Sulphuric acid production with recycle of desulphurized gas

1. A process for oxidation of SO 2 to SO 3 comprising the steps of

(a) directing a stream of feed gas comprising SO 2 and O 2 , to a catalytically active material;

(b) oxidizing an amount of said SO 2 in said process gas to SO 3 in the presence of at least 0.1% water and the catalytically active material, thereby providing a first oxidized process gas;

(c) reacting SO 3 with water in a condensing column;

(d) condensing H 2 SO 4 in a condenser;

(e) withdrawing a first desulphurized process gas and a first stream of sulphuric acid;

(f) from the desulphurized process gas, withdrawing a recycle stream of desulphurized process gas, wherein the recycle stream is added to said stream of feed gas or said first oxidized process gas;

(g) reheating the first desulphurized process gas;

(h) oxidizing an amount of the remaining SO 2 in said first desulphurized process gas to SO 3 in the presence of a second catalytically active material, thereby providing a second oxidized process gas;

(i) reacting SO 3 with water;

(j) condensing H 2 SO 4 ; and

(k) withdrawing a second desulphurized process gas and a second stream of sulphuric acid.

2. A process according to claim 1 further comprising the process step of

(l) heating the recycle stream of desulphurized process gas to a temperature at least 50° C. above the dew point of sulphuric acid in the recycle stream.

3. A process according to claim 1 further comprising one or more secondary sulphur removal process steps taken from the group consisting of:

(m) removal of sulphuric acid mist by collection of droplets in a mist filter and withdrawal of collected sulphuric acid droplets;

(n) removal of sulphuric acid mist by electrostatic precipitation, and removal of precipitated sulphuric acid;

(o) removal of sulphur oxides by absorption in a scrubber, wherein said scrubber contains an alkaline solution and/or an oxidative solution.

4. A process according to claim 1 , wherein the recycle stream is withdrawn downstream of the secondary sulphur removal process step.

5. A process according to claim 1 , wherein the recycle stream is withdrawn upstream of the secondary sulphur removal process step.

6. A process according to claim 1 , wherein the concentration of SO 2 in the feed gas is above 5% vol, and below 30% vol.

7. A process according to claim 1 in which at least 99% of the sulfur comprised in the feed gas is in oxidised form such as SO 2 or SO 3 or the corresponding acids.

8. A process according to claim 1 in which less than 50% of said desulphurized process gas is withdrawn as a recycle stream of desulphurized process gas.

9. A process according to claim 1 in which the volumetric concentration of oxygen in said process gas being directed to contact the catalytically active material is at least the same as the volumetric concentration of sulphur dioxide.

10. A process according to claim 1 in which the temperature of said recycled gas is above 200° C.

11. A process plant for desulphurisation of a feed gas said process plant comprising

a first bed and a second bed of catalytically active material,

a first condensation unit and a second condensation unit and a downstream desulphurisation plant, configured for recycling of a stream being withdrawn downstream said first condensation unit and optionally downstream the downstream desulphurisation unit, and being recycled to a process position upstream the first condensation unit, and optionally upstream said first bed of catalytically active material.

12. A process plant according to claim 11 , in which said downstream desulphurization unit is taken from the group consisting of (i) a process unit comprising a catalytic material for oxidation of SO 2 to SO 3 and a condenser for condensation of sulphuric acid, (ii) a mist filter for removal of sulphuric acid mist by collection of droplets and withdrawal of collected sulphuric acid droplets, (iii) an electrostatic precipitator, for collection of liquid sulphuric acid from sulphuric acid mist, and (iv) a scrubber for removal of sulphur oxides by absorption in an alkaline solution and/or an oxidative solution.

13. A process plant for desulphurisation of a feed gas according to claim 11 , configured for recycling of a stream being withdrawn downstream said first condensation unit, and upstream the downstream desulphurisation unit.

14. A process plant for production of sulphuric acid comprising a combustion unit for a sulphur rich material upstream a desulphurisation process plant according to claim 12 , in which the stream being recycled is directed to a position upstream or downstream said combustion unit.