Basic Petroleum refining, Chemistry tutorial

Introduction:

The Crude oil as it is found in nature comprises of complex mixtures of compounds having hydrogen and carbon (hydrocarbons). Moreover to the hydrocarbons, compounds of sulphur, nitrogen and oxygen are present in small amounts. Moreover, there are generally traces of vanadium, nickel, chlorine and arsenic. Such compounds are injurious unless they are eliminated from crude oil by refining. These comprise fractional distillation, vacuum distillation and other methods like solvent extraction, absorption, thermal diffusion, crystallization, absorption and stripping.

Historical Aspect:

Petroleum refining started in the United States of America and Russia in the second half of the 19th Century, following the discovery in the year 1859 of 'rock oil' in Pennsylvania. In the earliest refineries, simple distillation separated crude oil to impure gasoline, kerosene, lubricating oil and fuel oil fraction. Kerosene or lamp oil was the main marketable product. To enhance its odor and appearance, kerosene was treated chemically by caustic soda or sulphuric acid.

The earliest automobile fuel was comprised of those fractions of crude oil which were too light to be comprised in kerosene. Before the invention of automobile, this fraction had been virtually not stable as demand for it rose and there was increase in production and quality enhancement. Processes for the continuous distillation of crude oil were introduced.

After World War I a main enhancement in refining came by the growth of the cracking process, comprising of heating excess heavier oils under pressure and thus cracking or splitting, their large molecules to the smaller ones which form the lighter and more expensive fractions. After suitable chemical treatment, gasoline prepared via cracking performed better in automobile engines as compare to gasoline derived from the straight distillation.

Throughout the year 1930s and World War II, sophisticated refining methods comprising the use of catalysts led to further enhancements in the quality of fuels and increase in supply. These enhanced processes, comprising catalytic cracking, polymerisation, alkylation and isomerization, permitted the petroleum industry to meet up the high-performance demands of combat aircraft and, after the war, to supply the increasing quantities needed via commercial aviation.

The year 1950 and 1960 brought a large-scale demand for jet fuel and for high-quality lubricating oils. The catalytic reforming was established as the leading method for upgrading automotive motor gasoline for use in higher compression engines. Hydro-cracking, accomplished by addition of hydrogen throughout refining, as well enhanced the crude-oil fractions.

Separation into Components:

The different hydrocarbon compounds which are mixed altogether in crude oil have different   boiling  points,  but  apart  from  the  lightest,  the differences between the boiling points of neighboring members in the rising scale of molecular weight are thus small, only fraction of a degree that they can't  be separated via ordinary distillation. Luckily separation is not generally essential. Most of the common petroleum products comprise of mixtures of compounds whose boiling points fall in a specified range.

Four main kinds of hydrocarbons are present in the crude oil. They are normal paraffins, iso-paraffins, cycloparaffins (naphthenes) and aromatics. Some of the crude oil likes the heavy Mexican and Venezuelan crude oil, are predominantly naphthenic and are rich in asphalt (that is, a high boiling semi solid material). Wax is generally, however not always, related by paraffin-base crude oil. Moreover to the hydrocarbons, compounds of sulphur, nitrogen and oxygen are present in small amounts in crude oil. These are injurious unless they are eliminated. As well, there are generally traces of vanadium, nickel, chlorine and arsenic.

Whenever a liquid mixture of compounds is heated it boils, and the vapor above the liquid is richer in the lower boiling components. Whenever the vapor is led away from the liquid and condensed, the mixture can be separated to components boiling at various temperatures. This method is termed as simple distillation. Chemists, working in a laboratory, generally carry out this method in an apparatus termed as distillation apparatus.

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Fig: Simple Distillation Apparatus

The temperature of vapors condensing on the thermometer is generally taken to be the boiling temperature of any specific fraction. By changing the receiver flask if the reading on the thermometer changes; then pure compounds can frequently be separated.

The crude oil consists of so many components having similar boiling temperatures which the temperature slowly increases all via a distillation and as there is no clear separation, just mixtures  of compounds can be obtained as a outcome of the fact that the boiling temperature of the constituents of crude oil are quite close to one other therefore, making the isolation of individual constituents difficult.

Thus, simple distillation is not proficient enough to separate the crude oil. A related method termed as fractional distillation or fractionation is employed. The industrial plant required for fractionation of crude oil is very dissimilar from the laboratory equipment; although only different in scale however the principles are similar. 

