Patent Application
20100176063
The present invention relates to separations of immiscible liquids. More specifically, the present invention relates to method for coalescence induced liquid-liquid separations and apparatus thereof.
Many industrial processes involve mixtures of immiscible liquids. In some cases, the mixing of two liquids is necessary to obtain mass transfer between the phases or to promote a chemical reaction, but in others, it is an unintended or unavoidable result of the process.
Almost always, a full separation of the liquids may be important for efficient and cost effective performance of the downstream process.
Physical properties such as the size of the droplets (dispersed phase), the pH of the liquid mixture, and temperature variations within the liquid mixture are important and may completely alter the characteristics and ease of the separation process. Furthermore, the presence of solids and/or trace impurities within the liquid mixture may also increase the complexity of the separation process, and equally important is the size of the droplets since a given liquid-liquid separator has a minimum droplet size (dmin) that can efficiently be separated in the device.
There is thus provided, in accordance with some embodiments of the present invention, a method for coalescing droplets having a diameter d, said diameter d having a value of d* or larger in a given coalescing apparatus. The method includes: (a) mixing at least one first liquid with a second liquid in a coalescing apparatus for substantially residence time, tres, (b) defining a breakage probability, Pbreak, of said droplets to be Pbreak=f(μd,μc,σ,d,εvolume) where μd is the viscosity of said at least one first liquid, μc is the viscosity of said second liquid, σ is the interfacial surface tension of said droplets, and εvolume is the turbulent energy dissipation distribution per volume, (c) defining a coalescence probability, Pcoalescence, of said droplets to be Pcoalescence=f(μd,μc,σ,φ,Pr,d,εvolume) where φ is the concentration of said at least one first liquid in said second liquid, and Pr is the restricting pressure at the interface of said droplets, (d) defining a multiplication variable to be equal to (d/dav)x where d<dav, and (e) controlling said mixing so that a maximum value of the energy dissipation value, εmax, is greater than
where ρc is the density of the continuous phase.
Furthermore, according to embodiments of the present invention, (d/dav)x is smaller than 1 at all times.
Furthermore, according to embodiments of the present invention, a value obtained by multiplying said coalescence probability, Pcoalescence, by said multiplication variable, is greater than said breakage probability, Pbreak.
Furthermore, according to embodiments of the present invention, d* is the minimal coalescable diameter of droplets for said given coalescing apparatus, and the residence time tres is greater than 2/(Pcoalescence(μd,μc,σ,φ,Pr,εvolume,d*)·(d/dav)x−Pbreak(μd,μc,σ,d*,εvolume)).
Furthermore, according to embodiments of the present invention, x may range between ⅓ to ⅔.
The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.
A method and apparatus for separating immiscible liquids effectively in accordance to embodiments of the present invention are provided herein. Said method and apparatus may allow coalescing of relatively small-sized droplets having a diameter d, said diameter d having a value of d* or larger in a given coalescing apparatus for easing and improving the degree of separation thereafter.
The method may be defined by a system of equations describing the requirements and conditions imposed on the kinetics of drop breaking and coalescence as functions of properties of the involved liquids, involved energy and means for inducing mixing energy into the mixture, such as, but not limited to, viscosity, interfacial tension, droplet diameter distribution, average droplet diameter, average volumetric droplet diameter, concentration of the dispersed liquid in the coalescing apparatus, restricting pressure of the electrostatic double layer surrounding the interfacial boundary of the droplets, and turbulent energy dissipation distribution per volume within the coalescing apparatus.
Referring now to
It is to be understood that d* is the minimal coalescable diameter of droplets for a given coalescing apparatus in accordance to embodiments of the present invention.
The method may comprise mixing an immiscible mixture of at least one first liquid (dispersed phase) with a second liquid (continuous phase) in a coalescing apparatus for substantially residence time, tres, (block 102).
For the at least one first liquid which is dispersed in the second liquid and includes droplets of various diameters, the breakage probability, Pbreak, of droplets having a diameter, d, may be defined as Pbreak=f(μd,μc,σ,d,εvolume) (block 104), where μd is the viscosity of the dispersed liquid phase i.e., the viscosity of the at least one first liquid, μc is the viscosity of the continuous liquid phase i.e., the viscosity of the second liquid, σ is the interfacial surface tension of the droplets, and εvolume is the turbulent energy dissipation distribution per volume.
