PRODUCTION OF SULFURIC ACID EMPLOYING AN O2 RICH STREAM

Abstract

A process and a process plant for conversion of SO.sub.2 to H.sub.2SO.sub.4 including a. directing a process gas stream including at least 15 vol % SO.sub.2, and an amount of O.sub.2 originating from a source of purified O.sub.2 or O.sub.2 enriched air to contact a first material catalytically active in oxidation of SO.sub.2 to SO.sub.3 under oxidation conditions involving a maximum steady state temperature of the catalytically active material above 700 C., to provide an oxidized process gas stream, wherein the material catalytically active in oxidation of SO.sub.2 to SO.sub.3 includes an active phase in which the weight ration of vanadium to other metals is at least 2:1 supported on a porous carrier comprising at least 25 wt % crystalline silica, b. absorbing at least an amount of the produced SO.sub.3 in a stream of lean sulfuric acid to provide a stream of liquid sulfuric acid.

Claims

1. A process for conversion of SO.sub.2 to H.sub.2SO.sub.4 comprising the steps of a. directing a process gas stream comprising at least 15 vol % SO.sub.2, and an amount of O.sub.2 originating from a source of purified O.sub.2 or O.sub.2 enriched air to contact a first material catalytically active in oxidation of SO.sub.2 to SO.sub.3 under oxidation conditions involving a maximum steady state temperature of the catalytically active material above 700 C., to provide an oxidized process gas stream, wherein said material catalytically active in oxidation of SO.sub.2 to SO.sub.3 comprises an active phase in which the weight ration of vanadium to other metals is at least 2:1 supported on a porous carrier comprising at least 25 wt % crystalline silica, b. absorbing at least an amount of the produced SO.sub.3 in a stream of lean sulfuric acid to provide a stream of liquid sulfuric acid and optionally a desulfurized process gas stream.

2. A process according to claim 1, further comprising the step of recycling an amount of oxidized process gas or desulfurized process gas to contact said first material catalytically active in oxidation of SO.sub.2 to SO.sub.3.

3. A process according to claim 1, wherein oxidation conditions involve a pressure above 2 Barg.

4. A process according to claim 1, wherein oxidation conditions involve a pressure below 100 Barg.

5. A process according to claim 1, wherein less than 100 Nm.sup.3 process gas per ton sulfuric acid produced is released to the atmosphere.

6. A process according to claim 1, wherein the first material catalytically active in oxidation of SO.sub.2 to SO.sub.3 comprises vanadium pentoxide (V.sub.2O.sub.5), sulfur in the form of sulfate, pyrosulfate, tri- or tetrasulfate, one or more alkali metals on a porous carrier comprising at least 50 wt % crystalline silica.

7. A process according to claim 1, wherein an amount of the process gas stream is provided from an O.sub.2 enriched gas stream comprising at least 50 vol % O.sub.2.

8. A process according to claim 1, further comprising the step of directing an amount of elemental sulfur and the O.sub.2 enriched gas stream to a sulfur incinerator, to provide said process gas comprising SO.sub.2.

9. A process according to claim 1, wherein at least an amount of the O.sub.2 enriched gas stream is provided by electrolysis of H.sub.2O.

10. A process according to claim 9 wherein electrolysis of H.sub.2O is carried out in a process at a temperature above 400 C.

11. A process according to claim 1, wherein said process gas stream comprising at least 15 vol % SO.sub.2, originates from incineration of sulfur or sulfur recuperation from smelter operation.

12. A process according to claim 1, wherein an amount of the O2 is provided by separation of atmospheric air.

13. A process for co-production of NH.sub.3 and H.sub.2SO.sub.4 involving a process for production of H.sub.2SO.sub.4 according to claim 12, where the separation of atmospheric air further provides an N.sub.2 enriched gas stream which is directed to a plant for production of NH.sub.3, said process optionally involving the production of ammonium sulfate from NH.sub.3 and H.sub.2SO.sub.4.

14. A process according to claim 13, wherein heat is released during oxidation of SO.sub.2 to SO.sub.3 and directed to be used in NH.sub.3 production.

15. A process for production of fertilizer comprising ammonium and phosphate, comprising a process for co-production of NH.sub.3 and H.sub.2SO.sub.4 according to claim 13 and the process of producing phosphate from a phosphor source, employing the produced H.sub.2SO.sub.4.