A schematic diagram of the fractionating tower of crude oil on industrial level is illustrated in figure shown below. The oil is first heated very strongly via gas burners (generally the gas employed is a by-product of the fractionation). The hot, vaporized oil then passes up the tower where there is a sequence of trays. The higher boiling fractions collect in the lower trays and are piped out.

The lower boiling fractions pass via to higher trays before being taken off. Gases like methane, ethane and propane are led off at the top of the tower and recycled to the burners. The boiling range of the fractions can be regulated very accurately via adjusting the amount of heating in the first phase. Throughout fractionation the hydrocarbon is not chemically changed however are separated physically.

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Fig: Fractional Distillation Column

The hydrocarbon fractions are, for numerous purposes, employed unchanged. Typical uses of fractions, at times with other hydrocarbons added are in ascending order of boiling temperature. 

The primary refinery procedure is fractional distillation, which might be followed via other   physical separation processes, like solvent extraction, in which superior lubricating stocks are extracted via means of a solvent, generally furfural obtained from the oat hulls.

Fractional Distillation and Vacuum Distillation:

Modern petroleum distillation units function constantly over a long time period and are large compared by those that carry out the similar method in other industries. Units having 100,000 barrels per day capacities are common place, and units of over 200,000 barrels per day are now in operation. The American barrel is the most broadly used unit in the oil industry and consists of 42 U.S. gallons, or 35 imperial gallons that is around 160 litres.

The figure above exhibits the principles of operation of a fractional distillation unit. The crude oil is pumped at a constant rate via steel alloy tubes in a furnace, fired via gas oil, and heated to a temperature between 315 and 370oC based on the kind of crude oil and the end product desired. A mixture of vapors and un-vaporized oil passes from the furnace to the fractionating column, a vertical cylindrical tower as much as 150 feet (45 meters) high, provided by 30 or 40 perforated fractionating trays spaced at regular intervals. The bubble-cap tray is the commonest kind used; however the sieve tray (less expensive, however having a narrower operating range) is at times used. It comprises of a simple perforated plate having small hole of around 3/16 to 1/4 inch in diameter.

The oil vapors rise up via the column and are condensed to a liquid in a water-cooled condenser at the top. A small quantity of gas remains uncondensed; this is piped away to the refinery fuel-gas system. A pressure control valve on this line maintains the fractionating column pressure at the needed pressure, generally close to atmospheric pressure.

Portion of the condensed liquid, termed as reflux, and is pumped back to the top of the column and runs down from tray to tray, contacting the rising vapors as they pass via the slots in the bubble caps. The liquid progressively absorbs heavier constituents from the vapors and in turn loses its lighter components. Condensation and re-evaporation occur on each tray. Ultimately, equilibrium is reached in which there is a continuous gradation of temperature and of oil properties all through the column, having the lightest constituent on the top tray and the heaviest at the bottom. The utilization of reflux and a column of this kind differentiate fractional distillation from the simple distillation.

In the column illustrated in the figure above, fractions termed as side streams are withdrawn at some points. These products have properties intermediate from the top and base of the column.  Typical  boiling ranges for different products are as follows; light gasoline (overhead) 25 - 95oC; naphtha (no 1 side stream) 95-150oC; kerosene (no 2 side stream) 150 -230oC; and gas oil (no 3 side stream) 230-340oC.

In practice, the boiling ranges of such products can be differed in broad limits according to needs. This is accomplished either by choosing different draw off points in the column or via varying the quantity of oil withdrawn and consists of the effect of changing the equilibrium concentration of the liquid on the tray concerned.

The degree of fractionation, or sharpness of separation between hydrocarbons, based on the number of trays and their effectiveness in accomplishing equilibrium between vapors and liquid. It as well based on the reflux ratio, that is, the volume of liquid pumped back divided via the volume of overhead product. Reflux ratios in the crude oil distillation columns are generally between 1:1 and 3:1.

Un-vaporized oil entering the column flows downward over the other set of trays in the lower portion of column known as stripping trays that remove any light constituents remaining in the liquid. To help in this, steam is injected at the bottom of the column. The residue which passes from the base of the fractionating column is appropriate for blending to fuel oils. Alternatively, it might be distilled a second time under vacuum conditions and further quantities of distillate recovered for use as a beginning material for manufacturing lubricating oil or as feed-stock for the catalytic cracking.

