Chemistry of Petrochemical Processes
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The book reviews and describes the reactions and processes involved in transforming petroleum-based hydrocarbons into the chemicals that form the basis of the multi-billion dollar petrochemical industry. In addition, the book includes information on new process developments for the production of raw materials and intermediates for petrochemicals that have surfaced since the book's first edition.
Provides a quick understanding of the chemical reactions associated with oil and gas processing Contains insights into petrochemical reactions and products, process technology, and polymer synthesis. Chapter Four Nonhydrocarbon Intermediates. Chapter Five Chemicals Based on Methane. Chapter Seven Chemicals Based on Ethylene.
Chapter Eight Chemicals Based on Propylene.
Alkylation of benzene with ethylene produces ethyl benzene, which is dehydrogenated to styrene. Ethylene can be polymerized to different grades of polyethylenes or copolymerized with other olefins. Catalytic oxidation of ethylene produces ethylene oxide, which is hydrolyzed to ethylene glycol. Ethylene glycol is a monomer for the production of synthetic fibers. The main source for ethylene is the steam cracking of hydrocarbons Chapter 3. Table shows the world ethylene production by source until the year U.
Many important chemicals are based on propylene such as isopropanol, allyl alcohol, glycerol, and acrylonitrile. The three nbutenes are 1-butene and cis- and trans- 2-butene. The following shows the four butylene isomers: When the double bonds are separated by only one single bond, the compound is a conjugated diene conjugated diolefin. Nonconjugated diolefins have the double bonds separated isolated by more than one single bond.
This latter class is of little industrial importance. Each double bond in the compound behaves independently and reacts as if the other is not present. It can be polymerized to polybutadiene or copolymerized with styrene to styrene-butadiene rubber SBR. Butadiene is an important intermediate for the synthesis of many chemicals such as hexamethylenediamine and adipic acid. Both are monomers for producing nylon. Chloroprene is another butadiene derivative for the synthesis of neoprene rubber. The unique role of butadiene among other conjugated diolefins lies in its high reactivity as well as its low cost.
Other sources of butadiene are the catalytic dehydrogenation of butanes and butenes, and dehydration of 1,4-butanediol. Isoprene 2-methyl-1,3-butadiene is a colorless liquid, soluble in alcohol but not in water.
Its boiling temperature is Isoprene is the second important conjugated diene for synthetic rubber production. The main source for isoprene is the dehydrogenation of C5 olefins tertiary amylenes obtained by the extraction of a C5 fraction from catalytic cracking units. It can also be produced through several synthetic routes using reactive chemicals such as isobutene, formaldehyde, and propene. The main use of isoprene is the production of polyisoprene.
It is also a comonomer with isobutene for butyl rubber production. They are important precursors for many commercial chemicals and polymers such as phenol, trinitrotoluene TNT , nylons, and plastics. Accordingly, they do not easily add to reagents such as halogens and acids as do alkenes. Aromatic hydrocarbons are generally nonpolar. They are not soluble in water, but they dissolve in organic solvents such as hexane, diethyl ether, and carbon tetrachloride. The product reformate is rich in C6, C7, and C8 aromatics, which could be extracted by a suitable solvent such as sulfolane or ethylene glycol.
These solvents are characterized by a high affinity for aromatics, good thermal stability, and rapid phase separation. The Tetra extraction process by Union Carbide Figure uses tetraethylene glycol as a solvent. The feed reformate , which contains a mixture of aromatics, paraffins, and naphthenes, after heat exchange with hot raffinate, is countercurrentIy contacted with an aqueous tetraethylene lycol solution in the extraction column.
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The aromatics extract is then purified by extractive distillation and recovered from the solvent by steam stripping. Extractive distillation has been reviewed by Gentry and Kumar. The raffinate constituted mainly of paraffins, isoparaffins and cycloparaffins is washed with water to recover traces of solvent and then sent to storage.
