ACETALDEHYDE

Acetaldehyde (ethanal, CH3CH=O, melting point –123.5°C, boiling point: 20.1o C, density: 0.7780, flash point: –38o C, ignition temperature: 165o C) is a colorless, odorous liquid.

Acetaldehyde has a pungent, suffocating odor that is somewhat fruity and quite pleasant in dilute concentrations. Acetaldehyde is miscible in all proportions with water and most common organic solvents, e.g., acetone, benzene, ethyl alcohol, ether, gasoline, toluene, xylenes, turpentine, and acetic acid.

Because of its versatile chemical reactivity, acetaldehyde is widely used as a commencing material in organic syntheses, including the production of resins, dyestuffs, and explosives. It is also used as a reducing agent, preservative, and medium for silvering mirrors. In resin manufacture, paraldehyde [(CH3CHO)3] sometimes is preferred because of its higher boiling and flash points.

Acetaldehyde was first prepared by Scheele in 1774, by the action of manganese dioxide (MnO2) and sulfuric acid (H2SO4) on ethyl alcohol (ethanol, CH3CH2OH).

CH3CH2OH + [O] ? CH3CH=O + H2O

Commercially, passing alcohol vapors and preheated air over a silver catalyst at 480oC carries out the oxidation. With a multitubular reactor, conversions of 74 to 82 percent per pass can be obtained while generating steam to be used elsewhere in the process.

The formation of acetaldehyde by the addition of water to acetylene was observed by Kutscherow in 1881.

HC?CH + H2O ? CH3CH=O

In this hydration process, high-purity acetylene under a pressure of 15 psi (103.4 kPa) is passed into a vertical reactor containing a mercury catalyst dissolved in 18 to 25% sulfuric acid at 70 to 90oC. Fresh catalyst is fed to the reactor periodically; the catalyst may be added in the mercurous (Hg+) form, but the catalytic species has been shown to be a mercuric ion complex. The excess acetylene sweeps out the dissolved acetaldehyde, which is condensed by water and refrigerated brine and then scrubbed with water; this crude acetaldehyde is purified by distillation; the unreacted acetylene is recycled. The catalytic mercuric ion is reduced to catalytically inactive mercurous sulfate (Hg2SO4) and metallic mercury. Sludge, consisting of reduced catalyst and tars, is drained from the reactor at intervals and resulfated. The rate of catalyst depletion can be reduced by adding ferric or other suitable ions to the reaction solution. These ions reoxidize the mercurous ion to the mercuric ion; consequently, the quantity of sludge that must be recovered is reduced.

In one variation of the process, acetylene is completely hydrated with water in a single operation at 68 to 73o C using the mercuric-iron salt catalyst. The acetaldehyde is partially removed by vacuum distillation and the mother liquor recycled to the reactor. The aldehyde vapors are cooled to about 35o C, compressed to 37 psi (253 kPa), and condensed. It is claimed that this combination of vacuum and pressure operations substantially reduces heating and refrigeration costs.

The commercial process of choice for acetaldehyde production is the direct oxidation of ethylene.

CH2=CH2 + [O] ? CH3CH=O

There are two variations for this commercial production route: the two-stage process and the one-stage process.

In the one-stage process (Fig. 1), ethylene, oxygen, and recycle gas are directed to a vertical reactor for contact with the catalyst solution under slight pressure. The water evaporated during the reaction absorbs the heat
evolved, and makeup water is fed as necessary to maintain the desired catalyst concentration. The gases are water scrubbed, and the resulting acetaldehyde solution is fed to a distillation column. The tail gas from the scrubber is recycled to the reactor. Inert materials are eliminated from the recycle gas in a bleed stream that flows to an auxiliary reactor for additional ethylene conversion.

In the two-stage process (Fig. 2), ethylene is almost completely oxidized by air to acetaldehyde in one pass in a tubular plug-flow reactor made of titanium. The reaction is conducted at 125 to 130o C and 150 psi (1.03 MPa) with the palladium and cupric chloride catalysts. Acetaldehyde produced in the first reactor is removed from the reaction loop by adiabatic flashing in a tower. The flash step also removes the heat of reaction. The catalyst solution is recycled from the flash-tower base to the second stage (or oxidation reactor), where the cuprous salt is oxidized to the cupric state with air. The high-pressure off-gas from the oxidation reactor, mostly nitrogen, is separated from the liquid catalyst solution and scrubbed to remove acetaldehyde before venting. A small portion of the catalyst stream is heated in the catalyst regenerator to destroy any undesirable copper oxalate. The flasher overhead is fed to a distillation system where water is removed for recycle to the reactor system and organic impurities, including chlorinated aldehydes, are separated from the purified acetaldehyde product. Synthesis techniques purported to reduce the quantity of chlorinated by-products generated have been patented.

