Organic Industrial Chemistry

Petroleum


Industrial organic chemistry today can be divided roughly into four major areas. In order of their current economic importance they are polymers, petrochemicals, synthetic materials (other than polymers), and miscellaneous organic materials lumped together under the general heading of "fine chemicals". The historic development and present industrial structure of each of these areas are different.
Most modern industrial organic materials are derived from petroleum, whose modern production dates from about 1860, or from natural gas. Less important sources include coal tar, wood, and agricultural waste materials. Oil refineries are generally large installations having some flexibility in both input and output, and traditionally switch output between fuel, lubricating oils, and petrochemicals depending on prices and markets.
Petroleum, or oil, is a naturally occurring liquid with widely different composition of very great complexity. While there are a few su***ce seepages, the vast majority of petroleum is found well below the su***ce of the earth and can be reached only by drilling. Oil wells tap into pools of oil, or into porous rock containing the oil, called reservoirs or fields. The oil is sometimes found under sufficiently high pressure to flow to the su***ce without pumping, but for most wells pumping is required. The amount of oil recoverable from a field by pumping may be only 5%,more frequently 25-30%, of the oil believed to be present. Addition of liquids to the field down wells, often salt water from brine wells or local fresh su***ce water with a small amount of su***ctant added, is used to provide enhanced recovery of oil. In fields where the oil is very heavy steam injection may be used. Complete removal of the oil from a field is not possible even with enhanced recovery methods.
Petroleum as obtained from wells, crude oil, is a complex mixture of hydrocarbons. Its elemental composition is primarily carbon-hydrogen, with variable quantities of oxygen and sulfur, and trace amounts of nitrogen, metals and other elements. Crude oil is classified by the carbon-hydrogen ratio, which is lower for the more desirable crudes containing smaller molecules (light crude) and higher for the less desirable crudes containing primarily larger molecules (heavy crude). Crude oil lowin sulfur is called sweet crude while the less desirable crude oil with higher sulfur content is called sour crude.
Crude oil is found in many parts of the world. Major producing areas include the southern United States, western Canada, Mexico, Venezuela, the Middle East, the eastern Soviet Union, Rumania, Nigeria, and Indonesia. Crude oil can be transported economically long distances overland by pipeline and overseas in very large tanker vessels. Refineries are located on seacoasts with tanker docks or are connected to the production areas or tanker ports by pipeline.
Petroleum Refining

Crude oil refining usually begins with washing with water to remove salt and other inorganic impurities, followed by fractional distillation. The fractions into which crude oil is traditionally distilled is shown in the Table below.
Table: Typical Fractions Obtained on Distillation of Crude Oil

Output % Boiling Range Carbon Atoms Product
(deg Celsius)

2 <<30 1 to 4 light hydrocarbons
15 to 30 30-200 4 to 12 naptha
5 to 20 200-300 12 to 15 kerosene
10 to 40 300-400 15 to 25 gas oil
residue 400+ 25+ residual oil
The initial distillation or topping of the crude oil separates the fractions shown in the above Table. Each of these fractions is usually subjected to additional processes and portions of them may be combined to give final desired products. Modern refineries produce primarily fuels, especially motor gasoline, kerosene (jet fuel), fuel oil (heating oil), and heavier oils (residual oil), as well as a variety of minor products such as lubricating oils, paraffin, and asphalt.
The fraction indicated as light hydrocarbons includes methane through the butanes. The fraction or cut designated naptha is also called straight-run gasoline and can be used as a motor fuel, although modern motor gasolines are more sophisticated blends. The kerosene fraction, once in demand for lighting, now finds its primary modern use as jet aircraft fuel. The heavier gas oil fraction is blended into several grades of heating oil and bunker fuel (for ships). Both gas oil and residual oil are feedstocks for further processes. Residual oil is not the only form of residue; paraffins and asphalt are also left undistilled.
Natural gas, on compression, will also condense out heavier alkanes than methane as liquids (liquified petroleum gases, L.P.G.), and these are a useful source of propanes and butanes for polymers. In a modern refinery, unused "cuts" or fractions can be cycled to cracking units, which heat the material above 230oC at different pressures. Cracking may be done in the presence (catalytic cracking) or absence (thermal cracking) of a catalyst or in the presence of water (steam cracking). The reactions occurring in a cracking process are complex. Most, but not all, convert more complex hydrocarbons to simpler ones of lower boiling point with loss of hydrogen gas. Among them are 2CH4 --> HC=CH + 6H2, producing ethyne, and the production of ethene from ethane, C2H6 --> H2C=CH2 + H2.
Propane also can be cracked to ethene, C3H8(g) --> H2C=CH2 + CH4, or to propene, C3H8(g) --> H2C=CHCH3 + H2.
Heavier fractions of oil are cracked to gasoline. Propene, CH3-CH=CH2, arises as a byproduct of the cracking process which yields ethene or is a byproduct of gases liberated elsewhere in the refinery. Ethane, propane, and butane obtained from natural gas are cracked to ethene and propene in the same manner. Butenes and butadiene, produced in smaller quantities, are used in the production of synthetic rubber.