The principles of vacuum distillation look like those of fractional distillation, and apart from that larger diameter columns are employed to maintain the comparable vapors velocities at reduced pressures, the equipment is as well identical.  The vacuum is generated via steam ejectors in vacuum distillation. The components which are less volatile can be distilled without increasing the temperature to the range at which cracking takes place, as it would at atmospheric pressure. Firing conditions in the furnace are adjusted in such a way that the oil temperature doesn't surpass around 400oC. The residue after vacuum distillation is asphalt or bitumen.

Super fractionation is an extension of the fractional distillation using columns having a much larger number of trays (example: 100) and reflux ratios exceeding 5:1 having such equipment,  it is possible to get fractions having only some hydrocarbons or even to separate pure compounds. Via this process, isopentane of over 90% purity is produced for the aviation gasoline. Isohexane and isoheptane concentrates are as well prepared for the similar purpose; these isoparaffins encompass much higher octane numbers than corresponding normal paraffins.

Absorption and Stripping:

Absorption and stripping are methods employed to get valuable light products like propane or propylene and butane/butylene from the gasoline vapors which pass out of the top of the fractionating tower. In the absorption method, gasoline vapors are bubbled via absorption oil like kerosene or heavy naphtha in equipment looking like a fractionating column. The light products dissolve in the oil whereas dry gases like hydrogen, methane, ethane and ethylene pass via undissolved. Absorption is more efficient under pressure of around 100-150 pounds per square inch (7-11kg/cm) as compare to it is at atmospheric pressure.

The light products are separated from the absorption oil in the stripping method. The solution of the absorption oil and light products is boiled via steam and passes to stripping column where the light product vapors pass upward and are recovered via condensation by water cooling under pressure. The un-vaporized oil passes from the base of the column for reuse.

Solvent Extraction and Adsorption:

Solvent extraction procedure is primarily used for the elimination of constituents which would have an adverse effect on the performance of the product in use. The quality of kerosene is enhanced by the extraction of aromatic compounds that burn by a smoky flame. The other significant operation is the elimination of heavy aromatic compounds from lubricating oils. Removal enhances viscosity-temperature relationship of the oil, extending the temperature range over which the satisfactory lubrication is obtained. The usual solvents for extraction of lubricating oil are phenol and furfural. The other solvents are dichloroethylether, nitrobenzene, and a mixture of liquid propane and cresylic acid.

Some highly porous, solid materials have the capability to choose and adsorb particular kinds of molecules, therefore separating them from other kinds. Silica gel is employed in this manner to separate the aromatics from other hydrocarbons, and activated charcoal is employed to eliminate liquid components from gases.

Adsorption is therefore rather analogous to the method of absorption with oil, however the principles are different. Layers of the absorbed material having only some molecules thick are made on the extensive interior surface of the adsorbent; this interior surface might amount to some acres per pound of material.

Recent years have brought latest growths in the use of adsorbents of a very selective nature known as molecular sieves. Molecular sieves are generated via dehydration of naturally taking place or synthetic zeolites (that is, crystalline alkali metal aluminosilicates). The dehydration leaves inter-crystalline cavities which have pore openings of definite size, based on the alkali metal of the zeolite. Beneath adsorptive conditions, normal paraffins molecules can enter the crystalline lattice and be selectively retained, while the other molecules are excluded. This principle is employed commercially for the elimination of normal paraffins from gasoline fuels, therefore increasing their octane number. The use of molecular sieves has as well been extended to the separation of hydrocarbons of higher molecular weight.

Thermal Diffusion and Crystallization:

Whenever a mixture of hydrocarbons is passed via a narrow gap, of the order of 1/100 inch, between hot and cold surfaces, a few constituents concentrate close to the hot surface and others close to the cold.

The phenomenon is termed as thermal diffusion, it is not clearly comprehended, and however it is believed that separation takes place as a result of differences in the shapes of the molecules. However this method has been applied in the laboratory as an analytical tool, it is unlikely to determine much use in industry as the thermodynamic efficiency is low.

