Steam cracking

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Steam cracking or steam cracking is a petrochemical process in which long -chain hydrocarbons ( naphtha , but also propane , butane and ethane as well as gas oil and hydrowax ) are converted into short-chain hydrocarbons in the presence of steam by thermal cracking . There arise primarily hydrogen , methane , ethene and propene as the main product, butenes , pyrolysis gasoline (engl .: pygas ), and a tar-like residue (ECR: ethylene cracker residue , engl .: ethylene cracker residue ). A steam cracker is one of the most complicated systems in the petrochemical industry. It is used to manufacture raw materials that are mainly processed into plastics, paints, solvents or pesticides.

raw materials

• Naphtha ("standard" feed, also recycle (see pyrolysis gasoline ), boiling range can vary widely: ≈25 - ≈180 ° C)
• Refinery gas , feed for the gas stoves (with a high proportion of ethane, also recycle / see products)
• Propane, feed for the gas stoves (if economically feasible)
• Butane, feed for the gas stoves (if economically feasible)
• Heavy gas oil, feed for special furnaces for cracking heavy feedstock (boiling range: ≈300 - ≈370 ° C, possibly hydrogenated)
• Hydrowax, feed for special ovens for cracking heavy feedstock (boiling range: ≈340 - ≈560 ° C)

Procedure

The naphtha (or naphtha fractions) occurring in the crude oil refinery is often used as the starting material , along with light hydrocarbon fractions from accompanying gases from oil production or separated heavier fractions (C ₂₊ ) from natural gas. Gas oil and hydrowax are also used. However, a special furnace design is required for this.

The actual cracker is a tubular reactor with a pipe coil with an internal diameter of around 90-120 mm and a length of 60-80 m. The tube consists of a chromium / nickel - alloy and is located in a furnace which is heated by flames. The design of the tubular reactor (furnace) is tailored to the properties of the feed. A distinction is made between gas ovens (for ethane, propane, butane), naphtha ovens and ovens for heavy feedstock (gas oil, hydrowax). The 100 ° C hot bottom product is preheated to 550-600 ° C at around 12 bar in the convection zone of the oven. In this zone, process steam at a temperature of 180–200 ° C is also added (hence the name steam cracking). The process steam serves to bring about a partial pressure reduction of the individual reactants. In addition, it partially prevents the finished reaction products ( polymerisation of the alkenes ) from accumulating (due to the volume it occupies ) and cools the bundle of warmers in the convection zone. After the convection zone, the now completely gaseous bottom product reaches the radiation zone. In this it is cracked at around 805–850 ° C to form the low molecular weight hydrocarbons. The residence time τ is about 0.2-0.4 s. This creates important basic products such as ethene (ethylene), propene (propylene), 1,2- and 1,3-butadiene , n- and i- butene, benzene , toluene and xylenes . In addition, hydrogen and methane as well as other - sometimes disruptive - by-products such as ethyne (acetylene), propyne (in traces), propadiene (in traces) and - as a component of pyrolysis gasoline - n -, i - and cyclo - paraffins or - olefins , C ₉ - and C ₁₀ - aromatics . The heaviest fraction is the so-called ethylene cracker residue , a highly aromatic, highly olefinic residue that covers the boiling range ≈210–500 ° C.

So that the newly formed reaction products do not reassemble into much larger products ( oligomerization ), the hot cracked gas is suddenly cooled to around 350-400 ° C in a heat exchanger (this is often a high-pressure feed water cooler). The hot cracked gas is then additionally cooled to 150-170 ° C. with quench oil for the subsequent fractionation.

Products

A typical naphtha steam cracker delivers the following product streams (in mass%)

• Hydrogen-rich gas 2.0% (approx. 50% , 50% methane) ${\displaystyle {\ce {H2}}}$
• Methane-rich gas 15.4% (almost pure methane)
• Ethene 29.1% (desired main product, including the hydrogenated ethyne , see below)
• Ethane 3.9% (is normally recycled; requires higher temperatures compared to naphtha)
• Propene 16.1% (most important "by-product")
• Propane 1.2% (is usually recycled, if there is no propane / propene separation, it is sold together with the propene)
• 1,3-butadiene 4.3% (by-product)
• Butene mixture 5.5% (by-product)
• Pyrolysis gasoline 19.6% (by-product, boiling range: ~ 25–210 ° C, includes benzene, toluene, xylenes, ethylbenzene , isoprene , cyclopentadiene )
• Ethylene cracker residue 0.9% (by-product, boiling range: ~ 210–500 ° C, contains sulfur, highly olefinic, highly aromatic)
• Carbon monoxide ≈100–1000 ppm (partly in the hydrogen-rich gas)
• Carbon dioxide ≈30-500 ppm
• Hydrogen sulfide (traces, depending on the sulfur content of the feedstock)

