DK202100694A1 - Process for removal of sulfur dioxide from off gases - Google Patents

Abstract

The present disclosure relates to a process and a process plant for conversion of SO2 in a process gas to H2SO4, comprising a first step of spraying a first aqueous stream comprising H2O2 directly into the process gas enclosure for a first conversion of SO2 to provide a first converted process gas, and a second step of directing said first converted process gas and a second aqueous stream comprising H2O2 to a scrubber tower to provide a further converted process gas, wherein the concentration of H2O2 of said first aqueous stream is higher than the concentration of H2O2 of said second aqueous stream. This has the associated benefit of such a process being efficient in conversion of SO2 while also minimizing consumption and environmental release of H2O2.

Description

DK 2021 00694 A1 1 Title: Process for removal of sulfur dioxide from off gases The present invention relates to a process and a process plant for removal of sulfur di- oxide from off gases by reaction with hydrogen peroxide, in two steps.

Environmental legislation is continuously being tightened, allowing decreasing amounts of pollutants to become emitted to the atmosphere, water and/or soil.

For the sulfuric acid industry and environmental industries treating sulfurous waste streams, this typically means that emissions of H2SO4, usually in the form of mist/aero- sol), and SO, must be minimized, while not creating secondary pollution.

The minimum SO. emission from a plant producing sulfuric acid from SO, is typically limited by thermodynamic equilibrium between SO. and SO; in the catalytic con- verter(s). For that reason, it is common practice to install so-called tail gas plants to treat the off gases from such sulfuric acid plants, where the SO. emissions otherwise would exceed the environmental limits.

Such tail gas treatment technologies are nu- merous, but the most common are scrubbers in which the SO, containing off gas from the sulfuric acid plant is contacted with a liquid containing a chemical to react with the SO, and remove it from the gas phase.

Such chemicals can be alkaline compounds, such as NaOH, Ca(OH)., CaCO; and NHs, which are effective but can form undesired waste products.

HO: is also a widely used chemical, which converts the SO, in to H2SO4, which may be blended with the H.SO4 produced in the sulfuric acid plant, i.e. the product can be used.

The use of H>O; has a cost, and is environmentally regulated and therefore it is important to ensure that as much of the HO: is used for reaction with SO» and as little as possible leaves the plant via the treated gas or waste streams.

The objective of a H20> based SO, tail gas treatment plant is therefore to ensure that SO, is efficiently converted and the formed H>SO, is not emitted to the atmosphere as an aerosol, while minimizing the emission of H>0» to the atmosphere and H>0» in any waste streams.

A broad aspect of the present disclosure relates to a process for conversion of SO; in a process gas to H>SO4, comprising a first step of spraying a first aqueous stream com- prising H>0» directly into the process gas enclosure for a first conversion of SO» to

DK 2021 00694 A1 2 provide a first converted process gas, and a second step of directing said first con- verted process gas and a second aqueous stream comprising H>O» to a scrubber tower to provide a further converted process gas, wherein the concentration of H>O- of said first aqueous stream is higher than the concentration of H>0, of said second aqueous stream.

This has the associated benefit of such a process being efficient in conversion of SO; while also minimizing consumption and environmental release of H20.. In a further embodiment the first aqueous stream comprises at least 0.5 %w/w HO, or at least 1 %w/w H>O; and less than 10 %w/w HO; or less than 5 %w/w H>0».

This has the associated benefit of providing a stoichiometric amount of H>0. with re- spect to SO, conversion to H>S04 at the process inlet. In a further embodiment the second aqueous stream comprises at least 0.01 %w/w H20, or at least 0.05 %w/w H>0 and less than 1 %w/w HO, or less than 0.5 %w/w H202.

This has the associated benefit of minimizing the concentration and thus the environ- mental release of H>0, at the process outlet. In a further embodiment the first aqueous stream is made by mixing from an aqueous stream, such as pure water with a concentrated H>O» solution, preferably with a con- centration of 20-50 %w/w H20.

This has the associated benefit of providing independent control of water addition, in response to temperature and of HO, addition in response to inlet SO, concentration. In a further embodiment the first aqueous stream is added as droplets having a D90% diameter below 200 um, such as by use of an atomizing nozzle, preferably by use of twin-fluid nozzle(s), using a pressurized atomizing medium.

This has the associated benefit of providing droplets with a surface area, and thus a high potential for cooling as well as conversion of SO» in a short time. In a further embodiment the pressurized atomization medium is air or steam at the feed pressure of 2-10 bar. This has the associated benefit of such atomization media being available at the pro- cess plant while being compatible with the process.

DK 2021 00694 A1 3 In a further embodiment the volume flow of atomization medium is less than 2 % of the process gas volume flow, preferably less than 1 % of the process gas volume flow. This has the associated benefit of minimizing the additional volume required by the pro- cess.