16. A process plant for production of H.sub.2SO.sub.4 comprising a means for production of an O.sub.2 enriched stream having an O.sub.2 enriched stream outlet, an optional sulfur incinerator having at least one inlet and an outlet, a reactor containing a material catalytically active in SO.sub.2 oxidation at temperatures above 700 C. having an inlet and an outlet in fluid communication with the gas inlet of an absorber, having a liquid inlet for lean sulfuric acid, a liquid outlet for withdrawal of concentrated sulfuric acid and a gas outlet, wherein the O.sub.2 enriched stream outlet is in fluid communication with the inlet of the reactor, or if the optional sulfur incinerator is present, with an inlet to the sulfur incinerator, having the outlet in fluid communication with the inlet of the reactor.

Description

FIGURES

[0043] FIG. 1 illustrates a sulfuric acid plant according to the present disclosure.

[0044] FIG. 2 illustrates a sulfuric acid plant according to the prior art.

[0045] In FIG. 1, a process according to the present disclosure is shown. A stream of elemental sulfur (102) and a recycle stream (104) are directed to an incinerator (INC) also receiving an O.sub.2 enriched stream (108) and optionally also atmospheric air (110). The hot incinerated process gas (112) is cooled in a heat exchanger (HX1) which may be a waste heat boiler, connected to a steam circuit (not shown). The resulting process gas comprising SO.sub.2 (116) is directed to an SO.sub.2 converter (CONV), containing 4 beds of catalytically active material (B1-B4), with interbed cooling (not shown). The first bed (B1) and optionally the second bed (B2) will contain a heat stable SO.sub.2 oxidation catalyst, comprising V.sub.2O.sub.5 and at least partially crystalline silica, such as the proprietors product VK-HT. The following beds (B3 and B4) will contain regular SO.sub.2 oxidation catalyst, comprising V.sub.2O.sub.5 and a high surface non-crystalline silica, e.g. diatomaceous earth, such as the proprietors VK-38, VK-48 and VK-59. The oxidized process gas (120) is directed to cooling in a heat exchanger (HX2), and the cooled oxidized process gas (122) is directed to a sulfuric acid absorber tower (ABS), receiving weak sulfuric acid (126) and providing concentrated sulfuric acid (128). The desulfurized process gas is directed as recycle stream (104), optionally after withdrawal of amount of gas as purge (158).

[0046] In FIG. 2, a process according to the prior art is shown. Dried air is provided by directing a stream of atmospheric air (205) to a drying column (DRY) receiving concentrated sulfuric acid (206) and providing a weaker sulfuric acid (207) having captured water in the atmospheric air (205) to provide a stream of dried atmospheric air (210) which is combined with a stream of elemental sulfur (202) are directed to an incinerator (INC). The hot incinerated process gas (212) is cooled in a heat exchanger (HX1) which may be a waste heat boiler, connected to a steam circuit (not shown). The resulting process gas comprising SO.sub.2 (216) is directed to an SO.sub.2 converter (CONV), containing 5 beds of catalytically active material (B1-B5), with interbed cooling (not shown). The first three beds (B1-B3) constitute a first stage and contains regular SO.sub.2 oxidation catalyst, comprising V.sub.2O.sub.5 and at non-crystalline silica, e.g. diatomaceous earth, such as the proprietors VK-38, VK-48 and VK-59, and provides a first stage oxidized process gas (220). The first stage oxidized process gas (220) is directed to cooling in a heat exchanger (HX2), and the cooled oxidized process gas (222) is directed to a first sulfuric acid absorber tower (ABS1), receiving weak sulfuric acid (226) and providing concentrated sulfuric acid (228). The first stage desulfurized process gas (242) is directed as feed stream (244) for the second stage, constituted by bed 4 (B4) and bed 5 (B5). The final oxidized process gas (246) is cooled (HX4) and directed to a second sulfuric acid absorber tower (ABS2), receiving weak sulfuric acid (254) and providing concentrated sulfuric acid (256). The final desulfurized process gas (258) is directed to be released to the environment via a stack (STACK).

EXAMPLES

[0047] Two examples are presented, for comparison of traditional operation with operation according to the present disclosure.

[0048] In both examples 27 t/h sulfur is directed to the process which provides 83 t/h sulfuric acid.

[0049] Table 1 shows an example corresponding to FIG. 1, according to the present disclosure. This example assumes that 100% pure oxygen is used for incineration and SO.sub.2 oxidation, and that SO.sub.2 oxidation is carried out in 4 beds, of which 2 have a temperatures of 740 C. and 640 C., thus exceeding the common limit around 630 C. Concentrations are shown with reference to Figure captions (with B1,B2,B3 and B4 referring to the outlet of the beds of catalytically active material) in volume %, and total gas flows in Nm.sup.3/h, and the flow of sulfur in t/h.

