Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter alkenes (or commonly olefins), including ethene (or ethylene) and propene (or propylene). Steam cracker units are facilities in which a feedstock such as naphtha, liquefied petroleum gas (LPG), ethane, propane or butane is thermally cracked through the use of steam in steam cracking furnaces to produce lighter hydrocarbons. The propane dehydrogenation process may be accomplished through different commercial technologies. The main differences between each of them concerns the catalyst employed, design of the reactor and strategies to achieve higher conversion rates.[1]
A higher cracking temperature (also referred to as severity) favors the production of ethene and benzene, whereas lower severity produces higher amounts of propene, C4-hydrocarbons and liquid products. The process also results in the slow deposition of coke, a form of carbon, on the reactor walls. This degrades the efficiency of the reactor, so reaction conditions are designed to minimize this. Nonetheless, a steam cracking furnace can usually only run for a few months at a time between de-cokings. Decokes require the furnace to be isolated from the process and then a flow of steam or a steam/air mixture is passed through the furnace coils. This converts the hard solid carbon layer to carbon monoxide and carbon dioxide. Once this reaction is complete, the furnace can be returned to service.[citation needed]
cracking of propane to form ethene
Even though the thorough energy integration within a steam cracking plant, this process produces an unsurmountable amount of carbon dioxide. Per tonne of ethylene, 1 - 1.6 tonne of carbon dioxide (depending on the feedstock) is being produced.[4] Resulting in a staggering amount of more than 300 million tonnes of carbon dioxide that is annually emitted into the atmosphere of which 70-90% is directly attributed to the combustion of fossil fuel. In the last few decades, several advances in steam cracking technology have been implemented to increase its energy efficiency. These changes include oxy-fuel combustion, new burner technology, and 3D reactor geometries.[4] However, as is common within mature technologies these changes only led to marginal gains in energy efficiency. To drastically curb the greenhouse gas emission of steam cracking, electrification does offer a solution as renewable electricity can be directly transformed into heat by, for example, resistive and inductive heating.[4] As a result, several petrochemical companies joined forces resulting in the development of several joint agreements in which they combine R&D efforts to investigate how naphtha or gas steam crackers could be operated using renewable electricity instead of fossil fuel combustion.[5][6]
The process shown in Figure 1 is a steam-cracking process for ethylene production from an ethane-propane mixture. The process can be divided into three main parts: cracking and quenching; compression and drying; and separation.
Cracking and quenching. Initially, an ethane-propane mixture is fed to furnaces in which, under high-severity conditions, it is cracked, forming ethylene, propylene and other byproducts. The furnace outlet stream is subsequently fed to a water-based quench, to prevent further reactions and formation of undesirable byproducts.
Natural Gas Liquids (NGLs) include ethane, propane, butane, isobutene, and pentane. They also include a small amount of heavier hydrocarbons, such as hexane, heptane, and octane. Ethane is a major component of NGLs, especially in the Marcellus, Utica and Eagle Ford formations. While all of these NGLs can be cracked and used to produce petrochemicals, ethane is often the least expensive to use to create ethylene in places like the Appalachian Basin and the Gulf Coast.
Purity ethane (at least 90% ethane, but usually higher) then travels in a pipeline to its destination, an ethane cracker plant. At the cracker plant, which has access to a large energy source, ethane is heated to about 1500 degrees Fahrenheit. This process is called cracking, because heat energy is used to break apart or crack molecules to form new molecules. At that temperature ethane (C2H6) molecules lose two hydrogen molecules, which split off to form a separate, stable hydrogen molecule (H2), leaving molecules which are about 80 percent ethylene (C2H4).
The ethylene formed in the cracking process is next transported by pipeline to another facility to be converted to usable products, the most common of which is polyethylene. Ethylene is at this point still a gas and needs pressure and a catalyst to turn it into polyethylene, a resin. The process by which polyethylene is made from ethylene is known as polymerization.
Refinery grade propylene is the only liquid form of propylene. It is part of a shipment of propane. It is created alongside propane at either a refinery or an NGLs fractionation plant. RGP has three very different uses:
So, companies that had been cracking propane and heavier feedstocks like gasoil for decades were accustomed to producing a certain amount of propylene when they made ethylene. And switching to ethane made the propylene output nearly disappear.
