WO2012171824A1

WO2012171824A1 - High flow capacity condenser tube for sulphuric acid condensation

Info

Publication number: WO2012171824A1
Application number: PCT/EP2012/060514
Authority: WO
Inventors: Per Morsing; Kurt Agerbaek Christensen; Lusi Hindiyarti LØJ
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 2011-06-15
Filing date: 2012-06-04
Publication date: 2012-12-20
Anticipated expiration: 2013-12-15
Also published as: ZA201307913B; EA027599B1; CN202945060U; CN104591101A; EA201490001A1; CN102826517A

Abstract

A condenser for condensing vapours of sulphuric acid contained in a process gas comprising a tube of an acid resistant material, configured for having a process gas inlet proximate to one end, a process gas outlet proximate to the other end and an acid outlet proximate to the bottom end, said tubes configured for extending through a cooling zone and said cooling zone configured for having a cooling medium inlet and a cooling medium outlet, for a gaseous cooling medium being passed counter-currently to the process gas, characterized in that the tube has a length of 7.5 m to 12 m.

Description

Title: High Flow Capacity Condenser Tube for Sulphuric Acid Condensation

The present invention relates to a condenser for sulphuric acid having an increased gas flow capacity.

This is especially useful in connection with purification of sulphur containing flue gasses and off-gasses, where sulphur is present as sulphur trioxide and is removed as sulphuric acid, which is formed by condensation of the sul^¬ phur trioxide / water containing gas. This process is called Wet gas Sulphuric Acid process, WSA.

The WSA process has proven its value in industries like oil refining, metallurgy, petrochemicals production, coking, coal gasification, non-ferrous roasters and smelters, power plants and production of viscose fibers.

It is known to remove sulphur from flue gas or off-gas by oxidising sulphur compounds to sulphur trioxide, and then cool the sulphur trioxide in the presence of the water in the gas to form sulphuric acid followed by condensation and concentration of the formed sulphuric acid. Oleum may be produced from a gas with a deficit of water compared to sulphur trioxide (less than one mole of water per mole of sulphur trioxide) or by subsequently absorbing sulphur trioxide in the produced sulphuric acid.

In patent US 5,198,206 a desulphurisation process with ad- dition of particles to the process gas is disclosed and in US 5,108,731 a demister for use in condensation tubes for condensation of sulphuric acid mist is disclosed. In the experimental data presented, the tubes are up to 6 m long and configured for evaluating the effect of the length of the cooling zone at 4.05 m, 4.45 m, 4.95 m and 5.4 m. With the length of the cooling zone being 5.4 m, it was found possible to obtain a satisfactory level of H₂SO₄ after the tube in the range below 10 ppm. Furthermore, the use of a demister was found to effectively remove acid mist from the flow outlet.

In the process according to the prior art, the capacity of a condenser was considered a function of the number of tubes provided in the condenser. With twice the number of tubes, the flow area will be doubled, and thus the associ^¬ ated flow capacity will be doubled.

Providing a WSA plant with extra tubes is naturally costly and space consuming too, and therefore there is a need to identify means of providing increased capacity with little or no increase in the space required.

The limiting factor for the glass tube condenser capacity is the heat exchange capacity - i.e. how much gas that ef^¬ fectively may be cooled in the condenser. In the prior art the tube length has been evaluated according to whether the removal of sulphur trioxide and sulphuric acid was suffi^¬ cient .

The tube length can be used also to increase the flow capa^¬ city of a tube, since an increase in tube length will also increase the tube surface area, and thus the heat transfer area. The same heat transfer contact time can therefore be obtained with a higher flow rate, and therefore an in- creased process gas capacity per tube proportional to the tube length was expected. However, with increasing length of the glass tube challenges relating to production and handling of the glass tubes are also increased, and there- fore glass tubes for sulphuric acid condensers have only been produced in 6 m and 7 m lengths corresponding to an active cooling zone of 5.45 m or 6.45 m.

