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second partCatalytic Hydrocracking

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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.

Nitrogen and sulfur compounds are removed by conversion to ammonia and hydrogen sulfide.

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

FEED PREPARATION

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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.

Hydrogenation reactions, such as olefin saturation and aromatic ring saturation, take place, but cracking is almost insignificant at the operating conditions used.

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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).

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Composition is tailored to the process, feed material, and the products desired.

Consist of a crystalline mixture of silica-alumina with a small uniformly distributed amount of rare earth metals 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.

HYDROCRACKING CATALYST

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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.

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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

PROCESS VARIABLES

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Reactor TemperaturePrimary 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.

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Reactor Pressure

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.

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Space VelocityThe 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.

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Nitrogen ContentHydrocracking catalyst is deactivated by contact

with organic nitrogen compounds. An increase in organic nitrogen content of the

feed causes a decrease in conversion.

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Hydrogen SulfideAt 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

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.

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Heavy Polynuclear Aromatics (HPNA)HPNA 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

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