Acetaldehyde was first used extensively during World War I as a starting material for making acetone (CH3COCH3) from acetic acid (CH3CO2H) and is currently an important intermediate in the production of acetic acid, acetic anhydride (CH3CO-O-OCCH3), ethyl acetate (CH3CO-OC2H5), peracetic acid (CH3CO-O-OH), and a variety of other chemicals such as pentaerythritol, chloral, glyoxal, alkylamines, and pyridines.

In aqueous solutions, acetaldehyde exists in equilibrium with the acetaldehyde hydrate [CH3CH(OH)2]. The enol form, vinyl alcohol (CH2=CHOH) exists in equilibrium with acetaldehyde to the extent of 0.003% (1 molecule in approximately 30,000) and can be acetylated with ketene (CH2=C=O) to form vinyl acetate (CH2 =CHOCOCH 3).

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VINYLATION

Unlike ethynylation, in which acetylene adds across a carbonyl group and the triple bond is retained, in vinylation a labile hydrogen compound adds to acetylene, forming a double bond.

XH + HC?CH ? CH2=CHX

Catalytic vinylation has been applied to the manufacture of a wide range of alcohols, phenols, thiols, carboxylic acids, and certain amines and amides. Vinyl acetate is no longer prepared this way in many countries, although some minor vinyl esters such as vinyl stearate may still be manufactured by this route. However, the manufacture of vinyl-pyrrolidinone and vinyl ethers still depends on acetylene as the starting material.

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SULFONATION

Sulfonation is the introduction of a sulfonic acid group (–SO3H) into an organic compound as, for example, in the production of an aromatic sulfonic acid from the corresponding aromatic hydrocarbon.

ArH + H2SO4 ? ArSO3H + H2O

The usual sulfonating agent is concentrated sulfuric acid, but sulfur trioxide, chlorosulfonic acid, metallic sulfates, and sulfamic acid are also occasionally used. However, because of the nature and properties of sulfuric acid, it is desirable to use it for nucleophilic substitution wherever possible.

For each substance being sulfonated, there is a critical concentration of acid below which sulfonation ceases. The removal of the water formed in the reaction is therefore essential. The use of a very large excess of acid, while expensive, can maintain an essentially constant concentration as the reaction progresses. It is not easy to volatilize water from concentrated solutions of sulfuric acid, but azeotropic distillation can sometimes help.

The sulfonation reaction is exothermic, but not highly corrosive, so sulfonation can be conducted in steel, stainless-steel, or cast-iron sulfonators. A jacket heated with hot oil or steam can serve to heat the contents sufficiently to get the reaction started, then carry away the heat of reaction. A good agitator, a condenser, and a fume control system are usually also provided.

1- and 2-naphthalenesulfonic acids are formed simultaneously when naphthalene is sulfonated with concentrated sulfuric acid. The isomers must be separated if pure ?- or ?-naphthol are to be prepared from the product mix. Variations in time, temperature, sulfuric acid concentration, and acid/hydrocarbon ratio alter the yields to favor one particular isomer, but a pure single substance is never formed. Using similar acid/hydrocarbon ratios, sulfonation at 40oC yields 96% alpha isomer, 4% beta, while at 1600C the proportions are 15% ?-naphthol, 8.5% ?-naphthol.

The ?-sulfonic acid can be hydrolyzed to naphthalene by passing steam at 160o C into the sulfonation mass. The naphthalene so formed passes out with the steam and can be recovered. The pure ?-sulfonic acid left behind can be hydrolyzed by caustic fusion to yield relatively pure ?-naphthol.

In general, separations are based on some of the following consideration:
1. Variations in the rate of hydrolysis of two isomers
2. Variations in the solubility of various salts in water
3. Differences in solubility in solvents other than water
4. Differences in solubility accentuated by common-ion effect (salt additions)
5. Differences in properties of derivatives
6. Differences based on molecular size, such as using molecular sieves or absorption.

