Stagnant pockets

There are cases where W/D = 1/8 and J/D = 1/10 for some agitator correlations. Usually, 4 baffles are used and the clearance between the baffles and the wall is about 0.1-0.15 J. This ensures that the liquid does not form stagnant pockets between the baffle and the wall. The number of impeller blades varies from 4 to 16, but is generally between 6 and 8. 

In addition, they are usually constructed without isolation valves on the fuel supply lines. As a result the final connection in the pipework cannot be leak-tested. In practice, it is tested as far as possible at the manufacturer s works but often not leak-tested on-site. Reference 32 reviews the fuel leaks that have occurred, including a major explosion at a CCGT plant in England in 1996 due to the explosion of a leak of naphtha from a pipe joint. One man was seriously injured, and a 600-m chamber was lifted off its foundations. The reference also reviews the precautions that should be taken. They include. selecting a site where noise reduction is not required or can be achieved w ithout enclosure. If enclosure is essential, then a high ventilation rate is needed it is often designed to keep the turbine cool and is far too low to disperse gas leaks. Care must be taken to avoid stagnant pockets. 

Taylor (T4, T6), in two other articles, used the dispersed plug-flow model for turbulent flow, and Aris s treatment also included this case. Taylor and Aris both conclude that an effective axial-dispersion coefficient Dzf can again be used and that this coefficient is now a function of the well known Fanning friction factor. Tichacek et al. (T8) also considered turbulent flow, and found that Dl was quite sensitive to variations in the velocity profile. Aris further used the method for dispersion in a two-phase system with transfer between phases (All), for dispersion in flow through a tube with stagnant pockets (AlO), and for flow with a pulsating velocity (A12). Hawthorn (H7) considered the temperature effect of viscosity on dispersion coefficients he found that they can be altered by a factor of two in laminar flow, but that there is little effect for fully developed turbulent flow. Elder (E4) has considered open-channel flow and diffusion of discrete particles. Bischoff and Levenspiel (B14) extended Aris s theory to include a linear rate process, and used the results to construct comprehensive correlations of dispersion coefficients. 

Turner (T14) has proposed two detailed models of packed beds which try to closely approximate the true physical picture. The first model considers channels of equal diameter and length but with stagnant pockets of various lengths opening into the channels. There is no flow into or out of these pockets, and all mass transfer occurs only by molecular diffusion. The second model considers a collection of channels of various lengths and diameters. We will briefly discuss each of these models, which are probably more representative of consolidated porous materials than packed unconsolidated beds. 

In a model for the structure of packed beds, Turner (T14, T15) and Aris (A9, AlO) have also used stagnant pockets with crossflow by only molecular diffusion. 

The longitudinal diffusion coefficient in flow through a tube with stagnant pockets. Chem. Eng. Sci 11,194-198 (1959). 

Fig. 4.19. Trickle-flow of liquid over a bed of particles, showing possible complications of stagnant pockets of liquid and dry zones due to poor wetting of the solid surface...

Residence time distribution measurements, together with a theoretical model, provide a method to calculate the rate of mass transfer between the liquid flowing through the column, the dynamic holdup, and the stagnant pockets of liquid in between the particles. We have chosen the cross flow model (10). It has to be noted that the model starts from the assumption that the flow pattern has a steady-state character, which is in conflict with reality. Nevertheless, average values of the number of mass transfer units can be calculated as well as the part of the liquid being in the stagnant situation. 

Another model worth considering is to assume all the deadwater resides in the stagnant pockets in bubble wakes. Here, a moving coordinate system would be used, taking the bubble swarm velocity to be U. For this model, equation (13) is replaced by the distributed parameter equation ... 

The precipitated mud is removed in a settling device known as a clarifier. The better known clarifiers in the market are Rapi Dorr 444, the Graver clarifier, and Prima sap, Bach clarifier. Poly-cell, and BMA clarifier. Basically, a clarifier consists of a vertical cylindrical vessel composed of a number of trays with conical bottoms stacked one over the other. The limed raw juice enters near the center of each tray and flows toward a circumference. A sweep arm in each tray turns quite slowly and sweeps the settled mud toward a central mud outlet. The clear juice from the top circumference overflows into a collection compartment. With more uniform juice takeoff, the potential stagnant pockets are eliminated. 

Case 2. The space-time X is unknown. This situation arises when there are dead or stagnant pockets that exist in the reactor along with the dispersion effects. To analyze this situation we first calculate tm and from the data as in case 1. Next we solve Eqn. (14-31) for X and substitute for x in Eqn. (14-32) to solve for Pe. Knowing Pe, we can solve Eqn. (14-31) for X, and hence V. The dead volume is the difference between the measured volume (i.e, with a yardstick) and the volume calculated from the RTD. 

DEHUMIDIFYING CONDENSERS. A condenser for mixtures of vapors and noncondensable gases is shown in Fig. 15.9. It is set vertically, not horizontally like most condensers for vapor containing no noncondensable gas also, vapor is condensed inside the tubes, not outside, and the coolant flows through the shell. This provides a positive sweep of the vapor gas mixture through the tubes and avoids the formation of any stagnant pockets of inert gas that might blanket the heat-transfer surface. The modified lower head acts to separate the condensate from the uncondensed vapor and gas. 

Similar observations were made by Agarwal et al. (I) in operating a fluidized coal dryer. They noted that combustion occurred on the air distributor deck when stagnant pockets of coal caused by poor gas distribution reached temperatures of about 800°F. Consequently the practical pretreatment temperature range is quite narrow—between 725° and 750°F. The necessary close temperature control can be maintained with a fluidized bed. 

Based on this coefficient and assuming the validity of Pick s law, it is calculated that at 400°P. and for humid-gas concentrations in the 9 to 50% water range, approximately 400 hours would be required for a 1-inch thick pocket of sodium to react. This order of magnitude calculation indicates the desirability of extreme turbulence in recleaning. To avoid high temperatures, such turbulence should best be introduced by a humid gas of low-water content or after the bulk of the thin sodium films of a system have been reacted. The diffusion rate also points out the undesirability of relatively deep, stagnant pockets. 

The flow path should preferably be tapered. This maintains high velocities and avoids collection of noncondensables in stagnant pockets. 

For chemical reaction to control, 0.01 and, in that case, any increase in agitation over and above that necessary just to completely suspend the particles so that all of the surface area. A, is available for reaction fails to produce any further increases in reaction rate. Indeed, for an extremely slow reaction step , even complete suspension of the particles may be unnecessary with a general and gentle turnover of the solids to prevent stagnant pockets being all that is required. 

If sea water is to be used a difficult choice of materials for the condenser may present itself since, although the commonly used 316SS is satisfactory for both shell and tube sides, this would only be true for the tube side if a high flow rate were maintained at all times with no stagnant pockets. 

Super Intelox Toiwer Paddng (Rg. 8-2I g This packing was developed by the Norton Company, llie smooth ec es of the Intalox saddle wexe scalloped in the Super Intalox and holes were inserted. These changes promote drainage of liquid, eliminate stagnant pockets, and provide more open area for vapor rise. The Super Intalox was shown (2) to have higher capacity and h er efikieiuy compared to the Intalox saddle. 

In some cases = 5 for agitator correlations. The number of baffles is 4 in most uses. The clearance or gap between the baffles and the wall is usually 0.10 to 0.15 J to ensure that liquid does not form stagnant pockets next to the baffle and wall. In a few correlations the ratio of baffle to tank diameter is JID, = instead of... 

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