WO2008100317A1

WO2008100317A1 - Scrubber system for the desulfurization of gaseous streams

Info

Publication number: WO2008100317A1
Application number: PCT/US2007/062339
Authority: WO
Inventors: Steven F. Meyer
Original assignee: MECS Inc
Current assignee: MECS Inc
Priority date: 2007-02-16
Filing date: 2007-02-16
Publication date: 2008-08-21
Anticipated expiration: 2009-08-16

Abstract

A wet scrubbing process for treating sulfur-containing gaseous industrial process streams for the selective removal of sulfur-containing species, and particularly the simultaneous removal of sulfur dioxide (SO2) and a sulfur specie selected from sulfur trioxide (SO3), H2SO4 and mixtures thereof, is disclosed. The process is useful in removing sulfuric acid mists and/or reducing the opacity exhibited by the treated gas stream.

Description

SCRUBBER SYSTEM FOR THE DESULFURIZATION OF GASEOUS STREAMS

FIELD OF THE INVENTION

[0001] The present invention relates generally to treatment of gaseous industrial process streams for the selective removal of sulfur-containing species, and particularly the removal of sulfur dioxide (SO₂) , sulfur trioxide (SO3) , H₂SO₄ and mixtures thereof. The disclosed invention is useful in removing sulfuric acid mists and/or reducing the opacity exhibited by the treated gas stream.

BACKGROUND OF THE INVENTION

[0002] Various industrial processes produce gaseous streams containing sulfur oxides and other sulfur compounds.

Such processes include, but are not limited to, for example, fossil fuel-fired power plants, natural gas treatment plants, refineries (e.g., fluid catalytic cracking (FCC) units) , sulfur recovery units (SRUs) , sulfuric acid plants, metal roasting operations, cement kilns and synthesis gas plants. Before sulfur-containing gas streams can be vented to the atmosphere, they must often be treated to remove the sulfur-containing compounds including, for example, SO₂, SO₃, H₂S and H₂SO₄.

[0003] A wet scrubber system is one type of flue gas desulfurization process employed to remove sulfur-containing compounds from such process gas streams. When using a wet gas scrubber, the sulfur compounds present are typically first oxidized to SO₂ and the SO₂ then selectively removed by contacting the gas with an aqueous solution containing an alkaline or basic reagent circulating in a gas-liquid contacting device. Typical reagents include, for example, alkali metal and alkaline earth metal hydroxides, carbonates, bicarbonates, lime (CaO) , hydrated or slaked lime (Ca(OH)₂) limestone (CaCOs), and waste streams containing these compounds including, for example, cement kiln dust and soda ash. The SO₂ in the gas is absorbed or dissolved into the scrubbing solution where it reacts with the reagent. The resulting liquid solution containing the reaction products can be purged from the system for disposal or regenerated for recycle. Wet gas scrubbing systems can operate over a wide range of feed conditions, as is often found in refineries and gas plants.

[0004] Caustic (NaOH) is an example of a reagent commonly used to remove SO₂ in wet scrubbing operations. Depending on the pH of the solution, caustic removes SO₂ according to the following reactions that involve the formation of sodium sulfite (Na₂SC>3) and/or bisulfite (NaHSO₃) :

SO₂ (g) ~ SO₂ (aq) (1)

SO₂ (aq) + H₂O ~ H₂SO₃ (aq) (2)

2 NaOH (aq) + H₂SO₃ (aq) ~ Na₂SO₃ (aq) + 2H₂O (1) (3)

Na₂SO₃ (aq) + H₂SO₃ (aq) ~ 2NaHSO₃ (aq) (4)

The formation of sodium bisulfite in accordance with equation (4) is dependent upon the temperature and total dissolved solids concentration in the scrubbing solution and occurs primarily when the pH of the scrubbing solution is below about 7. The sodium sulfite and bisulfite salts formed in the above reactions are typically further oxidized to sodium sulfate in order to reduce the chemical oxygen demand of the liquid scrubber stream discharged from the system to an acceptable level. Other scrubbing reagents exhibit a similar chemistry.

[0005] Most gas streams containing SO₂ also contain SO₃. Gaseous streams containing SO₃ can create difficulties in the design and operation of a wet scrubber system. SO₃ may be present in the process stream from the base process from which it originates or may be produced during subsequent incineration of the process stream to convert sulfur compounds to SO₂ upstream of the wet scrubber operation. SO3 contacts and reacts with water in the scrubber to form sulfuric acid that can condense and form a sulfuric acid mist. Sulfuric acid mist formed in this fashion exhibits a very small, submicron droplet size, that renders it difficult to remove from the gas stream. Most wet scrubbers cannot effectively remove the sulfuric acid mist droplets because, as a result of the very small droplet size, the chance of contact between the mist droplets and the scrubbing solution is diminished and the droplets tend to pass through the scrubber unabated. If the gaseous stream to be treated contains significant concentrations of SO₃, the treated gas stream vented to the atmosphere from the wet scrubber may become opaque, forming a distinctive visible blue plume.

