WO1991015287A1

WO1991015287A1 - Apparatus and method for sparging a gas into a liquid

Info

Publication number: WO1991015287A1
Application number: PCT/US1991/002129
Authority: WO
Inventors: Alan T. Y. Cheng
Original assignee: Union Carbide Industrial Gases Technology Corporation
Priority date: 1990-04-03
Filing date: 1991-04-03
Publication date: 1991-10-17
Also published as: CA2056418A1; KR920700751A; BR9105678A; EP0474835A1; JPH05500772A

Abstract

An improved apparatus and method for sparging a gas into a body of liquid wherein the gas is discharge at sonic velocity into the liquid producing a shock wave and creating a jet of dispersed gas and liquid. The jet induces cocurrent flow in contiguous liquid preventing coalescence and enhancing mass transfer between the phases. In apparatus for the practice of the method, the gas discharge nozzle (3) is directed into an acceleration tube (5) through which the jet and induced liquid flow pass and are discharged into a larger draft tube (12) thereby increasing the induced liquid flow.

Description

APPARATUS AND METHOD FOR SPARGING A GAS INTO A LIQUID

Technical Field This invention relates to an apparatus and method for sparging a gas into a body of liquid.

Background Art

The distribution of a gas stream into a body of liquid, an operation known as sparging, has many mass transfer applications in chemical processing. These include dissolving a gas into a liquid, stripping a dissolved gas or vapor from a liquid with a gas insoluble in the liquid, stripping a fraction of a liquid mixture leaving a fraction richer in the least volatile component in the mixture, and reacting a gas with a liquid. Particulate matter can also be removed from a liquid if the particulate matter is susceptible to being suspended in ether the sparging gas or in the fraction of the liquid volatilized into the sparging gas. Dissolving or reacting oxygen or ozone into a liquid is a common operation. Mixing of a liquid can also be accomplished by sparging. Another application of sparging is suspending gas bubbles in a liquid to achieve a foamed product such as in the sparging of liquified margarine with gaseous nitrogen. Subsequent cooling produces a solidified, fluffed product which spreads more easily, and covers more surface than unfluffed margarine. Still another specific application is the sparging of gas into a liquified reactant such as a polyol prior to introducing another reactant such as isocyanate and injecting the reacting mixture into a mold. The degree of sparging and the resulting sparging gas content strongly influence the density of the polyurethane foam product.

Mass transfer between liquid and gas phases can only occur across their interfacial area. Hence dispersing a liquid and gas to increase the interfacial area directly increases the transfer rate achieved.

The simplest method of sparging a gas into a liquid is through a single open pipe, which is an inefficient and slow method for a sizeable body of liquid in a pond or a tank. Multiple orifices in a straight pipe protruding into the tank begin to achieve some limited distribution of gas across the tank. Ring spargers and branched network spargers are common and provide greater distribution of discharge orifices across the vessel. Their disadvantages are that gas bubbles emerging from the orifices are fairly large, and equal flows from the multiple orifices in a complex system are often difficult to achieve.

Finer dispersion of gas in liquid can be achieved through a perforated plate or porous diffuser, but the large number of such devices needed to cover the cross section of a sizeable tank is mechanically awkward and economically unattractive. Moreover with the small orifices found in a perforated plate and the fine tortuous passages found in porous diffusers, clogging from dirt and biological growth is a problem. Since the passages in a diffuser vary in size, the bubbles produced will also vary in size. The finer passages will clog first resulting in the flow channeling through the larger passages. Thus bubble size will also increase with service time. Eductors using liquid as motive fluid serve effectively for mixing liquids in vessels. However, air as a motive fluid in such eductors results in large bubbles and little dispersion and thus little attractiveness for mixing or mass transfer operations.

Greater dispersion and mass transfer rates are achieved by employing an external pipe loop ^• through which liquid is circulated by a pump. Gas is introduced in the loop creating a flowing mixture which is subsequently contracted and expanded to further disperse the phases. Such an in-line dispersion apparatus and method are described by A.T.Y. Cheng in U.S. Patent No. 4,861,352 and a continuation-in-part application thereof and by K. Kiyonaga et al in U.S. Patent No. 4,867,918.

