Chapter 7:Catalytic Hydrocracking • The interest in the use of hydrocracking has been caused by several factors, including • The demand for petroleum products has shifted to high ratios of gasoline and jet fuel compared with the usages of diesel fuel and home heating oils, • By-product hydrogen at low cost and in large amounts has become available from catalytic reforming operations, and • Environmental concerns limiting sulfur and aromatic compound concentrations in motor fuels have increased.
Some of the advantages of hydrocracking are: • Better balance of gasoline and distillate production • Greater gasoline yield • Improved gasoline pool octane quality and sensitivity • Production of relatively high amounts of isobutane in the butane fraction • Supplementing of ﬂuid catalytic cracking to upgrade heavy cracking stocks, aromatics, cycle oils, and coker oils to gasoline, jet fuels, and light fuel oils In a modern reﬁnery catalytic cracking and hydrocracking work as a team. The catalytic cracker takes the more easily cracked parafﬁnic atmospheric and vacuum gas oils as charge stocks, while the hydrocracker uses more aromatic cycle oils and coker distillates as feed
These streams are very refractory and resist catalytic cracking, while the higher pressures and hydrogen atmosphere make them relatively easy to hydrocrack. • The new zeolite cracking catalysts help improve the gasoline yields and octanes from catalytic crackers as well as reduce the cycle stock and gas make. • However, the cycle oil still represents a difﬁcult fraction to crack catalytically to extinction. • One alternative is to use the cycle stock as a component for fuel oil blending, but this is limited as it is a relatively poor burning stock and burns with a smoky ﬂame. • For this reason a limit is placed on the percentage that can be blended into distillate fuel oils. • The cycle oils that result from cracking operations with zeolite catalysts tend to be highly aromatic and therefore make satisfactory feedstocks for hydrocracking. • Vacuum and coker gas oils are also used as hydrocracker feed.
Sometimes diesel boiling range material is included in hydrocracker feed to make jet and motor gasoline products. Both straight-run and FCC LCO can be used and in some cases 100% LCO is used. • In cases where 100% LCO is the feed, there is a follow-up with a high-pressure hydrotreater to reduce the aromatic content and increase the smoke point to meet speciﬁcations. • When the feed contains large amounts of LCO, the major effects are increased heat release and lower smoke point of the jet fuel product • In addition to middle distillates and cycle oils used as feed for hydrocrack- ing units, it is also possible to process residual fuel oils and reduced crude by hydrocracking.
HYDROCRACKING REACTIONS • Hydrocracking = catalytic cracking + hydrogenation • Catalytic cracking is the scission of a carbon–carbon single bond, and hydrogenation is the addition of hydrogen to a carbon–carbon double bond.
Cracking and hydrogenation are complementary, for cracking provides oleﬁns for hydrogenation, while hydrogenation in turn provides heat for cracking. • The cracking reaction is endothermic and the hydrogenation reaction is exothermic. • The overall reaction provides an excess of heat because the amount of heat released by the exothermic hydrogenation reactions is much greater than the amount of heat consumed by the endothermic cracking reactions • This surplus of heat causes the reactor temperature to increase and accelerate the reaction rate. This is controlled by injecting cold hydrogen as quench into the reactors to absorb the excess heat of reaction
Another reaction that occurs and illustrates the complementary operation of the hydrogenation and cracking reactions is the initial hydrogenation of a condensed aromatic compound to a cycloparafﬁn • This allows subsequent cracking to proceed to a greater extent and thus converts a low-value component of catalytic cycle oils to a useful product • Isomerization is another reaction type that occurs in hydrocracking and accompanies the cracking reaction. • The oleﬁnic products formed are rapidly hydrogenated, thus maintaining a high concentration of high octane isoparafﬁns and preventing the reverse reaction back to straight-chain molecules. • An interesting point in connection with the hydrocracking of these compounds is the relatively small amounts of propane and lighter materials that are produced as compared with normal cracking processes.
Hydrocracking reactions are normally carried out at average catalyst temperatures between 550 and 750°F (290 to 400°C) and at reactor pressures between 1200 and 2000 psig (8275 and 13,800 kPa). • The circulation of large quantities of hydrogen with the feedstock prevents excessive catalyst fouling and permits long runs without catalyst regeneration. • Careful preparation of the feed is also necessary in order to remove catalyst poisons and to give long catalyst life. • Frequently the feedstock is hydrotreated to remove sulfur and nitrogen compounds as well as metals before it is sent to the ﬁrsthydrocracking stage or, sometimes, the ﬁrst reactor in the reactor train can be used for this purpose.
