WO1992012786A1

WO1992012786A1 - Stripping method and apparatus

Info

Publication number: WO1992012786A1
Application number: PCT/CA1992/000025
Authority: WO
Inventors: Donald R. Spink
Original assignee: Turbotak Technologies Inc.
Priority date: 1991-01-22
Filing date: 1992-01-22
Publication date: 1992-08-06
Also published as: GB9101336D0; ZA92455B; AU1180192A; CA2100768A1; IE920185A1

Abstract

Solute gas-rich absorbing media formed in scrubbing a solute gas from an off-gas stream prior to discharge of the same are regenerated to solute-lean absorbing medium for recycle to the scrubbing operation. Hot rich absorbing media is formed into one or more spray patterns of very small liquid droplets in a flowing purge steam stream in an elongate conduit into which the solute gas is desorbed from the droplets. The droplets are coalesced to form a regenerated absorption medium, the solute gas-containing gas stream is cooled to condense out the steam and a pure solute gas stream is recovered. The operation may be effected in multiple stripping stages, which may comprise countercurrent flow of solute gas laden absorbing medium and purge steam within a single conduit or in multiple conduits. Multiple stripping steps may be affected within a single stripping stage.

Description

STRIPPING METHOD AND APPARATUS FI ELD OF INVENTION

The present invention relates to a novel form of stripping operation for the removal of dissolved gases from liquid solvents or absorbents therefor.

BACKGROUND TO THE INVENTION

In previously issued United States Patents Nos. 4,865,817, 4,963,329 and 5,023,064 as well as concurrently pending United States patent applications Serial Nos. 646,197, 672,021 and 754,643, all assigned to the assignee hereof and the disclosures of which are incorporated herein by reference, there is disclosed the substance of a gas reacting apparatus and method for the wet mass transfer of solute gases from or a gas stream to a liquid reacting medium capable of chemisorption of the solute gases from or contained in the gas stream.

The basis of the above patents and applications and also why the unique approach to gas absorption described therein works so effectively relates to the exceptionally large surface area of liquid absorption medium in the form of very small droplets of the liquid absorption medium generated in-duct with two-phase atomizing nozzles or certain hydraulic nozzles. Accordingly, there have been developed commercial applications of such technology for the removal of acidic gases, such as SO₂, H₂S, Cl₂, CIO₂, NO_x, HCl, HF, SO₃, etc. using a variety of absorption media. In most of these applications, the absorption media chemically react with the acidic gas, sometimes to oxidize it or reduce it or otherwise to form a stable reaction product, that may be disposed of or otherwise treated for disposal. Apparatus embodying such techniques is known by our assignee as the "Waterloo Scrubber". In some cases of such solute gas absorption process, the absorption media can be regenerated. Often, the regeneration step is accomplished by steam stripping in more or less conventional tray or packed columns. This steam stripping regeneration procedure is an industrial process step that is widely used throughout the chemical industry for many differing desorbing or separating requirements, and not simply for the regeneration of absorption media used to remove solute gases by the procedures described in the above patents and applications.

The steam which is used in such stripping operations usually is generated in a reboiler located at the base of the conventional tray or packed column and rises in counter-current flow to the loaded liquid absorption medium, which normally is fed to the middle of the column and passes tray-to-tray or over the packing down the column. Overhead product reflux generally is employed to further purify the overhead product. The steam (gas) -liquid contact in such operations is confined to the interfacial area generated as the bubbles of steam pass through the shallow layer of liquid absorption medium covering each tray in the column. In normal practice, each equilibrium stage represents a number of trays. The number of equilibrium stages required to effect separation of the absorbed component varies and is dependent on the particular system under consideration.

SUMMARY OF INVENTION

The unique features that have enabled the Waterloo Scrubber to be especially efficient at absorption also are employed to reverse the absorption step in a desorption or steam-stripping operation. Accordingly, the present invention relates to the removal of absorbed gases from absorbing media therefor. In view of the large number of more-or-less conventional steam stripping operations carried out in the chemical industry, the apparatus and process described below and provided in accordance with this invention is considered to have broad applications in this industrial segment as well as other industrial segments wherein steam stripping is carried out.

Accordingly, in one aspect, the present invention provides a method for the removal of a solute gas from a solute gas-laden aqueous absorbing medium, which comprises :

(a) passing a flowing gas stream comprising steam through an elongate conduit having an inlet thereto and an outlet therefrom,

(b) injecting said absorbing medium at an elevated temperature directly into said flowing gas stream to form at least one spray pattern of said absorbing medium in said conduit containing liquid droplets ranging in size from about 5 to about 100 microns,

(c) desorbing dissolved solute gas from said liquid droplets of absorbing medium into said flowing gas stream,

(d) agglomerating said liquid droplets at said downstream end of said conduit to remove entrained liquid droplets from said gas stream to form an at least partially regenerated aqueous absorbing medium, and

(e) discharging a gaseous mixture comprising solute gas and steam from said downstream end of said conduit and recovering said solute gas from said gaseous mixture.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a schematic flow sheet of an SO₂-absorbing operation;

Figure 2 is a schematic flow sheet of an SO₂-stripping operation effected in accordance with one embodiment of the present invention, the two operations being interacted to provide a cycle absorption- desorption operation; and

Figure 3 is a schematic flow sheet of an alternative stripping operation to that illustrated in Figure 2, provided in accordance with another embodiment of the invention.

