CA2004652C - Method and apparatus for effecting gas-liquid contact reactions - Google Patents
Method and apparatus for effecting gas-liquid contact reactionsInfo
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- CA2004652C CA2004652C CA 2004652 CA2004652A CA2004652C CA 2004652 C CA2004652 C CA 2004652C CA 2004652 CA2004652 CA 2004652 CA 2004652 A CA2004652 A CA 2004652A CA 2004652 C CA2004652 C CA 2004652C
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Abstract
Gaseous components are removed from gas streams and chemically converted into an insoluble phase in a liquid medium. Specifically, hydrogen sulfide is removed from gas streams by oxidation in aqueous chelated iron hydroxide solution in a modified agitated flotation cell. A combined chemical reactor and solid product separation device comprising such modified agitated flotation cell also is described. In order to effect efficient mass transfer and rapid reaction, gas bubbles containing hydrogen sulfide and oxygen are formed by rotating an impeller at a blade tip velocity of at least about 300 in/sec. to achieve the required shear. To assist in the reaction, a surrounding shroud has a plurality of circular openings of equal diameter and arranged in uniform pattern such as to provide a gas flow therethrough less than about 0.02 lb/min/opening in the shroud. Each of the openings has a diameter less than about one inch.
Description
METHOD AND APPARATUS FOR EFFECTING GAS-LI UID
CONTACT REACTIONS
The present invention relates to method and apparatus for carrying out chemical reactions involving removal of gaseous components from gas streams by chemical conversion to an insoluble phase while in contact with a liquid phase.
Hydrogen sulfide occurs in varying quantities in a variety of gas streams, for example, in sour natural l0 gas streams and in tail gas streams from various industrial operations. Hydrogen sulfide is odiferous and highly toxic and hence it is desirable and often necessary to remove hydrogen sulfide from such gas streams.
There exist several commercial processes for effecting hydrogen sulfide removal. These include processes, such as absorption in solvents, in which the hydrogen sulfide first is removed as such and then converted into elemental sulfur in a second distinct step, such as a Claus plant. Such commercial processes also include liquid phase oxidation processes, such as Stretford, LoCat, Unisulf and others, whereby the hydrogen sulfide removal and conversion to elemental sulfur are effected in a single process.
In Canadian Patent No. 1,212,819, there is described a process for the removal of hydrogen sulfide from gas streams by oxidation of the hydrogen sulfide at a submerged location in an agitated flotation cell in intimate contact with an iron chelate solution and flotation of sulfur particles produced in the oxidation from the iron chelate solution by hydrogen sulfide-depleted gas bubbles.
The present invention is directed towards improving the process of the prior patent by modification to the physical structure of the agitated flotation cell and of the operating conditions employed therein, so as to improve the overall efficiency and thereby decrease operating and capital costs, while, at the same time, retaining a high removal efficiency for removal of hydrogen sulfide from the gas stream.
In the present invention, an efficient contact of gases is carried out for the purpose of effecting a reaction which removes a component of the gases and converts that component to an insoluble phase while in contact with a liquid phase. These multiple operations contrast markedly with the conventional objective of the design of a flotation cell, which is to separate a slurry or suspension into a concentrate and a gangue or barren stream in minerals beneficiation.
There are a variety of processes to which the principles of the present invention can be applied.
The processes generally involve reaction of the component with another gaseous species in a liquid phase, usually an aqueous phase, often an aqueous catalyst system.
One example of such a process is in the oxidative removal of mercaptans from gas streams in contact with a suitable aqueous catalyst system to form immiscible liquid disulfides.
Another example of such a process is the oxidative removal of hydrogen sulfide from gas streams using chlorine in contact with an aqueous sodium hydroxide solution, to form sodium sulphate, which, after first saturating the solution, precipitates.
The term "insoluble phase" as used herein, therefore, encompasses a solid insoluble phase, an immiscible liquid phase and a component which becomes insoluble when reaching its solubility limit in the liquid medium after start up.
Accordingly, in one aspect of the present invention, there is provided a method of removing a gaseous component from a gas stream containing the same by chemical conversion of the gaseous phase into an insoluble phase in a liquid phase, comprising a plurality of steps. A gaseous component-containing gas stream is fed to an enclosed reaction zone in which is located a liquid medium and a chemical conversion agent.
An impeller comprising a plurality of blades is rotated about a vertical axis at a submerged location in the liquid medium so as to induce flow of the gaseous component-containing gas stream along a vertical path from external to the reaction zone to the submerged location.
The impeller is surrounded by a stationary cylindrical shroud through which are formed a plurality of openings. The impeller is rotated at a speed corresponding to a blade tip velocity of at least about 350 in/sec., preferably about 500 to about 700 in/sec., so as to generate sufficient shear forces between the impeller blades and the plurality of openings in the cylindrical shroud to distribute the gas streams as fine bubbles of diameter no more than about ; inch, in the liquid medium, thereby achieving intimate contact of gaseous component, chemical conversion agent and liquid medium at the submerged location and chemical conversion of the gaseous component to form an insoluble phase.
Materials are permitted to flow from the interior of the stationary shroud through the openings therein into the body of the liquid medium external to the shroud at a gas flow rate of less than about 0.02 lb/min/opening in the shroud, whereby any chemical conversion of gaseous component not effected in the interior of the shroud is completed in the region adjacent to the exterior of the shroud.
A gaseous component-depleted gas stream is vented from a gas atmosphere above the liquid level in said reaction zone to exterior of the enclosed reaction zone.
While the present invention, in its method aspect, is described specifically with respect to the removal of hydrogen sulfide from gas streams containing the same by oxidation to sulfur and recovery of the so-formed sulfur by flotation, it will be apparent from the foregoing and subsequent discussion that both the apparatus provided in accordance with a further aspect of the present invention and the method are useful for effecting other procedures where a gaseous component of a gas stream is chemically converted to an insoluble phase in a liquid medium.
In a preferred aspect of the invention, the hydrogen sulfide is converted to solid sulfur particles by oxygen in an aqueous transition metal chelate solution. The oxygen is present in an oxygen-containing gas stream which is introduced to the same submerged location in the catalyst as the hydrogen sulfide-containing gas stream, either in admixture therewith or as a separate gas stream. The oxygen-containing gas stream similarly is distributed as fine bubbles by the rotating impeller, which achieves intimate contact of oxygen and hydrogen sulfide to effect the oxidation.
The solid sulfur particles are permitted to grow in size in the body of reaction medium external to the shroud until they are of a size which enables them to be floated from the body of the reaction medium by hydrogen sulfide-depleted gas bubbles.
The sulfur particles are of orthorhombic crystalline form and are transported when having a particle size of from about 10 to about 30 microns from the body of reaction medium by the hydrogen sulfide-depleted gas bubbles to form a sulfur froth floating on the surface of the aqueous medium and a hydrogen sulfide-depleted gas atmosphere above the froth, from 5 which is vented a hydrogen sulfide-depleted gas stream.
The sulfur froth is removed from the surface of the aqueous medium to exterior of the enclosed reaction zone.
According to another aspect of the present invention, there is provided a chemical reactor comprising an enclosed tank means. Inlet gas manifold means is provided for feeding at least one gas stream through an inlet in an upper closure to the tank means.
Standpipe means communicates with the inlet and extends downwardly within the tank from said upper closure.
Impeller means comprising a plurality of blades is located below the lower end of said standpipe means and is mounted to a shaft for rotation about a vertical axis. Drive means is provided for rotating the shaft.
Cylindrical stationary shroud means surrounds the impeller means and has a plurality of circular openings, preferably of equal diameter and arranged in a uniform pattern, and extending through the wall of the shroud means. Each of the openings through the shroud means has a diameter less than about 1 inch. By modification to the shroud in this way, the apparatus can be operated to provide a gas flow rate of less than about 0.02 lb/min/opening in the shroud. Vent means from the tank means also is provided.
One embodiment of the present invention is directed towards removing hydrogen sulfide from gas streams. High levels of hydrogen sulfide removal efficiency are attained, generally in excess of 99.99%, from gas streams containing any concentration of hydrogen sulfide. Residual concentrations of hydrogen sulfide less than 0.1 ppm can be attained.
The process of the invention is able to remove effectively hydrogen sulfide from a variety of 5 different source gas streams containing the same, provided there is sufficient oxygen present to oxidize the hydrogen sulfide. The oxygen may be present in the hydrogen sulfide-containing gas stream to be treated or may be separately fed, as is desirable where natural l0 gas or other combustible gas stream is treated.
Hydrogen sulfide-containing gas streams which may be processed in accordance with the invention include fuel gas and other hydrogen-containing streams, gas streams formed by air stripping hydrogen sulfide from 15 aqueous phases produced in oil refineries, mineral wool plants, kraft pulp mills, rayon manufacturing, heavy oil and tar sands processing, and a foul gas stream produced in the manufacture of carborundum. The gas stream may be one containing solids particulates or may 20 be one from which particulates are absent. The ability to handle a particulate-laden gas stream without plugging may be beneficial, since the necessity for upstream cleaning of the gas is obviated. The hydrogen sulfide-containing gas stream may contain mercaptans in 25 which case the hydrogen sulfide and mercaptans may be removed in separate reaction vessels.