In a further embodiment the process gas residence time from the spraying the first aqueous stream comprising H>O to the entry of the scrubber tower is at least 0.2 sec- onds or at least 0.5 seconds, and less than 3 seconds or less than 1 second.

This has the associated benefit of minimizing the additional volume required by the pro- cess, while still being sufficient for a high conversion of SO..

In a further embodiment the temperature of the process gas prior to spraying the first aqueous stream comprising H>O, is at least 80°C and less than 120 °C and the tem- perature of the first converted process gas entering scrubber tower is at least 45°C and less than 75 °C.

This has the associated benefit of matching sulfuric acid process outlet conditions, re- action kinetics and also being compatible with inexpensive materials such as polypro- pylene.

In a further embodiment the step of collecting droplets in the process gas leaving the first stage and directing them as the second aqueous stream comprising H.O; option- ally in combination with liquid collected from the scrubber tower.

This has the associated benefit of only requiring addition of H>0. solution in a single position.

In a further embodiment the conversion of SO, from spraying the first aqueous stream comprising H>0» to the entry of the scrubber tower is at least 70%, preferably at least 80% This has the associated benefit of consuming a high amount of HO; at the process in- let and thus only requiring a low conversion in the scrubber. In addition, the concentra- tion of H>0» is low in the scrubber.

In a further embodiment the overall degree conversion of SO, is at least 96%, prefera- bly at least 98%. This has the associated benefit of a minimal release of SO, to the environment.

DK 2021 00694 A1 4 In a further embodiment the scrubber tower is a packed bed scrubber and in which the second aqueous stream comprising H>0» further comprises at least 20%w/w H2SO4 and less than 60 %w/w H2SO4. This has the associated benefit of the second aqueous stream be sulfuric acid for fur- ther use, such as addition to a highly concentrated sulfuric acid.

In a further embodiment the process further comprises the step of directing the further converted process gas to a mist eliminator, such as a Brownian diffusion type candle filter or wet electrostatic precipitator.

This has the associated benefit of minimizing the release of droplets comprising H>SO4 and HO; to the environment.

The present disclosure may beneficially employ a process gas enclosure for combining an aqueous solution with a corrosive process gas, comprising an atomization unit hav- ing an outlet, inside said process gas enclosure and a means for directing a protective flow of non-corrosive gas to the surface of the atomization unit.

This has the associated benefit of providing small droplets of liquid for reaction with the process gas.

Such a process gas enclosure may be a gas duct or process vessel.

In a further embodiment the atomization unit outlet is a nozzle such as a twin-fluid noz- zle.

This has the associated benefit of such an atomization unit being highly effective in providing small droplets.

In a further embodiment the atomization unit further comprises an injection lance and the nozzle is positioned within the process gas enclosure, at the end of the injection lance.

This has the associated benefit of such a unit providing the added droplets in a position remote from the wall of the process gas enclosure.

In a further embodiment the direction of the atomization unit outlet is in between 0° (in- line) and 90 (perpendicular) in relation to the general flow direction of the process gas.

This has the associated benefit of the flow of process gas and droplets being well mixed with low impact on process gas flow.

In a further embodiment the spray angle of the nozzle is 15°-120°, preferably 15-90.

DK 2021 00694 A1 This has the associated benefit of the flow of process gas and droplets being well mixed. In a further embodiment the process enclosure further comprises a corrosion resistant sleeve between the process gas and the atomization unit. 5 This has the associated benefit of reducing the risk of corrosion of the atomization unit by process gas. In a further embodiment the protection sleeve is made of glass fiber reinforced plastic (GRP) lined with a fluoropolymer such as ECTFE, PFA, PTFE or fluoropolymer coated steel.

This has the associated benefit of such materials being highly resistive to corrosion, e.g. by sulfuric acid and hydrogen peroxide. In a further embodiment the sleeve and atomization unit are configured to have a gas inlet outside the process gas enclosure, configured to allow a flow of non-corrosive gas between sleeve and the atomization unit and configured to allow the gas to exit into the process gas enclosure proximate to the nozzle. This has the associated benefit of continuously purging corrosive process gas away from the structural material of the atomization unit, which may have a propensity for corrosion. A further aspect of the present disclosure relates to a process plant for conversion of SO, to H.SO4 comprising a process gas enclosure, a liquid reservoir, a means of recir- culation and a atomization nozzle, each having an inlet and an outlet, a scrubber tower having a gas inlet, a gas outlet, a liquid inlet and a liquid outlet, wherein the atomization nozzle outlet is positioned inside the process gas enclosure, said process gas enclo- sure is configured for receiving a process gas, said atomization nozzle inlet is config- ured for receiving an amount of an aqueous stream comprising H>O, and for releasing droplets of said aqueous stream comprising H20,, the outlet of said process gas enclo- sure is configured for being in fluid communication with the gas inlet of said scrubber tower and the liquid outlet of said scrubber tower, wherein the liquid outlet of said scrubber is in fluid communication with said liquid reservoir, wherein the inlet of the means of recirculation is configured for being in fluid communication with said liquid reservoir and the outlet of the means of recirculation is in fluid communication with the

DK 2021 00694 A1 6 liquid inlet of the scrubber tower, and wherein the liquid outlet of the scrubber is config- ured for withdrawal of a stream of sulfuric acid. This has the associated benefit of being a process plant which is efficient in conversion of SO, while also minimizing consumption and environmental release of H20.. In a further embodiment the process gas enclosure is constructed of GRP lined with a fluoropolymer, such as ECTFE, PFA or PTFE. This has the associated benefit of minimizing the corrosion of the process gas enclo- sure. In a further embodiment the scrubber tower is constructed in GRP with PP lining, GRP with ECTFE lining or PP.

