HK1181344B - Flow control method and apparatus for a continuous multiple zone mass transfer - Google Patents

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 in fluid communication with each other, including intimately contacting the liquid with a process gas in co-current flow in a downstream mass transfer zone to effect mass transfer between the liquid and the process gas, and introducing the liquid into an upstream mass transfer zone with a second gas, different from the process gas, thereby effecting mass transfer between the liquid and the second gas. The rate of flow of the liquid from the upstream mass transfer zone to downstream mass transfer zone is controlled by the controlled addition of a third gas into one or more downcomers separating each mass transfer zone such that the specific density of the liquid in the downcomers provides a driving force that controls flow.

Description

Flow control method and apparatus for continuous multi-zone mass transfer

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims U.S. application No. 12/794,875, filed on 7/6/2010, which is incorporated herein by reference in its entirety.

Technical Field

The invention relates to a gas-liquid reactor for the sequential contacting of a liquid with a plurality of different gases in a gas-liquid reactor having a plurality of gas-liquid contact zonesMethods and apparatus for contacting in a contacting apparatus, each gas-liquid contacting zone being in fluid communication with at least one other gas-liquid contacting zone within a single vessel for mass transfer between the gas and liquid, in such a way as to create solution circulation by the density difference that always exists between the gas-liquid contacting zone and an intermediate liquid-filled downcomer, without the use of a solution circulation pump. More particularly, the present invention relates to a process for the continuous removal of hydrogen sulfide gas (H) from a fluid stream₂S) method and apparatus for removing H by reacting a fluid stream with a catalytic redox polyvalent metal solution₂S gas and is continuously regenerated by reacting the catalytic solution with an oxygen-containing gas. The process is particularly suitable for processing a material having a relatively high H content at a relatively low pressure (e.g., less than 1 bar gauge)₂S concentration (e.g., at least 1 vol%) and can be used for process gas streams at any pressure.

Background

The process and apparatus of the present invention improves over the automatic circulation processes and apparatus set forth in the previous U.S. patent application nos. 4,238,462 and 5,160,714 and can be used for gas-liquid mass transfer in the case of liquid contacting two different gases in separate contacting zones. The series of reactions involved in the catalytic oxidation of sulfur contaminants (e.g., hydrogen sulfide) to elemental sulfur using iron chelate catalysts can be represented by the following reactions, where L generally represents two or more specific ligands selected for formulating a mixture of metal chelate catalysts:

(1)H₂S_(gas)+H₂O_(liquid)→H₂S_{(Water-based)}+H₂O_(liquid)

(2)H₂S_{(Water-based)}→H⁺+HS^-

(3)HS^-+2(Fe³⁺L2)→S_(solid)+2(Fe²⁺L₂)+H⁺

By combining equations (1) to (3), the resulting equations are:

(4)H₂S_(gas)+2(Fe³⁺L2)→2H⁺+2(Fe²⁺L2)+S_(solid)

In order to obtain an economically viable process for the removal of hydrogen sulphide from a gaseous stream when using a polyvalent metal chelate mixture to achieve catalytic oxidation of hydrogen sulphide, it is necessary to continuously regenerate the formed ferrous chelates (as exemplified above) by contacting the reaction solution with dissolved oxygen, preferably in the form of ambient air, to ferric chelates in the same or a separate contact zone. In regenerating the metal chelate catalyst, the series of reactions that occur in the oxidizer of the present invention can be represented by the following reaction formula:

(5)O_{2 (gas)}+2H₂O→O_{2 (Water-based)}+2H₂O

(6)O_{2 (Water-based)}+2H₂O+4(Fe²⁺L₂)→4(OH^-)+4(Fe³⁺L₂)

By combining equations (5) to (6), equation (7) is obtained:

(7)1/2O₂+H₂O+2(Fe²⁺L₂)→2(OH^-)+2(Fe³⁺L₂)

also, when equations (4) and (7) are combined, the entire process can be represented by the following equations:

(8)H₂S_(gas)+1/2O_{2 (gas)}→S_(solid)+H₂O_(liquid)

