Houdry Catalytic Cracking

Dr. Semih Eser: As I mentioned before, the evolution of catalytic cracking processes is really a good example of process engineering or reactor design to maximize the thermal efficiency of a process. The first cat cracking process was a batch process introduced in 1915, the McAfee process, from a Lewis acid. Aluminum chloride was used in the batch reactor system. But for commercial operation, you would need a floor reactor system. So Houdry cat cracking was the first commercial continuous process introduced in 1936.

The process used the natural aluminum silica solids particles as catalysts. And in the process, we're going to look at a configuration that has three reactors in parallel. These are fixed bed reactors essentially filled with the alumina silica pellets, packed with the alumina silica palettes as catalysts. So the gas oil feed is heated in a furnace, in a fired furnace to 800 degrees Fahrenheit, and the feed is fed from the bottom of this packed bed to go through cracking on catalyst surfaces, and the products are fed to a fractionater. In the fractionater, the products are separated into gas and gasoline and LCO, light cycle oil, as the major products from cat cracking.

As the cracking goes on, the catalysts are deactivated by coke buildup. And this happens pretty fast, within 10 minutes of the introduction of the feed into the reactor. Now, since this is an endothermic reaction, you would also imagine or could visualize that the temperature will go down as the feed moves from the bottom to the top of the reactor. So cracking isn't really very homogeneous because of this decreasing temperature.

Now, once the catalysts are deactivated within 10 minutes, all coked up, you bring in steam to strip the liquids, somewhat heavier liquids that are sticking onto the catalyst surfaces to remove them into the fractionater. And some of this will end up as heavy oil. Once this operation is finished, within about five minutes, then you bring in hot air to burn off the coke to reactivate the catalyst.

But to continue this cracking process in the meantime, while you are stripping the heavier products and reactivating the customers by burning the coke, you need to switch the feed to another reactor. So that is the second in the series. So you have a continuous feed and continuous production through the system using the swing reactor configuration, as we refer to.

The same thing happens in the second reactor. The catalyst is deactivated within 10 minutes, so you bring in the steam to strip the heavier products. And while you're doing that, obviously, and then later on, burning off the coke, you should switch the feed to another reactor. That would be the third reactor in these areas. And the products are all sent to a fractionater.

So by the time the third reactor is coked up, you can now switch the feed back to the first reactor. You will have enough time to steam strip and coke or decoke the first reactor, having these additional two reactors in the series. So that's essentially the cycle in the Houdry cat cracking reaction to enable a continuous flow system while the reactors are taken out of service for decoking operations.

You see the problem here. The endothermic reaction of cracking is decoupled, really, from the exothermic reaction of burning of the coke. So for that reason, the thermal efficiency of Houdry cat cracking is not very high, and it certainly needs to be improved.

Long Description (Houdry Catalytic Cracking Process Diagram)

This diagram illustrates the flow and operation of the Houdry catalytic cracking process, an early continuous commercial method for converting gas oil into lighter hydrocarbon products using fixed-bed reactors.

At a high level, the process shows feed preparation, reaction in multiple reactors, product separation, and catalyst regeneration occurring in a cyclic sequence across several vessels.

Overall process flow (left to right):
Gas oil feed is first heated in a furnace to approximately 800°F. The heated feed then flows into one of several vertical fixed-bed reactors packed with alumina/silica catalyst pellets. Cracked hydrocarbon products exit the reactor system and are sent to a fractionator, where they are separated into gas, gasoline, light cycle oil (LCO), and heavy oil fractions.

Feed and heating section (left side):

• A heater unit is shown at the lower left, labeled “Heater, T = 800°F.”
• Gas oil enters the heater and is raised to reaction temperature before being directed into the reactor system.

Reactor section (center):

• Three tall, vertical reactors are shown in parallel, representing “swing reactors.”
• These reactors operate alternately rather than simultaneously on feed.
• Arrows indicate that feed enters one reactor at a time, while others are in different stages of the cycle.
• Inside each reactor, catalyst beds facilitate cracking reactions that break large hydrocarbons into smaller molecules.

Reaction and deactivation:

• During operation, coke deposits form rapidly on the catalyst surface.
• The diagram notes “coke build-up on catalysts in 10 min,” indicating that each reactor can only operate for about 10 minutes before deactivation.

Steam stripping stage:

• After a reactor is taken offline, it is stripped with steam for approximately 5 minutes.
• This removes adsorbed hydrocarbons from the catalyst before regeneration.

Catalyst regeneration stage:

• Hot air is introduced into the spent reactor to burn off coke deposits.
• This regeneration step also takes about 5 minutes.
• The diagram labels this as “Hot air to regenerate catalyst after steam stripping.”
• Combustion produces flue gases, shown exiting the system.

Cyclic (swing) operation:

• The three reactors operate in rotation: one reactor is cracking feed, another is being steam-stripped, and another is undergoing regeneration.
• This staggered cycle allows continuous processing despite the short active life of each catalyst bed.

Product separation (right side):

• Cracked products from the active reactor are routed to a vertical fractionator column.
• The fractionator separates the mixture into:
  • Gas (top outlet)
  • Gasoline (upper side outlet)
  • Light cycle oil (LCO) (mid outlet)
  • Heavy oil (bottom outlet)