The crystallization of wax from lubricating oil fractions is necessary to make the oils appropriate for use. A solvent, for illustration, a mixture of benzene and methyl ethyl ketone is first added to the oil and the solution is chilled to around 20oC. The function of the benzene is to keep the oil in solution and maintain its fluidity at low temperatures, while the methyl ethyl ketone (that is, butanone) acts as the wax precipitant. Rotary filters are employed to filter off the wax crystals on a specially woven canvas cloth stretched over a perforated cylindrical drum. A vacuum, maintained in the drum, sucks the oil to it. The wax crystals are eliminated from the cloth via metal scrapers, after washing with solvent to eliminate traces of oil. The solvents are later distilled from the oil and reused.

Alteration of Molecular Structure:

The separation methods illustrated above are mainly based on the differences in physical properties of the components of crude oil.  By chemically changing their molecular structure, it is possible to transform less valuable hydrocarbon compound into those in demand.

The first of these conversion methods is cracking or thermal decomposition of long chain hydrocarbon molecules to shorter molecules having lower boiling points, for illustration, paraffin molecule like dodecane (C12H26) has such poor antiknock properties that it can't be employed in a modern automobile engine, however under intense heat it breaks down to shorter molecules like paraffins (C6H14) or olefins that is, alkene (C6H12, C5H10), that are appropriate for motor fuels.  The chemical reactions which occur in a cracking operation are complex. The product derived from the cracking method is, in effect, a synthetic crude oil.

Cracking:

Cracking is the term given to breaking up large hydrocarbon molecules into smaller and more helpful bits. This is accomplished by using high pressures and temperatures devoid of a catalyst, or lower temperatures and pressures in the presence of a catalyst. The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (that is, petroleum). Such fractions are obtained from the distillation method as liquids, however are re-vaporized before cracking.

There is no any single exceptional reaction which happens throughout cracking. The hydrocarbon molecules are broken up in a fairly arbitrary manner to produce the mixtures of smaller hydrocarbons, some of which encompass carbon-carbon double bonds. One possible reaction comprising the hydrocarbon C15H32 may be:

C15H32 → 2C2H4 + C3H6 + C8H18

               Ethene  Propene Octane

This is only one manner in which this specific molecule might break up. The ethene and propene are significant materials for making plastics or making other organic chemicals. The octane is one of the molecules found in the petrol (that is, gasoline).

Thermal Cracking and Reforming:

The earliest cracking methods are typified by methods in which kerosene or gas oil materials were transformed via heating to temperatures between 450-540oC at pressure of 250 to 500 pounds per square inch.

This method generated gasoline having octane number 70, which is low by modern standard. In thermal cracking, high temperatures (generally in the range of 450-750°C) and pressures (up to around 70 atmospheres) are employed to break the large hydrocarbons into smaller ones.  Thermal cracking provides mixtures of products having high proportions of hydrocarbons having double bonds-alkenes. 

Visbreaking, the other thermal cracking method, decreases the viscosity of heavy crude oil residues to make them more appropriate for inclusion in the fuel oils.

The steam cracking method through which ethylene and other olefins are made up from naphtha differs from the thermal cracking in that it is taken out at low pressures and higher temperatures.

Steam cracking is a petrochemical method in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. This is the principal industrial process for producing the lighter alkenes (or generally olefins), comprising ethene (or ethylene) and propene (or propylene).

In steam cracking, a gaseous or liquid hydrocarbon feed similar to naphtha, LPG or ethane is diluted by steam and briefly heated in a furnace devoid of the presence of oxygen. Usually, the reaction temperature is very high, at around 850°C; however the reaction is only allowed to take place very briefly. In modern cracking furnaces, the residence time is even decreased to milliseconds, resultant in gas velocities faster than the speed of sound, to enhance yield. After the cracking temperature has been reached, the gas is rapidly quenched to stop the reaction in the transfer line heat exchanger.

The products generated in the reaction based on the composition of the feed, the hydrocarbon to steam ratio and on the cracking temperature and furnace residence time. Light hydrocarbon feeds like ethane, LPGs or light naphtha give product streams rich in the lighter alkenes, comprising ethylene, propylene and butadiene. Heavier hydrocarbon (full range and heavy naphtha's and also other refinery products) feeds give some of these, however as well give products rich in aromatic hydrocarbons and hydrocarbons appropriate for inclusion in gasoline or fuel oil. The higher cracking temperature (as well termed to as severity) favors the production of ethene and benzene, while lower severity generates higher amounts of propene, C4-hydrocarbons and liquid products. The method as well yields in the slow deposition of coke, a form of carbon, on the reactor walls. This degrades the efficiency of the reactor, therefore reaction conditions are designed to minimize this. However, a steam cracking furnace can merely run for a few months at a time between de-cokings. Decoking needs the furnace to be isolated from the method and then a flow of steam or a steam/air mixture is passed via the furnace coils. This transforms the hard solid carbon layer to carbon monoxide and carbon-dioxide. Once this reaction is complete, the furnace can be returned to the service.