Petrochemicals: Introduction and Aliphatic Compounds

The petrochemicals industry is broadly defined as that industrial activity which uses petroleum or natural gas as a source of raw materials and whose products are neither fuels nor fertilizer. The petrochemical industry begins with oil refineries or extracting plants built to remove ethane and higher hydrocarbons from natural gas streams; sometimes methane itself is used as a source material or feedstock. The industry is so varied that analysis by specific compound or class of compound is the most effective method of presentation.
Present world data on production of organic industrial compounds is not easily obtainable. The Table below gives production figures for the United States for the fifteen organic chemicals produced in greatest quantity. This list excludes fuels, such as methane, ethane, propane, and butane, and also gasoline which includes aromatic compounds (toluene, xylene, etc) to raise its octane rating. Only the major sources are given; reference to another compound included in the Table is indicated by an asterisk.




Methane

Methane is obtained from natural gas, from oil refinery processes, or as a byproduct in other processes. Its major non-petrochemical use is in the production of hydrogen for use in the Haber synthesis of ammonia. Ammonia synthesis requires nitrogen, obtained from air, and hydrogen. The most common modern source of the hydrogen consumed in ammonia production, about 95% of it, is methane. Methane undergoes two useful reactions at 90oC in the presence of Fe3O4 as a catalyst: CH4 + H2O --> CO + 3H2 and CO + H2O --> CO2 + H2.
Alternatively, partial combustion of methane can be used to provide the required heat and steam. The carbon dioxide produced then reacts with methane at 900oC in the presence of a nickel catalyst: CH4 + 2O2 --> CO2 + 2H2O and CO2 + CH4 --> 2CO + 2H2, as does the water: CH4 + H2O --> CO + 3H2.

Methanol, CH3OH, is the second major product produced from methane. Synthetic methanol has virtually completely replaced methanol obtained from the distillation of wood, its original source material. One of the older trivial names used for methanol was wood alcohol. The synthesis reaction takes place at 350oC and 300 atm in the presence of ZnO as a catalyst: 2CH4 + O2 --> 2CH3OH.

Most of the methanol is then oxidized by oxygen from air to methanal, which is more commonly known as formaldehyde: 2CH3OH + O2 --> 2CH2O + 2H2O. Formaldehyde is used to form synthetic resins either alone or with phenol, urea, or melamine; other uses are minor.

Methane yields four compounds upon chlorination in the presence of heat or light: CH4 + Cl2 --> CH3Cl, CH2Cl2, CHCl3, CCl4. These compounds, known as chloromethane or methyl chloride, dichloromethane or methylene chloride, trichloromethane or chloroform, and tetrachloromethane or carbon tetrachloride, are used as solvents or in the production of chlorinated materials.

Methane reacts with sulfur in the presence of a catalyst to give the carbon disulfide used in the rayon industry: CH4 + 4S(g) --> CS2 + 2H2S. We will refer to this again in a following section.

