Vinyl acetate process

Today acetic acid is produced mainly from methanol and carbon monoxide. This process, along with the Tennessee Eastman acetic anhydride plant using syn gas, are the current standards in the industry when considering new expansion regardless of the price of ethylene. The vinyl acetate process described here may achieve this stature if its commercial development is permitted to occur. 

Reis, Thomas, "Compare Vinyl Acetate Processes, Hydrocarbon Processing (1966) 45 (11), 171. 

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... 

Chen, R. and McAvoy, T. J. (2003). Plantwide control system design methodology and application to a vinyl acetate process. Ind. Eng. Chem. Res., 42, 4753-4771. 

A nonlinear dynamic model of a vinyl acetate process, Ind. Eng. Chem. Res.,... 

The case study on Vinyl Acetate Process, developed in Chapter 10, demonstrates the benefit of solving a process design and plantwide control problem based on the analysis of the reactor/separation/recycles structure. In particular, it is demonstrated that the dynamic behavior of the chemical reactor and the recycle policy depend on the mechanism of the catalytic process, as well as on the safety constraints. Because low per pass conversion of both ethylene and acetic acid is needed, the temperature profile in the chemical reactor becomes the most important means for manipulating the reaction rate and hence ensuring the plant flexibility. The inventory of reactants is adapted accordingly by fresh reactant make-up directly in recycles. 

The vinyl acetate process is described in Chap. 11. The reactor inlet temperature is 148.5 C and oxygen is the limiting reactant (yA0 = 0.075). The heat of reaction is -42.1 kcal/mol vinyl acetate or -84.2 kcal/mol oxygen. The average heat capacity of the feed is computed from data provided in Chap. 11 ... 

Figure 2.2 contrasts a typical column with a variable feed rate as set by an upstream unit to an on-demand column with bottoms flowrate set by a downstream operation. Further examples are given in later chapters when specific plants are considered (Chap. 8 for the Eastman process and Chap. 11 for the vinyl acetate process). 

The study was conveyed as if we had been assigned the task of designing the control system for a proposed new vinyl acetate process that is to be built. A particular preliminary design was given that had not been optimized. The data provided are what would typically be available or easily obtainable (1) kinetic reaction parameters and physical property data, (2) a flowsheet structure with stream and equipment information, and (3) the location of control valves included in the preliminary design. 

The industrial process for the vapor-phase manufacture of vinyl acetate monomer is quite common (Daniels, 1989) and utilizes widely available raw materials. Vinyl acetate is used chiefly as a monomer to make polyvinyl acetate and other copolymers. Hoechst-Celanese, Union Carbide, and Quantum Chemical are reported U.S. manufacturers. DuPont also currently operates a vinyl acetate process at its plant in LaPorte, Texas. To protect any proprietary DuPont information, all of the physical property and kinetic data, process flowsheet information, and modeling formulation in the published paper come from sources... 

Figure 11.1 shows the eleven basic unit operations in the reaction section of the vinyl acetate process. Three raw materials, ethylene (C2H4), oxygen (O,), and acetic acid (HAc), are converted into the vinyl acetate (VAc) product. Water (H,0) and carbon dioxide (CO,) are byproducts. We assume that an inert component, ethane (C2H6), enters with the fresh ethylene feed stream. We consider the following two reactions ... 

TABLE 11.4 Economic Data for Vinyl Acetate Process... 

Figure 11.3 Vinyl acetate process control strategy. 

Some over-all reactions of the ethylene-to-vinyl acetate process with either PdCl2 + CuCl2 or with Pd(OCOCH.3)o + Cu(OCOCH3)2 as catalysts are formulated in Table XV. Simplified flowsheets of the one-stage and two-stage processes are shown in Figures 1 and 2. 

The vinyl acetate process, proceeding similarly in solutions of Pd and Cu complexes ... 

The vinyl acetate process shown contains seven components. The chemistry consists of two reactions, which produce vinyl acetate (C4H6O2) from ethylene, oxygen, and acetic acid (C2H4O2) and form by-products of water and carbon dioxide. 

The ethylene-based version of the vinyl acetate process was also developed by Wacker Chemie. The process is similar to the Wacker process for acetaldehyde from ethylene which was developed about the same time. In the vinyl acetate process, ethylene is reacted with high purity oxygen and acetic acid in the presence of a palladium chloride catalyst. National Distillers and Chemicals, which later became USI chemicals and is now a division of Quantum Chemicals, developed a similar vapor phase ethylene-based technology in the United States. Both versions of the process are presently used commercially [25,26]. 

The overall reaction is shown in equation (29). This reaction is similar to the Wacker acetaldehyde process. The same catalyst system is used, except that the vinyl acetate process is carried out in the vapor phase over a heterogeneous solid catalyst, whereas in the acetaldehyde process the catalyst is in solution in the liquid phase. 

A simplified process flow diagram of the ethylene-based vinyl acetate process is shown in Figure 20 [25]. Acetic acid feedstock is vaporized in the presence of fresh ethylene feed and unreacted ethylene recycle gas in the acetic acid vaporizer. The stream is preheated and high purity oxygen is added with a special mixing nozzle. The quantity of reactants and oxygen are carefully controlled to ensure that the mixture is outside of the explosive limits. 

The feedstock requirements for the oxygen based vinyl acetate process are calculated based on the stoichiometry and catalyst selectivity for the reaction of ethylene and acetic acid with oxygen to vinyl acetate. Catalyst selectivity is about 90% for ethylene to vinyl acetate. A small amount of acetaldehyde and other hydrocarbon byproducts are formed. However, for simplification of the calculation it is assumed that the remainder of the ethylene reacts to form carbon dioxide and water vapor. This gives a slightly higher quantity of oxygen than accounting for all of the by-... 

The chemistry is more complex than would appear on the surface. When acetic anhydride reacts with acetaldehyde, it forms EDA in an equilibrium that favors EDA (Equation [8]), but a subsequent disfavorable equilibrium follows which forms acetic acid and vinyl acetate (Equation [9]). These equilibrium constants indicate that EDA is the most thermodynamically favored product. In the presence of acids, these equilibria occur rapidly. Using LeChatelier s principle, if the most volatile product, acetaldehyde, is continuously removed you can shift the equilibrium until only acetic anhydride remains which can then be distilled independently. (The equilibrium between EDA and vinyl acetate would prove to be pivotal in the later development of a vinyl acetate process.)... 

The first new process was introduced by Celanese in the late 1950 s. The Celanese vinyl acetate process took advantage of the equilibria ... 

In 1960, quickly after the introduction of the Celanese process, Wacker-Chemie commercialized a liquid phase vinyl acetate process which represented and extension of its earlier acetaldehyde process wherein acetic acid was simply substituted for water. (See equation [19]. This chemical transformation is also referred to as oxidative acetoxylation.) As shown in Figure 2, wherein R=Ac, the liquid phase oxidative acetoxylation of ethylene utilized the same catalytic cycle as the Wacker-Chemie acetaldehyde process. 

The liquid phase vinyl acetate process dominated new installations until 1970 when Hoechst and Bayer commercialized a jointly developed heterogeneous vapor phase process for the addition of acetic acid to ethylene in the presence of oxygen (Equation [19]) in 1970. National Distillers (now Quantum Chemical) developed a similar process about the same time and commercialized the process rapidly thereafter. The processes (which were developed during the late 1960 s) used a Pd-Au or Pd/Cd catalyst with an alkali... 

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