Transport reactors

The transport reactor (Figure 4-22) is widely used in the production of gasoline from heavier petroleum fractions. In this reactor, either an... 

Adding a recirculating loop to the transport reactor, a well-mixed condition is achieved provided the recirculation rate is large with... 

Table 4-4 summarizes the ratings of the various reactors. The CFSTR and the recirculating transport reactor are the best choices because they are satisfactory in every category except for construction. The stirred batch and contained solid reactors are satisfactory if the catalyst under study does not decay. If the system is not limited by internal diffusion in the catalyst pellet, larger pellets could be used and the stirred-contained solids reactor is the better choice. However,... 

Figure 23.2 depicts some of the essential features of (a) fluidized-bed, (b) fast-fluidized-bed, and (c) pneumatic-transport reactors (after Yates, 1983, p. 35). 

Figure 23.2 Some features of (a) a fluidized-bed reactor (b) a fast-fluidized-bed reactor and (c) a pneumatic-transport reactor...

Transport Reactor Gasifier Coal (sub-bituminous), KY and IL No. 6, coke breeze LHV IGCC 26.8-64... 

Technologically, the Alkylene process is a break-through. Several significant inventions were required to make it technically feasible. The development of a unique solid-acid catalyst and transport reactor by UOP allows for the potential elimination of hazardous liquid acid processes. About 1 MM gallons of hydrofluoric acid inventory could be eliminated, transport of 33 MM lbs (4 MM gallons) of hydrofluoric acid per year would be stopped, and ca. 20 MM lbs per year of other fluoride containing solids would not have to be land filled. 

Catalytic cracking Zeolite in Si02 A1203 matrix plus other ingredients (transport reactor)... 

Figure 4-23. Recirculationg transport reactors. (Source V. W. Weekman, Laboratory Reactors and Their Limitations/ AlChEJ, Vol. 20, p. 833, 1974. Used with permission of the AlChEJ.)...

The catalyst support may either be inert or play a role in catalysis. Supports typically have a high internal surface area. Special shapes (e.g., trilobed particles) are often used to maximize the geometric surface area of the catalyst per reactor volume (and thereby increase the reaction rate per unit volume for diffusion-limited reactions) or to minimize pressure drop. Smaller particles may be used instead of shaped catalysts however, the pressure drop increases and compressor costs become an issue. For fixed beds, the catalyst size range is 1 to 5 mm (0.04 to 0.197 in). In reactors where pressure drop is not an issue, such as fluidized and transport reactors, particle diameters can average less than 0.1 mm (0.0039 in). Smaller particles improve fluidization however, they are entrained and have to be recovered. In slurry beds the diameters can be from about 1.0 mm (0.039 in) down to 10 Jim or less. 

Transport Reactors The superficial velocity of the gas exceeds the terminal velocity of the solid particles, and the particles are transported along with the gas. Usually, there is some slip between the gas and the solids—the solid velocity is slightly lower than the gas velocity. Transport reactors are typically used when the required residence time is small and the fluid reactant (or the solid reactant) can be substantially converted (consumed). They may also be used when the catalyst is substantially deactivated during its time in the reactor and has to be regenerated. 

Advantages of transport reactors include low gas and solid backmix-ing (compared to fluidized beds) and the ability to continuously remove deactivated catalyst (and add fresh catalyst), thereby maintaining catalyst activity. The fluid and catalyst are separated downstream by using settlers, cyclones, or filters. 

A transport reactor is also used in the Sasol Fischer-Tropsch process. The catalyst is promoted iron. It circulates through the 1.0-m (3.28-ft) ID riser at 72,600 kg/h (160,000 lbm/h) at 340°C (644°F) and 23 atm (338 psi) and has a life of about 50 days. Figure 19-23a shows an in-line heat exchanger in the Sasol unit. 

The mathematical models of the reacting polydispersed particles usually have stiff ordinary differential equations. Stiffness arises from the effect of particle sizes on the thermal transients of the particles and from the strong temperature dependence of the reactions like combustion and devolatilization. The computation time for the numerical solution using commercially available stiff ODE solvers may take excessive time for some systems. A model that uses K discrete size cuts and N gas-solid reactions will have K(N + 1) differential equations. As an alternative to the numerical solution of these equations an iterative finite difference method was developed and tested on the pyrolysis model of polydispersed coal particles in a transport reactor. The resulting 160 differential equations were solved in less than 30 seconds on a CDC Cyber 73. This is compared to more than 10 hours on the same machine using a commercially available stiff solver which is based on Gear s method. 

In the following sections some background information on stiff ordinary differential equations will be given and the general finite difference approximations for particle temperatures will be derived. Later, the technique will be applied to coal pyrolysis in a transport reactor where the difference equations for reaction kinetics will be discussed and the calculation results will be compared with those obtained by the previously established techniques. 

---