Turbulence promotion

Pulsed Columns. The efficiency of sieve-plate or packed columns is increased by the appHcation of sinusoidal pulsation to the contents of the column. The weU-distributed turbulence promotes dispersion and mass transfer while tending to reduce axial dispersion in comparison with the unpulsed column. This leads to a substantial reduction in HETS or HTU values. 

Spiral-wound cartridges are inserted ia series into cylindrical pressure vessels. Feed flows parallel to the membrane surfaces ia the channel defined by the mesh spacer which acts as a turbulence promoter. Permeate flows into the center permeate-withdrawal tube which is sealed through the housing end caps. 

Note that the group on the left side of Eq. (14-182) is dimensionless. When turbulence promoters are used at the inlet-gas seclion, an improvement in gas mass-transfer coefficient for absorption of water vapor by sulfuric acid was obsei ved by Greenewalt [Ind. Eng. Chem., 18, 1291 (1926)]. A falhug off of the rate of mass transfer below that indicated in Eq. (14-182) was obsei ved by Cogan and Cogan (thesis, Massachusetts Institute of Technology, 1932) when a cauTiiug zone preceded the gas inlet in ammonia absorption (Fig. 14-76). 

Narezhnyy, E. G., and Sudarev, A. V. (1971). Local heat transfer in air flowing in tubes with a turbulence promoter at the inlet. Heat Transfer 3, 62-66. 

Spiral-wound modules consist of several flat membranes separated by turbulence-promoting mesh separators and formed into a Swiss roll (Figure 16.18). The edges of the membranes are sealed to each other and to a central perforated tube. This produces a cylindrical module which can be installed within a pressure tube. The process feed enters at one end of the pressure tube and encounters a number of narrow, parallel feed channels formed between adjacent sheets of membrane. Permeate spirals roward the perforated central tube for collection. A standard size spiral-wound module has a diameter of about 0.1m, a length of about 0.9 m and contains about 5 m2 of membrane area. Up to six such modules may be installed in series in a single pressure tube. These modules make better use of space than tubular or flat sheet types, but they are rather prone to fouling and difficult to clean. 

The constant coefficient for float C arises from turbulence promotion, and for this reason the coefficient is also substantially independent of the fluid viscosity. The meter can be made relatively insensitive to changes in the density of the fluid by selection of the density of the float, pf. Thus the flowrate for a given meter will be independent of p when dG/dp = 0. 

Pratt, H.R.C. Trans. Inst. Chem. Eng. 28 (1950) 77. The application of turbulent How theory to transfer processes in tubes containing turbulence promoters and packings. 

The spacer grid in a rod bundle is also a turbulence promoter that enhances liquid-vapor exchange and bubble condensation. The local intensity of such turbulence is a function of the grid pressure loss coefficient, K, and the distance from the grid, t D. Thus an empirical spacer factor, Fs, can be defined as... 

Turbulent mass-transfer relations concerning forced convection are of interest for two main reasons (see Table VII, Part D) (a) because of their practical importance, since turbulence promotes increase of transfer rates and (b) they afford an indirect means of gaining insight into the mechanism... 

Hicks and Mandersloot (H4) investigated flow systems with turbulence promoters, where the orientation of the promoters (or packed irregular particles) is an important parameter. In such systems the Reynolds number,... 

They all improve mass tansport within the cell via the mechanism of efficient turbulence promotion and also greatly increase the electrode area. 

The spherical packings are too large to serve as effective targets for the deposition of fine dust particles. In dust-collection service, the packings actually serve as turbulence promoters, while the dust particles are collected primarily by the liquid droplets. 

The density of crude oil is on the order of 0.85 g/cm, so if the sea surface is calm, an oil spill will initially form a slick. The slick is subject to physical processes, such as advection and turbulence, causing it to move vertically and/or horizontally. Advection tends to lead to dispersal or, if land is nearby, shoreline stranding. Turbulence promotes the formation of emulsions, called chocolate mousse, which can be transformed via weathering into tarballs. The lower-molecular-weight compounds tend to evaporate or dissolve. Some fractions of petroleum have solubilities in seawater on the order of tens of milligrams per liter. Some are also photochemically oxidized. 

Considerable interest has been generated in turbulence promoters for both RO and UF. Equations 4 and 5 show considerable improvements in the mass-transfer coefficient when operating UF in turbulent flow. Of course the penalty in pressure drop incurred in a turbulent flow system is much higher than in laminar flow. Another way to increase the mass-transfer is by introducing turbulence promoters in laminar flow. This procedure is practiced extensively in enhanced heat-exchanger design and is now exploited in membrane hardware design. 

Thomas and Watson ( 9) of the Oak Ridge National Laboratory have shown that detached spiral wire turbulence promoters, positioned away from an RO membrane surface by small wire runners, markedly increased the rejection of salts and the permeation rate through the membrane. "Detached promoters" of this type may be designed to minimize stagnant regions in addition, they are relatively easy to install. 

Figure 26 presents their results for the rejection of 0.01 M MgCJl2 400 psi on a dynamic membrane as a function of where U is the linear velocity down the tube, v is the permeation rate, and N is the Reynolds number. It will be noted that the greatest effect of the turbulence promoter was observed at the lowest velocities, where the rejection increased from 25 to 72 percent. At the highest tangential velocities, the improvement was much less, from 90 to 93 percent. In addition, Thomas and Watson observed an increase in permeation rate varying from 10 to 50 percent. Thus, with turbulence promoters, the same rejection and flux, as in an unpromoted system, may be obtained at a considerable reduction. 

Probstein et al (10) Investigated the use of detached strip type turbulence promoters in the ultrafiltration of bovine serum albumin in laminar flow. His apparatus is shown in Figure 27 the detached strip type promoters tested were circular cylinders with a diameter (D) approximately one-half (0.46) of the channel height and were across the center of the channel cross-section, transverse to the flow. 

Typical flux data with two interpromoter spacings (AL) are shown in Figure 28 as a function of the cross-flow rate. The flux Increased by a factor of 3 for the best case. Though Probstein did not plot his data in this way, it is Interesting to note that the empty channel flux has a predictable 0.33 power dependence on tangential velocity. With the turbulence promoters, the slope shifts closer to the 0.7-0.8 power dependence normally observed in turbulent flow. Unfortunately, data are not available in Probstein s paper on the increased pressure drop associated with the turbulence promoters, but it would appear that the flux to power ratio is greatly improved with turbulence promoters. 

In current practice, turbulence promoters most often take the form of a net or screen material which also serves as a feed channel spacer between two membranes. For example, the familiar spiral wound modules (Figures 29) used extensively in reverse osmosis and to a lesser extent in ultrafiltration use a plastic screen material as the feed channel spacer. This is also used in some plate and frame systems (Figure 30). 

Spherical turbulence promoters have also been used in tubular systems (Figure 31). In the author s experience, these spherical promoters are not as effective as detached spiral wire promoters (9). Particles collect in the dead stagnant areas between the spheres and foul the system. The spheres serve more effectively as volume displacement spheres than as turbulence promoters. 

Figure 26. Effect of detached spiral turbulence promoters on rejection of 0.01...

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