[0006] To reduce plume opacity problems and comply with environmental regulations, wet scrubbers are often equipped with mist eliminators or similar abatement apparatus in the upper portion of the gas-liquid disengagement vessel. Mist eliminators collect, coalesce and drain mist droplets entrained in the gas stream exiting the scrubber before it is vented to the atmosphere. However, mist eliminators add to the cost of a wet scrubber system through increased initial capital cost and increased energy consumption due to the pressure drop associated with the mist eliminator.

[0007] A need persists for wet scrubber systems and processes for treating gaseous streams that are capable of efficiently removing SO₂, SO3, H₂SO₄ and mixtures thereof and that effectively remove sulfuric acid mists and reduce the opacity exhibited by the treated gas stream. SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of the present invention to provide a process for the effective removal of a sulfur specie selected from sulfur trioxide (SO₃) , H₂SO₄ and mixtures thereof from a gas stream. It is a further object of the present invention to provide a process for the simultaneous removal of sulfur dioxide (SO₂) and another sulfur specie selected from sulfur trioxide (SO₃) , H₂SO₄ and mixtures thereof from a gas stream. It is a further object of the present invention to provide a process useful in removing sulfuric acid mists and/or reducing the opacity exhibited by the treated gas stream. It is a still further object of the present invention to provide a process for the removal of one or more sulfur species from a gas stream in a wet scrubber system capable of utilizing a variety of basic reagents and one or more gas-liquid contacting devices or stages .

[0009] Briefly, therefore, the present invention is directed to a process for removing at least one sulfur specie selected from the group consisting of SO₃, H₂SO₄ and mixtures thereof from a gas stream. The process comprises contacting the gas stream in at least one scrubbing zone of a gas-liquid contacting device with a scrubbing liquid comprising an aqueous solution containing a basic reagent and having a pH of at least about 8.5. As a result of such contacting, SO₃ and/or H₂SO₄ is absorbed and reacted with the basic reagent in the liquid phase to produce a treated gas stream depleted in the sulfur specie (s) and a spent scrubbing solution comprising the liquid phase reaction product. Additional basic reagent is added to the spent scrubbing solution to regenerate the scrubbing liquid having a pH of at least about 8.5 and the regenerated scrubbing liquid is reintroduced into the scrubbing zone into contact with the gas stream. In accordance with some embodiments, the gas stream to be treated comprises SO₂, and the process further comprises simultaneous removal of SO₂ from the gas stream along with S03 and/or H₂SO₄. In such an embodiment, the SO₂ is also absorbed and reacted with the basic reagent in the liquid phase such that the treated gas stream produced is likewise depleted in SO2 and the spent scrubbing solution comprises the liquid phase reaction product of the basic reagent and SO₂.

[0010] Other objects and features will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Fig. 1 shows a plot of the theoretical Na⁺ ion concentration versus pH in a representative aqueous scrubbing solution containing NaOH as the basic reagent as determined at a temperature of about 71°C using electrolyte simulation software.

[0012] Fig. 2 is a schematic diagram of a wet scrubber system for removing sulfur-containing species and particulate impurities from a gaseous stream in accordance with a preferred embodiment of the present invention utilizing a reverse jet scrubber as a gas-liquid contacting device .

DETAILED DESCRIPTION OF THE INVENTION

[0013] In accordance with the present invention, a wet scrubber system has been discovered that provides for effective, removal of at least one sulfur specie selected from the group consisting of SO3, H₂SO₄ and mixtures thereof from a gaseous stream. SO₃ and/or H₂SO₄ present in the gaseous stream to be treated, and known to sometimes plague wet scrubber systems with opacity problems in the treated gas, are effectively removed by proper design and operation of the system. More particularly, the removal of SO₃ and/or H₂SO₄ from gaseous streams in a wet scrubber system employing a scrubbing liquid or medium comprising an aqueous solution of a basic reagent is enhanced by maintaining an elevated pH in the scrubbing liquid contacted with the gaseous stream. Removal of SO₃ and/or H₂SO₄ from process gas streams is further enhanced by design and operation of the scrubbing zone of the gas-liquid contacting device at increased liquid to gas (L/G) volumetric ratios. In some embodiments, the wet scrubber system of the present invention can be used to simultaneously remove SO₂ and at least one other sulfur specie selected from the group consisting of SO₃, H₂SO₄ and mixtures thereof from a gaseous stream.

[0014] Non-limiting examples of the types of sulfur- containing process gas streams that may be treated in the practice of the present invention include those issuing from fossil fuel-fired power plants or other flue gases generated in the combustion of sulfurous fuels, natural gas treatment plants, fluid catalytic cracking (FCC) units, cokers, calciners, incinerators, sulfur recovery units (SRUs) , the sulfur trioxide absorber of a contact sulfuric acid plant, metal roasting operations, synthesis gas plants, cement kilns, and the incinerator of a Claus plant. The gaseous streams may contain sulfur oxides (SO₂ and SO₃) hydrogen sulfide (H₂S) , carbon disulfide (CS₂) , dimethyl sulfide, carbonyl sulfide as well as other sulfur-containing compounds generated in the base process from which it originates. In addition to sulfur oxides and other sulfur compounds, the gaseous stream to be treated may also contain carbon dioxide, carbon monoxide, water vapor, oxygen, nitrogen and other compounds.

