Agitating – By injecting gas into mixing chamber
Reexamination Certificate
1998-03-23
2001-07-10
Cooley, Charles E. (Department: 1723)
Agitating
By injecting gas into mixing chamber
C261S124000
Reexamination Certificate
active
06257751
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for maintaining solids in suspension in a liquid and to a container for use in the method. One example of application of the present invention lies in its potential use in slurry-fermentation processes in which organic or other slurries, such as animal or other wastes are caused, or allowed, to react to produce for example methane and a residue which may be used as a fertilizer and is better for spreading on fields than the original material.
It is known that the fermentation, or digestion, of organic wastes takes place under anaerobic conditions, being caused by anaerobic bacteria in the slurry, and the material must therefore be kept in closed containers for the duration of the fermentation period. This is a relatively long period, which may be of the order of several weeks, during which solids tend to come out of suspension if the slurry is not kept continuously agitated. This can cause various problems, not the least being difficulties in the removal of the solids if, for example, they compact at the bottom of the container or form a floating mass or crust.
DESCRIPTION OF THE PRIOR ART
At present various means are known for keeping the solids in such slurries in suspension, some of which involve mechanical action, such as stirring or the turning of the container itself about an axis to churn its contents. Sometimes gas is bubbled through the slurry to agitate it.
Some of the known methods entail the input of a substantial quantity of energy into the slurry which, in addition to the cost of the equipment itself, militates against the cost-effectiveness of the process as a whole, whilst others are not entirely effective in maintaining the solids in suspension. Digesters are not therefore in such widespread use as might be wished in an age in which fuel economy and recycling of waste products are particularly valued.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to provide a method for maintaining solid particles in suspension which can be effected with a smaller input of energy, and therefore at lower cost, than current methods without compromising the effectiveness with which the solids are maintained in suspension. A further object is to provide apparatus for carrying out such a method, which can be manufactured relatively simply and cheaply and is economical to run. A further and, in many respects, more important object is to provide a method and apparatus which will perform reliably over long periods of use to maintain solids of many different types in suspension.
The present invention, in one aspect, provides a method of maintaining solids in suspension in a liquid comprising driving the suspension to follow a circulatory path about a non-vertical axis in a container and constraining the flow to follow the line of minimum energy so as to maintain the solids in suspension.
In another aspect, the invention provides a method of maintaining solids in suspension in a liquid comprising driving the suspension to follow a circulatory path about a non-vertical axis in a container and providing restraint for the flow which forces it to follow a downward curve along at least part of its flow path so as to maintain the solids mixed in suspension in the liquid.
In another aspect, there is provided a slurry container having means for maintaining solids in suspension in a liquid by causing forced circulation thereof with the container stationary, in which part (preferably an upper part) of the container wall is so shaped as to guide the forced fluid flow to follow a curved path with a vertical component so as to maintain the solids in suspension. Alternatively or, more preferably in addition, a lower part of the container wall may be so shaped as to guide the forced fluid flow to follow a curved path with a vertical component so as to maintain the solids in suspension.
It will be appreciated that the method is particularly suitable for maintaining slurries of organic wastes in suspension during fermentation processes and reference will generally be made to this use below but the method is equally applicable to other suspensions.
The method and container of the invention have resulted from the realization that solids and liquids of different specific gravities tend to separate vertically if left stationary but if the liquid and the solids in a slurry are constrained to follow the same path simultaneously, then this relative separating movement will be prevented and the flow will remain substantially uniform or at least more uniform and homogeneous than is achieved in current slurry digesters for the same energy input. Hence the energy input required to achieve a desired degree of homogeneity is reduced.
The energy required to drive the circulatory motion of the slurry can conveniently be provided by the introduction of gas into the lower part of the slurry container, the gas being allowed to bubble up through the slurry and cause the slurry itself, in the vicinity of the gas bubbles, to rise. The gas is allowed to escape at the top of the container while the slurry circulates within it, the movement of the flow being constrained as indicated above, preferably by the shape of the upper part of the container wall, which preferably has a convex curvature towards the interior of the container. The rate of flow of the slurry and the shape of the container wall are preferably such as to achieve substantially laminar flow of the slurry along the wall in use.
In the upper part of its travel the slurry first follows a path with an upward component and a horizontal component directed away from the rising stream of gas bubbles. It is then constrained to move horizontally and downwardly until, eventually, its path becomes substantially vertical. The container may also have a vertical wall component adjacent this region but more preferably the wall is continuously curved so as to direct the flow downwardly and back towards the gas introduction zone.
The gas introduction zone need not be in the lowest part of the container; preferably the container wall is curved to give the flow a downward and horizontal component of movement to the bottom of the container followed by an upward and horizontal component to the gas-introduction zone where the flow becomes substantially vertical. Almost throughout its movement the slurry is thus constrained to follow a curved path, with a smooth flow, that energy is not wasted in creating turbulence and the process may be made more energy-efficient than prior art processes.
The circulatory flow may be driven by gas supplied into a lower or median part of the container. The container could have a circular horizontal section with at least one gas inlet in its base and a domed top but, for simplicity of construction, the container is preferably elongate in a horizontal direction in its position of use and has a uniform cross-section with gas inlets spaced longitudinally thereof. The container may be operated with a batch feed, only one inlet/outlet being provided for the slurry but, more preferably, the container is operated continuously with slurry being fed in and withdrawn through separate inlet and outlet apertures. In this case the slurry is forced to flow longitudinally through the container as well as being subject to the forced circulatory flow.
The gas inlets could be along or adjacent one side of the container but, again for ease of manufacture, it is preferred for the container to be mirror-imaged about a longitudinal plane of symmetry, the gas being introduced along the plane of symmetry so as to set up two circulatory flows in the two halves of the container, one on either side of the plane of symmetry. The two flows are driven in opposite senses by the rising gas stream, in use, with a vertically upward flow in the center of the container which diverges in the upper layers of the liquid into the two halves, each flow being constrained by the shape of the container wall to fall along an outer wall portion and then being directed back to the center along the bo
Cooley Charles E.
Gifford, Krass, Groh Sprinkle, Anderson & Citkowski, P.C.
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