Sulfonation reactions may be carried out in batch reactors or in continuous reactors. Continuous sulfonation reactions are feasible only when the organic compounds possess certain chemical and physical properties, and are practical in only a relatively few industrial processes. Most commercial sulfonation reactions are batch operations.

Continuous operations are feasible and practical (1) where the organic compound (benzene or naphthalene) can be volatilized, (2) when reaction rates are high (as in the chlorosulfonation of paraffins and the sulfonation of alcohols), and (3) where production is large (as in the manufacture of detergents, such as alkylaryl sulfonates).

Water of reaction forms during most sulfonation reactions, and unless a method is devised to prevent excessive dilution because of water formed during the reaction, the rate of sulfonation will be reduced. In the interests of economy in sulfuric acid consumption, it is advantageous to remove or chemically combine this water of reaction. For example, the use of reduced pressure for removing the water of reaction has some technical advantages in the sulfonation of phenol and of benzene.

The use of the partial-pressure distillation is predicated upon the ability of the diluent, or an excess of volatile reactant, to remove the water of reaction as it is formed and, hence, to maintain a high concentration of sulfuric acid. If this concentration is maintained, the necessity for using excess sulfuric acid is eliminated, since its only function is to maintain the acid concentration above the desired value. Azeotropic removal of the water of reaction in the sulfonation of benzene can be achieved by using an excess of vaporized benzene.

The use of oleum (H 2SO4.SO3) for maintaining the necessary sulfur trioxide concentration of a sulfonation mixture is a practical procedure. Preferably the oleum and organic compound should be added gradually and concurrently to a large volume of cycle acid so as to take up the water as rapidly as it is formed by the reaction. Sulfur trioxide may be added intermittently to the sulfonation reactor to maintain the sulfur trioxide concentration above the value for the desired degree of sulfonation.

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POLYMERIZATION

Polymerization is a process in which similar molecules (usually olefins) are linked to form a high-molecular-weight product; such as the formation of polyethylene from ethylene

nCH2CH2 ? H–( CH2CH2)n–H

The molecular weight of the polyethylene can range from a few thousand to several hundred thousand.

Polymerization of the monomer in bulk may be carried out in the liquid or vapor state. The monomers and activator are mixed in a reactor and heated or cooled as needed. As most polymerization reactions are exothermic, provision must be made to remove the excess heat. In some cases, the polymers are soluble in their liquid monomers, causing the viscosity of the solution to increase greatly. In other cases, the polymer is not soluble in the monomer and it precipitates out after a small amount of polymerization occurs.

In the petroleum industry, the term polymerization takes on a different meaning since the polymerization processes convert by-product hydrocarbon gases produced in cracking into liquid hydrocarbons suitable (of limited or specific molecular weight) for use as high-octane motor and aviation fuels and for petrochemicals.

To combine olefinic gases by polymerization to form heavier fractions, the combining fractions must be unsaturated. Hydrocarbon gases, particularly olefins, from cracking reactors are the major feedstock of polymerization.

(CH3)2C=CH2 ? (CH3)3CH2C(CH3)=CH2
(CH3)3CH2C(CH3)=CH2 ? C12H24

Vapor-phase cracking produces considerable quantities of unsaturated gases suitable as feedstocks for polymerization units.

Catalytic polymerization is practical on both large and small scales and is adaptable to combination with reforming to increase the quality of the gasoline. Gasoline produced by polymerization contains a smog-producing olefinic bond. Polymer oligomers are widely used to make detergents.

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OXO REACTION

The oxo reaction is the general or generic name for a process in which an unsaturated hydrocarbon is reacted with carbon monoxide and hydrogen to form oxygen function compounds, such as aldehydes and alcohols.

In a typical process for the production of oxo alcohols, the feedstock comprises an olefin stream, carbon monoxide, and hydrogen. In a first step, the olefin reacts with CO and H2 in the presence of a catalyst (often cobalt) to produce an aldehyde that has one more carbon atom than the originating olefin:

RCH=CH2 + CO + H2 ? RCH2CH2CH=O

This step is exothermic and requires an ancillary cooling operation.