[0015] In some applications, depending upon the source and composition of the stream to be treated, the sulfur content may first be oxidized to convert the sulfur compounds present to sulfur oxides, primarily to SO₂. For example, the gas stream may be fed to a forced draft thermal incinerator or similar apparatus along with sufficient combustion oxygen (e.g., air) and a fuel source for the burners and maintained at a sufficiently high temperature for a given residence time to oxidize the sulfur compounds present. Although the mechanism for SO₂ to SO3 conversion within a thermal incinerator are not fully understood, at least a portion of the sulfur contained in the gas stream will typically be converted to SO3 in the incinerator such that the oxidized gas stream contains SO₃ in addition to SO₂. The gas stream may also be conditioned prior to being introduced into the wet scrubber system, for example, to cool the gas and maintain the desired operating temperature in the scrubber system and/or remove entrained particulate impurities from the gas. Conditioning can include, for example, indirect heat exchange, quenching through contact with a countercurrent flow of water (e.g., by passing the gaseous stream through a reverse jet scrubber of the type sold by MECS, Inc. (Chesterfield, Missouri U.S.A. 63017) under the trademark DYNAWAVE) and removal of particulate impurities from the gas stream in an electrostatic precipitator.

[0016] The composition and temperature of the gaseous stream contacted with the aqueous scrubbing liquid will vary depending upon its origin and any pre-treatment or conditioning measures employed prior to its introduction into the gas-liquid contacting device of the wet scrubber system. Generally, the temperature of the gas introduced into the gas-liquid contacting device will be less than about 1,400⁰C, typically from about 100⁰C to about 320⁰C. In many applications, the gaseous stream introduced into the gas-liquid contacting device will typically comprise, on a dry basis, at least about 500 ppmv SO₂ and up to about 100,000 ppmv SO₂ and at least one other sulfur specie selected from SO3, H₂SO₄ and mixtures thereof along with various other components such as nitrogen, oxygen, carbon dioxide, carbon monoxide and/or other sulfur compounds. However, the present invention is also useful in the removal SO3 and/or H₂SO₄ from gas streams independent of the SO₂ content of the gas and does not require the simultaneous removal of SO₂ from the gas stream. For example and without limitation, SO2 may be preferentially removed from the gas stream prior to application of the process of this invention such that the gas to be treated comprises a sulfur specie selected from SO₃, H₂SO₄ and mixtures thereof without significant SO₂ content. Regardless of the SO₂ content of the gas stream treated in the wet scrubber system of the present invention, in a particular application, the gas stream contacted with the aqueous scrubbing liquid contains SO₃ and/or H₂SO₄ at concentrations that might otherwise result in the formation of appreciable quantities of sulfuric acid mist and opacity problems in the treated gas stream exiting the wet scrubber system. For example, the gaseous stream introduced into the gas- liquid contacting device will typically comprise, on a dry basis, a concentration of SO₃ and/or H₂SO₄ (typically reported as H₂SO₄) of at least about 5 ppmv and up to about 1,000 ppmv, and more typically, from about 10 ppmv to about 200 ppmv, or from about 10 ppmv to about 50 ppmv.

[0017] In accordance with the process of the present invention, at least one sulfur specie selected from the group consisting of SO₃, H₂SO₄ and mixtures thereof are selectively removed from the gaseous stream. The gaseous stream is contacted with a scrubbing liquid in a scrubbing zone of a suitable gas-liquid contacting device of the wet scrubber system. The scrubbing liquid comprises an aqueous solution containing a basic reagent. Contact with the aqueous scrubbing liquid absorbs the SO₃ and/or H₂SO₄ along with any SO₂ present in the gaseous stream into the scrubbing liquid where these components then react with the basic reagent in the liquid phase to produce a treated gas stream depleted in SO₃ and/or H₂SO₄ and optionally SO₂ as well if present in the gas stream to be treated, and a spent scrubbing solution comprising the liquid phase reaction products. Other components of the gas stream largely pass through the scrubbing zone and are retained in the treated gas stream. Additional basic reagent is added to the spent scrubbing solution to regenerate the scrubbing liquid and the regenerated scrubbing liquid is reintroduced into the scrubbing zone into contact with the gas stream.

