Liquid purification or separation – Processes – Including controlling process in response to a sensed condition
Reexamination Certificate
1999-10-12
2001-06-05
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Including controlling process in response to a sensed condition
C210S614000, C210S627000, C210S628000, C210S758000, C261S026000, C261S063000, C261SDIG007
Reexamination Certificate
active
06241897
ABSTRACT:
This invention relates to a method and apparatus for dissolving a gas, in particular oxygen-containing gas, in a liquid, such as water or an aqueous medium such as waste water. The invention is limited neither to the example of oxygen as the gas nor to the example of an aqueous medium but may be used to dissolve other gases having a greater or lesser solubility than oxygen in aqueous or non-aqueous media.
Our UK patent specification No 1 455 567 discloses a process for treating liquid including the steps of taking a stream of the liquid, pressurising the stream, introducing a treatment gas into the pressurised stream so as to dissolve therein some of the gas, and introducing the stream containing dissolved and undissolved gas into a volume of the liquid under turbulent conditions such that the undissolved gas enters the volume of liquid in the form of fine bubbles that either dissolve or are consumed within the volume of liquid. The method is typically used to dissolve oxygen in an aqueous medium. The oxygenated stream is typically introduced back into the main volume of liquid through one or more orifices or jets. Undissolved oxygen bubbles are transported in the stream to the orifices or jets. The length of the conduit through which the stream flows from the point of oxygenation to the jet or jets is chosen so as to facilitate dissolution of some of the gas in the liquid without being so long that the gas bubbles tend to coalesce into separate pockets or slugs of gas or that the pressure drop is disproportionate to the amount of gas that dissolves in the conduit.
The process described in UK patent specification 1 455 567 has been commercially successful particularly in the oxygenation of water to improve the treatment of sewage. We attribute this success at least in part to the fact that the process admits much more gas to be retained in the body of the liquid than prior process operating with same power consumption. By using the stream as a carrier of gas bubbles a much higher quantity of gas can be successfully dissolved than if the amount of gas carried in the stream is merely limited to that which can be dissolved therein.
Another advantage afforded by the process according to UK patent specification 1 455 567 is that the stream returning to the main volume of liquid agitates the liquid thereby helping to keep solids in suspension and assisting in the distribution of oxygen throughout the volume. However, a difficulty arises when a large volume of liquid, for example an activated sludge tank used in the treatment of sewage having a volume of more than 1000 m
3
cubic meters, a sewage lagoon, or a long stretch of river requires oxygenation. The difficulty is that more than one oxygenation apparatus with its own separate pump is required. The requirement arises out of the fact that if several subsidiary oxygenated streams are returned to the main volume of liquid at widely spaced locations, there is a high frictional loss in the associated pipework with a result that the subsidiary streams do not impart sufficient energy to the main volume of the liquid to cause the desired degree of agitation.
A solution to these problems is set out in our European Patent No. 389230 B1, which discloses a method and apparatus for dissolving gas in a volume of liquid in which liquid is pressurised and formed into a main stream which is distributed into a plurality of subsidiary streams each having a velocity greater than that of the main stream. Each subsidiary is passed through a venturi such that turbulence is created therein, and gas is introduced into the turbulence. The gas-containing subsidiary streams are introduced into the volume of the liquid at a velocity sufficient to create further turbulence causing bubbles of gas to shear into smaller bubbles.
Such an arrangement is particularly suited for oxygenating waste water, and is capable of dealing with the frequently highly fluctuating oxygen demands often found in industrial waste processing plants, but only at unnecessary expense. Where the oxygenating gas is oxygen, then the oxygen supply system must be capable of satisfying peak oxygen demand; this results in over-capacity of oxygen supply capability, since the oxygen demand peaks are intrinsically transient, and the oxygen source is under utilised. Moreover, the use of oxygen to meet the oxygen demand is inherently more expensive than is the use of air. However, the use of air is also economically disadvantageous, because more air is required for a given oxygenation level compared to oxygen. And again, because of the erratic levels of oxygen demand, the air supply system must be capable of meeting peak demand levels which occur infrequently, which means that for much of the time the air supply system is not used to its full capacity. In treating wastes with a high oxygen demand or with a fluctuating demand it may not be practicable or cost effective to utilise air alone for oxygenation, as the cost of providing sufficient air to achieve the necessary oxygenation level may be prohibitive.
Accordingly, the present invention provides a method of dissolving oxygen-containing gas in a volume of liquid having a variable oxygen demand comprising pressurising the liquid, forming a main stream of the liquid, distributing the main stream into a plurality of subsidiary streams each having a velocity greater than that of the main stream, passing each of the subsidiary streams through means configured to create turbulence therein, introducing gas into the turbulence and introducing the gas-containing subsidiary streams into the volume of the liquid at a velocity sufficient to create further turbulence causing bubbles of said gas to shear into smaller bubbles, characterised by introducing oxygen into a first set of the subsidiary streams and air into a second set of the subsidiary streams, by sensing the instantaneous oxygen demand in the volume of liquid, and by controlling the relative proportion of oxygen and air introduced so as to meet the sensed instantaneous oxygen demand.
The invention also provides an apparatus for dissolving oxygen-containing gas in a volume of liquid having an oxygen demand, the apparatus comprising a pump, a main conduit, a plurality of first and second subsidiary conduits in communication with the main conduit each of which includes means configured to create turbulence in the flowing liquid, means for introducing gas into the turbulence and a nozzle terminating in the volume of the liquid and having an outlet configured such that the liquid is introduced into the volume of liquid at a velocity sufficient to create further turbulence, and means for sensing the oxygen demand in the volume of liquid, characterised in that the gas introducing means in the first and second subsidiary conduits are adapted for introducing oxygen and air, respectively, and in that control means responsive to the sensed oxygen demand are provided to vary the relative proportions of oxygen and air introduced.
The methods and apparatus according to the invention are particularly suited for use in oxygenating waste water at significantly greater efficiency and less cost than in the prior art. Satisfying a proportion of the oxygen demand by the use of air as an oxygenating gas reduces the capacity of oxygen supply required, whilst the oxygenation of the remaining proportion utilising oxygen is highly efficient for tracking accurately variations in oxygen demand. Moreover, the replacement of at least some of the oxygen formerly used for oxygenation with air leads to significant reductions in cost, because the provision of a supply of air is generally less expensive than the provision of an equivalent supply of oxygen. This apparatus is also particularly suited to injecting air to enable stripping of carbon dioxide, which is known to cause problems through pH depression and/or toxicity in pure oxygen based systems. A further use of the apparatus is to inject other liquids or gases e.g. sodium hypochlorite, magnesium hydroxide, chlorine, carbon dioxide and ozone (for example, to suppress filam
Hanson Cedric
Race Michael Jack
Hruskoci Peter A.
Neida Philip H. Von
Pace Salvatore P.
The BOC Group plc
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