An analysis of the available means for increasing the gas flow capacity, while ensuring sufficient heat exchange with the gas has now surprisingly revealed that the effect of increasing the length of condenser glass tubes upon the heat exchange is significantly stronger than would be ex^¬ pected from the increased tube surface area alone. An in- creased area allows for an increased flow rate inside the tube, and calls for an increased flow rate on the outside of the tube to provide sufficient cooling air. The combined effect of these factors is surprisingly that with an in^¬ crease in tube length of e.g. 18% from 7 m to 8.25 m and a corresponding 19% increase in active cooling zone from 6.45 m to 7.7m, the thermal gas flow capacity is increased by 32-58%, which is far more than the expected proportional increase . The effect of this super-proportional effect of increased length is that the number of tubes for a WSA plant, e.g. for ¾S removal from viscose production may be decreased without negative effects on the desulphurisation . This al^¬ lows a reduction of the footprint of the condenser and may be related to a lower production cost too. As used herein gas flow capacity of a condenser tube shall be understood as the mass flow rate of process gas in a tube, which allows sufficient cooling of the process gas. As used herein, process gas is to be understood as a gas comprising sulphur trioxide and water.

As used herein, cooling medium is to be understood as a me^¬ dium used for heat transfer without significant chemical reaction such as air. A cooling medium may in a different position in the process also be involved in chemical reac^¬ tions .

As used herein cooling zone shall be understood as a sec- tion of tube which is configured for having its outer surface contacted with a cooling medium during operation.

As used herein concentrations of sulphur trioxide in gas form are stated as mole% under the assumption that all hexavalent sulphur is present as sulphur trioxide, and therefore it includes sulphur trioxide as well as sulphur trioxide hydrated to gaseous sulphuric acid.

The present invention provides a condenser for condensing vapours of sulphuric acid contained in a process gas com^¬ prising a tube of an acid resistant material configured for having a process gas inlet proximate to one end, a process gas outlet proximate to the other end and an acid outlet proximate to the bottom end, said tubes configured for ex- tending through a cooling zone and said cooling zone configured for having a cooling medium inlet and a cooling medium outlet, for a gaseous cooling medium being passed counter-currently to the process gas, characterized in that the tube has a length of 7.5 m to 12 m, a tube of an acid resistant material, configured for having a process gas inlet proximate to one end, a process gas outlet proximate to the other end and an acid outlet proximate to the bottom end, said tubes configured for extending through a cooling zone and said cooling zone configured for having a cooling medium inlet and a cooling medium outlet for a gaseous cooling medium being passed counter-currently to the proc- ess gas, characterized in that the tube has a length of 7.5 m to 12 m, preferably 8 m to 11 m and even more preferably having a length of 8 m to 9 m, with the associated benefit of an increased heat exchange with increasing length, ena^¬ bling an increased gas flow, while avoiding handling chal- lenges of excessive lengths.

In an embodiment of the present disclosure further compris^¬ ing a high velocity aerosol filter mounted in substantially tight connection with the condenser tube in the end of the cooling zone proximate to the process gas outlet, said fil^¬ ter comprising fibres or filaments with a diameter of 0.05 to 0.5 mm, the fibres or filaments so being present in an amount, a layer thickness and a configuration such as to ensure that the pressure drop through the filter at a gas velocity of 1-7 m/sec will be between 2 and 20 mbar with the associated benefit that acid mist is condensed as drop^¬ lets by contact with the aerosol filter.

In a further embodiment, the present disclosure further comprises a turbulence generation means inside the tube, such as a spiral, glass indents or glass protrusions with the associated benefit that increased turbulence increases the heat transfer at the inner tube surface.

In a further embodiment the present disclosure is further configured for having a re-heating zone proximate to the process gas outlet with the associated benefit of providing a process gas downstream the condenser, having a temperature above the sulphuric acid dew point, and thus substan^¬ tially without presence of corrosive liquid sulphuric acid.

In a further embodiment, the present disclosure is further configured for the condensed sulphuric acid flowing

counter-currently to the process gas in the condenser by configuring the process gas inlet to be proximate to the bottom end of the tube with the associated benefit of a high sulphuric acid concentration, due to the counter- current flow.

