Solids circulation systems

There have been attempts to use catalysts in order to reduce the maximum temperature of thermal decomposition of methane. In the 1960s, Universal Oil Products Co. developed the HYPROd process for continuous production of hydrogen by catalytic decomposition of a gaseous hydrocarbon streams.15 Methane decomposition was carried out in a fluidized bed catalytic reactor from 815 to 1093°C. Supported Ni, Fe and Co catalysts (preferably Ni/Al203) were used in the process. The coked catalyst was continuously removed from the reactor to the regeneration section where carbon was burned off by air, and the regenerated catalyst returned to the reactor. Unfortunately, the system with two fluidized beds and the solids-circulation system was too complex and expensive and could not compete with the SR process. 

Generally, the cracking process has two sections One is for the heat sink, where cracking reactions and coke deposition take place the other is to burn off the coke deposited on the catalyst, which acts as a heat source and catalyst regenerator. The circulating solids are the means for catalysis and heat transport. The only way to carry out the process efficiently is by use of a solids circulation system involving one or more fluidized beds. Usually, the cracking reaction takes place in an adiabatic riser reactor. 

Matsen, J.M. Some characteristics of large solids circulation systems. In Fluidization Technology Keairns, D.L., Ed. Hemisphere Washington, 1976 Vol. 2, 135-149. 

Matsen, J. M., Some Characteristics of Large Solids Circulation Systems , in Fluidization Technology Vol. 2 (D.L. Keairns, ed.). Hemisphere, New York,... 

Readers wishing to learn more about solids circulation systems, standpipe flow and non-mechanical valves are referred to Kunii and Levenspiel (1990) or the chapters by Knowlton in either Geldart (1986) or Grace et al. (1997). 

Part A gives general guidelines for the design of large commercial fluidized bed reactors with respect to the following aspects (1) solids properties and their effect on the quality of fluidization (2) bubble size control through small solid particle size or baffles (3) particle recovery by means of cyclones (4) heat transfer tubes (5) solids circulation systems (6) instrumentation, corrosion and erosion, mathematical models, pilot plants and scale-up techniques. 

Erosion is a serious problem in solids circulation systems It is minimized by good design and the use of anti-erosion plates at critical points. Without precautions, fluid coke and iron ore, for instance, can erode a transport line in a matter of hours. 

The centrifugal separation of solid impurities is adopted either as an alternative to filtration or combined with it. For example, having fixed-element filters that arrest larger particles, and a centrifuge system that removes the finer solids in suspension together with any water contained in the oil can clean a lubricant circulating system. 

Chemicals are added to the circulation system to prevent fouling. Dispersants are added to prevent deposit of solids, corrosion inhibitors to prevent corrosion and biocides to inhibit biological growth. 

New applications and novel reactor configurations or operational modes for three-phase systems are continually being reported. These include the operation of a three-phase fluidized bed in a circulatory mode (Liang et al., 1995), similar to the commonly applied gas-solid circulating fluidized bed the application of a three-phase fluidized bed electrode that can be used as a fuel cell (Tanaka et al., 1990) magnetically stabilized three-phase fluidized beds centrifugal three-phase reactors and airlift reactors. 

Many circulating systems are characterized by an external cyclone return system that usually has as large a footprint as the reactor itself. The axial solids density profile is relatively flat, as indicated in Fig. 17-6. There is a parabolic radial solids density profile that is termed core... 

The 1-butene isomerization and the ethylene oligomerization were investigated as test reactions for solid acid sites of the Ni2+-substituted smectite catalysts. Both reactions were carried out in a closed circulating system. After pre-evacuation of a catalyst in the reactor at various temperatures for 1 h, the reactions were conducted by circulating the reactants. Products were analyzed by a gas chromatograph. 

In the present study, the effects of the initial TCE concentration, wavelength of the UV light, H2O concentration, O2 concentration, the ratio of TiCVsilica gel, superficial gas velocity (C/g), solid circulation rate (Gs) on photodegradation of TCE have been determined in a CFB photoreactor system. 

The solid circulation rate (Gs, kgm-2s-1) can be controlled by aeration gas in the loop-seal to adjust pressure balance in the CFB loop. Solid circulation rate is a unique operating variable in the CFB system as a function of pressure drop across loop-seal of a CFB photoractor [10] and directly related to solid holdup in the riser where the photodegradation of TCE occurs. 

Standpipes are used in the gas-solid circulating flow systems in which a cyclone-standpipe-valve configuration for the separation and reintroduction of particles, as shown in Fig. 8.18, is constantly encountered. The satisfactory operation of the cyclone-standpipe system depends on a small leakage flow of gas up the standpipe. A simple method to estimate... 

Example 8.3 In an operation of gas-solid circulating flow in a cyclone-standpipe-valve system, the particles are 100 p.m glass beads with a density of 2,500 kg/m3. The particle volume fraction in the standpipe is 0.55. The gas is air with a viscosity of 1.8 x 10-5 kg/m s and a density of 1.2 kg/m3. The particle mass flow is 70 kg/m2 s. The height of solids in the standpipe is 1.4 m. The total pressure head over the standpipe and valve is 4,500 Pa. Estimate the leakage flow of air in the standpipe. If the particle volume fraction at minimum fluidization is 0.5 and the area ratio of valve opening to pipe cross section is 0.6, what is the orifice coefficient of this valve ... 

The entrance and exit geometries have significant effects on the gas and solid flow behavior in the riser. The efficiency of the gas-solid separator can affect the particle size distribution and solids circulation rate in the system. In a CFB system, particle separation is typically achieved by cyclones (see Chapter 7). The downcomer provides hold volume... 

The key to smooth operation of a CFB system is the effective control of the solids recirculation rate to the riser. The solids flow control device serves two major functions, namely, sealing riser gas flow to the downcomer and controlling solids circulation rate. Both mechanical valves or feeders (see Figs. 10.1(a) and (d)) and nonmechanical valves (see Figs. 10.1(b) and (c)) are used to perform these functions. Typical mechanical valves are rotary, screw, butterfly, and sliding valves. Nonmechanical valves include L-valves, J-valves (see Chapter 8), V-valves, seal pots, and their variations. Blowers and compressors are commonly used as the gas suppliers. Operating characteristics of these gas suppliers which are directly associated with the dynamics and instability of the riser operation must be considered (see 10.3.3.2). 

The flow behavior in the riser varies with gas velocity, solids circulation rate, and system geometry. On the basis of the flow behavior, the fast fluidization regime can be distinguished from neighboring regimes. 

For a given system, t/tf and Ua are functions of the solids circulation rate. Rearranging Eq. (10.2) gives... 

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