[0018] Suitable basic reagents for use in the practice of the present invention include, for example, but are not limited to, sodium hydroxide, sodium carbonate, sodium bicarbonate, calcium oxide or lime (CaO) , calcium hydroxide or hydrated lime (Ca(OH)₂), calcium carbonate or limestone (CaCO₃) , calcium bicarbonate, potassium oxide, potassium hydroxide, potassium carbonate, potassium bicarbonate, magnesium oxide, magnesium hydroxide, magnesium carbonate, magnesium bicarbonate, zinc oxide, zinc hydroxide, zinc carbonate, zinc bicarbonate, ammonia, and ammonium hydroxide. The basic reagent can also be suitably derived from materials containing these reagents, such as cement kiln dust and soda ash. The selection of the basic reagent to employ in the aqueous scrubbing zone is based upon a number of factors including, for example, initial capital cost associated with the wet scrubber system, operating cost, cost of the reagent and the composition of the gaseous stream to be treated. In accordance with a preferred embodiment, the basic reagent used in the process of the present invention comprises sodium hydroxide or caustic (NaOH) or lime (CaO) . Most preferably, the basic reagent comprises NaOH. The use of NaOH as the basic reagent in the wet scrubbing operation provides relatively fast reaction times with the absorbed sulfur species and, subsequently, relatively high removal efficiencies. Also, use of NaOH allows for simpler scrubber design and operation because the sulfur-binding reactants are readily soluble in the aqueous scrubbing liquid.

[0019] As noted above, in accordance with the present invention, the removal of SO₃ and/or H₂SO₄ from the gaseous stream contacted with the aqueous scrubbing liquid comprising the basic reagent is enhanced by maintaining an elevated pH in the scrubbing liquid contacted with the gaseous stream. By maintaining a sufficiently high pH in the scrubbing liquid contacting the gaseous stream in the scrubbing zone of the gas-liquid contacting device, significant concentrations of SO₃ and/or H₂SO₄ can be readily removed simultaneously with any SO₂ present to produce a treated gas depleted in SO₂ and SO₃ and/or H₂SO₄ with reduced formation of sulfuric acid mist such that opacity problems in the treated gas can be minimized or substantially eliminated under a wide range of operating conditions. [0020] The pH of the aqueous scrubbing liquid is controlled by the addition of basic reagent to the spent scrubbing solution used to regenerate the scrubbing liquid prior to recycling the regenerated liquid to the scrubbing zone of the gas-liquid contacting device. In accordance with a preferred embodiment, the pH of the aqueous scrubbing liquid contacted with the gaseous stream in the scrubbing zone should be maximized in order to effectively remove SO₃ and/or H₂SO₄ from the gaseous stream, while the pH of the spent scrubbing solution collected in the scrubber sump should be minimized in order to avoid excessive consumption of reagent. The removal of SO₃ and/or H₂SO₄ is enhanced at relatively high pH . Therefore, in accordance with the present invention the scrubbing liquid contacted with the gas stream in the scrubbing zone of the gas-liquid contacting device generally has a pH of at least about 8.5, preferably at least about 9, and even more preferably at least about 9.5. In order to provide satisfactory removal of SO₃ and/or H₂SO₄, the pH of the aqueous scrubbing liquid contacted with the gas stream in the scrubbing zone of the gas-liquid contacting device should generally be increased as the concentration of SO₃ and/or H₂SO₄ as well as any SO₂ in the gas to be treated increases. In accordance with some preferred embodiments of the present invention, the scrubbing liquid contacted with the gas stream in the scrubbing zone of the gas-liquid contacting device has a pH of at least about 9.75, at least about 10, at least about 10.25, at least about 10.5, at least about 10.75, more preferably at least about 11, at least about 11.25, at least about 11.5, at least about 11.75, even more preferably at least about 12 or higher when the incoming gas stream to be treated contains appreciable quantities of these sulfur species and/or when a higher removal efficiency is required.

[0021] However, in order to avoid excessive consumption of the basic reagent while still attaining satisfactory SO₃ and/or H₂SO₄ removal efficiencies, the pH of the spent aqueous scrubbing liquid collected from the scrubbing zone of the gas-liquid contacting device is generally maintained from about 6 to about 12, preferably less than about 10, more preferably less than about 9, and even more preferably less than about 8.5. The pH differential between the regenerated scrubbing liquid contacting the gas to be treated in the scrubbing zone of the gas-liquid contacting device and the collected scrubbing liquid from the scrubbing zone is dependent upon a variety of factors including the composition of the incoming gas and the concentration of SO₂, SO₃ and/or H₂SO₄.

[0022] The effect of maintaining a relative high pH in the scrubbing liquid in enhancing the removal of SO₃ and/or H₂SO₄ from the gas contacted with the scrubbing liquid is not presently well understood. Without being bound to any particular theory, it is believed that the concentration of the basic reagent in the scrubbing liquid may increase significantly at relatively high pH . For example, Fig. 1 shows a plot of the theoretical Na⁺ ion concentration versus pH in a representative aqueous scrubbing solution containing NaOH as the basic reagent as determined at a temperature of about 71°C using electrolyte simulation software. As shown in Fig. 1, the concentration of sodium ion (Na⁺) in the aqueous scrubbing solution increases sharply at a pH of at least about 9. Thus, at relatively high pH, absorbed SO3 gas and/or H₂SO₄ in the liquid phase is much more likely to react with the basic reagent and become bound in a more soluble reaction product (e.g., sodium sulfate (Na₂SO₄) ) during a given residence time within the scrubbing zone and allow more SO3 gas and/or H₂SO₄ to be absorbed in the liquid phase. Alternatively or additionally, and without being bound to any particular theory, it is also believed that at relatively high pH, the viscosity and/or surface tension characteristics of the scrubbing liquid are more favorable for rapid absorption of SO₃ gas and/or H₂SO₄ into the scrubbing liquid.

