主要成分，但还要加进诸如铼这样的其他金属，它们能在苛刻的操作条件下延长催化剂的寿命。
反应温度在490℃~540℃区间、压力在10~30巴范围内进行。还有另外一些副反应，往往由于催化剂上的焦炭而使催化剂的活性降低。这些反应能用高压氢来复原；在装置操作压力下，循环氢与进料配合维持氢的操作压。不过在低压情况下，重整产品的产量较高，而高速循环氢成本昂贵。新型催化剂能使该工艺在10~15巴压力范围内、氢与进料之比为4~5:1（克分子、克分子）条件下运行，而原来的条件则为大约30巴、氢与进料之比约为8:1。
在前三个反应里进行的环烷脱氢是一种高吸热反应（即吸收热量）。因此，必须将催化剂分散到单独的反应器中。因此，当反应开始时，温度下降太大，结果反应速度变得太慢，而且产品必须重新加热才能使反应得以完成。
催化重整是供给脱硫装置所用氢的主要的宝贵的来源。这种工艺还用来生产芳烃（譬如，苯、甲苯、二甲苯），用作石油化工生产的原料。
为保持催化剂的高度活性，对进料必须精心加以净化。最好将硫、水之比保持在约百万分之二以下，并且要把微量铅除去。必须定期地将催化剂中的焦炭烧去，这通常要停机进行。不过，可通过轮换再生备用反应器，轮流切断各个反应器的蒸汽进行催化剂再生。在另一种情况下，少量的催化剂在再生后能够不断地移除，并回到反应器里。最新的再生程序包括再生或再活化的各个阶段，在这些阶段里，催化剂上的金属分布得到调整，使催化剂保持在最佳状态。
英语文献翻译
Petroleum Refining Processes
Desulphurisation
Sulphur occurs in crude oils combined in a variety of ways, from the simplest compound H2S to complex ring structures. H2S is produced during distillation of the crude oil by decomposition of higher boiling sulphur compounds and appears in the LPG from which it must be removed because of its poisonous and corrosive nature. This is done by counter current washing with an amine (e.g.diethanolamine), the H2S being removed for sulphur recovery by heating the amine solution in a separate vessel thus regenerating the amine for recycle to the washing stage . Mercaptans can be considered derivatives of H2S, in which one hydrogen atom is replace by a carbon /hydrogen group, and share some of its unpleasant properties of bad smell and corrosivity . Those mercaptans boiling below about 80℃ are readily dissolved in alkaline solutions but the solubility decreases rapidly above that temperature .For LPG and light gasolines therefore the mercaptans can be removed by counter current washing with caustic soda solution.
The UOP Merox process uses caustic soda to extract the mercaptans which are then oxidised with air to disulphides and the caustic soda regenerated for further use. The oxidation step is assisted by a metal complex catalyst dissolved in the caustic soda.
The process can be represented as follow:
4C2H5SH+4NaOH= 4C2H5SNa+2H2O +2H2O
+O2
2C2H5S—SC2H5+4NaOH
The disulphides are not soluble in caustic soda and form an oil layer which can be removed.
Mercaptans in fraction boiling between80℃ and 250℃ cannot be oxidised to disulphides in the Merox solution with air. The disulphides,which are non-corrosive and have little smell, remain dissolved in the oil so that no actual desulphurisation has been achived but the products have been “sweetened”. Another process for the oxidation of mercaptans uses copper chloride as a catalyst. Both processes can be used in the production of aviation jet fuels.
As the cuts taken from crude oil increase in boiling point it is found that the sulphur increases. In the 250-350℃ range which is used for both diesel fuel and domestic central-heating fuel the sulphur content is about 1 per cent weight from most Middle East crudes. When this material is burnt the sulphur is oxidised to SO2 which, being easily oxidised to sulphuric acid, causes atmospheric pollution and corrosion of metals. The sulphur cannot be treated by the methods previously outlined as it is mainly combined with carbon and hydrogen in forms much more complicated than the simple mercaptans. These complex compounds have to be broken down to get at the sulphur which is done by passing the oil together with hydrogen at high temperature (320-420℃) and high pressure (25-70 bar), over a catalyst containing cobalt and molybdenum oxides on an alumina base, made in the form of small pellets or extrudates. The reaction is easier and the catalyst life better when the ratio of hydrogen to feed is several times higher than that necessary to complete the reaction chemically. Under these conditions the sulphur compounds decompose and the sulphur combines with the hydrogen to give H2S. Almost all of the sulphur compounds can be decomposed in this way without significantly affecting the remaining hydrocarbons.
