Rabu, 19 Desember 2012

Cracking, Coking, Hydrocracking, and Reforming

The basic processes introduced to bring about thermal decomposition of the higher boiling streams are known as cracking. In these processes, the higher boiling fractions are converted to lower boiling products. Catalytic cracking is the most common cracking process, in which heavy feedstock or cuts are broken down or changed by being heated, and reacted with catalysts.

The concept behind thermal cracking is the thermal decomposition of higher molecular weight constituents of petroleum to produce lower molecular weight, normally more valuable, products. The first commercial process was in 1913, which is known as the Burton Process. Even though catalyst cracking generally replaced thermal cracking in the 1940s, noncatalytic cracking processes using high temperature to achieve the decomposition are still in operation. Through catalytic processes more gasoline having higher octane, less heavy fuel oils, and light gases are produced. The light gases produced by catalytic cracking contain more olefins than those produced by thermal cracking.

In the thermal cracking process, a feedstock (e.g., gas oil) is fed to the fractionator with their thermal reactivity to separate gasoline and light and heavy oil. The light oil is then fed to the heater at 540-5950C (1000-11000F) and a pressure of 350-700 psi, the light oil transforms to the vapor phase and is sent to the soaker. If the feedstock is heavy oil, temperatures on the order of 400-4800C (750-9000F) are used and higher pressures (350—700 psi) are used to maintain the feedstock in the liquid phase, then it is fed to the soaker. The liquid and vapor phase mix in the soaker and are sent to the separator, with the products coming out on the bottom as fuel oil and the light recycle back to the fractionator. Coking in the reactor is the main problem when heavy oil is heated at high temperatures.

In many refineries, coking processes, which use high temperatures and low pressures, have superseded thermal cracking processes.

The principal application of catalytic cracking (Table 13.2) is the production of high-octane gasoline, to supplement the gasoline produced by distillation and other processes. Catalytic cracking also produces heating oil components and hydrocarbon feedstocks, such as propene, and butene, for polymerization, alkylation, and petrochemical operations. The main process used in catalytic cracking involves the use of fluidized bed units (FCC).

TABLE 13.2 Summary of Catalytic Cracking Processes 

Figure 13.5 A fluid catalytic cracking (FCC) unit.

Fluid catalytic cracking (FCC) (Fig. 13.5) was first introduced in 1942 and uses a fluidized bed of catalyst with continuous feedstock flow. The catalyst is usually a synthetic alumina or zeolite used as a catalyst. Compared to thermal cracking, the catalytic cracking process (1) uses a lower temperature, (2) uses a lower pressure, (3) is more flexible, (4) and the reaction mechanism is controlled by the catalysts. Feedstocks for catalytic cracking include: straight-run gas oil, vacuum gas oil, atmospheric residuum, deasphalted oil, and vacuum residuum. Coke inevitably builds up on the catalyst over time and the issue can be circumvented by continuous replacement of the catalyst or the feedstock pretreated before it is used by deasphalting (removes coke precursors), demetallation (removes nickel and vanadium and prevents catalyst deactivation), or by feedstock hydrotreating (that also prevents excessive coke formation).

The catalyst, which may be an activated natural or synthetic material, is employed in bead, pellet, or microspherical form and can be used as a fixed bed, moving bed, or fluid bed. The fixed-bed process was the first process to be used commercially and uses a static bed of catalyst in several reactors that allows a continuous flow of feedstock to be maintained. Thus, the cycle of operations comprises (1) flow of feedstock through the catalyst bed, (2) discontinuance of feedstock flow and removal of coke from the catalyst by burning, and (3) insertion of the reactor on stream. The moving-bed process uses a reaction vessel (in which cracking takes place) and a kiln (in which the spent catalyst is regenerated) and catalyst movement between the vessels is provided by various means.

The fluid-bed process differs from the fixed-bed and moving-bed processes, insofar as the powdered catalyst is circulated essentially as a fluid with the feedstock. The several fluid catalytic cracking processes in use differ primarily in mechanical design. Side-by-side reactor-regenerator construction along with unitary vessel construction (the reactor either above or below the regenerator) are the two main mechanical variations.

