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It is simply a hypothetical equilibrium stage. However, the theoretical plate in packed beds, chromatography and other applications is defined as having a height. The empirical formula known as Van Winkle's Correlation can be used to predict the Murphree plate efficiency for distillation columns separating binary systems. [4]
For a packed column, the cross-sectional area of the column exit flow path is usually taken as 0.6 times the cross-sectional area of the column. Alternatively, the linear velocity can be taken as the ratio of the column length to the dead time. If the mobile phase is a gas, then the pressure correction must be applied.
A Raschig ring is a piece of tube, approximately equal in length and diameter, used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. They are usually ceramic, metal, or glass and provide a large surface area within the volume of the column for interaction between liquid and gas vapours.
In 1943 Dr Olaf George Dixon of ICI applied for a patent of a new product for column distillation. [1] He used stainless steel mesh instead of sheet steel in the Lessing ring in order to improve the pressure drop of the packed column (in fact, they were called "wire gauze Lessing rings" in a 1949 publication [2]).
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Proper column packing is important for maximum resolution: An over-packed column can collapse the pores in the beads, resulting in a loss of resolution. An under-packed column can reduce the relative surface area of the stationary phase accessible to smaller species, resulting in those species spending less time trapped in pores.
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A packed bed used to perform separation processes, such as absorption, stripping, and distillation is known as a packed column. [1] Columns used in certain types of chromatography consisting of a tube filled with packing material can also be called packed columns and their structure has similarities to packed beds.