High dispersion and transfer rates can also be achieved by more elaborate and more costly apparatus such as a bubble or packed column adjunct to the vessel. Liquid is circulated by a pump through the adjunct column where the gas is introduced and dispersed.

While adjunct loop and adjunct column recirculation methods are efficient for dispersion and mixing, situations arise where the solids content of the liquid is too high for pumping, a suitable circulation pump is not available, the liquid cannot readily be transferred out of a reactor, or a vessel with an adjunct recirculation loop or column is not available. In such situations sparging is preferable. Another application is sparging into a natural body of liquid such as a pond, stream or lake. Accordingly, an object of this invention is to provide an efficient method of sparging gas into a body of liquid with relatively simple and inexpensive apparatus.

A further object of this invention is to provide an efficient sparging method and apparatus that are operable in a tank.

. Still another object is to provide an efficient method and apparatus for mixing a tank of liquid by sparging. Yet other objects are made apparent in this specification.

Summary of the Invention

The above objects are achieved in this novel method of sparging by discharging gas at sonic velocity into a body of liquid, thereby forming a shock wave and a jet of dispersion of gas and liquid and inducing a flow in the liquid in the direction of the discharge. In another embodiment, the jet of dispersion of gas and liquid is subsequently caused to contract and then to expand effecting still further dispersion of the phases.

The apparatus comprises a tube ending in a nozzle for discharging the gas at sonic velocity into the liquid to form the jet of dispersion. An acceleration tube open at both ends is positioned to receive the jet at one end opening, induce and accelerate liquid, and discharge the mixture at the other. In another embodiment, the mixture from the acceleration tube is further received into one end opening of a draft tube, and discharged from the other end opening. The acceleration tube and draft tube induce and augment liquid flow past the gas discharge nozzle thereby inhibiting coalescence of gas bubbles and enhancing mass transfer across the interfacial area.

In yet a third embodiment, the jet of dispersion from the gas discharge nozzle is caused to expand and contract in the draft tube, which causes additional dispersion of the gas and liquid.

Brief Description of the Drawings

Fig. 1 is a cross-sectional representation of a first embodiment of apparatus for carrying out the method of this invention. Fig. 2 is a top view of a second embodiment installed in a tank.

Fig. 3 is a cross sectional representation of a third embodiment of apparatus for carrying out the method of this invention. Fig. 4 is a cross sectional representation of a fourth embodiment of apparatus for carrying out the method of this invention.

Fig. 5 is a schematic representation of equipment used for evaluating the first embodiment. Fig. 6 is a top view of the tank in which the first embodiment was mounted for evaluation. Fig. 7 is a graphical comparison of the sparging performance of the first embodiment with prior art sparging devices. Fig. 8 is a graphical representation of the effect of gas discharge nozzle locations relative to the acceleration tube in the practice of the invention. Fig. 9 is a graphical representation of the effect of acceleration tube length in the practice of the invention.

Fig. 10 is a graphical representation of the effect of acceleration tube diameter in the practice of the invention.

Fig. 11 is a graphical representation of the effect of sparging gas flow rate on stripping performance in the practice of the invention. Fig. 12 is a graphical representation of the effect of sparging gas pressure on stripping performance in the practice of the invention.

Fig. 13 is a graphical representation showing the improvement in stripping performance obtained with an embodiment including a draft tube.

Detailed Description Fig. 1 depicts a sonic jet sparger in accordance with this invention in its simplest embodiment. Within a body of liquid 1 is immersed a gas discharge tube 2 terminating in a flow discharge nozzle 3. Into liquid 1 from nozzle 3 is discharged gas at sufficient velocity to produce a sonic shock wave and create a jet of dispersed gas bubbles 4 and entrained liquid. An acceleration tube 5 is disposed to receive the jet within or near an entrance opening to the tube 5. The acceleration tube 5 serves to confine the jet thereby promoting entrainment of liquid by the jet, and, by viscous drag, a cocurrent flow of liquid contiguous to the jet. This induces a flow of liquid, denoted by arrow 6, into the entrance opening of the acceleration tube 5 and past the nozzle 3. The liquid flow energing from the acceleration tube mixes the liquid body outside the tube. Induced liquid flow past the nozzle 3 also promotes shearing of the discharging gas into smaller bubbles and inhibits coalescence of the bubbles. An important feature of this invention is the discharge (or injection) of gas at sufficient velocity into the liquid so that a sonic shock wave is produced which disperses the gas into the liquid. The shock wave will occur if the gas is discharged at sonic or supersonic velocity from the nozzle. It is well known that air at 70°F, for example, will discharge from an orifice or a converging nozzle at a velocity equal to the speed of sound in air if a pressure ratio of or greater than about 2 is provided across the orifice or nozzle. Supersonic velocities can be developed with appropriately shaped diverging flow nozzles, but they have not shown an advantage for the purposes of this invention.