Ethene

Ethene or ethylene, H2C=CH2, is produced from ethane, propane, butane, and from cracking of other refinery streams such as naptha, kerosene, and gas-oil. The ethene is obtained in part from refinery gases but primarily from stripper plants, which extract the ethane from natural gas. Ethane is generally more valuable as a chemical raw material than as a fuel.
Ethene is produced from ethane by cracking, followed by separation of fractions where necessary: C2H6 --> H2C=CH2 + H2

There are many uses for ethene. In North America about 38% of it is used directly to produce polyethylene, and this is the largest single use. About 22% of the ethene is oxidized in the presence of a silver catalyst to ethylene oxide: 2H2C=CH2 + O2 --> C2H4O. The vast majority of the ethylene oxide produced is hydrolyzed at 100oC to ethylene glycol. The oxidation reaction is: C2H4O + H2O --> HO-CH2-CH2-OH. Some 70% of the ethylene glycol produced is used as an automotive antifreeze and much of the rest is used in the synthesis of polyesters.

About 13% of the ethene is chlorinated to 1,2-dichloroethane (dichloroethane) or to ethylene dichloride. The reaction forming dichloroethane is: H2C=CH2 + Cl2 --> H2ClC-CH2Cl. There are some minor uses for ethylene dichloride, but about 90% of it is cracked to the monomer of polyvinyl chloride (PVC), chloroethene or vinyl chloride. The simplified cracking reaction is: H2ClC-CH2Cl --> HCl + H2C=CHCl.

About 10% of the ethene is used in the production of ethylbenzene. Another 10% is hydrated to ethanol; the reaction takes place at 400oC and 70 atm in the presence of phosphoric acid: H2C=CH2 + H2O --> C2H5OH. The remaining 7% of the ethene has many minor uses.

Ethyne

Ethyne (acetylene) is the only petrochemical produced in significant quantity which contains a triple bond, and is a major intermediate species. It is not easily shipped, and as a consequence its consumption is close to the point of origin. It can be made by hydrolysis of calcium carbide produced in the electric furnace from CaO and carbon. The reaction is CaC2 + 2H2O --> HCCH + Ca(OH)2.This is the original industrial process for ethyne production and is still significant, but requires a large input of electrical power.
An alternative method of manufacturing ethyne by cracking of methane is, in simplified form, 2CH4 --> HCCH + 6H2. This process produces only one-third of the methane input as ethyne, the remainder being burned in the reactor. Similar reactions employing heavier fractions of crude oil are being used increasingly since the price of methane relative to heavy crude is rising.

Ethyne is used as a special fuel gas (oxyacetylene torches) and as a chemical raw material.

Propene

Virtually all propene or propylene is made from propane, which is obtained from natural gas stripper plants or from refinery gases. Some 80% of the propene produced in North America is a refinery byproduct, the rest is a byproduct of cracking to ethene. Propane is converted to propene by cracking, followed by separation of fractions where necessary. The simplified cracking reaction is C3H8 --> CH3-CH=CH2 + H2.
The uses of propene include gasoline (80%), polypropylene, isopropanol, trimers and tetramers for detergents, propylene oxide, cumene, and glycerine.

Butene and Butadiene

Two butenes or butylenes are industrially significant, 1-butene and2-butene. The latter has end uses in the production of butyl rubber and polybutene plastics. The 1-butene is used in the production of 1,3-butadiene for the synthetic rubber industry. Butenes arise primarily from refinery gases or from the cracking of other fractions of crude oil.
Butadiene can be recovered from refinery streams as butadiene, as butenes, or as butanes; the latter two on appropriate heated catalysts dehydrogenate to give 1,3-butadiene. The dehydrogenation reaction is CH2=CH-CH2-CH3 --> CH2=CH-CH=CH2 + H2.

An alternative source is ethanol, which on appropriate catalytic treatment also gives the compound, but this is of little current industrial importance in North America.

Isoprene

Alkenes containing more than four carbon atoms are in little demand as petrochemicals and thus are generally used as fuel. The single exception to this is 2-methyl-1,3-butadiene or isoprene, which has a significant use in the synthetic rubber industry. It is more difficult to make than is 1,3-butadiene. Some is available in refinery streams, but more is manufactured from refinery stream 2-butene by reaction with methanal. The reaction is (CH3)2C=C2 + HCHO --> CH2=CH(CH3)-CH=CH2 + H2O.