This process of desulphurisation, also called “hydrofining”, is effective in attacking all forms of sulphur compounds, and can be used to treat any part of crude oil.
In principle the equipment used for all feeds is basically simlar and will contain means for carrying out the following steps:
1 Supplythe feed and hydrogen to the reactor at the correct temperature and pressure.
2 Cool the reactor product to condense the oil and allow the separation of the excess hydrogen so that it can be recycled to the reactor.
3 Remove the H2S and small quantity (2-3 per cent) of low-boiling hydrocarbons producted in the reaction.
A pump takes the feed and raises it to the repuired pressure and passes it through tubes in a furnace where it is heated to the required temperature before being mixed with the hydrogen and passing into the reactor. The reactor product is cooled, partially by the fresh feed in a heat exchanger to save fuel, and partially by water in another heat exchanger. Excess hydrogen is separated from the condensed oil in a drum and recirculated back to the reactor by a compressor together with fresh hydrogen to replace the amount consumed in the reaction. The liquid from the drum is passed into a distillation column where the H2S and low-boiling breakdown products are removed and the desulphurised oil taken from the bottom of the column.
Much of the crude oil boiling above 350℃ is used to make heavy fuel oil for power-stations, ships and large industrial plants and can have a sulphur content of 2.5-4 per cent weight from most Middle East crudes. Buring this material releases SO2 and very high chimneys have to be used (a number in the 500-600 foot range have been built, one of 800 feet in the USA) so that the SO2 can be dispered widely in the atmosphere thus avoiding localised pollution. The ideal solution would be to desulphurise all parts of the crude oil. Unfortunately, although the desulphurisation of distillates boiling up to about 550℃ cab be relatively easily accomplished, the treatment of heavy crude-oil residues poses many difficult problems. With increasing boiling-point the difficulty of desulphurisation increases and also the proportion of molecules containing sulphur becomes high (possibly up to 50 per cent) which means that a high proportion of the molecules present must be decomposed. Trace metals in the oil tend to deactivate the most effective desulphurisation catalysts and high pressures (up to 170 bar) must bs used. All these factors result in high costs for fuel-oil desulphurisation. Also recent developments in the crude-oil sopply situation worldwide have placed a stong emphasis on energy conservation. Consequently, fuel intensive processes would be employed only as alast resort when alternative means of miniming pollution are not viable.
In a refinery where desulphurisation is used extensively the production of H2S can easily reach 100 tonnes per day. Although the H2S could be burnt to SO2 and vented from all stacks,it is very undesirable because of the atmospheric pollution caused and additional plant is instslled to recover the sulphur. The H2S is burnt to SO2 with the oxygen supply limited so that about one-thirt of the H2S burns. This gives a mixture of two-thirds H2S and one-third SO2 which will combine to form sulphur and water:
2H2S+SO2=3S+2H2O
The sulphur is collected and is usually sold to chemical companies mainly for the manufacture of sulphuricacid.
Thermal Cracking
When hydrocarbons are heated to temperatures exceeding about 450℃ they begin to decompose. The large molecules breaking or “cracking” into smaller ones. Paraffins are the most easily cracked followed by naphthenes, aromatics being extremely refractory . At one time thermal-cracking processes were widely used to improve the octane number of naphthas or to produce gasoline and gas oil from heavy fractions. The quality of the gasoline from the thermal cracking of naphtha is not high enough for present motor gasolines and the process has fallen out of use. The products from heavy oil cracking requirements and while at present the process is little used it could be of interest should conversion of heavy distillates (up to 550℃) to gas oils be required. One thermalcracking process presently in common use is visbreaking which is the thermalcracking of viscous crude-oil residues to reduce their viscousity by breaking down the large complex molecules to smaller ones.A satisfactory fuel oil can them be made without the necessity of using gas oils or kerosine to blend with the viscous residue.
