книги / Переработка нефти и газа

molecules, therefore part of carbon transforms into coke consisting nearly entirely of carbon atoms stuck together. Large molecules decompose producing a set of smaller ones i.e. from methane and upwards. For lack of hydrogen, many of newly formed molecules represent olefins. If certain raw material molecules consist of several aromatic or naphthenic cycles linked together, they decompose forming smaller aromatic or naphthenic molecules and olefins. Eventually, molecules consisting of several aromatic or naphthenic cycles and long lateral chains would normally lose their lateral chains. Resultant molecules, although containing less carbon atoms, turn out to be heavier, i.e. having higher relative density. Apart from this, their boiling temperature would usually be higher too. Catalytic cracking equipment consists of three parts: reactor, regenerator and fractionating tower.

Process Parameters

Catalytic cracking unit would normally remain serviceable unless incapable of burning-off. This could happen differently, but usually becomes obvious, when gasoline yield drops, while the amount of gases С4- or heavy gasoil at the same time starts increasing. Cracking unit product yield values depend on a variety of factors including raw material quality, reactor temperature, raw material feed rate, circulation rate, time of day and ambient temperature.

Raw material quality. Cracking reaction is rather complicated, and there is a lot of data that could be used to predict the yield on the basis of various raw material properties. Among significant properties there are raw material density and content of paraffins, naphthenes and aromatics.

Reactor temperature. The higher the temperature, the higher cracking reaction intensity. But at a certain moment, gas generation amount sharply increases due to the decrease of gasoline or light gasoil amount. Optimum reactor temperature is based on economic considerations.

Raw material feed and circulation rate. An excessively high feed rate adversely affects yield values, therefore, it is necessary to observe balance with the amount of fractionation residue that should either be directed to circulation or left within the heavy cracking gasoil cut.

Day time and temperature. To regenerate dead catalyst, air should continuously pass through the regenerator. If air temperature beyond the unit limit drops, air density increases. As air supply pumps delivery rate is continuous, amount of cold air

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entering the regenerator is higher than that of warm air. The more oxygen supply, the more coke is burned off from catalyst surface. The fresher the catalyst, the higher reaction efficiency. The higher reaction efficiency, the larger gasoline production amount. Automatic record of process parameters would really enable to register deviations of pointers: for example, at night, when air temperature is lower, product yield is higher. At day, when it gets hotter, yield drops. The same applies to results obtained in winter and summer, which is bad as demand for gasoline is higher in summer, when yields decline.

A task of catalytic cracking consists in transforming heavy gasoil into gasoline and lighter cuts. Raw materials represent: heavy gasoil, light cut of vacuum distillation, gasoil recycle stock. Cracking reaction products: coke, gases, cracking gasoil, light cracking gasoil, heavy cracking gasoil and gasoil recycle stock.

REFORMING

Catalytic reforming units presently constitute an almost mandatory part of each oil refinery. This process intends to obtain high-aromatic gasoline distillates used as a high-octane component to extract individual aromatic hydrocarbons such as benzene, toluene and xylene out of them.

Catalytic reforming raw materials mainly represent straight-run naphtha and rarer oil distillates such as for example gasoline of thermal cracking, coking and hydrocracking. These cuts normally contain high concentrations of paraffins and naphthenes. Catalytic reforming turns many of such components into aromatic compounds with considerably higher octane numbers.

This is mainly accompanied by the following useful chemical reactions:

1.Paraffins transform into iso-paraffin (isomerization reaction).

2.Paraffins transform into naphthenes (ring formation reaction).

3.Naphthenes transform into aromatics (dehydrogenization reaction). A number of certain side reactions take place:

1.Part of paraffins and naphthenes undergo cracking transforming into hydrocarbon gases.

2.Part of naphthenes and aromatic hydrocarbons lose their lateral chains that transform into hydrocarbon gases.

The most important point to be kept in mind is that paraffins and naphthenes transform into aromatic compounds.

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Equipment

You might think that these complicated transformations need any extraordinary equipment. In fact, they require an unusual catalyst consisting of aluminum oxide (А12О3), silica gel (SiO2) and platinum (Рt). The required amount of platinum is not so small (a few million dollars worth for a single reforming unit), therefore the catalyst deserves great attention.

There are a number of techniques used to bring hydrocarbon raw material into contact with the catalyst. Below, we are going to consider an option termed fixedbed catalyst process as in such case hydrocarbons infiltrates through the reactor catalyst bed.

The most efficient progress of each of the reactions needs various conditions of the unit operation i.e. various values of pressure, temperature and duration of raw material stay in the reactor. Therefore, the unit uses three sequential reactors (fig. 2). Each of them discharges its function. The reactors pressure is 200–500 psi (14–35 atmospheres), and temperature – 480–520°С

Fig. 2. Reforming unit layout:

1 – reactor; 2 – separator; 3 – tower;

I – raw material; II – hydrogen-containing gas; III – gas, IV – reforming product

Reactors have usually characteristic spherical shape. Raw material is brought to a certain pressure, heated and fed to reactor I, where it infiltrates through the catalyst bed and escapes from the lower part of the reactor. This procedure takes place twice in two subsequent reactors. Then, raw material passes through the cooler, where the bulk of it is liquefied. Liquefaction is required to separate hydrogen-rich gas and send it to recirculation. It is a rather significant moment that deserves a couple of words.

