Continuous autocirculation, multiple zone mass transfer apparatus and method

Abstract: An apparatus and method of contacting a liquid with different gases sequentially in separate mass transfer zones within a single vessel, the mass transfer zones operatively connected in liquid communication with each other, including intimately contacting the liquid with a first process gas in cocurrent flow in a first mass transfer zone to effect mass transfer between the first liquid and the process gas, and introducing the liquid into a second mass transfer zone with a second gas, different from the first gas, in cocurrent flow with the second liquid, thereby effecting mass transfer between the second liquid and the second gas. The rate of flow of the liquid from said first mass transfer zone to second mass transfer zone and recirculation to the first mass transfer zone is motivated by the difference in density of the contents of the liquid phase portion of each mass transfer zone and a following liquid downcomer or flow channel connecting it to the next sequential mass transfer zone, such that after passing through the last mass transfer zone, the liquid recirculates to the first mass transfer zone. The apparatus and method are ideally suited to carrying out the process of hydrogen sulfide removal from gases by means of an air-regenerated liquid redox catalyst, such as chelated iron.

Description: FIELD OF THE INVENTION The present invention is directed to a method and apparatus for contacting a liquid with a plurality of different gases, sequentially, in a gas-liquid contact apparatus having a plurality of gas-liquid contact zones, each in fluid communication with at least one other gas-liquid contact zone within a single vessel for mass transfer between the gases and the liquid, in such a manner that solution circulation is created by density differences between gas-liquid contact zones and intermediate liquid-filled communication chambers, without the use of a solution circulation pump. More particularly, the present invention is directed to a method and apparatus for continuously removing hydrogen sulfide gas (H 2 S) from a fluid stream by reacting the fluid stream with a catalytic redox polyvalent metal solution for H 2 S gas removal, and continuously regenerating the catalytic solution by reaction with an oxygen-containing gas. The process is particularly suited to treating process gas streams having a relatively high H 2 S concentration, e.g., at least 1% by weight, at a relatively low pressure, e.g., less than 1 bar gage, but is useful for process gas streams at any pressure. BACKGROUND OF THE INVENTION AND PRIOR ART The method and apparatus of the present invention are improvements over the autocirculation method and apparatus disclosed in U.S. Patent No. 4,238,462 and is useful for gas-liquid mass transfer where a liquid is contacted with two different gases in separate contact zones. In a catalytic process for removing hydrogen sulfide gas (H 2 S) from a fluid stream using a ferric iron catalyst, such as a ferric iron chelate, the ferric iron is reduced to the ferrous state when reacted with hydrogen sulfide. The series of reactions involved in catalytically oxidizing hydrogen sulfide gas to form elemental sulfur using a ferric iron chelate catalytic reagent can be represented by the following equations, as set forth in U.S. Patent No. 4,238,462: H 2 S(Gas) + H 2 O(Liquid) ⇄ H 2 S(Aqueous) + H 2 O(Liquid) H 2 S(Aqueous) ⇄ H + + HS - HS - ⇄ H + + S = S = + 2(Fe*Chelate) +3 → S° (Solid) + (Fe*Chelate) +2 By combining these equations, the resulting equation is: H 2 S(gas) + 2(Fe*Chelate) +3 → 2H + + 2(Fe*Chelate) +2 + S° In order to have an economical, workable process to effect catalytic oxidation of the hydrogen sulfide, it is essential that the hydrogen sulfide gas be brought continuously into intimate contact with the chelated iron solution and that the ferrous iron chelate formed in the above described manner be continuously regenerated by oxidizing to ferric iron chelate by intimate contact with dissolved oxygen, preferably derived from ambient air. The series of reactions that take place when regenerating the required ferric iron chelate can be represented by the following equations: O 2 (Gas) + 2H 2 O ⇄ O 2 (Aqueous) + 2H 2 O O 2 (Aqueous) + 2H 2 O + 4(Fe*Chelate)+2 → 4 (OH - ) + 4(Fe*Chelate) + By combining these equations, the resulting equation is: 1/2 O 2 (Gas) + H 2 O + 2(Fe*Chelate) +2 → 2(OH) - + 2(Fe*Chelate) +3 It will be evident from the foregoing equations that theoretically two moles of ferric iron must be supplied to the reaction zone in which the hydrogen sulfide gas is oxidized to form elemental sulfur for each mole of hydrogen sulfide gas treated, and in actual practice considerably more than the