Crossflow structure

For the cases when efficient mixing has to be coupled with a solid-catalyzed reaction a whole family of open-crossflow-structure catalysts has been developed. The best known of them are the so-called KATAPAK s, commercialized by Sulzer. One of them, KATAPAK-M is shown in Figure 10. It has good mixing properties and can simultaneously be used as the support for catalytic material. 

Sulzer SMR static mixer, which has mixing elements made of heat-transfer tubes, and Sulzer s open-crossflow structure catalysts, so-called Katapaks Sulzer (www. sulzer.com) is a Swiss company active in the field of machinery and equipments ... 

Water quality is important, not only from an environmental point of view but also in relation to the type of packing to be specified. Analysis of the circulating water is simple to obtain, but it is very seldom offered to the cooling tower designer. The quality, or lack of it, will determine the type of pack to be used, the selection of structural materials and whether the tower should be induced or forced draft, counterflow or crossflow. Water treatment, in the shape of chemicals to control pH and to act as counter-corrosion agents or as biocides, all has a bearing on tower selection. 

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. 

Crossflow technology is increasing, as it proves practical. Micioliltration membranes are of an isotropic and homogeneous morphology, i.e., the pore structure is consistent throughout. There is some movement, however, toward ihe use of "skinned" anisotropic membranes. Microliltration membranes are available in a wide variety ol polymers, including some that arc quite chemically inert. They also tire available as tubular, hollow fiber, or capillary fiber elements. 

Certain parameters affect membrane fouling particle nature particle size and size distribution membrane type and structure surface interactions and the clogging mechanism. An important parameter is the method applied to the filtration technique, namely, crossflow or deadend filtration. The latter requires less pumping energy but tends to clog the membrane faster. 

Asymmetric hollow fibers provide an interesting support for enzyme immobilization, in this case the membrane structure allows the retention of the enzyme into the sponge layer of the fibers by crossflow filtration. The amount of biocatalyst loaded, its distribution and activity through the support and its lifetime are very important parameters to properly orientate the development of such systems. The specific effect that the support has upon the enzyme, however, greatly depend upon both the support and the enzyme involved in the immobilization as well as the method of immobilization used. 

A special case arises when the "skin" (membrane) layer of a normal composite membrane element is immobilized with a catalyst and not intended for separating reaction species. Consider the example of an enzyme, invertase, for the reaction of sucrose inversion. Enzyme is immobilized within a two<layer alumina membrane element by filtering an invertase solution from the porous support side. After enzyme immobilization, the sucrose solution is pumped to the skin or the support side of the membrane element in a crossflow fashion. By the action of an applied pressure difference across the element, the sucrose solution is forced to flow through the composite porous structure. Nakajima et al. [1988] found that the permeate direction of the sucrose solution has pronounced effects on the reaction rate and the degree of conversion. Higher reaction rates and conversions occur when the sucrose solution is supplied from the skin side. The effect on the reaction rate is consistently shown in Figure 11.6 for two different membrane elements membrane A is immobilized by filtering the enzyme solution from the support layer side while membrane B from the skin layer side. 

Table 1) Is due, at least in part, to a tighter colloid layer structure. Membranes formed at the same pressure but different crossflow velocities show similar values of Hint (Trials 5-8 in Table 1), although at the higher velocities there is some decrease, which may be due to imperfections in the thinner dynamically formed layer.

Advantages-Crossflow Trays versus High-Efficiency (Random or Structured) Packings... 

In down-wash configuration, the flame is established in the wake of the burner tube. A recirculation vortex in the wake of a burner tube appears as a flame sheet. When R is further reduced, the flame tip is severely deflected by the crossflow. A small recirculation bubble was observed by Huang and Chang [16] atR = 0.04. For a value of R between 0.1 and 1, the impact of a cross-flow stream is dominant. An axisymmetric tail flame forms downstream of the recirculation vortex and the flame widens. This structure is characterized by several features such as flickering and bifurcation. In jet-dominated mode, the recirculation vortex disappears and only the tail part remains attached to the burner. The transition from crossflow-dominated to jet-dominated conditions occurs from 1 = 1 to 3. For R>3, the effect of crossflow becomes negligible the jet fluid mechanics dictate the flame characteristics. For R > 10, the flame detaches from the burner and stabilizes above the exit plane of the burner tip. Depending upon the jet exit velocity and burner diameter, the flame is either attached to the burner tip or stabilizes as a lifted flame until it blows out. 

Goh and Gollahalli [20] measured temperature profiles in piloted and nonpiloted propane and propylene flames in crossflow at R = 32-97. The profiles are generally characterized by off-axis single peak structure for all flames. The nonpiloted flames produced higher peak temperatures signifying increased oxidation of O2 under the same flow conditions. Tsue, Kadota, and Kono [71] measured the structure of propane diffusion flame in crossflow, including the flame temperature, velocity and concentration fields. Further, Tsue, Kadota, and... 

Poudenx, P, Howell, L., Wilson, D. J., and Kostiuk, L. W. "Downstream Similarity of Thermal Structure in Plumes from Jet Diffusion Flames in a Crossflow." Combustion Science and Technology 176, no. 3 (2004) 409-35. 

Savas, O., Huang, R. R, and Gollahalli, S. R. "Structure of the Flow Field of a Nonpremixed Gas Jet Flame in a Crossflow." ASME Journal of Energy Resources Technology 119 (1997) 137-44. 

Further distinction has to be made between conventional filtration of fine particle less than 10 pm in diameter, and microfiltration. It would be unusual for the filtration of such particles on a conventional fiher cloth to be described as ndcrofiltratian. Thus microfihration is constituted by the filtration of small particles and by the medimn which is used for the filtration. Conventional fihration is undertaken on filter cloths with a very open structure, see Chapter 4, whereas membrane fihration is usua% concerned with fihration enq>loying membrane media where the equivalent pore size is of the order of 10 pm, or less. These definitions are, however, becoming less distinct as it is now possible to obtain conventional fihration equ ment employing membrane-type fiher media, as discussed in Chapter 4, and crossflow microfilters enqploying conventional filter cloth. 

Thus, at the start of a crossflow menoibrane filtration the inverse flux rate is proportional to the cumulative fihrate volume, in much the same way as described in Section 2.6.1. The intercept of such a plot can be used to provide an in situ value for the membrane resistance, via Equation (10.15). This resistance is usually much greater than the clean water permeation st value for the same membrane. This is due to the effect of the interaction of the initial layers of deposit within the membrane structure. These layers add substantially to the effective membrane resistance, ie. additional resistances due to adsorption and blocldng, see Section 10.4. This situation is again very similar to that for conventional filtration, where the filter medium resistance increases at the start of the filtration. If the membrane was a true surface filter this would not happen, but almost all membrane filters do permit some initial penetration of particulates. 

Crossflow ventilation is a technology that has been designed into the cellhouse structure in the past to reduce acid mist. Amplats Rustenberg plant in South Africa applied a cross flow principle first and this was applied on a larger scale at the zinc EW cellhouses in Canada (Cominco Trail in early 1980 s) and Kidd Creek. From the mid 1990 s until the more commonplace application of cell hoods, the large tankhouses in Chile typically used combinations of positive crossflow ventilation, balls and foams. The DESOM crossflow ventilation configuration, which became common practice in Chile, was first applied to the El Teniente SX-EW operation in 1985. Cerro Colorado, Quebrada Blanca, Chuqui SBL, Quebrada M and Escondida Coloso followed in 1994. 

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