Improved gas dispersion process

Abstract: Gas dispersion in a liquid is enhanced by combining the gas and liquid in close proximity to a venturi or other flow constriction means (2) used to create supersonic flow velocities and subsequent deacceleration to sub-sonic velocity. When a venturi is employed, the gas-liquid mixing occurs in the converging portion (3) of the venturi or upstream but in close proximity to said venturi. Turbulent flow conditions upstream of the venturi or other flow constriction means adequate to disperse the gas uniformly in the liquid are not required and, surprisingly, the gas and liquid need not be uniformly dispersed prior to acceleration to supersonic velocity.

Description: This invention relates to the mixing of gases and liquids. More particularly, it relates to enhancing the dispersion of gases in liquids. The dispersion of gases in liquids is an important feature of a wide variety of industrial operations. Thus, gases are dispersed in liquids for numerous gas dissolution, gas-liquid reaction and gas stripping of dissolved gas applications. As the gas is more finely dispersed in the liquid in the form of very small gas bubbles, the interfacial surface area between the gas and liquid is appreciably increased as compared to said surface area between the liquid and a like quantity of gas in the form of larger gas bubbles. In turn, an increase in the interfacial surface area between the gas and liquid is known to increase the mass transfer of the gas from the gas bubbles into the liquid, as well as the transfer of dissolved gas from the liquid into the gas bubble. Thus, by providing much higher interfacial area, all gas-liquid processes, such as gas dissolution, gas stripping and reactions between the gas phase and substances in the liquid phase will be improved. As a consequence of a change in flow velocity of a gas-liquid mixture crossing the velocity of sound in said mixture, gas bubbles in the liquid are very substantially broken up by the sonic shock wave created thereby. This phenomenon occurs upon an acceleration of the flow velocity of the gas-liquid mixture from a velocity of less than the speed of sound to one exceeding said speed of sound in the gas-liquid mixture, and/or when the flow velocity is decreased from above to below said supersonic velocity. As a result of said shock wave effect, fine gas bubbles having diameters in the range of from about 0.1 to 0.01 mm are produced. For gas-liquid mixtures initially having larger sized gas bubbles typically having a bubble diameter of about 2 mm, the formation of such fine gas bubbles corresponds to an increase in bubble surface area on the order of 20 to 200 times compared to the surface area of the initial larger diameter bubbles. The use of sonic shock waves to reduce the size of gas bubbles dispersed in a liquid is disclosed in the Garrett patent, U.S. 4,639,340 and the corresponding EP-A-0 152 202, which is directed particularly to the dissolving of oxygen in waste water. Oxygen is introduced from a side pipe into a pressurized stream of waste water and is uniformly dispersed in said stream at a flow velocity less than the velocity of sound in the gas-liquid mixture. For this purpose, Garrett indicates that a flow velocity of at least about 2 meters per second should be used to establish a turbulent flow condition such as to create and maintain a uniform dispersion of gas bubbles in the liquid, apart from the oxygen that is dissolved in the waste water stream, immediately downstream of the point at which the side pipe for oxygen introduction joins the conduit through which the pressurized stream of waste water is being passed. Typically, such uniform dispersion of gas in liquid can be developed in a turbulent flow condition in a distance of at least about three times the diameter of the conduit. It will be appreciated, however, that the distance between the point of oxygen introduction and the positioning of means for acceleration of the gas- liquid mixture to supersonic velocity may be considerably longer than this distance in practical commercial applications. Garrett discloses the passage of the uniformly dispersed gas-liquid mixture formed at said velocity in the sub-sonic, but turbulent flow, range to a venturi for the acceleration of the flow velocity to a velocity in excess of the speed of sound in said gas-liquid mixture. Garrett states that, in the region of the venturi between its upstream end and its throat portion of minimum diameter, the velocity of said uniformly dispersed gas-liquid mixture increases and reaches a velocity in excess of the velocity of sound in said dispersion, and that a sonic shock wave is created within said region of the venturi. As a result, the relatively coarse bubbles of oxygen in the dispersion are sheared into smaller or finer bubbles by the turbulence resulting from the shock wave. After passing through the throat of the venturi, the pressurized stream is deaccelerated as the venturi widens until it is returned to a sub-sonic velocity, which is nevertheless sufficiently high to maintain turbulent flow and the uniform