Stripping counter-current

G. Absorption, stripping, distillation, counter-current. Hi, and Hq, random packings, Cornell et al. correlation, and Belles and Fair correlation... 

Fig. 122. General scheme of liquid-liquid extraction process using the multistage counter-current method. The three main steps are extraction, washing and stripping.

Stripping can equally be done in single-stage, cross-flow, or counter-current systems. To illustrate how the overall concepts remain valid, the performance of a countercurrent cascade accepting as feed the scrubbed extract... 

The GS enriching process is a counter-current gas-liquid extraction done at a pressure of 2000 kPa in a sieve tray tower with the upper half operating at 30 C and the lower at 130 C. ( 5) In the top half of the tower, feedwater extracts deuterium from the upflowing cold H2S, reaching a maximum at the centre of the tower. The recycled lean H2S entering the lower hot half of the tower strips deuterium from the water, which then leaves the system depleted in deuterium. A cascade of several stages is used to reach the desired feed concentration for the final water distillation or finishing unit. Transfer between cascades can be either by gas or liquid from the centre of the tower. 

The countercurrent DBBP z 1hn extraction process was operated on a plant-scale for about six years. During that time, it provided excellent recovery ( vl00%) of soluble plutonium in the feed and adequate decontamination of 21flAm from plutonium and other metallic impurities. The Am/Pu ratio in the WS-1 Column (Am strip column) product was 2.5/1 compared to only 1/1 for the batch extraction process americium product. Concentrations of calcium, aluminum, and other metallic impurities in the counter-current Am product stream were also much lower than in the Am product from the batch extraction process. 

The PVC slurry, containing VCM, is continuously fed to the stripping column (1). The slurry passes counter-currently to steam, which is fed into the base of the column. The proprietary internals of the column are specially designed to ensure intimate contact between the steam and the PVC slurry and to ensure that no PVC particles remain inside the column. All process operations, including grade change, are automatically done in a completely closed system. 

A reflux is used where lighter liquid is injected at the top of the distillation column and this strips by counter-current absorption the heavy components from the rich vapour rising up the packing inside the column. 

The following extraction-stripping operation provides for complete Am and Cm extraction, together however with the RE fraction of the FP and substantial amounts of Ru. It acts in the 1st option as a back-up cycle for Pu and Np separation, otherwise these elements too must be separated here completely (option 2). The utilisation of pulsed columns remains the first choice also for this operation and the aqueous raffinate must meet the final specifications for "alpha free" HAW. Again it is therefore of highest importance to run the extraction section at convenient conditions, i.e. at a sufficient stage number and phase ratio. To an A/O ratio of 0.33 corresponds 33% TBP saturation (RE). Only experimental counter-current tests may be able to show if this phase ratio can be increased and if perhaps a scrub could provide for a better separation mainly from Ru, without compromising the complete Am-Cm extraction. 

Figure 9. Five-stage counter-current liquid-liquid extraction flow diagram for the stripping of HDBP and H2MBP from 2-ethyl-l-hexanol (2-EHOH) T = 50°C.

The third column (118) was a simple rectification column in which decane was separated from THF/ ethylacetate. Decane was recycled into the extraction column 116. Compared to different alternatives, which were simulated, this process has the following advantages. Water was eliminated from the ethylacetate/ THF-mixtures before their rectification. This approach takes advantage of the fact that the VLE-data of ethylacetate/ THF are more favorable than the ones of ethylacetate/ THF/ water. The counter current extraction with decane allows an efficient separation of the polar impurities such as methanol, ethanol, and acetic acid. Furthermore decane eliminated the water from the recovered solvent mixture (extractive rectification in column 117). Methylacetate posed a further problem and a rectification column was necessary to separate it from THF. The stripping column 117 combined the dewatering and the elimination of methylacetate. 

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