The coalescence probability, Pcoalescence, of droplets having a diameter d, may be defined as Pcoalescence=f(μd,μc,σ,φ,Pr,d,εvolume) (block 106), where φ is the concentration of the dispersed liquid phase in the continuous liquid phase i.e., the concentration of the at least one first liquid in the second liquid, and Pr is the restricting pressure at the interface of the droplets.
The method may further comprise defining the average droplet diameter, dav, as dav=f(μd,μc,σ,φ,Pr,εvolume) (block 107).
It should be noted that the average diameter of the droplets, the breakage probability, Pbreak, and the coalescence probability, Pcoalescence, of the droplets may be calculated using known methods described in Leonid N. Braginsky and Yury V. Kokotov, “Kinetics of Break-Up Coalescence of Drops In Mixing Vessels”, International Symposium on Liquid-Liquid Two Phase Flow and Transport Phenomena, Antalya, Turkey, Nov. 3-7, 1997.
The mixing of said at least one first liquid with said second liquid according to embodiments of the present invention may be done by controlling the degree of turbulence so that a maximum value of the energy dissipation value, εmax may be greater than
(Condition I, block 108) where εmax is the turbulent energy dissipation value in regions of most intensive turbulence within the coalescing apparatus, ρc is the density of the continuous phase.
Controlling physical parameters such as, but not limited to the concentration of the at least one first liquid in the second liquid, φ, and the turbulent energy dissipation distribution per volume, εvolume, so that the coalescence probability value Pcoalescence (μd,μc,σ,φ,Pr,d,εvolume) multiplied by a multiplication variable of (d/dav)x (where d<dav) is maintained greater than the breakage probability, Pbreak (μd,μc,σr,d,εvolume) for any d greater than d* (Condition II, block 110) where dav is the average diameter of the droplets in the coalescing apparatus, (d/dav)x is smaller than 1 at all times, and x lies in the range between ⅓ to ⅔ in accordance to embodiments of the present invention.
The method may further comprise controlling the residence time, tres, to be greater than: 2/(Pcoalescence(μd,μc,σ,φ,Pr,d*)·f(d*, dav)−Pbreak(μd,μc,σ,d*))(Condition III, block 112).
In accordance to embodiments of the present invention, turbulence may be induced by any means, including but not necessarily by the use of at least one agitator having a plurality of blades.
It should be noted, however, that in case that at least one agitator is used for inducing turbulence, the agitator's blades or other surfaces used for the mixing of said at least one first liquid and said second liquid should have a surface curvature smaller than 4 divided by the blade or surface width in order to reduce/minimize the breakup of droplets during the coalescence process (Condition IV, block 114).
It should be noted that for a blade with a varying width, the local surface curvature, should be smaller than k divided by the respective local blade width, where k=4.
While the above-mentioned conditions I-III must be satisfied for coalescing droplets with diameter d having a a value of d* or larger, satisfying condition IV may not be necessary for all emulsions in accordance with embodiments of the present invention.
Referring now to
Inlet stream 202 of at least one first liquid dispersed in a second liquid may enter coalescing apparatus 204 wherein the dispersed at least one first liquid contains droplets of various sizes including droplets having diameters ranging between d* and dmin where dmin is a minimal separable diameter in a given separator. After treatment, outlet stream 206 may contain a greater number of droplets having a diameter equal to or greater than dmin compared with the relative situation in inlet stream 202. Furthermore, outlet stream 206 may contain a substantially smaller number of droplets with diameters smaller than dmin compared with the relative situation in inlet stream 202.
Stream 206 exits coalescing apparatus 204 and may enter separator 210 to separate outlet stream 206 into two separated streams e.g., into a first separated stream 212 and a second separated stream 214.
First separated stream 212 may comprise a liquid that is producible from droplets having a diameter equal to or greater than a minimal separable diameter, dmin, where dmin is the minimal diameter of droplets that may be separated in separator 210, i.e., in a given separator. According to embodiments of the present invention the minimal separable diameter dmin, is greater than d*. Therefore, in order to increase the degree of separation, the efficiency of the coalescing process should be optimized so that as many as possible droplets with diameters ranging from d* to dmin should coalesce into larger droplets.
After separation, a portion of first separated stream 212, e.g., stream 208 may be returned back (i.e., recycled) into coalescing apparatus 204, as will be explained in more detail hereinafter.
