System and a method for producing aqueous sulphuric acid

A system for producing aqueous sulphuric acid is provided, the system including a first heat exchanger configured to cool aqueous sulphuric for producing cooled aqueous sulphuric acid; a pre-cooling unit comprising an inlet or inlets for receiving the gas containing sulphur trioxide and the cooled aqueous sulphuric acid, an outlet for letting out aqueous sulphuric acid and the gas containing sulphur trioxide, and a first nozzle for spraying the cooled aqueous sulphuric acid onto the gas containing sulphur trioxide. The system further includes a condensation tower comprising a first inlet for receiving the cooled gas containing sulphur trioxide and aqueous sulphuric acid from the pre-cooling unit and means for circulating the aqueous sulphuric acid within the condensation tower by spraying. An associated method and pre-cooling unit suitable for cooling gas comprising sulphur trioxide from at least 400° C. to at most 150° C. are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Application, filed under 35 U.S.C. § 371, of International Application No. PCT/FI2017/050579, filed Aug. 17, 2017, which claims priority to Finnish Application No. 20165665, filed Sep. 7, 2016; the contents of both of which as are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to methods and systems for producing sulphuric acid. The invention relates to methods and systems for producing aqueous sulphuric acid. The invention relates to methods and systems for producing sulphuric acid from sulphur trioxide and water. The invention relates to a pulp mill, which in use and as a side product produces sulphur dioxide, and a system for producing sulphuric acid from the sulphur dioxide.

BACKGROUND

A method for producing sulphuric acid (H2SO4) from gas comprising sulphur dioxide (SO2) is known from the patent SE 510 171. In the method, SO2is catalytically oxidized to sulphur trioxide (SO3). By contacting SO3with water (H2O) or aqueous sulphuric acid, the liquid can be strengthened, i.e. its sulphuric acid concentration can be increased. However, in the catalytic oxidization process, the temperature of SO3is high, typically over 450° C. Moreover both SO3and H2SO4are chemically extremely active. Therefore, the materials used in the equipment need to be both heat and corrosion resistant. Such materials are very expensive. Since resources are typically limited, this limits the size of the equipment, whereby, for given resources, the H2SO4production capacity may remain less than desired.

Moreover, the corrosive nature of sulphuric acid depends on its strength. Sulphuric acid is extremely corrosive in the strength range from about 20 w-% to 85 w-%. However, when the strength is even higher, such as 93 w-% or more, the corrosive nature of H2SO4is less harsh. This is one reason, why conventional production plants include a strengthening tower configured to strengthen the sulphuric acid to a strength of at least 93 w-%. However, if materials are selected to withstand only substantially pure H2SO4, the process needs to be run in such a way that weaker sulphuric acid is not produced. Otherwise corrosion problems would occur, which could lead to leakage of strong H2SO4posing health and environmental problems. A process may be hard to run in such a way, whereby such a process is considerably risky. Moreover, in many chemical processes involving sulphuric acid, only weaker sulphuric acid (i.e. aqueous sulphuric acid) is needed, whereby such a strengthening tower is not needed, provided that the system is configured to handle aqueous sulphuric acid. Due to the corrosive nature of aqueous sulphuric acid, suitable materials for the known systems are expensive.

Furthermore, even if corrosion resistant materials are used, the material wear or corrode to some extent during use. Thus, such systems may run to leakages and/or there may be a constant need for maintenance. Maintenance increases the operating costs, and leakages pose safety and environmental risks.

SUMMARY

A method for producing aqueous sulphuric acid is disclosed. The method allows for using less expensive materials, which are still durable in use. A corresponding system is also disclosed. In this way, the safety and environment risks can be reduced, while keeping the investment costs low. In addition, the equipment is configured in such a way that corroding parts can be easily replaced. In addition, a certain part of the system can be configured to take in most corrosion, and such sacrificial parts may also be easily replaceable.

In the method and the system, the gas containing SO3is pre-cooled in a pre-cooling unit before it is introduced in a condensation tower, wherein sulphuric acid H2SO4(and/or HSO4−and/or SO42−) is produced. Because of the pre-cooling, the temperature of the condensation tower can be retained at a low level, whereby cheaper materials can be used therein. Moreover, the pre-cooling unit, which still requires heat and corrosion resistant materials, can be made much smaller than the condensation tower, in effect keeping the overall material costs low. Furthermore, when applied in connection with a pulp mill releasing some gas containing SO2, the remaining SO2can be recovered in a scrubber as a reaction product usable in the pulp mill. The invention is more specifically disclosed in the claims.

DETAILED DESCRIPTION

FIG. 1ashows an embodiment of a system100for producing aqueous sulphuric acid. In the figures, Sz denotes a vertical direction. Both the directions Sx and Sy are perpendicular to each other and horizontal. The system100is connected to a source configured to produce some gas comprising sulphur trioxide SO3. An inlet212for this gas is shown in the figure. The system100is configured to receive [i] water H2O and [ii] the gas comprising sulphur trioxide SO3. The water that is received may be in the form of substantially pure water or in the form of aqueous sulphuric acid comprising water. Furthermore, the system100is configured to react the water H2O with the sulphur trioxide SO3to produce sulphuric acid H2SO4. In particular the system is configured to produce aqueous sulphuric acid.

In this description, the term “aqueous sulphuric acid” refers to an aqueous solution of sulphuric acid, in which the content of sulphuric acid is at most 80 w-%. As discussed below, in a typical process, the strength of the aqueous sulphuric acid is at most 70 w-%. At a given temperature, the strength of aqueous sulphuric acid correlates with its density and pH, which can be used as evidence on the strength of the aqueous sulphuric acid. In alternative terms, the term “aqueous sulphuric acid” refers to a liquid that can be made by mixing only water H2O and sulphuric acid H2SO4, wherein the weight percentage of H2SO4is at most 80 or at most 70. As known to a skilled person the aqueous solution of H2SO4will result in the following reactions:
H2SO4+H2O⇔H3O++HSO4−
and
HSO4−+H2O⇔H3O++SO42−

In both these reactions, the balance is strongly to the right side. Throughout this description, the terms “aqueous sulphuric acid” and “aqueous H2SO4” refer to an aqueous solution comprising at least H2O, H3O+, and HSO4−or SO42−. The aqueous sulphuric acid may be free from H2SO4or HSO4−or SO42−as indicated by the reactions above.