Natural clays have long been known to exert a catalytic influence on the cracking of oils, but it was not until about 1936 that the process using silica-alumina catalysts was developed sufficiently for commercial use. Since then, catalytic cracking has progressively supplanted thermal cracking as the most advantageous means of converting distillate oils into gasoline. The main reason for the wide adoption of catalytic cracking is the fact that a better yield of higher-octane gasoline can be obtained than by any known thermal operation. At the same time the gas produced mostly consists of propane and butane with less methane and ethane. The production of heavy oils and tars, higher in molecular weight than the charge material, is also minimized, and both the gasoline and the uncracked cycle oil are more saturated than the products of thermal cracking.

Coking units convert heavy feedstock into a solid coke and lower boiling hydrocarbon products that are suitable as feedstock to other refinery units for conversion into higher value transportation fuels. From a chemical reaction viewpoint, coking can be considered as a severe thermal cracking process in which (unlike visbreaking, q.v.) the reactions are allowed to proceed to completion. As coke is a by-product of this process, the sulfur and metal contents of the coke are high (sometimes as high as 8 percent by weight). There are three major coking processes: (1) delayed coking, (2) fluid coking, and (3) flexicoking.

Delayed coking (Fig. 13.6) involves the use of an insulated surge drum to accommodate the heater effluent that allows sufficient time for the coking reactions to proceed to completion. Delayed coking is a semi-continuous continuous, where as feed is introduced continuously into one of a pair of coking drums, the other is off-stream undergoing coke removal. Coke is removed from the drums by using high-pressure water jets (3000 psi) to bore a hole through the center of coke, and then inserting a probe to use high-pressure water jets to cut the coke radially. Feedstocks for delayed cokers include atmospheric residua, vacuum residua, heavy oils, and tar sand bitumen.

The fluid coking process (Fig. 13.7) is a continuous process in which coke is also made but only enough coke is burned to satisfy the heat requirements of the reactor and the feed preheating operations. The process is also (like delayed coking) a residuum conversion process that uses noncatalytic, thermal chemistry to achieve high conversion levels with even the heaviest refinery feedstock. Because a majority of the sulfur, nitrogen, and metals are rejected with the coke, the full-range of lighter products makes adequate feed for a fluid catalytic cracking unit. The process avoids process load swings and frequent thermal cycles that are typical of batch processes such as delayed coking. The configurations available with fluid coking include extinction recycle, once through, and once through with hydrocyclones.

Figure 13.6 A delayed coker

Figure 13.7 A Fluid coker

The flexicoking process is an adaptation of the fluid coking process that uses the same reactor as a fluid coker but has an integrated gasification unit available for coke gasification to produce, in addition to the typical fluid coking slate of products, a low-BTU gas.

Visbreaking is a thermal cracking process in which the thermal reactions are not allowed to proceed to completion. It is used for partial conversion of the higher boiling fractions, usually the residua. Visbreaking, unlike the coking processes (Table 13.3), is a relatively mild thermal cracking operation mainly used to reduce the viscosity and pour point of a vacuum residuum (vacuum tower bottoms) to meet No. 6 fuel oil specifications, or to reduce the amount of cutting stock required to dilute the residua to meet these specifications. Long paraffinic side chains attached to aromatic rings are the primary cause of high pour points and viscosities for paraffinic base residues. Visbreaking is carried out at conditions to optimize the breaking off of these long side chains and their subsequent cracking to shorter molecules with lower viscosities and pour point. 

TABLE 13.3 Comparison of Visbreaking with Delayed Coking and Fluid Coking

Figure 13.8 A soaker visbreaker.

TABLE 13.4 Summary of Hydrocracking Process Operations 

Hydroprocesses use the principle that the presence of hydrogen during a thermal reaction of a petroleum feedstock will terminate many of the coke-forming reactions and enhance the yields of the lower-boiling components such as gasoline, kerosene, and jet fuel.

