Petrochemicals from Natural Gas, Chemistry tutorial

Introduction:

Natural gas and crude oil are the fundamental raw materials for the manufacture of petrochemicals. Secondary raw materials or intermediates are acquired from natural gas and crude oil via different processing methods. The intermediates might be light hydrocarbon compounds like methane and ethane, or heavier hydrocarbon mixtures like naphtha or gas oil.

Chemical based on direct reactions of Methane:

Some chemicals are mainly based on the direct reaction of methane by other reagents. These are carbon disulphide, hydrogen cyanide, chloromethane and synthesis gas mixture. Presently, a redox fuel cell based on methane is being developed.

Carbon Disulphide (CS2):

Methane reacts with sulphur (that is, an active non-metal element of group 6A) at high temperatures to form carbon disulphide. The reaction is endothermic, and activation energy of around 160 KJ is needed. Activated alumina or clay is employed as the catalyst at around 675°C and 2 atmospheres. The method begins by vaporizing pure sulphur, mixing it by methane and passing the mixture over the alumina catalyst. The reaction could be symbolized as:

CH4 (g) + 2S2 (g) → CS2 (g) + 2H2S (g)       ΔH°298 = +150 KJ/mol

Hydrogen sulphide, a co-product, is employed to recover the sulphur by the Claus reaction. A CS2 yield of 85 to 90% dependent on methane is anticipated. The alternative route for CS2 is through the reaction of liquid sulphur by charcoal. Though, this process is not employed very much.

Uses of Carbon disulphide:

Carbon disulphide is mainly utilized to produce rayon and cellophane (that is, re-generated cellulose). CS2 is as well employed to produce carbon tetrachloride by using iron powder as a catalyst at 30°C:

CS2 + 3Cl2 → CCl4 + S2Cl2

Sulphur monochloride is an intermediate which is then reacted by carbon disulphide to generate more carbon tetrachloride and sulphur:

2S2Cl2 + CS2 → CCl4 + 6S

The total reaction is:

CS2 + 2Cl2 → CCl4 + 2S

Carbon disulphide is as well employed to produce xanthate ROC(S) SNa as an ore flotation agent and ammonium thiocyanate as the corrosion inhibitor in ammonia handling systems.

Hydrogen Cyanide:

Hydrogen cyanide (that is, hydrocyanic acid) is a colourless liquid (boiling point 25.6°C) that is miscible with water, generating a weakly acidic solution. This is a highly toxic compound, however a very helpful chemical intermediate with high reactivity. This is employed in the synthesis of acrylonitrile and adiponitrile that are significant monomers for plastic and synthetic fiber production. Hydrogen cyanide is prepared through the Andrussaw method by using ammonia and methane in the presence of air. The reaction is exothermic, and the heat discharged is employed to supplement the requisite catalyst-bed energy:

2CH4 + 2NH3 + 3O2 → 2HCN + 6H2O

A platinum-rhodium alloy is employed as a catalyst at 1100°C. Around equivalent amounts of ammonia and methane having 75 vol % air are introduced to the preheated reactor. The catalyst consists of some layers of wire gauze by a special mesh size (around 100 mesh). In Degussa procedure on the other hand, ammonia reacts by methane in absence of air by employing platinum, aluminum-ruthenium alloy as a catalyst at around 1200°C. The reaction generates hydrogen cyanide and hydrogen, and the outcome is over 90%. The reaction is endothermic and needs 251 KJ/mol.

CH4 + NH3 + 251 KJ → HCN + 3H2

Hydrogen cyanide might as well be produced via the reaction of ammonia and methanol in the presence of oxygen:

NH3 + CH3OH + O2 → HCN + 3H2O

Hydrogen cyanide is a reactant in the production of acrylonitrile, methyl methacrylate (from acetone), adiponitrile and sodium cyanide. It is as well employed to make oxamide, a long-lived fertilizer which discharges nitrogen steadily over the vegetation period. Oxamide is generated by the reaction of hydrogen cyanide having water and oxygen by employing a copper nitrate catalyst at around 70°C and atmospheric pressure. 

