The Liquid Phase Process

Pump 11 takes the depressurized slurry into an overflow sedimentation centrifuge 12 where most of the catalyst particles are separated from the liquid phase. Removal of the solids from the bowl is effected manually at appropriate intervals. The liquid phase flows into a still 13 topped by a rectification column 14. The head vapors of the column are liquefied in condenser 15, the resulting distillate being partly returned to the column to effect rectification and partly collected in tank 16. This distillate is pure furfuryl alcohol. Vacuum pump 17 maintains a reduced pressure to permit distillation at moderate temperatures. Catalyst fines and high-boiling polymers inevitably formed in the reactor remain in the still and are discarded. 

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The process is similar to the catalytic liquid-phase oxidation of ethylene to acetaldehyde. The difference hetween the two processes is the presence of acetic acid. In practice, acetaldehyde is a major coproduct. The mole ratio of acetaldehyde to vinyl acetate can he varied from 0.3 1 to 2.5 1. The liquid-phase process is not used extensively due to corrosion problems and the formation of a fairly wide variety of by-products. 

In the liquid-phase process, high pressures in the range of 80-100 atmospheres are used. A sulfonated polystyrene cation exchange resin is the catalyst commonly used at about 150°C. An isopropanol yield of 93.5% can be realized at 75% propylene conversion. The only important byproduct is diisopropyl ether (about 5%). Figure 8-4 is a flow diagram of the propylene hydration process. ... 

The metathetic reaction occurs in the gas phase at relatively high temperatures (150°-350°C) with molybdenum or tungsten supported catalysts or at low temperature (=50°C) with rhenium-based catalyst in either liquid or gas-phase. The liquid-phase process gives a better conversion. Equilibrium conversion in the range of 55-65% could be realized, depending on the reaction temperature. ... 

Either a liquid or a gas-phase process is used for the alkylation reaction. In the liquid-phase process, low temperatures and pressures (approximately 50°C and 5 atmospheres ) are used with sulfuric acid as a catalyst. 

Chemical vapor deposition competes directly with other coating processes which, in many cases, are more suitable for the application under consideration. These competing processes comprise the physical vapor deposition (PVD) processes of evaporation, sputtering, and ion plating, as well as the molten-material process of thermal spray and the liquid-phase process of solgel. A short description of each process follows. For greater detail, the listed references should be consulted. 

The flow diagram for the vapor phase looks about the same as Figure 8—3. But unlike the liquid phase process, in the reactor both alkylation and transalkylation take place simultaneously so there is no need for a separate reactor to convert PEB to EB. Virtually no PEB shows up as by-product. 

Processes involving oxygen and nitrogen oxides as catalysts have been operated commercially using either vapor- or liquid-phase reactors. The vapor-phase reactors require particularly close control because of the wide explosive limit of dimethyl sulfide in oxygen (1—83.5 vol %) plants in operation use liquid-phase reactions. Figure 2 is a schematic diagram for the liquid-phase process. The product stream from the reactor is neutralized with aqueous caustic and is vacuum-evaporated, and the DMSO is dried in a distillation column to obtain the product. 

Manufacture of Sodium Azide was conducted at the Kankakee Ordnance Works, Joliet, Illinois, (Ref 144) by the "liquid phase process as follows For this five 12-lb bricks of sodium were melted in an electrically heated melter and the molten Na at 350°F (176.7°) dropped to a highrpressure autoclave contg 375 lbs liq ammonia and 1 lb ferric nitrate catalyst. The Na reacted to form Na amide and hydrogen ... 

Ethyl Chloride. Hydrochlorination of ethylene with HC1 is carried out in either the vapor or the liquid phase, in the presence of a catalyst.182-184 Ethyl chloride or 1,2-dichloroethane containing less than 1% A1C13 is the reaction medium in the liquid-phase process operating under mild conditions (30-90°C, 3-5 atm). In new plants supported AlClj or ZnCl2 is used in the vapor phase. Equimolar amounts of the dry reagents are reacted in a fluidized- or fixed-bed reactor at elevated temperature and pressure (250-400°C, 5-15 atm). Both processes provide ethyl chloride with high (98-99%) selectivity. 