Purposes of producing only aqueous sulphuric acid are twofold. First, when sulphuric acid is only needed in a process that only needs aqueous sulphuric acid, the aqueous sulphuric acid can be produced in a resource efficient manner compared to producing substantially pure H2SO4and diluting it in the process. Second, when strong sulphuric acid is not needed, also a strengthening tower is not needed. This helps to keep the investment costs low.

In the method and system, sulphuric acid is produced in an exothermic reaction of SO3with water. The water may be contained in aqueous sulphuric acid. The reaction can be formally written as
SO3+H2O⇔H2SO4+heat.

With reference toFIG. 1a, the system100comprises a pre-cooling unit200configured to pre-cool the gas containing sulphur trioxide SO3. The pre-cooling unit200comprises a first inlet212for receiving the gas containing sulphur trioxide, a second inlet214for receiving cooled aqueous sulphuric acid or cooled water (i.e. water that is free from HSO4−or SO42−). The pre-cooling unit200comprises a first nozzle220configured to spray the cooled aqueous sulphuric acid and/or the cooled water onto the gas containing sulphur trioxide to cool the gas containing sulphur trioxide. Simultaneously the water reacts with SO3as indicated above. When the water reacts with sulphur trioxide, some of the sulphur trioxide forms sulphuric acid. In this way, the aqueous sulphuric acid strengthens to some extent in the pre-cooling unit200; or some of the water is converted to aqueous sulphuric acid in the pre-cooling unit200. The pre-cooling unit200comprises also an outlet216for letting out the aqueous sulphuric acid and the gas comprising sulphur trioxide.

With reference toFIG. 1ain a corresponding method, the gas containing sulphur trioxide is received such that at the first inlet212, the temperature of the gas containing sulphur trioxide is at least 300° C. Typically, SO3is produced in a catalytic oxidizing process, wherein the temperature may rise above 450° C., such as from 460° C. to 480° C. However, the gas may cool while conveyed to the pre-cooling unit200. Typically the gas comprising sulphur trioxide, when received in the pre-cooling unit200(i.e. at the inlet212), comprises from 0.1 vol-% to 3 vol-% such as from 0.2 vol-% to 2 vol-% sulphur trioxide. As is evident, when SO3reacts with water, the SO3content decreases. Thus, the processed gas, denoted by “flue” inFIG. 1a, may be essentially free from SO3. In case the gas containing sulphur trioxide further contains sulphur dioxide, the processed gas (“flue”) may comprise sulphur dioxide.

If needed, the processed gas (i.e. “flue”) can be further washed in a scrubber700e.g. with scrubbing solution including alkali, such as NaOH, to remove essentially all sulphur from the flue (FIG. 5). This may happen e.g. when the flue is released to atmosphere in locations where environmental restrictions are high. This may happen in particular when the gas containing sulphur trioxide further contains sulphur dioxide, as will be discussed in detail below.

In the pre-cooling unit200, the gas containing sulphur trioxide is cooled to a temperature of at most 120° C., preferably at most 100° C., by spraying cooled liquid (e.g. the aqueous sulphuric acid and/or water) onto the gas containing sulphur trioxide. The temperature refers to the temperature of the gas at the outlet216of the pre-cooling unit200. As indicated above, least some of the sulphur trioxide forms sulphuric acid by reacting with water, optionally the water of the aqueous sulphuric acid, thereby producing aqueous sulphuric acid or stronger aqueous sulphuric acid.

The system100further comprises a first heat exchanger410. The first heat exchanger410comprises a first inlet412for hot aqueous sulphuric acid and/or water, a first outlet414for cooled aqueous sulphuric acid and/or cooled water, a second inlet416for coolant (i.e. cooling medium) and an second outlet418for the coolant. The system further comprises a pipeline450configured to convey the cooled aqueous sulphuric acid and/or the cooled water from the first outlet414of the first heat exchanger410to the second inlet214of the pre-cooling unit200. As evident, from the second inlet214of the pre-cooling unit200the aqueous sulphuric acid and/or water is conveyed to the first nozzle220in a pipeline.

When the process starts, and optionally also later on, water H2O is fed into the process for making the aqueous sulphuric acid. InFIG. 2the liquid water is designated as H2O(l) to distinct from steam, i.e. gaseous water, H2O(g). Thus, in the beginning of the process the liquid that is conveyed from the first heat exchanger210to the pre-cooling unit200may be free from sulphuric acid or essentially free from sulphuric acid. However, as the process is driven for some time, the water is reacted to form the aqueous sulphuric acid. Moreover, in the method, the aqueous sulphuric acid from the condensation tower300and optionally some make-up water (H2O(l) e.g. through the valve465) is fed via the heat exchanger410to the pre-cooling unit200. Such make-up water is not needed after the start of the process, provided that the gas containing SO3further contains steam (H2O(g)) to a sufficient amount.

Correspondingly, the method comprises cooling water and/or aqueous sulphuric acid thereby producing cooled water and/or cooled aqueous sulphuric acid. The cooling is done in the first heat exchanger410as indicated above. Preferably the water and/or aqueous sulphuric acid is cooled in such a way that the temperature of the water and/or aqueous sulphuric acid at the first outlet414of the heat exchanger410is at most 80° C.

The system100comprises a condensation tower300. The condensation tower300comprises a wall310or walls310limiting a reaction chamber312for strengthening the aqueous sulphuric acid. The wall310may be an outer wall of the condensation tower300. The condensation tower300comprises an inlet302for receiving the aqueous sulphuric acid and the gas comprising sulphur trioxide from the outlet216of the pre-cooling unit200. As shown in theFIG. 1a, the system100comprises a pipeline290configured to convey the aqueous sulphuric acid and the gas comprising sulphur trioxide from the outlet216of the pre-cooling unit200to the condensation tower300. The condensation tower300comprises means320for circulating the aqueous sulphuric acid within the reaction chamber312by spraying in order to strengthen the aqueous sulphuric acid. The means320may comprise a second nozzle326, a pipeline324configured to convey aqueous sulphuric acid to the second nozzle326, and a pump322configured to pump the aqueous sulphuric acid to the pipeline324. The aqueous sulphuric acid may be pumped e.g. from the bottom of the reaction chamber312.