GENERAL DESCRIPTION OF INVENTION

For a solvent or other liquid absorbing medium (loaded with SO₂ or other gas) that can be reversably stripped by heating of the solvent by steam or other means to attain the desired temperature, one or more steps of adiabatic flashing coupled with some degree of heat stripping, where exceptionally large surface areas of liquid absorbing medium are produced according to the invention, may be incorporated into very compact equipment and effect optimal separation of the SO₂ or other dissolved gas from the solvent in a minimal number of stages, in contrast to conventional tray columns.

One class of absorption media of interest in the present invention is water-soluble single salts of secondary and tertiary di-amines, as described in U.S.

Patent No. 5,019,361, and water-soluble members of the hydroxyalkyl 2-piperazinone family, as described in published EP 303,501, both of which represent stable, high boiling chemical compounds useful as absorbing media for the removal of sulfur dioxide from gas streams using the techniques described above. These latter compounds are characterized with an especially high degree of selectivity for the chemisorption of SO₂ from industrial gas streams at temperatures below about 100°C and normally at the adiabatic dewpoint of the gas stream or, preferably, at lower or at ambient temperatures.

With these organic solvents, the chemisorption process which removes the SO₂ from the gas stream can readily be reversed at some higher temperature to effect desorption of the SO₂ and regeneration of the absorption media. In conventional practice, steam distillation is employed to reverse the absorption process and regenerate the absorption media. In such instances, the off-gas stream from the regeneration step contains only SO₂ and steam. After condensation by cooling of the steam and removal of the resulting water, a clean flow of SO₂ can be produced, which, after drying, is the pure product of the cyclic absorption-desorption operation. The same result is achieved by the process of the invention. In this way, a furnace or process off-gas stream containing SO₂ may be contacted by absorption media to remove SO₂ contaminant and any particulate present prior to venting the clean gas stream to a suitable stack and the absorption medium may be regenerated for reuse while recovering the SO₂ as a pure gas stream. The SO₂-free absorption medium resulting from the regeneration operation and having a substantially decreased SO₂ content is recycled to the absorption step. The by-product pure SO₂ stream can be used or sold as such or can be easily converted to sulfuric acid or, where specific reductants are available, be reduced to elemental sulfur.

In U.S. Patent No. 4,963,329, referred to above, Figure 7 presents a two-stage scrubber concept where fresh or regenerated reagent (absorption medium) is employed in the second absorption stage of the solute gas absorption scrubber in a recycle mode, wherein the absorption medium contacts an SO₂-containing gas stream containing lesser amounts of SO₂. To obtain a mass balance on the scrubbing reagent, the amount of fresh or regenerated solvent entering the second absorption stage must be balanced by an equivalent amount of reagent collected from the second absorption stage passing on to the first absorption stage where it contacts a higher level of SO₂ in the entering gas stream, thereby ensuring that the absorption medium removed from this first stage may be completely loaded with absorbed SO₂, depending on the L/G ratio employed and characteristics of the sorbent system.

It now has been found that certain specific reagents are much better than others, in that the kinetics in the absorption systems described in the above-mentioned patents and applications utilizing the Waterloo Scrubber is very fast so that equilibrium is very quickly achieved.

Nozzles employed in the duct to achieve formation of the sprays of very fine liquid droplets preferably are of the dual-fluid type wherein atomizing gas and liquid to be sprayed are combined into a gas-liquid mixture which is ejected from the nozzle. Preferably, the nozzles are cluster nozzles, which result in a plurality of sprays being obtained from a single nozzle, as described in U.S. Patent No. 4,893,752, assigned to the assignee hereof, and U.S. Patent application Serial No. 753,404 filed August 30, 1991, the disclosures of which are incorporated herein by reference. However, any nozzle technology or design which is capable of generating fine sprays with the characteristics of droplet size distribution and surface area may be used.

In most instances, only two such absorption stages are required to achieve 98% removal of the SO₂ in the entering gas stream. By varying the liquid-to-gas (L/G) ratio (expressed in US gallons of liquor sprayed/stage per 1000 ft³ of gas being scrubbed), higher or lower percent removal of the SO₂ can readily be achieved. Nevertheless, more than two solute gas absorption stages may be used as the need arises.

The excellent kinetics observed in this proprietary approach to absorption of SO₂ and other solute gases from off-gas streams depends on two factors, namely, first, the properties of the specific liquid absorption reagent employed and second, the amount of surface area of reagent generated by the spray nozzles within the ducts which comprise the Waterloo Scrubber.

With the countercurrent or co-current flow of liquid absorption medium and solute gas-containing stream, the first and second stages of absorption are preferably separated to avoid completely partially- loaded liquor from passing from the first stage to the second stage. While there are many ways of achieving such separation, the preferred method employs banks of special chevron-type mist eliminators located between the two stages, as described in the aforementioned U.S. Patent application Serial No. 646,197. These mist eliminators have been found to be most suitable for the purposes at hand, because 100% removal of the loaded reagent is not essential while greater than about 99% removal is readily achievable. Such interstage mist eliminators are not required when the absorbing medium is fed in parallel to two or more stages of solute gas removal, as described in U.K. Patent application No. 9123999.6 filed November 12, 1991.

