Liquid purification or separation – Processes – Separating
Reexamination Certificate
2000-03-21
2002-08-27
Drodge, Joseph W. (Department: 1723)
Liquid purification or separation
Processes
Separating
C210S369000, C210S771000, C210S781000
Reexamination Certificate
active
06440316
ABSTRACT:
BACKGROUND
Centrifugal filters are widely used for solid-liquid separation for a variety of particulate materials. In the coal and minerals industry, one type of particulate material is separated from another using various solid-solid separation methods. Since the separation is usually carried out in aqueous media, it is necessary to dewater the products before shipping to customers or downstream processes. In the coal industry, basket centrifuges are used to dewater the particles that are larger than approximately 1 mm, while finer particles are dewatered by means of screen bowl centrifuges. The latter is capable of providing considerably lower moistures than the more traditional vacuum filters, partly due to the loss of finer particles as effluent during filtration. In general, the moisture of dewatered product increases with decreasing particle size due to increased surface area. Therefore, elimination of the finest particles as effluent should help lower the dewatered product; however, it entails loss of valuables, which is not desirable.
When an aqueous suspension of particles is introduced to a batch centrifuge whose wall is made of a porous medium, the heavier solids settle quickly on the medium while the lighter water form a layer over the cake. As centrifugation continues, water begins to flow through the cake. The initial dewatering process, in which water flows through the cake while the cake is covered with a layer of water, is referred to as filtration. In time, the layer of water disappears from the surface of the cake, and the capillaries in the cake become saturated with water. The dewatering process that occurs with no water over the cake is referred to as drainage. For the reasons given below, the drainage process is much slower than the filtration process. Control of the rate of drainage is critical in controlling the final cake moisture.
The rate of drainage through the cake can be predicted by Darcy's law:
Q
=
K
Δ
PA
μ
L
[
1
]
where Q is the flow rate, K the permeability of the cake, &Dgr;P the pressure drop across the cake, A the filtration area, &mgr; the dynamic viscosity of water, and L is the cake thickness. During the filtration period, the pressure drop across the cake is determined by the following relationship:
Δ
P
=
1
2
ρω
2
(
r
S
2
-
r
0
2
)
,
[
2
]
where &rgr; is the density of the liquid, &ohgr; the angular velocity, and r
0
and r
s
are the radial distances of the free water and the cake surface from the rotational axis of a centrifuge, respectively. From Eqs. [1] and [2], one can see that the rate of filtration should increase with &ohgr; and the thickness (r
s
−r
0
) of the water over a filter cake.
According to Eq. [2], &Dgr;P becomes zero, when the water over the cake disappears, i.e., r
0
=r
s
. As the water level in the cake decreases further, i.e., r
0
>r
s
, the pressure within the cake becomes lower than the ambient pressure, as shown by the mathematical model developed by Zeitsch (in
Solid
-
liquid Separation
, 3rd Edition, edited by L. Svarovsky, Buttreworth, London, 1990, p.476). The model calculations show that the pressure in the cake becomes increasingly negative with increasing cake thickness.
Despite the lack of positive pressure drop in the cake, dewatering occurs during the drainage period inasmuch as the centrifugal force within the cake exceeds the sum of the forces holding the water in the capillaries, the forces created by the negative pressure, and the forces due to hydrodynamic drag. The process of drainage relying solely on the centrifugal force entails high energy consumption and requires high maintenance to obtain low cake moistures. Energy consumption and maintenance are the major concerns in using centrifugal filters for solid-liquid separation. In the present invention, methods of overcoming these problems are disclosed. They include methods of increasing the gas pressure inside a centrifuge and/or reducing the air pressure outside. These provisions are designed to increase the pressure drop across a filter cake, so that one can take advantage of the Darcy's law (Eq. [1]), which suggests that dewatering rate should increase with increasing pressure drop. The extraneous methods of increasing the pressure drop, as disclosed in the present invention, is particularly useful for increasing the rate of dewatering during the drainage period, which is critical in achieving lower cake moistures. The methods disclosed in the present invention are useful for obtaining low cake moistures without causing high energy consumption and maintenance problems.
A series of U.S. patents (U.S. Pat. Nos. 3,943,056 and 4,052,303) awarded to Hultch disclosed a method of creating a negative pressure on the outside wall of a centrifuge and thereby increasing filtration rate. This is accomplished by creating a chamber outside the filter medium, in which filtrate water is collected. Since the water in this chamber is subjected to a larger centrifugal force that that remaining in the cake, a negative (or vacuum) pressure is created due to a siphon effect. This technique is, therefore, referred to as the method of using rotating siphon. However, the effectiveness of this method breaks down as soon as air enters the filtrate chamber through the filter cake. This will not allow a sufficiently long drainage period, which is often necessary for producing low cake moistures.
The U.S. Pat. No. 4,997,575 teaches a method of using rotating siphons in a pressure housing with superatmospheric pressure, which is controlled by a difference in filtrate liquid levels in the filtrate liquid chamber and the annular space following the filter. This liquid control prevents the penetration of filtrate liquid into the gas exhaust line.
The U.S. Pat. Nos. 5,771,601 and 5,956,854 teach a method of injecting a gas stream such as air into the bed of particles during centrifugation and thereby reducing the surface moisture of the particles. The turbulent flow created by the gas flow strips the water from the surface of the particles. This technique is useful for the particles in the range of 0.5 to 30 mm that are dewatered in basket centrifuges. In this invention, the stream of gas is injected into an open space. Therefore, it cannot significantly increase the pressure drop across the bed of particles. Also, it would be difficult to increase the pressure drop, when a cake is continually disturbed by a scrawl, which is widely used to move the particles in basket centrifuges. Furthermore, the airflow is created by a blower rather than a compressor, which should make it difficult to create a high pressure drop across a filter cake.
SUMMARY OF THE INVENTION
According to the theoretical considerations given above, the rate of dewatering is low during the drainage period of a centrifugal filtration process, which in turn can be attributed to the lack of positive pressure drop across filter cake. This problem can be overcome by increasing the pressure drop using extraneous means such increasing the gas pressure inside a centrifugal filter and/or reducing the pressure of the gas (air) outside. It has been found that these provisions greatly enhance the rate of drainage and, thereby, lower the cake moistures.
In effect, the present invention suggests methods of combining the conventional centrifugal filtration with pressure and/or vacuum filtration. However, the moisture reductions that can be achieved using the combined method are substantially lower than the sum of the moisture reductions achieved using the different dewatering methods individually. Thus, the combined method exhibits synergism. Although the increase in drainage rate induced by the extraneous means of increasing the pressure drop can provide an explanation for the observed improvement, there may be other mechanisms that are responsible for the synergism.
In a typical operation, a slurry is introduced to a basket-type centrifuge whose side
Asmatulu Ramazan
Yoon Roe-Hoan
Drodge Joseph W.
Grossman Tucker Perreault & Pfleger PLLC
Virginia Tech Intellectual Properties Inc.
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