theoretical amount of iron is used. In a continuous process of removing hydrogen sulfide by contact with a catalytic ferric iron solution, the catalytic solution is circulated continuously between an absorber zone, where the H 2 S is absorbed by the catalytic ferric iron chelate solution, and the solution reduced to ferrous iron in an oxidizer zone where the reduced ferrous iron is oxidized back to the ferric iron state. In order to avoid using high concentrations of iron in the catalytic solution, the rate of circulation should be high. In the past, vanadium redox catalysts could not be used in an autocirculation process and apparatus, such as disclosed in US-A-4,238,462 because the vanadium redox reactions require more time to complete the H 2 S oxidation reaction in the absence of dissolved oxygen before the solution exiting the absorber is exposed to oxygen. The economics and workability of the Stretford process have depended upon a large volume of the metal vanadate redox solution. The reduced metal vanadate, after absorption of the H 2 S (as HS - or S = ) to form the metal vanadate in the +4 valance state is continuously regenerated to the +5 valance state by contact with dissolved oxygen for continuous use of the oxidized metal vanadate in the absorption zone to remove additional H 2 S as elemental sulfur. The Stretford process chemistry is typically summarized according to the following steps: Absorption and dissociation of H 2 S into alkali: 2H 2 S(g) + 2Na 2 CO 3 → 2NaHS + 2NaHCO 3 ; Bisulfide oxidation with metavanadate to form elemental sulfur and reduced vanadium: 2NaHS + 4NaVO 3 + H 2 O → Na 2 V 4 O 9 + 4NaOH + 2S; and Vanadium reoxidation by dissolved molecular oxygen in the presence of ADA: Prior to the autocirculation method and apparatus disclosed in U.S. Patent No. 4,238,462, the catalytic oxidation-reduction reactions for continuously removing hydrogen sulfide, or the like, from a fluid stream were carried out concurrently in the same reaction vessel by means of a process which can be referred to as an aerobic operation, or by means of a process in which the oxidation and reduction steps were carried out in separate reaction vessels in what can be referred to as an anaerobic operation (see U.S. Patent No. 3,897,219). While an anaerobic operation may have certain advantages over an aerobic operation for treating some gas streams which must be recovered after H 2 S removal, there is the extra expense involved in providing additional equipment, and the continuous pumping of large volumes of liquid from one vessel to the other increases operating costs. The method and apparatus described in U.S. Patent No. 4,238,462 has been commercially successful, but the commercial use of that method and apparatus suffers from several disadvantages including some lack of control of residence time for gas-liquid contact in each of the reaction zones; no provision for liquid flow control; a relatively high thiosulfate production rate - 4 to 6 percent by weight of the sulfur being converted to thiosulfate; and relatively high iron losses when the method and apparatus are used in the preferred embodiment for removing hydrogen sulfide from a gas stream. Further, one skilled in the art will see that the autocirculation process as practiced commercially (US-A-4,238,462) comprises a well-stirred oxidation section, in which the composition of the bulk of the solution in the oxidizer is of necessity substantially the same as that circulating into the absorption zone. That is, it is nearly completely oxidized, so has a relatively high oxygen partial pressure and presents a minimum driving force for dissolution of oxygen in the solution. In accordance with the process and apparatus of the present invention, the quantity of oxygen dispersed in a last oxidation stage can be controlled to prevent a substantial excess of dissolved oxygen in the polyvalent metal redox solution entering the absorption stage of the process so that sulfate, thiosulfate and other salts will be formed in the absorption zone of the process to a much lesser extent, enabling the process to be carried out at a more efficient, higher pH with little or no need for periodic addition of alkali to the polyvalent metal redox solution. Finally, the autocirculation apparatus as presently practiced is not applicable to the Stretford process and such redox processes which rely on relatively slow reactions between the oxidized metal redox catalyst and dissolved HS - or S = ions because there is no effective residence time for the solution leaving the absorber before it enters the oxidizer. The absence of sufficient residence time contributes significantly to the formation of thiosulfate and sulfate salts by permitting the residual HS - and S = ions to enter the oxygen-rich oxidation