Transport bed reactor

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. 

We will develop the rest of this chapter assuming that the catalyst is in a sohd phase with the reactants and products in a gas or liquid phase. In Chapter 12 we will consider some of the more complex reactor types, called multiphase reactors, where each phase has a specific residence time. Examples are the riser reactor, the moving bed reactor, and the transport bed reactor. 

Elf Atochem (now Arkema) and Du Pont have claimed a cycle process for the oxidation of propene to acrolein [70a]. In a first transport-bed reactor (a riser, where the catalyst is transported upwards by the gas) propene is put in contact with the catalyst, a Bi/Mo/W/... 

Circulating fluid beds and transported bed reactors have been developed to commercial status by Ensyn and their process is used commercially by Red Arrow in the USA for food flavourings in several plants of 1 to l.S t/h (19). Ensyn have also supplied a 650 kg/h unit to ENEL in Italy (20) and a 20 kg/h system to VTT in Finland (21). In some... 

Particular features of circulating fluid bed and transported bed reactors include ... 

The vapour product from fluid bed and transported bed reactors has a low partial pressure of collectible products due to the large volumes of fluidising gas, and this is an important design consideration in liquid collection. 

However, it is not always easy to distinguish between the flow behavior encountered in the fast fluidization and the transport bed reactors [56]. The transport reactors are sometimes called dilute riser (transport) reactors because they are operated at very low solids mass fluxes. The dense riser transport reactors are operated in the fast fluidization regime with higher solids mass fluxes. The transition between these two flow regimes appears to be gradual rather than abrupt. However, fast fluidization generally applies to a higher overall suspension density (typically 2 to 15% by volume solids) and to a situation wherein the flow continues to develop over virtually the entire... 

In vertical pneumatic transport the radial particle concentration distribution is almost uniform, but some particle strands may still be identified near the wall. Little or no axial variation of solids concentration except in the bottom acceleration section is observed [58]. The flow associated with transport bed reactors tends to be dilute (typically 1 to 5 % by volume solids) and uniform. By virtue of the smaller reflux and density of the suspension within the dilute pneumatic conveying regime, there might be larger temperature gradients than within the fast fluidization regime [56]. 

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 new process to manufacture THF and 1,4 butanediol from maleic anhydride is currently slated for start-up by DuPont in Asturias, Spain in 1996. The process involves the oxidation of n-butane in a transport bed reactor to form maleic anhydride. Recovery of maleic anhydride is accomplished by scrubbing with water which converts the anhydride immediately to maleic acid. The maleic acid is then hydrogenated to tetrahydrofuran in a bubble column reactor. By varying operating conditions in the hydrogenation reactor the alternate or coproduction of 1,4 butanediol can be accomplished. 

The maleic anhydride catalyst is vanadium pentoxide spray coated with porous silica to impart attrition resistance. The catalyst needs to be specially treated for attrition resistance for use in the transport bed reactor. Reactor operating conditions are approximately 500°C (930 F) and 1 atm pressure. 

Kahney, R. H. and McMinn, T. D., Laboratory Transported Bed Reactors , presented at 66th Annual AIChE Meeting, Philadelphia, 1973. 

Formation of the catalysts in such a way as to achieve the best mechanical resistance for use in fixed-, fluid- or transported-bed reactors. 

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