Flow control solids

Disturbances in both throughput and feed composition were used to evaluate the effectiveness of this alternative control stmcture. Figure 15.11 shows the responses to 20% step changes in the setpoint of the methanol feed flow controller. Solid lines are increases, and dashed lines are decreases. The increase is handled qirite well. It is surprising that the decrease is not handled well. There is a significant transient drop in MTBE purity in the bottoms product (middle left graph. Fig. 15.11), whieh lasts for several hours. In addition and of more importance, there is a very large increase in the isobutoie lost in the distillate. 

Standpipes, Solids Feeders, and Solids Flow Control. 17-10... 

FIG. 17-16 Solids-flow-control devices, a) Slide valve, (h) Rotary valve, (c) Table feeder, (d) Screw feeder, (e) Cone valve, (f ) L Valve. 

When the solid is one of the reactants, such as in ore roasting, the flow must be continuous and precise in order to maintain constant conditions in the reactor. Feeding of free-flowing granular solids into a fluidized bed is not difficult. Standard commercially available sohds-weighiug and -conveying equipment can be used to control the rate and dehver the solids to the feeder. Screw conveyors, dip pipes, seal legs, and injectors are used to introduce the solids into the reactor... 

Seal legs are frequently used in conjunction with solids-flow-control valves to equ ize pressures and to strip trapped or adsorbed gases from the sohds. The operation of a seal leg is shown schemati-caUy in Fig. 17-19. The sohds settle by gravity from the fluidized bed into the seal leg or standpipe. Seal and/or stripping gas is introduced near the bottom of the leg. This gas flows both upward and downward. Pressures indicated in the ihustratiou have no absolute value but are only relative. The legs are designed for either fluidized or settled solids. 

The L valve is shown schematicaUy in Fig. 17-20. It can act as a seal and as a sohds-flow control valve. However, control of sohds rate is only practical for solids that deaerate quickly (Geldart B and D). The... 

Sedimentation Tanks These tanks are an integral part of any activated-sludge system. It is essential to separate the suspended solids from the treated liquid if a high-quality effluent is to be produced. Circular sedimentation tanks with various types of hydraulic sludge collectors have become the standard secondary sedimentation system. Square tanks have been used with common-wall construc tion for compact design with multiple tanks. Most secondary sedimentation tanks use center-feed inlets and peripheral-weir outlets. Recently, efforts have been made to employ peripheral inlets with submerged-orifice flow controllers and either center-weir outlets or peripheral-weir outlets adjacent to the peripheral-inlet channel. 

In metals the situation is quite the opposite. The spherical atoms move easily from liquid to solid and the interface moves quickly in response to very small undercoolings. Latent heat is generated rapidly and the interface is warmed up almost to T, . The solidification of metals therefore tends to be heat-flow controlled rather than interface controlled. 

Primarily for flow controlled requirements, such as solids suspensions, heat transfer, and other high pumping efficiency applications. 

Drexel, C. F., Digital Mass Flow Controller Come of AgeJ Solid State Technology, pp. 99-106 (Nov. 1996)... 

To address these challenges, chemical engineers will need state-of-the-art analytical instruments, particularly those that can provide information about microstmctures for sizes down to atomic dimensions, surface properties in the presence of bulk fluids, and dynamic processes with time constants of less than a nanosecond. It will also be essential that chemical engineers become familiar with modem theoretical concepts of surface physics and chemistry, colloid physical chemistry, and rheology, particrrlarly as it apphes to free surface flow and flow near solid bormdaries. The application of theoretical concepts to rmderstanding the factors controlling surface properties and the evaluation of complex process models will require access to supercomputers. 

Figure 25-8. Control of adipose tissue lipolysis. (TSH, thyroid-stimulating hormone FFA, free fatty acids.) Note the cascade sequence of reactions affording amplification at each step. The lipolytic stimulus is "switched off" by removal of the stimulating hormone the action of lipase phosphatase the inhibition of the lipase and adenylyl cyclase by high concentrations of FFA the inhibition of adenylyl cyclase by adenosine and the removal of cAMP by the action of phosphodiesterase. ACTFI,TSFI, and glucagon may not activate adenylyl cyclase in vivo, since the concentration of each hormone required in vitro is much higher than is found in the circulation. Positive ( ) and negative ( ) regulatory effects are represented by broken lines and substrate flow by solid lines.

The L valve is shown schematically in Fig. 17-21. It can act as a seal and as a solids-flow control valve. However, control of solids rate is only practical for solids that deaerate quickly (Geldart B and D solids). The height at which aeration is added in Fig. 17-21 is usually one exit pipe diameter above the centerline of the exit pipe. For L-valve design equations, see Yang and Knowlton [Powder Tech., 77, 49-54 (1993)])... 

Flow Measurements Measurement of flow rates of clean gases presents no problem. Flow measurement of gas streams containing solids is almost always avoided. The flow of solids is usually controlled but not measured except solids flows added to or taken from the system. Solids flows in the system are usually adjusted on an inferential basis (temperature, pressure level, catalyst activity, gas analysis, heat balance, etc.). In many roasting operations, the color of the calcine discharge material indicates whether the solids feed rate is too high or too low. 

When in plug flow all solids stay in the reactor for the same length of time. From this and the kinetics for whatever resistance controls, the conversion X R) for any size of particle R can be found. Then the mean conversion of the solids leaving the reactor can be obtained by properly summing to find the overall contribution to conversion of all sizes of particles. Thus,... 

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