It will be apparent from the above equation that for every mole of hydrogen sulfide gas treated, theoretically 2 moles of ferric iron must be applied to the reaction zone where the hydrogen sulfide gas is oxidized to form elemental sulfur, and that in practical applications much more than the theoretical amount of iron is used. In the passage of and catalysis ofIn a continuous process for removing hydrogen sulfide by contacting a ferric solution, a catalytic solution is placed in an absorber zone (where H is₂S is absorbed by the catalytic ferric chelate solution and the solution is reduced to ferrous iron) and an oxidizer zone for oxidizing the reduced ferrous iron back to a ferric state. To avoid using high concentrations of iron in the catalytic solution, the circulation rate will be higher.

The methods and apparatus set forth in the early automated cycle references have been commercially successful, but commercial use of the methods and apparatus has many disadvantages, including lack of some control over residence time for gas-liquid contact in each reaction zone and poor liquid flow control. U.S. patent No. 5,160,714 provides a method of contacting liquid with different gases sequentially in separate mass transfer zones within a single vessel, whereby the rate of liquid flow from one mass transfer zone to another is promoted by the difference in the density of the aerated liquid in the mass transfer zone and the density of the non-aerated liquid in the previous liquid downcomer. This density difference acts as a "pump" to generate the driving force. In this patent, it is expected that the liquid flow rate will be controlled only by adjusting the gas rate to one or more mass transfer zones; however, it has proven impractical because the rate of gas to each mass transfer zone is generally governed by process requirements rather than liquid flow rates. The amount and composition of the acid gas entering the plant is typically controlled by the upstream process and therefore is not dependent on the operation of the plant. The operation of the plant must be capable of adjusting the inlet acid gas conditions. The amount of air injected into the oxidation zone is dependent on the H contained in the acid gas₂The amount of S, and the minimum liquid circulation rate, must supply enough moles of iron to satisfy equation 3 in the reaction zone. If not controlled, the actual solution circulation rate will be determined by the physical characteristics of the equipment and the aeration density of the reaction and oxidizer zones. If the solution circulation rate is too high, oxygen can be transferred from the oxidation zone to the reaction zone, resulting in the production of undesirable byproducts, such as sulfates. If the solution circulation rate is too low, the iron supplied to the reaction zone will be insufficient to satisfy reaction 3, resulting inIron sulfide is formed and precipitated.

To compensate for this lack of control, various items (e.g., butterfly valves, throttling wedges, and sliding gates) are installed in the liquid piping line that recirculates the used reagent from the absorber to the oxidizer. Unfortunately, all of the proposed solutions to control the flow of liquid through a mass transfer zone have proven impractical due to clogging by solids (i.e., elemental sulfur) entrained in the liquid.

The present invention addresses these needs in the art and in particular provides apparatus and process steps that alleviate all of the problems and difficulties in previous flow control devices. These and other advantages will become more apparent from the following more detailed description of the invention.

Disclosure of Invention

The present invention overcomes the disadvantages of the previously known processes by providing a continuous process for contacting a liquid reagent sequentially with a process gas and a second gas, the continuous process comprising the step of introducing the process gas into a downstream mass transfer zone containing the liquid reagent, wherein the mass transfer zone is in fluid communication with a second downcomer. The process gas preferably contains H₂S and the second gas preferably contains oxygen. Liquid reagent from the downstream mass transfer zone can optionally flow into a surge downcomer where it is removed and recycled to the upstream mass transfer zone in fluid communication with the first downcomer. A second gas is introduced into the upstream mass transfer zone at a first flow rate where it mixes with the liquid reagent before flowing into the first downcomer. A third gas is introduced into the first downcomer, wherein the third gas mixes with the liquid reagent flowing from the upstream mass transfer zone. The third gas may (but need not) be the same gas as the second gas. Liquid reagent from the first downcomer is flowed into an intermediate mass transfer zone in fluid communication with the second downcomer. A second gas may also be introduced into this intermediate mass transfer zone.