dispersion of oxygen bubbles in the waste water stream. Garrett also discloses that, in the event the oxygen were introduced into the waste water stream through the throat of the venturi, as is conventionally accomplished for certain gas-liquid mixing operations, no sonic shock wave would be produced in the upstream, converging portion of the venturi. CA-A-1 033 078 discloses a method for aerating a body of liquid with small bubbles comprising establishing a flow of liquid through a pipe; constricting said flow through a convergent venturi section thereby to increase its velocity; adding a flow of gas either under pressure or by natural aspiration into the convergent venturi section precisely at or adjacent the area of maximum constriction in sufficient quantity to create sonic velocity at the minimum constriction; and expanding the area of the resultant two-phase flow from the minimum of said convergent venturi section at a rate of between 0.5 and 3.5 degrees per unit of axial distance equal to the minimum diameter of said convergent venturi section thereby to cause supersonic flow velocities downstream from said minimum constriction and an attendant shock wave to reduce the gaseous phase to small bubbles. Despite such advantageous use of venturi devices to enhance the dispersion of a gas in a liquid, there remains a desire and a need to further enhance the process for dispersing gases in liquids. Such requirements pertain to gas-liquid processing operations in general, and are related to the continual desire in the art for improvement in industrial processing operations. There is also a general desire in the art for a more efficient use of oxygen, nitrogen and other industrial gases in the wide variety of commercial applications in which industrial gases are presently employed or could be advantageously employed to improve the current practice in the art. It is an object of the invention, therefore, to provide an improved process for the dispersion of gases in liquids. It is another object of the invention to provide a process for enhancing the interfacial surface area between a gas and the liquid in which it is dispersed so as to enhance the mass transfer between said gas and liquid. It is a further object of the invention to provide a process capable of enhancing the efficiency of gas-liquid dispersion operations. With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims. A process for the dispersion of a gas in a liquid which is passed through a flow conduit, comprising: (a) introducing the gas into the stream of liquid to form a gas bubble-liquid mixture; and (b) passing said gas bubble-liquid mixture through the throat section of a venturi; in conformity with the present invention is characterized by passing the stream of liquid through said flow conduit upstream of said venturi at a velocity less than that required to create and maintain turbulent flow conditions upstream of the venturi sufficient to achieve uniform dispersion of the gas in the liquid; injecting said gas into said liquid at an injection point which is disposed in the flow conduit at the upstream side of the converging section up to a point spaced from the upstream inlet end of said converging section by about one diameter in length, based on the diameter of the flow conduit, to thereby form a gas bubble-liquid mixture having a flow velocity of less than the velocity of sound in said gas-liquid mixture; accelerating the flow velocity of the gas-liquid mixture from sub-sonic velocity to supersonic velocity in an upstream converging section of the venturi; and subsequently deaccelerating the flow velocity of the gas-liquid mixture from supersonic velocity to sub-sonic velocity in a downstream diverging section of the venturi. Surprisingly, the gas and liquid need not be uniformly dispersed prior to the acceleration of the gas-liquid mature to supersonic velocity, and its subsequent deacceleration to sub-sonic velocity, to achieve an enhanced dispersion of the gas in the liquid. The invention is further described herein with reference to the accompanying drawings in which: Fig. 1 represents a side-elevational view, partly in cross-section, of a gas-liquid dispersion system adapted to carry out the process of the invention; and Fig. 2 is a chart illustrating the effect of the distance of the oxygen injection point from the throat of a venturi on the mass transfer rate of the oxygen in a stream of water. In the present gas-liquid dispersion process the gas and liquid are mixed in close proximity to the venturi capable of accelerating the flow velocity of the gas-liquid mixture from below to above supersonic velocity, with subsequent deacceleration to below said supersonic velocity of the gas-liquid mixture. The flow velocity of the mixture prior to such acceleration need not be maintained sufficiently high to establish turbulent flow conditions, and the gas need not be uniformly dispersed in the liquid prior to such acceleration. Nevertheless, the mass transfer rate between the gas and the liquid is significantly and surprisingly enhanced over that obtainable using the prior