All of these design features, coupled with kinetically-aggressive reagent molecules in the absorption medium, enable SO₂ or other solute gas removal from a gas stream to be effected at duct space velocities of up to as high as about 40 to 50 feet per second.

The desorption and regeneration operation of the present invention employs similar techniques to those used to remove solute gases from gas streams described above and in more detail in the aforementioned patents and applications, except that desorption of dissolved solute gas is effected in at least one stage by spraying heated solute gas-loaded liquid absorption medium at an elevated temperature as fine liquid droplets of large surface area using steam as the atomizing gas into a duct through which passes a purge steam stream into which is desorbed the solute gas. Although it is preferred herein to effect the formation of the fine liquid droplets of solute-loaded absorption medium using steam as the atomizing medium, any other convenient procedure may be employed. In such spray pattern of fine liquid droplets, the droplets have a size ranging from about 5 to about 100 microns, preferably about 5 to about 30 microns. The loaded liquid absorption medium is sprayed into the duct through which the purge steam passes at an elevated temperature to facilitate mass transfer of solute gas from the liquid droplets to the gaseous phase. The temperature employed varies with the particular absorption medium and may vary generally from about 170° to about 270°F.

However, simply because fine liquid droplets of liquid absorbing medium are very effective in removing solute gases from gas streams does not mean that a somewhat analogous technique is effective for regenerating an absorption medium and, in fact, it is surprising that the regeneration procedure provided herein is so effective.

The regeneration operation may be effected by passing the loaded absorption medium first to a second stage of steam stripping, removing partially- regenerated solvent from the downstream end of the second stage, forwarding the partially-regenerated solvent to a first stage of steam stripping and recovering the regenerated solvent for recycle to the solute gas absorption operation.

An alternative regeneration procedure involves removal of a percentage of the volume of partially- regenerated solvent from the downstream end of the second regeneration stage and forwarding the same to the first stage of the absorber scrubber for reloading, while the remaining volume of partially-regenerated solvent is forwarded to a second stage of stripping, to recover a much leaner sorbent for application to the second or third stage of the absorber.

The desorbed sulfur dioxide or other solute gas, which may be, for example, CI₂, HCl, SO₃, or VOCs, is removed from the downstream end of the duct following removal of the partially regenerated solvent by suitable coalescing means, such as a mist eliminator, in a stream of water vapor. Following cooling and condensation of water of the gas stream followed by drying, a pure stream of SO₂ or other solute gas is recovered, and may be further processed, as desired.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the drawings. Figures 1 and 2 illustrate an integrated absorption-desorption operation for removing SO₂ from a gas stream containing the same using a suitable regenerable solvent, with the absorption stage specifically being illustrated in Figure 1 and the desorption stage specifically being illustrated in Figure 2. While this specific embodiment is described with respect to the removal of the SO₂ from flue gas or other waste and off-gas streams, the invention has broad application to the removal of any solute gas from a gas stream containing the same in any regenerable liquid absorbing medium and the subsequent regeneration of the absorbing medium for reuse in the absorption stage and for recovery of pure solute gas. One example of such a process is the removal of H₂S from "sour" natural gas streams by using a liquid amine absorbent and the subsequent steam stripping step whereby the H₂S is recovered in relatively pure form for further processing. The recovery of CO₂ from gas streams by similar techniques represents another example.

In Figure 1, there is illustrated a solute gas removal apparatus 10. This apparatus 10 and its operation are generally described in the aforementioned

US patent application Serial No. 646,197. The apparatus 10 comprises a generally horizontal duct 12 having an inlet end 14 for receipt of a sulfur dioxide- containing off-gas stream from which the SO₂ is to be removed prior to venting to atmosphere. The duct 12 also may be arranged vertically, if desired. The gas stream also may contain particulate matter, which is removed along with the SO₂ in the duct 12. Heavily particulate-contaminated gas stream first may be subjected to a particulate removal operation prior to passage to the duct 12, such as described in the aforementioned US Patent No. 5,023,064.

A pair of mist eliminators 16 and 18 of any convenient construction, such as the chevron type, to effect coalescence of gas stream borne liquid droplets passing therethrough is provided, defining in the interior of the duct 12 two gas absorption stages or chambers 20 and 22, separated by the mist eliminator 16. A pair of dual-fluid nozzles 24 and 26 is located one in each of the gas absorption stages 20 and 22. The dual- fluid nozzles 24 and 26 are constructed to produce a spray of very fine liquid droplets of high surface area of regenerable liquid absorbing medium in the duct 12 and preferably comprise cluster nozzles, such as those described in the aforementioned U.S. Patent No. 4,893,752 and USSN 753,404, although any nozzle capable of generating similar fine droplet sprays may be used.

The nozzles 24 and 26 are illustrated as spraying the absorbing medium countercurrent to the direction of flow of the SO₂-containing gas stream through the duct, since this orientation is the most convenient to obtain a high rate of gas-liquid transfer. However, co-current spraying of absorbing medium into the duct 12 from both or one of the nozzles 24 or 26 can be effected. The downstream end 28 of the duct 12 is connected to an I.D. fan 30, which maintains the flow of gas through the duct 12 and discharges the purified gas stream, now SO₂- and particulate-free by duct 32 to a discharge stack 34. An SO₂ analyzer 36 may be provided in association with the discharge stack 34 to monitor SO2 content of the discharged gas stream to ensure that the ultimate discharge is within allowable limits or meets any other level as desired.