The solvent is recycled to the extraction tower. The extract, which is composed of BTX and ethylbenzene, is then fractionated. Benzene and toluene are recovered separately, and ethylbenzene and xylenes are obtained as a mixture C8 aromatics. A superfractionation technique is used to segregate ethylbenzene from the xylene mixture. Because p-xylene is the most valuable isomer for producing synthetic fibers, it is usually recovered from the xylene mixture. Currently, industry uses continuous liquid-phase adsorption separation processes. The overall yield of p-xylene is increased by incorporating an isomerization unit to isomerize o- and m-xylenesto p-xylene.
Figure is a flow diagram of the Mobil isomerization process. In this process, partial conversion of ethylbenzene to benzene also occurs. The catalyst used is shape selective and contains ZSM-5 zeolite. Before , the main source of benzene and substituted benzene was coal tar. Currently, it is mainly obtained from catalytic reforming. Other sources are pyrolysis gasolines and coal liquids. Aromatic compounds react by addition only under severe conditions. For example, electrophilic substitution of benzene using nitric acid produces nitrobenzene under normal conditions, while the addition of hydrogen to benzene occurs in presence of catalyst only under high pressure to give cyclohexane: It can be obtained by intensive fractionation of the aromatic extract, but only a small quantity of the demanded ethylbenzene is produced by this route.
Most ethylbenzene is obtained by the alkylation of benzene with ethylene. Those presently of commercial importance are toluene, o-xylene, p-xylene, and to a much lesser extent m-xylene. The primary sources of toluene and xylenes are reformates from catalytic reforming units, gasoline from catcracking, and pyrolysis gasoline from steam reforming of naphtha and gas oils. As mentioned earlier, solvent extraction is used to separate these aromatics from the reformate mixture. Only a small amount of the total toluene and xylenes available from these sources is separated and used to produce petrochemicals.
Naphtha from atmospheric distillation is characterized by an absence of olefinic compounds. Its main constituents are straight and branchedchain paraffins, cycloparaffins naphthenes , and aromatics, and the ratios of these components are mainly a function of the crude origin.
Naphthas obtained from cracking units generally contain variable amounts of olefins, higher ratios of aromatics, and branched paraffins. Due to presence of unsaturated compounds, they are less stable than straight-run naphthas. On the other hand, the absence of olefins increases the stability of naphthas produced by hydrocracking units.
Selecting the naphtha type can be an important processing procedure. For example, a paraffinic-base naphtha is a better feedstock for steam cracking units because paraffins are cracked at relatively lower temperatures than cycloparaffins. Alternately, a naphtha rich in cycloparaffins would be a better feedstock to catalytic reforming units because cycloparaffins are easily dehydrogenated to aromatic compounds.
Chapter 10 discusses aromatics-based chemicals. Naphtha is also a major feedstock to steam cracking units for the production of olefins. This route to olefins is especially important in places such as Europe, where ethane is not readily available as a feedstock because most gas reservoirs produce non-associated gas with a low ethane content.
Naphtha could also serve as a feedstock for steam reforming units forthe production of synthesis gas for methanol. Currently, kerosine is mainly used to produce jet fuels,. Butadiene, a conjugated diolefin, is normally coproduced with C2—C4 olefins from different cracking processes. Separation of these olefins from catalytic and thermal cracking gas streams could be achieved using physical and chemical separation methods. However, the petrochemical demand for olefins is much greater than the amounts these operations produce.
Most olefins and butadienes are produced by steam cracking hydrocarbons. Figure is a block diagram for ethylene from ethane. The furnace effluent is quenched in a heat exchanger and further cooled by direct contact in a water quench tower where steam is condensed and recycled to the pyrolysis furnace.
After the cracked gas is treated to remove acid gases, hydrogen and methane are separated from the pyrolysis products in the demethanizer. Ethane is then recycled to the pyrolysis furnace.
Chemistry of Petrochemical Processes
Feed characteristics are also considered, since they influence the process severity. Steam cracking reactions are highly endothermic. Increasing temperature favors the formation of olefins, high molecular weight olefins, and aromatics.
Optimum temperatures are usually selected to maximize olefin production and minimize formation of carbon deposits. In steam cracking processes, olefins are formed as primary products. Aromatics and higher hydrocarbon compounds result from secondary reactions of the formed olefins. Accordingly, short residence times are required for high olefin yield.