The process of the present invention for effecting removal of hydrogen sulfide from a gas stream containing the same employs a transition metal chelate 30 in aqueous medium as the catalyst for the oxidation of hydrogen sulfide to sulfur. The transition metal usually is iron, although other transition metals, such as chromium, manganese, nickel and cobalt may be employed.
Any desired chelating agent may be used but generally, the chelating agent is ethylenediaminetetraaceticacid (EDTA) for iron. The iron chelate catalyst may be employed in hydrogen or salt form. The operative range of pH for the process generally is about 7 to about 11.5.
The hydrogen sulfide removal process of the invention is conveniently carried out at ambient temperatures of about 20~ to 25~C, although higher and lower temperatures may be adopted and still achieve efficient operation. The temperature generally ranges from about 10~ to about 80~C.
The minimum catalyst concentration to hydrogen sulfide concentration ratio for a given gas throughput may be determined from the rates of the various reactions occurring in the process and is influenced by the temperature and the degree of agitation or turbulence in the reaction vessel. This minimum value may be determined for a given set of operating conditions by decreasing the catalyst concentration until the removal efficiency with respect to hydrogen sulfide begins to drop sharply. Any concentration of catalyst above this minimum may be used, up to the catalyst loading limit of the system.
The removal of hydrogen sulfide by the process of the present invention is carried out in an enclosed reaction zone in which is located an aqueous medium containing transition metal chelate catalyst. A
hydrogen sulfide-containing gas stream and an oxygen-containing gas stream are induced to flow, either separately or as a mixture, along a vertical flow path from external to the reaction zone to a submerged location in the aqueous catalyst medium out of fluid flow contact with aqueous medium by rotating, at the ~~ 2004652 submerged location, an impeller comprising a plurality of vertically-extending blades about a vertical axis.
The gas streams are distributed as fine bubbles at the submerged location by the combined action of the rotating impeller and a surrounding stationary cylindrical shroud which has a plurality of openings therethrough. To achieve good gas-liquid contact and hence efficient oxidation of hydrogen sulfide to sulfur, the impeller is rotated rapidly so as to achieve a blade top velocity of at least about 300 in/sec, preferably about 500 to about 700 in/sec. In addition, shear forces between the impeller and the stationary shroud assist in achieving the good gas-liquid contact by providing a gas flow rate through the openings in the shroud which is less than about 0.02 lb/min/opening in the shroud, generally down to about 0.004, and preferably in the range of about 0.005 to about 0.007 lb/min/opening in the shroud.
The distribution of the gases as fine bubbles in the reaction medium in the region of the impeller enables a high rate of mass transfer to occur. In the catalyst solution, a complicated series of chemical reactions occurs resulting in an overall reaction which is represented by the equation:
H2S + ;02 ' S + H20 The overall reaction thus is oxidation of hydrogen sulfide to sulfur.
The solid sulfur particles grow in size in the body of the reaction medium exterior to the shroud until of a size which can be floated. The flotable sulfur particles are floated by the hydrogen sulfide-depleted gas bubbles rising through the body of catalyst solution and collected as a froth on the surface of the aqueous medium. The sulfur particles range in size from about 10 to about 30 microns in diameter and are in orthorhombic crystalline form.
The series of reactions which is considered to occur in the metal chelate solution to achieve the overall reaction noted above is as follows:
HzS ~ H+ HS-HS- + FeEDTA ~ [Fe . HS . EDTA]
[Fe.HS.EDTA]- ~ FeEDTA + S + H' + 2e 2 a + ~ 02 + H20 ~ 2 OH-The invention is described further, by way of illustration, with reference to the accompanying drawings, wherein:
Figure 1 is an upright sectional view of a novel reactor provided in accordance with one embodiment of the invention;
Figure 2 is a detailed perspective view of the impeller and shroud of the apparatus of Figure l; and Figure 3 is a close-up perspective view of a portion of the shroud of Figure 2.
Referring to the drawings, a combined chemical reactor and solid product separation device 10 provided in accordance with one embodiment of the invention is a modified form of an agitated flotation cell. The design of the chemical reactor 10 is intended to serve the purpose of efficiently contacting gases to effect a reaction which removes a component of the gas and produces a flotable insoluble phase. This design differs from that of an agitated flotation cell whose objective is to separate a slurry or suspension into a concentrate and a gangue or barren stream.
There are significant differences between a 3o conventional agitated flotation cell and the modified flotation cell 10 of the present invention which arise from the differences in requirements of the two designs. In the present invention, the substances which are treated are contained in the gas stream 9a 2 0 0 4 6 5 2 whereas, in an agitated flotation cell, the substances which are treated are contained within the slurry and the gas is employed to float the particles out of the slurry.
5 An agitated flotation cell is designed to process a slurry or suspension. The capacity of the cell is measured as the volume of treated slurry in a given time and the efficiency is measured as the mass fraction of desired mineral separated relative to that 10 in the entering slurry or suspension. Normally, a number of stages is required, including a roughing stage to effect the non-reactive separation. In contrast, a chemical reactor which removes a component from a gas stream, as in the case of device 10, is engineered to process and treat a flow of gas. Capacity is measured in volume of gas throughput and efficiency is measured in terms of 5 the relative conversion as compared to the desired conversion. Normally, only one reaction step is required.
In addition, an agitated flotation cell is designed to generate a multiplicity of small air bubbles which 10 are distributed uniformly by means of a shroud to ensure good contacting between gas bubbles and desired mineral particle. Normally, no chemical reaction takes place in the cell but surface-active agents may be added to change the flotability of the concentrate. In contrast, in a chemical reactor, such as device 10, the contacting and reaction chemistry are of paramount importance and directly affect the efficiency of the unit. Effective contacting between gas phase and liquid phase is achieved by rotation of the impeller at rates well in excess of those used in an agitated flotation cell. The H2S reactor utilizes a chemical reaction in which hydrogen sulfide is oxidized through the medium of a catalyst by oxygen. The flotation of sulfur is a very significant additional benefit in the operation of the reactor but is not a primary design criterion.
In a conventional agitated flotation cell, the impeller is small relative to the size of the flotation cell, since its purpose is to produce a myriad of small bubbles and not to promote efficient gas-liquid contacting. The shroud is designed with relatively few large holes to distribute the small bubbles uniformly in the cell, ensuring good contacting between the bubbles and the desired contacting phase. The bubbles are maintained within a relatively narrow size range to ensure a large surface area for gas-solid contacting, not gas-liquid contacting, and the bubbles are active throughout the entire volume of the cell. In contrast, in the chemical reactor herein, the impeller may be larger relative to the size of the reactor and its design may be altered to increase the efficiency of gas-liquid contacting. Most of the chemical reaction occurs very close to the impeller, so that the reactive zone is only a small fraction of cell volume. The shroud is designed with a large number of smaller holes with sharp edges to promote secondary contacting by which gas shearing further improves the efficiency of the reaction.
In the chemical reactor, the gas inlets and outlets are much larger than in a conventional flotation cell to accommodate an increase flow of gas. Similarly, liquid inlets and outlets are sufficient for the purposes of filling and draining the vessel, but not for the continuous flow of slurry as in the case of the agitated flotation cell.
The reactor 10, constructed in accordance with one embodiment of the invention and useful in chemical reactions for removing a component from a gas stream, such as hydrogen sulfide, comprises an enclosed housing 12 having a standpipe 14 extending from exterior to the upper wall 16 of the housing 12 downwardly into the housing 12. Inlet pipes 18,20 communicate with the standpipe 14 through an inlet manifold at its upper end for feeding a hydrogen sulfide-containing gas stream and air to reactor 10.
The inlet pipes 18,20 have inlet openings 22,24 through which the gas flows. The openings are designed to provide a lower pressure drop than in a conventional agitated flotation cell.
Generally, the flow rate of gas streams ranges from about 50 to about 500 cu.ft/min. and the pressure drop across the unit varies from about -5 to about +10 in.
H20, preferably from about 0 to less than about 5 in.
H20.
A shaft 26 extends through the standpipe 14 and has an impeller 28 mounted at its lower end just below the lower extremity of the standpipe 14. A drive motor 30 is mounted to drive the shaft 26. Although there is illustrated in the drawings a reactor with a single impeller 28, it is possible to provide more than one impeller and hence more than one oxidative reaction location in the same enclosed tank. The gas flow ratio to the reaction referred to above represents the flow rate per impeller.
The impeller 28 comprises a plurality of radially extending blades 32. The number of such blades may vary and generally at least four blades are employed, with the individual blades being equi-angularly spaced apart.