Chloromethane:

The successive substitution of methane hydrogen having chlorine generates a mixture of four chloromethane:

CH4 + Cl2 - CH3Cl + GCl

CH3Cl + Cl2 - CH2 Cl2+ HCl

CH2Cl2 + Cl2 - CHCl3 + HCl

CHCl3+Cl2-CCl4 + HCl

Each of such four compounds has numerous industrial applications which will be dealt with separately.

Production of Chloromethane:

Methane is the most difficult alkane to chlorinate. The reaction is initiated through chlorine free radicals obtained through the application of heat (that is, thermal) or light (hv). Thermal chlorination (more broadly used industrially) takes place at around 350 to 370°C and atmospheric pressure. A typical product distribution for a CH4/Cl2 feed ratio of 1.7 is: mono- (58.7%), di-(29.3%) tri- (9.7%) and tetra- (2.3%) chloromethane. The highly exothermic chlorination reaction generates around 95 KJ/mol of HCl. The primary step is the breaking of the Cl-Cl bond (bond energy = + 584.2 KJ) that forms two chlorine free radicals (that is, Cl atoms):

Cl2 → (hv) → 2Cl.

The Cl atom attacks methane and makes a methyl free radical plus HCl. The methyl radical reacts in the later step by a chlorine molecule, making methyl chloride and a Cl atom:

Cl. + CH4 → CH3. + HCl

CH3. + Cl2 → CH3Cl + Cl.

The new Cl atom either attacks the other methane molecule to repeat the above reaction, or reacts by a methyl chloride molecule to make a chloromethyl free radical CH2Cl- and HCl.

Cl. + CH3Cl → CH2Cl. + HCl

The chloromethyl free radical then attacks the other chlorine molecule and generates dichloromethane all along by a Cl atom:

This formation of Cl free radicals continues till all chlorine is used. The chloroform and carbon tetrachloride are made in an identical manner by the reaction of CHCl2 and CCl3 free radicals with chlorine.

The product distribution among the chloromethane based mainly on the mole ratio of the reactants. For illustration, the yield of monochloromethane could be raised to 80% via increasing the CH4/Cl2 mole ratio to 10:1 at 450°C. If dichloromethane is desired, the CH4/Cl2 ratio is lowered and the monochloromethane recycled.

Reducing the CH4/Cl2 ratio normally raises poly-substitution and the chloroform and carbon tetrachloride yield. An alternative manner to form methyl chloride (that is, monochloromethane) is the reaction of methanol by HCl. Methyl chloride could be further chlorinated to provide a mixture of chloromethane (that is, dichloromethane, chloroform and carbon tetrachloride).

Uses of Chloromethane:

The main use of methyl chloride is to generate silicon polymers. The other uses comprise the synthesis of tetramethyl lead as a gasoline octane booster, a methylating agent in methyl cellulose production, a solvent and a refrigerant. Methylene chloride consists of a broad variety of markets. One main use is as a paint remover. It is as well employed as a degreasing solvent, a blowing agent for polyurethane foams, and a solvent for cellulose acetate. Chloroform is mostly employed to produce chlorodifluoromethane (that is, Fluorocarbon22) by the reaction with hydrogen fluoride:

CHCl3 + 2HF → CHClF2Cl + 2HCl

This compound is employed as a refrigerant and as an aerosol propellant. It is as well employed to synthesize tetrafluoroethylene that is polymerized to a heat resistant polymer (that is, Teflon):

2CHClF2 → CF2=CF2 + 2HCl

Carbon tetrachloride is employed to produce chlorofluorocarbons via the reaction with hydrogen fluoride by using an antimony pentachloride (SbCl5) catalyst:

CCl4 + HF → CCl3F + HCl 

CCl4 + 2HF → CCl2F2 + 2HCl

The formed mixture is comprised of trichlorofluoromethane (Freon-11) and dichlorodifluoromethane (Freon-12). Such compounds are employed as aerosols and as refrigerants. Because of the depleting effect of chlorofluorocarbons (CFCs) on the ozone layer, the production of such compounds might be reduced noticeably. Much research is being conducted to determine the alternatives to CFCs having little or no effect on the ozone layer. Among such is HCFC-123 (HCCl2CF3) to substitute Freon-11 and HCFC-22 (CHClF2) to substitute Freon-12 in such employs as air conditioning, refrigeration, aerosol and foam. These compounds encompass a much lower ozone depletion value as compared to Freon-11 that was assigned a value of 1. The Ozone depletion values for HCFC-123 and HCFC-22 relative to Freon-11 equivalents 0.02 and 0.055, correspondingly.