The system100comprises a pump arrangement (322,402) configured [i] to pump aqueous sulphuric acid from the condensation tower300to the first nozzles220via the first heat exchanger410and [ii] to pump aqueous sulphuric acid from the condensation tower300to the second nozzles326. InFIGS. 1 and 5, a pump322is configured to pump aqueous sulphuric acid from the condensation tower300to the second nozzles326via the pipeline324. In addition, the same pump322is, in the embodiment ofFIGS. 1 and 5, configured to pump aqueous sulphuric acid from the condensation tower300to the first nozzles220via the first heat exchanger410. However, as indicated inFIG. 2, the system may comprise another pump402configured to pump aqueous sulphuric acid from the condensation tower300to the first nozzles220via the first heat exchanger410. Either of the pump arrangements may be used in connection with any embodiment of the invention. The system further comprises a pipeline405configured to convey aqueous sulphuric acid from the condensation tower300to the first heat exchanger410. Correspondingly, an embodiment of method comprises conveying aqueous sulphuric acid from the condensation tower300to the first nozzles220via the first heat exchanger410.

Correspondingly, the method comprises conveying [i] the cooled gas containing sulphur trioxide and [ii] the cooled water and/or aqueous sulphuric acid from the pre-cooling unit200to the condensation tower300. The method further comprises strengthening, in the condensation tower300, the aqueous sulphuric acid by circulating the aqueous sulphuric acid in the condensation tower300and by spraying the aqueous sulphuric acid onto the gas containing sulphur trioxide. In this way, sulphuric acid is produced into the aqueous sulphuric acid. In particular, by selecting the flow velocities within the pre-cooling device200and the condensation tower300properly, e.g. by selecting the cross-sectional areas of the pre-cooling device200and the condensation tower300properly, a large part of the reactions of SO3with H2O may occur in the condensation tower300.

The method has the beneficial effect, that since the temperature of the gas containing sulphur trioxide is cooled before the condensation tower300, the material of the condensation tower300need not be very heat resistant. In particular some plastics may be suitable for the material of the wall(s)310of the condensation tower300. In an embodiment of the system, the wall310or the walls310of the condensation tower300comprise plastic material. Preferably the wall310comprises a part that extends through the wall310in the direction of thickness thereof, and that part consists of plastic material and optionally fibrous reinforcing material. Correspondingly, in an embodiment of the method, gas or gases are only conveyed into the condensation tower300in such a way that the temperature of the gas or gases is at most 120° C. at an inlet of the condensation tower300.

Throughout this description, the term plastic refers to a synthetic or semi-synthetic organic compounds that are malleable and can be molded into solid objects. The term may refer to a material comprising an organic polymer (or comprising organic polymers), which may comprise also other substances. The term may refer to a synthetic material comprising an organic polymer.

As for the plastic material, preferably the wall310comprises weldable plastic material. Weldable materials can be easily joined to other weldable materials by welding. Thus, with such materials, manufacturing the condensation tower300becomes easier. However, the plastic material should be reasonably resistant to heat and corrosion. The plastic material may be heat resistant to at least 130° C. Therefore, preferably the plastic material comprises fluorinated plastic material, even if some other plastic materials are also heat resistant to a sufficient degree. In an embodiment, the wall310or the walls310of the condensation tower300comprises plastic material. In an embodiment, the wall310or the walls310of the condensation tower300comprises fluorinated plastic material. In an embodiment, the wall310or the walls310of the condensation tower300further comprise reinforcing material. In an embodiment, the reinforcing material is a synthetic fibrous reinforcing material. Examples of synthetic fibrous reinforcing material include glass fibres, carbon fibres, para-aramid synthetic fibres (Kevlar®), and/or aramid fibres. Preferably, the synthetic fibrous material is heat resistant to at least 130° C. In an embodiment, the wall310or the walls310of the condensation tower300comprise at least 10 w-% weldable plastic material in addition to fibrous reinforcing material. The weldable plastic material may be heat resistant to at least 130° C. The plastic material may be a fluorinated plastic material.

As indicated inFIGS. 1aand 1b, the system100comprises a pipeline290configured to convey the gas comprising SO3and the aqueous H2SO4from the pre-cooling unit200to the condensation tower300. Since the gas comprising SO3and the aqueous H2SO4have been cooled already before the pipeline290, in an embodiment, also the pipeline290comprises plastic material. What has been said above about the materials of the wall(s)310of the condensation tower300applies, in an embodiment, also to the material of the pipeline290.

Regarding the temperature of the gas comprising SO3entering the condensation tower300, it is pointed out that typically the gas comprising SO3further comprises steam. Therefore, when the temperature of the gas comprising SO3is as low as discussed above, strong sulphuric acid cannot be produced, since the steam of the gas comprising SO3becomes condensed. If stronger acid would be needed, a pre-cooling device could not be used, and the material requirements for a strengthening tower would be significantly stricter, since a strengthening tower needs to receive SO3at a high temperature, in order to avoid condensation of water and/or dilute H2SO4.

In an embodiment of the method, aqueous sulphuric acid is produced in such a way that the sulphuric acid concentration of the aqueous sulphuric acid does not exceed 80 w-%. This allows for selection of simple materials for the condensation tower300. As indicated below, the production of stronger H2SO4from SO3typically requires a higher temperature than indicated above.

Correspondingly, a system is free from such a strengthening tower that would be configured to strengthen the aqueous sulphuric acid to a strength of more than 80 w-%. As indicated above, since the present invention solves problems related to corrosion, there is typically no need for a strengthening tower. Omitting such a tower decreases the investment costs, because, as indicated above, plastic materials cannot be used in a strengthening tower because of the high temperatures involved with strengthening.