When ethane and light hydrocarbon gases are used as feeds, shorter residence times are used to maximize olefin production and minimize BTX and liquid yields; residence times of 0. Cracking liquid feedstocks for the dual purpose of producing olefins plus BTX aromatics requires relatively longer residence times than for ethane. However, residence time is a compromise between the reaction temperature and other variables.
Steam reduces the partial pressure of the hydrocarbon mixture and increases the yield of olefins. Heavier hydrocarbon feeds require more steam than gaseous feeds to additionally reduce coke deposition in the furnace tubes. Liquid feeds such as gas oils and petroleum residues have complex polynuclear aromatic compounds, which are coke precursors. Steam to hydrocarbon weight ratios range between 0. Feeds to steam cracking units vary appreciably, from light hydrocarbon gases to petroleum residues.
Due to the difference in the cracking rates of the various hydrocarbons, the reactor temperature and residence time vary. As mentioned before, long chain hydrocarbons crack more easily than shorter chain compounds and require lower cracking temperatures. Feedstock composition also determines operation parameters.
The rates of cracking hydrocarbons differ according to structure. Isoparaffins such as isobutane and isopentane give high yields of propylene. This is expected, because cracking at a tertiary carbon is easier. The ratio of olefins produced from steam cracking of these feeds depends mainly on the feed type and, to a lesser extent, on the operation variables. For example, steam cracking light naphtha produces about twice the amount of ethylene obtained from steam cracking vacuum gas oil under nearly similar conditions.
Liquid feeds are usually cracked with lower residence times and higher steam dilution ratios than those used for gas feedstocks. This is necessitated by the greater variety and quantity of coproducts. An additional pyrolysis furnace for cracking coproduct ethane and propane and an effluent quench exchanger are required for liquid feeds. Also, a propylene separation tower and a methyl acetylene removal unit are incorporated in the process. Figure is a flow diagram for cracking naphtha or gas oil for ethylene production.
As with gas feeds, maximum olefin yields are obtained at lower hydrocarbon partial pressures, pressure drops, and residence times. These variables may be adjusted to obtain higher BTX at the expense of higher olefin yield. For example, steam cracking naphtha produces, in addition to olefins and diolefins, pyrolysis gasoline rich in BTX. Table shows products from steam cracking naphtha at low and at high severities. It should be noted that operation at a higher severity increased ethylene product and by-product methane and decreased propylene and butenes. Acetaldehyde has been suggested as an intermediate: Isoprene 2-methyl 1,3-butadiene is the second most important conjugated diolefin after butadiene.
Most isoprene production is used for the manufacture of cis-polyisoprene, which has a similar structure to natural rubber. It is also used as a copolymer in butyl rubber formulations. The amylenes are extracted from a C5 fraction with aqueous sulfuric acid. Dehydrogenation of t-amylenes over a dehydrogenation catalyst produces isoprene. The C5 olefin mixture can also be produced by the reaction of ethylene and propene using an acid catalyst.
A three-step process developed by Snamprogetti is based on the reaction of acetylene and acetone in liquid ammonia in the presence of an alkali metal hydroxide. The addition of carbon black to tires lengthens its life extensively by increasing the abrasion and oil resistance of rubber. Carbon black consists of elemental carbon with variable amounts of volatile matter and ash.
Chemistry of Petrochemical Processes - Sami Matar, Ph.D., Lewis F. Hatch, Ph.D. - Google Книги
There are several types of carbon blacks, and their characteristics depend on the particle size, which is mainly a function of the production method. Carbon black is produced by the partial combustion or the thermal decomposition of natural gas or petroleum distillates and residues. Coke produced from delayed and fluid coking units with low sulfur and ash contents has been investigated as a possible substitute for carbon black.
Three processes are currently used for the manufacture of carbon blacks. These are the channel, the furnace, and the thermal processes. The feed is first preheated and then combusted in the reactor with a limited amount of air. The hot gases containing carbon particles from the reactor are quenched with a water spray and then further cooled by heat exchange with the air used for the partial combustion.
The type of black produced depends on the feed type and the furnace temperature.