Hydrogenation processes for the conversion of petroleum fractions and petroleum products are classified as destructive and nondestructive (Table 13.4). Destructive hydrogenation (hydrogenolysis or hydro-cracking) is characterized by the conversion of the higher molecular weight constituents in a feedstock to lower-boiling products. Such treatment requires severe processing conditions and the use of high hydrogen pressures to minimize polymerization and condensation reactions that lead to coke formation. Nondestructive or simple hydrogenation is generally used for the purpose of improving product quality without appreciable alteration of the boiling range. Mild processing conditions are employed so that only the more unstable materials are attacked. Nitrogen, sulfur, and oxygen compounds undergo reaction with the hydrogen to remove ammonia, hydrogen sulfide, and water, respectively. Unstable compounds that might lead to the formation of gums, or insoluble materials, are converted to more stable compounds. 

Figure 13.9 A distillate hydrotreater for hydrodesulfurization.

Hydrotreating (Fig. 13.9) is carried out by charging the feed to the reactor, together with hydrogen in the presence of catalysts such as tungsten-nickel sulfide, cobalt-molybdenum-alumina, nickel oxidesilica-alumina, and platinum-alumina. Most processes employ cobaltmolybdena catalysts that generally contain about 10 percent of molybdenum oxide and less than 1 percent of cobalt oxide supported on alumina. The temperatures employed are in the range of 300-3450C (570-6550F), while the hydrogen pressures are about 500-1000 psi (Scott and Bridge, 1971).

The reaction generally takes place in the vapor phase but, depending on the application, may be a mixed-phase reaction. Generally it is more economical to hydrotreat high-sulfur feedstocks prior to catalytic cracking than to hydrotreat the products from catalytic cracking. The advantages are that: (1) sulfur is removed from the catalytic cracking feedstock, and corrosion is reduced in the cracking unit; (2) carbon formation during cracking is reduced so that higher conversions result; and (3) the cracking quality of the gas oil fraction is improved. 

Figure 13.10 A two stage hydrocracking unit

Hydrocracking (Fig. 13.10) is similar to catalytic cracking, with hydrogenation superimposed and with the reactions taking place either simultaneously or sequentially. Hydrocracking was initially used to upgrade low-value distillate feedstocks, such as cycle oils (high aromatic products from a catalytic cracker, which are usually not recycled to extinction for economic reasons), thermal and coker gas oils, and heavy-cracked and straight-run naphtha. These feedstocks are difficult to process by either catalytic cracking or reforming, because they are characterized usually by a high polycyclic aromatic content, or by high concentrations, or by both the two principal catalyst poisons—sulfur and nitrogen compounds.

The older hydrogenolysis type of the hydrocracking practiced in Europe during and after World War II used tungsten or molybdenium sulfides as catalysts and required high reaction temperatures and operating pressures, sometime in access of about 3000 psi (203 atmospheres), for continues operation. The modern hydrocracking processes were initially developed for converting refactory feedstocks (such as gas oil) to gasoline and jet fuel, but process and catalyst improvements and modifications have made it possible to yield products from gases and naphta to furnace oils and catalytic cracking feedstocks.

A comparison of hydrocracking with hydrotreating is useful in assessing the parts played by these two processes  in refinery operations. Hydrotreating of distillates may be defined simply as the removal of nitrogen, sulfur, and oxygen-containing compounds by selective hydrogenation. The hydrotreating catalysts are usually cobalt plus molybdenium or nickel plus molybdenium (in the sulfide form) impregnated on an alumina base. The hydrotreated operating conditions are such that appreciable hydrogenation of aromatics will not occur—1000-2000 psi hydrogen and about 3700C (7000F). The desulfurization reactions are usually accompanied by small amounts of hydrogenation and hydrocracking.