The raw aldehyde exiting from the oxo reactor then is subjected to a higher temperature to convert the catalyst to a form for easy separation from the reaction products. The subsequent treatment also decomposes unwanted by-products. The raw aldehyde then is hydrogenated in the presence of a catalyst (usually nickel) to form the desired alcohol:

RCH2CH2CH=O + H2 ? RCH2CH2CH2OH

The raw alcohol then is purified in a fractionating column. In addition to the purified alcohol, by-products include a light hydrocarbon stream and a heavy oil. The hydrogenation step takes place at about 150°C under a pressure of about 1470 psi (10.13 MPa). The olefin conversion usually is about 95 percent.

Among important products manufactured in this manner are substituted propionaldehyde from corresponding substituted ethylene, normal and iso-butyraldehyde from propylene, iso-octyl alcohol from heptene, and trimethylhexyl alcohol from di-isobutylene.

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OXIDATION

Oxidation is the addition of oxygen to an organic compound or, conversely, the removal of hydrogen.

Reaction control is the major issue during oxidation reactions. Only partial oxidation is required for conversion of one organic compound into another or complete oxidation to carbon dioxide and water will ensue.

The most common oxidation agent is air, but oxygen is frequently used.
Chemical oxidizing agents (nitric acid, dichromates, permanganates, chromic
anhydride, chlorates, and hydrogen peroxide) are also often used.

As examples of oxidation processes, two processes are available for the manufacture of phenol, and both involve oxidation. The major process involves oxidation of cumene to cumene hydroperoxide, followed by decomposition to phenol and acetone. A small amount of phenol is also made by the oxidation of toluene to benzoic acid, followed by decomposition of the benzoic acid to phenol.

Benzoic acid is synthesized by liquid-phase toluene oxidation over a cobalt naphthenate catalyst with air as the oxidizing agent. An older process involving halogenation of toluene to benzotrichloride and its decomposition into benzoic acid is still used available.

Maleic acid and anhydride are recovered as by-products of the oxidation of xylenes and naphthalenes to form phthalic acids, and are also made specifically by the partial oxidation of benzene over a vanadium pentoxide (V2O5) catalyst. This is a highly exothermic reaction, and several modifications of the basic process exist, including one using butylenes as the starting materials.

Formic acid is made by the oxidation of formamide or by the liquid-phase oxidation of n-butane to acetic acid. The by-product source is expected to dry up in the future, and the most promising route to replace it is through carbonylation of methanol.

Caprolactam, adipic acid, and hexamethylenediamine (HMDA) are all made from cyclohexane. Almost all high-purity cyclohexane is obtained by hydrogenating benzene, although some for solvent use is obtained by careful distillation of selected petroleum fractions.

Several oxidative routes are available to change cyclohexane to cyclohexanone, cyclohexanol, and ultimately to adipic acid or caprolactam. If phenol is hydrogenated, cyclohexanone can be obtained directly; this will react with hydroxylamine to give cyclohexanone oxime that converts to caprolactam on acid rearrangement. Cyclohexane can also be converted to adipic acid, then adiponitrile, which can be converted to hexamethylenediamine. Adipic acid and hexamethylenediamine are used to form nylon 6,6. This route to hexamethylenediamine is competitive with alternative routes through butene.

Acetaldehyde is manufactured by one of several possible processes: (1) the hydration of acetylene, no longer a significant process. (2) the Wacker process, in which ethylene is oxidized directly with air or 99% oxygen (Fig. 1) in the presence of a catalyst such as palladium chloride with a copper chloride promoter. The ethylene gas is bubbled, at atmospheric pressure, through the solution at its boiling point. The heat of reaction is removed by boiling of the water. Unreacted gas is recycled following condensation of the aldehyde and water, which are then separated by distillation, (3) passing ethyl alcohol over a copper or silver gauze catalyst gives a 25 percent conversion to acetaldehyde, with recirculation making a 90 to 95 percent yield possible, and (4) a process in which lower molecular weight paraffin hydrocarbons are oxidized noncatalytically to produce mixed compounds, among them acetaldehyde and acetic acid.