The recycling scheme, which returns a portion 208 of said first separated stream 212 back to coalescing apparatus 204 may be intended for keeping the concentration φ of the dispersed phase in the continuous phase i.e., the concentration of said at least one first liquid in said second liquid in the coalescing apparatus, within a certain concentration range. The defined concentration may be expressed in normalized value and may be kept, according to embodiments of the present invention, in a substantially predetermined normalized range such as, for example, 0.2-0.3. However, it should be mentioned that for some liquids, it is possible that a different normalized concentration range may also be effective.
A predetermined normalized concentration range such as, for instance, 0.2-0.3 may be required for satisfying the above mentioned condition II. However, it should be noted that keeping the concentration value equal to a predetermined concentration value in coalescing apparatus 204 may not necessarily require the use of a recycle scheme. Instead, an external source of liquid, for example said at least one first liquid, may be used for supplying the dispersed phase to the coalescing apparatus as needed for keeping the concentration value as desired.
Referring now to
Curve 302 defines the droplet size distribution of an emulsion prior to entering into a coalescing apparatus. As seen in the figure, the emulsion contains droplets smaller than the minimal separable diameter, dmin, 304 of a given separator.
Referring now to
Curve 354 defines the droplet size distribution of an emulsion exiting a coalescing apparatus. As seen in the figure, the quantity of droplets having a diameter smaller than the minimal separable diameter, dmin, 304 has substantially decreased as a result of the coalescing process. Similarly, as a result of the coalescing process, a relatively large number of droplets having a diameter larger than the minimal separable diameter, dmin, 304 has formed.
It should be noted that satisfying condition III, i.e. keeping tres greater than: 2/(Pcoalescence(μd,μc,σ,φ,Pr,d*)·f(d*,dav)−Pbreak(μd,μc,σ,d*)) may be achieved either by manipulating the flow rate of inlet stream 202 into coalescing apparatus 204 and/or the flow rate of treated stream 206 exiting coalescing apparatus 204, or by controlling the volume of liquid in the coalescing apparatus 204. Other methods and means for satisfying the requirements of condition III above may be used, alternatively or concurrently.
In order to keep the residence time, tres, and the size of the coalescing apparatus not exceedingly large, design specifications, such as, but not limited to, the concentration of the dispersed phase and the turbulence profile in the coalescing apparatus should be such that the value of coalescence probability, Pcoalescence, multiplied by a multiplication variable of (d/dav)x where x lies in the range between ⅓ to ⅔ in accordance to embodiments of the present invention, is significantly larger than the breakage probability, Pbreak.
It should be noted that in accordance to embodiments of the present invention, a coalescing system may include at least one coalescing apparatus 204 and/or at least one separator 210. Similarly, coalescing apparatus 204 may include at least one vessel and at least one agitator.
Furthermore, coalescing apparatus 204 may include at least one baffle for easing and improving the mixing of said at least one first liquid with said second liquid in accordance to embodiments of the present invention.
Separator 210, in accordance with embodiments of the present invention, may be designed based on, but not limited to, various techniques such as centrifugal separation, hydrocyclonic separation, gravitational separation, electrostatic separation and any combination thereof.
Referring now to
The average diameter of the droplets of the at least one first liquid dispersed in the second liquid may be estimated by using the following equation.
d
av(t)=f(μd,μc,σ,φ,Pr,εvolume,t)
where μd, μc, and σ are measured experimentally, (block 408).
The breakage probability, Pbreak, of droplets having a diameter d, is defined as Pbreak=f(μd,μc,σ,d,εvolume) (block 410).
Referring now to
(condition I, block 412). The coalescence probability, Pcoal, of droplets having a diameter, d, may be defined as Pcoalescence=f(μd,μc,σ,φ,Prd,εvolume) (block 414).
Physical parameters such as, but not limited to the concentration of the at least one first liquid in the second liquid, φ, and the turbulent energy dissipation distribution per volume, εvolume, may be controlled for having the coalescence probability value Pcoalescence (μd,μc,σ,φ,Pr,d,εvolume) multiplied by (d/dav)x greater than the breakage probability, Pbreak(μd,μc,σr,d,εvolume) for any d greater than d* (Condition II, block 416).
The residence time, tres, may be controlled to be greater than: 2/(Pcoalescence(μd,μc,σ,φ,Pr,d*)·f(d*,dav)−Pbreak(μd,μc,σ,d*)) which satisfies Condition III (block 418)
The method may further comprise defining a recycling flow rate for keeping the concentration of the at least one first liquid dispersed in the second liquid within a concentration range expressed in normalized values such as but not limited to 0.2-0.3 in coalescing apparatus 420.
