Rectification structured packings

D. Rectification in vertical wetted wall column with turbulent vapor flow, Johnstone and Pigford correlation =0.0.328(Wi) Wi P>vP 3000 < NL < 40,000, 0.5 < Ns. < 3 N=, v,.gi = gas velocity relative to R. liquid film = — in film -1 2 " [E] Use logarithmic mean driving force at two ends of column. Based on four systems with gas-side resistance only, = logarithmic mean partial pressure of nondiffusing species B in binary mixture. p = total pressure Modified form is used for structured packings (See Table 5-28-H). 

The multiple phase contact inside the column is promoted by internal mass transfer equipment. Three groups of mass transfer equipment are commonly differentiated, which are separation trays, random packings and structured packings. Besides mass transfer equipment, further column internals are required in rectification to ensure the proper operation of the mass transfer equipment. Such internals may include support and hold-down plates, liquid distributors and redistributors, vapour distributor devices, gas-liquid phase separators and liquid collectors that usually do not participate on mass transfer. 

The advantages of using modern lattice-type and structured packings are particularly significant in the vacuum rectification of thermally unstable mixtures or separation processes with a high number of theoretical stages [1,2] as well as in absorption technology. 

In the 1990s, packings were also successfully used for applications in pressure absorption and pressure rectification in the oil industry, where tray columns were being upgraded to structured packings. 

Table 4-4d. Determination of pressure drop in irrigated packed column in counter-current flow acc. to Eq. (4-48) or (4-50) and (4-54) throughout the entire operating range up to flooding point. List of test points and relative mean errors <5 (Ap/H) for tested structured packing, valid for system rectification systems acc. to Table 2-2... 

This work has looked at calculation principles for approx. 200 classic as well as modern randomly filled and stacked lattice packing elements, tube columns and structured packings of various types and sizes. Today, these packings are used not only in rectification in the vacuum and normal pressure range, but also in many areas of high-pressure rectification and high-pressure absorption as well as in waste-air and wastewater treatment and groundwater treatment. 

The first German edition concentrates on the description of the fluid dynamics of columns with random and structured packings in the vacuum and normal pressure range of up to approx. 2 bar and for specific liquid loads of up to 100 m m h for gas-liquid systems. This range covers a majority of the applications and tasks relevant in the absorption and desorption of highly and/or moderately soluble gases as well as in rectification under vacuum and normal pressure. 

The autiior fiilly agrees with Schultes [283], who considers that in future both types of mass transfer devices, random and structured packings, will be largely used in columns for absorption, desorption and rectification processes. Only economical considerations will determine the optimal solution in each case. 

Type B1 Type Cl Packing made of 0.2 mm embossed metal sheet with special surface structure and round form flow channels Similar to type Bl, but made from IT FE or polypropylene Rectification under overpressure, normal pressure and vacuum Absorption 100-500 0.6-3.0 Bl-300 to 30 Bl-300 ca. 4 Bl-300 0.1-2.0 Bl-300... 

Separation trays and structured and random packings are the prevailing mass transfer internals at choice for rectification columns and, in first deciding between them, a comparative performance design needs to be prepared including examinations on capacity limitations, pressure drop and separation performance. 

In thermal process engineering, packed columns as well as tray columns are often used for heat and mass transfer processes in rectification, absorption and extraction as well as for the cooling of gases and liquids and wastewater and groundwater treatment. They are mainly used for counter-current gas/liquid flow. Figure 1-1 shows the schematic structure of a packed column with random packing elements. 

For many years now, there has been a constant demand for low pressure drop column internals in rectification, absorption, stripping and liquid/hquid extraction. The prevailing trend in the chemical industry is to replace tray columns with those containing modem structured and random packings. When planning the design of packed columns, it therefore particularly important to use reliable methods for predicting the mass transfer and hydrodynamic behaviour of the two-phase flow. 

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