In physical terms, it is the momentum of the discharging gas stream that produces the desired jet of gas and liquid dispersion and cocurrent flow of contiguous liquid. Thus, the effectiveness of the invention action is promoted by increasing the momentum of the discharging gas, i.e., the product of its mass flow and its velocity. With sonic gas flow in the nozzle, as prescribed in the practice of this invention, the gas mass flow is proportional to its absolute pressure and inversely proportional to the square root of its absolute temperature. However, the speed of sound in a gas is also approximately proportional to the square root of the absolute temperature of the gas. Thus, following the mathematical relationships through, it develops that the momentum of the gas flow in a sonic nozzle is solely a function of the absolute pressure of the gas supplied to the nozzle. Accordingly, the momentum supplied and the effectiveness of the action of the invention is increased by increasing the gas supply pressure, which will, of course, also increase the gas consumption. However, it is apparent that the momentum supplied, and the effectiveness of the action of the invention, will be maintained by increasing the gas supply temperature, which will decrease the gas consumption. Thus the invention can be most economically practiced by preheating the gas supplied to the nozzle, which, as is well known, can readily be accomplished by various means such as an electrical heater or a steam jacket.

A second embodiment of the invention is shown in Fig. 2. Acceleration tube 5 is comprised of a constant area tubular section 7 to which is affixed a frusto-conical entry section 8 which serves to increase the amount of liquid flow (shown by arrow 6) induced into the acceleration tube 5. The gas discharge nozzle in Fig. 2 is shown located within the constant area section. However the-gas discharge nozzle could also be located within the frusto-conical section of the acceleration tube, or somewhat outside and upstream of the entrance to the frusto-conical entry section. The jet from the gas discharge nozzle must enter the acceleration tube. Therefore the gas discharge nozzle cannot be located so far upstream of the acceleration tube that the jet is appreciably dissipated before entering the acceleration tube. In Fig. 2, acceleration tube 5 is shown installed in a tank 9 containing liquid 1 and an agitator 11. In this particular application, the acceleration tube 5 is oriented in line with the direction of the flow produced by the agitator. This arrangement augments the flow of liquid into the acceleration tube 5, and the flow from the acceleration tube augments the flow produced by the agitator. A third embodiment is shown in Fig. 3. In this third embodiment, a draft tube 12 positioned to receive the dispersion of liquid and gas emerging from the acceleration tube 5 is provided to increase the induced liquid flow 6 into the acceleration tube 5. While the acceleration tube 5 is shown located within the draft tube 12 in Fig. 3, the acceleration tube 5 also could be located somewhat outside and upstream of draft tube 12. The third embodiment is shown immersed in a body of liquid 1 contained in a tank 9. A baffle 13 is positioned to deflect the flow from the draft tube 12 and prevent it from breaking through the surface of the liquid in the tank and dissipating into the space above the liquid. The baffle serves to redirect the flow from the draft tube back into the body of liquid thereby enhancing the mixing of the liquid and the mass transfer across the interfacial area.

In a fourth embodiment, shown in Figure 4, the shear tube 5 is provided with a frusto-conical entrance section 8, a constant area section 7, and frusto-conical exit section 14 to form a venturi. The entrance area and exit area of the frusto-conical sections respectively coincide with the inside diameter of the draft tube 12. The venturi configuration induces greater liquid flow 6 into the acceleration tube 5 and the draft tube 12 thereby further enhancing sparging performance.

