PROCESS FOR THE REMOVAL OF PARTICULATE MATTER FROM AN AQUEOUS STREAM

Abstract

Process for the removal of particulate matter from an aqueous stream containing a concentrated acid, preferably concentrated sulfuric acid, the process including mechanical filtration by passing the aqueous stream through a filter unit, the filter unit including a metallic, ceramic or polymeric filter, or a filter including a filter aid on a septum. The aqueous stream is the exit stream of a sulfuric acid condenser, optionally the exit stream of a sulfuric acid concentrator arranged downstream the sulfuric acid condenser.

Claims

1. A process for the removal of particulate matter from an aqueous stream containing a concentrated acid, wherein the average particle size of said particulate matter is in the range 0.05-10 μm, and the concentration of the sulfuric acid in said aqueous stream is above 60% wt., said process comprising mechanical filtration by passing said aqueous stream through a filter unit, and said filter unit comprising a metallic, ceramic or polymeric filter, or a filter comprising a filter aid on a septum, wherein the aqueous stream is the exit stream of a sulfuric acid condenser, optionally the exit stream of a sulfuric acid concentrator arranged downstream the sulfuric acid condenser, of a process plant for producing sulfuric acid from a process gas stream, said process gas stream containing sulfur and said particulate matter, said process plant including: converting the sulfur in the form of SO2 of said process gas stream into a SO3-rich gas stream in a SO2-conversion unit, said SO2-conversion unit comprising a catalyst bed and passing said process gas through said catalyst bed; converting the SO3-rich gas stream into said exit stream of said sulfuric acid condenser, optionally said exit stream of said sulfuric acid concentrator; and optionally providing an acid cooling step for reducing the temperature of said exit stream.

2. The process according to claim 1, wherein said filter unit is provided: downstream the acid cooling step; or inside the acid cooling step as part of or integrated within the acid cooling circuit.

3. The process according to claim 1, wherein the metallic, ceramic or polymeric filter, or the filter comprising a filter aid on a septum, has a filter media grade of 0.1 to 14 μm.

4. The process according to claim 1, wherein the metallic filter is a sintered metal and the filter media grade is in the range 3-7 μm.

5. The process according to claim 1, wherein the septum is a polymeric, ceramic or steel material and the filter aid comprises diatomaceous earth.

6. The process according to claim 1, wherein the ceramic of said ceramic filter or said septum comprises one or more elements taken from the group comprising aluminum, calcium, potassium, sodium, magnesium, tungsten, iron, and silicon; and the polymeric material of said polymeric filter or said septum is polypropylene, a fluorinated polymer, polyvinyl chloride, polyphenylene sulfide, polyphenylene oxide, or combinations thereof.

7. The process according to claim 7, wherein the polymeric material of said polymeric filter is PTFE and the polymeric filter is a PTFE membrane, optionally pleated, and optionally also in a propylene housing to form a filter cartridge.

8. The process according to claim 1, wherein the average particle size of said particulate matter is in the range 0.1-5 μm.

9. The process according to claim 1, wherein the content of particulate matter in the aqueous stream is 0.1-500 ppm-wt.

10. The process according to claim 1, wherein the particulate matter is soot.

11. The process according to claim 1, wherein the particulate matter is carbon black.

12. The process according to claim 1, wherein said process gas stream contains O2 and more than 200 ppm vol SO2 together with said particulate matter, wherein said particulate matter is soot and/or carbon black which is present in said process gas stream in a concentration of >2 mg/Nm3.

13. The process according to claim 1, wherein the process gas stream is off-gas from a carbon black producing plant.

14. The process according to claim 1, wherein said catalyst bed of said SO2-conversion unit comprises a catalyst comprising vanadium pentoxide, sulfur in the form of sulfate, pyrosulfate, tri- or tetrasulfate, and one or more alkali metals on a porous carrier, wherein the vanadium pentoxide content in the catalyst is 1-15 wt %, the sulfur content in the catalyst is 1-25 wt %, and the alkali metal in the catalyst is 2-25 wt %, and wherein the porous carrier is diatomaceous earth or silica, optionally containing up to 10 wt % alumina.

15. The process according to claim 1, wherein the concentration of the sulfuric acid in said aqueous stream is 85% wt. or higher.

Description

EXAMPLES

Example 1: Metallic and Ceramic Filters

[0052] Commercial metallic and ceramic filter samples of varying media grade were obtained.

[0053] Previous experiments have shown that the behavior of carbon black particles in sulfuric acid and water is similar, both are polar liquids that will not react with the carbon black particulates and their interactions with the carbon black particulate will therefore be identical. Thus, a suspension based on carbon black and water was used for the filtration tests.