In a further embodiment, the present disclosure is config- ured for the condensed sulphuric acid flowing co-currently with the process gas in the condenser by configuring the process gas inlet to be proximate to the top end of the tube with the associated benefit of reducing the risk of flooding in the condenser tube and in the demister.

In a further embodiment of the present disclosure, the con^¬ denser comprises multiple condenser tubes having a distance between tubes of at least 1/3 of the tube diameter with the associated benefit of a lower pressure drop on the cooling medium side of the condenser. In a further embodiment, the present disclosure relates to a process with two sulphuric acid condensers in series with an intermediate sulphur dioxide oxidation step with the as^¬ sociated benefit of treating process gases with an extra high concentration of sulphur trioxide or with an extra low sulphur dioxide emission.

In a further embodiment the present disclosure relates to a process with two sulphuric acid condensers in series with the first condenser operating with a deficit of water compared to the sulphur trioxide (less than one mole of water per mole of sulphur trioxide) and the second downstream condenser operating with a surplus of water compared to sulphur trioxide (more than one mole of water per mole of sulphur trioxide) with the associated benefit that oleum can be produced in the first upstream condenser.

In an embodiment, the diameter of the condenser tube may range from 20 to 70 mm, preferably between 25 and 60 mm and even more preferably between 30 and 50 mm. With narrow tubes the benefit is related to increased heat exchange surface per cross sectional area, whereas wider tubes pro^¬ vide lower pressure drops. In an embodiment, the process gas contains less than 2.0 % sulphur trioxide, and preferably less than 1.0 % sulphur trioxide with the associated benefit of an even increased super proportional effect of tube length with low concentrations of sulphur trioxide.

Figure 1 shows a condenser according to the present disclo^¬ sure . The present disclosure relates to a condenser 2, in which a process gas 20 flows inside tubes 4 and a cooling medium 22 flows outside the tubes 4. The inside of the condenser tubes 4 can be equipped with a spiral 8 or other turbu^¬ lence-enhancing element. The tubes 4 can be made from glass, such as borosilicate glass, and are connected to the process gas inlet of the condenser by means of a bottom plate 10 having substantially airtight contact with the outside of the tubes 4 and their position is stabilised by baffle plates 12 across the condenser on the cooling medium side. At the process gas outlet 16 from the condenser, the tubes 4 are fixed and sealed against a top plate 14. The cooling medium 22, which typically can be air, will thus flow substantially in cross flow across the tubes 4. The cooling medium can be at a slightly higher pressure than the process gas to avoid leaks of corrosive process gas to the cooling medium side. At the end of the tubes an op^¬ tional aerosol filter 30 may also be present.

According to the prior art, the typical length of the cool^¬ ing section of the tubes has been 4 to 6.5 m with the corresponding length of the tubes being 5 to 7 m. According to the present disclosure, the tubes are 7.5 m or longer. When the length of the condenser tubes is increased, it becomes possible to increase the flow rate inside the tube, while having the same contact time between the process gas and the tube wall. With an increased gas flow rate in the tubes, it is possible to reduce the number of tubes for the same volumetric flow. With the present disclosure, we have further found that the effect of the increased volumetric flow inside the tubes is an increased heat transfer from the process gas to the tube wall thus contributing to a su^¬ per-proportional effect of the increased tube length. To be able to cool the process gas, it is also required that an increased flow of cooling medium is provided. The increased flow rate of cooling medium also increases the heat trans^¬ fers on the outside of the tubes and thus contributes fur^¬ ther to the super-proportional effect with respect to the tube length, while maintaining the ratio between the mass flows of process gas and cooling medium. Therefore, with increased tube length a surprising super-proportional in^¬ crease of the heat transfer due to an increased flow rate of process gas and cooling medium is observed.