Liquid-phase reactions in which oxidation is secured by the use of oxidizing compounds need no special apparatus in the sense of elaborate means for temperature control and heat removal. There is usually provided a kettle form of apparatus, closed to prevent the loss of volatile materials and fitted with a reflux condenser to return vaporized materials to the reaction zone, with suitable means for adding reactants rapidly or slowly as may be required and for removing the product, and provided with adequate jackets or coils through which heating or cooling means may be circulated as required.

In the case of liquid-phase reactions in which oxidation is secured by means of atmospheric oxygen—for example, the oxidation of liquid hydrocarbons to fatty acids—special means must be provided to secure adequate mixing and contact of the two immiscible phases of gaseous oxidizing agent and the liquid being oxidized. Although temperature must be controlled and heat removed, the requirements are not severe, since the temperatures are generally low and the rate of heat generation controllable by regulation of the rate of air admission.

Heat may be removed and temperature controlled by circulation of either the liquid being oxidized or a special cooling fluid through the reaction zone and then through an external heat exchanger. Mixing may be obtained by the use of special distributor inlets for the air, designed to spread the air throughout the liquid and constructed of materials capable of withstanding temperatures that may be considerably higher at these inlet ports than in the main body of the liquid. With materials that are sensitive to overoxidation and under conditions where good contact must be used partly to offset the retarding effect of necessarily low temperatures, thorough mixing may be provided by the use of mechanical stirring or frothing of the liquid.

By their very nature, the vapor-phase oxidation processes result in the concentration of reaction heat in the catalyst zone, from which it must be removed in large quantities at high-temperature levels. Removal of heat is essential to prevent destruction of apparatus, catalyst, or raw material, and maintenance of temperature at the proper level is necessary to ensure the correct rate and degree of oxidation. With plant-scale operation and with reactions involving deep-seated oxidation, removal of heat constitutes a major problem. With limited oxidation, however, it may become necessary to supply heat even to oxidations conducted on a plant scale.

In the case of vapor-phase oxidation of aliphatic substances such as methanol and the lower molecular weight aliphatic hydrocarbons, the ratio of reacting oxygen is generally lower than in the case of the aromatic hydrocarbons for the formation of the desired products, and for this reason heat removal is simpler. Furthermore, in the case of the hydrocarbons, the proportion of oxygen in the reaction mixture is generally low, resulting in low per-pass conversions and, in some instances, necessitating preliminary heating of the reactants to reaction temperature.

Equipment for the oxidation of the aromatic hydrocarbons requires that the reactor design permit the maintenance of elevated temperatures, allow the removal of large quantities of heat at these elevated temperatures, and provide adequate catalyst surface to promote the reactions.

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NITRATION

Nitration is the insertion of a nitro group (–NO2) into an organic compound, usually through the agency of the reaction of a hydrocarbon with nitric acid. Concentrated sulfuric acid may be used as a catalyst.

ArH + HNO3 ? ArNO2 + H2O

More than one hydrogen atom may be replaced, but replacement of each succeeding hydrogen atom represents a more difficult substitution.

The nitrogen-bearing reactant may be:

1. Strong nitric acid
2. Mixed nitric and sulfuric acid
3. A nitrate plus sulfuric acid
4. Nitrogen pentoxide (N2O5)
5. A nitrate plus acetic acid

Both straight chain and ring-type carbon compounds can be nitrated; alkanes yield nitroparaffins.

The process for the production of nitrobenzene from benzene involves the use of mixed acid (Fig. 1), but there are other useful nitrating agents, e.g., inorganic nitrates, oxides of nitrogen, nitric acid plus acetic anhydride, and nitric acid plus phosphoric acid. In fact, the presence of sulfuric acid in quantity is vital to the success of the nitration because it increases the solubility of the hydrocarbon in the reaction mix, thus speeding up the reaction, and promotes the ionization of the nitric acid to give the nitronium ion (NO2+), which is the nitrating species. Absorption of water by sulfuric acid favors the nitration reaction and shifts the reaction equilibrium to the product.

Nitration offers a method of making unreactive paraffins into reactive substances without cracking. Because nitric acid and nitrogen oxides are strong oxidizing agents, oxidation always accompanies nitration. Aromatic nitration reactions have been important particularly for the manufacture of explosives. Nitrobenzene is probably the most important nitration product.

Certain esters of nitric acid (cellulose nitrate, glyceryl trinitrate) are often referred to as nitro compounds (nitrocellulose, nitroglycerin), but this terminology should be avoided.