Conversion per pass is reported to be low, 10—20%, with equally low yields, 30—50% (5). The vapor-phase oxidation of toluene was the dominant toluene oxidation process in the 1950s and eady 1960s, but is no longer of industrial importance. The liquid-phase process now dominates. 

In the liquid-phase process, both benzaldehyde and benzoic acid are recovered. This process was introduced and developed in the late 1950s by the Dow Chemical Company, as a part of their toluene-to-phenol process, and by Snia Viscosa for their toluene-to-caprolactam process. The benzaldehyde recovered from the liquid-phase air oxidation of toluene may be purified by either batch or continuous distillation. Liquid-phase air oxidation of toluene is covered more fully (see Benzoic acid). 

Vapor-phase oxidation of toluene to produce benzoic acid and benzaldehyde has been tried utilizing several different catalysts, but yields are low and the process cannot compete with the liquid-phase process (see Benzaldehyde). Other processes for the production of benzoic acid are presently of little commercial importance. 

Two selective processes are important in the oxidation of ethylene the production of ethylene oxide and acetaldehyde. The first process is specifically catalyzed by silver, the second one by palladium-based catalysts. Silver catalysts are unique and selective for the oxidation of ethylene. No similar situation exists for higher olefins. The effect of palladium catalysts shows a resemblance to the liquid phase oxidation of ethylene in the Wacker process, in which Pd—C2H4 coordination complexes are involved. The high selectivity of the liquid phase process (95%), however, is not matched by the gas phase route at present. 

The reaction of adipic acid with ammonia in either liquid or vapor phase produces adipamide as an intermediate which is subsequently dehydrated to adiponitrile. The most widely used catalysts are based on phosphorus-containing compounds, but boron compounds and silica gel also have been patented for this use (52—56). Vapor-phase processes involve the use of fixed catalyst beds whereas, in liquid—gas processes, the catalyst is added to the feed. The reaction temperature of the liquid-phase processes is ca 300°C and most vapor-phase processes mn at 350—400°C. Both operate at atmospheric pressure. Yields of adipic acid to adiponitrile are as high as 95% (57). 

The vinyl acetate process exists in both homogeneous and heterogeneous versions. The liquid-phase process developed by ICI is essentially a Wacker reaction performed in acetic acid ethylene, 02 and AcOH are reacted at 110 °C in the presence of PdCl2, Cu(OAc)2 and HC1. Overall yields are greater than 90%. Acetaldehyde is formed as a coproduct in the reaction, owing to the presence... 

The liquid-phase processes are more energy efficient than the vapor-phase processes, however, they incur cosdy high pressure equipment investment and also produce waste streams containing used catalyst (213). Both methods produce substantial quantities of by-products which cause refining difficulties. The by-products consist primarily of mesitylene [108-67-8], phorone [504-20-1], and the following xylitone isomers (215) ... 

The removal of potassium cations from the reaction sphere can be accomplished by their binding with 18-crown-6-ether (Scheme 2-13). The removal of potassium cations makes the results of the liquid-phase and electrode reactions similar. In the presence of the crown ether, the liquid-phase process also leads to the azodianion. The azodianion was indeed identified via benzidine after protonation and rearrangement (Scheme 2-14). 

Pd(OAc)2 with ethylene forms vinyl acetate. Industrial production of vinyl acetate from ethylene and AcOH has been developed by Imperial Chemical Industries, initially in the liquid phase [55]. However, due to operational problems caused mainly by corrosion, the liquid-phase process was abandoned. Then a gas-phase process using a supported Pd catalyst was developed [56]. Vinyl acetate is now produced commercially, based on this reaction in the gas phase, using Pd supported on alumina or silica as a catalyst in the absence of any Cu(II) salt [57]. It seems likely that Pd(OAc)2 is generated from the supported Pd by the reaction with AcOH and 02 at high temperature. 