This has the associated benefit of minimizing the corrosion of the scrubber in a cost ef- fective way.

Patent US 7,776,299 describes a combined SO, and H>SO4 mist removal system, in which an aqueous H>O» solution is sprayed into the process gas enclosure — the water in the solution cools the gas by evaporative cooling and the HO, react with the SO; to form an aerosol of H2SO4. The converted gas mixture then passes through a mist filter, in which the H>SO4 aerosol is captured and withdrawn from the bottom of the mist filter vessel and the cleaned process gas is directed to the atmosphere.

The process is simple and inexpensive but the potential SO, conversion is limited and the utilization of the H>O, will only be partial, due to a limited degree of contact be- tween liquid droplets and process gas. As little as 50% utilization of the supplied H20» may be seen, which result in high operating costs and possibly environmental problems with the unused H>0,, being emitted to the atmosphere or found in the sulfuric acid product.

The efficiency of this method depends on the HO; being available in small droplets, to ensure a good contact with the gas phase, and also an elevated concentration of H.O» to increase the reaction rate in a process with short residence time.

Scrubbers for SO» removal by H>0» often has process gas cooling systems installed upstream the reaction zone. By cooling the process gas to a temperature below 70°C, the packing and construction material of the scrubber can withstand the corrosive

DK 2021 00694 A1 7 solution of dilute H>SO4 with H20> present, and HO» evaporation from the liquid phase is at the same time decreased. Polypropylene, PP, is the preferred material for these conditions, but require a temperature below 60-70 °C. For higher temperatures, the fluoropolymers such as PFA, PTFE and ECTFE may be required.

The cooling system can be a corrosion resistant heat exchanger or a quench/cooling tower in which evaporation of water cools the process gas from the upstream sulfuric acid plant.

The process gas is cooled, typically to 60-70 C, by contact with droplets of circulating dilute H2SO4, from which water evaporate and cool the gas. The cooled process gas is then passed to the packed bed scrubber tower in which the packed bed is irrigated with circulating dilute H>SO4 containing H20, the H>SO4 concentration is typically 40-60 %w/w and the H>0> concentration is typically 0.3-5 %w/w, depending on the SO, con- centration and desired removal efficiency. The H>0» is supplied into this dilute acid cir- culating loop and reacts with the SO, to form H>SO4, which by gravity is drained to the quench tower, where it will be circulated in the cooling process and the HO, present will contribute to conversion of SO» to H>SO4 although only to a low extent, as the H20> concentration is low. Therefore, the overall SO. conversion is in practice limited by the height and performance of the packed bed and the amount of H.O, added to the sys- tem, to below 98% SO» conversion.

The presented process combines very high SO. conversion with very high utilization of the HO, supplied, while providing a cost-effective solution regarding both capital and operating costs. Furthermore, the system is designed to provide minimal emissions of H>0) to the atmosphere.

One embodiment of the process is described in the following section; however, many variants of the preferred solutions could also work well and will not be covered in full detail as they are assumed to be clear to a person, experienced in the field.

In an embodiment involving revamping an existing process, the already existing pro- cess gas duct is used as a reaction chamber by inserting an atomization nozzle, mounted on a lance, into the duct and spraying a first aqueous stream, comprising H>O: into the process gas. Atomization of the H>0» solution, typically enabled by pres- surized air, provides a sufficiently fine droplet distribution, which due to the high droplet surface area ensures rapid cooling and reaction, such that, in less than a second, the

DK 2021 00694 A1 8 process gas is cooled from e.g. 100°C to 55 °C by water evaporation and 80-90 % of the SO, is oxidized to H>SO4 by reaction with H.O,. The high degree of oxidation of SO, requires presence of a sufficient amount of HO, in the first aqueous stream.

The pre-cooled and pre-converted process gas enters a packed bed scrubber, where the remaining SO» oxidation takes place using a second aqueous stream comprising H20>. This second aqueous stream comprising H20> may contain less H>0» than the first aqueous stream, and it may be a stream of H25O4 and H>0» collected from the pre- cooled and pre-converted process gas, including the H>20> supplied from the atomiza- tion nozzle, ensuring both a very high SO» removal and H>O, utilization.

