Method of Operation

A distinction needs to be made between analytical and preparative-scale chromatography. Analytical processes are used to identify and quantify tiny amounts of unknown materials. This type of process can be classified into 2 groups: gas chromatography and liquid chromatography, depending on whether the mobile phase material is a gas or a liquid.

Preparative-scale chromatographic systems generally consist of a large cylindrical column within which the stationary material is packed. The mobile phase is invariably a liquid, and the stationary phase is either a solid, or a liquid supported by an adsorbent solid. Since the column is packed with stationary phase material, a pressure must be applied in order to force the liquid mobile phase through the column.

Batch Elution Chromatography

The most common large-scale batch (or elution) chromatography is based on a scaled-up version of an analytical chromatograph. The process for separating a binary mixture is shown in the Figure below:

As shown in the Figure, a recycled solvent or carrier gas is fed continuously into the sorbent packed column. The feed mixture and recycle is pulsed into the column by an injector. A timer or detector (not shown) splits the effluent from the column, sending it to different separators (condensers, evaporators, distillation columns, etc depending on the number of components to be separated). Each separator is designed to remove a particular feed component from the carrier fluid. An additional cleanup step is required to purify the carrier fluid before it is recycled to the column. Separator 1 produces no products because it handles an effluent pulse that contains the carrier fluid and 2 or more of the feed components, which are recovered and recycled to the column.

The stationary phase is located in the annular space, about 2-cm wide and 30-cm in diameter, located between an inner and an outer cylinder. See the Figure below. Mobile phase (also known as eluant) is introduced uniformly to all points of the annular stationary phase at the top of the column. Feed is introduced via a fixed inlet point. The column is rotated about its axis at a speed of 1 - 2 revolutions per hours.

Consider the separation of 2 components A and B. A interacts weakly with the stationary phase while B interacts more strongly compared to A.

Whilst the components of the feed stream are in the mobile phase, they will be carried downwards with the mobile phase towards the base of the column. Components of the feed material that interact with the stationary phase will move horizontally with the stationary phase in the direction of the rotation of the column. Component that interact more with the stationary phase will be more "retarded" in their movement. Therefore the different components will follow a helical path as they pass down the length of the column.

Since component A is interacting only weakly with the stationary phase, it will preferentially flow with the mobile phase and therefore will follow Path 1. Component B is more strongly retained by the stationary phase and its flow will follow Path 2.

In this manner, a mixture of A and B can be can fed continuously to the top of the column, and 2 high purity product streams of A and B can be recovered from the bottom of the column.

Moving-bed Chromatography

This is method of operation, the feed is introduced into the mid-point of a column where the "stationary" phase (the moving-bed) is flowing down the column and the mobile phase material is flowing up the column. See the Figure below:

Feed components that are more strongly retained by the moving-bed will hence move with this material towards the base of the column. Similarly, feed components that have a greater affinity for the mobile phase will leave with this material at the top of the column.

Practical problems associated with this method is the leakage into both streams. More sophisticated system consists up to 12 columns connected in series. By switching the flow of feed and eluant to these columns at periodic intervals, it has been possible to simulate the movement of stationary phase material relative to the mobile phase and achieve continuous feed component separation.

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