zone where the non-specific side reactions between these species and dissolved oxygen can take place. The present method and apparatus overcomes this limitation, and makes possible full use of the autocirculation principle for Stretford and any other H 2 S removal process that uses a catalytic redox polyvalent metal solution that is reduced and oxidized for converting H 2 S to elemental sulfur, because the present invention provides for the insertion of a residence chamber through which the liquid exiting the absorber passes before entering the oxidizer. In the case of the LO-CAT® process, a residence time of 10 to 60 seconds may be adequate, whereas in the case of the Stretford chemistry 10 to 60 minutes may be required. SUMMARY OF THE INVENTION The above and other disadvantages have been overcome in accordance with the method according to claim 1 and the apparatus according to claim 6. According to the present invention there is provided an apparatus having a plurality of gas-liquid contact mass transfer zones or chambers in fluid communication with one another, including a first gas-liquid mass transfer zone or chamber for contact of a liquid with a first or process gas, and a residence chamber in fluid communication with the first gas-liquid mass transfer zone or chamber to aid in controlling circulation of the mass transferred and/or mass transferring solution from the first mass transfer zone to a second mass transfer zone. Cross flow of solution from the residence chamber through a valve or other circulation control device to the second gas-liquid mass transfer zone or chamber provides controlled, continuous and sequential gas-liquid contact with two or more different gases in separate contact zones within a single vessel. Each of the first and second gas-liquid mass transfer zones or chambers may be divided into two or more separate contact stages for continued gas-liquid contact. The separate stages of each gas-liquid mass transfer zone or chamber are in sequential fluid communication, one with another through intermediate flow channels or communication chambers in which no additional gas is dispersed in the liquid. The flow channels are formed by an inverted weir extending downwardly from an upper portion of a downstream end of one stage and a submerged weir extending upwardly from an upstream end of the succeeding stage, the weirs being horizontally spaced and vertically overlapping within the liquid. The residence chamber separates the first gas-liquid mass transfer zone from the second gas-liquid mass transfer zone to provide controlled residence time and controlled circulation of solution between the two mass transfer zones for sequential contact with a different gas in each zone. In this manner, reliable, controlled circulation of solution is achieved regardless of the desired rate of flow of each process gas in each zone. In the application to liquid redox processes for oxidation of H 2 S to sulfur and water, there is little or no thiosulfate production since substantially all of the HS - and S = ions are converted to elemental sulfur in the residence chamber or in the cross flow conduit by reaction with the oxidized polyvalent metal ions prior to oxidation of the polyvalent metal ions in the oxidizer zone, and due to the ability to control the dissolved oxygen in the polyvalent metal redox solution entering the absorption chamber; and the first and/or second gas-liquid mass transfer zones can be separated into stages to minimize the total volume required for each mass transfer zone. Further advantages over the method and apparatus set forth in U.S. Patent No. 4,238,462 include ease of segregation of the various gases. In the liquid redox H 2 S oxidation process, spent air and spent process gas flows can be easily separated so the CO 2 in the case of sweetening of a CO 2 -H 2 S gas as might be derived from an amine gas sweetening process in a natural gas treating plant or other hydrocarbon or inert gas content of a clean process gas can, when economically feasible, be recovered, and to minimize the amount of spent air that is combined with sour process gas needing scrubbing or incineration; adequate driving forces for circulation of the solution are achieved at lower air rates per square foot of oxidation zone area without requiring pumping between gas-liquid contact zones; there is no possibility of circulation going the wrong direction if one or the other of the gas flows varies from the design rate; and there can be a substantial pressure difference between the various gas-liquid contact chambers to accommodate pressure requirements of downstream processing equipment. There is a further advantage which may be significant in terms of application to other chemical systems. With the method and apparatus