Hydrocracking

Question 1

What is it

Answer 1

Cracking and hydrogenation at same time. For streams which cannot be broken down using fluid cat cracking. Very heavy streams.
The feed to the unit is middle distillates and cycle (aromatic) oils.
- heating is not needed because cracking is endothermic and hydrogenation is exothermic hence they complement themselves
- since hydrogenation produces more heat than needed for cracking, cold hydrogen is added into the system

• cracking produces olefins for hydrogenation while hydrogenation produces heat for cracking
• Need pretreatment t remove metals, sulphur, nitrogen compounds. Use nickel or cobalt with molybdenum on silica support

Question 2

The Process

Answer 2

If the feed to the unit is very dirty, a guard reactor is used to reduce the nitrogen and sulphur contents to ammonia and hydrogen sulphide.

• The fresh feed is brought into contact with the hydrogen gas, and then taken through a finance before entering the fixed bed.
• In the fixed bed, the catalyst used is alumina silica impregnated a rare earth metal [where the silica alumni promotes cracking while rare earth metal promotes hydrogenation] . Join the fixed bed, both cracking and hydrogenation take place with cracking providing olefins for hydrogenation and hydrogenation providing reaction heat for cracking.
• The product from the fixed bed is then taken to a high pressure separator where hydrogen rich gas is recovered to be used in the process again. The liquid stream from the separator is then taken to a distillation column
• The distillation column, gases are extracted from the overhead while the liquid product is taken to a fractionation column for separation into LSR gasoline, naphtha and diesel.
• The bottoms for the fractionation column is taken for a second stage or recycled to the reactor.

Question 3

Process Variables

Answer 3

• With an increase in temperature, the reaction rate increases but conversion does not.
• With pressure, an increase caused by the addition of gas increases partial pressure hence increasing conversion. But if pressure is too high hater process process backwards.
• Increasing space velocity reduces contact time hence reducing reaction rate and conversion.
• Increase nitrogen concentration reduces conversion
• H2S prevents saturation of aromatics.