Thermal and catalytic cracking

Unfortunately, just sorting the molecules in crude oil isn't good enough for most refineries. The principal outputs of these refineries are transportation fuels and there is comparatively little market for the other molecules in crude oil. Since less than half of the molecules of crude oil are suitable for transportation fuels, the refinery has a problem. Moreover, it can't store the unmarketable molecules indefinitely. While the refinery burns some of the less useful molecules to provide its own power, it must sell everything else to make room for incoming crude oil. So large integrated refineries have facilities for converting the less useful molecules in crude oil into ones it can sell.

The original method for converting larger molecules into smaller molecules is ther­mal cracking. Above about 360°C, hydrocarbon molecules decompose into fragments. At that temperature, the random thermal energy in a hydrocarbon molecule is occasion­ally large enough to break that molecule into two pieces. After a short time as a free radical, each fragment rearranges into something that's chemically stable. Most of that time the new molecules are smaller than the old molecules.

The higher the temperature, the more often such decompositions occur and the faster the petroleum cracks. While thermal cracking is a nuisance to be avoided in distil­lation, it's valuable when done in a controlled manner in a cracking tank. The big mol­ecules that aren't suitable for gasoline generally decompose into smaller ones that are.

Moreover, thermal cracking produces many olefin molecules that have higher oc­tane numbers than the usual contents of crude oil. These olefin molecules are made when the free radical fragments of original hydrocarbon molecules rearrange internally to form double bonds. If the last carbon atom in a chain has only three neighbors, it can complete its electronic shell by forming a double bond with the carbon atom next to it. This rearrangement causes the neighboring carbon to abandon a hydrogen atom, which immediately becomes part of a hydrogen molecule. So thermal cracking creates many smaller molecules, with double bonds at their ends, and hydrogen molecules.

But thermal cracking is difficult to control and also creates many large and useless molecules. As a rule, the higher the temperature in the cracking tank, the higher the octane of the gasoline it produces but the smaller the yield. To make premium gasoline by thermal cracking, the refinery might have to waste all but 20% of the hydrocarbons it feeds to the cracking tank. Because this waste is intolerable, thermal cracking has been replaced almost completely by fluid catalytic cracking and reforming.

In these processes, hot hydrocarbon molecules are brought alumina catalysts. Like all catalysts, these materials facilitate reducing the activation energies needed to complete them. When a hydrocarbon mol­ecule attaches to the surface of the catalyst, the catalyst helps it rearrange (Fig. 1). The catalyst reduces the potential energies of the partially rearranged molecules so that less overall energy is needed to complete the rearrangement. Catalyzed rearrangements are proceeded at lower temperatures.

 

 

These catalysts also help to control the rearrangements. A particular catalyst will assist certain rearrangements more than others. Catalysts are particularly helpful in cracking larger molecules into smaller ones so that yields of gasoline molecules are much higher with catalysts than without.

Because all of the catalyst's work is done by its surface, most commercial catalysts are designed to have lots of surface area. The silica-alumina catalysts used in fluid cata­lytic cracking are actually small particles of porous materials. These particles are only about 50 microns in diameter and they swirl around with the fluid they are cracking.

The reactions take only a few seconds to complete, after which the catalyst particles must be separated from the fluid. The mixture passes through a cyclone separator, where it moves very rapidly around in a circle. The acceleration causes the denser catalyst par­ticles to migrate to the outside of the separator and the clear fluid can then be extracted from the middle of the device.

Unfortunately, the catalyst particles quickly accumulate a coating of very targe molecules that don't react and can't be removed easily. Like most catalysts, they lose their catalytic activity when their surfaces become dirty. The only effective way to clean the surfaces of these particles is to burn the residue off them. That's just what the oil refinery does. This burning regenerates the catalyst particles and prepares them for their next trip through the fluid.