At the immediate downstream end 28 of the duct 12 is a further mist eliminator 38 which serves, in conjunction with hot water sprays 39, to scavenge any residual entrained droplets of absorbing medium from the purified gas stream. Liquid collected in the mist eliminator 38 is returned by line 41 to the water tank 42, from which it is pumped to the sprays 39 by line 43. Any liquid droplets coalesced by the fan blades in the I.D. fan 30 are returned by line 40 to the wash water tank 42.

A liquid regenerable solvent or sorbent for sulfur dioxide, such as an aqueous solution of an aliphatic, alicyclic or heterocyclic amine, is fed to the dual- fluid spray nozzle 26 in chamber 22. Such solvent is fed by line 44 to the nozzle 26 and comprises make-up quantities of fresh solvent in an amount required to make up losses and regenerated solvent produced employing the procedure of Figure 2 described below. Atomizing air or other gas is fed to the nozzle 26 by line 46. The atomizing air generally is applied to the dual-fluid spray nozzles 24, 26 at a pressure of about 20 to about 100 psi, preferably about 20 to about 70 psi and more preferably about 25 to about 75 psi.

The air and liquid solvent form an intimate mixture in the nozzle 26 which is sprayed as a mass 48 of fine liquid droplets of high surface area into the duct 12, which contact the gas stream flowing through chamber 22. Such liquid droplets range in size from about 5 to about 100 microns, preferably about 5 to about 30 microns.

The liquid solvent is low in dissolved SO₂ concentration (or contains no SO₂, depending on the efficiency of removal of SO₂ in the stripping operation) while the gas stream is depleted in SO2 content as a result of an initial removal in chamber 20. Accordingly, the SO₂ is rapidly and substantially completely dissolved in the liquid droplets.

The entrained liquid droplets in the flowing gas stream in chamber 22 are removed and coalesced by the mist eliminator 18 and partially-loaded solvent passes by line 50 from the mist eliminator 18 to a "tank 52. The SO₂-free gas passes through the mist eliminator 38, through the outlet 28 to the fan 30 and then to the vent stack 34.

The partially-loaded solvent is forwarded by line 54 to the dual-fluid nozzle 24 located in chamber 20. Atomizing air also is fed to the nozzle 24 by line 56. The air and liquid solvent form an intimate mixture in the nozzle 24 which is sprayed as a mass 58 of fine liquid droplets of high surface area, which contact the gas stream flowing through chamber 20. The fine liquid droplets range in size from about 5 to about 100 microns, preferably about 5 to about 30 microns.

The partially-loaded liquid solvent contacts a high concentration of sulfur dioxide in the gas stream passing through the duct 12. Sulfur dioxide is rapidly chemisorbed in the liquid droplets, up to saturated loading of the solvent by SO2.

The entrained liquid droplets in the flowing gas stream in chamber 20 are removed and coalesced by mist eliminator 16 and fully-loaded solvent is removed from the mist eliminator 16 by line 60. The partially depleted SO₂-containing gas stream then passes from the mist eliminator 16 into chamber 22 for removal of the remainder of the SO₂ in the manner described above.

In place of the countercurrent flow of gas stream and absorbing medium employed in the embodiment of Figure 1, with an intermediate mist eliminator 16, there may be employed a parallel . flow of liquid absorbing medium to the two dual-fluid nozzles 24 and 26 with no mist eliminator 16, as described in our aforementioned UK patent application No. 9123999.6.

While two stages of gas-liquid contact are illustrated in Figure 1, additional stages may be employed, as desired, depending on the concentration of solute gas present in the gas stream, the degree of removal required, the L/G ratio employed, and the nature of the solvent employed.

Referring now to Figure 2, there is illustrated therein a loaded absorbing medium regeneration apparatus 110 which comprises a horizontal duct 112 having an inlet end 114 for receipt of a low pressure steam purge stream. The duct 112 may be arranged vertically, if desired. The duct 112 is provided with an outer heating jacket 116 to heat the duct sufficiently to avoid condensation of steam therein. The duct 112 may have a slight (e.g. about 1°) incline towards its downstream end to facilitate removal of any condensate from the walls of the duct 112.

A pair of mist eliminators 118 and 120 of any convenient construction to effect coalescence of liquid droplets passing therethrough is provided, defining in the interior of the duct 112 two desorption stages or chambers 122 and 124, separated by the mist eliminator 118. A pair of dual fluid nozzles 126 and 128 is provided one in each of the gas desorption chambers 122 and 124. The dual-fluid nozzles 126 and 128 are constructed to produce very fine liquid droplets of high surface area of the heated loaded liquid absorbing medium and preferably comprise cluster nozzles, such as those described in the aforementioned US Patent No. 4,893,752 and USSN 753,404, although any other suitable nozzle may be employed.

The atomizing gas used in the desorption process is steam employed at a pressure necessary to generate a proper droplet size distribution. The entire system, however, is maintained at approximately 100°C regardless of steam temperature and liquid temperature, since the excess heat is rapidly spent in evaporating water from the amine solution and expelling some of the SO₂. In cases where the sorbent employed cannot withstand temperatures of 100°C, the system must be operated at a reduced pressure to maintain the desired temperature.