Synthesis Gas (Steam Reforming of Natural Gas):

Synthetic gas might be generated from a variety of feedstock. Natural gas is the favored feedstock whenever it is available from gas fields (that is, non associated gas) or from oil wells (that is, associated gas). The primary step in the production of synthetic gas is to treat the natural gas to eliminate hydrogen sulphide. The purified gas is then mixed by steam and introduced to the first reactor (that is, primary reformer). The reactor is constructed from vertical  stainless  steel tubes lined in a refractory furnace. The steam to natural gas ratio differs from 4 to 5 based on natural gas composition (that is, natural gas might have ethane and heavier hydrocarbons) and the pressure employed. A promoted nickel type catalyst contained in the reactor tubes is employed at temperature and pressure ranges of 700 to 800°C and 30 to 50 atmospheres, correspondingly. The reforming reaction is equilibrium limited. This is favored at high temperatures, low pressures and a high steam to carbon ratio. Such conditions minimize methane slip at the reformer outlet and result an equilibrium mixture which is rich in hydrogen.

The product gas from the primary reformer is a mixture of H2, CO, CO2, unreacted CH4 and steam.

The major stream reforming reactions are:

CH4 (g) + H2O (g) → CO (g) + 3H2 (g)      H° = +206 KJ

H°800°C = +226 KJ

CH4 (g) + 2H2O (g) → CO2 (g) + 4H2 (g)     H° = +164.8 KJ

For the manufacture of methanol, this mixture could be employed directly by no further treatment apart from the adjustment of H2/(CO + CO2) ratio to around 2:1. For producing hydrogen for ammonia synthesis, though, further treatment steps are required. First, the requisite amount of nitrogen for ammonia should be obtained from the atmospheric air. This is done partly by oxidizing unreacted methane in the exit gas mixture from the first reactor in the other reactor (that is, secondary reforming). The main reaction taking place in the secondary reformer is the partial oxidation of methane by a limited amount of air. The product is a mixture of hydrogen, carbon-dioxide, carbon monoxide, plus nitrogen, which doesn't react under such conditions. The reaction is symbolized as follows:

CH4 + 1/2(O2 + 3.76 N2) → CO + 2H2 + 1.88 N2        H° = -32.1 KJ

The reactor temperature can reach above 900°C in the secondary reformer because of the exothermic reaction heat. Typical analysis of the exit gas from the primary and secondary reformers is illustrated in the table below. The second step after secondary reforming is eliminating carbon monoxide that poisons the catalyst employed for ammonia synthesis. This is completed in three further steps, shift conversion, carbon-dioxide elimination and methanation of the remaining CO and CO2.

Table: Typical analysis of Effluent from Primary and Secondary reformers

Constituent     Primary Reformer    Secondary Reformer

H2                          47                            39.0

CO                        10.2                          12.2

CO2                       6.3                            4.2

CH4                       7.0                            0.6

H2O                       29.4                          27.0

N2                          0.02                         17.0

Shift Conversion:

The product gas mixture from secondary reformer is cooled then subjected to the shift conversion. In the shift converter, carbon monoxide is reacted by steam to provide carbon-dioxide and hydrogen. The reaction is exothermic and independent of pressure:

CO (g) + H2O (g) → CO2 (g) + H2 (g)     H° = -41 KJ

The feed to the shift converter includes big amounts of carbon monoxide that must be oxidized. The iron catalyst promoted by chromium oxide is employed at a temperature range of 425 to 500°C to improve the oxidation. Exit gases from the shift conversion are treated to eliminate carbon-dioxide. This might be completed by absorbing carbon-dioxide in a physical or chemical absorption solvent or through adsorbing it employing a special kind of molecular sieves. Carbon dioxide, recovered from the treatment agent as the byproduct, is mostly employed by ammonia to produce urea. The product is a pure hydrogen gas having small amounts of carbon monoxide and carbon-dioxide that are further eliminated through methanation.