of the present invention, the Stretford process, or any other mass transfer process requiring liquid contact with two or more different gases can be used, such as Shafer, Unisulf, and the like, or any other gas-liquid contact mass transfer process that requires substantial residence time. Accordingly, one aspect of the present invention is to provide a method and apparatus for causing mass transfer between a liquid and a plurality of different gases or gases within separate mass transfer zones within a single vessel. Another aspect of the present invention is to provide an autocirculation method and apparatus for continuously removing hydrogen sulfide (H 2 S) gas from a process gas stream by intimate contact with a catalytic polyvalent metal redox solution in one mass transfer zone and continuously regenerating the catalytic solution within the same vessel by intimate contact with an oxidizing gas in a separate mass transfer zone. Another aspect of the present invention is to provide an autocirculation method and apparatus for continuously removing hydrogen sulfide (H 2 S) gas from a sour process gas stream without conversion of the H 2 S to significant levels of a metal thiosulfate (e.g., Na 2 S 2 O 3 ), generally less than about 5% by weight of the sulfur converted to thiosulfate, and preferably less than 1% by weight. A further aspect of the present invention is to provide an autocirculation method and apparatus for continuously removing hydrogen sulfide (H 2 S) gas from an H 2 S containing process gas by intimate contact of the gas with an iron or vanadium catalyst solution, without significant iron or vanadium losses. Still another aspect of the present invention is to provide an autocirculation method and apparatus for continuously removing hydrogen sulfide (H 2 S) gas from an H 2 S containing process gas by intimate contact of the process gas with a polyvalent metal redox catalyst solution in one mass transfer zone for absorption of the H 2 S gas into the catalyst solution, and regenerating the polyvalent metal catalyst solution in a separate mass transfer zone in the same vessel, while providing a residence chamber between the two mass transfer zones and liquid flow control between the two mass transfer zones. BRIEF DESCRIPTION OF THE DRAWINGS The above and other aspects and advantages of the present invention will become more apparent from the following detailed description of the invention, taken in conjunction with the drawings, wherein: Figure 1 is a side view of one embodiment of the gas-liquid mass transfer oxidizer/absorber vessel of the present invention; Figure 2 is a top view of another embodiment of the gas-liquid mass transfer vessel of the present invention as a circular design; Figure 3 is a top view of another embodiment of the gas-liquid mass transfer vessel of the present invention, as an alternative rectangular design providing additional oxidation stages; Figure 4 is a calibration graph of liquid flow rate vs. valve setting for an air flow rate of 198 dm 3 /min (7.0 SCFM); Figure 5 is a graph of thiosulfate for Example 1 in comparison to a conventional autocirculation unit at 6% Na 2 S 2 O 3 ; and Figure 6 is a graph showing oxidation reduction potential (ORP) vs. sample location wherein the H 2 S flow rate is 286 g/h (0.63 lb/hr). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to the drawings, and initially to Figure 1, a cross-flow autocirculator, generally designated 10, as shown in Figure 1, was designed for a treating capacity of about 454 g/h (1 pound/hr) sulfur, to test a 3-stage oxidation autocirculator having a liquid flow control ball valve 12 disposed in a conduit 14 between a first mass transfer zone, or absorber chamber, generally designated 16, and a second mass transfer zone, or oxidizer chamber. The oxidizer chamber includes three separated oxidation stages 18A, 18B and 18C, for regeneration of the catalyst solution. A residence chamber 20 is disposed between and in fluid communication with both the first mass transfer zone, or absorber chamber 16, and the second mass transfer zone, or oxidizer zone, via conduit 14. The autocirculator 10, shown in Figure 1, is a 416 ℓ (110 gallon) capacity rectangular acrylic tank, divided by baffles into absorber chamber 16, three oxidizer chambers 18A, 18B, 18C, and residence chamber 20. The absorber chamber 16 is connected in fluid communication through the residence chamber 20 to the first stage 18A of the oxidizer zone via a 7.6 cm (3 inch) internal diameter transparent hose 14. Circulation is driven by the liquid density differences induced by aeration, as disclosed in US-A-4,238,462. H 2 S rich gas is introduced at the bottom of the absorber through sparger 22 in the absorber chamber 16 for intimate contact with liquid ferric iron chelate