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The packed column is a gas-liquid mass transfer device in which a large amount of filler is installed in the column as a phase-to-phase contact member. The packed column was used in industrial production in the mid-19th century. Since then, it has competed with the plate column to form two different types of gas-liquid mass transfer equipment.
The column of the packed column is an upright vertical cylinder with a support plate at the bottom. The filler is placed on the support plate in a random or integral manner. A packing platen is placed over the packing to limit the movement of the packing with the updraft. The liquid is added from the top of the column, sprayed onto the packing through the liquid distributor, and flows down the surface of the packing. The gas is fed from the bottom of the column and distributed through the gas distribution device (the small diameter column generally does not have a gas distribution device), and then flows through the gap of the packing layer in a countercurrent flow with the liquid. Mass transfer is achieved by close contact of gas-liquid two phases on the surface of the packing. The packed column belongs to a continuous contact type gas-liquid mass transfer equipment, and the two-phase composition continuously changes along the column height. Under normal operating conditions, the gas phase is a continuous phase, and the liquid phase is a dispersed phase.
When the liquid flows down the packing layer, there is a tendency to gradually concentrate toward the column wall, so that the liquid flow near the column wall gradually increases. This phenomenon is called wall flow, and the wall flow effect causes uneven distribution of gas-liquid two phases in the packing layer. Therefore, the mass transfer efficiency is lowered. For this reason, when the filler layer is high, it is necessary to perform segmentation with a redistribution device in the middle. The liquid redistribution device comprises a liquid collector and a liquid redistributor. The liquid under the upper filler is collected by the liquid collector, sent to the liquid redistributor, and after being redistributed, sprayed onto the lower layer of the filler.
The packing is the core component of the packed column, which provides the phase interface of gas-liquid two-phase contact mass transfer, which is the main factor determining the performance of the packed column. There are many types of fillers, which can be divided into two types: bulk packing and structured packing depending on the filling method.
Bulk filler
A bulk packing is a granule of a certain geometry and size. Generally stacked in the column in bulk, also known as random packing or granular packing. Bulk fillers can be divided into annular packing, saddle packing, ring saddle packing and spherical packing according to different structural characteristics. The more typical bulk fillers are: pull two ring packing, Pall ring packing, step ring packing, arc saddle packing, saddle packing, metal ring saddle packing, spherical packing.
2. Structured packing
The structured packing is a packing that is arranged in a uniform geometric pattern in the column and neatly stacked. The packing is characterized by a gas-liquid flow path, which improves the gas-liquid distribution in the packing layer, and provides a larger specific surface area at a very low pressure drop, so that both the processing capacity and the mass transfer performance are relatively large. Improvement.
There are many kinds of structured packings. According to their geometric structure, they can be divided into grid packing, corrugated packing, pulse packing, etc. Several typical structured packings are introduced.
(1) Grill filler
The grid filler is a combination of strip-shaped unit bodies according to a certain rule, and the structure thereof varies depending on the form and combination rule of the strip-shaped unit body, and thus has various structural forms. The earliest grid filler used in the industry is wood grating filler. Griffic grid fillers, mesh grid fillers, honeycomb grid fillers, etc. are commonly used in applications, among which Griche grid fillers are the most representative. The grid filler has a low specific surface area and is therefore mainly used for applications requiring low pressure drop, large load and anti-blocking.
(2) corrugated packing
Corrugated packing is a general-purpose structured packing, and most of the structured packing used in industry belongs to this category. The corrugated packing is a disc-shaped packing composed of a plurality of corrugated sheets, and the inclination of the corrugations to the column shaft is 30. And 45. Two, when assembled, two adjacent corrugated plates are stacked in opposite directions. Each of the disc packings is vertically installed in the column, and the adjacent two disc packings are staggered 90. arrangement.
The corrugated packing has the advantages of compact structure, large specific surface area, and specific surface area which can be adjusted by the shape of the corrugated structure, and commonly used are 125, 150, 250, 350, 500, 700 and the like. The adjacent two plates of packing are perpendicular to each other, so that the ascending air current continuously changes direction, and the descending liquid is continuously redistributed, so the mass transfer efficiency is high. The regular arrangement of the fillers reduces the flow resistance and thus the processing capacity is improved. The disadvantage of corrugated packing is that it is not suitable for processing materials with large viscosity, easy to polymerize or have suspended matter. In addition, it is difficult to load and unload and clean the packing, and the cost is also high.
According to the material structure, corrugated packing can be divided into two types: mesh corrugated packing and corrugated packing. The material is divided into metal, plastic and ceramic.
(3) Pulse packing
The pulse packing is a structured packing which is assembled by a certain method by a hollow prismatic single-seat with a constricted neck. After the pulse packing is assembled, a porous prismatic channel with a constricted neck is formed, and the longitudinal flow path alternately contracts and expands, and a strong turbulence occurs when the gas-liquid two phases pass. In the neck-recession section, the gas velocity is faster and the turbulence is intense, thereby enhancing mass transfer. In the enlarged section, the gas velocity is minimized to achieve separation of the two phases. The flow path shrinks and expands alternately, achieving a "pulse" mass transfer process.
Pulsed packing is characterized by high throughput and low pressure drop, making it an ideal filler for vacuum distillation. Because of its excellent liquid distribution performance, the amplification effect is reduced, so it is especially suitable for large column diameters.
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