Other types of scrubbers, such as venturi type scrubbers and empty towers with one or more grid lay- ers of spray nozzles, will also work well.

The fully converted process gas then passes through an optional demister before the final mist filter.

The mist filter removes droplets of H>S04, either originating from the off gas from the upstream sulfuric acid plant or being formed by the reaction between SO. and H20>. The mist filter must be designed to remove very small droplets and Brownian diffusion type candle filters and wet electrostatic precipitators are suitable technologies for removal of such small droplets.

The fully converted and droplet free process gas leaves the mist filter and can be fur- ther processed or be emitted directly to the atmosphere.

A slight under-pressure can be maintained in the scrubber unit, by a blower or fan downstream the filter, which re- duces the risk of leaks to the surroundings.

There are several advantages of such a two-stage SO. conversion process, in which a first aqueous stream comprising a high amount of H>O, contacts the process gas, be- fore a second aqueous stream comprising a lower amount of H>20>, compared to the single-stage processes as presented in the prior art.

By ensuring a high amount of H>O, available to react in small droplets with a high surface area, rapid and efficient use of HO, is ensured.

By further treating the pre-treated process gas with a second aqueous stream comprising a lower amount of H>O, further conversion takes place, and due to the lower amount of H>0, less slip of H.O- is observed, benefitting the envi- ronment as well as process economy, especially if the second aqueous stream com- prising a lower amount of H>0, is the collected liquid product of the first stage.

The two- stage process will also be more robust than a single stage process as a minor SO»

DK 2021 00694 A1 9 conversion reduction in one of the steps will to a large degree be compensated for in the other step. Furthermore, it will be possible to achieve a higher overall SO. removal efficiency in a two-stage process than in a single stage, or alternatively, the same over- all SO, conversion efficiency can be maintained, while only designing each of the two stages with lower individual efficiencies, which results in smaller equipment which is simpler and less costly. If the process gas originates from a sulfuric acid plant, it may comprise H2SO4, found as small droplets (an aerosol). From a wet type sulfuric acid plant with an off-gas tem- perature of around 100 °C, the concentration of these sulfuric acid droplets is in the 60- 80 %w/w range and the combination of temperature and concentration limits the choice of metals and alloys for construction as few materials can withstand the corrosiveness of the acid. Therefore, polymers and especially fluoropolymers are the preferred construction mate- rials and the process gas enclosure is typically made of glass fiber reinforced plastics (GRP) with a lining of ECTFE, PFA, PTFE, PP or similar. For mechanical stability, the atomization lance and atomization nozzle are made of steel or other alloys and thus they need to be protected from the H>SO4 aerosols in the process gas. In principle, the lance and nozzle could be coated with an acid resistant coating, but especially at the nozzle outlet such a coating would interfere with the de- sired function of the nozzle. Therefore, in the present invention, corrosion protection is carried out by surrounding the lance and nozzle with a corrosion resistant sleeve, made either by lined GRP or coated steel. Between the inner shell of the sleeve and the outer part of lance and nozzle, there will be a (small) flow of non-corrosive gas, typically air, which will be released to the process gas very close to the atomization nozzle. This flow of non-corrosive gas will ensure that there is no ingress of corrosive process gas into the slit between the protection sleeve and lance and nozzle; without this non-corro- sive gas flow, process gas may enter by either diffusion or by means of gas recirculat- ing zones initiated by the turbulence around the spray nozzle and in the end cause cor- rosion to the lance and nozzle.

The preferred position of the lance and nozzle in the process gas enclosure will depend on factors such as length and width of the process gas enclosure and presence of flow disturbances such as bends and changes in enclosure diameter. An enclosure length

DK 2021 00694 A1 10 corresponding to 4-6 enclosure diameters is preferred for proper mixing of the gas and droplets, while avoiding an excessive enclosure size.

The process gas enclosure may also be a tower, with a number nozzles present, especially for larger plants.

The gas velocity in the process gas enclosure may be 10-20 meter/second, which is a compromise between cost of the process gas enclosure and pressure loss in the enclo- sure, i.e. related to blower cost.

It may be beneficial to decrease the gas velocity to in- crease the gas residence time to 0.5-3 seconds.

The orientation of the nozzle in relation to the process gas also depend on the factors mentioned.

The nozzle orientation may in principle be chosen freely, from direct oppo- site to the process gas flow direction, perpendicular to the flow and to in-line flow in the direction of the process gas flow.

An orientation between in-line (0° compared to pro- cess gas flow) and perpendicular (90° compared to process gas flow) is generally pre- ferred.

The nozzle itself also has a spray angle, determined by the geometry of the nozzle head, spray angles in the range 15° to 120° are available.

For the present invention, the range from 15° to 90 °C is preferred to reduce the risk of spraying the H>O- solution directly out on the enclosure wall, where gas/liquid contact is minimal and reduce the degree of SO; pre-conversion and thus H>O» utilization.

