Solids recycle rate

Note that in simplified case, the velocity scaling is not uniquely tied to just the gas properties as it is in the full scaling relationship. With uQ and ps set, the solids recycle rate can be determined by... 

Biological solids retention time (Oc) has been suggested in this paper as the kinetic based parameter of choice for use in design and control of fiuidized culture continuous fiow biological processes. The value of Oc selected for design of the process, (Oc ), directly determines the volume of reactor needed for a conventional digester system and a given waste fiow (Q) since the value of Oc is equal to the hydraulic retention time (0). The relationship between Oc and the reactor volume for the system with recycle is more complex and involves consideration of the effects of solids recycle rate and recycle solids concentration. 

Ideally. CFB risers operate at a relatively uniform temperature, which is achieved by a high solids recycle rate renewing the inventory of the riser. Catalytic reactions are generally carried out at relatively low temperatures (250-650 C) compared to combustion processes (>800°C). Low-temperature operation permits the use of mechanical devices to control solids mass flux. In combustion processes, the rate is controlled by non-mechanical devices. Fluid mechanics of CFB catalytic reactors and combustors are significantly different, as shown in Table 2. 

Zhang et al. (1991) carried out investigations with three different fast bed systems with diameters of 32, 90, and 300 mm. They found that the radial voidage distribution, given as a ratio to the cross-sectional average, was independent of bed diameter and solids recycle rate. The similarity does not hold at transition... 

Glicksman et al. (1995) used the simplified scaling parameters to construct a one-half linear scale model of a Foster Wheeler circulating bed combustor pressurized to 14 bar. The combustor has a 20.3 cm inner diameter with an overall height of 8.3 m with both a primary and a secondary air supply. The solids recycle rate was accurately determined by a calorimetric balance of a fluidized bed heat exchanger in the return loop of the circulating bed. The cold model, one-half scale, used polyethylene plastic particles to match the dimensionless particle size distribution as well as the gas-to-solid density ratio. 

Gs = Solids recycle rate per unit area g = Acceleration of gravity... 

Basu, P., J. Greenblatt, S. Wu and D. Briggs (1989), "Effect of Solid Recycle Rate, Bed Density and Sorbent Size on the Sulfur Capture in a Circulating Huidized Bed Combustor", in Proc. of 10th Int. Conf. on Fluidized Bed Combustion, San Francisco, 701-707... 

Of the 200 million tons of municipal solid waste collected in the United States in 1993 (1), 22% was recycled while 62% was placed in landfills and 16% incinerated (2). Plastics comprised 9.3% of these materials. The number of U.S. residential collection programs increased from 1,000 in 1988 to more than 7,000 involving more than 100 million people in 1993 (2). Approximate 1994 U.S. recycling rates are given in Table 1. 

Solid flow rate Recycling flow rate Eluent flow rate Extract flow rate Eeed flow rate Raffinate flow rate ... 

Increasing the switch time interval is equivalent to decrease the solid flow rate and the net fluxes of components in all sections of the TMB unit will be pushed in the same direction of the liquid phase. This implies that, first, the more retained species will move upwards in section III and will contaminate the raffinate stream and the less retained species will move upwards in section IV, will be recycled to section I, and will contaminate also the extract stream. The decrease of the switch time interval will have similar consequences. The equivalent solid flow rate will increase and the net fluxes of component in all four sections of the TMB unit will be pushed in the opposite direction of the liquid phase. This implies that, first, the less-retained species will move downwards in section II and will contaminate the extract stream and the more retained component will also move downwards in section I, will be recycled with the solid to the section IV, and will contaminate the raffinate stream. It is possible to obtain simultaneously high purities and recoveries in a SMB, but the tuning must be carefully carried out. 

In addition, both the recycling and the solid flow rates are also kept constant. 

Figure 11.10(b) can be modeled as a piston flow reactor with recycle. The fluid mechanics of spouting have been examined in detail so that model variables such as pressure drop, gas recycle rate, and solids circulation rate can be estimated. Spouted-bed reactors use relatively large particles. Particles of 1 mm (1000 pm) are typical, compared with 40-100 pm for most fluidizable catalysts. 

Solid catalysts can be conveniently studied in loop reactors, which allow measuring the reaction rates by concentration difference measurement across the catalyst bed. When operated continuously with high recycle rate, the entire loop usually can be modelled as a single well-stirred tank. Here the case of catalyst deactivation is studied. 

The experiments were repeated with a mixture of 60% small-diameter pellets and 40% low-density GPPS recycle material. The bulk density for this feedstock was measured at 0.10 g/cm a bulk density that was about 40% less than that for the commercial pellet-low-density recycle blend. This relatively large difference in density was attributed to the variability of the recycle material density. As indicated by the data in Table 12.11, the rate with no ledge was 20 kg/h, a rate that was about 30% less than that for the commercial pellet blend. Like before, the rate difference is primarily due to the differences between the feedstock bulk densities. When the ledge plates were positioned in the equipment, the solids-conveying rate was about 75% of the original rate. This rate decrease is very similar to the rate decrease that was experienced with the 114 mm diameter commercial extruder. Recall that the commercial extruder was operating at the maximum screw speed and at a rate that was only about 60% of the expected rate. 

Table 12.11 Solids-Conveying Rates for a Mixture of 60% Small-Diameter GPPS Pellets and 40% of a Low-Density GPPS Recycle at 50 rpm... 

Figure 5 shows typical test facilities used by different investigators. In configuration, they all consist of a fast column or riser, a gas-solids separator, a downcomer for solids recycle, and a loop seal valve and/or an additional controlling device for adjusting solids circulation rate, mounted in positions appropriate with the inlet and outlet geometry. These facilities can be grouped into three types. 

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