Thermal reforming, a modification of the thermal cracking method, reforms or modifies the properties of low grade components like naphtha via transforming the molecules into those of higher octane number, Pressures employed are rather higher as compare to those in cracking. At a temperature from 950 to 1050oF, it is possible to get gasoline having octane numbers of between 70 and 80 from components of less than 40.

Hydrocracking:

Hydrocracking is a catalytic cracking method assisted via the presence of an elevated partial pressure of hydrogen gas. Identical to the hydrotreater, the function of hydrogen is the purification of the hydrocarbon stream from sulphur and nitrogen hetero-atoms. 

The products of this method are saturated hydrocarbons, based on the reaction conditions (that is, temperature, pressure, catalyst activity) these products range from ethane, LPG to heavier hydrocarbons consisting mostly of isoparaffins. Hydrocracking is generally facilitated via a bi-functional catalyst which is capable of rearranging and breaking hydrocarbon chains and also adding hydrogen to aromatics and olefins to generate naphthenes and alkanes.

Main products from hydrocracking are jet fuel and diesel, whereas as well high octane rating gasoline fractions and LPG are generated. All such products encompass a very low content of sulphur and other contaminants.

This is very common in India, Europe and Asia as those regions have high demand for diesel and kerosene. In the US, Fluid Catalytic Cracking is more common as the demand for gasoline is higher.

Catalytic Cracking:

By the year 1950, a reforming method was introduced that used a catalyst to enhance the yield of the most desirable gasoline components whereas minimizing the formation of unwanted heavy products and coke. (A catalyst is a substance which promotes a chemical reaction however doesn't take part in it.)  In  catalytic  reforming,  as  in  the thermal  reforming,  a naphtha-type  material serves as the feedstock,  however the reactions are taken out in the presence of hydrogen, that inhibits the formation of unstable unsaturated compounds which polymerize to  the higher-boiling materials.

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Fig: Catalytic Cracking unit

In most of the catalytic reforming methods, platinum is the active catalyst; it is distributed on the surface of the aluminum oxide carrier. Small amounts of rhenium, chlorine and fluorine act as catalyst promoters. Despite of the high cost of platinum, the method is economical due to the long life of the catalyst and the high quality and outcome of the products obtained. The main reactions comprise the breaking down of long- chain hydrocarbons into smaller saturated chains and the formation of isoparaffins, made up of branched-chain molecules. The formation of ring compounds (that is, technically, the cyclisation of paraffins into naphthenes) as well occurs, and the naphthenes are   then dehydrogenated to aromatic compounds (that is, ring-shaped unsaturated compounds having fewer hydrogen atoms bonded to the carbon). The hydrogen discharged in this process makes a valuable by-product of catalytic reforming. The desirable end products are isoparaffins and aromatics, both having the high octane numbers.

In a usual reforming unit the naphtha charge is first passed over the catalyst bed in the   presence of hydrogen to eliminate any sulphur impurities. The desulphurised feed is then mixed by hydrogen (around five molecules of hydrogen to one of hydrocarbon) and heated to a temperature of 500 to 540° C (930 to 1,000° F). The gaseous mixture passes downward via catalyst pellets in a sequence of three or more reactor vessels. Early reactors were designed to operate at around 25 kilograms per square centimeter (that is, 350 pounds per square inch), however current units often operate at less than 7 kg/cm2 (that is, 100 pounds per square inch). As heat is absorbed in the reforming reactions, the mixture should be reheated in the intermediate furnaces between the reactors.

After departing the final reactor, the product is condensed to a liquid, separated from the hydrogen stream and passed to a fractionating column, where the light hydrocarbons generated in the reactors are eliminated by distillation. The reformate product is then available for blending to gasoline devoid of further treatment. The hydrogen leaving the product separator is compressed and returned to the reactor system.