With increasing SO₃ content, the contribution to the heat transfer from heat of condensation is increased. Therefore, the relative effect of increased flow rate is lower for high SO₃ levels. On the other hand for tubes with process gas flowing vertically upwards and with low SO₃ levels, the flooding in the demister or in the tubes becomes limiting, i.e. the process gas drives the condensed sulphuric acid towards the gas outlet of the condenser and the flow rate may no longer be increased. However, in processes where flooding becomes a limiting factor, it may be chosen to operate the condenser such that the process gas flows down- wards. This overcomes problems with flooding, but can re^¬ sult in a lower concentration of sulphuric acid. Alterna^¬ tively, with an upward gas flow the flooding limit in the demister may be increased by increasing the demister diameter or by using a more open demister material with a higher void fraction. It is also preferred to balance the flow rate, the tube diameter of the demister section and the void fraction of the demister such that the linear flow rate is in the range 1-7 m/sec, and the associated demister pressure drop is less than 20 mbar.

An upper limit of the increased flow rate inside the tubes exists due to potential flooding in the condenser tube i.e. the effect that with an increasing flow rate of gas, the condensed sulphuric acid may be drawn upwards and out of the glass tubes by the gas flow. The flooding limit inside the tubes may be increased by increasing the tube diameter or altering the shape or size of any turbulence generation means installed inside the tubes.

Since the flow rate of the cooling medium is increased, the pressure drop over the condenser on the cooling medium side will also increase. To reduce or avoid this pressure drop, the distance between condenser tubes, may be increased.

In a further embodiment, an optional demister may be posi^¬ tioned at the exit of the cooling section of the condenser tube to prevent acid mist from leaving the condenser. This demister may preferably be positioned in a zone of the con^¬ denser tube having a larger diameter to provide a demister zone with minimal pressure drop, which otherwise may be an increasing problem with increased gas flow rate.

A further embodiment involves a condenser in which the process gas is re-heated in the last part of the condenser, such that corrosive liquid sulphuric acid not captured in the condenser is evaporated, and the risk of corrosion is reduced in the downstream equipment. EXAMPLES

In a validation of the performance of the disclosure, the heat transfer of different tube condensers has been evalu^¬ ated for different gas composition. A zone for fastening the tubes in the condenser is necessary such that for a 6 m tube the effective cooling length is reduced by about 0.55 m to 5.45 m, a 7m tube has an effective cooling zone length of 6.45 m and an 8.25 m tube has an effective cooling zone length 7.7 m etc. Investigations of the heat exchange per- formance were made for 7 effective cooling zone lengths in the range 5.45 m to 11.45 m.

The heat exchange was evaluated for compositions with 12% 0₂, 2% C0₂, 7-10% H₂0, 1.0%-5.5% S0₃ and N₂ as balance. For all tube lengths for each SO₃ concentration, the flow was adjusted to obtain the same cooling medium outlet tempera^¬ ture. The cooling medium outlet temperature varied slightly between the three investigated SO₃ concentrations. From Table 1 it is clearly seen that with increased tube length the flow may be increased to a higher extent than that expected from an assumption of proportionality.

With increasing flow rate there will also be an increase in the pressure drop over the condenser both on the process gas side and on the cooling medium side. On the cooling me^¬ dium side the pressure drop is 16-200 mbar and it is as^¬ sumed that a pressure drop below 100 mbar is acceptable, but for higher pressure drops there may be a need for coun- teracting the pressure drop by increasing the pitch, i.e. the distance between tubes in the condenser. On the process gas side the pressure drop in the tube ranges from 5 to 200 mbar. Here the pressure drop may be problematic already from values around 60 mbar, and there^¬ fore the tube diameter may have to be increased already with flows around 30 Nm³/h per tube.

For tubes equipped with a demister, an additional pressure drop ranging from 4 to 347 mbar over the demister is observed. Again, this is mainly an issue of operational cost and to some extent, it may be ignored, but it may be bene^¬ ficial to counteract this increased pressure drop. This may be done by increasing the tube area in the section of the demister or by providing a more open demister material with higher void fraction.

