Internal Reflux Distillation
The internal reflux distillation technique is based on a particular type of column which combines mass transfer and heat transfer, necessary for the fractionation of mixed vapours generated by boiling of a multi-component liquid mixture, in an equipment basically made of a number of vertical finned coils, internally cooled by a cooling fluid. The heat transfer thus carried out, thanks to a specific surface of about 500 m2/m3, is associated to an extremely efficient mass transfer, also advantaged by the uniform wetting of the surface at all points. The technology (US patent no. 5296103 and European patent no. 92107868.9) has resulted particularly efficient in batch distillation of compounds where it is required a good quality of distillate, as well as in solvent recovery from complex mixtures, where other available techniques result inadequate or bring to economically inconvenient recovery.
With the Polaris column, it is possible to design the recovery process according to various known modes, adopting case by case the more convenient one for the mixture to be treated and depending on the quality specifications of products.
- distillation at pressure less, equal or more than atmospheric
- distillation under high vacuum
- azeotropic distillation
- extractive distillation
- de-hydration by distillation with phase separation
For each of the above mentioned processes, it is possible to fractionate with variable or constant internal reflux, depending on convenience to be evaluated case by case.
The plant configuration and the recovery method are established depending on the characteristics and quantities of the mixtures to be treated, and on quality specifications defined for the distilled products.
The multi-component mixture to be treated is loaded into the still T-01 and circulated, by means of pump P-01, through the reboiler E-01, where it is heated by steam under flow control. When the mixture starts boiling, the mixed vapours developed pass through the internal reflux column C-01, where they are partially condensed, by indirect heat transfer with the cooling fluid (usually water), that passes through the internal coils at controlled flowrate. Such partial condensation, which occurs on the whole surface of the column, establishes dynamic and continuous equilibria between the condensed components and those in vapour phase, at various position in the column, according to a temperature and composition gradient. The flowrate of cooling fluid is controlled in order to get the preferential condensation of the less volatile compounds, that are refluxed internally towards the column bottom, more than the more volatile compound, that remains in vapour phase at the top exit of the column. For this purpose it is used a control system with high precision and sensitivity, set at boiling point of the more volatile compound. In this way the vapour coming out from the column top is only made by one compound, which can be condensed (and eventually subcooled) and quantitatively sent to the recovered product tank, as it corresponds to product specification.
On the other hand, in conventional columns, the re-condensation of the less volatile compounds to get fractionation is carried out by reflux at column top of balanced quantities of the more volatile compound that has come out from the column top itself, and that has been condensed in excess in an external condenser and only partially sent to recovery.
In the Polaris column, the distillate vapours are condensed in condenser E-02, and subcooled in cooler E-03, and then sent to the distillate storage tank. The control system selects and accepts only the product corresponding to specification. In time succession, the more volatile compound is separated by the mixture, and consequently the still temperature increases and the total temperature profile in the column increases. The above described separation process is carried out until fairly complete separation in the still of the more volatile fraction. At this moment the control parameters are changed automatically in order to get as column top temperature the boiling point of the next volatile compound to be separated. The temperature shift is carried out with a ramp, during this time the distillate is sent to the feed tank in order to be treated with next batch. The quantity of recycled distillate is anyway low.
In case of azeotropic distillation, the distillate fractions are sent to the phase separator S-01, and from here to the relative tanks, proceeding with the separation process until product specification is met.
1) The steady-state running of the internal reflux column is established instantaneously just as the boiling vapours reach the column top, i.e. few minutes after the mixture starts boiling in the still. In conventional columns, with external reflux, the start-up phase of the unit requires longer time, usually some hours, just to bring to specification appreciable quantities of product, which is until this moment totally refluxed. This advantage is also more evident when the product quality specifications are more stringent, like in pharmaceutical industry.
2) In industrial practice, several cases with different difficulties are met, due to interactions and affinity of chemico-physical attitude between the compounds that are present in the mixture: the stronger this affinity is, the more difficult the separation will be, even for compounds which are present in minor quantities. Significant quantities of off-spec fractions are thus produced, reducing the recovery yield and the overall system net capacity, and increasing the costs of recovery. The Polaris technology minimises these recycled fractions, because the distillate circuit has no dead points nor hold-up of distillate fractions, which contaminate the next distillate fractions (like external condenser, reflux accumulator, column distributor, column trays, reflux pump and piping, etc.). This allows to reduce to minimum the transitory phases from one product to next one, and to increase the recovery yields.
Due to construction and functional characteristics above described, with the internal reflux column it is possible to get selective the separation of components of extremely complex mixtures, with similar chemico-physical characteristics, not economically separable otherwise.
3) When one product is removed from the original mixture, it is possible to introduce in the automatic process sequence a cleaning cycle, by introducing steam in the column coils (inside tubes), so that the very little tails of distillate product adhering to the column surface are physically evaporated and drained at column bottom to separate tank (to be recycled).
The overall quantities of “heads” and “tails” to be recycled result thus reduced to minimum percentage, even less than 5 % than all other conventional distillation techniques.
Thanks to this “cleaning in place” system, without production of waste water, the validation of the unit by internal Quality Assurance as well as by external agencies results easier, in particular for multi-purpose units in pharmaceutical applications.
4) When the internal reflux column is installed directly on the vapour line on reactor top, for organic synthesis, it is possible to separate vapours of solvents during distillation phases (even under vacuum or during reaction), removing the same quantitatively and selectively based on their volatility, leaving inside the reactor only the solvents required by the process functionality. This result cannot be obtained with a simple condenser, which is able to carry out a mass transfer only corresponding to one theoretical stage maximum. As example, it can be considered the selective removal of one solvent from the reactor, leaving in the mass only water or another solvent present, in order to make easier crystallisation of the reaction product, without changing reactor or without more separation steps. In many cases the application of the internal reflux column has increased yield, quality and production capacity.
5) The Polaris internal reflux column can reduce the costs of disposal of waste mixtures of solvents and water, with a recovery otherwise not economically justified with the application of conventional systems. In this case the system:
- separates the organic solvents from water, producing mixtures which can be sold in the market of diluents or as fuel;
- separates water with COD levels below the law limits in order to send the same to public sewer or water collection;
- minimises the residual fraction of non recovered mixture to be disposed by third parties.
6) The cooling fluid that circulates in the column coils exits at a temperature close to the still temperature (at boiling point), i.e. an interesting thermal grade that can be considered for heat recovery. On the other hand, specially in batch distillation systems, the heat recovery in conventional column is usually not applicable due to very low thermal grade.
The plant results as a multi-purpose rectification unit, which can be used for solvent recovery from several mixtures (mother liquors of synthesis processes, centrifugation, drying, etc.), easy to operate and very flexible in use. The unit, in its more complete configuration, includes the still, the reboiler, the rectification column, one or more condenser, one gravity phase separator, one vacuum pump, and of course instrumentation and control system for the automation of all the process operation.
The plant, skid mounted, is supplied as packaged unit.
Other accessories can be included, e.g. liquid-liquid extraction column for pretreatment of other unit operations for specific applications or product specifications.
Polaris makes the preliminary process analysis and optimisation in order to define and to propose to the customer the more adequate system configuration and method to recover products with higher yield and quality, and better cost/benefits ratio.