Maleic anhydride production, fluidized

Lonza/ABB Lummus Global Maleic anhydride Butane Fluidized bed technology, organic solvent product recovery 9 1993... 

Most fluidized bed partial oxidation processes are operated in the turbulent flow regime of fluidization. However, DuPont operated a circulating fluidized bed catalytic reactor process for maleic anhydride production in Spain, featuring regeneration of the catalyst (by oxidation) on the downcomer side of the circulating system. 

Selective oxidation of n-butane in either fixed-bed or fluidized-bed reactors is the commercialized method of maleic anhydride production. Because of the important role of this material as a chemical intermediate, much attention has been focused on finding a process alternative not only to increase the selectivity and yield of the desired product but also to enhance reactor safety. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Extension of the Kunii-Levenspiel bubbling-bed model for first-order reactions to complex systems is of practical significance, since most of the processes conducted in fluidized-bed reactors involve such systems. Thus, the yield or selectivity to a desired product is a primary design issue which should be considered. As described in Chapter 5, reactions may occur in series or parallel, or a combination of both. Specific examples include the production of acrylonitrile from propylene, in which other nitriles may be formed, oxidation of butadiene and butene to produce maleic anhydride and other oxidation products, and the production of phthalic anhydride from naphthalene, in which phthalic anhydride may undergo further oxidation. 

Oxidation of n-butane and butenes requires higher reaction temperatures (400-480°C). Since more water is produced in this reaction, most of the product is recovered in the form of maleic acid. The currently best process is the Alma (Alusuisse) process.1015-1018 Its main features are a fluidized-bed reactor and an anhydrous product recovery. Because of the better temperature control, a lower air hydrocarbon ratio can be employed (4 mol% of n-butane). Instead of absorption in water, maleic anhydride is recovered from the reactor effluent gas by a high-boiling organic... 

The oxidation of butane (or butylene or mixtures thereof) to maleic anhydride is a successful example of the replacement of a feedstock (in this case benzene) by a more economical one (Table 1, entry 5). Process conditions are similar to the conventional process starting from aromatics or butylene. Catalysts are based on vanadium and phosphorus oxides [11]. The reaction can be performed in multitubular fixed bed or in fluidized bed reactors. To achieve high selectivity the conversion is limited to <20 % in the fixed bed reactor and the concentration of C4 is limited to values below the explosion limit of approx. 2 mol% in the feed of fixed bed reactors. The fluidized-bed reactor can be operated above the explosion limits but the selectivity is lower than for a fixed bed process. The synthesis of maleic anhydride is also an example of the intensive process development that has occurred in recent decades. In the 1990s DuPont developed and introduced a so called cataloreactant concept on a technical scale. In this process hydrocarbons are oxidized by a catalyst in a high oxidation state and the catalyst is reduced in this first reaction step. In a second reaction step the catalyst is reoxidized separately. DuPont s circulating reactor-regenerator principle thus limits total oxidation of feed and products by the absence of gas phase oxygen in the reaction step of hydrocarbon oxidation [12]. 

Type of Reaction and Application. An increased emphasis on gas-solid reactions has been evident for about a decade. Three of the papers in this symposium treat gas-solid reactions, two (13,18) dealing with coal combustion and the other (11) with catalyst regeneration. Of the four papers which consider solid-catalysed gas-phase reactions, one (15) deals with a specific application (production of maleic anhydride), and one (12) treats an unspecified consecutive reaction of the type A B C the other two (14,16) are concerned with unspecified first order irreversible reactions. The final paper (17) considers a relatively recent application, fluidized bed aerosol filtration. Principles of fluid bed reactor modeling are directly applicable to such a case Aerosol particles disappear by adsorption on the collector (fluidized) particles much as a gaseous component disappears by reaction in the case of a solid-catalysed reaction. 

Simulation of a Fluidized Bed Reactor for the Production of Maleic Anhydride... 

The production of maleic anhydride by the catalytic oxidation of benzene is an established industrial process. While hydrocarbons are often suggested as a feedstock, it has been pointed out recently by De Maio (1) that they are an alternative but not necessarily a substitute. The benzene oxidation is done commercially in fixed bed reactors and, because of its exothermicity, is difficult to control in any optimal sense. The process is thus a natural candidate for a fluidized-bed reactor. The reaction has been studied in both fixed bed (2, 3) and fluidized bed (4-7) reactors. These studies, with the exception of that of Kizer et al (7) do not give sufficient information for simulation purposes. The availability of the reaction data of Kizer et al and the kinetic studies of Quach et al ( ) using a similar catalyst suggested the possibility of simulating the process. 

