(740a) Countercurrent Centrifugal Extraction: A New and Highly Efficient Extraction Apparatus Based On Centrifugal Partition Chromatography

Centrifugal partition chromatography (CPC) is a kind of liquid-liquid partition chromatography in which the separation mechanism is based on different distribution of the components between two immiscible liquid phases. As an advantage compared to other chromatographic devices no supporting solid phase is used in CPC. Thus, CPC provides gentle separation conditions (e.g. no irreversible adsorption) and a high amount of stationary phase (60-80% of the total column volume) which is accessible to the sample solutes and thus, providing a high capacity.

To immobilize one liquid phase without a supporting solid phase, the column of a CPC consists of a cascade of chambers connected by ducts in serial and aligned around a central axis of rotation. Caused by the rotation of the rotor, a centrifugal force is generated which is used to immobilize the liquid (stationary) phase in each chamber, while the mobile phase is pumped through the stationary one along the whole chamber cascade. Due to different distribution behavior between the two phases, the components of a mixture injected at the beginning of the cascade will elute at different times. E.g., a component that distributes more to the stationary phase will be retained for a longer time in the rotor.

Depending on the density ratio of the two liquid phases two different operation modes can be distinguished, namely “ascending mode” (ρ_stat/ρ_mob > 1) and “descending mode” (ρ_stat/ρ_mob < 1). For both modes, the rotor is initially filled with the phase, which is chosen to be the stationary one. Then, the mobile phase is pumped into the rotor and some part of stationary phase is replaced until equilibrium is reached and no further stationary phase is eluted (name: retention of stationary phase). In general, a high retention of stationary phase in the chambers is desired for CPC operations. The retention of stationary phase is mainly influenced by the selection of the phase system with its physical properties (densities, viscosities and interfacial tension) and the operating parameters of the CPC (mode of operation, volume flow of mobile phase and rotational speed of the rotor).

For the operation in CPC basically all liquid systems which form two immiscible liquid phases can be chosen. Aqueous organic systems are common for the separation of natural products and there are guidelines for the selection of suitable phase and operating conditions. Aqueous two phase systems (ATPS), however, are relatively new as phase system for CPC operation. Here, both phases are mainly composed of water enriched with phase forming components (two incompatible polymers, one polymer and one salt or two different salts) above a critical amount in each phase. ATPS provide gentle conditions for the separation of biomolecules (e.g. proteins) and thus, allow the CPC technique to deal with another application field. However, due to their physical properties (higher viscosities, lower density difference and lower interfacial tension when compared to common aqueous organic systems) the efficient operation in CPC is difficult and up to now it has not been well investigated.

The separation of a substance mixture with CPC is generally a discontinuous process and its set-up and application is similar to preparative chromatography. To increase the capacity for industrial applications other CPC operations were developed. A new CPC concept, the sequential CPC (sCPC), derives from the fact, that each of the liquid phases used can be the mobile phase and that the mode of operation (ascending or descending) can be switched during a separation. By this technique the sample is feed continuously between two conventional CPC rotors, which are connected in serial. The mode of operation in sCPC is sequentially switched between ascending and descending mode for a defined period of time. The mobile phase in each sequence is pumped in the inlet of rotor one and a product stream is collected at the outlet of rotor two, or vice versa. Thus, comparable to other cyclic chromatographic technics (e.g. simulated moving bed chromatography), two product streams can be collected discontinuously, each stream enriched with some components of the mixture to be separated. By this purification technology a higher capacity of separation, compared to classical CPC can be achieved.

To further increase the capacity for industrial applications the principle of countercurrent centrifugal extraction (CCE) was developed and patented (PCT/EP2012/068999) by the chair of plant and process design at TU Dortmund in Germany. With the CCE technology it is possible to pump both liquid phases simultaneously in counter flow direction over the whole chamber cascade allowing a continuous separation. The continuous working principle is achieved by connecting each chamber with two channels, one for each liquid phase. Additionally, rotary joints providing five channels in total are necessary (in CPC only two channels) to maintain a continuous flow of both phases as well as the sample feed. The whole set-up consists of one rotor only, which simplifies the set-up compared to sCPC. The substance mixture to be separated can be fed continuously at the middle of the chamber cascade. According to the distribution coefficients between the two liquid phases the separation of the target substances occurs in direction of the heavy or light phase flow. The principle of CCE is similar to liquid-liquid extraction processes. However, a particularly advantage of CCE technology is the design of combined “mixer-settler-units” in small scale. By use of the centrifugal field it is possible to use instable phase systems, which form more stable emulsions with long settling times. This is mainly important for the use of ATPS, where the process efficiency is often limited by the settling time of the phase system. Additionally, a high number of separation stages can be realized in CCE and it is possible to adjust the number of stages by the number of CCE discs used.  Another advantage is the easy scale-up by variation of the chamber size.

In the presentation, the construction and operating principle of the CCE technology will be explained in detail. Additionally, experimental results for the separation of substances using organic-aqueous systems will demonstrate its efficiency and thus, will show the applicability of this new technology.