As evidenced byFIG. 1a, the concentration of H2SO4can be controlled by the rate of water H2O(l) fed to the process and by the rate of aqueous sulphuric acid (H2O+H2SO4) taken out from the process. The rate at which sodium trioxide SO3is fed to the process and the steam concentration of the gas containing SO3affect the rates of water and aqueous sulphuric acid. In an embodiment, water H2O(l) is fed to the process and aqueous sulphuric acid H2SO4is taken out from the process in such a way that the sulphuric acid concentration of the aqueous sulphuric acid does not exceed 80 w-%.

For controlling the strength of the aqueous sulphuric acid H2SO4, the system100may comprise a first regulator460, such as a valve460, configured to regulate the flow of aqueous sulphuric acid H2SO4from the process. Referring toFIG. 1a, the first regulator460may be configured to regulate the flow of aqueous sulphuric acid H2SO4out from the system100. Referring toFIG. 2, the first regulator460may be configured to regulate the flow of aqueous sulphuric acid H2SO4within the system, e.g. to the pulp mill600of the system. Referring toFIG. 1a, the first regulator460may be configured to regulate the flow of such aqueous sulphuric acid H2SO4that is taken out from the condensation tower300without returning it directly back to the condensation tower300or the pre-heating unit200.

For controlling the strength of the aqueous sulphuric acid H2SO4, the system100may comprise a second regulator465, such as a valve465, configured to regulate the flow of water into the process. Referring toFIG. 1a, the second regulator465may be configured to regulate the flow of additional water into the pre-cooling unit200. In the alternative or in addition, the water may be fed into the condensation tower300, whereby the second regulator465may be configured to regulate the flow of additional water into the condensation tower300, as indicated by a dotted line inFIG. 1a. Depending on the temperature of the added water, it may be beneficial to feed the water before the first exchanger410(as indicated inFIG. 1a), whereby also the water will be cooled; and/or to feed water (i.e. the water or some other water at some other temperature) after the first exchanger410(not shown inFIG. 1a), whereby that part of the water would not be cooled in the first heat exchanger410.

In addition, the system100may comprise a sensor470configured to give information indicative of the strength of the aqueous sulphuric acid. The sensor470may be configured to measure at least one of electrical conductivity, electrical resistivity, density (i.e. specific mass), and molarity (e.g. by titration), of the aqueous sulphuric acid. As known, the electrical resistivity (and conductivity) correlates with the pH. The system may comprise a processing unit475configured to receive the information indicative of the strength of the aqueous sulphuric acid from the sensor470, and by using this information control at least one of the first regulator460and the second regulator465.

In a typical process, the gas containing SO3further contains steam to such an amount that water needs not to be added to process after the process has started. Moreover, in such a case the molar amount of aqueous sulphuric acid that is removed from the process is substantially the same as the molar amount of condensed steam. Typically, the steam to SO3ratio of the gas containing SO3is such that the final strength of the aqueous sulphuric acid is in the range of from 60 w-% to 75 w-%, when no additional water is fed to the process e.g. through the second valve465after the process has started. As indicated above, the strength may be controlled (i.e. decreased) by feeding some water or steam to the process.

In an embodiment, the system comprises a sensor configured to give information on the surface level of the aqueous sulphuric acid within the condensation tower300. Provided that the surface level rises above a limit, aqueous sulphuric acid may be removed from the process. Aqueous sulphuric acid may be removed from the process to such an amount that the surface level of the aqueous sulphuric acid within the condensation tower300lowers below the limit or another limit. In a corresponding embodiment of the method, water is fed to the condensation tower300only initially, when the process is started.

Because the reaction of SO3with H2O is exothermic, if no further cooling was done, the aqueous sulphuric acid within the condensation tower300would heat up. This could pose problems related to safety, when plastic materials are used in the condensation tower300.

Therefore, an embodiment of the system100comprises a second heat exchanger420. The second heat exchanger420comprises a first inlet422for hot aqueous sulphuric acid, a first outlet424for cooled aqueous sulphuric acid, a second inlet426for coolant (i.e. cooling medium) and an second outlet428for the coolant. In the embodiment, the pump322is pump configured to pump aqueous sulphuric acid to the first inlet422of the second heat exchanger420. Moreover, the pipeline324is configured to convey cooled aqueous sulphuric acid from the first outlet424of the second heat exchanger420to the second nozzle326; and a pipeline430is configured to convey aqueous sulphuric acid from the reaction chamber312of the condensation tower300to the first inlet422of the second heat exchanger420. When present, the second heat exchanger420can be considered to be a part of the means320for circulating the aqueous sulphuric acid within the reaction chamber312by spraying in order to strengthen the aqueous sulphuric acid.

A corresponding embodiment of the method comprises cooling the aqueous sulphuric acid before spraying it onto the gas containing sulphur trioxide in the condensation tower300. In particular, a corresponding embodiment of the method comprises cooling the aqueous sulphuric acid in between [i] taking the aqueous sulphuric acid out from the condensation tower300and [ii] spraying it onto the gas containing sulphur trioxide in the condensation tower300. As indicated above, the aqueous sulphuric acid is cooled in the second heat exchanger420and conveyed through the pipelines430and324from the condensation tower300to the second nozzle326.

To prevent droplets of aqueous sulphuric acid from escaping from the condensation tower300, in an embodiment, the condensation tower300comprises a first droplet separator330. The first droplet separator is arranged above the reaction chamber312or in the upper part of the reaction chamber312of the condensation tower300. The droplet separator330is configured to arrest some droplets of the aqueous sulphuric acid sprayed through the second nozzles326, in order to prevent these from flowing with the flue gas out of the condensation tower300.

Referring toFIG. 2, in an embodiment, the gas comprising sulphur trioxide SO3is produced in a converter500configured [i] to receive some gas containing at least some sulphur dioxide SO2and [ii] to convert (i.e. oxidize) at least some of the SO2to SO3by reacting SO2with oxygen O2. Formally the oxidization reaction can be written as
SO2+½O2⇔SO3+heat

The oxidization reaction is preferably catalysed using a catalyst, such as vanadium pentoxide V2O5and/or platinum Pt.

