Larger diameter

One potential problem with this approach is that heat loss from a small scale column is much greater than from a larger diameter column. As a result, small columns tend to operate almost isotherm ally whereas in a large column the system is almost adiabatic. Since the temperature profile in general affects the concentration profile, the LUB may be underestimated unless great care is taken to ensure adiabatic operation of the experimental column. 

Particle Attrition. Distributor jets are a potential source of particle attrition. Particles are swept into the jet, accelerated to a high velocity, and smash into other particles as they leave. To reduce attrition at distributors, a shroud or larger-diameter pipe is often added concentric to the jet hole, as shown in Figure 15. The required length of the concentric shroud is given by the relation... 

The second type of hoUow-fiber module is the bore-side feed type illustrated in Figure 23b. The fibers in this type of unit are open at both ends, and the feed fluid is usually circulated through the bore of the fibers. To minimize pressure drops inside the fibers, the fibers often have larger diameters than the very fine fibers used in the shell-side feed system and are generally made by solution spinning. These so-called capillary fibers are used in ultrafiltration, pervaporation, and in some low to medium pressure gas appHcations. Feed pressures are usually limited to less than 1 MPa (150 psig) in this type of module. 

Thermoforming and Extrusion. Improved equipment and polymers have increased the capabiUty to extmde and thermoform polypropylene however, consumption of polypropylene in these areas has not grown dramatically. Drinking straws are commonly extmded from polypropylene, however most larger diameter tubes, such as pipes and conduits, are predominantly extmded from other thermoplastics. Extmded sheet is thermoformed into food containers and trays polypropylene is used when microwavabiUty is desired. 

Reboilers need to be located next to the tower they serve, except for the pump-through types, which can be located elsewhere. Fired heater reboilers are always located away from the associated tower and use a pump to circulate the bottoms. Ketde-type reboders are preferred from an operational and hydraulic standpoint because they can be designed without the worry of having to ensure sufficient head for circulation required by thermosyphon reboders. However, ketde reboders require a larger-diameter shed that is more cosdy, and the reboder must be supported at a sufficient elevation to get the product to the bottoms pump with adequate NPSH. 

Multielevation piperacks are usually needed to handle all the required services for piping, electrical, utilities, and instmmentation. The two-level rack is one of the most common but three-level ones are also used. The utility lines are usually mn in the upper level and the process lines in the lower levels. The larger-diameter lines are located to the outside of the rack to be closest to the column supports. Access platforms are required at the battery limit to provide operators access to the block valves and blinds. If long mns of hot pipe are required, a portion of the pipe rack needs to be dedicated to an expansion loop. A horizontal space in the piperack is provided for a set of lines to be flat-turned into a set of expansion loops with the large pipes located on the outside. AH of the pipe turns are in the same horizontal plane, which is an exception to normal piping practice. A flat turn takes up and blocks space for other pipes. Flat turns are generally only made from the outside of the rack to minimize this blockage. 

The smaller reactor approaches plug-flow behavior and exhibits a large temperature gradient. In this case, external recycle provides the same degree of back-mixing as is provided by internal circulation in the larger diameter reactor. 

The center-drive mechanism and feed launder are usually supported by a walkway that extends across one-half or the whole diameter of the basin. Devices having drive mechanisms and rakes supported by a tmss across the diameter of the thickener are referred to as bridge machines. The bridge thickeners usually do not exceed 25—45 m in diameter. In thickeners with larger diameters, the drive mechanism is supported by a central column or pier and the rates are driven and supported by a drive cage. The sediment is discharged into an aimular trench around the bottom of the column. 

Welded pipe is made from rolled strips formed into cylinders and seam-welded by various methods. The welds are credited with 60 to 100 percent of the strength of the pipe wall depending on welding and inspec tion procedures. Larger diameters and lower ratios of wall thickness to diameter can be obtained in welded pipe than can be... 

For larger diameter columns, and for low liquid rates, the distributor must be almost exactly level (e.g., within 6 mm for a 3-m diameter) or all pour points will not function. On the other hand, the rises must be high enough to accommodate the backup caused by high liquid rates. The needed head can be estimated from the orifice equation, with a discharge coefficient of 0.5. In some cases the orinces discharge directly into tubes that extend to the packed bed (the Tubed drip-pan distributor ). 

At low concentrations, the type of sedimentation encountered is called particulate settling. Regardless of their nature, particles are sufficiently far apart to settle freely. Faster-setthug particles may collide with slower-setthug ones and, if they do not cohere, continue downward at their own specific rate. Those that do cohere will form floc-cnles of a larger diameter that will settle at a rate greater than that of the individual particles. 

---