Another thermal-cracking process presently employed is Delayed Coking, which is normally applied toatmospheric or vacuum residues from low sulphur crudes for the production of electrode grade coke (used mostly in aluminium production). The residue is heated to about 500℃ and passed to the bottom of a large drum where the cracking reaction proceeds which breaks down the high-boiling materials, The lower boiling materials formed vaporise at the high temperature in the drum and pass out of the top to the fractionation system where they are separated into gas, gasoline and gas oil and leave behind in the drum a porous mass of coke. When the drum is full of coke the feed is switched to another drum which is filled while the full one is steamedout and the coke removed. As in the thermal-cracking process the liquid products require hydrogenation for use as naphtha or gas oil.
Catalytic Cracking
Thermal cracking of heavy distillates for gasoline production is not selective and produces substantial quantities of gas and fuel oil together with the gasoline, which is also not of very good quality. About thirty years ago it was found that fuller’s earth and simillar materials could act as cracking catalysts and give a good yield of high octane number gasline (catalytic cracking). Unfortunately, the fuller’s earth became quickly covered in carbon and no longer acted as a catalyst was returned to its previous activity and thus to operate the process continuously it was necessary to devise methods of alternatively using and regenerating the catalyst continuously on a large scale.
One of the most successful methods of achieving this depends on the use of fluidisation. When a gas is passed up through a bed of fine power the behaviour of the power depends on the velocity of the gas. If it is high (about 1 m/sec.) the particles are moved about by the gas and the bed of power acts like a fluid and can be transported, find its own level, ect. just like a liquid. By using this property, the catalyst, in power form, can be circulated continuously between a reaction stage and a regeneration stage.
The reactor and regenerator vessels are each designed so that the upward vapour velocity is sufficient to fluidise the catalyst. The oil feed (normally boiling 350-550℃) meets hot (620-740℃) regenerated catalyst, which is substantially free of carbon, and the vaporised oil and catalyst pass through a transfer line to the reactor, where the catalyst forms a fluidised bed. The cracking reaction proceeds as soon as catalyst meets the oil and is completed within the reactor at 480-540℃, depositing carbon on the catalyst. The spent catalyst is steam stripped to remove entrained hydrocarbons and returned to the regenerator where air is used to burn the carbon from the catalyst. The oil products leave the reactor, via cyclones to reduce catalyst entrainment, and are separated into fuel gas C3/C4, gasoline and gas oils. The gasoline octane number can be as high as 95.
The catalyst loses activity as a result of hydrothermal deactivation and the accumulation of metals from the feed, which can contain up to 1 p.p.m. of vanadium plus nickel. Catalyst activity is maintained by continuous sddition oof fresh catalyst and withdrawal of equilibrium catalyst to maintain a constant inventory.
In addition to straight-run and vacuum gas oils, coker gas oil,etc. the feed to a catalytic cracker can include atmospheric residue provided the metals content is low enough. The Kellogg heavy oil cracking (HOC) process is designed for high metals content atmospheric residue.
Great improvement have been made in the manufacture of catalysts which now incorporate molecular sieve materials (zeolite) and have a very high activity. It has been found that the long residence time of the vapours in the reactor gives rise to secondary reactions which reduce the selectivity of the conversion to gasoline and produce more gas and coke. New designs dispese with the fluid bed in the reactor and carry out the reaction in the transfer line; the oil and catalyst are quickly separated at the end of the transfer line, for instance in a cyclone, and the catalyst drops into a stripper as previously to the remove entrained hydrocarbons before transfer to the regenerator. This is called Short Contact Time (SCT) cracking and has markedly improved the yield of gasoline obtainable in the process.
Hydrocracking
As hydrocarbons increase in the number of carbonatoms they contain, so there is a decrease in the ratio of the number of hydrogen atoms per carbon atom, e.g. methane, CH4, has a ratio of 4; pentane, C5H12, has a ratio of 2.4; decane, C10H22, has a ratio of 2.2. If we wish to produce low-boiling hydrocarbons (e.g. gasoline containing 5-10 carbon atoms) from highboiling hydrocarbons containing say 20 carbon atoms we must find some means of increasing the ratio of hydrogen to carbon. In thermal cracking olefines (which have a lower hydrogen/carbon ratio than paraffins) are produced and also carbon eliminated by deposition in the catalyst . The alternative approach is to add hydrogen and this is done in the hydrocracking process by cracking at a very high pressure in hydrogen.
