Solids residence time

The average composition of the stream leaving a continuous mixer is the same as the average of the added entering streams. Variations in proportions of the entering streams will be damped out by the mixing action of a continuous mixer. These effluent-stream variations will become smaller as average solids residence time is increased and the frequency of the variations increases. 

The Effect of Residence Time The final parameter that was studied was the solid residence time. In the semi-continuous reactor used for this study, the volatile product is swept from the reactor by a continuous stream of hydrogen and, therefore, there is both a vapour and solid residence time. It is this latter parameter that has been studied here and the solid residence time was considered to be the time that the reactor spends at temperature. For the study of the residence time, tin (1 % of the coal) as stannous chloride was used as the catalyst and the other conditions are given in Table II. 

The changes that occur with solid residence time in the hot-rod reactor were not very pronounced because only the nonvolatile portion of the oil remaining on the coal bed would be expected to undergo secondary reactions such as aromatization and loss of heteroatoms. However, the oils from the hot-rod reactor were also compared with those obtained in a rotating autoclave with much longer solid and vapour residence times and the changes with residence time were more noticeable in this case as can be seen in Table II. 

Figure 4.2 presents a simplified flow diagram of the ENCOAL Liquid from Coal (LFC) process. The process upgrades low-rank coals to two fuels, Process-Derived Coal (PDF ) and Coal-Derived Liquid (CDL ). Coal is first crushed and screened to about 50 mm by 3 mm and conveyed to a rotary grate dryer, where it is heated and dried by a hot gas stream under controlled conditions. The gas temperature and solids residence time are controlled so that the moisture content of the coal is reduced but pyrolysis reactions are not initiated. Under the drier operating conditions most of the coal moisture content is released however, releases of methane, carbon dioxide, and monoxide are minimal. The dried coal is then transferred to a pyrolysis reactor, where hot recycled gas heats the coal to about 540°C. The solids residence time... 

Add a calculation of the operating parameters substrate consumption rate to micro-organisms ratio (Load), solids residence time (0m) and sludge age ((),) to the program. Experiment with variations in the values of these parameters under various conditions of flow and feed concentration to test their value as process control parameters. 

In comparison to incineration, the STRATEX technology does not remove nonvolatile organics such as plastics from treated material. Among the factors listed as items of concern for the scaled-up model of the STRATEX system are the depth of the soil (or other solids) found in the STRATEX device during or after treatment and maintenance of the solids residence time at the proper temperature. 

The E-concept permits scale-up between similar centrifuges solely on the basis of sedimentation performance. Other criteria and limitations, however, should also be investigated. Scale-up analysis for a specified solids concentration, for instance, requires knowledge of solids residence time, permissible accumulation of solids in the bowl, G level, solids conveyability, flowability, compressibility, limitations of torque, and solids loading. Extrapolation of data from one size centrifuge to another calls for the application of specific scale-up mechanisms for the particular type of centrifuge and performance requirement. 

Possibility of optimizing independently in each step (reaction and regeneration) the reaction parameters, such as the reactant concentration, the gas residence time and the solid residence time and the temperature. 

CFBs as reactors, the solids residence time is an important parameter. Previous studies mostly assess operations at moderate values of the solids circulation rates (<100kg/m2s), whereas gas-catalytic reactions and, e.g., biomass pyrolysis require completely different operating conditions. 

Hydrocarbonization processes are characterized by three primary independent variables - temperature, hydrogen pressure, and coal type - and five other, important independent variables -solid residence time, gas residence time, reactor configuration, coal pretreatment, and catalyst impregnation. Control of these variables permits control, over a wide range, of (1) the relative yields of liquid, gaseous, and solid products, (2) the quality of one or more of these products, (3) hydrogen consumption, and, ultimately (4) product cost. 

In a similar way, the composition of product gas is influenced by process conditions. The percentage of carbon dioxide in the gas decreases with increasing temperature, hydrogen pressure, or solid residence time. The percentage of carbon... 