FEED PREPARATION • Hydrocracking catalyst is susceptible to poisoning by metallic salts, oxygen, organic nitrogen compounds, and sulfur in the feedstocks. • The feedstock is hydrotreated to saturate the oleﬁns and remove sulfur, nitrogen, and oxygen compounds. • Molecules containing metals are cracked and the metals are retained on the catalyst. • The nitrogen and sulfur compounds are removed by conversion to ammonia and hydrogen sulﬁde. • Although organic nitrogen compounds are thought to act as permanent poisons to the catalyst, the ammonia produced by reaction of the organic nitrogen compounds with hydrogen does not affect the catalyst permanently
For some types of hydrocracking catalysts, the presence of hydrogen sulﬁde in low concentrations acts as a catalyst to inhibit the saturation of aromatic rings. • This is a beneﬁcial effect when maximizing gasoline production as it conserves hydrogen and produces a higher octane product. • In the hydrotreater a number of hydrogenation reactions, such as oleﬁn saturation and aromatic ring saturation, take place, but cracking is almost insigniﬁcant at the operating conditions used.
In addition to the removal of nitrogen and sulfur compounds and metals, it is also necessary to reduce the water content of the feed streams to less than 25 ppm because, at the temperatures required for hydrocracking, steam causes the crystalline structure of the catalyst to collapse and the dispersed rare-earth atoms to agglomerate. • Water removal is accomplished by passing the feed stream through a silica gel or molecular sieve dryer • On the average, the hydrogen treating process requires approximately 150 to 300 ft3 of hydrogen per barrel of feed (27 to 54 m3 hydrogen per m3 feed).
THE HYDROCRACKING PROCESS • The hydrocracking process may require either one or two stages, depending upon the process and the feed stocks used. • The GOFining process is a ﬁxed-bed regenerative process employing a molecular-sieve catalyst impregnated with a rare-earth metal. • The process employs either single-stage or two-stage hydrocracking with typical operating conditions ranging from 660 to 785°F and from 1000 to 2000 psig (350–420°C and 6900–13,800 kPa). • The temperature and pressure vary with the age of the catalyst, the product desired, and the properties of the feedstock.
The decision to use a single- or two-stage system depends upon the size of the unit and the product desired. • For most feedstocks the use of a single stage will permit the total conversion of the feed material to gasoline and lighter products by recycling the heavier material back to the reactor. • The process ﬂow for a two-stage reactor is shown in Figure 7.2. If only one stage is used, the process ﬂow is the same as that of the ﬁrst stage of the two-stage plant except the fractionation tower bottoms is recycled to the reactor feed.
The fresh feed is mixed with makeup hydrogen and recycle gas (high in hydrogen content) and passed through a heater to the ﬁrst reactor. If the feed has not been hydrotreated, there is a guard reactor before the ﬁrsthydrocracking reactor. • The guard reactor usually has a modiﬁedhydrotreating catalyst such as cobalt-molybdenum on silica-alumina to convert organic sulfur and nitrogen compounds to hydrogen sulﬁde, ammonia, and hydrocarbons to protect the precious metals catalyst in the following reactors. • The hydrocracking reactor(s) is operated at a sufﬁciently high temperature to convert 40 to 50 vol% of the reactor efﬂuent to material boiling below 400°F (205°C).
The reactor efﬂuent goes through heat exchangers to a high-pressure separator where the hydrogen-rich gases are separated and recycled to the ﬁrst stage for mixing both makeup hydrogen and fresh feed. • The liquid product from the separator is sent to a distillation column where the C4 and lighter gases are taken off overhead, and the light and heavy naphtha, jet fuel, and diesel fuel boiling range streams are removed as liquid sidestreams. • The fractionator bottoms are used as feed to the second-stage reactor system. • The unit can be operated to produce all gasoline and lighter products or to maximize jet fuel or diesel fuel products.