Methanation:

Catalytic methanation is the reverse of steam reforming reaction. Hydrogen reacts by carbon monoxide and carbon dioxide, transforming them to methane. Methanation reactions are exothermic and methane result is favored at lower temperatures:

3H2 (g) + CO (g) → CH4 (g) + H2O (g)      H° = -206 KJ

4H2 (g) + CO2 (g) → CH4 (g) + 2H2O (g)    H° = -164.8 KJ

The forward reactions are as well favored at higher pressures. Though, the space velocity becomes high with increased pressures, and contact time becomes shorter, reducing the yield. The actual process conditions of temperature, pressure and space velocity are practically a compromise of some factors. Raney nickel is the preferred catalyst. Typical methanation reactor operating conditions are 200 to 300°C and around 10 atmospheres. The product is a gas mixture of hydrogen and nitrogen having an estimated ratio of 3:1 for ammonia production. The figure illustrates the ICI process for the production of synthesis gas for the manufacture of ammonia.

142_ICI Process for Producing Synthetic Gas and Ammonia.jpg

Fig: ICI process for Producing Synthetic Gas and Ammonia; (1) desulphurization, (2) feed gas  saturator,  (3)  primary  reformer,  (4) secondary reformer, (5) shift converter, (6) methanator, (7) ammonia reactor.

Chemicals based on Synthetic Gas:

Most of the chemicals are generated from synthetic gas. This is an effect of the high reactivity related with hydrogen and carbon monoxide gases, the two constituents of synthetic gas.

905_Chemicals Based on Synthetic Gas.jpg

Fig: Chemicals based on synthetic gas

The reactivity of this mixture was illustrated throughout World War II, when it was employed to produce alternative hydrocarbon fuels by using Fischer Tropsch technology. The synthesis gas mixture was generated then via gasifying coal. Synthesis gas is as well a significant building block for aldehydes from olefins. The catalytic hydroformylation reaction (that is, Oxo reaction) is employed by numerous olefins to produce aldehydes and alcohols of commercial significance.  The two main chemicals dependent on synthesis gas are ammonia and methanol. Each and every compound is a precursor for numerous other chemicals. From ammonia, urea, nitric acid, acrylonitrile, hydrazine, methylamines and many other minor chemicals are generated (figure shown above). Each of such chemicals is as well a precursor of more chemicals. Methanol, the second main product from synthesis gas, is an exclusive compound of high chemical reactivity and also good fuel properties. This is a building block for numerous reactive compounds like formaldehyde, acetic acid and methylamine. It as well offers an alternative way to generate hydrocarbons in the gasoline range (that is, Mobil to Gasoline MTG process). It might prove to be a competitive source for generating light olefins in the future.

Ammonia (NH3)

This colorless gas consists of an irritating odor, and is very soluble in water, making a weakly basic solution. Ammonia could be simply liquefied under pressure (that is, liquid ammonia) and it is a significant refrigerant. Anhydrous ammonia is a fertilizer via direct application to the soil. Ammonia is obtained via the reaction of hydrogen and atmospheric nitrogen,   the synthetic gas for ammonia.  The 1994 U.S. ammonia production was around 40 billion pounds (6th highest volume chemical).

Ammonia Production (Haber process):

The production of ammonia is of historical interest as it symbolizes the first significant   application of thermodynamics to an industrial method. Considering the synthesis reaction of ammonia from its elements, the computed reaction heat (ΔH) and free energy change (ΔG) at room temperature are around -46 and -16.5KJ/mol, correspondingly. However the computed equilibrium constant Kc = 3.6 × 108 at room temperature is substantially high, no reaction takes place under such conditions, and the rate is practically zero. The ammonia synthesis reaction could be symbolized as follows:

N2 (g) + 3H2 (g) → 2NH3 (g)    ΔH°= - 46.1 KJ/mol

Increasing the temperature raises the reaction rate, however reduces the equilibrium (Kc at 500°C = 0.08). According to Le Chatelier's principle, the equilibrium is favored at high pressures and at lower temperatures.

Much of Haber's research was to determine a catalyst that favored the formation of ammonia at a reasonable rate at lower temperatures. Iron oxide promoted having other oxides like potassium and aluminum oxides is presently employed to generate ammonia in good yield at relatively low temperatures.