Example I The first embodiment, shown in Fig. 1, was evaluated by immersion into a tank of liquid as shown in Fig. 5 and Fig. 6, and measuring its performance in stripping out a gas dissolved in the liquid.

The dimensions of the components cited in the following description are illustrative and not intended to restrict or limit the scope of the invention. Referring to Fig. 1, the gas discharge tube 2 was 1/2 inch in outside diameter and terminated in a converging nozzle having a minimum flow area with a diameter of 0.072 inches. The gas discharge tube 2 was inserted through and mounted in a reducing bushing closing one opening of a copper pipe tee of 1 inch nominal pipe size. In the opening opposite the gas discharge tube was screwed a matching-size, copper pipe nipple 3 inches long. The discharge nozzle 3 of the gas discharge tube 2 was positioned so that its end extended 1/4 inches into the pipe nipple.

As shown in Fig. 5 and 6 this embodiment of the invention was inserted into a 200 gallon tank comprised of an upright cylinder about 38 inches in diameter with a conical bottom. The embodiment was fixed near the tank wall with the acceleration tube oriented horizontally. Instrumentation for controlling and measuring the pressure and flow rate of the sparging gas was provided in the piping leading to the embodiment. Opposite the embodiment, near the tank wall, was mounted an oxygen sensor 15. The tank was charged with 92 gallons of soybean oil at 46°C immersing the embodiment and the oxygen sensor. An agitator propeller 11 was immersed in the oil and rotated to impart a slow circulation to the oil. Air was sparged into the oil until the oxygen meter 16 indicated that the oil was saturated with oxygen.

Nitrogen gas at a pressure of 140 psig was then supplied to the embodiment producing a discharge of 4.8 scfm. Inasmuch as the pressure ratio across the converging flow discharge nozzle in the embodiment exceeded the critical pressure ratio for sonic flow, the nitrogen discharged from the nozzle at sonic velocity and produced a shock wave. The described arrangement, conditions and embodiment dimensions were used in all of the experimentation to be described below with exceptions as noted.

The performance of embodiment 1 and three other spargers known in the art were determined for comparison. Results are depicted in Fig. 7. The lowest curve depicts the performance of a tube of 1/2 inch outside diameter formed into a ring and perforated with 17 holes of 1/8 inch diameter. The second lowest curve is for a porous metal disc 5 1/2 inches in diameter. The third lowest curve is for a single nozzle having a discharge diameter of 0.072 inches. The top curve is for the identical nozzle fitted with an acceleration tube to comprise the first embodiment. Comparing at a constant sparging time of 20 minutes and at the same sparging gas flow rate of 4.8 scf , it is seen that the degree of oxygen removal accomplished by the ring sparger was 48%, by the porous metal disc 63%, by the single nozzle 75%, and by the first invention embodiment 89%. The performance of the invention embodiment was clearly superior over the prior art spargers.

The penetration of the nozzle 3 into the acceleration tube 5 was found to have an influence as depicted in Fig. 8 on the performance of the first embodiment 1. Sparging performance is shown with the gas discharge nozzle 3 positioned: (1) 1 1/2 inches upstream of the entrance edge of the acceleration tube, (2) 1/4 inches within the entrance edge of the acceleration tube and (3) 2 3/4 inches within the acceleration tube. Best performance was obtained with the gas discharge nozzle within the acceleration tube, but little variation in performance was observed in varying the length of penetration. A penetration of 1/4 inches was adopted as a standard for measuring the influence of other geometric variables on sparging performance.

Fig. 9 depicts oxygen removal obtained at sparging times of 10, 20 and 30 minutes for the first embodiment alternately fitted with several lengths of acceleration tube. With the standard nozzle penetration of 1/4 inches into the acceleration tube, the optimum extension of the acceleration tube past the nozzle was about 2 3/4 inches.