The converter500comprises an inlet arrangement502for receiving sulphur dioxide SO2and oxygen O2. The inlet arrangement502may comprise a first inlet for receiving gas comprising SO2but free from O2and a second inlet for receiving other gas comprising O2. The inlet arrangement502may comprise only a first inlet for receiving gas comprising SO2and O2. The inlet arrangement502may comprise a first inlet for receiving gas comprising SO2(optionally comprising also O2) and a second inlet for receiving other gas comprising O2(optionally free from SO2). The converter500comprises an outlet504for gas containing sulphur trioxide SO3. The converter500comprises catalyst510, such as V2O5and/or Pt, for converting at least some of the sulphur dioxide SO2and the oxygen O2to sulphur trioxide SO3. The system100comprises a pipeline440configured to convey gas containing sulphur trioxide SO3from the outlet504of the converter500to first inlet212of the pre-cooling unit200.

The corresponding method comprises receiving sulphur dioxide SO2and oxygen O2, both of them contained in some gas or gases. The method comprises catalytically oxidizing at least some of the sulphur dioxide SO2to sulphur trioxide SO3in the converter500, thereby producing the gas containing sulphur trioxide SO3. The method comprises conveying the gas containing sulphur trioxide SO3to the pre-cooling unit200. An embodiment comprises using V2O5and/or Pt to catalytically oxidize SO2to SO3.

The oxidization reaction is somewhat sensitive to temperature. Typical catalysts perform well at temperature above 400° C. However, the reaction favours formation of SO3at low temperatures. Therefore, in an embodiment of the method, a reaction temperature in the converter500is adjusted to be from 350° C. to 480° C., such as from 400° C. to 480° C. The reaction temperature refers to the temperature within the converter500at a point that is in the middle between the inlet arrangement502and the outlet504along the flow path of the gas within the converter500. The temperature in the converter500or of the converter500may be adjusted by using steam H2O(g) from a boiler610. Preferably, some steam from the boiler610is fed to the converter500for adjusting the temperature within the converter500. As indicated inFIG. 2, the steam may be mixed with the gas comprising SO2before they are fed to the converter500. Preferably, steam is added to the gas comprising SO2in such a way that the temperature of the gas comprising SO2is 400° C. (given with one or two significant digits), such as from 370° C. to 430° C., at the inlet arrangement502of the converter500. As indicated above, using excess steam decreases the strength of H2SO4obtainable by the process. Thus, if used, the temperature of the steam should be high. In addition or alternatively, other means (e.g. a heater, such as a burner) may be used to control the temperature.

The balance of the oxidization reaction depends also on the ratio of oxygen to sulphur dioxide. Typically the molar ratio of O2to SO2is more than 3, such as at least 5.

A corresponding system100comprises means for adjusting the temperature of the converter500. Such means may comprise a heater configured to heat the gas containing SO2and/or the converter500. A corresponding system100may comprise a boiler610configured to produce steam and a pipeline612for conveying the steam and from bringing the steam in contact with the gas containing SO2. The system100may comprise a control unit614configured to control the amount of steam in such a way that the reaction temperature in the converter500is within the aforementioned limits.

Because the oxidization reaction is exothermic, the temperature of the gas rises in the converter500. Typically, the temperature of the gas at the outlet504of the converter500is from 460° C. to 480° C.

Even if the catalyst, the process temperature, and the molar ratio are suitably selected, the conversion efficiency of the converter500is not necessarily 100%. Typically, the conversion efficiency is at least 90%, i.e. at least 90 v-% of SO2is converted to SO3. Even more typically, the conversion efficiency is in the range of from 93% to 98%. Therefore, the gas indicated by “Flue” inFIG. 2may contain e.g. at most 3000 ppm SO2, or more typically at most 1500 ppm SO2. The flue may be scrubbed with a scrubber700, as will be discussed in more detail below in connection withFIG. 5.

In an embodiment, circulation of the gas comprising SO3is enhanced with a pump or pumps480a,480b(seeFIG. 5). A pump480amay be arranged upstream from the pre-cooling unit200, such as upstream from the catalytic reactor500. Such a pump480ais arranged to increase the pressure of the gas containing SO3and/or the gas containing SO2, and thereby drive the gas towards the condensation tower300. A pump480bmay be arranged downstream from the condensation tower300, such as downstream from the scrubber700. Such a pump is arranged to decrease the pressure of the flue, and thereby suck the gas containing SO3and/or the gas containing SO2towards the condensation tower300.

When the pump (480aor480b) or the pumps (480aand480b) is/are used, one does not need to use additional carrier gas for conveying the gas containing SO2or SO3. Therefore, the obtainable strength of the aqueous sulphuric acid is reasonably high. For example, in case medium-pressure steam was used to carry the gas containing SO2or SO3, the steam would dilute the aqueous sulphuric acid.

Examples of suitable pumps and pump configurations in such systems are disclosed e.g. in the patent WO2010/019079. The document discloses liquid ring pumps (1aand1btherein), equivalent to the pumps480a,480bdiscussed above. Such pump solutions are incorporated by reference to the present embodiments.

In an embodiment, the system100further comprises a pulp mill600. A pulp mill600refers to an arrangement that converts wood chips or other plant fibre source into fibre boards. The fibre boards can be shipped to a paper mill for further processing. The pulp mill600may be a Kraft mill (i.e. a sulphate mill) or a sulphite mill. In an embodiment, the pulp mill600is a sulphate pulp mill. The aforementioned boiler610may be a part of the pulp mill600such as the sulphate pulp mill600.

As a side product, the pulp mill600is configured to produce at least some gas containing sulphur dioxide SO2. The system100comprises a pipeline602configured to convey the gas containing sulphur dioxide from the pulp mill600to the inlet arrangement502of the converter500. In such a system, the benefits are three-fold. First the sulphur dioxide, which has a pungent odour, can be safely removed from the pulp mill600, thus reducing environmental disadvantages. Second, by using SO2, aqueous sulphuric acid can be made. Third, since the pulp mill600requires some dilute sulphuric acid for operation, the produced aqueous sulphuric acid can be utilized in the pulp mill600. To this end, an embodiment of the system100comprises a pipeline604for conveying the aqueous sulphuric acid from the condensation tower300to the pulp mill600.

