Column diameter increasing

It Is seen that If the length of the column is constant, the minimum column diameter does not change much with particle diameter once it exceeds about 7 microns. However, the minimum column diameter increases rapidly and becomes very large when the particle diameter is reduced to 3 micron or less. It is also seen that the extra column dispersion has a profound effect on the minimum column diameter for particles of all sizes. 

The most common vertical vessel internals are trays or packing. Tray cost increases considerably with increase in column diameter. Packing likewise increases with column diameter increase, due to column volume increase. (See Tables 8.19 to 8.21.)... 

The number of stages decreases (Fig. 3.5), making the column shorter, but column diameter increases. Near minimum reflux, small increases in reflux ratio considerably shorten the column, but only marginally increase its diameter. Capital cost declines. Upon further increases in reflux, the height reduction slows down while the diameter increases accelerate. Eventually, the savings from the shorter column become less than the cost of increasing... 

Effect of column diameter (at constant UV and percent of flood). As column diameter increases, both the liquid and vapor flow rates increase as the square of the diameter. The area for vapor flow also increases as the square of the diameter, so the vapor load remains unaffected. On the other hand, the area available for liquid flow only increases in proportion to the diameter. Therefore, the liquid rate per unit of weir length increases, the increase being proportional to the column diameter. The operating point on Fig. 6.29 will therefore shift horizontally to the right, toward the emulsion regime. Increasing the number of liquid passes on the tray reverses the above action, and shifts the operating point back to the left. 

Redistribution of vapor depends on a balance between the vertical and horizontal pressure gradients (157). The horizontal pressure gradient depends on column diameter, and diminishes rapidly as column diameter increases. This explains the strong effect of diameter in item 4 above. Another important factor cited by Porter and Ali (157) is vortex formation. This can cause downward flows of vapor in the bed. Downward flows were actually measured by Kabakov and Rozen (154) and Porter and Ali (157). 

For random packings there are many reports (3,56,98,136,140a, 162-167) of an increase in HETP with column diameter, fewer in which column diameter hed little effect on HETP (3,56,162,163), and an odd case in which increasing column diameter led to a decrease in HETP (56). Billet and Mackowiak s (3,62,166) scaleup chart for Pall rings implies that efficiency decreases as column diameter increases. 

If the throughput triples and the velocity remains the same, by what factor must the column diameter increase or decrease For a column with a throughput of 100 ft /min, the diameter is 3.6 feet. A new column with a throughput of 260 ft /min is to be built for a similar separation. What must the diameter be ... 

It is recommended that operating capacity and column efficiency be run initially on a small, laboratory scale to determine if the reaction desired can be made to proceed in the desired direction and manner. The column should be at least 2.5 cm in diameter to minimize wall effects. The preferential flow in a resin column is along the wall of the column. The percentage of the total flow along the wall of the column decreases as the column diameter increases and as the resin particle size decreases. 

Some common terms used in preparative-scale liquid chromatography are summarized in Table 11.4, The production rate, specific production, or the recovery yield provide suitable objective functions to judge the relative success of individual methods. For efficient use of the separation system, the production rate and the recovery yield should be maximized. Invariably, this results in operating the column in an overloaded condition. Unfortunately, column operation under nonlinear conditions is complex, and optimum conditions are not as easy to predict as the less demanding, although less powerful, scale-up approach. To scale up an analytical separation, the same column packing, column length, and mobile phase velocity are used, and the column diameter increased... 

Wall wipers (or "rosette ) redistributors (Fig. 3.86) This is a collection ring equipped with short projections extending toward the tower center. Liquid removed from the wall is deflected into the projections ("fingers ), which transport it to a desired location in the bed. Wall wipers effectively remove liquid from the wall, but they are only partially effective for counteracting composition pinches. Their ability to counteract composition pinches diminishes as column diameter increases. Therefore, they are only suitable for small columns [< 2 to 3 ft in diameter (74, 305)], where deflection of liquid and vapor by packing particles is sufficient to counteract pinching effects, and where wall flow formation is the main problem. 

Sample capacity increases as column diameter increases. Samples that have components present in the same concentration range can be analyzed on a column of any diameter. The choice depends on the resolution required. In general, the sample capacity of any capillary column is proportional to the square of the column radius. 

For many years up to the late 1960s, the use of packed columns in rectification and absorption processes was limited to relatively small plants with column diameters of up to 1 m. This was due to the properties of Raschig rings, commonly used at the time, which became less effective as the column diameter increased. It was not until Pall rings were introduced by BASF in the 1960s, that these limitations were partly reduced. In addition, little consideration was given to an even distribution of liquid at the column top and the pre-distribution of gas on inlet below the packed bed. 

For preparative applications, however, the infinite diameter mode is not so desirable as it reduces the effective capacity of the column. Here, complete utilization of the whole packed bed is required and so it is necessary to ensure uniform presentation of the sample across the whole cross section of the column, together with uniform flow. Although this will result in some peak broadening, due to the wall effect, this is reduced as the column diameter increases. 

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