The effect of acceleration tube diameter was explored next. Three different sizes of copper pipe were evaluated having 1/2, 1 and 2 inches nominal outside diameter and 0.626, 1.063 and 2.063 inches inside diameter, respectively. The equipment in each evaluation comprised a pipe nipple 3 inches long screwed into a tee of matching pipe size. Resu-lts presented in Fig. 10 indicate that under the conditions employed, an acceleration tube of about 1 inch diameter provided best performance in stripping oxygen from the 92 gallons of soybean oil. This optimum was also confirmed in stripping oxygen from 153 gallons of carboxy-ethylcellulose solution.

The optimum acceleration tube length and diameter are functions of the sparging gas flow rate. With higher gas flow rates, acceleration tubes of greater length and diameter are preferred for favorable performance. The choice of these variables will be influenced by the tank size and shape, the allowable time for the sparging operation, and the cost of the sparging gas.

It is anticipated that good sparging results will be obtained by scaling the acceleration tube length and diameter and sparging gas flow with the tank size and inversely with the desired sparging time. Thus acceleration tube inside diameters ranging from 1/2 to 10 inches and lengths ranging from 1 to 10 acceleration tube diameters will be useful in practice. With these acceleration tube diameters, draft tube inside diameters ranging from 3 inches to 72 inches and lengths ranging from 1 to 20 draft tube diameters will be compatible and useful.

Example 2 Evaluations were also conducted upon the second embodiment, which was comprised of a 3 inch length of stainless pipe of 1 inch nominal pipe size welded to a 45° frusto-conical entry section with a maximum diameter of 4 inches. This sparger embodiment was oriented vertically upward in the center and just above the bottom of an upright cylindrical tank 30.5 inches in diameter. The tank was charged with 85.4 gallons of water at 17.9°C which provided about 24 inches of water above the sparger. The gas discharge tube was 0.5 inches in outside diameter, protruded 1 inch into the constant diameter portion of the acceleration tube and terminated in an orifice 0.072 inches in diameter. At midheight the tank was provided with an inlet and outlet through which its contents were circulated at about 5 gallons per minute by a pump. An oxygen sensor was positioned near the outlet. The tank contents were sparged with air until the sensor indicated saturation with oxygen, and then sparging with nitrogen gas was initiated. Oxygen concentration was monitored with sparging time for various nitrogen flow rates. Results are presented in Fig. 11 as percent of oxygen removed versus nitrogen flow rate in parameters of sparging time. Sonic flow was calculated to occur with about 2.37 scfm passing through the gas discharge orifice. The curves in Fig. 11 show that as the sparging gas flow rate was increased, the oxygen removal rate increased rapidly until sonic flow was reached after which the removal rate increase is reduced. Thus the advantage of operating the sparger with at least sonic gas discharge is apparent. The transition in removal rates, which appeared to occur over some range in flow rate encompassing the calculated sonic flow rate, is considered within the experimental accuracy.

Example 3

Using the second embodiment described in Example 3, the gas discharge tube orifice was increased to a diameter of 0.15 inches. With the identical experimental apparatus as in Example 2, the oxygen removal was measured as a function of sparging time with a sparging nitrogen gas flow of 5.12 scfm. This required a pressure upstream of the orifice of 14 psig and subsonic flow through the orifice. Fig. 12 compares the oxygen removal obtained with the prior orifice which had a diameter of 0.072 inches and passed 4.8 scfm. This required an upstream pressure of 64 psig and sonic flow through the orifice. As can be seen, at about the same sparging gas flow and same sparging time, the oxygen removal rate was greater by about 10% for the smaller orifice with the higher pressure gas supply which produced sonic flow. Thus supplying sparging gas at higher pressures and particularly at a pressure which produces sonic flow results in greater removal rates, other conditions being equal.

Example 4 Evaluations were conducted on the third embodiment, as generally depicted in Fig. 3, comprised of the acceleration tube described in Example 2 with a draft tube 6.36 inches in inside diameter and 22 inches in length. The embodiment was oriented vertically in the tank described in Example 2 with a clearance at the bottom of 1.5 inches. A flat circular baffle 8.6 inches in diameter was located 4 inches above the outlet end of the draft tube and about 1 inch below the surface of the water. As shown in Fig. 13, the third embodiment provided about 8 percent greater oxygen removal rate than the second embodiment at the same sparging nitrogen gas flow of 5 scfm supplied at a pressure of

86 psig which exceeded the critical for sonic flow.

