Liquid-liquid Extraction Equipment's and Working

In liquid-liquid extraction (extraction equipment's) operation, the two phases must be brought into intimate contact for mass transfer to occur,, and then the phases are separated. This operation may be carried out batch-wise Or continuously. 

Whenever we are dealing with simple systems and the quantities to be handled are small, the batch-wise procedure is adopted, and when large quantities are to be handled and several contacts are required continuous operation becomes economical. 

Most of the equipment is operated continuously with either successive stage contacts or differential contacts.

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Classifications of Extraction Equipment

Industrial extractions are two types 1. Stage-type extractions and 2. Differential extractions (continuous contacts extraction). 

Stage-type extractions

In stage-type extractors, the two phases are allowed to mix together to reach equilibrium, and then the phases are separated before they pass countercurrent to each other. Advantages of stage-type contractors include simplicity of design no axial mixing high stage efficiency etc. These units are large and bulky owing to the requirement of a separate phase separation after each stage. 

Mixer settler (single and multistage) plate columns, pulsed sieve plate columns, etc. are examples of stage-type extractors. 

Differential extractions (continuous contacts extraction) 

In the differential extractors, the two-phase is always in continuous contact. These are compact for a given throughput and require small floor space. In plate columns, packed columns, and spray columns used for extraction operation, the phase interspersion and countercurrent flow are produced by the force of gravity. In mixer settlers, and rotating disc contactors, the phase interspersion is produced by mechanical agitation, and countercurrent flow is produced by gravity. 

Packed columns, spray columns, pulsed packed columns, rotating disc contactors, etc. are examples of differential extractors. 

Liquid Extraction Working Principle

1. Mixer settlers, 2. Sieve tray tower/Column, 3. Spray tower/Column, 4. Packed columns/towers, 5. Rotating Disk contactors (R.D.C.). 

Mixer Settlers, working principle

A mixer settler is a single-stage extraction device comprising of a mixture for contacting the two liquid phases to effect the mass transfer and a settler for mechanical separation of the phases. 

For extraction operations carried out in a batch-wise fashion/manner, the mixer and settler may be the same unit. Image

It consists of a vertical vessel incorporating a turbine Or propeller agitator. It is provided with charging nozzles at the top and a discharge connection provided with a slight glass at the bottom. The feed solution to be extracted is taken into the agitated vassal, the required amount of the solvent is added, and the whole mass is agitated for a predetermined time. At the end of the mixing, cycle agitation is stopped, and settling is applied for the phase separation. Afterward, the raffinate and extract phases are withdrawn from the bottom discharge connection into separate receivers. 

For continuous extraction operation, the mixer and settler are separate units. The mixer is a small baffled-agitated tank provided with inlet-outlet connections and the settler is often a continuous gravity decanter. In this extractor, the phases are continuously in contact with each other in the mixer under thought agitation before glowing to the settler for phase separation. 

One phase is usually dispersed into the other in the form of small droplets to produce large interfacial areas resulting in faster extraction. 

When several contacts are required, a battery/train of mixer settlers operated in a counter-current fashion is used. Generally, the liquids are pumped from one stage to the next, but if sufficient headroom is available gravity flow is arranged. A feed solution is introduced in the first mixer and a fresh solvent is fed to the last mixer so that the raffinate from each settler becomes the feed to the next settler, and the extract from the last settler acts as the solvent for the second last mixer and so no. The final extract leaves the first settler while the final raffinate is taken out from the last settler. 

In the case of liquids that emulsify easily and have nearly the same density, it is necessary to pass a mixer discharge through a pad of glass fiber to coalesce the droplets of the dispersed phase before gravity settling is practicable. Since pumps and piping systems are involved in the installation of a train of mixer-settler, the initial investment in equipment is high. 

Advantages of mixer-settler

(i) High-stage efficiency

(ii) Good flexibility

(iii) Capacity to handle liquids of high viscosity and 

(iv) High capacity. 

These units are employed in industries such as petrochemical, fertilizer, metallurgical, etc. 

Perforated plate/Sieve Tray Tower/Column

These are multistage, counter-current contactors wherein the axial mixing of the continuous phase is confined to the region between trays, and redispersion occurs at each tray resulting in effective mass transfer. These are very effective, especially for systems of law interfacial tension both concerning the liquid handling capacity and extraction efficiency. The performances in the plates are about 1.5 to 4.5 mm in diameter and the plate/tray spacings are 150 to 600 mm.

The upper picture shows a perforated plate tower wherein the light liquid is dispersed. The general arrangement of plates and downcomers is the same as that used for the gas-liquid contact except that no weir is needed. 

The light liquid is introduced at the bottom, and phases through the performance in the plate in the form of fine droplets, which rise through the heavy continuous phase, coalesce into the layer beneath the plate, and again redisperses through the plate above. The principle interface is maintained at the top, the light liquid is removed from the top, and the heavy liquid from the bottom. The heavy liquid can be dispersed in which case the tower is turned so that the downcomer of each plate becomes upcoming.

Spray Tower/Column

Spray towers are the simplest of differential contactors. In its simplest form, it consists of an empty tower provided with inlet and outlet connections at the top and bottom, for introducing and removing the heavy and light liquid phases. Due to maximum freedom for liquid movement, as the shell is empty, there will be severe axial mixing in these extractors and hence it is practically very difficult to obtain the equipment of more than one or two theoretical stages even with tall towers. 

In these towers, mixing and settling proceed simultaneously, and there is a continuous transfer of material between phases, and the composition of each phase changes as it flows through the tower/column.

Read more 

Solvent Selection for Extraction and Distribution coefficient 

Liquid-Liquid Extraction Principle | Liquid-Liquid Extraction Application 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane