78.5 (most polar)
have to be utilized. The co-current system is illustrated in Fig. 10.25. There are n mixer/separator vessels in line and the raffinate goes from vessel 1 to vessel n. Fresh solvent is added to each stage, the feed and extracting solvent pass through the cascade in the same direction. Extract is recovered from each stage. Although a relatively large amount of solvent is used, a high degree of extraction is achieved.
A counter-current system is illustrated in Fig. 10.26. There are a number of mixer/separators connected in series. The extracted raffinate passes from vessel 1 to vessel n while the product-enriched solvent is flowing from vessel n to vessel 1. The feed and extracting solvent pass through the cascade in opposite directions. The most efficient system for solvent utilization is counter-current operation, showing a considerable advantage over batch and co-current systems. Unless there are special reasons the counter-current system should be used. In practice, the series of counter-current extractions are conducted in a single continuous extractor using centrifugal forces to separate the two liquid phases. The two liquid streams are forced to flow counter-current to each other through a long spiral of channels within the rotor.
The Podbielniak centrifugal extractor (Fig. 10.27) consists of a horizontal cylindrical drum revolving at up to 5000 rpm about a shaft passing through its axis. The liquids to be run counter-current are introduced into the shaft, with the heavy liquid entering the drum at the shaft while the light liquid is led by an internal route to the periphery of the drum. As the drum rotates, the heavy liquid is forced to the periphery of the drum by centrifugal action where it contacts the light liquid. The solute is transferred between the liquids and the light liquid is displaced back towards the axis of the drum. The heavy liquid is returned to the drum's axis via internal channels. The two liquid streams are then discharged via the shaft. Flow rates in excess of 100,000 dm3 h 1 are possible in the largest models. Probably the most useful property of this type of extractor is the low hold-up volume of liquid in the machine compared with the throughput.
Penicillin G is an antibiotic which is recovered from fermentation broths by centrifugal counter-current solvent extraction. At neutral pHs in water penicillin is ionized:
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