Eluent recycling

Another important parameter is the eluent composition. Binary mixtures (and obviously pure solvents) should be preferred to complex mixtures, since new systems perform an on-line analysis of the composition of binary eluents. These eluent systems allow the automatic eluent recycling, with a reduced number of controls. 

Most process scale chromatographic separations are run under isocratic conditions, and therefore robust solvent recycling processes need to be designed. Figure 12.16 presents a simplified scheme coupling chromatography with eluent recycling [11]. The solvent is recovered from both evaporators and dryers while pure dry compounds are recovered. The recycled solvent is reused to elute the column and to dissolve the dry feed mixture. Only a small amount of fresh solvent is automatically added to the recycled solvent in order to adjust the eluent composition. 

Figure 12.16 Scheme of purification solution integrating isocratic chromatography coupled with eluent recycling. 

The preferred solution is CO2 recycling at a pressure of about 40-50 bar, slightly below the typical pressure inside CO2 cylinders stored at ambient temperature. Eluent recycling and condensation at this pressure requires cooling uhlities for a temperature range of 0-5 °C. Under these condihons the solute-eluent separation... 

The step responses of the extract and raffinate purities, resp. yj and (Fig. 2) are obtained for 0.05% steps of respective eluent, recycle, extract and solid flow rates,resp. M , 2, U and U4, used as manipulated inputs. The steps are performed after the process reaches a steady state purity of 95% for both products. Most of the responses are close to first order step responses and present similar time constants, which is suitable for further control. Only the step response of the extract purity with respect to the eluent... 

Figure 2 Step response coefficients of the extract and raffinate purities with respect to the flow rates (from top to bottom eluent, recycle, extract and solid ).

Easier eluent recycling due to constant eluent composition... 

Improved performance is achieved with a more complex operation and layout of the process, which requires precise process design and control - especially when a recycling of the section IV outlet is applied and a second point of mixing at the desorbent port is implemented. An open-loop operation without eluent recycling between sections I and IV is more robust, especially when regeneration of the adsorbent is necessary. 

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