The nozzles 126 and 128 are illustrated as spraying the absorbing medium countercurrent to the direction of flow of purge steam gas stream through the duct 112, although co-current flow may be preferred. Thus, co- current spraying of absorbing medium and purge gas steam into the duct 112 from both or one of the nozzles 126 and 128 can be effected.

The downstream end 130 of the duct 112 is connected by line 131 to a cooler-condenser 132 of any convenient construction wherein steam is condensed and removed as water in line 134, resulting in a clean saturated flow of pure SO₂ in line 136. This by-product pure SO₂ gas stream may be used as such, may be converted into other useful chemicals, such as sulfuric acid, may be reduced to elemental sulfur, or otherwise processed. A solvent analyzer 138 may be provided between the downstream end

130 of the duct 112 and the cooler-condenser 132 to monitor solvent content of the gas stream to ensure the absence of such material from the gas stream exiting the duct 112.

At the immediate-downstream end 130 of the duct 112 is a further mist eliminator 140 which serves, in combination with hot water sprays 142 to scavenge any residual entrained droplets of absorbing medium from the gas stream exiting the duct 112. Liquid collected in the mist eliminator 140 is returned by line 144 to a hot water tank 146, from which it is pumped by line 146 to the sprayers 142.

The loaded liquid solvent in line 60 (Figure 1) is forwarded to a heated solvent holding tank 152 and then is forwarded by line 154 and pump 155 to the nozzle 128 located in the chamber 124. Atomizing steam also is forwarded to the nozzle 128 by line 156 via a solvent heater 150. The atomizing steam generally is applied to the dual-fluid spray nozzles 126, 128 at a pressure of about 20 to about 100 psi, preferably about 20 to about 70 psi and more preferably about 25 to about 75 psi.

The steam and loaded liquid solvent form an intimate saturated mixture in nozzle 128 which is sprayed as a mass 158 of fine liquid droplets of high surface area into the duct 112 in contact with the purge gas stream passing therethrough. The liquid droplets generally are sized from about 5 to about 100 microns, preferably about 5 to about 30 microns. The high surface area of liquid droplets contained in the flowing hot purge gas stream in the chamber 124 and the relatively high temperature of the droplets results in a rapid mass transfer of SO₂ gas to the gas phase, resulting in partial regeneration of the liquid absorbent medium.

The entrained liquid droplets in the flowing purge gas stream in chamber 124 are removed and coalesced by mist eliminator 120. The resulting partially regenerated solvent passes by line 160 from the mist eliminator 120 to a tank 162. The solvent-free gas stream exits the downstream end 130 of the duct 112 by line 131 and passes through the solvent analyzer 138 and an optional vacuum pump 164 to the cooler-condenser 132. The vacuum pump 164 may be employed to maintain the duct 112 under a reduced pressure, to enable operation at a lower temperature to be effected.

The partially-regenerated solvent is forwarded by line 166 to the dual-fluid nozzle 126 located in chamber

122 via a second solvent heater 167. Atomizing steam is fed to the dual-fluid nozzle 126 by line 168. The steam and partially-stripped solvent form an intimate mixture within the nozzle 126 which is sprayed into the chamber 122 as a mass 170 of fine liquid droplets of high surface area in the purge gas stream flowing through chamber 122. The liquid droplets generally are sized from about 5 to about 100 microns, preferably about 5 to about 30 microns. The high surface area of the liquid droplets and the relatively high temperature of the droplets results in a rapid mass transfer of SO₂ gas to the hot purge gas stream, resulting in further regeneration of the liquid absorbent medium.

The entrained liquid droplets in the flowing gas stream are removed and coalesced by mist eliminator 118 and regenerated solvent is removed from the mist eliminator 118 by line 172 and passes to a regenerated solvent holding tank 174. The regenerated solvent, after cooling by heat exchanger 176, may be passed by line 178 to regenerated solvent feed line 44 in Figure 1. Energy recovered from the regenerated solvent by heat exchanger 176 may be used to provide at least part of the heat requirements of solvent heater 150.

Although the procedure of Figure 2 is described with respect to countercurrent flow of loaded solvent and purge gas with an intermediate mist eliminator 118, there may be employed a parallel flow of loaded solvent to the dual-fluid nozzles 126, 128 with no mist eliminator 118, in analogous manner to that described in the aforementioned UK patent application No. 9123999.6 for absorbing the solute gas. In such operation, the duct or ducts in which the regeneration is effected may be vertical rather than horizontal and the spray nozzles may be oriented to effect spraying countercurrent to or co-current with the direction of flow of the gas stream.

While two stages of steam stripping are illustrated in Figure 2, additional stages may be employed, as desired, depending on the concentration of dissolved gas in the loaded absorbing medium, the nature of the absorbing medium, and the degree of regeneration required.

In a closed cycle (absorption-desorption) operation combining the operations of Figures 1 and 2, it may be preferred to strip less than 100% of the absorbed SO₂ during the regeneration operation from the absorbing medium as long as this does not have a deleterious effect on the effectiveness of the absorbing medium to remove SO₂ to the desired level from the incoming gas stream. The choice of action may be more a function of the absorbing medium, so each situation must be handled as appropriate.