[0023] The flow of the gas stream to be treated and the aqueous scrubbing liquid through the scrubbing zone of the gas-liquid contacting device may be co-current or countercurrent . However, in order to maximize transfer of components from the gaseous stream to the aqueous scrubbing liquid, the stream is preferably initially contacted countercurrently with the scrubbing liquid in the gas-liquid contacting device. Various gas-liquid contacting devices may be used in the wet scrubber system including, for example, towers containing means for promoting mass transfer between the gas and liquid phases (e.g., a bed of random packing such as saddles or rings, structured packing or trays) . In such an embodiment, the incoming gas stream is preferably introduced through an inlet near the bottom of the tower and the aqueous scrubbing liquid is introduced through a liquid inlet near the top of the tower and distributed over a bed of packing or other means for promoting mass transfer. The spent scrubbing solution comprising the liquid phase reaction products collects in the bottom or sump of the tower and the treated gas stream is removed from an outlet near the top of the tower.

[0024] The amount of scrubbing liquid circulating through the gas-liquid contacting device versus the flowrate of the incoming gaseous stream is referred to as the liquid to gas volumetric ratio, or L/G ratio, and is typically expressed in gallons per minute (gpm) divided by the gas flow through the device in actual cubic feet per minute (acfm) , divided by 1000. The L/G ratio is a key parameter for the scrubbing operation in accordance with the present invention and is preferably sufficiently high to fully quench the incoming gaseous stream (if necessary) and to absorb the SO₂ and other components of the stream gas without unduly suppressing the pH of the aqueous scrubbing solution. In general, higher L/G ratios will provide higher SO₂ removal efficiencies. In addition, it is believed that the removal of SO3 and/or H₂SO₄ from the gaseous stream and the removal of sulfuric acid mist may be further enhanced by operating the gas-liquid contacting device at high L/G ratios. Accordingly, the gas-liquid contacting device in which the gaseous stream contacts the aqueous scrubbing liquid is preferably capable of operating at high L/G ratios and, in a particularly preferred embodiment, comprises a reverse jet scrubber of the type disclosed in U.S. Patent No. 3,803,805 and sold under the trademark DYNAWAVE by MECS, Inc. (Chesterfield, Missouri U.S.A. 63017). Reverse jet scrubbers are particularly suited for effective separation and removal of particulate and gaseous components from gas streams .

[0025] For the purposes of illustration and better understanding of the present invention, and in accordance with certain preferred embodiments, the present invention will be described with reference to the wet scrubber system shown schematically in Fig. 2 comprising a reverse jet scrubber as a gas-liquid contacting device and utilizing NaOH as the basic reagent in the aqueous scrubbing liquid. [0026] The wet scrubber system of Fig. 2 comprises a gas-liquid disengagement vessel 9 with a sump 1 in which the spent aqueous scrubbing solution is collected, a vertical inlet gas duct 2 connected with the vessel through which the gaseous stream is introduced and an outlet gas duct 3 running out of the vessel through which the treated gas stream is discharged. At least one reverse jet nozzle 4 is disposed and arranged within the inlet gas duct 2 for directing a flow of the aqueous scrubbing liquid countercurrent to the direction of the incoming gas flowing vertically downward into the vessel. A pump 5 transfers scrubbing liquid from the sump 1 to the nozzle (s) 4. Preferably, the wet scrubber system is constructed of materials that are resistant to corrosion caused by continual contact with acidic gases and the basic reagent. If used to treat the gaseous effluent from a Claus sulfur recovery unit, it is preferred that the inlet gas duct 2 be made from a high alloy material, such as for example, AL6XN or DUPLEX 2205. When NaOH is used as the basic reagent and if the caustic is membrane grade, the gas-liquid disengagement vessel 9 can be suitably constructed from 316 stainless steel because of the low level of chloride. However use of caustic with higher concentrations of chlorides (e.g., diaphragm grade caustic) may require the use of more corrosion resistant materials such as DUPLEX 2205 or fiberglass reinforced plastic (FRP) in the construction of the vessel. Preferably, the reverse jet nozzles 4 are non-atomizing and can operate at relatively low liquid pressures and are manufactured from abrasion resistant material.

