Info

/ Breakpoint

Bed Volumes

Bed Volumes

Figure 18. Concentration of adsorbed species in column effluent during column loading.

Figure 19. Elution curve showing concentration of adsorbed species eluted during resin regeneration.

Feed concentrations, flow rates, and regenerant dosages may be varied to develop the relationship between resin utilization and regenerant efficiency so that the optimum operating conditions can be selected for the system.

The first portion of the breakthrough curve in Fig. 18 shows the quality of product that can be obtained under the processing conditions. An integration of the area up to the breakthrough point provides an estimate for commercial column capacities for the space velocities used in the experiment. The velocity at which the mass transfer zone is moving through the column is given by dividing the length of the column by the time it takes to detect the solute in the column effluent. The difference between that time and the time at which the selected breakthrough concentration appears in the effluent, when multiplied by the velocity of the mass transfer zone, results in an approximation of the mass transfer zone.

For simple molecules with large differences in distribution coefficients, a single eluting solution may be used to develop the chromatogram. However, more complex materials, such as peptides and proteins, require a shift in the ionic strength of the eluent. This can be done step-wise or as a gradient. Sememza[65] has proposed the following rules for the proper choice of eluent:

1. Use cationic buffers (Tris-HCI, piperazine-HC104, etc.) with anion resins and anionic buffers (phosphate, acetate, etc.) with cation exchange resins.

2. With anion resins use decreasing pH gradients and with cation resins use rising pH gradients.

3. Avoid using buffers whose pH lies near the pK of the adsorbent.

If the chromatographed solutes are to be isolated by solvent evaporation, the use of volatile buffers, such as carbonic acid, carbonates, acetates and formates of ammonium should be used.

If better resolution is required, it may be obtained by changing the type of gradient applied. A convex gradient may be useful in improving the resolution during the last portion of a chromatogram or to speed up separation when the first peaks are well separated and the last few are not adequately spaced. A concave gradient can be used if it is necessary to improve resolution in the first part of the chromatogram or to shorten the separation time when peaks in the latter portion are more than adequately spaced.

During the column test, the starting volume and the final volume of the resin should be measured. If there is a change of more than 5%, progressive volume changes as the resin is operated through several cycles should be recorded. These changes may be significant enough to affect the placement of laterals or distributors in the design of commercial equipment. For instance, carboxylic resins may expand by 90% when going from the hydrogen form to the sodium form. This type of volume change may dictate how the resin must be regenerated to prevent the breakage of glass columns due to the pressure from the swelling resin.

Gassing, the formation of air pockets, within the resin bed is to be avoided. Gassing may occur because of heat released during the exchange reaction. It will also occur if a cold solution is placed in a warm bed or if the liquid level falls below the resin level. Keeping the feed solution 5°C warmer than the column temperature should prevent the gassing due to thermal differences.

It is necessary to configure the experimental apparatus to insure that the feedstream moves through the column at a steady rate to maintain a well-defined mass transfer zone. Possible methods of maintaining constant flow are shown in Fig. 20.

Once it is determined that the action will proceed as desired, subsequent optimization of the system in the laboratory calls for setting a packed resin column of approximately the bed depth to be used in the final equipment, typically one to three meters.

Large Volume Constant Head Device

Pump Operation

Liquid Laval

Hon Clamp

Pump

Liquid Laval

Pump

Figure 20. Equipment for laboratory evaluation of ion exchange resins.

Pump Operation

Hon Clamp

Sintarad GIm Bad Support

Faad, EliMnt Fraction or Rlnaa Solution Collector

Sintarad GIm Bad Support

Faad, EliMnt Fraction or Rlnaa Solution Collector

Figure 20. Equipment for laboratory evaluation of ion exchange resins.

Was this article helpful?

0 0

Post a comment