Vapor-phase nitration of paraffin hydrocarbons, particularly propane, can be brought about by uncatalyzed contact between a large excess of hydrocarbon and nitric acid vapor at around 400 oC, followed by quenching.

A multiplicity of nitrated and oxidized products results from nitrating propane; nitromethane, nitroethane, nitropropanes, and carbon dioxide all appear, but yields of useful products are fair. Materials of construction must be very oxidation-resistant and are usually of ceramic-lined steel. The nitroparaffins have found limited use as fuels for race cars, submarines, and model airplanes. Their reduction products, the amines, and other hydroxyl compounds resulting from aldol condensations have made a great many new aliphatic syntheses possible because of their ready reactivity.

Nitration reactions are carried out in closed vessels that are provided with an agitating mechanism and means for controlling the reaction temperature. The nitration vessels are usually constructed of cast iron and steel, but often acid-resistant alloys, particularly chrome-nickel steel alloys, are used.

Plants may have large (several hundred gallon capacity) nitration vessels operating in a batch mode or small continuous units. The temperature is held at about 50o C, governed by the rate of feed of benzene. Reaction is rapid in well-stirred and continuous nitration vessels. The reaction products are decanted from the spent acid and are washed with dilute alkali. The spent acid is sent to some type of recovery system and yields of 98 percent can be anticipated.

Considerable heat evolution accompanies the nitration reaction, oxidation increases it, and the heat of dilution of the sulfuric acid increases it still further. Increased temperature favors dinitration arid oxidation, so the reaction must be cooled to keep it under control. Good heat transfer can be assured by the use of jackets, coils, and good agitation in the nitration vessel. Nitration vessels are usually made of stainless steel, although cast iron stands up well against mixed acid.

When temperature regulation is dependent solely on external jackets, a disproportional increase in nitration vessel capacity as compared with jacket surface occurs when the size of the machine is enlarged. Thus, if the volume is increased from 400 to 800 gallons, the heat-exchange area increases as the square and the volume as the cube of the expanded unit.

To overcome this fault, internal cooling coils or tubes are introduced, which have proved satisfactory when installed on the basis of sound calculations that include the several thermal factors entering into this unit process.

A way of providing an efficient agitation inside the nitration vessel is essential if local overheating is to be mitigated. Furthermore, the smooth-ness of the reaction depends on the dispersion of the reacting material as it comes in contact with the change in the nitration vessel so that a fairly uniform temperature is maintained throughout the vessel.

Nitration vessels are usually equipped with one of three general types of agitating mechanism: (1) single or double impeller, (2) propeller or turbine, with cooling sleeve, and (3) outside tunnel circulation.

The single-impeller agitator consists of one vertical shaft containing horizontal arms. The shaft may be placed off center in order to create rapid circulation past, or local turbulence at, the point of contact between the nitrating acid and the organic compound.

The double-impeller agitator consists of two vertical shafts rotating in opposite directions, and each shaft has a series of horizontal arms attached.

The lower blades have an upward thrust, whereas the upper ones repel the liquid downward. This conformation provides a reaction mix that is essentially homogeneous.

The term sleeve-and-propeller agitation is usually applied when the nitration vessel is equipped with a vertical sleeve through which the charge is circulated by the action of a marine propeller or turbine. The sleeve is usually made of a solid bank of acid-resisting cooling coils through which cold water or brine is circulated at a calculated rate. In order to obtain the maximum efficiency with this type of nitration vessel, it is essential to maintain a rapid circulation of liquid upward or downward in the sleeves and past the coils.

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HYDROGENATION

In its simplest interpretation, hydrogenation is the addition of hydrogen to a chemical compound. Generally, the process involves elevated temperature and relatively high pressure in the presence of a catalyst.

Hydrogenation yields many useful chemicals, and its use has increased phenomenally, particularly in the petroleum refining industry. Besides saturating double bonds, hydrogenation can be used to eliminate other elements from a molecule. These elements include oxygen, nitrogen, halogens, and particularly sulfur. Cracking (thermal decomposition) in the presence of hydrogen is particularly effective in desulfurizing high-boiling petroleum fractions, thereby producing lower-boiling and higher-quality products.