 

WORDLIST:

 

Transportation fuel топливо для транспортных средств

decomposition распад, разложение

double bond двойная связь

electronic shell электронная оболочка

cracking tank (зд) крекинг - установка

fluid catalyst cracking флюидизирбванный крекинг

silica-alumina алюмосиликатный

cyclone separator циклонный разделитель

coke нефтяной кокс

coker коксовая установка, коксовик

 

EXPRESSIONS:

То store unmarketable molecules - хранить невостребованные (нереализован­ные) нефтепродукты

То complete electronic shell by forming a double bond - дополнять электрон­ную оболочку путем формирования двойной связи

Thermal cracking is difficult to control and also creates many large and useless molecules - Процессом термического крекинга трудно управлять, и в ходе него образуется большое количество крупных и ненужных молекулярных соединений.

То make premium gasoline by thermal cracking, the refinery might have to waste all but 20% of the hydrocarbons it feeds to the cracking tank. - При про­изводстве бензина марки «премиум» с использованием термического крекинга, выход нефтепродукта составляет около 20% (углеводородных соединений) от об­щего количества сырья, подаваемого в крекинг - установку.

 

EXERCISES

/. Give equivalents to the following words and word combinations:

 

Hydrogen atom, branched paraffin,сырая нефть, расщеплять молекулы, raw gaso­line, электронная оболочка, термический крекинг, октановое число, catalytic crack­ing, cracking tank, distillation, двойные связи, catalyzed rearrangement, пористый материал, восстанавливать катализатор.

II. Answer the following questions:

a) What is the thermal cracking?

b) Try to formulate the principle difference between the thermal and the catalytic cracking.

c) Why has the thermal cracking been replaced by the catalytic one?

d) What is the role of a catalyst in the cracking process?

e) Can you give and explain the principle advantage of the catalytic one?

f) What are main requirements for the commercial catalysts?

g) What method is used to regenerate catalysts?

III. Translate the following sentences:

 

a) Для получения бензинов с хорошими характеристиками (октановое число более 80) нужна дополнительная вторичная переработка бензиновой фракции. Ката­литический крекинг — один из основных процессов вторичной нефтепереработки.

b) Ger hydrocarbons split into shorter ones at low temperatures if a catalyst is used - a process called the catalytic cracking. The main use of the catalytic cracking in a refinery is to covert surplus fuel oil into gasoline and diesel fuels.

c) Distilling crude oil provides different amounts of each fraction. But the demand for a fraction such as gasoline is higher than the amount supplied by the distillation alone. Rather than distilling more crude oil, an alternative is to crack oil fractions with longer hydrocarbons.

d) Каталитический крекинг - это процесс разложения тяжелых молекул уг­леводородов на более легкие, и протекает он при высоких температурах (500°С) в присутствии катализатора.

e) The other product in this reaction is an alkane called octane used in gasoline. During the catalytic cracking, the catalyst quickly accumulates a coating of carbon. The only effective way to clean the surfaces of the carbon is to bum it off.

f) Нефтяной реформинг состоит в разложении больших молекул углеводоро­дов на меньшие для получения более легких углеводородов. Процесс перегонки включает каталитический крекинг, который понижает вязкость нефтепродукта.

IV. Translate the following text into English:

 

Для улучшения антидетонационных свойств бензинов прямой перегонки их перерабатывают в условиях каталитического крекинга. Такая переработка бензи­на называется риформингом.

С помощью каталитического риформинга низкооктановые бензины превра­щаются в высокооктановые.

При прямой перегонке нефти, термическом и каталитическом крекинге поми­мо бензина, керосина и масел образуются также тяжелые остатки. Они представ­лены различными сортами мазута, гудрона и крекинг-остатками. Часть этих остат­ков используется как котельное топливо, а также для получения битумов. Другая часть подвергается дополнительной переработке с целью получения бензина и других легких фракций. Для переработки тяжелых остатков применяется способ, называемый коксованием. Тяжелый нефтяной остаток расщепляется при высокой температуре, в результате образуются газ, бензиновая и газойлевая фракции и в остатке ~ нефтяной кокс. Кокс представляет собой твердое пористое вещество серебристо-серого цвета, состоящее из чистого углерода и зольной части угля.