Gas-Moving Solids Reactors

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

Henry s law constant, Kh, for carbon dioxide in water at 30°C is 11.1 10 mm of Hg per mole fraction. What is the dimensionless value for Kh so that a and Og have the same units  

Complete Example 11.4 for the case where Vg/Qg = 20s. Solve the governing ODEs analytically or numerically as you prefer. How does this more rigorous approach change the 95% response time calculated in Example 11.3  

A reactive gas is slowly bubbled into a column of liquid. The bubbles are small, approximately spherical, and are well separated from each other. Assume Stokes law and ignore the change in gas density due to elevation. The gas is pure and reacts in the liquid phase with first-order kinetics. Derive an expression for the size of the bubbles as a function of height in the column. Carefully specify any additional assumptions you need to make. 

Example 11.5 treats a reaction that is catalyzed by a stagnant liquid phase. Find the outlet concentration of component A for the limiting case of high catalytic activity, k oo. Repeat for the limiting case of high mass transfer, kiAi oo. 

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A fundamental division of multiphase reactors may be made, depending on whether the solid phase is present as a moving-or as a fixed bed. In principle, one gas-liquid-solid reactor with the fixed bed of solids can be operated in three ways, depending upon the relative orientation of the superficial gas-mass G and superficial liquid-mass L flow-rates (see Figure 5.2-1). 

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

Figure 5 shows the axial gas and solid temperature profiles during start-up operation. Notice that the hot spot in the reactor moves down the bed as the heat of reaction increases the temperature of the catalyst particles. Also note the significant temperature difference between the catalyst and gas in the early part of the reactor, where conversion is rapid due to the heat of reaction being generated on the catalyst surface. These differences are even more pronounced (over 20 K) near the center of the bed and near the outer wall.11... 

Reactors with moving solid phase Three-phase fluidized-bed (ebullated-bed) reactor Catalyst particles are fluidized by an upward liquid flow, whereas the gas phase rises in a dispersed bubble regime. A typical application of this reactor is the hydrogenation of residues. 

A parametric study of moving bed behaviour has been undertaken. The solid pellets are assumed to be preheated to the appropriate reduction temperatures before entering the reaction zone of the reactor. Although this neglects the solids preheat zone, this can easily be included in the model if required. The present study therefore is focussed on the reaction zone itself where the important parameters of gas and solid flow rates, gas inlet temperature and gas mixture composition are considered. Reactor length is also of major importance but in the present paper this has been fixed at lm in order to obtain comparative data. 

The proposed model takes another approach. It was developed for multistage semibatch reactors with stationary solids and continuous co-current reactors with moving solids. It also allows for a crosscurrent stream such as gas sparged separately into any number of stages. The residence time of each stage is divided into a number of finite time intervals. Within each interval, the individual reactions are treated as successive rather than simultaneous. The model accuracy is controlled by selecting the number of intervals. 

A feasibility study, carried out with mock-ups of various sizes, ensured that the catalyst bed could flow in countercurrent to the fluids (gas and liquid) and that the flow of the particulate solids was sufficiently close to plug flow. A moving bed reactor (4O0 cm x 20 m high) was succesfully operated as a demonstration plant for several months, whereby different heavy residues were treated. 

Bubble column reactors (BCR) are widely used in chemical process industries to carry out gas-liquid and gas--liquid-solid reactions, the solid suspended in the liquid phase being most frequently a finely divided catalyst (slurry reactor). The main advantages of BCR are their simple construction, the absence of any moving parts, ease of maintenance, good mass transfer and excellent heat transfer properties. These favorable properties have lead to their application in various fields production of various chemical intermediates, petroleum engineering, Fischer-Tropsch synthesis, fermentations and waste water treatment. 

One further note, the University of Delaware gasifier model used in the pseudo steady state approximation assumes that the gas and solids temperatures are the same within the reactor. That assumption removes an important dynamic feedback effect between the countercurrent flowing gas and solids streams. This is particularly important when the burning zone moves up and down within the reactor in an oscillatory manner in response to a step change in operating conditions. 

A moving bed gasifier is a vertical reactor with countercurrent flowing gas and solids streams. Coal enters the top of the reactor and ash (and/or clinker or molten slag) is removed from the bottom. A mixture of steam and oxygen (or air) enters the bottom of the reactor and the raw product gas exits from the top. 

Fluid-solid systems, especially in situations where the fluid is a gas, are very frequently encountered in various important industrial processes such as packed-bed reactors, moving-bed reactors, fluidized-bed reactors, and entrained reactors. 

E.H.P. Wolff. P. Veenstra. and L.A. Chewier, A novel circulating cross-flow moving bed reactor system for gas-solids contacting, Chem, Eng. Sci. 49 5427 (1994). 

Heat Effects in Moving Beds. We shall consider two cases for modeling the temperature profile in the moving-bed reactor. In one case the temperature of the solid catalyst and the temperature of the gas are different and in the other case they are the same. 

Gas-solid reactors Fluidized bed reactors, fixed bed reactors, moving bed reactors. 

The counter-current moving bed reactor was first mentioned by Takeuchi et al. (1978). However, so far only gas-phase investigations are known. The lack of applications of the TMBR are for the same reasons as with the TMB process, namely difficulties with particle movement, back mixing of the solid, and abrasion of the particles. 

The types of reactors used for catalytic and noncatalytic gas-solid reactions are also often similar. Moving-bed reactors are used in blast furnaces and cement kilns. Fluidized-bed reactors are used for the roasting of sulflde ores and regeneration of catalytic cracking catalyst, and fixed-bed reactors are used to remove sulfur compounds from ammonia synthesis feed gas. When regeneration of the solid reactant is desired, two or more reactors operating in parallel are required if continuous, steady-state operation is to be achieved. 

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