Although occasionally hydrogen for a reaction is provided by donor solvents and a few older reactions use hydrogen generated by acid or alkali acting upon a metal, gaseous hydrogen is the usual hydrogenating agent. Hydrogenation is generally carried out in the presence of a catalyst and under elevated temperature and pressure. Noble metals, nickel, copper, and various metal oxide combinations are the common catalysts.

Nickel, prepared in finely divided form by reduction of nickel oxide in a stream of hydrogen gas at about 300°C, was introduced by 1897 as a catalyst for the reaction of hydrogen with unsaturated organic substances to be conducted at about 175°C. Nickel proved to be one of the most successful catalysts for such reactions. The unsaturated organic substances that are hydrogenated are usually those containing a double bond, but those containing a triple bond also may be hydrogenated. Platinum black, palladium black, copper metal, copper oxide, nickel oxide, aluminum, and other materials have subsequently been developed as hydrogenation catalysts. Temperatures and pressures have been increased in many instances to improve yields of desired product. The hydrogenation of methyl ester to fatty alcohol and methanol, for example, occurs at about 290 to 315°C and 3000 psi (20.7 MPa). In the hydrotreating of liquid hydrocarbon fuels to improve quality, the reaction may take place in fixed-bed reactors at pressures ranging from 100 to 3000 psi (690 kPa to 20.7 MPa).

Many hydrogenation processes are of a proprietary nature, with numerous combinations of catalysts, temperature, and pressure possible.

Lower pressures and higher temperatures favor dehydrogenation, but the catalysts used are the same as for hydrogenation.

Methyl alcohol (methanol) is manufactured from a mixture of carbon monoxide and hydrogen (synthesis gas), using a copper-based catalyst.

CO + 2H2 ? CH3OH

In the process (Fig. 1), the reactor temperature is 250 to 260o C at a pressure of 725 to 1150 psi (5 to 8 MPa). High- and low-boiling impurities are removed in two columns and the unreacted gas is recirculated.

New catalysts have helped increase the conversion and yields. The older, high-pressure processes used zinc-chromium catalysts, but the low-pressure units use highly active copper catalysts. Liquid-entrained micrometer-sized catalysts have been developed that can convert as much as 25 percent per pass. Contact of the synthesis gases with hot iron catalyzes competing reactions and also forms volatile iron carbonyl that fouls the copper catalyst. Some reactors are lined with copper.

Because the catalyst is sensitive to sulfur, the gases are purified by one of several sulfur-removing processes, then are fed through heat exchangers into one of two types of reactors. With bed-in-place reactors, steam at around 4.5 kPa, in quantity sufficient to drive the gas compressors, can be generated. A tray-type reactor with gases introduced just above every bed for cooling offers more nearly isothermal operation but does not give convenient heat recovery.

Reaction vessels are usually of two types: one in which the contents are agitated or stirred in some way and the other in which the reactor and contents are stationary. The first is used with materials such as solids or liquids that need to be brought into intimate contact with the catalyst and the hydrogen. The second type is used where the substance may have sufficient vapor pressure at the temperature of operation so that a gas-phase as well as a liquid-phase reaction is possible. It is also most frequently used in continuous operation where larger quantities of material need to be processed than can be done conveniently with batch methods.

In hydrogenation processes, heating of the ingoing materials is best accomplished by heat exchange with the outgoing materials and adding additional heat by means of high-pressure pipe coils. A pipe coil is the only convenient and efficient method of heating, for the reactor is usually so large that heating it is very difficult. It is usually better practice to add all the heat needed to the materials before they enter the reactor and then simply have the reactor properly insulated thermally. Hydrogenation reactions are usually exothermic, so that once the process is started, the problem may be one of heat removal. This is accomplished by allowing the heat of reaction to flow into the ingoing materials by heat exchange in the reactor, or, if it is still in excess, by recycling and cooling in heat exchangers the proper portion of the material to maintain the desired temperature.

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HYDROFORMYLATION

The hydroformylation (oxo) reactions offer ways of converting a-olefins to aldehydes and/or alcohols containing an additional carbon atom.

CH3CH=CH2 + CO + H2 ? CH 3CH2CH2CHO
CH3CH2CH2CHO + H2 ? CH3CH2CH2CH2OH

In the process (Fig. 1), the olefin in a liquid state is reacted at 27 to 30 MPa and 150 to 170oC in the presence of a soluble cobalt catalyst. The aldehyde and a lesser amount of the alcohol are formed and flashed off along with steam, and the catalyst is recycled. Conversions of over 97 percent are obtained, and the reaction is strongly exothermic. The carbon monoxide and hydrogen are usually in the form of synthesis gas.