Generally, the impeller has a diameter of about 6 to about 30 inches, for a standpipe diameter of about 8 to about 45 inches. In addition, the impeller 28 generally has a height which corresponds to an approximately l:l ratio with the diameter, but the ratio may vary from about 0.5:1 to about 2:1. As the gas is drawn down through the standpipe 14 by the action of the rotary impeller, the action of gas flow and rotary motion produce a vortex of liquid phase in the upper region of the impeller 28.
The ratio of the volume of the shrouded impeller 28 to the capacity of the cell may vary widely, and often is less than in a conventional agitated flotation cell, since the reaction is confined to a small volume of the reaction medium. The ratio may be as little as about 2:1.
Another function of the impeller 28 is to distribute the induced gases as small bubbles. This result is achieved by rotation of the impeller 28, resulting in shear of liquid and gases to form fine bubbles dimensioned no more than about ; inch. The critical parameter in determining an adequate shearing is the velocity of the outer tip of the blades 32. A
blade tip velocity of at least about 350 in/sec is required to achieve efficient (i.e., 99.99%+) removal of hydrogen sulfide, preferably about 500 to about 7o0 in/sec. This blade tip velocity is much higher than typically used in a conventional agitated flotation cell, wherein the velocity is about 275 in/sec.
The impeller 28 is surrounded by a cylindrical stationary shroud 34 having a uniform array of small diameter openings 36 through the wall thereof. The shroud 34 generally has a diameter slightly greater than the standpipe 14.
The shroud 34 serves a multiple function in the device. Thus, the shroud 34 prevents gases from by-passing the impeller, assists in the formation of the vortex in the liquid necessary for gas induction, assists in achieving shearing and maintains the turbulence produced by the impeller.
The shroud 34 is spaced only a short distance from the extremity of the impeller blades 30, in order to provide the above-noted functions. Generally, the diameter of the shroud 34 generally is in a ratio of about 2:1 to about 1.2:1, preferably approximately 1.5:1, of the diameter of the impeller 28.
In contrast to the shroud in a conventional agitated flotation cell, the openings 36 are larger in number and smaller in diameter, in order to provide an increased area for shearing. The openings 36 are uniformly distributed over the wall of the shroud 34 and are of equal diameter. The diameter of the openings 36 is less than one inch and generally should be as small as possible without plugging, preferably about 3/8 to about 5/8 inch in diameter, in order to provide for the required gas flow therethrough. The openings have sharp corners to promote shearing.
The openings 36 are dimensioned to permit a gas flow therethrough corresponding to less than about 0.02 lb/min/shroud opening, generally down to about 0.004.
Preferably, the gas flow through the shroud openings is about 0.005 to about 0.007 lb/min/opening in the shroud.
As a typical example, in a conventional agitated flotation cell, 48 openings 1.25 inches in diameter for a circumferential length of 188 inches may be employed while, in the same size unit constructed as a reactor in accordance with the present invention, 670 openings each 3/8 inch in diameter are used for a total circumferential length of 789 inches. In addition, in the present invention the gas flow through the openings is typically 0.007 lb/min/opening in the shroud, while in a conventional agitated flotation cell of the same unit size the same parameter is 0.03 lb/min/opening in the shroud. As may be seen from this typical comparison, the physical dimensions of the openings and the gas flow are significantly different in the chemical reactor of this invention from those in an agitated flotation cell.
The spacing between openings is largely dictated by considerations of adequacy of structural strength.
Generally, each opening is spaced from about 0.25 to about 0.75 of the diameter of the opening from each other, typically about 0.5. Generally, the plurality of openings is arranged at a density of less than about 2 per square inch in a regular array.
The shroud 34 is illustrated as extending downwardly for the height of the impeller 28. It is possible for the shroud 34 to extend below the height of the impeller 28 or for less than its full height, if desired.
.. , In addition, in the illustrated embodiment, the impeller 28 is located a distance corresponding approximately half the diameter of the impeller 28 from the bottom wall of the reactor 10. It is possible for 5 this dimension to vary from no less than about 0.25:1 to about 1:1 of the proportion of the diameter dimension of the impeller. This spacing of the impeller 28 from the lower wall allows liquid phase to be drawn into the area between the impeller 28 and the shroud 34 from the mass 10 in the reactor.
By distributing the gases in the form of tiny bubbles and effecting shearing of the bubbles in contact with the iron chelate solution, rapid mass transfer occurs and the hydrogen sulfide is rapidly oxidized to 15 sulfur. The reaction occurs largely in the region of the impeller 28 and shroud 34 and forms sulfur and hydrogen sulfide-depleted gas bubbles.
The sulfur particles initially remain suspended in the turbulent reaction medium but grow in the body of the reaction medium to a size which enables them to be floated by the hydrogen sulfide-depleted gas bubbles.
When the sulfur particles have reached a size in the range of about 10 to about 30 microns, they possess sufficient inertia to penetrate the gas bubbles to thereby enable them to be floated by the upwardly flowing hydrogen sulfide-depleted gas bubbles.
At the surface of the aqueous reaction medium, the floated sulfur accumulates as a froth 38 and the hydrogen sulfide-depleted gas bubbles enter an atmosphere 40 of such gas above the reaction medium 42.
The presence of the froth 38 tends to inhibit entrainment of an aerosol of reaction medium in the atmosphere 40.
A hydrogen sulfide-depleted gas flow outlet 44 is provided in the upper closure 16 to permit the treated gas stream to pass out of the reactor vessel 12.
An adequate freeboard above the liquid level in the reaction vessel is provided greater than the thickness of the sulfur froth 38, to further inhibit aerosol entrainment.
Paddle wheels 46 are provided adjacent the edges of the vessel 12 in operative relation with the sulfur froth 38, so as to skim the sulfur froth from the surface of the reaction medium 42 into collecting launders 48 provided at each side of the vessel 12. The skimmed sulfur is removed periodically or continuously from the launders 48 for further processing.
The sulfur is obtained in the form of froth containing about 15 to about 50 wt.% sulfur in reaction medium. Since the sulfur is in the form of particles of a relatively narrow particle size, the sulfur is readily separated from the entrained reaction medium, which is returned to the reactor 10.
The reactor 10 provides a very compact unit which rapidly and efficiently removes hydrogen sulfide from gas streams containing the same. Such gas streams may have a wide range of concentrations of hydrogen sulfide.
The compact nature of the unit leads to considerable economies, both in terms of capital cost and operating cost, when compared to conventional hydrogen sulfide removal systems.
There has previously been described in U.S. Patent No. 3,993,563 a gas ingestion and mixing device of the general type described herein. In that reference, it is indicated that, for the device described therein, if an increase in the rotor speed is made in an attempt to obtain greater gas-liquid mixing action, then it is necessary to employ a baffle in the standpipe in order to obtain satisfactory gas ingestion. As is apparent from the description herein, such a baffle is not required in the present invention.
Example 1 2 0 0 4 6 5 2 A pilot plant apparatus was constructed as schematically shown in Figure 1 and was tested for efficiency of removal of hydrogen sulfide from a gas stream containing the same.
The overall liquid capacity of the tank was 135 L.
The standpipe had a diameter of 8 in., and the impeller consisted of six blades and had a diameter of 5i~ in.
and a height of 61/ in. and was positioned 21/ in. from l0 the base of the tank.
The pilot plant apparatus, fitted with a standard froth flotation shroud and impeller combination, was charged with 110 L of an aqueous solution which contained 0.016 mol/L of ethylenediaminetetraacetic acid, iron-ammonium complex and 0.05 mol/L of sodium hydrogen carbonate. The pH of the aqueous medium was 8.5. The shroud consisted of a cylinder of diameter 12 in., height 5 3/4 in. and thickness 3/4 in. in which was formed 48 openings each 1.25 in. in diameter, for a total circumferential length of 188 inches.
Air containing 4000 ppm by volume of hydrogen sulfide was passed through the apparatus via the standpipe at a rate of 835 L/min. at room temperature while the impeller in the aqueous medium rotated at a rate of 733 rpm., corresponding to a blade tip velocity of about 211 in/sec. The gas flow rate through the shroud openings was 0.05 lb/min/opening in the shroud.
Over the one and a half hour test period, 99.5% of the hydrogen sulfide was removed from the gas stream, leaving a residual amount of HZS in the gas stream of 20 ppm. Sulphur was formed and appeared as a froth on the surface of the aqueous solution and was skimmed from the surface using the paddle wheels. Simultaneous removal of hydrogen sulfide from the gas stream and 1$ 2004652 recovery of the sulfur produced thereby, therefore, was effected.
During the test period, the pH of the aqueous solution dropped to 8.3 but no additional alkali was added during this period. Further, no additional catalyst was added during the period of the test.
Example 2 The procedure of Example 1 was repeated with an increased impeller rotation rate and higher gas flow rate.
Air containing 4000 ppm by volume of hydrogen sulfide was passed through the apparatus via the standpipe at a rate of 995 L/min. at room temperature while the impeller in the aqueous medium rotated at a rate of 1772 rpm corresponding to a blade tip velocity of about 510 in/sec. The gas flow rate through the shroud openings was 0.06 lb/min/opening in the shroud.