art practice referred to above, in which the flow velocity is to be maintained high enough to establish and maintain turbulent flow conditions, with consequent uniform dispersion of the gas in the liquid at a relatively large distance upstream of a venturi used to accelerate the mixture to supersonic velocity. In the practice of various practical commercial embodiments of the invention, the gas to be dispersed in the liquid is injected with a liquid stream as said liquid stream passes with close proximity with the venturi used to accelerate the flow velocity of the gas-liquid mixture from a sub-sonic to a supersonic velocity, with subsequent deacceleration to a sub-sonic velocity. The acceleration of the gas-liquid mixture from sub-sonic to supersonic flow velocities, with subsequent deacceleration to sub-sonic levels with its consequent sonic shock wave effect on the gas-liquid mixture is accomplished by a venturi. The venturi achieves the desired acceleration-deacceleration to above and below the velocity of sound in the gas-liquid mixture at the prevailing pressure conditions while tending to minimize the accompanying pressure loss in the flow conduit. A particular embodiment of the gas-liquid dispersion system used for carrying out the process of the invention, employing a venturi for gas-liquid acceleration and deacceleration purposes, is illustrated in Fig. 1 of the drawings, in which numeral 1 represents a gas-liquid dispersion body in which said venturi section is machined for use as described herein. The venturi comprises throat section 2 of minimum diameter, upstream converging section 3 and downstream diverging section 4. At said upstream end of dispersion body 1, an inlet conduit 5 is provided for the introduction of a stream of liquid into dispersing body 1. In this embodiment, the direction of said flow of liquid is perpendicular to upstream liquid flow conduit 6 through which the liquid is passed to upstream converging section 3 of the venturi, although those skilled in the art will appreciate that other configurations of liquid flow can be employed to minimize pressure losses in the system. The gas-liquid mixture discharged from the venturi through downstream diverging section 4 passes into downstream flow conduit 7 for passage from said dispersion body 1 through gas-liquid outlet line 8. For the introduction of gas into the liquid stream passing through dispersion body 1, gas inlet tube 9 is positioned within upstream liquid flow conduit 6 and extends up to an injection point which is disposed in the flow conduit 6 at the upstream side of the converging section 3 up to a point spaced from the upstream inlet end of said converging section by about one diameter in length, based on the diameter of the flow conduit. Gas injection tip 10 is provided to facilitate the injection of gas into the liquid. The upstream end of gas inlet tube 9 will be seen to extend through liquid chamber 11, into which liquid passes from liquid inlet 5 for discharge into upstream liquid flow conduit 6, and inlet cover plate 12 for communication with a source of supply for said gas. O-rings 13 are positioned between cover plate 12 and the upstream end portion of dispersion body 1 to provide a fluid-tight seal therebetween. O-rings 14 are similarly provided to assure a fluid-tight seal between gas inlet tube 9 and cover plate 12. Seal plates 15 and 16 are provided at the downstream end of said cover plate 12, respectively, to assure proper deformation of O-rings 14 into a fluid-type position. Suitable clamping means 17 and 18 are used to secure end plate 12 to said seal plates 15 and 16 and to dispersion body 1. Those skilled in the art will appreciate that any convenient means, manual, mechanical or otherwise, can be used to slide the gas inlet tube into any desired position relative to the position of the venturi. In an embodiment of the apparatus for carrying out the process of the invention as illustrated in Fig. 1, throat section 2 of the venturi was 4.8 mm (0.19") in inside diameter and 9.1 mm (0.36") long. The inside diameters of upstream and downstream flow conduits 6 and 7 were 16.0 mm (0.63") and 21.1 mm (0.83"), respectively. Converging section 3 was 18.3 mm (0.72") long, and diverging section 4 was 25.4 mm (1") long. Thus, it will be appreciated that the converging and diverging sections of the venturi need not be of the same dimensions. The included angle of diverging section 4 of the venturi in this embodiment is 34°. The included angle of converging section 3 is 35°, with gas inlet tube 9 being 6.35 mm (1/4") in inside diameter with gas injection tip 10 being 1.59 mm (1/16") in inside diameter. As indicated above, the injection of gas into a liquid stream, is carried out in close proximity to the venturi used to create acceleration to supersonic velocities and deacceleration to sub-sonic velocities. For purposes of the invention, such close proximity to the venturi is intended to mean that the injection point of gas into liquid, should be positioned in the flow conduit at the upstream side of the converging venturi section 3 up to a point spaced