Process for removal of aerosol droplets

Abstract

The present disclosure relates to a process for removal of an aerosol, comprising the steps of directing a process gas comprising an aerosol to contact an inertial demister providing a first demisted process gas, and directing the first demisted process gas to contact a coalescing demister providing a second demisted process gas, characterized in said first inertial demister being more open than said coalescing demister, where more open is defined as having a higher void fraction or a lower density with the associated benefit of such a process providing an efficient removal of a large volume of liquid from the inertial demister, while avoiding flooding of the demister system. It further relates to a process plant for sulfuric acid production employing such a pair of demisters.

Claims

1. A process for removal of an aerosol from a demister system and avoiding flooding of the demister system, comprising: a. cooling a process gas comprising at least 5,000 ppm wt of a condensable species under condensing conditions to provide a process gas comprising an aerosol, b. directing said process gas comprising an aerosol to contact an inertial demister providing a first demisted process gas, and c. directing the first demisted process gas to contact a coalescing demister providing a second demisted process gas, wherein d. said first inertial demister being more open than said coalescing demister, where more open is defined as having a higher void fraction or a lower density e. and the process gas entering the demister having an aerosol content being more than 5,000 ppm wt.

2. The process according to claim 1, wherein said inertial demister has a void fraction of more than 0.90 and a void fraction of less than 0.99.

3. The process according to claim 1, wherein said coalescing demister has a void fraction of more than 0.60 and a void fraction of less than 0.90.

4. The process according to claim 1, wherein said inertial demister has a density of more than 30 kg/m.sup.3 and a density of less than 100 kg/m.sup.3.

5. The process according to claim 1, wherein said coalescing demister has a density of more than 170 kg/m.sup.3 and a density of less than 300 kg/m.sup.3.

6. The process according to claim 1, wherein the inertial demister and coalescing demister both have a dimension in the direction of gas flow, wherein the ratio between the dimension in the direction of gas flow of the inertial demister and the dimension in the direction of gas flow of the coalescing demister is more than 0.5 and less than 10.0.

7. The process according to claim 1, wherein the gas entering the inertial demister has a superficial velocity more than 1.0 m/s and less than 7.0 m/s.

8. The process according to claim 1, wherein the difference of the temperature of the process gas entering the demister and the process gas exiting the demister is less than 5° C.

9. The process according to claim 1, wherein the process gas comprises a sulfuric acid aerosol with a concentration of sulfuric acid in the aerosol droplets at the contact with the inertial demister is above 50 wt % and below 99 wt %.

10. The process according to claim 9, wherein at least one demister is made from an inorganic material or a polymeric material.

11. The process according to claim 9, wherein the demister is positioned in a heat and acid resistant vertical tube, in which the process gas flows inside the tube and a cooling medium flow outside the tube.

12. The process according to claim 9, wherein the temperature of the process gas entering said inertial demister is above 60° C. and below 120° C.

Description

FIGURES

(1) FIG. 1 shows a sulfuric acid plant comprising a condenser with process gas inside the tubes.

(2) FIG. 2 shows a condenser with heat exchange medium inside the tubes.

(3) FIG. 3 shows a condenser tube with a single layer demister

(4) FIG. 4 shows a condenser tube with a two-layer demister

(5) A process as shown in FIG. 1, for removal of SO.sub.2 from process gases, with associated production of sulfuric acid is known from the prior art. In the process a feed gas 2 containing SO.sub.2 is provided at a temperature sufficient for catalytic oxidation of SO.sub.2 to SO.sub.3 to be initiated such as around 370-420° C., to a catalytic reactor 4 in which oxidation of SO.sub.2 to SO.sub.3 takes place in the presence of an appropriate sulfuric acid catalyst. A range of such sulfuric acid catalysts are known to the person skilled in the art. One possible catalyst is vanadium oxide supported on a silica carrier material and promoted with alkali metals. Preferred alkali metals are potassium, sodium, and/or cesium.

(6) To avoid pushing the SO.sub.2/SO.sub.3 equilibrium towards SO.sub.2 while enjoying the benefit from high reaction rates at high temperatures, the oxidation is often carried out in two or three beds with intermediate heat exchangers, and followed by a further heat exchanger.

(7) At the outlet from the catalytic reactor an oxidized process gas 6 is available. This oxidized process gas contains water vapor which as temperature is reduced hydrates SO.sub.3 to form gaseous H.sub.2SO.sub.4, sulfuric acid. The oxidized and (partly) hydrated process gas is directed to a condensation unit 8 with the process gas enclosure formed by vertical glass tubes 10, in which the temperature is reduced to below the dew point of sulfuric acid, by heat exchange with a cold heat exchange medium, such as atmospheric air provided in heat exchange medium inlet 12 and withdrawn from heat exchange medium outlet 14. The sulfuric acid condenses and may be collected in concentrated form at the liquid outlet 16 at the bottom of the condensation unit. At the cold end of the vertical glass tubes 10, a demister 18 is provided in the tubes. Demisted process gas 20 is directed to the stack 22.

(8) FIG. 2 shows an alternative condenser. Here an oxidized and (partly) hydrated process gas 6 is directed to a condensation unit 8 in which the process gas enclosure surrounds a heating medium enclosure formed by horizontal glass tubes 30. The temperature of the process gas is reduced to below the dew point of the condensable liquid, by heat exchange with a cold heat exchange medium, such as atmospheric air provided in heat exchange medium inlet 12 and withdrawn from heat exchange medium outlet 14.