Over the two hour test period 99.7% of the hydrogen sulfide was removed from the gas stream, leaving a residual amount of H2S of 11 ppm. Sulfur was formed and appeared as a froth on the surface of the aqueous solution and was skimmed from the surface.
Simultaneous removal of hydrogen sulfide from the gas stream and recovery of the sulfur produced thereby, therefore, was effected.
During the test period, the pH of the aqueous solution dropped to 8.3 but no additional alkali was added during this period. Further, no additional catalyst was added during this period of the test.
Example 3 The pilot plant apparatus was modified and fitted with a shroud and impeller combination as illustrated in Figure 2, was charged with 110 L of an aqueous solution which contained 0.016 mol/L of ethylenediaminetetra-acetic acid, iron-ammonium complex and 0.05 mol/L of sodium hydrogen carbonate. The pH of the aqueous solution was 8.5. The shroud consisted of a cylinder of diameter 12 3/4 in., height 8'~ in., and thickness z in.
in which was formed 670 openings each of 3/8 in.
diameter for a total circumferential length of 789 inches. The impeller was replaced by one having a diameter of 6; in. The other dimensions remained the same.
Air containing 4000 ppm by volume of hydrogen sulfide was passed through the apparatus via the standpipe at a rate of 995 L/min. at room temperature while the impeller in the aqueous medium rotated at a rate of 1754 rpm., corresponding to a blade tip velocity of about 597 in/sec. The gas flow rate through the shroud was 0.004 lb/min/opening. Over the two hour test period 99.998% of the hydrogen sulfide was removed from the gas stream, leaving a residual amount of H2S of less than 0.1 ppm. Sulphur was formed and appeared as a froth on the surface of the aqueous solution and was skimmed from the surface. Simultaneous removal of hydrogen sulfide from the gas stream and recovery of the sulfur produced thereby, therefore, was effected.
During the test period, the pH of the aqueous solution remained relatively constant at 8.5. No additional alkali or catalyst was added during the period of this test.
As may be seen from a comparison of the results presented in Examples 1, 2 and 3, it is possible to remove hydrogen sulfide with greater than 99% efficiency using an agitated flotation cell which is provided with a conventional shroud and impeller construction (Examples 1 and 2), as already described in Canadian Patent No. 1,212,819. However, by employing a higher blade tip velocity, as in Example 2, a modest increase in efficiency can be achieved.
However, as seen in Example 3, with a shroud modified as described therein to provide the critical _ 2~ 2004652 gas flow rate and using the critical blade tip velocity, efficiency values over 99.99% can be achieved, leaving virtually no residual hydrogen sulfide in the gas stream.
In summary of this disclosure, the present invention provides novel method and apparatus for effecting chemical reactions and, if desired, for separating flotable by-products of such reactions using an agitated flotation cell, modified in certain critical respects to function as an efficient reactor.
Modifications are possible within the scope of this invention.
CONTACT REACTIONS
The present invention relates to method and apparatus for carrying out chemical reactions involving removal of gaseous components from gas streams by chemical conversion to an insoluble phase while in contact with a liquid phase.
Hydrogen sulfide occurs in varying quantities in a variety of gas streams, for example, in sour natural l0 gas streams and in tail gas streams from various industrial operations. Hydrogen sulfide is odiferous and highly toxic and hence it is desirable and often necessary to remove hydrogen sulfide from such gas streams.
There exist several commercial processes for effecting hydrogen sulfide removal. These include processes, such as absorption in solvents, in which the hydrogen sulfide first is removed as such and then converted into elemental sulfur in a second distinct step, such as a Claus plant. Such commercial processes also include liquid phase oxidation processes, such as Stretford, LoCat, Unisulf and others, whereby the hydrogen sulfide removal and conversion to elemental sulfur are effected in a single process.
In Canadian Patent No. 1,212,819, there is described a process for the removal of hydrogen sulfide from gas streams by oxidation of the hydrogen sulfide at a submerged location in an agitated flotation cell in intimate contact with an iron chelate solution and flotation of sulfur particles produced in the oxidation from the iron chelate solution by hydrogen sulfide-depleted gas bubbles.
The present invention is directed towards improving the process of the prior patent by modification to the physical structure of the agitated flotation cell and of the operating conditions employed therein, so as to improve the overall efficiency and thereby decrease operating and capital costs, while, at the same time, retaining a high removal efficiency for removal of hydrogen sulfide from the gas stream.
In the present invention, an efficient contact of gases is carried out for the purpose of effecting a reaction which removes a component of the gases and converts that component to an insoluble phase while in contact with a liquid phase. These multiple operations contrast markedly with the conventional objective of the design of a flotation cell, which is to separate a slurry or suspension into a concentrate and a gangue or barren stream in minerals beneficiation.
There are a variety of processes to which the principles of the present invention can be applied.
The processes generally involve reaction of the component with another gaseous species in a liquid phase, usually an aqueous phase, often an aqueous catalyst system.
One example of such a process is in the oxidative removal of mercaptans from gas streams in contact with a suitable aqueous catalyst system to form immiscible liquid disulfides.
Another example of such a process is the oxidative removal of hydrogen sulfide from gas streams using chlorine in contact with an aqueous sodium hydroxide solution, to form sodium sulphate, which, after first saturating the solution, precipitates.
The term "insoluble phase" as used herein, therefore, encompasses a solid insoluble phase, an immiscible liquid phase and a component which becomes insoluble when reaching its solubility limit in the liquid medium after start up.
Accordingly, in one aspect of the present invention, there is provided a method of removing a gaseous component from a gas stream containing the same by chemical conversion of the gaseous phase into an insoluble phase in a liquid phase, comprising a plurality of steps. A gaseous component-containing gas stream is fed to an enclosed reaction zone in which is located a liquid medium and a chemical conversion agent.
An impeller comprising a plurality of blades is rotated about a vertical axis at a submerged location in the liquid medium so as to induce flow of the gaseous component-containing gas stream along a vertical path from external to the reaction zone to the submerged location.
The impeller is surrounded by a stationary cylindrical shroud through which are formed a plurality of openings. The impeller is rotated at a speed corresponding to a blade tip velocity of at least about 350 in/sec., preferably about 500 to about 700 in/sec., so as to generate sufficient shear forces between the impeller blades and the plurality of openings in the cylindrical shroud to distribute the gas streams as fine bubbles of diameter no more than about ; inch, in the liquid medium, thereby achieving intimate contact of gaseous component, chemical conversion agent and liquid medium at the submerged location and chemical conversion of the gaseous component to form an insoluble phase.
Materials are permitted to flow from the interior of the stationary shroud through the openings therein into the body of the liquid medium external to the shroud at a gas flow rate of less than about 0.02 lb/min/opening in the shroud, whereby any chemical conversion of gaseous component not effected in the interior of the shroud is completed in the region adjacent to the exterior of the shroud.
A gaseous component-depleted gas stream is vented from a gas atmosphere above the liquid level in said reaction zone to exterior of the enclosed reaction zone.
While the present invention, in its method aspect, is described specifically with respect to the removal of hydrogen sulfide from gas streams containing the same by oxidation to sulfur and recovery of the so-formed sulfur by flotation, it will be apparent from the foregoing and subsequent discussion that both the apparatus provided in accordance with a further aspect of the present invention and the method are useful for effecting other procedures where a gaseous component of a gas stream is chemically converted to an insoluble phase in a liquid medium.
In a preferred aspect of the invention, the hydrogen sulfide is converted to solid sulfur particles by oxygen in an aqueous transition metal chelate solution. The oxygen is present in an oxygen-containing gas stream which is introduced to the same submerged location in the catalyst as the hydrogen sulfide-containing gas stream, either in admixture therewith or as a separate gas stream. The oxygen-containing gas stream similarly is distributed as fine bubbles by the rotating impeller, which achieves intimate contact of oxygen and hydrogen sulfide to effect the oxidation.
The solid sulfur particles are permitted to grow in size in the body of reaction medium external to the shroud until they are of a size which enables them to be floated from the body of the reaction medium by hydrogen sulfide-depleted gas bubbles.
The sulfur particles are of orthorhombic crystalline form and are transported when having a particle size of from about 10 to about 30 microns from the body of reaction medium by the hydrogen sulfide-depleted gas bubbles to form a sulfur froth floating on the surface of the aqueous medium and a hydrogen sulfide-depleted gas atmosphere above the froth, from 5 which is vented a hydrogen sulfide-depleted gas stream.
The sulfur froth is removed from the surface of the aqueous medium to exterior of the enclosed reaction zone.
According to another aspect of the present invention, there is provided a chemical reactor comprising an enclosed tank means. Inlet gas manifold means is provided for feeding at least one gas stream through an inlet in an upper closure to the tank means.
Standpipe means communicates with the inlet and extends downwardly within the tank from said upper closure.
Impeller means comprising a plurality of blades is located below the lower end of said standpipe means and is mounted to a shaft for rotation about a vertical axis. Drive means is provided for rotating the shaft.
