Packed columns support plates

The typical packed column has one or more beds, each consisting of packing, a support plate, a hold-down support plate, and a liquid distributor. 

The separation operation called distillation utihzes vapor and hquid phases at essentially the same temperature and pressure for the coexisting zones. Various lands of devices such as r andom or sti uctui ed packings and plates or tr ays are used to bring the two phases into intimate contact. Trays are stacked one above the other and enclosed in a cyhndrical shell to form a column. Pacldngs are also generally contained in a cyhndrical shell between hold-down and support plates. A typical tray-type distillation column plus major external accessories is shown schematically in Fig. 13-1. 

As indicated above, packed column internals include hqiiid distributors, packing support plates, redistributors (as needed), and holddown plates (to prevent movement of packing under flow conditions). Costs of these internals for columns with random packing are given in Fig. 14-80, based on early 1976 prices, and a Marshall and Swift cost index of 460. 

FIG. 14-80 Cost of internal devices for columns containing dumped packings, a) Holddown plates and support plates, (h) Redistributors, (c) Liquid distributors. [Pikulik and Diaz, Cbem. Eng., 84(21), 106 (Oct. 10, 1977).]... 

Each SynChropak column is tested chromatographically to assure that it has been packed according to specifications. For SynChropak GPC columns, a mixture of a high molecular weight DNA and glycyltyrosine, a dipeptide, is used to evaluate internal volume and efficiency. The mobile phase used for the test is 0.1 M potassium phosphate, pH 7, and the flow rate is 0.5 ml/min for 4.6-mm i.d. columns. Minimum plate count values and operational flow rates are listed in Table 10.4 for 4.6-mm i.d. columns of all supports and the various diameters of the SynChropak GPC 100 columns. 

Design the column internals plates, distributors, packing supports. 

Figure 3.11 — (A) Immobilized peroxidase sensor. Glass-immobilized peroxidase is packed in the flow-cell shown. The plastic support plate fits the top surface of the photomultiplier chamber of the immunometer so as to support the vertically held flow-cell in front of the photomultiplier itself. (B) Flow system for hydrogen peroxide/ethanol determinations. For ethanol determinations, the immobilized alcohol oxidase column is inserted immediately after the injection valve (shown by the arrows). Luminol (62 /zM) and 4-iodophenoI (0.4 M) are dissolved in 200 mM borate buffer (pH 8.9) and pumped at a flow-rate of 0.8 mL/min. Phosphate buffer (10 mM, pH 7.0) is pumped at 1.6 ml/min. (Reproduced from [78] with permission of Elsevier Science Publishers).

The support can then be mechanically coated with a variety of liquid stationary phases. The mobile phase most commonly used in packed column GC is nitrogen with a flow rate of ca 20 ml/min. Packed column GC affords a relatively low degree of resolution compared to capillary GC typically 4000-6000 plates for a 2 m column compared to > 100 000 plates for a 25 m capillary column. The high temperature limit of packed columns is ca 280°C beyond this temperature the liquid stationary phase evaporates at a rate which creates a large background signal. However, for many routine quality control operations, they are quite adequate. 

Figure 1335. Packed column and internals, (a) Example packed column with a variety of internals [Chen, Chem. Eng. 40, (5 Mar. 1984)]. (b) Packing support and redistributor assembly, (c) Trough-type liquid distributor, (d) Perforated pipe distributor, (e) Rosette redistributor for small towers. (0 Hold-down plate, particularly for low density packing.

Packed columns and thin layer plates have similar characteristics in that they contain a solid support or packing that is not present in OT columns. We need to examine more closely the nature of packed beds and will use a packed column as an example. 

In a packed column the liquid comes in at the top, or near the top, and enters the column via a series of nozzles or through a distributor plat. The gas enters below the packing and passes upward. Packing placed in the column is held in place by support plates. 

End Effects Analysis of the mass-transfer efficiency of a packed column should take into account that transfer which takes place outside the bed, i.e., at the ends of the packed sections. Inlet gas may very well contact exit liquid below the bottom support plate, and exit gas can contact liquid from some types of distributors (e.g., spray nozzles). The bottom of the column is the more likely place for transfer, and Sil-vey and Keller [Chem. Eng. Prog., 62(1), 68 (1966)] found that the... 

In open tubular columns, the stationary phase is held on the inner surface of a capillary, whereas in packed columns, the stationary phase is supported on particles that are contained in a glass or metal tube. Open tubular columns contain an enormous number of plates that permit rapid separations of closely related species. They suffer from small sample capacities. 

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