Vinyl acetate liquid phase

Liquid- and vapor-phase processes have been described the latter appear to be advantageous. Supported cadmium, zinc, or mercury salts are used as catalysts. In 1963 it was estimated that 85% of U.S. vinyl acetate capacity was based on acetylene, but it has been completely replaced since about 1982 by newer technology using oxidative addition of acetic acid to ethylene (2) (see Vinyl polymers). In western Europe production of vinyl acetate from acetylene stiU remains a significant commercial route. 

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. 

The chemistry of vinyl acetate synthesis from the gas-phase oxidative coupling of acetic acid with ethylene has been shown to be facilitated by many co-catalysts. Since the inception of the ethylene-based homogeneous liquid-phase process by Moiseev et al. (1960), the active c ytic species in both the liquid and gas-phase process has always been seen to be some form of palladium acetate [Nakamura et al, 1971 Augustine and Blitz, 1993]. Many co-catalysts which help to enhance the productivity or selectivity of the catalyst have appeared in the literature over the years. The most notable promoters being gold (Au) [Sennewald et al., 1971 Bissot, 1977], cadmium acetate (Cd(OAc)j) [Hoechst, 1967], and potassium acetate (KOAc) [Sennewald et al., 1971 Bissot, 1977]. 

In 1969, 90% of vinyl acetate was manufactured by this process. By 1975 only 10% was made from acetylene, and in 1980 it was obsolete. Instead, a newer method based on ethylene replaced this old acetylene chemistry. A Wacker catalyst is used in this process similar to that for acetic acid. Since the acetic acid can also be made from ethylene, the basic raw material is solely ethylene, in recent years very economically advantageous as compared to acetylene chemistry. An older liquid-phase process has been replaced by a vapor-phase reaction run at 70-140 psi and 175-200°C. Catalysts may be (1) C—PdCb—CuCb, (2) PdClj—AI2O3, or (3) Pd—C, KOAc. The product is distilled water, acetaldehyde, and some polymer are... 

Figure 7.17 Ternary A/B/C phase diagram (at 25°C, 1 atm) for A = acetic acid, B = vinyl acetate, C = water, showing nonhorizontal tie-lines in the immiscible two-phase region (organic liquid + aqueous liquid), culminating at a plait point (x). Concentration grid values (dotted lines) are in wt% at 10% intervals.

A report on the change in molecular orientation after grafting is presented by Hayakawa et al. (28), through the angular distribution of polarization of fluorescence on Nylon films grafted with methyl methacrylate, vinyl acetate, and vinyl pyrrolidone, respectively, in liquid phase. 

The last of the direct methods for graft initiation in liquid phase presented in this review involves chemical additives. Either free radical or ionic initiators can be chosen. Benzoyl peroxide is reported for grafting styrene on Nylon fibers in methanol media (71,105-107), as well as vinyl acetate (106). Azoisobutyro-nitrile has been employed in systems where the graft monomer is styrene (71,106) or vinyl acetate (106). Redox systems involving hydrogen peroxide and monomers like styrene (106,108,109). vinyl acetate (106), acrylic acid (108,109), methyl... 

By chemical agents, indirect grafting on Nylon in liquid phase is frequently referred to in the bibliography. The most common reagent is air (144) or ozone, under controlled conditions, in order to avoid deterioration on the mechanical properties of the fiber, which is then immersed in the monomer. Hence, styrene (145-149), vinylidene chloride (146), vinyl acetate (146), acrylic and methacrylic acids (149), methyl methacrylate (146), acrylonitrile (146,148,149), 2-methyl-5-vinylpyridine (149) were successfully employed as grafting comonomers. 

Industrial problems have, in some instances, been solved either by a proper choice of construction materials and suitable process design or by development of heterogeneous catalytic systems using supported complexes or by generating active complexes in situ on a support material which avoid some of the problems of liquid-phase operation. For example, a number of the problems in liquid-phase vinyl acetate processing have been overcome by development of supported Pd catalysts (106). Vapor-phase hydroformylation has been carried out on supported rhodium complexes (107). 

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... 

Most of the vinyl acetate produced in the United States is made by the vapor-phase ethylene process. In this process, a vapor-phase mixture of ethylene, acetic acid, and oxygen is passed at elevated temperature and pressures over a fixed-bed catalyst consisting of supported palladium (85). Less than 70% oxygen, acetic acid, and ethylene conversion is realized per pass. Therefore, these components have to be recovered and returned to the reaction zone. The vinyl acetate yield using this process is typically in the 91—95% range (86). Vinyl acetate can be manufactured also from acetylene, acetaldehyde, and the liquid-phase ethylene process (see Vinyl polymers). 

DMTA is a very interesting tool for characterizing heterogeneous materials in which domains of distinct Tg values coexist. The most interesting cases involve modified thermosets of different types (see Chapter 8). Examples are the use of rubbers (e.g., liquid polybutadiene and random copolymers), or thermoplastics (e.g., polyethersulphone or polyetherimide in epoxy matrices or poly(vinyl acetate) in unsaturated polyesters), as impact modifier (epoxies), or low-profile additives (polyesters). The modifier-rich phase may be characterized by the presence of a new a peak (Fig. 11.10). But on occasions there may be superposition of peaks and the presence of the modifier cannot be easily detected by these techniques. If part of the added polymer is soluble in the thermoset matrix, its eventual plasticizing effect can be determined from the corresponding matrix Tg depletion, and the... 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

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 older process for the production of vinyl acetate (melting point -93.2°C, boiling point 72.3°C, density 0.9317) involved the reaction of acetylene with acetic acid in the liquid phase with zinc amalgam as the catalyst. 