[0027] As shown in Fig. 2, in one preferred embodiment, the wet scrubber system comprises two reverse jet nozzles 4 in series in the inlet gas duct 2. The scrubbing zone(s) or contacting stages in which the countercurrent flows of gas and scrubbing liquid initially collide comprise a "froth zone" 7 and 8, respectively. The froth zone(s) generated by reverse jet scrubbers are characterized by intense mixing and mass transfer from the gas to the liquid phase and operate to simultaneously quench the incoming gas, absorb SO₂ and SO3 and/or H₂SO₄ in the scrubbing liquid and remove entrained particulates . Although the initial contact between the gaseous stream and the scrubbing liquid that generates the froth zone is countercurrent, the flow of gas and liquid exiting the froth zone is co-current, vertically downward and generally along the walls of inlet gas duct 2. In the first froth zone 7, the incoming gaseous stream is initially contacted with the scrubbing liquid and provides rapid quenching of the gas stream and an initial absorption of components of the gas stream into the scrubbing liquid. The gas typically exits the first froth zone 7 at its adiabatic saturation temperature (generally less than about 100⁰C) and passes further down inlet gas duct 2 where it again contacts a countercurrent flow of the scrubbing liquid in the second froth zone 8 wherein further absorption of components of the gas stream occurs.

[0028] The wet scrubber system of the present invention illustrated in Fig. 2 is typically operated at an overall liquid to gas ratio (L/G) in the range of from about 35 to about 600 gpm of liquid per 1000 acfm of gas. Each reverse jet scrubbing zone or contacting stage of the scrubber system is preferably operated at an L/G ratio in the range of from about 40 to about 200 gpm of liquid per 1000 acfm of gas, more preferably from about 60 to about 200 gpm of liquid per 1000 acfm of gas, even more preferably from about 80 to about 200 gpm of liquid per 1000 acfm of gas.

[0029] Gas exiting the second froth zone 8 is passed into gas-liquid disengagement vessel 9 where it reverses vertical direction resulting in separation of the spent scrubbing solution from the treated gas stream depleted in SO3 and/or H₂SO₄ and optionally SO₂. The spent aqueous scrubbing solution collects in sump 1 and comprises excess reagent, reaction products [sodium sulfite (Na₂SOs), sodium sulfate (Na₂SO₄) and/or sodium bisulfite (NaHSOs) ] and solid particulates. The treated gas stream optionally passes through a gas/liquid separation device 10 (e.g., a vertical flow chevron demister) to remove larger entrained liquid droplets before it enters the outlet gas duct 3 and is discharged from the vessel. The outlet gas duct 3 can be connected to further gas processing equipment or vented to the atmosphere. In most applications, it is unnecessary to provide for downstream treatment to remove finer liquid mist particles (e.g., conventional fiber bed mist eliminators installed within gas-liquid disengagement vessel 9) since the wet scrubbing process in accordance with the present invention is capable of adequately removing sulfuric acid mists and/or sufficiently reducing the opacity exhibited by the treated gas stream. However, the treated gas stream may optionally be subjected to additional mist elimination treatment without departing from the scope of the present invention.

[0030] When the gas to be treated contains high levels of SO₂ (e.g., at least about 5,000 ppmv) , as shown in Fig. 2, gas-liquid disengagement vessel 9 of the wet scrubber system may optionally include a further gas-liquid contacting stage 14 comprising a bed of packing material (e.g., 5 cm nominal diameter ceramic saddles or other types of mass transfer packing) , a froth column or similar means for promoting mass transfer between the gas and liquid phases. In such embodiments, the scrubber system is further equipped with a distributor 15 to supply scrubbing liquid to the further gas-liquid contacting stage 14. Treated gas flows upwardly through contacting stage 14 countercurrent to the flow of descending scrubbing liquid, wherein additional SO2 and SO3 and/or H₂SO₄ are absorbed and reacted in the liquid phase.

[0031] As the gas is treated, the pH of the spent scrubbing solution collected in sump 1 decreases as NaOH is consumed in the reaction with absorbed SO₂ and SO3 and/or H₂SO₄. In order to maintain the pH of the scrubbing liquid contacting the incoming gas in the scrubbing zone(s) of the reverse jet scrubber (s), and optionally any further contacting stage, additional NaOH is added to the spent scrubbing solution to regenerate the scrubbing liquid to the desired pH before it is reintroduced into the scrubbing zone(s). Addition of basic reagent to the circulating scrubbing liquid and pH control can be achieved in various ways. For example, as shown in Fig. 2, NaOH may be metered into the wet scrubber system through reagent feed line 6 into the circulation line connecting the sump with reverse jet nozzles 4 in response to the measured pH of the circulating liquid downstream of the reagent feed line and upstream of the nozzles. In such an embodiment, the pH of the spent scrubbing solution in the sump is usually lower than the pH of the regenerated scrubbing liquid reintroduced into the scrubbing zone(s), the magnitude of the pH differential being dependent upon the composition of the incoming gas, the L/G ratio in the reverse jet scrubber and other operating parameters . Alternatively, pH control can be attained by measuring the pH at various other locations in the wet scrubber system, such as the pH of the spent scrubbing solution collected in the sump, and additional NaOH can be introduced directly into the sump.