The first is called thermal cracking. In the absence of air, steam and raw material are mixed and heated to temperatures around 800oC inside a furnace. During this process, lighter unsaturated hydrocarbons are produced from the thermal decomposition of heavier saturated hydrocarbons. Two examples are given below. The gases are in the furnace for less than one second in order to prevent continued decomposition of ethene into carbon (coke). As the products leave the furnace they are quickly liquefied at temperatures around -100oC and separated by fractional distillation. Carbon dioxide and hydrogen sulfide are unwanted byproducts of thermal cracking. These gases must be removed before they escape into the atmosphere by reaction with sodium hydroxide solution. 2NaOH(aq) + H2S(g) => Na2S(aq) + 2H2O(l) 2NaOH(aq) + CO2(g) => Na2CO3(aq) + H2O(l)
The second method is known as catalytic cracking. In this process a catalyst, zeolite, is used to crack heavy fractions at temperatures around 500oC. The products are a mixture of saturated and unsaturated hydrocarbons. Octane is a valuable product and is used as a major component of petrol. The other products are used as feed stock for thermal cracking to yield more ethene. The benefits of catalytic cracking as opposed to thermal cracking include:
Note: If you are interested in other examples of catalysis in the petrochemical industry, you should follow this link. It will lead you to information on reforming and isomerisation (as well as a repeat of what you have just read about catalytic cracking).
Thermal cracking doesn't go via ionic intermediates like catalytic cracking. Instead, carbon-carbon bonds are broken so that each carbon atom ends up with a single electron. In other words, free radicals are formed.
Note: If you are interested, there is a lot of really useful information on cracking on this page from the Essential Chemical Industry. You will be reading that out of interest, not because you need to know it all!
The example compounds of ethene or ethylene and pentene are shown on the LEFT. Ethylene is the number one organic chemical synthesized in the U. S. and the world. The small quantities of ethane, propane, and butane found in natural gas are converted into ethene. It can be produced by thermal cracking of ethane to produce ethene and a hydrogen molecule.
Ethene is made industrially by cracking hydrocarbons (notably ethane and propane) from petroleum and is an important raw material for making other organic chemicals, including ethanal, ethanol, ethyl chloride, diethyl ether, and ethane-1,2-diol. It is also obtained from ethanol by vapor-phase dehydration using an activated alumina catalyst at 350C. It may be prepared by dropping ethanol on to syrupy phosphoric acid heated to 220C.
Chloroethene (CH2CHCl), also known as vinyl chloride, is a gas with an ether-like odor. It is manufactured by the chlorination of ethene (ethylene). It polymerizes to form polychloroethene, or polyvinyl chloride (PVC), and is widely used in this form for making electric wire insulation and vinyl records.
In the case of catalytic cracking, the source of the large hydrocarbon molecules is usually the gas oil fraction of crude oil (petroleum). Useful "straight run" products, such as gasoline, kerosene, diesel, butane and propane are separated from the crude oil mixture in the Atmospheric Distillation Unit (at atmospheric pressure). These "straight run" products only account for 30-50% of the crude oil, depending on its origin, the balance is in the form of atmospheric residue, typically boiling above about 350C/650F. Atmospheric residue is typically routed to the Vacuum Distillation unit for separation into gas oil (usually termed vacuum gas oil, or VGO for short) and vacuum residue; via distillation at reduced pressure. VGO is the principle feed used in the Fluid Catalytic Cracking (FCC) process, though many FCC units also co-feed some portion of lower cost atmospheric residue too, typically 10-20% depending operational constraints.
The FCC unit employs a sophisticated powdered catalyst to lower the activation energy of cracking and thereby drive the cracking process. The particle size and density of the powder are set such that the catalyst powder is fluidizable - the powder behaves like a fluid when aerated. This allows the catalyst to form fluidized beds and be transported between the FCC reactor and regenerator with ease. FCC catalyst are considered to have two catalytically active parts: (a) the "zeolite", and (b) what is termed the "matrix".
Ethene is not a catalytic product in the FCC, a small amount is formed via thermal cracking reactions which are considered undesirable in the FCC because they are non-selective and limit operations by increasing coke and light gas make. Every effort is made in the design and operation of an FCC unit to minimize thermal cracking because coke drives up the regenerator temperature and increases its air demand, while light gases are more difficult to compress in the product recovery section and limit product recovery. Ethene is a very minor product in the FCC that is not generally economically worth recovering. 2ff7e9595c
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