The operating conditions are set to get the needed octane level, generally between 90 and 100. At the higher octane levels, product outcomes are smaller, and more common catalyst regenerations are needed. Throughout the course of reforming method, minute amounts of carbon are deposited on the catalyst, causing a gradual deterioration of the product yield pattern. Several units are semi-regenerative facilities that is, they should be eliminated from service periodically (once or two times annually) to burn off the carbon and rejuvenate the catalyst system-however increased demand for high-octane fuels has as well led to the growth of continuous regeneration systems, that avoid the periodic unit shutdowns and maximize the yield of high-octane reformate (figure shown above). Continuous regeneration uses a moving bed of catalyst particles which is gradually withdrawn from the reactor system and passed via a regenerator vessel, where the   carbon is eliminated and the catalyst rejuvenated for reintroduction to the reactor system.

Fluid Catalytic Cracking:

Fluid Catalytic Cracking (FCC, introduced by Tom Barnthouse) is the most significant conversion method employed in petroleum refineries. This is broadly employed to transform the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oil to more valuable gasoline, olefinic gases and other products. The cracking of petroleum hydrocarbons was originally completed by thermal cracking which has been nearly fully replaced by the catalytic cracking as it generates more gasoline having a higher octane rating. It as well generates byproduct gases which are more olefinic, and therefore more valuable, than those generated by the thermal cracking.

The feedstock to an FCC is generally that part of the crude oil that consists of an initial boiling point of 340 °C or higher at atmospheric pressure and an average molecular weight ranging from around 200 to 600 or higher.

This part of crude oil is often termed to as heavy gas oil. The FCC method vaporizes and breaks the long-chain molecules of the high-boiling hydrocarbon liquids to much shorter molecules via contacting the feedstock, at high temperature and moderate pressure, having a fluidised powdered catalyst.  In effect, refineries make use of fluid catalytic cracking to correct the imbalance between the market demand for gasoline and the surplus of heavy, high boiling range products resultant from the distillation of crude oil.

The modern FCC units are all continuous methods that operate 24 hours a day for as much as 2 to 3 years between shutdowns for schedule maintenance. There are a number of various proprietary designs that have been developed for modern FCC units. Each and every design is available under a license which should be purchased from the design developer via any petroleum refining company desiring to construct and operate an FCC of the given design.

Fundamentally, there are two different configurations for an FCC unit, the 'stacked' type where the reactor and the catalyst regenerator are contained in a single vessel having the reactor above the catalyst regenerator and the 'side-by-side' kind where the reactor and catalyst regenerator are in two separate vessels. These are the main FCC designers and licensors. Each of the proprietary design licensors claims to encompass exclusive features and benefits.

Reactor and Regenerator:

The schematic flow diagram of a general modern FCC unit below is mainly based on the 'side-by-side' configuration. The preheated high-boiling petroleum feedstock (at around 315 - 430 °C) comprising of long-chain hydrocarbon molecules is joined with recycle slurry oil from the bottom of the distillation column and injected to the catalyst riser  where it is vaporized and  cracked  to smaller molecules of vapors via contact and mixing by very hot powdered catalyst from the regenerator. All of the cracking reactions occur in the catalyst riser. The hydrocarbon vapors 'fluidise' the powdered catalyst and the mixture of hydrocarbon vapors and catalyst flows upward to enter the reactor at a temperature of around 535 °C and a pressure of  around 1.72 barg.

The reactor is however just a vessel in which the cracked product vapors are: (a) separated from the so-called spent catalyst via flowing via a set of two-phase cyclones in the reactor and (b) the spent catalyst flows downward via a steam stripping  section to eliminate any hydrocarbon vapors before the spent catalyst returns to the catalyst regenerator. The flow of spent catalyst to the regenerator is controlled via a slide valve in the spent catalyst line.

As the cracking reactions generate a few carbonaceous materials (termed to as coke) which deposits on the catalyst and very quickly decreases the catalyst reactivity, the catalyst is regenerated via burning off the deposited coke having air blown into the regenerator. The regenerator functions at a temperature of around 715 °C and a pressure of around 2.41 barg. The combustion of the coke is exothermic as it generates a huge amount of heat which is partly absorbed via the regenerated catalyst and gives the heat needed for the vaporization of the feedstock and the endothermic cracking reactions that occur in the catalyst riser. For that reason, FCC units are frequently termed to as being heat balanced.