Varying the flow rate at which the third gas is introduced into the first downcomer controls the flow of liquid reagent from the upstream mass transfer zone to the intermediate mass transfer zone. This may be due to the fact that the density of the aerated solution varies proportionally with the non-aerated density of the solution and inversely with the velocity of the gas through the solution, i.e., the higher the gas velocity, the lower the aerated density. Thus, the lower the density (or specific gravity) of the liquid in the first downcomer, the slower the flow of liquid from the upstream mass transfer zone to the intermediate mass transfer zone. Adjusting the rate at which the third gas is added to the first downcomer allows the flow of liquid to be increased or decreased for optimum sulfur removal. As previously stated, the ideal solution circulation rate is to supply enough moles of ferric iron to oxidize the sulfide ions and at the same time supply enough moles of oxygen to oxidize the ferrous ions. If the solution circulation rate is too high, then the sulfur ions can be transported to the oxidizer zone, resulting in the formation of by-products, such as thiosulfate and sulfate. If the solution circulation rate is too low, there will not be enough ferric ions to oxidize the sulfide ions. In this case, the iron will be over-reduced, resulting in the formation of iron sulfide which is extremely detrimental to the process.

In other embodiments of the invention, a flow of a second gas may also be introduced into the liquid reagent in the intermediate mass transfer zone, and a flow of a third gas may be introduced into the second downcomer to control the flow of the liquid reagent from the intermediate mass transfer zone to the downstream mass transfer zone. Again, the second gas and the third gas may be the same.

The flow rates of the second gas and the third gas may be controlled and varied using controllers known in the art. The flow rate of the second gas containing oxygen is generally not precisely controlled. The system is typically designed with 2 or 3 "air blowers" and the air flow to the unit is controlled by turning the blowers on or off. This is determined by analyzing the "REDOX" potential of the solution, a routine analytical procedure well known to those skilled in the art. If the redox potential is too low, which means that the iron ions are insufficiently oxidized, the flow rate of the oxygen-containing gas can be increased. If the "redox" potential is too high, this means thatThe iron ions are excessively oxidized, and the flow rate of the oxygen-containing gas should be reduced. Another reason for the low redox potential is the insufficiently oxidized iron (Fe)⁺⁺⁺) Is supplied to the reaction zone to satisfy equation 3. If this happens, the iron content of the solution can be increased; however, this will result in higher operating costs due to higher iron losses from the system associated with solution losses. Another way to meet the iron requirement of the reaction zone is to increase the solution circulation rate by increasing the flow rate of the third gas. This will increase the density difference between the reaction and/or oxidation zones and the corresponding downcomers. The adjustment amount is measured again from the change in the oxidation-reduction potential of the solution. If the solution circulation rate is increased to meet the iron requirement of the reaction zone and the by-product formation is increased by carrying oxygen into the reaction zone, the circulation rate should be reduced by increasing the flow of the third gas into the downcomer in combination with a corresponding increase in the iron content of the solution.

Each of the mass transfer zones or chambers may be divided into two or more separate contacting sections for continuous gas-liquid contacting. The separate sections of each gas-liquid mass transfer zone are in sequential fluid communication with each other through an intermediate downcomer formed by a weir extending downwardly from an upper portion of the downstream end of one section and a submerged weir extending upwardly from the upstream end of a subsequent section, the weirs being horizontally spaced and vertically overlapping within the liquid. According to a preferred embodiment, at least one downcomer containing a gas distributor separates one gas-liquid mass transfer zone from another gas-liquid mass transfer zone to provide a controlled residence time between the two mass transfer zones and a controlled circulation of solution, preferably for contacting different gases in each zone in sequence. In a preferred arrangement, the upstream mass transfer zone is a first zone, the intermediate mass transfer zone is a second zone, and the downstream mass transfer zone is a third zone. However, it is also within the scope of the invention to have more than three mass transfer zones, each separated by one or more downcomers.