The corresponding method comprises separating some gas containing sulphur dioxide from a liquor circulation of a pulp mill600and conveying the gas containing sulphur dioxide to the converter500. For the details of the liquor circulation of the sulphate pulp mill, see the patent SE 510 171. An embodiment comprises conveying at least some of the produced aqueous sulphuric acid to a pulp mill. An embodiment comprises conveying at least some of the produced aqueous sulphuric acid to the same pulp mill600from which the gas containing SO2, which is converted to SO3in the converter500and to H2SO4in the condensation tower300, is received in to the converter500.

As indicated above, the temperature in the pre-cooling unit200is high. Moreover, both SO3and H2SO4are very corrosive. Therefore, the corrosion and heat resistance requirements of the materials of the pre-cooling unit200are high. Thus, the materials of the pre-cooling unit200are expensive. For this reason alone, the pre-cooling unit200should be relatively small, i.e. smaller than the condensation tower300. Moreover, when the pre-cooling unit200is small in comparison to the condensation tower300, the formation of H2SO4can be shifted from the pre-cooling unit200mainly to the condensation tower300. This helps the selection of the materials for the pre-cooling unit200, as less H2SO4will be produced therein.

For these reasons and with reference toFIG. 1b, in an embodiment, the pre-cooling unit200has a first cross sectional area A200on a plane having a surface normal that is parallel to the direction of flow of gases within the pre-cooling unit200. The cross sectional area A200refers to the area in the aforementioned plane limited by such wall(s) of the pre-cooling unit200that limit the flow of the gas comprising SO3. Correspondingly, the first cross sectional area A200refers to the area of the flow channel of the pre-cooling unit200, in which the gas comprising SO3is configured to flow. Within the pre-cooling unit200, the direction of flow of gases is parallel to the direction of the flow velocity v1of the gas containing SO3in the pre-cooling unit200. The velocity v1shown inFIG. 1aby the corresponding arrow.

The condensation tower300has a second cross sectional area A300on a plane having a surface normal that is parallel to the direction of flow of gases within the condensation tower300. The second cross sectional area A300refers to the area in the aforementioned plane limited by such wall(s) of the condensation tower300that limit the flow of the gas comprising SO3. Such wall may be the walls310, or the flow may be further limit by additional walls limiting the reaction chamber312. Correspondingly, the second cross sectional area A300refers to the area of the flow channel of the condensation tower300, in which the gas comprising SO3is configured to flow. Within the condensation tower300, the direction of flow of gases is parallel to the direction of the flow velocity v2of the gas containing SO3in the condensation tower300. The velocity v2shown inFIG. 1aby the corresponding arrow.

To have a lot of H2SO4being produced in the condensation tower300, the flow velocity of the gas comprising sulphur trioxide should be larger in the pre-cooling unit200than in the condensation tower300. Therefore, in an embodiment, the second cross sectional area A300is greater than the first cross sectional area A200(i.e. A300>A200). In an embodiment, the second cross sectional area A300is at least two times or at least three times the first cross sectional A200area (i.e. A300≥2×A200or A300≥3×A200). As indicated inFIG. 1b, the cross sections may be circular. In an embodiment, the diameter of the cross section of the pre-cooling unit200is from 0.3 m to 2.4 m. In an embodiment, the diameter of the cross section of the condensation tower300is at least 1.5 m, such as from 1.5 m to 8 m. In case the pre-cooling unit200is not circular, the aforementioned values apply to an effective diameter 2×√(A200/π). In case the condensation tower300is not circular, the aforementioned values apply to an effective diameter 2×√(A300/π).

When using such a system100, i.e. in an embodiment of a method, the gas comprising sulphur trioxide has a first flow velocity v1(seeFIG. 1a) in the pre-cooling unit200and the gas comprising sulphur trioxide has a second flow velocity v2(seeFIG. 1a) in the condensation tower300. In an embodiment the magnitude of the second flow velocity v2is less than the magnitude of the first flow velocity v1(i.e. v2<v1). Preferably, the magnitude of the second flow velocity is at most half or at most one third of the magnitude of the first flow velocity (i.e. v2≤v1/2 or v2≤v1/3). The term magnitude is used, because velocities in general are vectors.

Referring toFIG. 3, because of the corrosive conditions within the pre-cooling unit200, preferably at least part of the pre-cooling unit200of the system100is changeable. For example, the whole pre-cooling unit200may be connected to the pipeline290with first openable locking means233, such as nuts and bolts. The pre-cooling unit200may comprise a flange230that is arranged to be joined to the pipeline290with the first openable fastening means233. The shape of the pre-cooling unit200may be adapted to function in combination with the first openable fastening means233. For example, the pre-cooling unit200, such as the flange230thereof, may limit holes232for a nut and bolt233, which serve as openable fastening means233.

As an alternative or in addition to the solution ofFIG. 3, the pipeline290may be connected to the condensation tower300in a similar, openable, manner. Correspondingly, the pre-heating unit200may comprise the pipeline290. Thus, in an embodiment, the pre-cooling unit200is connected with first openable fastening means233to the condensation tower300. More specifically, in an embodiment, the pre-cooling unit200is connected with first openable fastening means233directly to the condensation tower300(not shown).

However, it has been noticed that the most corrosive points within the pre-cooling unit200are the ones, where the aqueous sulphuric acid condensates on the walls of the pre-cooling unit200, in the inner side thereof. It has been found that most of the interior of the pre-cooling unit can be made acid proof in such a way that only a small corrosive part of the pre-cooling unit200needs to be replaced for every now and then for maintenance. The part that is designed to corrode during use will be referred to as a sacrificial lid240(seeFIGS. 3 and 4).