In a commercial method, a mixture of hydrogen and nitrogen (that is, exit gas from the methanator) in a ratio of 3:1 is compressed to the desired pressure (150 to 1,000 atmospheres). The compressed mixture is then pre-heated through heat exchange by the product stream before entering the ammonia reactor. The reaction takes place over the catalyst bed at around 450°C. The exit gas having ammonia is passed via a cooling chamber where ammonia is condensed to a liquid, whereas unreacted hydrogen and nitrogen are recycled. Generally, a conversion of around 15% per pass is achieved under such conditions.

Uses of Ammonia:

The key end use of ammonia is the fertilizer field for the manufacture of urea, ammonium   nitrate and ammonium phosphate and sulphate. Anhydrous ammonia could be directly applied to the soil as a fertilizer. Urea is gaining broad acceptance as a slow-acting fertilizer. Ammonia is the pioneer for numerous other chemicals like nitric acid, hydrazine, acrylonitrile and hexamethylenediamine. Ammonia, containing three hydrogen atoms per molecule, might be viewed as an energy source. It has been stated that anhydrous liquid ammonia might be employed as a clean fuel for the automotive industry. Compared by hydrogen, anhydrous ammonia is more manageable. This is stored in iron or steel containers and could be transported commercially through pipeline, tanker, cars, railroad and highway tanker trucks. The oxidation reaction could be symbolized as:

4NH3 + 3O2 → 2N2 + 6H2O       ΔH = -316.9 KJ/mol

Merely nitrogen and water are produced. Though, most of the factors should be considered such as the coproduction of nitrogen oxides, the economics associated to retrofitting of auto engines, and so on. The given illustrates the significant chemicals based on ammonia.

Urea:

The highest fixed nitrogen-containing fertilizer 46.7 wt %, urea is a white solid which is soluble in water and alcohol. This is generally sold in the form of crystals, prills, flakes or granules. Urea is the active compound which reacts with numerous reagents. It forms adducts and clathrates with numerous substances like phenol and salicylic acid. By reacting by formaldehyde, it generates a significant commercial polymer (that is, urea formaldehyde resins) which is employed as glue for particle board and plywood.

Production of Urea:

The technical production of urea is mainly based on the reaction of ammonia by carbon-dioxide.  The reaction takes place in two steps: Ammonium carbamate is made first, followed through a decomposition step of the carbamate to urea and water. The primary reaction is exothermic and the equilibrium is favored at lower temperatures and higher pressures. The higher operating pressures are as well desirable for the separation absorption step which results in the higher carbamate solution concentration. The higher ammonia ratio than stoichiometric is employed to compensate for the ammonia which dissolves in the melt. The reactor temperature ranges between 170 to 220°C at a pressure of around 200 atmospheres.

965_Snamprogetti Process for Producing Urea.jpg

Fig: Snamprogetti process for producing Urea

The second reaction symbolizes the decomposition of the carbamate. The reaction conditions are 200°C and 30 atmospheres. Decomposition in the presence of surplus ammonia limits corrosion problems and slows down the decomposition of the carbamate to ammonia and carbon-dioxide.

The urea solution leaving the carbamate decomposer is expanded through heating at low pressures and ammonia recycled. The resulting solution is further concentrated to a melt, that is then prilled via passing it by special sprays in an air stream. The figure above illustrates the Snamprogetti procedure for urea production.

Uses of Urea:

The main use of urea is the fertilizer field that accounts for around 80% of its production (around 16.2 billion pounds were produced all through 1994 in U.S.). Around 10% of urea is employed for the production of adhesives and plastics (that is, urea formaldehyde and melamine formaldehyde resins). Animal feed accounts for around 5% of the urea produced.

Methyl alcohol (CH3OH):

Methyl alcohol (or methanol) is the first member of the aliphatic alcohol family. It ranks among the top twenty (20) organic chemicals used in the U.S.  The present world demand for methanol is around 25.5 million tons/year (1998) and is anticipated to reach 30 million tons by the year 2002. The 1994 U.S. production was around 10.8 billion pounds. Methanol was originally manufactured by the destructive distillation of wood (that is, wood alcohol) for charcoal production. Presently, it is mostly produced from the synthetic gas. As a chemical compound, methanol is highly polar and hydrogen bonding is evidenced through its relatively high boiling temperature (65°C), its high heat of vaporization, and its low volatility. Because of the high oxygen content of methanol (50% wt), it is being considered as the gasoline blending compound to decrease carbon monoxide and hydrocarbon emissions in the automobile exhaust gases. It was as well tested for blending by gasoline because of its high octane number.

