Saltation

Figure 6-32, taken from Govier and Aziz, schematically indicates four flow pattern regions superimposed on a plot of pressure gradient vs. mixture velocity = Vl -t- V5 = Qj + ( s)/A where and Vs are the superficial liquid and solid velocities, Qi, and ( 5 are liquid and solid volumetric flow rates, and A is the pipe cross-sectional area. is the transition velocity above which a bed exists in the bottom of the pipe, part of which is stationary and part of which moves by saltation, with the upper particles tumbling and bouncing over one another, often with Formation of dunes. With a broad particle-size distribution, the finer particles may be fully suspended. Near V 4, the pressure gra-... 

The lift pipe design was tapered to a larger diameter at the top. This minimized the effects of erosion and catalyst attrition, and also prevented the instantaneous total collapse of circulations when the saltation concentration, or velocity, of solids is experienced (i.e. the slump veloeity-that velocity helow which particles drop out of the flowing gas stream). In a typical operation, 2 % to 4 % eoke can he deposited on the catalyst in the reactor and burned in the regenerator. Catalyst circulation is generally not sufficient to remove all the heat of eombustion. This facilitated the need for steam or pressurized water coils to be located in the regeneration zone to remove exeess heat. 

As shown in Fig. 14.2, the material conveying region is bounded by the air-only curve and the stationary-plug curve, where the air merely percolates through or flows above a packed bed of stationary particles. Dense-phase conveying occurs when v, the velocity of air, is below the so-called saltation velocity. 

The dense-phase regime can be further subdivided into three distinct regions,which are shown in Fig. 14.3. In continuous dense-phase flow the material moves by saltation over a stable creeping bed, in discontinuous dense-phase flow particles move as groups, and in the solid dense-phase the solids are extruded through the pipe as a continuous slag. 

Since the pre-Socrates philosophers, who two and half thousand years ago tried to understand the world with reason, there has been a winged word, that was especially known in its Latin form Natura non saltat Leukippfith centuty BC) — nature doesn t jump. All experience has shown that in nature one thing always leads to another. Changes occur smoothly and "flow" into each other. Of course, occasionally there is some turbulence, but mostly at the edges and without any influence on the main-... 

But it was Max Planck who shattered the paradigm of the steadiness of nature. He showed that atoms could not absorb energy in all forms and quantities, but only in so-called quanta, that is, in defined amounts. Thus, electrons jump, as we explain it today, from one energy level to another. Natura saltat Albert Einstein s theory was even more groundbreaking space and time form a continuum, matter and energy, in contrast, are quantized, essentially "grainy", so to speak. In this case, nature cannot but jump. 

Dunes and sand plains form where strong winds carry sand grains in saltation over short distances. Particles finer than sand are transported in suspension and over greater distances until they settle as loess , predominantly in the steppe regions adjacent to the desert zone. 

One major difference between pneumatic transport and hydraulic transport is that the gas-solid interaction for pneumatic transport is generally much smaller than the particle-particle and particle-wall interaction. There are two primary modes of pneumatic transport dense phase and dilute phase. In the former, the transport occurs below the saltation velocity (which is roughly equivalent to the minimum deposit velocity) in plug flow, dune flow, or sliding bed flow. Dilute phase transport occurs above the saltation velocity in suspended flow. The saltation velocity is not the same as the entrainment or pickup velocity, however, which is approximately 50% greater than the saltation velocity. The pressure gradient-velocity relationship is similar to the one for hydraulic transport, as shown in... 

There are a variety of correlations for the saltation velocity, one of the most popular being that of Rizk (1973) ... 

Here ps is the solids loading (mass of solids/mass of gas), Vgs is the saltation gas velocity, and d is the particle diameter in mm. (It should be pointed out that correlations such as this are based, of necessity, on a finite range of conditions and have a relatively broad range of uncertainty, e.g., 50-60% is not unusual.)... 

Material flow characteristics particles suspended in the airstream saltated flow plug flow... 

However, the PVC powder was tested in a 52 mm internal diameter pipeline, 71 m in length, and found to exhibit unstable plugging in the vicinity of saltation or minimum pressure (i.e., prior to the fluidized dense-phase region). That is, dilute-phase transport was only possible on this test rig. Also, solids/gas loadings were quite low (e.g., max m 20). Note that the unstable plugging was accompanied by sudden increases in pressure and severe pipe vibrations. 

As a result of some recent investigations into the deposition (saltation) of small particles in large diameter ducts (Cable, 1994 Miletich, 1994) and analogous work in pneumatic conveying (Wypych and Reed, 1990 Zenz, 1964 Cabrejos andKlinzing, 1994), some other interesting characteristics of Vfmin are listed below. 

Cabrejos, F. J., and Klinzing, G. E., Minimum Conveying Velocity in Horizontal Pneumatic Transport and the Pickup and Saltation Mechanisms of Solid Particles, Bulk Solids Handling, 14(3) 541-550 (1994)... 

Sterk G, Lopez MV, Arrue JL (1999) Saltation transport on a silt loam soil in northeast Spain. Land Degrad Dev 10 545-554... 

Saltation is also used to transport settling slurries through pipelines [Condolios and Chapus (1963a)]. In this case the solid particles bounce and roll along the bottom of a horizontal pipe. 

Wind (aeolian) transport (relocation by wind) can also occur and is particularly relevant when catalyst dust and coke dust are considered. Dust becomes airborne when winds traversing arid land with httle vegetation cover pick up small particles such as catalyst dust, coke dust, and other refinery debris and send them skyward. Wind transport may occur through suspension, saltation, or creep of the particles. 

A polyketide/fatty acid-type metabolic route using three propionate units has been shown by stable isotope-labeled probes and MS to the biosynthetic route towards the production of the insect pheromone (Sl-d-methyl-d-heptanone in the ant Harpegnathos saltator. 

Mechanism for saltational shifts in pheromone communication system. Proceedings of the National Academy of Sciences, USA 99 13368-13370. 

Saltation The movement of sediment particles through a series of bounces caused by flowing wind or liquid water. 

Sklar LS, Dietrich WE (2004) A mechanistic model for river incision into bedrock by saltating bed load. Water... 

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