[0054] A suspension of 100 ppm wt. carbon black in water was prepared by mixing carbon black powder with water on a dissolver.

[0055] Sintered metal filter discs of varying filter grade were acquired from Mott. The discs had a diameter of 1 inch (25.4 mm) and were manufactured from stainless steel 316 L. The ceramic discs tested were acquired from Sigma Aldrich and comprised glass fiber and sintered glass filters in the form of fused silica.

TABLE-US-00001 Particulate media grade Thickness removal Filter # Type [μm] [mm] efficiency [%] 1 Sintered metal 0.2 1.0 >99.99 2 Sintered metal 0.5 1.2 >99.9 3 Sintered metal 1 1.6 >99.9 4 Sintered metal 2 1.6 99.9 5 Sintered metal 5 1.6 99.2 6 Sintered metal 10 1.6 48.6 7 Sintered metal 20 1.6 16.7 8 Sintered metal 40 2.0 7.2 9 Sintered metal 100 2.4 6.1 10 Glass fiber 1 0.7 99.5 11 Fused silica 10 4.0 97.0 12 Fused silica 16 4.0 60.5 All filter tests were performed by placing the filter samples in a custom-made funnel that was placed in a 1 L Büchner flask.

[0056] Subsequently, the 100 ppm wt carbon black suspension was poured on the filter and sucked through the filter by a pump providing a trans-filter pressure difference of 0.6 bar. For filter 1 to 3 and 11 to 12, the applied trans-filter pressure was 0.8 bar. The liquid passing through the filter was collected and subjected to determination of the obscuration using a Malvern Mastersize 3000. A standard curve for the obscuration was established by measuring the obscuration for known concentrations of carbon black suspensions in water.

[0057] It was observed that particularly ceramic or polymeric filters having a media grade of 15 μm or higher allowed the carbon black to pass through with the aqueous stream whereas a denser filter such as one having a filter media grade of 14 or 12 or 10 μm or much lower such as 1 or 0.5 or 0.1 μm, in particular the range 0.1-14 μm or 0.5-12 μm, would remove the particulate matter efficiently, albeit with some increase in pressure drop.

[0058] When using the metallic filter in the form of a sintered metal with filter media grade in the range 3-7 μm, in particular 5 μm, high particulate removal efficiency was observed (as shown in the Table), without significant penalties in terms of pressure drop.

[0059] When using the ceramic filter in the form of sintered glass, particularly fused silica, and with a filter media grade in the range 1-10 μm, high particulate removal efficiency was also observed without significant penalties in terms of pressure drop.

[0060] The term “particulate removal efficiency” represents the percentage of the carbon black in the suspension retained by the filter, calculated by converting the obscuration to wt-ppm by the standard curve.

Example 2: Polymeric Filters

[0061] Commercial cartridge filters (filter cartridges) with three different filter media grades, herein also referred as pore ratings, were obtained. The filters were tested in the laboratory to determine their particulate removal efficiency when removing carbon black from a test liquid.

[0062] The cartridge filters consist of a pleated ePTFE membrane housed in a polypropylene housing. Three different filter media grades (pore ratings) were supplied: 1.0, 0.45, and 0.2 μm. The filters were 254 mm (10″) in length with an outer diameter of 70 mm. Two different measurements were done on the filtrate following a filtration test; determination of the particle size distribution and determination of the remaining carbon black content. Both measurements were carried out using a Malvern Mastersizer 3000.

[0063] The particle size distribution was measured for the filtrate from the filter cartridges with a pore rating of 1.0 and 0.45 μm, as well as 0.2 μm pore rating. In both cases the size distribution of the carbon black remaining in the filtrate was below the pore rating, confirming that the filter cartridges can remove carbon black particles at least down to their pore rating. The filtrate from the filter cartridge with a pore rating of 0.2 μm was clear enough that it was not possible to measure a particle size distribution.

[0064] The particle size distribution was measured for the filtrate from the filter cartridges with a 1.0 and 0.45 μm pore rating. In both cases the size distribution of the carbon black remaining in the filtrate was below the pore rating, confirming that the filter cartridges can remove carbon black particles at least down to their pore rating. The filtrate from the filter cartridge with a pore rating of 0.2 μm was clear enough that it was not possible to measure a particle size distribution.

[0065] The tests show that the cartridge filters are efficient at removing carbon black particles down to the above pore rating of the individual filters. The tests also confirmed that nearly all the carbon black needs to be removed for the liquid to be visibly clear.