from the upstream inlet end of said converging section by about one diameter in length, based on the diameter of the flow conduit 6 in which the venturi is positioned. The chart of Fig. 2 illustrates the effect of the positioning of tip 10 of gas inlet tube 9 on the oxygen mass transfer in an oxygen dispersion in water application carried out in a gas-liquid dispersion system essentially as shown in Fig. 1. The oxygen mass transfer is expressed in moles/(1-hr), and the distance of the tip of gas inlet tube 9 from the throat of the venturi is expressed in mm (inches). Thus, the distance from the upstream beginning of throat 2 to the upstream end of converging section 3 was 46.0 mm (1.81"). The inside diameter of the flow conduit upstream of converging section 3 was 21.1 mm (0.83"). Thus, the positioning of tip 10 of gas inlet tube 9 can extend, in the practice of the invention, about 21.1 mm (0.83") from the upstream end of converging section 3, i.e. about 66.0 mm (2.6") upstream from the beginning of throat section 2. The water flow velocity in the 21.1 mm (0.83") conduit 6 was 0.73 meters per second. The operation was carried out at a pressure of 205 kPa (15 psig) downstream of the venturi, with about 320 mg of oxygen being added per kilogram of water. It will be appreciated from the results illustrated in Fig. 2 that, from a mass transfer viewpoint, it is desirable to position the gas injection means as close to the throat of the venturi as practical. As the positioning of the gas injection means is moved further from the throat, but nevertheless within the converging section of the venturi, the mass transfer rate decreases but remains relatively high. Movement of the gas injection means outside the converging section of the venturi, but within the close proximity of a length not exceeding about one diameter of the liquid flow conduit, results in lower mass transfer rates. Movement of the gas injection means further upstream, however, results in even lower oxygen mass transfer rate characteristics. It should be noted that the included angles of the converging and diverging sections of the venturi, sometimes referred to as cone angles, can vary widely in the practice of the invention. Those skilled in the art will appreciate that smaller, or shallower, included angles on the converging side of the venturi will result in lower amounts of pressure loss than if a larger or wider included angle is employed. Such smaller angled venturi sections are, however, correspondingly longer in overall length and thus require more complex and costly machining in the manufacture thereof. In addition, the positioning of the gas injection means cannot be as close to the throat section of the venturi as may be desired, from a mass transfer viewpoint, in smaller angled venturi sections because the positioning of the gas injection means close to the throat of the venturi would tend to cut off, or seriously reduce, the flow of fluid to the venturi throat. It will be appreciated that such an arrangement would also result in a significant pressure loss within the venturi and in the operating cost of the system. Such positioning of the gas injection means on a wider angled venturi will result in a lesser pressure loss than that occurring if the same gas injection means were positioned close to the throat of a more shallow angled venturi. While the mass transfer characteristics of a desired gas-liquid mixing operation are thus increasingly enhanced by the positioning of the gas injection means closer to the venturi throat, an overall consideration of the mass transfer and pressure loss effects results in the positioning of the gas injection means in close proximity to the venturi, but at a distance farther from the throat than would be suggested from a consideration of mass transfer effects alone. It will be appreciated that the optimum positioning of the gas injection means in accordance with the practice of the invention will depend upon a wide variety of pertinent operating factors, including the nature of the gases and liquids being mixed, the operating temperatures and pressures involved, the flow rates desired, the venturi configuration desired, the nature of the gas dispersion operation, the overall economics pertaining to a given gas dispersion operation, and the like. While the gas injection means has been illustrated as being a gas inlet tube having a suitable gas injection tip, it is within the scope of the invention to employ a simple gas injection tube without incorporating a discharge tip or nozzle on the end portion thereof. In light of the disclosure above with respect to the flow encumbrance and pressure effects of the gas injection means, particularly as related to the movement of said gas injection means close to the venturi throat for enhanced mass transfer, it should be noted that it is desirable to make the gas injection tube as small and as streamlined as possible to reduce the pressure effects of its positioning at closer distances to the throat of the venturi. By incorporating an injection nozzle having a small tip portion, it is possible to position