The commercial processes for treating, or finishing, petroleum fractions with hydrogen all operate in essentially the same manner. The feedstock is heated and passed with hydrogen gas through a tower or reactor filled with catalyst pellets. The reactor is maintained at a temperature of 260-4250C (500-8000F) at pressures from 100-1000 psi, depending on the particular process, the nature of the feedstock and the degree of hydrogenation required. After leaving the reactor, excess hydrogen is separated from the treated product and recycled through the reactor after the removal of hydrogen sulfide. The liquid product is passed into a stripping tower where steam removes dissolved hydrogen and hydrogen sulfide and, after cooling, the product is taken to product storage or, in the case of feedstock preparation, pumped to the next processing unit.

Reforming processes are used to change the inherent chemical structures of the hydrocarbons that exist in distillation fractions crude oil into different compounds. Catalytic reforming (Fig. 13.11) is one of the most important processes in a modern refinery, altering straight-run fraction or fractions from a catalytic cracker into new compounds through a combination of heat and pressure in the presence of a catalyst. 

Figure 13.11 Catalytic reforming.

Reforming processes are particularly important in producing high-quality gasoline fuels. Reforming processes are classified as continuous, cyclic, or semi-regenerative, depending upon the frequency of catalyst regeneration.

In carrying out thermal reforming, a feedstock such as 205°C (4000F) end-point naphtha or a straight-run gasoline is heated to 510-5950C (950-11000F) in a furnace, much the same as a cracking furnace, with pressures from 400-1000 psi (27—68 atmospheres). As the heated naphtha leaves the furnace, it is cooled or quenched by the addition of cold naphtha. The material then enters a fractional distillation tower where any heavy products are separated. The remainder of the reformed material leaves the top of the tower to be separated into gases and reformate. The higher octane of the reformate is due primarily to the cracking of longer-chain paraffins into higher-octane olefins.

The products of thermal reforming are gases, gasoline, and residual oil or tar, the latter being formed in very small amounts (about 1 percent). The amount and quality of the gasoline, known as reformate, is very dependent on the temperature. A general rule is: the higher the reforming temperature, the higher the octane number, but the lower the yield of reformate.

Thermal reforming is less effective and less economical than catalytic processes and has been largely supplanted. As it used to be practiced, a single-pass operation was employed at temperatures in the range of 540-7600C (1000-11400F) and pressures of about 500-1000 psi (34-68 atmospheres). The degree of octane number improvement depended on the extent of conversion but was not directly proportional to the extent of crack per pass. However, at very high conversions, the production of coke and gas became prohibitively high. The gases produced were generally olefinic; and the process required either a separate gas polymerization operation or one in which C3 to C4 gases were added back to the reforming system.

More recent modifications of the thermal reforming process because of the inclusion of hydrocarbon gases with the feedstock are known as gas reversion and polyforming. Thus, olefinic gases produced by cracking and reforming can be converted into liquids boiling in the gasoline range by heating them under high pressure. As the resulting liquids (polymers) have high octane numbers, they increase the overall quantity and quality of gasoline produced in a refinery.

Like thermal reforming, catalytic reforming converts low-octane gasoline into high-octane gasoline (reformate). Whereas thermal reforming could produce reformate with research octane numbers of 65 to 80 depending on the yield, catalytic reforming produces reformate with octane numbers on the order of 90 to 95. Catalytic reforming is conducted in the presence of hydrogen over hydrogenation-dehydrogenation catalysts, which may be supported on alumina or silica-alumina. Depending on the catalyst, a definite sequence of reactions takes place, involving structural changes in the feedstock. This more modern concept actually rendered thermal reforming somewhat obsolescent.

The commercial processes available for use can be broadly classified as the moving-bed, fluid-bed, and fixed-bed types. The fluid- and moving-bed processes use mixed nonprecious metal oxide catalysts in units equipped with separate regeneration facilities. Fixed-bed processes use predominantly platinum-containing catalysts in units equipped for cycle, occasional, or no regeneration.

Catalytic reformer feeds are saturated (i.e., not olefinic) materials; in the majority of cases that feed may be a straight-run naphtha but in others by-product low-octane naphtha (e.g., coker naphtha) can be processed after treatment to remove olefins and other contaminants. Hydrocracker naphtha that contains substantial quantities of naphthenes is also a suitable feed.