[0032] Contact between the circulating scrubbing liquid and the gas causes evaporation of a portion of the water from the aqueous solution of the basic reagent and is vented as part of the treated gas stream. Typically, the amount of liquid lost exceeds the amount supplied through reagent feed line 6. Thus, the system may be supplied with make-up water as needed through make-up water feed line 13.

The amount of make-up water can be controlled to respond to the liquid level in sump 1.

[0033] Oxygen, hydrogen peroxide or other suitable oxidizing agent may be introduced into sump 1 through an oxidant inlet 12 to further oxidize the reaction products in the spent scrubbing solution resulting from the reaction between the reagent and absorbed SO₂ and SO3 and/or H₂SO₄ (e.g., Na₂SOs) . Oxidation treatment may be used to stabilize the reaction products and reduce the chemical oxygen demand of the spent scrubbing solution to a level acceptable by wastewater treatment facilities. The reaction products (preferably in an oxidized state) and particulate contaminate captured in the spent solution may be removed from the wet scrubber system through purge stream 11. The amount of liquid purged can be controlled in response to changes in particulate solids content. For example, it is preferred that liquid be purged when the spent scrubbing solution in the sump reaches a specific gravity of from about 1.07 to about 1.20.

[0034] The wet scrubber system operated in accordance with the present invention provides for effective, simultaneous removal of SO₂ and SO3 and/or H₂SO₄ from a gaseous stream. The proportion of SO₂ and SO₃ and/or H₂SO₄ removed from the gaseous stream and the concentrations of these components in the treated gas stream will depend upon the composition of the incoming gas and the design parameters established for the wet scrubber system. Typically, at least about 90% and up to about 99% or greater of SO₂ and at least about 50% of SO₃ and/or H₂SO₄ contained in the gaseous effluent may be removed. Preferably at least about 55%, more preferably at least about 60%, more preferably at least about 65%, more preferably at least about 70%, more preferably at least about 75%, more preferably at least about 80%, more preferably at least about 85%, more preferably at least about 85%, more preferably at least about 90%, more preferably at least about 95% and most preferably from about 95% to about 99% of SO₃ and/or H₂SO₄ contained in the gaseous effluent is removed. The concentration of H₂SO₄ in the treated gas stream may be reduced to an extent such that the treated gas stream does not exhibit a visible plume. More particularly, the concentration of H₂SO₄ in the treated gas stream may be reduced such that the opacity of the treated gas stream is less than about 20%, more preferably, less than about 10% (as determined using U.S. EPA Method 9) .

[0035] Those skilled in the art will appreciate the applicability of the present invention to other wet scrubber designs including multi-stage (e.g., two-stage) scrubber systems such as those shown and disclosed in U.S. Patent Nos. 4,834,959 and 4,201,755 the entire contents of which are incorporated herein by reference. More particularly, the removal of SO₃ and/or H₂SO₄ from a gas stream in accordance with the present invention by contacting the gas with a scrubbing liquid at elevated pH can be conducted in the second or downstream stage or tower, while the first or upstream stage or tower operating at a lower pH is utilized primarily for the removal of SO₂ from the gas stream.

[0036] Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.

EXAMPLE

[0037] The following non-limiting example is provided to further illustrate the present invention.

[0038] A tailgas stream of about 13,500 acfm containing, on a dry basis, about 7,100 ppmv SO₂ and about 20 ppmv acid vapor (SO₃ and/or H₂SO₄) was treated in accordance with the present invention. The wet scrubber system utilized was similar to that shown in Fig. 2 and included two reverse jet (DYNAWAVE) contacting stages and a subsequent packed column contacting stage. The reverse jet stages were each operated at an L/G ratio of about 100 gpm of liquid per 1000 acfm of gas. The aqueous scrubbing solution comprising an aqueous caustic solution of NaOH was collected in the sump of the wet scrubber system. The sump liquid was maintained at a pH range of from about 6.5 to about 10.5. Significant removal of SO₃ and/or H₂SO₄ from the tailgas was observed during intervals when the pH of the spent scrubbing liquid collected in the sump of the wet scrubber system was in excess of about 9, corresponding to an estimated pH of the regenerated scrubbing liquid fed to the reverse jet contacting stages of at least about 12. During these intervals, the treated gas from the wet scrubber outlet was found to contain about 1 ppmv dry basis SO₂ and less than about 6 ppmv dry basis H₂SO₄. Thus, the removal efficiency for SO₂ was greater than 99.9% and about

75% for SO₃/H₂SO₄.

[0039] In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

[0040] As various changes could be made in the above methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

[0041] When introducing elements of the present invention or the preferred embodiments (s) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

Claims

WHAT IS CLAIMED IS:

1. A process for removing at least one sulfur specie selected from the group consisting of SO₃, H₂SO₄ and mixtures thereof from a gas stream, the process comprising: contacting the gas stream with a scrubbing liquid in at least one scrubbing zone of a gas-liquid contacting device, the scrubbing liquid comprising an aqueous solution containing a basic reagent and having a pH of at least about 8.5 to thereby absorb and react the at least one sulfur specie with the basic reagent in the liquid phase and produce a treated gas stream depleted in the at least one sulfur specie and a spent scrubbing solution comprising the liquid phase reaction product of the basic reagent and the at least one sulfur specie; adding additional basic reagent to the spent scrubbing solution to regenerate the scrubbing liquid having a pH of at least about 8.5; and reintroducing the regenerated scrubbing liquid into the scrubbing zone.

