Liquid distributor performance

Figure 7-5 (see Chapter 7) demonstrates the effect of liquid distributor performance on the separation efficiency of a packed bed. Curve II shows the performance typical of commercial liquid distributors available in the 1970s. Curve I represents the operation of high-performance liquid distributors now available. In evaluating distributor performance, the following points should be considered ... 

C. Liquid Distributor (see Figures 4-9). According to Strigle, This is the most important column internal from a process standpoint. A liquid distributor is required at all locations in the tower where an external liquid stream is introduced. High-performance distributors provide a flow rate variation per irrigation point that is a maximum of 5% of the average flow. The geometric uniformity of... 

Nutter offers an improved high performance random packing identified as Nutter Ring, see Figure 9-6K. To achieve the best performance from any random packing, the liquid distributor must be level and the distributor points of the discharging liquid to the packing must be uniformly distributed, see earlier discussion on this topic. 

Liquid distribution may be an important parameter, as demonstrated in the HOC1 process, where different liquid distributors provided significantly different results (8). The initial contact of the liquid with the rotor influences the mass transfer performance of the RPB in gas continuous operations (15). Although the use of a packing support at the inside diameter of the rotor would be expected to impact this initial liquid contact with the rotor, experiments did not show any reduced mass transfer performance (36). 

The perforated-pipe distributor is suitable for liquid feeds only and should be avoided when vapor is present. This distributor also needs to be running full if uniform distribution is to be achieved. A method to check this is presented elsewhere (111). A case where this tjq)e of distributor performed poorly with a partially vaporized feed has been reported (76). 

Distributor performance should always be water-tested prior to startup. A similar recommendation was made by others (318a), with emphasis on critical services and large-dieuneter (> 8 ft) towers. This test can be performed in situ or at the manufacturer s shop. If not performed in situ, the piping supplying liquid to the distributor should be closely duplicated at the test rig. If maldistribution is apparent, it is best to seek the manufacturer s advice. The author is familiar with experiences where severe maldistribution problems could have been detected and rectified prior to startup if a water test had been performed. One experience has been reported (349) where a water test led to the solution of an absorber separation problem which resulted from maldistribution. 

Poor distributor performance due to motion may escalate the efficiency loss. Pilot tests (323) showed a smaller efficiency loss with pipe distributors than with orifice-trough distributors. Inclination, liquid sloshing in the distributor, and liquid overflowing the trough may all contribute to pcwr performance of some distributors imder motion conditions. 

Miscellaneous Six columns ddnitanizer Qflene tower, ethylene oodde absorber Selexol towers, 3-to 14-itID In each case, a standard liquid distributor was replaced by a hi -performanoe distributor. In aU cases, substantial improvements in separation efficiency resulted. Well-designed hi -performance distributors can substantially improve column efficiency. 

Specialty chemicals 6.5-ft-diameter column, fouling service The column, which used conventional pan-type liquid distributors with large orifices, could not achieve design separation efficiency. Problem was solved replacing aU distributors with a proprietary two-stage hi -performance distributor. Orifice distributors can be troublesome in fouling services. 

Following replacement of trays with structured packing, the product failed to meet specs and tamdown was poor. Glycol rate had to be doubled to achieve dehydration. The most likely cause was gas maldistribution induced by an inlet vapor velocity head of 56 in of water. The column achieved design performance after new structured packing as well as new vapor and liquid distributors were installed. 

The products did not meet design specs. Replacing the liquid distributor by a hi -performance distributor solv the problem. 

Installed-cost data for high-performance liquid distributors and redistributors are also not widely available. Distributors should be placed at every feed point and, conservatively, redistributors should be used every 20 ft. In the absence of a vendor quote and for a very approximate estimate, the installed cost of a liquid distributor can be taken as 100/ft of column cross-sectional area. 

For fast reactions, change in the flow regime has dramatic effects on performance. Prevent foaming. Liquid distributor design is very important. Carefully plan the liquid redistribution along the walls for columns < 2-3 m diameter. For these small columns place redistributors at distances = 8-10 times the column diameter. The critical surface tension of the solid packing should be greater than the surface tension of the liquid to ensure that the liquid film remains intact in a packed contactor. 

Grid scans can also be used to investigate mechanical construction problems such as collapsed packed beds or the correct installation of distributors as well as the correct distribution of incoming liquid feed. An irregular distributor can undermine the performance of the entire packed bed and column. Liquid distributors must spread liquid uniformly on top of a bed, resist plugging and fouling, and also provide free space for gas flow. An incorrectly water level installed distributor, that is a tilted distributor, could cause liquid to flow preferentially on one side of the column. 

The improvement in vapor-liquid contact can enhance the performance of distillate hydrotreaters. As an example, in testing of an improved vapor-liquid distributor in commercial use, Haldor-Topsoe and Phillips Petroleum found that the new Topsoe Dense Pattern Flexible Distribution Tray (installed in 1996 to replace a chimney type distributor installed in 1995 in a refiney) allowed a 30% higher sulfur feed to be processed at 25°C lower temperatures, while reducing the sulfur content of the product from 500 to 350 ppmw . Albemarle estimates that an improved vapor-liquid distributor can reduce the temperature necessary to meet a 50 ppmw sulfur level by 10 °C, which in turn would increase catalyst life and allow an increase in cycle length from 10 to 18 months. Based on the above data from Haldor-Topsoe, if temperature were maintained, the final sulfur level could be reduced by 50%. Maintaining temperature should have allowed an additional reduction in sulfur of more than two-thirds. Thus, ensuring adequate vapor-liquid contact can have a major impact on final sulfur levels. 

Unfortunately, the heat transfer coefficient is not completely independent of the packed depth. This occurs because the spray-type liquid distributor typically used in these towers also provides some amount of heat transfer. This results in proportionally better performance from short packed beds than from deeper beds. By correlating operating data from such towers, Graf reports that vapor-to-liquid temperature differences of 50T are typical [13]. A deep packed bed is required to obtain a 20 F temperature difference between vapor and liquid streams. Figure 6-4 shows the efiective packed depth as compared to an actual packed depth if each foot of bed depth greater than 4 ft were equally efficient. 

With the advancements in liquid distribution technology, packed bed depths are being increased in order to better utilize the available column height. Packed depths up to 40 ft, which develop over 25 theoretical stages, are becoming more common with the use of high-performance liquid distributors. 

Fig. 116. Performance of Mellapak 250Y sheet metal sbuctured packing, (Fig. lll at different liquid distributors.

Spray Towers These are simple gravity extractors, consisting of empty towers with provisions for introducing and removing liquids at the ends (see Fig. 15-32). The interface can be run above the top distributor, below the bottom distributor, or in the middle, depending on where the best performance is achieved. Because of severe axial back mixing, it is difficult to achieve the equivalent of more than one or two theoretical stages or transfer units on one side of the interface. For this reason they have only rarely been applied in extraction applications. 

Ov Crall, the careful design of a distributor for liquid in the top of a packed tower, and for the redistribution of liquid flowing dow n multi-section packing in the tower, is essential to good consistent tower performance. However, the liquid flow is not alone, the uniformity of vapor distribution is likewise essential, because non-uniform vapor distribution can cause non-uniform liquid downflow. Then, there is the selection of the packing itself and its characteristics and requirements/sensitivity to the uniform distribution of the liquid and vapor. As earlier emphasized, the level of the distributor tray or trough can be critical to the consistent uniform liquid distribution. 

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