Scale-Up of Elution Chromatography

In liquid elution chromatography, separation is based on adsorption on the solid or on partition to a stationary or bonded liquid phase. Distribution coefficients are modest so that solutes migrate through the column as shown in Figs. 14.1-2 and 14.1-3. Gradients can be used but usually are not since the column has to be reequilibrated afterward. Two somewhat different approaches have bMn taken  

Relatively small-scale—say less than IS cm (6 in.) in diameter—highly efficient columns based on the scale-up of analytical columns. A standard sorbent is used and a may be as low as 1.1. Standard equipment applicable to a variety of separation problems is available commercially. In the range up to a few centimeters in diameter these meth s have been extensively review.  

Large-scale, custom built systems using a packing with relatively large a (preferably 2). Equip-ment is dedicated to one separation. A few applications were repotted as early as 1947, ° but interest has increased recently. The packing material may be specially developed to be highly selective.  

Absolutely even plug flow is desired. Any deviations from plug flow cause zone spreading and decrease resolution. To achieve the desired plug flow the sorbent must be packed carefully and the distribution system must be designed carefully. A low // is desired and external dead volumes must be minimized. 

FIGURE 14.2-1 General system for large-scale chromatography. 

The distribution system should put in and withdraw liquid without disturbing the plug flow. As the size increases this becomes more important. A combination of a distribution system plus a firh to hold flie packing seems to work well. The feed pulses should be input across the entire column diameter in as close to a square wave as possible. Otherwise, the obtained resolution is decreased. The distribution system and all piping and valves should have a minimum volume and distance of travel between the injection device and the downstream valves which isolate the products. This minimizes mixing and makes control easier. 

Chromatographic packings act as depth filters if the feed or solvent is ditty. Since this increases pressure drop and decreases capacity, the feed, solvent, and recycle streams all should be filtered. In addition, the liquids need to be deaeraeted to prevent air bubbles in the column. 

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The scale-up of chromatography operated in isocratic elution can be carried out as follows. 

Frontal analysis chromatography allows greater flexibility for continuous scale-up than elution or displacement chromatography. The primary difference is that the sample to be separated is introduced as a step change in frontal analysis. As the sample mixture is fed continuously into one end of the stationary phase bed, the least retained solute begins to emerge at the outlet... 

A typical chemical engineering approach to a large-scale separation problem is to try to devise a steady-state, oounteroussent system. This has led to methods that are quite different from the scale-up of analytical elution chromatography. First, approaches where the solid is moved are considered and iheo methods where the solid does not move but movement is simulated are discussed. Finally, the different systems are compared. In all cases the countercurrent or simulated countercurrent Systran replaces the column in Figure 14.2-1. The equipment for product and solvent recovery still is required. 

Isolation procedures for many biochemicals are based on chromatography. Practically any substance can be selected from a crude mixture and eluted at relatively high purity from a chromatographic column with the right combination of adsorbent, conditions, and eluant. For bench scale or for a small pilot plant, such chromatography has rendered alternate procedures such as electrophoresis nearly obsolete. Unfortunately, as size increases, dispersion in the column ruins resolution. To produce small amounts or up to tens of kilograms per year, chromatography is an excellent choice. When the scale-up problem is solved, these procedures should displace some of the conventional steps in the chemical process industries. 

Liquid chromatography can be operated under mild conditions in terms of pH, ionic strength, polarity of liquid, and temperature. The apparatus used is simple in construction and easily scaled up. Moreover, many types of interaction between the adsorbent (the stationary phase) and solutes to be separated can be utilized, as shown in Table 11.1. Liquid chromatography can be operated isocratically, stepwise, and with gradient changes in the mobile phase composition. Since the performance of chromatography columns was discussed, with use of several models and on the basis of retention time and the width of elution curves, in Chapter 11, we will at this point discuss some of the factors that affect the performance of chromatography columns. 

The technique of boxcar injections (not to be confused with boxcar chromatography) can be extremely productive for iso-cratic elution in any mode of chromatography and should always be considered when scaling up a separation. The preparative HPLC of an enantiomeric mixture utilising a chiral stationary phase is described here to demonstrate the approach for separation of a binary mixture. 

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