Cylindrical stationary shroud means surrounds the impeller means and has a plurality of circular openings, preferably of equal diameter and arranged in a uniform pattern, and extending through the wall of the shroud means. Each of the openings through the shroud means has a diameter less than about 1 inch. By modification to the shroud in this way, the apparatus can be operated to provide a gas flow rate of less than about 0.02 lb/min/opening in the shroud. Vent means from the tank means also is provided.
One embodiment of the present invention is directed towards removing hydrogen sulfide from gas streams. High levels of hydrogen sulfide removal efficiency are attained, generally in excess of 99.99%, from gas streams containing any concentration of hydrogen sulfide. Residual concentrations of hydrogen sulfide less than 0.1 ppm can be attained.
The process of the invention is able to remove effectively hydrogen sulfide from a variety of 5 different source gas streams containing the same, provided there is sufficient oxygen present to oxidize the hydrogen sulfide. The oxygen may be present in the hydrogen sulfide-containing gas stream to be treated or may be separately fed, as is desirable where natural l0 gas or other combustible gas stream is treated.
Hydrogen sulfide-containing gas streams which may be processed in accordance with the invention include fuel gas and other hydrogen-containing streams, gas streams formed by air stripping hydrogen sulfide from 15 aqueous phases produced in oil refineries, mineral wool plants, kraft pulp mills, rayon manufacturing, heavy oil and tar sands processing, and a foul gas stream produced in the manufacture of carborundum. The gas stream may be one containing solids particulates or may 20 be one from which particulates are absent. The ability to handle a particulate-laden gas stream without plugging may be beneficial, since the necessity for upstream cleaning of the gas is obviated. The hydrogen sulfide-containing gas stream may contain mercaptans in 25 which case the hydrogen sulfide and mercaptans may be removed in separate reaction vessels.
The process of the present invention for effecting removal of hydrogen sulfide from a gas stream containing the same employs a transition metal chelate 30 in aqueous medium as the catalyst for the oxidation of hydrogen sulfide to sulfur. The transition metal usually is iron, although other transition metals, such as chromium, manganese, nickel and cobalt may be employed.
Any desired chelating agent may be used but generally, the chelating agent is ethylenediaminetetraaceticacid (EDTA) for iron. The iron chelate catalyst may be employed in hydrogen or salt form. The operative range of pH for the process generally is about 7 to about 11.5.
The hydrogen sulfide removal process of the invention is conveniently carried out at ambient temperatures of about 20~ to 25~C, although higher and lower temperatures may be adopted and still achieve efficient operation. The temperature generally ranges from about 10~ to about 80~C.
The minimum catalyst concentration to hydrogen sulfide concentration ratio for a given gas throughput may be determined from the rates of the various reactions occurring in the process and is influenced by the temperature and the degree of agitation or turbulence in the reaction vessel. This minimum value may be determined for a given set of operating conditions by decreasing the catalyst concentration until the removal efficiency with respect to hydrogen sulfide begins to drop sharply. Any concentration of catalyst above this minimum may be used, up to the catalyst loading limit of the system.
The removal of hydrogen sulfide by the process of the present invention is carried out in an enclosed reaction zone in which is located an aqueous medium containing transition metal chelate catalyst. A
hydrogen sulfide-containing gas stream and an oxygen-containing gas stream are induced to flow, either separately or as a mixture, along a vertical flow path from external to the reaction zone to a submerged location in the aqueous catalyst medium out of fluid flow contact with aqueous medium by rotating, at the ~~ 2004652 submerged location, an impeller comprising a plurality of vertically-extending blades about a vertical axis.
The gas streams are distributed as fine bubbles at the submerged location by the combined action of the rotating impeller and a surrounding stationary cylindrical shroud which has a plurality of openings therethrough. To achieve good gas-liquid contact and hence efficient oxidation of hydrogen sulfide to sulfur, the impeller is rotated rapidly so as to achieve a blade top velocity of at least about 300 in/sec, preferably about 500 to about 700 in/sec. In addition, shear forces between the impeller and the stationary shroud assist in achieving the good gas-liquid contact by providing a gas flow rate through the openings in the shroud which is less than about 0.02 lb/min/opening in the shroud, generally down to about 0.004, and preferably in the range of about 0.005 to about 0.007 lb/min/opening in the shroud.
The distribution of the gases as fine bubbles in the reaction medium in the region of the impeller enables a high rate of mass transfer to occur. In the catalyst solution, a complicated series of chemical reactions occurs resulting in an overall reaction which is represented by the equation:
H2S + ;02 ' S + H20 The overall reaction thus is oxidation of hydrogen sulfide to sulfur.
The solid sulfur particles grow in size in the body of the reaction medium exterior to the shroud until of a size which can be floated. The flotable sulfur particles are floated by the hydrogen sulfide-depleted gas bubbles rising through the body of catalyst solution and collected as a froth on the surface of the aqueous medium. The sulfur particles range in size from about 10 to about 30 microns in diameter and are in orthorhombic crystalline form.
The series of reactions which is considered to occur in the metal chelate solution to achieve the overall reaction noted above is as follows:
HzS ~ H+ HS-HS- + FeEDTA ~ [Fe . HS . EDTA]
[Fe.HS.EDTA]- ~ FeEDTA + S + H' + 2e 2 a + ~ 02 + H20 ~ 2 OH-The invention is described further, by way of illustration, with reference to the accompanying drawings, wherein:
Figure 1 is an upright sectional view of a novel reactor provided in accordance with one embodiment of the invention;
Figure 2 is a detailed perspective view of the impeller and shroud of the apparatus of Figure l; and Figure 3 is a close-up perspective view of a portion of the shroud of Figure 2.
Referring to the drawings, a combined chemical reactor and solid product separation device 10 provided in accordance with one embodiment of the invention is a modified form of an agitated flotation cell. The design of the chemical reactor 10 is intended to serve the purpose of efficiently contacting gases to effect a reaction which removes a component of the gas and produces a flotable insoluble phase. This design differs from that of an agitated flotation cell whose objective is to separate a slurry or suspension into a concentrate and a gangue or barren stream.
There are significant differences between a 3o conventional agitated flotation cell and the modified flotation cell 10 of the present invention which arise from the differences in requirements of the two designs. In the present invention, the substances which are treated are contained in the gas stream 9a 2 0 0 4 6 5 2 whereas, in an agitated flotation cell, the substances which are treated are contained within the slurry and the gas is employed to float the particles out of the slurry.
5 An agitated flotation cell is designed to process a slurry or suspension. The capacity of the cell is measured as the volume of treated slurry in a given time and the efficiency is measured as the mass fraction of desired mineral separated relative to that 10 in the entering slurry or suspension. Normally, a number of stages is required, including a roughing stage to effect the non-reactive separation. In contrast, a chemical reactor which removes a component from a gas stream, as in the case of device 10, is engineered to process and treat a flow of gas. Capacity is measured in volume of gas throughput and efficiency is measured in terms of 5 the relative conversion as compared to the desired conversion. Normally, only one reaction step is required.
In addition, an agitated flotation cell is designed to generate a multiplicity of small air bubbles which 10 are distributed uniformly by means of a shroud to ensure good contacting between gas bubbles and desired mineral particle. Normally, no chemical reaction takes place in the cell but surface-active agents may be added to change the flotability of the concentrate. In contrast, in a chemical reactor, such as device 10, the contacting and reaction chemistry are of paramount importance and directly affect the efficiency of the unit. Effective contacting between gas phase and liquid phase is achieved by rotation of the impeller at rates well in excess of those used in an agitated flotation cell. The H2S reactor utilizes a chemical reaction in which hydrogen sulfide is oxidized through the medium of a catalyst by oxygen. The flotation of sulfur is a very significant additional benefit in the operation of the reactor but is not a primary design criterion.
In a conventional agitated flotation cell, the impeller is small relative to the size of the flotation cell, since its purpose is to produce a myriad of small bubbles and not to promote efficient gas-liquid contacting. The shroud is designed with relatively few large holes to distribute the small bubbles uniformly in the cell, ensuring good contacting between the bubbles and the desired contacting phase. The bubbles are maintained within a relatively narrow size range to ensure a large surface area for gas-solid contacting, not gas-liquid contacting, and the bubbles are active throughout the entire volume of the cell. In contrast, in the chemical reactor herein, the impeller may be larger relative to the size of the reactor and its design may be altered to increase the efficiency of gas-liquid contacting. Most of the chemical reaction occurs very close to the impeller, so that the reactive zone is only a small fraction of cell volume. The shroud is designed with a large number of smaller holes with sharp edges to promote secondary contacting by which gas shearing further improves the efficiency of the reaction.
In the chemical reactor, the gas inlets and outlets are much larger than in a conventional flotation cell to accommodate an increase flow of gas. Similarly, liquid inlets and outlets are sufficient for the purposes of filling and draining the vessel, but not for the continuous flow of slurry as in the case of the agitated flotation cell.