The composition of the cracking gas can vary significantly, depending on the use and cracking conditions. Heavier feedstocks like gas oil or hydrowax produce less ethene and hydrogen, but u. a. more pyrolysis gasoline and ethylene cracker residue (up to 10% and more ECR).

Separation of products

The product flow at the furnace outlet contains a large number of substances that are now separated from one another. The products of value must generally be produced in a very high purity. The substances that one does not want to obtain as a product are returned to the cracker, burned (gases) or sold as a semi-finished product. The processing is extremely complex and is shown here in a greatly simplified manner.

Work-up begins with oil washing and water washing, in which the still hot gas cools down further and heavy contaminants such as coke and tar are separated out. The most important separation steps are rectifications (multi-stage distillations ). For the separation of the light hydrocarbons, low temperature rectifications at high pressure are necessary. For this purpose, the cracked gas is initially compressed to approx. 30 bar in stages. The acid gases are absorbed in a caustic wash. An adsorptive dryer removes water. The separation of traces of ethine would be extremely difficult, so that ethyne is catalytically hydrogenated to ethene instead .

This is followed by the gradual cooling of the cracked gas and a sequence of rectifications in which the hydrocarbon mixture is broken down into fractions of different carbon numbers. The sequence of the separating cuts is of decisive importance for the integration of heat , the expenditure on equipment and thus the economic efficiency of the process. Depending on the local conditions, the composition of the cracked gas and the desired products, very different solutions are possible and useful. The individual fractions are separated into saturated and unsaturated hydrocarbons in further distillations.

The main products, especially ethene and propene, are now available in pure form. The butane derivatives can be used for a variety of petrochemical processes (eg. As the iso- butene for the production of MTBE or ETBE , n butenes to produce alkylate ). The pyrolysis gasoline is the starting material for the production of benzene and toluene (see pyrolysis gasoline ). Fractions that are not wanted as a product, especially alkanes , can be returned to the cracker. The fractions that are not suitable for cracking, such as methane or hydrogen, are burned in the cracking ovens and provide the energy required for the process (hydrogen is a very valuable intermediate product and is normally returned to the refinery). The tar-like residue (ECR) is either incinerated in a power plant, sold as a binder for the production of graphite electrodes (for aluminum production ) or used to produce industrial soot .

Others

• The naphtha steam cracker from BASF FINA Petrochemicals Limited Partnership in Port Arthur , Texas, is the largest naphtha cracker in the world. It has been producing since December 2001. This steam cracker has an annual capacity of 920,000 tons of ethene. 210 people are employed at the plant.

• The largest ethane steam cracker in the world, with an annual capacity of 1.5 million tons of ethene, is operated by Borouge, a joint venture of Borealis and the Abu Dhabi National Oil Company , in Ruwais , Abu Dhabi . A second ethane steam cracker of the same capacity is currently under construction there. Both systems were supplied by Linde Engineering, a division of Linde AG .

• BASF operates a total of five steam crackers worldwide: two in the main plant in Ludwigshafen am Rhein , one in the Antwerp plant , the largest in Port Arthur and another in Nanjing , China.

• In Germany, Dow Olefinverbund GmbH operates a naphtha steam cracker in Böhlen (Saxony) .

literature

• Arno Behr , David W. Agar, Jakob Jörissen: Introduction to Technical Chemistry. Spektrum Akademischer Verlag, Heidelberg 2009, ISBN 978-3-8274-2073-2 , p. 178 ( limited preview in Google book search).

Web links

• Crude oil processing - steam cracking in ChemgaPedia

Individual evidence

1. ↑ Data from the Port Arthur Refinery (PDF; 146 kB).
2. ↑ The Heart of the Verbund. In: basf.com. Retrieved June 17, 2020 .