the nozzle at an optimal point farther in toward the venturi throat, with the increase in pressure drop resulting therefrom being adequately compensated for by the enhanced mass transfer characteristics obtained by such advantageous positioning of the injection nozzle near the venturi throat. To the extent permissible in light of the applicable flow requirements of a given gas-liquid dispersion operation, it would be desirable to employ a gas injection tube having an extended needle portion on the downstream end thereof. High oxygen mass transfer rates were achieved using a venturi device to accelerate the flow velocity from less than that of sound to one where the velocity exceeds that of sound in the gas-liquid mixture, thereby enabling the oxygen bubbles to be very substantially broken up by the sonic shock wave effect accompanying the acceleration-deacceleration sequence to supersonic and back to sub-sonic velocity. The water velocity was about 0.73 meters per second upstream of the venturi, which is considerably less than that required for turbulent flow conditions adequate to disperse the oxygen uniformly in the water. Such relatively low water velocity is, however, desirable from an overall processing viewpoint. A uniform dispersion of oxygen in water was not formed prior to the acceleration of the oxygen-water mixture to supersonic velocity as it passed to the throat section of the venturi. The placing of the gas introduction means in close proximity to the venturi used to create supersonic flow velocities will enable the gas mass transfer rate to be enhanced, and highly advantageous mass transfer/power requirement balances to be achieved in the practice of the invention. The improved results obtainable in the practice of the invention are deemed surprising, particularly since a uniform dispersion of the gas in the liquid prior to acceleration of the gas-liquid mixture to supersonic velocity need not be achieved, and typically will not be achieved, in the practice of the invention. The advantageous results of the invention are, nevertheless, associated with the supersonic shock wave effect obtained as a result of the passage of the gas-liquid mixture at a velocity deaccelerating from supersonic velocity to sub-sonic velocity in the downstream diverging portion of the venturi, and/or accelerating from a sub-sonic velocity to a supersonic velocity in the upstream converging portion of the venturi in which the gas and liquid are mixed. It should be understood that, depending upon the overall conditions pertaining to a given gas-liquid mixing operation, a dual sonic shock wave effect, i.e. upon deacceleration and upon initial acceleration of the gas-liquid dispersion in the venturi device used to create supersonic flow velocities, may possibly be realized, and any such dual shock effect is within the scope of the invention. Those skilled in the art will appreciate that various changes and modifications can be made in the details of the invention, without departing from the scope of the invention as recited in the claims. It will be understood, for example, that the speed of sound in a gas-liquid mixture will depend upon the components of said mixture and the operating conditions pertaining thereto. In general, however, the sonic velocity of a mixture will be very considerably lower than the speed of sound in either the liquid or the gas itself. It is this characteristic of sonic velocity that enables the invention and prior art operations based on acceleration to supersonic velocities and subsequent deacceleration to be carried out under practical, commercial operating conditions. Kieffer, "Sound Speed in Liquid-Gas Mixtures: Water-Air and Water-Steam", Journal of Geophysical Research, Vol. 82, No. 20, July 10, 1977, pp. 2895-2903, confirm this effect which is used to advantage in practical commercial embodiments of the invention. Depending on the particular gas/liquid ratio employed, the velocity of sound in an air/water mixture may be on the order of about 20 meters per second. The upstream velocity needed to achieve the results desired in the Garrett patent referred to above, i.e. less than that of the velocity of sound in the gas-liquid dispersion, but sufficient to enable the gas bubbles to remain uniformly dispersed by turbulence to avoid slug flow or stratification, is generally at least on the order of about 2 meters per second. Those skilled in the art will appreciate that slug flow is disadvantageous and would result in undesired discontinuity of operation and results. In view of the surprising improvement of mass transfer and mass transfer/pressure results obtainable in the practice of the invention without the necessity of assuring that turbulent flow conditions adequate to disperse the gas uniformly in the liquid are created prior to acceleration to supersonic velocities, the water or other liquid flow velocity upstream of the venturi can be maintained at a considerably lower level than that pertaining in the prior art approach of Garrett. Thus, water flow velocities of from about 0.3 to 1 meter per second can conveniently be