The bottoms stream from the fractionator is mixed with recycle hydrogen from the second stage and sent through a furnace to the second-stage reactor. • Here the temperature is maintained to bring the total conversion of the unconverted oil from the ﬁrst-stage and second-stage recycle to 50 to 70 vol% per pass. • The second-stage product is combined with the ﬁrst-stage product prior to fractionation. • Both the ﬁrst- and second-stage reactors contain several beds of catalysts. • The major reason for having separate beds is to provide locations for injecting old recycled hydrogen into the reactors for temperature control. • In addition, redistribution of the feed and hydrogen between the beds helps to maintain a more uniform utilization of the catalyst.
HYDROCRACKING CATALYST • There are a number of hydrocracking catalysts available and the actual composition is tailored to the process, feed material, and the products desired. • Most of the hydrocracking catalysts consist of a crystalline mixture of silica-alumina with a small uniformly distributed amount of rare earths contained within the crystalline lattice. • The silica-alumina portion of the catalyst provides cracking activity while the rare-earth metals promote hydrogenation. • Catalyst activity decreases with use, and reactor temperatures are raised during a run to increase reaction rate and maintain conversion. • The catalyst selectivity also changes with age and more gas is made and less naphtha produced as the catalyst temperature is raised to maintain conversion.
With typical feedstocks it will take from two to four years for catalyst activity to decrease from the accumulation of coke and other deposits to a level which will require regeneration. • Regeneration is accomplished by burning off the catalyst deposits, and catalyst activity is restored to close to its original level. • The catalyst can undergo several regenerations before it is necessary to replace it. • Almost all hydrocracking catalysts use silica-alumina as the cracking base but the rare-earth metals vary according to the manufacturer. • Those in most common use are platinum, palladium, tungsten, and nickel.
PROCESS VARIABLES • The severity of the hydrocracking reaction is measured by the degree of conversion of the feed to lighter products. • Conversion is deﬁned as the volume percent of the feed which disappears to form products boiling below the desired product end point • The primary reaction variables are reactor temperature and pressure, space velocity, hydrogen consumption, nitrogen content of feed, and hydrogen sulﬁde content of the gases
The effects of these are as follows: • Reactor Temperature • Reactor temperature is the primary means of conversion control. • At normal reactor conditions a 20°F (10°C) increase in temperature almost doubles the reaction rate, but does not affect the conversion level as much because a portion of the reaction involves material that has already been converted to materials boiling below the desired product end point. • As the run progresses it is necessary to raise the average temperature about 0.1 to 0.2°F per day to compensate for the loss in catalyst activity.
Reactor Pressure • The primary effect of reactor pressure is in its effects on the partial pressures of hydrogen and ammonia. • An increase in total pressure increases the partial pressures of both hydrogen and ammonia. • Conversion increases with increasing hydrogen partial pressure and decreases with increasing ammonia partial pressure. • The hydrogen effect is greater, however, and the net effect of raising total pressure is to increase conversion.
Space Velocity • The volumetric space velocity is the ratio of liquid ﬂow rate, in barrels per hour, to catalyst volume, in barrels. • The catalyst volume is constant, therefore the space velocity varies directly with feed rate. • As the feed rate increases, the time of catalyst contact for each barrel of feed is decreased and conversion is lowered. • In order to maintain conversion at the proper level when the feed rate is increased, it is necessary to increase the temperature.
Nitrogen Content • The organic nitrogen content of the feed is of great importance as the hydrocrackingcatalyst is deactivated by contact with organic nitrogen compounds. • An increase in organic nitrogen content of the feed causes a decrease in conversion.
Hydrogen Sulﬁde • At low concentrations the presence of hydrogen sulﬁde acts as a catalyst to inhibit the saturation of aromatic rings. • This conserves hydrogen and produces a product with a higher octane number because the aromatic naphtha has a higher octane than does its naphthenic counterpart. • However, hydrocracking in the presence of a small amount of hydrogen sulﬁde normally produces a very low-smoke-point jet fuel. • At high hydrogen sulﬁde levels corrosion of the equipment becomes important and the cracking activity of the catalyst is also affected adversely.
Heavy Polynuclear Aromatics (HPNA) • Heavy polynuclear aromatics are formed in small amounts from hydrocracking reactions and, when the fractionator bottoms is recycled, can build up to concentrations that cause fouling of heat exchanger surfaces and equipment. • Steps such as reducing feed end point or removal of a drag stream may be necessary to control this problem