The choice of the most optimal position, orientation and angle of the nozzle may be found by support from computational fluid dynamics and/or pilot testing can be carried out.

Numerous nozzle types for atomization of liquids exist, each with their own characteris- tics.

So-called twin-fluid, air-assisted or pneumatic nozzles are characterized by using a pressurized gaseous stream accelerated to high velocity to “cut” the liquid phase into small droplets.

The gaseous stream can be steam, Na, process gas or other pressur- ized streams, however pressurized air may be preferred as it is often available on the plant.

Generally, the higher the pressure and flow of gaseous stream, the smaller the liquid droplets.

The droplet size distribution from such nozzles will be in the range 50- 300 um, depending on the pressure and flow of gas and liquid.

DK 2021 00694 A1 11 For droplet evaporation purposes, it is common to use the so-called D90% characteris- tics of the spray nozzle. D90% is the droplet diameter in which 90% of the mass(or vol- ume) is found in droplets having a smaller diameter than D90%. The flow of atomizing gas will typically be less than 0.5-1% of the total process gas flow. The pressure of the atomizing gas and liquid will typically be in the 3-8 bar range. For most sulfuric acid plants, a single atomization nozzle will be sufficient, but for larger plants it may be necessary to use two or more nozzles. These nozzles can then be evenly positioned on the periphery of the process gas enclosure and/or in one or more stages in the process gas enclosure.

The atomization of the H>0» solution serves both the purpose of cooling the process gas by water evaporation and SO, removal by reaction with the H>O-. In one disclosed layout , the HO: solution for the injection is mixed from a H>O» concentration with high concentration, typically 20-50 %w/w HO, and pure water. This enables separate con- trol of the SO. conversion and process gas cooling, while adding a dilute H>0 solution with fixed concentration will only be able to control either the SO. conversion or the temperature. A control loop for control of addition of H20 and H>O, may be configured based on measurement of one or more of the acid concentration, liquid level, SO slip and temperatures in the first stage or the second stage.

The HO, flow is preferably controlled from a signal from a SO» analyzer located down- stream the packed bed scrubber tower but using a feed forward signal is also possible. The HO flow is controlled to ensure proper process gas cooling, but also that the liquid level and/or H2SO4 concentration in the packed bed scrubber tower stay within prede- termined limits. Which parameter that control the water flow will depend on the actual process conditions, i.e. process gas temperature, SO, and water concentration in the gas and the process gas flow.

In the preferred solution, water and H>O: is only supplied to the nozzle in the process gas enclosure, but in some applications, it may be preferred to control the process fur- ther by adding water and/or H>0» directly in the circulation loop in the packed bed scrubber tower.

Over time, the nozzles may get worn and that will influence the liquid atomization, such that the droplet size will increase and the cooling and SO» conversion efficiency in the

DK 2021 00694 A1 12 process gas enclosure will decrease.

This higher process gas temperature and SO. concentration will be significantly dampened in the downstream packed bed scrubber as it also provides efficient gas/liquid contact for both cooling and reaction.

The state of the nozzle can be monitored by combining flow and pressure readings of both liquid and air to the nozzle into a nozzle parameter, which only depends on the geometry of the nozzle.

If the geometry changes, e.g. due to corrosion or erosion, the nozzle parameter will change and at a given critical value of the parameter, the nozzle must be replaced.

By proper design of lance mounting and plant operation, it will be possible to withdraw the lance and replace the nozzle without shutting down the plant.

Description of figures Figure 1 shows an example according to the prior art with a spray nozzle positioned in the process gas enclosure upstream a mist filter.

Figure 2 shows an example according to the prior art with a quench tower followed by a packed bed scrubber and mist filter.

Figure 3 shows an example of the present invention with a spray nozzle positioned in the process gas enclosure, followed by a packed bed scrubber and mist filter.

Figure 4 shows an example of the spray nozzle system including lance and protection sleeve of the present invention.

In Figure 1 a process gas (102), containing SO, is directed to a mist filter vessel (160) via a process gas enclosure (104). Inside this process gas enclosure, one or more noz- zles (106) are arranged to spray an aqueous H»>O» solution (108) into the process gas to produce a fine droplet dispersion.

If droplets are small water may evaporate and cool the process gas, and H>O; will react with SO» to form an H>SO4 aerosol.

The process gas and H>SO4 aerosol will enter the mist filter vessel (160) and pass through a num- ber of filter candles (162), in which the H2SO4 aerosol is captured, flow to the bottom of the filter vessel by gravity and drain out of the bottom outlet (166). The aerosol free process gas leaves the mist filter vessel (160) via the outlet enclosure (164). The pro- cess gas can then be further processed or be emitted to the atmosphere via a stack.

