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Packed columns are used for distillation, gas absorption, and liquid-liquid extraction. The gas liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column. The liquid flows down the column over the packing surface and the gas or vapor, counter-currently, up the column. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution throughout the packed bed, and this is an important consideration in packed-column design.
Types of packed column include structured packing and random packing. Structured packing refers to packing elements made up from wire mesh or perforated sheets. The material is folded and arranged with a regular geometry, to give a high surface area with a high void fraction. Structured packings are usually made of metal, ceramic, or plastic and their basic construction and performance are similar.
Fig. 1 Structured packing column
The principal requirements of a structured packing are as follows:
The uniform arrangement of structured packing provides many advantages compared to random packing or fractionation trays, including: i) lower pressure drop through the column; ii) higher efficiency in the same tower height; iii) higher capacity; iv) excellent liquid spreading characteristics; v) reduced liquid hold-up. While structured packing columns are not suitable for very low liquid rates, difficult in the provision of cooling arrangement, and not possible in the removal of side stream.
The performance of packed columns, for distillation or absorption services, is frequently expressed in terms of HETP (height equivalent to a theoretical plate). According to the double Film Theory, HETP can be calculated by the following expression
(λ-stripping factor; uGs-vapor phase superficial velocity m/s; kG-vapor phase mass transfer coefficient m/s; ae-effective interfacial area m2/m3; uLs-liquid phase superficial velocity m/s; kL-liquid phase mass transfer coefficient m/s)
Therefore, the precision of HETP evaluation by equation depends on the accuracy of correlations used to predict the effective interfacial area and the vapor and liquid mass transfer coefficients. For structured packing, HETP varies with circulation rate, gas rate, gas density, and packing characteristics.
As with decades of experience in chemical products and technical service, Alfa Chemistry has become a trustable supplier in petroleum and chemical industry, food and beverage industry, pharmaceutical and bio-technology industry, etc. We can offer revamping of structured packing services including equipment improvement and process optimization. Please feel free to contact us for a better solution to your several of columns.
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