(9) The liquid condenses and may be collected in concentrated form at the liquid outlet 16 at the bottom of the condensation unit. Proximate to the process gas outlet of the condensation unit a two-layer demister pad 32 according to the present disclosure is provided. Demised process gas 20 is directed to the stack.

(10) FIG. 3 shows a demister being an embodiment according to the prior art in which the process gas enclosure is tubular, e.g. corresponding to the demister 18 of FIG. 1.

(11) The process gas flows upward through the narrow section of the tube 52 and widens out when reaching the demister enclosure. The widening of the tube serves several purposes, i.e. reducing the gas velocity in the demister to avoid flooding, ensuring that the demister is fixed at the position within the enclosure and to provide a fixation point for the glass tube 52 in a tube sheet.

(12) The process gas containing droplets flows upwards with a typical vertical superficial velocity between 1 and 7 m/s in a wide section of the tube 54. An inertial demister 56 is positioned in a wide section of the tube 54. When the droplets meet the demister 56, droplets with high inertia are collected. The smallest droplets will follow the gas path through the demister and will be collected at a much lower efficiency.

(13) Had a demister with a more dense structure been chosen, the smallest droplets would be collected, but this would be associated with an increased risk of flooding, an increased pressure loss and a risk of entrainment of droplets from flooding at the exit of the demister.

(14) FIG. 4 shows a demister being an embodiment of the present disclosure in which the process gas enclosure is tubular, similar to FIG. 3.

(15) The process gas flows upward through the narrow section of the tube 52 and widens out when reaching the demister enclosure. The widening of the tube serves several purposes, i.e. reducing the gas velocity in the demister to avoid flooding, ensuring that the demister is fixed at the position within the enclosure and to provide a fixation point for the glass tube 52 in a tube sheet.

(16) The process gas containing droplets flows upwards with a typical vertical superficial velocity between 1 and 7 m/s in a wide section of the tube 54.

(17) An inertial demister 56 is positioned below a coalescing demister 58 in a wide section of the tube 54. When the droplets meet the open inertial demister, a high fraction of large droplets are collected, forming a liquid film on the demister material and will drain from the demister. The smallest aerosol droplets follow the gas to the upper denser coalescing demister 58, in which they are efficiently collected, forming a liquid film and drain to the open demister below. The liquid loading of the coalescing filter is low and hence flooding of the demister is avoided.

EXAMPLES

(18) Three examples are presented, to document the effect of the present invention.

(19) Table 1 shows an estimate of droplet size distribution at the entrance to the demister, for a gas with a concentration of sulfuric acid aerosol out of the condenser of 50,000 ppm wt, calculated as 100% w/w H.sub.2SO.sub.4.

(20) TABLE-US-00001 TABLE 1 Droplet size Wt % of sulfuric acid mist <0.3 μm 0.05 0.3-1.0 μm 0.5 1.0-3.0 μm 2 3.0-10 μm 10 >10 μm 87.45

(21) The first example according to the prior art shows a demister designed to remove only a moderate amount of the droplets having a size below 1 μm, corresponding to a condenser designed to remove acid mist to below 50 ppm wt, based on an inertial demister, with a void fraction of 90%.

(22) The second example according to the prior art shows a demister designed to remove a high amount of the droplets having a size below 1 μm, based on a coalescing demister, with a void fraction of 86%. Due to high liquid loading, this demister is very likely to experience flooding due to entrainment of droplets, which may result in liquid carry over.

(23) The third example according to the present disclosure shows a demister designed to remove a high amount of the droplets having a size below 1 μm, based on an inertial demister, with a void fraction of 90%, followed by a coalescing demister, with a void fraction of 86%. This solution provides efficient aerosol removal, while reducing the pressure drop and risk of demister flooding.

(24) The void fraction was measured by determining the water displacement resulting from immersing a demister with a known volume, V, into a 250 ml graduated cylinder partly filled with water, V.sub.w, at a temperature of 20° C. The sample was allowed to rest for 10 min before the new water level, V.sub.L, was noted. The void was subsequently calculated as

(25) $\mathrm{Void}=1-\frac{V_L-V_W}{V}$

(26) Density refers to the density of the demister in the position it is installed in during operation, i.e. the mass of the demister, divided by the volume of process gas enclosure taken up by the demister.

(27) TABLE-US-00002 TABLE 2 Example 1 Example 2 Example 3 0.1-0.3 μm droplet removal 0 80 80 0.3-1.0 μm droplet removal 94 98 98 1.0-3.0 μm droplet removal 99 100 100 3.0-10 μm droplet removal 100 100 100 Inertial demister void 90% — 90% Inertial demister density 90 kg/m.sup.3 — 90 kg/m.sup.3 Inertial demister height 190 mm — 100 mm Coalescing demister void — 86% 86% Coalescing demister density 200 kg/m.sup.3 200 kg/m.sup.3 Coalescing demister height — 100 mm 60 mm Temperature upstream demister 100° C. 100° C. 100° C. Temperature downstream demister 100° C. 100° C. 100° C. Pressure drop 60 mm H.sub.2O 400 mm H.sub.2O 120 mm H.sub.2O ppmwt H.sub.2SO.sub.4 inlet to demister 50000 50000 50000 ppmwt H.sub.2SO.sub.4 in outlet 50 10 10

(28) From the three examples it is clear that increased removal efficiency may be achieved by operating a condenser with a dual demister according to the invention, with lower pressure drop compared to the denser demister alone. In addition the increased pressure drop, Example 2 would be associated with an increased risk of flooding, as practically all of the sulfuric acid droplets will be collected in the dense coalescing demister.