Referring toFIG. 4, in an embodiment, the pre-cooling unit200comprises a frame260, such as an outer wall260of the pre-cooling unit200.FIG. 4shows the part IV ofFIG. 3in more detail. The frame260can be made of a material that is suitably heat resistant. The frame need260not be acid proof provided that it is sufficiently protected from the aqueous H2SO4inside the pre-cooling unit200. To insulate the frame260from the aqueous H2SO4the pre-cooling unit200comprises a first inner wall252laterally surrounding a reaction chamber202of the pre-cooling unit200. The first inner wall252is made of acid proof material, preferably from acid resistant bricks.

For better acid proofness, in an embodiment, the pre-cooling unit200comprises a second inner wall254laterally surrounding the first inner wall252. The second inner wall254is made of acid proof material, preferably from acid resistant bricks. The pre-cooling unit200may comprise mortar in between the first inner wall252and the second inner wall254.

When the first inner wall252is made from acid free bricks, the thickness of the first inner wall252is preferably from 80 mm to 150 mm, such as from 90 mm to 110 mm, such as 100 mm. When the second inner wall254is made from acid free bricks, the thickness of the second inner wall254is preferably from 80 mm to 150 mm, such as from 90 mm to 110 mm, such as 100 mm. These thicknesses may be applicable also to other suitably acid proof materials.

To thermally insulate the inner wall(s)252,254from the outer wall260, and in this way helping to maintain the proper reaction temperature within the reaction chamber202, the pre-cooling unit200comprises a thermally insulating inner wall256laterally surrounding the reaction chamber202of the pre-cooling unit200and laterally surrounded by an outer wall260of the pre-cooling unit200. Preferably, the thermally insulating inner wall256laterally surrounds the first inner wall252, and if present, also the second inner wall254.

To have the thermally insulating inner wall256reasonably thermally insulating, the material of the thermally insulating inner wall256may have a thermal conductivity κ of at most 0.1 W/m·K, preferably at most 0.05 W/m·K at a temperature 10° C. The thermally insulating inner wall256may be made of foam glass. In an embodiment, the thermally insulating inner wall256is made of foam glass and the thickness t256of the thermally insulating inner wall256is from 60 mm to 160 mm, such as from 70 mm to 100 mm, such as 80 mm; for it to have suitable thermal insulation properties. Preferably, the ratio (κ/t256) of the thermal conductivity κ and the thickness t256is at most 5 W/m2·K, more preferably at most 1 W/m2·K, as calculated at the temperature 10° C. The pre-cooling unit200may comprise mortar in between the walls252and256and/or in between the walls254and256.

To further protect the outer wall260from corrosion, the outer wall260may comprise, on the inner side thereof, a first lining258. The first lining258may be e.g. a heat resistant polymer lining, such as a rubber lining. The heat resistance of the first lining258may be e.g. at least 100° C. It should be noted that because the outer wall260is in contact with the environment and may conduct heat reasonably well, the temperature of the second lining258, in use, may be reasonably low. Moreover, the thermal insulation provided by the other walls (252,254,256) will help to keep the temperature of the first lining258reasonably low. The pre-cooling unit200may comprise mortar in between the first lining258and the thermally insulating inner wall256.

In general, acid proof brick walls having an arbitrary shape are hard to manufacture. Therefore, the ceiling of the reaction chamber202of the pre-cooling unit200may be hard to insulate from the reaction chamber in an acid proof manner. To simplify the construction, in an embodiment, the pre-cooling unit200comprises a sacrificial lid240. The sacrificial lid240is configured to corrode during use, and is thus arranged to be easily replaceable.

As indicated inFIG. 3, the sacrificial lid240is connected to the frame260with second openable fastening means243, such as nuts and bolts, hooks, or clasps. The shape of the sacrificial lid240is adapted to the shape of the second openable fastening means243. For example, as shown inFIG. 3, the sacrificial lid240limits holes242for fastening the sacrificial lid240to the frame260of the pre-cooling unit with nuts and bolts243(see alsoFIG. 4).

As indicated inFIG. 4, the sacrificial lid240is only weakly thermally insulated from the reaction chamber202. Thus, the sacrificial lid240comprises material241that is heat resistant to at least 500° C. Preferably, all such parts of the sacrificial lid that extend through the sacrificial lid240in the direction of the thickness of the sacrificial lid240comprise material241that is heat resistant to at least 500° C. Preferably, the sacrificial lid240comprises a layer of a metal, such as steel, e.g. acid proof steel. This layer may be heat resistant as indicated above.

In addition, the sacrificial lid240comprises a second lining244. The second lining244need not fully cover a side of the material241. The purpose of the second lining is to protect at least a part of the layer241from liquid aqueous sulphuric acid. Therefore, the second lining244is arranged in between the interior of the pre-cooling unit200and the material241. In other words, the second lining244faces towards the interior of the pre-cooling unit200. The second lining244should be heat resistant to at least the boiling point of the aqueous sulphuric acid. In an embodiment, the second lining244is heat resistant to at least 230° C., preferably at least 300° C. In an embodiment, the second lining244comprises plastic material. In an embodiment, the second lining244comprises fluorinated plastic material. In an embodiment, the second lining244consists of plastic material. In an embodiment, the second lining244consists of fluorinated plastic material.

As indicated inFIG. 4, in an embodiment, the sacrificial lid240comprises a metal layer241that is partly coated with a second lining244that is heat resistant to at least 240° C. The second lining244may be made of e.g. a fluorinated plastic, such as polytetrafluoroethylene (i.e. Teflon), or non-fluorinated plastic having suitably high thermal resistance (e.g. some polyamides such as Nylon 66 or parylene).

Furthermore, in an embodiment, the pre-cooling unit200comprises a cooling channel248that is arranged in contact with the sacrificial lid240. The cooling channel248is arranged in contact with the sacrificial lid240in a thermally conductive manner. In use, some liquid coolant may flow in the cooling channel248. The pre-cooling unit200comprises an inlet248afor this coolant and an outlet248bfor this coolant (seeFIG. 3), through which the coolant can be circulated within the channel248. The cooling channel248is arranged in contact with the sacrificial lid240in such a thermally conductive manner, that, in use, the temperature difference between [i] a liquid flowing inside the cooling channel248and [ii] a part of the sacrificial lid240is at most 50° C.

