Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2001-01-04
2002-07-02
Drodge, Joseph W. (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S511000, C210S513000, C210S799000, C210S800000, C210S804000, C210SDIG005
Reexamination Certificate
active
06413429
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention pertains to a process for extracting a component dissolved in a liquid by means of liquid-liquid extraction using an extraction liquid which is immiscible or only partially miscible with the liquid, in which process the extraction liquid is dispersed in the liquid in a dispersing apparatus and then coalesced in a coalescer, after which the extraction liquid, the specific weight of which differs by at least 5% from the specific weight of the liquid be extracted in which the component to be extracted is to be found, is separated from the liquid by gravity in a phase separator, as well as to an apparatus for implementing such as process.
The literature describes a large number of apparatuses for carrying out processes of the above-mentioned type. Particularly when high demands are made on the efficiency per extraction stage at a comparatively high throughput, generally use is made of columns or centrifuges. These as a rule require much greater investment than the well-known mixer-settlers. However, the latter have the drawback of a comparatively low throughput and hence are less suitable for use on a commercial schale.
A description of a mixer-settler is to be found in for example GB-A-1 443 704. The liquid to be extracted and the extraction liquid are continuously charged to a mixing chamber, where a dispersion is created by vigorous stirring. This is then passed through a perforate, coalescence promoting packing which encloses the chamber wholly or in part, on the opposite side of which packing a phase separator is provided.
One drawback to the known process consists in that the coalescence of very fine dispersions (<100 &mgr;m) through the indicated packings generally proceeds with great difficulty. Only when dispersions with a comparatively large droplet size are employed is it possible to achieve a high throughput. However, this is at the expense of the efficiency per extraction stage, the result of which is that a large number of these units have to be arranged in series, which calls for a high capital outlay.
It has also been found that the known process is subject to many restrictions, in particular because of the limited range of suitable extracting agents. Thus the viscosity of the extracting agent may be a restrictive factor when implementing the described dispersion method.
SUMMARY OF THE INVENTION
The invention now provides a process and an apparatus with the aid of which far higher efficiencies can be obtained with a comparatively small number of extraction units than is the case with the known apparatuses, while the selection of the type of extracting agent is subject to Far fewer restrictions.
The invention consists in that in a process of the known type mentioned in the opening paragraph to obtain an efficiency per extraction stage of at least 0.9 at an average residence time of at most 15 seconds in the dispersion apparatus and of at most 300 seconds in the phase separator, at a linear velocity in the coalescer related to its cross-section of at least 30 m/hour:
a. the dispersion apparatus used is a centrifugal pump which will give droplets having an average diameter of 5 to 500 &mgr;m,
b. the coalescer used is a packed bed of 5-100 cm high composed of dimensionally stable particles wettable by the extraction liquid having an average particle size of 0.05 to 2 mm, and
c. the coagulated phase in the phase separator is separated at a linear velocity related to the cross-section of the phase separator of 0.1 to 0.7 times the linear velocity in the coalescer.
In this case preference is given to a process where the average residence time in the dispersion apparatus is at most 10 seconds.
By efficiency per extraction stage is meant the ratio of the actual concentration of the substance to be extracted in the effluent, x
effl
, to the concentration which can be realised in a state of complete equilibrium, x
effl
*. The efficiency per extraction stage ranges from 0 to 1. A stage efficiency of 1 signifies that in the extraction stage in question a state of complete equilibrium has been achieved.
The number of equilibrium stages N required to achieve a pre-set extraction efficiency can be calculated with the aid of the Kremser-Souders-Brown equation:
N
=
ln
{
(
1
-
1
/
Λ
)
(
x
infl
-
x
effl
*
)
/
(
x
effl
-
x
effl
*
)
+
1
/
Λ
}
ln
Λ
,
wherein &Lgr; represent the extraction factor, x
infl
is the concentration of the component to be extracted in the influent, and x
effl
and x
effl
* are as indicated above.
The extraction factor &Lgr;=(m*&PHgr;
d
*&rgr;
d
)/&PHgr;
c
*&rgr;
c
, wherein
m=the mass based distribution coefficient in [kg
c
/kg
d
],
&PHgr;
c,d
=the flow rates of the continuous and the disperse phase, respectively, in [m
3
/hour], and
&rgr;
c,d
=the density of the continuous and the disperse phase, respectively, in [kg/m
3
].
To obtain an efficiency per extraction stage of at least 0.9, as a rule, to obtain a large specific surface area, preference is given to a dispersion in which the average droplet size of the disperse phase <<100 &mgr;m. According to the invention, preference is given in that case to the use of a modified or unmodified centrifugal pump in which at the fan tips rates of shear of 150,000 to 2,000 s
−1
can be realised, resulting in a Reynolds number (Re) of 25,000 to 500,000, corresponding to the formula:
Re=&rgr;ND
2
/&eegr;, wherein N represents the rotational speed of the fans in numbers of revolutions per second, D stands for the diameter of the fans in m, &rgr; stands for the density of the continuous phase in kg/m
3
, and &eegr; stands for the viscosity of the continuous phase in Pa.s, with the difference in surface tension between the disperse and the continuous phase ranging from 0.01 to 0.3 N/m and the viscosity ratio of the disperse phase (&eegr;
d
) to the continuous phase (&eegr;
c
) ranging from 0.1 to 10.
Examples of suitable centrifugal pump mixers have been disclosed, int. al., in
Ullman
, Vol. B3, Part 6, pp. 20 and 21. Preference is given in this case to a centrifugal pump mixer of the type described in U.S. Pat. No. 3,973,759. Various methods can be used for coalescing the dispersions obtained with these mixers. When they make mention of a packed column, it will generally be filled with, e.g., Raschig rings with a size ranging from 0.5″ to 1.5″, which corresponds to 12.7 to 38.1 mm. They make no mention whatsoever of the potential advantages of using a packed filter bed filled with extraction liquid-wettable particles having an average particle size of 0.05 to 2 mm. Such advantages are mentioned in EP-A-0 685 249, where a disperse liquid phase is separated from a gas or a liquid. However is document is not concerned with extraction, as is clear from the examples, which deal only with collecting very finely distributed droplets of liquid from an aerosol. Nor can any suggestion be derived from it to the effect that a liquid-liquid dispersion containing droplets of an average diameter ranging from 5 to 500 &mgr;m can be separated to a sufficient degree in such a column at a rate of 30 m/hour and that, in consequence, such a column would be pre-eminently suitable for use in a highly efficient liquid-liquid extraction process.
For that reason it has to be considered extremely surprising, especially in view of the very large dimensions of the packing material used up to now in commercially available mixer-settlers, that a much higher efficiency can be obtained at a much larger throughput than was customary up to now when the aforesaid packing material is employed.
It has been found that when the linear velocity in the coalescer is increased to more than 60 m/hour, favourable results cain still be obtained at a residence time in the phase separator of less than 150 and preferably not more than 60 seconds.
The height of the packed bed in the coalescer may range from 5 to 100 cm. However, a bed with a height selected in the range of 8 to 50 cm is prefer
Breman Berthold Berend
Visser Jan
Akzo Nobel NV
Drodge Joseph W.
Fennelly Richard P.
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