CATALYTIC CRACKING

INTRODUCTION

                 Cracking essentially involves a chemical cleavage reaction, in which a saturated aliphatic hydrocarbon molecule splits into one paraffinic and one olefinic molecule. This is the primary cracking reaction. The molecular species thus formed participate in secondary cracking reactions, which takes place at different sites on the hydrocarbon chain and yield a variety of gas and olefin-rich gasoline’s whose compositions and yields vary with the operating conditions.

Cracking (chemistry)

         In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products are strongly dependent on the temperature and presence of catalysts. Cracking is the breakdown of a large alkane into smaller, more useful alkanes and an alkene. Simply put, hydrocarbon cracking is the process of breaking long-chain hydrocarbons into short ones.

Catalytic CRACKING

Definition:

               Petroleum refinery process in which heavy oil is passed through metal chambers (called catalytic crackers or cat crackers) under pressure and high temperature in the presence of catalysts such as alumina, silica, or zeolites. This boiling breaks up heavy, large, and more complex long-chain oil molecules into lighter, smaller, and simpler short-chain molecules such as those of gasoline

Cracking processes are widely used in the chemical industry to convert heavy oil fractions into light liquid hydrocarbons (C₅ to _C15). Several cracking processes exist, each having its own typical product composition. Catalytic cracking operates at more suitable conditions than other cracking processes.

Reactions:

             The catalytic cracking of hydrocarbons is not restricted to one single reaction. Instead, several reactions occur, all resulting in different product compositions. Here the catalytic cracking of diesel is investigated. Diesel mainly consists of saturated hydrocarbons (C_nH_2n+2) ranging from C₁₀H₂₂ to C₁₅H₃₂. Therefore, in this study the cracking of dodecane (C₁₂H₂₆) has been taken as the model reaction. Because no hydrogen is added during the cracking process the product mostly comprises of unsaturated hydrocarbons (C_nH_2n), called olefins.

Burning:

The direct combustion of dodecane is given by:

Cracking:

            Four possible cracking reactions of dodecane are mentioned here. These were chosen because of the important difference in products. The first reaction gives relatively large hydrocarbons and the last reaction produces only small ones.

CATALYT IC CRACKER

Examples

Advantages:

          Advantages of using catalytic cracking of liquid hydrocarbons on-board are:

·     Increased lower heating value of the fuel by about 6%

·     Better burning due to usage of gaseous hydrocarbons

·     Reaction conditions comparable with exhaust temperature

Disadvantages:

          Disadvantages of using catalytic cracking of liquid hydrocarbons on-board are:

·     Incomplete conversion of the fuel

·     Polymerizing by olefins in the system

·     Coking on the catalyst, which leads to deactivation

Solutions:

           To overcome the problem of coking on the catalyst in industrial processes the catalyst is regenerated by heating it up and burning of the cokes. In a car this has to be applied continuously. In another process called hydro cracking the problem of coking is circumvented by adding a high pressure hydrogen stream. This also might work on-board when the cracking system is combined with a hydrogen-generating process. Hydro cracking would provide a solution for the polymerization problem as well.

FLUID CATALYTIC CRACKING

            Fluid catalytic cracking is a commonly used process, and a modern oil refinery will typically include a cat cracker, particularly at refineries in the U.S., due to the high demand for gasoline. The process was first used in around 1942 and employs a powdered catalyst. During the Second World War, in contrast to the Axis Forces which suffered severe shortages of gasoline and artificial rubber, the Allied Forces were supplied with plentiful supplies of the materials. Initial process implementations were based on low activity alumina catalyst and a reactor where the catalyst particles were suspended in a rising flow of feed hydrocarbons in a fluidized bed.

            In newer designs, cracking takes place using a very active zeolite-based catalyst in a short-contact time vertical or upward sloped pipe called the "riser". Pre-heated feed is sprayed into the base of the riser via feed nozzles where it contacts extremely hot fluidized catalyst at 1230 °F to 1400 °F (665 °C to 760 °C). The hot catalyst vaporizes the feed and catalyzes the cracking reactions that break down the high molecular weight oil into lighter components including LPG, gasoline, and diesel. The catalyst-hydrocarbon mixture flows upward through the riser for just a few seconds and then the mixture is separated via cyclones. The catalyst-free hydrocarbons are routed to main fractionators for separation into fuel gas, LPG, gasoline, naphtha, light cycle oils used in diesel and jet fuel, and heavy fuel oil.

Cracking unit in petroleum industries :

Catalytic cracker:     

          The feedstock of long-chain hydrocarbons

(1) Is mixed with hot catalyst.

(2) And vaporized. The vapour/powder mixture is carried to the reactor where the cracking reactions occur. Cyclones

(3) Extract the cracked hydrocarbon vapour and pass it to the fractionating column where it is fractionated, yielding petroleum gases and gasoline

(4) Light gas oil

(5) Medium gas oil

(6)  And heavy gas oil

(7) Spent catalyst meanwhile is mixed with steam

(8) And carried in a current of hot air

(9) To the catalyst regenerator where it is cleaned and recycled

(10) Waste gases are drawn off

(11) And vented.

                   Petroleum refiners use fluid catalytic cracking (FCC) technology to convert crude oil to blending stocks for use in gasoline, diesel, and heating oil. Construction and operation of the 200-foot tall FCC units are expensive, and process control Improvements are slow to be adopted. The new process does the conversion more efficiently with less emissions and dramatic savings in costs. Oil refinery cracking processes allow the production of "light" products such as liquefied petroleum gas (LPG) and gasoline from heavier crude oil distillation fractions such as gas oils and residues. Fluid catalytic cracking produces a high yield of gasoline and LPG, while hydro cracking is a major source of jet fuel, diesel, naphtha, and LPG.

Catalytic cracking product

v Gasoline.                                      

v Fuel oil.

v  Petrochemicals.

v Diesel