Preferably, the cooling channel248is arranged on an opposite side of the sacrificial lid240with respect to the second lining244. Moreover, preferably the cooling channel248is arranged on the sacrificial lid240to such a location that a straight line L that is parallel to a surface normal N of the sacrificial lid240penetrates both the cooling channel248and the second lining244. This has the technical effect, that as the coolant flowing in the cooling channel248cools the sacrificial lid240locally near the channel248itself, the aqueous sulphuric acid will condense at the point, where the cooling channel248is located; however, on the other side of the lid240. Thus, the aqueous sulphuric acid will condense at a point comprising second lining244, which protects the material241from corrosion at that point. As indicated inFIG. 4, the second lining244can be arranged also to locations that are cooler. Thus, the second lining244can be arranged also at all locations that are, in a lateral direction, further away from a centre of the reaction chamber202than the cooling channel248. As indicated in the figure, typically the surface normal N is parallel to the thickness of the sacrificial lid240.

However, as indicated inFIG. 4, at some of such locations that are in a lateral direction nearer to the centre of the reaction chamber202than the cooling channel248, no second lining is necessarily present. In practice, an initially uniform second lining244may burn or melt at such locations. However, when the second lining244is resistant to temperatures higher than the boiling point of the aqueous sulphuric acid, at such locations, the metal layer241of the sacrificial lid240is only exposed to gaseous sulphuric acid, which is less corrosive than aqueous sulphuric acid. In this way, the corrosion resistance of the sacrificial lid240can be improved with the lining244even if the lining is not uniformly applied onto the sacrificial lid240.

As indicated inFIG. 4, most preferably at least a part of the cooling channel248, at least some second lining244, and at least part of the thermally insulating inner wall256are arranged on a same straight line L that is parallel to a surface normal N of the sacrificial lid240; such as on a same straight line L that is parallel to the direction of thickness of the sacrificial lid240. This helps to control the location in which the condensation of aqueous sulphuric acid occurs, since also the thermally insulating inner wall256imposes a high temperature gradient within the pre-cooling unit200.

Since the pre-cooling unit200may be replaceable, it is evident, that the pre-cooling unit200can be sold even without the rest of the system100. In this way, a pre-cooling unit200may be seen as an embodiment independent of the other components of the system.

Referring toFIG. 5, in particular when the gas containing SO3further contains SO2, e.g. due to incomplete catalytic reactions in the converter500, the system may comprise a scrubber700. InFIG. 5, a pipeline340is configured to convey flue from the condensation tower300to the scrubber. The scrubber700is configured to remove at least some SO2from the flue. Such SO2removal processes are known as flue-gas desulphurization.

In connection with a pulp mill, an extremely usable type of scrubber is a bisulphite scrubber. In a bisulphite scrubber, the sulphur dioxide is reacted with an aqueous solution of alkaline, thereby producing some bisulphite. As an example, an aqueous solution of NaOH may be used to scrub SO2, resulting in sodium sulphite Na2SO3and/or sodium bisulphite NaHSO3, depending on the alkalinity of the scrubbing liquid. If the pH of the scrubbing liquid is about 10 or more, substantially only Na2SO3and water will be produced by the reaction of SO2with NaOH.

Other possible alkalis include potassium hydroxide KOH and ammonia water NH3(aq). Using them in the scrubber700produces corresponding sulphite and/or bisulphite.

As known to a skilled person, such sulphites and/or bisulphites are used in a pulp mill. More precisely, such sulphites and/or bisulphites are needed in both a sulphate pulp mill (i.e. a Kraft pulp mill) and a sulphite pulp mill; even if a sulphite mill uses sulphites in larger amounts. For example, in a sulphate mill, sulphites and/or bisulphites may be utilized is the process of scrubbing vent gases containing chlorine dioxide ClO2. In this way, the reaction products of the scrubber700are usable in the pulp mill600regardless of its type. A system comprises a channel752for conveying some reaction products from the scrubber700to the pulp mill600. Such a means may comprise a pipeline752configured for the purpose. The reaction product may comprise at least one of a sulphite and a bisulphite. The reaction product may comprise at least one of sodium sulphite, sodium bisulphite, potassium sulphite, potassium bisulphite, ammonium sulphite (NH4)2SO3, and ammonium bisulphite NH4HSO3.

FIG. 5also shows a structure of a typical scrubber700, which is wet scrubber, e.g. a bisulphite scrubber. The scrubber700comprises nozzles710for spraying scrubbing solution onto the flue gas. The scrubbing solution may comprise water and the aforementioned additive; which may be fed to the scrubber700, or to a circulation of scrubbing solution, as indicated by the corresponding arrows. In addition, some oxidizing gas may be fed to the scrubber if needed. When sprayed with the nozzles710, most of the scrubbing liquid falls to the bottom of the scrubber700. To prevent droplets from escaping the scrubber700, the scrubber700may comprise a second droplet separator730. The second droplet separator730arrests most of also the small droplets, which, when forming larger droplets, fall to the bottom of the scrubber700. The scrubber700comprises a pump740and a pipeline742for circulating the scrubbing solution from the bottom of the scrubber700to the nozzles710. The scrubber700may comprise a packed bed720for improving the contact between the flue gas and the scrubbing solution.

A corresponding method comprises removing at least some SO2from the flue (i.e. the remaining gas) that is removed from the condensation tower300. The SO2may be removed in a scrubber700. The SO2may be removed in a wet scrubber700. The SO2may be removed in a bisulphite scrubber700using an alkaline scrubbing solution.

The scrubber700comprises an outlet750for letting out at least some of the liquid reaction products. An embodiment comprises a channel752, such as a pipeline752, configured to convey at least some of the solid and/or liquid reaction products from the outlet750to a pulp mill600. Another channel754may be used to convey another part of the reaction products e.g. to a waste treatment plant, e.g. if the scrubber is a two stage scrubber.