[0050] According to the example a purge is not carried out, but a presence of nitrogen is assumed. If the oxygen source is air separation, the oxygen enriched gas may comprise an amount of nitrogen, which increases with recycle. In this case a small purge is necessary. In practice the place of the nitrogen may be taken by recycled SO.sub.3, but for computational convenience a presence of nitrogen as diluent is assumed.

[0051] Depending on the impurities (which in addition to nitrogen, also may include CO.sub.2 and H.sub.2O from combustion of hydrocarbon impurities in the sulfur) of a small purge may be required. Assuming an impurity corresponding to 7% of the O.sub.2, the purge will be 3270 Nm.sup.3/h (12% of the recycle), and assuming 0.5%, the purge will be 234 Nm.sup.3/h (0.85% of the recycle), which is 40 Nm.sup.3/t sulfuric acid and 2.8 Nm.sup.3/t sulfuric acid respectively. The purged stream must be purified by scrubbing or other means, as it will contain some SO.sub.2 and SO.sub.3.

[0052] The total amount of catalyst is 206 m.sup.3, and due to the temperature the catalyst of beds 1 and 2 is of the type V.sub.2O.sub.5 sulfate on crystalline silica whereas the rest is V.sub.2O.sub.5 sulfate on diatomaceous earth.

[0053] The process pressure is 10 bar, and thus the volume of the incinerator, heat exchangers and the absorber may be reduced, but a pressure shell must be provided.

[0054] For comparison, Table 2 shows an example corresponding to FIG. 2, according to the prior art. This example assumes that atmospheric air is used for incineration and SO.sub.2 oxidation, and that SO.sub.2 oxidation is carried out in a so-called 3+2 dual converter and dual absorber (DCDA) configuration, with 3 beds in the first converter and 2 beds in the second. None of the beds exceed the common limit of 630 C. Concentrations are shown with reference to Figure captions (with B1,B2,B3, B4 and B5) referring to the outlet of the beds of catalytically active material) in vo Western Australia, [0055] lume %, and total gas flows in Nm.sup.3/h, and the flow of sulfur in t/h.

[0056] The total amount of catalyst is 405 m.sup.3, and all catalyst is of the type of V.sub.2O.sub.5 on non-crystalline silica, such as diatomaceous earth.

[0057] The process pressure is 1.3 bar, and thus the need for a pressure shell is avoided. The volume of purified process gas released to the environment is 137,217 Nm.sup.3/h, which is 1658 Nm.sup.3/t sulfuric acid.

[0058] When comparing the two examples, the use of a heat stable catalytically active material enables use of pure oxygen as oxidant. The result is a reduction of catalyst volume to almost half, due to a combination of increased reaction rate due to increased temperature, increased SO.sub.2 partial pressure and an acceptable lower conversion, due to the recycle. Furthermore, the use of pure oxygen reduces the volume of process gas released to the environment by 99.8%, which also is relevant for the size of the stack used in the plant.

[0059] As the temperatures of the process with pure oxygen are higher, the value of heat integration to other plants will also increase. This will be beneficial for ammonia production or methanol, and generally if the pure oxygen is obtained from a process plant, where hydrogen is produced electrolytically by high temperature electrolysis in a solid oxide electrolyzer.

TABLE-US-00001 TABLE 1 102 108 116 B1 B2 B3 B4 104 158 Temperature 380 C. 740 C. 640 C. 560 C. 480 C. S 100% O.sub.2 99.5% 35% 30% 26% 23% 22% 35% 35% SO.sub.2 0% 35% 21% 11% 5% 1% 2% 2% SO.sub.3 0% 0% 17% 29% 37% 41% 4% 4% N.sub.2 0.5% 30% 33% 34% 36% 36% 59% 59% Total flow 26.97 28,377 56,127 51,760 49,087 47,396 46,605 28,660 234 [t/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h]

TABLE-US-00002 TABLE 2 202 210 216 B1 B2 B3 242 B4 B5 258 Temperature 425 C. 620 C. 520 C. 470 C. 430 C. 380 C. S 100% O.sub.2 21% 9% 6% 5% 4% 5% 4% 4% 4% SO.sub.2 0% 12% 5% 2% 1% 1% 0% 0% 0% SO.sub.3 0% 0% 7% 10% 11% 0% 1% 0% 0% N.sub.2 79% 79% 82% 83% 84% 94% 95% 95% 95% Total flow 27.25 165,670 165,670 159,764 157,454 156,692 138,756 138,224 137,217 137,217 [t/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h] [Nm.sup.3/h]