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Fig: Fluid Catalytic Cracking Unit as used in Petroleum Refineries

The hot catalyst (at around 715 °C) leaving the regenerator flows to a catalyst withdrawal well where any entrained combustion flue gases are allowed to escape and flow back to the upper portion to the regenerator. The flow of regenerated catalyst to the feedstock injection point beneath the catalyst riser is controlled via a slide valve in the regenerated catalyst line. The hot flue gas exits in the regenerator after passing via multiple sets of two-phase cyclones which eliminate entrained catalyst from the flue gas. 

The amount of catalyst circulating between the regenerator and the reactor amounts to around 5 kg per kg of feedstock that is equivalent to around 4.66 kg per litre of feedstock. Therefore, an FCC unit processing 75,000 barrels/day (12,000,000 litres/day) will circulate around 55,900 metric tons per day of catalyst.

Distillation Column:

The reaction product vapors (at 535 °C and a pressure of 1.72 barg) flow from the top of the reactor to the bottom part of the distillation column (generally termed to as the main fractionators) where they are distilled to the FCC end products of cracked naphtha, fuel oil and off gas.

After further processing for the elimination of sulphur compounds, the cracked naphtha becomes a high-octane component of the refinery's blended gasoline.

The major fractionators off gas is sent to what is termed as a gas recovery unit where it is separated to butanes and butylenes, propane and propylene and  lower molecular weight gases   (that is, hydrogen, methane, ethylene and ethane). Some of the FCC gas recovery units might as well separate out some of the ethane and ethylene. 

However the schematic flow diagram (figure shown above) overleaf depicts the major fractionator as having merely one side cut stripper and one fuel oil product, numerous FCC main fractionators encompass two strippers and produce a light fuel oil and a heavy fuel oil. Similarly, numerous FCC major fractionators produce light cracked naphtha and a heavy cracked naphtha.  The terminology light and heavy in this circumstance refers to the product boiling ranges, having light products having a lower boiling range than heavy products.

The bottom product oil from the major fractionators includes residual catalyst particles that were not fully eliminated by the cyclones in the top of the reactor. For that reason, the bottom product oil is termed to as slurry oil. Part of that slurry oil is recycled back to the major fractionator above the entry point of the hot reaction product vapors so as to cool and partly condense the reaction product vapors as they enter the major fractionators. The remainder of the slurry oil is pumped via a slurry settler. The bottom oil from the slurry settler includes most of the slurry oil catalyst particles and is recycled back to the catalyst riser through joining it by the FCC feedstock oil. The so-called clarified slurry oil or decant oil is withdrawn from the top of slurry settler for use elsewhere in the refinery or as the heavy fuel oil blending component.

Regenerator Flue Gas:

Based on the choice of FCC design, the combustion in the regenerator of the coke on the spent catalyst might or might not be complete combustion to carbon-dioxide (CO2). The combustion air flow is controlled so as to give the desired ratio of carbon monoxide (CO) to carbon-dioxide (CO2) for each and every specific FCC design.

In the design illustrated in the figure above, the coke has only been partly combusted to CO2. The combustion flue gas (having CO and CO2) at 715 °C and at a pressure of 2.41 barg is routed via a secondary catalyst separator having swirl tubes designed to eliminate 70 to 90% of the  particulates in the flue gas leaving the regenerator. This is needed to prevent erosion damage to the blades in the turbo-expander which the flue gas is subsequent routed through.

The expansion of flue gas via a turbo-expander gives sufficient power to drive the regenerator's combustion air compressor. The electrical motor-generator can use or generate electrical power. If the expansion of the flue gas doesn't give enough power to drive the air compressor, the electric motor or generator gives the needed additional power. If the flue gas expansion gives more power than required to drive the air compressor, than the electric motor or generator transforms the surplus power to electric power and exports it to the refinery's electrical system. 

The expanded flue gas is then routed via a steam-generating boiler (termed to as a CO boiler) where the carbon monoxide in the flue gas is burned as fuel to give steam for use in the refinery and also to comply by any applicable environmental regulatory limits on carbon monoxide emissions. 

The flue gas is finally processed via an electrostatic precipitator (ESP) to remove residual particulate matter to comply with any applicable environmental regulations concerning particulate emissions. The ESP eliminates particulates in the size range of 2 to 20 microns from the flue gas.