The reactor 10, constructed in accordance with one embodiment of the invention and useful in chemical reactions for removing a component from a gas stream, such as hydrogen sulfide, comprises an enclosed housing 12 having a standpipe 14 extending from exterior to the upper wall 16 of the housing 12 downwardly into the housing 12. Inlet pipes 18,20 communicate with the standpipe 14 through an inlet manifold at its upper end for feeding a hydrogen sulfide-containing gas stream and air to reactor 10.
The inlet pipes 18,20 have inlet openings 22,24 through which the gas flows. The openings are designed to provide a lower pressure drop than in a conventional agitated flotation cell.
Generally, the flow rate of gas streams ranges from about 50 to about 500 cu.ft/min. and the pressure drop across the unit varies from about -5 to about +10 in.
H20, preferably from about 0 to less than about 5 in.
H20.
A shaft 26 extends through the standpipe 14 and has an impeller 28 mounted at its lower end just below the lower extremity of the standpipe 14. A drive motor 30 is mounted to drive the shaft 26. Although there is illustrated in the drawings a reactor with a single impeller 28, it is possible to provide more than one impeller and hence more than one oxidative reaction location in the same enclosed tank. The gas flow ratio to the reaction referred to above represents the flow rate per impeller.
The impeller 28 comprises a plurality of radially extending blades 32. The number of such blades may vary and generally at least four blades are employed, with the individual blades being equi-angularly spaced apart.
Generally, the impeller has a diameter of about 6 to about 30 inches, for a standpipe diameter of about 8 to about 45 inches. In addition, the impeller 28 generally has a height which corresponds to an approximately l:l ratio with the diameter, but the ratio may vary from about 0.5:1 to about 2:1. As the gas is drawn down through the standpipe 14 by the action of the rotary impeller, the action of gas flow and rotary motion produce a vortex of liquid phase in the upper region of the impeller 28.
The ratio of the volume of the shrouded impeller 28 to the capacity of the cell may vary widely, and often is less than in a conventional agitated flotation cell, since the reaction is confined to a small volume of the reaction medium. The ratio may be as little as about 2:1.
Another function of the impeller 28 is to distribute the induced gases as small bubbles. This result is achieved by rotation of the impeller 28, resulting in shear of liquid and gases to form fine bubbles dimensioned no more than about ; inch. The critical parameter in determining an adequate shearing is the velocity of the outer tip of the blades 32. A
blade tip velocity of at least about 350 in/sec is required to achieve efficient (i.e., 99.99%+) removal of hydrogen sulfide, preferably about 500 to about 7o0 in/sec. This blade tip velocity is much higher than typically used in a conventional agitated flotation cell, wherein the velocity is about 275 in/sec.
The impeller 28 is surrounded by a cylindrical stationary shroud 34 having a uniform array of small diameter openings 36 through the wall thereof. The shroud 34 generally has a diameter slightly greater than the standpipe 14.
The shroud 34 serves a multiple function in the device. Thus, the shroud 34 prevents gases from by-passing the impeller, assists in the formation of the vortex in the liquid necessary for gas induction, assists in achieving shearing and maintains the turbulence produced by the impeller.
The shroud 34 is spaced only a short distance from the extremity of the impeller blades 30, in order to provide the above-noted functions. Generally, the diameter of the shroud 34 generally is in a ratio of about 2:1 to about 1.2:1, preferably approximately 1.5:1, of the diameter of the impeller 28.
In contrast to the shroud in a conventional agitated flotation cell, the openings 36 are larger in number and smaller in diameter, in order to provide an increased area for shearing. The openings 36 are uniformly distributed over the wall of the shroud 34 and are of equal diameter. The diameter of the openings 36 is less than one inch and generally should be as small as possible without plugging, preferably about 3/8 to about 5/8 inch in diameter, in order to provide for the required gas flow therethrough. The openings have sharp corners to promote shearing.
The openings 36 are dimensioned to permit a gas flow therethrough corresponding to less than about 0.02 lb/min/shroud opening, generally down to about 0.004.
Preferably, the gas flow through the shroud openings is about 0.005 to about 0.007 lb/min/opening in the shroud.
As a typical example, in a conventional agitated flotation cell, 48 openings 1.25 inches in diameter for a circumferential length of 188 inches may be employed while, in the same size unit constructed as a reactor in accordance with the present invention, 670 openings each 3/8 inch in diameter are used for a total circumferential length of 789 inches. In addition, in the present invention the gas flow through the openings is typically 0.007 lb/min/opening in the shroud, while in a conventional agitated flotation cell of the same unit size the same parameter is 0.03 lb/min/opening in the shroud. As may be seen from this typical comparison, the physical dimensions of the openings and the gas flow are significantly different in the chemical reactor of this invention from those in an agitated flotation cell.
The spacing between openings is largely dictated by considerations of adequacy of structural strength.
Generally, each opening is spaced from about 0.25 to about 0.75 of the diameter of the opening from each other, typically about 0.5. Generally, the plurality of openings is arranged at a density of less than about 2 per square inch in a regular array.
The shroud 34 is illustrated as extending downwardly for the height of the impeller 28. It is possible for the shroud 34 to extend below the height of the impeller 28 or for less than its full height, if desired.
.. , In addition, in the illustrated embodiment, the impeller 28 is located a distance corresponding approximately half the diameter of the impeller 28 from the bottom wall of the reactor 10. It is possible for 5 this dimension to vary from no less than about 0.25:1 to about 1:1 of the proportion of the diameter dimension of the impeller. This spacing of the impeller 28 from the lower wall allows liquid phase to be drawn into the area between the impeller 28 and the shroud 34 from the mass 10 in the reactor.
By distributing the gases in the form of tiny bubbles and effecting shearing of the bubbles in contact with the iron chelate solution, rapid mass transfer occurs and the hydrogen sulfide is rapidly oxidized to 15 sulfur. The reaction occurs largely in the region of the impeller 28 and shroud 34 and forms sulfur and hydrogen sulfide-depleted gas bubbles.
The sulfur particles initially remain suspended in the turbulent reaction medium but grow in the body of the reaction medium to a size which enables them to be floated by the hydrogen sulfide-depleted gas bubbles.
When the sulfur particles have reached a size in the range of about 10 to about 30 microns, they possess sufficient inertia to penetrate the gas bubbles to thereby enable them to be floated by the upwardly flowing hydrogen sulfide-depleted gas bubbles.
At the surface of the aqueous reaction medium, the floated sulfur accumulates as a froth 38 and the hydrogen sulfide-depleted gas bubbles enter an atmosphere 40 of such gas above the reaction medium 42.
The presence of the froth 38 tends to inhibit entrainment of an aerosol of reaction medium in the atmosphere 40.
A hydrogen sulfide-depleted gas flow outlet 44 is provided in the upper closure 16 to permit the treated gas stream to pass out of the reactor vessel 12.
An adequate freeboard above the liquid level in the reaction vessel is provided greater than the thickness of the sulfur froth 38, to further inhibit aerosol entrainment.
Paddle wheels 46 are provided adjacent the edges of the vessel 12 in operative relation with the sulfur froth 38, so as to skim the sulfur froth from the surface of the reaction medium 42 into collecting launders 48 provided at each side of the vessel 12. The skimmed sulfur is removed periodically or continuously from the launders 48 for further processing.
The sulfur is obtained in the form of froth containing about 15 to about 50 wt.% sulfur in reaction medium. Since the sulfur is in the form of particles of a relatively narrow particle size, the sulfur is readily separated from the entrained reaction medium, which is returned to the reactor 10.
The reactor 10 provides a very compact unit which rapidly and efficiently removes hydrogen sulfide from gas streams containing the same. Such gas streams may have a wide range of concentrations of hydrogen sulfide.
The compact nature of the unit leads to considerable economies, both in terms of capital cost and operating cost, when compared to conventional hydrogen sulfide removal systems.
There has previously been described in U.S. Patent No. 3,993,563 a gas ingestion and mixing device of the general type described herein. In that reference, it is indicated that, for the device described therein, if an increase in the rotor speed is made in an attempt to obtain greater gas-liquid mixing action, then it is necessary to employ a baffle in the standpipe in order to obtain satisfactory gas ingestion. As is apparent from the description herein, such a baffle is not required in the present invention.
Example 1 2 0 0 4 6 5 2 A pilot plant apparatus was constructed as schematically shown in Figure 1 and was tested for efficiency of removal of hydrogen sulfide from a gas stream containing the same.
The overall liquid capacity of the tank was 135 L.
The standpipe had a diameter of 8 in., and the impeller consisted of six blades and had a diameter of 5i~ in.
and a height of 61/ in. and was positioned 21/ in. from l0 the base of the tank.