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Hydrogen is an essential co-product of the catalytic reforming. Look once again at the chemical reactions. The majority of them are accompanied by hydrogen release as its content in aromatic hydrocarbons is less than that in paraffins and naphthenes. But hydrogen is consumed at this very stage. It should be added to raw material so that to maintain its high concentration in reactors. In this case, carbon atoms would never precipitate on the catalyst as it takes place during catalytic cracking. Instead, carbon interacts with hydrogen forming hydrocarbon gases.

Process Parameters

Reforming unit controls that could be handled by an engineer are temperature, pressure and raw material duration time in the reactor. Task of such handling consists in maintaining a balance between quantity and quality of reforming product. Ratio between these two parameters is shown in fig. 3: the higher octane number, the lower reforming product yield by % of volume. Accordingly, yield of gaseous products increases. Thus, catalytic reforming process should be brought into an ideal compliance with gasoline blending operations and with the work of other units that deal with such products as gasoline components.

Fig. 3. Reforming Product Yield and Octane Number Relationship

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Lecture 5

HYDROCRACKING

Hydrocracking is a process of a later generation than catalytic cracking and catalytic reforming, due to which it efficiently handles the same tasks as of the two above processes. Hydrocracking allows for increasing the yield of gasoline components normally through the transformation of a raw material such as gasoil. Gasoline components quality reachable in this case can never be reached with repeated submission of gasoil through cracking process, as a result of which it has been obtained. Hydrocracking would also allow for transforming heave gasoil into light distillates (jet and diesel fuel). And probably, what is more important that hydrocracking leaves no heavy undistillable residue (coke, pitch or still bottoms), but volatile cuts.

Production Process

The term hydrocracking meaning is very simple. It is catalytic cracking in the presence of hydrogen. The combination of hydrogen, catalyst and appropriate process conditions allows for cracking poor quality gasoil that is produced by other cracking units and is normally used as diesel fuel component. Hydrocracking unit produces high-quality gasoline.

The greatest advantage of hydrocracking consists in its capability of switching over the refinery capacities from producing large amounts of gasoline (when hydrocracking unit operates) to producing large amounts of diesel fuel (when the unit is idle).

Worth mentioning is another moment: hydrocracking process results in significant amounts of iso-butane, which turns out to be useful for controlling raw material quality in alkylation process.

Equipment and Chemical Reaction

Hydrocracking catalysts are fortunately less valuable and expensive than those of reforming. Normally they represent sulfur compounds with cobalt, molybdenum and nickel (СоS, МоS2, NiS) and aluminum oxide. As distinct from catalytic cracking, but as it takes place in the course of catalytic reforming, catalyst constitutes a static bed. As it takes place during catalytic reforming, hydrocracking would most commonly undergo in two reactors, as shown in fig. 1.

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Raw materials is mixed with hydrogen brought to 290— 400°С and pressure of 1,200— 2,000 psi (84—140 atmospheres), and is directed to reactor I. When passing through the catalyst bed, a portion of raw materials accounting nearly for 40–50 % is subjected to cracking with the formation of products corresponding to gasoline in terms of boiling temperatures (boil-off point under 200 °С ).

Catalyst and hydrogen complement each other in a number of aspects. At first, catalyst provides cracking. Further cracking requires heat supply, which means that this is an endothermal process. At the same time, hydrogen interacts with molecules that result from cracking saturating them, and at the same time, heat is generated. In other words, this reaction referred to as hydrogenization, is an exothermal reaction. Thus, hydrogen provides heat required for cracking to progress. Another aspect, when they complement each other is the formation of iso-paraffins. Cracking produces olefins that could combine with each other leading to normal paraffins. Hydrogenization makes double bonds quickly saturate, which frequently results in iso-paraffins. This prevents repeated generation of undesirable molecules (octane numbers of iso-paraffins are higher than in the case of normal paraffins).

When the hydrocarbon mixture leaves reactor I, it is cooled down, liquefied and passed through a separator for hydrogen separation. Hydrogen is again mixed with raw material and directed into the process, whereas liquid is fed to the distillation. Products obtained in reactor I are separated in fractionation tower and, depending on what is required as a result (components of gasoline, jet fuel or gasoil), their part is separated. Kerosene cut could be singled out as a side cut or left together with gasoil as the distillation residue.

The distillation residue is mixed again with hydrogen stream and introduced into reactor II. As the substance has already be subjected to hydrogenization, cracking and reforming in reactor I, reactor II process takes place under more severe conditions (higher temperatures and pressures). As the products of stage I, the mixture leaving reactor II separates from hydrogen and directed to fractionation.