In particular, the present invention provides an automatic cycle method and apparatus for: by mass transfer inContinuous removal of hydrogen sulfide (H) from a process gas stream in a zone in intimate contact with a catalytic polyvalent metal redox solution₂S) gas and continuously regenerating the catalytic solution in the same vessel by intimate contact with an oxidizing gas in a separate mass transfer zone. Where H is introduced₂During the liquid redox process in which S is oxidized to sulfur and water, the liquid reagent contacts the second gas causing an oxidation reaction to occur to form elemental sulfur. Specifically, in a preferred embodiment, the process gas comprises hydrogen sulfide gas and the liquid reagent is an oxidation-reduction solution, whereby oxidation of the hydrogen sulfide gas and reduction of the oxidation-reduction solution is effected to form a reduced oxidation-reduction solution and elemental sulfur in the third mass transfer zone. The second gas is capable of oxidizing the reduced solution to form an oxidized solution in the first mass transfer zone and the second mass transfer zone such that the oxidized solution is capable of further absorbing the process gas in the third mass transfer zone.

To carry out the process of the present invention, an apparatus for continuously contacting a liquid reagent with a process gas and a second gas in succession is set forth comprising a first mass transfer zone in fluid communication with a first downcomer and a second downcomer. The third mass transfer zone is in fluid communication with the second downcomer and is downstream of the first mass transfer zone and the second mass transfer zone. A process gas is introduced in the third mass transfer zone. The surge downcomer may be downstream of the third mass transfer zone and a liquid reagent recirculation conduit connecting the surge downcomer with the first mass transfer zone may be used to recirculate used liquid reagent to the mass transfer zone for regeneration. A separate gas distributor may be provided in each mass transfer zone, but must be incorporated into at least one downcomer. Alternatively, gas distributors can be provided in other downcomers to control liquid flow between other mass transfer zones. Any type of mechanical or electrical controller can be used to vary the flow rate of the gas fed to the mass transfer zone and distributor in the downcomer. Likewise, any gas distributor design known in the art may be used to inject gas into the liquid-filled downcomer.

The present invention has been described so far with particular emphasis on the use of iron as the selected polyvalent metal; however, polyvalent metals which form chelates with the ligands set forth above may also be used. Such additional polyvalent metals include copper, cobalt, vanadium, manganese, platinum, tungsten, nickel, mercury, tin, and lead. The chelating agent is typically an aminopolycarboxylic acid family (e.g., EDTA, HEDTA, MGDA, and NTA) or any other chelating agent that may be used in conjunction with the present invention.

These and other embodiments will become more apparent from the detailed description of the preferred embodiments contained below.

Drawings

The above and other aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side view of an embodiment of a gas-liquid mass transfer oxidizer/absorber vessel of the present invention;

FIG. 2 is a top view of another embodiment of the gas-liquid mass transfer vessel of the present invention in a circular design;

FIG. 3 is a graph showing the relationship of aerated density to gas velocity for 4 non-aerated solutions of different specific gravities; and is

FIG. 4 is a graph showing the effective frictional pressure drop versus gas velocity in a downcomer.

Detailed Description

Turning now to the drawings and first to FIG. 1, there is illustrated one embodiment of the present invention showing the insertion of a gas distributor 10 into one or more downcomers 12 connecting two mass transfer zones 13. The distributor 10 is located above the bottom of the inverted weir 11. Gas is then injected into the sparger 10. As the amount of injected gas increases, the gas-filled density of the liquid in the downcomer 12 will approach the gas-filled density of the liquid in the mass transfer zone 13. The distributor 10 can be inserted into a downcomer 12 that separates two or more oxidation mass transfer zones and/or two or more process gas mass transfer zones. The following is an explanation of how the device controls the flow of solution.

As mentioned above, the density of an aerated solution varies proportionally with the non-aerated density of the solution and inversely with the velocity of gas through the solution. In other words, the higher the gas velocity, the lower the charge density and the lower the driving force, and thus the lower the flow rate. This relationship for aqueous solutions containing various amounts of inorganic salts that change the specific gravity of the solution is illustrated in fig. 3.