When propylene is used as the hydrocarbon, n- and iso-butyraldehyde are formed. This reaction is most frequently run with the C3 and C7 to C12 olefins. When C7 olefins are used, a series of dimethyl- and ethylhexanols and methyl heptanols are formed that are used as octyl alcohols to make plasticizers and esters.

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HYDRATION AND HYDROLYSIS

Ethyl alcohol is a product of fermentation of sugars and cellulose but the alcohol is manufactured mostly by the hydration of ethylene.

An indirect process for the manufacture of ethyl alcohol involves the dissolution of ethylene in sulfuric acid to form ethyl sulfate, which is hydrolyzed to form ethyl alcohol (Fig. 1). There is always some by-product diethyl ether that can be either sold or recirculated.

3CH2=CH2 + 2H2SO4 ? C2H5HSO4 + (C2H5)2SO4
C2H5HSO4 + (C2H5)2SO4 + H2O ? 3C2H5OH + 2H2SO4
C2H5OH + C2 H5HSO4 ? C2H5OC2H5

The conversion yield of ethylene to ethyl alcohol is 90 percent with a 5 to 10 percent yield of diethyl ether (C2H5OC 2H5).

A direct hydration method using phosphoric acid as a catalyst at 300o C is also available (Fig. 2):

CH2=CH2 + H2O ? C2H5OH

and produces ethyl alcohol in yields in excess of 92 percent. The conversion per pass is 4 to 25 percent, depending on the activity of the catalyst used.

In this process, ethylene and water are combined with a recycle stream in the ratio ethylene/water 1/0.6 (mole ratio), a furnace heats the mixture to 300o C, and the gases react over the catalyst of phosphoric acid absorbed on diatomaceous earth. Unreacted reagents are separated and recirculated. By-product acetaldehyde (CH3CHO) is hydrogenated over a catalyst to form more ethyl alcohol.

Iso-propyl alcohol is a widely used and easily made alcohol. It is used in making acetone, cosmetics, chemical derivatives, and as a process solvent. There are four processes that are available for the manufacture of iso-propyl alcohol:

1. A sulfuric acid process similar to the one described for ethanol hydration
2. A gas-phase hydration using a fixed-bed-supported phosphoric acid catalyst
3. A mixed-phase reaction using a cation exchange resin catalyst
4. A liquid-phase hydration in the presence of a dissolved tungsten catalyst

The last three processes (2, 3, and 4) are all essentially direct hydration processes.

CH3CH=CH2 + H2 O ? CH3CHOHCH3

Per-pass conversions vary from a low of 5 to a high of 70 percent for the gas-phase reaction.

Secondary butanol (CH3CH2CHOHCH3) is manufactured by processes similar to those described for ethylene and propylene.

Hydrolysis usually refers to the replacement of a sulfonic group (–SO3H) or a chloro group (–Cl) with an hydroxyl group (–OH) and is usually accomplished by fusion with alkali. Hydrolysis uses a far wider range of reagents and operating conditions than most chemical conversion processes. Polysubstituted molecules may be hydrolyzed with less drastic conditions. Enzymes, acids, or sometimes water can also bring about hydrolysis alone.

ArSO3Na + 2NaOH ? ArONa + Na2SO3 + H2O
ArCl + 2NaOH ? ArONa + NaCl + H2O

Acidification will give the hydroxyl compound (ArOH). Most hydrolysis reactions are modestly exothermic.

The more efficient route via cumene has superceded the fusion of benzene sulfonic acid with caustic soda for the manufacture of phenol, and the hydrolysis of chlorobenzene to phenol requires far more drastic conditions and is no longer competitive. Ethylene chlorohydrin can be hydrolyzed to glycol with aqueous sodium carbonate.

ClCH2CH2OH ? HOCH2CH2OH

Cast-iron or steel open fusion pots heated to the high temperatures required (200 to 325oC) with oil, electricity, or directly with gas, are standard equipment.

12. November 2021 by Jack
Categories: Chemical Process | Leave a comment

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