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Fig. 1. Two-stage hydrocracking unit:

1, 5 – furnaces; 2 – reactor; 3 – separator; 4 – tower;

I – raw material; II – hydrogen-containing gas; III – hydrocarbon gas; IV – light products; V – kerosene cuts; VI – heavy cracking product

Products and yields. Another notable property of hydrocracking process is 25 % increase of product volume. The combination of cracking and hydrogenation gives products, relative density of which is significantly lower than density of raw material. Hydrocracking products represent two major cuts used as gasoline components. Normal flow rate of hydrogen is 2,500 standard cubic feet per barrel. Heavy hydrocracking product is ligroin (naphtha) that contains many precursors of aromatics (i.e. compounds easily transformable into aromatics). This product is frequently directed to reforming unit for upgrading. Kerosene cuts are a good jet fuel or a raw material for distillate (diesel) fuel as they contain little aromatics (as a result of hydrogen saturation of double bonds).

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Lecture 6

HYDROTREATING

Oil cuts containing hydrocarbons С6 and heavier ones and organic compounds of sulfur. Sulfur atoms could be attached to carbon atoms in various positions of molecules and therefore, from viewpoint of chemistry, sulfur comprises the cut. Hydrotreating enables to break sulfur atoms loose from hydrocarbons molecules.

At present, submitted to hydrotreating are light straight-run distillates boiling off at temperatures below 350°С including distillates directed to platforming similar to distillates obtained from raw materials of secondary origin (gasoils of catalytic cracking and coking), heavy gasoils coming to catalytic cracking and other products as well.

The petrochemical flow is mixed with hydrogen stream and brought to 260–425 °С, then the mixture of the petroleum product and hydrogen is directed to the reactor filled with catalyst in the form of pellets (fig. 1). Hydrotreating of petrochemicals to make them free from sulfur compounds is normally performed with the use of cobaltmolybdenum or nickel-molybdenum catalyst on aluminum oxide support. In the presence of catalyst, a number of chemical reactions would take place:

1.Hydrogen combines with sulfur to form hydrogen sulfide (Н2S).

2.Certain nitrogen compounds transform into ammonia.

3.Any metals contained in oil precipitate on the catalyst,

4.Certain olefins and aromatic hydrocarbons saturate with hydrogen, apart from this hydrocracking of naphthenes takes place to a certain extent, and a certain amount of methane, ethane, propane and butanes would form.

Stream leaving the reactor is directed to the evaporator, where gaseous hydrocarbon, as well as Н2S with a small amount of ammonia would at once come up. So that to completely separate all these light products, the reactor outlet is provided with a small rectification tower.

Significance of hydrotreating process continuously increases for two major reasons:

1.Removal of sulfur and metals from the cuts directed for further treatment is an important protection for catalysts used in reforming, cracking and hydrocracking processes.

2.According to environmental regulations, content of sulfur in petrochemicals continuously decreases, which requires desulfuring of distillates and jet fuels.

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Hydrotreating of jet fuel. Hydrotreating is used to improve combustion indices of distillate fuels and particularly of jet fuel. Kerosene cut could contain many aromatic hydrocarbons characterized by high carbon-to-hydrogen ratio. Combustion of these compounds could produce large amount of smoke due to lack of hydrogen. By the way, one of jet fuel rated components is smoke point.

Fig. 1. Hydrotreating unit:

1 – furnace; 2 – reactor; 3 – separator; 4 – tower;

I – raw material; II – hydrogen-containing gas; III – hydrocarbon gas; IV – hydrotreating product

Hydrotreating allows for improving kerosene with low sootless flame. Such process makes benzene rings in molecules of aromatic hydrocarbons saturate with hydrogen and thus transform into naphthenes the flame of which is as not as sooting.

Layout of a fuel-producing refinery

The majority of domestic and foreign oils cannot yield fairly high-quality motor gasoline without catalytic reforming and cracking as straight-run gasolines extractable from such oils have the following octane numbers: under 120°С – 56.4; under 150°С – 50.4 and under 200°С — 41.6.

If the refinery is included units of coking, catalytic cracking, catalytic reforming, alkylation of iso-butane with butylenes and polymerization of cracking gases propylene cut, this could produce motor gasoline (under 205oC) with octane number of 80, while its yield will account for 30.5 % versus oil. With the same oil refining

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option, the refinery obtains about 6.4 % (versus oil) of valuable hydrocarbon gases that could be used as chemical industry raw materials (aside from 0.6 % of hydrogen for making either elemental sulfur or sulfuric acid).

Such option of oil refining is shown in fig. 2.

Fig. 2. Hydroforming of light cuts (simple oil refining):

1 – oil distillation process at atmospheric pressure; 2 – vacuum distillation; 3 – visbreaking process; 4 – catalytic cracking process; 5 – alkylation process; 6 – hydrotreatment process; 7 – reforming process; 8 – gasoline cuts separation process; 9 – isomerization process; 10 – gas-fractionation unit;

I – oil; II – gasoline; III – sulfur; IV – fuel gas; V – jet fuel; VI – diesel fuel; VII – residual fuel

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