DK 2021 00694 A1 13 In Figure 2 a process gas, containing SO; (202) is directed, from an upstream unit to a quench tower (220) via a process gas enclosure (204). The process gas is contacted with a shower of dilute H2>SO, sprayed into the quench tower by means of a number of liquid spray nozzles (222). The majority of the dilute H2SO4 will fall into the sump (231) of the quench tower (220), flow out of the bottom outlet line (232) and be mixed with water from water feed line (210a), before the mixture (230) is passed to the circulation pump (228). The pump outlet line (226) is split into a bleed line (234) of dilute sulfuric acid and the main line (224) to the spray nozzles (222). In the quench tower, the pro- cess gas is cooled and some of the H>0, in the dilute H2SO4 react with the SO» to form H2SO4. The cooled process gas pass through an interconnecting enclosure (238), op- tionally with a droplet separator installed, and enters the packed bed scrubber tower (240). In the scrubber tower, dilute H.SO4 containing H20:> (246) is circulated as dilute H2S0O4 is withdrawn from the bottom of the scrubber tower (240) via the outlet line (254). The dilute H>S0O4 is then optionally mixed with water from the water feed line (210b) and the mixture (252) is passed to the scrubber circulation pump (250). The out- let stream (248) from the circulation pump (250) is mixed with a H202 solution from the feed line (212) and the H>0» containing H2S0O4 is directed to the top of the packing in the scrubber (242) and distributed over the entire packing area by means of a nozzle arrangement or trough distributor system (244).

The surplus liquid produced in the scrubber flows by gravity to the sump (231) of the quench tower (220) via the overflow line (236). The process gas from the quench tower passes through the bed of packing material (242) with countercurrent flow of dilute H2SO4 containing HO», the H202 will react with SO, to form H>S04. The process gas then passes through an optional demister pad (256) before it exits the scrubber tower outlet nozzle. The cleaned process gas is then passed to the mist filter vessel (260) via process gas enclosure (258). The candle filters (262) in the mist filter will separate droplets and aerosols, which drain by gravity and leave the filter vessel via the liquid outlet line (266). The cleaned process gas leaves the filter vessel via the outlet duct (264) for further procession or emission to the atmos- phere via a stack. In Figure 3 process gas, containing SO» (302), is directed from the upstream unit to the packed bed scrubber tower (340) via the process gas enclosure (304). At a given

DK 2021 00694 A1 14 position in the enclosure, an atomization nozzle (306) sprays an amount of a first aque- ous solution containing HO: into the process gas.

A stream of concentrated H,O; solu- tion (312) is mixed with water (310) and the diluted H>O- solution (308) is led to the lig- uid inlet of the injection lance (313) on which the atomization nozzle (306) is mounted.

A stream of pressurized gas (314) may be led to the gas side of the injection lance (313). The process gas and partly evaporated HO; solution enter the scrubber tower (340) and are directed to the packed bed layer (342), where the gas is contacted with an amount of a second aqueous solution containing H>0, flowing countercurrent to the process gas.

The second aqueous solution containing H>0> which here comprises H>S0O4, is supplied via the dilute acid distributor system (344), which e.g. can be a num- ber of liquid nozzles or trough type distributors.

Dilute H>SO4 is withdrawn from the bottom of the scrubber tower (340) via the outlet line (354) and passed to the acid circulation pump (350) to increase pressure of the lig- uid.

The outlet line from the pump (348) is split into a dilute acid bleed line (334) and the feeding line (346) to the acid distributor system (344). After passage through the packed bed (342), the process gas pass through an optional demister (356) before the gas via the process gas enclosure (358) is directed to the mist filter vessel (360). Here candle type filters (362) separate the droplets and aero- sols from the gas phase and the liquid phase drain by gravity to the bottom of the mist filter vessel and is withdrawn via the outlet line (366), but other types of mist control, such as electrostatic precipitators, may also be employed.

The cleaned process gas leaves via the gas outlet duct (364) for further processing or emission to the atmos- phere via a stack.

In Figure 4 a detail of the process gas enclosure is shown, corresponding to the nozzle (306) in the process gas enclosure (304) of Figure 3, with focus on an injection lance (413) and the atomization nozzle (406) is found.

The lance is inserted into the enclo- sure via the connecting nozzle (476), which here is flanged at the end.

In the nozzle (406), the diluted HO» solution (408) is atomized into a fine droplet distri- bution by means of a stream of pressurized gas supplied via the gas line (414). The di- luted HO» solution (408) is formed by mixing a stream of concentrated H>O» solution (412) with a stream of water (410).

DK 2021 00694 A1 15 To protect the lance (413) and nozzle (406) from the corrosive process gas in the en- closure, the lance and nozzle is surrounded by an acid resistant protection sleeve (472). Between the inner of the protection sleeve (472) and the outer of the injection lance (413) and nozzle (406), there is a flow of shielding air, supplied via the air line (474). Optionally, this air line (474) can be mounted on the spool piece (478), separat- ing the lance from the protection sleeve. A bushing limiting the air flow between the in- jection lance and the protection sleeve may be provided and configured to ensure a controlled flow rate of purge air. Example 1 In Example 1, reported in Table 1, the process layout described in Figure 3 is com- pared to the process layout described in Figure 2 regarding overall SO conversion effi- ciency and robustness.