The pressure balance near point a in fig. 1 yields the following equation:

(ρ_d)(H2)+(ρ_na)(H1-H2)-(F_d)＝(ρ_tz)(H1)-(F_tz)

or

(F_d-F_tz)＝(ρ_d)(H2)+ρ_na(H1-H2)-(ρ_tz)(H1)

Wherein:

ρ_dpounds per cubic foot of solution density in the downcomer

ρ_naPounds per cubic foot of non-aerated solution density

ρ_tzPounds per cubic foot of solution density in the mass transfer zone

H1-the height of solution in the mass transfer zone, feet above the distributor

H2-the height of solution in the downcomer, foot above the distributor

F_dPounds per square foot of frictional pressure drop in the downcomer due to solution flow

F_tzIn the mass transfer region due toFrictional pressure drop caused by solution flow, pounds per square foot

Term (F)_d-F_tz) Is the difference in frictional pressure drop created by the flow of the solution through the downcomer and the mass transfer zone. Which is related to the squared difference of the two solution velocities. For example, assuming a solution velocity of 5 feet per minute in the mass transfer zone, a solution height of 8 feet for H1 and 6 feet for H2, the effect of increasing the gas velocity in the downcomer is illustrated in fig. 4. As the effective frictional pressure drop increases, the solution flow rate through the downcomer required for pressure equalization of the system increases. In contrast, as more gas is injected into the downcomer distributor, the effective frictional pressure drop or driving force is reduced, resulting in less solution flow.

When the apparatus of fig. 1 is used for a sulfur removal process, the absorber chamber 32 is connected in fluid communication to the first mass transfer zone 13 of the oxidizer 34 through the surge downcomer 4 and via conduit 5. The circulation is driven by the difference in liquid density caused by aeration (as set forth above), in particular by controlling the flow of gas by the control valve 6 to one or more distributors 10 located in one or more downcomers 12. Rich in H₂The gas of S is introduced into the bottom of the absorber via line 1 and passes through sparger 9 in absorber chamber 32 to be in intimate contact with liquid ferric chelate catalyst solution 14. H₂S rises from the third or last stage of the oxidizer zone to an absorption zone along with the oxidized catalyst solution and H is absorbed from the process gas in the absorption zone in the oxidized catalyst solution₂S and converts it to sulfur, the sulfur-laden liquid catalyst solution flows over the baffle 50 separating the absorber chamber 32 from the surge downcomer 4 and through conduit 5 to the first oxidizer stage 34. In a preferred embodiment, an inclined plate (not shown) forms the floor of surge downcomer 4 to direct any settled sulfur to an outlet (not shown). The sulfur is finally removed from the system by filtration.

The first oxidizer stage is loaded with H₂The liquid catalyst of S is delivered through line 2 and pump 3 and then flowsThe oxygen-containing gas passing through the sparger 15 is oxidized and flows over the baffle 8 and under the baffle 11 to the intermediate oxidizer. The partially oxidized solution in the intermediate oxidizer is further oxidized by the oxygen-containing gas flowing through the sparger and finally flows to the absorber 32. Spent oxidizing gas is discharged from the top of each oxidizer mass transfer zone through an outlet conduit (not shown). All of the vented gases may be treated before being released into the atmosphere.

The circular design of the process and apparatus shown in fig. 2, generally designated 30, includes an absorber chamber 32 and an oxidizer chamber divided into three sections 34, 36 and 38 separated by gas. Horizontally and vertically spaced baffles or weirs 40 and 42 disposed between the absorber chamber 32 and the first oxidation section 34 define a downcomer 44 therebetween for containing the H2S-loaded polyvalent metal chelate solution prior to oxidation of the chelate solution. Similar downcomers are shown as 45, 47 and 48, each defined by horizontally and vertically spaced baffles such that 46 and 48 are disposed between the first and second oxidizer sections 34 and 36; horizontally and vertically spaced baffles 41 and 43 are disposed between the second and third oxidizer sections 36 and 38; and horizontally and vertically spaced baffles 49 and 50 are disposed between the third oxidizer section 38 and the absorber chamber 32. The spaced baffles or weirs (weirs) are configured as shown in fig. 1 so that liquid from the absorber flows above baffle 40 and below baffle 42, and liquid from each successive oxidizer section flows above its adjacent baffles 46, 41 and 49 and below the baffle 48, 43 or 50 adjacent the next successive zone. Although not shown in fig. 2, gas distributors are disposed in the oxidizer mass transfer zone, in the absorber mass transfer zone, and in one or more downcomers. Preferably, the cylindrical absorber/oxidizer vessel shown in fig. 2 has a planar, horizontally disposed floor.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation.