In the development of these processes and their transference into an industrial-scale, dimensional analysis and scale-up based on it play only a subordinate role. This is reasonable, because one is often forced to perform experiments in a demonstration plant which copes in its scope with a small produdion plant ( mock-up plant, ca. 1/10-th of the industrial scale). Experiments in such plants are costly and often time-consuming, but they are often indispensable for the lay-out of a technical plant. This is because the experiments performed in them deliver a valuable information about the scale-dependent hydrodynamic behavior (arculation of liquids and of dispersed solids, residence time distributions). As model substances hydrocarbons as the liquid phase and nitrogen or air as the gas phase are used. The operation conditions are ambient temperature and atmospheric pressure ( cold-flow model ). As a rule, the experiments are evaluated according to dimensional analysis. 

Figure 7. Flash hydrogasification of coal maximum percentage of carbon conversion to CHk vs. pressure (at 900°C). Solid residence time (ts) gas residence time (tg) North Dakota lignite (O) New Mexico subbituminous (O).

The model was developed from experiments conducted in a downflow reactor, where gas residence times are greater than solid residence times. The model may not be able to predict residence time effect in a fluid bed or upflow reactor where solid residence times are greater than gas residence times. But it does give useful information for pressure and temperature effects in these reactors. 

Primary air flow, supplied at the bottom of the bed, is insufficient for complete combustion of the fuel. The zone below the elevation of secondary-air jets is endothermic, cracking oil to yield carbon and fuel gas species. These burned in the upper, exothermic zone. Alumina product is withdrawn from a standpipe receiving solid from the upper zone, and burn-off of carbon in this zone is sufficient to yield a product that is acceptably white. Fluidizing-gas velocity being lower in the primary combustion zone than in the secondary, density is higher. Provision of the two zones accomplishes two purposes (1) affording a sufficient solid residence time in the primary zone and (2) reducing horsepower needed for air compression. 

In the Lurgi fast-bed calciner, in contrast, Reh wished to achieve the lowest practicable horsepower for compression. He wished to maintain a powder inventory at the lowest level necessary to achieve both the desired solid residence time and complete combustion of fuel. He welcomed a decline in density with bed height. 

Patience, G. S., Chaouki, J., and Kennedy, G. Solids residence time distribution in CFB reactors, in Circulating Fluidized Bed Technology III (P. Basu, M. Horio and M. Hasatani, eds.), pp. 599-604. Pergamon Press, 1991. 

The expansion of the bed height is determined by bubbles behavior and bed temperature. The residence time of gases in the fluidized bed is merely estimated with the gas flow rate and the cross sectional area of the reactor bed. The char is recirculated in each compartment to adjust solids residence times. A small fraction of char, found from the eluiriation model [18], is allowed to escape to the above stage. 

Batch Dryers If the batch size is stipulated, the requirement is simply that the dryer be able to physically contain the volume of the solids, and the dryer volume and dimensions can thus be calculated directly. Solids residence time must then be calculated. Equation (12-61) can be reversed and modified to give... 

Papadakis et al. [Dry. Tech. 12(1 2) 259 (1994)] rearranged the Matchett and Baker model from its original parallel form into a more computationally convenient series form. The sum of the calculated residence times in the airborne and dense phases, Xg and Xj, respectively, is the total solids residence time. The dryer length is simply the sum of the distances travelled in the two phases. 

The method gives no information about solids residence time or dryer length. A minimum drying time min can be calculated by evaluating the maximum (unhindered) drying rate Ncr, assuming gas-phase heat-transfer control and estimating a gas-to-solids heat-transfer coefficient. The simple equation (12-60) then applies ... 

Drying kinetics (drying too slowly, or solids residence time in dryer too short)... 

For reactions where the particles themselves react, substantial particle mixing, greatly broadening solid residence time distributions relative to moving beds... 

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