Throughout the late seventies and early eighties, most of experiments tested the possible utilization of pure (straight) methanol as an alternative fuel for the gasoline cars. Some problems were encountered, though, in its use as a fuel, like the cold engine startability because of its high heat of vaporization (that is, heat of vaporization is 3.7 times as compare to gasoline), its lower heating value, that is around half that of gasoline, and its corrosive properties. The subject has been reviewed via Keller. Though, methanol is a potential fuel for gas turbines as it burns smoothly and consists of exceptionally low nitrogen oxide emission levels. Because of the high reactivity of methanol, most of the chemicals could be derived from it. For illustration, it could be oxidized to formaldehyde, a significant chemical building block, carbonylated to acetic acid, and dehydrated and polymerized to hydrocarbons in the gasoline range (that is, MTG process).

Methanol reacts nearly quantitatively having isobutene and isoamylenes, making methylt-butylether (or MTBE) and tertiary amylmethylether (TAME), correspondingly. Both are significant gasoline additives for increasing the octane number and reducing carbon monoxide and hydrocarbon exhaust emissions. Moreover, much of the present work is centered on the use of shape-selective catalysts to transform to light olefins as a possible future source of ethylene and propylene.

Production of Methanol:

Methanol is produced via the catalytic reaction of carbon monoxide and hydrogen (that is, synthesis gas). As the ratio of CO: H2 in synthesis gas from natural  gas is around 1:3, and the  stoichiometric ratio needed for methanol synthesis is around 1:2, carbon-dioxide is added to decrease the excess hydrogen. The energy-efficient alternative to adjusting the CO: H2 ratio is to join the steam reforming method having auto thermal reforming (that is, combined reforming) in such a way that the  amount of natural gas fed is that needed to generate a synthesis gas having a stoichiometric ratio of around 1:2.05. Figure below is a combined reforming diagram. Whenever an auto thermal reforming step is added, pure oxygen must be employed. (This is a main difference between the secondary reforming in case of ammonia production, where air is employed to supply the required nitrogen).

1950_Block Flow Diagram Showing the Combined Reforming for Methanol.jpg

Fig: Block flow diagram showing the Combined Reforming for Methanol synthesis

The added benefit of combined reforming is the decrease in NO emission. Though, a capital cost raise (for air separation unit) of around 15 % is anticipated whenever employing combined reforming in comparison to plants by employing a single train steam reformer. The method scheme is viable and is commercially confirmed. The given reactions are representative for the methanol synthesis.

CO (g) + 2H2 (g) → CH3OH (l)     ΔH° = -128 KJ/mol

CO2 + 3H2 → CH3OH + H2O

Old methods make use of a zinc-chromium oxide catalyst at a high-pressure range of around 270 to 420 atmospheres for methanol production. A low-pressure method has been introduced by ICI operating at around 50 atm (700 psi) by using a new active copper-based catalyst at 240°C. The synthesis reaction takes place over a bed of heterogeneous catalyst arranged in either sequential adiabatic beds or positioned in heat transfer tubes. The reaction is limited via equilibrium and methanol concentration at the converter's exit rarely surpasses 7%. The converter effluent is cooled to 40°C to condense the product methanol, and the unreacted gases are recycled. The crude methanol from the separator consists of water and low levels of by-products, which are eliminated by using a two-column distillation system.

Uses of Methanol:

Methanol has numerous significant uses as a chemical, a fuel and a building block. Around 50% of methanol production is oxidized to formaldehyde. As a methylating agent, it is employed by numerous organic acids to generate the methyl esters like methyl acrylate, methylmethacrylate, methyl acetate and methyl terephthalate. Methanol is as well employed to form the dimethyl carbonate and methyl-t-butylether, a significant gasoline additive. It is as well utilized to produce synthetic gasoline by employing a shape selective catalyst (that is, MTG process). Olefins from methanol might be a future route for ethylene and propylene in competition by steam cracking of hydrocarbons.  The utilization of methanol in fuel cells is being investigated. The fuel cells are theoretically able of transforming the free energy of oxidation of a fuel to electrical work. In one kind of fuel cells, the cathode is made up of vanadium that catalyses the reduction of oxygen, whereas the anode is iron (III) that oxidizes methane to CO2 and iron (II) is made in aqueous H2SO4. The advantages of low emission might be offset via the high cost. The given illustrates the main chemicals based on methanol.