Claims (15)

1. A method for continuous automated circulation of a liquid reagent in sequential contact with a process gas and a second gas comprising the steps of:

(a) introducing a process gas into a downstream mass transfer zone containing a liquid reagent, wherein the downstream mass transfer zone is in fluid communication with a second downcomer and a downstream downcomer;

(b) removing liquid reagent from the downstream downcomer and recycling the removed liquid reagent to an upstream mass transfer zone in fluid communication with the first downcomer, wherein a first flow rate of a second gas is introduced into and mixed with the liquid reagent in the upstream mass transfer zone;

(c) introducing a third gas into the first downcomer at a second flow rate where the third gas mixes with the liquid reagent flowing from the upstream mass transfer zone; and

(d) flowing the liquid reagent from the first downcomer into an intermediate mass transfer zone in fluid communication with the second downcomer,

wherein the flow of the liquid reagent from the upstream mass transfer zone to the intermediate mass transfer zone is controlled by varying the second flow rate of the third gas introduced into the first downcomer.

2. The method of claim 1, wherein the second gas and the third gas are the same gas.

3. The method of claim 1, wherein the second gas is introduced into the liquid reagent in the intermediate mass transfer zone using a third flow rate.

4. The method of claim 1, wherein the third gas is introduced into the liquid reagent in the second downcomer using a fourth flow rate.

5. The method of claim 4, wherein flow of the liquid reagent from the intermediate mass transfer zone to the downstream mass transfer zone is controlled by varying the fourth flow rate of the third gas.

6. The method of claim 5, wherein the second gas and the third gas are the same gas.

7. The method of claim 1, wherein the first flow rate is different from the second flow rate.

8. The method of claim 6, wherein the third flow rate is different from the fourth flow rate.

9. The method of claim 3, wherein the first flow rate and third flow rate of the second gas are different.

10. The method of claim 1, wherein contacting the liquid reagent with the second gas causes an oxidation reaction to occur.

11. The method of claim 1, wherein contacting the liquid reagent with the second gas causes an oxidation reaction to occur to form elemental sulfur.

12. The method of claim 1, wherein the process gas comprises hydrogen sulfide gas and the liquid reagent is an oxidation-reduction solution, thereby effecting oxidation of the hydrogen sulfide gas and reduction of the oxidation-reduction solution and forming a reduced oxidation-reduction solution and elemental sulfur in the third mass transfer zone; and wherein the second gas is capable of oxidizing the reduced solution to form an oxidized solution in the first mass transfer zone and the second mass transfer zone such that the oxidized solution is capable of further absorbing the process gas in the third mass transfer zone.

13. An apparatus for a continuous automated cycle method for sequentially contacting a liquid reagent with a process gas and a second gas comprising, in combination:

(a) a first mass transfer zone in fluid communication with the first downcomer;

(b) a second mass transfer zone in fluid communication with the first downcomer and the second downcomer;

(c) a third mass transfer zone in fluid communication with the second downcomer; and

(d) a respective gas distributor located in each mass transfer zone and in at least one of the first downcomer or the second downcomer,

wherein the gas distributor located in the at least one of the first downcomer or the second downcomer is configured to introduce gas into the at least one of the first downcomer or the second downcomer to control flow of liquid reagent between the first mass transfer zone and the second mass transfer zone or between the second mass transfer zone and the third mass transfer zone.

14. The apparatus of claim 13, further comprising a controller operably connected to the gas distributor located at least in the first downcomer or the second downcomer.

15. The apparatus of claim 13, further comprising a liquid agent recirculation conduit connecting a surge downcomer in fluid communication with the third mass transfer zone to the first mass transfer zone.