Petrochemicals: Introduction and Aliphatic Compounds

James Richard Fromm


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.


Table: Production of Industrial Organic Compounds, U.S.A.
Compound Production (Tg, 1985) Production (Tg, 1975) Source
       
Ethene 13.54 9.99 Ethene
Propene 6.75 4.43 Propane
Dichloroethane 5.49 3.59 Ethene*, Chlorine
Vinyl Chloride 4.29 2.60 Dichloroethane*
Benzene 4.26 4.81 Refinery
Styrene 3.46 2.86 Ethylbenzene*
Ethylbenzene 3.35 2.78 Benzene*, Ethene*
Terephthalic Acid 2.94 2.29 Xylene*
Formaldehyde 2.54 2.55 Methanol*
Ethylene Oxide 2.46 1.90 Ethene*, Oxygen
Xylene 2.41 3.30 Refinery
Toluene 2.30 3.73 Refinery
Methanol 2.27 2.83 Methane
Ethylene Glycol 1.90 1.52 Ethylene Oxide*
Butadiene 1.06 1.47 Butanes
       
The unit of one Tg used in this Table is also one million Metric Tons (MMT).      

Methane

Methane is obtained from natural gas, from oil refinery processes, or as a by-product 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 rarrow.gif (63 bytes) CO + 3H2 and CO + H2O rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) CO2 + 2H2O and CO2 + CH4 rarrow.gif (63 bytes) 2CO + 2H2,

as does the water:

CH4 + H2O rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 2CH3OH.

Most of the methanol is then oxidized by oxygen from air to methanal, which is more commonly known as formaldehyde:

2CH3OH + O2 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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) rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) C2H4O.

The vast majority of the ethylene oxide produced is hydrolyzed at 100oC to ethylene glycol. The oxidation reaction is:

C2H4O + H2O rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) 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 by-product, the rest is a by-product of cracking to ethene. Propane is converted to propene by cracking, followed by separation of fractions where necessary. The simplified cracking reaction is

C3H8 rarrow.gif (63 bytes) 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 and 2-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 rarrow.gif (63 bytes) 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 rarrow.gif (63 bytes) CH2=CH(CH3)-CH=CH2 + H2O.


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Copyright 1997 James R. Fromm