Table 1 shows SO, conversion and other parameters for 5 cases. For Figure 2, it is as- sumed that the quench tower utilizes the carry-over H,O, from the packed bed scrub- ber, to provide a minor SO» pre-conversion in the quench tower. The design case and the same configuration, in a situation in which the packed bed scrubber is not operating optimally, e.g. due to liquid maldistribution and/or gas channeling in the packing mate- rial and resulting in a lower SO» conversion of 90% are reported. It is seen that the re- sult is an increase of emission by a factor 4.55.

For the layout of Figure 3, three further cases are presented. A design case and a mal- distribution case similar to those for the layout of Figure 2, as well as a high conversion case. For the two first cases, as the conversion in the packed bed is reduced, the pack- ing height may also be reduced, with significant savings. For the maldistribution case, the large amount of SO. which was removed in the first stage, means an increase of emission by a factor of 2.5 and not 4.55 — so the impact is close to half, compared to the configuration of Figure 2. In the high conversion case, the same packing height as in the configuration of Figure 2 is used and the benefit of lower emissions is realized. Comparing the scenarios of Example 1, it is seen that the high degree of pre-conver- sion will allow for a smaller and thus cheaper packed bed, while maintaining an overall SO, removal efficiency of 98 %, while obtaining a more robust operation, or alterna- tively that the pre-conversion of the layout of Figure 3 can provide extremely low SO» emissions, not achievable with the layout of Figure 2.

DK 2021 00694 A1 16 Table 1 SO, conversion in 97.8 % 90 % 90 % 80 % 98 % am mej | | |||

DK 2021 00694 A1 17 Example 2 Example 2 describes the benefits of the configuration of Figure 3 over the configuration of Figure 1 and Figure 2, on H>O; utilization and extent of HO, emissions to the atmos- phere.

The HO; utilization is calculated as the degree of H>0> supplied compared to what has reacted with the SO, in the process gas.

The extent of HO, emissions is calculated as the relative concentration of H>0, in the liquid phase closest to the outlet of the scrubber system, i.e. the top of the packed bed in the scrubber and in the mist filter of the single step nozzle solution (see figure 1, 2 and 3). To achieve 98% SO, conversion, requires that a slight excess of HO» is added to the system for the Figure 2 configuration and a larger excess for the layout of Figure 3. The excess H>0» is drained out of the system with the dilute acid product, evaporated and emitted via the cleaned off-gas.

For simplicity no decomposition of H>0» into H20 and O2 in the scrubbing units have been assumed, but inclusion of such decomposition would increase the required H20/SO, ratios in a similar manner in the three cases.

The highest H>O, concentration in the layout of Figure 2 is found in the dilute acid di- rected to the top of the packed bed in scrubber, i.e. closest to the gas outlet of the sys- tem.

The H>O. concentration is still rather low due to a high degree of dilution with the circulating acid, the index concentration is defined as 100. For comparison, the performance of the layout of Figure 3 is shown, where the same SO» conversion has been chosen for easy comparison.

The notable differences is in the H20->/SO, ratios and the HO» concentration closest to the process gas outlet, which is only 5% of the of the configuration of Figure 2 and thus the extent of evapora- tion of H>0, is decreased by a factor of 20. The H>0> concentration at the outlet of the scrubber system as described in Figure 3 is dependent on whether the HO; in the droplets from the SO. pre-scrubber travel upwards with the process gas and react with SO, before the droplets travel to the mist filter or are collected on the packing or if the droplets are so big that they will fall into the sump of the scrubber tower and will be transported to the top of the packed bed by means of the circulation pump.

Both mech- anisms will be in force, but the contribution from each mechanism will depend on the

DK 2021 00694 A1 18 droplet size distribution from the atomization nozzle; small droplets will follow the gas and larger droplets will fall out of the gas phase and become a part of the sump liquid, and the H20» concentration index may thus be in the range 2-10 of the solution of Fig- ure 2.

For the Figure 1 layout, i.e. the single step nozzle solution, it is necessary to add a higher H>0 surplus to obtain the high SO» conversion. This again reduces the HO» utilization, which drops to 82%. The highest H>0> concentration is found in the mist fil- ter of the solution and as there is a high H202 surplus and no circulating acid to dilute the H20,, the relative H20> concentration becomes 275 and thus the extent of H202 evaporation and emissions to the atmosphere is significantly increased. Some SO» oxi- dation in the mist filter is foreseen as the filter provide surface for intimate contact be- tween the aqueous H>O» solution and the process gas comprising SO».