Unsaturated vinyl esters for use in polymerization reactions are made by the esterification of olefins. The most important ones are vinyl esters vinyl acetate, vinyl chloride, acrylonitrile, and vinyl fluoride. The addition reaction may be carried out in either the liquid, vapor, or mixed phases, depending on the properties of the acid. Care must be taken to reduce the polymerization of the vinyl ester produced. 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

Supported liquid-phase catalysts (SLPCs) combine the salient features of both homogeneous and heterogeneous catalysis for enhanced catalytic and/or process efficiency (337). SLPC catalysts, in which a liquid-phase (homogeneous) catalyst is dispersed within a porous support, have been used in Wacker-type ethylene oxidation for acetaldehyde and vinyl acetate production (337, 338). In the former case, a traditional homogeneous Wacker catalyst (vide supra) consisting of a chlorinated solution of Pd and Cu chlorides retained on a support with monomodal pore size distribution... 

Liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes,3 "16 including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p-xylene, acetaldehyde and vinyl acetate from ethylene, propylene oxide from propylene, and many others. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. 

Activation of olefins to nucleophilic attack, by 7r-complex formation at soft metal centers (Section III.D), can also occur with heterogeneous catalysts. Thus, the oxidation of ethylene to acetaldehyde or vinyl acetate, as described earlier for homogeneous Pd(II) catalysts, can also be carried out heterogeneously in either the liquid or gas phase.512... 

To get an idea about the relative volatilities of components we proceed with a simple flash of the outlet reactor mixture at 33 °C and 9 bar. The selection of the thermodynamic method is important since the mixture contains both supercritical and condensable components, some highly polar. From the gas-separation viewpoint an equation of state with capabilities for polar species should be the first choice, as SR-Polar in Aspen Plus [16]. From the liquid-separation viewpoint liquid-activity models are recommended, such as Wilson, NRTL or Uniquac, with the Hayden O Connell option for handling the vapor-phase dimerization of the acetic acid [3]. Note that SR-Polar makes use of interaction parameters for C2H4, C2H6 and C02, but neglects the others, while the liquid-activity models account only for the interactions among vinyl acetate, acetic acid and water. To overcome this problem a mixed manner is selected, in which the condensable components are treated by a liquid-activity model and the gaseous species by the Henry law. 

In poly(vinyl acetate) copolymer emulsions, the properties are significantly affected by the composition of the aqueous phase and by the stabilizers and buffers used in the preparation of these materials, along with the process conditions (eg, monomer concentrations, pH, agitation, and temperature). The emulsions are milk-white liquids containing ca 55 wt % PVAc, the balance being water and small quantities of wetting agents or protective colloids. 

The liquid-phase oxidation (LPO) of light saturated hydrocarbons yields acetic acid and a spectrum of coproduct acids, ketones, and esters. Although propane and pentanes have been used, n-butane is the most common feedstock because it can ideally yield two moles of acetic acid. The catalytic LPO process consumes more than 500 million lb of n-butane to produce about 500 million lb of acetic acid, 70 million lb of methyl ethyl ketone, and smaller amounts of vinyl acetate and formic acid. The process employs a liquid-phase, high-pressure (850 psi), 160-180°C oxidation, using acetic acid as a diluent and a cobalt or manganese acetate catalyst. 

In most industrially relevant reacting systems, one main reaction typically makes the desired products and several side reactions make byproducts. The specific rate of production or consumption of a particular component in such a reaction set depends upon the stoichiometry and the rates. For example, assume that the main reaction for making vinyl acetate, Eq. (4.4.1, proceeds with a rate r< (mol/L s) and that the side reaction, Eq. (4.8), proceeds with rate r2 (mol/L s). Then the net consumption of ethylene is (-l)r1 - (-1 )r2 (mol/L s). Similarly, the net consumption of oxygen is (-0.5)fi + (— 3)r2, and the net production of water is (l)r-, + (2)ra. For a given chemistry (stoichiometry), our ability to control the production or consumption of any one component in the reactor is thus limited to how well we can influence the various rates. This boils down to manipulating the reactor temperature and/ or the concentrations of the dominant components. Occasionally, the reaction volume for liquid-phase reactions or the pressure for gas-phase reactions can also be manipulated for overall production control. These are the fundamentals of reactor control. 

Another feature of a residue map we would like to illustrate is the representation of systems that form two liquid phases. In Fig, 6.3 we show how mixtures of vinyl acetate and water form two liquid phases with drastically different compositions. We can take advantage of this nonideality to help produce pure acetic acid from a single distillation column. In Fig. 6.4 we show how the net feed to a column can be changed by mixing the original feed with the vinyl acetate rich reflux. The new feed composition contains less acetic acid acid and water and more vinyl acetate. When we look at the residue curves that pertain to the new feed composition, we find that they move over areas with little water. Most of the feed water is rejected with the overhead vapors... 

The final example to illustrate our plantwide control design procedure comes from Luyben and Tyreus (1998), who present design details of an industrial process for the vapor-phase manufacture of vinyl acetate monomer. This process is uniquely suited for researchers pursuing process simulation, design, and control studies. It has common real chemical components in a realistically large process flowsheet with standard chemical unit operations, gas and liquid recycle streams, and energy integration. 

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