DK201900999A1

DK201900999A1 - Solid absorbent and method for the selective removal of sulfur trioxide from gases

Info

Publication number: DK201900999A1
Application number: DKPA201900999A
Authority: DK
Inventors: Theilgaard Madsen Anders; Bengt Jakobsson Niklas; Aggerholm SØRENSEN Per
Original assignee: Haldor Topsøe A/S
Priority date: 2019-08-26
Filing date: 2019-08-26
Publication date: 2020-05-25

Abstract

Solid selective sulfur trioxide absorbent comprising a porous alumina and method for the selective removal of sulfur trioxide from a gas comprising sulfur trioxide and sulfur dioxide, wherein the gas is contacted with a solid absorbent comprising porous alumina.

Description

Title: Solid absorbent and method for the selective removal of sulfur trioxide from gases

The present invention relates to an absorbent and method for the selective removal of sulfur trioxide from gases. In particular, the invention provides an absorbent selectively adsorbing sulfur trioxide (SO₃) with very limited adsorption activity and with low binding ability for sulfur dioxide (SO₂) and a method for the selective removal of sulfur trioxide from gasses further containing sulfur dioxide.

Thermal formation of SO₃ during combustion of various sulfur containing fuels and the unintended formation of SO₃ from sulfur containing compounds over various, in fact essentially all, oxidation catalysts is a very well known phenomena.

The formation of SO₃ results in several issues such as acid mist emission from the stack, corrosion of heat exchangers and piping due to formation of diluted sulfuric acid if the piping metal temperature falls below the sulfuric acid dew point.

Formation of SO₃ also causes formation of ammonium bisulphate if ammonia is present in the gas. Ammonium bisulfate forms a sticky crust on heat exchanger surfaces and piping causing clogging and excessive pressure drop as well as corrosion.

There is essentially no standard solution in the industry today to remove SO₃ from gases having been treated by catalytic oxidation beside choosing oxidation catalysts with

DK 2019 00999 A1 the lowest possible formation of SO₃ and the use of protective coatings or even glass lined tubing in heat exchangers are to mitigate SO₃ formation.

Oxidation of organic sulfur compounds and H₂S, results in formation of SO₂ and only trace amounts of SO₃. The latter will react with water vapor in the gas and form sulfuric acid. The sulfuric acid will lead to corrosion of the equipment making the proccess unfeasable. Hence, SO₃ must be removed before the process gas is cooled below the acid dew point.

Removal of sulfur oxides is typically achieved by scrubbing with an alkaline solution or by dry scrubbing, i.e. injection of reactive powders which subsequently must be filtered out.

There are many materials such as Ca(OH)₂, sodium bicarbonate, KOH etc. that absorb SO3 but they also absorb SO₂ which makes the volume of absorbent needed unreasonably large and thereby too expensive.

A solution for this problem is to use an absorbent or scavenger to selectively absorbs SO₃ without absorbing SO₂, which is typically present in 1000 or more times higher concentration. In many applications the content of SO₂ in a process gas is acceptable.

We have found that highly porous materials are prone to a higher degree of utilization of the absorbent i.e. the percentage of the active substance in the material that can be accessed by SO₃ because the porous structure allows SO₃ not

DK 2019 00999 A1 only to be absorbed on the surface but also within the absorbent material.

In particular, we found that porous alumina has a very high absorption activity for SO₃ with a limited binding and absorption activity for SO₂.

Pursuant to these findings, this invention provides in a first aspect a solid selective sulfur trioxide absorbent comprising porous alumina.

In a second aspect, the invention provides a method for the selective removal of sulfur trioxide from a gas comprising sulfur trioxide and sulfur dioxide, wherein the gas is contacted with a solid absorbent comprising porous alumina.

In an embodiment of the invention, the porous alumina is activated alumina.

Activated alumina can be manufactured from aluminium hydroxide by dehydroxylating that produces a highly porous material. Activated alumina is further characterized by a high surface area.

The morphology and surface activity of the solid absorbent may also be altered by thermal, chemical treatment or different preparation routes to increase its SO₃ absorption capacity and selectivity.

Preferably, the porous alumina is gamma alumina.

DK 2019 00999 A1

It is further preferred that the porous alumina has a surface area of at least 20 m²/g, such as more than 50 m²/g and more than 150 m²/g, such as more than 200 m2/g

The temperature operation window for porous alumina sulfur trioxide absorbent is between 200 and 450^o C, such as between 250 and 350^o C.

In further an embodiment, the absorbent is in form of pellets, beads or extrudates.

To reduce pressure drop over the solid absorbent it may be preferred to support the absorbent on a ceramic monolithic substrate, metal corrugated monolith or a fibrous corrugated carrier.