employed, although higher or lower velocities can also be employed. In any event, it is not necessary to create and maintain turbulent flow conditions upstream of the venturi sufficient to achieve uniform dispersion. In the practice of the invention, the gas injected but not fully dispersed into the liquid stream does not have an opportunity to grow into large bubbles, which are more difficult to uniformly disperse. Such bubble growth would tend to occur in prior art practice in which the gas is injected into the liquid at a greater distance upstream from the venturi, particularly if relatively low liquid flow velocities were employed. Those skilled in the art will appreciate gas will be injected into the liquid at a pressure greater than the pressure of the liquid in the flow conduit and such as to provide the desired ratio of gas to liquid. The gas-liquid dispersion process of the invention can be applied to a wide variety of gas-liquid mixing operations. Thus, the process of the invention can be conveniently employed not only in the dissolution of oxygen in water application referred to above, but in a variety of operations in which it may be desirable to dissolve air or oxygen, nitrogen, chlorine or other industrial gas in a variety of inorganic or organic liquids. In addition, the process of the invention can be applied to advantage for gas-liquid reaction operations, as for the oxidation of iron present in aqueous solution or for the oxidation of organic liquids. The invention can also be employed for desirable dissolved gas stripping operations, as for the stripping of dissolved oxygen or other volatiles from liquids by the injection of nitrogen therein in accordance with the practice of the invention. The invention will thus be seen to provide a very desirable and beneficial advance in the art of dispersing gases in liquids. By enabling the mass transfer characteristics to the dispersion operation to be enhanced, the invention enables the benefits of employing venturi to be more fully realized than was heretofore obtainable in the art. By enabling the mass transfer to be significantly improved in advantageous balance with the power requirements of the gas-liquid dispersion operation, the invention further benefits the gas-liquid dispersion art by providing a desirable processing flexibility enabling the overall benefits of the invention to be more fully related to the practical requirements of any particular commercial application. As the flow velocities employed in conjunction with the use of the venturi can be maintained at a considerably lower level than in the prior art practice wherein turbulent flow conditions are required, the practical commercial feasibility of the invention is further enhanced, as is its benefit in a wide range of important commercial gas-liquid dispersion operations. The invention will thus be seen as serving to greatly increase the prospects for employing industrial gases in a wide variety of industrial activities in which the enhanced dispersion achieved in the practice of the invention provides a needed incentive for the desirably increased use of such gases in such industrial activities.

Claims: A process for the dispersion of a gas in a liquid which is passed through a flow conduit (6), comprising: (a) introducing the gas into the stream of liquid to form a gus bubble-liquid mixture; and (b) passing said gas bubble-liquid mixture through the throat section (2) of a venturi; characterized by passing the stream of liquid through said flow conduit (6) upstream of said venturi at a velocity less than that required to create and maintain turbulent flow conditions upstream of the venturi, but sufficient to achieve uniform dispersion of the gas in the liquid; injecting said gas into said liquid at an injection point which is disposed in the flow conduit at the upstream side of the converging section up to a point spaced from the upstream inlet end of said converging section by about one diameter in length, based on the diameter of the flow conduit, to thereby form a gas bubble-liquid mixture having a flow velocity of less than the velocity of sound in said gas-liquid mixture; accelerating the flow velocity of the gas-liquid mature from subsonic velocity to supersonic velocity in the upstream converging section (3) of the venturi; and subsequently decelerating the flow velocity of the gas-liquid mature from supersonic velocity to sub-sonic velocity in the downstream diverging section (4) of the venturi. Application of the process of claim 1 for the dissolution of the gas in the liquid. Application of the process of claim 2 in which oxygen is used as said gas, and water is used as said liquid. Application of the process of claim 1 for the reaction of the gas and the liquid. Application of the process of claim 1 in which the gas is used to strip dissolved gas from the liquid.

Inventors: BERGMAN THOMAS JOHN, KIYONAGA KAZUO, LITZ LAWRENCE MARVIN, BERGMAN, THOMAS JOHN, KIYONAGA, KAZUO, LITZ, LAWRENCE MARVIN
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1988
Main classification: B01F3
IPC codes: B01J10, B01F3, B01F5, B01J19