In describing the above approach applied specifically to in-duct scrubbing to remove SO2 from various emitting sources, as depicted schematically in Figure 1, those skilled in the art can readily perceive that such an approach results in relatively smaller equipment than any conventional absorption process can adopt. This result infers significant advantages where retrofit applications exist as well as much smaller capital needs for a variety of purposes. In addition, the liquid-to-gas ratio used to achieve high removal efficiency of the SO₂ as shown in the absorption procedure of Example 1 is consistently much lower than previously found in any other system.

The fact that an in-duct absorption process is so successful encouraged us to investigate the use of similar technology to perform the stripping step which, in effect, reverses the absorption step. The basis of this approach, according to the invention, depends again on the creation of a very large liquid surface area, found to be as high as 50,000 ft² per gallon of liquid sprayed, in the duct 112 of the desorption apparatus 110. Such generation of a large surface area is coupled with heating the loaded absorbing medium by steam to a temperature where the vapour pressure of SO₂ over the solvent is sufficient to completely release the SO₂ under the dynamic conditions of the operation into the purge stream but below a temperature that would be deleterious to the absorbing medium. The temperature required to achieve the release of SO₂ or other solute gas from the absorbing medium varies be with the specific reagent used and the solute gas removed and is limited solely by the stability of the absorbent medium used. This efficient stripping of SO₂ or other solute gas from the sorbent medium is effected at relatively lower steam consumption, shorter exposure time to elevated temperature, with consequent decreased chemical oxidation or degradation of the solvent, increased inlet temperature of sorbent and steam than in conventional steam stripping operations.

In the desorption system of Figure 2, low gas velocities are employed, equivalent to the amount of SO₂ generated per unit time plus the amount of low pressure steam introduced to purge the system plus the amount of steam employed in the dual-fluid nozzles to effect atomization of the loaded and heated solvent. Accordingly, the size of the stripping equipment (duct 112) can be much smaller than the size of the related absorption equipment (duct 12) since the total gas flow through the duct 112 is considerably smaller than that flowing through the duct 12.

Specific solvents may require more than one stage to effect the degree of regeneration (desorption) desired in the procedure of Figure 2. While one may consider employing a higher temperature to achieve better stage-wise separation (desorption), the ability to proceed in this manner also depends on the stability of the specific reagent to elevated temperature, as this relates to oxidation and/or disproportionation of the solvent or to the formation of heat-stable salts in the solvent, which must be removed to retain the absorption capability of the solvent system.

In Figure 3, there is illustrated an alternative and currently preferred stripping operation to that described with respect to Figure 2, comprising multiple serial stages of stripping with multiple spray stages in parallel within each stage of stripping. This stripping operation may be integrated with a solute gas removal operation, such as that described above with respect to Figure 1.

As seen in Figure 3, a stripping apparatus 210 comprises three separate stripping stages 212, 214, 216. Each stripping stage 212, 214, 216 comprising an elongate duct 218 having an inlet 220 for a steam purge stream at one end and a mist eliminator 222 for removal and coalescence of droplets from the gaseous phase at the opposite end. In the illustrated embodiment, the ducts 218 are oriented horizontally and may have a slight (e.g. about 1°) downward incline towards the downstream end to facilitate removal of condensate from the duct 218. However, each or one or more of the ducts 218 may be provided in a vertical orientation.

Within each duct 218 are located three dual-fluid spray nozzles 224, 226, 228, which are fed in parallel by solvent gas loaded absorption medium, as well as steam, to form sprays 230 of very fine liquid droplets countercurrent or co-current to the flow of the steam purge through the duct 218. One or more of the spray nozzles 224, 226, 228 may be employed per stage, depending on the system requirements. The liquid droplets in such sprays may be sized from about 5 to about 100 microns, preferably about 5 to about 30 microns. The spray nozzles 224, 226, 228 are illustrated oriented to spray absorption medium countercurrent to the flow of the purge gas stream in each duct 218. One or more of such groups of nozzles or individual members of the groups of nozzles may be oriented to spray co-current with the flow of the purge gas stream.

Solute gas-loaded solvent is passed from a holding tank 232 by line 234 through a steam heater 236 and via line 238 to the series of spray nozzles 224, 226, 228 and into the duct 218 of the first stripping stage 212. Steam is fed to the dual-fluid spray nozzles 224, 226, 228 in the duct 218 of the first stripping stage 212 by line 240. At the downstream end of the first stripping stage 212, the liquid droplets in the gas stream are coalesced in the mist eliminator 222 and the resulting partially-stripped absorbing medium is forwarded by line 242 through heater 244 to parallel feed 246 to the dual- fluid spray nozzles 224, 226, 228 in duct 218 of the second stripping stage 214. Steam also is fed to the nozzles 224, 226, 228 in stripping stage 214 by line 248.

Further partially-stripped absorbing medium collected from mist eliminator 222 at the downstream end of the second stripping stage 214 is passed by line 250 via steam heater 252 to parallel feed 254 to the dual- fluid spray nozzles 224, 226, 228 in duct 218 of the third stripping stage 218. Steam is fed to those nozzles by line 256.

The lean regenerated solvent is removed by line 258 from mist eliminator 222 at the downstream end of the third stripping stage 216 and passed through a cooler^"

260 to provide in line 262 a final discharge of cooled lean regenerated solvent for utilization in an absorption operation.

