Moving-bed transport

Li, H., and Kwauk, M., Vertical Pneumatic Moving-Bed Transport -1. Analysis of Flow Dynamics, Chem. Engr. Sci., 44 249-259 (1989)... 

Point D stands for co-up moving-bed transport with voidage e0 and at terminal fluid velocity u 0 = 1. 

Point D Moving-bed transport at terminal fluid velocity So 1 (... 

In moving-bed transport, solid particles are in contiguous contact with one another and are pushed along by the transporting fluid more or less in plug flow, with a voidage equivalent to that of the loosest fixed bed. The velocity of solids movement can be varied from zero to any practical upper limit. 

Kwauk, M., A Tentative Analysis of Moving-Bed Transport of Granular Materials with Compressible Media (unpublished), Inst. Chem. Metall., 1963-7-2 (1963c). 

The basic character of non-fluidized gas-particle flow is the existence of contact pressure (or stress) among particles and between particles and the pipe wall. Both theoretical analyses and experimental results (Terzaghi, 1954 Johanson and Jenike, 1972 Li and Kwauk, 1989) showed that the pressure of the interstitial fluid in particulate material neither compresses nor increases the shear resistance of the particulate material. After Walker (1966) and Walters (1973), Li and Kwauk (1989) analyzed the stresses in a vertical pneumatic moving-bed transport tube by using the stress theory of particulate media mechanics and Mohr circles, shown in Figs. 19 and 20, and gave the following stress ratios at any point in the flow field ... 

Butane-Based Transport-Bed Process Technology. Du Pont aimounced the commercialization of a moving-bed recycle-based technology for the oxidation of butane to maleic anhydride (109,149). Athough maleic anhydride is produced in the reaction section of the process and could be recovered, it is not a direct product of the process. Maleic anhydride is recovered as aqueous maleic acid for hydrogenation to tetrahydrofuran [109-99-9] (THF). 

FIG. 23-24 Reactors with moving catalysts, a) Transport fluidized type for the Sasol Fischer-Tropsch process, nonregenerating, (h) Esso type of stable fluidized bed reactor/regeuerator for cracldug petroleum oils, (c) UOP reformer with moving bed of platinum catalyst and continuous regeneration of a controlled quantity of catalyst, (d) Flow distribution in a fluidized bed the catalyst rains through the bubbles. 

Fluid cracking retained the moving bed concept of the catalyst transported regularly between the reactor and regenerator. And as with the air-lift systems, the fluid plant rejected mechanical carrying devices (elevators) in favor of standpipes through which the catalyst fluid traveled. 

The technical realization of these considerations failed in the movement of the molecular sieves from the top to the bottom of the tower. Aside from the enormous abrasion caused by mechanical exposure there is no known conveyor which would be able to transport the molecular sieve from top to bottom. The problem was solved through the simulation of the moving bed (Fig. 4) [32]. 

There are three basic modes of transport which are employed. The first, and most common, is termed dilute phase or lean phase transport in which the volume fraction of solids in this suspension does not exceed about 0.05 and a high proportion of the particles spend most of their time in suspension. The second is transport which takes place largely in the form of a moving bed in which the solids volume fraction may be as high as 0.6 this is relevant only for horizontal or slightly inclined pipelines. The third form is dense phase transport in which fairly close packed slugs of particles, with volume fractions of up to... 

Knowlton has cautioned on the difference between small diameter and large diameter systems for pressure losses. The difference between these systems is especially apparent for dense phase flow where recirculation occurs and wall friction differs considerably. Li and Kwauk (1989, 1989) have also studied the dense phase vertical transport in their analysis and approach to recirculating fluid beds. Li and Kwauk s analysis included the dynamics of a vertical pneumatic moving bed upward transport using the basic solid mechanics formulation. Some noncircular geometries were treated including experimental verification. The flows have been characterized into packed and transition flows. Accurate prediction of the discharge rates from these systems has been obtained. 

In fluidized beds, the temperature is uniform within a few degrees even in the largest vessels, but variation of comnposition is appreciable in large vessels, and is not well correlated for design purposes. One currently successful moving bed process is the UOP "Stacked Reactor" platforming where the catalyst is transported and regenerated in a separate zone. When the activity of the catalyst declines fairly rapidly, its variation with time and position must be taken into account by the mathematical formulation. 

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. 

Equation (8.62) is the general momentum balance equation. For moving bed solids transport in the standpipe, a is constant with a value of amb. Thus, Eq. (8.62) can be simplified to... 

A model for transient simulation of radial and axial composition and temperature profiles In pressurized dry ash and slagging moving bed gasifiers Is described. The model Is based on mass and energy balances, thermodynamics, and kinetic and transport rate processes. Particle and gas temperatures are taken to be equal. Computation Is done using orthogonal collocation In the radial variable and exponential collocation In time, with numerical Integration In the axial direction. 

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