The pilot plant apparatus, fitted with a standard froth flotation shroud and impeller combination, was charged with 110 L of an aqueous solution which contained 0.016 mol/L of ethylenediaminetetraacetic acid, iron-ammonium complex and 0.05 mol/L of sodium hydrogen carbonate. The pH of the aqueous medium was 8.5. The shroud consisted of a cylinder of diameter 12 in., height 5 3/4 in. and thickness 3/4 in. in which was formed 48 openings each 1.25 in. in diameter, for a total circumferential length of 188 inches.
Air containing 4000 ppm by volume of hydrogen sulfide was passed through the apparatus via the standpipe at a rate of 835 L/min. at room temperature while the impeller in the aqueous medium rotated at a rate of 733 rpm., corresponding to a blade tip velocity of about 211 in/sec. The gas flow rate through the shroud openings was 0.05 lb/min/opening in the shroud.
Over the one and a half hour test period, 99.5% of the hydrogen sulfide was removed from the gas stream, leaving a residual amount of HZS in the gas stream of 20 ppm. Sulphur was formed and appeared as a froth on the surface of the aqueous solution and was skimmed from the surface using the paddle wheels. Simultaneous removal of hydrogen sulfide from the gas stream and 1$ 2004652 recovery of the sulfur produced thereby, therefore, was effected.
During the test period, the pH of the aqueous solution dropped to 8.3 but no additional alkali was added during this period. Further, no additional catalyst was added during the period of the test.
Example 2 The procedure of Example 1 was repeated with an increased impeller rotation rate and higher gas flow rate.
Air containing 4000 ppm by volume of hydrogen sulfide was passed through the apparatus via the standpipe at a rate of 995 L/min. at room temperature while the impeller in the aqueous medium rotated at a rate of 1772 rpm corresponding to a blade tip velocity of about 510 in/sec. The gas flow rate through the shroud openings was 0.06 lb/min/opening in the shroud.
Over the two hour test period 99.7% of the hydrogen sulfide was removed from the gas stream, leaving a residual amount of H2S of 11 ppm. Sulfur was formed and appeared as a froth on the surface of the aqueous solution and was skimmed from the surface.
Simultaneous removal of hydrogen sulfide from the gas stream and recovery of the sulfur produced thereby, therefore, was effected.
During the test period, the pH of the aqueous solution dropped to 8.3 but no additional alkali was added during this period. Further, no additional catalyst was added during this period of the test.
Example 3 The pilot plant apparatus was modified and fitted with a shroud and impeller combination as illustrated in Figure 2, was charged with 110 L of an aqueous solution which contained 0.016 mol/L of ethylenediaminetetra-acetic acid, iron-ammonium complex and 0.05 mol/L of sodium hydrogen carbonate. The pH of the aqueous solution was 8.5. The shroud consisted of a cylinder of diameter 12 3/4 in., height 8'~ in., and thickness z in.
in which was formed 670 openings each of 3/8 in.
diameter for a total circumferential length of 789 inches. The impeller was replaced by one having a diameter of 6; in. The other dimensions remained the same.
Air containing 4000 ppm by volume of hydrogen sulfide was passed through the apparatus via the standpipe at a rate of 995 L/min. at room temperature while the impeller in the aqueous medium rotated at a rate of 1754 rpm., corresponding to a blade tip velocity of about 597 in/sec. The gas flow rate through the shroud was 0.004 lb/min/opening. Over the two hour test period 99.998% of the hydrogen sulfide was removed from the gas stream, leaving a residual amount of H2S of less than 0.1 ppm. Sulphur was formed and appeared as a froth on the surface of the aqueous solution and was skimmed from the surface. Simultaneous removal of hydrogen sulfide from the gas stream and recovery of the sulfur produced thereby, therefore, was effected.
During the test period, the pH of the aqueous solution remained relatively constant at 8.5. No additional alkali or catalyst was added during the period of this test.
As may be seen from a comparison of the results presented in Examples 1, 2 and 3, it is possible to remove hydrogen sulfide with greater than 99% efficiency using an agitated flotation cell which is provided with a conventional shroud and impeller construction (Examples 1 and 2), as already described in Canadian Patent No. 1,212,819. However, by employing a higher blade tip velocity, as in Example 2, a modest increase in efficiency can be achieved.
However, as seen in Example 3, with a shroud modified as described therein to provide the critical _ 2~ 2004652 gas flow rate and using the critical blade tip velocity, efficiency values over 99.99% can be achieved, leaving virtually no residual hydrogen sulfide in the gas stream.
In summary of this disclosure, the present invention provides novel method and apparatus for effecting chemical reactions and, if desired, for separating flotable by-products of such reactions using an agitated flotation cell, modified in certain critical respects to function as an efficient reactor.
Modifications are possible within the scope of this invention.
Claims (30)
1. A method of removal of a gaseous component from a gas stream containing the same by chemical conversion to an insoluble phase in a liquid medium, which comprises:
feeding said gaseous component-containing gas stream to an enclosed reaction zone in which is located said liquid medium and a chemical conversion agent for converting said gaseous component to the insoluble phase, rotating an impeller comprising a plurality of blades about a vertical axis at a submerged location in said liquid medium so as to induce flow of said gaseous component-containing gas stream along a vertical flow path from external to said reaction zone to said submerged location, surrounding said impeller with a stationary cylindrical shroud through which are formed a plurality of openings while said impeller is rotated at a speed corresponding to a blade tip velocity of at least about 300 in/sec so as to generate sufficient shear forces between said impeller blades and said plurality of openings in said cylindrical shroud to distribute said gas stream as fine gas bubbles of diameter no more than about 1/4 inch, and to effect intimate contact of gaseous component, chemical conversion agent, and liquid medium at said submerged location so as to chemically convert the gaseous component to an insoluble phase, permitting flow of materials from the interior of the shroud through the openings therein into the body of the liquid medium external to the shroud at a gas flow rate of less than about 0.02 lb/min/opening in said shroud, whereby any chemical conversion of gaseous component to insoluble phase not effected in the interior of the shroud is completed in the region adjacent to the exterior of the shroud, venting a gaseous component-depleted gas stream from a gas atmosphere above the liquid level in said reaction zone to exterior of said enclosed reaction zone.
feeding said gaseous component-containing gas stream to an enclosed reaction zone in which is located said liquid medium and a chemical conversion agent for converting said gaseous component to the insoluble phase, rotating an impeller comprising a plurality of blades about a vertical axis at a submerged location in said liquid medium so as to induce flow of said gaseous component-containing gas stream along a vertical flow path from external to said reaction zone to said submerged location, surrounding said impeller with a stationary cylindrical shroud through which are formed a plurality of openings while said impeller is rotated at a speed corresponding to a blade tip velocity of at least about 300 in/sec so as to generate sufficient shear forces between said impeller blades and said plurality of openings in said cylindrical shroud to distribute said gas stream as fine gas bubbles of diameter no more than about 1/4 inch, and to effect intimate contact of gaseous component, chemical conversion agent, and liquid medium at said submerged location so as to chemically convert the gaseous component to an insoluble phase, permitting flow of materials from the interior of the shroud through the openings therein into the body of the liquid medium external to the shroud at a gas flow rate of less than about 0.02 lb/min/opening in said shroud, whereby any chemical conversion of gaseous component to insoluble phase not effected in the interior of the shroud is completed in the region adjacent to the exterior of the shroud, venting a gaseous component-depleted gas stream from a gas atmosphere above the liquid level in said reaction zone to exterior of said enclosed reaction zone.
2. The method of claim 1 wherein said blade tip velocity is 500 to 700 in/sec.
3. The method of claim 1 or 2 wherein said gas flow rate is 0.005 to 0.007 lb/min/opening in said shroud.
4. The method of any one of claims 1 to 3 wherein said insoluble phase is flotable by said gas bubbles when depleted of reacted gaseous components thereof, and said depleted gas bubbles are permitted to rise through the liquid medium to float said insoluble phase on the surface of liquid medium in the respective individual reaction zone.
5. The method of any one of claims 1 to 4 wherein said chemical conversion agent is an oxidation-component-containing gas stream introduced to the liquid medium at the same submerged location as the gaseous component-containing gas stream and distributed as fine gas bubbles of diameter no more than about 1/4 inch, and said liquid medium is an aqueous medium containing a catalyst for oxidative conversion of said gaseous component to said insoluble phase.
6. The method of any one of claims 1 to 4 wherein said gas stream is a hydrogen sulfide-containing gas stream from which hydrogen sulfide is to be removed as the removed gaseous component, said chemical conversion agent is an oxygen-containing gas stream introduced to the same submerged location as the hydrogen sulfide-containing gas stream, either as a mixed gas stream or as separate gas streams, and distributed thereat as fine gas bubbles of diameter no more than about 1/4 inch, said liquid medium is a transition metal chelate solution, and said insoluble phase is solid sulfur particles.
7. The method of claim 6 wherein said solid sulfur particles are permitted to grow in said body of metal chelate solution until they are of a size to be floated from the body of the metal chelate solution by hydrogen sulfide-depleted gas bubbles, and the sulfur particles of orthorhombic crystalline form and having a particle size from about 10 to about 30 microns are transported from the body of metal chelate solution by the hydrogen sulfide-depleted gas bubbles to form a sulfur froth floating on the surface of the metal chelate solution and a hydrogen sulfide-depleted gas atmosphere above the sulfur froth.