Effective

S0₃ conGas Expected Δρ deΔρ toΔρ cootube T air Δρ tube

centraflow gas flow mister tal ling air length out °C mbar

tion Nm³/h Nm³/h mbar mbar mbar

(m)

1.00% 5. 45 10 .78 10 .78 205 5 3 8 16

1.00% 6. 45 16 .24 12 .76 205 12 7 18 26

1.00% 7. 70 25 .73 15 .23 205 31 14 44 44

1.00% 8. 45 33 .34 16 .71 205 52 21 73 60

1.00% 9. 45 46 .43 18 .69 205 102 35 137 89

1.00% 10 .45 64 .20 20 .67 205 196 59 254 134

1.00% 11 .45 88 .30 22 .65 205 347 98 444 200

2.50% 5. 45 10 .74 10 .74 203 5 3 9 26

2.50% 6. 45 14 .65 12 .71 203 10 4 14 33

2.50% 7. 70 20 .84 15 .17 203 22 10 32 46

2.50% 8. 45 25 .43 16 .65 203 34 14 47 56

2.50% 9. 45 32 .89 18 .62 203 58 21 79 73

2.50% 10 .45 42 .31 20 .59 203 98 31 129 97

2.50% 11 .45 54 .30 22 .56 203 164 46 210 129

5.50% 5. 45 9. 43 9. 43 208 4 3 7 35

5.50% 6. 45 12 .07 11 .16 208 8 4 12 41

5.50% 7. 70 15 .93 13 .32 208 14 7 21 49

5.50% 8. 45 18 .57 14 .62 208 21 9 29 55

5.50% 9. 45 22 .55 16 .35 208 32 12 43 64

5.50% 10 .45 27 .09 18 .08 208 48 16 63 74

5.50% 11 .45 32 .37 19 .81 208 71 21 92 87

Claims

1. A condenser for condensing vapours of sulphuric acid contained in a process gas comprising a tube of an acid re- sistant material, configured for having a process gas inlet proximate to one end, a process gas outlet proximate to the other end and an acid outlet proximate to the bottom end, said tubes configured for extending through a cooling zone, and said cooling zone configured for having a cooling me- dium inlet and a cooling medium outlet for a gaseous cool^¬ ing medium being passed counter-currently to the process gas, characterized in that the tube has a length of 7.5 m to 12 m, preferably 8 m to 11 m and even more preferably having a length of 8 m to 9 m.

2. A condenser according to claim 1 further comprising a high velocity aerosol filter mounted in substantially tight connection with the tube in the end of the cooling zone proximate to the process gas outlet, said filter comprising fibres or filaments with a diameter of 0.05 to 0.5 mm, the fibres or filaments so being present in an amount, a layer thickness and a configuration such as to ensure that the pressure drop through the filter at a gas velocity of 1-7 m/sec will be between 2 and 20 mbar.

3. A condenser according to claim 1 or 2 further comprising turbulence generation means inside the tube.

4. A condenser according to claim 1, 2 or 3 further configured for having a re-heating zone proximate to the proc^¬ ess gas outlet.

5. A condenser according to claim 1, 2, 3 or 4 further configured for the condensed sulphuric acid flowing

counter-currently with the process gas in the condenser, by configuring the process gas inlet proximate to be at the bottom end of the tube.

6. A condenser according to claim 1, 2, 3 or 4 further configured for the condensed sulphuric acid flowing co- currently with the process gas in the condenser, by config- uring the process gas inlet proximate to be at the top end of the tube.

7. A condenser according to any claim above having a condenser tube diameter between 20 and 70 mm, preferably be- tween 25 and 60 mm and even more preferably between 30 and 50 mm.

8. A condenser according to any claim above comprising multiple condenser tubes having a distance between tubes of at least 1/3 of the tube diameter.

9. A condenser according to any claim of 1 to 7 condensing sulphuric acid from a process gas containing less than 2.0 % sulphur trioxide, and preferably less than 1.0 % sul- phur trioxide.

10. A plant for production of oleum comprising two sulphuric acid condensers in series with the first condenser configured for operating with less than one mole of water per mole of sulphur trioxide and the second downstream con^¬ denser configured for operating with more than one mole of water per mole of sulphur trioxide.