General description of the process
Whereas distillation takes advantage of different volatilities means different distributions of a product in the liquid and the gas phase the liquid/liquid extraction is based on different solubilities means different distributions of a product in 2 co-existing liquid phases.
For the extraction of a product (white dots) out of the so called feed liquor (blue liquid with white dots) a suitable solvent (yellow liquid) has therefore to be found. The first step of an extraction process is mixing for an intensive contact of both liquid phases to enable the mass transfer of the product (white dots) from the (blue) feed liquor into the (yellow) solvent. The second step is the phase separation or settling of the 2 liquid phases. After the extraction of the product the feed liquor is called raffinate (blue liquid with less white dots) whereas the solvent containing the product is called extract (yellow liquid with white dots). For the recovery of the product the solvent has to be separated in a subsequent third step from the product which is mostly done by distillation.
Solvent+Feed Mixing Settling Extract+Raffinate
Requirements for the solvent
The solvent will have to be checked for the following characteristics:
- Maximum solubility of the product in the solvent
- Minimum solubility of the solvent in the raffinate
- Minimum solubility of the feed liquid in the solvent
- Fast phase separation of the extract from the raffinate
- Easy separation of the product from the extract/solvent
With the right solvent there are numerous very different applications which can be realized advantageously by an extraction process as described below.
When to use the extraction process
The liquid/liquid extraction process is favorable for separations as for:
- products having similar volatilities
- products forming azeotropes
- products which would require high energy input in distillation
- products being temperature sensitive
- non-volatile products as e.g. metal salts
Separation of products with similar volatilities but different liquid/liquid phase distributions
The separation by distillation of mixtures with components having similar volatilities require high columns means high investment costs and high reflux ratios means high operating costs so that they may become economically not advantageous.
A very common mixture of a binary mixture with a low vapour/liquid separating factor is e.g. acetic acid in water. The acetic acid can be easily separated from water by extracting it with Methyl tert-butyl ether (MTBE) as solvent which can be evaporated easily.
- Acetic Acid /Water
- Aliphatic /Aromatic Compounds
Separation of products forming azeotropes
Products forming a homogenous azeotrope cannot be separated from each other by a conventional distillation above the azeotropic point. Sometimes this binary azeotropic mixture can be split by adding a specific third component or changing the operating pressure affecting the volatility of one product more than of the other. If this is not feasible a liquid/liquid extraction is the method of choice. Examples of such common mixtures are:
- Formic Acid /Water
- Alkylchlorides/Alcohols as Dichloromethane/Ethanol
Separation of products which would require high energy input in distillation
There are many cases where aqueous effluents contain organic components having a higher boiling point than water. The separation of the organics by evaporation of the water would require a lot of energy and is hence in most cases not economic. The alternative is the extraction of the organic components with a solvent out off the water followed by an evaporation of the solvent to get the product. The evaporation heat of a solvent is mostly smaller than of water and the concentration of the product in the solvent is higher than it was in the water so that much less energy for the evaporation of the solvent is required than for the water of the feed liquor.
Such aqueous effluents are commonly either industrial waste waters or reaction mixtures.
Waste waters with high boiling organic compounds:
- Phenols, cresols and aniline or other aromatic derivates thereof
Reaction mixtures requiring the removal of organic compounds:
- Oxidation of organic products
- Production of caprolactam
- Syntheses of organic acids
Separation of temperature sensitive products
Another necessity for an extraction process can be the temperature sensitivity of the products themselves which can make a distillation process either impossible or economically disadvantageous.
Food, pharmaceutical and green chemistry bio-molcules as:
- Flavors and fragrances
- Certains aldehydes and organic acids
Separation of non-volatile products as e.g. metal salts
The production or recovery of valuable metals requires often the separation of their metal ions from others in an aqueous solution. An option is the extraction of a metal ion if the metal ion forms a chelate complex in the organic phase leaving the other metal ions back in the aqueous phase.
Metals being recovered or purified are for example:
- Precious metals
- Rare earths
Equipment to carry out extraction processes
Extractions can be carried out batch wise in e.g. mixers or in continuous mode either in e.g. mixer-settlers or counter-currently operate columns. Beside theoretical calculations and extraction expertise gathered along the last 50 years De Dietrich Process Systems will be pleased to carry out trials in our experimental hall to develop the optimum process and to select the most suitable extraction equipment for you. Borosilicate glass 3.3 is an ideal material for extraction equipment as the process can be optimized while visually observing the process.
Batch mode - Mixers/Reactors
Using a mixer in batch mode offers a huge flexibility to optimize the mass transfer by varying ratio of the liquid phases, the type of stirrer, stirrer speed and mixing time. The settling period can also be easily influenced by varying the time. Such a batch operation is labor intensive and requires sufficiently big equipment.
What can be done in the laboratory scale with a separation funnel can be realized in a bigger scale with larger quantities in a more defined way by using tempered mixers having basically the same functionality as batch reactors for reactions in the liquid phase. This is especially advantageous for the extraction of reaction mixture as it can be carried out in the same equipment as the reaction. Mixing can be adjusted by the stirrer shape, speed and mixing time. The phase separation is a question of time and can be observed advantageously in glass reactors. For the phase separation it is necessary to determine the location of the interphase. In the reactor this can be measured by systems based on a floater, a radar signal or the conductivity depending on the system to be examined. Outside the reactor it is also possible to detect the interphase while draining the lower means heavier phase through the bottom outlet by visual inspection through a glass pipe or by the sudden change of media property.
Continuous mode - Mixer-Settlers and Columns
Operating in continuous mode is less labor sensitive and the equipment is smaller but the equipment has to be fitted more exactly to the mixing and settling processes as e.g. the parameter “time” can be adjusted only by adjusting the throughput. This results in various types of mixer-settlers and extraction columns to suit the numerous different extraction applications as described above.
How to find the best solution ?
The phase distribution equilibrium coefficients are the basis for the selection of an extraction process and the calculation of the minimum number of theoretical extraction stages. The type and size of the real extraction stages are determined by the mass transfer and phase separation processes. Mass transfer and phase separation are strongly dependent on the characteristics of the interface between the 2 liquid phases. Any surfactants even present only in small amounts can have an important impact on the extraction process. Therefore results from extraction trials with your real feed liquors are important for a reliable process design. De Dietrich Process Systems will be pleased to carry out these trials in our experimental hall to develop the optimum process. They are the bases for selection of the most suitable extraction system and the theoretical calculations to dimension their size.
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