Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1999-10-20
2001-12-18
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S650000, C210S651000, C426S425000
Reexamination Certificate
active
06331253
ABSTRACT:
DESCRIPTION
The invention relates to a method of tangential filtration of a viscous fluid or liquid in which a third substance is dissolved in a supercritical state in this viscous fluid or liquid so as to lower the viscosity.
The invention also relates to the installation for the implementation of the method.
The invention is applicable particularly to the filtration of heat sensitive organic liquids and used motor oils.
The technological field of the invention is the filtration, more precisely the filtration of viscous fluids or liquids, and in particular the tangential filtration of these viscous fluids or liquids.
Filtration in the traditional sense of the word is head-on filtration in which a flow of liquid meets a porous obstacle perpendicular to its path which retains all the particles greater than the size of the pores. The retained particles, the size of which is traditionally of the order of from a millimeter to a micrometer, then form a cake which, in its turn contributes to the performance of the filtration. The particles which are retained are smaller and smaller while the flow of filtrate reduces, this being the phenomenon of clogging up. The filtration conditions never reach a quasi-stationary state.
In tangential filtration, these disadvantages are done away with, since the fluid is carried in a dynamic fashion and the main flow of liquid runs parallel to the filtering surface which prevents it clogging up.
Because of this and provided that the operating conditions are well chosen, the performance of the filtration in relation to retention and to flow can be considered to be constant over a sufficiently large period of time, in other words the conditions are quasi-stationary.
In the case of viscous fluids, this filtration is hindered by the rheological properties of these compounds.
In effect, the friction forces induced by the flow of a fluid through a porous body are very high in conformity with POISEUILLE's Law since the flow rate of an uncompressible liquid is in inverse proportion to the viscosity.
In tangential filtration, where the fluid, as has been seen above, is carried in a dynamic fashion in order to avoid clogging up the filtering surface, this disadvantage is even more important since the viscous drag through the porous body is added to the frictional forces due to the turbulent flow in the piping.
This is one of the reasons why the traditional methods of tangential filtration, such as, for example micro-filtration, ultra-filtration or nano-filtration essentially relate to aqueous liquids, the viscosity of which is close to that of pure water, namely 1 mPa·s at 20° C.
With regard to organic liquids, their viscosity, when they are not pure solvents, can be very high, which makes their filtration impossible. This is notably the case for mineral oils that arise from petroleum fractions or vegetable or animal oils.
Hence various solutions have been proposed in the literature so as to optimize or simply to make the filtration of viscous fluids possible, in particular by tangential filtration.
The first solution consists of increasing the temperature of the fluid or liquid to be filtered so as to lower the viscosity.
Hence the document EP-A-0 041 013 describes a method known under the name of the REGELUB® method, in which used motor oils are treated, with a view to recycling them, at a temperature of from 250 to 300° C. to lower their kinematic viscosity from 150 cSt to 1 to 2 cSt. The used oils, previously decanted to remove the water and distilled to remove the diesel oil or gasoline, are then filtered on ceramic mineral membranes made of &agr; alumina or on carbon based membranes, meaning that two batches are obtained: a first batch representing 10% of the initial volume where the contaminants are concentrated by a factor of 10, and a second purified batch, representing 90% of the initial volume, of commercial quality that simply requires a discoloration and an adjustment of the concentration of the additives.
The contaminants are essentially solid particles in the form of sediments, sulfur containing compounds, metals stemming from wear in the engines such as iron, lead stemming from the fuel etc., as well as calcium, magnesium, phosphorus etc.
The essential disadvantages of this method are:
the very high operating temperature which necessitates precautions because of the dangers of the inflammability of the treated product; and
the mechanical stresses applied to the ceramic membranes. These stresses stem from the differential expansions between materials, caused by the heat during the start-up and the shut-down of the equipment.
At the present time and for techno-economic reasons, the industry prefers the sulfuric acid process to this method, despite its negative environmental impact, notably due to the large amount of solid waste in the form of sludges.
More generally, the solution that consists of heating the liquids to be filtered in order to reduce their viscosity cannot be used for heat sensitive products which would be degraded by the heating, and, on the other hand, it is restricted by the technology of “standard range” equipment. Because of this, temperatures of from 120 to 150° C. are not exceeded, which only causes a limited decrease in the viscosity.
A second solution that allows the filtration of viscous fluids consists of adding at atmospheric pressure, a low viscosity solvent or “third substance”, soluble in the product to be filtered, in order to lower the viscosity.
For example, as described in the document FR-A-2 453 211, hexane can be added to a motor oil.
In this way, the viscosity of the used oil will be reduced by a factor of 2 to 3 which allows a real improvement in tangential filtration equal to the ratio of the viscosities divided by the volumes of each of the constituents. Therefore it is necessary for the viscosity to be strongly reduced on adding small quantities of a third substance. Put another way, there is interest in adding a third substance of the lowest possible viscosity.
The main disadvantage of such a method is to have to separate the two miscible phases in an extra process step. For example, distillation would be used to separate hexane from the oil phase. In addition to it representing an additional step, this operation is costly in energy which makes the method rather uncompetitive.
A third solution which makes the tangential filtration of viscous liquids possible consists of increasing the parietal stress by the filtration of a two phase, gas/liquid mixture.
In this case, the working pressure is of the same order as for traditional ultra-filtration since it reaches a maximum of 5 to 6 bars. This can qualify as atmospheric filtration since the permeate is at atmospheric pressure.
The gas is not dissolved in the liquid phase but is injected in co-current with the liquid, generally vertically. Because of the difference in the density of the two phases, the bubbles of gas will have an ascending speed greater than that of the liquid flow. They will create a “blockage” and the steric volume it occupies will destabilize the polarization layer by lamination. In absolute terms, the velocity gradient will become very high locally, causing a significant increase in the flow of the permeate. This method, on a laboratory scale, has also been described in the document by M. MERCIER and C. DELORME—“Influence d'un écoulement diphasique gaz/liquide sur les performances de la filtration tangential”—Colloque Université Industrie—Toulouse 13.06.96, and is only applicable when the product to be filtered does not foam.
A third method, which consists of using a “third substance” under pressure to treat hydrocarbons is described, for example, in the document FR-A-2 598 717 in which hydrocarbon oils are treated with a solvent to remove bituminous materials and to form a light oily phase and a heavy bituminous phase. The solvent is recovered from the oily phase by subjecting it to super-critical conditions for the solvent so as to form a new light phase enriched in solvent and a new heavy phase. This new light phase, very
Perre Christian
Sarrade Stephane
Schrive Luc
Commissariat A l'Energie Atomique
Fortuna Ana
Pearne & Gordon LLP
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