DK 2021 00694 A1 19 Table 2 SO,» concentration at inlet 500 ppmv 500 ppmv 500 ppmv (102,202,302) H>0> concentration at injection 600 ppmv 50 ppmv 495 ppmv (1086, 222, 306) SO» concentration before 50 ppmv 450 ppmv 50 ppmv packed bed (138,238,338) H>0> concentration before 150 ppmv <10 ppmv 45 ppmv packed bed (138,238,338) SO» concentration at outlet 10 ppmv 10 ppmv 10 ppmv (164,264,364) H>0> concentration at outlet 55 ppmv 20 ppmv 1 ppmv (164,264,364) H>0> concentration index close 275 100 5 to outlet

Claims (15)

DK 2021 00694 A1 20 Claims

1. A process for conversion of SO, in a process gas to H>SO4, comprising a first step of spraying a first aqueous stream comprising H>0» directly into the pro- cess gas enclosure for a first conversion of SO, to provide a first converted pro- cess gas, and a second step of directing said first converted process gas and a second aqueous stream comprising H>0» to a scrubber tower to provide a fur- ther converted process gas, wherein the concentration of H20, of said first aqueous stream is higher than the concentration of H>0. of said second aque- ous stream.

2. A process according to claim 1 in which the first aqueous stream comprises at least 0.5 %w/w H>0» or at least 1 %w/w H>0, and less than 10 %w/w HO, or less than 5 %w/w H20.

3. A process according to claim 1 or 2, in which the second aqueous stream com- prises at least 0.01 %w/w H>O- or at least 0.05 %w/w HO; and less than 1 %w/w H>0, or less than 0.5 %w/w H20.

4. A process according to claim 1, 2 or 3, in which the first aqueous stream is made by mixing from an aqueous stream, such as pure water with a concen- trated H>0» solution, preferably with a concentration of 20-50 %w/w H>0..

5. A process according to claim 1, 2, 3 or 4 in which the first aqueous stream is added as droplets having a D90% diameter below 200 um, such as by use of an atomizing nozzle, preferably by use of twin-fluid nozzle(s), using a pressur- ized atomizing medium.

6. A process according to claim 5, in which the pressurized atomization medium is air or steam at the feed pressure of 2-10 bar.

7. A process according to claim 5 or 6, in which the volume flow of atomization medium is less than 2 % of the process gas volume flow, preferably less than 1 % of the process gas volume flow.

8. A process according to claim 1, 2, 3, 4, 5, 6 or 7, in which the process gas resi- dence time from the spraying the first aqueous stream comprising HO to the entry of the scrubber tower is at least 0.2 seconds or at least 0.5 seconds, and less than 3 seconds or less than 1 second.

9. A process according to claim 1, 2, 3, 4, 5, 6, 7 or 8, in which the temperature of the process gas prior to spraying the first aqueous stream comprising H-O- is at

DK 2021 00694 A1 21 least 80°C and less than 120 °C and the temperature of the first converted pro- cess gas entering scrubber tower is at least 45°C and less than 75 °C.

10. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, including the step of col- lecting droplets in the process gas leaving the first stage and directing them as the second aqueous stream comprising H>0» optionally in combination with lig- uid collected from the scrubber tower.

11. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, in which the conver- sion of SO, from spraying the first aqueous stream comprising HO: to the entry of the scrubber tower is at least 70%, preferably at least 80%

12. A process according to claim 1, 2, 3, 4, 5,6, 7, 8, 9, 10 or 11, in which the over- all degree of conversion of SO; is at least 96%, preferably at least 98%.

13. A process according to claim 1, 2, 3, 4, 5,6, 7, 8, 9, 10, 11 or 12, in which the scrubber tower is a packed bed scrubber and in which the second aqueous stream comprising H>O: further comprises at least 20%w/w H>SO4 and less than 60 %w/w H2SO4.

14. A process according to claim 1, 2, 3,4, 5,6, 7, 8, 9, 10, 11, 12 or 13 further comprising the step of directing the further converted process gas to a mist eliminator, such as a Brownian diffusion type candle filter or wet electrostatic precipitator.

15. A process plant for conversion of SO, to H2SO4 comprising a process gas en- closure, a liquid reservoir, a means of recirculation and an atomization nozzle, each having an inlet and an outlet, a scrubber tower having a gas inlet, a gas outlet, a liquid inlet and a liquid outlet, wherein the atomization nozzle outlet is positioned inside the process gas enclosure, said process gas enclosure is con- figured for receiving a process gas, said atomization nozzle inlet is configured for receiving an amount of an aqueous stream comprising H>20. and for releas- ing droplet of said aqueous stream comprising H>0,, the outlet of said process gas enclosure is configured for being in fluid communication with the gas inlet of said scrubber tower and the liquid outlet of said scrubber, wherein the liquid outlet of said scrubber is in fluid communication with said liquid reservoir, wherein the inlet of the means of recirculation is configured for being in fluid communication with said liquid reservoir and the outlet of the means of

DK 2021 00694 A1 22 recirculation is in fluid communication with the liquid inlet of the scrubber tower and is configured for withdrawal of a stream of sulfuric acid.