The success of FCC encouraged applications of the fluidized catalyst reactor to other catalytic reactions. Successful applications can be found in fluid catalytic reforming, production of alkyl chloride by oxychlorina-tion, production of phthalic anhydride, acrylonitrile synthesis by ammox-idation, and production of maleic anhydride. 

This class of reactions, carried out in fluidized beds, involves parallel and series reactions, with reaction intermediates being the desired products. Industrial examples include partial oxidation of n-butane to maleic anhydride and o-xylene to phthalic anhydride. The vigorous solid mixing of fluidized beds is valuable for these reactions because they are highly exothermic. However, gas backmixing must be minimized to avoid extended gas residence times that lead to the formation of products of total combustion (i.e., CO2 and H2O). For this reason, fluidized bed catalytic partial oxidation reactors are operated in the higher velocity regimes of turbulent and fast-fluidization. 

Substantial improvements in the performance of several processes of hydrocarbon selective oxidation can be achieved solely by developing new reactor configurations. An important step in this direction is exemplified by the circulating fluidized bed reactor, which over the years has been proposed for use in several selective oxidation reactions and has, finally, found application in w-butane selective oxidation to maleic anhydride. Although production at the plant (built in Spain) was later stopped, because it was uneconomic, it remains an interesting example that may find application in other reactions. 

CDP6-24g The production of maleic anhydride by oxidation with air can be carried out over a vanadium catalyst in (a) a "fluidized" CSTR and tb) a PBR at different temperatures. 

DuPont has commercialized a transport bed process for the production of maleic anhydride. The transport bed reactor is a special fluidized bed reactor similar to a riser cracker reactor used in the petroleum refining industry. In this process, maleic anhydride is converted to maleic acid which is hydrogenation to tetrahydrofuran. 

A major advantage of the riser reactor for catalytic cracking is that the gas and solid move in nearly plug flow, which gives more uniform catalytic activity and better selectivity than with a bubbling or turbulent fluidized bed. A riser reactor can be used for other rapid catalytic reactions, such as the production of acrolein from propylene [3] or the partial oxidation of n-butane to make maleic anhydride. In DuPont s butane oxidation process... 

Figure 2.16 shows the trend of selectivity versus conversion for the partial oxidation of -butane to maleic anhydride in a fluidized bed reactor. Besides the by product acids (acrylic and acetic), CO and CO2 are the principal products either formed directly from butane (parallel reaction) or via maleic... 

Figure 5.41 Production of maleic anhydride by the catalytic oxidation of butane in a fluidized bed...

Although the traditional production method of maleic anhydride was the oxidation of benzene or other aromatic compounds, butane partial oxidation has recently been used for this purpose. This substitution is mainly because of the lower price, flammability risk, and toxicity of butane. In this process, the reactant stream of n-butane and air mixture enters the packed bed or fluidized bed reactor at a temperature of 120—150 °C and a pressure of 2—3 atm and undergoes the following reaction ... 

To summarize the model calculation, the concentrations of each species (reactant and products) in the bed are calculated using Eqs. (20) and (21). Equation (23) is used to calculate the concentration profiles of each species in the upper dilute region. The total gas flow rate can be calculated by addition of the molar flows of each species at any given axial location within the bed. The total flow rate can be converted to the volumetric gas flow rate, and the superficial gas velocity may be determined using Eqs. (24) and (25). The values for hydrodynamic parameters are then calculated for the obtained gas velocity and the set of equations is solved numerically. Figure 27 shows the axial profiles of butane and maleic anhydride concentrations calculated based on the model equations. The effects of fine particle (<7p < 45 pm) contents on butane conversion are also predicted as shown in Fig. 28. As surmised from the figure, the content of fine particles affects fluidization properties and reactor conversion. 

Many catalytic reactions which have been attempted in fluidized beds (e.g. manufacture of phthalic and maleic anhydride and the production of acrylonitrile) involve reactions of the type A B C, with B being the desired product. Expressions for the concentration of B in the case where both reactions are first order and irreversible have been given by Grace (15,19) for the Orcutt model (17,18) (see Table 2) in which the gas is assumed to be well mixed in the dense phase and for the two-phase bubbling bed model featured in section 4.3. Predictions for the latter model appear in Figure 6 for and for representative... 

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