The steam turbine in the flue gas processing system (illustrated in the above diagram) is employed to drive the regenerator's combustion air compressor throughout start-ups of the FCC unit until there is adequate combustion flue gas to take over that task.

Catalysts:

Modern FCC catalysts are fine powders having a bulk density of 0.80 to 0.96 g/cc and having a particle size distribution ranging from 10 to 150 µ and an average particle size of around 60 to 100 µm. The design and operation of an FCC unit is largely based on the chemical and physical properties of the catalyst. The desirable properties of an FCC catalyst are as follows:

  • Good stability to high temperature and to steam
  • High activity
  • Large pore sizes
  • Good resistance to attrition
  • Low coke production  

A modern FCC catalyst consists of four main components: crystalline zeolite, matrix, binder and filler. Zeolite is the main active component and can range from around 15 to 50 weight percent of the catalyst. The zeolite employed in FCC catalysts is termed to as faujasite or as Type Y and is includes of silica and alumina tetrahedra by each tetrahedron having either aluminium or a silicon atom at the center and four oxygen atoms at the corners. This is a molecular sieve having a distinctive lattice structure which lets only a certain size range of hydrocarbon molecules to enter the lattice. In common, the zeolite doesn't allow molecules bigger than 8 to 10 nm (that is, 80 to 90 angstroms) to enter the lattice.

The catalytic sites in the zeolite are strong acids (equal to 90% sulphuric acid) and give most of the catalytic activity. The acidic sites are given by the alumina tetrahedra. The aluminum atom at the center of each and every alumina tetrahedra is at a +3 oxidation state surrounded by four oxygen atoms at the corners that are shared by the neighboring tetrahedra. Therefore, the total charge of the alumina tetrahedra is -1 that is balanced via a sodium ion throughout the production of the catalyst. The sodium ion is later substituted via an ammonium ion that is vapourised when the catalyst is subsequently dried, resultant in the formation of Lewis and Bronsted acidic sites. In certain FCC catalysts, the Bronsted sites might be later substituted by rare earth metals like cerium and lanthanum to give alternative activity and stability levels.

The matrix component of an FCC catalyst includes amorphous alumina that as well provides catalytic activity sites and in larger pores which lets entry for larger molecules than does the zeolite. That lets the cracking of higher-boiling, larger feedstock molecules than are cracked via the zeolite.

The binder and filler components give the physical strength and integrity of the catalyst. The binder is generally silica sol and the filler is generally clay (kaolin). Nickel, vanadium, iron, copper and other metal contaminants, present in the FCC feedstock in the parts per million ranges, all encompass detrimental effects on the catalyst activity and performance.

Nickel and Vanadium are principally troublesome. There are a number of processes for mitigating the effects of the contaminant metals listed as follows: 

- Avoid feedstock having high metals content:   This seriously hampers a refinery's flexibility to process different crude oil or purchased FCC feedstock.

- Feedstock feed pretreatment: Hydro-desulphurization of the FCC feedstock eliminates some of the metals and as well decreases the sulphur content of the FCC products. Though, this is an expensive option.

- Increasing fresh catalyst addition:  The whole FCC units withdraw some of the circulating equilibrium catalyst as spent catalyst and substitute it with fresh catalyst in order to keep a desired level of activity. By increasing the rate of such exchange lowers the level of metals in the circulating equilibrium catalyst; however this is as well an expensive option.

- Demetalisation:  The commercial proprietary Demet method eliminates nickel and vanadium from the withdrawn spent catalyst. The nickel and vanadium are transformed to chlorides that are then washed out of the catalyst. After drying, the demetalized catalyst is recycled to the circulating catalyst. The removals of around 95% nickel and 67 to 85% vanadium have been reported. In spite of that, the use of Demet process has not become widespread, perhaps due to the high capital expenditure needed.

- Metals passivation: Some materials can be employed as additives that can be impregnated to the catalyst or added to the FCC feedstock in the form of metal-organic compounds. These materials react by the metal contaminants and passivate the contaminants via forming less harmful compounds which remain on the catalyst. For illustration, antimony and bismuth are efficient in passivating the nickel and tin is effective in passivating vanadium. A number of proprietary passivation methods are available and fairly broadly employed.

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