catalyst solution 29. The H 2 S rises along with oxidized catalyst solution from the third or last stage 18C of the oxidizer zone to absorption zone 16 and, after the oxidized catalyst solution has absorbed H 2 S from the process gas in absorption zone 16, the H 2 S-laden liquid catalyst solution flows over the baffle 30 separating the absorber chamber 16 from the residence chamber 20, and flows through outlet 32, conduit 14, and ball valve 12, to the first oxidizer stage 18A, through inlet 33. In a preferred embodiment, a sloped plate (not shown) forms a floor of residence chamber 20 to direct any settled sulfur to the residence chamber outlet 32. The H 2 S-laden liquid catalyst in the first oxidizer stage 18A is oxidized by air flowing through sparger 36 and flows over baffle 38 and under baffle 40 to the second stage 18B of the oxidizer. The partially oxidized solution in the second stage 18B is further oxidized by air flowing through sparger 42 and flows over baffle 44, and under baffle 46 to the third stage 18C of the oxidizer, for further oxidation with air from sparger 47. From the third oxidation stage 18C, the catalyst flows over baffle 48, and under baffle 50 to the absorber chamber 16. The catalyst enters the first oxidation stage 18A through an equal area transition plate 52 to spread the flow from the 7.6 cm (3 inch) hose along the bottom center of the first oxidation stage 18A. Spent oxidation air is vented from the top of each oxidizer stage 18A, 18B and 18C through outlet conduits (not shown). Also, an overhead space 59 above absorber 16 is flushed with air and the overhead space vented. All vented gases can be treated prior to release to the atmosphere. Ambient H 2 S data showed that H 2 S concentrations were less than 2 parts per million, and usually less that 0.25 parts per million at steady state. The circular design of the process and apparatus shown in Figure 2, generally designated 60, includes an absorber chamber 62 and an oxidizer chamber that is divided into three gas-separated stages, 64, 66 and 68. Horizontally and vertically spaced baffles 70 and 72, disposed between the absorber chamber 62 and the first oxidation stage 64 define a residence chamber 74 therebetween, for holding the H 2 S-laden polyvalent metal chelate solution prior to oxidation of the chelate solution. The dimensions of the residence chamber 74 can be enlarged as desired to provide any desired residence time prior to oxidation of the polyvalent metal redox solution in the first oxidizer stage 64. Similarly, horizontally and vertically spaced baffles 76 and 78 are disposed between the first and second oxidizer stages 64 and 66; horizontally and vertically spaced baffles 80 and 82 are disposed between the second and third oxidizer stages 66 and 68; and horizontally and vertically spaced baffles 84 and 86 are disposed between the third oxidizer stage 68 and the absorber chamber 62. The spaced baffles 70 and 72; 76 and 78; 80 and 82; and 84 and 86 are constructed like those shown in Figure 1 such that liquid from the absorber flows over baffle 70 and under baffle 72, and liquid from each successive oxidizer stage flows over its adjacent baffle 76, 80 and 84, and under the baffle 78, 82 or 86 adjacent the next succeeding zone. Like Figure 1, the baffle that the liquid flows under prevents fluid communication above the liquid level between adjacent zones. Although not shown in Figure 2, air spargers are disposed in oxidizer stages 64, 66 and 68, and a process gas sparger is disposed in absorber chamber 62. Preferably, the cylindrical absorber/oxidizer vessel shown in Figure 2 has a planar, horizontally disposed floor. The absorber/oxidizer vessel construction of Figure 3, generally designated 90, is an enlarged version of that shown in Figure 1 that provides additional oxidizer zones, and includes an absorber chamber 92 and a total of five oxidizer zones, 94, 96, 98, 100 and 102. Vertically and horizontally spaced baffles 104 and 106 separate absorber chamber 92 from the first oxidizer stage 94 and define a residence chamber 107 therebetween. Reduced polyvalent metal redox solution from absorber 92 flows over baffle 104 into residence chamber 107 and under baffle 106 into the first oxidizer stage 94. Baffle 106 prevents fluid communication above the liquid level between absorber chamber 92 and the first oxidizer stage 94. Similarly, horizontally and vertically spaced baffles 108 and 110; 112 and 114; 116 and 118; and 120 and 122 are constructed like those of Figure 1 to prevent fluid communication above the liquid level between adjacent zones and to allow liquid from one zone or chamber to flow over its adjacent baffle 104, 108, 112, 116, 120 and 124, and under the baffle 106, 110, 114, 118, 122 or 126 adjacent the next succeeding