Example 5

Another evaluation demonstrated the mixing capability of the invention. The third embodiment, comprised of an acceleration tube and draft tube, were submerged in vertical orientation into 3000 gallons of water contained in a tank. The acceleration tube was positioned just within the bottom end of the draft tube located 12 inches above the tank bottom in the center of the tank cross section. The acceleration tube comprised a tube about 1 inch in diameter and 2 1/2 inches in length with an entry frusto-conical section having an included angle of 45° and a maximum (opening) diameter of 4 inches. The sparging gas discharge tube was 1/2 inch in outside diameter and terminated in an orifice 0.15 inch in diameter. The draft tube was about 12 inches in diameter and 66 inches long. A flat circular baffle was positioned 4 inches above the upper end of the draft tube. A flow of 1500 scfm of nitrogen gas was passed at a pressure ratio in excess of the critical ratio for sonic flow. With a small volume of dye injected near the inlet of the acceleration tube, the dye was dispersed throughout the tank in 1.2 minutes, thereby demonstrating the effectiveness of the sonic sparger for mixing.

The described embodiments are illustrative and not restrictive, there being others which may be made without departing from the spirit or essential character of the invention, its true scope being indicated by the appended claims, which are intended to embrace all changes which come within their meaning and range of equivalency.

Claims

What is claimed is:

1. An gas sparger comprising: means for discharging a gas at not less than sonic velocity into a body of liquid whereby a jet of dispersion of gas and liquid is formed and a cocurrent flow of contiguous liquid is induced; and an acceleration tube having an opening at each end positioned to receive the jet of dispersion and induced liquid flow at one end opening and discharge the combined flow at the other end opening.

2. The invention of claim 1 further comprising a draft tube having an opening at each end, positioned to receive the combined flow from the acceleration tube and induce additional liquid flow into one end opening of the draft tube and discharge the total flow at the other end opening of the draft tube.

3. The invention of claim 2 further comprising a baffle positioned within the liquid to - -

redirect the flow from the draft tube into the body of liquid.

4. The invention of claim 1 wherein the means of discharging a gas is a gas discharge tube having an opening at one end for discharging the gas into the liquid.

5. The invention of claim 4 wherein the gas discharge tube at its discharge end includes a nozzle of reduced flow area for discharging the gas.

6. The invention of claim 4 wherein the acceleration tube includes a frusto-conical entry section of reducing flow area in the direction of the jet flow followed by a section of constant flow

7. The invention of claim 6 wherein the acceleration tube includes a frusto-conical section of expanding flow area following the section of constant flow area.

8. The invention of claim 6 wherein the discharge end of the gas discharge tube is located within the reducing section of the acceleration tube.

9. The invention of claim 2 wherein the acceleration tube is located within the draft tube near the opening in the draft tube for receiving the combined flow from the acceleration tube.

10. The invention of claim 1 wherein the acceleration tube has an inside diameter ranging from 1/2 inches to 10 inches and a length ranging from 1 to 10 acceleration tube inside diameters.

11. The invention of claim 1 wherein the draft tube has an inside diameter ranging from 3 inches to 72 inches and a length ranging from 1 to 20 draft tube inside diameters.

12. A method of sparging a gas into a body of liquid comprising: discharging a gas at not less than sonic velocity into the liquid; inducing a cocurrent flow in the liquid contiguous to the gas discharge; and forming a jet of dispersion of gas and liquid which mixes with the induced flow of liquid.

13. The method of claim 12 wherein the gas is to be reacted with the liquid.

14. The method of claim 12 wherein the gas is to be dissolved, absorbed or suspended in the liquid.

15. The method of claim 12 wherein the sparging gas is discharged into the liquid to strip out another gas or a vapor contained in the liquid.

16. The method of claim 12 wherein the gas is sparged into the liquid to mix the liquid.

17. The method of claim 12 wherein the gas is sparged into the liquid to remove particulate matter from the liquid.

18. The method of claim 12 wherein the liquid is a mixture of at least two liquid components having different volatilities and the gas is sparged into the mixture to remove a fraction richer in the most volatile component than the fraction remaining.