2. The process as set forth in claim 1 wherein the gas stream comprises SO2, the process further comprising simultaneously removing SO₂ from the gas stream, wherein the SO₂ is absorbed and reacted with the basic reagent in the liquid phase such that the treated gas stream produced is depleted in SO₂ and the spent scrubbing solution comprises the liquid phase reaction product of the basic reagent and SO₂.

3. The process as set forth in claim 1 or claim 2 wherein the basic reagent is selected from the group consisting of sodium hydroxide, sodium carbonate, sodium bicarbonate, calcium oxide, calcium hydroxide, calcium carbonate, calcium bicarbonate, potassium oxide, potassium hydroxide, potassium carbonate, potassium bicarbonate, magnesium oxide, magnesium hydroxide, magnesium carbonate, magnesium bicarbonate, zinc oxide, zinc hydroxide, zinc carbonate, zinc bicarbonate, ammonia, and ammonium hydroxide.

4. The process as set forth in claim 1 or claim 2 wherein the basic reagent is sodium hydroxide.

5. The process as set forth in any one of claims 1 to 4 wherein the scrubbing liquid contacted with the gaseous stream in the scrubbing zone of the gas-liquid contacting device has a pH of at least about 9.

6. The process as set forth in any one of claims 1 to 4 wherein the scrubbing liquid contacted with the gaseous stream in the scrubbing zone of the gas-liquid contacting device has a pH of at least about 9.5.

7. The process as set forth in any one of claims 1 to

6 wherein the temperature of the gas stream introduced into the scrubbing zone of the gas-liquid contacting device is from about 100⁰C to about 320⁰C.

8. The process as set forth in any one of claims 1 to

7 wherein the temperature of the treated gas stream exiting the scrubbing zone of the gas-liquid contacting device is less than about 100⁰C.

9. The process as set forth in any one of claims 1 to

8 wherein the treated gas stream exiting the scrubbing zone of the gas-liquid contacting device is adiabatically saturated .

10. The process as set forth in any one of claims 1 to 9 wherein the gas stream is contacted with the scrubbing liquid in the scrubbing zone of a plurality of gas-liquid contacting devices .

11. The process as set forth in any one of claims 1 to 9 wherein the gas stream and the scrubbing liquid are contacted countercurrently in the scrubbing zone of the gas-liquid contacting device.

12. The process as set forth in claim 11 wherein the scrubbing liquid is sprayed upward countercurrent to the flow of the gas stream in the scrubbing zone of the gas- liquid contacting device.

13. The process as set forth in claim 12 wherein the scrubbing zone of the gas-liquid contacting device comprises the froth zone of a reverse jet scrubber.

14. The process as set forth in claim 13 wherein the volumetric ratio of the scrubbing liquid to the gas stream

(L/G) introduced into the reverse jet scrubber is from about 60 to about 200 gallons per minute of liquid per 1000 actual cubic feet per minute of gas.

15. The process as set forth in claim 13 wherein the volumetric ratio of the scrubbing liquid to the gas stream

(L/G) introduced into the reverse jet scrubber is from about 80 to about 200 gallons per minute of liquid per 1000 actual cubic feet per minute of gas.

16. The process as set forth in claim 2 wherein the gas stream introduced into the scrubbing zone of the gas- liquid contacting device comprises, on a dry basis, at least about 500 ppmv SO₂.

17. The process as set forth in claim 16 wherein at least about 90% of SO₂ contained in the gas stream introduced into the scrubbing zone of the gas-liquid contacting device is removed.

18. The process as set forth in any one of claims 1 to 17 wherein the gas stream introduced into the scrubbing zone of the gas-liquid contacting device comprises, on a dry basis, at least about 5 ppmv SO₃ and/or H₂SO₄.

19. The process as set forth in claim 18 wherein at least about 60% of SO₃ and/or H₂SO₄ contained in the gas stream introduced into the scrubbing zone of the gas-liquid contacting device is removed.

20. The process as set forth in any one of claims 1 to 19 wherein the opacity of the treated gas stream is less than about 20%.

21. The process as set forth in any one of claims 1 to 19 wherein the treated gas stream does not exhibit a visible plume.

22. The process as set forth in any one of claims 1 to 21 wherein the gas stream is derived from a sulfur- containing process gas issuing from a fossil fuel-fired power plant, a natural gas treatment plant, a fluid catalytic cracking unit, a coker, a calciner, a sulfur recovery unit, a sulfur trioxide absorber of a contact sulfuric acid plant, a metal roasting operation, a synthesis gas plant, a cement kiln or the incinerator of a Claus plant.