This process, which is very flexible and can produce high yields of either gasoline or gas oil from wax distillaate, or gasoline from gas oil, operates at pressures of 150-170 bar and temperatures of around 430℃. Reactors capable of withstanding these severe conditions may be 150-200 mm thick and pose many difficult engineering problems in design and construction. Hydrogen requirements for hydrocracking are very high, up to about 300 m3 per m3 of oil processsd which is far in excess of that available from catalytic reformers so that a large hydrogen production plant must be built to supply the hydrocracker.
When designed to produce gas oil a hydrocracker will use one reactor and the basic-flow diagram appears very similar to a hydrofiner, but two reactors containing different catalyst are used when gasoline production is required . Unreacted feed is recycled to the reactor so that complete conversion of the feed to lower boiling products may be achieved. It will be appreciated that the severe operating conditions required in this process necessitate high-duty equipment to withstand the high temperatures and pressures, large gas compressors and pumps, and a hydrogen production unit which makes the capital cost very high.
Catalytic Reforming
Catalytic reforming is now one of the most important processes for the production of motor gasolines taking straight-run materials in the boiling range of about 70-190℃ as feed and raising the octane number from about 40 to 95-100 .
The main reactions taking place are: Dehydrogenation of naphthenes to aromatics
CH2 CH3 CH2
CH2 CH CH2 CH CH3 
+ 3H2CH2 CH2 CH2 CH2
CH2 CH2
a paraffin isomerisation
n hexane 2 methylpentaneb naphthene isomerisation
CH3 CH2
CH2 CH CH2 CH2
CH2 CH2 CH2 CH2
CH2 CH2
The cyclohexane can then dehydrogenate by reaction (1)
Dehydrocyclisation
n heptane methylcyclohexane + H2The straight-chain paraffin can cyclise to the naphthene with production of hydrogen and the naphthene then dehydrogenate to an aromatic by reaction(1)
Hydrocracking
C10H22 + H2 C6H14 + C4H10Hydrocracking involves the breaking of a carbon chain to give two smaller molecules. Paraffins are produced because of the addition of hydrogen to the olefinic fragments resulting from the cracking.
All four reactions result in an increase in octane number as in the first three reactions aromatics are produced which have much greater octane numbers than the corresponding paraffins and napthenes, and in the fourth reaction low octane number long-chain paraffins are destroyed.
The catalysts originally developed consisted of platinium (about 0.3-0.75 per cent weight) on highly purified alumina . In the past few years new catalysts have been developed still using platinum as a major component but also adding other metas such as rthenium which improve the life of the catalyst under severe operating conditions.
The reactions are carried out at temperatures in the region 490-540℃ and pressure 10-30 bar. There are other side reactions which tend to deactivate the catclyst by the foemation of carbon on it. These reactions can be reduced by a high pressure of hydrogen which is maintained by a combination of unit operating pressure and recycle of hydrogen with the feed. However, yields of reformate are higher at lower pressures whilst a high recycle of hydrogen/feed ratio of 4-5:1 (mol/mol) compared with the previous conditions of about 30 bar and about 8:1 hydrogen/feed ratio.
Dehydrogenation of naphthenes which occurs in the first three reactions is a highly endothermic procss (absorbs heat) and it is necessary to divide the catalyst into a number of separate reactors because the temperature drops so much as the reaction proceeds that its rate becomes too slow and the products have to be reheated to enable the reaction to be completed.
Catalytic reforming is a valuable source of hydrogen which is used mainly in desulphurisation units. The process is also used for the production of aromatics (e.g.benzene , toluene, xvlenes) for use as petrochemical feedstocks.
In order to maintain the catalyst at a high level of activity the feedstock must be carefully purified; it is preferable to maintain levels of sulphur and water below about two parts per million and eliminate traces of lead. It is necessary to burn carbon off the catelyst periodically. This is normally done by shutting down the plant. However, there are versions of the process in which each reactor is taken off stream in turn for regeneration by interchanging with a swing reactor. In another case small amounts of catalyst can be removed from, and returned to, the reactors continuously after regeneration or reactivation steps in which the distribution of metals on the catalyst are adjusted to maintain the catalyst in the most active state.