The gas exiting the mist eliminators 222 at the downstream ends of the ducts 218 of the three stripping stages, comprising steam and SO2, passes through ducting 264 to a further mist eliminator 266, which is fed with hot water showers 268, to ensure that any residual solvent is removed from the gas stream. Liquid removed from the gases by the mist eliminator 266 passes by line 268 to storage tank 270 for recycle to the showers 268 by line 272.

The gas stream exiting the mist eliminator 266 passes by line 274 to a cooler-condenser 276, wherein the steam component of gas stream is condensed out, leaving a pure saturated SO2 gas stream in line 278 for recovery. The condensed water is removed from the cooler-condenser 276 by line 280 and normally is added back to regenerated sorbent to maintain a water balance.

EXAMPLES

An experimental stripping unit was set up with a single stage of stripping to test the feasibility of the stripping procedure and to test the effect of various parameters on the efficiency of stripping of absorbed SO2 from two aqueous amine absorbents, namely triethanolamine (TEA) and a proprietary amine (PA) , which was a proprietary blend consisting primarily of an aqueous amine salt solution.

In the experiments, a single dual-fluid spray nozzle was axially located in an insulated, jacketed horizontal duct of 12 inches I.D. and a length of 12 feet to spray co-current with the direction of flow of the purge stream and a chevron-type demister was located at the downstream end to remove entrained partially stripped liquid droplets. A steam purge stream was passed through the duct from the upstream end to the downstream end. Steam also was fed to the dual-fluid spray nozzle.

The data which has been obtained is set forth in the following Table I:

As may be seen from this data, the best single stripping efficiency obtained was that in Example 10 of 50%. Attaining this degree of stripping in a single stage of stripping indicates that substantially complete stripping is attainable using multiple stripping stages. It is also considered that improved single-stage stripping can be achieved via an optimization process.

From this data, several conclusions can be drawn with respect to the effect of various variables on the stripping operation. For example, at higher steam pressures to the nozzles, smaller droplets are produced and a higher surface area is generated per unit of flow. This operation improves the efficiency of SO₂ stripping at any one temperature large volume nozzles -spraying high liquid flows use large volumes of steam and may eliminate or significantly decrease the need for a steam purge. Upon stripping, the highest SO₂ concentration occurs approximately 1 to 2 feet downstream from the nozzle. Means may be provided to rapidly dilute and disperse the SO₂ and thereby decrease the potential for readsorption. If steam is employed as this means, further stripping of the amine is quite likely. In addition, the greater the volume of steam purge, the greater is the efficiency.

A relationship appears to exist between amine concentration and SO₂ loading. At any amine concentration, a higher SO₂ loading provides a more easily stripped solution and for each solvent there is a lower stripping limit which is approached asymmetrically.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides a novel regeneration procedure for solute gas-loaded liquid absorption media which is simple and effective and of low capital cost as compared to conventional column-type steam strippers. Modifications are possible within the scope of this invention.

Claims

CLAIMS What I claim is:

1. A method for the removal of a solute gas from a solute gas-laden aqueous absorbing medium, which comprises:

2. The method of claim 1, wherein said absorbing medium is injected into said conduit from at least one dual-fluid spray nozzle disposed in said conduit and to which steam is fed to effect atomization of said absorbing medium to form said spray pattern.

3. The method of claim 2, wherein said absorbing medium is injected directly into said flowing gas stream to form a plurality of said spray patterns in said conduit which do not substantially overlap one another.

4. The method of claim 2 which is effected to form a partially-regenerated aqueous absorbing medium, and including:

(i) injecting said partially-regenerated aqueous absorbing medium into a further elongate conduit through which passes a flowing gas stream comprising steam, by atomization from a dual-fluid nozzle in said further conduit using steam to form a spray pattern of the partially-regenerated aqueous absorbing medium in said further conduit containing liquid droplets ranging in size from about 5 to about 100 microns,

(ii) desorbing dissolved solute gas from said liquid droplets of partially-regenerated absorbing medium into said flowing gas stream,

(iii) agglomerating said liquid droplets at said downstream end of said further conduit to remove entrained liquid droplets from said gas stream to form a regenerated aqueous absorbing medium, and

(iv) discharging a gaseous mixture comprising solute gas and steam from said downstream end of said further conduit and recovering said solute gas from said from gaseous mixture.

5. The method of claim 4 wherein said conduit and said further conduit comprise a single contiguous conduit with said further conduit being located upstream of said conduit with respect to said flowing gas stream, and said flowing gas stream passes from the upstream end of said further conduit to the downstream end of said conduit.

6. The method of claim 2 which is effected to form a partially-regenerated aqueous absorbing medium, and including:

(i) injecting said partially-regenerated aqueous absorbing medium into a further elongate conduit through which passes a flowing gas stream comprising steam, by atomization from at least one dual-fluid nozzle in said further conduit using steam to form a spray pattern of the partially-regenerated aqueous absorbing medium in said further conduit which contains liquid droplets ranging in size from about 5 to about 100 microns,

(ii) desorbing dissolved solute gas from said liquid droplets of partially-regenerated aqueous absorbing medium in said spray pattern into said flowing gas stream,

(iii) agglomerating said liquid droplets at said downstream end of said further conduit to remove entrained liquid droplets from said gas stream to form an at least further partially-regenerated absorbing medium, and

(iv) discharging a gaseous mixture comprising solute gas and steam from said downstream end of said further conduit and recovering said solute gas from said gaseous mixture.