Naphtha-based Chemicals:

Light naphtha having hydrocarbons in the C5-C7 range is the preferred feedstock for making acetic acid by oxidation. Identical to the catalytic oxidation of n-butane, the   oxidation of light naphtha is performed at around the similar temperature and pressure ranges (170 to 200°C and ≈ 50 atmospheres) in the presence of manganese acetate catalyst. The result of acetic acid is around 40% wt.

Light naphtha + O2 → CH3COOH + by-products + H2O

The product mixture includes basically oxygenated compounds (that is, acids, alcohols, esters, aldehydes, ketones and so on). As many as 13 distillations columns are employed to separate the complex mixture. The number of products could be decreased via recycling most of them to extinction.

Manganese naphthenate might be employed as an oxidation catalyst. Rouchaud and Lutete have prepared an in-depth study of the liquid phase oxidation of n-hexane by utilizing manganese naphthenate. A result of 83% of C1-C5 acids relative to n-hexane was reported. The maximum result of such acids was for acetic acid followed through formic acid. The lowest yield was noticed for pentanoic acid. In Europe, naphtha is the favored feedstock for the production of synthesis gas that is employed to synthesize methanol and ammonia. The other significant role for naphtha is its use as a feedstock for steam cracking units for light olefins production. Heavy naphtha, on the other hand, is a main feedstock for the catalytic reforming. The product reformates having a high percentage of C6-C8 aromatic hydrocarbons are employed to make gasoline. Reformates are as well extracted to separate the aromatics as the intermediates for petrochemicals.

Chemicals from high molecular weight n-Paraffins:

The high molecular weight n-paraffins are obtained from various petroleum fractions via physical separation methods. Those in the range of C8-C14 are generally recovered from kerosene containing a high ratio of such compounds. Vapor stage adsorption by using molecular sieve 5A is employed to accomplish the separation. The n-paraffins are then desorbed via the action of ammonia. Continuous operation is possible by employing two adsorption sieve columns, one bed on stream whereas the other bed is being desorbed. n- Paraffins could as well be separated through forming adduct with urea. For a paraffinic hydrocarbon to make adduct beneath ambient temperature and atmospheric pressure, the compound should have a long unbranched chain of at least six carbon atoms. Ease of adduct formation and adduct stability rises with the increase of chain length. As with the shorter-chain n-paraffins, the longer chain compounds are not highly reactive. Though, they might be oxidized, chlorinated, dehydrogenated, sulphonated and fermented under special conditions. The C9-C17 paraffins are employed to form olefins or monochlorinated paraffins for the production of detergents.

Oxidation of Paraffins (Fatty Acids and Fatty Alcohols):

The catalytic oxidation of long-chain paraffins (Cl8 -30) over manganese salts forms a mixture of fatty acids having different chain lengths.  Temperature and pressure ranges of 105 to 120°C and 15 to 60 atmospheres are employed. Around 60% wt result of fatty acids in the range of Cl2-Cl4 is obtained. These acids are employed for making soaps. The major source of fatty acids for soap manufacture, though, is the hydrolysis of fats and oils (that is, a non petroleum source). The oxidation of paraffins to fatty acids might be described as:

RCH2(CH2)nCH2CH2R + 5/4 O2 → R(CH2)nCOOH + RCH2COOH + H2O

Oxidation of C12 - C14 n-paraffins by employing boron trioxide catalysts was extensively studied for the production of fatty alcohols. Typical reaction conditions are 120 to 130°C at atmospheric pressure. Ter-butyl hydroperoxide (0.5%) was employed to initiate the reaction. The result of the alcohols was 76.2% wt at 30.5% conversion. The fatty acids (8.9% wt) were as well obtained.  Product alcohols were necessarily secondary by the similar number of carbons and the same structure 1/2 per molecule as the parent paraffin hydrocarbon. This exhibits that no cracking has occurred under the conditions utilized. The oxidation reaction could be symbolized as:

RCH2CH2 R + 1/2O2 → R-CH2CHOHR'

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