Example 1:

The absorption of SO₂ and SO₃ was investigated in a laboratory scale setup in a tubular absorber reactor made of quartz glass fitted in a temperature controlled tubular oven. Porous alumina was tested for absorbing gaseous SO₂ and SO₃ as determined by analysis of outlet and inlet gas flows.

The porous alumina absorbent was exposed to a gas consisting of H₂O, O₂, SO₂, SO₃ and N₂ to evaluate the SO₂ and SO₃ absorption efficiency.

SO₂ was supplied from a gas cylinder and diluted with N₂ and O₂ (in air) supplied from a central pressurized system and were dosed through mass flow controllers. SO₃ was supplied by oxidizing a part of the SO₂ with O₂ in the flow

DK 2019 00999 A1 over a pre-oxidation catalyst. A second stream of N₂ was bubbled through liquid water to evaporate H₂O to the process gas. The two streams were mixed before being added to an absorption reaction chamber, which contained 25 g of solid porous alumina absorbent material.

Table 1 shows the results of the experiment with SO₂ and

SO₃ absorption on porous alumina at a residence time of about 2 seconds. The inlet gas to and outlet gas from the absorption reaction chamber was analyzed for SO₂ and SO₃. The results show that the Al₂O₃ absorbent according to the invention efficiently absorbs SO₃ and only very small amounts of SO₂.

Table 1:

Material	Al2O3
Gas normal residence time [s]	2
O₂ inlet concentration [vol%]	1
Absorber temperature [°C]	300
Inlet SO₂ concentration [ppmv]	360
Inlet SO₃ concentration [ppmv]	1.5
Outlet SO₂ concentration [ppmv]	358
Outlet SO₃ concentration [ppmv]	0.0
SO₂ capture [%]	0.8
SO₃ capture [%]	~100

Example 2:

Various absorption materials where tested in the same manner as in Example 1.

DK 2019 00999 A1

Table 2 shows the results with SO2 absorption of various materials at residence times of about 2 seconds. The inlet gas to and outlet gas from the absorption reaction chamber was analyzed for SO₂. Table 2 shows that very small amounts of SO₂ are absorbed by the porous Al₂O₃ according to the invention, while K₂CO₃/Al₂O₃ absorbed 25%. The more alkaline combination of CaO/NaOH absorbed >60% of the SO₂.

This demonstrates that porous alumina possesses a unique selectivity towards selective SO₃ absorption in an SO₂ containing gas.

Table 2:

Material	Al2O3	K2CO3/Al2O3	CaO/NaOH
Residence time [s]	0.5	0.5	0.5
Absorber temperature [°C]	300	300	300
Inlet O₂ concentration [vol%]	1	1	1
Inlet SO₂ concentration [ppmv]	2005	1950	1980
Outlet SO₂ concentration [ppmv]	1700	1460	775
Uptake of SO₂ [%]	15	25	60

Claims

1. Solid selective sulfur trioxide absorbent compris- ing porous alumina.

2. The solid selective sulfur trioxide absorbent of claim 1, wherein the porous alumina is activated alumina.

3. The solid selective sulfur trioxide absorbent of claim 1 or 2, wherein the alumina is gamma-alumina.

4. The solid selective sulfur trioxide absorbent of any one of claims 1 to 3 having a surface area of at least

20 m²/g, such as more than 50 m²/g and more than 150m such as more than 200 m2/g

5. The solid selective sulfur trioxide absorbentof any one of claims 1 to 4, wherein the absorbent is in form of pellets, beads or extrudates.

6. The solid selective sulfur trioxide absorbentof any one of claims 1 to 5, wherein the absorbent is coated onto a ceramic monolith, metal corrugated monolith or a fibrous corrugated carrier.

7. Method for the selective removal of sulfur trioxide from a gas comprising sulfur trioxide and sulfur dioxide, wherein the gas is contacted with a solid porous absorbent comprising porous alumina.

DK 2019 00999 A1

8. The method of claim 7, wherein the alumina is acti- vated alumina. 9. The method of claim 7 or 8, wherein the alumina is gamma- alumina. 10. The method of any one of claims 7 to 9, wherein the

solid selective sulfur trioxide absorbent has a surface area of at least 20 m²/g, such as more than 50 m²/g, and more than 150 m²/g, such as more than 200 m²/g.

11. The method of any one of claims 7 to 10, wherein the gas is contacted with the solid selective sulfur trioxide absorbent at a temperature of between 200^o C and 450^o

C, preferably between 250 and 350^o C.

12. The method of any one of claims 7 to 11, wherein the solid selective sulfur trioxide absorbent is in form of pellets, beads or extrudates.

13. The method of any one of claims 7 to 12, wherein the solid selective sulfur trioxide absorbent is coated onto a ceramic monolith, metal corrugated monolith or a fibrous corrugated carrier.