7. The method of claim 6 which is effected to form a further partially-regenerated absorbing medium and wherein said further partially-regenerated absorbing medium is subjected to steps (i), (ii), (iii) and (iv) of claim 6 in a yet further conduit to form a regenerated absorbing medium.

8. The method of claim 1 wherein said solute gas is sulfur dioxide and said absorbing medium is an aqueous amine solution or aqueous amine salt solution.

9. The method of claim 2 wherein said liquid droplets in said spray pattern are sized from about 5 to about 30 microns.

10. The method of claim 2 wherein said atomizing is effected at a pressure of about 20 to about 100 psi.

11. The method of claim 10 wherein said atomizing is effected at a pressure of about 20 to about 70 psi.

12. The method of claim 11 wherein said atomizing is effected at a pressure of about 25 to about 75 psi.

13. The method of claim l wherein said absorbing medium has a temperature of about 170° to about 270°F when injected into said conduit.

14. The method of claim 1 wherein said solute gas-laden aqueous absorbing medium is formed in a solute gas-removal process for removing solute gas from an off-gas stream prior to venting the same using said absorbing medium.

15. A method for the removal of a solute gas from a gas stream containing the same using a regenerable aqueous solvent for said solute gas, which comprises:

A. effecting removal of solute gas from said gas stream to form a purified gas stream by the steps of:

(i) passing said gas stream through an elongate conduit having an inlet thereto and an outlet therefrom,

(ii) injecting said regenerable aqueous solvent directly into said gas stream from at least two dual-fluid spray nozzles located in longitudinally spaced-apart relationship in said conduit by atomizing gas to form a spray pattern of said regenerable aqueous solvent in said conduit from each said nozzle and containing liquid droplets ranging in size from about 5 to about 100 microns,

(iii) absorbing solute gas from said gas stream into said liquid droplets at a first temperature,

(iv) agglomerating said liquid droplets contained in said conduit to remove entrained liquid droplets from said gas stream to form a solute gas-laden aqueous solvent, and

(v) discharging said purified gas stream from said downstream end of said conduit;

B. effecting regeneration of said solute gas-laden aqueous solvent to remove and recover dissolved solute gas and regenerate the solvent for recycle to step A for use as said regenerable aqueous solvent therein by steps of:

(i) passing a flowing gas stream comprising steam through a further elongate conduit having an inlet thereto and an outlet therefrom,

(ii) injecting said solute gas-laden aqueous solvent directly into said flowing gas stream from at least one dual-fluid spray nozzle located in said further conduit by atomizing steam to form a spray pattern of said solute gas-laden liquid solvent in said further conduit from each said nozzle and containing liquid droplets ranging in size from about 5 to about 100 microns,

(iii) desorbing dissolved solute gas from said liquid droplets of solute gas-laden liquid solvent into said flowing gas stream at a second temperature greater than said first temperature,

(iv) agglomerating said liquid droplets contained in said further conduit to remove entrained liquid droplets from said flowing gas stream to form a regenerated aqueous solvent, and

(v) discharging a gaseous mixture comprising solute gas and steam from said downstream end of said further conduit and recovering said solute gas from said aqueous mixture; and

C. recycling said regenerated aqueous solvent to step A as said regenerable aqueous solvent.

16. The method of claim 15 wherein steps A (ii) to (iv) include:

(a) agglomerating said liquid droplets contained in said conduit to remove entrained liquid droplets from said gas stream at a location intermediate between said dual-fluid spray nozzles to form said solute gas-laden aqueous solvent,

(b) agglomerating liquid droplets contained in said conduit to remove entrained liquid droplets from said gas stream at said downstream end of said conduit to form a partially solute gas-laden aqueous solvent,

(c) injecting said regenerable aqueous solvent only from said dual-fluid spray nozzle immediately upstream of said downstream end of said conduit, and

(d) injecting said partially solute gas-laden aqueous solvent from said dual-fluid spray nozzle immediately downstream of said upstream end of said conduit.

17. The method of claim 15 wherein steps B (ii) to (iv) include:

(a) injecting said solute gas-laden aqueous solvent directly into said flowing gas stream from at least two dual-fluid spray nozzles located in longitudinally spaced-apart relationship in said further conduit and each forming a spray pattern in said further conduit;

(b) agglomerating said liquid droplets contained in said further conduit to remove entrained liquid droplets from said flowing gas stream to form at a location intermediate between said dual-fluid spray nozzles said regenerated aqueous solvent,

(c) agglomerating liquid droplets contained in said further conduit to remove entrained liquid droplets from said flowing gas stream at said downstream end of said further conduit to form a partially regenerated aqueous solvent,

(d) injecting said solute gas-laden aqueous solvent only from said dual-fluid spray nozzle immediately upstream of said downstream end of said further conduit, and

(e) injecting said partially regenerated aqueous medium from said dual-fluid spray nozzle immediately downstream of said upstream end of said further conduit.

18. The method of claim 15 wherein said solute gas is sulfur dioxide and said absorbing medium is an aqueous amine solution.