zone. Although not shown in Figure 3, air spargers are disposed in oxidizer stages 94, 96, 98, 100 and 102, and a process gas sparger is disposed in absorber chamber 92, the same as the spargers shown in Figure 1. The unit shown in Figure 1 was constructed for experimental purposes to handle 454 g/h (1 pound/hr). sulfur and data were collected to show the marked improvement using the apparatus and method of the present invention, particularly utilizing the liquid flow control afforded by the valve 12 in conduit 14 between the first mass transfer zone 16 and the second mass transfer zone 18. This liquid flow control using the valve 12 was found to be a marked improvement over attempts to control liquid flow using only control of air flow through the air spargers 36, 42 and 47. The ball valve 12 allows the catalyst flow rate to be adjusted. A calibration graph of liquid flow rate versus valve setting is given for various air flow rates in Figure 4. Overhead sample ports (not shown) allow for vented H 2 S concentration to be determined and sample ports (not shown) at the bottom of each chamber allow for liquid sampling. A continuous 3.8-18.9 ℓ/min (1-5 gpm) stream was removed from the third oxidation stage 18C through a sample port and gravity filtered through a cloth bag for sulfur removal. The filtrate is automatically returned from the collection vessel by a pump activated by a level control (not shown). Solution temperatures were determined by installing a thermocouple in the first oxidation stage 18A in an overhead sample port. The air spargers 36, 42, and 47 in each of the three oxidizer stages 18A, 18B and 18C of the oxidizer zone 18 were formed from pieces of slit and plugged silicone rubber oxidizer hoses attached to a capped 2.54 cm (one inch) internal diameter pipe disposed along the bottom of each oxidizer stage 18A, 18B, and 18C. In the first stage 18A of the oxidizer 18, the sparger 36 is disposed immediately overlying the perforated distribution plate 52. Air was initially supplied by a Becker pump type DT 3.25 with an 0.51 m 3 /min (18 CFM) capacity. A second such pump was added for later work. Air was regulated and measured using a King Instrument 0-227 dm 3 /min (0-8 SCFM) rotameter for each oxidizer stage 18A, 18B and 18C. Overhead air in the absorber chamber was regulated with a Dwyer 1.4·10 2 -1.4·10 3 dm 3 /h (5-50 SCFH) rotameter. Air pressures were periodically determined using a quick-connect test gauge. Air temperature was determined temporarily by fastening a thermocouple to the air hose. An H 2 S sparger 22, in the absorber zone 16, was used to supply H 2 S as a pure gas from a cylinder supplied by Air Products. The regulator delivery pressure was set at 1.4·10 5 Pa (20 psig). Flow was regulated with Dwyer series RMB Rate-Master rotameters calibrated for air. Rotameters with ranges of 0-1.4·10 2 , 0-2.8·10 2 , and (0.85-5.7)·10 2 dm 3 /h (0-5, 0-10, and 3-20 SCFH) were used, as required. During the testing, a pressure gauge was installed between the rotameter and spargers, and barometric pressure was read from a mercury barometer. Samples were periodically withdrawn from each pilot plant chamber. The pH and oxidation reduction potential (ORP) were measured using a Cole-Parmer pH meter. A 20% or 50% NaOH solution was pumped into the catalyst via a tube inserted in a standpipe (not shown) in the conduit 14 flowing between the absorber zone 16 and the first stage 18A of the oxidizer zone 18. The NaOH pump rate was manually adjusted as necessary to maintain the pH between 8.0 and 8.3. The oxidation reduction potential (ORP) probe was an Orion platinum redox electrode model 96-78, used with a 90-00-01 reference electrode filling solution. EXPERIMENTAL OPERATION Catalyst Formulation - The catalyst was prepared with a target Fe content of 500 mg/kg. Just prior to use, after concentration adjustments with water, the soluble Fe concentration was determined to be 445 mg/kg at the start of the experiment. The liquid flow rate was determined at several air flow rates and control valve positions. The pilot plant (Figure 1) was filled with water and the air sparger turned on to achieve flow. Dye was injected at one end of the straight, 7.6 cm (3 inch) I.D., transparent pipe installed between the absorber and oxidizer 10. Its time of travel through 83.1 cm (32.7 inches), which was the length equivalent to 3.8 ℓ (1 gallon), was determined. Flow was laminar; dye travel time was measured at the center of the pipe. From the velocity and pipe diameter, flow rates were determined in ℓ/min (gpm). Valve positions were marked. It should be noted that setting this valve give a rather coarse flow adjustment with an accuracy of about ± 7.56 ℓ/min (± 2 gpm). With the valve wide open, the flow was 79.4 ℓ/min (21 gpm) with 0.19 ℓ/min (5.3 SCFM) air and 100.2 ℓ/min (26.5 gpm) with 215 ℓ/min (7.6 SCFM) air set on each rotameter. A calibration curve is shown in Figure 3. EXAMPLE 1 The 454 g/h (1 pound/hr) sulfur autocirculation unit was run for 16 days with an initial thiosulfate concentration of about 1.7% Na 2 S 2 O 3 . Air flow was maintained at about 147 ℓ/min (5.2 SCFM) on each oxidizer stage rotometer. Conclusions Example 1 (1.7% Na 2 S 2 O 3 ) 1. No measurable thiosulfate was produced in the crossflow autocirculator. Less than 1% of the H 2 S was converted to thiosulfate. 