Dehydrogenation is a main chemical reaction in catalytic reforming, and hydrogen gas is consequently produced in large quantities. The hydrogen is recycled though the reactors where the reforming takes place to provide the atmosphere necessary for the chemical reactions and also prevents the carbon from being deposited on the catalyst, thus extending its operating life. An excess of hydrogen above whatever is consumed in the process is produced, and, as a result, catalytic reforming processes are unique in that they are the only petroleum refinery processes to produce hydrogen as a by-product.

Catalytic reforming usually is carried out by feeding a naphtha (after pretreating with hydrogen if necessary) and hydrogen mixture to a furnace where the mixture is heated to the desired temperature, 4505200C (840-9650F), and then passed through fixed-bed catalytic reactors at hydrogen pressures of 100—1000 psi (7-68 atmospheres). Normally pairs of reactors are used in series and heaters are located between adjoining reactors to compensate for the endothermic reactions taking place. Sometimes as many as four or five reactors are kept on-stream in series while one or more being regenerated.

The on-stream cycle of any one reactor may vary from several hours to many days, depending on the feedstock and reaction conditions.

The composition of a reforming catalyst is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally molybdena-alumina, chromia-alumina, or platinum on a silica-alumina or alumina base. The nonplatinum catalysts are widely used in regenerative process for feeds containing, for example, sulfur, which poisons platinum catalysts, although pretreatment processes (e.g., hydrodesulfurization) may permit platinum catalysts to be employed.

The purpose of platinum on the catalyst is to promote dehydrogenation and hydrogenation reactions, that is, the production of aromatics, participation in hydrocracking, and rapid hydrogenation of carbon-forming precursors. For the catalyst to have an activity for isomerization of both paraffins and naphthenes—the initial cracking step of hydrocracking—and to participate in paraffin dehydrocyclization, it must have an acid activity. The balance between these two activities is most important in a reforming catalyst. In fact, in the production of aromatics from cyclic saturated materials (naphthenes), it is important that hydrocracking be minimized to avoid loss of the desired product and, thus, the catalytic activity must be moderated relative to the case of gasoline production from a paraffinic feed, where dehydrocyclization and hydrocracking play an important part.

Other processes to maximize the production of gasoline products include isomerization (Fig. 13.12) that is used for reforming or recombining lighter cuts into new products, polymerization (not in the true sense of the word) processes (Fig. 13.13), and alkylation processes (Fig. 13.14).

Present isomerization applications in petroleum refining are used with the objective of providing additional feedstock for alkylation units or high-octane fractions for gasoline blending. Straight-chain paraffins (n-butane, n-pentane, n-hexane) are converted to respective isohydrocarbons by continuous catalytic (aluminum chloride, noble metals) processes. Natural gasoline or light straight-run gasoline can provide feed by first fractionating as a preparatory step. High volumetric yields while aluminum chloride plus hydrogen chloride are universally used for the low-temperature processes.
Figure 13.12 A butane isomerization unit

Figure 13.13 A polymerization unit

Figure 13.14 An alkylation unit (sulfuric acid catalyst)

A nonregenerable aluminum chloride catalyst is employed with various carriers in a fixed-bed or liquid contactor. Platinum or other metal catalyst processes use fixed-bed operations and can be regenerable or nonregenerable. The reaction conditions vary widely depending on the particular process and feedstock, 40-4800C (100-9000F) and 150-1000 psi (10-68 atmospheres).

The purpose of the polymerization reaction is to polymerize low molecular weight olefins to form a high-octane product boiling in the gasoline boiling range. Phosphoric acid on an inert support is used as the catalyst but sulfur in the feedstock poisons the catalyst and basic compounds neutralize the acid. Oxygen dissolved in the feedstock strongly affects the reactions and must be removed. Propene and butene are used as feedstock and the reaction is highly exothermic, hence, the temperature is controlled either by propane quench or by generating steam.