The most dramatic results obtained so far with gold catalysts have been with the liquid phase processes. They are conducted with oxygen or air, often using water as solvent, and are therefore felt to be environmentally benign. Particular success has been obtained with reducing sugars (Section 8.3.2) and other aldehydes (Section 8.3.3), and with alcohols and other hydroxy-compounds (Sections 8.3.4-8.3.7). Reactions that use soluble gold complexes to catalyse selective oxidation are reported in Chapter 12. 

The liquid phase processes resembled Wacker-Hoechst s acetaldehyde process, i.e., acetic acid solutions of PdCl2 and CuCl2 are used as catalysts. The water produced from the oxidation of Cu(I) to Cu(II) (Figure 27) forms acetaldehyde in a secondary reaction with ethylene. The ratio of acetaldehyde to vinyl acetate can be regulated by changing the operating conditions. The reaction takes place at 110-130°C and 30-40 bar. The vinyl acetate selectivity reaches 93% (based on acetic acid). The net selectivity to acetaldehyde and vinyl acetate is about 83% (based on ethylene), the by-products being CO2, formic acid, oxalic acid, butene and chlorinated compounds. The reaction solution is very corrosive, so that titanium must be used for many plant components. After a few years of operation, in 1969-1970 both ICI and Celanese shut down their plants due to corrosion and economic problems. 

Ethylene acetoxylation was also developed as a gas phase process following the liquid phase process and has been in commercial use since 1968. There is a notable difference between the two processes in the liquid phase the presence of palladium salts and redox systems results in the formation of both vinyl acetate and acetaldehyde, whereas in the gas phase process, using palladium metal,... 

The liquid and gas phase catalyst systems for vinyl acetate are based on the same components no coincidence as the latter was developed after the discovery of the former. They differ mainly in the reoxidation of Pd(0), which is carried out by Cu(II) in the liquid phase process and is not necessary in the gas phase process. It therefore seems tempting to suggest that the chemistry is similar in both cases, at least as far as the vinyl acetate formation is concerned. 

An ion pair of the type, H" AlCl4, is believed to be the actual catalyst in the liquid-phase process operating at ca. 90 °C. As is well known in electrophilic aromatic substitution, electron-releasing substituents favour alkylation thus ethylbenzene can undergo further alkylation to diethylbenzenes. To limit overalkylation, benzene must be used in large excess. The peralkylated benzenes, however, can be recycled to obtain ethylbenzene by exchange with... 

Ethylbenzene is commercially produced almost entirely as an intermediate for the manufacture of styrene. Since only a limited amount can be made by the superfractionation of Ce petroleum aromatics, most ethylbenzene is produced by the alkylation of benzene with ethylene. The alkylation reaction can occur eUher in the vapor phase or the liquid phase. A number of proven processes exist. The liquid phase processes using aluminum chloride catalysts are currently the most widely used. The purpose of this paper is to describe a new and improved version of this latter process which has been commercialized. 

In addition to the liquid-phase processes discussed above, vapor-phase halogen-exchange processes have also been developed. A variety of metal... 

The crucial step of the new phenol synthesis is oxidizing the obtained benzoic acid to phenol. Early literature data indicated that heating copper benzoate or benzoic acid in the presence of copper salts gave various phenol precursors—e.g., phenyl benzoate and salicylic acid, as well as phenol itself (3, 10, 13, 24, 26, 36). In one of the initial approaches, by Dow Chemical Co., mixtures of benzoic acid vapors, air, and steam were passed over a CuO catalyst promoted with metal salts, giving phenol and phenyl benzoate (5). However, much tar was produced, probably because of the high reaction temperature, which led to excessive decomposition. Because of this, the vapor-phase method was abandoned in favor of the liquid-phase process. Next, benzoic acid was oxidized in aqueous solution with inorganic copper salts, as shown below (18) ... 

The liquid phase process conveys the advantages of better heat transfer characteristics relative to the traditional gas phase chemistry, which is prone to developing hot spots, resulting in depressed yields. In addition, the phosgene is more readily separated from... 

Just as with the liquid-phase process, the catalyst requires regeneration from time to time. The usual problem is coking, which requires treatment of the catalyst bed with oxygen or air at high temperatures. There have been some successful attempts to prolong... 

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