2. The treatment capacity of the unit, when it was run with 401.6 ℓ/min (14.2 SCFM) of air, was 212 ℓ/min (7.49 SCFH) of H 2 S or 286 g (0.63 lb) of sulfur per hour. 3. The pH and ORP were measured in each chamber regularly throughout Example 1. The pH was maintained between 7.9 and 8.3. There was little difference among absorber and oxidizer pHs. ORP's over the H 2 S flow range are plotted in Figure 5. In accordance with another feature of the present invention, some rich solution from any of the oxidizer stages 18B or 18C can be directed back into an earlier oxidizer stage, e.g., 18A, to allow for a desired initial build-up of thiosulfate to, e.g., about 10 to about 20% by weight from an initial concentration of, for example, about 1.5% by weight. Suitable pumps and by-pass conduits can be provided from oxidizer stage 18B and/or 18C back to oxidizer stage 18A to by-pass the residence chamber 59. Alternatively, air can be sparged into the solution within residence chamber 59 to allow for an initial build-up of thiosulfate to a desired initial concentration in the range of about 10% to about 20% by weight thiosulfate. Thereafter, the by-passing of residence chamber 59 or air sparging within residence chamber 59 can be stopped to maintain the thiosulfate level at a desired level, e.g., 10-20% by weight of the solution.

Claims: A continuous process for intimate contact of a liquid reagent sequentially with first and second different gases in a vessel including a first mass transfer zone and a second mass transfer zone laterally spaced and in liquid communication, with said first and second mass transfer zones being in open liquid flow communication at their upper and lower ends and containing a level of liquid reagent in each mass transfer zone, such that liquid reagent is introduced into the first mass transfer zone in intimate contact with the first gas and the liquid reagent flows from the first mass transfer zone into the second mass transfer zone for intimate contact with the second gas, including the steps of introducing the first gas into said first mass transfer zone for intimate contact and mass transfer with said liquid reagent; and introducing the second gas into said second mass transfer zone for intimate contact and mass transfer with said liquid reagent after said liquid reagent has been intimately contacted and mass transferred with the first gas in the first mass transfer zone, the process comprising: establishing the flow of said liquid reagent in the two mass transfer zones by means of a static pressure difference induced by aeration between the liquid inlet and the liquid outlet of each mass transfer zone to effect continuous circulation of liquid reagent from one mass transfer zone to the other mass transfer zone while the liquid reagent and the gas flow cocurrently in each of said mass transfer zones without mechanical pumping means for the liquid and controlling the flow rate of the reacted liquid reagent from the first mass transfer zone to the second mass transfer zone, further introducing the liquid reagent from the outlet of the first mass transfer zone into a residence chamber through its inlet for retaining a quantity of the liquid reagent between the first mass transfer zone and the second mass transfer zone, and continuously flowing the liquid reagent from the residence chamber through its outlet to the second mass transfer zone at a controlled rate to provide a substantially constant reaction time in the residence chamber, wherein there is essentially no gas dispersed through the liquid reagent in the residence chamber, except for the first and second gases dissolved in the liquid reagent in the first and second mass transfer zones, the inlet of the residence chamber being positioned opposite the outlet of the residence chamber. The method of claim 1 wherein the first gas includes hydrogen sulfide gas and said liquid reagent is a catalytic oxidation - reduction solution to effect oxidation of the hydrogen sulfide gas and reduction of the catalytic solution and to form a reduced catalytic solution in the first mass transfer zone and to form elemental sulfur; and wherein the second gas is capable of oxidizing the reduced catalytic solution to form