Polymerization may be accomplished thermally or in the presence of a catalyst at lower temperatures. Thermal polymerization is regarded as not being as effective as catalytic polymerization but has the advantage that it can be used to polymerize saturated materials that cannot be induced to react by catalysts. The process comprises vapor-phase cracking of, for example, propane and butane followed by prolonged periods at high temperature (510-5950C, 950-11000F) for the reactions to proceed to near completion.

Olefins can also be conveniently polymerized by means of an acid catalyst. Thus, the treated, olefin-rich feed stream is contacted with a catalyst (sulfuric acid, copper pyrophosphate, phosphoric acid) at 150 to 2200C (300-4250F) and 150-1200 psi (10-81 atmospheres), depending on the feedstock and product requirement.

Phosphates are the principal catalysts used in polymerization units; the commercially used catalysts are liquid phosphoric acid, phosphoric acid on kieselguhr, copper pyrophosphate pellets, and phosphoric acid film on quartz. The latter is the least active, but the most used and the easiest one to regenerate simply by washing and recoating; the serious disadvantage is that tar must occasionally be burned off the support. The process using the liquid phosphoric acid catalyst is far more responsible for attempts to raise production by increasing temperature compared to other processes.

The purpose of the alkylation reaction is to produce an iso-paraffin by the reaction of low molecular weight olefin with an iso-paraffin to produce constituents for high-octane gasoline. By proper choice of operating conditions, most of the product can be made to fall within the gasoline boiling range. Although alkylation is possible without catalysts, commercial processes use aluminum chloride, sulfuric acid, or hydrogen fluoride as catalysts, when the reactions can take place at low temperatures, minimizing undesirable side reactions, such as polymerization of olefins.

Alkylate is composed of a mixture of iso-paraffins, which have octane numbers that vary with the olefins from which they were made. Butenes produce the highest octane numbers, propene the lowest, and pentenes the intermediate values. All alkylates, however, have high octane numbers (>87) that make them particularly valuable.

The alkylation reaction as now practiced in petroleum refining is the union, through the agency of a catalyst, of an olefin (ethylene, propene, butene, and pentene) with iso-butane to yield high-octane branched-chain hydrocarbons in the gasoline boiling range. Olefin feedstock is derived from the gas produced in a catalytic cracker, whereas iso-butane is recovered by refinery gases or produced by catalytic butane isomerization.

To accomplish this either ethylene or propene is combined with isobutane at 50-200C (125-4500F) and 300-1000 psi (20-68 atmospheres) in the presence of metal halide catalysts such as aluminum chloride. Conditions are less stringent in catalytic alkylation; olefins (propene, butene, or pentene) are combined with iso-butane in the presence of an acid catalyst (sulfuric acid or hydrofluoric acid) at low temperatures and pressures (1-400C, 30-1050F, and 14.8-150 psi, 1-10 atmospheres).

Sulfuric acid, hydrogen fluoride, and aluminum chloride are the general catalysts used commercially. Sulfuric acid is used with propene and higher-boiling feeds, but not with ethylene, because it reacts to form ethyl hydrogen sulfate. The acid is pumped through the reactor and forms an air emulsion with reactants, and the emulsion is maintained at 50 percent acid. The rate of deactivation varies with the feed and isobutane charge rate. Butene feeds cause less acid consumption than the propene feeds.

Aluminum chloride is not widely used as an alkylation catalyst but when employed, hydrogen chloride is used as a promoter and water is injected to activate the catalyst as an aluminum chloride or hydrocarbon complex. Hydrogen fluoride is used for alkylation of higher-boiling olefins and the advantage of hydrogen fluoride is that it is more readily separated and recovered from the resulting product.

The most important variables in the process are: (1) reaction temperature, (2) acid strength, (3) iso-butane concentration, and (4) olefin space velocity. 

Farhat AIi, M., El Ali, B. M., & Speight, J. G.. 2005. Handbook of industrial chemistry:
organic chemicals. New York: McGraw-Hill  

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