Achieve Good Packed Tower Efficiency

Proper design of liquid distributors plays a key role in getting the most from packings.

By Frank Rukovena and Tony Cai, Fractionation Research, Inc.

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The distribution of vapor entering the bottom of a packed bed normally isn’t as great a concern as liquid distribution. However, vapor must be reasonably evenly distributed before it enters the bottom of the bed. Adequate vapor distribution can be a concern especially when a tower is converted from trays to packing. The pressure drop per theoretical packed stage is one-tenth to one-fifth that of a typical tray, so it’s important to make sure that the kinetic energy of the incoming vapor is dissipated sufficiently by the packed bed resistance or a vapor distributor/diffuser. Refs. 1 and 5 provide design considerations for this. The resistance necessary for vapor distribution should be provided by about one TS of packing. If it isn’t, consider a vapor distributor — otherwise pinching effects as discussed above may reduce separation efficiency.

Flow variation
This can occur on both micro and macro scales. Micro flow variation is the flow differences among orifices and may be expressed by Cv, as suggested in Refs. 2 and 6. Macro flow variation is between significant distributor areas, which is defined in Ref. 1 as one-twelfth of the tower area. Ref. 7 provides a detailed discussion of liquid distributor evaluation.

A number of factors can cause flow variation:
• shape of the orifice;
• shape of the edge of the orifice;
• orifice layout pattern;
• orifice density;
• depth of the liquid pool above the orifice;
• horizontal liquid velocity above the orifice;
• velocity through the orifice; and
• feed pipe design.

All must be considered when designing or reviewing the design of a liquid distributor.

Shape of the orifice. It’s normally assumed that the orifice is round. The flow through the orifice is a function, among other things, of the ratio of the orifice perimeter to the open area, which is incorporated in the orifice coefficient. Standard industrial practice is to punch, not drill, the liquid distributor orifices; drilled holes usually are lobed-shaped not round. Non-circular orifices also are employed — in such cases, the hydraulic diameter for the non-circular orifice needs to be used for the distributor design and flow capacity calculation, etc.

Shape of the edge of the orifice. The orifice is assumed to be sharp-edged. Depending upon the edge condition, the direction of flow relative to the punched direction, and the ratio of plate thickness to orifice diameter, the orifice coefficient will vary from about 0.52 to 0.98 [8]. Liquid flow always should be in the direction of the punching [8]. The punch creates a sharp edge as it enters and a torn rounded edge as it passes through the material. If good quality control isn’t maintained during punching operations, the orifice shape and, consequently, the orifice coefficient will change from the beginning to the end of manufacturing. This will result in the orifice coefficient not matching the design, which may cause a problem in tightly designed distributors at the low or high liquid rate extremes of the operating range. For long production runs, this could lead to macro maldistribution, which we’ll address later. Micro flow variations among orifices have little effect on random packing efficiency if the orifices with the variations are randomly distributed across the tower area. Similar variations do have an effect on structured packing efficiency [2, 3].

Orifice layout pattern. This should ensure that liquid entering the top of the packed bed is evenly distributed across the tower cross-sectional area. Liquid streams leaving the distributor shouldn’t impinge upon or be interfered with by the bed limiter. Don’t rely on the packing to markedly improve initial liquid maldistribution. The packing can enhance liquid distribution on a micro scale but not on a macro scale. The spreading coefficient of the packing may suffice to spread the liquid 6 in. to 12 in. from its entry point on the top of the bed but it can’t even out the liquid flow for several feet or more across the tower. If the L/V ratio is wrong, while the packing is evening out liquid flow the desired separation won’t be achieved unless a large safety factor is applied to allow for this decrease in apparent packing efficiency.

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