an oxidized catalytic solution so that the oxidized catalytic solution is capable of further oxidation of the first gas in the first mass transfer zone. The method of claim 2 wherein the catalytic solution contains a ferric iron chelate that is reduced by hydrogen sulfide gas to a ferrous iron chelate in said first mass transfer zone. The method of claim 1 wherein the second mass transfer zone includes a plurality of mass transfer stages in open liquid communication, one with another, and further including introducing the second gas into a first stage of the second mass transfer zone in contact with said liquid reagent; and thereafter contacting the liquid reagent received from the first stage of the second mass transfer zone with the second gas in a second stage of the second mass transfer zone. The method of claims 1 to 4 further including the step of controlling the rate of flow of the liquid reagent from said residence chamber to said second mass transfer zone to control the amount of mass transfer in said second mass transfer zone. Apparatus for effecting the method of claim 1 for mass transfer of a liquid reagent sequentially with two different gases in a separate mass transfer zone for each gas without permitting both gases to contact the same portion of the liquid simultaneously, the separate mass transfer zones being laterally spaced and in open liquid flow communication at their upper and lower end comprising: a first mass transfer zone (16) for intimate contact and mass transfer between the liquid reagent and a first gas; means (48,50,22) for directing the liquid reagent and the first gas into the first mass transfer zone in cocurrent flow to form a treated gas and a reacted liquid reagent; a second mass transfer zone (18) in liquid, but not vapor, communication with the first mass transfer zone; means (36) for directing a second gas into the second mass transfer zone in cocurrent flow with the reacted liquid reagent for intimate contact and mass transfer therebetween, said first and second mass transfer zones disposed in a single mass transfer vessel (10) including baffles (48,50) separating the mass transfer zones to prevent gas from the first mass transfer zone from contacting reacted liquid reagent in the second mass transfer zone; means operatively connected between the first mass transfer zone and the second mass transfer zone for establishing the flow rate of reacted liquid reagent from the first mass transfer zone to the second mass transfer zone, while maintaining the continuous operation of both mass transfer zones in a single vessel; and means for circulation of liquid continuously between the mass transfer zones by means of solution density differences between the liquid-gas mixture in the mass transfer zones and gas-free liquid in interconnecting flow channels, wherein the means for establishing the liquid reagent flow from the first mass transfer zone (16) to the second mass transfer zone (18) includes a residence chamber (20) disposed between the first mass transfer zone and the second mass transfer zone for retaining a supply of liquid sufficient to carry out chemical reactions while maintaining a continuous flow of liquid reagent, the inlet of the residence chamber being positioned opposite the outlet of the residence chamber. The apparatus of claim 6 wherein the baffles separating the first (16) and second mass transfer zones (18) are two spaced, overlapping vertical baffles (48, 50) overlapping within the liquid reagent to allow liquid reagent to flow between the mass transfer zones while preventing gas introduced into the first mass transfer zone from entering the second mass transfer zone. The apparatus according to claim 6 or 7 further including a conduit (14) operatively connected between the residence chamber (20) and the second mass transfer zone (18), said first mass transfer zone (16) being in liquid communication with both the residence chamber and the second mass transfer zone, said conduit including means for adjusting the flow of said reacted liquid reagent from said residence chamber to said second mass transfer zone. The apparatus of claim 8 wherein the residence chamber (20) includes an inclined floor sloped downwardly toward the conduit for directing elemental sulfur into the conduit for removal from said reacted liquid reagent. The apparatus of claim 8 further including a plurality of stages (18A, 18B, 18C) of said second mass transfer zone in liquid communication, one with the other, for effecting mass transfer between said liquid reagent and said second gas.

Inventors: HARDISON LESLIE C, HARDISON, LESLIE C.
Applicants: WHEELABRATOR CLEAN AIR SYSTEMS, WHEELABRATOR CLEAN AIR SYSTEMS INC.
Application year: 1992
Main classification: B01D53
IPC codes: B01D53, C02F1, B01J10, C10K1