It should be noted that keeping the concentration of the at least one first liquid in said second liquid in the coalescing apparatus within a predetermined normalized range such as, for instance, 0.2-0.3 may be essential for the coalescence process since the lower limit of the range, i.e., 0.2 in this case, signifies a value below which condition II may not be satisfied, and the upper limit of the range, i.e., 0.3 in this case, signifies an approximate value above which the emulsion mixture may undergo an inversion meaning that the at least one first liquid may no longer be dispersed in the second liquid, but instead, the second liquid may be dispersed in the at least one first liquid.
It should be noted that for some liquids phase inversion does not occur until normalized concentrations approaching 0.5 or higher.
Since the restricting pressure of the electrostatic double layer, Pr, is not directly measurable, a curve fitting technique may be used for estimating the value of the restricting pressure in accordance with embodiments of the present invention. More specifically, a droplet average diameter versus time profile obtained from experimental measurements may be compared to profiles generated from numerical simulations for various values of the restricting pressure. Then, the correct restricting pressure value may be assumed to be the restricting pressure value used for simulating the profile which best fits the experimental profile.
Referring now to
Dotted curve 502 was generated from experimental measurements while curves 504, 506, 508, 510 and 512 were generated by numerical simulations for restricting pressure values of 4.5 N/m2, 10 N/m2, 20 N/m2, 40 N/m2, and 90 N/m2 respectively. As seen, curve 508 best fits dotted curve 502; therefore, the restricting pressure value in this case may be assumed to be 20 N/m2.
A coalescing apparatus-separator system in accordance to embodiments of the present invention was tested for the removal of oil from an aqueous phase. The flow rate of a continuous liquid phase (i.e., aqueous phase) containing 1% oil in the form of droplets of approximately 5 μm into a coalescing apparatus equaled 10 l/h. The oil (dispersed phase) and the aqueous phase (continuous phase) possessed the following properties:
The coalescing apparatus included:
A recycle scheme with a recycle flow rate of 4.3 l/h was used so that the concentration of the dispersed phase in the continuous phase in the coalescing apparatus possesses a normalized value of 0.3.
A commercially available software suitable for hydrodynamic-type calculations was used for calculating the following turbulence properties:
equals 200 W/kg (i.e., smaller than the maximum value of the turbulent energy dissipation, εmax, which, as noted earlier, equals 450 w/kg;
2/(Pcoalescence(μd,μc,σ,φ,Pr,d*)·(d*/dav)2/3−Pbreak(μd,μc,σ,d*,εvolume))=500 sec;
The separation process was carried out in a settler and took about 0.5 hour. The resulting first separated stream contained oil droplets with an average diameter of about 5 μm. The resulting second separated stream included about 0.21% of oil in aqueous phase, e.g., a percent concentration that is about 5 times lower than the percent concentration of oil in aqueous phase typically obtained in separation processes not including at least one coalescing apparatus.
The process characteristics described above in example I also apply to the current example as well, except the impeller rotation speed which is 500 Rpm in this example. In this case, the energy dissipation in zone of maximum turbulence is 110 W/kg (i.e., smaller than
which equals 200 W/kg. Therefore, condition I is not satisfied.
The resulting second separated stream included about 0.83% of oil in aqueous phase, e.g., a concentration which is close to the concentration of the oil phase in the aqueous phase of the emulsion prior to entering into the coalescing apparatus. Therefore, the effect of the coalescing process in this case is relatively poor.
The process characteristics described above in example I apply to the current example as well excluding the average droplet diameter which is 2 μm in this case. Energy dissipation in zone of maximum turbulence is 500 W/kg (i.e., greater than:
which equals 200 W/kg.
2/(Pcoalescence(μd,μc,σ,φ,Pr,d*)·(d*/dav)2/3−Pbreak(μd,μc,σ,d*,εvolume))=1800 sec;
The resulting second separated stream included about 1.012% of oil in aqueous phase, e.g., a percent concentration that is greater than the percent concentration of the oil phase in the aqueous phase in the emulsion prior to entering into the coalescing apparatus.
Therefore, in this case, the inclusion of a coalescing apparatus in the separation process worsens the situation.
As illustrated in examples II and III, when at least one of the 3 specified conditions I-III is not satisfied, the coalescing process may not be efficient and may fail to produce the desired results. Therefore, conditions I-III must be satisfied for having an efficient coalescing process in accordance to embodiments of the present invention.
While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