8. The method of claim 7 wherein the sulfur froth is removed from the surface of the metal chelate solution to exterior of the enclosed reaction zone.
9. The method of any one of claims 6 to 8 wherein said transition metal chelate is an iron chelate formed with a chelating agent.
10. The method of claim 9 wherein the chelating agent is EDTA.
11. The method of any one of claims 6 to 10 wherein said process is effected at a temperature of 10° to 80°C in an iron chelate solution having a pH of 7 to 11.5.
12. The method of any one of claims 6 to 10 wherein said gas stream is a sour natural gas stream and said oxygen-containing gas stream is fed separately from said hydrogen sulfide-containing gas stream to said submerged location.
13. The method of any one of claims 1 to 4 wherein said gas stream is a hydrogen sulfide-containing gas stream from which hydrogen sulfide is to be removed as the removed gaseous component, said chemical conversion agent is a chlorine-containing gas stream introduced at the same submerged location as the hydrogen sulfide-containing gas stream and distributed thereat as fine gas bubbles of diameter no more than about 1/4 inch, said liquid medium is an aqueous sodium hydroxide solution, and said insoluble phase is sodium sulfate crystals formed after saturation of the aqueous solution after start-up.
14. The method of any one of claims 1 to 4 wherein said gas stream is a mercaptan-containing gas stream from which mercaptans are to be removed as the removed gaseous component, the chemical conversion agent is an oxygen-containing gas stream introduced to the liquid medium as the mercaptan-containing gas stream and distributed thereat as fine gas bubbles of diameter no more than about 1/4 inch, and the insoluble phase is immiscible liquid disulfides.
15. The method of any one of claims 1 to 4 wherein said gaseous component-containing gas stream contains mercaptans and hydrogen sulfide and said mercaptans and hydrogen sulfide are removed from said gas streams in separate reaction vessels.
16. The method of any one of claims 1 to 15 wherein said gas stream is fed to said submerged location at a flow rate of 50 to 500 cu.ft/min.
17. The method of any one of claims 1 to 16 wherein the rotation of the impeller induces a gas flow of said gas stream to said submerged location with a pressure drop across the reactor of -5 to +10 in H2O.
18. The method of claim 17 wherein the pressure drop is 0 to less than 5 in H2O.
19. A chemical reactor comprising:
enclosed tank means, inlet gas manifold means for feeding at least one gas stream through an inlet in an upper closure to said tank means, standpipe means communicating with said inlet and extending downwardly within said tank from said upper closure, impeller means comprising a plurality of blades located below the lower end of said standpipe means and mounted to a shaft for rotation about a vertical axis, drive means for rotating said shaft, cylindrical stationary shroud means surrounding said impeller means and having a plurality of circular openings and extending through the wall of said shroud means, each of said openings through said shroud means having a diameter less than 1 inch, and vent means from said tank means.
enclosed tank means, inlet gas manifold means for feeding at least one gas stream through an inlet in an upper closure to said tank means, standpipe means communicating with said inlet and extending downwardly within said tank from said upper closure, impeller means comprising a plurality of blades located below the lower end of said standpipe means and mounted to a shaft for rotation about a vertical axis, drive means for rotating said shaft, cylindrical stationary shroud means surrounding said impeller means and having a plurality of circular openings and extending through the wall of said shroud means, each of said openings through said shroud means having a diameter less than 1 inch, and vent means from said tank means.
20. The apparatus of claim 19 wherein said impeller has a diameter from 6 to 30 inches and said shroud has a diameter corresponding to 2:1 to 1.2:1 times the diameter of the impeller.
21. The apparatus of claim 20 wherein said impeller has a height corresponding to 0.5:1 to 2:1 times the diameter of the impeller.
22. The apparatus of any one of claims 19 to 21 wherein said impeller has at least 4 equally-angularly spaced blades and said shroud has a diameter which is approximately 1.5 times that of the impeller.
23. The apparatus of any one of claims 19 to 22 wherein said standpipe has a diameter of 8 to 45 inches and said shroud has a diameter not less than that of said standpipe.
24. The apparatus of any one of claims 19 to 23 wherein said impeller is spaced from a bottom wall of the vessel at least 0.25 times the diameter of the impeller.
25. The apparatus of any one of claims 19 to 24 wherein said plurality of openings is arranged to provide a gas flow rate of 0.005 to 0.007 lb/min/opening in the shroud.
26. The apparatus of any one of claims 19 to 25 wherein the openings are spaced 0.25 to 0.75 times the diameter of the openings from each other.
27. The apparatus of claim 26 wherein the openings are spaced about 0.5 times the diameter of the opening from each other.
28. The apparatus of any one of claims 19 to 27 wherein the plurality of openings is arranged at a density of less than 2 per square inch in a uniform array.
29. The apparatus of any one of claims 19 to 28 wherein each of said openings is dimensioned from 3/8 to 5/8 in. in diameter.
30. The apparatus of any one of claims 19 to 29 wherein said openings are all of the same diameter.
Priority Applications (19)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2004652 CA2004652C (en) | 1989-12-05 | 1989-12-05 | Method and apparatus for effecting gas-liquid contact reactions |
| PCT/CA1990/000431 WO1991008038A1 (en) | 1989-12-05 | 1990-12-04 | Method and apparatus for effecting gas-liquid contact |
| DK91900143.8T DK0504203T3 (en) | 1989-12-05 | 1990-12-04 | Method and apparatus for conducting a gas-liquid contact |
| ES91900143T ES2048007T3 (en) | 1989-12-05 | 1990-12-04 | METHOD AND APPARATUS TO MAKE GAS-LIQUID CONTACT. |
| KR1019920701328A KR0152244B1 (en) | 1989-12-05 | 1990-12-04 | Method and apparatus for effecting gas-liquid contact |
| EP91900143A EP0504203B1 (en) | 1989-12-05 | 1990-12-04 | Method and apparatus for effecting gas-liquid contact |
| AT91900143T ATE97013T1 (en) | 1989-12-05 | 1990-12-04 | METHOD AND DEVICE FOR CARRYING OUT GAS-LIQUID CONTACT. |
| DE91900143T DE69004578T2 (en) | 1989-12-05 | 1990-12-04 | METHOD AND DEVICE FOR PERFORMING A GAS-LIQUID CONTACT. |
| AU78954/91A AU636495B2 (en) | 1989-12-05 | 1990-12-04 | Method and apparatus for effecting gas-liquid contact |
| CA002070630A CA2070630C (en) | 1989-12-05 | 1990-12-04 | Method and apparatus for effecting gas-liquid contact |
| HU9201886A HU215365B (en) | 1989-12-05 | 1990-12-04 | Method and apparatus for removing gas-components in the course of contacting carrying out with fluid |
| JP3500610A JPH0677664B2 (en) | 1989-12-05 | 1990-12-04 | Method and apparatus for effective gas-liquid contact |
| HU188590A HU9201885D0 (en) | 1989-12-05 | 1990-12-05 | Dual impeller method and apparatus for effecting chemical conversion |
| US07/622,485 US5174973A (en) | 1989-12-05 | 1990-12-05 | Method and apparatus for effecting gas-liquid contact |
| US07/863,720 US5352421A (en) | 1989-12-05 | 1992-04-03 | Method and apparatus for effecting gas-liquid contact |
| US07/863,692 US5366698A (en) | 1989-12-05 | 1992-05-20 | Apparatus for effecting gas liquid contact |
| NO922210A NO178845C (en) | 1989-12-05 | 1992-06-04 | Method and apparatus for providing gas-liquid contact |
| FI922597A FI922597A0 (en) | 1989-12-05 | 1992-06-04 | FOERFARANDE OCH ANORDNING FOER ATT AOSTADKOMMA GAS-VAETSKAKONTAKT. |
| US08/230,230 US5413765A (en) | 1989-12-05 | 1994-04-20 | Method and apparatus for effecting gas-liquid contact |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2004652 CA2004652C (en) | 1989-12-05 | 1989-12-05 | Method and apparatus for effecting gas-liquid contact reactions |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2004652A1 CA2004652A1 (en) | 1991-06-05 |
| CA2004652C true CA2004652C (en) | 1999-08-31 |
Family
ID=4143720
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2004652 Expired - Lifetime CA2004652C (en) | 1989-12-05 | 1989-12-05 | Method and apparatus for effecting gas-liquid contact reactions |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA2004652C (en) |
| HU (1) | HU9201885D0 (en) |
-
1989
- 1989-12-05 CA CA 2004652 patent/CA2004652C/en not_active Expired - Lifetime
-
1990
- 1990-12-05 HU HU188590A patent/HU9201885D0/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| CA2004652A1 (en) | 1991-06-05 |
| HU9201885D0 (en) | 1992-12-28 |
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| EEER | Examination request | ||
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